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GCC(1) GNU GCC(1) NAME gcc - GNU project C and C++ compiler SYNOPSIS gcc [-c|-S|-E] [-std=standard] [-g] [-pg] [-Olevel] [-Wwarn...] [-pedantic] [-Idir...] [-Ldir...] [-Dmacro[=defn]...] [-Umacro] [-foption...] [-mmachine-option...] [-o outfile] [@file] infile... Only the most useful options are listed here; see below for the remain- der. g++ accepts mostly the same options as gcc. DESCRIPTION When you invoke GCC, it normally does preprocessing, compilation, as- sembly 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 stage of processing. Some options control the preprocessor and others the compiler itself. Yet other op- tions control the assembler and linker; most of these are not docu- mented 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 gcc program accepts options and file names as operands. Many op- tions have multi-letter names; therefore multiple single-letter options may not be grouped: -dr is very different from -d -r. 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. 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 would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default. OPTIONS 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 -combine -pipe -pass-exit-codes -x language -v -### --help --target-help --version @file C Language Options -ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fopenmp -fms-extensions -trigraphs -no-integrated-cpp -tradi- tional -traditional-cpp -fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -fun- signed-char C++ Language Options -fabi-version=n -fno-access-control -fcheck-new -fconserve-space -ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-tem- plates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fno-nonansi-builtins -fno-operator-names -fno-op- tional-diags -fpermissive -frepo -fno-rtti -fstats -ftem- plate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo Objective-C and Objective-C++ Language Options -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector Language Independent Options -fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnostics-show-option Warning Options -fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waggregate-return -Wno-attributes -Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconver- sion -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror=* -Werror-im- plicit-function-declaration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-se- curity -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-at- tribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredun- dant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wstrict-overflow -Wstrict-overflow=n -Wswitch -Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitial- ized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings C-only Warning Options -Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement -Wpointer-sign Debugging Options -dletters -dumpspecs -dumpmachine -dumpversion -fdump-noaddr -fdump-unnumbered -fdump-translation-unit[-n] -fdump-class-hierar- chy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-in- lined[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyre- name[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -feliminate-un- used-debug-types -feliminate-unused-debug-symbols -femit-class-de- bug-always -fmem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-prog-name=program -print-search-dirs -Q -save-temps -time Optimization Options -falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n -fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-tar- get-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks -fcx-limited-range -fdata-sections -fde- layed-branch -fdelete-null-pointer-checks -fearly-inlining -fex- pensive-optimizations -ffast-math -ffloat-store -fforce-addr -ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload -fcrossjumping -fif-conversion -fif-conver- sion2 -finline-functions -finline-functions-called-once -fin- line-limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmodulo-sched -fno-branch-count-reg -fno-default-inline -fno-defer-pop -fmove-loop-invariants -fno-function-cse -fno-guess-branch-proba- bility -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -funsafe-loop-optimizations -ffi- nite-math-only -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-reg- ister-move -foptimize-sibling-calls -fprefetch-loop-arrays -fpro- file-generate -fprofile-use -fregmove -frename-registers -fre- order-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frounding-math -frtl-abstract-sequences -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=n -fsched-stalled-insns-dep=n -fsched2-use-superblocks -fsched2-use-traces -fsee -fresched- ule-modulo-scheduled-loops -fsection-anchors -fsignaling-nans -fsingle-precision-constant -fstack-protector -fstack-protec- tor-all -fstrict-aliasing -fstrict-overflow -ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expan- sion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-op- timize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -ftree-vect-loop-version -ftree-salias -fipa-pta -fweb -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fw- hole-program --param name=value -O -O0 -O1 -O2 -O3 -Os Preprocessor Options -Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option Assembler Option -Wa,option -Xassembler option Linker Options object-file-name -llibrary -nostartfiles -nodefaultlibs -nost- dlib -pie -rdynamic -s -static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol Directory Options -Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir Target Options -V version -b machine Machine Dependent Options ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-data-section ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-pro- log -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-lit- tle-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-sin- gle-pic-base -mpic-register=reg -mnop-fun-dllimport -mcir- rus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-func- tion-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack -mint8 Blackfin Options -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mid-shared-library -mno-id-shared-library -mshared-library-id=n -mlong-calls -mno-long-calls 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 CRX Options -mmac -mpush-args Darwin Options -all_load -allowable_client -arch -arch_er- rors_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_in- stall_names -image_base -init -install_name -keep_private_ex- terns -multi_module -multiply_defined -multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -no- multidefs -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 -mma- cosx-version-min=version -mkernel -mone-byte-bool DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -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 DEC Alpha/VMS Options -mvms-return-codes 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-mem- bar -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 -muclibc H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300 HPPA Options -march=architecture-type -mbig-switch -mdis- able-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-big-switch -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 i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfp- math=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-dou- ble -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -m3dnow -mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mregparm=num -msseregparm -mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata -mcon- stant-gp -mauto-pic -minline-float-divide-min-latency -min- line-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -minline-sqrt-min-latency -min- line-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64 -mno-sched-br-data-spec -msched-ar-data-spec -mno-sched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-con- trol-spec -msched-ldc -mno-sched-control-ldc -mno-sched-spec-ver- bose -mno-sched-prefer-non-data-spec-insns -mno-sched-pre- fer-non-control-spec-insns -mno-sched-count-spec-in-critical-path 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 -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -mcfv4e -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mshort -msoft-reg-count=count MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-im- mediates -mno-relax-immediates -mwide-bitfields -mno-wide-bit- fields -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 MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mdsp -mpaired-single -mips3d -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -membed- ded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mto- plevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit -mno-sin- gle-exit MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax MT Options -mno-crt0 -mbacc -msim -march=cpu-type PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mab- shi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm PowerPC Options See RS/6000 and PowerPC Options. RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-pow- erpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopc- ntb -mno-popcntb -mfprnd -mno-fprnd -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -ma- lign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -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 -maltivec -mswdiv -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-re- turn -mabi=abi-type -msecure-plt -mbss-plt -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -ms- mall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -madjust-un- roll -mindexed-addressing -mgettrcost=number -mpt-fixed -minvalid-symbols SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mimpure-text -mno-im- pure-text -mlittle-endian -mstack-bias -mno-stack-bias -mu- naligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads -pthread System V Options -Qy -Qn -YP,paths -Ym,dir TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmem- parm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mpro- log-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-dis- able-callt -mv850e1 -mv850e -mv850 -mbig-switch VAX Options -mg -mgnu -munix x86-64 Options See i386 and x86-64 Options. Xstormy16 Options -msim Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls zSeries Options See S/390 and zSeries Options. Code Generation Options -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check -fs- tack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias -fargument-noalias -fargument-noalias-global -fargu- ment-noalias-anything -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility 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 in- put file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an exe- cutable file. For any given input file, the file name suffix determines what kind of compilation is done: file.c C source code which must be preprocessed. file.i C source code which should not be preprocessed. file.ii C++ source code which 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 which should not be preprocessed. file.mm file.M Objective-C++ source code. Note that you must link with the li- bobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M. file.mii Objective-C++ source code which should not be preprocessed. file.h C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header. file.cc file.cp file.cxx file.cpp file.CPP file.c++ file.C C++ source code which 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 which must be preprocessed. file.mii Objective-C++ source code which should not be preprocessed. file.hh file.H C++ header file to be turned into a precompiled header. file.f file.for file.FOR Fixed form Fortran source code which should not be preprocessed. file.F file.fpp file.FPP Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor). file.f90 file.f95 Free form Fortran source code which should not be preprocessed. file.F90 file.F95 Free form Fortran source code which must be preprocessed (with the traditional preprocessor). file.ads Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic in- stantiation), 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 Assembler code which 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 c-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 f95 f95-cpp-input java treelang -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). -pass-exit-codes Normally the gcc program will exit 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 will instead return with numeri- cally highest error produced by any phase that returned an error indication. The C, C++, and Fortran frontends return 4, if an in- ternal compiler error is encountered. 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 re- placing 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-assem- bler 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 which don't require preprocessing are ignored. -o file Place output in file file. This applies regardless 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 com- piler driver program and of the preprocessor and the compiler proper. -### Like -v except the commands are not executed and all command argu- ments are quoted. This is useful for shell scripts to capture the driver-generated command lines. -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 as- sembler has no trouble. -combine If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropri- ate for it. For those languages that do not support IMA this op- tion will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in con- junction with -save-temps, the compiler will generate multiple pre- processed files (one for each source file), but only one (combined) .o or .s file. --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 will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept. If the -Wextra option is also specified then command line options which have no documentation associated with them will also be dis- played. --target-help Print (on the standard output) a description of target specific command line options for each tool. --version Display the version number and copyrights of the invoked GCC. @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 charac- ter 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 in- cluded 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 or .H; 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 com- piler 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 treats .c, .h and .i files as C++ source files in- stead of C source files unless -x is used, and automatically specifies linking against the C++ library. This program is also useful when pre- compiling a C header file with a .h extension for use in C++ compila- tions. On many systems, g++ is also installed with the name c++. When you compile C++ programs, you may specify many of the same com- mand-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or op- tions 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, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++. 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, -pedantic 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 de- claring 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 which would normally be built in but do not have seman- tics defined by ISO C (such as "alloca" and "ffs") are not built-in functions with -ansi is used. -std= Determine the language standard. This option is currently only supported when compiling C or C++. A value for this option must be provided; possible values are c89 iso9899:1990 ISO C90 (same as -ansi). iso9899:199409 ISO C90 as modified in amendment 1. c99 c9x iso9899:1999 iso9899:199x ISO C99. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/gcc-4.2/c99status.html> for more infor- mation. The names c9x and iso9899:199x are deprecated. gnu89 Default, ISO C90 plus GNU extensions (including some C99 fea- tures). gnu99 gnu9x ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is depre- cated. c++98 The 1998 ISO C++ standard plus amendments. gnu++98 The same as -std=c++98 plus GNU extensions. This is the de- fault for C++ code. Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use "__restrict__" even when -std=c99 is not specified. The -std options specifying some version of ISO C have the same ef- fects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the "in- line" keyword in ISO C99) are not disabled. -fgnu89-inline The option -fgnu89-inline tells GCC to use the traditional GNU se- mantics for "inline" functions when in C99 mode. Using this option is roughly equivalent to adding the "gnu_in- line" function attribute to all inline functions. This option is accepted by GCC versions 4.1.3 and up. In GCC ver- sions prior to 4.3, C99 inline semantics are not supported, and thus this option is effectively assumed to be present regardless of whether or not it is specified; the only effect of specifying it explicitly is to disable warnings about using inline functions in C99 mode. Likewise, the option -fno-gnu89-inline is not supported in versions of GCC before 4.3. It will be supported only in C99 or gnu99 mode, not in C89 or gnu89 mode. The preprocesor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_IN- LINE__" may be used to check which semantics are in effect for "in- line" functions. -aux-info filename Output to the given filename prototyped declarations for all func- tions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any lan- guage 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 de- claration. -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 be- come single instructions that 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 this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-func- tion 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)) -fhosted Assert that compilation takes place in 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 takes place in 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. -fopenmp Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is specified, the compiler gen- erates parallel code according to the OpenMP Application Program Interface v2.5 <http://www.openmp.org/>. -fms-extensions Accept some non-standard constructs used in Microsoft header files. Some cases of unnamed fields in structures and unions are only ac- cepted with this option. -trigraphs Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs. -no-integrated-cpp Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (inter- nal cpp) The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged. -traditional -traditional-cpp Formerly, these options caused GCC to attempt to emulate a pre- standard C compiler. They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details. -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++. -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 "un- signed 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 ma- chines 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. Options Controlling C++ Dialect This section describes the command-line options that are only meaning- ful for C++ programs; but you can also use most of the GNU compiler op- tions regardless of what language your program is in. For example, you might compile a file "firstClass.C" like this: g++ -g -frepo -O -c firstClass.C In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC. Here is a list of options that are only for compiling C++ programs: -fabi-version=n Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specifica- tion. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed. The default is version 2. -fno-access-control Turn off all access checking. This switch is mainly useful for working around bugs in the access control code. -fcheck-new Check that the pointer returned by "operator new" is non-null be- fore attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that "oper- ator new" will only return 0 if it is declared throw(), in which case the compiler will always check 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). -fconserve-space Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two definitions were merged. This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common. -ffriend-injection Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are de- clared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function which is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be in- jected as they were in earlier releases. This option is for compatibility, and may be removed in a future release of G++. -fno-elide-constructors The C++ standard allows an implementation to omit creating a tempo- rary which 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. -fno-enforce-eh-specs Don't generate code to check for violation of exception specifica- tions at runtime. 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 com- piler will still optimize based on the specifications, so throwing an unexpected exception will result in undefined behavior. -ffor-scope -fno-for-scope If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++. The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior. -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__" in- stead. -ansi implies -fno-gnu-keywords. -fno-implicit-templates Never emit code for non-inline templates which are instantiated im- plicitly (i.e. by use); only emit code for explicit instantiations. -fno-implicit-inline-templates Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that com- piles with and without optimization will need the same set of ex- plicit instantiations. -fno-implement-inlines To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This will cause linker er- rors if these functions are not inlined everywhere they are called. -fms-extensions Disable pedantic warnings about constructs used in MFC, such as im- plicit int and getting a pointer to member function via non-stan- dard syntax. -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. -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 will allow some nonconforming code to compile. -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++ runtime 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 it will generate it as needed. The dynamic_cast operator can still be used for casts that do not require runtime type information, i.e. casts to "void *" or to unambiguous base classes. -fstats Emit statistics about front-end processing at the end of the compi- lation. This information is generally only useful to the G++ de- velopment team. -ftemplate-depth-n Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect end- less recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17. -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 will only work if your C library supports "__cxa_atexit". -fno-use-cxa-get-exception-ptr Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause "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 methods where the addresses of the two functions were taken in different shared objects. The effect of this is that GCC may, effectively, mark inline meth- ods 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 mas- sively reduces the size of the dynamic export table when the li- brary makes heavy use of templates. The behaviour of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static vari- ables 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 will have no effect. Explicitly instantiated inline methods are unaffected by this op- tion as their linkage might otherwise cross a shared library bound- ary. -fno-weak Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are avail- able. This option exists only for testing, and should not be used by end-users; it will result 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: -fno-default-inline Do not assume inline for functions defined inside a class scope. Note that these functions will have linkage like inline func- tions; they just won't be inlined by default. -Wabi (C++ only) Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. 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 will be 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 bi- nary compatible with code generated by other compilers. The known incompatibilities at this point include: * Incorrect handling of tail-padding for bit-fields. G++ may at- tempt to pack data into the same byte as a base class. For ex- ample: struct A { virtual void f(); int f1 : 1; }; struct B : public A { int f2 : 1; }; In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can avoid this prob- lem by explicitly padding "A" so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout "B" identically. * Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For exam- ple: struct A { virtual void f(); char c1; }; struct B { B(); char c2; }; struct C : public A, public virtual B {}; In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can avoid this problem by ex- plicitly padding "A" so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout "C" identically. * Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example: union U { int i : 4096; }; Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits in an "int". * Empty classes can be placed at incorrect offsets. For example: struct A {}; struct B { A a; virtual void f (); }; struct C : public B, public A {}; G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the "A" data member of "B" is already at offset zero. * Names of template functions whose types involve "typename" or template template parameters can be mangled incorrectly. template <typename Q> void f(typename Q::X) {} template <template <typename> class Q> void f(typename Q<int>::X) {} Instantiations of these templates may be mangled incorrectly. -Wctor-dtor-privacy (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. -Wnon-virtual-dtor (C++ only) Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warn- ing is also enabled if -Weffc++ is specified. -Wreorder (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 will rearrange 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. The following -W... options are not affected by -Wall. -Weffc++ (C++ only) Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book: * Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory. * Item 12: Prefer initialization to assignment in constructors. * Item 14: Make destructors virtual in base classes. * Item 15: Have "operator=" return a reference to *this. * Item 23: Don't try to return a reference when you must return an object. Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book: * Item 6: Distinguish between prefix and postfix forms of incre- ment and decrement operators. * Item 7: Never overload "&&", "||", or ",". 