GCC(1)
💖 GCC(1) 💖
Okay bestie, this is the GCC(1) page from the GNU manual, and it is giving everything for anyone who compiles C and C++ 🔥
✨ NAME ✨
gcc - GNU project C and C++ compiler 💅
✨ SYNOPSIS ✨
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...Okay so this is only the most useful options 🌸 The rest are waiting for you further down. And g++ accepts mostly the same options as gcc, so you're not learning a whole new vibe 🦋
✨ DESCRIPTION ✨
When you invoke GCC, it normally does preprocessing, compilation, assembly, and linking, all in one go 🧚 The "overall options" let you stop this process at an intermediate stage. For example, -c says not to run the linker, so the output is the object files the assembler spits out 💜
Other options get passed on to one or more stages of processing. Some control the preprocessor, some control the compiler itself, and some control the assembler and linker. Most of those last ones aren't documented here, because you rarely need to touch them, so no stress 🫶
Most of the command-line options 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 an option's description doesn't mention a source language, then you can use it with all supported languages ✨
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version if you want a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead 👑
The gcc program takes options and file names as operands. Lots of options have multi-letter names, so multiple single-letter options cannot be grouped together. -dv is very different from -d -v, so don't try to be clever 💀
You can mix options and other arguments. For the most part, the order doesn't matter. Order does matter when you use several options of the same kind: if you specify -L more than once, the directories are searched in the order you specified them. Also, where you place the -l option is significant 🙈
Lots of options have long names that start with -f or with -W, like -fmove-loop-invariants or -Wformat. Most of these have both positive and negative forms, and the negative form of -ffoo is -fno-foo. This manual only documents one of the two forms, whichever one is not the default 🌈
Some options take one or more arguments, typically separated from the option name either by a space or by the equals sign (=). Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix 🍓
Arguments to options that specify a size threshold of some sort can be arbitrarily large decimal or hexadecimal integers, followed by a byte size suffix that designates a multiple of bytes. So you've got kB and KiB for kilobyte and kibibyte, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. These arguments are called byte-size in the rest of the text. For the full listing and explanation of the binary and decimal byte size prefixes, check the NIST, IEC, and other relevant national and international standards 🤓
✨ OPTIONS ✨
✨ Option Summary ✨
Here's a summary of all the options, grouped by type. The explanations live in the following sections, so keep scrolling 📖
Overall Options
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -ffile-prefix-map=old=new -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=fileC Language Options
-ansi -std=standard -fgnu89-inline
-fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fgimple -fhosted -ffreestanding -fopenacc
-fopenacc-dim=geom -fopenmp -fopenmp-simd -fms-extensions
-fplan9-extensions -fsso-struct=endianness -fallow-single-precision
-fcond-mismatch -flax-vector-conversions -fsigned-bitfields
-fsigned-char -funsigned-bitfields -funsigned-charC++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fchar8_t -fcheck-new -fconstexpr-depth=n
-fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fno-elide-constructors
-fno-enforce-eh-specs -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules-ts
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive -fno-pretty-templates -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value -Wcatch-value=n
-Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++ -Wno-exceptions
-Wextra-semi -Wno-inaccessible-base -Wno-inherited-variadic-ctor
