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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | BUGS | FOOTNOTES | SEE ALSO | AUTHOR | COPYRIGHT | COLOPHON |
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GCC(1) GNU GCC(1)
gcc - GNU project C and C++ compiler
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop
this process at an intermediate stage. For example, the -c
option says not to run the linker. Then the output consists of
object files output by the assembler.
Other options are passed on to one or more stages of processing.
Some options control the preprocessor and others the compiler
itself. Yet other options control the assembler and linker; most
of these are not documented here, since you rarely need to use
any of them.
Most of the command-line options that you can use with GCC are
useful for C programs; when an option is only useful with another
language (usually C++), the explanation says so explicitly. If
the description for a particular option does not mention a source
language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or machine-gcc-version to run a
specific version of GCC. When you compile C++ programs, you
should invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the
order you use doesn't matter. Order does matter when you use
several options of the same kind; for example, if you specify -L
more than once, the directories are searched in the order
specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for
example, -fmove-loop-invariants, -Wformat and so on. Most of
these have both positive and negative forms; the negative form of
-ffoo is -fno-foo. This manual documents only one of these two
forms, whichever one is not the default.
Some options take one or more arguments typically separated
either by a space or by the equals sign (=) from the option name.
Unless documented otherwise, an argument can be either numeric or
a string. Numeric arguments must typically be small unsigned
decimal or hexadecimal integers. Hexadecimal arguments must
begin with the 0x prefix. Arguments to options that specify a
size threshold of some sort may be arbitrarily large decimal or
hexadecimal integers followed by a byte size suffix designating a
multiple of bytes such as "kB" and "KiB" for kilobyte and
kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte,
"GB" and "GiB" for gigabyte and gigibyte, and so on. Such
arguments are designated by byte-size in the following text.
Refer to the NIST, IEC, and other relevant national and
international standards for the full listing and explanation of
the binary and decimal byte size prefixes.
Option Summary
Here is a summary of all the options, grouped by type.
Explanations are in the following sections.
Overall Options
-c -S -E -o file -x language -v -###
--help[=class[,...]] --target-help --version
-pass-exit-codes -pipe -specs=file -wrapper @file
-ffile-prefix-map=old=new -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline
-fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm
-fno-builtin -fno-builtin-function -fgimple -fhosted
-ffreestanding -fopenacc -fopenacc-dim=geom -fopenmp
-fopenmp-simd -fms-extensions -fplan9-extensions
-fsso-struct=endianness -fallow-single-precision
-fcond-mismatch -flax-vector-conversions -fsigned-bitfields
-fsigned-char -funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fchar8_t -fcheck-new -fconstexpr-depth=n
-fconstexpr-loop-limit=n -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs
-fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive -fno-pretty-templates
-frepo -fno-rtti -fsized-deallocation
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak
-nostdinc++ -fvisibility-inlines-hidden
-fvisibility-ms-compat -fext-numeric-literals -Wabi=n
-Wabi-tag -Wconversion-null -Wctor-dtor-privacy
-Wdelete-non-virtual-dtor -Wdeprecated-copy
-Wdeprecated-copy-dtor -Wliteral-suffix
-Wmultiple-inheritance -Wno-init-list-lifetime -Wnamespaces
-Wnarrowing -Wpessimizing-move -Wredundant-move -Wnoexcept
-Wnoexcept-type -Wclass-memaccess -Wnon-virtual-dtor
-Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel
-Wtemplates -Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions
-Wno-class-conversion -Wno-terminate -Wsign-promo
-Wvirtual-inheritance
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime
-fnext-runtime -fno-nil-receivers -fobjc-abi-version=n
-fobjc-call-cxx-cdtors -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1
-fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls
-Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-
line] -fdiagnostics-color=[auto|never|always]
-fdiagnostics-format=[text|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w
-Wextra -Wall -Waddress -Waddress-of-packed-member
-Waggregate-return -Waligned-new -Walloc-zero
-Walloc-size-larger-than=byte-size -Walloca
-Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warray-bounds
-Warray-bounds=n -Wno-attributes -Wattribute-alias=n
-Wbool-compare -Wbool-operation
-Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat
-Wc99-c11-compat -Wc11-c2x-compat -Wc++-compat
-Wc++11-compat -Wc++14-compat -Wc++17-compat -Wcast-align
-Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts -Wcatch-value -Wcatch-value=n -Wclobbered
-Wcomment -Wconditionally-supported -Wconversion
-Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time
-Wdelete-incomplete -Wno-attribute-warning -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-qualifiers
-Wno-discarded-array-qualifiers -Wno-div-by-zero
-Wdouble-promotion -Wduplicated-branches -Wduplicated-cond
-Wempty-body -Wenum-compare -Wno-endif-labels
-Wexpansion-to-defined -Werror -Werror=* -Wextra-semi
-Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-overflow=n -Wformat-security
-Wformat-signedness -Wformat-truncation=n -Wformat-y2k
-Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wjump-misses-init -Whsa
-Wif-not-aligned -Wignored-qualifiers -Wignored-attributes
-Wincompatible-pointer-types -Wimplicit
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wimplicit-function-declaration -Wimplicit-int -Winit-self
-Winline -Wno-int-conversion -Wint-in-bool-context
-Wno-int-to-pointer-cast -Winvalid-memory-model
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=byte-size
-Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain
-Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-attributes -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wmissing-profile
-Wno-multichar -Wmultistatement-macros -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd
-Woverride-init-side-effects -Woverlength-strings -Wpacked
-Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wplacement-new
-Wplacement-new=n -Wpointer-arith -Wpointer-compare
-Wno-pointer-to-int-cast -Wno-pragmas -Wno-prio-ctor-dtor
-Wredundant-decls -Wrestrict -Wno-return-local-addr
-Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar
-Wshadow=global, -Wshadow=local, -Wshadow=compatible-local
-Wshift-overflow -Wshift-overflow=n -Wshift-count-negative
-Wshift-count-overflow -Wshift-negative-value -Wsign-compare
-Wsign-conversion -Wfloat-conversion
-Wno-scalar-storage-order -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wsizeof-array-argument
-Wstack-protector -Wstack-usage=byte-size -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wstringop-overflow=n -Wstringop-truncation
-Wsubobject-linkage
-Wsuggest-attribute=[pure|const|noreturn|format|malloc]
-Wsuggest-final-types -Wsuggest-final-methods
-Wsuggest-override -Wswitch -Wswitch-bool -Wswitch-default
-Wswitch-enum -Wswitch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines
-Wtrigraphs -Wtype-limits -Wundef -Wuninitialized
-Wunknown-pragmas -Wunsuffixed-float-constants -Wunused
-Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-macros -Wunused-parameter -Wno-unused-result
-Wunused-value -Wunused-variable -Wunused-const-variable
-Wunused-const-variable=n -Wunused-but-set-parameter
-Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-
size -Wvolatile-register-var -Wwrite-strings
-Wzero-as-null-pointer-constant
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes
-Wnested-externs -Wold-style-declaration
-Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement
-Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstabs
-gstabs+ -gstrict-dwarf -gno-strict-dwarf -gas-loc-support
-gno-as-loc-support -gas-locview-support
-gno-as-locview-support -gcolumn-info -gno-column-info
-gstatement-frontiers -gno-statement-frontiers
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views
-gno-internal-reset-location-views -ginline-points
-gno-inline-points -gvms -gxcoff -gxcoff+ -gz[=type]
-gsplit-dwarf -gdescribe-dies -gno-describe-dies
-fdebug-prefix-map=old=new -fdebug-types-section
-fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-feliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations
-falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]]
-falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fassociative-math
-fauto-profile -fauto-profile[=path] -fauto-inc-dec
-fbranch-probabilities -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcombine-stack-adjustments -fconserve-stack
-fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules
-fcx-limited-range -fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only
-ffloat-store -fexcess-precision=style -fforward-propagate
-ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm
-fgraphite-identity -fgcse-sm -fhoist-adjacent-loads
-fif-conversion -fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once
-finline-limit=n -finline-small-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fipa-stack-alignment -fipa-icf
-fira-algorithm=algorithm -flive-patching=level
-fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto
-flto-compression-level -flto-partition=alg
-fmerge-all-constants -fmerge-constants -fmodulo-sched
-fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fno-branch-count-reg -fno-defer-pop
-fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -fno-printf-return-value
-fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays
-fprofile-correction -fprofile-use -fprofile-use=path
-fprofile-values -fprofile-reorder-functions
-freciprocal-math -free -frename-registers
-freorder-blocks -freorder-blocks-algorithm=algorithm
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsave-optimization-record
-fsched2-use-superblocks -fsched-pressure -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n]
-fsched-stalled-insns[=n] -fsched-group-heuristic
-fsched-critical-path-heuristic -fsched-spec-insn-heuristic
-fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-fusion
-fschedule-insns -fschedule-insns2 -fsection-anchors
-fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap
-fshrink-wrap-separate -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-loops -fsplit-paths -fsplit-wide-types
-fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging
-fstrict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-fcode-hoisting -ftree-loop-if-convert -ftree-loop-im
-ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops=n -ftree-pre
-ftree-partial-pre -ftree-pta -ftree-reassoc
-ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra
-ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp -funconstrained-commons
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt
-fweb -fwhole-program -fwpa -fuse-linker-plugin --param
name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-abs-path -fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fprofile-update=method
-fprofile-filter-files=regex -fprofile-exclude-files=regex
-fsanitize=style -fsanitize-recover
-fsanitize-recover=style -fasan-shadow-offset=number
-fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcf-protection=[full|branch|return|none] -fstack-protector
-fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC
-Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers
-fexec-charset=charset -fextended-identifiers
-finput-charset=charset -fmacro-prefix-map=old=new
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros
file -include file -M -MD -MF -MG -MM -MMD -MP -MQ
-MT -no-integrated-cpp -P -pthread -remap -traditional
-traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles
-nodefaultlibs -nolibc -nostdlib -e entry --entry=entry
-pie -pthread -r -rdynamic -s -static -static-pie
-static-libgcc -static-libstdc++ -static-libasan
-static-libtsan -static-liblsan -static-libubsan -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker
option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib
dir -iplugindir=dir -iprefix file -iquote dir -isysroot dir
-isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir
-no-canonical-prefixes --no-sysroot-suffix -nostdinc
-nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions
-funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt
-fno-jump-tables -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fleading-underscore -ftls-model=model
-fstack-reuse=reuse_level -ftrampolines -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion
-dumpfullversion -fchecking -fchecking=n -fdbg-cnt-list
-fdbg-cnt=counter-value-list -fdisable-ipa-pass_name
-fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-debug -fdump-earlydebug -fdump-noaddr
-fdump-unnumbered -fdump-unnumbered-links
-fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph
-fdump-ipa-inline -fdump-lang-all -fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename -fdump-passes
-fdump-rtl-pass -fdump-rtl-pass=filename -fdump-statistics
-fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options
-fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass
-fenable-kind-pass=range-list -fira-verbose=n -flto-report
-flto-report-wpa -fmem-report-wpa -fmem-report
-fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info
-fopt-info-options[=file] -fprofile-report
-frandom-seed=string -fsched-verbose=n -fsel-sched-verbose
-fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats
-fstack-usage -ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps -save-temps=cwd
-save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian
-mgeneral-regs-only -mcmodel=tiny -mcmodel=small
-mcmodel=large -mstrict-align -mno-strict-align
-momit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size
-mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt
-mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope
-mbranch-protection=none|standard|pac-ret[+leaf]|bti
-mharden-sls=opts -march=name -mcpu=name -mtune=name
-moverride=string -mverbose-cost-dump
-mstack-protector-guard=guard
-mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation
-moutline-atomics
Adapteva Epiphany Options -mhalf-reg-file
-mprefer-short-insn-regs -mbranch-cost=num -mcmove
-mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc
-mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode
-mvect-double -max-vect-align=num -msplit-vecmove-early
-m1reg-reg
AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes
ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6
-mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic
-mnorm -mspfp -mspfp-compact -mspfp-fast -msimd
-msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock
-mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy
-misize -mannotate-align -marclinux -marclinux_prof
-mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved
-mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache
-mtp-regno=regno -malign-call -mauto-modify-reg
-mbbit-peephole -mno-brcc -mcase-vector-pcrel
-mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact mlra-priority-noncompact -mmillicode
-mmixed-code -mq-class -mRcq -mRcw -msize-level=level
-mtune=cpu -mmultcost=num -mcode-density-frame
-munalign-prob-threshold=probability -mmpy-option=multo
-mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16
-mbranch-index
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant
-mno-apcs-reentrant -mgeneral-regs-only -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name
-mtune=name -mprint-tune-info -mstructure-size-boundary=n
-mabort-on-noreturn -mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mpoke-function-name -mthumb -marm
-mflip-thumb -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations
-mfix-cortex-m3-ldrd -munaligned-access -mneon-for-64bits
-mslow-flash-data -masm-syntax-unified -mrestrict-it
-mverbose-cost-dump -mpure-code -mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mgas-isr-prologues
-mint8 -mn_flash=size -mno-interrupts -mmain-is-OS_task
-mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -mshort-calls -nodevicelib
-nodevicespecs -Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library
-mshared-library-id=n -mleaf-id-shared-library
-mno-leaf-id-shared-library -msep-data -mno-sep-data
-mlong-calls -mno-long-calls -mfast-fp -minline-plt
-mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align -m32-bit -m16-bit
-m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout
-melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32
-mbit-ops -mdata-model=model
C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB
-mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu
-mdouble-float -mfdivdu -melrw -mistack -mmp -mcp
-mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv
-msmart -mhigh-registers -manchor -mpushpop
-mmultiple-stld -mconstpool -mstack-size -mccrt
-mbranch-cost=n -mcse-cc -msched-prolog
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign -sectobjectsymbols -whyload
-seg1addr -sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr
-seg_addr_table -seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches
-whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode
-mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx
-mmax -mfix -mcix -mfloat-vax -mfloat-ieee
-mexplicit-relocs -msmall-data -mlarge-data -msmall-text
-mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress
-mnopm
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword
-mno-dword -mdouble -mno-double -mmedia -mno-media
-mmuladd -mno-muladd -mfdpic -minline-plt -mgprel-ro
-multilib-library-pic -mlinked-fp -mlong-calls
-malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack
-mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic
-mandroid -tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr
-mint32 -malign-300
HPPA Options -march=architecture-type -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls
-mgas -mgnu-ld -mhp-ld -mfixed-range=register-range
-mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store
-mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1
-mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type
-mspace-regs -msio -mwsio -munix=unix-std -nolibdld
-static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as
-mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names
-msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-
type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec
-msched-br-in-data-spec -msched-ar-in-data-spec
-msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type
-mno-flush-func -mflush-func=name -mno-flush-trap
-mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000
-m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e
-mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield
-mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short
-mhard-float -m68881 -msoft-float -mpcrel -malign-int
-mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library
-mno-id-shared-library -mxgot -mno-xgot
-mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian
-m210 -m340 -mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n
-mbitops -mc=n -mclip -mconfig=name -mcop -mcop32
-mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml
-mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms
-msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float
-msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul
-mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare
-mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-
model -mpic-data-is-text-relative
MIPS Options -EL -EB -march=arch -mtune=arch -mips1
-mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3
-mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5
-mips64r6 -mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed
-minterlink-mips16 -mno-interlink-mips16 -mabi=abi
-mabicalls -mno-abicalls -mshared -mno-shared -mplt
-mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx
-mfp64 -mhard-float -msoft-float -mno-float -msingle-float
-mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode
-mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu
-mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa
-mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips
-mmsa -mno-msa -mloongson-mmi -mno-loongson-mmi
-mloongson-ext -mno-loongson-ext -mloongson-ext2
-mno-loongson-ext2 -mfpu=fpu-type -msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata
-mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt
-mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable=setting -msplit-addresses
-mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division
-mdivide-traps -mdivide-breaks -mload-store-pairs
-mno-load-store-pairs -mmemcpy -mno-memcpy -mlong-calls
-mno-long-calls -mmad -mno-mad -mimadd -mno-imadd
-mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000
-mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely -mcompact-branches=policy
-mfp-exceptions -mno-fp-exceptions -mvr4130-align
-mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1
-mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls
-mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt
-mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon
-mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf -mbranch-predict
-mno-branch-predict -mbase-addresses -mno-base-addresses
-msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33
-mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge
-msmall -mrelax -mwarn-mcu -mcode-region= -mdata-region=
-msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf
-mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string
-mv3push -mno-v3push -m16bit -mno-16bit
-misr-vector-size=num -mcache-block-size=num -march=arch
-mcmodel=code-model -mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt
-mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb
-mno-bypass-cache -mbypass-cache -mno-cache-volatile
-mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul
-mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div
-mcustom-insn=N -mno-custom-insn -mcustom-fpu-cfg=name -mhal
-msmallc -msys-crt0=name -msys-lib=name -march=arch -mbmx
-mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
OpenRISC Options -mboard=name -mnewlib -mhard-mul
-mhard-div -msoft-mul -msoft-div -mcmov -mror -msext
-msfimm -mshftimm
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40
-m45 -m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit
-munix-asm -mdec-asm -mgnu-asm -mlra
picoChip Options -mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt
-mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div
-march=ISA-string -mtune=processor-string
-mpreferred-stack-boundary=num -msmall-data-limit=N-bytes
-msave-restore -mno-save-restore -mstrict-align
-mno-strict-align -mcmodel=medlow -mcmodel=medany
-mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
-mriscv-attribute -mmo-riscv-attribute
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14
-mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13
-mg14 -m64bit-doubles -m32bit-doubles
-msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb
-mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd
-mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp
-mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig
-mbig-endian -mdynamic-no-pic -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type
-maix-struct-return -msvr4-struct-return -mabi=abi-type
-msecure-plt -mbss-plt -mlongcall -mno-longcall -mpltseq
-mno-pltseq -mblock-move-inline-limit=num
-mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb
-mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt
-mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt
-mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto
-mhtm -mno-htm -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic
-mcompat-align-parm -mno-compat-align-parm -mfloat128
-mno-float128 -mfloat128-hardware -mno-float128-hardware
-mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu
-mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data
-msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax
-mmax-constant-size= -mint-register= -mpid
-mallow-string-insns -mno-allow-string-insns -mjsr
-mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain
-mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31
-mdebug -mno-debug -mesa -mzarch -mhtm -mvx -mzvector
-mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard -mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5
-mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single
-m4a -m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd
-mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee
-mbitops -misize -minline-ic_invalidate -mpadstruct
-mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range
-maccumulate-outgoing-args -matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mno-zdcbranch
-mcbranch-force-delay-slot -mfused-madd -mno-fused-madd
-mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap
-mimpure-text -mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-
model -mmemory-model=mem-model -m32 -m64 -mapp-regs
-mno-app-regs -mfaster-structs -mno-faster-structs -mflat
-mno-flat -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mstack-bias
-mno-stack-bias -mstd-struct-return -mno-std-struct-return
-munaligned-doubles -mno-unaligned-doubles -muser-mode
-mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2
-mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b
-mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld
-mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc
-mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra
-mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma
-munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem
-mstdmain -mfixed-range=register-range -mea32 -mea64
-maddress-space-conversion -mno-address-space-conversion
-mcache-size=cache-size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian
-mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n
-msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es
-mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps
-msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode
-muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type
-mtune-ctrl=feature-list -mdump-tune-features -mno-default
-mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float
-mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num
-mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt
-mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt
-mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2
-msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er
-mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma
-mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd
-mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mprefetchwt1
-mclflushopt -mclwb -mxsavec -mxsaves -msse4a -m3dnow
-m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx
-mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle
-mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes
-mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call
-mavx512vbmi2 -mvpclmulqdq -mavx512bitalg -mmovdiri
-mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni
-mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx -mcldemote
-mms-bitfields -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm
-mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80
-mstackrealign -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name
-maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu
-mlarge-data-threshold=num -msse2avx -mfentry
-mrecord-mcount -mnop-mcount -m8bit-idiv
-minstrument-return=type -mfentry-name=name
-mfentry-section=name -mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing,
compilation proper, assembly and linking, always in that order.
GCC is capable of preprocessing and compiling several files
either into several assembler input files, or into one assembler
input file; then each assembler input file produces an object
file, and linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what
kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the
libobjc library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note
that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned
into a precompiled header (default), or C, C++ header file to
be turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in
.cxx, the last two letters must both be literally x.
Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada
spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be
preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed
(with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be
preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with
the traditional preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.d
D source code.
file.di
D interface file.
file.dd
D documentation code (Ddoc).
file.ads
Ada source code file that contains a library unit declaration
(a declaration of a package, subprogram, or generic, or a
generic instantiation), or a library unit renaming
declaration (a package, generic, or subprogram renaming
declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a
subprogram or package body). Such files are also called
bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file
name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on
the file name suffix). This option applies to all following
input files until the next -x option. Possible values for
language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that subsequent
files are handled according to their file name suffixes (as
they are if -x has not been used at all).
If you only want some of the stages of compilation, you can use
-x (or filename suffixes) to tell gcc where to start, and one of
the options -c, -S, or -E to say where gcc is to stop. Note that
some combinations (for example, -x cpp-output -E) instruct gcc to
do nothing at all.
-c Compile or assemble the source files, but do not link. The
linking stage simply is not done. The ultimate output is in
the form of an object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or
assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble.
The output is in the form of an assembler code file for each
non-assembler input file specified.
By default, the assembler file name for a source file is made
by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler
proper. The output is in the form of preprocessed source
code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of
output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable
file in a.out, the object file for source.suffix in source.o,
its assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on standard
output.
-v Print (on standard error output) the commands executed to run
the stages of compilation. Also print the version number of
the compiler driver program and of the preprocessor and the
compiler proper.
-###
Like -v except the commands are not executed and arguments
are quoted unless they contain only alphanumeric characters
or "./-_". This is useful for shell scripts to capture the
driver-generated command lines.
--help
Print (on the standard output) a description of the command-
line options understood by gcc. If the -v option is also
specified then --help is also passed on to the various
processes invoked by gcc, so that they can display the
command-line options they accept. If the -Wextra option has
also been specified (prior to the --help option), then
command-line options that have no documentation associated
with them are also displayed.
--target-help
Print (on the standard output) a description of target-
specific command-line options for each tool. For some
targets extra target-specific information may also be
printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-
line options understood by the compiler that fit into all
specified classes and qualifiers. These are the supported
classes:
optimizers
Display all of the optimization options supported by the
compiler.
warnings
Display all of the options controlling warning messages
produced by the compiler.
target
Display target-specific options. Unlike the
--target-help option however, target-specific options of
the linker and assembler are not displayed. This is
because those tools do not currently support the extended
--help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where
language is the name of one of the languages supported in
this version of GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an
equal sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a
separate word following the original option, such as: -o
output-file.
Thus for example to display all the undocumented target-
specific switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with
the ^ character, so for example to display all binary warning
options (i.e., ones that are either on or off and that do not
take an argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to
display. One case where it does work, however, is when one
of the classes is target. For example, to display all the
target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options,
skipping those that have already been displayed. If --help
is also specified anywhere on the command line then this
takes precedence over any --help= option.
If the -Q option appears on the command line before the
--help= option, then the descriptive text displayed by
--help= is changed. Instead of describing the displayed
options, an indication is given as to whether the option is
enabled, disabled or set to a specific value (assuming that
the compiler knows this at the point where the --help= option
is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-
line options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are
enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any
phase of the compiler returns a non-success return code. If
you specify -pass-exit-codes, the gcc program instead returns
with the numerically highest error produced by any phase
returning an error indication. The C, C++, and Fortran front
ends return 4 if an internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication
between the various stages of compilation. This fails to
work on some systems where the assembler is unable to read
from a pipe; but the GNU assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs
file, in order to override the defaults which the gcc driver
program uses when determining what switches to pass to cc1,
cc1plus, as, ld, etc. More than one -specs=file can be
specified on the command line, and they are processed in
order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The name of
the wrapper program and its parameters are passed as a comma
separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus
the invocation of cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any
references to them in the result of the compilation as if the
files resided in directory new instead. Specifying this
option is equivalent to specifying all the individual
-f*-prefix-map options. This can be used to make
reproducible builds that are location independent. See also
-fmacro-prefix-map and -fdebug-prefix-map.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared
object to be dlopen'd by the compiler. The base name of the
shared object file is used to identify the plugin for the
purposes of argument parsing (See -fplugin-arg-name-key=value
below). Each plugin should define the callback functions
specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the
plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate
corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate
Ada specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type",
"var", and "func" declarations which may be a useful way to
start writing a Go interface to code written in some other
language.
@file
Read command-line options from file. The options read are
inserted in place of the original @file option. If file does
not exist, or cannot be read, then the option will be treated
literally, and not removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the
entire option in either single or double quotes. Any
character (including a backslash) may be included by
prefixing the character to be included with a backslash. The
file may itself contain additional @file options; any such
options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc,
.cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh,
.hpp, .H, or (for shared template code) .tcc; and preprocessed
C++ files use the suffix .ii. GCC recognizes files with these
names and compiles them as C++ programs even if you call the
compiler the same way as for compiling C programs (usually with
the name gcc).
However, the use of gcc does not add the C++ library. g++ is a
program that calls GCC and automatically specifies linking
against the C++ library. It treats .c, .h and .i files as C++
source files instead of C source files unless -x is used. This
program is also useful when precompiling a C header file with a
.h extension for use in C++ compilations. On many systems, g++
is also installed with the name c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages
derived from C, such as C++, Objective-C and Objective-C++) that
the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible
with ISO C90 (when compiling C code), or of standard C++
(when compiling C++ code), such as the "asm" and "typeof"
keywords, and predefined macros such as "unix" and "vax" that
identify the type of system you are using. It also enables
the undesirable and rarely used ISO trigraph feature. For
the C compiler, it disables recognition of C++ style //
comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" continue to work despite -ansi.
You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that
might be included in compilations done with -ansi. Alternate
predefined macros such as "__unix__" and "__vax__" are also
available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be
rejected gratuitously. For that, -Wpedantic is required in
addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi
option is used. Some header files may notice this macro and
refrain from declaring certain functions or defining certain
macros that the ISO standard doesn't call for; this is to
avoid interfering with any programs that might use these
names for other things.
Functions that are normally built in but do not have
semantics defined by ISO C (such as "alloca" and "ffs") are
not built-in functions when -ansi is used.
-std=
Determine the language standard. This option is currently
only supported when compiling C or C++.
The compiler can accept several base standards, such as c90
or c++98, and GNU dialects of those standards, such as gnu90
or gnu++98. When a base standard is specified, the compiler
accepts all programs following that standard plus those using
GNU extensions that do not contradict it. For example,
-std=c90 turns off certain features of GCC that are
incompatible with ISO C90, such as the "asm" and "typeof"
keywords, but not other GNU extensions that do not have a
meaning in ISO C90, such as omitting the middle term of a
"?:" expression. On the other hand, when a GNU dialect of a
standard is specified, all features supported by the compiler
are enabled, even when those features change the meaning of
the base standard. As a result, some strict-conforming
programs may be rejected. The particular standard is used by
-Wpedantic to identify which features are GNU extensions
given that version of the standard. For example -std=gnu90
-Wpedantic warns about C++ style // comments, while
-std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C
code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely
supported, modulo bugs and floating-point issues (mainly
but not entirely relating to optional C99 features from
Annexes F and G). See
<http://gcc.gnu.org/c99status.html > for more information.
The names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo
bugs, floating-point issues (mainly but not entirely
relating to optional C11 features from Annexes F and G)
and the optional Annexes K (Bounds-checking interfaces)
and L (Analyzability). The name c1x is deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard
(published in 2018). This standard is same as C11 except
for corrections of defects (all of which are also applied
with -std=c11) and a new value of "__STDC_VERSION__", and
so is supported to the same extent as C11.
c2x The next version of the ISO C standard, still under
development. The support for this version is
experimental and incomplete.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17. This is the default for C code.
gnu2x
The next version of the ISO C standard, still under
development, plus GNU extensions. The support for this
version is experimental and incomplete.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical
corrigendum and some additional defect reports. Same as
-ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name
c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is
deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name
c++1y is deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for C++
code. The name gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name
c++1z is deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. The name gnu++1z is
deprecated.
c++2a
The next revision of the ISO C++ standard, tentatively
planned for 2020. Support is highly experimental, and
will almost certainly change in incompatible ways in
future releases.
gnu++2a
GNU dialect of -std=c++2a. Support is highly
experimental, and will almost certainly change in
incompatible ways in future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional
GNU semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the
C99 semantics for "inline" when in C99 or gnu99 mode (i.e.,
it specifies the default behavior). This option is not
supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics
are in effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that operations and constants
with a semantic type that is an interchange or extended
format should be evaluated to the precision and range of that
type. These new values are a superset of those permitted
under C99/C11, which does not specify the meaning of other
positive values of "FLT_EVAL_METHOD". As such, code
conforming to C11 may not have been written expecting the
possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler
should allow only the values of "FLT_EVAL_METHOD" specified
in C99/C11, or the extended set of values specified in
ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or
similar) is -fpermitted-flt-eval-methods=c11. The default
when in a GNU dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit,
including those in header files. This option is silently
ignored in any language other than C.
Besides declarations, the file indicates, in comments, the
origin of each declaration (source file and line), whether
the declaration was implicit, prototyped or unprototyped (I,
N for new or O for old, respectively, in the first character
after the line number and the colon), and whether it came
from a declaration or a definition (C or F, respectively, in
the following character). In the case of function
definitions, a K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is
not very useful as it is not possible to read the arguments.
This is only supported for C as this construct is allowed by
C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so
that code can use these words as identifiers. You can use
the keywords "__asm__", "__inline__" and "__typeof__"
instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want to
use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode (-std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since "inline"
is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-
in functions more efficiently; for instance, calls to
"alloca" may become single instructions which adjust the
stack directly, and calls to "memcpy" may become inline copy
loops. The resulting code is often both smaller and faster,
but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change
the behavior of the functions by linking with a different
library. In addition, when a function is recognized as a
built-in function, GCC may use information about that
function to warn about problems with calls to that function,
or to generate more efficient code, even if the resulting
code still contains calls to that function. For example,
warnings are given with -Wformat for bad calls to "printf"
when "printf" is built in and "strlen" is known not to modify
global memory.
With the -fno-builtin-function option only the built-in
function function is disabled. function must not begin with
__builtin_. If a function is named that is not built-in in
this version of GCC, this option is ignored. There is no
corresponding -fbuiltin-function option; if you wish to
enable built-in functions selectively when using -fno-builtin
or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with
"__GIMPLE". This is an experimental feature that allows unit
testing of GIMPLE passes.
-fhosted
Assert that compilation targets a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the
entire standard library is available, and in which "main" has
a return type of "int". Examples are nearly everything
except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment.
This implies -fno-builtin. A freestanding environment is one
in which the standard library may not exist, and program
startup may not necessarily be at "main". The most obvious
example is an OS kernel. This is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++
and "!$acc" in Fortran. When -fopenacc is specified, the
compiler generates accelerated code according to the OpenACC
Application Programming Interface v2.0
<https://www.openacc.org >. This option implies -pthread, and
thus is only supported on targets that have support for
-pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload
regions that do not explicitly specify. The geom value is a
triple of ':'-separated sizes, in order 'gang', 'worker' and,
'vector'. A size can be omitted, to use a target-specific
default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++
and "!$omp" in Fortran. When -fopenmp is specified, the
compiler generates parallel code according to the OpenMP
Application Program Interface v4.5 <https://www.openmp.org >.
This option implies -pthread, and thus is only supported on
targets that have support for -pthread. -fopenmp implies
-fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with "#pragma
omp" in C/C++ and "!$omp" in Fortran. Other OpenMP directives
are ignored.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates
code for the Linux variant of Intel's current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009).
This is an experimental feature whose interface may change in
future versions of GCC, as the official specification
changes. Please note that not all architectures are
supported for this feature.
For more information on GCC's support for transactional
memory,
Note that the transactional memory feature is not supported
with non-call exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header
files.
In C++ code, this allows member names in structures to be
similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are
only accepted with this option.
Note that this option is off for all targets but x86 targets
using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect
pointers to elements of the type of the field, and permits
referring to anonymous fields declared using a typedef.
This is only supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in the
second and third arguments. The value of such an expression
is void. This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing
numbers of elements and/or incompatible element types. This
option should not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be.
It is either like "unsigned char" by default or like "signed
char" by default.
Ideally, a portable program should always use "signed char"
or "unsigned char" when it depends on the signedness of an
object. But many programs have been written to use plain
"char" and expect it to be signed, or expect it to be
unsigned, depending on the machines they were written for.
This option, and its inverse, let you make such a program
work with the opposite default.
The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its behavior is
always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is
the negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or
unsigned, when the declaration does not use either "signed"
or "unsigned". By default, such a bit-field is signed,
because this is consistent: the basic integer types such as
"int" are signed types.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions
to the specified endianness. The accepted values are big-
endian, little-endian and native for the native endianness of
the target (the default). This option is not supported for
C++.
Warning: the -fsso-struct switch causes GCC to generate code
that is not binary compatible with code generated without it
if the specified endianness is not the native endianness of
the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only
meaningful for C++ programs. You can also use most of the GNU
compiler options regardless of what language your program is in.
For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for
C++ programs; you can use the other options with any language
supported by GCC.
Some options for compiling C programs, such as -std, are also
relevant for C++ programs.
Here is a list of options that are only for compiling C++
programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to
the C++ ABI specification. Therefore, the ABI obtained using
version 0 will change in different versions of G++ as ABI
bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared
in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared
in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as
a template argument.
Version 4, which first appeared in G++ 4.5, implements a
standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the
mangling of attribute const/volatile on function pointer
types, decltype of a plain decl, and use of a function
parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the
promotion behavior of C++11 scoped enums and the mangling of
template argument packs, const/static_cast, prefix ++ and --,
and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats
nullptr_t as a builtin type and corrects the mangling of
lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the
substitution behavior of function types with function-cv-
qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the
alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling
convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the
mangling of sizeof... expressions and operator names. For
multiple entities with the same name within a function, that
are declared in different scopes, the mangling now changes
starting with the twelfth occurrence. It also implies
-fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the
calling conventions for empty classes on the x86_64 target
and for classes with only deleted copy/move constructors. It
accidentally changes the calling convention for classes with
a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the
accidental change in version 12.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around
mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled
name. This switch specifies which ABI version to use for the
alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC
7 compatibility). If another ABI version is explicitly
selected, this defaults to 0. For compatibility with GCC
versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version
is used for compatibility aliases. If this option is
provided along with -Wabi (without the version), the version
from this option is used for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful
for working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more
alignment than "void* ::operator new(std::size_t)" provides.
A numeric argument such as "-faligned-new=32" can be used to
specify how much alignment (in bytes) is provided by that
function, but few users will need to override the default of
"alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++2a. This
includes the addition of a new "char8_t" fundamental type,
changes to the types of UTF-8 string and character literals,
new signatures for user-defined literals, associated standard
library updates, and new "__cpp_char8_t" and
"__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary
and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character
literals introduces incompatibilities with ISO C++11 and
later standards. For example, the following code is well-
formed under ISO C++11, but is ill-formed when -fchar8_t is
specified.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This
check is normally unnecessary because the C++ standard
specifies that "operator new" only returns 0 if it is
declared "throw()", in which case the compiler always checks
the return value even without this option. In all other
cases, when "operator new" has a non-empty exception
specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless
recursion during constant expression evaluation. The minimum
specified by the standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14
constexpr functions to n. A limit is needed to detect
infinite loops during constant expression evaluation. The
default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single
constexpr evaluation. Even when number of iterations of a
single loop is limited with the above limit, if there are
several nested loops and each of them has many iterations but
still smaller than the above limit, or if in a body of some
loop or even outside of a loop too many expressions need to
be evaluated, the resulting constexpr evaluation might take
too long. The default is 33554432 (1<<25).
-fdeduce-init-list
Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed initializer
list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was
not part of the final standard, so it is disabled by default.
This option is deprecated, and may be removed in a future
version of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of
the same type. Specifying this option disables that
optimization, and forces G++ to call the copy constructor in
all cases. This option also causes G++ to call trivial
member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries,
but this option still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++
standard, but may be useful for reducing code size in
production builds, much like defining "NDEBUG". This does
not give user code permission to throw exceptions in
violation of the exception specifications; the compiler still
optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run
time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a variable
goes through a wrapper function that performs any necessary
initialization. When the use and definition of the variable
are in the same translation unit, this overhead can be
optimized away, but when the use is in a different
translation unit there is significant overhead even if the
variable doesn't actually need dynamic initialization. If
the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization
(either because the variable is statically initialized, or a
use of the variable in the defining TU will be executed
before any uses in another TU), they can avoid this overhead
with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol
aliases, the default is -fno-extern-tls-init.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use
this word as an identifier. You can use the keyword
"__typeof__" instead. This option is implied by the strict
ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are
instantiated implicitly (i.e. by use); only emit code for
explicit instantiations. If you use this option, you must
take care to structure your code to include all the necessary
explicit instantiations to avoid getting undefined symbols at
link time.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline
templates, either. The default is to handle inlines
differently so that compiles with and without optimization
need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline
functions controlled by "#pragma implementation". This
causes linker errors if these functions are not inlined
everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such
as implicit int and getting a pointer to member function via
non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11
constructor inheritance. This is part of C++17 but also
considered to be a Defect Report against C++11 and C++14.
This flag is enabled by default unless -fabi-version=10 or
lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template
template parameters and default arguments: this allows a
template with default template arguments as an argument for a
template template parameter with fewer template parameters.
This flag is enabled by default for -std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not
mandated by ANSI/ISO C. These include "ffs", "alloca",
"_exit", "index", "bzero", "conjf", and other related
functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception
specification. If the function has local variables of types
with non-trivial destructors, the exception specification
actually makes the function smaller because the EH cleanups
for those variables can be optimized away. The semantic
effect is that an exception thrown out of a function with
such an exception specification results in a call to
"terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand",
"bitor", "compl", "not", "or" and "xor" as synonyms as
keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does
not need to issue. Currently, the only such diagnostic
issued by G++ is the one for a name having multiple meanings
within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive allows some
nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a
function template, the compiler normally prints the signature
of the template followed by the template arguments and any
typedefs or typenames in the signature (e.g. "void f(T) [with
T = int]" rather than "void f(int)") so that it's clear which
template is involved. When an error message refers to a
specialization of a class template, the compiler omits any
template arguments that match the default template arguments
for that template. If either of these behaviors make it
harder to understand the error message rather than easier,
you can use -fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This
option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with
virtual functions for use by the C++ run-time type
identification features ("dynamic_cast" and "typeid"). If
you don't use those parts of the language, you can save some
space by using this flag. Note that exception handling uses
the same information, but G++ generates it as needed. The
"dynamic_cast" operator can still be used for casts that do
not require run-time type information, i.e. casts to "void *"
or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with
-fno-rtti may not work. For example, programs may fail to
link if a class compiled with -fno-rtti is used as a base for
a class compiled with -frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example, use the
size of the object to make deallocation faster. Enabled by
default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want to
add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a
value of enumerated type can only be one of the values of the
enumeration (as defined in the C++ standard; basically, a
value that can be represented in the minimum number of bits
needed to represent all the enumerators). This assumption
may not be valid if the program uses a cast to convert an
arbitrary integer value to the enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift
expressions in left-to-right order, and evaluate assignment
in right-to-left order, as adopted for C++17. Enabled by
default with -std=c++17. -fstrong-eval-order=some enables
just the ordering of member access and shift expressions, and
is the default without -std=c++17.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a
single warning or error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to
n. A limit on the template instantiation depth is needed to
detect endless recursions during template class
instantiation. ANSI/ISO C++ conforming programs must not
rely on a maximum depth greater than 17 (changed to 1024 in
C++11). The default value is 900, as the compiler can run
out of stack space before hitting 1024 in some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in
the C++ ABI for thread-safe initialization of local statics.
You can use this option to reduce code size slightly in code
that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration
with the "__cxa_atexit" function rather than the "atexit"
function. This option is required for fully standards-
compliant handling of static destructors, but only works if
your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine.
This causes "std::uncaught_exception" to be incorrect, but is
necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to
compare pointers to inline functions or methods where the
addresses of the two functions are taken in different shared
objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so
that they do not appear in the export table of a DSO and do
not require a PLT indirection when used within the DSO.
Enabling this option can have a dramatic effect on load and
link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of
templates.
The behavior of this switch is not quite the same as marking
the methods as hidden directly, because it does not affect
static variables local to the function or cause the compiler
to deduce that the function is defined in only one shared
object.
You may mark a method as having a visibility explicitly to
negate the effect of the switch for that method. For
example, if you do want to compare pointers to a particular
inline method, you might mark it as having default
visibility. Marking the enclosing class with explicit
visibility has no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared
library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's
C++ linkage model compatible with that of Microsoft Visual
Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without
explicit visibility specifications that are defined in
more than one shared object: those declarations are
permitted if they are permitted when this option is not
used.
In new code it is better to use -fvisibility=hidden and
export those classes that are intended to be externally
visible. Unfortunately it is possible for code to rely,
perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects are different, so changing one does
not change the other; and that pointers to function members
defined in different shared objects may not compare equal.
When this flag is given, it is a violation of the ODR to
define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ uses weak symbols if they are
available. This option exists only for testing, and should
not be used by end-users; it results in inferior code and has
no benefits. This option may be removed in a future release
of G++.
-nostdinc++
Do not search for header files in the standard directories
specific to C++, but do still search the other standard
directories. (This option is used when building the C++
library.)
In addition, these optimization, warning, and code generation
options have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not
compatible with the vendor-neutral C++ ABI. Since G++ now
defaults to updating the ABI with each major release,
normally -Wabi will warn only if there is a check added later
in a release series for an ABI issue discovered since the
initial release. -Wabi will warn about more things if an
older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to
warn about compatibility with a particular -fabi-version
level, e.g. -Wabi=2 to warn about changes relative to
-fabi-version=2.
If an explicit version number is provided and
-fabi-compat-version is not specified, the version number
from this option is used for compatibility aliases. If no
explicit version number is provided with this option, but
-fabi-compat-version is specified, that version number is
used for ABI warnings.
Although an effort has been made to warn about all such
cases, there are probably some cases that are not warned
about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even
though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you
are concerned about the fact that code generated by G++ may
not be binary compatible with code generated by other
compilers.
Known incompatibilities in -fabi-version=2 (which was the
default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of
reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way that
does not allow for overloading of functions taking
vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as
type qualifiers, and "decltype" of a plain declaration
was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic
function are promoted like unscoped enumerators, causing
"va_arg" to complain. On most targets this does not
actually affect the parameter passing ABI, as there is no
way to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument
packs, "const_cast", "static_cast", prefix
increment/decrement, and a class scope function used as a
template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled
incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-
qualifiers, the un-qualified function type was
incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC
5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1,
leading to unaligned accesses. Note that this did not
affect the ABI of a function with a "nullptr_t"
parameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC
5.2.
* Target-specific attributes that affect the identity of a
type, such as ia32 calling conventions on a function type
(stdcall, regparm, etc.), did not affect the mangled
name, leading to name collisions when function pointers
were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC
6.1.
It also warns about psABI-related changes. The known psABI
changes at this point include:
* For SysV/x86-64, unions with "long double" members are
passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that
does not have that ABI tag. See C++ Attributes for more
information about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors
or destructors in that class are private, and it has neither
friends nor public static member functions. Also warn if
there are no non-private methods, and there's at least one
private member function that isn't a constructor or
destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class
that has virtual functions and non-virtual destructor. It is
unsafe to delete an instance of a derived class through a
pointer to a base class if the base class does not have a
virtual destructor. This warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or
copy assignment operator is deprecated if the class has a
user-provided copy constructor or copy assignment operator,
in C++11 and up. This warning is enabled by -Wextra. With
-Wdeprecated-copy-dtor, also deprecate if the class has a
user-provided destructor.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are
likely to result in dangling pointers. Since the underlying
array for an "initializer_list" is handled like a normal C++
temporary object, it is easy to inadvertently keep a pointer
to the array past the end of the array's lifetime. For
example:
* If a function returns a temporary "initializer_list", or
a local "initializer_list" variable, the array's lifetime
ends at the end of the return statement, so the value
returned has a dangling pointer.
* If a new-expression creates an "initializer_list", the
array only lives until the end of the enclosing full-
expression, so the "initializer_list" in the heap has a
dangling pointer.
* When an "initializer_list" variable is assigned from a
brace-enclosed initializer list, the temporary array
created for the right side of the assignment only lives
until the end of the full-expression, so at the next
statement the "initializer_list" variable has a dangling
pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from
the "initializer_list" argument, this doesn't extend the
lifetime of the array, so if a class variable is
constructed from a temporary "initializer_list", the
pointer is left dangling by the end of the variable
declaration statement.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-
suffix which does not begin with an underscore. As a
conforming extension, GCC treats such suffixes as separate
preprocessing tokens in order to maintain backwards
compatibility with code that uses formatting macros from
"<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
Additionally, warn when a user-defined literal operator is
declared with a literal suffix identifier that doesn't begin
with an underscore. Literal suffix identifiers that don't
begin with an underscore are reserved for future
standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type mismatches
in global declarations from different compilation units.
Requires -flto to be enabled. Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are
diagnosed by default, as required by the standard. A
narrowing conversion from a constant produces an error, and a
narrowing conversion from a non-constant produces a warning,
but -Wno-narrowing suppresses the diagnostic. Note that this
does not affect the meaning of well-formed code; narrowing
conversions are still considered ill-formed in SFINAE
contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion
prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of
a call to a function that does not have a non-throwing
exception specification (i.e. "throw()" or "noexcept") but is
known by the compiler to never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a
function type changes the mangled name of a symbol relative
to C++14. Enabled by -Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls
"f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function
such as "memset" or "memcpy" is an object of class type, and
when writing into such an object might bypass the class non-
trivial or deleted constructor or copy assignment, violate
const-correctness or encapsulation, or corrupt virtual table
pointers. Modifying the representation of such objects may
violate invariants maintained by member functions of the
class. For example, the call to "memset" below is undefined
because it modifies a non-trivial class object and is,
therefore, diagnosed. The safe way to either initialize or
clear the storage of objects of such types is by using the
appropriate constructor or assignment operator, if one is
available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly
casting the pointer to the class object to "void *" or to a
type that can be safely accessed by the raw memory function
suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible
non-virtual destructor itself or in an accessible polymorphic
base class, in which case it is possible but unsafe to delete
an instance of a derived class through a pointer to the class
itself or base class. This warning is automatically enabled
if -Weffc++ is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier,
except when it is part of the GNU Explicit Register Variables
extension. The use of the "register" keyword as storage
class specifier has been deprecated in C++11 and removed in
C++17. Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code
does not match the order in which they must be executed. For
instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and
"j" to match the declaration order of the members, emitting a
warning to that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy
elision. A typical scenario when copy elision can occur is
when returning in a function with a class return type, when
the expression being returned is the name of a non-volatile
automatic object, and is not a function parameter, and has
the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy
elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that
is, when a move operation would have been performed even
without the "std::move" call. This happens because the
compiler is forced to treat the object as if it were an
rvalue in certain situations such as returning a local
variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++
implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the
type of the expression being returned and the function return
type differ, yet G++ treats the return value as if it were
designated by an rvalue.
This warning is enabled by -Wextra.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal
number suffixes as GNU extensions. When this option is
turned off these suffixes are treated as C++11 user-defined
literal numeric suffixes. This is on by default for all
pre-C++11 dialects and all GNU dialects: -std=c++98,
-std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off
by default for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from
Scott Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an
object.
* Distinguish between prefix and postfix forms of increment
and decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also
one of the effective C++ recommendations. However, the check
is extended to warn about the lack of virtual destructor in
accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard
library headers do not obey all of these guidelines; use grep
-v to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL"
is defined to "__null". Although it is a null pointer
constant rather than a null pointer, it is guaranteed to be
of the same size as a pointer. But this use is not portable
across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are
declared within a template. In very old versions of GCC that
predate implementation of the ISO standard, declarations such
as friend int foo(int), where the name of the friend is an
unqualified-id, could be interpreted as a particular
specialization of a template function; the warning exists to
diagnose compatibility problems, and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is
used within a C++ program. The new-style casts
("dynamic_cast", "static_cast", "reinterpret_cast", and
"const_cast") are less vulnerable to unintended effects and
much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from
a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to
member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from
unsigned or enumerated type to a signed type, over a
conversion to an unsigned type of the same size. Previous
versions of G++ tried to preserve unsignedness, but the
standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered.
Some coding rules disallow templates, and this may be used to
enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL.
One may also instantiate or specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base
classes. Some coding rules disallow multiple inheritance,
and this may be used to enforce that rule. The warning is
inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that
indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base
class. Some coding rules disallow multiple inheritance, and
this may be used to enforce that rule. The warning is
inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that
indirectly use virtual inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding
rules disallow namespaces, and this may be used to enforce
that rule. The warning is inactive inside a system header
file, such as the STL, so one can still use the STL. One may
also use using directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will
immediately result in a call to "terminate".
-Wno-class-conversion (C++ and Objective-C++ only)
Disable the warning about the case when a conversion function
converts an object to the same type, to a base class of that
type, or to void; such a conversion function will never be
called.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and
Objective-C++ languages themselves.
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs. You can
also use most of the language-independent GNU compiler options.
For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for
Objective-C and Objective-C++ programs; you can use the other
options with any language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C
front-end (e.g., -Wtraditional). Similarly, Objective-C++
compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C
and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for
each literal string specified with the syntax "@"..."". The
default class name is "NXConstantString" if the GNU runtime
is being used, and "NSConstantString" if the NeXT runtime is
being used (see below). The -fconstant-cfstrings option, if
also present, overrides the -fconstant-string-class setting
and cause "@"..."" literals to be laid out as constant
CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU
Objective-C runtime. This is the default for most types of
systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is
the default for NeXT-based systems, including Darwin and Mac
OS X. The macro "__NEXT_RUNTIME__" is predefined if (and
only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the
receiver is not "nil". This allows for more efficient entry
points in the runtime to be used. This option is only
available in conjunction with the NeXT runtime and ABI
version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected
runtime. This option is currently supported only for the
NeXT runtime. In that case, Version 0 is the traditional
(32-bit) ABI without support for properties and other
Objective-C 2.0 additions. Version 1 is the traditional
(32-bit) ABI with support for properties and other Objective-
C 2.0 additions. Version 2 is the modern (64-bit) ABI. If
nothing is specified, the default is Version 0 on 32-bit
target machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance
variables is a C++ object with a non-trivial default
constructor. If so, synthesize a special "- (id)
.cxx_construct" instance method which runs non-trivial
default constructors on any such instance variables, in
order, and then return "self". Similarly, check if any
instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which runs all such default
destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct"
methods thusly generated only operate on instance variables
declared in the current Objective-C class, and not those
inherited from superclasses. It is the responsibility of the
Objective-C runtime to invoke all such methods in an object's
inheritance hierarchy. The "- (id) .cxx_construct" methods
are invoked by the runtime immediately after a new object
instance is allocated; the "- (void) .cxx_destruct" methods
are invoked immediately before the runtime deallocates an
object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4
and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this
is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++. This option
is required to use the Objective-C keywords @try, @throw,
@catch, @finally and @synchronized. This option is available
with both the GNU runtime and the NeXT runtime (but not
available in conjunction with the NeXT runtime on Mac OS X
10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and
Objective-C++ programs. This option is only available with
the NeXT runtime; the GNU runtime has a different garbage
collection implementation that does not require special
compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a
nil receiver in method invocations before doing the actual
method call. This is the default and can be disabled using
-fno-objc-nilcheck. Class methods and super calls are never
checked for nil in this way no matter what this flag is set
to. Currently this flag does nothing when the GNU runtime,
or an older version of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the
language recognized by GCC 4.0. This only affects the
Objective-C additions to the C/C++ language; it does not
affect conformance to C/C++ standards, which is controlled by
the separate C/C++ dialect option flags. When this option is
used with the Objective-C or Objective-C++ compiler, any
Objective-C syntax that is not recognized by GCC 4.0 is
rejected. This is useful if you need to make sure that your
Objective-C code can be compiled with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically
link in the resulting object file, and allow dyld(1) to load
it in at run time instead. This is used in conjunction with
the Fix-and-Continue debugging mode, where the object file in
question may be recompiled and dynamically reloaded in the
course of program execution, without the need to restart the
program itself. Currently, Fix-and-Continue functionality is
only available in conjunction with the NeXT runtime on Mac OS
X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of
the class is known at compile time) with static class
references that get initialized at load time, which improves
run-time performance. Specifying the -fzero-link flag
suppresses this behavior and causes calls to
"objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual
class implementations to be modified during program
execution. The GNU runtime currently always retains calls to
"objc_get_class("...")" regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed
as if they were local variables from within the methods of
the class they're declared in. This can lead to shadowing
between instance variables and other variables declared
either locally inside a class method or globally with the
same name. Specifying the -fno-local-ivars flag disables
this behavior thus avoiding variable shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified
option so that instance variables declared outside the scope
of any access modifier directives default to the specified
visibility.
-gen-decls
Dump interface declarations for all classes seen in the
source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted
by the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is
issued for every method in the protocol that is not
implemented by the class. The default behavior is to issue a
warning for every method not explicitly implemented in the
class, even if a method implementation is inherited from the
superclass. If you use the -Wno-protocol option, then
methods inherited from the superclass are considered to be
implemented, and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same
selector are found during compilation. The check is
performed on the list of methods in the final stage of
compilation. Additionally, a check is performed for each
selector appearing in a "@selector(...)" expression, and a
corresponding method for that selector has been found during
compilation. Because these checks scan the method table only
at the end of compilation, these warnings are not produced if
the final stage of compilation is not reached, for example
because an error is found during compilation, or because the
-fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or
return types are found for a given selector when attempting
to send a message using this selector to a receiver of type
"id" or "Class". When this flag is off (which is the default
behavior), the compiler omits such warnings if any
differences found are confined to types that share the same
size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an
undeclared selector is found. A selector is considered
undeclared if no method with that name has been declared
before the "@selector(...)" expression, either explicitly in
an @interface or @protocol declaration, or implicitly in an
@implementation section. This option always performs its
checks as soon as a "@selector(...)" expression is found,
while -Wselector only performs its checks in the final stage
of compilation. This also enforces the coding style
convention that methods and selectors must be declared before
being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is
passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted
irrespective of the output device's aspect (e.g. its width, ...).
You can use the options described below to control the formatting
algorithm for diagnostic messages, e.g. how many characters per
line, how often source location information should be reported.
Note that some language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of
about n characters. If n is zero, then no line-wrapping is
done; each error message appears on a single line. This is
the default for all front ends.
Note - this option also affects the display of the #error and
#warning pre-processor directives, and the deprecated
function/type/variable attribute. It does not however affect
the pragma GCC warning and pragma GCC error pragmas.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit source location
information once; that is, in case the message is too long to
fit on a single physical line and has to be wrapped, the
source location won't be emitted (as prefix) again, over and
over, in subsequent continuation lines. This is the default
behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the
diagnostic messages reporter to emit the same source location
information (as prefix) for physical lines that result from
the process of breaking a message which is too long to fit on
a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto.
The default depends on how the compiler has been configured,
it can be any of the above WHEN options or also never if
GCC_COLORS environment variable isn't present in the
environment, and auto otherwise. auto means to use color
only when the standard error is a terminal. The forms
-fdiagnostics-color and -fno-diagnostics-color are aliases
for -fdiagnostics-color=always and -fdiagnostics-color=never,
respectively.
The colors are defined by the environment variable
GCC_COLORS. Its value is a colon-separated list of
capabilities and Select Graphic Rendition (SGR) substrings.
SGR commands are interpreted by the terminal or terminal
emulator. (See the section in the documentation of your text
terminal for permitted values and their meanings as character
attributes.) These substring values are integers in decimal
representation and can be concatenated with semicolons.
Common values to concatenate include 1 for bold, 4 for
underline, 5 for blink, 7 for inverse, 39 for default
foreground color, 30 to 37 for foreground colors, 90 to 97
for 16-color mode foreground colors, 38;5;0 to 38;5;255 for
88-color and 256-color modes foreground colors, 49 for
default background color, 40 to 47 for background colors, 100
to 107 for 16-color mode background colors, and 48;5;0 to
48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold
cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red.
Setting GCC_COLORS to the empty string disables colors.
Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be
inserted or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be
deleted.
"diff-filename="
SGR substring for filename headers within generated
patches.
"diff-hunk="
SGR substring for the starts of hunks within generated
patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated
patches.
"type-diff="
SGR substring for highlighting mismatching types within
template arguments in the C++ frontend.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating
the command-line option that directly controls the diagnostic
(if such an option is known to the diagnostic machinery).
Specifying the -fno-diagnostics-show-option flag suppresses
that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original
source line and a caret ^ indicating the column. This option
suppresses this information. The source line is truncated to
n characters, if the -fmessage-length=n option is given.
When the output is done to the terminal, the width is limited
to the width given by the COLUMNS environment variable or, if
not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via
-fdiagnostics-show-caret), diagnostics can label ranges of
source code with pertinent information, such as the types of
expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long int
This option suppresses the printing of these labels (in the
example above, the vertical bars and the "char *" and "long
int" text).
-fno-diagnostics-show-line-numbers
By default, when printing source code (via
-fdiagnostics-show-caret), a left margin is printed, showing
line numbers. This option suppresses this left margin.
-fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin
printed by -fdiagnostics-show-line-numbers. It defaults to
6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for
consumption by IDEs. For each fix-it, a line will be printed
after the relevant diagnostic, starting with the string "fix-
it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as
a count of bytes, starting at byte 1 for the initial column.
In the above example, bytes 3 through 20 of line 45 of
"test.c" are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\",
tab as "\t", newline as "\n", double quotes as "\"", non-
printable characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is
to be removed. An empty range (e.g. "45:3-45:3") indicates
that the string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after
any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same
rules as for diagnostics (see -fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing
mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a
tree-like structure showing the common and differing parts of
the types, such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double"
and "float" in this case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing
mismatching template types, common parts of the types are
printed as "[...]" to simplify the error message. For
example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior.
This flag also affects the output of the
-fdiagnostics-show-template-tree flag.
-fno-show-column
Do not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program that
does not understand the column numbers, such as dejagnu.
-fdiagnostics-format=FORMAT
Select a different format for printing diagnostics. FORMAT
is text or json. The default is text.
The json format consists of a top-level JSON array containing
JSON objects representing the diagnostics.
The JSON is emitted as one line, without formatting; the
examples below have been formatted for clarity.
Diagnostics can have child diagnostics. For example, this
error and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^
might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"message": "...this statement, but the latter is ..."
}
]
},
...
]
where the "note" is a child of the "warning".
A diagnostic has a "kind". If this is "warning", then there
is an "option" key describing the command-line option
controlling the warning.
A diagnostic can contain zero or more locations. Each
location has up to three positions within it: a "caret"
position and optional "start" and "finish" positions. A
location can also have an optional "label" string. For
example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}
has three locations. Its primary location is at the "+"
token at column 23. It has two secondary locations,
describing the left and right-hand sides of the expression,
which have labels. It might be printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"message": "invalid operands to binary + ..."
}
If a diagnostic contains fix-it hints, it has a "fixits"
array, consisting of half-open intervals, similar to the
output of -fdiagnostics-parseable-fixits. For example, this
diagnostic with a replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| color
might be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"message": "\u2018struct s\u2019 has no member named ..."
}
where the fix-it hint suggests replacing the text from
"start" up to but not including "next" with "string"'s value.
Deletions are expressed via an empty value for "string",
insertions by having "start" equal "next".
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything
beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which
point GCC bails out rather than attempting to continue
processing the source code. If n is 0 (the default), there
is no limit on the number of error messages produced. If
-Wfatal-errors is also specified, then -Wfatal-errors takes
precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for
a warning is appended; for example -Werror=switch turns the
warnings controlled by -Wswitch into errors. This switch
takes a negative form, to be used to negate -Werror for
specific warnings; for example -Wno-error=switch makes
-Wswitch warnings not be errors, even when -Werror is in
effect.
The warning message for each controllable warning includes
the option that controls the warning. That option can then
be used with -Werror= and -Wno-error= as described above.
(Printing of the option in the warning message can be
disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the
first error occurred rather than trying to keep going and
printing further error messages.
You can request many specific warnings with options beginning
with -W, for example -Wimplicit to request warnings on implicit
declarations. Each of these specific warning options also has a
negative form beginning -Wno- to turn off warnings; for example,
-Wno-implicit. This manual lists only one of the two forms,
whichever is not the default. For further language-specific
options also refer to C++ Dialect Options and Objective-C and
Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options,
such as -Wunused, which may turn on further options, such as
-Wunused-value. The combined effect of positive and negative
forms is that more specific options have priority over less
specific ones, independently of their position in the command-
line. For options of the same specificity, the last one takes
effect. Options enabled or disabled via pragmas take effect as if
they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the
option is not recognized. However, if the -Wno- form is used,
the behavior is slightly different: no diagnostic is produced for
-Wno-unknown-warning unless other diagnostics are being produced.
This allows the use of new -Wno- options with old compilers, but
if something goes wrong, the compiler warns that an unrecognized
option is present.
The effectiveness of some warnings depends on optimizations also
being enabled. For example -Wsuggest-final-types is more
effective with link-time optimization and -Wmaybe-uninitialized
will not warn at all unless optimization is enabled.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some
other programs that do not follow ISO C and ISO C++. For ISO
C, follows the version of the ISO C standard specified by any
-std option used.
Valid ISO C and ISO C++ programs should compile properly with
or without this option (though a rare few require -ansi or a
-std option specifying the required version of ISO C).
However, without this option, certain GNU extensions and
traditional C and C++ features are supported as well. With
this option, they are rejected.
-Wpedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with __.
Pedantic warnings are also disabled in the expression that
follows "__extension__". However, only system header files
should use these escape routes; application programs should
avoid them.
Some users try to use -Wpedantic to check programs for strict
ISO C conformance. They soon find that it does not do quite
what they want: it finds some non-ISO practices, but not
all---only those for which ISO C requires a diagnostic, and
some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be
useful in some instances, but would require considerable
additional work and would be quite different from -Wpedantic.
We don't have plans to support such a feature in the near
future.
Where the standard specified with -std represents a GNU
extended dialect of C, such as gnu90 or gnu99, there is a
corresponding base standard, the version of ISO C on which
the GNU extended dialect is based. Warnings from -Wpedantic
are given where they are required by the base standard. (It
does not make sense for such warnings to be given only for
features not in the specified GNU C dialect, since by
definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be
nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic)
requires a diagnostic, in some cases where there is undefined
behavior at compile-time and in some other cases that do not
prevent compilation of programs that are valid according to
the standard. This is not equivalent to -Werror=pedantic,
since there are errors enabled by this option and not enabled
by the latter and vice versa.
-Wall
This enables all the warnings about constructions that some
users consider questionable, and that are easy to avoid (or
modify to prevent the warning), even in conjunction with
macros. This also enables some language-specific warnings
described in C++ Dialect Options and Objective-C and
Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value
(C++ and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat -Wint-in-bool-context -Wimplicit (C and Objective-C
only) -Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -Wlogical-not-parentheses -Wmain
(only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation (only for
C/C++) -Wmissing-attributes -Wmissing-braces (only for
C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++)
-Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses
-Wpessimizing-move (only for C++) -Wpointer-sign -Wreorder
-Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare
(only in C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess
-Wstrict-aliasing -Wstrict-overflow=1 -Wswitch
-Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label
-Wunused-value -Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some
of them warn about constructions that users generally do not
consider questionable, but which occasionally you might wish
to check for; others warn about constructions that are
necessary or hard to avoid in some cases, and there is no
simple way to modify the code to suppress the warning. Some
of them are enabled by -Wextra but many of them must be
enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is
still supported, but the newer name is more descriptive.)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3
-Wmissing-field-initializers -Wmissing-parameter-type (C
only) -Wold-style-declaration (C only) -Woverride-init
-Wsign-compare (C only) -Wredundant-move (only for C++)
-Wtype-limits -Wuninitialized -Wshift-negative-value (in
C++11 to C++17 and in C99 and newer) -Wunused-parameter (only
with -Wunused or -Wall) -Wunused-but-set-parameter (only with
-Wunused or -Wall)
The option -Wextra also prints warning messages for the
following cases:
* A pointer is compared against integer zero with "<",
"<=", ">", or ">=".
* (C++ only) An enumerator and a non-enumerator both appear
in a conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared
"register".
* (C++ only) Taking the address of a variable that has been
declared "register".
* (C++ only) A base class is not initialized in the copy
constructor of a derived class.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common
cause of error, as programmers often forget that this type is
signed on some machines. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the
-fprofile-use option. If a source file is changed between
compiling with -fprofile-generate and with -fprofile-use, the
files with the profile feedback can fail to match the source
file and GCC cannot use the profile feedback information. By
default, this warning is enabled and is treated as an error.
-Wno-coverage-mismatch can be used to disable the warning or
-Wno-error=coverage-mismatch can be used to disable the
error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the case of very
minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly
promoted to "double". CPUs with a 32-bit "single-precision"
floating-point unit implement "float" in hardware, but
emulate "double" in software. On such a machine, doing
computations using "double" values is much more expensive
because of the overhead required for software emulation.
It is easy to accidentally do computations with "double"
because floating-point literals are implicitly of type
"double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double"
because the floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile",
"restrict" or "_Atomic" specifier. This warning is enabled
by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that
the arguments supplied have types appropriate to the format
string specified, and that the conversions specified in the
format string make sense. This includes standard functions,
and others specified by format attributes, in the "printf",
"scanf", "strftime" and "strfmon" (an X/Open extension, not
in the C standard) families (or other target-specific
families). Which functions are checked without format
attributes having been specified depends on the standard
version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or
-fno-builtin.
The formats are checked against the format features supported
by GNU libc version 2.2. These include all ISO C90 and C99
features, as well as features from the Single Unix
Specification and some BSD and GNU extensions. Other library
implementations may not support all these features; GCC does
not support warning about features that go beyond a
particular library's limitations. However, if -Wpedantic is
used with -Wformat, warnings are given about format features
not in the selected standard version (but not for "strfmon"
formats, since those are not in any version of the C
standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and
-Wno-format is equivalent to -Wformat=0. Since -Wformat
also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull. Some aspects
of this level of format checking can be disabled by the
options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length.
-Wformat is enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format
strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess
arguments to a "printf" or "scanf" format function. The
C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments
that are specified with $ operand number specifications,
normally warnings are still given, since the
implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the
case of "scanf" formats, this option suppresses the
warning if the unused arguments are all pointers, since
the Single Unix Specification says that such unused
arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such
as "sprintf" and "vsprintf" that might overflow the
destination buffer. When the exact number of bytes
written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that
depend on the level argument and on optimization. While
enabling optimization will in most cases improve the
accuracy of the warning, it may also result in false
positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat
employs a conservative approach that warns only about
calls that most likely overflow the buffer. At this
level, numeric arguments to format directives with
unknown values are assumed to have the value of one,
and strings of unknown length to be empty. Numeric
arguments that are known to be bounded to a subrange
of their type, or string arguments whose output is
bounded either by their directive's precision or by a
finite set of string literals, are assumed to take on
the value within the range that results in the most
bytes on output. For example, the call to "sprintf"
below is diagnosed because even with both a and b
equal to zero, the terminating NUL character ('\0')
appended by the function to the destination buffer
will be written past its end. Increasing the size of
the buffer by a single byte is sufficient to avoid
the warning, though it may not be sufficient to avoid
the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow
the destination buffer given an argument of
sufficient length or magnitude. At level 2, unknown
numeric arguments are assumed to have the minimum
representable value for signed types with a precision
greater than 1, and the maximum representable value
otherwise. Unknown string arguments whose length
cannot be assumed to be bounded either by the
directive's precision, or by a finite set of string
literals they may evaluate to, or the character array
they may point to, are assumed to be 1 character
long.
At level 2, the call in the example above is again
diagnosed, but this time because with a equal to a
32-bit "INT_MIN" the first %i directive will write
some of its digits beyond the end of the destination
buffer. To make the call safe regardless of the
values of the two variables, the size of the
destination buffer must be increased to at least 34
bytes. GCC includes the minimum size of the buffer
in an informational note following the warning.
An alternative to increasing the size of the
destination buffer is to constrain the range of
formatted values. The maximum length of string
arguments can be bounded by specifying the precision
in the format directive. When numeric arguments of
format directives can be assumed to be bounded by
less than the precision of their type, choosing an
appropriate length modifier to the format specifier
will reduce the required buffer size. For example,
if a and b in the example above can be assumed to be
within the precision of the "short int" type then
using either the %hi format directive or casting the
argument to "short" reduces the maximum required size
of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length
formats. The C standard specifies that zero-length
formats are allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string
is not a string literal and so cannot be checked, unless
the format function takes its format arguments as a
"va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At
present, this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal
and there are no format arguments, as in "printf (foo);".
This may be a security hole if the format string came
from untrusted input and contains %n. (This is currently
a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that
are not included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string
requires an unsigned argument and the argument is signed
and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such
as "snprintf" and "vsnprintf" that might result in output
truncation. When the exact number of bytes written by a
format directive cannot be determined at compile-time it
is estimated based on heuristics that depend on the level
argument and on optimization. While enabling
optimization will in most cases improve the accuracy of
the warning, it may also result in false positives.
Except as noted otherwise, the option uses the same logic
-Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat
employs a conservative approach that warns only about
calls to bounded functions whose return value is
unused and that will most likely result in output
truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions
whose return value is used and that might result in
truncation given an argument of sufficient length or
magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime"
formats that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function
attribute.
-Wnonnull is included in -Wall and -Wformat. It can be
disabled with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull"
function attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled
with the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This
option is only active when -fdelete-null-pointer-checks is
active, which is enabled by optimizations in most targets.
The precision of the warnings depends on the optimization
options used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This
warning is enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being
declared. In C99 mode (-std=c99 or -std=gnu99), this warning
is enabled by default and it is made into an error by
-pedantic-errors. This warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration.
This warning is enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as
-Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the
same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case
cannot fall through, e.g. when there is a return statement or
a call to function declared with the noreturn attribute.
-Wimplicit-fallthrough= also takes into account control flow
statements, such as ifs, and only warns when appropriate.
E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is
desirable, GCC provides an attribute, "__attribute__
((fallthrough))", that is to be used along with a null
statement to suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the
-Wimplicit-fallthrough warning using "[[fallthrough]];"
instead of the GNU attribute. In C++11 or C++14 users can
use "[[gnu::fallthrough]];", which is a GNU extension.
Instead of these attributes, it is also possible to add a
fallthrough comment to silence the warning. The whole body
of the C or C++ style comment should match the given regular
expressions listed below. The option argument n specifies
what kind of comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of
the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S |
|-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of
the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the
warning.
The comment needs to be followed after optional whitespace
and other comments by "case" or "default" keywords or by a
user label that precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wif-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warning triggered by the "warn_if_not_aligned"
attribute should be issued. This is enabled by default. Use
-Wno-if-not-aligned to disable it.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier
such as "const". For ISO C such a type qualifier has no
effect, since the value returned by a function is not an
lvalue. For C++, the warning is only emitted for scalar
types or "void". ISO C prohibits qualified "void" return
types on function definitions, so such return types always
receive a warning even without this option.
This warning is also enabled by -Wextra.
-Wignored-attributes (C and C++ only)
Warn when an attribute is ignored. This is different from
the -Wattributes option in that it warns whenever the
compiler decides to drop an attribute, not that the attribute
is either unknown, used in a wrong place, etc. This warning
is enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a
function with external linkage, returning int, taking either
zero arguments, two, or three arguments of appropriate types.
This warning is enabled by default in C++ and is enabled by
either -Wall or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the
block structure. Specifically, a warning is issued for "if",
"else", "while", and "for" clauses with a guarded statement
that does not use braces, followed by an unguarded statement
with the same indentation.
In the following example, the call to "bar" is misleadingly
indented as if it were guarded by the "if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the
-ftabstop= option to determine if the statements line up
(defaulting to 8).
The warning is not issued for code involving multiline
preprocessor logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since
this typically indicates autogenerated code, and no
assumptions can be made about the layout of the file that the
directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more
attributes that a related function is declared with and whose
absence may adversely affect the correctness or efficiency of
generated code. For example, the warning is issued for
declarations of aliases that use attributes to specify less
restrictive requirements than those of their targets. This
typically represents a potential optimization opportunity.
By contrast, the -Wattribute-alias=2 option controls warnings
issued when the alias is more restrictive than the target,
which could lead to incorrect code generation. Attributes
considered include "alloc_align", "alloc_size", "cold",
"const", "hot", "leaf", "malloc", "nonnull", "noreturn",
"nothrow", "pure", "returns_nonnull", and "returns_twice".
In C++, the warning is issued when an explicit specialization
of a primary template declared with attribute "alloc_align",
"alloc_size", "assume_aligned", "format", "format_arg",
"malloc", or "nonnull" is declared without it. Attributes
"deprecated", "error", and "warning" suppress the warning..
You can use the "copy" attribute to apply the same set of
attributes to a declaration as that on another declaration
without explicitly enumerating the attributes. This attribute
can be applied to declarations of functions, variables, or
types.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function
template below makes use of both attribute "malloc" and
"alloc_size" the declaration of the explicit specialization
of the template is diagnosed because it is missing one of the
attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully
bracketed. In the following example, the initializer for "a"
is not fully bracketed, but that for "b" is fully bracketed.
This warning is enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++
only)
Warn if a user-supplied include directory does not exist.
-Wmissing-profile
Warn if feedback profiles are missing when using the
-fprofile-use option. This option diagnoses those cases
where a new function or a new file is added to the user code
between compiling with -fprofile-generate and with
-fprofile-use, without regenerating the profiles. In these
cases, the profile feedback data files do not contain any
profile feedback information for the newly added function or
file respectively. Also, in the case when profile count data
(.gcda) files are removed, GCC cannot use any profile
feedback information. In all these cases, warnings are
issued to inform the user that a profile generation step is
due. -Wno-missing-profile can be used to disable the
warning. Ignoring the warning can result in poorly optimized
code. Completely disabling the warning is not recommended
and should be done only when non-existent profile data is
justified.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be
guarded by a clause such as "if", "else", "for", "switch", or
"while", in which only the first statement is actually
guarded after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds.
The can usually be fixed by wrapping the macro in a do-while
loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as
when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence
people often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:"
with omitted middle operand. When the condition in the "?":
operator is a boolean expression, the omitted value is always
1. Often programmers expect it to be a value computed inside
the conditional expression instead.
For C++ this also warns for some cases of unnecessary
parentheses in declarations, which can indicate an attempt at
a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++
standards.
The C and C++ standards define the order in which expressions
in a C/C++ program are evaluated in terms of sequence points,
which represent a partial ordering between the execution of
parts of the program: those executed before the sequence
point, and those executed after it. These occur after the
evaluation of a full expression (one which is not part of a
larger expression), after the evaluation of the first operand
of a "&&", "||", "? :" or "," (comma) operator, before a
function is called (but after the evaluation of its arguments
and the expression denoting the called function), and in
certain other places. Other than as expressed by the
sequence point rules, the order of evaluation of
subexpressions of an expression is not specified. All these
rules describe only a partial order rather than a total
order, since, for example, if two functions are called within
one expression with no sequence point between them, the order
in which the functions are called is not specified. However,
the standards committee have ruled that function calls do not
overlap.
It is not specified when between sequence points
modifications to the values of objects take effect. Programs
whose behavior depends on this have undefined behavior; the C
and C++ standards specify that "Between the previous and next
sequence point an object shall have its stored value modified
at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine
the value to be stored.". If a program breaks these rules,
the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;",
"a[n] = b[n++]" and "a[i++] = i;". Some more complicated
cases are not diagnosed by this option, and it may give an
occasional false positive result, but in general it has been
found fairly effective at detecting this sort of problem in
programs.
The C++17 standard will define the order of evaluation of
operands in more cases: in particular it requires that the
right-hand side of an assignment be evaluated before the
left-hand side, so the above examples are no longer
undefined. But this warning will still warn about them, to
help people avoid writing code that is undefined in C and
earlier revisions of C++.
The standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules
in subtle cases. Links to discussions of the problem,
including proposed formal definitions, may be found on the
GCC readings page, at <http://gcc.gnu.org/readings.html >.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a
reference) to a variable that goes out of scope after the
function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that
defaults to "int". Also warn about any "return" statement
with no return value in a function whose return type is not
"void" (falling off the end of the function body is
considered returning without a value).
For C only, warn about a "return" statement with an
expression in a function whose return type is "void", unless
the expression type is also "void". As a GNU extension, the
latter case is accepted without a warning unless -Wpedantic
is used. Attempting to use the return value of a non-"void"
function other than "main" that flows off the end by reaching
the closing curly brace that terminates the function is
undefined.
Unlike in C, in C++, flowing off the end of a non-"void"
function other than "main" results in undefined behavior even
when the value of the function is not used.
This warning is enabled by default in C++ and by -Wall
otherwise.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled by
default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is enabled
by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is
enabled by -Wextra in C99 (and newer) and C++11 to C++17
modes.
-Wshift-overflow
-Wshift-overflow=n
Warn about left shift overflows. This warning is enabled by
default in C99 and C++11 modes (and newer).
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is
enabled by default in C99 and C++11 modes (and newer).
This warning level does not warn about left-shifting 1
into the sign bit. (However, in C, such an overflow is
still rejected in contexts where an integer constant
expression is required.) No warning is emitted in C++2A
mode (and newer), as signed left shifts always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into
the sign bit, unless C++14 mode (or newer) is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated
type and lacks a "case" for one or more of the named codes of
that enumeration. (The presence of a "default" label
prevents this warning.) "case" labels outside the
enumeration range also provoke warnings when this option is
used (even if there is a "default" label). This warning is
enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default"
case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated
type and lacks a "case" for one or more of the named codes of
that enumeration. "case" labels outside the enumeration
range also provoke warnings when this option is used. The
only difference between -Wswitch and this option is that this
option gives a warning about an omitted enumeration code even
if there is a "default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of boolean
type and the case values are outside the range of a boolean
type. It is possible to suppress this warning by casting the
controlling expression to a type other than "bool". For
example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wswitch-unreachable
Warn whenever a "switch" statement contains statements
between the controlling expression and the first case label,
which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between
the controlling expression and the first case label is just a
declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed
semantics in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but
otherwise unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with
-Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise
unused (aside from its declaration). This warning is enabled
by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
-Wunused-function
Warn whenever a static function is declared but not defined
or a non-inline static function is unused. This warning is
enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning
is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++
only)
Warn when a typedef locally defined in a function is not
used. This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The
default is -Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from
its declaration. This option implies
-Wunused-const-variable=1 for C, but not for C++. This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from
its declaration. -Wunused-const-variable=1 is enabled by
-Wunused-variable for C, but not for C++. In C this declares
variable storage, but in C++ this is not an error since const
variables take the place of "#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by
-Wunused-variable for C. It warns only about unused
static const variables defined in the main compilation
unit, but not about static const variables declared in
any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static
variables in headers (excluding system headers). This is
the warning level of -Wunused-const-variable and must be
explicitly requested since in C++ this isn't an error and
in C it might be harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is
explicitly not used. To suppress this warning cast the unused
expression to "void". This includes an expression-statement
or the left-hand side of a comma expression that contains no
side effects. For example, an expression such as "x[i,j]"
causes a warning, while "x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter,
you must either specify -Wextra -Wunused (note that -Wall
implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp"
call. In C++, warn if a non-static reference or non-static
"const" member appears in a class without constructors.
If you want to warn about code that uses the uninitialized
value of the variable in its own initializer, use the
-Winit-self option.
These warnings occur for individual uninitialized or
clobbered elements of structure, union or array variables as
well as for variables that are uninitialized or clobbered as
a whole. They do not occur for variables or elements
declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there
are warnings depends on the precise optimization options and
version of GCC used.
Note that there may be no warning about a variable that is
used only to compute a value that itself is never used,
because such computations may be deleted by data flow
analysis before the warnings are printed.
-Winvalid-memory-model
Warn for invocations of __atomic Builtins, __sync Builtins,
and the C11 atomic generic functions with a memory
consistency argument that is either invalid for the operation
or outside the range of values of the "memory_order"
enumeration. For example, since the "__atomic_store" and
"__atomic_store_n" built-ins are only defined for the
relaxed, release, and sequentially consistent memory orders
the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic (i.e. local) variable, if there exists a
path from the function entry to a use of the variable that is
initialized, but there exist some other paths for which the
variable is not initialized, the compiler emits a warning if
it cannot prove the uninitialized paths are not executed at
run time.
These warnings are only possible in optimizing compilation,
because otherwise GCC does not keep track of the state of
variables.
These warnings are made optional because GCC may not be able
to determine when the code is correct in spite of appearing
to have an error. Here is one example of how this can
happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.
This option also warns when a non-volatile automatic variable
might be changed by a call to "longjmp". The compiler sees
only the calls to "setjmp". It cannot know where "longjmp"
will be called; in fact, a signal handler could call it at
any point in the code. As a result, you may get a warning
even when there is in fact no problem because "longjmp"
cannot in fact be called at the place that would cause a
problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not
understood by GCC. If this command-line option is used,
warnings are even issued for unknown pragmas in system header
files. This is not the case if the warnings are only enabled
by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas.
See also -Wunknown-pragmas.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for
constructor or destructor. The use of constructor and
destructor attributes allow you to assign a priority to the
constructor/destructor to control its order of execution
before "main" is called or after it returns. The priority
values must be greater than 100 as the compiler reserves
priority values between 0--100 for the implementation.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing
rules that the compiler is using for optimization. The
warning does not catch all cases, but does attempt to catch
the more common pitfalls. It is included in -Wall. It is
equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing
rules that the compiler is using for optimization. Higher
levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the
way -O works. -Wstrict-aliasing is equivalent to
-Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly
useful when higher levels do not warn but -fstrict-aliasing
still breaks the code, as it has very few false negatives.
However, it has many false positives. Warns for all pointer
conversions between possibly incompatible types, even if
never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have
many false positives (not as many as level 1 though), and few
false negatives (but possibly more than level 1). Unlike
level 1, it only warns when an address is taken. Warns about
incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few
false positives and few false negatives. Slightly slower
than levels 1 or 2 when optimization is enabled. Takes care
of the common pun+dereference pattern in the front end:
"*(int*)&some_float". If optimization is enabled, it also
runs in the back end, where it deals with multiple statement
cases using flow-sensitive points-to information. Only warns
when the converted pointer is dereferenced. Does not warn
about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined.
It warns about cases where the compiler optimizes based on
the assumption that signed overflow does not occur. Note
that it does not warn about all cases where the code might
overflow: it only warns about cases where the compiler
implements some optimization. Thus this warning depends on
the optimization level.
An optimization that assumes that signed overflow does not
occur is perfectly safe if the values of the variables
involved are such that overflow never does, in fact, occur.
Therefore this warning can easily give a false positive: a
warning about code that is not actually a problem. To help
focus on important issues, several warning levels are
defined. No warnings are issued for the use of undefined
signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will
be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to
avoid. For example the compiler simplifies "x + 1 > x"
to 1. This level of -Wstrict-overflow is enabled by
-Wall; higher levels are not, and must be explicitly
requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is
simplified to a constant. For example: "abs (x) >= 0".
This can only be simplified when signed integer overflow
is undefined, because "abs (INT_MIN)" overflows to
"INT_MIN", which is less than zero. -Wstrict-overflow
(with no level) is the same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is
simplified. For example: "x + 1 > 1" is simplified to "x
> 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the
above cases. For example: "(x * 10) / 5" is simplified
to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison. For
example: "x + 2 > y" is simplified to "x + 1 >= y". This
is reported only at the highest warning level because
this simplification applies to many comparisons, so this
warning level gives a very large number of false
positives.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as
"memcpy" and "strcpy" that are determined to overflow the
destination buffer. The optional argument is one greater
than the type of Object Size Checking to perform to determine
the size of the destination. The argument is meaningful only
for functions that operate on character arrays but not for
raw memory functions like "memcpy" which always make use of
Object Size type-0. The option also warns for calls that
specify a size in excess of the largest possible object or at
most "SIZE_MAX / 2" bytes. The option produces the best
results with optimization enabled but can detect a small
subset of simple buffer overflows even without optimization
in calls to the GCC built-in functions like
"__builtin_memcpy" that correspond to the standard functions.
In any case, the option warns about just a subset of buffer
overflows detected by the corresponding overflow checking
built-ins. For example, the option will issue a warning for
the "strcpy" call below because it copies at least 5
characters (the string "blue" including the terminating NUL)
into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object
Size Checking to determine the sizes of destination
objects. This is the default setting of the option. At
this setting the option will not warn for writes past the
end of subobjects of larger objects accessed by pointers
unless the size of the largest surrounding object is
known. When the destination may be one of several
objects it is assumed to be the largest one of them. On
Linux systems, when optimization is enabled at this
setting the option warns for the same code as when the
"_FORTIFY_SOURCE" macro is defined to a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object
Size Checking to determine the sizes of destination
objects. At this setting the option will warn about
overflows when writing to members of the largest complete
objects whose exact size is known. It will, however, not
warn for excessive writes to the same members of unknown
objects referenced by pointers since they may point to
arrays containing unknown numbers of elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object
Size Checking to determine the sizes of destination
objects. At this setting the option warns about
overflowing the smallest object or data member. This is
the most restrictive setting of the option that may
result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object
Size Checking to determine the sizes of destination
objects. At this setting the option will warn about
overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of
them to decide whether to issue a warning. Similarly to
-Wstringop-overflow=3 this setting of the option may
result in warnings for benign code.
-Wstringop-truncation
Warn for calls to bounded string manipulation functions such
as "strncat", "strncpy", and "stpncpy" that may either
truncate the copied string or leave the destination
unchanged.
In the following example, the call to "strncat" specifies a
bound that is less than the length of the source string. As
a result, the copy of the source will be truncated and so the
call is diagnosed. To avoid the warning use "bufsize -
strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results
in copying to "d" just the characters preceding the
terminating NUL, without appending the NUL to the end.
Assuming the result of "strncpy" is necessarily a NUL-
terminated string is a common mistake, and so the call is
diagnosed. To avoid the warning when the result is not
expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the
size of the destination buffer as the bound. If the length
of the source string is equal to or greater than this size
the result of the copy will not be NUL-terminated.
Therefore, the call is also diagnosed. To avoid the warning,
specify "sizeof buf - 1" as the bound and set the last
element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a
sequence of bytes with no terminating "NUL" such an array may
be annotated with attribute "nonstring" to avoid this
warning. Such arrays, however, are not suitable arguments to
functions that expect "NUL"-terminated strings. To help
detect accidental misuses of such arrays GCC issues warnings
unless it can prove that the use is safe.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
Warn for cases where adding an attribute may be beneficial.
The attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for
attributes "pure", "const" or "noreturn" or "malloc". The
compiler only warns for functions visible in other
compilation units or (in the case of "pure" and "const")
if it cannot prove that the function returns normally. A
function returns normally if it doesn't contain an
infinite loop or return abnormally by throwing, calling
"abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy
of the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible
candidates, not absolute ones. GCC guesses that function
pointers with "format" attributes that are used in
assignment, initialization, parameter passing or return
statements should have a corresponding "format" attribute
in the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter
variable, or the return type of the containing function
respectively should also have a "format" attribute to
avoid the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are
only possible candidates. GCC guesses that "format"
attributes might be appropriate for any function that
calls a function like "vprintf" or "vscanf", but this
might not always be the case, and some functions for
which "format" attributes are appropriate may not be
detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold"
attribute. This is based on static detection and
generally will only warn about functions which always
leads to a call to another "cold" function such as
wrappers of C++ "throw" or fatal error reporting
functions leading to "abort".
-Wsuggest-final-types
Warn about types with virtual methods where code quality
would be improved if the type were declared with the C++11
"final" specifier, or, if possible, declared in an anonymous
namespace. This allows GCC to more aggressively devirtualize
the polymorphic calls. This warning is more effective with
link time optimization, where the information about the class
hierarchy graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be
improved if the method were declared with the C++11 "final"
specifier, or, if possible, its type were declared in an
anonymous namespace or with the "final" specifier. This
warning is more effective with link-time optimization, where
the information about the class hierarchy graph is more
complete. It is recommended to first consider suggestions of
-Wsuggest-final-types and then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked
with the override keyword.
-Walloc-zero
Warn about calls to allocation functions decorated with
attribute "alloc_size" that specify zero bytes, including
those to the built-in forms of the functions "aligned_alloc",
"alloca", "calloc", "malloc", and "realloc". Because the
behavior of these functions when called with a zero size
differs among implementations (and in the case of "realloc"
has been deprecated) relying on it may result in subtle
portability bugs and should be avoided.
-Walloc-size-larger-than=byte-size
Warn about calls to functions decorated with attribute
"alloc_size" that attempt to allocate objects larger than the
specified number of bytes, or where the result of the size
computation in an integer type with infinite precision would
exceed the value of PTRDIFF_MAX on the target.
-Walloc-size-larger-than=PTRDIFF_MAX is enabled by default.
Warnings controlled by the option can be disabled either by
specifying byte-size of SIZE_MAX or more or by
-Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disable -Walloc-size-larger-than= warnings. The option is
equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=byte-size
This option warns on calls to "alloca" with an integer
argument whose value is either zero, or that is not bounded
by a controlling predicate that limits its value to at most
byte-size. It also warns for calls to "alloca" where the
bound value is unknown. Arguments of non-integer types are
considered unbounded even if they appear to be constrained to
the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000"
would not issue a warning because the call to "alloca" is
known to be at most 1000 bytes. However, if
"-Walloca-larger-than=500" were passed, the compiler would
emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with
no controlling predicate constraining its integer argument.
For example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would
trigger a warning, but this time because of the lack of
bounds checking.
Note, that even seemingly correct code involving signed
integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger
than expected argument to be implicitly cast into the
"alloca" call.
This option also warns when "alloca" is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is
usually only effective when -ftree-vrp is active (default
for -O2 and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is
equivalent to -Walloca-larger-than=SIZE_MAX or larger.
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default
for -O2 and above). It warns about subscripts to arrays that
are always out of bounds. This warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is
enabled by -Wall; higher levels are not, and must be
explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds access
for arrays at the end of a struct and for arrays accessed
through pointers. This warning level may give a larger
number of false positives and is deactivated by default.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar
attributes whose target is incompatible with the type of the
alias.
-Wattribute-alias=1
The default warning level of the -Wattribute-alias option
diagnoses incompatibilities between the type of the alias
declaration and that of its target. Such
incompatibilities are typically indicative of bugs.
-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases
where the attributes of the alias declaration are more
restrictive than the attributes applied to its target.
These mismatches can potentially result in incorrect code
generation. In other cases they may be benign and could
be resolved simply by adding the missing attribute to the
target. For comparison, see the -Wmissing-attributes
option, which controls diagnostics when the alias
declaration is less restrictive than the target, rather
than more restrictive.
Attributes considered include "alloc_align",
"alloc_size", "cold", "const", "hot", "leaf", "malloc",
"nonnull", "noreturn", "nothrow", "pure",
"returns_nonnull", and "returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This
is the default. You can disable these warnings with either
-Wno-attribute-alias or -Wattribute-alias=0.
-Wbool-compare
Warn about boolean expression compared with an integer value
different from "true"/"false". For instance, the following
comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean
type. For instance, bitwise negation of a boolean is very
likely a bug in the program. For C, this warning also warns
about incrementing or decrementing a boolean, which rarely
makes sense. (In C++, decrementing a boolean is always
invalid. Incrementing a boolean is invalid in C++17, and
deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning
detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null
statement. This warning also warn for conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For
instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or
__builtin_return_address is called with an argument greater
than 0. Such calls may return indeterminate values or crash
the program. The warning is included in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being
discarded. Typically, the compiler warns if a "const char *"
variable is passed to a function that takes a "char *"
parameter. This option can be used to suppress such a
warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer
targets are being discarded. Typically, the compiler warns if
a "const int (*)[]" variable is passed to a function that
takes a "int (*)[]" parameter. This option can be used to
suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that
have incompatible types. This warning is for cases not
covered by -Wno-pointer-sign, which warns for pointer
argument passing or assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer
to integer conversions. This warning is about implicit
conversions; for explicit conversions the warnings
-Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be
used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero.
Floating-point division by zero is not warned about, as it
can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header
files. Warnings from system headers are normally suppressed,
on the assumption that they usually do not indicate real
problems and would only make the compiler output harder to
read. Using this command-line option tells GCC to emit
warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with
this option does not warn about unknown pragmas in system
headers---for that, -Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false.
This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always
evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested
functions. A trampoline is a small piece of data or code
that is created at run time on the stack when the address of
a nested function is taken, and is used to call the nested
function indirectly. For some targets, it is made up of data
only and thus requires no special treatment. But, for most
targets, it is made up of code and thus requires the stack to
be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality
comparisons.
The idea behind this is that sometimes it is convenient (for
the programmer) to consider floating-point values as
approximations to infinitely precise real numbers. If you
are doing this, then you need to compute (by analyzing the
code, or in some other way) the maximum or likely maximum
error that the computation introduces, and allow for it when
performing comparisons (and when producing output, but that's
a different problem). In particular, instead of testing for
equality, you should check to see whether the two values have
ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that
have no traditional C equivalent, and/or problematic
constructs that should be avoided.
* Macro parameters that appear within string literals in
the macro body. In traditional C macro replacement takes
place within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not
exist. Traditional preprocessors only considered a line
to be a directive if the # appeared in column 1 on the
line. Therefore -Wtraditional warns about directives
that traditional C understands but ignores because the #
does not appear as the first character on the line. It
also suggests you hide directives like "#pragma" not
understood by traditional C by indenting them. Some
traditional implementations do not recognize "#elif", so
this option suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-
point constant suffixes. (Traditional C does support the
L suffix on integer constants.) Note, these suffixes
appear in macros defined in the system headers of most
modern systems, e.g. the _MIN/_MAX macros in
"<limits.h>". Use of these macros in user code might
normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid
warning in these cases.
* A function declared external in one block and then used
after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static"
one. This construct is not accepted by some traditional
C compilers.
* The ISO type of an integer constant has a different width
or signedness from its traditional type. This warning is
only issued if the base of the constant is ten. I.e.
hexadecimal or octal values, which typically represent
bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
* Initialization of unions. If the initializer is zero,
the warning is omitted. This is done under the
assumption that the zero initializer in user code appears
conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization
to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point
values and vice versa. The absence of these prototypes
when compiling with traditional C causes serious
problems. This is a subset of the possible conversion
warnings; for the full set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features appear in
your code when using libiberty's traditional C
compatibility macros, "PARAMS" and "VPARAMS". This
warning is also bypassed for nested functions because
that feature is already a GCC extension and thus not
relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is
different from what would happen to the same argument in the
absence of a prototype. This includes conversions of fixed
point to floating and vice versa, and conversions changing
the width or signedness of a fixed-point argument except when
the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a
block. This construct, known from C++, was introduced with
ISO C99 and is by default allowed in GCC. It is not
supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration shadows
another variable, parameter, type, class member (in C++), or
instance variable (in Objective-C) or whenever a built-in
function is shadowed. Note that in C++, the compiler warns if
a local variable shadows an explicit typedef, but not if it
shadows a struct/class/enum. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance
variable in an Objective-C method.
-Wshadow=global
The default for -Wshadow. Warns for any (global) shadowing.
-Wshadow=local
Warn when a local variable shadows another local variable or
parameter. This warning is enabled by -Wshadow=global.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or
parameter whose type is compatible with that of the shadowing
variable. In C++, type compatibility here means the type of
the shadowing variable can be converted to that of the
shadowed variable. The creation of this flag (in addition to
-Wshadow=local) is based on the idea that when a local
variable shadows another one of incompatible type, it is most
likely intentional, not a bug or typo, as shown in the
following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have
incompatible types, enabling only -Wshadow=compatible-local
will not emit a warning. Because their types are
incompatible, if a programmer accidentally uses one in place
of the other, type checking will catch that and emit an error
or warning. So not warning (about shadowing) in this case
will not lead to undetected bugs. Use of this flag instead of
-Wshadow=local can possibly reduce the number of warnings
triggered by intentional shadowing.
This warning is enabled by -Wshadow=local.
-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-
size. -Wlarger-than=PTRDIFF_MAX is enabled by default.
Warnings controlled by the option can be disabled either by
specifying byte-size of SIZE_MAX or more or by
-Wno-larger-than.
-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to
-Wlarger-than=SIZE_MAX or larger.
-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The
computation done to determine the stack frame size is
approximate and not conservative. The actual requirements
may be somewhat greater than byte-size even if you do not get
a warning. In addition, any space allocated via "alloca",
variable-length arrays, or related constructs is not included
by the compiler when determining whether or not to issue a
warning. -Wframe-larger-than=PTRDIFF_MAX is enabled by
default. Warnings controlled by the option can be disabled
either by specifying byte-size of SIZE_MAX or more or by
-Wno-frame-larger-than.
-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is
equivalent to -Wframe-larger-than=SIZE_MAX or larger.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not
allocated on the heap.
-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size.
The computation done to determine the stack usage is
conservative. Any space allocated via "alloca", variable-
length arrays, or related constructs is included by the
compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the
specified amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded,
it's:
warning: stack usage might be unbounded
-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings
controlled by the option can be disabled either by specifying
byte-size of SIZE_MAX or more or by -Wno-stack-usage.
-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to
-Wstack-usage=SIZE_MAX or larger.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler
cannot assume anything on the bounds of the loop indices.
With -funsafe-loop-optimizations warn if the compiler makes
such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without
GNU extensions, this option disables the warnings about non-
ISO "printf" / "scanf" format width specifiers "I32", "I64",
and "I" used on Windows targets, which depend on the MS
runtime.
-Waligned-new
Warn about a new-expression of a type that requires greater
alignment than the "alignof(std::max_align_t)" but uses an
allocation function without an explicit alignment parameter.
This option is enabled by -Wall.
Normally this only warns about global allocation functions,
but -Waligned-new=all also warns about class member
allocation functions.
-Wplacement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior,
such as constructing an object in a buffer that is smaller
than the type of the object. For example, the placement new
expression below is diagnosed because it attempts to
construct an array of 64 integers in a buffer only 64 bytes
large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At
this level the warning is not issued for some strictly
undefined constructs that GCC allows as extensions for
compatibility with legacy code. For example, the
following "new" expression is not diagnosed at this level
even though it has undefined behavior according to the
C++ standard because it writes past the end of the one-
element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same
constructs as at level 1, a diagnostic is also issued for
placement new expressions that construct an object in the
last member of structure whose type is an array of a
single element and whose size is less than the size of
the object being constructed. While the previous example
would be diagnosed, the following construct makes use of
the flexible member array extension to avoid the warning
at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wpointer-arith
Warn about anything that depends on the "size of" a function
type or of "void". GNU C assigns these types a size of 1,
for convenience in calculations with "void *" pointers and
pointers to functions. In C++, warn also when an arithmetic
operation involves "NULL". This warning is also enabled by
-Wpedantic.
-Wpointer-compare
Warn if a pointer is compared with a zero character constant.
This usually means that the pointer was meant to be
dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to
the limited range of the data type, but do not warn for
constant expressions. For example, warn if an unsigned
variable is compared against zero with "<" or ">=". This
warning is also enabled by -Wextra.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the
absolute value of an argument when a more appropriate
standard function is available. For example, calling
"abs(3.14)" triggers the warning because the appropriate
function to call to compute the absolute value of a double
argument is "fabs". The option also triggers warnings when
the argument in a call to such a function has an unsigned
type. This warning can be suppressed with an explicit type
cast and it is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a //
comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the
meaning of the program. Trigraphs within comments are not
warned about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if"
directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a
macro (including the case where the macro is expanded by an
#if directive). Such usage is not portable. This warning is
also enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused.
A macro is used if it is expanded or tested for existence at
least once. The preprocessor also warns if the macro has not
been used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and
macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as
unused. To avoid the warning in such a case, you might
improve the scope of the macro's definition by, for example,
moving it into the first skipped block. Alternatively, you
could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed
by text. This sometimes happens in older programs with code
of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This
warning is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an
integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in
ISO C99. For instance, warn about use of variable length
arrays, "long long" type, "bool" type, compound literals,
designated initializers, and so on. This option is
independent of the standards mode. Warnings are disabled in
the expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in
ISO C11. For instance, warn about use of anonymous
structures and unions, "_Atomic" type qualifier,
"_Thread_local" storage-class specifier, "_Alignas"
specifier, "Alignof" operator, "_Generic" keyword, and so on.
This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in
ISO C2X. For instance, warn about omitting the string in
"_Static_assert". This option is independent of the
standards mode. Warnings are disabled in the expression that
follows "__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common
subset of ISO C and ISO C++, e.g. request for implicit
conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO
C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998
that are keywords in ISO C++ 2011. This warning turns on
-Wnarrowing and is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO
C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO
C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type
qualifier from the target type. For example, warn if a
"const char *" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier
in an unsafe way. For example, casting "char **" to "const
char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required
alignment of the target is increased. For example, warn if a
"char *" is cast to an "int *" on machines where integers can
only be accessed at two- or four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required
alignment of the target is increased. For example, warn if a
"char *" is cast to an "int *" regardless of the target
machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible
function pointer. In a cast involving function types with a
variable argument list only the types of initial arguments
that are provided are considered. Any parameter of pointer-
type matches any other pointer-type. Any benign differences
in integral types are ignored, like "int" vs. "long" on ILP32
targets. Likewise type qualifiers are ignored. The function
type "void (*) (void)" is special and matches everything,
which can be used to suppress this warning. In a cast
involving pointer to member types this warning warns whenever
the type cast is changing the pointer to member type. This
warning is enabled by -Wextra.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a
non-"const" "char *" pointer produces a warning. These
warnings help you find at compile time code that can try to
write into a string constant, but only if you have been very
careful about using "const" in declarations and prototypes.
Otherwise, it is just a nuisance. This is why we did not make
-Wall request these warnings.
When compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning is enabled by
default for C++ programs.
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference.
With -Wcatch-value=1 (or -Wcatch-value for short) warn about
polymorphic class types that are caught by value. With
-Wcatch-value=2 warn about all class types that are caught by
value. With -Wcatch-value=3 warn about all types that are not
caught by reference. -Wcatch-value is enabled by -Wall.
-Wclobbered
Warn for variables that might be changed by "longjmp" or
"vfork". This warning is also enabled by -Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs])
constructs.
-Wconversion
Warn for implicit conversions that may alter a value. This
includes conversions between real and integer, like "abs (x)"
when "x" is "double"; conversions between signed and
unsigned, like "unsigned ui = -1"; and conversions to smaller
types, like "sqrtf (M_PI)". Do not warn for explicit casts
like "abs ((int) x)" and "ui = (unsigned) -1", or if the
value is not changed by the conversion like in "abs (2.0)".
Warnings about conversions between signed and unsigned
integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for
user-defined conversions; and conversions that never use a
type conversion operator: conversions to "void", the same
type, a base class or a reference to them. Warnings about
conversions between signed and unsigned integers are disabled
by default in C++ unless -Wsign-conversion is explicitly
enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer
types. -Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This
can be useful to facilitate the conversion to "nullptr" in
C++11.
-Wsubobject-linkage (C++ and Objective-C++ only)
Warn if a class type has a base or a field whose type uses
the anonymous namespace or depends on a type with no linkage.
If a type A depends on a type B with no or internal linkage,
defining it in multiple translation units would be an ODR
violation because the meaning of B is different in each
translation unit. If A only appears in a single translation
unit, the best way to silence the warning is to give it
internal linkage by putting it in an anonymous namespace as
well. The compiler doesn't give this warning for types
defined in the main .C file, as those are unlikely to have
multiple definitions. -Wsubobject-linkage is enabled by
default.
-Wdangling-else
Warn about constructions where there may be confusion to
which "if" statement an "else" branch belongs. Here is an
example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost
possible "if" statement, which in this example is "if (b)".
This is often not what the programmer expected, as
illustrated in the above example by indentation the
programmer chose. When there is the potential for this
confusion, GCC issues a warning when this flag is specified.
To eliminate the warning, add explicit braces around the
innermost "if" statement so there is no way the "else" can
belong to the enclosing "if". The resulting code looks like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__"
are encountered as they might prevent bit-wise-identical
reproducible compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which may
cause undefined behavior at runtime. This warning is enabled
by default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while"
statement. This warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different
enumerated types. In C++ enumerated type mismatches in
conditional expressions are also diagnosed and the warning is
enabled by default. In C this warning is enabled by -Wall.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolon after in-class function
definition.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps
forward across the initialization of a variable, or jumps
backward to a label after the variable has been initialized.
This only warns about variables that are initialized when
they are declared. This warning is only supported for C and
Objective-C; in C++ this sort of branch is an error in any
case.
-Wjump-misses-init is included in -Wc++-compat. It can be
disabled with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values
could produce an incorrect result when the signed value is
converted to unsigned. In C++, this warning is also enabled
by -Wall. In C, it is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to
an unsigned integer variable. An explicit cast silences the
warning. In C, this option is enabled also by -Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a
real value. This includes conversions from real to integer,
and from higher precision real to lower precision real
values. This option is also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar
storage order.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation
function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with
-fsized-deallocation.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that
divide the pointer size by the element size, which is the
usual way to compute the array size but won't work out
correctly with pointers. This warning warns e.g. about
"sizeof (ptr) / sizeof (ptr[0])" if "ptr" is not an array,
but a pointer. This warning is enabled by -Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and
memory built-in functions if the argument uses "sizeof".
This warning triggers for example for "memset (ptr, 0, sizeof
(ptr));" if "ptr" is not an array, but a pointer, and
suggests a possible fix, or about "memcpy (&foo, ptr, sizeof
(&foo));". -Wsizeof-pointer-memaccess also warns about calls
to bounded string copy functions like "strncat" or "strncpy"
that specify as the bound a "sizeof" expression of the source
array. For example, in the following function the call to
"strncat" specifies the size of the source string as the
bound. That is almost certainly a mistake and so the call is
diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a parameter
that is declared as an array in a function definition. This
warning is enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function,
if the first argument references an array, and the third
argument is a number equal to the number of elements, but not
equal to the size of the array in memory. This indicates
that the user has omitted a multiplication by the element
size. This warning is enabled by -Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function
where the second argument is not zero and the third argument
is zero. For example, the call "memset (buf, sizeof buf, 0)"
is diagnosed because "memset (buf, 0, sizeof buf)" was meant
instead. The diagnostic is only emitted if the third
argument is a literal zero. Otherwise, if it is an
expression that is folded to zero, or a cast of zero to some
type, it is far less likely that the arguments have been
mistakenly transposed and no warning is emitted. This
warning is enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include
using the address of a function in a conditional expression,
such as "void func(void); if (func)", and comparisons against
the memory address of a string literal, such as "if (x ==
"abc")". Such uses typically indicate a programmer error:
the address of a function always evaluates to true, so their
use in a conditional usually indicate that the programmer
forgot the parentheses in a function call; and comparisons
against string literals result in unspecified behavior and
are not portable in C, so they usually indicate that the
programmer intended to use "strcmp". This warning is enabled
by -Wall.
-Waddress-of-packed-member
Warn when the address of packed member of struct or union is
taken, which usually results in an unaligned pointer value.
This is enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in
expressions. This includes using logical operators in
contexts where a bit-wise operator is likely to be expected.
Also warns when the operands of a logical operator are the
same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of
a comparison. This option does not warn if the right operand
is considered to be a boolean expression. Its purpose is to
detect suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS
into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are
defined or called. (In languages where you can return an
array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the
compiler detects undefined behavior in some statement during
one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to
variables, etc. This does not stop errors for incorrect use
of supported attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible
signature or as a non-function, or when a built-in function
declared with a type that does not include a prototype is
called with arguments whose promoted types do not match those
expected by the function. When -Wextra is specified, also
warn when a built-in function that takes arguments is
declared without a prototype. The
-Wno-builtin-declaration-mismatch warning is enabled by
default. To avoid the warning include the appropriate header
to bring the prototypes of built-in functions into scope.
For example, the call to "memset" below is diagnosed by the
warning because the function expects a value of type "size_t"
as its argument but the type of 32 is "int". With -Wextra,
the declaration of the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying
the argument types. (An old-style function definition is
permitted without a warning if preceded by a declaration that
specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in
a declaration. For example, warn if storage-class specifiers
like "static" are not the first things in a declaration.
This warning is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning
is given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous
prototype declaration. This warning is issued even if the
definition itself provides a prototype. Use this option to
detect global functions that do not have a matching prototype
declaration in a header file. This option is not valid for
C++ because all function declarations provide prototypes and
a non-matching declaration declares an overload rather than
conflict with an earlier declaration. Use
-Wmissing-declarations to detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that
are not declared in header files. In C, no warnings are
issued for functions with previous non-prototype
declarations; use -Wmissing-prototypes to detect missing
prototypes. In C++, no warnings are issued for function
templates, or for inline functions, or for functions in
anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing.
For example, the following code causes such a warning,
because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so
the following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero
initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty {
} initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in
portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they
are different sequences of characters. However, sometimes
when characters outside the basic ASCII character set are
used, you can have two different character sequences that
look the same. To avoid confusion, the ISO 10646 standard
sets out some normalization rules which when applied ensure
that two sequences that look the same are turned into the
same sequence. GCC can warn you if you are using identifiers
that have not been normalized; this option controls that
warning.
There are four levels of warning supported by GCC. The
default is -Wnormalized=nfc, which warns about any identifier
that is not in the ISO 10646 "C" normalized form, NFC. NFC
is the recommended form for most uses. It is equivalent to
-Wnormalized.
Unfortunately, there are some characters allowed in
identifiers by ISO C and ISO C++ that, when turned into NFC,
are not allowed in identifiers. That is, there's no way to
use these symbols in portable ISO C or C++ and have all your
identifiers in NFC. -Wnormalized=id suppresses the warning
for these characters. It is hoped that future versions of
the standards involved will correct this, which is why this
option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do
this if you are using some other normalization scheme (like
"D"), because otherwise you can easily create bugs that are
literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially
once formatting has been applied. For instance "\u207F",
"SUPERSCRIPT LATIN SMALL LETTER N", displays just like a
regular "n" that has been placed in a superscript. ISO 10646
defines the NFKC normalization scheme to convert all these
into a standard form as well, and GCC warns if your code is
not in NFKC if you use -Wnormalized=nfkc. This warning is
comparable to warning about every identifier that contains
the letter O because it might be confused with the digit 0,
and so is not the default, but may be useful as a local
coding convention if the programming environment cannot be
fixed to display these characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions declared with "warning"
attribute. By default, this warning is enabled.
-Wno-attribute-warning can be used to disable the warning or
-Wno-error=attribute-warning can be used to disable the error
when compiled with -Werror flag.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types
marked as deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant
expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time
optimization. Requires -flto-odr-type-merging to be enabled.
Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd
directive set by user. The -fsimd-cost-model=unlimited
option can be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is
overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra -Wno-override-init.
-Woverride-init-side-effects (C and Objective-C only)
Warn if an initialized field with side effects is overridden
when using designated initializers. This warning is enabled
by default.
-Wpacked
Warn if a structure is given the packed attribute, but the
packed attribute has no effect on the layout or size of the
structure. Such structures may be mis-aligned for little
benefit. For instance, in this code, the variable "f.x" in
"struct bar" is misaligned even though "struct bar" does not
itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed"
attribute on bit-fields of type "char". This has been fixed
in GCC 4.4 but the change can lead to differences in the
structure layout. GCC informs you when the offset of such a
field has changed in GCC 4.4. For example there is no longer
a 4-bit padding between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment
in a packed struct or union is misaligned. For example, a
warning will be issued on "struct S", like, "warning:
alignment 1 of 'struct S' is less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align
an element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the
fields of the structure to reduce the padding and so make the
structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same
scope, even in cases where multiple declaration is valid and
changes nothing.
-Wno-restrict
Warn when an object referenced by a "restrict"-qualified
parameter (or, in C++, a "__restrict"-qualified parameter) is
aliased by another argument, or when copies between such
objects overlap. For example, the call to the "strcpy"
function below attempts to truncate the string by replacing
its initial characters with the last four. However, because
the call writes the terminating NUL into "a[4]", the copies
overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple
overlap even without optimization but works best at -O2 and
above. It is included in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a
function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors
when the base class inherited from has a C variadic
constructor; the warning is on by default because the
ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be
inlined. Even with this option, the compiler does not warn
about failures to inline functions declared in system
headers.
The compiler uses a variety of heuristics to determine
whether or not to inline a function. For example, the
compiler takes into account the size of the function being
inlined and the amount of inlining that has already been done
in the current function. Therefore, seemingly insignificant
changes in the source program can cause the warnings produced
by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a
non-POD type. According to the 2014 ISO C++ standard,
applying "offsetof" to a non-standard-layout type is
undefined. In existing C++ implementations, however,
"offsetof" typically gives meaningful results. This flag is
for users who are aware that they are writing nonportable
code and who have deliberately chosen to ignore the warning
about it.
The restrictions on "offsetof" may be relaxed in a future
version of the C++ standard.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean
values are expected, such as conditional expressions (?:)
using non-boolean integer constants in boolean context, like
"if (a <= b ? 2 : 3)". Or left shifting of signed integers
in boolean context, like "for (a = 0; 1 << a; a++);".
Likewise for all kinds of multiplications regardless of the
data type. This warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of
a different size. In C++, casting to a pointer type of
smaller size is an error. Wint-to-pointer-cast is enabled by
default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer
type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but
cannot be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To
inhibit the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the
GNU alternate syntax is used in ISO C99 mode. This is
enabled by either -Wpedantic or -Wtraditional. To inhibit
the warning messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle
variable arguments like "va_start". This is default. To
inhibit the warning messages, use -Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD
capabilities of the architecture. Mainly useful for the
performance tuning. Vector operation can be implemented
"piecewise", which means that the scalar operation is
performed on every vector element; "in parallel", which means
that the vector operation is implemented using scalars of
wider type, which normally is more performance efficient; and
"as a single scalar", which means that vector fits into a
scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator. This is
dangerous because if the virtual base is reachable along more
than one path, it is moved multiple times, which can mean
both objects end up in the moved-from state. If the move
assignment operator is written to avoid moving from a moved-
from object, this warning can be disabled.
-Wvla
Warn if a variable-length array is used in the code.
-Wno-vla prevents the -Wpedantic warning of the variable-
length array.
-Wvla-larger-than=byte-size
If this option is used, the compiler will warn for
declarations of variable-length arrays whose size is either
unbounded, or bounded by an argument that allows the array
size to exceed byte-size bytes. This is similar to how
-Walloca-larger-than=byte-size works, but with variable-
length arrays.