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. -Wno-deprecated (C++ only) Do not warn about usage of deprecated features. -Wstrict-null-sentinel (C++ only) Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is de- fined to "__null". Although it is a null pointer constant not a null pointer, it is guaranteed to of the same size as a pointer. But this use is not portable across different compilers. -Wno-non-template-friend (C++ only) Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specifi- cation support in G++, if the name of the friend is an unqualified- id (i.e., friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary, nontemplate func- tion. (Section 14.5.3). Before G++ implemented explicit specifi- cation, unqualified-ids could be interpreted as a particular spe- cialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-tem- plate-friend allows the compiler to check existing code for poten- tial trouble spots and is on by default. This new compiler behav- ior can be turned off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warning. -Wold-style-cast (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, sta- tic_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. -Woverloaded-virtual (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(); will fail to compile. -Wno-pmf-conversions (C++ only) Disable the diagnostic for converting a bound pointer to member function to a plain pointer. -Wsign-promo (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++ would try to pre- serve unsignedness, but the standard mandates the current behavior. struct A { operator int (); A& operator = (int); }; main () { A a,b; a = b; } In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =. Options Controlling Objective-C and Objective-C++ Dialects (NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See This section describes the command-line options that are only meaning- ful for Objective-C and Objective-C++ programs, but 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 lan- guage supported by GCC. Note that since Objective-C is an extension of the C language, Objec- tive-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 be- low). The -fconstant-cfstrings option, if also present, will over- ride the -fconstant-string-class setting and cause "@"..."" liter- als 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 de- fault 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 (e.g., "[receiver message:arg]") in this translation unit ensure that the receiver is not "nil". This allows for more efficient entry points in the run- time to be used. Currently, this option is only available in con- junction with the NeXT runtime on Mac OS X 10.3 and later. -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 that will run 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 destruc- tor, and if so, synthesize a special "- (void) .cxx_destruct" method that will run all such default destructors, in reverse or- der. The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will only operate on instance variables declared in the current Objective-C class, and not those inherited from su- perclasses. 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 will be invoked by the runtime im- mediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods will be invoked immediately before the run- time 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 ac- complished via the comm page. -fobjc-exceptions Enable syntactic support for structured exception handling in Ob- jective-C, similar to what is offered by C++ and Java. This option is unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier. @try { ... @throw expr; ... } @catch (AnObjCClass *exc) { ... @throw expr; ... @throw; ... } @catch (AnotherClass *exc) { ... } @catch (id allOthers) { ... } @finally { ... @throw expr; ... } The @throw statement may appear anywhere in an Objective-C or Ob- jective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown. Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any). The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java. There are several caveats to using the new exception mechanism: * Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided by the "NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime. * As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it in C++, or vice versa (i.e., "throw ... @catch"). The -fobjc-exceptions switch also enables the use of synchroniza- tion blocks for thread-safe execution: @synchronized (ObjCClass *guard) { ... } Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making "guard" available to other threads). Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @syn- chronized blocks is allowed, and will cause the guarding object to be unlocked properly. -fobjc-gc Enable garbage collection (GC) in Objective-C and Objective-C++ programs. -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-Con- tinue debugging mode, where the object file in question may be re- compiled and dynamically reloaded in the course of program execu- tion, 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 re- places 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 im- plementations to be modified during program execution. -gen-decls Dump interface declarations for all classes seen in the source file to a file named sourcename.decl. -Wassign-intercept Warn whenever an Objective-C assignment is being intercepted by the garbage collector. -Wno-protocol 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 implemen- tation is inherited from the superclass. If you use the -Wno-pro- tocol option, then methods inherited from the superclass are con- sidered to be implemented, and no warning is issued for them. -Wselector 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 "@selec- tor(...)" 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 ex- ample because an error is found during compilation, or because the -fsyntax-only option is being used. -Wstrict-selector-match Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a mes- sage using this selector to a receiver of type "id" or "Class". When this flag is off (which is the default behavior), the compiler will omit such warnings if any differences found are confined to types which share the same size and alignment. -Wundeclared-selector 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 decla- ration, 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 conven- tion 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, ...). The options de- scribed below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly. -fmessage-length=n Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line. -fdiagnostics-show-location=once Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; 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-show-option This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option di- rectly controls that diagnostic, when such an option is known to the diagnostic machinery. Options to Request or Suppress Warnings Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error. You can request many specific warnings with options beginning -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. The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Di- alect Options and Objective-C and Objective-C++ Dialect Options. -fsyntax-only Check the code for syntax errors, but don't do anything beyond that. -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 ver- sion 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 will 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++ fea- tures are supported as well. With this option, they are rejected. -pedantic 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__". How- ever, only system header files should use these escape routes; ap- plication programs should avoid them. Some users try to use -pedantic 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 diag- nostics 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 -pedantic. 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 gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic are given where they are required by the base standard. (It would 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 Like -pedantic, except that errors are produced rather than warn- ings. -w Inhibit all warning messages. -Wno-import Inhibit warning messages about the use of #import. -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. -Wcomment 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. -Wfatal-errors This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing fur- ther error messages. -Wformat 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 with- out format attributes having been specified depends on the standard version selected, and such checks of functions without the at- tribute 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 -pedantic is used with -Wformat, warnings will be given about for- mat features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C standard). Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. -Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-ex- tra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wfor- mat-security, and -Wformat=2 are available, but are not included in -Wall. -Wformat-y2k If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year. -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 will suppress the warning if the unused arguments are all pointers, since the Single Unix Speci- fication says that such unused arguments are allowed. -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-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 cur- rently a subset of what -Wformat-nonliteral warns about, but in fu- ture warnings may be added to -Wformat-security that are not in- cluded in -Wformat-nonliteral.) -Wformat=2 Enable -Wformat plus format checks not included in -Wformat. Cur- rently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. -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. -Winit-self (C, C++, Objective-C and Objective-C++ only) Warn about uninitialized variables which are initialized with them- selves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above. For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has been specified: int f() { int i = i; return i; } -Wimplicit-int Warn when a declaration does not specify a type. This warning is enabled by -Wall. -Wimplicit-function-declaration -Werror-implicit-function-declaration Give a warning (or error) whenever a function is used before being declared. The form -Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning, not an error). -Wimplicit Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. -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 en- abled by -Wall. -Wmissing-braces Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully brack- eted, but that for b is fully bracketed. 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. -Wparentheses Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is ex- pected, or when operators are nested whose precedence people often get confused about. Only the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are only supported when compiling C. 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 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, 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 will issue 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" could belong to the enclosing "if". The resulting code would look like this: { if (a) { if (b) foo (); else bar (); } } This warning is enabled by -Wall. -Wsequence-point Warn about code that may have undefined semantics because of viola- tions of sequence point rules in the C and C++ standards. The C and C++ standards defines the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which rep- resent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those exe- cuted after it. These occur after the evaluation of a full expres- sion (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 evalu- ation of its arguments and the expression denoting the called func- tion), 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 func- tion 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 de- termine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpre- dictable. Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more complicated cases are not di- agnosed by this option, and it may give an occasional false posi- tive result, but in general it has been found fairly effective at detecting this sort of problem in programs. 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++. -Wreturn-type Warn whenever a function is defined with a return-type that de- faults to "int". Also warn about any "return" statement with no return-value in a function whose return-type is not "void". For C, also warn if the return type of a function has a type quali- fier such as "const". Such a type qualifier has no effect, since the value returned by a function is not an lvalue. ISO C prohibits qualified "void" return types on function definitions, so such re- turn types always receive a warning even without this option. For C++, a function without return type always produces a diagnos- tic message, even when -Wno-return-type is specified. The only ex- ceptions are main and functions defined in system headers. This warning is enabled by -Wall. -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 enu- meration. (The presence of a "default" label prevents this warn- ing.) "case" labels outside the enumeration range also provoke warnings when this option is used. 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 enu- meration. "case" labels outside the enumeration range also provoke warnings when this option is used. -Wtrigraphs Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning 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-parameter Warn whenever a function parameter is unused aside from its decla- ration. To suppress this warning use the unused attribute. -Wunused-variable Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall. To suppress this warning use the unused attribute. -Wunused-value Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by -Wall. To suppress this warning cast the expression to void. -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 initial- ized or if a variable may be clobbered by a "setjmp" call. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when opti- mizing. If you do not specify -O, you will not get these warnings. Instead, GCC will issue a warning about -Wuninitialized requiring -O. If you want to warn about code which uses the uninitialized value of the variable in its own initializer, use the -Winit-self option. These warnings occur for individual uninitialized or clobbered ele- ments of structure, union or array variables as well as for vari- ables which 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 will depend on the precise optimiza- tion 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 warn- ings are printed. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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 ini- tialized, but GCC doesn't know this. Here is another common case: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } This has no bug because "save_y" is used only if it is set. This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". These warnings as well are pos- sible only in optimizing compilation. 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 which would cause a problem. Some spurious warnings can be avoided if you declare all the func- tions you use that never return as "noreturn". This warning is enabled by -Wall. -Wunknown-pragmas Warn when a #pragma directive is encountered which is not under- stood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were 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 -Wun- known-pragmas. -Wstrict-aliasing This option is only active when -fstrict-aliasing is active. It warns about code which 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. -Wstrict-aliasing=2 This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. This warning catches more cases than -Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe. -Wstrict-overflow -Wstrict-overflow=n This option is only active when -fstrict-overflow is active. It warns about cases where the compiler optimizes based on the assump- tion 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 which 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 which 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 will require, in particular when determining whether a loop will be executed at all. @option<-Wstrict-overflow=1> Warn about cases which are both questionable and easy to avoid. For example: "x + 1 > x"; with -fstrict-overflow, the compiler will simplify this to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested. @option<-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 -fstrict-overflow is in effect, because "abs (INT_MIN)" overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-over- flow=2. @option<-Wstrict-overflow=3> Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" will be simplified to "x > 0". @option<-Wstrict-overflow=4> Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" will be simplified to "x * 2". @option<-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" will be simplified to "x + 1 >= y". This is reported only at the highest warning level because this simplification ap- plies to many comparisons, so this warning level will give a very large number of false positives. -Wall All of the above -W options combined. This enables all the warn- ings 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 Ob- jective-C++ Dialect Options. The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider question- able, 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. -Wextra (This option used to be called -W. The older name is still sup- ported, but the newer name is more descriptive.) Print extra warn- ing messages for these events: * A function can return either with or without a value. (Falling off the end of the function body is considered returning with- out a value.) For example, this function would evoke such a warning: foo (a) { if (a > 0) return a; } * An expression-statement or the left-hand side of a comma ex- pression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not. * An unsigned value is compared against zero with < or >=. * Storage-class specifiers like "static" are not the first things in a declaration. According to the C Standard, this usage is obsolescent. * If -Wall or -Wunused is also specified, warn about unused argu- ments. * A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to un- signed. (But don't warn if -Wno-sign-compare is also speci- fied.) * An aggregate has an initializer which does not initialize all members. This warning can be independently controlled by -Wmissing-field-initializers. * An initialized field without side effects is overridden when using designated initializers. This warning can be indepen- dently controlled by -Woverride-init. * A function parameter is declared without a type specifier in K&R-style functions: void foo(bar) { } * An empty body occurs in an if or else statement. * A pointer is compared against integer zero with <, <=, >, or >=. * A variable might be changed by longjmp or vfork. *<(C++ only)> An enumerator and a non-enumerator both appear in a conditional expression. *<(C++ only)> A non-static reference or non-static const member appears in a class without constructors. *<(C++ only)> Ambiguous virtual bases. *<(C++ only)> Subscripting an array which has been declared register. *<(C++ only)> Taking the address of a variable which has been declared regis- ter. *<(C++ only)> A base class is not initialized in a derived class' copy con- structor. -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 legiti- mate 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 as- sumption 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 con- junction with this option will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used. -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 would check to see whether the two values have ranges that overlap; and this is done with the relational op- erators, so equality comparisons are probably mistaken. -Wtraditional (C only) Warn about certain constructs that behave differently in tradi- tional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs which 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 does not in ISO C. * In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C under- stands but would ignore because the # does not appear as the first character on the line. It also suggests you hide direc- tives like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognize #elif, so it 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 de- fined 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 is- sued 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 sepa- rate 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 would cause serious problems. This is a subset of the possible conversion warnings, for the full set use -Wconversion. * Use of ISO C style function definitions. This warning inten- tionally is not issued for prototype declarations or variadic functions because these ISO C features will 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 exten- sion and thus not relevant to traditional C compatibility. -Wdeclaration-after-statement (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 and was not supported by GCC versions before GCC 3.0. -Wundef Warn if an undefined identifier is evaluated in an #if directive. -Wno-endif-labels Do not warn whenever an #else or an #endif are followed by text. -Wshadow Warn whenever a local variable shadows another local variable, pa- rameter or global variable or whenever a built-in function is shad- owed. -Wlarger-than-len Warn whenever an object of larger than len bytes is defined. -Wunsafe-loop-optimizations Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices. With -fun- safe-loop-optimizations warn if the compiler made such assumptions. -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 conve- nience in calculations with "void *" pointers and pointers to func- tions. -Wbad-function-cast (C only) Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast to "anything *". -Wc++-compat 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. -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 *". -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. -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 will get a warning; when compiling C++, warn about the deprecated conversion from string literals to "char *". This warning, by default, is enabled for C++ programs. These warnings will 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 us- ing "const" in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make -Wall request these warnings. -Wconversion Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a proto- type. 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. Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assign- ment "x = -1" if "x" is unsigned. But do not warn about explicit casts like "(unsigned) -1". -Wsign-compare Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare. -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 in- dicate 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. -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-attributes Do not warn if an unexpected "__attribute__" is used, such as un- recognized attributes, function attributes applied to variables, etc. This will not stop errors for incorrect use of supported at- tributes. -Wstrict-prototypes (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 which specifies the argument types.) -Wold-style-definition (C only) Warn if an old-style function definition is used. A warning is given even if there is a previous prototype. -Wmissing-prototypes (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. The aim is to detect global functions that fail to be declared in header files. -Wmissing-declarations (C only) Warn if a global function is defined without a previous declara- tion. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. -Wmissing-field-initializers Warn if a structure's initializer has some fields missing. For ex- ample, the following code would cause 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 would not trigger a warning: struct s { int f, g, h; }; struct s x = { .f = 3, .g = 4 }; This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. -Wmissing-noreturn Warn about functions which might be candidates for attribute "nore- turn". Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the "noreturn" attribute, otherwise sub- tle code generation bugs could be introduced. You will not get a warning for "main" in hosted C environments. -Wmissing-format-attribute Warn about function pointers which might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC will guess that function pointers with "format" attrib- utes 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 re- turn type of the containing function respectively should also have a "format" attribute to avoid the warning. GCC will also warn about function definitions which might be candi- dates for "format" attributes. Again, these are only possible can- didates. GCC will guess that "format" attributes might be appro- priate 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. -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 implementa- tion-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 charac- ters 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 which have not been normalized; this option controls that warning. There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnor- malized=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 -Wnor- malized=none. You would only want to do this if you were 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 iden- tical in some fonts or display methodologies, especially once for- matting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", will display just like a regular "n" which has been placed in a superscript. ISO 10646 defines the NFKC nor- malization scheme to convert all these into a standard form as well, and GCC will warn 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 is unable to be fixed to display these characters distinctly. -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. -Woverride-init 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. -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 in- stance, in this code, the variable "f.x" in "struct bar" will be 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; }; -Wpadded Warn if padding is included in a structure, either to align an ele- ment 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. -Wnested-externs (C only) Warn if an "extern" declaration is encountered within a function. -Wunreachable-code Warn if the compiler detects that code will never be executed. This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a proce- dure that never returns. It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-un- reachable code. For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function. This option is not made part of -Wall because in a debugging ver- sion of a program there is often substantial code which checks cor- rect functioning of the program and is, hopefully, unreachable be- cause the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time. -Winline Warn if a function can not be inlined and it was declared as in- line. Even with this option, the compiler will 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 ac- count the size of the function being inlined and the amount of in- lining that has already been done in the current function. There- fore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. -Wno-invalid-offsetof (C++ only) Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, how- ever, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.) This flag is for users who are aware that they are writing non- portable code and who have deliberately chosen to ignore the warn- ing about it. The restrictions on offsetof may be relaxed in a future version of the C++ standard. -Wno-int-to-pointer-cast (C only) Suppress warnings from casts to pointer type of an integer of a different size. -Wno-pointer-to-int-cast (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 can't be used. -Wlong-long Warn if long long type is used. This is default. To inhibit the warning messages, use -Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used. -Wvariadic-macros Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode. This is de- fault. To inhibit the warning messages, use -Wno-variadic-macros. -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. -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 were unable to han- dle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of time. -Wpointer-sign 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 -pedantic, which can be disabled with -Wno-pointer-sign. -Werror Make all warnings into errors. -Werror= Make the specified warning into an errors. 