-Wno-init-list-lifetime -Winvalid-imported-macros
-Wno-invalid-offsetof -Wno-literal-suffix
-Wno-mismatched-new-delete -Wmismatched-tags -Wmultiple-inheritance
-Wnamespaces -Wnarrowing -Wnoexcept -Wnoexcept-type
-Wnon-virtual-dtor -Wpessimizing-move -Wno-placement-new
-Wplacement-new=n -Wrange-loop-construct -Wredundant-move
-Wredundant-tags -Wreorder -Wregister -Wstrict-null-sentinel
-Wno-subobject-linkage -Wtemplates -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsized-deallocation -Wsuggest-final-methods
-Wsuggest-final-types -Wsuggest-override -Wno-terminate
-Wuseless-cast -Wno-vexing-parse -Wvirtual-inheritance
-Wno-virtual-move-assign -Wvolatile
-Wzero-as-null-pointer-constantObjective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selectorDiagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin✨ Warning Options ✨
okay bestie, this list is absolutely unhinged in length 💀 but I promise I'm lining up every single flag so nothing gets dropped 🫶
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w
-Wextra -Wall -Wabi=n -Waddress -Wno-address-of-packed-member
-Waggregate-return -Walloc-size-larger-than=byte-size -Walloc-zero
-Walloca -Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds
-Warray-bounds=n -Wno-attributes -Wattribute-alias=n
-Wno-attribute-alias -Wno-attribute-warning -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c2x-compat -Wc++-compat -Wc++11-compat -Wc++14-compat
-Wc++17-compat -Wc++20-compat -Wcast-align -Wcast-align=strict
-Wcast-function-type -Wcast-qual -Wchar-subscripts -Wclobbered
-Wcomment -Wconversion -Wno-coverage-mismatch -Wno-cpp
-Wdangling-else -Wdate-time -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers
-Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wno-endif-labels -Wenum-compare -Wenum-conversion -Werror
-Werror=* -Wexpansion-to-defined -Wfatal-errors -Wfloat-conversion
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n
-Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Wimplicit -Wimplicit-fallthrough
-Wimplicit-fallthrough=n -Wno-implicit-function-declaration
-Wno-implicit-int -Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch -Wjump-misses-init
-Wlarger-than=byte-size -Wlogical-not-parentheses -Wlogical-op
-Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs
-Wmissing-noreturn -Wno-missing-profile -Wno-multichar
-Wmultistatement-macros -Wnonnull -Wnonnull-compare
-Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr
-Wopenmp-simd -Wno-overflow -Woverlength-strings
-Wno-override-init-side-effects -Wpacked
-Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas
-Wno-prio-ctor-dtor -Wredundant-decls -Wrestrict
-Wno-return-local-addr -Wreturn-type -Wno-scalar-storage-order
-Wsequence-point -Wshadow -Wshadow=global -Wshadow=local
-Wshadow=compatible-local -Wno-shadow-ivar -Wno-shift-count-negative
-Wno-shift-count-overflow -Wshift-negative-value
-Wno-shift-overflow -Wshift-overflow=n -Wsign-compare
-Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector
-Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wstring-compare
-Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation
-Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch
-Wno-switch-bool -Wswitch-default -Wswitch-enum
-Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs
-Wtsan -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas
-Wunsuffixed-float-constants -Wunused -Wunused-but-set-parameter
-Wunused-but-set-variable -Wunused-const-variable
-Wunused-const-variable=n -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros -Wunused-parameter
-Wno-unused-result -Wunused-value -Wunused-variable -Wno-varargs
-Wvariadic-macros -Wvector-operation-performance -Wvla
-Wvla-larger-than=byte-size -Wno-vla-larger-than
-Wvolatile-register-var -Wwrite-strings -Wzero-length-bounds✨ Static Analyzer Options ✨
the analyzer crew 🔍 is up next, and these flags are giving main character energy 👑
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-state-merge -fanalyzer-state-purge
-fanalyzer-transitivity -fanalyzer-verbose-edges
-fanalyzer-verbose-state-changes -fanalyzer-verbosity=level