Note that GCC may optimize small variable-length arrays of a
known value into plain arrays, so this warning may not get
triggered for such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is
typically only effective when -ftree-vrp is active (default
for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is
equivalent to -Wvla-larger-than=SIZE_MAX or larger.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The
volatile modifier does not inhibit all optimizations that may
eliminate reads and/or writes to register variables. This
warning is enabled by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This
warning does not generally indicate that there is anything
wrong with your code; it merely indicates that GCC's
optimizers are unable to handle the code effectively. Often,
the problem is that your code is too big or too complex; GCC
refuses to optimize programs when the optimization itself is
likely to take inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with
different signedness. This option is only supported for C
and Objective-C. It is implied by -Wall and by -Wpedantic,
which can be disabled with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active.
It warns about functions that are not protected against stack
smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern
compilers generally allow string constants that are much
longer than the standard's minimum limit, but very portable
programs should avoid using longer strings.
The limit applies after string constant concatenation, and
does not count the trailing NUL. In C90, the limit was 509
characters; in C99, it was raised to 4095. C++98 does not
specify a normative minimum maximum, so we do not diagnose
overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled
with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have
a suffix. When used together with -Wsystem-headers it warns
about such constants in system header files. This can be
useful when preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the decimal floating-
point extension to C99.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to
initialize a structure that has been marked with the
"designated_init" attribute.
-Whsa
Issue a warning when HSAIL cannot be emitted for the compiled
function or OpenMP construct.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in
almost all cases you need only to add -g to your other options.
GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally be surprising: some variables you
declared may not exist at all; flow of control may briefly move
where you did not expect it; some statements may not be executed
because they compute constant results or their values are already
at hand; some statements may execute in different places because
they have been moved out of loops. Nevertheless it is possible
to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
If you are not using some other optimization option, consider
using -Og with -g. With no -O option at all, some compiler
passes that collect information useful for debugging do not run
at all, so that -Og may result in a better debugging experience.
-g Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF). GDB can work
with this debugging information.
On most systems that use stabs format, -g enables use of
extra debugging information that only GDB can use; this extra
information makes debugging work better in GDB but probably
makes other debuggers crash or refuse to read the program.
If you want to control for certain whether to generate the
extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff,
or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This means to
use the most expressive format available (DWARF, stabs, or
the native format if neither of those are supported),
including GDB extensions if at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is
supported). The value of version may be either 2, 3, 4 or 5;
the default version for most targets is 4. DWARF Version 5
is only experimental.
Note that with DWARF Version 2, some ports require and always
use some non-conflicting DWARF 3 extensions in the unwind
tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments
for maximum benefit.
GCC no longer supports DWARF Version 1, which is
substantially different than Version 2 and later. For
historical reasons, some other DWARF-related options such as
-fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in
their names, but apply to all currently-supported versions of
DWARF.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used
by DBX on most BSD systems. On MIPS, Alpha and System V
Release 4 systems this option produces stabs debugging output
that is not understood by DBX. On System V Release 4 systems
this option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to
make other debuggers crash or refuse to read the program.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on
IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to
make other debuggers crash or refuse to read the program, and
may cause assemblers other than the GNU assembler (GAS) to
fail with an error.
-gvms
Produce debugging information in Alpha/VMS debug format (if
that is supported). This is the format used by DEBUG on
Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify
how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0
negates -g.
Level 1 produces minimal information, enough for making
backtraces in parts of the program that you don't plan to
debug. This includes descriptions of functions and external
variables, and line number tables, but no information about
local variables.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support
macro expansion when you use -g3.
If you use multiple -g options, with or without level
numbers, the last such option is the one that is effective.
-gdwarf does not accept a concatenated debug level, to avoid
confusion with -gdwarf-level. Instead use an additional
-glevel option to change the debug level for DWARF.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in
only one object file, emit it in all object files using the
class. This option should be used only with debuggers that
are unable to handle the way GCC normally emits debugging
information for classes because using this option increases
the size of debugging information by as much as a factor of
two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the
debugging information that are identical in different object
files. Merging is not supported by all assemblers or
linkers. Merging decreases the size of the debug information
in the output file at the cost of increasing link processing
time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record
debugging information describing them as if the files resided
in directory new instead. This can be used to replace a
build-time path with an install-time path in the debug info.
It can also be used to change an absolute path to a relative
path by using . for new. This can give more reproducible
builds, which are location independent, but may require an
extra command to tell GDB where to find the source files. See
also -ffile-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are
stored at each position in code. Better debugging
information is then generated (if the debugging information
format supports this information).
It is enabled by default when compiling with optimization
(-Os, -O, -O2, ...), debugging information (-g) and the debug
info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the
compilation and attempt to carry the annotations over
throughout the compilation all the way to the end, in an
attempt to improve debug information while optimizing. Use
of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which
case annotations are created and maintained, but discarded at
the end. By default, this flag is enabled together with
-fvar-tracking, except when selective scheduling is enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible into
a separate output file with the extension .dwo. This option
allows the build system to avoid linking files with debug
information. To be useful, this option requires a debugger
capable of reading .dwo files.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no
name attribute, such as artificial variables, external
references and call site parameter DIEs.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes"
sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in
a format suitable for conversion into a GDB index. This
option is only useful with a linker that can produce GDB
index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put
into their own ".debug_types" section instead of making them
part of the ".debug_info" section. It is more efficient to
put them in a separate comdat section since the linker can
then remove duplicates. But not all DWARF consumers support
".debug_types" sections yet and on some objects
".debug_types" produces larger instead of smaller debugging
information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke
the compiler that may affect code generation to be appended
to the DW_AT_producer attribute in DWARF debugging
information. The options are concatenated with spaces
separating them from each other and from the compiler
version. It is enabled by default. See also
-frecord-gcc-switches for another way of storing compiler
options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version
than selected with -gdwarf-version. On most targets using
non-conflicting DWARF extensions from later standard versions
is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc"
directives. It may then use them for the assembler to
generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated
line-number tables are a lot more compact than those the
compiler can generate itself.
This option will be enabled by default if, at GCC configure
time, the assembler was found to support such directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally,
if DWARF2+ line number tables are to be generated.
gas-locview-support
Inform the compiler that the assembler supports "view"
assignment and reset assertion checking in ".loc" directives.
This option will be enabled by default if, at GCC configure
time, the assembler was found to support them.
gno-as-locview-support
Force GCC to assign view numbers internally, if
-gvariable-location-views are explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging
information, rather than just file and line. This option is
enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal
representation at the beginning of statements, and to keep
them roughly in place throughout compilation, using them to
guide the output of "is_stmt" markers in the line number
table. This is enabled by default when compiling with
optimization (-Os, -O, -O2, ...), and outputting DWARF 2
debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers
implied from the line number table. This enables debug
information consumers to inspect state at certain points of
the program, even if no instructions associated with the
corresponding source locations are present at that point. If
the assembler lacks support for view numbers in line number
tables, this will cause the compiler to emit the line number
table, which generally makes them somewhat less compact. The
augmented line number tables and location lists are fully
backward-compatible, so they can be consumed by debug
information consumers that are not aware of these
augmentations, but they won't derive any benefit from them
either.
This is enabled by default when outputting DWARF 2 debug
information at the normal level, as long as there is
assembler support, -fvar-tracking-assignments is enabled and
-gstrict-dwarf is not. When assembler support is not
available, this may still be enabled, but it will force GCC
to output internal line number tables, and if
-ginternal-reset-location-views is not enabled, that will
most certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is
not backward compatible with the location list format
introduced in DWARF 5, that can be enabled with
-gvariable-location-views=incompat5. This option may be
removed in the future, is only provided as a reference
implementation of the proposed representation. Debug
information consumers are not expected to support this
extended format, and they would be rendered unable to decode
location lists using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from
location view lists. This requires the compiler to have very
accurate insn length estimates, which isn't always the case,
and it may cause incorrect view lists to be generated
silently when using an assembler that does not support
location view lists. The GNU assembler will flag any such
error as a "view number mismatch". This is only enabled on
ports that define a reliable estimation function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions.
Location view tracking markers are inserted at inlined entry
points, so that address and view numbers can be computed and
output in debug information. This can be enabled
independently of location views, in which case the view
numbers won't be output, but it can only be enabled along
with statement frontiers, and it is only enabled by default
if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is
supported. If type is not given, the default type depends on
the capabilities of the assembler and linker used. type may
be one of none (don't compress debug sections), zlib (use
zlib compression in ELF gABI format), or zlib-gnu (use zlib
compression in traditional GNU format). If the linker
doesn't support writing compressed debug sections, the option
is rejected. Otherwise, if the assembler does not support
them, -gz is silently ignored when producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the
base name of the compilation source file matches the base
name of file in which the struct is defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type
information to the debugger. See -femit-struct-debug-reduced
for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the
base name of the compilation source file matches the base
name of file in which the type is defined, unless the struct
is a template or defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type information to
the debugger. See -femit-struct-debug-baseonly for a more
aggressive option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler
generates debug information. The intent is to reduce
duplicate struct debug information between different object
files within the same program.
This option is a detailed version of
-femit-struct-debug-reduced and -femit-struct-debug-baseonly,
which serves for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs
that are used directly (dir:) or used indirectly (ind:). A
struct type is used directly when it is the type of a
variable, member. Indirect uses arise through pointers to
structs. That is, when use of an incomplete struct is valid,
the use is indirect. An example is struct one direct; struct
two * indirect;.
The optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:). Generic structs
are a bit complicated to explain. For C++, these are non-
explicit specializations of template classes, or non-template
classes within the above. Other programming languages have
generics, but -femit-struct-debug-detailed does not yet
implement them.
The third word specifies the source files for those structs
for which the compiler should emit debug information. The
values none and any have the normal meaning. The value base
means that the base of name of the file in which the type
declaration appears must match the base of the name of the
main compilation file. In practice, this means that when
compiling foo.c, debug information is generated for types
declared in that file and foo.h, but not other header files.
The value sys means those types satisfying base or declared
in system or compiler headers.
You may need to experiment to determine the best settings for
your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame"
section instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing
debug symbol output for types that are nowhere used in the
source file being compiled. Sometimes it is useful to have
GCC emit debugging information for all types declared in a
compilation unit, regardless of whether or not they are
actually used in that compilation unit, for example if, in
the debugger, you want to cast a value to a type that is not
actually used in your program (but is declared). More often,
however, this results in a significant amount of wasted
space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce
the cost of compilation and to make debugging produce the
expected results. Statements are independent: if you stop the
program with a breakpoint between statements, you can then assign
a new value to any variable or change the program counter to any
other statement in the function and get exactly the results you
expect from the source code.
Turning on optimization flags makes the compiler attempt to
improve the performance and/or code size at the expense of
compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has
of the program. Compiling multiple files at once to a single
output file mode allows the compiler to use information gained
from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at -O0 or if an -O
level is not set on the command line, even if individual
optimization flags are specified. Similarly, -Og suppresses many
optimization passes.
Depending on the target and how GCC was configured, a slightly
different set of optimizations may be enabled at each -O level
than those listed here. You can invoke GCC with -Q
--help=optimizers to find out the exact set of optimizations that
are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time,
and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great
deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop
-fdelayed-branch -fdse -fforward-propagate
-fguess-branch-probability -fif-conversion -fif-conversion2
-finline-functions-called-once -fipa-profile -fipa-pure-const
-fipa-reference -fipa-reference-addressable -fmerge-constants
-fmove-loop-invariants -fomit-frame-pointer -freorder-blocks
-fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types
-fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp
-ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both compilation time
and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also
turns on the following optimization flags:
-falign-functions -falign-jumps -falign-labels
-falign-loops -fcaller-saves -fcode-hoisting -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations -fgcse
-fgcse-lm -fhoist-adjacent-loads -finline-small-functions
-findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf
-fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging
-fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce
-ftree-pre -ftree-switch-conversion -ftree-tail-merge
-ftree-vrp
Please note the warning under -fgcse about invoking -O2 on
programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified
by -O2 and also turns on the following optimization flags:
-fgcse-after-reload -finline-functions -fipa-cp-clone
-floop-interchange -floop-unroll-and-jam -fpeel-loops
-fpredictive-commoning -fsplit-paths
-ftree-loop-distribute-patterns -ftree-loop-distribution
-ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model
-fversion-loops-for-strides
-O0 Reduce compilation time and make debugging produce the
expected results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations except
those that often increase code size:
-falign-functions -falign-jumps -falign-labels
-falign-loops -fprefetch-loop-arrays
-freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to
tune for code size rather than execution speed, and performs
further optimizations designed to reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all
-O3 optimizations. It also enables optimizations that are
not valid for all standard-compliant programs. It turns on
-ffast-math and the Fortran-specific -fstack-arrays, unless
-fmax-stack-var-size is specified, and -fno-protect-parens.
-Og Optimize debugging experience. -Og should be the
optimization level of choice for the standard edit-compile-
debug cycle, offering a reasonable level of optimization
while maintaining fast compilation and a good debugging
experience. It is a better choice than -O0 for producing
debuggable code because some compiler passes that collect
debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization
passes so that individual options controlling them have no
effect. Otherwise -Og enables all -O1 optimization flags
except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp
-ftree-pta -ftree-sra
If you use multiple -O options, with or without level numbers,
the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags.
Most flags have both positive and negative forms; the negative
form of -ffoo is -fno-foo. In the table below, only one of the
forms is listed---the one you typically use. You can figure out
the other form by either removing no- or adding it.
The following options control specific optimizations. They are
either activated by -O options or are related to ones that are.
You can use the following flags in the rare cases when "fine-
tuning" of optimizations to be performed is desired.
-fno-defer-pop
For machines that must pop arguments after a function call,
always pop the arguments as soon as each function returns.
At levels -O1 and higher, -fdefer-pop is the default; this
allows the compiler to let arguments accumulate on the stack
for several function calls and pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to
combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are
performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O,
-O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression
contraction. -ffp-contract=fast enables floating-point
expression contraction such as forming of fused multiply-add
operations if the target has native support for them.
-ffp-contract=on enables floating-point expression
contraction if allowed by the language standard. This is
currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one.
This avoids the instructions to save, set up and restore the
frame pointer; on many targets it also makes an extra
register available.
On some targets this flag has no effect because the standard
calling sequence always uses a frame pointer, so it cannot be
omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame
pointer is used in all functions. Several targets always
omit the frame pointer in leaf functions.
Enabled by default at -O and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen",
"strchr" or "strcpy") and their "_FORTIFY_SOURCE"
counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those marked
with the "always_inline" attribute. This is the default when
not optimizing.
Single functions can be exempted from inlining by marking
them with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is
smaller than expected function call code (so overall size of
program gets smaller). The compiler heuristically decides
which functions are simple enough to be worth integrating in
this way. This inlining applies to all functions, even those
not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has
any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-finline-functions
Consider all functions for inlining, even if they are not
declared inline. The compiler heuristically decides which
functions are worth integrating in this way.
If all calls to a given function are integrated, and the
function is declared "static", then the function is normally
not output as assembler code in its own right.
Enabled at levels -O3, -Os. Also enabled by -fprofile-use
and -fauto-profile.
-finline-functions-called-once
Consider all "static" functions called once for inlining into
their caller even if they are not marked "inline". If a call
to a given function is integrated, then the function is not
output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining
Inline functions marked by "always_inline" and functions
whose body seems smaller than the function call overhead
early before doing -fprofile-generate instrumentation and
real inlining pass. Doing so makes profiling significantly
cheaper and usually inlining faster on programs having large
chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates,
removal of unused parameters and replacement of parameters
passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be
inlined. This flag allows coarse control of this limit. n
is the size of functions that can be inlined in number of
pseudo instructions.
Inlining is actually controlled by a number of parameters,
which may be specified individually by using --param
name=value. The -finline-limit=n option sets some of these
parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these
parameters.
Note: there may be no value to -finline-limit that results in
default behavior.
Note: pseudo instruction represents, in this particular
context, an abstract measurement of function's size. In no
way does it represent a count of assembly instructions and as
such its exact meaning might change from one release to an
another.
-fno-keep-inline-dllexport
This is a more fine-grained version of
-fkeep-inline-functions, which applies only to functions that
are declared using the "dllexport" attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into
the object file, even if the function has been inlined into
all of its callers. This switch does not affect functions
using the "extern inline" extension in GNU C90. In C++, emit
any and all inline functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the
function is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization
isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if a variable is referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and
floating-point constants) across compilation units.
This option is the default for optimized compilation if the
assembler and linker support it. Use -fno-merge-constants to
inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables with
integral or floating-point types. Languages like C or C++
require each variable, including multiple instances of the
same variable in recursive calls, to have distinct locations,
so using this option results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and
reorders their instructions by overlapping different
iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with
register moves allowed. By setting this flag certain anti-
dependences edges are deleted, which triggers the generation
of reg-moves based on the life-range analysis. This option
is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to
use "decrement and branch" instructions on a count register
instead of instruction sequences that decrement a register,
compare it against zero, and then branch based upon the
result. This option is only meaningful on architectures that
support such instructions, which include x86, PowerPC, IA-64
and S/390. Note that the -fno-branch-count-reg option
doesn't remove the decrement and branch instructions from the
generated instruction stream introduced by other optimization
passes.
The default is -fbranch-count-reg at -O1 and higher, except
for -Og.
-fno-function-cse
Do not put function addresses in registers; make each
instruction that calls a constant function contain the
function's address explicitly.
This option results in less efficient code, but some strange
hacks that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts
variables that are initialized to zero into BSS. This can
save space in the resulting code.
This option turns off this behavior because some programs
explicitly rely on variables going to the data
section---e.g., so that the resulting executable can find the
beginning of that section and/or make assumptions based on
that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump branches to
a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the
destination of the second branch or a point immediately
following it, depending on whether the condition is known to
be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as
"long long" on a 32-bit system, split the registers apart and
allocate them independently. This normally generates better
code for those types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by
any other path. For example, when CSE encounters an "if"
statement with an "else" clause, CSE follows the jump when
the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps that conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop
optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This
pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you
disable the global common subexpression elimination pass by
adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression
elimination attempts to move loads that are only killed by
stores into themselves. This allows a loop containing a
load/store sequence to be changed to a load outside the loop,
and a copy/store within the loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after
global common subexpression elimination. This pass attempts
to move stores out of loops. When used in conjunction with
-fgcse-lm, loops containing a load/store sequence can be
changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after
stores to the same memory location (both partial and full
redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load
elimination pass is performed after reload. The purpose of
this pass is to clean up redundant spilling.
Enabled by -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language
constraints to derive bounds for the number of iterations of
a loop. This assumes that loop code does not invoke
undefined behavior by for example causing signed integer
overflows or out-of-bound array accesses. The bounds for the
number of iterations of a loop are used to guide loop
unrolling and peeling and loop exit test optimizations. This
option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in
common blocks (e.g. Fortran) may later be overridden with
longer trailing arrays. This prevents certain optimizations
that depend on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation
unifies equivalent code and saves code size. The resulting
code may or may not perform better than without cross-
jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory
accesses. This pass is always skipped on architectures that
do not have instructions to support this. Enabled by default
at -O and higher on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by
default at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by
default at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This includes use of conditional moves, min,
max, set flags and abs instructions, and some tricks doable
by standard arithmetics. The use of conditional execution on
chips where it is available is controlled by
-fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors
and destructors: one for a base subobject, one for a complete
object, and one for a virtual destructor that calls operator
delete afterwards. For a hierarchy with virtual bases, the
base and complete variants are clones, which means two copies
of the function. With this option, the base and complete
variants are changed to be thunks that call a common
implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers,
and that no code or data element resides at address zero.
This option enables simple constant folding optimizations at
all optimization levels. In addition, other optimization
passes in GCC use this flag to control global dataflow
analyses that eliminate useless checks for null pointers;
these assume that a memory access to address zero always
results in a trap, so that if a pointer is checked after it
has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not
true. Use -fno-delete-null-pointer-checks to disable this
optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On Nios
II ELF, it defaults to off. On AVR, CR16, and MSP430, this
option is completely disabled.
Passes that use the dataflow information are enabled
independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct
calls. This is done both within a procedure and
interprocedurally as part of indirect inlining
(-findirect-inlining) and interprocedural constant
propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative
direct calls. Based on the analysis of the type inheritance
graph, determine for a given call the set of likely targets.
If the set is small, preferably of size 1, change the call
into a conditional deciding between direct and indirect
calls. The speculative calls enable more optimizations, such
as inlining. When they seem useless after further
optimization, they are converted back into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive
devirtualization when running the link-time optimizer in
local transformation mode. This option enables more
devirtualization but significantly increases the size of
streamed data. For this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture, which
implicitly zero-extends in 64-bit registers after writing to
their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes
within its lifetime: when the constructor begins, the object
has an indeterminate value, and any changes during the
lifetime of the object are dead when the object is destroyed.
Normally dead store elimination will take advantage of this;
if your code relies on the value of the object storage
persisting beyond the lifetime of the object, you can use
this flag to disable this optimization. To preserve stores
before the constructor starts (e.g. because your operator new
clears the object storage) but still treat the object as dead
after the destructor you, can use -flifetime-dse=1. The
default behavior can be explicitly selected with
-flifetime-dse=2. -flifetime-dse=0 is equivalent to
-fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live
range shrinkage. This is helpful for fast processors with
small or moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated
register allocator. The algorithm argument can be priority,
which specifies Chow's priority coloring, or CB, which
specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring
is not implemented for all architectures, but for those
targets that do support it, it is the default because it
generates better code.
-fira-region=region
Use specified regions for the integrated register allocator.
The region argument should be one of the following:
all Use all loops as register allocation regions. This can
give the best results for machines with a small and/or
irregular register set.
mixed
Use all loops except for loops with small register
pressure as the regions. This value usually gives the
best results in most cases and for most architectures,
and is enabled by default when compiling with
optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically
results in the smallest code size, and is enabled by
default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting
pass for decisions to hoist expressions. This option usually
results in smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions
to move loop invariants. This option usually results in
generation of faster and smaller code on machines with large
register files (>= 32 registers), but it can slow the
compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames
are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-
registers. Each pseudo-register that does not get a hard
register gets a separate stack slot, and as a result function
stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of
loading values of spilled pseudos, LRA tries to rematerialize
(recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after
delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os, but not at -Og.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required
data being unavailable. This helps machines that have slow
floating point or memory load instructions by allowing other
instructions to be issued until the result of the load or
floating-point instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass
of instruction scheduling after register allocation has been
done. This is especially useful on machines with a
relatively small number of registers and where memory load
instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is
normally enabled when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is
normally enabled when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before
register allocation. This only makes sense when scheduling
before register allocation is enabled, i.e. with
-fschedule-insns or at -O2 or higher. Usage of this option
can improve the generated code and decrease its size by
preventing register pressure increase above the number of
available hard registers and subsequent spills in register
allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This
only makes sense when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This
only makes sense when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from
the queue of stalled insns into the ready list during the
second scheduling pass. -fno-sched-stalled-insns means that
no insns are moved prematurely, -fsched-stalled-insns=0 means
there is no limit on how many queued insns can be moved
prematurely. -fsched-stalled-insns without a value is
equivalent to -fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a
dependency on a stalled insn that is a candidate for
premature removal from the queue of stalled insns. This has
an effect only during the second scheduling pass, and only if
-fsched-stalled-insns is used. -fno-sched-stalled-insns-dep
is equivalent to -fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock
scheduling. This allows motion across basic block
boundaries, resulting in faster schedules. This option is
experimental, as not all machine descriptions used by GCC
model the CPU closely enough to avoid unreliable results from
the algorithm.
This only makes sense when scheduling after register
allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic
favors the instruction that belongs to a schedule group.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This
heuristic favors instructions on the critical path. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the
scheduler. This heuristic favors speculative instructions
with greater dependency weakness. This is enabled by default
when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic
favors the instruction belonging to a basic block with
greater size or frequency. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on
the last instruction scheduled. This is enabled by default
when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or -fschedule-insns2
or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling.
If a loop is modulo scheduled, later scheduling passes may
change its schedule. Use this option to control that
behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler
pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler
pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during
selective scheduling. This option has no effect unless one
of -fselective-scheduling or -fselective-scheduling2 is
turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also
pipeline outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols
by the dynamic linker. This means that for symbols exported
from the DSO, the compiler cannot perform interprocedural
propagation, inlining and other optimizations in anticipation
that the function or variable in question may change. While
this feature is useful, for example, to rewrite memory
allocation functions by a debugging implementation, it is
expensive in the terms of code quality. With
-fno-semantic-interposition the compiler assumes that if
interposition happens for functions the overwriting function
will have precisely the same semantics (and side effects).
Similarly if interposition happens for variables, the
constructor of the variable will be the same. The flag has no
effect for functions explicitly declared inline (where it is
never allowed for interposition to change semantics) and for
symbols explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function
that need it, rather than at the top of the function. This
flag is enabled by default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue
separately, so that those parts are only executed when
needed. This option is on by default, but has no effect
unless -fshrink-wrap is also turned on and the target
supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered
by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is
done only when it seems to result in better code.
This option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers
are not used by any called function. In that case it is not
necessary to save and restore them around calls. This is
only possible if called functions are part of same
compilation unit as current function and they are compiled
before it.
Enabled at levels -O2, -O3, -Os, however the option is
disabled if generated code will be instrumented for profiling
(-p, or -pg) or if callee's register usage cannot be known
exactly (this happens on targets that do not expose prologues
and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to
use less stack space, even if that makes the program slower.
This option implies setting the large-stack-frame parameter
to 100 and the large-stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by
default at -O and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the
evaluation of expressions executed on all paths to the
function exit as early as possible. This is especially
useful as a code size optimization, but it often helps for
code speed as well. This flag is enabled by default at -O2
and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This
flag is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive.
This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled
by default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The
difference between FRE and PRE is that FRE only considers
expressions that are computed on all paths leading to the
redundant computation. This analysis is faster than PRE,
though it exposes fewer redundancies. This flag is enabled
by default at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees.
This pass is enabled by default at -O and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-
else if the loads are from adjacent locations in the same
structure and the target architecture has a conditional move
instruction. This flag is enabled by default at -O2 and
higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default
at -O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by
default at -O and higher.
-fipa-reference
Discover which static variables do not escape the compilation
unit. Enabled by default at -O and higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static
variables. Enabled by default at -O and higher.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by
default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation
units. It is not enabled by default at any optimization
level.
-fipa-profile
Perform interprocedural profile propagation. The functions
called only from cold functions are marked as cold. Also
functions executed once (such as "cold", "noreturn", static
constructors or destructors) are identified. Cold functions
and loop less parts of functions executed once are then
optimized for size. Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This
optimization analyzes the program to determine when values
passed to functions are constants and then optimizes
accordingly. This optimization can substantially increase
performance if the application has constants passed to
functions. This flag is enabled by default at -O2, -Os and
-O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-cp-clone
Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation performs function cloning when externally visible
function can be called with constant arguments. Because this
optimization can create multiple copies of functions, it may
significantly increase code size (see --param
ipcp-unit-growth=value). This flag is enabled by default at
-O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant
propagation. This flag is enabled by default at -O2 and by
-fprofile-use and -fauto-profile. It requires that -fipa-cp
is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value
ranges. This flag is enabled by default at -O2. It requires
that -fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only
variables. The optimization reduces code size and may
disturb unwind stacks by replacing a function by equivalent
one with a different name. The optimization works more
effectively with link-time optimization enabled.
Although the behavior is similar to the Gold Linker's ICF
optimization, GCC ICF works on different levels and thus the
optimizations are not same - there are equivalences that are
found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for
live-patching.
If the compiler's optimization uses a function's body or
information extracted from its body to optimize/change
another function, the latter is called an impacted function
of the former. If a function is patched, its impacted
functions should be patched too.
The impacted functions are determined by the compiler's
interprocedural optimizations. For example, a caller is
impacted when inlining a function into its caller, cloning a
function and changing its caller to call this new clone, or
extracting a function's pureness/constness information to
optimize its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the
number of impacted functions for each function. In order to
control the number of impacted functions and more easily
compute the list of impacted function, IPA optimizations can
be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which
includes inlining, cloning, interprocedural scalar
replacement of aggregates and partial inlining. As a
result, when patching a function, all its callers and its
clones' callers are impacted, therefore need to be
patched as well.
-flive-patching=inline-clone disables the following
optimization flags: -fwhole-program -fipa-pta
-fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions
-fipa-icf-variables -fipa-bit-cp -fipa-vrp
-fipa-pure-const -fipa-reference-addressable
-fipa-stack-alignment
inline-only-static
Only enable inlining of static functions. As a result,
when patching a static function, all its callers are
impacted and so need to be patched as well.
In addition to all the flags that
-flive-patching=inline-clone disables,
-flive-patching=inline-only-static disables the following
additional optimization flags: -fipa-cp-clone -fipa-sra
-fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the
default value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time
optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due
to dereferencing a null pointer. Isolate those paths from
the main control flow and turn the statement with erroneous
or undefined behavior into a trap. This flag is enabled by
default at -O2 and higher and depends on
-fdelete-null-pointer-checks also being enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due
to a null value being used in a way forbidden by a
"returns_nonnull" or "nonnull" attribute. Isolate those
paths from the main control flow and turn the statement with
erroneous or undefined behavior into a trap. This is not
currently enabled, but may be enabled by -O2 in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled
by default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees
and propagate pointer alignment information. This pass only
operates on local scalar variables and is enabled by default
at -O1 and higher, except for -Og. It requires that
-ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on
trees. This pass only operates on local scalar variables and
is enabled by default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the definition
chain in order to simplify the definitions. For example,
this pass strips sign operations if the sign of a value never
matters. The flag is enabled by default at -O and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize
conditional code. This pass is enabled by default at -O1 and
higher, except for -Og.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one
with a jump to the other. This optimization is known as tail
merging or cross jumping. This flag is enabled by default at
-O2 and higher. The compilation time in this pass can be
limited using max-tail-merge-comparisons parameter and max-
tail-merge-iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is
enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to
built-in functions that may set "errno" but are otherwise
free of side effects. This flag is enabled by default at -O2
and higher if -Os is not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree
traversal. This also performs jump threading (to reduce
jumps to jumps). This flag is enabled by default at -O and
higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store
is a store into a memory location that is later overwritten
by another store without any intervening loads. In this case
the earlier store can be deleted. This flag is enabled by
default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial
since it increases effectiveness of code motion
optimizations. It also saves one jump. This flag is enabled
by default at -O and higher. It is not enabled for -Os,
since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as
-floop-nest-optimize. To use this code transformation, GCC
has to be configured with --with-isl to enable the Graphite
loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every
SCoP we generate the polyhedral representation and transform
it back to gimple. Using -fgraphite-identity we can check
the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE
transformation. Some minimal optimizations are also
performed by the code generator isl, like index splitting and
dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic
loop nest optimizer based on the Pluto optimization
algorithms. It calculates a loop structure optimized for
data-locality and parallelism. This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops
that can be parallelized. Parallelize all the loops that can
be analyzed to not contain loop carried dependences without
checking that it is profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation,
attempt to reduce copying by coalescing versions of different
user-defined variables, instead of just compiler temporaries.
This may severely limit the ability to debug an optimized
program compiled with -fno-var-tracking-assignments. In the
negated form, this flag prevents SSA coalescing of user
variables. This option is enabled by default if optimization
is enabled, and it does very little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops
to branch-less equivalents. The intent is to remove control-
flow from the innermost loops in order to improve the ability
of the vectorization pass to handle these loops. This is
enabled by default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache
performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to take
place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at -O3. It is also enabled
by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code
generated with calls to a library. This flag is enabled by
default at -O3, and by -fprofile-use and -fauto-profile.
This pass distributes the initialization loops and generates
a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to
memset zero. This flag is enabled by default at -O3. It is
also enabled by -fprofile-use and -fauto-profile.
-floop-interchange
Perform loop interchange outside of graphite. This flag can
improve cache performance on loop nest and allow further loop
optimizations, like vectorization, to take place. For
example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3. It is also enabled
by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a
loop nest this unrolls the outer loop by some factor and
fuses the resulting multiple inner loops. This flag is
enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that are hard to handle at RTL level (function
calls, operations that expand to nontrivial sequences of
insns). With -funswitch-loops it also moves operands of
conditions that are invariant out of the loop, so that we can
use just trivial invariantness analysis in loop unswitching.
The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops
for which determining number of iterations requires
complicated analysis. Later optimizations then may determine
the number easily. Useful especially in connection with
unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified
in a loop in such a way that its value when exiting the loop
can be determined using only its initial value and the number
of loop iterations, replace uses of the final value by such a
computation, provided it is sufficiently cheap. This reduces
data dependencies and may allow further simplifications.
Enabled by default at -O and higher.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable
elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run
in n threads. This is only possible for loops whose
iterations are independent and can be arbitrarily reordered.
The optimization is only profitable on multiprocessor
machines, for loops that are CPU-intensive, rather than
constrained e.g. by memory bandwidth. This option implies
-pthread, and thus is only supported on targets that have
support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This
flag is enabled by default at -O1 and higher, except for -Og.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by
default at -O1 and higher, except for -Og.
-fstore-merging
Perform merging of narrow stores to consecutive memory
addresses. This pass merges contiguous stores of immediate
values narrower than a word into fewer wider stores to reduce
the number of instructions. This is enabled by default at
-O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the
SSA->normal phase. Single use/single def temporaries are
replaced at their use location with their defining
expression. This results in non-GIMPLE code, but gives the
expanders much more complex trees to work on resulting in
better RTL generation. This is enabled by default at -O and
higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This
recognizes related expressions involving multiplications and
replaces them by less expensive calculations when possible.
This is enabled by default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not
explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by
default at -O3 and by -ftree-vectorize, -fprofile-use, and
-fauto-profile.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is
enabled by default at -O3 and by -ftree-vectorize,
-fprofile-use, and -fauto-profile.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model
argument should be one of unlimited, dynamic or cheap. With
the unlimited model the vectorized code-path is assumed to be
profitable while with the dynamic model a runtime check
guards the vectorized code-path to enable it only for
iteration counts that will likely execute faster than when
executing the original scalar loop. The cheap model disables
vectorization of loops where doing so would be cost
prohibitive for example due to required runtime checks for
data dependence or alignment but otherwise is equal to the
dynamic model. The default cost model depends on other
optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked
with the OpenMP simd directive. The model argument should be
one of unlimited, dynamic, cheap. All values of model have
the same meaning as described in -fvect-cost-model and by
default a cost model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to
the constant propagation pass, but instead of values, ranges
of values are propagated. This allows the optimizers to
remove unnecessary range checks like array bound checks and
null pointer checks. This is enabled by default at -O2 and
higher. Null pointer check elimination is only done if
-fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead
code elimination and common subexpression elimination. This
is enabled by default at -O3 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus
improving efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain
the same effect. However, that is not reliable in cases
where the loop body is more complicated than a single basic
block. It also does not work at all on some architectures
due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of
some local variables when unrolling a loop, which can result
in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed
in previous iterations of loops.
This option is enabled at level -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that
access large arrays.
This option may generate better or worse code; results are
highly dependent on the structure of loops within the source
code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of
formatted output functions such as "sprintf", "snprintf",
"vsprintf", and "vsnprintf" (but not "printf" of "fprintf").
This transformation allows GCC to optimize or even eliminate
branches based on the known return value of these functions
called with arguments that are either constant, or whose
values are known to be in a range that makes determining the
exact return value possible. For example, when
-fprintf-return-value is in effect, both the branch and the
body of the "if" statement (but not the call to "snprint")
can be optimized away when "i" is a 32-bit or smaller integer
because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other
optimizations and yields best results with -O2 and above. It
works in tandem with the -Wformat-overflow and
-Wformat-truncation options. The -fprintf-return-value
option is enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in how
they are implemented in the compiler; some targets use one,
some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at
levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are
not provided by profiling feedback (-fprofile-arcs). These
heuristics are based on the control flow graph. If some
branch probabilities are specified by "__builtin_expect",
then the heuristics are used to guess branch probabilities
for the rest of the control flow graph, taking the
"__builtin_expect" info into account. The interactions
between the heuristics and "__builtin_expect" can be complex,
and in some cases, it may be useful to disable the heuristics
so that the effects of "__builtin_expect" are easier to
understand.
It is also possible to specify expected probability of the
expression with "__builtin_expect_with_probability" built-in
function.
The default is -fguess-branch-probability at levels -O, -O2,
-O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to
reduce number of taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The
algorithm argument can be simple, which does not increase
code size (except sometimes due to secondary effects like
alignment), or stc, the "software trace cache" algorithm,
which tries to put all often executed code together,
minimizing the number of branches executed by making extra
copies of code.
The default is simple at levels -O, -Os, and stc at levels
-O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled
function, in order to reduce number of taken branches,
partitions hot and cold basic blocks into separate sections
of the assembly and .o files, to improve paging and cache
locality performance.
This optimization is automatically turned off in the presence
of exception handling or unwind tables (on targets using
setjump/longjump or target specific scheme), for linkonce
sections, for functions with a user-defined section attribute
and on any architecture that does not support named sections.
When -fsplit-stack is used this option is not enabled by
default (to avoid linker errors), but may be enabled
explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code
locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering
is done by the linker so object file format must support
named sections and linker must place them in a reasonable
way.
This option isn't effective unless you either provide profile
feedback (see -fprofile-arcs for details) or manually
annotate functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++),
this activates optimizations based on the type of
expressions. In particular, an object of one type is assumed
never to reside at the same address as an object of a
different type, unless the types are almost the same. For
example, an "unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than
the one most recently written to (called "type-punning") is
common. Even with -fstrict-aliasing, type-punning is
allowed, provided the memory is accessed through the union
type. So, the code above works as expected. However, this
code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the
resulting pointer and dereferencing the result has undefined
behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3,
-Os.
-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater
than n, skipping up to m-1 bytes. This ensures that at least
the first m bytes of the function can be fetched by the CPU
without crossing an n-byte alignment boundary.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next
32-byte boundary, -falign-functions=24 aligns to the next
32-byte boundary only if this can be done by skipping 23
bytes or less, -falign-functions=32:7 aligns to the next
32-byte boundary only if this can be done by skipping 6 bytes
or less.
The second pair of n2:m2 values allows you to specify a
secondary alignment: -falign-functions=64:7:32:3 aligns to
the next 64-byte boundary if this can be done by skipping 6
bytes or less, otherwise aligns to the next 32-byte boundary
if this can be done by skipping 2 bytes or less. If m2 is
not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of
two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent
and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent
default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid
unnecessarily overaligning functions. It attempts to instruct
the assembler to align by the amount specified by
-falign-functions, but not to skip more bytes than the size
of the function.
-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the
-falign-functions option. -fno-align-labels and
-falign-labels=1 are equivalent and mean that labels are not
aligned.
If -falign-loops or -falign-jumps are applicable and are
greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent
default which is very likely to be 1, meaning no alignment.
The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are
executed many times, this makes up for any execution of the
dummy padding instructions.
Parameters of this option are analogous to the
-falign-functions option. -fno-align-loops and
-falign-loops=1 are equivalent and mean that loops are not
aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent
default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch
targets where the targets can only be reached by jumping. In
this case, no dummy operations need be executed.
Parameters of this option are analogous to the
-falign-functions option. -fno-align-jumps and
-falign-jumps=1 are equivalent and mean that loops are not
aligned.
If n is not specified or is zero, use a machine-dependent
default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons.
-funit-at-a-time has no effect, while -fno-unit-at-a-time
implies -fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear
in the input file. When this option is used, unreferenced
static variables are not removed. This option is intended to
support existing code that relies on a particular ordering.
For new code, it is better to use attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also
at -O0 if -fsection-anchors is explicitly requested.
Additionally -fno-toplevel-reorder implies
-fno-section-anchors.
-fweb
Constructs webs as commonly used for register allocation
purposes and assign each web individual pseudo register.
This allows the register allocation pass to operate on
pseudos directly, but also strengthens several other
optimization passes, such as CSE, loop optimizer and trivial
dead code remover. It can, however, make debugging
impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole
program being compiled. All public functions and variables
with the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect
are optimized more aggressively by interprocedural
optimizers.
This option should not be used in combination with -flto.
Instead relying on a linker plugin should provide safer and
more precise information.
-flto[=n]
This option runs the standard link-time optimizer. When
invoked with source code, it generates GIMPLE (one of GCC's
internal representations) and writes it to special ELF
sections in the object file. When the object files are
linked together, all the function bodies are read from these
ELF sections and instantiated as if they had been part of the
same translation unit.
To use the link-time optimizer, -flto and optimization
options should be specified at compile time and during the
final link. It is recommended that you compile all the files
participating in the same link with the same options and also
specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode
representation of GIMPLE into special ELF sections inside
foo.o and bar.o. The final invocation reads the GIMPLE
bytecode from foo.o and bar.o, merges the two files into a
single internal image, and compiles the result as usual.
Since both foo.o and bar.o are merged into a single image,
this causes all the interprocedural analyses and
optimizations in GCC to work across the two files as if they
were a single one. This means, for example, that the inliner
is able to inline functions in bar.o into functions in foo.o
and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes
them as usual to produce myprog.
The important thing to keep in mind is that to enable link-
time optimizations you need to use the GCC driver to perform
the link step. GCC automatically performs link-time
optimization if any of the objects involved were compiled
with the -flto command-line option. You can always override
the automatic decision to do link-time optimization by
passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary
to make certain whole program assumptions. The compiler
needs to know what functions and variables can be accessed by
libraries and runtime outside of the link-time optimized
unit. When supported by the linker, the linker plugin (see
-fuse-linker-plugin) passes information to the compiler about
used and externally visible symbols. When the linker plugin
is not available, -fwhole-program should be used to allow the
compiler to make these assumptions, which leads to more
aggressive optimization decisions.
When a file is compiled with -flto without
-fuse-linker-plugin, the generated object file is larger than
a regular object file because it contains GIMPLE bytecodes
and the usual final code (see -ffat-lto-objects. This means
that object files with LTO information can be linked as
normal object files; if -fno-lto is passed to the linker, no
interprocedural optimizations are applied. Note that when
-fno-fat-lto-objects is enabled the compile stage is faster
but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time
optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries
with GIMPLE bytecodes and final object code. GCC
automatically selects which files to optimize in LTO mode and
which files to link without further processing.
Generally, options specified at link time override those
specified at compile time, although in some cases GCC
attempts to infer link-time options from the settings used to
compile the input files.
If you do not specify an optimization level option -O at link
time, then GCC uses the highest optimization level used when
compiling the object files. Note that it is generally
ineffective to specify an optimization level option only at
link time and not at compile time, for two reasons. First,
compiling without optimization suppresses compiler passes
that gather information needed for effective optimization at
link time. Second, some early optimization passes can be
performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the
final link. Currently, the following options and their
settings are taken from the first object file that explicitly
specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions,
-fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all
compilation units, and trying to override this at link time
with a conflicting value is ignored. This includes options
such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow,
-fwrapv, -fno-trapv or -fno-strict-aliasing are passed
through to the link stage and merged conservatively for
conflicting translation units. Specifically
-fno-strict-overflow, -fwrapv and -fno-trapv take precedence;
and for example -ffp-contract=off takes precedence over
-ffp-contract=fast. You can override them at link time.
When you need to pass options to the assembler via -Wa or
-Xassembler make sure to either compile such translation
units with -fno-lto or consistently use the same assembler
options on all translation units. You can alternatively also
specify assembler options at LTO link time.
If LTO encounters objects with C linkage declared with
incompatible types in separate translation units to be linked
together (undefined behavior according to ISO C99 6.2.7), a
non-fatal diagnostic may be issued. The behavior is still
undefined at run time. Similar diagnostics may be raised for
other languages.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++
runtime libraries and -lgfortran is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO
mode, you should use the same link command options as when
mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a
library archive, say libfoo.a, it is possible to extract and
use them in an LTO link if you are using a linker with plugin
support. To create static libraries suitable for LTO, use
gcc-ar and gcc-ranlib instead of ar and ranlib; to show the
symbols of object files with GIMPLE bytecode, use gcc-nm.
Those commands require that ar, ranlib and nm have been
compiled with plugin support. At link time, use the flag
-fuse-linker-plugin to ensure that the library participates
in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the
needed GIMPLE files from libfoo.a and passes them on to the
running GCC to make them part of the aggregated GIMPLE image
to be optimized.
If you are not using a linker with plugin support and/or do
not enable the linker plugin, then the objects inside
libfoo.a are extracted and linked as usual, but they do not
participate in the LTO optimization process. In order to
make a static library suitable for both LTO optimization and
usual linkage, compile its object files with -flto
-ffat-lto-objects.
Link-time optimizations do not require the presence of the
whole program to operate. If the program does not require
any symbols to be exported, it is possible to combine -flto
and -fwhole-program to allow the interprocedural optimizers
to use more aggressive assumptions which may lead to improved
optimization opportunities. Use of -fwhole-program is not
needed when linker plugin is active (see
-fuse-linker-plugin).
The current implementation of LTO makes no attempt to
generate bytecode that is portable between different types of
hosts. The bytecode files are versioned and there is a
strict version check, so bytecode files generated in one
version of GCC do not work with an older or newer version of
GCC.
Link-time optimization does not work well with generation of
debugging information on systems other than those using a
combination of ELF and DWARF.
If you specify the optional n, the optimization and code
generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program
used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job
server mode to determine the number of parallel jobs. This is
useful when the Makefile calling GCC is already executing in
parallel. You must prepend a + to the command recipe in the
parent Makefile for this to work. This option likely only
works if MAKE is GNU make.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time
optimizer. The value is either 1to1 to specify a
partitioning mirroring the original source files or balanced
to specify partitioning into equally sized chunks (whenever
possible) or max to create new partition for every symbol
where possible. Specifying none as an algorithm disables
partitioning and streaming completely. The default value is
balanced. While 1to1 can be used as an workaround for various
code ordering issues, the max partitioning is intended for
internal testing only. The value one specifies that exactly
one partition should be used while the value none bypasses
partitioning and executes the link-time optimization step
directly from the WPA phase.
-flto-odr-type-merging
Enable streaming of mangled types names of C++ types and
their unification at link time. This increases size of LTO
object files, but enables diagnostics about One Definition
Rule violations.
-flto-compression-level=n
This option specifies the level of compression used for
intermediate language written to LTO object files, and is
only meaningful in conjunction with LTO mode (-flto). Valid
values are 0 (no compression) to 9 (maximum compression).
Values outside this range are clamped to either 0 or 9. If
the option is not given, a default balanced compression
setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time
optimization. This option relies on plugin support in the
linker, which is available in gold or in GNU ld 2.21 or
newer.
This option enables the extraction of object files with
GIMPLE bytecode out of library archives. This improves the
quality of optimization by exposing more code to the link-
time optimizer. This information specifies what symbols can
be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries
(and shared libraries that use hidden visibility) are similar
to -fwhole-program. See -flto for a description of the
effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is
enabled and GCC was configured for use with a linker
supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the
intermediate language and the object code. This makes them
usable for both LTO linking and normal linking. This option
is effective only when compiling with -flto and is ignored at
link time.
-fno-fat-lto-objects improves compilation time over plain
LTO, but requires the complete toolchain to be aware of LTO.
It requires a linker with linker plugin support for basic
functionality. Additionally, nm, ar and ranlib need to
support linker plugins to allow a full-featured build
environment (capable of building static libraries etc). GCC
provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the
right options to these tools. With non fat LTO makefiles need
to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism.
Installing the linker plugin into $libdir/bfd-plugins has the
same effect as usage of the command wrappers (gcc-ar, gcc-nm
and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker
plugin support.
-fcompare-elim
After register allocation and post-register allocation
instruction splitting, identify arithmetic instructions that
compute processor flags similar to a comparison operation
based on that arithmetic. If possible, eliminate the
explicit comparison operation.
This pass only applies to certain targets that cannot
explicitly represent the comparison operation before register
allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation
instruction splitting, perform a copy-propagation pass to try
to reduce scheduling dependencies and occasionally eliminate
the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-
threaded programs may be inconsistent due to missed counter
updates. When this option is specified, GCC uses heuristics
to correct or smooth out such inconsistencies. By default,
GCC emits an error message when an inconsistent profile is
detected.
This option is enabled by -fauto-profile.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the
following optimizations, many of which are generally
profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning
-fsplit-loops -funswitch-loops -fgcse-after-reload
-ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-reorder-functions
Before you can use this option, you must first generate
profiling information.
By default, GCC emits an error message if the feedback
profiles do not match the source code. This error can be
turned into a warning by using -Wno-error=coverage-mismatch.
Note this may result in poorly optimized code. Additionally,
by default, GCC also emits a warning message if the feedback
profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and
the following optimizations, many of which are generally
profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning
-fsplit-loops -funswitch-loops -fgcse-after-reload
-ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-correction
path is the name of a file containing AutoFDO profile
information. If omitted, it defaults to fbdata.afdo in the
current directory.
Producing an AutoFDO profile data file requires running your
program with the perf utility on a supported GNU/Linux target
system. For more information, see
<https://perf.wiki.kernel.org/ >.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data
to a format that can be used by GCC. You must also supply
the unstripped binary for your program to this tool. See
<https://github.com/google/autofdo >.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding
floating-point arithmetic. These options trade off between speed
and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and
inhibit other options that might change whether a floating-
point value is taken from a register or memory.
This option prevents undesirable excess precision on machines
such as the 68000 where the floating registers (of the 68881)
keep more precision than a "double" is supposed to have.
Similarly for the x86 architecture. For most programs, the
excess precision does only good, but a few programs rely on
the precise definition of IEEE floating point. Use
-ffloat-store for such programs, after modifying them to
store all pertinent intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision on
machines where floating-point operations occur in a format
with more precision or range than the IEEE standard and
interchange floating-point types. By default,
-fexcess-precision=fast is in effect; this means that
operations may be carried out in a wider precision than the
types specified in the source if that would result in faster
code, and it is unpredictable when rounding to the types
specified in the source code takes place. When compiling C,
if -fexcess-precision=standard is specified then excess
precision follows the rules specified in ISO C99; in
particular, both casts and assignments cause values to be
rounded to their semantic types (whereas -ffloat-store only
affects assignments). This option is enabled by default for
C if a strict conformance option such as -std=c99 is used.
-ffast-math enables -fexcess-precision=fast by default
regardless of whether a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages
other than C. On the x86, it has no effect if -mfpmath=sse
or -mfpmath=sse+387 is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter,
rounding is unpredictable.
-ffast-math
Sets the options -fno-math-errno,
-funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans, -fcx-limited-range
and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to
be defined.
This option is not turned on by any -O option besides -Ofast
since it can result in incorrect output for programs that
depend on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the
guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are
executed with a single instruction, e.g., "sqrt". A program
that relies on IEEE exceptions for math error handling may
want to use this flag for speed while maintaining IEEE
arithmetic compatibility.
This option is not turned on by any -O option since it can
result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for
programs that do not require the guarantees of these
specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno".
There is therefore no reason for the compiler to consider the
possibility that it might, and -fno-math-errno is the
default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a)
assume that arguments and results are valid and (b) may
violate IEEE or ANSI standards. When used at link time, it
may include libraries or startup files that change the
default FPU control word or other similar optimizations.
This option is not turned on by any -O option since it can
result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for
programs that do not require the guarantees of these
specifications. Enables -fno-signed-zeros,
-fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language
standard by possibly changing computation result. NOTE: re-
ordering may change the sign of zero as well as ignore NaNs
and inhibit or create underflow or overflow (and thus cannot
be used on code that relies on rounding behavior like "(x +
2**52) - 2**52". May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are
required. This option requires that both -fno-signed-zeros
and -fno-trapping-math be in effect. Moreover, it doesn't
make much sense with -frounding-math. For Fortran the option
is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of
dividing by the value if this enables optimizations. For
example "x / y" can be replaced with "x * (1/y)", which is
useful if "(1/y)" is subject to common subexpression
elimination. Note that this loses precision and increases
the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can
result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for
programs that do not require the guarantees of these
specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore
the signedness of zero. IEEE arithmetic specifies the
behavior of distinct +0.0 and -0.0 values, which then
prohibits simplification of expressions such as x+0.0 or
0.0*x (even with -ffinite-math-only). This option implies
that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by
zero, overflow, underflow, inexact result and invalid
operation. This option requires that -fno-signaling-nans be
in effect. Setting this option may allow faster code if one
relies on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications
for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for
all floating point to integer conversions, and round-to-
nearest for all other arithmetic truncations. This option
should be specified for programs that change the FP rounding
mode dynamically, or that may be executed with a non-default
rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be
affected by rounding mode) and arithmetic transformations
that are unsafe in the presence of sign-dependent rounding
modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee
to disable all GCC optimizations that are affected by
rounding mode. Future versions of GCC may provide finer
control of this setting using C99's "FENV_ACCESS" pragma.
This command-line option will be used to specify the default
state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate
user-visible traps during floating-point operations. Setting
this option disables optimizations that may change the number
of exceptions visible with signaling NaNs. This option
implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__"
to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee
to disable all GCC optimizations that affect signaling NaN
behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round"
and "trunc", and their "float" and "long double" variants, to
generate code that raises the "inexact" floating-point
exception for noninteger arguments. ISO C99 and C11 allow
these functions to raise the "inexact" exception, but ISO/IEC
TS 18661-1:2014, the C bindings to IEEE 754-2008, does not
allow these functions to do so.
The default is -ffp-int-builtin-inexact, allowing the
exception to be raised. This option does nothing unless
-ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions
generate a call to a library function then the "inexact"
exception may be raised if the library implementation does
not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step
is not needed when performing complex division. Also, there
is no checking whether the result of a complex multiplication
or division is "NaN + I*NaN", with an attempt to rescue the
situation in that case. The default is
-fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies
to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules.
Range reduction is done as part of complex division, but
there is no checking whether the result of a complex
multiplication or division is "NaN + I*NaN", with an attempt
to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This section
includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can
compile it a second time using -fbranch-probabilities, to
improve optimizations based on the number of times each
branch was taken. When a program compiled with
-fprofile-arcs exits, it saves arc execution counts to a file
called sourcename.gcda for each source file. The information
in this data file is very dependent on the structure of the
generated code, so you must use the same source code and the
same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on
each JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is most
likely to take, the REG_BR_PROB values are used to exactly
determine which path is taken more often.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some
data about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered
from profiling values of expressions for usage in
optimizations.
Enabled by -fprofile-generate, -fprofile-use, and
-fauto-profile.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects
first time of execution of a function and orders these
functions in ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the
compiler to add code to gather information about values of
expressions.
With -fbranch-probabilities, it reads back the data gathered
and actually performs the optimizations based on them.
Currently the optimizations include specialization of
division operations using the knowledge about the value of
the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers
Attempt to avoid false dependencies in scheduled code by
making use of registers left over after register allocation.
This optimization most benefits processors with lots of
registers. Depending on the debug information format adopted
by the target, however, it can make debugging impossible,
since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream
to schedule instructions of same type together because target
machine can execute them more efficiently if they are
adjacent to each other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops
implies -frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal
of loops with a small constant number of iterations). This
option makes code larger, and may or may not make it run
faster.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops
Unroll all loops, even if their number of iterations is
uncertain when the loop is entered. This usually makes
programs run more slowly. -funroll-all-loops implies the
same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they
do not roll much (from profile feedback or static analysis).
It also turns on complete loop peeling (i.e. complete removal
of loops with small constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop
optimizer. Enabled at level -O1 and higher, except for -Og.
-fsplit-loops
Split a loop into two if it contains a condition that's
always true for one side of the iteration space and false for
the other.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops
Move branches with loop invariant conditions out of the loop,
with duplicates of the loop on both branches (modified
according to result of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride,
create another version of the loop that assumes the stride is
always one. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in
Fortran where (for example) it allows better vectorization
assuming contiguous accesses. This flag is enabled by
default at -O3. It is also enabled by -fprofile-use and
-fauto-profile.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the
output file if the target supports arbitrary sections. The
name of the function or the name of the data item determines
the section's name in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the
instruction space. Most systems using the ELF object format
have linkers with such optimizations. On AIX, the linker
rearranges sections (CSECTs) based on the call graph. The
performance impact varies.
Together with a linker garbage collection (linker
--gc-sections option) these options may lead to smaller
statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the
quality of the debug information. There could be issues with
other object files/debug info formats.
Only use these options when there are significant benefits
from doing so. When you specify these options, the assembler
and linker create larger object and executable files and are
also slower. These options affect code generation. They
prevent optimizations by the compiler and assembler using
relative locations inside a translation unit since the
locations are unknown until link time. An example of such an
optimization is relaxing calls to short call instructions.
-fbranch-target-load-optimize
Perform branch target register load optimization before
prologue / epilogue threading. The use of target registers
can typically be exposed only during reload, thus hoisting
loads out of loops and doing inter-block scheduling needs a
separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after
prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization,
don't reuse branch target registers within any basic block.
-fstdarg-opt
Optimize the prologue of variadic argument functions with
respect to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by
using shared "anchor" symbols to address nearby objects.
This transformation can help to reduce the number of GOT
entries and GOT accesses on some targets.
For example, the implementation of the following function
"foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but
if you compile it with -fsection-anchors, it accesses the
variables from a common anchor point instead. The effect is
similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the
amount of optimization that is done. For example, GCC does
not inline functions that contain more than a certain number
of instructions. You can control some of these constants on
the command line using the --param option.
The names of specific parameters, and the meaning of the
values, are tied to the internals of the compiler, and are
subject to change without notice in future releases.
In order to get minimal, maximal and default value of a
parameter, one can use --help=param -Q options.