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 exam- ple -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect. You can use the -fdiagnostics-show-op- tion option to have each controllable warning amended with the op- tion which controls it, to determine what to use with this option. Note that specifying -Werror=foo automatically implies -Wfoo. How- ever, -Wno-error=foo does not imply anything. -Wstack-protector This option is only active when -fstack-protector is active. It warns about functions that will not be protected against stack smashing. -Woverlength-strings Warn about string constants which are longer than the "minimum max- imum" length specified in the C standard. Modern compilers gener- ally allow string constants which are much longer than the stan- dard'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 C89, 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 -pedantic, and can be disabled with -Wno-overlength-strings. Options for Debugging Your Program or GCC GCC has various special options that are used for debugging either your program or GCC: -g Produce debugging information in the operating system's native for- mat (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this de- bugging information. On most systems that use stabs format, -g enables use of extra de- bugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other de- buggers crash or refuse to read the program. If you want to con- trol for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below). GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: 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 were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GCC is generated with the ca- pability for more than one debugging format. -ggdb Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. -gstabs Produce debugging information in stabs format (if that is sup- ported), 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 which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler. -feliminate-unused-debug-symbols Produce debugging information in stabs format (if that is sup- ported), 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 be- cause using this option will increase the size of debugging infor- mation by as much as a factor of two. -gstabs+ Produce debugging information in stabs format (if that is sup- ported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debug- gers crash or refuse to read the program. -gcoff Produce debugging information in COFF format (if that is sup- ported). This is the format used by SDB on most System V systems prior to System V Release 4. -gxcoff Produce debugging information in XCOFF format (if that is sup- ported). This is the format used by the DBX debugger on IBM RS/6000 systems. -gxcoff+ Produce debugging information in XCOFF format (if that is sup- ported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debug- gers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error. -gdwarf-2 Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6. With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is upward compatible with version 2, but may still cause problems for older debuggers. -gvms Produce debugging information in VMS debug format (if that is sup- ported). This is the format used by DEBUG on VMS systems. -glevel -ggdblevel -gstabslevel -gcofflevel -gxcofflevel -gvmslevel Request debugging information and also use level to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This in- cludes descriptions of functions and external variables, but no in- formation about local variables and no line numbers. Level 3 includes extra information, such as all the macro defini- tions present in the program. Some debuggers support macro expan- sion when you use -g3. -gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug in- formation in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. In- stead use an additional -glevel option to change the debug level for DWARF2. -feliminate-dwarf2-dups Compress DWARF2 debugging information by eliminating duplicated in- formation about each symbol. This option only makes sense when generating DWARF2 debugging information with -gdwarf-2. -p Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking. -pg Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking. -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. -fmem-report Makes the compiler print some statistics about permanent memory al- location when it finishes. -fprofile-arcs Add code so that program flow arcs are instrumented. During execu- tion the program records how many times each branch and call is ex- ecuted and how many times it is taken or returns. When the com- piled program exits it saves this data to a file called aux- name.gcda for each source file. The data may be used for profile- directed optimizations (-fbranch-probabilities), or for test cover- age analysis (-ftest-coverage). Each object file's auxname is gen- erated 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 cov- erage 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 pro- file every source file in a program. * Link your object files with -lgcov or -fprofile-arcs (the lat- ter 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. Also "fork" calls are detected and correctly handled (double counting will not happen). * 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 instru- mented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only en- trance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the in- strumentation 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 aux- name.gcno. Refer to the -fprofile-arcs option above for a descrip- tion of auxname and instructions on how to generate test coverage data. Coverage data will match the source files more closely, if you do not optimize. -dletters -fdump-rtl-pass Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by ap- pending a pass number and a word to the dumpname. dumpname is gen- erated 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. Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch; here are the possible letters for use in letters and pass, and their meanings: -dA Annotate the assembler output with miscellaneous debugging in- formation. -dB -fdump-rtl-bbro Dump after block reordering, to file.148r.bbro. -dc -fdump-rtl-combine Dump after instruction combination, to the file file.129r.com- bine. -dC -fdump-rtl-ce1 -fdump-rtl-ce2 -dC and -fdump-rtl-ce1 enable dumping after the first if con- version, to the file file.117r.ce1. -dC and -fdump-rtl-ce2 en- able dumping after the second if conversion, to the file file.130r.ce2. -dd -fdump-rtl-btl -fdump-rtl-dbr -dd and -fdump-rtl-btl enable dumping after branch target load optimization, to file.31.btl. -dd and -fdump-rtl-dbr enable dumping after delayed branch scheduling, to file.36.dbr. -dD Dump all macro definitions, at the end of preprocessing, in ad- dition to normal output. -dE -fdump-rtl-ce3 Dump after the third if conversion, to file.146r.ce3. -df -fdump-rtl-cfg -fdump-rtl-life -df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to file.116r.cfg. -df and -fdump-rtl-cfg enable dumping dump after life analysis, to file.128r.life1 and file.135r.life2. -dg -fdump-rtl-greg Dump after global register allocation, to file.139r.greg. -dG -fdump-rtl-gcse -fdump-rtl-bypass -dG and -fdump-rtl-gcse enable dumping after GCSE, to file.114r.gcse. -dG and -fdump-rtl-bypass enable dumping after jump bypassing and control flow optimizations, to file.115r.by- pass. -dh -fdump-rtl-eh Dump after finalization of EH handling code, to file.02.eh. -di -fdump-rtl-sibling Dump after sibling call optimizations, to file.106r.sibling. -dj -fdump-rtl-jump Dump after the first jump optimization, to file.112r.jump. -dk -fdump-rtl-stack Dump after conversion from registers to stack, to file.152r.stack. -dl -fdump-rtl-lreg Dump after local register allocation, to file.138r.lreg. -dL -fdump-rtl-loop2 -dL and -fdump-rtl-loop2 enable dumping after the loop opti- mization pass, to file.119r.loop2, file.120r.loop2_init, file.121r.loop2_invariant, and file.125r.loop2_done. -dm -fdump-rtl-sms Dump after modulo scheduling, to file.136r.sms. -dM -fdump-rtl-mach Dump after performing the machine dependent reorganization pass, to file.155r.mach. -dn -fdump-rtl-rnreg Dump after register renumbering, to file.147r.rnreg. -dN -fdump-rtl-regmove Dump after the register move pass, to file.132r.regmove. -do -fdump-rtl-postreload Dump after post-reload optimizations, to file.24.postreload. -dr -fdump-rtl-expand Dump after RTL generation, to file.104r.expand. -dR -fdump-rtl-sched2 Dump after the second scheduling pass, to file.150r.sched2. -ds -fdump-rtl-cse Dump after CSE (including the jump optimization that sometimes follows CSE), to file.113r.cse. -dS -fdump-rtl-sched Dump after the first scheduling pass, to file.21.sched. -dt -fdump-rtl-cse2 Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to file.127r.cse2. -dT -fdump-rtl-tracer Dump after running tracer, to file.118r.tracer. -dV -fdump-rtl-vpt -fdump-rtl-vartrack -dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to file.10.vpt. -dV and -fdump-rtl-vartrack enable dumping after variable tracking, to file.154r.vartrack. -dw -fdump-rtl-flow2 Dump after the second flow pass, to file.142r.flow2. -dz -fdump-rtl-peephole2 Dump after the peephole pass, to file.145r.peephole2. -dZ -fdump-rtl-web Dump after live range splitting, to file.126r.web. -da -fdump-rtl-all Produce all the dumps listed above. -dH Produce a core dump whenever an error occurs. -dm Print statistics on memory usage, at the end of the run, to standard error. -dp Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruc- tion is also printed. -dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation. -dv For each of the other indicated dump files (either with -d or -fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg. -dx Just generate RTL for a function instead of compiling it. Usu- ally used with r (-fdump-rtl-expand). -dy Dump debugging information during parsing, to standard error. -fdump-noaddr When doing debugging dumps (see -d option above), 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. -fdump-unnumbered When doing debugging dumps (see -d option above), suppress instruc- tion numbers, line number note and address output. This makes it more feasible to use diff on debugging dumps for compiler invoca- tions with different options, in particular with and without -g. -fdump-translation-unit (C++ only) -fdump-translation-unit-options (C++ only) Dump a representation of the tree structure for the entire transla- tion unit to a file. The file name is made by appending .tu to the source file name. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options. -fdump-class-hierarchy (C++ only) -fdump-class-hierarchy-options (C++ only) Dump a representation of each class's hierarchy and virtual func- tion table layout to a file. The file name is made by appending .class to the source file name. If the -options form is used, op- tions controls the details of the dump as described for the -fdump-tree options. -fdump-ipa-switch 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. The following dumps are possible: all Enables all inter-procedural analysis dumps; currently the only produced dump is the cgraph dump. cgraph Dumps information about call-graph optimization, unused func- tion removal, and inlining decisions. -fdump-tree-switch -fdump-tree-switch-options Control the dumping at various stages of processing the intermedi- ate language tree to a file. The file name is generated by append- ing a switch specific suffix to the source file name. If the -op- tions form is used, options is a list of - separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be 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 envi- ronment. slim 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. 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). stats Enable dumping various statistics about the pass (not honored by every dump option). blocks Enable showing basic block boundaries (disabled in raw dumps). 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. all Turn on all options, except raw, slim and lineno. The following tree dumps are possible: original Dump before any tree based optimization, to file.original. optimized Dump after all tree based optimization, to file.optimized. inlined Dump after function inlining, to file.inlined. gimple Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name. cfg Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name. vcg Dump the control flow graph of each function to a file in VCG format. The file name is made by appending .vcg to the source file name. Note that if the file contains more than one func- tion, the generated file cannot be used directly by VCG. You will need to cut and paste each function's graph into its own separate file first. ch Dump each function after copying loop headers. The file name is made by appending .ch to the source file name. ssa Dump SSA related information to a file. The file name is made by appending .ssa to the source file name. salias Dump structure aliasing variable information to a file. This file name is made by appending .salias to the source file name. alias Dump aliasing information for each function. The file name is made by appending .alias to the source file name. ccp Dump each function after CCP. The file name is made by append- ing .ccp to the source file name. storeccp Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name. pre Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name. fre Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name. copyprop Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name. store_copyprop Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name. dce Dump each function after dead code elimination. The file name is made by appending .dce to the source file name. mudflap Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source file name. sra Dump each function after performing scalar replacement of ag- gregates. The file name is made by appending .sra to the source file name. sink Dump each function after performing code sinking. The file name is made by appending .sink to the source file name. dom Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name. dse Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name. phiopt Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name. forwprop Dump each function after forward propagating single use vari- ables. The file name is made by appending .forwprop to the source file name. copyrename Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name. nrv Dump each function after applying the named return value opti- mization on generic trees. The file name is made by appending .nrv to the source file name. vect Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name. vrp Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name. all Enable all the available tree dumps with the flags provided in this option. -ftree-vectorizer-verbose=n This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect. For n=0 no diag- nostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops. This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean either more information dumped for each reported loop, or same amount of information reported for more loops: If n=3, alignment related information is added to the re- ports. If n=4, data-references related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=5, the vectorizer reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e. may not be count- able, or may have complicated control-flow). If n=6, the vector- izer reports also non-vectorized nested loops. For n=7, all the information the vectorizer generates during its analysis and trans- formation is reported. This is the same verbosity level that -fdump-tree-vect-details uses. -frandom-seed=string This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to generate 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 should be different for every file you compile. -fsched-verbose=n On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This informa- tion is written to standard error, unless -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than nine, the out- put is always printed to standard error. For n greater than zero, -fsched-verbose outputs the same informa- tion as -dRS. 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. -save-temps 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 would produce files foo.i and foo.s, as well as foo.o. This creates a pre- processed 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. -time Report the CPU time taken by each subprocess in the compilation se- quence. For C source files, this is the compiler proper and assem- bler (plus the linker if linking is done). 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. -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 infor- mation). It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it. -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 op- tion, 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 @samp{-, without spaces between multiple switches. This is sup- posed to ease shell-processing. -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 will search---and don't do anything else. This is useful when gcc prints the error message installation prob- lem, 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 /. -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)---and don't do any- thing else. -dumpspecs Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.) -feliminate-unused-debug-types Normally, when producing DWARF2 output, GCC will emit debugging in- formation for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as 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 signifi- cant amount of wasted space. With this option, GCC will avoid pro- ducing debug symbol output for types that are nowhere used in the source file being compiled. 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 would 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. Optimization levels -O and above, in particular, enable unit- at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a- time 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 opti- mizations that have a flag are listed. -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 com- pilation time. -O turns on the following optimization flags: -fdefer-pop -fde- layed-branch -fguess-branch-probability -fcprop-registers -fif-con- version -fif-conversion2 -ftree-ccp -ftree-dce -ftree-domina- tor-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra -ftree-copyre- name -ftree-fre -ftree-ch -funit-at-a-time -fmerge-constants -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging. -O2 Optimize even more. GCC performs nearly all supported optimiza- tions that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you spec- ify -O2. 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: -fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse -fgcse-lm -fexpensive-optimizations -frerun-cse-after-loop -fcaller-saves -fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fregmove -fstrict-aliasing -fstrict-overflow -fdelete-null-pointer-checks -freorder-blocks -freorder-functions -falign-functions -falign-jumps -falign-loops -falign-labels -ftree-vrp -ftree-pre Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. -O2 doesn't turn on -ftree-vrp for the Ada compiler. This option must be explicitly specified on the command line to be enabled for the Ada compiler. -O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops and -fgcse-after-reload options. -O0 Do not optimize. This is the default. -Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimiza- tions designed to reduce code size. -Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version 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 would be -fno-foo. In the table below, only one of the forms is listed---the one you typically will 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 optimiza- tions to be performed is desired. -fno-default-inline Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are com- piled inline by default; i.e., you don't need to add inline in front of the member function name. -fno-defer-pop Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once. Disabled at levels -O, -O2, -O3, -Os. -fforce-mem Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not com- mon subexpressions, instruction combination should eliminate the separate register-load. This option is now a nop and will be re- moved in 4.3. -fforce-addr Force memory address constants to be copied into registers before doing arithmetic on them. -fomit-frame-pointer Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines. On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag. Enabled at levels -O, -O2, -O3, -Os. -foptimize-sibling-calls Optimize sibling and tail recursive calls. Enabled at levels -O2, -O3, -Os. -fno-inline Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline. -finline-functions Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be 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 level -O3. -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 assem- bler code in its own right. Enabled if -funit-at-a-time is enabled. -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. -finline-limit=n By default, GCC limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 600. In- creasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presum- ably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on re- cursive templates with C++. 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 I<n>/2. max-inline-insns-auto is set to I<n>/2. min-inline-insns is set to 130 or I<n>/4, whichever is smaller. max-inline-insns-rtl is set to I<n>. See below for a documentation of the individual parameters control- ling inlining. Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it repre- sent a count of assembly instructions and as such its exact meaning might change from one release to an another. -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 C. In C++, emit any and all inline func- tions into the object file. -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 com- piler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the -fno-keep-sta- tic-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 assem- bler 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-con- stants this considers e.g. even constant initialized arrays or ini- tialized constant variables with integral or floating point types. Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conform- ing behavior. -fmodulo-sched Perform swing modulo scheduling immediately before the first sched- uling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. -fno-branch-count-reg Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the re- sult. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. The default is -fbranch-count-reg. -fno-function-cse Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address ex- plicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimiza- tions 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 explic- itly 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. -fbounds-check For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran front-ends, where this option defaults to true and false respec- tively. -fmudflap -fmudflapth -fmudflapir For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate run- time library (libmudflap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the instru- mented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options. Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmud- flap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright mem- ory corrupting writes, but allows erroneously read data to propa- gate within a program. -fthread-jumps Perform optimizations where we 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. -fcse-follow-jumps In common subexpression elimination, 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 will follow 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 which 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 has been 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 exten- sion, you may get better runtime 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 will attempt to move loads which 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 gcse is enabled. -fgcse-sm When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt 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 be- fore 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 elimi- nation 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 cleanup redundant spilling. -funsafe-loop-optimizations If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop. -fcrossjumping Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping. Enabled at levels -O2, -O3, -Os. -fif-conversion Attempt to transform conditional jumps into branch-less equiva- lents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arith- metics. The use of conditional execution on chips where it is available is controlled by "if-conversion2". Enabled at levels -O, -O2, -O3, -Os. -fif-conversion2 Use conditional execution (where available) to transform condi- tional jumps into branch-less equivalents. Enabled at levels -O, -O2, -O3, -Os. -fdelete-null-pointer-checks Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null. In some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior. Enabled at levels -O2, -O3, -Os. -fexpensive-optimizations Perform a number of minor optimizations that are relatively expen- sive. Enabled at levels -O2, -O3, -Os. -foptimize-register-move -fregmove Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. Note -fregmove and -foptimize-register-move are the same optimiza- tion. Enabled at levels -O2, -O3, -Os. -fdelayed-branch If supported for the target machine, attempt to reorder instruc- tions to exploit instruction slots available after delayed branch instructions. Enabled at levels -O, -O2, -O3, -Os. -fschedule-insns If supported for the target machine, attempt to reorder instruc- tions to eliminate execution stalls due to required data being un- available. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be is- sued until the result of the load or floating point instruction is required. Enabled at levels -O2, -O3, -Os. -fschedule-insns2 Similar to -fschedule-insns, but requests an additional pass of in- struction 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 cy- cle. Enabled at levels -O2, -O3, -Os. -fno-sched-interblock Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. -fno-sched-spec Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register alloca- tion, i.e. with -fschedule-insns or at -O2 or higher. -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=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. -fsched-stalled-insns-dep=n Define how many insn groups (cycles) will be examined for a depen- dency on a stalled insn that is candidate for premature removal from the queue of stalled insns. Has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used and its value is not zero. -fsched2-use-superblocks When scheduling after register allocation, do use superblock sched- uling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on faster schedules. This option is ex- perimental, 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. -fsched2-use-traces Use -fsched2-use-superblocks algorithm when scheduling after regis- ter allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details on trace formation. This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with -fschedule-in- sns2 or at -O2 or higher. -fsee Eliminates redundant extension instructions and move the non redun- dant ones to optimal placement using LCM. -freschedule-modulo-scheduled-loops The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later schedul- ing passes from changing its schedule, we use this option to con- trol that. -fcaller-saves Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and re- store the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is always enabled by default on certain machines, usu- ally those which have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os. -ftree-pre Perform Partial Redundancy Elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3. -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 faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher. -ftree-copy-prop Perform copy propagation on trees. This pass eliminates unneces- sary copy operations. This flag is enabled by default at -O and higher. -ftree-store-copy-prop Perform copy propagation of memory loads and stores. This pass eliminates unnecessary copy operations in memory references (struc- tures, global variables, arrays, etc). This flag is enabled by de- fault at -O2 and higher. -ftree-salias Perform structural alias analysis on trees. This flag is enabled by default at -O and higher. -fipa-pta Perform interprocedural pointer analysis. -ftree-sink Perform forward store motion on trees. This flag is enabled by default at -O and higher. -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. -ftree-store-ccp Perform sparse conditional constant propagation (CCP) on trees. This pass operates on both local scalar variables and memory stores and loads (global variables, structures, arrays, etc). This flag is enabled by default at -O2 and higher. -ftree-dce Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher. -ftree-dominator-opts Perform a variety of simple scalar cleanups (constant/copy propaga- tion, redundancy elimination, range propagation and expression sim- plification) based on a dominator tree traversal. This also per- forms jump threading (to reduce jumps to jumps). This flag is en- abled 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 de- fault at -O and higher. -ftree-loop-linear Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place. -ftree-loop-im Perform loop invariant motion on trees. This pass moves only in- variants that would be 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 in- variant out of the loop, so that we can use just trivial invariant- ness analysis in loop unswitching. The pass also includes store motion. -ftree-loop-ivcanon Create a canonical counter for number of iterations in the loop for that determining number of iterations requires complicated analy- sis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling. -fivopts Perform induction variable optimizations (strength reduction, in- duction variable merging and induction variable elimination) on trees. -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 -O and higher. -ftree-copyrename Perform copy renaming on trees. This pass attempts to rename com- piler temporaries to other variables at copy locations, usually re- sulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher. -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-GIM- PLE 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-lrs Perform live range splitting during the SSA->normal phase. Dis- tinct live ranges of a variable are split into unique variables, allowing for better optimization later. This is enabled by default at -O and higher. -ftree-vectorize Perform loop vectorization on trees. -ftree-vect-loop-version Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vec- torized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled. -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 elimi- nation is only done if -fdelete-null-pointer-checks is enabled. -ftracer Perform tail duplication to enlarge superblock size. This trans- formation simplifies the control flow of the function allowing other optimizations to do better job. -funroll-loops Unroll loops whose number of iterations can be determined at com- pile time or upon entry to the loop. -funroll-loops implies -fre- run-cse-after-loop. This option makes code larger, and may or may not make it run faster. -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 -fun- roll-loops, -fsplit-ivs-in-unroller Enables expressing of values of induction variables in later itera- tions of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes. Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass. This optimization is enabled by default. -fvariable-expansion-in-unroller With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code. -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-peephole -fno-peephole2 Disable any machine-specific peephole optimizations. The differ- ence 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 will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuris- tics are based on the control flow graph. If some branch probabil- ities are specified by __builtin_expect, then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The in- teractions between the heuristics and __builtin_expect can be com- plex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand. 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 -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, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. -freorder-functions Reorder functions in the object file in order to improve code lo- cality. This is implemented by using special subsections ".text.hot" for most frequently executed functions and ".text.un- likely" 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. Also profile feedback must be available in to make this option ef- fective. See -fprofile-arcs for details. Enabled at levels -O2, -O3, -Os. -fstrict-aliasing Allows the compiler to assume the strictest aliasing rules applica- ble to the language being compiled. For C (and C++), this acti- vates optimizations based on the type of expressions. In particu- lar, 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 al- most 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() { 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 mem- ory is accessed through the union type. So, the code above will work as expected. However, this code might not: int f() { a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; } Every language that wishes to perform language-specific alias analysis should define a function that computes, given an "tree" node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function "c_get_alias_set". Enabled at levels -O2, -O3, -Os. -fstrict-overflow Allow the compiler to assume strict signed overflow rules, depend- ing on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is unde- fined, which means that the compiler may assume that it will not happen. This permits various optimizations. For example, the com- piler will assume that an expression like "i + 10 > i" will always be true for signed "i". This assumption is only valid if signed overflow is undefined, as the expression is false if "i + 10" over- flows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually in- volve overflow. See also the -fwrapv option. Using -fwrapv means that signed over- flow is fully defined: it wraps. When -fwrapv is used, there is no difference between -fstrict-overflow and -fno-strict-overflow. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on con- stants, the overflowed value can still be used with -fwrapv, but not otherwise. The -fstrict-overflow option is enabled at levels -O2, -O3, -Os. -falign-functions -falign-functions=n Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-func- tions=24 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less. -fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned. Some assemblers only support this flag when n is a power of two; in that case, it is rounded up. If n is not specified or is zero, use a machine-dependent default. Enabled at levels -O2, -O3. -falign-labels -falign-labels=n Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be 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. Enabled at levels -O2, -O3. -falign-loops -falign-loops=n Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that the loop will be executed many times, which will make up for any execution of the dummy opera- tions. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned. If n is not specified or is zero, use a machine-dependent default. Enabled at levels -O2, -O3. -falign-jumps -falign-jumps=n Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be executed. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned. If n is not specified or is zero, use a machine-dependent default. Enabled at levels -O2, -O3. -funit-at-a-time Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory (in general). There are some compatibility issues with unit-at-a-time mode: * enabling unit-at-a-time mode may change the order in which functions, variables, and top-level "asm" statements are emit- ted, and will likely break code relying on some particular or- dering. The majority of such top-level "asm" statements, though, can be replaced by "section" attributes. The fno- toplevel-reorder option may be used to keep the ordering used in the input file, at the cost of some optimizations. * unit-at-a-time mode removes unreferenced static variables and functions. This may result in undefined references when an "asm" statement refers directly to variables or functions that are otherwise unused. In that case either the variable/func- tion shall be listed as an operand of the "asm" statement operand or, in the case of top-level "asm" statements the at- tribute "used" shall be used on the declaration. * Static functions now can use non-standard passing conventions that may break "asm" statements calling functions directly. Again, attribute "used" will prevent this behavior. As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC. Enabled at levels -O, -O2, -O3, -Os. -fno-toplevel-reorder Do not reorder top-level functions, variables, and "asm" state- ments. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables will not be removed. This option is intended to support existing code which relies on a particular ordering. For new code, it is better to use attributes. -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 de- bugging impossible, since variables will no longer stay in a "home register". Enabled by default with -funroll-loops. -fwhole-program Assume that the current compilation unit represents whole program being compiled. All public functions and variables with the excep- tion of "main" and those merged by attribute "externally_visible" become static functions and in a affect gets more aggressively op- timized by interprocedural optimizers. While this option is equiv- alent to proper use of "static" keyword for programs consisting of single file, in combination with option --combine this flag can be used to compile most of smaller scale C programs since the func- tions and variables become local for the whole combined compilation unit, not for the single source file itself. -fno-cprop-registers After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Disabled at levels -O, -O2, -O3, -Os. -fprofile-generate Enable options usually used for instrumenting application to pro- duce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when com- piling and when linking your program. The following options are enabled: "-fprofile-arcs", "-fpro- file-values", "-fvpt". -fprofile-use Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available. The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer" The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correct- ness. 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. -ffast-math Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trap- ping-math, -ffinite-math-only, -fno-rounding-math, -fno-signal- ing-nans and fcx-limited-range. This option causes the preprocessor macro "__FAST_MATH__" to be de- fined. This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math func- tions. -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 should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math func- tions. 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 should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math func- tions. The default is -fno-unsafe-math-optimizations. -ffinite-math-only Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications. The default is -fno-finite-math-only. -fno-trapping-math Compile code assuming that floating-point operations cannot gener- ate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option implies -fno-signaling-nans. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for exam- ple. This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math func- tions. 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 transforma- tions 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". -frtl-abstract-sequences It is a size optimization method. This option is to find identical sequences of code, which can be turned into pseudo-procedures and then replace all occurrences with calls to the newly created subroutine. It is kind of an opposite of -finline-functions. This optimization runs at RTL level. -fsignaling-nans Compile code assuming that IEEE signaling NaNs may generate user- visible traps during floating-point operations. Setting this op- tion disables optimizations that may change the number of excep- tions visible with signaling NaNs. This option implies -ftrap- ping-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. -fsingle-precision-constant Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant. -fcx-limited-range -fno-cx-limited-range When enabled, this option states that a range reduction step is not needed when performing complex division. The default is -fno-cx-limited-range, but is enabled by -ffast-math. This option controls the default setting of the ISO C99 "CX_LIM- ITED_RANGE" pragma. Nevertheless, the option applies to all lan- guages. The following options control optimizations that may improve perfor- mance, 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 com- pile it a second time using -fbranch-probabilities, to improve op- timizations based on the number of times each branch was taken. When the 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 optimiza- tion. Currently, they are only used in one place: in reorg.c, in- stead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. -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 and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations. Enabled with -fprofile-generate and -fprofile-use. -fvpt If combined with -fprofile-arcs, it instructs the compiler to add a 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 operation using the knowledge about the value of the denominator. -frename-registers Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimiza- tion will most benefit processors with lots of registers. Depend- ing on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer stay in a "home register". Enabled by default with -funroll-loops. -ftracer Perform tail duplication to enlarge superblock size. This trans- formation simplifies the control flow of the function allowing other optimizations to do better job. Enabled with -fprofile-use. -funroll-loops Unroll loops whose number of iterations can be determined at com- pile time or upon entry to the loop. -funroll-loops implies -fre- run-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code larger, and may or may not make it run faster. Enabled with -fprofile-use. -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 -fun- roll-loops. -fpeel-loops Peels the loops for that there is enough information that they do not roll much (from profile feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). Enabled with -fprofile-use. -fmove-loop-invariants Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 -funswitch-loops Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to re- sult of the condition). -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 optimiza- tions to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors run- ning Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future. Only use these options when there are significant benefits from do- ing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof" on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g. -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 in within any basic block. -fstack-protector Emit extra code to check for buffer overflows, such as stack smash- ing 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. -fsection-anchors Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transfor- mation can help to reduce the number of GOT entries and GOT ac- cesses on some targets. For example, the implementation of the following function "foo": static int a, b, c; int foo (void) { return a + b + c; } would usually calculate the addresses of all three variables, but if you compile it with -fsection-anchors, it will access the vari- ables 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 will not inline func- tions that contain more that 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 each case, the value is an integer. The allowable choices for name are given in the following table: salias-max-implicit-fields The maximum number of fields in a variable without direct structure accesses for which structure aliasing will consider trying to track each field. The default is 5 salias-max-array-elements The maximum number of elements an array can have and its ele- ments still be tracked individually by structure aliasing. The default is 4 sra-max-structure-size The maximum structure size, in bytes, at which the scalar re- placement of aggregates (SRA) optimization will perform block copies. The default value, 0, implies that GCC will select the most appropriate size itself. sra-field-structure-ratio The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We say that if the ra- tio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used. The default is 75. max-crossjump-edges The maximum number of incoming edges to consider for crossjump- ing. 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 compile time increase with probably small improvement in executable size. min-crossjump-insns The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them. This value is ignored in the case where all instructions in the block being crossjumped from are matched. The default value is 5. 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. The default value is 8. 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. The default value is 8. 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 arbi- trary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. In- creasing values mean more aggressive optimization, making the compile time increase with probably small improvement in exe- cutable run time. max-delay-slot-live-search When trying to fill delay slots, the maximum number of instruc- tions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile 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 will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done. max-gcse-passes The maximum number of passes of GCSE to run. The default is 1. max-pending-list-length The maximum number of pending dependencies scheduling will al- low 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-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 will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 450. 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 will be investigated. To those functions, a different (more restrictive) limit compared to functions de- clared inline can be applied. The default value is 90. 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-lin- ear algorithms used by the backend. This parameter is ignored when -funit-at-a-time is not used. The default value is 2700. large-function-growth Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored when -funit-at-a-time is not used. The default value is 100 which limits large func- tion 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 (consider unit consisting of function A that is inline and B that just calls A three time. 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 inlininable functions however the overall unit growth limit is needed to avoid expo- nential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. The default is 10000 inline-unit-growth Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when -funit-at-a-time is not used. The default value is 50 which limits unit growth to 1.5 times the original size. max-inline-insns-recursive max-inline-insns-recursive-auto Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow into by performing re- cursive inlining. For functions declared inline --param max-inline-insns-recur- sive is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (in- cluded in -O3) is enabled and --param max-inline-insns-recur- sive-auto is used. The default value is 450. max-inline-recursive-depth max-inline-recursive-depth-auto Specifies maximum recursion depth used by the recursive inlin- ing. For functions declared inline --param max-inline-recursive- depth is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (in- cluded in -O3) is enabled and --param max-inline-recursive- depth-auto is used. The default value is 450. 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 probability that function will recurse via given call expression. This parame- ter limits inlining only to call expression whose probability exceeds given threshold (in percents). The default value is 10. inline-call-cost Specify cost of call instruction relative to simple arithmetics operations (having cost of 1). Increasing this cost disquali- fies inlining of non-leaf functions and at the same time in- creases size of leaf function that is believed to reduce func- tion size by being inlined. In effect it increases amount of inlining for code having large abstraction penalty (many func- tions that just pass the arguments to other functions) and de- crease inlining for code with low abstraction penalty. The de- fault value is 16. max-unrolled-insns The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it deter- mines 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 should have if that loop is un- rolled, and if the loop is unrolled, it 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 should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled. max-peel-times The maximum number of peelings of a single loop. 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-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 invari- ant motion. iv-consider-all-candidates-bound Bound on number of candidates for induction variables below that all candidates are considered for each use in induction variable optimizations. Only the most relevant candidates are considered if there are more candidates, to avoid quadratic time complexity. iv-max-considered-uses The induction variable optimizations give up on loops that con- tain more induction variable uses. iv-always-prune-cand-set-bound If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set. scev-max-expr-size Bound on size of expressions used in the scalar evolutions ana- lyzer. Large expressions slow the analyzer. vect-max-version-checks The maximum number of runtime checks that can be performed when doing loop versioning in the vectorizer. See option ftree- vect-loop-version for more information. max-iterations-to-track The maximum number of iterations of a loop the brute force al- gorithm for analysis of # of iterations of the loop tries to evaluate. 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 consid- ered hot. hot-bb-frequency-fraction Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be consid- ered hot max-predicted-iterations The maximum number of loop iterations we predict statically. This is useful in cases where function contain single loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the unknown number of it- erations average to roughly 10. This means that the loop with- out bounds would appear artificially cold relative to the other one. 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 is used only when profile feedback is available. The real profiles (as opposed to stati- cally 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 per- centage. This is rather hokey argument, as most of the dupli- cates will be 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-ratio tracer-min-branch-ratio-feedback Stop forward growth if the best edge do have probability lower than this threshold. Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compila- tion without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective. max-cse-path-length Maximum number of basic blocks on path that cse considers. The default is 10. max-cse-insns The maximum instructions CSE process before flushing. The de- fault is 1000. global-var-threshold Counts the number of function calls (n) and the number of call- clobbered variables (v). If nxv is larger than this limit, a single artificial variable will be created to represent all the call-clobbered variables at function call sites. This artifi- cial variable will then be made to alias every call-clobbered variable. (done as "int * size_t" on the host machine; beware overflow). max-aliased-vops Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping heuristic. Alias grouping reduces compile times and memory consumption needed for alias- ing at the expense of precision loss in alias information. ggc-min-expand GCC uses a garbage collector to manage its own memory alloca- tion. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand be- tween 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 particu- lar platform, the lower bound of 30% is used. Setting this pa- rameter 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 af- ter the heap expands by ggc-min-expand% beyond ggc-min-heap- size. 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 which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not ex- ceeded, 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 aggres- sive optimization, making the compile time increase with proba- bly slightly better performance. The default value is 100. max-cselib-memory-locations The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500. max-flow-memory-locations Similar as max-cselib-memory-locations but for dataflow live- ness. The default value is 100. reorder-blocks-duplicate reorder-blocks-duplicate-feedback Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program. The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than re- order-block-duplicate since information about the hot spots is more accurate. 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. The default value is 100. max-sched-region-blocks The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10. max-sched-region-insns The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100. min-spec-prob The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default value is 40. max-sched-extend-regions-iters The maximum number of iterations through CFG to extend regions. 0 - disable region extension, N - do at most N iterations. The default value is 0. max-sched-insn-conflict-delay The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3. sched-spec-prob-cutoff The minimal probability of speculation success (in percents), so that speculative insn will be scheduled. The default value is 40. 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. The default is 10000. integer-share-limit Small integer constants can use a shared data structure, reduc- ing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant's. The de- fault value is 256. min-virtual-mappings Specifies the minimum number of virtual mappings in the incre- mental SSA updater that should be registered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value is 100. virtual-mappings-ratio If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols to be updated, then the incremental SSA updater switches to a full update for those symbols. The default ratio is 3. ssp-buffer-size The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-protection is used. 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 we will treat in a field sensitive manner during pointer analysis. 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. You can use -Wp,option to bypass the compiler driver and pass op- tion directly through to the preprocessor. If option contains com- mas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the com- piler 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 in- stead. -Xpreprocessor option Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options which GCC does not know how to 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 argu- ment. -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 will be 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 will need to 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. -undef Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. -I dir Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the spe- cial treatment of system headers are not defeated . -o file Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpreta- tion of a second non-option argument, so you must use -o to specify the output file. -Wall Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warn- ing about integer promotion causing a change of sign in "#if" ex- pressions. Note that many of the preprocessor's warnings are on by default and have no options to control them. -Wcomment -Wcomments Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. (Both forms have the same effect.) -Wtrigraphs Most trigraphs in comments cannot affect the meaning of the pro- gram. However, a trigraph that would form an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment. This option is implied by -Wall. If -Wall is not given, this op- tion is still enabled unless trigraphs are enabled. To get tri- graph conversion without warnings, but get the other -Wall warn- ings, use -trigraphs -Wall -Wno-trigraphs. -Wtraditional Warn about certain constructs that behave differently in tradi- tional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and problematic constructs which should be avoided. -Wimport Warn the first time #import is used. -Wundef Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are re- placed with zero. -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 will also warn 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 de- fined in include files are not warned about. Note: If a macro is actually used, but only used in skipped condi- tional blocks, then CPP will report 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 -Wendif-labels Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form #if FOO ... #else FOO ... #endif FOO The second and third "FOO" should be in comments, but often are not in older programs. This warning is on by default. -Werror Make all warnings into hard errors. Source code which triggers warnings will be rejected. -Wsystem-headers Issue warnings for code in system headers. These are normally un- helpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them. -w Suppress all warnings, including those which GNU CPP issues by de- fault. -pedantic Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code. -pedantic-errors Issue all the mandatory diagnostics, and make all mandatory diag- nostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings. -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 basename of the source file with any suffix re- placed with object file suffix. 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. De- bug output will still be sent to the regular output stream as nor- mal. 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 will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier. -MF file When used with -M or -MM, specifies a file to write the dependen- cies to. If no -MF switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. -MG In conjunction with an option such as -M requesting dependency gen- eration, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The de- pendency filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses preprocessed out- put, 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, including any path, deletes any file suffix such as .c, and appends the plat- form's usual object suffix. The result is the target. An -MT option will set 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 take the basename of the input file 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. -fpch-deps When using precompiled headers, this flag will cause the depen- dency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be 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 "<file- name>"" 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 differ- ent location. The filename may be absolute or it may be relative to GCC's current directory. -x c -x c++ -x objective-c -x assembler-with-cpp Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recog- nize the extension, it will treat the file as C; this is the most generic mode. Note: Previous versions of cpp accepted a -lang option which se- lected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option. -std=standard -ansi Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the fu- ture. standard may be one of: "iso9899:1990" "c89" The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard. The -ansi option is equivalent to -std=c89. "iso9899:199409" The 1990 C standard, as amended in 1994. "iso9899:1999" "c99" "iso9899:199x" "c9x" The revised ISO C standard, published in December 1999. Before publication, this was known as C9X. "gnu89" The 1990 C standard plus GNU extensions. This is the default. "gnu99" "gnu9x" The 1999 C standard plus GNU extensions. "c++98" The 1998 ISO C++ standard plus amendments. "gnu++98" The same as -std=c++98 plus GNU extensions. This is the de- fault for C++ code. -I- Split the include path. Any directories specified with -I options before -I- are searched only for headers requested with "#in- clude "file""; they are not searched for "#include <file>". If ad- ditional 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"". This option has been deprecated. -nostdinc Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched. -nostdinc++ Do not search for header files in the C++-specific standard direc- tories, but do still search the other standard directories. (This option is used when building the C++ library.) -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 direc- tory 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 al- lows you to acquire all the macros from a header without also pro- cessing its declarations. All files specified by -imacros are processed before all files specified by -include. -idirafter dir Search dir for header files, but do it after all directories speci- fied with -I and the standard system directories have been ex- hausted. dir is treated as a system include directory. -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. -iwithpre- fixbefore 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. See the --sysroot option for more information. -imultilib dir Use dir as a subdirectory of the directory containing target-spe- cific C++ headers. -isystem dir Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. -iquote dir Search dir only for header files requested with "#include "file""; they are not searched for "#include <file>", before all directories specified by -I and before the standard system directories. -fdollars-in-identifiers Accept $ in identifiers. -fextended-identifiers Accept universal character names in identifiers. This option is experimental; in a future version of GCC, it will be enabled by de- fault for C99 and C++. -fpreprocessed Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, tri- graph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes com- ments, so that you can pass a file preprocessed with -C to the com- piler 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 exten- sions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps. -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. -fexec-charset=charset Set the execution character set, used for string and character con- stants. The default is UTF-8. charset can be any encoding sup- ported 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" li- brary 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 charac- ter 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 com- mand line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library rou- tine. -fworking-directory Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preproces- sor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's present in the preprocessed in- put, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly en- abled if debugging information is enabled, but this can be inhib- ited 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. -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 under- stand the column numbers, such as dejagnu. -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. -dCHARS CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined. M Instead of the normal output, generate a list of #define direc- tives 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 will show all the predefined macros. D Like M except in two respects: it does not include the prede- fined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file. N Like D, but emit only the macro names, not their expansions. I Output #include directives in addition to the result of prepro- cessing. -P Inhibit generation of linemarkers in the output from the preproces- sor. This might be useful when running the preprocessor on some- thing that is not C code, and will be sent to a program which might be confused by the linemarkers. -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 di- rective 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. -traditional-cpp Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors. -trigraphs Process trigraph sequences. 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 charac- ter constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. The nine trigraphs and their replacements are Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- Replacement: [ ] { } # \ ^ | ~ -remap Enable special code to work around file systems which only permit very short file names, such as MS-DOS. --help --target-help Print text describing all the command line options instead of pre- processing anything. -v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path. -H Print the name of each header file used, in addition to other nor- mal activities. Each name is indented to show how deep in the #in- clude 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 ...! . -version --version Print out GNU CPP's version number. With one dash, proceed to pre- process as normal. With two dashes, exit immediately. 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 com- mas, 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 which GCC does not know how to 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 in- put 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. -llibrary -l library Search the library named library when linking. (The second alter- native with the library as a separate argument is only for POSIX compliance and is not recommended.) 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. The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directo- ries plus any that you specify with -L. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fash- ion. The only difference between using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several directories. -lobjc You need this special case of the -l option in order to link an Ob- jective-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 or -nodefaultlibs is used. -nodefaultlibs Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. 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. -nostdlib Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to "mem- cmp", "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 -nodefault- libs is libgcc.a, a library of internal subroutines that 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 en- sures that you have no unresolved references to internal GCC li- brary subroutines. (For example, __main, used to ensure C++ con- structors will be called.) -pie Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option. -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 exe- cutable. -static On systems that support dynamic linking, this 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 that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this 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 con- figured, 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 li- braries as well as the application itself should use the shared libgcc. Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java 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 will not always be 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 will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize 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, with- out 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++ or GCJ driver, as appropriate for the languages used in the program, or using the op- tion -shared-libgcc, such that it is linked with the shared libgcc. -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 sup- port this option. -Xlinker option Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to rec- ognize. If you want to pass an option that takes an 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 -as- sert -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. -Wl,option Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. -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. Options for Directory Search These options specify directories to search for header files, for li- braries and for parts of the compiler: -Idir Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain ven- dor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left- to-right order; the standard system directories come after. If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option will be ig- nored. The directory will still be searched but as a system direc- tory 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. -iquotedir Add the directory dir to the head of the list of directories to be searched for header files only for the case of #include "file"; they are not searched for #include <file>, otherwise just like -I. -Ldir Add directory dir to the list of directories to be searched for -l. -Bprefix This option specifies where to find the executables, libraries, in- clude 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 each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /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 environ- ment variable. The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add 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 op- tions into -L options for the linker. They also apply to includes 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 run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two stan- dard 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 will be replaced by [dir/]include. This is to help with boot-strapping the com- piler. -specs=file Process file after the compiler reads in the standard specs file, in order to override the defaults that 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. --sysroot=dir Use dir as the logical root directory for headers and libraries. For example, if the compiler would normally search for headers in /usr/include and libraries in /usr/lib, it will instead search dir/usr/include and dir/usr/lib. If you use both this option and the -isysroot option, then the --sysroot option will apply to libraries, but the -isysroot option will apply to header files. The GNU linker (beginning with version 2.16) has the necessary sup- port for this option. If your linker does not support this option, the header file aspect of --sysroot will still work, but the li- brary aspect will not. -I- This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I-. Any directories you specify with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives. (Ordinarily all -I directories are used this way.) In addition, the -I- option inhibits the use of the current direc- tory (where the current input file came from) as the first search directory for #include "file". There is no way to override this effect of -I-. With -I. you can specify searching the directory which was current when the compiler was invoked. That is not ex- actly the same as what the preprocessor does by default, but it is often satisfactory. -I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc are independent. Specifying Target Machine and Compiler Version The usual way to run GCC is to run the executable called gcc, or <ma- chine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version. -b machine The argument machine specifies the target machine for compilation. The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For exam- ple, if a cross-compiler was configured with configure arm-elf, meaning to compile for an arm processor with elf binaries, then you would specify -b arm-elf to run that cross compiler. Because there are other options beginning with -b, the configuration must contain a hyphen. -V version The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, ver- sion might be 4.0, meaning to run GCC version 4.0. The -V and -b options work by running the <machine>-gcc-<version> exe- cutable, so there's no real reason to use them if you can just run that directly. Hardware Models and Configurations Earlier we discussed the standard option -b which chooses among differ- ent installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386. In addition, each of these target machine types can have its own spe- cial options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified. Some configurations of the compiler also support additional special op- tions, usually for compatibility with other compilers on the same plat- form. ARC Options These options are defined for ARC implementations: -EL Compile code for little endian mode. This is the default. -EB Compile code for big endian mode. -mmangle-cpu Prepend the name of the cpu to all public symbol names. In multi- ple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for an- other. No facility exists for handling variants that are "almost identical". This is an all or nothing option. -mcpu=cpu Compile code for ARC variant cpu. Which variants are supported de- pend on the configuration. All variants support -mcpu=base, this is the default. -mtext=text-section -mdata=data-section -mrodata=readonly-data-section Put functions, data, and readonly data in text-section, data-sec- tion, and readonly-data-section respectively by default. This can be overridden with the "section" attribute. ARM Options These -m options are defined for Advanced RISC Machines (ARM) architec- tures: -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 nec- essary for correct execution of the code. Specifying -fomit-frame-pointer with this option will cause the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. -mapcs This is a synonym for -mapcs-frame. -mthumb-interwork Generate code which supports calling between the ARM and Thumb in- struction sets. Without this option the two instruction sets can- not be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. -mno-sched-prolog Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the func- tion's body. This means that all functions will start with a rec- ognizable 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 if functions inside an executable piece of code. The default is -msched-prolog. -mhard-float Generate output containing floating point instructions. This is the default. -msoft-float Generate output containing library calls for floating point. Warn- ing: the requisite libraries are not available for all ARM 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 li- brary that comes with GCC, with -msoft-float in order for this to work. -mfloat-abi=name Specifies which ABI to use for floating point values. Permissible values are: soft, softfp and hard. soft and hard are equivalent to -msoft-float and -mhard-float re- spectively. softfp allows the generation of floating point in- structions, but still uses the soft-float calling conventions. -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 de- fault is to compile code for a little-endian processor. -mwords-little-endian This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8. -mcpu=name This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when gener- ating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm, stron- garm110, strongarm1100, 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, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312. -mtune=name This option is very similar to the -mcpu= option, except that in- stead of specifying the actual target processor type, and hence re- stricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instruc- tions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementations better performance can be ob- tained by using this option. -march=name 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: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5te, armv6, armv6j, iwmmxt, ep9312. -mfpu=name -mfpe=number -mfp=number This specifies what floating point hardware (or hardware emulation) is available on the target. Permissible names are: fpa, fpe2, fpe3, maverick, vfp. -mfp and -mfpe are synonyms for -mfpu=fpenum- ber, for compatibility with older versions of GCC. If -msoft-float is specified this specifies the format of floating point values. -mstructure-size-boundary=n The size of all structures and unions will be rounded up to a mul- tiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different tool- chains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it. Specifying the 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. -mabort-on-noreturn Generate a call to the function "abort" at the end of a "noreturn" function. It will be 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 sub- routine call on this register. This switch is needed if the target function will lie 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 will be turned into long calls. The heuristic is that static functions, functions which 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, will not be turned into long calls. The excep- tion to this rule is 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, will always be turned into long calls. This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will 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. -mnop-fun-dllimport Disable support for the "dllimport" attribute. -msingle-pic-base Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The run-time system is responsible for initializing this register with an appro- priate value before execution begins. -mpic-register=reg Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled, when R9 is used. -mcirrus-fix-invalid-insns Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations. This op- tion is only valid if the -mcpu=ep9312 option has been used to en- able generation of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by default, since the problem is only present in older Maverick implementations. The default can be re-enabled by use of the -mno-cirrus-fix-invalid-in- sns switch. -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 loca- tion "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 Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set. -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. -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. -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 (sup- ported in the arm6k architecture), and auto, which uses the best available method for the selected processor. The default setting is auto. AVR Options These options are defined for AVR implementations: -mmcu=mcu Specify ATMEL AVR instruction set or MCU type. Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28). Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, at- tiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535). Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711). Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85). Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k). -msize Output instruction sizes to the asm file. -minit-stack=N Specify the initial stack address, which may be a symbol or numeric value, __stack is the default. -mno-interrupts Generated code is not compatible with hardware interrupts. Code size will be smaller. -mcall-prologues Functions prologues/epilogues expanded as call to appropriate sub- routines. Code size will be smaller. -mno-tablejump Do not generate tablejump insns which sometimes increase code size. -mtiny-stack Change only the low 8 bits of the stack pointer. -mint8 Assume int to be 8 bit integer. This affects the sizes of all types: A char will be 1 byte, an int will be 1 byte, an long will be 2 bytes and long long will be 4 bytes. Please note that this option does not comply to the C standards, but it will provide you with smaller code size. Blackfin Options -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-frame-pointer removes the frame pointer for all functions which might make debugging harder. -mspecld-anomaly When enabled, the compiler will ensure that the generated code does not contain speculative loads after jump instructions. This option is enabled by default. -mno-specld-anomaly Don't generate extra code to prevent speculative loads from occur- ring. -mcsync-anomaly When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. This option is enabled by default. -mno-csync-anomaly Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch. -mlow-64k When enabled, the compiler is free to take advantage of the knowl- edge that the entire program fits into the low 64k of memory. -mno-low-64k Assume that the program is arbitrarily large. 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 im- plies -fPIC. -mno-id-shared-library Generate code that doesn't assume ID based shared libraries are be- ing used. This is the default. -mshared-library-id=n Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time effi- cient than omitting this option. -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 sub- routine call on this register. This switch is needed if the target function will lie 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 will restore the default behavior. Note these switches have no ef- fect on how the compiler generates code to handle function calls via function pointers. 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 ar- chitecture-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=archi- tecture-type. -mmax-stack-frame=n Warn when the stack frame of a function exceeds n bytes. -melinux-stacksize=n Only available with the cris-axis-aout target. Arranges for indi- cations in the program to the kernel loader that the stack of the program should be set to 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 as- sembly code. This option also has the effect to turn 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) arranges (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 align- ment. -mno-prologue-epilogue -mprologue-epilogue With -mno-prologue-epilogue, the normal function prologue and epi- logue that sets up the stack-frame are omitted and no return in- structions 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 variable needs to be allocated. -mno-gotplt -mgotplt With -fpic and -fPIC, don't generate (do generate) instruction se- quences 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. -maout Legacy no-op option only recognized with the cris-axis-aout target. -melf Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets. -melinux Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for -march=v8. -mlinux Legacy no-op option only recognized with the cris-axis-linux-gnu target. -sim This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output functions from a simulator li- brary. Code, initialized data and zero-initialized data are allo- cated consecutively. -sim2 Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. CRX Options These options are defined specifically for the CRX ports. -mmac Enable the use of multiply-accumulate instructions. Disabled by de- fault. -mpush-args Push instructions will be used to pass outgoing arguments when functions are called. Enabled by default. Darwin Options These options are defined for all architectures running the Darwin op- erating system. FSF GCC on Darwin does not create "fat" object files; it will create an object file for the single architecture that it 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 de- termined by the flags that specify the ISA that GCC is targetting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to over- ride this. The Darwin tools vary in their behavior when presented with an ISA mis- match. The assembler, as, will only permit instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in an ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and print 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, will quietly give 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 directo- ries to be searched for header files. These directories are inter- leaved 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 sib- lings if they occur in the same framework. A subframework should not have the same name as a framework, a warning will be issued if this is violated. Currently a subframework cannot have subframe- works, in the future, the mechanism may be extended to support this. The standard frameworks can be found in "/System/Li- brary/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 "Private- Headers" or "Headers" directory. -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. The default for this option is to make choices that seem to be most useful. -mkernel Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-cxa-atexit, -fno-exceptions, -fno-non-call-excep- tions, -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 com- piling 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 mod- ules 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 turn around development. Needed to enable gdb to dynamically load ".o" files into already running pro- grams. -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 archi- tecture 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 infor- mation. -bundle_loader executable This option specifies the executable that will be loading the build output file being linked. See man ld(1) for more information. -dynamiclib When passed this option, GCC will produce 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, func- tions in libgcc.a will be used to perform floating-point opera- tions. 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 will 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 in- teger 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 com- piled 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 regis- ters. -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 com- pliant code except that the inexact-flag is not maintained (see be- low). 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 num- bers and exceptional IEEE values such as not-a-number and plus/mi- nus 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 sig- nificantly 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 en- abled 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 to- wards zero. d Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture reference manual) con- trols 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 applica- tion, 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 func- tion that caused a floating point exception. i Instruction precision. The trap handler can determine the ex- act 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 gener- ated assembly file. Under DEC Unix, this has the effect that IEEE- conformant math library routines will be linked in. -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 instruc- tions. If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). You would typically use this option to build a shared library dy- namic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment. -malpha-as -mgas Select whether to generate code to be assembled by the vendor-sup- plied assembler (-malpha-as) or by the GNU assembler -mgas. -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 in- struction sets supported by the CPU type specified via -mcpu= op- tion or that of the CPU on which GCC was built if none was speci- fied. -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 reloca- tions 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 capabili- ties of the assembler when it is built and sets the default accord- ingly. -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 -ms- mall-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 will choose the default values for the instruction set from the processor you spec- ify. If you do not specify a processor type, GCC will default 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 exten- sions. ev67 21264a Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX ex- tensions. -mtune=cpu_type Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed. -mmemory-latency=time Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly de- pendent 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. DEC Alpha/VMS Options These -m options are defined for the DEC Alpha/VMS implementations: -mvms-return-codes Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes. 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, that uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it im- plies -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. -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 de- fault, 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 de- fault and can be disabled with -mno-linked-fp. -mlong-calls Use indirect addressing to call functions outside the current com- pilation 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 (de- fault). 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 com- piler 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 instead of uClibc. This is the default ex- cept on *-*-linux-*uclibc* targets. -muclibc Use uClibc instead of the GNU C library. This is the default on *-*-linux-*uclibc* targets. 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. -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 ar- chitecture-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 will run on higher num- bered 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. -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. -mjump-in-delay Fill delay slots of function calls with unconditional jump instruc- tions by modifying the return pointer for the function call to be the target of the conditional jump. -mdisable-fpregs Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort. -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 which performs faster indirect calls. This option will 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 can not 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 re- quired 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 sys- tems. -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. Warn- ing: the requisite libraries are not available for all HPPA tar- gets. 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 target hppa1.1-*-pro does 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 li- brary that comes with GCC, with -msoft-float in order for this to work. -msio Generate the predefine, "_SIO", for server IO. The default is -mw- sio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX. -mgnu-ld Use GNU ld specific options. This passes -shared to ld when build- ing a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not have any affect on which ld is called, it only changes what pa- rameters are passed to that ld. The ld that is called is deter- mined 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 HP ld specific options. 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 have any affect on 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 will degrade performance. However, it may be useful in large applications, par- ticularly when partial linking is used to build the application. The types of long calls used depends on the capabilities of the as- sembler and linker, and the type of code being generated. The im- pact on systems that support long absolute calls, and long pic sym- bol-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 speci- fied 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. -mu- nix=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 "_IN- CLUDE_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 op- tion. Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_ex- tended_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 -no- libdld 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. Intel 386 and AMD x86-64 Options These -m options are defined for the i386 and x86-64 family of comput- ers: -mtune=cpu-type Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for cpu-type are: 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 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, the code generated option will change to reflect the processors that were most common when that version of GCC was 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. native This selects the CPU to tune for at compilation time by deter- mining the processor type of the compiling machine. Using -mtune=native will produce code optimized for the local machine under the constraints of the selected instruction set. Using -march=native will enable all instruction subsets supported by the local machine (hence the result might not run on different machines). i386 Original Intel's i386 CPU. i486 Intel's i486 CPU. (No scheduling is implemented for this chip.) i586, pentium Intel Pentium CPU with no MMX support. pentium-mmx Intel PentiumMMX CPU based on Pentium core with MMX instruction set support. pentiumpro Intel PentiumPro CPU. i686 Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the code will run on all i686 family chips. pentium2 Intel Pentium2 CPU based on PentiumPro core with MMX instruc- tion set support. pentium3, pentium3m Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support. pentium-m Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks. pentium4, pentium4m Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set sup- port. prescott Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support. nocona Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 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 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! 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 Winchip2 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 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.) While picking a specific cpu-type will schedule things appropri- ately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=cpu-type op- tion being used. -march=cpu-type Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type. -mcpu=cpu-type A deprecated synonym for -mtune. -m386 -m486 -mpentium -mpentiumpro These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respectively. These synonyms are deprecated. -mfpmath=unit Generate floating point arithmetics for selected unit unit. The choices for unit are: 387 Use the standard 387 floating point coprocessor present major- ity of chips and emulated otherwise. Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precision instead of precision specified by the type resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed de- scription. This is the default choice for i386 compiler. sse Use scalar floating point instructions present in the SSE in- struction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too. For the i386 compiler, you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this op- tion effective. For the x86-64 compiler, these extensions are enabled by default. The resulting code should be considerably faster in the major- ity of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects tempo- raries to be 80bit. This is the default choice for the x86-64 compiler. sse,387 Attempt to utilize both instruction sets at once. This effec- tively double the amount of available registers and on chips with separate execution units for 387 and SSE the execution re- sources too. Use this option with care, as it is still experi- mental, because the GCC register allocator does not model sepa- rate functional units well resulting in instable performance. -masm=dialect Output asm instructions using selected dialect. Supported choices are intel or att (the default one). Darwin does not support intel. -mieee-fp -mno-ieee-fp Control whether or not the compiler uses IEEE floating point com- parisons. These handle correctly the case where the result of a comparison is unordered. -msoft-float Generate output containing library calls for floating point. Warn- ing: the requisite libraries are not part of GCC. Normally the fa- cilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compi- lation. 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" in- structions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden when -march indicates that the target cpu will always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, 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 will produce 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 contain- ing the above types will be aligned differently than the published application binary interface specifications for the 386 and will not be 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 i386 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) would prefer "long double" to be aligned to an 8 or 16 byte boundary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a -m128bit-long-double will align "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 to be 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, the structures and arrays containing "long double" variables will change their size as well as function calling convention for func- tion taking "long double" will be modified. Hence they will not be binary compatible with arrays or structures in code compiled with- out that switch. -mmlarge-data-threshold=number When -mcmodel=medium is specified, the data greater than threshold are placed in large data section. This value must be the same across all object linked into the binary and defaults to 65535. -msvr3-shlib -mno-svr3-shlib Control whether GCC places uninitialized local variables into the "bss" or "data" segments. -msvr3-shlib places them into "bss". These options are meaningful only on System V Release 3. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "ret" num in- struction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the ar- guments 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 nor- mally used on Unix, so you cannot use it if you need to call li- braries 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 will be generated for calls to those functions. In addition, seriously incorrect code will result 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 spe- cific 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. -mstackrealign Realign the stack at entry. On the Intel x86, the -mstackrealign option will generate an alternate prologue and epilogue that re- aligns the runtime stack. This supports mixing legacy codes that keep a 4-byte aligned stack with modern codes that keep a 16-byte stack for SSE compatibility. The alternate prologue and epilogue are slower and bigger than the regular ones, and the alternate pro- logue requires an extra scratch register; this lowers the number of registers available if used in conjunction with the "regparm" at- tribute. The -mstackrealign option is incompatible with the nested function prologue; this is considered a hard error. See also the attribute "force_align_arg_pointer", applicable to individual func- tions. -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). On Pentium and PentiumPro, "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 will most likely misalign 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 -mno-mmx -msse -mno-sse -msse2 -mno-sse2 -msse3 -mno-sse3 -m3dnow -mno-3dnow These switches enable or disable the use of instructions in the MMX, SSE, SSE2 or 3DNow! extended instruction sets. These exten- sions are also available as built-in functions: see X86 Built-in Functions, for details of the functions enabled and disabled by these switches. To have SSE/SSE2 instructions generated automatically from float- ing-point code (as opposed to 387 instructions), see -mfpmath=sse. These options will enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications which per- form runtime CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particu- lar, the file containing the CPU detection code should be compiled without these options. -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 dependen- cies. -maccumulate-outgoing-args If enabled, the maximum amount of space required for outgoing argu- ments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved schedul- ing and reduced stack usage when 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 Mingw32. Code that re- lies 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. -mno-align-stringops Do not align 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 destination is known to be aligned at least to 4 byte boundary. This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths. -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-frame-pointer removes the frame pointer for all 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 legal depends on the operating system, and whether it maps the segment to cover the entire TLS area. For systems that use GNU libc, the default is on. These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments. -m32 -m64 Generate code for a 32-bit or 64-bit environment. The 32-bit envi- ronment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture. For darwin only the -m64 option turns off the -fno-pic and -mdynamic-no-pic options. -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 loca- tion of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone dis- ables 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 but symbols can be located anywhere in the address space. Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model. -mcmodel=large Generate code for the large model: This model makes no assumptions about addresses and sizes of sections. Currently GCC does not im- plement this model. 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 de- fault. -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 re- sult 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 -mcon- stant-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. -minline-int-divide-min-latency Generate code for inline divides of integer values using the mini- mum latency algorithm. -minline-int-divide-max-throughput Generate code for inline divides of integer values using the maxi- mum throughput algorithm. -minline-sqrt-min-latency Generate code for inline square roots using the minimum latency al- gorithm. -minline-sqrt-max-throughput Generate code for inline square roots using the maximum throughput algorithm. -mno-dwarf2-asm -mdwarf2-asm Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when not using the GNU assem- bler. -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 instruc- tion 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 can not 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. -mt -pthread Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. It does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it. These are HP-UX specific flags. -milp32 -mlp64 Generate code for a 32-bit or 64-bit environment. The 32-bit envi- ronment sets int, long and pointer to 32 bits. The 64-bit environ- ment 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 will result in generation of the ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default is 'disable'. -msched-ar-data-spec -mno-sched-ar-data-spec (En/Dis)able data speculative scheduling after reload. This will result in generation of the ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default is 'enable'. -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 will result in generation of the ld.s instructions and the corre- sponding check instructions chk.s . The default is 'disable'. -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 ef- fective only with -msched-br-data-spec enabled. The default is 'enable'. -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 ef- fective only with -msched-ar-data-spec enabled. The default is 'enable'. -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 is 'enable'. -msched-ldc -mno-sched-ldc (En/Dis)able use of simple data speculation checks ld.c . If dis- abled, only chk.a instructions will be emitted to check data specu- lative loads. The default is 'enable'. -mno-sched-control-ldc -msched-control-ldc (Dis/En)able use of ld.c instructions to check control speculative loads. If enabled, in case of control speculative load with no speculatively scheduled dependent instructions this load will be emitted as ld.sa and ld.c will be used to check it. The default is 'disable'. -mno-sched-spec-verbose -msched-spec-verbose (Dis/En)able printing of the information about speculative motions. -mno-sched-prefer-non-data-spec-insns -msched-prefer-non-data-spec-insns If enabled, data speculative instructions will be chosen for sched- ule only if there are no other choices at the moment. This will make the use of the data speculation much more conservative. The default is 'disable'. -mno-sched-prefer-non-control-spec-insns -msched-prefer-non-control-spec-insns If enabled, control speculative instructions will be chosen for schedule only if there are no other choices at the moment. This will make the use of the control speculation much more conserva- tive. The default is 'disable'. -mno-sched-count-spec-in-critical-path -msched-count-spec-in-critical-path If enabled, speculative dependencies will be considered during com- putation of the instructions priorities. This will make the use of the speculation a bit more conservative. The default is 'disable'. 