-fdump-analyzer -fdump-analyzer-stderr -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3
-fdump-analyzer-feasibility -fdump-analyzer-json
-fdump-analyzer-state-purge -fdump-analyzer-supergraph
-Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file -Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap -Wno-analyzer-malloc-leak
-Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument
-Wno-analyzer-null-dereference -Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-array-index -Wanalyzer-too-complex
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal✨ C and Objective-C-only Warning Options ✨
these ones are only for C and Objective-C 🌸 so if you're not writing those, you can just vibe past them 🫶
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type
-Wmissing-prototypes -Wnested-externs -Wold-style-declaration
-Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement
-Wpointer-sign✨ Debugging Options ✨
ok and last but not least, the debugging squad 🐛 these are all the -g and -f flags that handle debug info, and they are a whole mood 🦋
-g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf
-gno-strict-dwarf -gas-loc-support -gno-as-loc-support
-gas-locview-support -gno-as-locview-support -gcolumn-info
-gno-column-info -gdwarf32 -gdwarf64 -gstatement-frontiers
-gno-statement-frontiers -gvariable-location-views
-gno-variable-location-views -ginternal-reset-location-views
-gno-internal-reset-location-views -ginline-points
-gno-inline-points -gvms -gxcoff -gxcoff+ -gz[=type]
-gsplit-dwarf -gdescribe-dies -gno-describe-dies
-fdebug-prefix-map=old=new -fdebug-types-section
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list]
-fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments✨ Optimization Options ✨
okay bestie, this is the BIG tuning list 💅 every single flag, zero cap
-faggressive-loop-optimizations— loops get a glow-up 🔥-falign-functions[=n[:m:[n2[:m2]]]]— function alignment, very precise 📏-falign-jumps[=n[:m:[n2[:m2]]]]— jump target alignment 🦋-falign-labels[=n[:m:[n2[:m2]]]]— label alignment 🏷️-falign-loops[=n[:m:[n2[:m2]]]]— loop alignment 🔁-fno-allocation-dce— the no-thanks version of allocation cleanup 🙅♀️-fallow-store-data-races— allowing store data races, ⚠️ spicy-fassociative-math— math reshuffling vibes 🧮-fauto-profile— auto profile, hands-off 🪄-fauto-profile[=path]— same, but you pick the path 📂-fauto-inc-dec— auto increment and decrement 🌸-fbranch-probabilities— branch odds, bestie 🎲-fcaller-saves— caller saves, very responsible 💜-fcombine-stack-adjustments— stack adjustments, combined 🧩-fconserve-stack— stack-saving mode 🌿-fcompare-elim— comparison elimination ✂️-fcprop-registers— register propagation 🎀-fcrossjumping— crossjumping, iconic 🦘-fcse-follow-jumps— CSE that follows the jumps 👣-fcse-skip-blocks— CSE that skips blocks ⏭️-fcx-fortran-rules— Fortran complex rules 📜-fcx-limited-range— limited range for complex math 🌈-fdata-sections— data gets its own sections 🗂️-fdce— dead code elimination, bye bye 💀-fdelayed-branch— delayed branches 🕰️-fdelete-null-pointer-checks— null pointer checks get deleted 🙈-fdevirtualize— devirtualization ✨-fdevirtualize-speculatively— speculative devirtualization 🔮-fdevirtualize-at-ltrans— devirtualize at LTRANS time 🕐-fdse— dead store elimination 🧹-fearly-inlining— inlining, early edition 🍓-fipa-sra— interprocedural SRA 🧠-fexpensive-optimizations— the pricey-but-worth-it ones 💎-ffat-lto-objects— fat LTO objects 🐘-ffast-math— fast math, ⚠️ not for the faint-hearted-ffinite-math-only— finite math only, no infinities 🧊-ffloat-store— float storage rules 💾-fexcess-precision=style— excess precision, your call 🎚️-ffinite-loops— loops that are finite 🔂-fforward-propagate— forward propagation ➡️-ffp-contract=style— floating-point contraction, pick a style 🪡-ffunction-sections— functions get their own sections 🧺-fgcse— global CSE 🌍-fgcse-after-reload— global CSE after reload 🔄-fgcse-las— GCSE load after store 🗃️-fgcse-lm— GCSE load motion 🚚-fgraphite-identity— Graphite identity transform 🪪-fgcse-sm— GCSE store motion 📦-fhoist-adjacent-loads— hoisting adjacent loads 🏗️-fif-conversion— if-conversion 