In each case, the value is an integer. The allowable choices
for name are:
predictable-branch-outcome
When branch is predicted to be taken with probability
lower than this threshold (in percent), then it is
considered well predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches
around a block and replace them with conditionally
executed instructions. This parameter gives the maximum
number of instructions in a block which should be
considered for if-conversion. The compiler will also use
other heuristics to decide whether if-conversion is
likely to be profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches
around a block and replace them with conditionally
executed instructions. These parameters give the maximum
permissible cost for the sequence that would be generated
by if-conversion depending on whether the branch is
statically determined to be predictable or not. The
units for this parameter are the same as those for the
GCC internal seq_cost metric. The compiler will try to
provide a reasonable default for this parameter using the
BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for
cross-jumping. The algorithm used by -fcrossjumping is
O(N^2) in the number of edges incoming to each block.
Increasing values mean more aggressive optimization,
making the compilation time increase with probably small
improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched
at the end of two blocks before cross-jumping is
performed on them. This value is ignored in the case
where all instructions in the block being cross-jumped
from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to
a jump instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a
block that jumps to a computed goto. To avoid O(N^2)
behavior in a number of passes, GCC factors computed
gotos early in the compilation process, and unfactors
them as late as possible. Only computed jumps at the end
of a basic blocks with no more than max-goto-duplication-
insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when
looking for an instruction to fill a delay slot. If more
than this arbitrary number of instructions are searched,
the time savings from filling the delay slot are minimal,
so stop searching. Increasing values mean more
aggressive optimization, making the compilation time
increase with probably small improvement in execution
time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with
valid live register information. Increasing this
arbitrarily chosen value means more aggressive
optimization, increasing the compilation time. This
parameter should be removed when the delay slot code is
rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that can be
allocated in order to perform the global common
subexpression elimination optimization. If more memory
than specified is required, the optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is
larger than this value for any expression, then RTL PRE
inserts or removes the expression and thus leaves
partially redundant computations in the instruction
stream.
max-pending-list-length
The maximum number of pending dependencies scheduling
allows before flushing the current state and starting
over. Large functions with few branches or calls can
create excessively large lists which needlessly consume
memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler
should make when modulo scheduling a loop. Larger values
can exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC.
This number sets the maximum number of instructions
(counted in GCC's internal representation) in a single
function that the tree inliner considers for inlining.
This only affects functions declared inline and methods
implemented in a class declaration (C++).
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot
of functions that would otherwise not be considered for
inlining by the compiler are investigated. To those
functions, a different (more restrictive) limit compared
to functions declared inline can be applied.
max-inline-insns-small
This is bound applied to calls which are considered
relevant with -finline-small-functions.
max-inline-insns-size
This is bound applied to calls which are optimized for
size. Small growth may be desirable to anticipate
optimization oppurtunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function
overhead such as function prologue and epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead
such as time needed to execute function prologue and
epilogue
uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param
uninlined-function-time but applied to function thunks
inline-min-speedup
When estimated performance improvement of caller + callee
runtime exceeds this threshold (in percent), the function
can be inlined regardless of the limit on --param max-
inline-insns-single and --param max-inline-insns-auto.
large-function-insns
The limit specifying really large functions. For
functions larger than this limit after inlining, inlining
is constrained by --param large-function-growth. This
parameter is useful primarily to avoid extreme
compilation time caused by non-linear algorithms used by
the back end.
large-function-growth
Specifies maximal growth of large function caused by
inlining in percents. For example, parameter value 100
limits large function growth to 2.0 times the original
size.
large-unit-insns
The limit specifying large translation unit. Growth
caused by inlining of units larger than this limit is
limited by --param inline-unit-growth. For small units
this might be too tight. For example, consider a unit
consisting of function A that is inline and B that just
calls A three times. If B is small relative to A, the
growth of unit is 300\% and yet such inlining is very
sane. For very large units consisting of small
inlineable functions, however, the overall unit growth
limit is needed to avoid exponential explosion of code
size. Thus for smaller units, the size is increased to
--param large-unit-insns before applying --param inline-
unit-growth.
inline-unit-growth
Specifies maximal overall growth of the compilation unit
caused by inlining. For example, parameter value 20
limits unit growth to 1.2 times the original size. Cold
functions (either marked cold via an attribute or by
profile feedback) are not accounted into the unit size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit
caused by interprocedural constant propagation. For
example, parameter value 10 limits unit growth to 1.1
times the original size.
large-stack-frame
The limit specifying large stack frames. While inlining
the algorithm is trying to not grow past this limit too
much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by
inlining in percents. For example, parameter value 1000
limits large stack frame growth to 11 times the original
size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-
line copy of a self-recursive inline function can grow
into by performing recursive inlining.
--param max-inline-insns-recursive applies to functions
declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled; --param max-inline-insns-
recursive-auto applies instead.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive
inlining.
--param max-inline-recursive-depth applies to functions
declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled; --param max-inline-
recursive-depth-auto applies instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having
deep recursion in average and can hurt for function
having little recursion depth by increasing the prologue
size or complexity of function body to other optimizers.
When profile feedback is available (see
-fprofile-generate) the actual recursion depth can be
guessed from the probability that function recurses via a
given call expression. This parameter limits inlining
only to call expressions whose probability exceeds the
given threshold (in percents).
early-inlining-insns
Specify growth that the early inliner can make. In
effect it increases the amount of inlining for code
having a large abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically
bounds the number of nested indirect calls the early
inliner can resolve. Deeper chains are still handled by
late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with
comdat visibility are shared across multiple compilation
units.
profile-func-internal-id
A parameter to control whether to use function internal
id in profile database lookup. If the value is 0, the
compiler uses an id that is based on function assembler
name and filename, which makes old profile data more
tolerant to source changes such as function reordering
etc.
min-vect-loop-bound
The minimum number of iterations under which loops are
not vectorized when -ftree-vectorize is used. The number
of iterations after vectorization needs to be greater
than the value specified by this option to allow
vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an
expression can be moved by GCSE optimizations. This is
currently supported only in the code hoisting pass. The
bigger the ratio, the more aggressive code hoisting is
with simple expressions, i.e., the expressions that have
cost less than gcse-unrestricted-cost. Specifying 0
disables hoisting of simple expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical
machine instruction, at which GCSE optimizations do not
constrain the distance an expression can travel. This is
currently supported only in the code hoisting pass. The
lesser the cost, the more aggressive code hoisting is.
Specifying 0 allows all expressions to travel
unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions
to hoist. This is used to avoid quadratic behavior in
hoisting algorithm. The value of 0 does not limit on the
search, but may slow down compilation of huge functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with.
This is used to avoid quadratic behavior in tree tail
merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the
function. This is used to limit compilation time in tree
tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned
stores if it is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into
wider stores in the store merging pass.
max-unrolled-insns
The maximum number of instructions that a loop may have
to be unrolled. If a loop is unrolled, this parameter
also determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by
probabilities of their execution that a loop may have to
be unrolled. If a loop is unrolled, this parameter also
determines how many times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have
to be peeled. If a loop is peeled, this parameter also
determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through
the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable
for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete
peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single
loop.
lim-expensive
The minimum cost of an expensive expression in the loop
invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables,
below which all candidates are considered for each use in
induction variable optimizations. If there are more
candidates than this, only the most relevant ones are
considered to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops
that contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than
this value, always try to remove unnecessary ivs from the
set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by
dead store elimination. Larger values may result in
larger compilation times.
dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per
store. Larger values result in larger compilation times
and may result in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar
evolutions analyzer. Large expressions slow the
analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the
analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if
conversion unless the loop is marked with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be
performed when doing loop versioning for alignment in the
vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be
performed when doing loop versioning for alias in the
vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access
alignment for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-
force algorithm for analysis of the number of iterations
of the loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it
contributes to the given permillage (i.e. 0...1000) of
the entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of
executions of basic block in function given basic block
needs to have to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict
statically. This is useful in cases where a function
contains a single loop with known bound and another loop
with unknown bound. The known number of iterations is
predicted correctly, while the unknown number of
iterations average to roughly 10. This means that the
loop without bounds appears artificially cold relative to
the other one.
builtin-expect-probability
Control the probability of the expression having the
specified value. This parameter takes a percentage (i.e.
0 ... 100) as input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin
string cmp call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of
a basic block in a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number
of iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the
given percentage of executed instructions is covered.
This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used
only when profile feedback is available. The real
profiles (as opposed to statically estimated ones) are
much less balanced allowing the threshold to be larger
value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given
percentage. This is a rather artificial limit, as most
of the duplicates are eliminated later in cross jumping,
so it may be set to much higher values than is the
desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best
edge is less than this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability
lower than this threshold.
Similarly to tracer-dynamic-coverage two parameters are
provided. tracer-min-branch-probability-feedback is used
for compilation with profile feedback and tracer-min-
branch-probability compilation without. The value for
compilation with profile feedback needs to be more
conservative (higher) in order to make tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack
guard as 2 raised to num bytes. Higher values may reduce
the number of explicit probes, but a value larger than
the operating system provided guard will leave code
vulnerable to stack clash style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it
is allocated. This param controls the maximum distance
between probes into the stack as 2 raised to num bytes.
Higher values may reduce the number of explicit probes,
but a value larger than the operating system provided
guard will leave code vulnerable to stack clash style
attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE
considers.
max-cse-insns
The maximum number of instructions CSE processes before
flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum
percentage by which the garbage collector's heap should
be allowed to expand between collections. Tuning this
may improve compilation speed; it has no effect on code
generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound
of 100% when RAM >= 1GB. If "getrlimit" is available,
the notion of "RAM" is the smallest of actual RAM and
"RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to
calculate RAM on a particular platform, the lower bound
of 30% is used. Setting this parameter and ggc-min-
heapsize to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be
useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it
begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-min-
expand% beyond ggc-min-heapsize. Again, tuning this may
improve compilation speed, and has no effect on code
generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a
limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS
are not exceeded, but with a lower bound of 4096 (four
megabytes) and an upper bound of 131072 (128 megabytes).
If GCC is not able to calculate RAM on a particular
platform, the lower bound is used. Setting this
parameter very large effectively disables garbage
collection. Setting this parameter and ggc-min-expand to
zero causes a full collection to occur at every
opportunity.
max-reload-search-insns
The maximum number of instruction reload should look
backward for equivalent register. Increasing values mean
more aggressive optimization, making the compilation time
increase with probably slightly better performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take
into account. Increasing values mean more aggressive
optimization, making the compilation time increase with
probably slightly better performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the
first scheduling pass. Increasing values mean more
thorough searches, making the compilation time increase
with probably little benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered
for interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered
for pipelining in the selective scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered
for interblock scheduling.
max-pipeline-region-insns
The maximum number of insns in a region to be considered
for pipelining in the selective scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a
source block for interblock speculative scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend
regions. A value of 0 disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered
for speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in
percents), so that speculative insns are scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the
scheduler to save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load
targeting same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions.
selsched-max-sched-times
The maximum number of times that an instruction is
scheduled during selective scheduling. This is the limit
on the number of iterations through which the instruction
may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list
that are considered for renaming in the selective
scheduler.
sms-min-sc
The minimum value of stage count that swing modulo
scheduler generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo
register as last known value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries
to combine.
integer-share-limit
Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its
speed. This sets the maximum value of a shared integer
constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive
stack smashing protection when -fstack-protection is
used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot
sharing when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that
needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a
field sensitive manner during pointer analysis.
prefetch-latency
Estimate on average number of instructions that are
executed before prefetch finishes. The distance
prefetched ahead is proportional to this constant.
Increasing this number may also lead to less streams
being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same
time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue
software prefetch hints for strides that are non-
constant. In some cases this may be beneficial, though
the fact the stride is non-constant may make it hard to
predict when there is clear benefit to issuing these
hints.
Set to 1 if the prefetch hints should be issued for non-
constant strides. Set to 0 if prefetch hints should be
issued only for strides that are known to be constant and
below prefetch-minimum-stride.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using
prefetch hints for. If the stride is less than this
threshold, prefetch hints will not be issued.
This setting is useful for processors that have hardware
prefetchers, in which case there may be conflicts between
the hardware prefetchers and the software prefetchers.
If the hardware prefetchers have a maximum stride they
can handle, it should be used here to improve the use of
software prefetchers.
A value of -1 means we don't have a threshold and
therefore prefetch hints can be issued for any constant
stride.
This setting is only useful for strides that are known
and constant.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for
interchange to be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and
the number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and
the number of memory references to enable prefetching in
a loop.
use-canonical-types
Whether the compiler should use the "canonical" type
system. Should always be 1, which uses a more efficient
internal mechanism for comparing types in C++ and
Objective-C++. However, if bugs in the canonical type
system are causing compilation failures, set this value
to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays
that are bigger than switch-conversion-max-branch-ratio
times the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during
the tree partial redundancy elimination optimization
(-ftree-pre) when optimizing at -O3 and above. For some
sorts of source code the enhanced partial redundancy
elimination optimization can run away, consuming all of
the memory available on the host machine. This parameter
sets a limit on the length of the sets that are computed,
which prevents the runaway behavior. Setting a value of
0 for this parameter allows an unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically.
When the limit hits the innermost rpo-vn-max-loop-depth
loops and the outermost loop in the loop nest are value-
numbered optimistically and the remaining ones not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when
looking for redundancies for loads and stores. If this
limit is hit the search is aborted and the load or store
is not considered redundant. The number of queries is
algorithmically limited to the number of stores on all
paths from the load to the function entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a
function contains more loops than the number given by
this parameter, only at most the given number of the most
frequently-executed loops form regions for regional
register allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress
the conflict table, the table can still require excessive
amounts of memory for huge functions. If the conflict
table for a function could be more than the size in MB
given by this parameter, the register allocator instead
uses a faster, simpler, and lower-quality algorithm that
does not require building a pseudo-register conflict
table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register
pressure in loops for decisions to move loop invariants
(see -O3). The number of available registers reserved
for some other purposes is given by this parameter.
Default of the parameter is the best found from numerous
experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in
subsequent insns. This optimization is called
inheritance. EBB is used as a region to do this
optimization. The parameter defines a minimal fall-
through edge probability in percentage used to add BB to
inheritance EBB in LRA. The default value was chosen
from numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-time
memory, with very large loops. Loops with more basic
blocks than this parameter won't have loop invariant
motion optimization performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large
loops. This parameter limits the number of data
references in loops that are considered for data
dependence analysis. These large loops are no handled by
the optimizations using loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during
variable tracking dataflow analysis of any function. If
this limit is exceeded with variable tracking at
assignments enabled, analysis for that function is
retried without it, after removing all debug insns from
the function. If the limit is exceeded even without
debug insns, var tracking analysis is completely disabled
for the function. Setting the parameter to zero makes it
unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting
to map variable names or debug temporaries to value
expressions. This trades compilation time for more
complete debug information. If this is set too low,
value expressions that are available and could be
represented in debug information may end up not being
used; setting this higher may enable the compiler to find
more complex debug expressions, but compile time and
memory use may grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g.
begin stmt markers) to avoid complexity explosion at
inlining or expanding to RTL. If a function has more
such gimple stmts than the set limit, such stmts will be
dropped from the inlined copy of a function, and from its
RTL expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns.
The range below the parameter is reserved exclusively for
debug insns created by -fvar-tracking-assignments, but
debug insns may get (non-overlapping) uids above it if
the reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or
more new parameters only when their cumulative size is
less or equal to ipa-sra-ptr-growth-factor times the size
of the original pointer parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and
IPA-SRA) aim to replace scalar parts of aggregates with
uses of independent scalar variables. These parameters
control the maximum size, in storage units, of aggregate
which is considered for replacement when compiling for
speed (sra-max-scalarization-size-Ospeed) or size (sra-
max-scalarization-size-Osize) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar
Replacement of Aggregates (SRA) will track, per one local
variable, in order to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a
transaction, this parameter specifies the size in bytes
after which variables are saved with the logging
functions as opposed to save/restore code sequence pairs.
This option only applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop
transforms, the number of parameters in a Static Control
Part (SCoP) is bounded. A value of zero can be used to
lift the bound. A variable whose value is unknown at
compilation time and defined outside a SCoP is a
parameter of the SCoP.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop
in the loop nest by a given number of iterations. The
strip length can be changed using the loop-block-tile-
size parameter.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types
passed to a function's parameter in order to propagate
them and perform devirtualization. ipa-cp-value-list-
size is the maximum number of values and types it stores
per one formal parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability
heuristics and performs those cloning opportunities with
scores that exceed ipa-cp-eval-threshold.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive
when they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to
another function will receive when they are evaluated for
cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar
values passed in an aggregate. ipa-max-agg-items controls
the maximum number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would
make the number of iterations of a loop known, it adds a
bonus of ipa-cp-loop-hint-bonus to the profitability
score of the candidate.
ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate would
make the index of an array access known, it adds a bonus
of ipa-cp-array-index-hint-bonus to the profitability
score of the candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs
alias analysis in order to track values pointed to by
function parameters. In order not spend too much time
analyzing huge functions, it gives up and consider all
memory clobbered after examining ipa-max-aa-steps
statements modifying memory.
lto-partitions
Specify desired number of partitions produced during
WHOPR compilation. The number of partitions should
exceed the number of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very
small programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated
instructions). to provide an upper bound for individual
size of partition. Meant to be used only with balanced
partitioning.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO
streaming.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for
suggestions when C++ name lookup fails for an identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of
the target block relative to a statement's original block
to allow statement sinking of a statement. Larger
numbers result in more aggressive statement sinking. A
small positive adjustment is applied for statements with
memory operands as those are even more profitable so
sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be
sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion
(-ftree-loop-if-convert) is disabled.
allow-store-data-races
Allow optimizers to introduce new data races on stores.
Set to 1 to allow, otherwise to 0.
case-values-threshold
The smallest number of different values for which it is
best to use a jump-table instead of a tree of conditional
branches. If the value is 0, use the default for the
machine.
tree-reassoc-width
Set the maximum number of instructions executed in
parallel in reassociated tree. This parameter overrides
target dependent heuristics used by default if has non
zero value.
sched-pressure-algorithm
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original
implementation and is the more likely to prevent
instructions from being reordered. Algorithm 2 was
designed to be a compromise between the relatively
conservative approach taken by algorithm 1 and the rather
aggressive approach taken by the default scheduler. It
relies more heavily on having a regular register file and
accurate register pressure classes. See haifa-sched.c in
the GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are
considered when seeking a basis for a new straight-line
strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects.
This kind of protection is enabled by default if you are
using -fsanitize=address option. To disable global
objects protection use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This
kind of protection is enabled by default when using
-fsanitize=address. To disable stack protection use
--param asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This
kind of protection is enabled by default when using
-fsanitize=address. To disable memory reads protection
use --param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This
kind of protection is enabled by default when using
-fsanitize=address. To disable memory writes protection
use --param asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of
protection is enabled by default when using
-fsanitize=address. To disable built-in functions
protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of
protection is enabled by default when using the
-fsanitize=address option. To disable it use --param
asan-use-after-return=0.
Note: By default the check is disabled at run time. To
enable it, add "detect_stack_use_after_return=1" to the
environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being
instrumented is greater or equal to this number, use
callbacks instead of inline checks. E.g. to disable
inline code use --param
asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or
equal to this number, directly poison (or unpoison)
shadow memory instead of using run-time callbacks.
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating
blocks on a finite state automaton jump thread path.
max-fsm-thread-length
Maximum number of basic blocks on a finite state
automaton jump thread path.
max-fsm-thread-paths
Maximum number of new jump thread paths to create for a
finite state automaton.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by
parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by
parloops (static, dynamic, guided, auto, runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an
innermost parallelized loop for which the parallelized
variant is preferred over the single threaded one. Note
that for a parallelized loop nest the minimum number of
iterations of the outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of
SSA names in things like fold routines. One level of
recursion corresponds to following a use-def chain.
hsa-gen-debug-stores
Enable emission of special debug stores within HSA
kernels which are then read and reported by libgomp
plugin. Generation of these stores is disabled by
default, use --param hsa-gen-debug-stores=1 to enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking
for a must-def specifying the dynamic type of an object
that invokes a virtual call we may be able to
devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default
edge of a switch statement during VRP.
unroll-jam-min-percent
The minimum percentage of memory references that must be
optimized away for the unroll-and-jam transformation to
be considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be
unrolled by the unroll-and-jam transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be
generated by the RTL if-conversion pass for a branch that
is considered unpredictable.
max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum
number of times that an individual variable will be
expanded during loop unrolling.
tracer-min-branch-probability-feedback
Stop forward growth if the probability of best edge is
less than this threshold (in percent). Used when profile
feedback is available.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in
percent relative to entry BB of the function) to make
partial inlining happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization
pass will track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy
elimination after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count
that permit performing redundancy elimination after
reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by
the copy loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector
size.
slp-max-insns-in-bb
Maximum number of instructions in basic block to be
considered for SLP vectorization.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the
swing modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers
when checking conflicts using DFA.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of
basic block in program given basic block needs to have to
be considered hot (used in non-LTO mode)
max-inline-insns-recursive-auto
The maximum number of instructions non-inline function
can grow to via recursive inlining.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo
scheduler uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered
during spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing
exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store
elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number
of iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be
added.
unlikely-bb-count-fraction
The minimum fraction of profile runs a given basic block
execution count must be not to be considered unlikely.
tracer-dynamic-coverage-feedback
The percentage of function, weighted by execution
frequency, that must be covered by trace formation. Used
when profile feedback is available.
max-inline-recursive-depth-auto
The maximum depth of recursive inlining for non-inline
functions.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a
threading path when comparing to the number of (scaled)
blocks.
fsm-maximum-phi-arguments
Maximum number of arguments a PHI may have before the FSM
threader will not try to thread through its block.
uninit-control-dep-attempts
Maximum number of nested calls to search for control
dependencies during uninitialized variable analysis.
indir-call-topn-profile
Track top N target addresses in indirect-call profile.
max-once-peeled-insns
The maximum number of insns of a peeled loop that rolls
only once.
sra-max-scalarization-size-Osize
Maximum size, in storage units, of an aggregate which
should be considered for scalarization when compiling for
size.
fsm-scale-path-blocks
Scale factor to apply to the number of blocks in a
threading path when comparing to the number of (scaled)
statements.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag.
Number of lookahead cycles the model looks into; at ' '
only enable instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can
have before the loop versioning pass considers it too big
to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can
have before the loop versioning pass considers it too big
to copy, discounting any instructions in inner loops that
directly benefit from versioning.
ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in
determining a property of a variable such as its value.
This limits the number of iterations or recursive calls
GCC performs when optimizing certain statements or when
determining their validity prior to issuing diagnostics.
Program Instrumentation Options
GCC supports a number of command-line options that control adding
run-time instrumentation to the code it normally generates. For
example, one purpose of instrumentation is collect profiling
statistics for use in finding program hot spots, code coverage
analysis, or profile-guided optimizations. Another class of
program instrumentation is adding run-time checking to detect
programming errors like invalid pointer dereferences or out-of-
bounds array accesses, as well as deliberately hostile attacks
such as stack smashing or C++ vtable hijacking. There is also a
general hook which can be used to implement other forms of
tracing or function-level instrumentation for debug or program
analysis purposes.
-p
-pg Generate extra code to write profile information suitable for
the analysis program prof (for -p) or gprof (for -pg). You
must use this option when compiling the source files you want
data about, and you must also use it when linking.
You can use the function attribute "no_instrument_function"
to suppress profiling of individual functions when compiling
with these options.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and
call is executed and how many times it is taken or returns.
On targets that support constructors with priority support,
profiling properly handles constructors, destructors and C++
constructors (and destructors) of classes which are used as a
type of a global variable.
When the compiled program exits it saves this data to a file
called auxname.gcda for each source file. The data may be
used for profile-directed optimizations
(-fbranch-probabilities), or for test coverage analysis
(-ftest-coverage). Each object file's auxname is generated
from the name of the output file, if explicitly specified and
it is not the final executable, otherwise it is the basename
of the source file. In both cases any suffix is removed
(e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for
output file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for
-fprofile-arcs -ftest-coverage (when compiling) and -lgcov
(when linking). See the documentation for those options for
more details.
* Compile the source files with -fprofile-arcs plus
optimization and code generation options. For test
coverage analysis, use the additional -ftest-coverage
option. You do not need to profile every source file in
a program.
* Compile the source files additionally with
-fprofile-abs-path to create absolute path names in the
.gcno files. This allows gcov to find the correct
sources in projects where compilations occur with
different working directories.
* Link your object files with -lgcov or -fprofile-arcs (the
latter implies the former).
* Run the program on a representative workload to generate
the arc profile information. This may be repeated any
number of times. You can run concurrent instances of
your program, and provided that the file system supports
locking, the data files will be correctly updated.
Unless a strict ISO C dialect option is in effect, "fork"
calls are detected and correctly handled without double
counting.
* For profile-directed optimizations, compile the source
files again with the same optimization and code
generation options plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human
readable information from the .gcno and .gcda files.
Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC
creates a program flow graph, then finds a spanning tree for
the graph. Only arcs that are not on the spanning tree have
to be instrumented: the compiler adds code to count the
number of times that these arcs are executed. When an arc is
the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a
new basic block must be created to hold the instrumentation
code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can
use to show program coverage. Each source file's note file
is called auxname.gcno. Refer to the -fprofile-arcs option
above for a description of auxname and instructions on how to
generate test coverage data. Coverage data matches the
source files more closely if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute
path names in the .gcno files. This allows gcov to find the
correct sources in projects where compilations occur with
different working directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to
path. This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs and used
by -fprofile-use and -fbranch-probabilities and its related
options. Both absolute and relative paths can be used. By
default, GCC uses the current directory as path, thus the
profile data file appears in the same directory as the object
file. In order to prevent the file name clashing, if the
object file name is not an absolute path, we mangle the
absolute path of the sourcename.gcda file and use it as the
file name of a .gcda file.
When an executable is run in a massive parallel environment,
it is recommended to save profile to different folders. That
can be done with variables in path that are exported during
run-time:
%p process ID.
%q{VAR}
value of environment variable VAR
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use -fprofile-generate
both when compiling and when linking your program.
The following options are enabled: -fprofile-arcs,
-fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the
profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile
information, use -fprofile-use.
-fprofile-update=method
Alter the update method for an application instrumented for
profile feedback based optimization. The method argument
should be one of single, atomic or prefer-atomic. The first
one is useful for single-threaded applications, while the
second one prevents profile corruption by emitting thread-
safe code.
Warning: When an application does not properly join all
threads (or creates an detached thread), a profile file can
be still corrupted.
Using prefer-atomic would be transformed either to atomic,
when supported by a target, or to single otherwise. The GCC
driver automatically selects prefer-atomic when -pthread is
present in the command line.
-fprofile-filter-files=regex
Instrument only functions from files where names match any
regular expression (separated by a semi-colon).
For example, -fprofile-filter-files=main.c;module.*.c will
instrument only main.c and all C files starting with
'module'.
-fprofile-exclude-files=regex
Instrument only functions from files where names do not match
all the regular expressions (separated by a semi-colon).
For example, -fprofile-exclude-files=/usr/* will prevent
instrumentation of all files that are located in /usr/
folder.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect out-of-
bounds and use-after-free bugs. The option enables
-fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer >
for more details. The run-time behavior can be influenced
using the ASAN_OPTIONS environment variable. When set to
"help=1", the available options are shown at startup of the
instrumented program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags >
for a list of supported options. The option cannot be
combined with -fsanitize=thread.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan/wiki > for more details.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer
operands. The option must be combined with either
-fsanitize=kernel-address or -fsanitize=address The option
cannot be combined with -fsanitize=thread. Note: By default
the check is disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable
ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects
invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option
must be combined with either -fsanitize=kernel-address or
-fsanitize=address The option cannot be combined with
-fsanitize=thread. Note: By default the check is disabled at
run time. To enable it, add "detect_invalid_pointer_pairs=2"
to the environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation
only when both pointers are non-null.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory
access instructions are instrumented to detect data race
bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer >
for more details. The run-time behavior can be influenced
using the TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags >
for a list of supported options. The option cannot be
combined with -fsanitize=address, -fsanitize=leak.
Note that sanitized atomic builtins cannot throw exceptions
when operating on invalid memory addresses with non-call
exceptions (-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option
only matters for linking of executables and the executable is
linked against a library that overrides "malloc" and other
allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer >
for more details. The run-time behavior can be influenced
using the LSAN_OPTIONS environment variable. The option
cannot be combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior
detector. Various computations are instrumented to detect
undefined behavior at runtime. See
<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html >
for more details. The run-time behavior can be influenced
using the UBSAN_OPTIONS environment variable. Current
suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift
operation is not undefined. Note that what exactly is
considered undefined differs slightly between C and C++,
as well as between ISO C90 and C99, etc. This option has
two suboptions, -fsanitize=shift-base and
-fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of
a shift operation is not negative and is smaller than the
precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within
range, check that the result of a shift operation is not
undefined. Note that what exactly is considered
undefined differs slightly between C and C++, as well as
between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1"
division.
-fsanitize=unreachable
With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics message
call instead. When reaching the "__builtin_unreachable"
call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size
of a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the
application built with this option turned on will issue
an error message when it tries to dereference a NULL
pointer, or if a reference (possibly an rvalue reference)
is bound to a NULL pointer, or if a method is invoked on
an object pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs
built with this option turned on will issue an error
message when the end of a non-void function is reached
without actually returning a value. This option works in
C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We
check that the result of "+", "*", and both unary and
binary "-" does not overflow in the signed arithmetics.
Note, integer promotion rules must be taken into account.
That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds.
Various out of bounds accesses are detected. Flexible
array members, flexible array member-like arrays, and
initializers of variables with static storage are not
instrumented.
-fsanitize=bounds-strict
This option enables strict instrumentation of array
bounds. Most out of bounds accesses are detected,
including flexible array members and flexible array
member-like arrays. Initializers of variables with
static storage are not instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers
when they are dereferenced, or when a reference is bound
to insufficiently aligned target, or when a method or
constructor is invoked on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references
using the "__builtin_object_size" function. Various out
of bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other
similar options, -fsanitize=float-divide-by-zero is not
enabled by -fsanitize=undefined, since floating-point
division by zero can be a legitimate way of obtaining
infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer
conversion checking. We check that the result of the
conversion does not overflow. Unlike other similar
options, -fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well
with "FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking
whether null values are not passed to arguments marked as
requiring a non-null value by the "nonnull" function
attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements
in functions marked with "returns_nonnull" function
attribute, to detect returning of null values from such
functions.
-fsanitize=bool
This option enables instrumentation of loads from bool.
If a value other than 0/1 is loaded, a run-time error is
issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum
type. If a value outside the range of values for the
enum type is loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member
function calls, member accesses and some conversions
between pointers to base and derived classes, to verify
the referenced object has the correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer
arithmetics. If the pointer arithmetics overflows, a
run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to
selected builtin functions. If an invalid value is
passed to such arguments, a run-time error is issued.
E.g. passing 0 as the argument to "__builtin_ctz" or
"__builtin_clz" invokes undefined behavior and is
diagnosed by this option.
While -ftrapv causes traps for signed overflows to be
emitted, -fsanitize=undefined gives a diagnostic message.
This currently works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be
used together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with
different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections.
si may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for
sanitizers mentioned in comma-separated list of opts.
Enabling this option for a sanitizer component causes it to
attempt to continue running the program as if no error
happened. This means multiple runtime errors can be reported
in a single program run, and the exit code of the program may
indicate success even when errors have been reported. The
-fno-sanitize-recover= option can be used to alter this
behavior: only the first detected error is reported and
program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined
(and its suboptions except for -fsanitize=unreachable and
-fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero, -fsanitize=bounds-strict,
-fsanitize=kernel-address and -fsanitize=address. For these
sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is also
accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers
that support it.
Even if a recovery mode is turned on the compiler side, it
needs to be also enabled on the runtime library side,
otherwise the failures are still fatal. The runtime library
defaults to "halt_on_error=0" for ThreadSanitizer and
UndefinedBehaviorSanitizer, while default value for
AddressSanitizer is "halt_on_error=1". This can be overridden
through setting the "halt_on_error" flag in the corresponding
environment variable.
Syntax without an explicit opts parameter is deprecated. It
is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-
scope bugs. The option sets -fstack-reuse to none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the
compiler to report undefined behavior using "__builtin_trap"
rather than a "libubsan" library routine. The advantage of
this is that the "libubsan" library is not needed and is not
linked in, so this is usable even in freestanding
environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts
a call to "__sanitizer_cov_trace_pc" into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts
a call to "__sanitizer_cov_trace_cmp1",
"__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or
"__sanitizer_cov_trace_cmp8" for integral comparison with
both operands variable or "__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4" or
"__sanitizer_cov_trace_const_cmp8" for integral comparison
with one operand constant, "__sanitizer_cov_trace_cmpf" or
"__sanitizer_cov_trace_cmpd" for float or double comparisons
and "__sanitizer_cov_trace_switch" for switch statements.
-fcf-protection=[full|branch|return|none]
Enable code instrumentation of control-flow transfers to
increase program security by checking that target addresses
of control-flow transfer instructions (such as indirect
function call, function return, indirect jump) are valid.
This prevents diverting the flow of control to an unexpected
target. This is intended to protect against such threats as
Return-oriented Programming (ROP), and similarly
call/jmp-oriented programming (COP/JOP).
The value "branch" tells the compiler to implement checking
of validity of control-flow transfer at the point of indirect
branch instructions, i.e. call/jmp instructions. The value
"return" implements checking of validity at the point of
returning from a function. The value "full" is an alias for
specifying both "branch" and "return". The value "none" turns
off instrumentation.
The macro "__CET__" is defined when -fcf-protection is used.
The first bit of "__CET__" is set to 1 for the value "branch"
and the second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which
functions and calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation
based on Intel Control-flow Enforcement Technology (CET)
which works for i686 processor or newer.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions
that call "alloca", and functions with buffers larger than 8
bytes. The guards are initialized when a function is entered
and then checked when the function exits. If a guard check
fails, an error message is printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are
protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to
be protected --- those that have local array definitions, or
have references to local frame addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions
which have the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the
boundary of the stack. You should specify this flag if you
are running in an environment with multiple threads, but you
only rarely need to specify it in a single-threaded
environment since stack overflow is automatically detected on
nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be
done; the operating system or the language runtime must do
that. The switch causes generation of code to ensure that
they see the stack being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent
to bare -fstack-check.
Old-style checking is a generic mechanism that requires no
specific target support in the compiler but comes with the
following drawbacks:
1. Modified allocation strategy for large objects: they are
always allocated dynamically if their size exceeds a
fixed threshold. Note this may change the semantics of
some code.
2. Fixed limit on the size of the static frame of functions:
when it is topped by a particular function, stack
checking is not reliable and a warning is issued by the
compiler.
3. Inefficiency: because of both the modified allocation
strategy and the generic implementation, code performance
is hampered.
Note that old-style stack checking is also the fallback
method for specific if no target support has been added in
the compiler.
-fstack-check= is designed for Ada's needs to detect infinite
recursion and stack overflows. specific is an excellent
choice when compiling Ada code. It is not generally
sufficient to protect against stack-clash attacks. To
protect against those you want -fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When
this option is enabled, the compiler will only allocate one
page of stack space at a time and each page is accessed
immediately after allocation. Thus, it prevents allocations
from jumping over any stack guard page provided by the
operating system.
Most targets do not fully support stack clash protection.
However, on those targets -fstack-clash-protection will
protect dynamic stack allocations. -fstack-clash-protection
may also provide limited protection for static stack
allocations if the target supports -fstack-check=specific.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a
certain value, either the value of a register or the address
of a symbol. If a larger stack is required, a signal is
raised at run time. For most targets, the signal is raised
before the stack overruns the boundary, so it is possible to
catch the signal without taking special precautions.
For instance, if the stack starts at absolute address
0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack
limit of 128KB. Note that this may only work with the GNU
linker.
You can locally override stack limit checking by using the
"no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it
overflows. The resulting program has a discontiguous stack
which can only overflow if the program is unable to allocate
any more memory. This is most useful when running threaded
programs, as it is no longer necessary to calculate a good
stack size to use for each thread. This is currently only
implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled
without -fsplit-stack, there may not be much stack space
available for the latter code to run. If compiling all code,
including library code, with -fsplit-stack is not an option,
then the linker can fix up these calls so that the code
compiled without -fsplit-stack always has a large stack.
Support for this is implemented in the gold linker in GNU
binutils release 2.21 and later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It
turns on (or off, if using -fvtable-verify=none) the security
feature that verifies at run time, for every virtual call,
that the vtable pointer through which the call is made is
valid for the type of the object, and has not been corrupted
or overwritten. If an invalid vtable pointer is detected at
run time, an error is reported and execution of the program
is immediately halted.
This option causes run-time data structures to be built at
program startup, which are used for verifying the vtable
pointers. The options std and preinit control the timing of
when these data structures are built. In both cases the data
structures are built before execution reaches "main". Using
-fvtable-verify=std causes the data structures to be built
after shared libraries have been loaded and initialized.
-fvtable-verify=preinit causes them to be built before shared
libraries have been loaded and initialized.
If this option appears multiple times in the command line
with different values specified, none takes highest priority
over both std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called.
This flag also causes the compiler to log information about
which vtable pointers it finds for each class. This
information is written to a file named vtv_set_ptr_data.log
in the directory named by the environment variable
VTV_LOGS_DIR if that is defined or the current working
directory otherwise.
Note: This feature appends data to the log file. If you want
a fresh log file, be sure to delete any existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes
the compiler to keep track of the total number of virtual
calls it encounters and the number of verifications it
inserts. It also counts the number of calls to certain run-
time library functions that it inserts and logs this
information for each compilation unit. The compiler writes
this information to a file named vtv_count_data.log in the
directory named by the environment variable VTV_LOGS_DIR if
that is defined or the current working directory otherwise.
It also counts the size of the vtable pointer sets for each
class, and writes this information to vtv_class_set_sizes.log
in the same directory.
Note: This feature appends data to the log files. To get
fresh log files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to
functions. Just after function entry and just before
function exit, the following profiling functions are called
with the address of the current function and its call site.
(On some platforms, "__builtin_return_address" does not work
beyond the current function, so the call site information may
not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current
function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded
inline in other functions. The profiling calls indicate
where, conceptually, the inline function is entered and
exited. This means that addressable versions of such
functions must be available. If all your uses of a function
are expanded inline, this may mean an additional expansion of
code size. If you use "extern inline" in your C code, an
addressable version of such functions must be provided.
(This is normally the case anyway, but if you get lucky and
the optimizer always expands the functions inline, you might
have gotten away without providing static copies.)
A function may be given the attribute
"no_instrument_function", in which case this instrumentation
is not done. This can be used, for example, for the
profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling
functions cannot safely be called (perhaps signal handlers,
if the profiling routines generate output or allocate
memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from
instrumentation (see the description of
-finstrument-functions). If the file that contains a
function definition matches with one of file, then that
function is not instrumented. The match is done on
substrings: if the file parameter is a substring of the file
name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames
contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of
sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the
single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list,
but this option sets the list of function names to be
excluded from instrumentation. The function name to be
matched is its user-visible name, such as "vector<int>
blah(const vector<int> &)", not the internal mangled name
(e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on
substrings: if the sym parameter is a substring of the
function name, it is considered to be a match. For C99 and
C++ extended identifiers, the function name must be given in
UTF-8, not using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with
the function entry point before the Mth NOP. If M is
omitted, it defaults to 0 so the function entry points to the
address just at the first NOP. The NOP instructions reserve
extra space which can be used to patch in any desired
instrumentation at run time, provided that the code segment
is writable. The amount of space is controllable indirectly
via the number of NOPs; the NOP instruction used corresponds
to the instruction emitted by the internal GCC back-end
interface "gen_nop". This behavior is target-specific and
may also depend on the architecture variant and/or other
compilation options.
For run-time identification, the starting addresses of these
areas, which correspond to their respective function entries
minus M, are additionally collected in the
"__patchable_function_entries" section of the resulting
binary.
Note that the value of "__attribute__
((patchable_function_entry (N,M)))" takes precedence over
command-line option -fpatchable-function-entry=N,M. This can
be used to increase the area size or to remove it completely
on a single function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before,
depending on M---the function entry address, even before the
prologue.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C
source file before actual compilation.
If you use the -E option, nothing is done except preprocessing.
Some of these options make sense only together with -E because
they cause the preprocessor output to be unsuitable for actual
compilation.
In addition to the options listed here, there are a number of
options to control search paths for include files documented in
Directory Options. Options to control preprocessor diagnostics
are listed in Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if
they appeared during translation phase three in a #define
directive. In particular, the definition is truncated by
embedded newline characters.
If you are invoking the preprocessor from a shell or shell-
like program you may need to use the shell's quoting syntax
to protect characters such as spaces that have a meaning in
the shell syntax.
If you wish to define a function-like macro on the command
line, write its argument list with surrounding parentheses
before the equals sign (if any). Parentheses are meaningful
to most shells, so you should quote the option. With sh and
csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given
on the command line. All -imacros file and -include file
options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or
provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the first
line of the primary source file. However, the first
directory searched for file is the preprocessor's working
directory instead of the directory containing the main source
file. If not found there, it is searched for in the
remainder of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are
included in the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by
scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a
header without also processing its declarations.
All files specified by -imacros are processed before all
files specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros.
The standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads
library. You should use this option consistently for both
compilation and linking. This option is supported on
GNU/Linux targets, most other Unix derivatives, and also on
x86 Cygwin and MinGW targets.
-M Instead of outputting the result of preprocessing, output a
rule suitable for make describing the dependencies of the
main source file. The preprocessor outputs one make rule
containing the object file name for that source file, a
colon, and the names of all the included files, including
those coming from -include or -imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the object
file name consists of the name of the source file with any
suffix replaced with object file suffix and with any leading
directory parts removed. If there are many included files
then the rule is split into several lines using \-newline.
The rule has no commands.
This option does not suppress the preprocessor's debug
output, such as -dM. To avoid mixing such debug output with
the dependency rules you should explicitly specify the
dependency output file with -MF, or use an environment
variable like DEPENDENCIES_OUTPUT. Debug output is still
sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings
with an implicit -w.
-MM Like -M but do not mention header files that are found in
system header directories, nor header files that are
included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double
quotes in an #include directive does not in itself determine
whether that header appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the preprocessor
sends the rules to the same place it would send preprocessed
output.
When used with the driver options -MD or -MMD, -MF overrides
the default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without
raising an error. The dependency filename is taken directly
from the "#include" directive without prepending any path.
-MG also suppresses preprocessed output, as a missing header
file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to depend
on nothing. These dummy rules work around errors make gives
if you remove header files without updating the Makefile to
match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency
generation. By default CPP takes the name of the main input
file, deletes any directory components and any file suffix
such as .c, and appends the platform's usual object suffix.
The result is the target.
An -MT option sets the target to be exactly the string you
specify. If you want multiple targets, you can specify them
as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special
to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were
given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not
implied. The driver determines file based on whether an -o
option is given. If it is, the driver uses its argument but
with a suffix of .d, otherwise it takes the name of the input
file, removes any directory components and suffix, and
applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if used
without -E, each -o is understood to specify a target object
file.
Since -E is not implied, -MD can be used to generate a
dependency output file as a side effect of the compilation
process.
-MMD
Like -MD except mention only user header files, not system
header files.
-fpreprocessed
Indicate to the preprocessor that the input file has already
been preprocessed. This suppresses things like macro
expansion, trigraph conversion, escaped newline splicing, and
processing of most directives. The preprocessor still
recognizes and removes comments, so that you can pass a file
preprocessed with -C to the compiler without problems. In
this mode the integrated preprocessor is little more than a
tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions that GCC
uses for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand
macros.
The option's behavior depends on the -E and -fpreprocessed
options.
With -E, preprocessing is limited to the handling of
directives such as "#define", "#ifdef", and "#error". Other
preprocessor operations, such as macro expansion and trigraph
conversion are not performed. In addition, the -dD option is
implicitly enabled.
With -fpreprocessed, predefinition of command line and most
builtin macros is disabled. Macros such as "__LINE__", which
are contextually dependent, are handled normally. This
enables compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed
take precedence. This enables full preprocessing of files
previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option
is enabled by default for C99 (and later C standard versions)
and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with
canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the
preprocessor report correct column numbers in warnings or
errors, even if tabs appear on the line. If the value is
less than 1 or greater than 100, the option is ignored. The
default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This
allows the compiler to emit diagnostic about the current
macro expansion stack when a compilation error occurs in a
macro expansion. Using this option makes the preprocessor and
the compiler consume more memory. The level parameter can be
used to choose the level of precision of token location
tracking thus decreasing the memory consumption if necessary.
Value 0 of level de-activates this option. Value 1 tracks
tokens locations in a degraded mode for the sake of minimal
memory overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have the
same location. Value 2 tracks tokens locations completely.
This value is the most memory hungry. When this option is
given no argument, the default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by
default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand
the "__FILE__" and "__BASE_FILE__" macros as if the files
resided in directory new instead. This can be used to change
an absolute path to a relative path by using . for new which
can result in more reproducible builds that are location
independent. This option also affects "__builtin_FILE()"
during compilation. See also -ffile-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and
character constants. The default is UTF-8. charset can be
any encoding supported by the system's "iconv" library
routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string
and character constants. The default is UTF-32 or UTF-16,
whichever corresponds to the width of "wchar_t". As with
-fexec-charset, charset can be any encoding supported by the
system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the
character set of the input file to the source character set
used by GCC. If the locale does not specify, or GCC cannot
get this information from the locale, the default is UTF-8.
This can be overridden by either the locale or this command-
line option. Currently the command-line option takes
precedence if there's a conflict. charset can be any
encoding supported by the system's "iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the
dependency-output flags to also list the files from the
precompiled header's dependencies. If not specified, only
the precompiled header are listed and not the files that were
used to create it, because those files are not consulted when
a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with
-E. It inserts a special "#pragma", "#pragma GCC
pch_preprocess "filename"" in the output to mark the place
where the precompiled header was found, and its filename.
When -fpreprocessed is in use, GCC recognizes this "#pragma"
and loads the PCH.
This option is off by default, because the resulting
preprocessed output is only really suitable as input to GCC.
It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it
is safe to edit the filename if the PCH file is available in
a different location. The filename may be absolute or it may
be relative to GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output
that let the compiler know the current working directory at
the time of preprocessing. When this option is enabled, the
preprocessor emits, after the initial linemarker, a second
linemarker with the current working directory followed by two
slashes. GCC uses this directory, when it's present in the
preprocessed input, as the directory emitted as the current
working directory in some debugging information formats.
This option is implicitly enabled if debugging information is
enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in the
command line, this option has no effect, since no "#line"
directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer
answer. This form is preferred to the older form -A
predicate(answer), which is still supported, because it does
not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer
answer.
-C Do not discard comments. All comments are passed through to
the output file, except for comments in processed directives,
which are deleted along with the directive.
You should be prepared for side effects when using -C; it
causes the preprocessor to treat comments as tokens in their
own right. For example, comments appearing at the start of
what would be a directive line have the effect of turning
that line into an ordinary source line, since the first token
on the line is no longer a #.
-CC Do not discard comments, including during macro expansion.
This is like -C, except that comments contained within macros
are also passed through to the output file where the macro is
expanded.
In addition to the side effects of the -C option, the -CC
option causes all C++-style comments inside a macro to be
converted to C-style comments. This is to prevent later use
of that macro from inadvertently commenting out the remainder
of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will be
sent to a program which might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors,
as opposed to ISO C preprocessors. See the GNU CPP manual
for details.
Note that GCC does not otherwise attempt to emulate a pre-
standard C compiler, and these options are only supported
with the -E switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character
sequences, all starting with ??, that are defined by ISO C to
stand for single characters. For example, ??/ stands for \,
so '??/n' is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only
permit very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other
normal activities. Each name is indented to show how deep in
the #include stack it is. Precompiled header files are also
printed, even if they are found to be invalid; an invalid
precompiled header file is printed with ...x and a valid one
with ...! .
-dletters
Says to make debugging dumps during compilation as specified
by letters. The flags documented here are those relevant to
the preprocessor. Other letters are interpreted by the
compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify letters whose
behavior conflicts, the result is undefined.
-dM Instead of the normal output, generate a list of #define
directives for all the macros defined during the
execution of the preprocessor, including predefined
macros. This gives you a way of finding out what is
predefined in your version of the preprocessor. Assuming
you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted
as a synonym for -fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include the
predefined macros, and it outputs both the #define
directives and the result of preprocessing. Both kinds
of output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dI Output #include directives in addition to the result of
preprocessing.
-dU Like -dD except that only macros that are expanded, or
whose definedness is tested in preprocessor directives,
are output; the output is delayed until the use or test
of the macro; and #undef directives are also output for
macros tested but undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from
CPP or with -E, it dumps debugging information about location
maps. Every token in the output is preceded by the dump of
the map its location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass
option directly through to the preprocessor. If option
contains commas, it is split into multiple options at the
commas. However, many options are modified, translated or
interpreted by the compiler driver before being passed to the
preprocessor, and -Wp forcibly bypasses this phase. The
preprocessor's direct interface is undocumented and subject
to change, so whenever possible you should avoid using -Wp
and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use
this to supply system-specific preprocessor options that GCC
does not recognize.
If you want to pass an option that takes an argument, you
must use -Xpreprocessor twice, once for the option and once
for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation.
By default, GCC performs preprocessing as an integrated part
of input tokenization and parsing. If this option is
provided, the appropriate language front end (cc1, cc1plus,
or cc1obj for C, C++, and Objective-C, respectively) is
instead invoked twice, once for preprocessing only and once
for actual compilation of the preprocessed input. This
option may be useful in conjunction with the -B or -wrapper
options to specify an alternate preprocessor or perform
additional processing of the program source between normal
preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option
contains commas, it is split into multiple options at the
commas.
-Xassembler option
Pass option as an option to the assembler. You can use this
to supply system-specific assembler options that GCC does not
recognize.
If you want to pass an option that takes an argument, you
must use -Xassembler twice, once for the option and once for
the argument.
Options for Linking
These options come into play when the compiler links object files
into an executable output file. They are meaningless if the
compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix
is considered to name an object file or library. (Object
files are distinguished from libraries by the linker
according to the file contents.) If linking is done, these
object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run,
and object file names should not be used as arguments.
-flinker-output=type
This option controls the code generation of the link time
optimizer. By default the linker output is determined by the
linker plugin automatically. For debugging the compiler and
in the case of incremental linking to non-lto object file is
desired, it may be useful to control the type manually.
If type is exec the code generation is configured to produce
static binary. In this case -fpic and -fpie are both
disabled.
If type is dyn the code generation is configured to produce
shared library. In this case -fpic or -fPIC is preserved, but
not enabled automatically. This makes it possible to build
shared libraries without position independent code on
architectures this is possible, i.e. on x86.
If type is pie the code generation is configured to produce
-fpie executable. This result in similar optimizations as
exec except that -fpie is not disabled if specified at
compilation time.
If type is rel the compiler assumes that incremental linking
is done. The sections containing intermediate code for link-
time optimization are merged, pre-optimized, and output to
the resulting object file. In addition, if -ffat-lto-objects
is specified the binary code is produced for future non-lto
linking. The object file produced by incremental linking will
be smaller than a static library produced from the same
object files. At link-time the result of incremental linking
will also load faster to compiler than a static library
assuming that majority of objects in the library are used.
Finally nolto-rel configure compiler to for incremental
linking where code generation is forced, final binary is
produced and the intermediate code for later link-time
optimization is stripped. When multiple object files are
linked together the resulting code will be optimized better
than with link time optimizations disabled (for example, the
cross-module inlining will happen), most of benefits of whole
program optimizations are however lost.
During the incremental link (by -r) the linker plugin will
default to rel. With current interfaces to GNU Binutils it is
however not possible to link incrementally LTO objects and
non-LTO objects into a single mixed object file. In the case
any of object files in incremental link cannot be used for
link-time optimization the linker plugin will output warning
and use nolto-rel. To maintain the whole program optimization
it is recommended to link such objects into static library
instead. Alternatively it is possible to use H.J. Lu's
binutils with support for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only
for POSIX compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer
to your linker documentation for exact details. The general
description below applies to the GNU linker.
The linker searches a standard list of directories for the
library. The directories searched include several standard
system directories plus any that you specify with -L.
Static libraries are archives of object files, and have file
names like liblibrary.a. Some targets also support shared
libraries, which typically have names like liblibrary.so. If
both static and shared libraries are found, the linker gives
preference to linking with the shared library unless the
-static option is used.
It makes a difference where in the command you write this
option; the linker searches and processes libraries and
object files in the order they are specified. Thus, foo.o
-lz bar.o searches library z after file foo.o but before
bar.o. If bar.o refers to functions in z, those functions
may not be loaded.
-lobjc
You need this special case of the -l option in order to link
an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless
-nostdlib, -nolibc, or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only
the libraries you specify are passed to the linker, and
options specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored. The standard
startup files are used normally, unless -nostartfiles is
used.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved
by entries in libc. These entry points should be supplied
through some other mechanism when this option is specified.
-nolibc
Do not use the C library or system libraries tightly coupled
with it when linking. Still link with the startup files,
libgcc or toolchain provided language support libraries such
as libgnat, libgfortran or libstdc++ unless options
preventing their inclusion are used as well. This typically
removes -lc from the link command line, as well as system
libraries that normally go with it and become meaningless
when absence of a C library is assumed, for example -lpthread
or -lm in some configurations. This is intended for bare-
board targets when there is indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries
when linking. No startup files and only the libraries you
specify are passed to the linker, and options specifying
linkage of the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved
by entries in libc. These entry points should be supplied
through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines
which GCC uses to overcome shortcomings of particular
machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify
-nostdlib or -nodefaultlibs you should usually specify -lgcc
as well. This ensures that you have no unresolved references
to internal GCC library subroutines. (An example of such an
internal subroutine is "__main", used to ensure C++
constructors are called.)
-e entry
--entry=entry
Specify that the program entry point is entry. The argument
is interpreted by the linker; the GNU linker accepts either a
symbol name or an address.
-pie
Produce a dynamically linked position independent executable
on targets that support it. For predictable results, you
must also specify the same set of options used for
compilation (-fpie, -fPIE, or model suboptions) when you
specify this linker option.
-no-pie
Don't produce a dynamically linked position independent
executable.
-static-pie
Produce a static position independent executable on targets
that support it. A static position independent executable is
similar to a static executable, but can be loaded at any
address without a dynamic linker. For predictable results,
you must also specify the same set of options used for
compilation (-fpie, -fPIE, or model suboptions) when you
specify this linker option.
-pthread
Link with the POSIX threads library. This option is
supported on GNU/Linux targets, most other Unix derivatives,
and also on x86 Cygwin and MinGW targets. On some targets
this option also sets flags for the preprocessor, so it
should be used consistently for both compilation and linking.
-r Produce a relocatable object as output. This is also known
as partial linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets
that support it. This instructs the linker to add all
symbols, not only used ones, to the dynamic symbol table.
This option is needed for some uses of "dlopen" or to allow
obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this overrides -pie
and prevents linking with the shared libraries. On other
systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other
objects to form an executable. Not all systems support this
option. For predictable results, you must also specify the
same set of options used for compilation (-fpic, -fPIC, or
model suboptions) when you specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these
options force the use of either the shared or static version,
respectively. If no shared version of libgcc was built when
the compiler was configured, these options have no effect.
There are several situations in which an application should
use the shared libgcc instead of the static version. The
most common of these is when the application wishes to throw
and catch exceptions across different shared libraries. In
that case, each of the libraries as well as the application
itself should use the shared libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc
whenever you build a shared library or a main executable,
because C++ programs typically use exceptions, so this is the
right thing to do.
If, instead, you use the GCC driver to create shared
libraries, you may find that they are not always linked with
the shared libgcc. If GCC finds, at its configuration time,
that you have a non-GNU linker or a GNU linker that does not
support option --eh-frame-hdr, it links the shared version of
libgcc into shared libraries by default. Otherwise, it takes
advantage of the linker and optimizes away the linking with
the shared version of libgcc, linking with the static version
of libgcc by default. This allows exceptions to propagate
through such shared libraries, without incurring relocation
costs at library load time.
However, if a library or main executable is supposed to throw
or catch exceptions, you must link it using the G++ driver,
or using the option -shared-libgcc, such that it is linked
with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program,
the GCC driver automatically links against libasan. If
libasan is available as a shared library, and the -static
option is not used, then this links against the shared
version of libasan. The -static-libasan option directs the
GCC driver to link libasan statically, without necessarily
linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program,
the GCC driver automatically links against libtsan. If
libtsan is available as a shared library, and the -static
option is not used, then this links against the shared
version of libtsan. The -static-libtsan option directs the
GCC driver to link libtsan statically, without necessarily
linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program,
the GCC driver automatically links against liblsan. If
liblsan is available as a shared library, and the -static
option is not used, then this links against the shared
version of liblsan. The -static-liblsan option directs the
GCC driver to link liblsan statically, without necessarily
linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a
program, the GCC driver automatically links against libubsan.
If libubsan is available as a shared library, and the -static
option is not used, then this links against the shared
version of libubsan. The -static-libubsan option directs the
GCC driver to link libubsan statically, without necessarily
linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it
normally automatically links against libstdc++. If libstdc++
is available as a shared library, and the -static option is
not used, then this links against the shared version of
libstdc++. That is normally fine. However, it is sometimes
useful to freeze the version of libstdc++ used by the program
without going all the way to a fully static link. The
-static-libstdc++ option directs the g++ driver to link
libstdc++ statically, without necessarily linking other
libraries statically.
-symbolic
Bind references to global symbols when building a shared
object. Warn about any unresolved references (unless
overridden by the link editor option -Xlinker -z -Xlinker
defs). Only a few systems support this option.