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 generat- ing 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 will use during code generation. These pseudo-registers will be 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 pro- gram must be compiled with the same value for this option. Because of that, you must not use this option with the default runtime li- braries gcc builds. 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 will generate "seth/add3" instructions to load their ad- dresses), 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 will generate "seth/add3" instructions to load their ad- dresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler will generate the much slower "seth/add3/jl" instruction sequence). -msdata=none Disable use of the small data area. Variables will be 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" at- tribute 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 gener- ate 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. Compil- ing with different values of num may or may not work; if it doesn't the linker will give an error message---incorrect code will not be generated. -mdebug Makes the M32R specific code in the compiler display some statis- tics 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 will be pre- ferred over conditional code, if it is 2, then the opposite will apply. -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 will only be 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 the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below. -m68000 -mc68000 Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. -m68020 -mc68020 Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. -m68881 Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless --nfp was speci- fied when the compiler was configured. -m68030 Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. -m68040 Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. 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. 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. 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" family cpu. This is the de- fault when the compiler is configured for 520X-based systems. Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202. -mcfv4e Generate output for a ColdFire V4e family cpu (e.g. 547x/548x). This includes use of hardware floating point instructions. -m68020-40 Generate output for a 68040, without using any of the new instruc- tions. This results in code which 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. -m68020-60 Generate output for a 68060, without using any of the new instruc- tions. This results in code which 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. -msoft-float Generate output containing library calls for floating point. Warn- ing: the requisite libraries are not available for all m68k tar- gets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets m68k-*-aout and m68k-*-coff do provide software floating point support. -mshort Consider type "int" to be 16 bits wide, like "short int". Addi- tionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. -mnobitfield 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 de- signed for a 68020. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "rtd" in- struction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the ar- guments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries com- piled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result 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. -malign-int -mno-align-int Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit boundary (-ma- lign-int) or a 16-bit boundary (-mno-align-int). Aligning vari- ables 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 will align struc- tures 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 will be handled by the system. -msep-data Generate code that allows the data segment to be located in a dif- ferent area of memory from the text segment. This allows for exe- cute 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 im- plies -fPIC. -mno-id-shared-library Generate code that doesn't assume ID based shared libraries are be- ing used. This is the default. -mshared-library-id=n Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time effi- cient than omitting this option. M68hc1x Options These are the -m options defined for the 68hc11 and 68hc12 microcon- trollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below. -m6811 -m68hc11 Generate output for a 68HC11. This is the default when the com- piler is configured for 68HC11-based systems. -m6812 -m68hc12 Generate output for a 68HC12. This is the default when the com- piler is configured for 68HC12-based systems. -m68S12 -m68hcs12 Generate output for a 68HCS12. -mauto-incdec Enable the use of 68HC12 pre and post auto-increment and auto- decrement addressing modes. -minmax -nominmax Enable the use of 68HC12 min and max instructions. -mlong-calls -mno-long-calls Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the "call" instruction to call a function and the "rtc" instruction for returning. -mshort Consider type "int" to be 16 bits wide, like "short int". -msoft-reg-count=count Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the pro- gram. The default is 4 for 68HC11 and 2 for 68HC12. 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 in- structions 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 four 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. 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 will run 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, and mips64. The processor names are: 4kc, 4km, 4kp, 5kc, 5kf, 20kc, 24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400 and vr5500. 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). In processor names, a final 000 can be abbreviated as k (for exam- ple, -march=r2k). Prefixes are optional, and vr may be written r. 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 will set _MIPS_ARCH to "r2000" and define the macro _MIPS_ARCH_R2000. Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full prefix and will 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 arith- metic operations. The list of arch values is the same as for -march. When this option is not used, GCC will optimize for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that will run 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. -mips32r2 Equivalent to -march=mips32r2. -mips64 Equivalent to -march=mips64. -mips16 -mno-mips16 Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE. -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>. -mabicalls -mno-abicalls Generate (do not generate) code that is suitable for SVR4-style dy- namic 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 op- tion only affects -mabicalls. All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an exten- sion, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initial- ization 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 ob- jects that can only be linked by the GNU linker. However, the op- tion does not affect the ABI of the final executable; it only af- fects the ABI of relocatable objects. Using -mno-shared will gen- erally make executables both smaller and quicker. -mshared is the default. -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 will only work if the GOT is smaller than about 64k. Anything larger will cause 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. It should then work with very large GOTs, although it will also be less efficient, since it will take 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 in- dependent 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. -mhard-float Use floating-point coprocessor instructions. -msoft-float Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead. -msingle-float Assume that the floating-point coprocessor only supports single- precision operations. -mdouble-float Assume that the floating-point coprocessor supports double-preci- sion operations. This is the default. -mdsp -mno-dsp Use (do not use) the MIPS DSP ASE. -mpaired-single -mno-paired-single Use (do not use) paired-single floating-point instructions. This option can only be used when generating 64-bit code and re- quires hardware floating-point support to be enabled. -mips3d -mno-mips3d Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. -mlong64 Force "long" types to be 64 bits wide. See -mlong32 for an expla- nation of the default and the way that the pointer size is deter- mined. -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, regard- less 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 global and static items less than or equal to num bytes into the small data or bss section instead of the normal data or bss section. This allows the data to be accessed using a single in- struction. All modules should be compiled with the same -G num value. -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. -msplit-addresses -mno-split-addresses Enable (disable) use of the "%hi()" and "%lo()" assembler reloca- tion operators. This option has been superseded by -mexplicit-re- locs 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-re- locs, 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 con- ditional 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. -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. -mfused-madd -mno-fused-madd Enable (disable) use of the floating point multiply-accumulate in- structions, when they are available. The default is -mfused-madd. 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 cir- cumstances. -nocpp Tell the MIPS assembler to not run its preprocessor over user as- sembler files (with a .s suffix) when assembling them. -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-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 will avoid 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 er- rata.) -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-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 which implement those architectures; for those, Branch Likely instructions will not be generated by de- fault because the MIPS32 and MIPS64 architectures specifically dep- recate their use. -mfp-exceptions -mno-fp-exceptions Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions 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. Other- wise, 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 will align 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. MMIX Options These options are defined for the MMIX: -mlibfuncs -mno-libfuncs Specify that intrinsic library functions are being compiled, pass- ing 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 as- sembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address re- quests within the range 0 to 255 from the value held in the regis- ter. 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 which uses features specific to the AM33 processor. -mno-am33 Do not generate code which uses features specific to the AM33 processor. This is the default. -mreturn-pointer-on-d0 When generating a function which 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 would 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 opti- mization pass to shorten branches, calls and absolute memory ad- dresses. This option only has an effect when used on the command line for the final link step. This option makes symbolic debugging impossible. MT Options These -m options are defined for Morpho MT architectures: -march=cpu-type Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are ms1-64-001, ms1-16-002, ms1-16-003 and ms2. When this option is not used, the default is -march=ms1-16-002. -mbacc Use byte loads and stores when generating code. -mno-bacc Do not use byte loads and stores when generating code. -msim Use simulator runtime -mno-crt0 Do not link in the C run-time initialization object file crti.o. Other run-time initialization and termination files such as startup.o and exit.o are still included on the linker command line. PDP-11 Options These options are defined for the PDP-11: -mfpu Use hardware FPP floating point. This is the default. (FIS float- ing point on the PDP-11/40 is not supported.) -msoft-float Do not use hardware floating point. -mac0 Return floating-point results in ac0 (fr0 in Unix assembler syn- tax). -mno-ac0 Return floating-point results in memory. This is the default. -m40 Generate code for a PDP-11/40. -m45 Generate code for a PDP-11/45. This is the default. -m10 Generate code for a PDP-11/10. -mbcopy-builtin Use inline "movmemhi" patterns for copying memory. This is the de- fault. -mbcopy Do not use inline "movmemhi" patterns for copying memory. -mint16 -mno-int32 Use 16-bit "int". This is the default. -mint32 -mno-int16 Use 32-bit "int". -mfloat64 -mno-float32 Use 64-bit "float". This is the default. -mfloat32 -mno-float64 Use 32-bit "float". -mabshi Use "abshi2" pattern. This is the default. -mno-abshi Do not use "abshi2" pattern. -mbranch-expensive Pretend that branches are expensive. This is for experimenting with code generation only. -mbranch-cheap Do not pretend that branches are expensive. This is the default. -msplit Generate code for a system with split I&D. -mno-split Generate code for a system without split I&D. This is the default. -munix-asm Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd. -mdec-asm Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than pdp11-*-bsd. PowerPC Options These are listed under IBM RS/6000 and PowerPC Options These -m options are defined for the IBM RS/6000 and PowerPC: -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mno-powerpc -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mpowerpc64 -mno-powerpc64 -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instruc- tions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx, 6xx, and follow-on microprocessors. Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ reg- ister is included in processors supporting the POWER architecture. 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. The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture. The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Spec- ifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Pur- pose group, including floating-point square root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also 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 -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ proces- sor and other processors that support the PowerPC V2.03 architec- ture. 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. If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ regis- ter. Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601. -mnew-mnemonics -mold-mnemonics Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assem- bler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified. GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default. -mcpu=cpu_type Set architecture type, register usage, choice of mnemonics, and in- struction scheduling parameters for machine type cpu_type. Sup- ported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5, power, power2, power3, power4, power5, power5+, power6, common, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64. -mcpu=common selects a completely generic processor. Code gener- ated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both ar- chitectures, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes. -mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 spec- ify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes. The other options specify a specific processor. Code generated un- der those options will run best on that processor, and may not run at all on others. The -mcpu options automatically enable or disable the following op- tions: -maltivec, -mfprnd, -mhard-float, -mmfcrf, -mmultiple, -mnew-mnemonics, -mpopcntb, -mpower, -mpower2, -mpowerpc64, -mpow- erpc-gpopt, -mpowerpc-gfxopt, -mstring, -mmulhw, -mdlmzb. The par- ticular options set for any particular CPU will vary between com- piler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hard- ware'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, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune. -mswdiv -mno-swdiv Generate code to compute division as reciprocal estimate and itera- tive refinement, creating opportunities for increased throughput. This feature requires: optional PowerPC Graphics instruction set for single precision and FRE instruction for double precision, as- suming divides cannot generate user-visible traps, and the domain values not include Infinities, denormals or zero denominator. -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 enhance- ments. -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-exec .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 exe- cutable. This is a PowerPC 32-bit SYSV ABI option. -misel -mno-isel This switch enables or disables the generation of ISEL instruc- tions. -misel=yes/no This switch has been deprecated. Use -misel and -mno-isel instead. -mspe -mno-spe This switch enables or disables the generation of SPE simd instruc- tions. -mspe=yes/no This option has been deprecated. Use -mspe and -mno-spe instead. -mfloat-gprs=yes/single/double/no -mfloat-gprs This switch enables or disables the generation of floating point operations on the general purpose registers for architectures that support it. The argument yes or single enables the use of single-precision floating point operations. The argument double enables the use of single and double-precision floating point operations. The argument no disables floating point operations on the general purpose registers. This option is currently only available on the MPC854x. -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 will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place 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 over- flowed 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 cal- culate 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 will produce 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 and -mpowerpc, 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 seman- tics 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 sig- nificant 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 docu- mented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than de- clared. IBM XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point argu- ments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subrou- tines compiled by IBM XL compilers without optimization. -mpe Support IBM RS/6000 SP Parallel Environment (PE). Link an applica- tion 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 appropri- ate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompati- ble. -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 align- ment defined in the ABI. On 64-bit Darwin, natural alignment is the default, and -ma- lign-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 in- structions and the store multiple word instructions. These in- structions are generated by default on POWER systems, and not gen- erated 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 the instructions usage in little endian mode. -mstring -mno-string Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by de- fault on POWER systems, and not generated on PowerPC systems. Do not use -mstring on little endian PowerPC systems, since those in- structions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode. -mupdate -mno-update Generate code that uses (does not use) the load or store instruc- tions that update the base register to the address of the calcu- lated memory location. These instructions are generated by de- fault. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the pre- vious frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. -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 is used. -mmulhw -mno-mulhw Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405 and 440 processors. These instructions are generated by default when targetting those processors. -mdlmzb -mno-dlmzb Generate code that uses (does not use) the string-search dlmzb in- struction on the IBM 405 and 440 processors. This instruction is generated by default when targetting those processors. -mno-bit-align -mbit-align On System V.4 and embedded PowerPC systems do not (do) force struc- tures 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 "un- signed" bit-fields of length 1 would be aligned to a 4 byte bound- ary and have a size of 4 bytes. By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size. -mno-strict-align -mstrict-align On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system. -mrelocatable -mno-relocatable On embedded PowerPC systems generate code that allows (does not al- low) the program to be relocated to a different address at runtime. If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable-lib -mno-relocatable-lib On embedded PowerPC systems generate code that allows (does not al- low) the program to be relocated to a different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with 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 ad- dresses 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 re- locatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared li- braries. -mprioritize-restricted-insns=priority This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The ar- gument priority takes the value 0/1/2 to assign no/highest/sec- ond-highest 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 depen- dence from store to load is costly, store_to_load: any dependence from store to load is costly, number: any dependence which latency >= number is costly. -minsert-sched-nops=scheme This option controls which nop insertion scheme will be 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 which 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 esti- mated processor grouping. number: Insert nops to force costly de- pendent 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 call- ing conventions that adheres 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 Specify both -mcall-sysv and -meabi options. -mcall-sysv-noeabi Specify both -mcall-sysv and -mno-eabi options. -mcall-solaris On System V.4 and embedded PowerPC systems compile code for the So- laris operating system. -mcall-linux On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system. -mcall-gnu On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system. -mcall-netbsd On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system. -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, spe, no-spe, ibm- longdouble, ieeelongdouble. -mabi=spe Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI. -mabi=no-spe Disable Booke SPE ABI extensions for the current ABI. -mabi=ibmlongdouble Change the current ABI to use IBM extended precision long double. This is a PowerPC 32-bit SYSV ABI option. -mabi=ieeelongdouble Change the current ABI to use IEEE extended precision long double. This is a PowerPC 32-bit Linux ABI option. -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 were passed in the floating point registers in case the function takes a variable arguments. With -mprototype, only calls to prototyped variable argument func- tions will 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. configura- tions. -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. -mwindiss Specify that you are compiling for the WindISS simulation environ- ment. -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 to 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, do not call an initializa- tion function from "main", and the -msdata option will only use "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the pow- erpc*-*-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, com- pile 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. -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 regis- ter 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 more expensive calling sequence is required. This is required for calls further than 32 megabytes (33,554,432 bytes) from the current loca- tion. A short call will be 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 gener- ating glue code on the fly. On these systems, long calls are un- necessary 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 Darwin/PPC systems, "#pragma longcall" will generate "jbsr callee, L42", plus a "branch island" (glue code). The two target addresses represent the callee and the "branch island". The Dar- win/PPC linker will prefer the first address and generate a "bl callee" if the PPC "bl" instruction will reach the callee directly; otherwise, the linker will generate "bl L42" to call the "branch island". The "branch island" is appended to the body of the call- ing 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, we may cause GCC to ignore all longcall specifica- tions when the linker is known to generate glue. -pthread Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker. S/390 and zSeries Options These are the -m options defined for the S/390 and zSeries architec- ture. -mhard-float -msoft-float Use (do not use) the hardware floating-point instructions and reg- isters for floating-point operations. When -msoft-float is speci- fied, functions in libgcc.a will be used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. -mlong-double-64 -mlong-double-128 These switches control the size of "long double" type. A size of 64bit 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-2 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 top- most 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, reg- ister 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 exe- cutable 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. -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 compil- ing. The default is to not print debug information. -march=cpu-type Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are g5, g6, z900, and z990. When generating code using the instructions available on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5. -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 which 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 These arguments always have to be used in conjunction. If they are present the s390 back end emits additional instructions in the function prologue which trigger a trap if the stack size is stack- guard bytes above the stack-size (remember that the stack on s390 grows downward). These options are intended to be used to help de- bugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in produc- tion 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 ef- ficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. 