-T script
Use script as the linker script. This option is supported by
most systems using the GNU linker. On some targets, such as
bare-board targets without an operating system, the -T option
may be required when linking to avoid references to undefined
symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to
supply system-specific linker options that GCC does not
recognize.
If you want to pass an option that takes a separate argument,
you must use -Xlinker twice, once for the option and once for
the argument. For example, to pass -assert definitions, you
must write -Xlinker -assert -Xlinker definitions. It does
not work to write -Xlinker "-assert definitions", because
this passes the entire string as a single argument, which is
not what the linker expects.
When using the GNU linker, it is usually more convenient to
pass arguments to linker options using the option=value
syntax than as separate arguments. For example, you can
specify -Xlinker -Map=output.map rather than -Xlinker -Map
-Xlinker output.map. Other linkers may not support this
syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains
commas, it is split into multiple options at the commas. You
can use this syntax to pass an argument to the option. For
example, -Wl,-Map,output.map passes -Map output.map to the
linker. When using the GNU linker, you can also get the same
effect with -Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking of
library modules to define it. You can use -u multiple times
with different symbols to force loading of additional library
modules.
-z keyword
-z is passed directly on to the linker along with the keyword
keyword. See the section in the documentation of your linker
for permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be
searched for header files during preprocessing. If dir
begins with = or $SYSROOT, then the = or $SYSROOT is replaced
by the sysroot prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote
form of the directive, "#include "file"". Directories
specified with -I, -isystem, or -idirafter apply to lookup
for both the "#include "file"" and "#include <file>"
directives.
You can specify any number or combination of these options on
the command line to search for header files in several
directories. The lookup order is as follows:
1. For the quote form of the include directive, the
directory of the current file is searched first.
2. For the quote form of the include directive, the
directories specified by -iquote options are searched in
left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in
left-to-right order.
4. Directories specified with -isystem options are scanned
in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned
in left-to-right order.
You can use -I to override a system header file, substituting
your own version, since these directories are searched before
the standard system header file directories. However, you
should not use this option to add directories that contain
vendor-supplied system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory
as a system directory, so that it gets the same special
treatment that is applied to the standard system directories.
If a standard system include directory, or a directory
specified with -isystem, is also specified with -I, the -I
option is ignored. The directory is still searched but as a
system directory at its normal position in the system include
chain. This is to ensure that GCC's procedure to fix buggy
system headers and the ordering for the "#include_next"
directive are not inadvertently changed. If you really need
to change the search order for system directories, use the
-nostdinc and/or -isystem options.
-I- Split the include path. This option has been deprecated.
Please use -iquote instead for -I directories before the -I-
and remove the -I- option.
Any directories specified with -I options before -I- are
searched only for headers requested with "#include "file"";
they are not searched for "#include <file>". If additional
directories are specified with -I options after the -I-,
those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the
current file directory as the first search directory for
"#include "file"". There is no way to override this effect
of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should
include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix,
and add the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would;
-iwithprefix puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to
header files (except for Darwin targets, where it applies to
both header files and libraries). See the --sysroot option
for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-nostdinc
Do not search the standard system directories for header
files. Only the directories explicitly specified with -I,
-iquote, -isystem, and/or -idirafter options (and the
directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard
directories. (This option is used when building the C++
library.)
-iplugindir=dir
Set the directory to search for plugins that are passed by
-fplugin=name instead of -fplugin=path/name.so. This option
is not meant to be used by the user, but only passed by the
driver.
-Ldir
Add directory dir to the list of directories to be searched
for -l.
-Bprefix
This option specifies where to find the executables,
libraries, include files, and data files of the compiler
itself.
The compiler driver program runs one or more of the
subprograms cpp, cc1, as and ld. It tries prefix as a prefix
for each program it tries to run, both with and without
machine/version/ for the corresponding target machine and
compiler version.
For each subprogram to be run, the compiler driver first
tries the -B prefix, if any. If that name is not found, or
if -B is not specified, the driver tries two standard
prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither
of those results in a file name that is found, the unmodified
program name is searched for using the directories specified
in your PATH environment variable.
The compiler checks to see if the path provided by -B refers
to a directory, and if necessary it adds a directory
separator character at the end of the path.
-B prefixes that effectively specify directory names also
apply to libraries in the linker, because the compiler
translates these options into -L options for the linker.
They also apply to include files in the preprocessor, because
the compiler translates these options into -isystem options
for the preprocessor. In this case, the compiler appends
include to the prefix.
The runtime support file libgcc.a can also be searched for
using the -B prefix, if needed. If it is not found there,
the two standard prefixes above are tried, and that is all.
The file is left out of the link if it is not found by those
means.
Another way to specify a prefix much like the -B prefix is to
use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the range 0 to 9, then
it is replaced by [dir/]include. This is to help with boot-
strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../
or /./, or make the path absolute when generating a relative
prefix.
--sysroot=dir
Use dir as the logical root directory for headers and
libraries. For example, if the compiler normally searches
for headers in /usr/include and libraries in /usr/lib, it
instead searches dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then
the --sysroot option applies to libraries, but the -isysroot
option applies to header files.
The GNU linker (beginning with version 2.16) has the
necessary support for this option. If your linker does not
support this option, the header file aspect of --sysroot
still works, but the library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory
specified with --sysroot, depending on the other options
used, so that headers may for example be found in
dir/suffix/usr/include instead of dir/usr/include. This
option disables the addition of such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface
conventions used in code generation.
Most of them have both positive and negative forms; the negative
form of -ffoo is -fno-foo. In the table below, only one of the
forms is listed---the one that is not the default. You can
figure out the other form by either removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared
local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none. all enables
stack reuse for all local variables and temporaries,
named_vars enables the reuse only for user defined local
variables with names, and none disables stack reuse
completely. The default value is all. The option is needed
when the program extends the lifetime of a scoped local
variable or a compiler generated temporary beyond the end
point defined by the language. When a lifetime of a variable
ends, and if the variable lives in memory, the optimizing
compiler has the freedom to reuse its stack space with other
temporaries or scoped local variables whose live range does
not overlap with it. Legacy code extending local lifetime is
likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well
defined by the C++ standard. When a lifetime of a temporary
ends, and if the temporary lives in memory, the optimizing
compiler has the freedom to reuse its stack space with other
temporaries or scoped local variables whose live range does
not overlap with it. However some of the legacy code relies
on the behavior of older compilers in which temporaries'
stack space is not reused, the aggressive stack reuse can
lead to runtime errors. This option is used to control the
temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations. The options -ftrapv
and -fwrapv override each other, so using -ftrapv -fwrapv on
the command-line results in -fwrapv being effective. Note
that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being
effective.
-fwrapv
This option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement
representation. This flag enables some optimizations and
disables others. The options -ftrapv and -fwrapv override
each other, so using -ftrapv -fwrapv on the command-line
results in -fwrapv being effective. Note that only active
options override, so using -ftrapv -fwrapv -fno-wrapv on the
command-line results in -ftrapv being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer
arithmetic overflow on addition and subtraction wraps around
using twos-complement representation. This flag disables
some optimizations which assume pointer overflow is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when
negated implies -fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC
generates frame unwind information for all functions, which
can produce significant data size overhead, although it does
not affect execution. If you do not specify this option, GCC
enables it by default for languages like C++ that normally
require exception handling, and disables it for languages
like C that do not normally require it. However, you may
need to enable this option when compiling C code that needs
to interoperate properly with exception handlers written in
C++. You may also wish to disable this option if you are
compiling older C++ programs that don't use exception
handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific
runtime support that does not exist everywhere. Moreover, it
only allows trapping instructions to throw exceptions, i.e.
memory references or floating-point instructions. It does
not allow exceptions to be thrown from arbitrary signal
handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but
don't otherwise contribute to the execution of the program
can be optimized away. This option is enabled by default for
the Ada front end, as permitted by the Ada language
specification. Optimization passes that cause dead
exceptions to be removed are enabled independently at
different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any
needed static data, but does not affect the generated code in
any other way. You normally do not need to enable this
option; instead, a language processor that needs this
handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target
machine. The table is exact at each instruction boundary, so
it can be used for stack unwinding from asynchronous events
(such as debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++
compiler uses the "STB_GNU_UNIQUE" binding to make sure that
definitions of template static data members and static local
variables in inline functions are unique even in the presence
of "RTLD_LOCAL"; this is necessary to avoid problems with a
library used by two different "RTLD_LOCAL" plugins depending
on a definition in one of them and therefore disagreeing with
the other one about the binding of the symbol. But this
causes "dlclose" to be ignored for affected DSOs; if your
program relies on reinitialization of a DSO via "dlclose" and
"dlopen", you can use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like
longer ones, rather than in registers. This convention is
less efficient, but it has the advantage of allowing
intercallability between GCC-compiled files and files
compiled with other compilers, particularly the Portable C
Compiler (pcc).
The precise convention for returning structures in memory
depends on the target configuration macros.
Short structures and unions are those whose size and
alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is
not binary compatible with code compiled with the
-freg-struct-return switch. Use it to conform to a non-
default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when
possible. This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention,
GCC defaults to -fpcc-struct-return, except on targets where
GCC is the principal compiler. In those cases, we can choose
the standard, and we chose the more efficient register return
alternative.
Warning: code compiled with the -freg-struct-return switch is
not binary compatible with code compiled with the
-fpcc-struct-return switch. Use it to conform to a non-
default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for
the declared range of possible values. Specifically, the
"enum" type is equivalent to the smallest integer type that
has enough room.
Warning: the -fshort-enums switch causes GCC to generate code
that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application
binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short
unsigned int" instead of the default for the target. This
option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code
that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application
binary interface.
-fno-common
In C code, this option controls the placement of global
variables defined without an initializer, known as tentative
definitions in the C standard. Tentative definitions are
distinct from declarations of a variable with the "extern"
keyword, which do not allocate storage.
Unix C compilers have traditionally allocated storage for
uninitialized global variables in a common block. This
allows the linker to resolve all tentative definitions of the
same variable in different compilation units to the same
object, or to a non-tentative definition. This is the
behavior specified by -fcommon, and is the default for GCC on
most targets. On the other hand, this behavior is not
required by ISO C, and on some targets may carry a speed or
code size penalty on variable references.
The -fno-common option specifies that the compiler should
instead place uninitialized global variables in the BSS
section of the object file. This inhibits the merging of
tentative definitions by the linker so you get a multiple-
definition error if the same variable is defined in more than
one compilation unit. Compiling with -fno-common is useful
on targets for which it provides better performance, or if
you wish to verify that the program will work on other
systems that always treat uninitialized variable definitions
this way.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else
that would cause trouble if the function is split in the
middle, and the two halves are placed at locations far apart
in memory. This option is used when compiling crtstuff.c;
you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly
code to make it more readable. This option is generally only
of use to those who actually need to read the generated
assembly code (perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information
to be omitted and is useful when comparing two assembler
files.
The added comments include:
* information on the compiler version and command-line
options,
* the source code lines associated with the assembly
instructions, in the form FILENAME:LINENUMBER:CONTENT OF
LINE,
* hints on which high-level expressions correspond to the
various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result
direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and
hence the precise format of the comments is subject to
change.
-frecord-gcc-switches
This switch causes the command line used to invoke the
compiler to be recorded into the object file that is being
created. This switch is only implemented on some targets and
the exact format of the recording is target and binary file
format dependent, but it usually takes the form of a section
containing ASCII text. This switch is related to the
-fverbose-asm switch, but that switch only records
information in the assembler output file as comments, so it
never reaches the object file. See also
-grecord-gcc-switches for another way of storing compiler
options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in
a shared library, if supported for the target machine. Such
code accesses all constant addresses through a global offset
table (GOT). The dynamic loader resolves the GOT entries
when the program starts (the dynamic loader is not part of
GCC; it is part of the operating system). If the GOT size
for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating
that -fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC, 28k on AArch64
and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and
therefore works only on certain machines. For the x86, GCC
supports PIC for System V but not for the Sun 386i. Code
generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 1.
-fPIC
If supported for the target machine, emit position-
independent code, suitable for dynamic linking and avoiding
any limit on the size of the global offset table. This
option makes a difference on AArch64, m68k, PowerPC and
SPARC.
Position-independent code requires special support, and
therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the
generated position-independent code can be only linked into
executables. Usually these options are used to compile code
that will be linked using the -pie GCC option.
-fpie and -fPIE both define the macros "__pie__" and
"__PIE__". The macros have the value 1 for -fpie and 2 for
-fPIE.
-fno-plt
Do not use the PLT for external function calls in position-
independent code. Instead, load the callee address at call
sites from the GOT and branch to it. This leads to more
efficient code by eliminating PLT stubs and exposing GOT
loads to optimizations. On architectures such as 32-bit x86
where PLT stubs expect the GOT pointer in a specific
register, this gives more register allocation freedom to the
compiler. Lazy binding requires use of the PLT; with
-fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to
avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls
to functions that are marked to not use the PLT to use the
GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it
would be more efficient than other code generation
strategies. This option is of use in conjunction with -fpic
or -fPIC for building code that forms part of a dynamic
linker and cannot reference the address of a jump table. On
some targets, jump tables do not require a GOT and this
option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated
code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names
accepted are machine-specific and are defined in the
"REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies
a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for
temporaries or variables that do not live across a call.
Functions compiled this way do not save and restore the
register reg.
It is an error to use this flag with the frame pointer or
stack pointer. Use of this flag for other registers that
have fixed pervasive roles in the machine's execution model
produces disastrous results.
This flag does not have a negative form, because it specifies
a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved
by functions. It may be allocated even for temporaries or
variables that live across a call. Functions compiled this
way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or
stack pointer. Use of this flag for other registers that
have fixed pervasive roles in the machine's execution model
produces disastrous results.
A different sort of disaster results from the use of this
flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies
a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members
together without holes. When a value is specified (which
must be a small power of two), pack structure members
according to this value, representing the maximum alignment
(that is, objects with default alignment requirements larger
than this are output potentially unaligned at the next
fitting location.
Warning: the -fpack-struct switch causes GCC to generate code
that is not binary compatible with code generated without
that switch. Additionally, it makes the code suboptimal.
Use it to conform to a non-default application binary
interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore,
forcibly change the way C symbols are represented in the
object file. One use is to help link with legacy assembly
code.
Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-
default application binary interface. Not all targets
provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model
argument should be one of global-dynamic, local-dynamic,
initial-exec or local-exec. Note that the choice is subject
to optimization: the compiler may use a more efficient model
for symbols not visible outside of the translation unit, or
if -fpic is not given on the command line.
The default without -fpic is initial-exec; with -fpic the
default is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested
functions, always generate them instead of using descriptors.
Otherwise, for targets that do not need them, like for
example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at run
time on the stack when the address of a nested function is
taken, and is used to call the nested function indirectly.
Therefore, it requires the stack to be made executable in
order for the program to work properly.
-fno-trampolines is enabled by default on a language by
language basis to let the compiler avoid generating them, if
it computes that this is safe, and replace them with
descriptors. Descriptors are made up of data only, but the
generated code must be prepared to deal with them. As of
this writing, -fno-trampolines is enabled by default only for
Ada.
Moreover, code compiled with -ftrampolines and code compiled
with -fno-trampolines are not binary compatible if nested
functions are present. This option must therefore be used on
a program-wide basis and be manipulated with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified
option---all symbols are marked with this unless overridden
within the code. Using this feature can very substantially
improve linking and load times of shared object libraries,
produce more optimized code, provide near-perfect API export
and prevent symbol clashes. It is strongly recommended that
you use this in any shared objects you distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared
object. protected and internal are pretty useless in real-
world usage so the only other commonly used option is hidden.
The default if -fvisibility isn't specified is default, i.e.,
make every symbol public.
A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by "How To Write
Shared Libraries" by Ulrich Drepper (which can be found at
<https://www.akkadia.org/drepper/ >)---however a superior
solution made possible by this option to marking things
hidden when the default is public is to make the default
hidden and mark things public. This is the norm with DLLs on
Windows and with -fvisibility=hidden and "__attribute__
((visibility("default")))" instead of "__declspec(dllexport)"
you get almost identical semantics with identical syntax.
This is a great boon to those working with cross-platform
projects.
For those adding visibility support to existing code, you may
find "#pragma GCC visibility" of use. This works by you
enclosing the declarations you wish to set visibility for
with (for example) "#pragma GCC visibility push(hidden)" and
"#pragma GCC visibility pop". Bear in mind that symbol
visibility should be viewed as part of the API interface
contract and thus all new code should always specify
visibility when it is not the default; i.e., declarations
only for use within the local DSO should always be marked
explicitly as hidden as so to avoid PLT indirection
overheads---making this abundantly clear also aids
readability and self-documentation of the code. Note that
due to ISO C++ specification requirements, "operator new" and
"operator delete" must always be of default visibility.
Be aware that headers from outside your project, in
particular system headers and headers from any other library
you use, may not be expecting to be compiled with visibility
other than the default. You may need to explicitly say
"#pragma GCC visibility push(default)" before including any
such headers.
"extern" declarations are not affected by -fvisibility, so a
lot of code can be recompiled with -fvisibility=hidden with
no modifications. However, this means that calls to "extern"
functions with no explicit visibility use the PLT, so it is
more effective to use "__attribute ((visibility))" and/or
"#pragma GCC visibility" to tell the compiler which "extern"
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage
entities. This means that, for instance, an exception class
that is be thrown between DSOs must be explicitly marked with
default visibility so that the type_info nodes are unified
between the DSOs.
An overview of these techniques, their benefits and how to
use them is at <http://gcc.gnu.org/wiki/Visibility >.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields
(or other structure fields, although the compiler usually
honors those types anyway) should use a single access of the
width of the field's type, aligned to a natural alignment if
possible. For example, targets with memory-mapped peripheral
registers might require all such accesses to be 16 bits wide;
with this flag you can declare all peripheral bit-fields as
"unsigned short" (assuming short is 16 bits on these targets)
to force GCC to use 16-bit accesses instead of, perhaps, a
more efficient 32-bit access.
If this option is disabled, the compiler uses the most
efficient instruction. In the previous example, that might
be a 32-bit load instruction, even though that accesses bytes
that do not contain any portion of the bit-field, or memory-
mapped registers unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied
to a structure field, it may not be possible to access the
field with a single read or write that is correctly aligned
for the target machine. In this case GCC falls back to
generating multiple accesses rather than code that will fault
or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write
accesses are not allowed to touch non bit-field members. It
is therefore recommended to define all bits of the field's
type as bit-field members.
The default value of this option is determined by the
application binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the
"__sync" family of functions may be used to implement the
C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only
useful form of the option is -fno-sync-libcalls. This option
is used in the implementation of the libatomic runtime
library.
GCC Developer Options
This section describes command-line options that are primarily of
interest to GCC developers, including options to support compiler
testing and investigation of compiler bugs and compile-time
performance problems. This includes options that produce debug
dumps at various points in the compilation; that print statistics
such as memory use and execution time; and that print information
about GCC's configuration, such as where it searches for
libraries. You should rarely need to use any of these options
for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file
take an optional =filename suffix. You can specify stdout or - to
dump to standard output, and stderr for standard error.
If =filename is omitted, a default dump file name is constructed
by concatenating the base dump file name, a pass number, phase
letter, and pass name. The base dump file name is the name of
output file produced by the compiler if explicitly specified and
not an executable; otherwise it is the source file name. The
pass number is determined by the order passes are registered with
the compiler's pass manager. This is generally the same as the
order of execution, but passes registered by plugins, target-
specific passes, or passes that are otherwise registered late are
numbered higher than the pass named final, even if they are
executed earlier. The phase letter is one of i (inter-procedural
analysis), l (language-specific), r (RTL), or t (tree). The
files are created in the directory of the output file.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times
specified by letters. This is used for debugging the RTL-
based passes of the compiler.
Some -dletters switches have different meaning when -E is
used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some
-d option letters. Here are the possible letters for use in
pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied
in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run
on architectures that have auto inc or auto dec
instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after
the two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable
dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after
the two common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after
the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping
after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping
after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent
reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and
epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping
after the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of
instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on
some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file"
registers to the x87's stack-like registers. This pass
is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping
after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard
registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous
debugging information.
-dD Dump all macro definitions, at the end of preprocessing,
in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating
which pattern and alternative is used. The length and
cost of each instruction are also printed.
-dP Dump the RTL in the assembler output as a comment before
each instruction. Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
-fdump-debug
Dump debugging information generated during the debug
generation phase.
-fdump-earlydebug
Dump debugging information generated during the early debug
generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This
makes it more feasible to use diff on debugging dumps for
compiler invocations with different compiler binaries and/or
different text / bss / data / heap / stack / dso start
locations.
-freport-bug
Collect and dump debug information into a temporary file if
an internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and
address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different
options, in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural
analysis language tree to a file. The file name is generated
by appending a switch specific suffix to the source file
name, and the file is created in the same directory as the
output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
inline
Dump after function inlining.
Additionally, the options -optimized, -missed, -note, and
-all can be provided, with the same meaning as for
-fopt-info, defaulting to -optimized.
For example, -fdump-ipa-inline-optimized-missed will emit
information on callsites that were inlined, along with
callsites that were not inlined.
By default, the dump will contain messages about successful
optimizations (equivalent to -optimized) together with low-
level details about the analysis.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The
options and filename portions behave as described in the
-fdump-tree option. The following switch values are
accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table
information is emitted unless 'slim' is specified. This
option is applicable to C++ only.
raw Dump the raw internal tree data. This option is
applicable to C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are
turned on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate
file. The file name is generated by appending a suffix
ending in .statistics to the source file name, and the file
is created in the same directory as the output file. If the
-option form is used, -stats causes counters to be summed
over the whole compilation unit while -details dumps every
event as the passes generate them. The default with no
option is to sum counters for each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the
intermediate language tree to a file. If the -options form
is used, options is a list of - separated options which
control the details of the dump. Not all options are
applicable to all dumps; those that are not meaningful are
ignored. The following options are available
address
Print the address of each node. Usually this is not
meaningful as it changes according to the environment and
source file. Its primary use is for tying up a dump file
with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl,
use that in the dump instead of "DECL_NAME". Its primary
use is ease of use working backward from mangled names in
the assembly file.
slim
When dumping front-end intermediate representations,
inhibit dumping of members of a scope or body of a
function merely because that scope has been reached.
Only dump such items when they are directly reachable by
some other path.
When dumping pretty-printed trees, this option inhibits
dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form
instead of the default LISP-like representation.
raw Print a raw representation of the tree. By default,
trees are pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump
option). Also include information from the optimization
passes.
stats
Enable dumping various statistics about the pass (not
honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw
dumps).
graph
For each of the other indicated dump files
(-fdump-rtl-pass), dump a representation of the control
flow graph suitable for viewing with GraphViz to
file.passid.pass.dot. Each function in the file is
pretty-printed as a subgraph, so that GraphViz can render
them all in a single plot.
This option currently only works for RTL dumps, and the
RTL is always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each
variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each
statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available
in certain passes).
missed
Enable showing missed optimization information (only
available in certain passes).
note
Enable other detailed optimization information (only
available in certain passes).
all Turn on all options, except raw, slim, verbose and
lineno.
optall
Turn on all optimization options, i.e., optimized,
missed, and note.
To determine what tree dumps are available or find the dump
for a pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output
look for a code that corresponds to the pass you are
interested in. For example, the codes "tree-evrp",
"tree-vrp1", and "tree-vrp2" correspond to the three
Value Range Propagation passes. The number at the end
distinguishes distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass
code to the -fdump- option prefix and invoke GCC with it.
For example, to enable the dump from the Early Value
Range Propagation pass, invoke GCC with the
-fdump-tree-evrp option. Optionally, you may specify the
name of the dump file. If you don't specify one, GCC
creates as described below.
3. Find the pass dump in a file whose name is composed of
three components separated by a period: the name of the
source file GCC was invoked to compile, a numeric suffix
indicating the pass number followed by the letter t for
tree passes (and the letter r for RTL passes), and
finally the pass code. For example, the Early VRP pass
dump might be in a file named myfile.c.038t.evrp in the
current working directory. Note that the numeric codes
are not stable and may change from one version of GCC to
another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes.
If the -options form is used, options is a list of -
separated option keywords to select the dump details and
optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be
emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be
included.
The options from each group can be freely mixed as they are
non-overlapping. However, in case of any conflicts, the later
options override the earlier options on the command line.
The following options control which kinds of messages should
be emitted:
optimized
Print information when an optimization is successfully
applied. It is up to a pass to decide which information
is relevant. For example, the vectorizer passes print the
source location of loops which are successfully
vectorized.
missed
Print information about missed optimizations. Individual
passes control which information to include in the
output.
note
Print verbose information about optimizations, such as
certain transformations, more detailed messages about
decisions etc.
all Print detailed optimization information. This includes
optimized, missed, and note.
The following option controls the dump verbosity:
internals
By default, only "high-level" messages are emitted. This
option enables additional, more detailed, messages, which
are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to
describe a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi
Processing) optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset
of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which
means to dump messages about successful optimizations from
all the passes, omitting messages that are treated as
"internals".
If the filename is provided, then the dumps from all the
applicable optimizations are concatenated into the filename.
Otherwise the dump is output onto stderr. Though multiple
-fopt-info options are accepted, only one of them can include
a filename. If other filenames are provided then all but the
first such option are ignored.
Note that the output filename is overwritten in case of
multiple translation units. If a combined output from
multiple translation units is desired, stderr should be used
instead.
In the following example, the optimization info is output to
stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities
from vectorization passes on stderr. Note that
-fopt-info-vec-missed is equivalent to -fopt-info-missed-vec.
The order of the optimization group names and message types
listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as
optimized locations from all the inlining passes into
inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in
conflict since only one output file is allowed. In this case,
only the first option takes effect and the subsequent options
are ignored. Thus only vec.miss is produced which contains
dumps from the vectorizer about missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what
optimizations were performed, for those optimizations that
support -fopt-info.
This option is experimental and the format of the data within
the compressed JSON file is subject to change.
It is roughly equivalent to a machine-readable version of
-fopt-info-all, as a collection of messages with source file,
line number and column number, with the following additional
data for each message:
* the execution count of the code being optimized, along
with metadata about whether this was from actual profile
data, or just an estimate, allowing consumers to
prioritize messages by code hotness,
* the function name of the code being optimized, where
applicable,
* the "inlining chain" for the code being optimized, so
that when a function is inlined into several different
places (which might themselves be inlined), the reader
can distinguish between the copies,
* objects identifying those parts of the message that refer
to expressions, statements or symbol-table nodes, which
of these categories they are, and, when available, their
source code location,
* the GCC pass that emitted the message, and
* the location in GCC's own code from which the message was
emitted
Additionally, some messages are logically nested within other
messages, reflecting implementation details of the
optimization passes.
-fsched-verbose=n
On targets that use instruction scheduling, this option
controls the amount of debugging output the scheduler prints
to the dump files.
For n greater than zero, -fsched-verbose outputs the same
information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For
n greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n
greater than two, it includes RTL at abort point, control-
flow and regions info. And for n over four, -fsched-verbose
also includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly
disable/enable optimization passes. These options are
intended for use for debugging GCC. Compiler users should
use regular options for enabling/disabling passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from 1. range-list is a comma-separated
list of function ranges or assembler names. Each range
is a number pair separated by a colon. The range is
inclusive in both ends. If the range is trivial, the
number pair can be simplified as a single number. If the
function's call graph node's uid falls within one of the
specified ranges, the pass is disabled for that function.
The uid is shown in the function header of a dump file,
and the pass names can be dumped by using option
-fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the
description of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option
argument description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the
description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on
the compiler configuration. -fchecking=2 enables further
internal consistency checking that might affect code
generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random
numbers in generating certain symbol names that have to be
different in every compiled file. It is also used to place
unique stamps in coverage data files and the object files
that produce them. You can use the -frandom-seed option to
produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or
an arbitrary string (in which case it's converted to a number
by computing CRC32).
The string should be different for every file you compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently;
place them in the current directory and name them based on
the source file. Thus, compiling foo.c with -c -save-temps
produces files foo.i and foo.s, as well as foo.o. This
creates a preprocessed foo.i output file even though the
compiler now normally uses an integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file.
The corresponding intermediate file may be obtained by
renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different
source files that share a common base name in different
subdirectories or the same source file compiled for multiple
output destinations, it is likely that the different parallel
compilers will interfere with each other, and overwrite the
temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously
by both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently.
If the -o option is used, the temporary files are based on
the object file. If the -o option is not used, the
-save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the
compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is
done).
Without the specification of an output file, the output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is
time spent executing the program itself. The second number
is "system time", time spent executing operating system
routines on behalf of the program. Both numbers are in
seconds.
With the specification of an output file, the output is
appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the
program name, and the options passed to the program are
displayed, so that one can later tell what file was being
compiled, and with which options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of
the dump file is determined by appending ".gkd" to the
compilation output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a
second time, adding opts and -fcompare-debug-second to the
arguments passed to the second compilation. Dump the final
internal representation in both compilations, and print an
error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-
empty and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a
dash, then it is used for opts, otherwise the default
-gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping
of the final representation and the second compilation,
preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden,
which GCC rejects as an invalid option in any actual
compilation (rather than preprocessing, assembly or linking).
To get just a warning, setting GCC_COMPARE_DEBUG to
-w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the
second compilation requested by -fcompare-debug, along with
options to silence warnings, and omitting other options that
would cause the compiler to produce output to files or to
standard output as a side effect. Dump files and preserved
temporary files are renamed so as to contain the ".gk"
additional extension during the second compilation, to avoid
overwriting those generated by the first.
When this option is passed to the compiler driver, it causes
the first compilation to be skipped, which makes it useful
for little other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The
position of this argument in the command line does not
matter; it takes effect after all other options are
processed, and it does so only once, no matter how many times
it is given. This is mainly intended to be used with
-fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that
-gtoggle toggles -g.
-Q Makes the compiler print out each function name as it is
compiled, and print some statistics about each pass when it
finishes.
-ftime-report
Makes the compiler print some statistics about the time
consumed by each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately
for each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated
register allocator. The default value is 5. If the value n
is greater or equal to 10, the dump output is sent to stderr
using the same format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the
link-time optimizer. The contents of this report vary from
version to version. It is meant to be useful to GCC
developers when processing object files in LTO mode (via
-flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of Link
Time Optimization.
-fmem-report
Makes the compiler print some statistics about permanent
memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent
memory allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent
memory allocation before or after interprocedural
optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of
the (estimated) profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the
program, on a per-function basis. The filename for the dump
is made by appending .su to the auxname. auxname is
generated from the name of the output file, if explicitly
specified and it is not an executable, otherwise it is the
basename of the source file. An entry is made up of three
fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates
the stack statically: a fixed number of bytes are allocated
for the frame on function entry and released on function
exit; no stack adjustments are otherwise made in the
function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates
the stack dynamically: in addition to the static allocation
described above, stack adjustments are made in the body of
the function, for example to push/pop arguments around
function calls. If the qualifier "bounded" is also present,
the amount of these adjustments is bounded at compile time
and the second field is an upper bound of the total amount of
stack used by the function. If it is not present, the amount
of these adjustments is not bounded at compile time and the
second field only represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This option is supported only by the C++ front
end, and the information is generally only useful to the G++
development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug
counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound.
counter-value-list is a comma-separated list of
name:lower_bound:upper_bound tuples which sets the lower and
the upper bound of each debug counter name. The lower_bound
is optional and is zero initialized if not set. All debug
counters have the initial upper bound of "UINT_MAX"; thus
"dbg_cnt" returns true always unless the upper bound is set
by this option. For example, with
-fdbg-cnt=dce:2:4,tail_call:10, "dbg_cnt(dce)" returns true
only for third and fourth invocation. For
"dbg_cnt(tail_call)" true is returned for first 10
invocations.
-print-file-name=library
Print the full absolute name of the library file library that
would be used when linking---and don't do anything else.
With this option, GCC does not compile or link anything; it
just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib
selected by any other switches present in the command line.
This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated
from the switches by ;, and each switch starts with an @
instead of the -, without spaces between multiple switches.
This is supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib,
relative to some lib subdirectory. If OS libraries are
present in the lib subdirectory and no multilibs are used,
this is usually just ., if OS libraries are present in
libsuffix sibling directories this prints e.g. ../lib64,
../lib or ../lib32, or if OS libraries are present in
lib/subdir subdirectories it prints e.g. amd64, sparcv9 or
ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch,
relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as
cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but
you do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a
list of program and library directories gcc searches---and
don't do anything else.
This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To
resolve this you either need to put cpp0 and the other
compiler components where gcc expects to find them, or you
can set the environment variable GCC_EXEC_PREFIX to the
directory where you installed them. Don't forget the
trailing /.
-print-sysroot
Print the target sysroot directory that is used during
compilation. This is the target sysroot specified either at
configure time or using the --sysroot option, possibly with
an extra suffix that depends on compilation options. If no
target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching
for headers, or give an error if the compiler is not
configured with such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or
7)---and don't do anything else. This is the compiler
version used in filesystem paths and specs. Depending on how
the compiler has been configured it can be just a single
number (major version), two numbers separated by a dot (major
and minor version) or three numbers separated by dots (major,
minor and patchlevel version).
-dumpfullversion
Print the full compiler version---and don't do anything else.
The output is always three numbers separated by dots, major,
minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything
else. (This is used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own
options---for example, to allow you to compile for a particular
processor variant or ABI, or to control optimizations specific to
that machine. By convention, the names of machine-specific
options start with -m.
Some configurations of the compiler also support additional
target-specific options, usually for compatibility with other
compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible
values are ilp32 for SysV-like data model where int, long int
and pointers are 32 bits, and lp64 for SysV-like data model
where int is 32 bits, but long int and pointers are 64 bits.
The default depends on the specific target configuration.
Note that the LP64 and ILP32 ABIs are not link-compatible;
you must compile your entire program with the same ABI, and
link with a compatible set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is
configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers.
This will prevent the compiler from using floating-point and
Advanced SIMD registers but will not impose any restrictions
on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is
configured for an aarch64-*-* but not an aarch64_be-*-*
target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its
statically defined symbols must be within 1MB of each other.
Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its
statically defined symbols must be within 4GB of each other.
Programs can be statically or dynamically linked. This is
the default code model.
-mcmodel=large
Generate code for the large code model. This makes no
assumptions about addresses and sizes of sections. Programs
can be statically linked only.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be
aligned on a natural object boundary as described in the
architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former
behavior is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard.
Supported locations are global for a global canary or sysreg
for a canary in an appropriate system register.
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify
which system register to use as base register for reading the
canary, and from what offset from that base register. There
is no default register or offset as this is entirely for use
within the Linux kernel.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard.
Supported locations are global for a global canary or sysreg
for a canary in an appropriate system register.
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify
which system register to use as base register for reading the
canary, and from what offset from that base register. There
is no default register or offset as this is entirely for use
within the Linux kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for
dynamic accesses of TLS variables. This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for
dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are
12, 24, 32, 48. This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53
erratum number 835769. This involves inserting a NOP
instruction between memory instructions and 64-bit integer
multiply-accumulate instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53
erratum number 843419. This erratum workaround is made at
link time and this will only pass the corresponding flag to
the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation.
This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this
reduces precision of reciprocal square root results to about
16 bits for single precision and to 32 bits for double
precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option
only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this
reduces precision of square root results to about 16 bits for
single precision and to 32 bits for double precision. If
enabled, it implies -mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option
only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this
reduces precision of division results to about 16 bits for
single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track
speculative execution through conditional branches. The
tracking state can then be used by the compiler when
expanding calls to "__builtin_speculation_safe_copy" to
permit a more efficient code sequence to be generated.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement
atomic operations. These helpers will, at runtime, determine
if the LSE instructions from ARMv8.1-A can be used; if not,
they will use the load/store-exclusive instructions that are
present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base
ARMv8.0 instruction set. If using a later revision, e.g.
-march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics
instructions will be used directly. The same applies when
using -mcpu= when the selected cpu supports the lse feature.
-march=name
Specify the name of the target architecture and, optionally,
one or more feature modifiers. This option has the form
-march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a,
armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a or native.
The value armv8.5-a implies armv8.4-a and enables compiler
support for the ARMv8.5-A architecture extensions.
The value armv8.4-a implies armv8.3-a and enables compiler
support for the ARMv8.4-A architecture extensions.
The value armv8.3-a implies armv8.2-a and enables compiler
support for the ARMv8.3-A architecture extensions.
The value armv8.2-a implies armv8.1-a and enables compiler
support for the ARMv8.2-A architecture extensions.
The value armv8.1-a implies armv8-a and enables compiler
support for the ARMv8.1-A architecture extension. In
particular, it enables the +crc, +lse, and +rdma features.
The value native is available on native AArch64 GNU/Linux and
causes the compiler to pick the architecture of the host
system. This option has no effect if the compiler is unable
to recognize the architecture of the host system,
The permissible values for feature are listed in the sub-
section on aarch64-feature-modifiers,,-march and -mcpu
Feature Modifiers. Where conflicting feature modifiers are
specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can
emit when generating assembly code. If -march is specified
without either of -mtune or -mcpu also being specified, the
code is tuned to perform well across a range of target
processors implementing the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should
tune the performance of the code. Permissible values for
this option are: generic, cortex-a35, cortex-a53, cortex-a55,
cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76,
ares, exynos-m1, emag, falkor, neoverse-e1, neoverse-n1,
neoverse-n2, neoverse-v1, neoverse-512tvb, qdf24xx, saphira,
phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83,
a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81,
tsv110, thunderxt83, thunderx2t99, zeus,
cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55 native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC
should tune for a big.LITTLE system.
The value neoverse-512tvb specifies that GCC should tune for
Neoverse cores that (a) implement SVE and (b) have a total
vector bandwidth of 512 bits per cycle. In other words, the
option tells GCC to tune for Neoverse cores that can execute
4 128-bit Advanced SIMD arithmetic instructions a cycle and
that can execute an equivalent number of SVE arithmetic
instructions per cycle (2 for 256-bit SVE, 4 for 128-bit
SVE). This is more general than tuning for a specific core
like Neoverse V1 but is more specific than the default tuning
described below.
Additionally on native AArch64 GNU/Linux systems the value
native tunes performance to the host system. This option has
no effect if the compiler is unable to recognize the
processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the
code is tuned to perform well across a range of target
processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed
by one or more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the permissible values for
cpu are the same as those available for -mtune. The
permissible values for feature are documented in the sub-
section on aarch64-feature-modifiers,,-march and -mcpu
Feature Modifiers. Where conflicting feature modifiers are
specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can
emit when generating assembly code (as if by -march) and to
determine the target processor for which to tune for
performance (as if by -mtune). Where this option is used in
conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
-mcpu=neoverse-512tvb is special in that it does not refer to
a specific core, but instead refers to all Neoverse cores
that (a) implement SVE and (b) have a total vector bandwidth
of 512 bits a cycle. Unless overridden by -march,
-mcpu=neoverse-512tvb generates code that can run on a
Neoverse V1 core, since Neoverse V1 is the first Neoverse
core with these properties. Unless overridden by -mtune,
-mcpu=neoverse-512tvb tunes code in the same way as for
-mtune=neoverse-512tvb.
-moverride=string
Override tuning decisions made by the back-end in response to
a -mtune= switch. The syntax, semantics, and accepted values
for string in this option are not guaranteed to be consistent
across releases.
This option is only intended to be useful when developing
GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files.
This option is provided for use in debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this
option literal pools are accessed using a single instruction
and emitted after each function. This limits the maximum
size of functions to 1MB. This is enabled by default for
-mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing
will be applied. Permissible values are none, which disables
return address signing, non-leaf, which enables pointer
signing for functions which are not leaf functions, and all,
which enables pointer signing for all functions. The default
value is none. This option has been deprecated by
-mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf]|bti
Select the branch protection features to use. none is the
default and turns off all types of branch protection.
standard turns on all types of branch protection features.
If a feature has additional tuning options, then standard
sets it to its standard level. pac-ret[+leaf] turns on
return address signing to its standard level: signing
functions that save the return address to memory (non-leaf
functions will practically always do this) using the a-key.
The optional argument leaf can be used to extend the signing
to include leaf functions. bti turns on branch target
identification mechanism.
-mharden-sls=opts
Enable compiler hardening against straight line speculation
(SLS). opts is a comma-separated list of the following
options:
retbr
blr
In addition, -mharden-sls=all enables all SLS hardening while
-mharden-sls=none disables all SLS hardening.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This
option only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length
agnostic" output that works with any size of vector register
and "vector-length specific" output that allows GCC to make
assumptions about the vector length when it is useful for
optimization reasons. The possible values of bits are:
scalable, 128, 256, 512, 1024 and 2048. Specifying scalable
selects vector-length agnostic output. At present
-msve-vector-bits=128 also generates vector-length agnostic
output. All other values generate vector-length specific
code. The behavior of these values may change in future
releases and no value except scalable should be relied on for
producing code that is portable across different hardware SVE
vector lengths.
The default is -msve-vector-bits=scalable, which produces
vector-length agnostic code.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the
following and their inverses nofeature:
crc Enable CRC extension. This is on by default for
-march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and
floating-point instructions.
fp Enable floating-point instructions. This is on by default
for all possible values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables
floating-point instructions. This is on by default for all
possible values for options -march and -mcpu.
sve Enable Scalable Vector Extension instructions. This also
enables Advanced SIMD and floating-point instructions.
lse Enable Large System Extension instructions. This is on by
default for -march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This
is on by default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point
instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16
extensions and floating-point instructions. This option is
enabled by default for -march=armv8.4-a. Use of this option
with architectures prior to Armv8.2-A is not supported.
rcpc
Enable the RcPc extension. This does not change code
generation from GCC, but is passed on to the assembler,
enabling inline asm statements to use instructions from the
RcPc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced
SIMD instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also
enables Advanced SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables
Advanced SIMD instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also
enables Advanced SIMD instructions. Use of this option with
architectures prior to Armv8.2-A is not supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables
Advanced SIMD instructions. Use of this option with
architectures prior to Armv8.2-A is not supported.
profile
Enable the Statistical Profiling extension. This option is
only to enable the extension at the assembler level and does
not affect code generation.
rng Enable the Armv8.5-a Random Number instructions. This option
is only to enable the extension at the assembler level and
does not affect code generation.
memtag
Enable the Armv8.5-a Memory Tagging Extensions. This option
is only to enable the extension at the assembler level and
does not affect code generation.
sb Enable the Armv8-a Speculation Barrier instruction. This
option is only to enable the extension at the assembler level
and does not affect code generation. This option is enabled
by default for -march=armv8.5-a.
ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction.
This option is only to enable the extension at the assembler
level and does not affect code generation. This option is
enabled by default for -march=armv8.5-a.
predres
Enable the Armv8-a Execution and Data Prediction Restriction
instructions. This option is only to enable the extension at
the assembler level and does not affect code generation.
This option is enabled by default for -march=armv8.5-a.
Feature crypto implies aes, sha2, and simd, which implies fp.
Conversely, nofp implies nosimd, which implies nocrypto, noaes
and nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That
allows code to run on hardware variants that lack these
registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short
instruction generation. This can result in increased
instruction count, so this may either reduce or increase
overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple"
instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an
"fsub" instruction and test the flags. This is faster than a
software comparison, but can get incorrect results in the
presence of NaNs, or when two different small numbers are
compared such that their difference is calculated as zero.
The default is -msoft-cmpsf, which uses slower, but IEEE-
compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack
pointer. E.g., a value of 8 means that the eight bytes in
the range "sp+0...sp+7" can be used by leaf functions without
stack allocation. Values other than 8 or 16 are untested and
unlikely to work. Note also that this option changes the
ABI; compiling a program with a different stack offset than
the libraries have been compiled with generally does not
work. This option can be useful if you want to evaluate if a
different stack offset would give you better code, but to
actually use a different stack offset to build working
programs, it is recommended to configure the toolchain with
the appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set
to truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls
might be beyond the offset range of the "b" / "bl"
instructions, and therefore load the function address into a
register before performing a (otherwise direct) call. This
is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct
calls are in the range of the "b" / "bl" instructions, so use
these instructions for direct calls. The default is
-mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values.
This does not apply to function addresses for which
-mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and
expected at function call and return time. Making this mode
match the mode you predominantly need at function start can
make your programs smaller and faster by avoiding unnecessary
mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or
restored when the function returns, and when it calls
other functions. This mode is useful for compiling
libraries or other compilation units you might want to
incorporate into different programs with different
prevailing FPU modes, and the convenience of being able
to use a single object file outweighs the size and speed
overhead for any extra mode switching that might be
needed, compared with what would be needed with a more
specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations
with truncating (i.e. round towards zero) rounding mode.
That includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations
with round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in
the FPU, e.g. integer multiply, or integer multiply-and-
accumulate.
The default is -mfp-mode=caller
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting
of 32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are
msplit-lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be
4 or 8. The default is 8. Note that this is an ABI change,
even though many library function interfaces are unaffected
if they don't use SIMD vector modes in places that affect
size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In
theory this can give better register allocation, but so far
the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes
loading small negative constants and certain bitmasks faster.
Allowable values for reg are r43 and r63, which specify use
of that register as a fixed register, and none, which means
that no register is used for this purpose. The default is
-m1reg-none.
AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for
gpu are
fiji
Compile for GCN3 Fiji devices (gfx803).
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
-mstack-size=bytes
Specify how many bytes of stack space will be requested for
each GPU thread (wave-front). Beware that there may be many
threads and limited memory available. The size of the stack
allocation may also have an impact on run-time performance.
The default is 32KB when using OpenACC or OpenMP, and 1MB
otherwise.
ARC Options
The following options control the architecture variant for which
code is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is
the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
-mjli-always
Force to call a function using jli_s instruction. This
option is valid only for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction
scheduling parameters for cpu. There are also shortcut alias
options available for backward compatibility and convenience.
Supported values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is
the default when configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision
floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision
floating-point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except
the atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit
multiply instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family
instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit
multiply instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family
instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring
reduced register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the
compact implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the
fast implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension
aux registers.
-mea
Generate extended arithmetic instructions. Currently only
"divaw", "adds", "subs", and "sat16" are supported. This is
always enabled for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This
option is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate
instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for
-mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if
-mcpu=ARC700 is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the
compact implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the
fast implementation.
-msimd
Enable generation of ARC SIMD instructions via target-
specific builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility
purposes only. Software floating-point code is emitted by
default, and this default can overridden by FPX options;
-mspfp, -mspfp-compact, or -mspfp-fast for single precision,
and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double
precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional
extension to implement atomic memory built-in functions. Not
available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is
on by default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can
specify the option using either a string or numeric value for
multo. wlh1 is the default value. The recognized values
are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following
instructions are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The
following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The
following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The
following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The
following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu",
"mpym", "mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware
extensions for ARCv2 cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point
hardware extensions.
fpud
Enables support for double-precision floating-point
hardware extensions. The single-precision floating-point
extension is also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point
extension is also enabled. This option is only available
for ARC EM.
fpuda_div
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point,
square-root, and divide extensions are also enabled.
This option is only available for ARC EM.
fpuda_fma
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. The single-precision floating-point and
fused multiply and add hardware extensions are also
enabled. This option is only available for ARC EM.
fpuda_all
Enables support for double-precision floating-point
hardware extensions using double-precision assist
instructions. All single-precision floating-point
hardware extensions are also enabled. This option is
only available for ARC EM.
fpus_div
Enables support for single-precision floating-point,
square-root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point,
square-root and divide hardware extensions. This option
includes option fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and
fused multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and
fused multiply and add hardware extensions. This option
includes option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point
hardware extensions.
fpud_all
Enables support for all single- and double-precision
floating-point hardware extensions. Not available for
ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor
automatically saves/restores on interrupt entry and exit.
register-range is specified as two registers separated by a
dash. The register range always starts with "r0", the upper
limit is "fp" register. blink and lp_count are optional.
This option is only valid for ARC EM and ARC HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second
register bank on entry to fast interrupt. Fast interrupts
are interrupts with the highest priority level P0. These
interrupts save only PC and STATUS32 registers to avoid
memory transactions during interrupt entry and exit
sequences. Use this option when you are using fast
interrupts in an ARC V2 family processor. Permitted values
are 4, 8, 16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values
for width are 8, 16, 20, 24, 28 and 32 bits. The default
width is fixed to 32 bits. If the width is less than 32, the
compiler does not attempt to transform loops in your program
to use the zero-delay loop mechanism unless it is known that
the "lp_count" register can hold the required loop-counter
value. Depending on the width specified, the compiler and
run-time library might continue to use the loop mechanism for
various needs. This option defines macro "__ARC_LPC_WIDTH__"
with the value of width.
-mrf16
This option instructs the compiler to generate code for a
16-entry register file. This option defines the
"__ARC_RF16__" preprocessor macro.
-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump
tables.
The following options are passed through to the assembler, and
also define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A
extensions. Also sets the preprocessor symbol
"__Xdsp_packa". This option is deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi
butterfly extension. Also sets the preprocessor symbol
"__Xdvbf". This option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store
conditional extension. Also sets the preprocessor symbol
"__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor
symbol "__Xxmac_d16". This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor
symbol "__Xxmac_24". This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp
counter extension instruction. Also sets the preprocessor
symbol "__Xrtsc". This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering
extension instruction. Also sets the preprocessor symbol
"__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-
operand instructions for telephony. Also sets the
preprocessor symbol "__Xtelephony". This option is
deprecated.
-mxy
Passed down to the assembler to enable the XY memory
extension. Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to
make an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the
"arclinux" emulation. This option is enabled by default in
tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the
"arclinux_prof" emulation. This option is enabled by default
in tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing
access to the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which
is the offset available for an unconditional branch-and-link
instruction. Conditional execution of function calls is
suppressed, to allow use of the 25-bit range, rather than the
21-bit range with conditional branch-and-link. This is the
default for tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
-G num
Put definitions of externally-visible data in a small data
section if that data is no bigger than num bytes. The
default value of num is 4 for any ARC configuration, or 8
when we have double load/store operations.
-mno-sdata
Do not generate sdata references. This is the default for
tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile
references. This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg to
generate compare-and-branch ("brcc") instructions. It has no
effect on generation of these instructions driven by the
combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table
shortening. This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for
-Os, and only available for ARCv1 cores. This option is
deprecated.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional
execution instructions.
Due to delay slot scheduling and interactions between operand
numbers, literal sizes, instruction lengths, and the support
for conditional execution, the target-independent pass to
generate conditional execution is often lacking, so the ARC
port has kept a special pass around that tries to find more
conditional execution generation opportunities after register
allocation, branch shortening, and delay slot scheduling have
been done. This pass generally, but not always, improves
performance and code size, at the cost of extra compilation
time, which is why there is an option to switch it off. If
you have a problem with call instructions exceeding their
allowable offset range because they are conditionalized, you
should consider using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into
"add.f", "adc" etc. This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic
because some optimizers then assume that indexed stores
exist, which is not the case.
-mlra
Enable Local Register Allocation. This is still experimental
for ARC, so by default the compiler uses standard reload
(i.e. -mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using -Os), prologues and epilogues
that have to save or restore a large number of registers are
often shortened by using call to a special function in
libgcc; this is referred to as a millicode call. As these
calls can pose performance issues, and/or cause linking
issues when linking in a nonstandard way, this option is
provided to turn on or off millicode call generation.
-mcode-density-frame
This option enable the compiler to emit "enter" and "leave"
instructions. These instructions are only valid for CPUs
with code-density feature.
-mmixed-code
Tweak register allocation to help 16-bit instruction
generation. This generally has the effect of decreasing the
average instruction size while increasing the instruction
count.
-mq-class
Enable q instruction alternatives. This is the default for
-Os.
-mRcq
Enable Rcq constraint handling. Most short code generation
depends on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly
depends on this. This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction
lengths and alignment. The recognized values for level are:
0 No size optimization. This level is deprecated and
treated like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after
barriers are dropped.
3 In addition, optional data alignment is dropped, and the
option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the
behavior when this is not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any
implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal
to a normal instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When
tuning for ARC700 and optimizing for speed, branches without
filled delay slot are preferably emitted unaligned and long,
unless profiling indicates that the probability for the
branch to be taken is below probability. The default is
(REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility,
but are now deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is
now deprecated. Big-endian code is supported by configuring
GCC to build "arceb-elf32" and "arceb-linux-uclibc" targets,
for which big endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options
is now deprecated. Little-endian code is supported by
configuring GCC to build "arc-elf32" and "arc-linux-uclibc"
targets, for which little endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are
replaced by ARC600, ARC601, ARC700 and ARC700-xmac
respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are:
apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM
Procedure Call Standard for all functions, even if this is
not strictly necessary for correct execution of the code.
Specifying -fomit-frame-pointer with this option causes the
stack frames not to be generated for leaf functions. The
default is -mno-apcs-frame. This option is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5
architectures, the two instruction sets cannot be reliably
used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is generated
when -mthumb-interwork is specified. In AAPCS configurations
this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function
prologue, or the merging of those instruction with the
instructions in the function's body. This means that all
functions start with a recognizable set of instructions (or
in fact one of a choice from a small set of different
function prologues), and this information can be used to
locate the start of functions inside an executable piece of
code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible
values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing
library calls for floating-point operations. softfp allows
the generation of code using hardware floating-point
instructions, but still uses the soft-float calling
conventions. hard allows generation of floating-point
instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration.
Note that the hard-float and soft-float ABIs are not link-
compatible; you must compile your entire program with the
same ABI, and link with a compatible set of libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers.
This will prevent the compiler from using floating-point and
Advanced SIMD registers but will not impose any restrictions
on the assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode.
This is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the
default is to compile code for a little-endian processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32
formats. The option has no effect for little-endian images
and is ignored. The default is dependent on the selected
target architecture. For ARMv6 and later architectures the
default is BE8, for older architectures the default is BE32.
BE32 format has been deprecated by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC
uses this name to determine what kind of instructions it can
emit when generating assembly code. This option can be used
in conjunction with or instead of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6,
armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7,
armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a,
armv8.4-a, armv8.5-a, armv7-r, armv8-r, armv6-m, armv6s-m,
armv7-m, armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and
iwmmxt2.
Additionally, the following architectures, which lack support
for the Thumb execution state, are recognized but support is
deprecated: armv4.
Many of the architectures support extensions. These can be
added by appending +extension to the architecture name.
Extension options are processed in order and capabilities
accumulate. An extension will also enable any necessary base
extensions upon which it depends. For example, the +crypto
extension will always enable the +simd extension. The
exception to the additive construction is for extensions that
are prefixed with +no...: these extensions disable the
specified option and any other extensions that may depend on
the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to
writing -march=armv7-a+vfpv4 since the +simd option is
entirely disabled by the +nofp option that follows it.
Most extension names are generically named, but have an
effect that is dependent upon the architecture to which it is
applied. For example, the +simd option can be applied to
both armv7-a and armv8-a architectures, but will enable the
original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a
and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each
architecture. Architectures not mentioned do not support any
extensions.
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension
+vfpv2 can be used as an alias for this extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M
architectures.
+fp The VFPv3 floating-point instructions, with 16
double-precision registers. The extension +vfpv3-d16
can be used as an alias for this extension. Note
that floating-point is not supported by the base
ARMv7-M architecture, but is compatible with both the
ARMv7-A and ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16
double-precision registers. The extension +vfpv3-d16
can be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-
point instructions. The extensions +neon and
+neon-vfpv3 can be used as aliases for this
extension.
+vfpv3
The VFPv3 floating-point instructions, with 32
double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16
double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32
double-precision registers and the half-precision
floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16
double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32
double-precision registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-
point instructions, with the half-precision floating-
point conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-
point instructions.
+nosimd
Disable the Advanced SIMD instructions (does not
disable floating point).
+nofp
Disable the floating-point and Advanced SIMD
instructions.
armv7ve
The extended version of the ARMv7-A architecture with
support for virtualization.
+fp The VFPv4 floating-point instructions, with 16
double-precision registers. The extension +vfpv4-d16
can be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-
point instructions. The extension +neon-vfpv4 can be
used as an alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16
double-precision registers.
+vfpv3
The VFPv3 floating-point instructions, with 32
double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16
double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32
double-precision registers and the half-precision
floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16
double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32
double-precision registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-
point instructions. The extension +neon-vfpv3 can be
used as an alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-
point instructions, with the half-precision floating-
point conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not
disable floating point).
+nofp
Disable the floating-point and Advanced SIMD
instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction
Instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions. This also enables
the Advanced SIMD and floating-point instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction
Instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD
and floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension.
This also enables the half-precision floating-point
extension and Advanced SIMD and floating-point
instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions. This also enables
the Advanced SIMD and floating-point instructions.
+dotprod
Enable the Dot Product extension. This also enables
Advanced SIMD instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction
Instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD
and floating-point instructions as well as the Dot
Product extension and the half-precision floating-
point fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point
instructions as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables
the Advanced SIMD and floating-point instructions as
well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction
Instructions.
armv8.5-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD
and floating-point instructions as well as the Dot
Product extension and the half-precision floating-
point fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point
instructions as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables
the Advanced SIMD and floating-point instructions as
well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point
instructions. The extension +vfpv3xd can be used as
an alias for this extension.
+fp The VFPv3 floating-point instructions with 16 double-
precision registers. The extension +vfpv3-d16 can be
used as an alias for this extension.
+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point
instructions with 16 double-precision registers and
the half-precision floating-point conversion
operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-
precision registers and the half-precision floating-
point conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point
instructions.
+fpv5
The single-precision FPv5 floating-point
instructions.