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 generate bcnz instruction. -muls Enable generate unaligned load and store instruction. -mmac Enable the use of multiply-accumulate instructions. Disabled by de- fault. -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. -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 sup- ports 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. -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 mo- ment. -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 will 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. -mfmovd Enable the use of the instruction "fmovd". -mhitachi Comply with the calling conventions defined by Renesas. -mrenesas Comply with the calling conventions defined by Renesas. -mno-renesas Comply with the calling conventions defined for GCC before the Re- nesas conventions were available. This option is the default for all targets of the SH toolchain except for sh-symbianelf. -mnomacsave Mark the "MAC" register as call-clobbered, even if -mhitachi is given. -mieee Increase IEEE-compliance of floating-point code. At the moment, this is equivalent to -fno-finite-math-only. When generating 16 bit SH opcodes, getting IEEE-conforming results for comparisons of NANs / infinities incurs extra overhead in every floating point comparison, therefore the default is set to -ffinite-math-only. -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. -mspace Optimize for space instead of speed. Implied by -Os. -mprefergot When generating position-independent code, emit function calls us- ing the Global Offset Table instead of the Procedure Linkage Table. -musermode Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline. This library function call doesn't assume it can write to the whole memory address space. This is the default when the target is "sh-*-linux*". -multcost=number Set the cost to assume for a multiply insn. -mdiv=strategy Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating point. This has a very high latency, but needs only a few instructions, so it might be a good choice if your code has enough easily exploitable ILP to allow the compiler to schedule the float- ing point instructions together with other instructions. Division by zero causes a floating point exception. "inv" uses integer op- erations to calculate the inverse of the divisor, and then multi- plies the dividend with the inverse. This strategy allows cse and hoisting of the inverse calculation. Division by zero calculates an unspecified result, but does not trap. "inv:minlat" is a vari- ant of "inv" where if no cse / hoisting opportunities have been found, or if the entire operation has been hoisted to the same place, the last stages of the inverse calculation are intertwined with the final multiply to reduce the overall latency, at the ex- pense of using a few more instructions, and thus offering fewer scheduling opportunities with other code. "call" calls a library function that usually implements the inv:minlat strategy. This gives high code density for m5-*media-nofpu compilations. "call2" uses a different entry point of the same library function, where it assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to cse / code hoisting optimiza- tions. "inv:call", "inv:call2" and "inv:fp" all use the "inv" al- gorithm for initial code generation, but if the code stays unopti- mized, revert to the "call", "call2", or "fp" strategies, respec- tively. Note that the potentially-trapping side effect of division by zero is carried by a separate instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the side effect stays where it is. A recombination to fp oper- ations or a call is not possible in that case. "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case that the inverse calculation was nor separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable), by inserting a test to skip a number of op- erations in this case; this test slows down the case of larger div- idends. inv20u assumes the case of a such a small dividend to be unlikely, and inv20l assumes it to be likely. -mdivsi3_libfunc=name Set the name of the library function used for 32 bit signed divi- sion to name. This only affect the name used in the call and inv:call division strategies, and the compiler will still expect the same sets of input/output/clobbered registers as if this option was not present. -madjust-unroll Throttle unrolling to avoid thrashing target registers. This op- tion only has an effect if the gcc code base supports the TAR- GET_ADJUST_UNROLL_MAX target hook. -mindexed-addressing Enable the use of the indexed addressing mode for SHmedia32/SHcom- pact. This is only safe if the hardware and/or OS implement 32 bit wrap-around semantics for the indexed addressing mode. The archi- tecture allows the implementation of processors with 64 bit MMU, which the OS could use to get 32 bit addressing, but since no cur- rent hardware implementation supports this or any other way to make the indexed addressing mode safe to use in the 32 bit ABI, the de- fault is -mno-indexed-addressing. -mgettrcost=number Set the cost assumed for the gettr instruction to number. The de- fault is 2 if -mpt-fixed is in effect, 100 otherwise. -mpt-fixed Assume pt* instructions won't trap. This will generally generate better scheduled code, but is unsafe on current hardware. The cur- rent architecture definition says that ptabs and ptrel trap when the target anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule ptabs / ptrel before a branch, or hoist it out of a loop. For example, __do_global_ctors, a part of libgcc that runs constructors at program startup, calls functions in a list which is delimited by -1. With the -mpt-fixed option, the ptabs will be done before testing against -1. That means that all the constructors will be run a bit quicker, but when the loop comes to the end of the list, the program crashes because ptabs loads -1 into a target register. Since this option is unsafe for any hardware implementing the current architecture specification, the default is -mno-pt-fixed. Unless the user specifies a specific cost with -mgettrcost, -mno-pt-fixed also implies -mgettrcost=100; this deters register allocation using target registers for storing ordinary integers. -minvalid-symbols Assume symbols might be invalid. Ordinary function symbols gener- ated by the compiler will always be valid to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible to generate symbols that will cause ptabs / ptrel to trap. This option is only meaningful when -mno-pt-fixed is in effect. It will then prevent cross-basic-block cse, hoisting and most scheduling of symbol loads. The default is -mno-invalid-symbols. 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. 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 sys- tem software with this option. -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. Warn- ing: the requisite libraries are not available for all SPARC tar- gets. 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 li- brary 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 dou- ble) 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. Be- cause of the trap handler overhead, this is much slower than call- ing 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 de- fault because it results in a performance loss, especially for floating point code. -mno-faster-structs -mfaster-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 in- tended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI. -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 ob- ject. -mimpure-text suppresses the "relocations remain against allocat- able but non-writable sections" linker error message. However, the necessary relocations will 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. This option is only available on SunOS and Solaris. -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, sparclite, f930, f934, hypersparc, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultra- sparc3, and niagara. 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 v8: supersparc, hypersparc sparclite: f930, f934, sparclite86x sparclet: tsc701 v9: ultrasparc, ultrasparc3, niagara 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 ap- propriate 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 com- piler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=super- sparc, 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, multi- ply/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 in- structions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the com- piler 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 Ul- traSPARC T1 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 would. 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, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, and niagara. -mv8plus -mno-v8plus With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The dif- ference from the V8 ABI is that the global and out registers are considered 64-bit 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 Ultra- SPARC Visual Instruction Set extensions. The default is -mno-vis. These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments: -mlittle-endian Generate code for a processor running in little-endian mode. It is only available for a few configurations and most notably not on So- laris and Linux. -m32 -m64 Generate code for a 32-bit or 64-bit environment. The 32-bit envi- ronment sets int, long and pointer to 32 bits. The 64-bit environ- ment sets int to 32 bits and long and pointer to 64 bits. -mcmodel=medlow Generate code for the Medium/Low code model: 64-bit addresses, pro- grams must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked. -mcmodel=medmid Generate code for 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. -mcmodel=medany Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data seg- ments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. -mcmodel=embmedany Generate code for the Medium/Anywhere code model for embedded sys- tems: 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. -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. These switches are supported in addition to the above on Solaris: -threads Add support for multithreading using the Solaris threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it. -pthreads Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it. -pthread This is a synonym for -pthreads. Options for System V These additional options are available on System V Release 4 for com- patibility 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 assem- bler uses this option. TMS320C3x/C4x Options These -m options are defined for TMS320C3x/C4x implementations: -mcpu=cpu_type Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the TMS320C40. -mbig-memory -mbig -msmall-memory -msmall Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run- time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big mem- ory model is the default and requires reloading of the DP register for every direct memory access. -mbk -mno-bk Allow (disallow) allocation of general integer operands into the block count register BK. -mdb -mno-db Enable (disable) generation of code using decrement and branch, DB- cond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maxi- mum iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on the C3x?). Note that GCC will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if there is more than one memory ref- erence in the loop. Thus a loop where the loop counter is decre- mented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized. -mdp-isr-reload -mparanoid Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library. -mmpyi -mno-mpyi For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are performed in- line instead of a library call. -mfast-fix -mno-fast-fix The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incor- rectly truncated an additional code is necessary to detect and cor- rect this case. This option can be used to disable generation of the additional code required to correct the result. -mrptb -mno-rptb Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2. -mrpts=count -mno-rpts Enable (disable) the use of the single instruction repeat instruc- tion RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count can- not be determined at compile time. Note that the repeated instruc- tion following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since interrupts are blocked by this instruction, it is disabled by default. -mloop-unsigned -mno-loop-unsigned The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^{31 + 1} maximum itera- tion count may be exceeded. This switch allows an unsigned itera- tion count. -mti Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers. -mregparm -mmemparm Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack. -mparallel-insns -mno-parallel-insns Allow the generation of parallel instructions. This is enabled by default with -O2. -mparallel-mpy -mno-parallel-mpy Allow the generation of MPY||ADD and MPY||SUB parallel instruc- tions, provided -mparallel-insns is also specified. These instruc- tions have tight register constraints which can pessimize the code generation of large functions. 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 will always load the functions address up into a register, and call 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 regis- ters 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 refer- ences). -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. -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 will cause r2 and r5 to be used in the code generated by the compiler. This setting is the default. -mno-app-regs This option will cause r2 and r5 to be treated as fixed registers. -mv850e1 Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will be defined if this option is used. -mv850e Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined if this option is used. If neither -mv850 nor -mv850e nor -mv850e1 are defined then a de- fault target processor will be chosen and the relevant __v850*__ preprocessor constant will be defined. The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. -mdisable-callt This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850 architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used. 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 you will assemble with the GNU assembler. -mg Output code for g-format floating point numbers instead of d-for- mat. x86-64 Options These are listed under 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 con- stant values. The "CONST16" instruction is currently not a stan- dard 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" instruc- tion 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 op- erations. 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 produc- ing 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. -mtext-section-literals -mno-text-section-literals 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 liter- als and improve code size. With -mtext-section-literals, the lit- erals 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. -mtarget-align -mno-target-align When this option is enabled, GCC instructs the assembler to auto- matically align instructions to reduce branch penalties at the ex- pense of some code density. The assembler attempts to widen den- sity instructions to align branch targets and the instructions fol- lowing call instructions. If there are not enough preceding safe density instructions to align a target, no widening will be per- formed. The default is -mtarget-align. These options do not af- fect the treatment of auto-aligned instructions like "LOOP", which the assembler will always align, either by widening density in- structions or by inserting no-op instructions. -mlongcalls -mno-longcalls When this option is enabled, GCC instructs the assembler to trans- late direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call in- struction. This translation typically occurs for calls to func- tions in other source files. Specifically, the assembler trans- lates 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 will still show di- rect call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler will use an indirect call for every cross-file call, not just those that really will be out of range. zSeries Options These are listed under 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 would be -fno-foo. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing no- or adding it. -fbounds-check For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran front-ends, where this option defaults to true and false respec- tively. -ftrapv This option generates traps for signed overflow on addition, sub- traction, multiplication operations. -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 opti- mizations and disables others. This option is enabled by default for the Java front-end, as required by the Java language specifica- tion. -fexceptions Enable exception handling. Generates extra code needed to propa- gate exceptions. For some targets, this implies GCC will generate frame unwind information for all functions, which can produce sig- nificant data size overhead, although it does not affect execution. If you do not specify this option, GCC will enable it by default for languages like C++ which normally require exception handling, and disable 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 excep- tions. 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 float- ing point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". -funwind-tables Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf. -fasynchronous-unwind-tables Generate unwind table in dwarf2 format, if supported by target ma- chine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as de- bugger or garbage collector). -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, particu- larly 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 in- terface. -freg-struct-return Return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-re- turn. 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 in- terface. -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 will be equivalent to the smallest integer type which 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-double Use the same size for "double" as for "float". Warning: the -fshort-double 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, allocate even uninitialized global variables in the data sec- tion of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without "extern") in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work 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 op- tion 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 (per- haps 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. -fpic Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code ac- cesses 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 ma- chine-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 and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the 386, 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 de- fined 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 the 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 de- fined to 2. -fpie -fPIE These options are similar to -fpic and -fPIC, but generated posi- tion independent code can be only linked into executables. Usually these options are used when -pie GCC option will be used during linking. -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 which forms part of a dynamic linker and cannot reference the ad- dress 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 clob- bered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed per- vasive roles in the machine's execution model will produce disas- trous 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 will save and restore the register reg if they use it. It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed per- vasive roles in the machine's execution model will produce disas- trous results. A different sort of disaster will result 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, rep- resenting the maximum alignment (that is, objects with default alignment requirements larger than this will be 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. -finstrument-functions Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the fol- lowing profiling functions will be called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current func- tion, so the call site information may not be available to the pro- filing 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 func- tion, 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 will indicate where, concep- tually, 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 addi- tional 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 anyways, but if you get lucky and the optimizer always expands the functions inline, you might have got- ten away without providing static copies.) A function may be given the attribute "no_instrument_function", in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high- priority interrupt routines, and any functions from which the pro- filing functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). -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 only rarely need to specify it in a single-threaded environment since stack overflow is auto- matically 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 must do that. The switch causes generation of code to ensure that the operating system sees the stack being ex- tended. -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit Generate code to ensure that the stack does not grow beyond a cer- tain value, either the value of a register or the address of a sym- bol. If the stack would grow beyond the value, a signal is raised. 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-sym- bol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to en- force a stack limit of 128KB. Note that this may only work with the GNU linker. -fargument-alias -fargument-noalias -fargument-noalias-global -fargument-noalias-anything Specify the possible relationships among parameters and between pa- rameters and global data. -fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.-fargument-noalias speci- fies that arguments do not alias each other, but may alias global storage.-fargument-noalias-global specifies that arguments do not alias each other and do not alias global storage. -fargu- ment-noalias-anything specifies that arguments do not alias any other storage. Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself. -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 in- terface. Not all targets provide complete support for this switch. -ftls-model=model Alter the thread-local storage model to be used. The model argu- ment should be one of "global-dynamic", "local-dynamic", "ini- tial-exec" or "local-exec". The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic". -fvisibility=default|internal|hidden|protected Set the default ELF image symbol visibility to the specified op- tion---all symbols will be 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 op- timized 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 ie; avail- able to be linked against from outside the shared object. "pro- tected" and "internal" are pretty useless in real-world usage so the only other commonly used option will be "hidden". The default if -fvisibility isn't specified is "default", i.e., make every sym- bol public---this causes the same behavior as previous versions of GCC. A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Li- braries" by Ulrich Drepper (which can be found at <http://peo- ple.redhat.com/~drepper/>)---however a superior solution made pos- sible 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 DLL's on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__de- clspec(dllexport)" you get almost identical semantics with identi- cal syntax. This is a great boon to those working with cross-plat- form projects. For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the de- clarations 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 ie; 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 sys- tem 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 modifica- tions. However, this means that calls to extern functions with no explicit visibility will 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 will be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes will be unified between the DSOs. An overview of these techniques, their benefits and how to use them is at <http://gcc.gnu.org/wiki/Visibility>. ENVIRONMENT 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 envi- ronment 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 localiza- tion information that allow 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 cate- gories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom en- coded in UTF-8. The LC_CTYPE environment variable specifies character classifica- tion. 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 would otherwise be 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 be- havior. 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 ex- ample, the output of the preprocessor, which is the input to the compiler proper. 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 will attempt to figure out an appropriate prefix to use based on the pathname it was 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 pre- fix 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 re- placing that beginning with the specified prefix to produce an al- ternate directory name. Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next. COMPILER_PATH The value of COMPILER_PATH is a colon-separated list of directo- ries, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it can't 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 can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary li- braries 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 char- acter 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 will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters. Some additional environments variables affect the behavior of the pre- processor. CPATH C_INCLUDE_PATH CPLUS_INCLUDE_PATH OBJC_INCLUDE_PATH Each variable's value is a list of directories separated by a spe- cial 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 speci- fied with -I, but after any paths given with -I options on the com- mand 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 direc- tories 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 depen- dencies 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 combin- ing the options -MM and -MF, with an optional -MT switch too. SUNPRO_DEPENDENCIES This variable is the same as DEPENDENCIES_OUTPUT (see above), ex- cept that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted. BUGS For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>. FOOTNOTES 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. SEE ALSO gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, as, ld, binu- tils and gdb. AUTHOR See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Con- tributors.html>, for contributors to GCC. COPYRIGHT Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Founda- tion, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 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 in- cluded 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. gcc-4.2.1 2007-07-19 GCC(1)
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | BUGS | FOOTNOTES | SEE ALSO | AUTHOR | COPYRIGHT
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