+fp.dp
The single- and double-precision FPv5 floating-point
instructions.
+nofp
Disable the floating-point extensions.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point
instructions.
+nofp
Disable the floating-point extension.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point
instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and
cryptographic instructions.
-march=native causes the compiler to auto-detect the
architecture of the build computer. At present, this feature
is only supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option
has no effect.
-mtune=name
This option specifies the name of the target ARM processor
for which GCC should tune the performance of the code. For
some ARM implementations better performance can be obtained
by using this option. Permissible names are: arm7tdmi,
arm7tdmi-s, arm710t, arm720t, arm740t, strongarm,
strongarm110, strongarm1100, 0strongarm1110, arm8, arm810,
arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s,
arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi,
arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a,
cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12,
cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75,
cortex-a76, ares, cortex-r4, cortex-r4f, cortex-r5,
cortex-r7, cortex-r8, cortex-r52, cortex-m0, cortex-m0plus,
cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23,
cortex-m33, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2,
neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626,
fa606te, fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune
the performance of the code for a big.LITTLE system.
Permissible names are: cortex-a15.cortex-a7,
cortex-a17.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a72.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55,
cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the
performance for a blend of processors within architecture
arch. The aim is to generate code that run well on the
current most popular processors, balancing between
optimizations that benefit some CPUs in the range, and
avoiding performance pitfalls of other CPUs. The effects of
this option may change in future GCC versions as CPU models
come and go.
-mtune permits the same extension options as -mcpu, but the
extension options do not affect the tuning of the generated
code.
-mtune=native causes the compiler to auto-detect the CPU of
the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option
has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC
uses this name to derive the name of the target ARM
architecture (as if specified by -march) and the ARM
processor type for which to tune for performance (as if
specified by -mtune). Where this option is used in
conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural
extensions. Where this is so the architectural extensions
are normally enabled by default. If implementations that
lack the extension exist, then the extension syntax can be
used to disable those extensions that have been omitted. For
floating-point and Advanced SIMD (Neon) instructions, the
settings of the options -mfloat-abi and -mfpu must also be
considered: floating-point and Advanced SIMD instructions
will only be used if -mfloat-abi is not set to soft; and any
setting of -mfpu other than auto will override the available
floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major
configurations: integer only, with just a floating-point unit
or with floating-point and Advanced SIMD. The default is to
enable all the instructions, but the extensions +nosimd and
+nofp can be used to disable just the SIMD or both the SIMD
and floating-point instructions respectively.
Permissible names for this option are the same as those for
-mtune.
The following extension options are common to the listed
CPUs:
+nodsp
Disable the DSP instructions on cortex-m33.
+nofp
Disables the floating-point instructions on arm9e,
arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e,
arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7,
cortex-r8, cortex-m4, cortex-m7 and cortex-m33. Disables
the floating-point and SIMD instructions on
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8,
cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32,
cortex-a35, cortex-a53 and cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-
point instructions on cortex-r5, cortex-r7, cortex-r8,
cortex-r52 and cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions
on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32,
cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1,
cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 and
cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to
VFPv3 with 16 double-precision registers. It supports the
following extension options: mp, sec, vfpv3-d16, vfpv3,
vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon,
neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same
as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch -mtune=generic-arch. See -mtune for more
information.
-mcpu=native causes the compiler to auto-detect the CPU of
the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option
has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware
emulation) is available on the target. Permissible names
are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16,
vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16,
vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16,
fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for
neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes
the compiler to select the floating-point and Advanced SIMD
instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON
extension (e.g. -mfpu=neon), note that floating-point
operations are not generated by GCC's auto-vectorization pass
unless -funsafe-math-optimizations is also specified. This
is because NEON hardware does not fully implement the IEEE
754 standard for floating-point arithmetic (in particular
denormal values are treated as zero), so the use of NEON
instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the
"target("fpu=")" function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-
point type. Permissible names are none, ieee, and
alternative; the default is none, in which case the "__fp16"
type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a
multiple of the number of bits set by this option.
Permissible values are 8, 32 and 64. The default value
varies for different toolchains. For the COFF targeted
toolchain the default value is 8. A value of 64 is only
allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient
code, but can also increase the size of the program.
Different values are potentially incompatible. Code compiled
with one value cannot necessarily expect to work with code or
libraries compiled with another value, if they exchange
information using structures or unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a
"noreturn" function. It is executed if the function tries to
return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading
the address of the function into a register and then
performing a subroutine call on this register. This switch
is needed if the target function lies outside of the
64-megabyte addressing range of the offset-based version of
subroutine call instruction.
Even if this switch is enabled, not all function calls are
turned into long calls. The heuristic is that static
functions, functions that have the "short_call" attribute,
functions that are inside the scope of a "#pragma
no_long_calls" directive, and functions whose definitions
have already been compiled within the current compilation
unit are not turned into long calls. The exceptions to this
rule are that weak function definitions, functions with the
"long_call" attribute or the "section" attribute, and
functions that are within the scope of a "#pragma long_calls"
directive are always turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior, as does
placing the function calls within the scope of a "#pragma
long_calls_off" directive. Note these switches have no
effect on how the compiler generates code to handle function
calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each function.
The runtime system is responsible for initializing this
register with an appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For
standard PIC base case, the default is any suitable register
determined by compiler. For single PIC base case, the
default is R9 if target is EABI based or stack-checking is
enabled, otherwise the default is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data
segments is fixed at static link time. This permits using
PC-relative addressing operations to access data known to be
in the data segment. For non-VxWorks RTP targets, this
option is enabled by default. When disabled on such targets,
it will enable -msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section,
directly preceding the function prologue. The generated code
is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value
of "pc" stored at "fp + 0". If the trace function then looks
at location "pc - 12" and the top 8 bits are set, then we
know that there is a function name embedded immediately
preceding this location and has length "((pc[-3]) &
0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb
states. The default for most configurations is to generate
code that executes in ARM state, but the default can be
changed by configuring GCC with the --with-mode=state
configure option.
You can also override the ARM and Thumb mode for each
function by using the "target("thumb")" and "target("arm")"
function attributes or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option
is provided for regression testing of mixed Thumb/ARM code
generation, and is not intended for ordinary use in compiling
code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all non-leaf functions. (A leaf
function is one that does not call any other functions.) The
default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb
Procedure Call Standard for all leaf functions. (A leaf
function is one that does not call any other functions.) The
default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being
compiled an ARM instruction set header which switches to
Thumb mode before executing the rest of the function. This
allows these functions to be called from non-interworking
code. This option is not valid in AAPCS configurations
because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual
functions) to execute correctly regardless of whether the
target code has been compiled for interworking or not. There
is a small overhead in the cost of executing a function
pointer if this option is enabled. This option is not valid
in AAPCS configurations because interworking is enabled by
default.
-mtp=name
Specify the access model for the thread local storage
pointer. The valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which fetches the thread pointer
from "cp15" directly (supported in the arm6k architecture),
and auto, which uses the best available method for the
selected processor. The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local
storage. Two dialects are supported---gnu and gnu2. The gnu
dialect selects the original GNU scheme for supporting local
and global dynamic TLS models. The gnu2 dialect selects the
GNU descriptor scheme, which provides better performance for
shared libraries. The GNU descriptor scheme is compatible
with the original scheme, but does require new assembler,
linker and library support. Initial and local exec TLS
models are unaffected by this option and always use the
original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets
(uClinux, SymbianOS) where the runtime loader imposes this
restriction, and when -fpic or -fPIC is specified. This
option conflicts with -mslow-flash-data.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd"
instructions with overlapping destination and base registers
are used. This option avoids generating these instructions.
This option is enabled by default when -mcpu=cortex-m3 is
specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit
values from addresses that are not 16- or 32- bit aligned.
By default unaligned access is disabled for all pre-ARMv6,
all ARMv6-M and for ARMv8-M Baseline architectures, and
enabled for all other architectures. If unaligned access is
not enabled then words in packed data structures are accessed
a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the
generated object file to either true or false, depending upon
the setting of this option. If unaligned access is enabled
then the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is
also defined.
-mneon-for-64bits
Enables using Neon to handle scalar 64-bits operations. This
is disabled by default since the cost of moving data from
core registers to Neon is high.
-mslow-flash-data
Assume loading data from flash is slower than fetching
instruction. Therefore literal load is minimized for better
performance. This option is only supported when compiling
for ARMv7 M-profile and off by default. It conflicts with
-mword-relocations.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The
default is currently off which implies divided syntax. This
option has no impact on Thumb2. However, this may change in
future releases of GCC. Divided syntax should be considered
deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of
ARMv8-A. IT blocks can only contain a single 16-bit
instruction from a select set of instructions. This option is
on by default for ARMv8-A Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file.
This is an option used only for regression testing of the
compiler and not intended for ordinary use in compiling code.
This option is disabled by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files.
This option is provided for use in debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections.
Additionally, when compiling for ELF object format give all
text sections the ELF processor-specific section attribute
"SHF_ARM_PURECODE". This option is only available when
generating non-pic code for M-profile targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions:
Requirements on Development Tools Engineering Specification",
which can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/ >.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU
type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8 KiB of program memory.
mcu = "attiny22", "attiny26", "at90s2313", "at90s2323",
"at90s2333", "at90s2343", "at90s4414", "at90s4433",
"at90s4434", "at90c8534", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and
with the "MOVW" instruction. mcu = "attiny13",
"attiny13a", "attiny24", "attiny24a", "attiny25",
"attiny261", "attiny261a", "attiny2313", "attiny2313a",
"attiny43u", "attiny44", "attiny44a", "attiny45",
"attiny48", "attiny441", "attiny461", "attiny461a",
"attiny4313", "attiny84", "attiny84a", "attiny85",
"attiny87", "attiny88", "attiny828", "attiny841",
"attiny861", "attiny861a", "ata5272", "ata6616c",
"at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program
memory. mcu = "at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu =
"atmega103", "at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program
memory and with the "MOVW" instruction. mcu =
"attiny167", "attiny1634", "atmega8u2", "atmega16u2",
"atmega32u2", "ata5505", "ata6617c", "ata664251",
"at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory.
mcu = "atmega48", "atmega48a", "atmega48p", "atmega48pa",
"atmega48pb", "atmega8", "atmega8a", "atmega8hva",
"atmega88", "atmega88a", "atmega88p", "atmega88pa",
"atmega88pb", "atmega8515", "atmega8535", "ata6285",
"ata6286", "ata6289", "ata6612c", "at90pwm1", "at90pwm2",
"at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program
memory. mcu = "atmega16", "atmega16a", "atmega16hva",
"atmega16hva2", "atmega16hvb", "atmega16hvbrevb",
"atmega16m1", "atmega16u4", "atmega161", "atmega162",
"atmega163", "atmega164a", "atmega164p", "atmega164pa",
"atmega165", "atmega165a", "atmega165p", "atmega165pa",
"atmega168", "atmega168a", "atmega168p", "atmega168pa",
"atmega168pb", "atmega169", "atmega169a", "atmega169p",
"atmega169pa", "atmega32", "atmega32a", "atmega32c1",
"atmega32hvb", "atmega32hvbrevb", "atmega32m1",
"atmega32u4", "atmega32u6", "atmega323", "atmega324a",
"atmega324p", "atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega325pa", "atmega328", "atmega328p",
"atmega328pb", "atmega329", "atmega329a", "atmega329p",
"atmega329pa", "atmega3250", "atmega3250a",
"atmega3250p", "atmega3250pa", "atmega3290",
"atmega3290a", "atmega3290p", "atmega3290pa",
"atmega406", "atmega64", "atmega64a", "atmega64c1",
"atmega64hve", "atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644", "atmega644a",
"atmega644p", "atmega644pa", "atmega644rfr2",
"atmega645", "atmega645a", "atmega645p", "atmega649",
"atmega649a", "atmega649p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega6490", "atmega6490a",
"atmega6490p", "ata5795", "ata5790", "ata5790n",
"ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831",
"ata8210", "ata8510", "ata5702m322", "at90pwm161",
"at90pwm216", "at90pwm316", "at90can32", "at90can64",
"at90scr100", "at90usb646", "at90usb647", "at94k",
"m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu =
"atmega128", "atmega128a", "atmega128rfa1",
"atmega128rfr2", "atmega1280", "atmega1281",
"atmega1284", "atmega1284p", "atmega1284rfr2",
"at90can128", "at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than
128 KiB of program memory. mcu = "atmega256rfr2",
"atmega2560", "atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of
program memory. mcu = "atxmega8e5", "atxmega16a4",
"atxmega16a4u", "atxmega16c4", "atxmega16d4",
"atxmega16e5", "atxmega32a4", "atxmega32a4u",
"atxmega32c3", "atxmega32c4", "atxmega32d3",
"atxmega32d4", "atxmega32e5".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program
memory and RAM, and with program memory visible in the
RAM address space. mcu = "attiny202", "attiny204",
"attiny212", "attiny214", "attiny402", "attiny404",
"attiny406", "attiny412", "attiny414", "attiny416",
"attiny417", "attiny804", "attiny806", "attiny807",
"attiny814", "attiny816", "attiny817", "attiny1604",
"attiny1606", "attiny1607", "attiny1614", "attiny1616",
"attiny1617", "attiny3214", "attiny3216", "attiny3217",
"atmega808", "atmega809", "atmega1608", "atmega1609",
"atmega3208", "atmega3209", "atmega4808", "atmega4809".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB
of program memory. mcu = "atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1", "atxmega64b3",
"atxmega64c3", "atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB
of program memory and more than 64 KiB of RAM. mcu =
"atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory.
mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1",
"atxmega128b3", "atxmega128c3", "atxmega128d3",
"atxmega128d4", "atxmega192a3", "atxmega192a3u",
"atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3", "atxmega384c3",
"atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory
and more than 64 KiB of RAM. mcu = "atxmega128a1",
"atxmega128a1u", "atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512 B up to 4 KiB of
program memory. mcu = "attiny4", "attiny5", "attiny9",
"attiny10", "attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and
supported for assembler only. mcu = "attiny11",
"attiny12", "attiny15", "attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS
/ STS instructions. This option has only an effect on
reduced Tiny devices like ATtiny40. See also the "absdata"
AVR Variable Attributes,variable attribute.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release
the needed stack space for outgoing function arguments once
in function prologue/epilogue. Without this option, outgoing
arguments are pushed before calling a function and popped
afterwards.
Popping the arguments after the function call can be
expensive on AVR so that accumulating the stack space might
lead to smaller executables because arguments need not be
removed from the stack after such a function call.
This option can lead to reduced code size for functions that
perform several calls to functions that get their arguments
on the stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to
cost. Reasonable values for cost are small, non-negative
integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to
appropriate subroutines. Code size is smaller.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr"
pseudo instruction supported by GNU Binutils. If this option
is on, the feature can still be disabled for individual ISRs
by means of the AVR Function Attributes,,"no_gccisr" function
attribute. This feature is activated per default if
optimization is on (but not with -Og, @pxref{Optimize
Options}), and if GNU Binutils support PR21683
("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of
all types: a "char" is 1 byte, an "int" is 1 byte, a "long"
is 2 bytes, and "long long" is 4 bytes. Please note that
this option does not conform to the C standards, but it
results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like
attaching attribute AVR Function Attributes,,"OS_task" to
"main". It is activated per default if optimization is on.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts.
Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter
"RCALL" resp. "RJMP" instruction if applicable. Setting
-mrelax just adds the --mlink-relax option to the assembler's
command line and the --relax option to the linker's command
line.
Jump relaxing is performed by the linker because jump offsets
are not known before code is located. Therefore, the
assembler code generated by the compiler is the same, but the
instructions in the executable may differ from instructions
in the assembler code.
Relaxing must be turned on if linker stubs are needed, see
the section on "EIND" and linker stubs below.
-mrmw
Assume that the device supports the Read-Modify-Write
instructions "XCH", "LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program
memory.
This option is used internally for multilib selection. It is
not an optimization option, and you don't need to set it by
hand.
-msp8
Treat the stack pointer register as an 8-bit register, i.e.
assume the high byte of the stack pointer is zero. In
general, you don't need to set this option by hand.
This option is used internally by the compiler to select and
build multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any
setting other than -mmcu=avr2 or -mmcu=avr25 the compiler
driver adds or removes this option from the compiler proper's
command line, because the compiler then knows if the device
or architecture has an 8-bit stack pointer and thus no "SPH"
register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware.
This means that "X" is only used in indirect, post-increment
or pre-decrement addressing.
Without this option, the "X" register may be used in the same
way as "Y" or "Z" which then is emulated by additional
instructions. For example, loading a value with "X+const"
addressing with a small non-negative "const < 64" to a
register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for
fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library
"lib<mcu>.a".
-nodevicespecs
Don't add -specs=device-specs/specs-<mcu> to the compiler
driver's command line. The user takes responsibility for
supplying the sub-processes like compiler proper, assembler
and linker with appropriate command line options.
-Waddr-space-convert
Warn about conversions between address spaces in the case
where the resulting address space is not contained in the
incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix.
Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of
a function or label is represented as word address so that
indirect jumps and calls can target any code address in the range
of 64 Ki words.
In order to facilitate indirect jump on devices with more than
128 Ki bytes of program memory space, there is a special function
register called "EIND" that serves as most significant part of
the target address when "EICALL" or "EIJMP" instructions are
used.
Indirect jumps and calls on these devices are handled as follows
by the compiler and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to
emulate an indirect call/jump by means of a "RET"
instruction.
* The compiler assumes that "EIND" never changes during the
startup code or during the application. In particular, "EIND"
is not saved/restored in function or interrupt service
routine prologue/epilogue.
* For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a
stub. The stub contains a direct jump to the desired
address.
* Linker relaxation must be turned on so that the linker
generates the stubs correctly in all situations. See the
compiler option -mrelax and the linker option --relax. There
are corner cases where the linker is supposed to generate
stubs but aborts without relaxation and without a helpful
error message.
* The default linker script is arranged for code with "EIND =
0". If code is supposed to work for a setup with "EIND !=
0", a custom linker script has to be used in order to place
the sections whose name start with ".trampolines" into the
segment where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice that
startup code is a blend of code from libgcc and AVR-LibC.
For the impact of AVR-LibC on "EIND", see the AVR-
LibC user manual ("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set up
"EIND" early, for example by means of initialization code
located in section ".init3". Such code runs prior to general
startup code that initializes RAM and calls constructors, but
after the bit of startup code from AVR-LibC that sets "EIND"
to the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker
script.
* Stubs are generated automatically by the linker if the
following two conditions are met:
-<The address of a label is taken by means of the "gs"
modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the
following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such
dispatch>
tables you can specify the -fno-jump-tables command-line
option.
-<C and C++ constructors/destructors called during
startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be
called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special
Function Registers
Some AVR devices support memories larger than the 64 KiB range
that can be accessed with 16-bit pointers. To access memory
locations outside this 64 KiB range, the content of a "RAMP"
register is used as high part of the address: The "X", "Y", "Z"
address register is concatenated with the "RAMPX", "RAMPY",
"RAMPZ" special function register, respectively, to get a wide
address. Similarly, "RAMPD" is used together with direct
addressing.
* The startup code initializes the "RAMP" special function
registers with zero.
* If a AVR Named Address Spaces,named address space other than
generic or "__flash" is used, then "RAMPZ" is set as needed
before the operation.
* If the device supports RAM larger than 64 KiB and the
compiler needs to change "RAMPZ" to accomplish an operation,
"RAMPZ" is reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it
with zero in case the ISR code might (implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR
targets. If you use inline assembler to read from locations
outside the 16-bit address range and change one of the "RAMP"
registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can
test for the presence or absence of features. Almost any of the
following built-in macros are deduced from device capabilities
and thus triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address
Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that
identifies the architecture and depends on the -mmcu=mcu
option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4",
"avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu
specifies a device, this built-in macro is set accordingly.
For example, with -mmcu=atmega8 the macro is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which
reflects the device's name. For example, -mmcu=atmega8
defines the built-in macro "__AVR_ATmega8__",
-mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__"
where Device is the device name as from the AVR user manual.
The difference between Device in the built-in macro and
device in -mmcu=device is that the latter is always
lowercase.
If device is not a device but only a core architecture like
avr51, this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the
device's name. For example, with -mmcu=atmega8 the macro is
defined to "atmega8".
If device is not a device but only a core architecture like
avr51, this macro is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of
devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit
register-register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is
the case for devices with more than 8 KiB of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This
is the case for devices with more than 128 KiB of program
memory. This also means that the program counter (PC) is 3
bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case
for devices with up to 128 KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit
respectively 16-bit register by the compiler. The definition
of these macros is affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special
function register or has an 8-bit stack pointer,
respectively. The definition of these macros is affected by
-mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by
-msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip
instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE".
The second macro is only defined if "__AVR_HAVE_JMP_CALL__"
is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS
and LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers
directly like "IN", "OUT", "SBI", etc. may use a different
address as if addressed by an instruction to access RAM like
"LD" or "STS". This offset depends on the device architecture
and has to be subtracted from the RAM address in order to get
the respective I/O address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of
"LD*" instructions. The flash memory is seen in the data
address space at an offset of "__AVR_PM_BASE_ADDRESS__". If
this macro is not defined, this feature is not available. If
defined, the address space is linear and there is no need to
put ".rodata" into RAM. This is handled by the default
linker description file, and is currently available for
"avrtiny" and "avrxmega3". Even more convenient, there is no
need to use address spaces like "__flash" or features like
attribute "progmem" and "pgm_read_*".
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc.
See the --with-avrlibc configure option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor.
Currently, cpu can be one of bf512, bf514, bf516, bf518,
bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532,
bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544,
bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m,
bf561, bf592.
The optional sirevision specifies the silicon revision of the
target Blackfin processor. Any workarounds available for the
targeted silicon revision are enabled. If sirevision is
none, no workarounds are enabled. If sirevision is any, all
workarounds for the targeted processor are enabled. The
"__SILICON_REVISION__" macro is defined to two hexadecimal
digits representing the major and minor numbers in the
silicon revision. If sirevision is none, the
"__SILICON_REVISION__" is not defined. If sirevision is any,
the "__SILICON_REVISION__" is defined to be 0xffff. If this
optional sirevision is not used, GCC assumes the latest known
silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For
the bfin-elf toolchain, this option causes the hardware BSP
provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by
default.
Note that support for bf561 is incomplete. For bf561, only
the preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator.
This causes the simulator BSP provided by libgloss to be
linked in. This option has effect only for bfin-elf
toolchain. Certain other options, such as
-mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up and
restore frame pointers and makes an extra register available
in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code
does not contain speculative loads after jump instructions.
If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is
defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code
does not contain CSYNC or SSYNC instructions too soon after
conditional branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC
instructions from occurring too soon after a conditional
branch.
-mlow64k
When enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of
memory.
-mno-low64k
Assume that the program is arbitrarily large. This is the
default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad
memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library
ID method. This allows for execute in place and shared
libraries in an environment without virtual memory
management. This option implies -fPIC. With a bfin-elf
target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries
are being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library
ID method, but assumes that this library or executable won't
link against any other ID shared libraries. That allows the
compiler to use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against
any ID shared libraries. Slower code is generated for jump
and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates
more compact code; specifying other values forces the
allocation of that number to the current library but is no
more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows
for execute in place in an environment without virtual memory
management by eliminating relocations against the text
section.
-mno-sep-data
Generate code that assumes that the data segment follows the
text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading
the address of the function into a register and then
performing a subroutine call on this register. This switch
is needed if the target function lies outside of the 24-bit
addressing range of the offset-based version of subroutine
call instruction.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior. Note these
switches have no effect on how the compiler generates code to
handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library. This library
relaxes some of the IEEE floating-point standard's rules for
checking inputs against Not-a-Number (NAN), in the interest
of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions
that are not known to bind locally. It has no effect without
-mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin
processors. This option causes proper start files and link
scripts supporting multicore to be used, and defines the
macro "__BFIN_MULTICORE". It can only be used with
-mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which
selects the one-application-per-core programming model.
Without -mcorea or -mcoreb, the single-application/dual-core
programming model is used. In this model, the main function
of Core B should be named as "coreb_main".
If this option is not used, the single-core application
programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when using
the one-application-per-core programming model. Proper start
files and link scripts are used to support Core A, and the
macro "__BFIN_COREA" is defined. This option can only be
used in conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using
the one-application-per-core programming model. Proper start
files and link scripts are used to support Core B, and the
macro "__BFIN_COREB" is defined. When this option is used,
"coreb_main" should be used instead of "main". This option
can only be used in conjunction with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files
and link scripts are used to put the application into SDRAM,
and the macro "__BFIN_SDRAM" is defined. The loader should
initialize SDRAM before loading the application.
-micplb
Assume that ICPLBs are enabled at run time. This has an
effect on certain anomaly workarounds. For Linux targets,
the default is to assume ICPLBs are enabled; for standalone
applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses
this name to determine what kind of instructions it can emit
when generating assembly code. Permissible names are: c62x,
c64x, c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the
default.
-msim
Choose startup files and linker script suitable for the
simulator.
-msdata=default
Put small global and static data in the ".neardata" section,
which is pointed to by register "B14". Put small
uninitialized global and static data in the ".bss" section,
which is adjacent to the ".neardata" section. Put small
read-only data into the ".rodata" section. The corresponding
sections used for large pieces of data are ".fardata", ".far"
and ".const".
-msdata=all
Put all data, not just small objects, into the sections
reserved for small data, and use addressing relative to the
"B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use
absolute addresses to access all data. Put all initialized
global and static data in the ".fardata" section, and all
uninitialized data in the ".far" section. Put all constant
data into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices
for architecture-type are v3, v8 and v10 for respectively
ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except
for cris-axis-linux-gnu, where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the
generated code, except for the ABI and the set of available
instructions. The choices for architecture-type are the same
as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for
-march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for
CPU models where it applies. This option is active by
default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the
assembly code. This option also has the effect of turning
off the #NO_APP formatted-code indicator to the assembler at
the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction;
always emit compare and test instructions before use of
condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing
modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements)
for the stack frame, individual data and constants to be
aligned for the maximum single data access size for the
chosen CPU model. The default is to arrange for 32-bit
alignment. ABI details such as structure layout are not
affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above,
these options arrange for stack frame, writable data and
constants to all be 32-bit, 16-bit or 8-bit aligned. The
default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and
epilogue which set up the stack frame are omitted and no
return instructions or return sequences are generated in the
code. Use this option only together with visual inspection
of the compiled code: no warnings or errors are generated
when call-saved registers must be saved, or storage for local
variables needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate)
instruction sequences that load addresses for functions from
the PLT part of the GOT rather than (traditional on other
architectures) calls to the PLT. The default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf
and cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-
gnu target.
-sim
This option, recognized for the cris-axis-elf, arranges to
link with input-output functions from a simulator library.
Code, initialized data and zero-initialized data are
allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data
at 0x40000000 and zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled
by default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+
architecture is default.
-msim
Links the library libsim.a which is in compatible with
simulator. Applicable to ELF compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates "sbit"/"cbit" instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or
medium. medium is default. However, far is not valid with
-mcr16c, as the CR16C architecture does not support the far
data model.
C-SKY Options
GCC supports these options when compiling for C-SKY V2
processors.
-march=arch
Specify the C-SKY target architecture. Valid values for arch
are: ck801, ck802, ck803, ck807, and ck810. The default is
ck810.
-mcpu=cpu
Specify the C-SKY target processor. Valid values for cpu
are: ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h,
ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et,
ck803eht, ck803ef, ck803efh, ck803ft, ck803eft, ck803efht,
ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1,
ck803er1, ck803ehr1, ck803etr1, ck803ehtr1, ck803efr1,
ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s,
ck803st, ck803se, ck803sf, ck803sef, ck803seft, ck807e,
ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv.
-mbig-endian
-EB
-mlittle-endian
-EL Select big- or little-endian code. The default is little-
endian.
-mhard-float
-msoft-float
Select hardware or software floating-point implementations.
The default is soft float.
-mdouble-float
-mno-double-float
When -mhard-float is in effect, enable generation of double-
precision float instructions. This is the default except
when compiling for CK803.
-mfdivdu
-mno-fdivdu
When -mhard-float is in effect, enable generation of
"frecipd", "fsqrtd", and "fdivd" instructions. This is the
default except when compiling for CK803.
-mfpu=fpu
Select the floating-point processor. This option can only be
used with -mhard-float. Values for fpu are fpv2_sf
(equivalent to -mno-double-float -mno-fdivdu), fpv2
(-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float
-mdivdu).
-melrw
-mno-elrw
Enable the extended "lrw" instruction. This option defaults
to on for CK801 and off otherwise.
-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the "interrupt" and
"isr" function attributes.
-mmp
Enable multiprocessor instructions; the default is off.
-mcp
Enable coprocessor instructions; the default is off.
-mcache
Enable coprocessor instructions; the default is off.
-msecurity
Enable C-SKY security instructions; the default is off.
-mtrust
Enable C-SKY trust instructions; the default is off.
-mdsp
-medsp
-mvdsp
Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions,
respectively. All of these options default to off.
-mdiv
-mno-div
Generate divide instructions. Default is off.
-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered
0-7 to allow use of 16-bit instructions. This option is
ignored for CK801 where this is the required behavior, and it
defaults to on for CK802. For other targets, the default is
off.
-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This
option is not supported on CK801, CK802, or CK803, and is
enabled by default for other processors.
-manchor
-mno-anchor
Generate code using global anchor symbol addresses.
-mpushpop
-mno-pushpop
Generate code using "push" and "pop" instructions. This
option defaults to on.
-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm
Generate code using "stm" and "ldm" instructions. This
option isn't supported on CK801 but is enabled by default on
other processors.
-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it
to the assembler. This option is the default and required
for correct code generation on CK801 and CK802, and is
optional on other processors.
-mstack-size
-mno-stack-size
Emit ".stack_size" directives for each function in the
assembly output. This option defaults to off.
-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of
libgcc. This option defaults to off.
-mbranch-cost=n
Set the branch costs to roughly "n" instructions. The
default is 1.
-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue
sequences. Using this option can result in code that is not
compliant with the C-SKY V2 ABI prologue requirements and
that cannot be debugged or backtraced. It is disabled by
default.
Darwin Options
These options are defined for all architectures running the
Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates
an object file for the single architecture that GCC was built to
target. Apple's GCC on Darwin does create "fat" files if
multiple -arch options are used; it does so by running the
compiler or linker multiple times and joining the results
together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686)
is determined by the flags that specify the ISA that GCC is
targeting, like -mcpu or -march. The -force_cpusubtype_ALL
option can be used to override this.
The Darwin tools vary in their behavior when presented with an
ISA mismatch. The assembler, as, only permits instructions to be
used that are valid for the subtype of the file it is generating,
so you cannot put 64-bit instructions in a ppc750 object file.
The linker for shared libraries, /usr/bin/libtool, fails and
prints an error if asked to create a shared library with a less
restrictive subtype than its input files (for instance, trying to
put a ppc970 object file in a ppc7400 library). The linker for
executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of
directories to be searched for header files. These
directories are interleaved with those specified by -I
options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it.
A framework is a directory with a Headers and/or
PrivateHeaders directory contained directly in it that ends
in .framework. The name of a framework is the name of this
directory excluding the .framework. Headers associated with
the framework are found in one of those two directories, with
Headers being searched first. A subframework is a framework
directory that is in a framework's Frameworks directory.
Includes of subframework headers can only appear in a header
of a framework that contains the subframework, or in a
sibling subframework header. Two subframeworks are siblings
if they occur in the same framework. A subframework should
not have the same name as a framework; a warning is issued if
this is violated. Currently a subframework cannot have
subframeworks; in the future, the mechanism may be extended
to support this. The standard frameworks can be found in
/System/Library/Frameworks and /Library/Frameworks. An
example include looks like "#include <Framework/header.h>",
where Framework denotes the name of the framework and
header.h is found in the PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system
directory. The main difference between this -iframework and
-F is that with -iframework the compiler does not warn about
constructs contained within header files found via dir. This
option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For
stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run
on is version. Typical values of version include 10.1, 10.2,
and 10.3.9.
If the compiler was built to use the system's headers by
default, then the default for this option is the system
version on which the compiler is running, otherwise the
default is to make choices that are compatible with as many
systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets
-static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and
-fno-rtti where applicable. This mode also sets
-mno-altivec, -msoft-float, -fno-builtin and -mlong-branch
for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1".
By default "sizeof(bool)" is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this
option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate
code that is not binary compatible with code generated
without that switch. Using this switch may require
recompiling all other modules in a program, including system
libraries. Use this switch to conform to a non-default data
model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such
as to allow GDB to dynamically load .o files into already-
running programs. -findirect-data and -ffix-and-continue are
provided for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1)
for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong
architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic
linker will bind all undefined references when the file is
loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loader executable
This option specifies the executable that will load the build
output file being linked. See man ld(1) for more
information.
-dynamiclib
When passed this option, GCC produces a dynamic library
instead of an executable when linking, using the Darwin
libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype,
instead of one controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin
linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that
emulate the floating-point operations, or compiled in such a
way as to call such emulations routines, these routines issue
floating-point operations. If you are compiling for an
Alpha without floating-point operations, you must ensure that
the library is built so as not to call them.
Note that Alpha implementations without floating-point
operations are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point
register set. -mno-fp-regs implies -msoft-float. If the
floating-point register set is not used, floating-point
operands are passed in integer registers as if they were
integers and floating-point results are passed in $0 instead
of $f0. This is a non-standard calling sequence, so any
function with a floating-point argument or return value
called by code compiled with -mno-fp-regs must also be
compiled with that option.
A typical use of this option is building a kernel that does
not use, and hence need not save and restore, any floating-
point registers.
-mieee
The Alpha architecture implements floating-point hardware
optimized for maximum performance. It is mostly compliant
with the IEEE floating-point standard. However, for full
compliance, software assistance is required. This option
generates code fully IEEE-compliant code except that the
inexact-flag is not maintained (see below). If this option
is turned on, the preprocessor macro "_IEEE_FP" is defined
during compilation. The resulting code is less efficient but
is able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and plus/minus
infinity. Other Alpha compilers call this option
-ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains
the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In
addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the
resulting code may execute significantly slower than the code
generated by default. Since there is very little code that
depends on the inexact-flag, you should normally not specify
this option. Other Alpha compilers call this option
-ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are
enabled. Other Alpha compilers call this option -fptm trap-
mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps
that are enabled are the ones that cannot be disabled in
software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps
are enabled as well.
su Like u, but the instructions are marked to be safe for
software completion (see Alpha architecture manual for
details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call
this option -fprm rounding-mode. The rounding-mode can be
one of:
n Normal IEEE rounding mode. Floating-point numbers are
rounded towards the nearest machine number or towards the
even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are
rounded towards zero.
d Dynamic rounding mode. A field in the floating-point
control register (fpcr, see Alpha architecture reference
manual) controls the rounding mode in effect. The C
library initializes this register for rounding towards
plus infinity. Thus, unless your program modifies the
fpcr, d corresponds to round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are
imprecise. This means without software assistance it is
impossible to recover from a floating trap and program
execution normally needs to be terminated. GCC can generate
code that can assist operating system trap handlers in
determining the exact location that caused a floating-point
trap. Depending on the requirements of an application,
different levels of precisions can be selected:
p Program precision. This option is the default and means
a trap handler can only identify which program caused a
floating-point exception.
f Function precision. The trap handler can determine the
function that caused a floating-point exception.
i Instruction precision. The trap handler can determine
the exact instruction that caused a floating-point
exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You
must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line
.eflag 48 in the function prologue of the generated assembly
file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see
if it can construct it from smaller constants in two or three
instructions. If it cannot, it outputs the constant as a
literal and generates code to load it from the data segment
at run time.
Use this option to require GCC to construct all integer
constants using code, even if it takes more instructions (the
maximum is six).
You typically use this option to build a shared library
dynamic loader. Itself a shared library, it must relocate
itself in memory before it can find the variables and
constants in its own data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional
BWX, CIX, FIX and MAX instruction sets. The default is to
use the instruction sets supported by the CPU type specified
via -mcpu= option or that of the CPU on which GCC was built
if none is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-
point arithmetic instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros
does not allow optimal instruction scheduling. GNU binutils
as of version 2.12 supports a new syntax that allows the
compiler to explicitly mark which relocations should apply to
which instructions. This option is mostly useful for
debugging, as GCC detects the capabilities of the assembler
when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed
via gp-relative relocations. When -msmall-data is used,
objects 8 bytes long or smaller are placed in a small data
area (the ".sdata" and ".sbss" sections) and are accessed via
16-bit relocations off of the $gp register. This limits the
size of the small data area to 64KB, but allows the variables
to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area
is limited to just below 2GB. Programs that require more
than 2GB of data must use "malloc" or "mmap" to allocate the
data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code
of the entire program (or shared library) fits in 4MB, and is
thus reachable with a branch instruction. When -msmall-data
is used, the compiler can assume that all local symbols share
the same $gp value, and thus reduce the number of
instructions required for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters
for machine type cpu_type. You can specify either the EV
style name or the corresponding chip number. GCC supports
scheduling parameters for the EV4, EV5 and EV6 family of
processors and chooses the default values for the instruction
set from the processor you specify. If you do not specify a
processor type, GCC defaults to the processor on which the
compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set
extensions.
ev5
21164
Schedules as an EV5 and has no instruction set
extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX
extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and
MAX extensions.
Native toolchains also support the value native, which
selects the best architecture option for the host processor.
-mcpu=native has no effect if GCC does not recognize the
processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine
type cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which
selects the best architecture option for the host processor.
-mtune=native has no effect if GCC does not recognize the
processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical
memory references as seen by the application. This number is
highly dependent on the memory access patterns used by the
application and the size of the external cache on the
machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock
cycles for "typical" EV4 & EV5 hardware for the Level 1,
2 & 3 caches (also called Dcache, Scache, and Bcache), as
well as to main memory. Note that L3 is only valid for
EV5.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller
code, but it does assume that all symbolic values and
addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is
no need to include the simulator library (libsim.a) on the
linker command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator.
This causes an alternate runtime startup and library to be
linked. You must not use this option when generating
programs that will run on real hardware; you must provide
your own runtime library for whatever I/O functions are
needed.
-mlra
Enable Local Register Allocation. This is still experimental
for FT32, so by default the compiler uses standard reload.
-mnodiv
Do not use div and mod instructions.
-mft32b
Enable use of the extended instructions of the FT32B
processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm
Do not generate code that reads program memory.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers,
only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to
represent pointers to functions. Without any PIC/PIE-related
options, it implies -fPIE. With -fpic or -fpie, it assumes
GOT entries and small data are within a 12-bit range from the
GOT base address; with -fPIC or -fPIE, GOT offsets are
computed with 32 bits. With a bfin-elf target, this option
implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions
that are not known to bind locally. It has no effect without
-mfdpic. It's enabled by default if optimizing for speed and
compiling for shared libraries (i.e., -fPIC or -fpic), or
when an optimization option such as -O3 or above is present
in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-
local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for
data that is known to be in read-only sections. It's enabled
by default, except for -fpic or -fpie: even though it may
help make the global offset table smaller, it trades 1
instruction for 4. With -fPIC or -fPIE, it trades 3
instructions for 4, one of which may be shared by multiple
symbols, and it avoids the need for a GOT entry for the
referenced symbol, so it's more likely to be a win. If it is
not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied
by -mlibrary-pic, as well as by -fPIC and -fpic without
-mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame
pointer whenever a stack frame is allocated. This option is
enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed
anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs
into the previous packet. This option only has an effect
when VLIW packing is enabled. It doesn't create new packets;
it merely adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution
(default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional
execution.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will
likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the
compiler-generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant
"membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code.
Possible values are frv, fr550, tomcat, fr500, fr450, fr405,
fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on
*-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android*
targets.
-muclibc
Use uClibc C library. This is the default on
*-*-linux-*uclibc* targets.
-mmusl
Use the musl C library. This is the default on
*-*-linux-*musl* targets.
-mbionic
Use Bionic C library. This is the default on
*-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is the
default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking, this
option makes the GCC driver pass Android-specific options to
the linker. Finally, this option causes the preprocessor
macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable
-mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard
Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible;
uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode.
This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used
with -ms.
-mexr
Extended registers are stored on stack before execution of
function with monitor attribute. Default option is -mexr.
This option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution
of function with monitor attribute. Default option is
-mno-exr. This option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for
the H8/300. The default for the H8/300H and H8S is to align
longs and floats on 4-byte boundaries. -malign-300 causes
them to be aligned on 2-byte boundaries. This option has no
effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices
for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and
2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on
an HP-UX system to determine the proper architecture option
for your machine. Code compiled for lower numbered
architectures runs on higher numbered architectures, but not
the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0
respectively.
-mcaller-copies
The caller copies function arguments passed by hidden
reference. This option should be used with care as it is not
compatible with the default 32-bit runtime. However, only
aggregates larger than eight bytes are passed by hidden
reference and the option provides better compatibility with
OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility
purposes only.
-mdisable-fpregs
Prevent floating-point registers from being used in any
manner. This is necessary for compiling kernels that perform
lazy context switching of floating-point registers. If you
use this option and attempt to perform floating-point
operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This
avoids some rather obscure problems when compiling MIG
generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers.
This allows GCC to generate faster indirect calls and use
unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space
boundaries. This allows GCC to emit code that performs
faster indirect calls.
This option does not work in the presence of shared libraries
or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel
code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be
specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes
required by the HP-UX 10 linker. This is equivalent to the
+k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF
systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine
type cpu-type. The choices for cpu-type are 700 7100,
7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models
on an HP-UX system to determine the proper scheduling option
for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this
makes symbolic debugging impossible. It also triggers a bug
in the HP-UX 8 and HP-UX 9 linkers in which they give bogus
error messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all
HPPA targets. Normally the facilities of the machine's usual
C compiler are used, but this cannot be done directly in
cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output
file; therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile
libgcc.a, the library that comes with GCC, with -msoft-float
in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default
is -mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These
options are available under HP-UX and HI-UX.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld
when building a shared library. It is the default when GCC
is configured, explicitly or implicitly, with the GNU linker.
This option does not affect which ld is called; it only
changes what parameters are passed to that ld. The ld that
is called is determined by the --with-ld configure option,
GCC's program search path, and finally by the user's PATH.
The linker used by GCC can be printed using which `gcc
-print-prog-name=ld`. This option is only available on the
64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when
building a shared library and passes +Accept TypeMismatch to
ld on all links. It is the default when GCC is configured,
explicitly or implicitly, with the HP linker. This option
does not affect which ld is called; it only changes what
parameters are passed to that ld. The ld that is called is
determined by the --with-ld configure option, GCC's program
search path, and finally by the user's PATH. The linker used
by GCC can be printed using which `gcc -print-prog-name=ld`.
This option is only available on the 64-bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures
that a call is always able to reach linker generated stubs.
The default is to generate long calls only when the distance
from the call site to the beginning of the function or
translation unit, as the case may be, exceeds a predefined
limit set by the branch type being used. The limits for
normal calls are 7,600,000 and 240,000 bytes, respectively
for the PA 2.0 and PA 1.X architectures. Sibcalls are always
limited at 240,000 bytes.
Distances are measured from the beginning of functions when
using the -ffunction-sections option, or when using the -mgas
and -mno-portable-runtime options together under HP-UX with
the SOM linker.
It is normally not desirable to use this option as it
degrades performance. However, it may be useful in large
applications, particularly when partial linking is used to
build the application.
The types of long calls used depends on the capabilities of
the assembler and linker, and the type of code being
generated. The impact on systems that support long absolute
calls, and long pic symbol-difference or pc-relative calls
should be relatively small. However, an indirect call is
used on 32-bit ELF systems in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the
specified UNIX standard. The choices for unix-std are 93, 95
and 98. 93 is supported on all HP-UX versions. 95 is
available on HP-UX 10.10 and later. 98 is available on HP-UX
11.11 and later. The default values are 93 for HP-UX 10.00,
95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11
and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o.
-munix=98 provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the
interfaces for various library routines. It also affects the
operational behavior of the C library. Thus, extreme care is
needed in using this option.
Library code that is intended to operate with more than one
UNIX standard must test, set and restore the variable
"__xpg4_extended_mask" as appropriate. Most GNU software
doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl
when the -static option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a
dependency on libdld.sl. There isn't an archive version of
libdld.sl. Thus, when the -static option is specified,
special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary
options to link with libdld.sl when the -static option is
specified. This causes the resulting binary to be dynamic.
On the 64-bit port, the linkers generate dynamic binaries by
default in any case. The -nolibdld option can be used to
prevent the GCC driver from adding these link options.
-threads
Add support for multithreading with the dce thread library
under HP-UX. This option sets flags for both the
preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64
architecture.
-mbig-endian
Generate code for a big-endian target. This is the default
for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the
default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the
default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the
default.
-mno-pic
Generate code that does not use a global pointer register.
The result is not position independent code, and violates the
IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after
volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the
stacked registers. This may make assembler output more
readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data
section. This may be useful for working around optimizer
bugs.
-mconstant-gp
Generate code that uses a single constant global pointer
value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values
using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values
using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point
values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the
minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the
maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum
latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum
throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or
multiply/subtract instructions. The default is to use these
instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line
number debugging info. This may be useful when not using the
GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately
preceding the instruction that triggered the stop bit. This
can improve instruction scheduling, but does not always do
so.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel
code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be
specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are
14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid
values are itanium, itanium1, merced, itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer
to 64 bits. These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This
results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a"). The
default setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This
results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a"). The
default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is
available only during region scheduling (i.e. before reload).
This results in generation of the "ld.s" instructions and the
corresponding check instructions "chk.s". The default
setting is disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that
are dependent on the data speculative loads before reload.
This is effective only with -msched-br-data-spec enabled.
The default setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that
are dependent on the data speculative loads after reload.
This is effective only with -msched-ar-data-spec enabled.
The default setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that
are dependent on the control speculative loads. This is
effective only with -msched-control-spec enabled. The
default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for
schedule only if there are no other choices at the moment.
This makes the use of the data speculation much more
conservative. The default setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for
schedule only if there are no other choices at the moment.
This makes the use of the control speculation much more
conservative. The default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during
computation of the instructions priorities. This makes the
use of the speculation a bit more conservative. The default
setting is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by
default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is
on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This
option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to
cause a conflict when placed into the same instruction group.
This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective
scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group,
giving lower priority to subsequent memory insns attempting
to schedule in the same instruction group. Frequently useful
to prevent cache bank conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard
limit, disallowing more than that number in an instruction
group. Otherwise, the limit is "soft", meaning that non-
memory operations are preferred when the limit is reached,
but memory operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one
of r8c for the R8C/Tiny series, m16c for the M16C (up to /60)
series, m32cm for the M16C/80 series, or m32c for the M32C/80
series.
-msim
Specifies that the program will be run on the simulator.
This causes an alternate runtime library to be linked in
which supports, for example, file I/O. You must not use this
option when generating programs that will run on real
hardware; you must provide your own runtime library for
whatever I/O functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC
uses during code generation. These pseudo-registers are used
like real registers, so there is a tradeoff between GCC's
ability to fit the code into available registers, and the
performance penalty of using memory instead of registers.
Note that all modules in a program must be compiled with the
same value for this option. Because of that, you must not
use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that
their addresses can be loaded with the "ld24" instruction),
and assume all subroutines are reachable with the "bl"
instruction. This is the default.
The addressability of a particular object can be set with the
"model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space
(the compiler generates "seth/add3" instructions to load
their addresses), and assume all subroutines are reachable
with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space
(the compiler generates "seth/add3" instructions to load
their addresses), and assume subroutines may not be reachable
with the "bl" instruction (the compiler generates the much
slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into
one of ".data", ".bss", or ".rodata" (unless the "section"
attribute has been specified). This is the default.
The small data area consists of sections ".sdata" and
".sbss". Objects may be explicitly put in the small data
area with the "section" attribute using one of these
sections.
-msdata=sdata
Put small global and static data in the small data area, but
do not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and
generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal
data or BSS sections. The default value of num is 8. The
-msdata option must be set to one of sdata or use for this
option to have any effect.
All modules should be compiled with the same -G num value.
Compiling with different values of num may or may not work;
if it doesn't the linker gives an error message---incorrect
code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display some
statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the
default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or
2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are
preferred over conditional code, if it is 2, then the
opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The
default is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call
to flush the cache. The default is _flush_cache, but a
function call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the
cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire
processors. The default settings depend on which architecture
was selected when the compiler was configured; the defaults for
the most common choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction
set architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020, 68030, 68040, 68060
and cpu32. ColdFire architectures are selected according to
Freescale's ISA classification and the permissible values
are: isaa, isaaplus, isab and isac.
GCC defines a macro "__mcfarch__" whenever it is generating
code for a ColdFire target. The arch in this macro is one of
the -march arguments given above.
When used together, -march and -mtune select code that runs
on a family of similar processors but that is optimized for a
particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor.
The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040,
68060, 68302, 68332 and cpu32. The ColdFire cpus are given
by the table below, which also classifies the CPUs into
families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe
51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483
5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with
cpu. Other combinations of -mcpu and -march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target
cpu is selected. It also defines "__mcf_family_family",
where the value of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the
constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020, 68030, 68040,
68060 and cpu32. The ColdFire microarchitectures are: cfv1,
cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run
relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets as
well. These two options select the same tuning decisions as
-m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when
tuning for 680x0 architecture arch. It also defines "mcarch"
unless either -ansi or a non-GNU -std option is used. If GCC
is tuning for a range of architectures, as selected by
-mtune=68020-40 or -mtune=68020-60, it defines the macros for
every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for
ColdFire microarchitecture uarch, where uarch is one of the
arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the
compiler is configured for 68000-based systems. It is
equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000
core, including the 68008, 68302, 68306, 68307, 68322, 68328
and 68356.
-m68010
Generate output for a 68010. This is the default when the
compiler is configured for 68010-based systems. It is
equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the
compiler is configured for 68020-based systems. It is
equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when the
compiler is configured for 68030-based systems. It is
equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the
compiler is configured for 68040-based systems. It is
equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that
have to be emulated by software on the 68040. Use this
option if your 68040 does not have code to emulate those
instructions.
-m68060
Generate output for a 68060. This is the default when the
compiler is configured for 68060-based systems. It is
equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882
instructions that have to be emulated by software on the
68060. Use this option if your 68060 does not have code to
emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the
compiler is configured for CPU32-based systems. It is
equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+
core, including the 68330, 68331, 68332, 68333, 68334, 68336,
68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default
when the compiler is configured for 520X-based systems. It
is equivalent to -mcpu=5206, and is now deprecated in favor
of that option.
Use this option for microcontroller with a 5200 core,
including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family.
The option is now deprecated in favor of the equivalent
-mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g.
547x/548x). This includes use of hardware floating-point
instructions. The option is equivalent to -mcpu=547x, and is
now deprecated in favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are
emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are
emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default
for 68020 and above, and for ColdFire devices that have an
FPU. It defines the macro "__HAVE_68881__" on M680x0 targets
and "__mcffpu__" on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library
calls instead. This is the default for 68000, 68010, and
68832 targets. It is also the default for ColdFire devices
that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and
remainder instructions. If -march is used without -mcpu, the
default is "on" for ColdFire architectures and "off" for
M680x0 architectures. Otherwise, the default is taken from
the target CPU (either the default CPU, or the one specified
by -mcpu). For example, the default is "off" for -mcpu=5206
and "on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is
enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
Additionally, parameters passed on the stack are also aligned
to a 16-bit boundary even on targets whose API mandates
promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the
default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32
and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option
implies -mbitfield. This is the default if you use a
configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return with
the "rtd" instruction, which pops their arguments while
returning. This saves one instruction in the caller since
there is no need to pop the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions
that take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments
are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020,
68030, 68040, 68060 and CPU32 processors, but not by the
68000 or 5200.
The default is -mno-rtd.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long",
"float", "double", and "long double" variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int).
Aligning variables on 32-bit boundaries produces code that
runs somewhat faster on processors with 32-bit busses at the
expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns
structures containing the above types differently than most
published application binary interface specifications for the
m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly,
instead of using a global offset table. At present, this
option implies -fpic, allowing at most a 16-bit offset for
pc-relative addressing. -fPIC is not presently supported
with -mpcrel, though this could be supported for 68020 and
higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are
handled by the system.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows
for execute-in-place in an environment without virtual memory
management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the
text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library
ID method. This allows for execute-in-place and shared
libraries in an environment without virtual memory
management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries
are being used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates
more compact code; specifying other values forces the
allocation of that number to the current library, but is no
more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire,
generate code that works if the GOT has more than 8192
entries. This code is larger and slower than code generated
without this option. On M680x0 processors, this option is
not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from
the GOT. While this is relatively efficient, it only works
if the GOT is smaller than about 64k. Anything larger causes
the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot.
It should then work with very large GOTs. However, code
generated with -mxgot is less efficient, since it takes 4
instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU
linker, can create multiple GOTs and sort GOT entries. If
you have such a linker, you should only need to use -mxgot
when compiling a single object file that accesses more than
8192 GOT entries. Very few do.
These options have no effect unless GCC is generating
position-independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core
processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in
two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment
operation. Large values can increase the speed of programs
that contain functions that need a large amount of stack
space, but they can also trigger a segmentation fault if the
stack is extended too much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute
difference between two registers.
-mall-opts
Enables all the optional instructions---average, multiply,
divide, bit operations, leading zero, absolute difference,
min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of
two registers.
-mbased=n
Variables of size n bytes or smaller are placed in the
".based" section by default. Based variables use the $tp
register as a base register, and there is a 128-byte limit to
the ".based" section.
-mbitops
Enables the bit operation instructions---bit test ("btstm"),
set ("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-
and-set ("tas").
-mc=name
Selects which section constant data is placed in. name may
be tiny, near, or far.
-mclip
Enables the "clip" instruction. Note that -mclip is not
useful unless you also provide -mminmax.
-mconfig=name
Selects one of the built-in core configurations. Each MeP
chip has one or more modules in it; each module has a core
CPU and a variety of coprocessors, optional instructions, and
peripherals. The "MeP-Integrator" tool, not part of GCC,
provides these configurations through this option; using this
option is the same as using all the corresponding command-
line options. The default configuration is default.
-mcop
Enables the coprocessor instructions. By default, this is a
32-bit coprocessor. Note that the coprocessor is normally
enabled via the -mconfig= option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near"
section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io"
attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by
default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by
default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate
instructions.
-mno-opts
Disables all the optional instructions enabled by -mall-opts.
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for
low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note
that there is a 65536-byte limit to this section. Accesses
to these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler
does not currently generate these itself, but this option is
included for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based
runtime.
-msim
Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in
support for reset and exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section.
Without this option, functions default to the ".near"
section.
-mtiny=n
Variables that are n bytes or smaller are allocated to the
".tiny" section. These variables use the $gp base register.
The default for this option is 4, but note that there's a
65536-byte limit to the ".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss
instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU.
Supported values are in the format vX.YY.Z, where X is a
major version, YY is the minor version, and Z is
compatibility code. Example values are v3.00.a, v4.00.b,
v5.00.a, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer
divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32
multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
-mpic-data-is-text-relative
Assume that the displacement between the text and data
segments is fixed at static link time. This allows data
to be referenced by offset from start of text address
instead of GOT since PC-relative addressing is not
supported.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based
software intrusive debug agent called xmdstub. This uses
startup file crt1.o and sets the start address of the
program to 0x800.
bootstrap
for applications that are loaded using a bootloader.
This model uses startup file crt2.o which does not
contain a processor reset vector handler. This is
suitable for transferring control on a processor reset to
the bootloader rather than the application.
novectors
for applications that do not require any of the
MicroBlaze vectors. This option may be useful for
applications running within a monitoring application.
This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for
-mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for
mips*el-*-* configurations.
-march=arch
Generate code that runs on arch, which can be the name of a
generic MIPS ISA, or the name of a particular processor. The
ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2,
mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3,
mips64r5 and mips64r6. The processor names are: 4kc, 4km,
4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc,
24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1,
34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc,
1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e,
loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k,
m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2,
octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000,
rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100,
vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr
and xlp. The special value from-abi selects the most
compatible architecture for the selected ABI (that is, mips1
for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value
native, which selects the best architecture option for the
host processor. -march=native has no effect if GCC does not
recognize the processor.
In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be
written r.
Names of the form nf2_1 refer to processors with FPUs clocked
at half the rate of the core, names of the form nf1_1 refer
to processors with FPUs clocked at the same rate as the core,
and names of the form nf3_2 refer to processors with FPUs
clocked a ratio of 3:2 with respect to the core. For
compatibility reasons, nf is accepted as a synonym for nf2_1
while nx and bfx are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option.
The first is "_MIPS_ARCH", which gives the name of target
architecture, as a string. The second has the form
"_MIPS_ARCH_foo", where foo is the capitalized value of
"_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to
"r2000" and defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names
given above. In other words, it has the full prefix and does
not abbreviate 000 as k. In the case of from-abi, the macro
names the resolved architecture (either "mips1" or "mips3").
It names the default architecture when no -march option is
given.
-mtune=arch
Optimize for arch. Among other things, this option controls
the way instructions are scheduled, and the perceived cost of
arithmetic operations. The list of arch values is the same
as for -march.
When this option is not used, GCC optimizes for the processor
specified by -march. By using -march and -mtune together, it
is possible to generate code that runs on a family of
processors, but optimize the code for one particular member
of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo",
which work in the same way as the -march ones described
above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting
a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e
ASE.
MIPS16 code generation can also be controlled on a per-
function basis by means of "mips16" and "nomips16"
attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option
is provided for regression testing of mixed MIPS16/non-MIPS16
code generation, and is not intended for ordinary use in
compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard
(uncompressed) MIPS ISA be link-compatible with MIPS16 and
microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump
directly to MIPS16 or microMIPS code; it must either use a
call or an indirect jump. -minterlink-compressed therefore
disables direct jumps unless GCC knows that the target of the
jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and
-mno-interlink-compressed. These options predate the
microMIPS ASE and are retained for backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC
normally generates 64-bit code when you select a 64-bit
architecture, but you can use -mgp32 to get 32-bit code
instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html >.
GCC supports a variant of the o32 ABI in which floating-point
registers are 64 rather than 32 bits wide. You can select
this combination with -mabi=32 -mfp64. This ABI relies on
the "mthc1" and "mfhc1" instructions and is therefore only
supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values
remain the same, but each scalar value is passed in a single
64-bit register rather than a pair of 32-bit registers. For
example, scalar floating-point values are returned in $f0
only, not a $f0/$f1 pair. The set of call-saved registers
also remains the same in that the even-numbered double-
precision registers are saved.
Two additional variants of the o32 ABI are supported to
enable a transition from 32-bit to 64-bit registers. These
are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The
FPXX extension mandates that all code must execute correctly
when run using 32-bit or 64-bit registers. The code can be
interlinked with either FP32 or FP64, but not both. The
FP64A extension is similar to the FP64 extension but forbids
the use of odd-numbered single-precision registers. This can
be used in conjunction with the "FRE" mode of FPUs in
MIPS32R5 processors and allows both FP32 and FP64A code to
interlink and run in the same process without changing FPU
modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for
SVR4-style dynamic objects. -mabicalls is the default for
SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-
independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-
independent, regardless of options like -fPIC and -fpic.
However, as an extension, the GNU toolchain allows
executables to use absolute accesses for locally-binding
symbols. It can also use shorter GP initialization sequences
and generate direct calls to locally-defined functions. This
mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates
objects that can only be linked by the GNU linker. However,
the option does not affect the ABI of the final executable;
it only affects the ABI of relocatable objects. Using
-mno-shared generally makes executables both smaller and
quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers
support PLTs and copy relocations. This option only affects
-mno-shared -mabicalls. For the n64 ABI, this option has no
effect without -msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the
global offset table.
GCC normally uses a single instruction to load values from
the GOT. While this is relatively efficient, it only works
if the GOT is smaller than about 64k. Anything larger causes
the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot.
This works with very large GOTs, although the code is also
less efficient, since it takes three instructions to fetch
the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have
such a linker, you should only need to use -mxgot when a
single object file accesses more than 64k's worth of GOT
entries. Very few do.
These options have no effect unless GCC is generating
position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions.
Implement floating-point calculations using library calls
instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the
program being compiled does not perform any floating-point
operations. This option is presently supported only by some
bare-metal MIPS configurations, where it may select a special
set of libraries that lack all floating-point support
(including, for example, the floating-point "printf"
formats). If code compiled with -mno-float accidentally
contains floating-point operations, it is likely to suffer a
link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports
single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-
precision operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-
point registers for the o32 ABI. This is the default for
processors that are known to support these registers. When
using the o32 FPXX ABI, -mno-odd-spreg is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-
number (NaN) IEEE 754 floating-point data with the "abs.fmt"
and "neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment
is selected. In this case these instructions are considered
arithmetic and avoided where correct operation is required
and the input operand might be a NaN. A longer sequence of
instructions that manipulate the sign bit of floating-point
datum manually is used instead unless the -ffinite-math-only
option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment.
In this case these instructions are considered non-arithmetic
and therefore operating correctly in all cases, including in
particular where the input operand is a NaN. These
instructions are therefore always used for the respective
operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-
number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this
case quiet NaNs (qNaNs) are denoted by the first bit of their
trailing significand field being 0, whereas signaling NaNs
(sNaNs) are denoted by the first bit of their trailing
significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In
this case qNaNs are denoted by the first bit of their
trailing significand field being 1, whereas sNaNs are denoted
by the first bit of their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured
with --with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement
atomic memory built-in functions. When neither option is
specified, GCC uses the instructions if the target
architecture supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by
configuring GCC with --with-llsc and --without-llsc
respectively. --with-llsc is the default for some
configurations; see the installation documentation for
details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp".
It also defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp"
and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be
enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions.
This option can only be used when generating 64-bit code and
requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies
-mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-
function basis by means of "micromips" and "nomicromips"
attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing
instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA)
instructions.
-mcrc
-mno-crc
Use (do not use) the MIPS Cyclic Redundancy Check (CRC)
instructions.
-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV)
instructions.
-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions
Instructions (MMI).
-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT)
instructions.
-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2)
instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an
explanation of the default and the way that the pointer size
is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on
the ABI. All the supported ABIs use 32-bit "int"s. The n64
ABI uses 64-bit "long"s, as does the 64-bit EABI; the others
use 32-bit "long"s. Pointers are the same size as "long"s,
or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in
combination with -mabi=64 and -mno-abicalls because it allows
GCC to generate shorter and faster references to symbolic
addresses.
-G num
Put definitions of externally-visible data in a small data
section if that data is no bigger than num bytes. GCC can
then generate more efficient accesses to the data; see
-mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too,
such as to static variables in C. -mlocal-sdata is the
default for all configurations.
If the linker complains that an application is using too much
small data, you might want to try rebuilding the less
performance-critical parts with -mno-local-sdata. You might
also want to build large libraries with -mno-local-sdata, so
that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a
small data section if the size of that data is within the -G
limit. -mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num
-mgpopt, and Mod references a variable Var that is no bigger
than num bytes, you must make sure that Var is placed in a
small data section. If Var is defined by another module, you
must either compile that module with a high-enough -G setting
or attach a "section" attribute to Var's definition. If Var
is common, you must link the application with a high-enough
-G setting.
The easiest way of satisfying these restrictions is to
compile and link every module with the same -G option.
However, you may wish to build a library that supports
several different small data limits. You can do this by
compiling the library with the highest supported -G setting
and additionally using -mno-extern-sdata to stop the library
from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are
known to be in a small data section; see -G, -mlocal-sdata
and -mextern-sdata. -mgpopt is the default for all
configurations.
-mno-gpopt is useful for cases where the $gp register might
not hold the value of "_gp". For example, if the code is
part of a library that might be used in a boot monitor,
programs that call boot monitor routines pass an unknown
value in $gp. (In such situations, the boot monitor itself
is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if
possible, then next in the small data section if possible,
otherwise in data. This gives slightly slower code than the
default, but reduces the amount of RAM required when
executing, and thus may be preferred for some embedded
systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data
section. This option is only meaningful in conjunction with
-membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from
executable sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This
is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access
executable sections, but other instructions must not do
so. This option is useful on 4KSc and 4KSd processors
when the code TLBs have the Read Inhibit bit set. It is
also useful on processors that can be configured to have
a dual instruction/data SRAM interface and that, like the
M4K, automatically redirect PC-relative loads to the
instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This
option can be useful on targets that are configured to
have a dual instruction/data SRAM interface but that
(unlike the M4K) do not automatically redirect PC-
relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler
relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards
compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing
with symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use
an assembler that supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either
a conditional trap or a break instruction. Using traps
results in smaller code, but is only supported on MIPS II and
later. Also, some versions of the Linux kernel have a bug
that prevents trap from generating the proper signal
("SIGFPE"). Use -mdivide-traps to allow conditional traps on
architectures that support them and -mdivide-breaks to force
the use of breaks.
The default is usually -mdivide-traps, but this can be
overridden at configure time using --with-divide=breaks.
Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load
or store instructions to enable load/store bonding. This
option is enabled by default but only takes effect when the
selected architecture is known to support bonding.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial
block moves. The default is -mno-memcpy, which allows GCC to
inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction.
Calling functions using "jal" is more efficient but requires
the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul"
instructions, as provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer
instructions. The default is -mimadd on architectures that
support "madd" and "msub" except for the 74k architecture
where it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-
accumulate instructions, when they are available. The
default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are
used, the intermediate product is calculated to infinite
precision and is not subject to the FCSR Flush to Zero bit.
This may be undesirable in some circumstances. On other
processors the result is numerically identical to the
equivalent computation using separate multiply, add, subtract
and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill)
errata. The workarounds are implemented by the assembler
rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer
division.
- A double-word or a variable shift may give an incorrect
result if executed while an integer multiplication is in
progress.
- An integer division may give an incorrect result if
started in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer
division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on
revisions prior to 3.0. They may deadlock on revisions
2.6 and earlier.
This option can only be used if the target architecture
supports branch-likely instructions. -mfix-r10000 is the
default when -march=r10000 is used; -mno-fix-r10000 is the
default otherwise.
-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the
delay slot of a branch instruction placed at the end of a
short loop of six instructions or fewer and always schedule a
"nop" instruction there instead. The short loop bug under
certain conditions causes loops to execute only once or
twice, due to a hardware bug in the R5900 chip. The
workaround is implemented by the assembler rather than by
GCC.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The
workarounds are implemented by the assembler rather than by
GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result
if one of the operands is negative.
The workarounds for the division errata rely on special
functions in libgcc.a. At present, these functions are only
provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between
certain pairs of instructions. These errata are handled by
the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds
are implemented by the assembler rather than by GCC, although
GCC avoids using "mflo" and "mfhi" if the VR4130 "macc",
"macchi", "dmacc" and "dmacchi" instructions are available
instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag
currently works around the SB-1 revision 2 "F1" and "F2"
floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the
side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the
outcome of a conditional branch and speculatively executes
instructions from the "taken" branch. It later aborts these
instructions if the predicted outcome is wrong. However, on
the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the
system, kernel loads. As an example, a speculatively-
executed store may load the target memory into cache and mark
the cache line as dirty, even if the store itself is later
aborted. If a DMA operation writes to the same area of
memory before the "dirty" line is flushed, the cached data
overwrites the DMA-ed data. See the R10K processor manual
for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before
every memory access that might be speculatively executed and
that might have side effects even if aborted.
-mr10k-cache-barrier=setting controls GCC's implementation of
this workaround. It assumes that aborted accesses to any
byte in the following regions does not have side effects:
1. the memory occupied by the current function's stack
frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-
constant address.
It is the kernel's responsibility to ensure that speculative
accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal
foo" to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It expects
non-GCC functions (such as hand-written assembly code) to do
the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might
be speculatively executed and that might have side
effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be
speculatively executed and that might have side effects
even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the
default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches,
or to not call any such function. If called, the function
must take the same arguments as the common "_flush_func",
that is, the address of the memory range for which the cache
is being flushed, the size of the memory range, and the
number 3 (to flush both caches). The default depends on the
target GCC was configured for, but commonly is either
"_flush_func" or "__cpu_flush".
mbranch-cost=num
Set the cost of branches to roughly num "simple"
instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases. A
zero cost redundantly selects the default, which is based on
the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions,
regardless of the default for the selected architecture. By
default, Branch Likely instructions may be generated if they
are supported by the selected architecture. An exception is
for the MIPS32 and MIPS64 architectures and processors that
implement those architectures; for those, Branch Likely
instructions are not be generated by default because the
MIPS32 and MIPS64 architectures specifically deprecate their
use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be
generated. The default is -mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact
branch instructions will never be generated.
The -mcompact-branches=always option ensures that a compact
branch instruction will be generated if available. If a
compact branch instruction is not available, a delay slot
form of the branch will be used instead.
This option is supported from MIPS Release 6 onwards.
The -mcompact-branches=optimal option will cause a delay slot
branch to be used if one is available in the current ISA and
the delay slot is successfully filled. If the delay slot is
not filled, a compact branch will be chosen if one is
available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects
how FP instructions are scheduled for some processors. The
default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and
we are emitting 64-bit code, then we can use both FP pipes.
Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only
issue two instructions together if the first one is 8-byte
aligned. When this option is enabled, GCC aligns pairs of
instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the
VR4130. It normally makes code faster, but at the expense of
making it bigger. It is enabled by default at optimization
level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on
architectures that support it. The "synci" instructions (if
enabled) are generated when "__builtin___clear_cache" is
compiled.
This option defaults to -mno-synci, but the default can be
overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is
generally safe to use "synci". However, on many multi-core
(SMP) systems, it does not invalidate the instruction caches
on all cores and may lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via
register $25 into direct calls. This is only possible if the
linker can resolve the destination at link time and if the
destination is within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use
an assembler and a linker that support the ".reloc" assembly
directive and -mexplicit-relocs is in effect. With
-mno-explicit-relocs, this optimization can be performed by
the assembler and the linker alone without help from the
compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the
calling function's return address. When enabled, this option
extends the usual "_mcount" interface with a new ra-address
parameter, which has type "intptr_t *" and is passed in
register $12. "_mcount" can then modify the return address
by doing both of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address
is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI.
When using the o32 ABI, calling functions will allocate 16
bytes on the stack for the called function to write out
register arguments. When enabled, this optimization will
suppress the allocation of the frame header if it can be
determined that it is unused.
This optimization is off by default at all optimization
levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1",
"swxc1", "ldxc1", "sdxc1" instructions. Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand
"madd.s", "madd.d" and related instructions. Enabled by
default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled,
passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare
with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return
values that (in the called function) are seen as registers $0
and up, as opposed to the GNU ABI which uses global registers
$231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits,
use (do not use) zero-extending load instructions by default,
rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the
same sign as the divisor. With the default, -mno-knuthdiv,
the sign of the remainder follows the sign of the dividend.
Both methods are arithmetically valid, the latter being
almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the
assembly code can be used with the "PREFIX" assembly
directive.
-melf
Generate an executable in the ELF format, rather than the
default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when
static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses.
Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to
be set up in a global register. The register is used for one
or more base address requests within the range 0 to 255 from
the value held in the register. The generally leads to short
and fast code, but the number of different data items that
can be addressed is limited. This means that a program that
uses lots of static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit
point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300
architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for
the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply
instructions for the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33
processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0
processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when
scheduling instructions. This does not change the targeted
processor type. The CPU type must be one of mn10300, am33,
am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the
pointer in both "a0" and "d0". Otherwise, the pointer is
returned only in "a0", and attempts to call such functions
without a prototype result in errors. Note that this option
is on by default; use -mno-return-pointer-on-d0 to disable
it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute
memory addresses. This option only has an effect when used
on the command line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word
instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro
"__LIW__".
-mno-liw
Do not allow the compiler to generate Long Instruction Word
instructions. This option defines the preprocessor macro
"__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions
if the target is the AM33 or later. This is the default.
This option defines the preprocessor macro "__SETLB__".
-mno-setlb
Do not allow the compiler to generate SETLB or Lcc
instructions. This option defines the preprocessor macro
"__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-*
configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default
for moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally
such constants are signed decimals, but this option is
available for testsuite and/or aesthetic purposes.
-mmcu=
Select the MCU to target. This is used to create a C
preprocessor symbol based upon the MCU name, converted to
upper case and pre- and post-fixed with __. This in turn is
used by the msp430.h header file to select an MCU-specific
supplementary header file.
The option also sets the ISA to use. If the MCU name is one
that is known to only support the 430 ISA then that is
selected, otherwise the 430X ISA is selected. A generic MCU
name of msp430 can also be used to select the 430 ISA.
Similarly the generic msp430x MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the
linker command line. The script's name is the name of the
MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc
command line defines the C preprocessor symbol "__XXX__" and
cause the linker to search for a script called xxx.ld.
This option is also passed on to the assembler.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts
between the MCU name specified by the -mmcu option and the
ISA set by the -mcpu option and/or the hardware multiply
support set by the -mhwmult option. It also toggles warnings
about unrecognized MCU names. This option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430,
msp430x and msp430xv2. This option is deprecated. The
-mmcu= option should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script.
Overrides any scripts that would be selected by the -mmcu=
option.
-mlarge
Use large-model addressing (20-bit pointers, 32-bit
"size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit
"size_t").
-mrelax
This option is passed to the assembler and linker, and allows
the linker to perform certain optimizations that cannot be
done until the final link.
mhwmult=
Describes the type of hardware multiply supported by the
target. Accepted values are none for no hardware multiply,
16bit for the original 16-bit-only multiply supported by
early MCUs. 32bit for the 16/32-bit multiply supported by
later MCUs and f5series for the 16/32-bit multiply supported
by F5-series MCUs. A value of auto can also be given. This
tells GCC to deduce the hardware multiply support based upon
the MCU name provided by the -mmcu option. If no -mmcu
option is specified or if the MCU name is not recognized then
no hardware multiply support is assumed. "auto" is the
default setting.
Hardware multiplies are normally performed by calling a
library routine. This saves space in the generated code.
When compiling at -O3 or higher however the hardware
multiplier is invoked inline. This makes for bigger, but
faster code.
The hardware multiply routines disable interrupts whilst
running and restore the previous interrupt state when they
finish. This makes them safe to use inside interrupt
handlers as well as in normal code.
-minrt
Enable the use of a minimum runtime environment - no static
initializers or constructors. This is intended for memory-
constrained devices. The compiler includes special symbols
in some objects that tell the linker and runtime which code
fragments are required.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and
data that do not have one of the "lower", "upper", "either"
or "section" attributes. Possible values are "lower",
"upper", "either" or "any". The first three behave like the
corresponding attribute. The fourth possible value - "any" -
is the default. It leaves placement entirely up to the
linker script and how it assigns the standard sections
(".text", ".data", etc) to the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the
fixes for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable
warning messages when a silicon errata might need to be
applied.
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or
16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power
of 2 between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within
512KB addressing space. The text segment must be within
16MB addressing space.
medium
The data segment must be within 512KB while the read-only
data segment can be within 4GB addressing space. The
text segment should be still within 16MB addressing
space.
large
All the text and data segments can be within 4GB
addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
-G num
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal
data or BSS sections. The default value of num is 8.
-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The
following option names are recognized:
none
Do not generate GP-relative accesses.
local
Generate GP-relative accesses for small data objects that
are not external, weak, or uninitialized common symbols.
Also use GP-relative addressing for objects that have
been explicitly placed in a small data section via a
"section" attribute.
global
As for local, but also generate GP-relative accesses for
small data objects that are external, weak, or common.
If you use this option, you must ensure that all parts of
your program (including libraries) are compiled with the
same -G setting.
data
Generate GP-relative accesses for all data objects in the
program. If you use this option, the entire data and BSS
segments of your program must fit in 64K of memory and
you must use an appropriate linker script to allocate
them within the addressable range of the global pointer.
all Generate GP-relative addresses for function pointers as
well as data pointers. If you use this option, the
entire text, data, and BSS segments of your program must
fit in 64K of memory and you must use an appropriate
linker script to allocate them within the addressable
range of the global pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is
equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is
specified to generate position-independent code. Note that
the Nios II ABI does not permit GP-relative accesses from
shared libraries.
You may need to specify -mno-gpopt explicitly when building
programs that include large amounts of small data, including
large GOT data sections. In this case, the 16-bit offset for
GP-relative addressing may not be large enough to allow
access to the entire small data section.
-mgprel-sec=regexp
This option specifies additional section names that can be
accessed via GP-relative addressing. It is most useful in
conjunction with "section" attributes on variable
declarations and a custom linker script. The regexp is a
POSIX Extended Regular Expression.
This option does not affect the behavior of the -G option,
and the specified sections are in addition to the standard
".sdata" and ".sbss" small-data sections that are recognized
by -mgpopt.
-mr0rel-sec=regexp
This option specifies names of sections that can be accessed
via a 16-bit offset from "r0"; that is, in the low 32K or
high 32K of the 32-bit address space. It is most useful in
conjunction with "section" attributes on variable
declarations and a custom linker script. The regexp is a
POSIX Extended Regular Expression.
In contrast to the use of GP-relative addressing for small
data, zero-based addressing is never generated by default and
there are no conventional section names used in standard
linker scripts for sections in the low or high areas of
memory.
-mel
-meb
Generate little-endian (default) or big-endian (experimental)
code, respectively.
-march=arch
This specifies the name of the target Nios II architecture.
GCC uses this name to determine what kind of instructions it
can emit when generating assembly code. Permissible names
are: r1, r2.
The preprocessor macro "__nios2_arch__" is available to
programs, with value 1 or 2, indicating the targeted ISA
level.
-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache
by using I/O variants of the instructions. The default is not
to bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O
variants of the load and store instructions. The default is
not to bypass the cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The
default is to use the fast divide at -O3 and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div" family of
instructions by the compiler. The default is to emit "mul"
and not emit "div" and "mulx".
-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit
manipulation) and CDX (code density) instructions. Enabling
these instructions also requires -march=r2. Since these
instructions are optional extensions to the R2 architecture,
the default is not to emit them.
-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom
instruction with encoding N when generating code that uses
insn. For example, -mcustom-fadds=253 generates custom
instruction 253 for single-precision floating-point add
operations instead of the default behavior of using a library
call.
The following values of insn are supported. Except as
otherwise noted, floating-point operations are expected to be
implemented with normal IEEE 754 semantics and correspond
directly to the C operators or the equivalent GCC built-in
functions.
Single-precision floating point:
fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.
fnegs
Unary negation.
fabss
Unary absolute value.
fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.
fmins, fmaxs
Floating-point minimum and maximum. These instructions
are only generated if -ffinite-math-only is specified.
fsqrts
Unary square root operation.
fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Double-precision floating point:
faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.
fnegd
Unary negation.
fabsd
Unary absolute value.
fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.
fmind, fmaxd
Double-precision minimum and maximum. These instructions
are only generated if -ffinite-math-only is specified.
fsqrtd
Unary square root operation.
fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Conversions:
fextsd
Conversion from single precision to double precision.
ftruncds
Conversion from double precision to single precision.
fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned
integer types, with truncation towards zero.
round
Conversion from single-precision floating point to signed
integer, rounding to the nearest integer and ties away
from zero. This corresponds to the "__builtin_lroundf"
function when -fno-math-errno is used.
floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to
floating-point types.
In addition, all of the following transfer instructions for
internal registers X and Y must be provided to use any of the
double-precision floating-point instructions. Custom
instructions taking two double-precision source operands
expect the first operand in the 64-bit register X. The other
operand (or only operand of a unary operation) is given to
the custom arithmetic instruction with the least significant
half in source register src1 and the most significant half in
src2. A custom instruction that returns a double-precision
result returns the most significant 32 bits in the
destination register and the other half in 32-bit register Y.
GCC automatically generates the necessary code sequences to
write register X and/or read register Y when double-precision
floating-point instructions are used.
fwrx
Write src1 into the least significant half of X and src2
into the most significant half of X.
fwry
Write src1 into Y.
frdxhi, frdxlo
Read the most or least (respectively) significant half of
X and store it in dest.
frdy
Read the value of Y and store it into dest.
Note that you can gain more local control over generation of
Nios II custom instructions by using the
"target("custom-insn=N")" and "target("no-custom-insn")"
function attributes or pragmas.
-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom
instruction encodings (see -mcustom-insn above). Currently,
the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254
-fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250
-mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
Custom instruction assignments given by individual
-mcustom-insn= options override those given by
-mcustom-fpu-cfg=, regardless of the order of the options on
the command line.
Note that you can gain more local control over selection of a
FPU configuration by using the
"target("custom-fpu-cfg=name")" function attribute or pragma.
These additional -m options are available for the Altera Nios II
ELF (bare-metal) target:
-mhal
Link with HAL BSP. This suppresses linking with the GCC-
provided C runtime startup and termination code, and is
typically used in conjunction with -msys-crt0= to specify the
location of the alternate startup code provided by the HAL
BSP.
-msmallc
Link with a limited version of the C library, -lsmallc,
rather than Newlib.
-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to use
when linking. This option is only useful in conjunction with
-mhal.
-msys-lib=systemlib
systemlib is the library name of the library that provides
low-level system calls required by the C library, e.g. "read"
and "write". This option is typically used to link with a
library provided by a HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m32
-m64
Generate code for 32-bit or 64-bit ABI.
-misa=ISA-string
Generate code for given the specified PTX ISA (e.g. sm_35).
ISA strings must be lower-case. Valid ISA strings include
sm_30 and sm_35. The default ISA is sm_30.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone
instead of offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the
default when any level of optimization is selected.
-msoft-stack
Generate code that does not use ".local" memory directly for
stack storage. Instead, a per-warp stack pointer is
maintained explicitly. This enables variable-length stack
allocation (with variable-length arrays or "alloca"), and
when global memory is used for underlying storage, makes it
possible to access automatic variables from other threads, or
with atomic instructions. This code generation variant is
used for OpenMP offloading, but the option is exposed on its
own for the purpose of testing the compiler; to generate code
suitable for linking into programs using OpenMP offloading,
use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all
threads in each warp, while maintaining memory state and side
effects as if only one thread in each warp was active outside
of OpenMP SIMD regions. All atomic operations and calls to
runtime (malloc, free, vprintf) are conditionally executed
(iff current lane index equals the master lane index), and
the register being assigned is copied via a shuffle
instruction from the master lane. Outside of SIMD regions
lane 0 is the master; inside, each thread sees itself as the
master. Shared memory array "int __nvptx_uni[]" stores all-
zeros or all-ones bitmasks for each warp, indicating current
mode (0 outside of SIMD regions). Each thread can bitwise-
and the bitmask at position "tid.y" with current lane index
to compute the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables
-msoft-stack and -muniform-simt options, and selects
corresponding multilib variant.
OpenRISC Options
These options are defined for OpenRISC:
-mboard=name
Configure a board specific runtime. This will be passed to
the linker for newlib board library linking. The default is
"or1ksim".
-mnewlib
For compatibility, it's always newlib for elf now.
-mhard-div
Generate code for hardware which supports divide
instructions. This is the default.
-mhard-mul
Generate code for hardware which supports multiply
instructions. This is the default.
-mcmov
Generate code for hardware which supports the conditional
move ("l.cmov") instruction.
-mror
Generate code for hardware which supports rotate right
instructions.
-msext
Generate code for hardware which supports sign-extension
instructions.
-msfimm
Generate code for hardware which supports set flag immediate
("l.sf*i") instructions.
-mshftimm
Generate code for hardware which supports shift immediate
related instructions (i.e. "l.srai", "l.srli", "l.slli",
"1.rori"). Note, to enable generation of the "l.rori"
instruction the -mror flag must also be specified.
-msoft-div
Generate code for hardware which requires divide instruction
emulation.
-msoft-mul
Generate code for hardware which requires multiply
instruction emulation.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS
floating point on the PDP-11/40 is not supported.) Implies
-m45.
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler
syntax).
-mno-ac0
Return floating-point results in memory. This is the
default.
-m40
Generate code for a PDP-11/40. Implies -msoft-float
-mno-split.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10. Implies -msoft-float
-mno-split.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-msplit
Target has split instruction and data space. Implies -m45.
-munix-asm
Use Unix assembler syntax.
-mdec-asm
Use DEC assembler syntax.
-mgnu-asm
Use GNU assembler syntax. This is the default.
-mlra
Use the new LRA register allocator. By default, the old
"reload" allocator is used.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction
scheduling parameters for array element type ae_type.
Supported values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code
generated with this option runs on any of the other AE types.
The code is not as efficient as it would be if compiled for a
specific AE type, and some types of operation (e.g.,
multiplication) do not work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE
type for compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this
option may suffer from poor performance of byte (char)
manipulation, since the DSP AE does not provide hardware
support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an
address in a load/store instruction, without first loading it
into a register. Typically, the use of this option generates
larger programs, which run faster than when the option isn't
used. However, the results vary from program to program, so
it is left as a user option, rather than being permanently
enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code.
These warnings can be generated, for example, when compiling
code that performs byte-level memory operations on the MAC AE
type. The MAC AE has no hardware support for byte-level
memory operations, so all byte load/stores must be
synthesized from word load/store operations. This is
inefficient and a warning is generated to indicate that you
should rewrite the code to avoid byte operations, or to
target an AE type that has the necessary hardware support.
This option disables these warnings.
PowerPC Options
These are listed under
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs.
Ignored for non-PIC. The default is -mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-
string contains two parts: the size of integer types and the
registers used for floating-point types. For example
-march=rv64ifd -mabi=lp64d means that long and pointers are
64-bit (implicitly defining int to be 32-bit), and that
floating-point values up to 64 bits wide are passed in F
registers. Contrast this with -march=rv64ifd -mabi=lp64f,
which still allows the compiler to generate code that uses
the F and D extensions but only allows floating-point values
up to 32 bits long to be passed in registers; or
-march=rv64ifd -mabi=lp64, in which no floating-point
arguments will be passed in registers.
The default for this argument is system dependent, users who
want a specific calling convention should specify one
explicitly. The valid calling conventions are: ilp32,
ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling
conventions are impossible to implement on some ISAs: for
example, -march=rv32if -mabi=ilp32d is invalid because the
ABI requires 64-bit values be passed in F registers, but F
registers are only 32 bits wide. There is also the ilp32e
ABI that can only be used with the rv32e architecture. This
ABI is not well specified at present, and is subject to
change.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square
root instructions. This requires the F or D extensions for
floating-point registers. The default is to use them if the
specified architecture has these instructions.
-mdiv
-mno-div
Do or don't use hardware instructions for integer division.
This requires the M extension. The default is to use them if
the specified architecture has these instructions.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA
strings must be lower-case. Examples include rv64i, rv32g,
rv32e, and rv32imaf.
-mtune=processor-string
Optimize the output for the given processor, specified by
microarchitecture name. Permissible values for this option
are: rocket, sifive-3-series, sifive-5-series,
sifive-7-series, and size.
When -mtune= is not specified, the default is rocket.
The size choice is not intended for use by end-users. This
is used when -Os is specified. It overrides the instruction
cost info provided by -mtune=, but does not override the
pipeline info. This helps reduce code size while still
giving good performance.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to
num byte boundary. If -mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128-bits).
Warning: If you use this switch, then you must build all
modules with the same value, including any libraries. This
includes the system libraries and startup modules.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a
special section (on some targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code
that uses library function calls. The default is to use fast
inline prologues and epilogues.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default
is set depending on whether the processor we are optimizing
for supports fast unaligned access or not.
-mcmodel=medlow
Generate code for the medium-low code model. The program and
its statically defined symbols must lie within a single 2 GiB
address range and must lie between absolute addresses -2 GiB
and +2 GiB. Programs can be statically or dynamically linked.
This is the default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and
its statically defined symbols must be within any single 2
GiB address range. Programs can be statically or dynamically
linked.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing
with symbolic addresses. The alternative is to use assembler
macros instead, which may limit optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of
instructions required to materialize symbol addresses. The
default is to take advantage of linker relaxations.
-memit-attribute
-mno-emit-attribute
Emit (do not emit) RISC-V attribute to record extra
information into ELF objects. This feature requires at least
binutils 2.32.
RL78 Options
-msim
Links in additional target libraries to support operation
within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division
support to be used. The simplest is "none", which uses
software for both multiplication and division. This is the
default. The "g13" value is for the hardware multiply/divide
peripheral found on the RL78/G13 (S2 core) targets. The
"g14" value selects the use of the multiplication and
division instructions supported by the RL78/G14 (S3 core)
parts. The value "rl78" is an alias for "g14" and the value
"mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the
setting of this option. Possible values are:
"__RL78_MUL_NONE__", "__RL78_MUL_G13__" or
"__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14
core, also known as an S3 core or just RL78. The G13 or S2
core does not have multiply or divide instructions, instead
it uses a hardware peripheral for these operations. The G10
or S1 core does not have register banks, so it uses a
different calling convention.
If this option is set it also selects the type of hardware
multiply support to use, unless this is overridden by an
explicit -mmul=none option on the command line. Thus
specifying -mcpu=g13 enables the use of the G13 hardware
multiply peripheral and specifying -mcpu=g10 disables the use
of hardware multiplications altogether.
Note, although the RL78/G14 core is the default target,
specifying -mcpu=g14 or -mcpu=rl78 on the command line does
change the behavior of the toolchain since it also enables
G14 hardware multiply support. If these options are not
specified on the command line then software multiplication
routines will be used even though the code targets the RL78
core. This is for backwards compatibility with older
toolchains which did not have hardware multiply and divide
support.
In addition a C preprocessor macro is defined, based upon the
setting of this option. Possible values are: "__RL78_G10__",
"__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They
are provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By
default registers "r24..r31" are reserved for use in
interrupt handlers. With this option enabled these registers
can be used in ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or
32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve
the MDUC registers. This is only necessary if normal code
might use the MDUC registers, for example because it performs
multiplication and division operations. The default is to
ignore the MDUC registers as this makes the interrupt
handlers faster. The target option -mg13 needs to be passed
for this to work as this feature is only available on the G13
target (S2 core). The MDUC registers will only be saved if
the interrupt handler performs a multiplication or division
operation or it calls another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are
available on the processor you are using. The default value
of these options is determined when configuring GCC.
Specifying the -mcpu=cpu_type overrides the specification of
these options. We recommend you use the -mcpu=cpu_type
option rather than the options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional
PowerPC architecture instructions in the General Purpose
group, including floating-point square root. Specifying
-mpowerpc-gfxopt allows GCC to use the optional PowerPC
architecture instructions in the Graphics group, including
floating-point select.
The -mmfcrf option allows GCC to generate the move from
condition register field instruction implemented on the
POWER4 processor and other processors that support the
PowerPC V2.01 architecture. The -mpopcntb option allows GCC
to generate the popcount and double-precision FP reciprocal
estimate instruction implemented on the POWER5 processor and
other processors that support the PowerPC V2.02 architecture.
The -mpopcntd option allows GCC to generate the popcount
instruction implemented on the POWER7 processor and other
processors that support the PowerPC V2.06 architecture. The
-mfprnd option allows GCC to generate the FP round to integer
instructions implemented on the POWER5+ processor and other
processors that support the PowerPC V2.03 architecture. The
-mcmpb option allows GCC to generate the compare bytes
instruction implemented on the POWER6 processor and other
processors that support the PowerPC V2.05 architecture. The
-mmfpgpr option allows GCC to generate the FP move to/from
general-purpose register instructions implemented on the
POWER6X processor and other processors that support the
extended PowerPC V2.05 architecture. The -mhard-dfp option
allows GCC to generate the decimal floating-point
instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional
64-bit instructions that are found in the full PowerPC64
architecture and to treat GPRs as 64-bit, doubleword
quantities. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction
scheduling parameters for machine type cpu_type. Supported
values for cpu_type are 401, 403, 405, 405fp, 440, 440fp,
464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e,
620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970,
8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500,
ec603e, G3, G4, G5, titan, power3, power4, power5, power5+,
power6, power6x, power7, power8, power9, powerpc, powerpc64,
powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify
pure 32-bit PowerPC (either endian), 64-bit big endian
PowerPC and 64-bit little endian PowerPC architecture machine
types, with an appropriate, generic processor model assumed
for scheduling purposes.
Specifying native as cpu type detects and selects the
architecture option that corresponds to the host processor of
the system performing the compilation. -mcpu=native has no
effect if GCC does not recognize the processor.
The other options specify a specific processor. Code
generated under those options runs best on that processor,
and may not run at all on others.
The -mcpu options automatically enable or disable the
following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple
-mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt
-mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto
-mhtm -mpower8-fusion -mpower8-vector -mquad-memory
-mquad-memory-atomic -mfloat128 -mfloat128-hardware
The particular options set for any particular CPU varies
between compiler versions, depending on what setting seems to
produce optimal code for that CPU; it doesn't necessarily
reflect the actual hardware's capabilities. If you wish to
set an individual option to a particular value, you may
specify it after the -mcpu option, like -mcpu=970
-mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled
or disabled by the -mcpu option at present because AIX does
not have full support for these options. You may still
enable or disable them individually if you're sure it'll work
in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type or register
usage, as -mcpu=cpu_type does. The same values for cpu_type
are used for -mtune as for -mcpu. If both are specified, the
code generated uses the architecture and registers set by
-mcpu, but the scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is
limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and
other static data may be up to a total of 4G in size. This
is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be
up to 4G in size. Other data and code is only limited by the
64-bit address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions,
and also enable the use of built-in functions that allow more
direct access to the AltiVec instruction set. You may also
need to set -mabi=altivec to adjust the current ABI with
AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec
intrinsics such as "vec_splat", "vec_extract", and
"vec_insert" match array element order corresponding to the
endianness of the target. That is, element zero identifies
the leftmost element in a vector register when targeting a
big-endian platform, and identifies the rightmost element in
a vector register when targeting a little-endian platform.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables
and shared libraries with non-executable ".plt" and ".got"
sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills
in, and requires ".plt" and ".got" sections that are both
writable and executable. This is a PowerPC 32-bit SYSV ABI
option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL
instructions.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions
that allow more direct access to the VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow
direct access to the cryptographic instructions that were
added in version 2.07 of the PowerPC ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow
direct access to the Hardware Transactional Memory (HTM)
instructions that were added in version 2.07 of the PowerPC
ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer
operations adjacent so that the instructions can be fused
together on power8 and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar
instructions that were added in version 2.07 of the PowerPC
ISA. Also enable the use of built-in functions that allow
more direct access to the vector instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad
word memory instructions. The -mquad-memory option requires
use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word
memory instructions. The -mquad-memory-atomic option
requires use of 64-bit mode.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit
floating point and use either software emulation for IEEE
128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx, -mcpu=power7, -mcpu=power8),
or -mcpu=power9 must be enabled to use the IEEE 128-bit
floating point support. The IEEE 128-bit floating point
support only works on PowerPC Linux systems.
The default for -mfloat128 is enabled on PowerPC Linux
systems using the VSX instruction set, and disabled on other
systems.
If you use the ISA 3.0 instruction set (-mpower9-vector or
-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating
point support will also enable the generation of ISA 3.0 IEEE
128-bit floating point instructions. Otherwise, if you do
not specify to generate ISA 3.0 instructions or you are
targeting a 32-bit big endian system, IEEE 128-bit floating
point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support
the __float128 data type.
The default for -mfloat128-hardware is enabled on PowerPC
Linux systems using the ISA 3.0 instruction set, and disabled
on other systems.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and
SVR4 targets (including GNU/Linux). The 32-bit environment
sets int, long and pointer to 32 bits and generates code that
runs on any PowerPC variant. The 64-bit environment sets int
to 32 bits and long and pointer to 64 bits, and generates
code for PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is
created for every executable file. The -mfull-toc option is
selected by default. In that case, GCC allocates at least
one TOC entry for each unique non-automatic variable
reference in your program. GCC also places floating-point
constants in the TOC. However, only 16,384 entries are
available in the TOC.
If you receive a linker error message that saying you have
overflowed the available TOC space, you can reduce the amount
of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc
options. -mno-fp-in-toc prevents GCC from putting floating-
point constants in the TOC and -mno-sum-in-toc forces GCC to
generate code to calculate the sum of an address and a
constant at run time instead of putting that sum into the
TOC. You may specify one or both of these options. Each
causes GCC to produce very slightly slower and larger code at
the expense of conserving TOC space.
If you still run out of space in the TOC even when you
specify both of these options, specify -mminimal-toc instead.
This option causes GCC to make only one TOC entry for every
file. When you specify this option, GCC produces code that
is slower and larger but which uses extremely little TOC
space. You may wish to use this option only on files that
contain less frequently-executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit
pointers, 64-bit "long" type, and the infrastructure needed
to support them. Specifying -maix64 implies -mpowerpc64,
while -maix32 disables the 64-bit ABI and implies
-mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler
semantics when using AIX-compatible ABI. Pass floating-point
arguments to prototyped functions beyond the register save
area (RSA) on the stack in addition to argument FPRs. Do not
assume that most significant double in 128-bit long double
value is properly rounded when comparing values and
converting to double. Use XL symbol names for long double
support routines.
The AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a
function that takes the address of its arguments with fewer
arguments than declared. IBM XL compilers access floating-
point arguments that do not fit in the RSA from the stack
when a subroutine is compiled without optimization. Because
always storing floating-point arguments on the stack is
inefficient and rarely needed, this option is not enabled by
default and only is necessary when calling subroutines
compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special
startup code to enable the application to run. The system
must have PE installed in the standard location
(/usr/lpp/ppe.poe/), or the specs file must be overridden
with the -specs= option to specify the appropriate directory
location. The Parallel Environment does not support threads,
so the -mpe option and the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the
option -malign-natural overrides the ABI-defined alignment of
larger types, such as floating-point doubles, on their
natural size-based boundary. The option -malign-power
instructs GCC to follow the ABI-specified alignment rules.
GCC defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and
-malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point
register set. Software floating-point emulation is provided
if you use the -msoft-float option, and pass the option to
GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and
not generated on PowerPC systems. Do not use -mmultiple on
little-endian PowerPC systems, since those instructions do
not work when the processor is in little-endian mode. The
exceptions are PPC740 and PPC750 which permit these
instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the address of
the calculated memory location. These instructions are
generated by default. If you use -mno-update, there is a
small window between the time that the stack pointer is
updated and the address of the previous frame is stored,
which means code that walks the stack frame across interrupts
or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of
indexed load or store instructions. These instructions can
incur a performance penalty on Power6 processors in certain
situations, such as when stepping through large arrays that
cross a 16M boundary. This option is enabled by default when
targeting Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The
machine-dependent -mfused-madd option is now mapped to the
machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply
and multiply-accumulate instructions on the IBM 405, 440, 464
and 476 processors. These instructions are generated by
default when targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search
dlmzb instruction on the IBM 405, 440, 464 and 476
processors. This instruction is generated by default when
targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned
to the base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte
boundary and has a size of 4 bytes. By using -mno-bit-align,
the structure is aligned to a 1-byte boundary and is 1 byte
in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume
that unaligned memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static
executable to be relocated to a different address at run
time. A simple embedded PowerPC system loader should
relocate the entire contents of ".got2" and 4-byte locations
listed in the ".fixup" section, a table of 32-bit addresses
generated by this option. For this to work, all objects
linked together must be compiled with -mrelocatable or
-mrelocatable-lib. -mrelocatable code aligns the stack to an
8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup"
section to allow static executables to be relocated at run
time, but -mrelocatable-lib does not use the smaller stack
alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with
any combination of the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume
that register 2 contains a pointer to a global area pointing
to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for
the processor in little-endian mode. The -mlittle-endian
option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for
the processor in big-endian mode. The -mbig-endian option is
the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is
not relocatable, but that its external references are
relocatable. The resulting code is suitable for
applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only,
rather than loading it in the prologue for each function.
The runtime system is responsible for initializing this
register with an appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to
dispatch-slot restricted instructions during the second
scheduling pass. The argument priority takes the value 0, 1,
or 2 to assign no, highest, or second-highest (respectively)
priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly
by the target during instruction scheduling. The argument
dependence_type takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or
equal to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used
during the second scheduling pass. The argument scheme takes
one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue
slots, according to the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate
groups. Insert exactly as many NOPs as needed to force
an insn to a new group, according to the estimated
processor grouping.
number
Insert NOPs to force costly dependent insns into separate
groups. Insert number NOPs to force an insn to a new
group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using
calling conventions that adhere to the March 1995 draft of
the System V Application Binary Interface, PowerPC processor
supplement. This is the default unless you configured GCC
using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for
the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for
the Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for
the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for
the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for
the OpenBSD operating system.
-mtraceback=traceback_type
Select the type of traceback table. Valid values for
traceback_type are full, part, and no.
-maix-struct-return
Return all structures in memory (as specified by the AIX
ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as
specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove
such extension. Valid values are altivec, no-altivec,
ibmlongdouble, ieeelongdouble, elfv1, elfv2.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long
double. This is not likely to work if your system defaults
to using IEEE extended-precision long double. If you change
the long double type from IEEE extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi
option. Requires -mlong-double-128 to be enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long
double. This is not likely to work if your system defaults
to using IBM extended-precision long double. If you change
the long double type from IBM extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi
option. Requires -mlong-double-128 to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the
default ABI for big-endian PowerPC 64-bit Linux. Overriding
the default ABI requires special system support and is likely
to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the
default ABI for little-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support
and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value
pairs in a .gnu.attributes section that specify ABI
variations in function parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all
calls to variable argument functions are properly prototyped.
Otherwise, the compiler must insert an instruction before
every non-prototyped call to set or clear bit 6 of the
condition code register ("CR") to indicate whether floating-
point values are passed in the floating-point registers in
case the function takes variable arguments. With
-mprototype, only calls to prototyped variable argument
functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module
is called sim-crt0.o and that the standard C libraries are
libsim.a and libc.a. This is the default for
powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libmvme.a
and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libads.a
and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module
is called crt0.o and the standard C libraries are libyk.a and
libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you
are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF
flags header to indicate that eabi extended relocations are
used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere
to the Embedded Applications Binary Interface (EABI), which
is a set of modifications to the System V.4 specifications.
Selecting -meabi means that the stack is aligned to an 8-byte
boundary, a function "__eabi" is called from "main" to set up
the EABI environment, and the -msdata option can use both
"r2" and "r13" to point to two separate small data areas.
Selecting -mno-eabi means that the stack is aligned to a
16-byte boundary, no EABI initialization function is called
from "main", and the -msdata option only uses "r13" to point
to a single small data area. The -meabi option is on by
default if you configured GCC using one of the
powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small
initialized "const" global and static data in the ".sdata2"
section, which is pointed to by register "r2". Put small
initialized non-"const" global and static data in the
".sdata" section, which is pointed to by register "r13". Put
small uninitialized global and static data in the ".sbss"
section, which is adjacent to the ".sdata" section. The
-msdata=eabi option is incompatible with the -mrelocatable
option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global
and static data in the ".sdata" section, which is pointed to
by register "r13". Put small uninitialized global and static
data in the ".sbss" section, which is adjacent to the
".sdata" section. The -msdata=sysv option is incompatible
with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is
used, compile code the same as -msdata=eabi, otherwise
compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global
data in the ".sdata" section. Put small uninitialized global
data in the ".sbss" section. Do not use register "r13" to
address small data however. This is the default behavior
unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and
static data in the ".data" section, and all uninitialized
data in the ".bss" section.
-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This
is the default.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or
structure copies) less than or equal to num bytes. The
minimum value for num is 32 bytes on 32-bit targets and 64
bytes on 64-bit targets. The default value is target-
specific.
-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such
as calls to "memcmp" or structure compares) less than or
equal to num bytes. If num is 0, all inline expansion (non-
loop and loop) of block compare is disabled. The default
value is target-specific.
-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block
compares that are less than or equal to num bytes, but
greater than the limit for non-loop inline block compare
expansion. If the block length is not constant, at most num
bytes will be compared before "memcmp" is called to compare
the remainder of the block. The default value is target-
specific.
-mstring-compare-inline-limit=num
Compare at most num string bytes with inline code. If the
difference or end of string is not found at the end of the
inline compare a call to "strcmp" or "strncmp" will take care
of the rest of the comparison. The default is 64 bytes.
-G num
On embedded PowerPC systems, put global and static items less
than or equal to num bytes into the small data or BSS
sections instead of the normal data or BSS section. By
default, num is 8. The -G num switch is also passed to the
linker. All modules should be compiled with the same -G num
value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic
forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a
longer and more expensive calling sequence is required. This
is required for calls farther than 32 megabytes (33,554,432
bytes) from the current location. A short call is generated
if the compiler knows the call cannot be that far away. This
setting can be overridden by the "shortcall" function
attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long
calls are unnecessary and generate slower code. As of this
writing, the AIX linker can do this, as can the GNU linker
for PowerPC/64. It is planned to add this feature to the GNU
linker for 32-bit PowerPC systems as well.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU
linkers, GCC can generate long calls using an inline PLT call
sequence (see -mpltseq). PowerPC with -mbss-plt and
PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr
callee, L42", plus a branch island (glue code). The two
target addresses represent the callee and the branch island.
The Darwin/PPC linker prefers the first address and generates
a "bl callee" if the PPC "bl" instruction reaches the callee
directly; otherwise, the linker generates "bl L42" to call
the branch island. The branch island is appended to the body
of the calling function; it computes the full 32-bit address
of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler
emit to the glue for every direct call, and the Darwin linker
decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications
when the linker is known to generate glue.
-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an
inline PLT call sequence that supports lazy linking and long
calls to functions in dlopen'd shared libraries. Inline PLT
calls are only supported on PowerPC64 ELFv2 and 32-bit
PowerPC systems with newer GNU linkers, and are enabled by
default if the support is detected when configuring GCC, and,
in the case of 32-bit PowerPC, if GCC is configured with
--enable-secureplt. -mpltseq code and -mbss-plt 32-bit
PowerPC relocatable objects may not be linked together.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a
relocation specifying the function argument. The relocation
allows the linker to reliably associate function call with
argument setup instructions for TLS optimization, which in
turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and
reciprocal square root estimate instructions with additional
Newton-Raphson steps to increase precision instead of doing a
divide or square root and divide for floating-point
arguments. You should use the -ffast-math option when using
-mrecip (or at least -funsafe-math-optimizations,
-ffinite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the
sequence is generally higher than the throughput of the non-
reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals
0.99999994) for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which
may be preceded by a "!" to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to
-mno-recip.
div Enable the reciprocal approximation instructions for both
single and double precision.
divf
Enable the single-precision reciprocal approximation
instructions.
divd
Enable the double-precision reciprocal approximation
instructions.
rsqrt
Enable the reciprocal square root approximation
instructions for both single and double precision.
rsqrtf
Enable the single-precision reciprocal square root
approximation instructions.
rsqrtd
Enable the double-precision reciprocal square root
approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the
reciprocal estimate instructions, except for the "FRSQRTE",
"XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the
double-precision reciprocal square root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate
instructions provide higher-precision estimates than is
mandated by the PowerPC ABI. Selecting -mcpu=power6,
-mcpu=power7 or -mcpu=power8 automatically selects
-mrecip-precision. The double-precision square root estimate
instructions are not generated by default on low-precision
machines, since they do not provide an estimate that
converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics
using an external library. The only type supported at
present is mass, which specifies to use IBM's Mathematical
Acceleration Subsystem (MASS) libraries for vectorizing
intrinsics using external libraries. GCC currently emits
calls to "acosd2", "acosf4", "acoshd2", "acoshf4", "asind2",
"asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4",
"atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4",
"cosd2", "cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4",
"erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4",
"expm1d2", "expm1f4", "hypotd2", "hypotf4", "lgammad2",
"lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4",
"sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4",
"tand2", "tanf4", "tanhd2", and "tanhf4" when generating code
for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS
libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize
rounding of floating-point values to 64-bit integer and back
to floating point. The "friz" instruction does not return
the same value if the floating-point number is too large to
fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain
register ("r11") when calling through a pointer on AIX and
64-bit Linux systems where a function pointer points to a
3-word descriptor giving the function address, TOC value to
be loaded in register "r2", and static chain value to be
loaded in register "r11". The -mpointers-to-nested-functions
is on by default. You cannot call through pointers to nested
functions or pointers to functions compiled in other
languages that use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the
reserved stack location in the function prologue if the
function calls through a pointer on AIX and 64-bit Linux
systems. If the TOC value is not saved in the prologue, it
is saved just before the call through the pointer. The
-mno-save-toc-indirect option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters
with a maximum alignment of 64 bits, for compatibility with
older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not
align a structure parameter on a 128-bit boundary when that
structure contained a member requiring 128-bit alignment.
This is corrected in more recent versions of GCC. This
option may be used to generate code that is compatible with
functions compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard.
Supported locations are global for global canary or tls for
per-thread canary in the TLS block (the default with GNU libc
version 2.4 or later).
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify
which register to use as base register for reading the
canary, and from what offset from that base register. The
default for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset
with a symbol reference to a canary in the TLS block.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or
32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX floating-point hardware only works
on 32-bit values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-
point hardware. The default is enabled for the RX600 series
and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit
floating-point values, however, so the FPU hardware is not
used for doubles if the -m64bit-doubles option is used.
Note If the -fpu option is enabled then
-funsafe-math-optimizations is also enabled automatically.
This is because the RX FPU instructions are themselves
unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three
types are supported, the generic RX600 and RX200 series
hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610
does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit
and so -nofpu is enabled by default when this type is
selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The
default is -mlittle-endian-data, i.e. to store data in the
little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static
variables which can be placed into the small data area.
Using the small data area can lead to smaller and faster
code, but the size of area is limited and it is up to the
programmer to ensure that the area does not overflow. Also
when the small data area is used one of the RX's registers
(usually "r13") is reserved for use pointing to this area, so
it is no longer available for use by the compiler. This
could result in slower and/or larger code if variables are
pushed onto the stack instead of being held in this register.
Note, common variables (variables that have not been
initialized) and constants are not placed into the small data
area as they are assigned to other sections in the output
executable.
The default value is zero, which disables this feature.
Note, this feature is not enabled by default with higher
optimization levels (-O2 etc) because of the potentially
detrimental effects of reserving a register. It is up to the
programmer to experiment and discover whether this feature is
of benefit to their program. See the description of the
-mpid option for a description of how the actual register to
hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the
libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is
compatible with Renesas's AS100 assembler. This syntax can
also be handled by the GAS assembler, but it has some
restrictions so it is not generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can
be used as an operand in a RX instruction. Although the RX
instruction set does allow constants of up to 4 bytes in
length to be used in instructions, a longer value equates to
a longer instruction. Thus in some circumstances it can be
beneficial to restrict the size of constants that are used in
instructions. Constants that are too big are instead placed
into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the
default) or 4 means that constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process
whereby the linker attempts to reduce the size of a program
by finding shorter versions of various instructions.
Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt
handler functions. The value N can be between 0 and 4. A
value of 1 means that register "r13" is reserved for the
exclusive use of fast interrupt handlers. A value of 2
reserves "r13" and "r12". A value of 3 reserves "r13", "r12"
and "r11", and a value of 4 reserves "r13" through "r10". A
value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve
the accumulator register. This is only necessary if normal
code might use the accumulator register, for example because
it performs 64-bit multiplications. The default is to ignore
the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When
enabled any access to constant data is done via an offset
from a base address held in a register. This allows the
location of constant data to be determined at run time
without requiring the executable to be relocated, which is a
benefit to embedded applications with tight memory
constraints. Data that can be modified is not affected by
this option.
Note, using this feature reserves a register, usually "r13",
for the constant data base address. This can result in
slower and/or larger code, especially in complicated
functions.
The actual register chosen to hold the constant data base
address depends upon whether the -msmall-data-limit and/or
the -mint-register command-line options are enabled.
Starting with register "r13" and proceeding downwards,
registers are allocated first to satisfy the requirements of
-mint-register, then -mpid and finally -msmall-data-limit.
Thus it is possible for the small data area register to be
"r8" if both -mint-register=4 and -mpid are specified on the
command line.
By default this feature is not enabled. The default can be
restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more
than one fast interrupt handler when it is compiling a file.
The default is to issue a warning for each extra fast
interrupt handler found, as the RX only supports one such
interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation
instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL"
"SWHILE" and also the "RMPA" instruction. These instructions
may prefetch data, which is not safe to do if accessing an
I/O register. (See section 12.2.7 of the RX62N Group User's
Manual for more information).
The default is to allow these instructions, but it is not
possible for GCC to reliably detect all circumstances where a
string instruction might be used to access an I/O register,
so their use cannot be disabled automatically. Instead it is
reliant upon the programmer to use the
-mno-allow-string-insns option if their program accesses I/O
space.
When the instructions are enabled GCC defines the C
preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it
defines the symbol "__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access
functions. This option can be used when code size exceeds
the range of "BSR" instructions. Note that -mno-jsr does not
mean to not use "JSR" but instead means that any type of
branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special
significance to the RX port when used with the "interrupt"
function attribute. This attribute indicates a function intended
to process fast interrupts. GCC ensures that it only uses the
registers "r10", "r11", "r12" and/or "r13" and only provided that
the normal use of the corresponding registers have been
restricted via the -ffixed-reg or -mint-register command-line
options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries
architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and
registers for floating-point operations. When -msoft-float
is specified, functions in libgcc.a are used to perform
floating-point operations. When -mhard-float is specified,
the compiler generates IEEE floating-point instructions.
This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point
instructions for decimal-floating-point operations. When
-mno-hard-dfp is specified, functions in libgcc.a are used to
perform decimal-floating-point operations. When -mhard-dfp
is specified, the compiler generates decimal-floating-point
hardware instructions. This is the default for -march=z9-ec
or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size
of 64 bits makes the "long double" type equivalent to the
"double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as
backchain pointer into the callee's stack frame. A backchain
may be needed to allow debugging using tools that do not
understand DWARF call frame information. When
-mno-packed-stack is in effect, the backchain pointer is
stored at the bottom of the stack frame; when -mpacked-stack
is in effect, the backchain is placed into the topmost word
of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible
with code compiled with -mmo-backchain; however, use of the
backchain for debugging purposes usually requires that the
whole binary is built with -mbackchain. Note that the
combination of -mbackchain, -mpacked-stack and -mhard-float
is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When
-mno-packed-stack is specified, the compiler uses the all
fields of the 96/160 byte register save area only for their
default purpose; unused fields still take up stack space.
When -mpacked-stack is specified, register save slots are
densely packed at the top of the register save area; unused
space is reused for other purposes, allowing for more
efficient use of the available stack space. However, when
-mbackchain is also in effect, the topmost word of the save
area is always used to store the backchain, and the return
address register is always saved two words below the
backchain.
As long as the stack frame backchain is not used, code
generated with -mpacked-stack is call-compatible with code
generated with -mno-packed-stack. Note that some non-FSF
releases of GCC 2.95 for S/390 or zSeries generated code that
uses the stack frame backchain at run time, not just for
debugging purposes. Such code is not call-compatible with
code compiled with -mpacked-stack. Also, note that the
combination of -mbackchain, -mpacked-stack and -mhard-float
is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras"
instruction to do subroutine calls. This only works reliably
if the total executable size does not exceed 64k. The
default is to use the "basr" instruction instead, which does
not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the
GNU/Linux for S/390 ABI. When -m64 is specified, generate
code compliant to the GNU/Linux for zSeries ABI. This allows
GCC in particular to generate 64-bit instructions. For the
s390 targets, the default is -m31, while the s390x targets
default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the
instructions available on z/Architecture. When -mesa is
specified, generate code using the instructions available on
ESA/390. Note that -mesa is not possible with -m64. When
generating code compliant to the GNU/Linux for S/390 ABI, the
default is -mesa. When generating code compliant to the
GNU/Linux for zSeries ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of
instructions available with the transactional execution
facility introduced with the IBM zEnterprise EC12 machine
generation S/390 System z Built-in Functions. -mhtm is
enabled by default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions
available with the vector extension facility introduced with
the IBM z13 machine generation. This option changes the ABI
for some vector type values with regard to alignment and
calling conventions. In case vector type values are being
used in an ABI-relevant context a GAS .gnu_attribute command
will be added to mark the resulting binary with the ABI used.
-mvx is enabled by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and
builtins using instructions available with the vector
extension facility introduced with the IBM z13 machine
generation. This option adds support for vector to be used
as a keyword to define vector type variables and arguments.
vector is only available when GNU extensions are enabled. It
will not be expanded when requesting strict standard
compliance e.g. with -std=c99. In addition to the GCC low-
level builtins -mzvector enables a set of builtins added for
compatibility with AltiVec-style implementations like Power
and Cell. In order to make use of these builtins the header
file vecintrin.h needs to be included. -mzvector is disabled
by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle"
instruction to perform block moves. When -mno-mvcle is
specified, use a "mvc" loop instead. This is the default
unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when
compiling. The default is to not print debug information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a
system representing a certain processor type. Possible
values for cpu-type are z900/arch5, z990/arch6, z9-109,
z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11,
z14/arch12, and native.
The default is -march=z900.
Specifying native as cpu type can be used to select the best
architecture option for the host processor. -march=native
has no effect if GCC does not recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated
code, except for the ABI and the set of available
instructions. The list of cpu-type values is the same as for
-march. The default is the value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific
branches to trace routines in the operating system. This
option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given
frame size. Because this is a compile-time check it doesn't
need to be a real problem when the program runs. It is
intended to identify functions that most probably cause a
stack overflow. It is useful to be used in an environment
with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses
dynamically-sized arrays. This is generally a bad idea with
a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits
additional instructions in the function prologue that trigger
a trap if the stack size is stack-guard bytes above the
stack-size (remember that the stack on S/390 grows downward).
If the stack-guard option is omitted the smallest power of 2
larger than the frame size of the compiled function is
chosen. These options are intended to be used to help
debugging stack overflow problems. The additionally emitted
code causes only little overhead and hence can also be used
in production-like systems without greater performance
degradation. The given values have to be exact powers of 2
and stack-size has to be greater than stack-guard without
exceeding 64k. In order to be efficient the extra code makes
the assumption that the stack starts at an address aligned to
the value given by stack-size. The stack-guard option can
only be used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function
prologue is generated for all functions in the compilation
unit. The funtion label is prepended with the given number
of two-byte NOP instructions (pre-halfwords, maximum
1000000). After the label, 2 * post-halfwords bytes are
appended, using the largest NOP like instructions the
architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with
the "hotpatch" attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store instructions.
-mmac
Enable the use of multiply-accumulate instructions. Disabled
by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the
default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in
such a way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-
precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point
unit is in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point
unit is in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that
only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is
in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-
point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is
in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-
precision floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the
floating-point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is
in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-
precision floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the
floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-
precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-
precision floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to
the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way
that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-
precision floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit
is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to
the assembler. GCC doesn't generate any DSP instructions at
the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes
the calling conventions, and thus some functions from the
standard C library do not work unless you recompile it first
with -mdalign.
-mrelax
Shorten some address references at link time, when possible;
uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign
for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before
the Renesas conventions were available. This option is the
default for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas
is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons,
which affects the handling of cases where the result of a
comparison is unordered. By default -mieee is implicitly
enabled. If -ffinite-math-only is enabled -mno-ieee is
implicitly set, which results in faster floating-point
greater-equal and less-equal comparisons. The implicit
settings can be overridden by specifying either -mieee or
-mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after
setting up nested function trampolines. This option has no
effect if -musermode is in effect and the selected code
generation option (e.g. -m4) does not allow the use of the
"icbi" instruction. If the selected code generation option
does not allow the use of the "icbi" instruction, and
-musermode is not in effect, the inlined code manipulates the
instruction cache address array directly with an associative
write. This not only requires privileged mode at run time,
but it also fails if the cache line had been mapped via the
TLB and has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of
4 bytes, which is incompatible with the SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters
as a comma separated list. For details on the atomic built-
in functions see __atomic Builtins. The following models and
parameters are supported:
none
Disable compiler generated atomic sequences and emit
library calls for atomic operations. This is the default
if the target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic
sequences for the atomic built-in functions. The
generated atomic sequences require additional support
from the interrupt/exception handling code of the system
and are only suitable for SH3* and SH4* single-core
systems. This option is enabled by default when the
target is "sh*-*-linux*" and SH3* or SH4*. When the
target is SH4A, this option also partially utilizes the
hardware atomic instructions "movli.l" and "movco.l" to
create more efficient code, unless strict is specified.
soft-tcb
Generate software atomic sequences that use a variable in
the thread control block. This is a variation of the
gUSA sequences which can also be used on SH1* and SH2*
targets. The generated atomic sequences require
additional support from the interrupt/exception handling
code of the system and are only suitable for single-core
systems. When using this model, the gbr-offset=
parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily
disable interrupts by setting "SR.IMASK = 1111". This
model works only when the program runs in privileged mode
and is only suitable for single-core systems. Additional
support from the interrupt/exception handling code of the
system is not required. This model is enabled by default
when the target is "sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l"
and "movco.l" instructions only. This is only available
on SH4A and is suitable for multi-core systems. Since
the hardware instructions support only 32 bit atomic
variables access to 8 or 16 bit variables is emulated
with 32 bit accesses. Code compiled with this option is
also compatible with other software atomic model
interrupt/exception handling systems if executed on an
SH4A system. Additional support from the
interrupt/exception handling code of the system is not
required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the
variable in the thread control block structure that
should be used by the generated atomic sequences when the
soft-tcb model has been selected. For other models this
parameter is ignored. The specified value must be an
integer multiple of four and in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic
models, even if they are compatible, and makes the
compiler generate atomic sequences of the specified model
only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set".
Notice that depending on the particular hardware and software
configuration this can degrade overall performance due to the
operand cache line flushes that are implied by the "tas.b"
instruction. On multi-core SH4A processors the "tas.b"
instruction must be used with caution since it can result in
data corruption for certain cache configurations.
-mprefergot
When generating position-independent code, emit function
calls using the Global Offset Table instead of the Procedure
Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode
code. Specifying -musermode also implies
-mno-inline-ic_invalidate if the inlined code would not work
in user mode. -musermode is the default when the target is
"sh*-*-linux*". If the target is SH1* or SH2* -musermode has
no effect, since there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division
operations. strategy can be one of:
call-div1
Calls a library function that uses the single-step
division instruction "div1" to perform the operation.
Division by zero calculates an unspecified result and
does not trap. This is the default except for SH4, SH2A
and SHcompact.
call-fp
Calls a library function that performs the operation in
double precision floating point. Division by zero causes
a floating-point exception. This is the default for
SHcompact with FPU. Specifying this for targets that do
not have a double precision FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for
small divisors and the "div1" instruction with case
distinction for larger divisors. Division by zero
calculates an unspecified result and does not trap. This
is the default for SH4. Specifying this for targets that
do not have dynamic shift instructions defaults to
"call-div1".
When a division strategy has not been specified the default
strategy is selected based on the current target. For SH2A
the default strategy is to use the "divs" and "divu"
instructions instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function
prologue rather than around each call. Generally beneficial
for performance and size. Also needed for unwinding to avoid
changing the stack frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed
division to name. This only affects the name used in the
call division strategies, and the compiler still expects the
same sets of input/output/clobbered registers as if this
option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel
code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be
specified separated by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher
numbers make the compiler try to generate more branch-free
code if possible. If not specified the value is selected
depending on the processor type that is being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional
branch instructions "bt" and "bf" are fast. If -mzdcbranch
is specified, the compiler prefers zero displacement branch
code sequences. This is enabled by default when generating
code for SH4 and SH4A. It can be explicitly disabled by
specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches,
which stuffs the delay slot with a "nop" if a suitable
instruction cannot be found. By default this option is
disabled. It can be enabled to work around hardware bugs as
found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The
machine-dependent -mfused-madd option is now mapped to the
machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction
for sine and cosine approximations. The option -mfsca must
be used in combination with -funsafe-math-optimizations. It
is enabled by default when generating code for SH4A. Using
-mno-fsca disables sine and cosine approximations even if
-funsafe-math-optimizations is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra"
instruction for reciprocal square root approximations. The
option -mfsrra must be used in combination with
-funsafe-math-optimizations and -ffinite-math-only. It is
enabled by default when generating code for SH4A. Using
-mno-fsrra disables reciprocal square root approximations
even if -funsafe-math-optimizations and -ffinite-math-only
are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional
move instruction patterns. This can result in faster code on
the SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware
capabilities generated by the Solaris assembler. This is
only necessary when object files use ISA extensions not
supported by the current machine, but check at runtime
whether or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the
compiler to not pass -z text to the linker when linking a
shared object. Using this option, you can link position-
dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error message.
However, the necessary relocations trigger copy-on-write, and
the shared object is not actually shared across processes.
Instead of using -mimpure-text, you should compile all source
code with -fpic or -fPIC.
These switches are supported in addition to the above on Solaris
2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global
registers 2 through 4, which the SPARC SVR4 ABI reserves for
applications. Like the global register 1, each global
register 2 through 4 is then treated as an allocable register
that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some
performance loss, specify -mno-app-regs. You should compile
libraries and system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore
instructions and uses a "flat" or single register window
model. This model is compatible with the regular register
window model. The local registers and the input registers
(0--5) are still treated as "call-saved" registers and are
saved on the stack as needed.
With -mno-flat (the default), the compiler generates
save/restore instructions (except for leaf functions). This
is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This
is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all
SPARC targets. Normally the facilities of the machine's
usual C compiler are used, but this cannot be done directly
in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation.
The embedded targets sparc-*-aout and sparclite-*-* do
provide software floating-point support.
-msoft-float changes the calling convention in the output
file; therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile
libgcc.a, the library that comes with GCC, with -msoft-float
in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-
point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long
double) floating-point instructions. The functions called
are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that
have hardware support for the quad-word floating-point
instructions. They all invoke a trap handler for one of
these instructions, and then the trap handler emulates the
effect of the instruction. Because of the trap handler
overhead, this is much slower than calling the ABI library
routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the
default.
With -munaligned-doubles, GCC assumes that doubles have
8-byte alignment only if they are contained in another type,
or if they have an absolute address. Otherwise, it assumes
they have 4-byte alignment. Specifying this option avoids
some rare compatibility problems with code generated by other
compilers. It is not the default because it results in a
performance loss, especially for floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode.
This is relevant only for the "casa" instruction emitted for
the LEON3 processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures
should have 8-byte alignment. This enables the use of pairs
of "ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st" pairs.
However, the use of this changed alignment directly violates
the SPARC ABI. Thus, it's intended only for use on targets
where the developer acknowledges that their resulting code is
not directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking
code in functions returning structures or unions to detect
size mismatches between the two sides of function calls, as
per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no
effect in 64-bit mode.
-mlra
-mno-lra
Enable Local Register Allocation. This is the default for
SPARC since GCC 7 so -mno-lra needs to be passed to get old
Reload.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Supported
values for cpu_type are v7, cypress, v8, supersparc,
hypersparc, leon, leon3, leon3v7, sparclite, f930, f934,
sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3,
niagara, niagara2, niagara3, niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the
value native, which selects the best architecture option for
the host processor. -mcpu=native has no effect if GCC does
not recognize the processor.
Default instruction scheduling parameters are used for values
that select an architecture and not an implementation. These
are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their
supported implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4, niagara7, m8
By default (unless configured otherwise), GCC generates code
for the V7 variant of the SPARC architecture. With
-mcpu=cypress, the compiler additionally optimizes it for the
Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer
3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the
SPARC architecture. The only difference from V7 code is that
the compiler emits the integer multiply and integer divide
instructions which exist in SPARC-V8 but not in SPARC-V7.
With -mcpu=supersparc, the compiler additionally optimizes it
for the SuperSPARC chip, as used in the SPARCStation 10, 1000
and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite
variant of the SPARC architecture. This adds the integer
multiply, integer divide step and scan ("ffs") instructions
which exist in SPARClite but not in SPARC-V7. With
-mcpu=f930, the compiler additionally optimizes it for the
Fujitsu MB86930 chip, which is the original SPARClite, with
no FPU. With -mcpu=f934, the compiler additionally optimizes
it for the Fujitsu MB86934 chip, which is the more recent
SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet
variant of the SPARC architecture. This adds the integer
multiply, multiply/accumulate, integer divide step and scan
("ffs") instructions which exist in SPARClet but not in
SPARC-V7. With -mcpu=tsc701, the compiler additionally
optimizes it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the
SPARC architecture. This adds 64-bit integer and floating-
point move instructions, 3 additional floating-point
condition code registers and conditional move instructions.
With -mcpu=ultrasparc, the compiler additionally optimizes it
for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally optimizes it for
the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips. With -mcpu=niagara2, the compiler
additionally optimizes it for Sun UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally optimizes it for
Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler
additionally optimizes it for Sun UltraSPARC T4 chips. With
-mcpu=niagara7, the compiler additionally optimizes it for
Oracle SPARC M7 chips. With -mcpu=m8, the compiler
additionally optimizes it for Oracle M8 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set
that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those that
select a particular CPU implementation. Those are cypress,
supersparc, hypersparc, leon, leon3, leon3v7, f930, f934,
sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara,
niagara2, niagara3, niagara4, niagara7 and m8. With native
Solaris and GNU/Linux toolchains, native can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The
difference from the V8 ABI is that the global and out
registers are considered 64 bits wide. This is enabled by
default on Solaris in 32-bit mode for all SPARC-V9
processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the
UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of
version 2.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis2 when targeting a cpu that
supports such instructions, such as UltraSPARC-III and later.
Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of
version 3.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis3 when targeting a cpu that
supports such instructions, such as niagara-3 and later.
Setting -mvis3 also sets -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of
version 4.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis4 when targeting a cpu that
supports such instructions, such as niagara-7 and later.
Setting -mvis4 also sets -mvis3, -mvis2 and -mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of
version 4.0 of the UltraSPARC Visual Instruction Set
extensions, plus the additional VIS instructions introduced
in the Oracle SPARC Architecture 2017. The default is
-mvis4b when targeting a cpu that supports such instructions,
such as m8 and later. Setting -mvis4b also sets -mvis4,
-mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the
UltraSPARC Compare-and-Branch-on-Condition instructions. The
default is -mcbcond when targeting a CPU that supports such
instructions, such as Niagara-4 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the
UltraSPARC Fused Multiply-Add Floating-point instructions.
The default is -mfmaf when targeting a CPU that supports such
instructions, such as Niagara-3 and later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the
Floating-point Multiply Single to Double (FsMULd)
instruction. The default is -mfsmuld when targeting a CPU
supporting the architecture versions V8 or V9 with FPU except
-mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the
UltraSPARC Population Count instruction. The default is
-mpopc when targeting a CPU that supports such an
instruction, such as Niagara-2 and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the
UltraSPARC Subtract-Extended-with-Carry instruction. The
default is -msubxc when targeting a CPU that supports such an
instruction, such as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of
the Atmel AT697F processor (which corresponds to erratum #13
of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point
errata and the data cache nullify errata of the UT699
processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store
errata of the UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store
errata of the GR712RC processor.
These -m options are supported in addition to the above on
SPARC-V9 processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer
to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs
must be linked in the low 32 bits of memory. Programs
can be statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs
must be linked in the low 44 bits of memory, the text and
data segments must be less than 2GB in size and the data
segment must be located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses,
programs may be linked anywhere in memory, the text and
data segments must be less than 2GB in size and the data
segment must be located within 2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems:
64-bit addresses, the text and data segments must be less
than 2GB in size, both starting anywhere in memory
(determined at link time). The global register %g4
points to the base of the data segment. Programs are
statically linked and PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating
system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the
SPARC-V9 architecture manual, as set in the processor's
"PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and
frame pointer if present, are offset by -2047 which must be
added back when making stack frame references. This is the
default in 64-bit mode. Otherwise, assume no such offset is
present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By
default, GCC gives an error when it generates code that
requires a dynamic relocation. -mno-error-reloc disables the
error, -mwarn-reloc generates a warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not
be reordered with respect to loads and stores of the memory
that is being accessed. With -munsafe-dma you must use the
"volatile" keyword to protect memory accesses, but that can
lead to inefficient code in places where the memory is known
to not change. Rather than mark the memory as volatile, you
can use -msafe-dma to tell the compiler to treat the DMA
instructions as potentially affecting all memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to avoid
pipeline stalls for always-taken or probably-taken branches.
A hint is not generated closer than 8 instructions away from
its branch. There is little reason to disable them, except
for debugging purposes, or to make an object a little bit
smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are
never larger than 18 bits. With -mlarge-mem code is
generated that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the
SPU-style main function interface (which has an
unconventional parameter list). With -mstdmain, GCC links
your program against startup code that assumes a C99-style
interface to "main", including a local copy of "argv"
strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel
code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be
specified separated by a comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address space
accessed via the "__ea" named address space qualifier are
either 32 or 64 bits wide. The default is 32 bits. As this
is an ABI-changing option, all object code in an executable
must be compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset
of the generic address space. This enables explicit type
casts between "__ea" and generic pointer as well as implicit
conversions of generic pointers to "__ea" pointers. The
default is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler
links to an executable and selects a software-managed cache
for accessing variables in the "__ea" address space with a
particular cache size. Possible options for cache-size are
8, 16, 32, 64 and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler
links to an executable and selects whether atomic updates to
the software-managed cache of PPU-side variables are used.
If you use atomic updates, changes to a PPU variable from SPU
code using the "__ea" named address space qualifier do not
interfere with changes to other PPU variables residing in the
same cache line from PPU code. If you do not use atomic
updates, such interference may occur; however, writing back
cache lines is more efficient. The default behavior is to
use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts NOPs to increase dual issue when it
expects it to increase performance. n can be a value from 0
to 10. A smaller n inserts fewer NOPs. 10 is the default, 0
is the same as -mno-dual-nops. Disabled with -Os.
-mhint-max-nops=n
Maximum number of NOPs to insert for a branch hint. A branch
hint must be at least 8 instructions away from the branch it
is affecting. GCC inserts up to n NOPs to enforce this,
otherwise it does not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint
to be within 256 instructions of the branch it is affecting.
By default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall
indefinitely. By default, GCC inserts the "hbrp" instruction
to make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or
-shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file
(this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries
specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The
assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for direct
calls is limited to 500M in either direction. PC-relative
addresses are 32 bits. Absolute addresses support the full
address range.
-mcmodel=large
Generate code for the large model. There is no limitation on
call distance, pc-relative addresses, or absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long, and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer
to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long,
and pointer to 32 bits. This is the only supported behavior
so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are
assumed to be far away, the compiler always loads the
function's address into a register, and calls indirect
through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same
index pointer 4 or more times to copy pointer into the "ep"
register, and use the shorter "sld" and "sst" instructions.
The -mep option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The
external functions are slower, but use less code space if
more than one function saves the same number of registers.
The -mprolog-function option is on by default if you
optimize.
-mspace
Try to make the code as small as possible. At present, this
just turns on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less
into the tiny data area that register "ep" points to. The
tiny data area can hold up to 256 bytes in total (128 bytes
for byte references).
-msda=n
Put static or global variables whose size is n bytes or less
into the small data area that register "gp" points to. The
small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less
into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The
preprocessor constant "__v850e3v5__" is defined if this
option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is
an alias for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The
preprocessor constant "__v850e2v3__" is defined if this
option is used.
-mv850e2
Specify that the target processor is the V850E2. The
preprocessor constant "__v850e2__" is defined if this option
is used.
-mv850e1
Specify that the target processor is the V850E1. The
preprocessor constants "__v850e1__" and "__v850e__" are
defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an
alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The
preprocessor constant "__v850e__" is defined if this option
is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor
-mv850e2v3 nor -mv850e3v5 are defined then a default target
processor is chosen and the relevant __v850*__ preprocessor
constant is defined.
The preprocessor constants "__v850" and "__v851__" are always
defined, regardless of which processor variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction
for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors
of the v850 architecture.
This option is enabled by default when the RH850 ABI is in
use (see -mrh850-abi), and disabled by default when the GCC
ABI is in use. If "CALLT" instructions are being generated
then the C preprocessor symbol "__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option
to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump
instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating
point instructions. This option is only significant when the
target architecture is V850E2V3 or higher. If hardware
floating point instructions are being generated then the C
preprocessor symbol "__FPU_OK__" is defined, otherwise the
symbol "__NO_FPU__" is defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of
this instruction is not enabled by default when the e3v5
architecture is selected because its use is still
experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This
is the default. With this version of the ABI the following
rules apply:
* Integer sized structures and unions are returned via a
memory pointer rather than a register.
* Large structures and unions (more than 8 bytes in size)
are passed by value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by
default. The -mno-disable-callt command-line option is
not supported.
When this version of the ABI is enabled the C preprocessor
symbol "__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI.
With this version of the ABI the following rules apply:
* Integer sized structures and unions are returned in
register "r10".
* Large structures and unions (more than 8 bytes in size)
are passed by reference.
* Functions are aligned to 32-bit boundaries, unless
optimizing for size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but
not enabled by default.
When this version of the ABI is enabled the C preprocessor
symbol "__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be
aligned on 8-byte boundaries. The default is to restrict the
alignment of all objects to at most 4-bytes. When
-m8byte-align is in effect the C preprocessor symbol
"__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this
option only if the assembler/linker complain about out of
range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated
by the compiler. This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed
registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on)
that the Unix assembler for the VAX cannot handle across long
ranges.
-mgnu
Do output those jump instructions, on the assumption that the
GNU assembler is being used.
-mg Output code for G-format floating-point numbers instead of
D-format.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on
an MCM target should be linked with this option. It causes
the libraries libc.a and libdebug.a to be linked. The
program should be run on the target under the control of the
GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run on
the simulator should be linked with option. This causes
libraries libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This
is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output
file; therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile
libgcc.a, the library that comes with GCC, with -msoft-float
in order for this to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Supported
values for cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code
for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the
Visium architecture. The only difference from GR5 code is
that the compiler will generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set
that the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no
restrictions on the access to general registers. This is the
default.
-muser-mode
Generate code for the user mode, where the access to some
general registers is forbidden: on the GR5, registers r24 to
r31 cannot be accessed in this mode; on the GR6, only
registers r29 to r31 are affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to
return POSIX-style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the
main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size
are 32 or short for 32 bit pointers, 64 or long for 64 bit
pointers, and no for supporting only 32 bit pointers. The
later option disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets.
Options specific to the target hardware are listed with the other
options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time
processes (RTPs). This option switches from the former to
the latter. It also defines the preprocessor macro
"__RTP__".
-non-static
Link an RTP executable against shared libraries rather than
static libraries. The options -static and -shared can also
be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are
defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is
equivalent to -Wl,-z,now and is defined for compatibility
with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the
default and is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In
contrast to -mtune=cpu-type, which merely tunes the generated
code for the specified cpu-type, -march=cpu-type allows GCC
to generate code that may not run at all on processors other
than the one indicated. Specifying -march=cpu-type implies
-mtune=cpu-type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation
time by determining the processor type of the compiling
machine. Using -march=native enables all instruction
subsets supported by the local machine (hence the result
might not run on different machines). Using
-mtune=native produces code optimized for the local
machine under the constraints of the selected instruction
set.
x86-64
A generic CPU with 64-bit extensions.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this
chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX
instruction set support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction set is
used, so the code runs on all i686 family chips. When
used with -mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX
instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX
and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III
CPU with MMX, SSE and SSE2 instruction set support. Used
by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction
set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE,
SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit
extensions, MMX, SSE, SSE2 and SSE3 instruction set
support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3 and SSSE3 instruction set support.
nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set
support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL
instruction set support.
sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and
PCLMUL instruction set support.
ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES,
PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
haswell
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and
F16C instruction set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED ADCX and PREFETCHW instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES
instruction set support.
bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3 and SSSE3 instruction set support.
silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL and RDRND instruction set support.
goldmont
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT
and FSGSBASE instruction set support.
goldmont-plus
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP
instruction set support.
tremont
Intel Tremont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE,
CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG
instruction set support.
knl Intel Knight's Landing CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF,
AVX512ER and AVX512CD instruction set support.
knm Intel Knights Mill CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF,
AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and
AVX512VPOPCNTDQ instruction set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2,
F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and
AVX512CD instruction set support.
cannonlake
Intel Cannonlake Server CPU with 64-bit extensions,
MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW,
CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP
instruction set support.
icelake-client
Intel Icelake Client CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2,
F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI,
AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES instruction set support.
icelake-server
Intel Icelake Server CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2,
F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI,
AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set
support.
cascadelake
Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES,
AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and
AVX512VNNI instruction set support.
tigerlake
Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES,
AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI,
AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B
and CLWB instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow!
instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE
prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow!
and full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64
instruction set support, including the AMD Opteron,
Athlon 64, and Athlon 64 FX processors. (This supersets
MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit
instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction
set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64
instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit
instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64
instruction set support. (This supersets FMA4, AVX, XOP,
LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction
set support. (This supersets BMI, TBM, F16C, FMA, FMA4,
AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction
set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction
set support. (This supersets BMI, TBM, F16C, FMA, FMA4,
FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE,
SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.
bdver4
AMD Family 15h core based CPUs with x86-64 instruction
set support. (This supersets BMI, BMI2, TBM, F16C, FMA,
FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.
znver1
AMD Family 17h core based CPUs with x86-64 instruction
set support. (This supersets BMI, BMI2, F16C, FMA,
FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT,
POPCNT, and 64-bit instruction set extensions.
znver2
AMD Family 17h core based CPUs with x86-64 instruction
set support. (This supersets BMI, BMI2, ,CLWB, F16C, FMA,
FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT,
POPCNT, and 64-bit instruction set extensions.)
btver1
CPUs based on AMD Family 14h cores with x86-64
instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set
extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64
instruction set support. This includes MOVBE, F16C, BMI,
AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A,
SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set
extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with
additional MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with
additional MMX and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support.
(No scheduling is implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction
set support. (No scheduling is implemented for this
chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is implemented
for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set
support. (No scheduling is implemented for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set
support. (No scheduling is implemented for this chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is implemented
for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is implemented
for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, AVX and AVX2 instruction set support.
(No scheduling is implemented for this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3
and SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling
is implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling
is implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling
is implemented for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow!
instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated
code, except for the ABI and the set of available
instructions. While picking a specific cpu-type schedules
things appropriately for that particular chip, the compiler
does not generate any code that cannot run on the default
machine type unless you use a -march=cpu-type option. For
example, if GCC is configured for i686-pc-linux-gnu then
-mtune=pentium4 generates code that is tuned for Pentium 4
but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In
addition, -mtune supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common
IA32/AMD64/EM64T processors. If you know the CPU on
which your code will run, then you should use the
corresponding -mtune or -march option instead of
-mtune=generic. But, if you do not know exactly what CPU
users of your application will have, then you should use
this option.
As new processors are deployed in the marketplace, the
behavior of this option will change. Therefore, if you
upgrade to a newer version of GCC, code generation
controlled by this option will change to reflect the
processors that are most common at the time that version
of GCC is released.
There is no -march=generic option because -march
indicates the instruction set the compiler can use, and
there is no generic instruction set applicable to all
processors. In contrast, -mtune indicates the processor
(or, in this case, collection of processors) for which
the code is optimized.
intel
Produce code optimized for the most current Intel
processors, which are Haswell and Silvermont for this
version of GCC. If you know the CPU on which your code
will run, then you should use the corresponding -mtune or
-march option instead of -mtune=intel. But, if you want
your application performs better on both Haswell and
Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace,
the behavior of this option will change. Therefore, if
you upgrade to a newer version of GCC, code generation
controlled by this option will change to reflect the most
current Intel processors at the time that version of GCC
is released.
There is no -march=intel option because -march indicates
the instruction set the compiler can use, and there is no
common instruction set applicable to all processors. In
contrast, -mtune indicates the processor (or, in this
case, collection of processors) for which the code is
optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit.
The choices for unit are:
387 Use the standard 387 floating-point coprocessor present
on the majority of chips and emulated otherwise. Code
compiled with this option runs almost everywhere. The
temporary results are computed in 80-bit precision
instead of the precision specified by the type, resulting
in slightly different results compared to most of other
chips. See -ffloat-store for more detailed description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by
Pentium III and newer chips, and in the AMD line by
Athlon-4, Athlon XP and Athlon MP chips. The earlier
version of the SSE instruction set supports only single-
precision arithmetic, thus the double and extended-
precision arithmetic are still done using 387. A later
version, present only in Pentium 4 and AMD x86-64 chips,
supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type,
-msse or -msse2 switches to enable SSE extensions and
make this option effective. For the x86-64 compiler,
these extensions are enabled by default.
The resulting code should be considerably faster in the
majority of cases and avoid the numerical instability
problems of 387 code, but may break some existing code
that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler,
Darwin x86-32 targets, and the default choice for x86-32
targets with the SSE2 instruction set when -ffast-math is
enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This
effectively doubles the amount of available registers,
and on chips with separate execution units for 387 and
SSE the execution resources too. Use this option with
care, as it is still experimental, because the GCC
register allocator does not model separate functional
units well, resulting in unstable performance.
-masm=dialect
Output assembly instructions using selected dialect. Also
affects which dialect is used for basic "asm" and extended
"asm". Supported choices (in dialect order) are att or intel.
The default is att. Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point
comparisons. These correctly handle the case where the
result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating
point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable
library functions for cross-compilation.
On machines where a function returns floating-point results
in the 80387 register stack, some floating-point opcodes may
be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of
types "float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should
emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be
returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid
generating those instructions. This option is overridden
when -march indicates that the target CPU always has an FPU
and so the instruction does not need emulation. These
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long
long" variables on a two-word boundary or a one-word
boundary. Aligning "double" variables on a two-word boundary
produces code that runs somewhat faster on a Pentium at the
expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures
containing the above types are aligned differently than the
published application binary interface specifications for the
x86-32 and are not binary compatible with structures in code
compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The
x86-32 application binary interface specifies the size to be
96 bits, so -m96bit-long-double is the default in 32-bit
mode.
Modern architectures (Pentium and newer) prefer "long double"
to be aligned to an 8- or 16-byte boundary. In arrays or
structures conforming to the ABI, this is not possible. So
specifying -m128bit-long-double aligns "long double" to a
16-byte boundary by padding the "long double" with an
additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default
choice as its ABI specifies that "long double" is aligned on
16-byte boundary.
Notice that neither of these options enable any extra
precision over the x87 standard of 80 bits for a "long
double".
Warning: if you override the default value for your target
ABI, this changes the size of structures and arrays
containing "long double" variables, as well as modifying the
function calling convention for functions taking "long
double". Hence they are not binary-compatible with code
compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size
of 64 bits makes the "long double" type equivalent to the
"double" type. This is the default for 32-bit Bionic C
library. A size of 128 bits makes the "long double" type
equivalent to the "__float128" type. This is the default for
64-bit Bionic C library.
Warning: if you override the default value for your target
ABI, this changes the size of structures and arrays
containing "long double" variables, as well as modifying the
function calling convention for functions taking "long
double". Hence they are not binary-compatible with code
compiled without that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type
are compat uses increased alignment value compatible uses GCC
4.8 and earlier, abi uses alignment value as specified by the
psABI, and cacheline uses increased alignment value to match
the cache line size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than
threshold are placed in the large data section. This value
must be the same across all objects linked into the binary,
and defaults to 65535.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return with
the "ret num" instruction, which pops their arguments while
returning. This saves one instruction in the caller since
there is no need to pop the arguments there.
You can specify that an individual function is called with
this calling sequence with the function attribute "stdcall".
You can also override the -mrtd option by using the function
attribute "cdecl".
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to
call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions
that take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments
are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer
arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can
control this behavior for a specific function by using the
function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you
must build all modules with the same value, including any
libraries. This includes the system libraries and startup
modules.
-msseregparm
Use SSE register passing conventions for float and double
arguments and return values. You can control this behavior
for a specific function by using the function attribute
"sseregparm".
Warning: if you use this switch then you must build all
modules with the same value, including any libraries. This
includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers.
This is the default on Solaris 8 and 9 and VxWorks to match
the ABI of the Sun Studio compilers until version 12. Later
compiler versions (starting with Studio 12 Update 1) follow
the ABI used by other x86 targets, which is the default on
Solaris 10 and later. Only use this option if you need to
remain compatible with existing code produced by those
previous compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits.
When -mpc32 is specified, the significands of results of
floating-point operations are rounded to 24 bits (single
precision); -mpc64 rounds the significands of results of
floating-point operations to 53 bits (double precision) and
-mpc80 rounds the significands of results of floating-point
operations to 64 bits (extended double precision), which is
the default. When this option is used, floating-point
operations in higher precisions are not available to the
programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less
than the default 80 bits can speed some programs by 2% or
more. Note that some mathematical libraries assume that
extended-precision (80-bit) floating-point operations are
enabled by default; routines in such libraries could suffer
significant loss of accuracy, typically through so-called
"catastrophic cancellation", when this option is used to set
the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign
option generates an alternate prologue and epilogue that
realigns the run-time stack if necessary. This supports
mixing legacy codes that keep 4-byte stack alignment with
modern codes that keep 16-byte stack alignment for SSE
compatibility. See also the attribute
"force_align_arg_pointer", applicable to individual
functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to
num byte boundary. If -mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture
with SSE extensions disabled, -mpreferred-stack-boundary=3
can be used to keep the stack boundary aligned to 8 byte
boundary. Since x86-64 ABI require 16 byte stack alignment,
this is ABI incompatible and intended to be used in
controlled environment where stack space is important
limitation. This option leads to wrong code when functions
compiled with 16 byte stack alignment (such as functions from
a standard library) are called with misaligned stack. In
this case, SSE instructions may lead to misaligned memory
access traps. In addition, variable arguments are handled
incorrectly for 16 byte aligned objects (including x87 long
double and __int128), leading to wrong results. You must
build all modules with -mpreferred-stack-boundary=3,
including any libraries. This includes the system libraries
and startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num
byte boundary. If -mincoming-stack-boundary is not
specified, the one specified by -mpreferred-stack-boundary is
used.
On Pentium and Pentium Pro, "double" and "long double" values
should be aligned to an 8-byte boundary (see -malign-double)
or suffer significant run time performance penalties. On
Pentium III, the Streaming SIMD Extension (SSE) data type
"__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the
stack boundary must be as aligned as that required by any
value stored on the stack. Further, every function must be
generated such that it keeps the stack aligned. Thus calling
a function compiled with a higher preferred stack boundary
from a function compiled with a lower preferred stack
boundary most likely misaligns the stack. It is recommended
that libraries that use callbacks always use the default
setting.
This extra alignment does consume extra stack space, and
generally increases code size. Code that is sensitive to
stack space usage, such as embedded systems and operating
system kernels, may want to reduce the preferred alignment to
-mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-mavx512vpopcntdq
-mavx5124fmaps
-mavx512vnni
-mavx5124vnniw
-mcldemote
These switches enable the use of instructions in the MMX,
SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX,
AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL,
AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL,
CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA,
PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1,
RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM,
ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES,
RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES,
WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B,
AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or
CLDEMOTE extended instruction sets. Each has a corresponding
-mno- option to disable use of these instructions.
These extensions are also available as built-in functions:
see x86 Built-in Functions, for details of the functions
enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from
floating-point code (as opposed to 387 instructions), see
-mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead,
it generates new AVX instructions or AVX equivalence for all
SSEx instructions when needed.
These options enable GCC to use these extended instructions
in generated code, even without -mfpmath=sse. Applications
that perform run-time CPU detection must compile separate
files for each supported architecture, using the appropriate
flags. In particular, the file containing the CPU detection
code should be compiled without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86
performance tuning features and default settings. The names
can be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code
generation features. feature-list is a comma separated list
of feature names. See also -mdump-tune-features. When
specified, the feature is turned on if it is not preceded
with ^, otherwise, it is turned off. -mtune-ctrl=feature-
list is intended to be used by GCC developers. Using it may
lead to code paths not covered by testing and can potentially
result in compiler ICEs or runtime errors.
-mno-default
This option instructs GCC to turn off all tunable features.
See also -mtune-ctrl=feature-list and -mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the
prologue of functions that use string instructions. String
instructions depend on the DF flag to select between
autoincrement or autodecrement mode. While the ABI specifies
the DF flag to be cleared on function entry, some operating
systems violate this specification by not clearing the DF
flag in their exception dispatchers. The exception handler
can be invoked with the DF flag set, which leads to wrong
direction mode when string instructions are used. This
option can be enabled by default on 32-bit x86 targets by
configuring GCC with the --enable-cld configure option.
Generation of "cld" instructions can be suppressed with the
-mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction
before a transfer of control flow out of the function to
minimize the AVX to SSE transition penalty as well as remove
unnecessary "zeroupper" intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions
instead of 256-bit AVX instructions in the auto-vectorizer.
-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in
instructions instead of default on the selected platform.
none
No extra limitations applied to GCC other than defined by
the selected platform.
128 Prefer 128-bit vector width for instructions.
256 Prefer 256-bit vector width for instructions.
512 Prefer 512-bit vector width for instructions.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions
in 64-bit code to implement compare-and-exchange operations
on 16-byte aligned 128-bit objects. This is useful for
atomic updates of data structures exceeding one machine word
in size. The compiler uses this instruction to implement
__sync Builtins. However, for __atomic Builtins operating on
128-bit integers, a library call is always used.
-msahf
This option enables generation of "SAHF" instructions in
64-bit code. Early Intel Pentium 4 CPUs with Intel 64
support, prior to the introduction of Pentium 4 G1 step in
December 2005, lacked the "LAHF" and "SAHF" instructions
which are supported by AMD64. These are load and store
instructions, respectively, for certain status flags. In
64-bit mode, the "SAHF" instruction is used to optimize
"fmod", "drem", and "remainder" built-in functions; see Other
Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to
implement "__builtin_bswap32" and "__builtin_bswap64".
-mshstk
The -mshstk option enables shadow stack built-in functions
from x86 Control-flow Enforcement Technology (CET).
-mcrc32
This option enables built-in functions
"__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to
generate the "crc32" machine instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions
(and their vectorized variants "RCPPS" and "RSQRTPS") with an
additional Newton-Raphson step to increase precision instead
of "DIVSS" and "SQRTSS" (and their vectorized variants) for
single-precision floating-point arguments. These
instructions are generated only when
-funsafe-math-optimizations is enabled together with
-ffinite-math-only and -fno-trapping-math. Note that while
the throughput of the sequence is higher than the throughput
of the non-reciprocal instruction, the precision of the
sequence can be decreased by up to 2 ulp (i.e. the inverse of
1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of
"RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the
above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional
Newton-Raphson step for vectorized single-float division and
vectorized "sqrtf(x)" already with -ffast-math (or the above
option combination), and doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which
may be preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to
-mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the
reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics
using an external library. Supported values for type are
svml for the Intel short vector math library and acml for the
AMD math core library. To use this option, both
-ftree-vectorize and -funsafe-math-optimizations have to be
enabled, and an SVML or ACML ABI-compatible library must be
specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2",
"vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2",
"vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2",
"vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2",
"vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4",
"vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4",
"vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4",
"vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4",
"vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding
function type when -mveclibabi=svml is used, and
"__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log",
"__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf",
"__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f"
and "__vrs4_powf" for the corresponding function type when
-mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention.
Permissible values are sysv for the ABI used on GNU/Linux and
other systems, and ms for the Microsoft ABI. The default is
to use the Microsoft ABI when targeting Microsoft Windows and
the SysV ABI on all other systems. You can control this
behavior for specific functions by using the function
attributes "ms_abi" and "sysv_abi".
-mforce-indirect-call
Force all calls to functions to be indirect. This is useful
when using Intel Processor Trace where it generates more
precise timing information for function calls.
-mmanual-endbr
Insert ENDBR instruction at function entry only via the
"cf_check" function attribute. This is useful when used with
the option -fcf-protection=branch to control ENDBR insertion
at the function entry.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function
that calls a System V ABI function must consider RSI, RDI and
XMM6-15 as clobbered. By default, the code for saving and
restoring these registers is emitted inline, resulting in
fairly lengthy prologues and epilogues. Using
-mcall-ms2sysv-xlogues emits prologues and epilogues that use
stubs in the static portion of libgcc to perform these saves
and restores, thus reducing function size at the cost of a
few extra instructions.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or
gnu2 conventions. gnu is the conservative default; gnu2 is
more efficient, but it may add compile- and run-time
requirements that cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This
method is shorter and usually equally fast as method using
SUB/MOV operations and is enabled by default. In some cases
disabling it may improve performance because of improved
scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing
arguments is computed in the function prologue. This is
faster on most modern CPUs because of reduced dependencies,
improved scheduling and reduced stack usage when the
preferred stack boundary is not equal to 2. The drawback is
a notable increase in code size. This switch implies
-mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs
that rely on thread-safe exception handling must compile and
link all code with the -mthreads option. When compiling,
-mthreads defines -D_MT; when linking, it links in a special
thread helper library -lmingwthrd which cleans up per-thread
exception-handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native
Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are
used, it may be that the Microsoft ABI lays out the structure
differently than the way GCC normally does. Particularly
when moving packed data between functions compiled with GCC
and the native Microsoft compiler (either via function call
or as data in a file), it may be necessary to access either
format.
This option is enabled by default for Microsoft Windows
targets. This behavior can also be controlled locally by use
of variable or type attributes. For more information, see
x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple
with the exception of the bit-field packing. The padding and
alignment of members of structures and whether a bit-field
can straddle a storage-unit boundary are determine by these
rules:
1. Structure members are stored sequentially in the order in
which they are
declared: the first member has the lowest memory address
and the last member the highest.
2. Every data object has an alignment requirement. The
alignment requirement
for all data except structures, unions, and arrays is
either the size of the object or the current packing size
(specified with either the "aligned" attribute or the
"pack" pragma), whichever is less. For structures,
unions, and arrays, the alignment requirement is the
largest alignment requirement of its members. Every
object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or
4-byte allocation
unit if the integral types are the same size and if the
next bit-field fits into the current allocation unit
without crossing the boundary imposed by the common
alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-
fields that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-
field. If the zero-length bit-field were removed, "t1"'s
size would be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field,
"foo", and the
alignment of the zero-length bit-field is greater than
the member that follows it, "bar", "bar" is aligned as
the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset
1. Accordingly, the size of "t2" is 4. For "t3", the
zero-length bit-field does not affect the alignment of
"bar" or, as a result, the size of the structure.
Taking this into account, it is important to note the
following:
1. If a zero-length bit-field follows a normal bit-field,
the type of the
zero-length bit-field may affect the alignment of the
structure as whole. For example, "t2" has a size of 4
bytes, since the zero-length bit-field follows a
normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a
normal bit-field, it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are
ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations.
This switch reduces code size and improves performance in
case the destination is already aligned, but GCC doesn't know
about it.
-minline-all-stringops
By default GCC inlines string operations only when the
destination is known to be aligned to least a 4-byte
boundary. This enables more inlining and increases code
size, but may improve performance of code that depends on
fast "memcpy", "strlen", and "memset" for short lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks
with inline code for small blocks and a library call for
large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular
algorithm to use for inlining string operations. The allowed
values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if
"__builtin_memcpy" should be inlined and what inline
algorithm to use when the expected size of the copy operation
is known. strategy is a comma-separated list of
alg:max_size:dest_align triplets. alg is specified in
-mstringop-strategy, max_size specifies the max byte size
with which inline algorithm alg is allowed. For the last
triplet, the max_size must be "-1". The max_size of the
triplets in the list must be specified in increasing order.
The minimal byte size for alg is 0 for the first triplet and
"max_size + 1" of the preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is
to control "__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf
functions. This avoids the instructions to save, set up, and
restore frame pointers and makes an extra register available
in leaf functions. The option -fomit-leaf-frame-pointer
removes the frame pointer for leaf functions, which might
make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets
from the TLS segment register (%gs for 32-bit, %fs for
64-bit), or whether the thread base pointer must be added.
Whether or not this is valid depends on the operating system,
and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions
with VEX prefix. The option -mavx turns this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call
before the prologue. Note: On x86 architectures the
attribute "ms_hook_prologue" isn't possible at the moment for
-mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section
that contains pointers to each profiling call. This is useful
for automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the
profiling functions as NOPs. This is useful when they should
be patched in later dynamically. This is likely only useful
together with -mrecord-mcount.
-minstrument-return=type
Instrument function exit in -pg -mfentry instrumented
functions with call to specified function. This only
instruments true returns ending with ret, but not sibling
calls ending with jump. Valid types are none to not
instrument, call to generate a call to __return__, or nop5 to
generate a 5 byte nop.
-mrecord-return
-mno-record-return
Generate a __return_loc section pointing to all return
instrumentation code.
-mfentry-name=name
Set name of __fentry__ symbol called at function entry for
-pg -mfentry functions.
-mfentry-section=name
Set name of section to record -mrecord-mcount calls (default
__mcount_loc).
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE
extensions disabled, -mskip-rax-setup can be used to skip
setting up RAX register when there are no variable arguments
passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily
saving vector registers on stack when passing variable
arguments, the impacts of this option are callees may waste
some stack space, misbehave or jump to a random location.
GCC 4.4 or newer don't have those issues, regardless the RAX
register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer
divide is much faster than 32-bit/64-bit integer divide.
This option generates a run-time check. If both dividend and
divisor are within range of 0 to 255, 8-bit unsigned integer
divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard.
Supported locations are global for global canary or tls for
per-thread canary in the TLS block (the default). This
option has effect only when -fstack-protector or
-fstack-protector-all is specified.
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify
which segment register (%fs or %gs) to use as base register
for reading the canary, and from what offset from that base
register. The default for those is as specified in the
relevant ABI.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers.
This prevents the compiler from using floating-point, vector,
mask and bound registers.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default is
keep, which keeps indirect call and jump unmodified. thunk
converts indirect call and jump to call and return thunk.
thunk-inline converts indirect call and jump to inlined call
and return thunk. thunk-extern converts indirect call and
jump to external call and return thunk provided in a separate
object file. You can control this behavior for a specific
function by using the function attribute "indirect_branch".
Note that -mcmodel=large is incompatible with
-mindirect-branch=thunk and -mindirect-branch=thunk-extern
since the thunk function may not be reachable in the large
code model.
Note that -mindirect-branch=thunk-extern is compatible with
-fcf-protection=branch since the external thunk can be made
to enable control-flow check.
-mfunction-return=choice
Convert function return with choice. The default is keep,
which keeps function return unmodified. thunk converts
function return to call and return thunk. thunk-inline
converts function return to inlined call and return thunk.
thunk-extern converts function return to external call and
return thunk provided in a separate object file. You can
control this behavior for a specific function by using the
function attribute "function_return".
Note that -mindirect-return=thunk-extern is compatible with
-fcf-protection=branch since the external thunk can be made
to enable control-flow check.
Note that -mcmodel=large is incompatible with
-mfunction-return=thunk and -mfunction-return=thunk-extern
since the thunk function may not be reachable in the large
code model.
-mindirect-branch-register
Force indirect call and jump via register.
These -m switches are supported in addition to the above on
x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment.
The -m32 option sets "int", "long", and pointer types to 32
bits, and generates code that runs on any i386 system.
The -m64 option sets "int" to 32 bits and "long" and pointer
types to 64 bits, and generates code for the x86-64
architecture. For Darwin only the -m64 option also turns off
the -fno-pic and -mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types to 32
bits, and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it
outputs the ".code16gcc" assembly directive at the beginning
of the assembly output so that the binary can run in 16-bit
mode.
The -miamcu option generates code which conforms to Intel MCU
psABI. It requires the -m32 option to be turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red
zone is mandated by the x86-64 ABI; it is a 128-byte area
beyond the location of the stack pointer that is not modified
by signal or interrupt handlers and therefore can be used for
temporary data without adjusting the stack pointer. The flag
-mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its
symbols must be linked in the lower 2 GB of the address
space. Pointers are 64 bits. Programs can be statically or
dynamically linked. This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in
the negative 2 GB of the address space. This model has to be
used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in
the lower 2 GB of the address space. Small symbols are also
placed there. Symbols with sizes larger than
-mlarge-data-threshold are put into large data or BSS
sections and can be located above 2GB. Programs can be
statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no
assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported
for 64-bit and x32 environments. It is the default address
mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported
for 32-bit and x32 environments. It is the default address
mode for 32-bit and x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows
targets:
-mconsole
This option specifies that a console application is to be
generated, by instructing the linker to set the PE header
subsystem type required for console applications. This
option is available for Cygwin and MinGW targets and is
enabled by default on those targets.
-mdll
This option is available for Cygwin and MinGW targets. It
specifies that a DLL---a dynamic link library---is to be
generated, enabling the selection of the required runtime
startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It
specifies that the "dllimport" attribute should be ignored.
-mthread
This option is available for MinGW targets. It specifies that
MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes
the "UNICODE" preprocessor macro to be predefined, and
chooses Unicode-capable runtime startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It
specifies that the typical Microsoft Windows predefined
macros are to be set in the pre-processor, but does not
influence the choice of runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be generated by
instructing the linker to set the PE header subsystem type
appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that
the executable flag for the stack used by nested functions
isn't set. This is necessary for binaries running in kernel
mode of Microsoft Windows, as there the User32 API, which is
used to set executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It
specifies that relocated-data in read-only section is put
into the ".data" section. This is a necessary for older
runtimes not supporting modification of ".rdata" sections for
pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE file format that
permits the correct alignment of COMMON variables should be
used when generating code. It is enabled by default if GCC
detects that the target assembler found during configuration
supports the feature.
See also under x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the
simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading
constant values. The "CONST16" instruction is currently not
a standard option from Tensilica. When enabled, "CONST16"
instructions are always used in place of the standard "L32R"
instructions. The use of "CONST16" is enabled by default
only if the "L32R" instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and
multiply/subtract instructions in the floating-point option.
This has no effect if the floating-point option is not also
enabled. Disabling fused multiply/add and multiply/subtract
instructions forces the compiler to use separate instructions
for the multiply and add/subtract operations. This may be
desirable in some cases where strict IEEE 754-compliant
results are required: the fused multiply add/subtract
instructions do not round the intermediate result, thereby
producing results with more bits of precision than specified
by the IEEE standard. Disabling fused multiply add/subtract
instructions also ensures that the program output is not
sensitive to the compiler's ability to combine multiply and
add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions
before "volatile" memory references to guarantee sequential
consistency. The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code
must be position-independent code (PIC), this option disables
PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The
default is -mno-text-section-literals, which places literals
in a separate section in the output file. This allows the
literal pool to be placed in a data RAM/ROM, and it also
allows the linker to combine literal pools from separate
object files to remove redundant literals and improve code
size. With -mtext-section-literals, the literals are
interspersed in the text section in order to keep them as
close as possible to their references. This may be necessary
for large assembly files. Literals for each function are
placed right before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The
default is -mno-auto-litpools, which places literals in a
separate section in the output file unless
-mtext-section-literals is used. With -mauto-litpools the
literals are interspersed in the text section by the
assembler. Compiler does not produce explicit ".literal"
directives and loads literals into registers with "MOVI"
instructions instead of "L32R" to let the assembler do
relaxation and place literals as necessary. This option
allows assembler to create several literal pools per function
and assemble very big functions, which may not be possible
with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties
at the expense of some code density. The assembler attempts
to widen density instructions to align branch targets and the
instructions following call instructions. If there are not
enough preceding safe density instructions to align a target,
no widening is performed. The default is -mtarget-align.
These options do not affect the treatment of auto-aligned
instructions like "LOOP", which the assembler always aligns,
either by widening density instructions or by inserting NOP
instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to
translate direct calls to indirect calls unless it can
determine that the target of a direct call is in the range
allowed by the call instruction. This translation typically
occurs for calls to functions in other source files.
Specifically, the assembler translates a direct "CALL"
instruction into an "L32R" followed by a "CALLX" instruction.
The default is -mno-longcalls. This option should be used in
programs where the call target can potentially be out of
range. This option is implemented in the assembler, not the
compiler, so the assembly code generated by GCC still shows
direct call instructions---look at the disassembled object
code to see the actual instructions. Note that the assembler
uses an indirect call for every cross-file call, not just
those that really are out of range.
zSeries Options
These are listed under
This section describes several environment variables that affect
how GCC operates. Some of them work by specifying directories or
prefixes to use when searching for various kinds of files. Some
are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options
such as -B, -I and -L. These take precedence over places
specified using environment variables, which in turn take
precedence over those specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses
localization information which allows GCC to work with
different national conventions. GCC inspects the locale
categories LC_CTYPE and LC_MESSAGES if it has been configured
to do so. These locale categories can be set to any value
supported by your installation. A typical value is
en_GB.UTF-8 for English in the United Kingdom encoded in
UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character
boundaries in a string; this is needed for some multibyte
encodings that contain quote and escape characters that are
otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language
to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the
value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and
LC_MESSAGES default to the value of the LANG environment
variable. If none of these variables are set, GCC defaults
to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for
temporary files. GCC uses temporary files to hold the output
of one stage of compilation which is to be used as input to
the next stage: for example, the output of the preprocessor,
which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the
documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in
the names of the subprograms executed by the compiler. No
slash is added when this prefix is combined with the name of
a subprogram, but you can specify a prefix that ends with a
slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an
appropriate prefix to use based on the pathname it is invoked
with.
If GCC cannot find the subprogram using the specified prefix,
it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where
prefix is the prefix to the installed compiler. In many cases
prefix is the value of "prefix" when you ran the configure
script.
Other prefixes specified with -B take precedence over this
prefix.
This prefix is also used for finding files such as crt0.o
that are used for linking.
In addition, the prefix is used in an unusual way in finding
the directories to search for header files. For each of the
standard directories whose name normally begins with
/usr/local/lib/gcc (more precisely, with the value of
GCC_INCLUDE_DIR), GCC tries replacing that beginning with the
specified prefix to produce an alternate directory name.
Thus, with -Bfoo/, GCC searches foo/bar just before it
searches the standard directory /usr/local/lib/bar. If a
standard directory begins with the configured prefix then the
value of prefix is replaced by GCC_EXEC_PREFIX when looking
for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it cannot find
the subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when
searching for special linker files, if it cannot find them
using GCC_EXEC_PREFIX. Linking using GCC also uses these
directories when searching for ordinary libraries for the -l
option (but directories specified with -L come first).
LANG
This variable is used to pass locale information to the
compiler. One way in which this information is used is to
determine the character set to be used when character
literals, string literals and comments are parsed in C and
C++. When the compiler is configured to allow multibyte
characters, the following values for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then
the compiler uses "mblen" and "mbtowc" as defined by the
default locale to recognize and translate multibyte
characters.
Some additional environment variables affect the behavior of the
preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a
special character, much like PATH, in which to look for
header files. The special character, "PATH_SEPARATOR", is
target-dependent and determined at GCC build time. For
Microsoft Windows-based targets it is a semicolon, and for
almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I options
on the command line. This environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when
preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if
specified with -isystem, but after any paths given with
-isystem options on the command line.
In all these variables, an empty element instructs the
compiler to search its current working directory. Empty
elements can appear at the beginning or end of a path. For
instance, if the value of CPATH is ":/special/include", that
has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored
in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing
the target name from the source file name. Or the value can
have the form file target, in which case the rules are
written to file file using target as the target name.
In other words, this environment variable is equivalent to
combining the options -MM and -MF, with an optional -MT
switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it
implies -M rather than -MM. However, the dependence on the
main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp
to be used in replacement of the current date and time in the
"__DATE__" and "__TIME__" macros, so that the embedded
timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,
defined as the number of seconds (excluding leap seconds)
since 01 Jan 1970 00:00:00 represented in ASCII; identical to
the output of @command{date +%s} on GNU/Linux and other
systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last
modification time of the source or package and it should be
set by the build process.
For instructions on reporting bugs, see
<https://gcc.gnu.org/bugs/ >.
1. On some systems, gcc -shared needs to build supplementary
stub code for constructors to work. On multi-libbed systems,
gcc -shared must select the correct support libraries to link
against. Failing to supply the correct flags may lead to
subtle defects. Supplying them in cases where they are not
necessary is innocuous.
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1),
gdb(1), dbx(1) and the Info entries for gcc, cpp, as, ld,
binutils and gdb.
See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html >, for
contributors to GCC.
Copyright (c) 1988-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with the Invariant Sections being "GNU General Public
License" and "Funding Free Software", the Front-Cover texts being
(a) (see below), and with the Back-Cover Texts being (b) (see
below). A copy of the license is included in the gfdl(7) man
page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
This page is part of the gcc (GNU Compiler Collection) project.
Information about the project can be found at
⟨http://gcc.gnu.org/⟩. If you have a bug report for this manual
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gcc-9.5.0 2022-05-27 GCC(1)
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