🔀-fif-conversion2— if-conversion, round two 🔀✨-findirect-inlining— inlining the indirect calls 🎯-finline-functions— inline all the functions 💅-finline-functions-called-once— inline the once-only functions 1️⃣-finline-limit=n— set the inline limit ton🚧-finline-small-functions— inline the small ones 🐣-fipa-modref— mod/ref analysis across functions 🪞-fipa-cp— interprocedural constant propagation 💬-fipa-cp-clone— constant propagation with clones 👯-fipa-bit-cp— bit-level constant propagation 🔢-fipa-vrp— interprocedural value range propagation 📈-fipa-pta— interprocedural pointer analysis 🧭-fipa-profile— interprocedural profile use 📊-fipa-pure-const— pure and const function detection 🧘-fipa-reference— reference analysis 🔗-fipa-reference-addressable— reference analysis for addressable stuff 📬-fipa-stack-alignment— stack alignment, interprocedural 📐-fipa-icf— identical code folding 👭-fira-algorithm=algorithm— pick the IRA algorithm 🧑🔬-flive-patching=level— live patching level 🩹-fira-region=region— IRA region choice 🗺️-fira-hoist-pressure— hoist pressure, IRA style ⛰️-fira-loop-pressure— loop pressure, IRA style 🌀-fno-ira-share-save-slots— don't share save slots 🙅-fno-ira-share-spill-slots— don't share spill slots 🙅♀️-fisolate-erroneous-paths-dereference— isolate erroneous dereference paths 🚧-fisolate-erroneous-paths-attribute— isolate erroneous paths by attribute 🏷️-fivopts— induction variable optimizations 🔁✨-fkeep-inline-functions— keep the inline functions around 🧸-fkeep-static-functions— keep the static functions 🧸-fkeep-static-consts— keep the static consts 💎-flimit-function-alignment— limit function alignment 📏-flive-range-shrinkage— live range shrinkage 🤏-floop-block— loop blocking 🧱-floop-interchange— loop interchange 🔁-floop-strip-mine— loop strip mining 🪚-floop-unroll-and-jam— unroll and jam loops 🍯-floop-nest-optimize— loop nest optimization 🪆-floop-parallelize-all— parallelize all the loops 🧵-flra-remat— LRA rematerialization 🔁-flto— link-time optimization 🚀-flto-compression-level— LTO compression level 🗜️-flto-partition=alg— LTO partitioning algorithm 🍰-fmerge-all-constants— merge all the constants 🤝-fmerge-constants— merge constants 🤝-fmodulo-sched— modulo scheduling 🎶-fmodulo-sched-allow-regmoves— modulo scheduling with register moves 🚚-fmove-loop-invariants— move loop invariants out 🏃-fno-branch-count-reg— no branch count register 🙅-fno-defer-pop— don't defer pops 🙅♀️-fno-fp-int-builtin-inexact— no inexact floating-point to integer builtins 🙅-fno-function-cse— no CSE on function addresses 🙅-fno-guess-branch-probability— don't guess branch probabilities 🙅♀️-fno-inline— no inlining at all 🙅-fno-math-errno— no errno for math 🙅♀️-fno-peephole— no peephole optimizer 🙅-fno-peephole2— no second peephole optimizer 🙅♀️-fno-printf-return-value— no printf return value tracking 🙅-fno-sched-interblock— no interblock scheduling 🙅♀️-fno-sched-spec— no speculative scheduling 🙅-fno-signed-zeros— no signed zeros 🙅♀️-fno-toplevel-reorder— no top-level reordering 🙅-fno-trapping-math— no trapping math 🙅♀️-fno-zero-initialized-in-bss— no zero-initialized data in BSS 🙅-fomit-frame-pointer— omit the frame pointer 🫥-foptimize-sibling-calls— optimize sibling calls 👯♀️-fpartial-inlining— partial inlining 🧩-fpeel-loops— peel loops 🍊-fpredictive-commoning— predictive commoning 🔮-fprefetch-loop-arrays— prefetch loop arrays 🏃♀️-fprofile-correction— profile correction 🩺-fprofile-use— use profile data 📈-fprofile-use=path— use profile data from a path 📂-fprofile-partial-training— partial training profiles 🏋️-fprofile-values— profile values 🔢-fprofile-reorder-functions— reorder functions by profile 🪄-freciprocal-math— reciprocal math 🔁-free— FREE, the SSA-based redundancy elimination 🪶-frename-registers— rename registers 🏷️-freorder-blocks— reorder blocks 🧱-freorder-blocks-algorithm=algorithm— pick the block reordering algorithm 🧮-freorder-blocks-and-partition— reorder blocks and partition 🍕-freorder-functions— reorder functions 🛍️-frerun-cse-after-loop— rerun CSE after loops 🔄-freschedule-modulo-scheduled-loops— reschedule modulo-scheduled loops 🗓️-frounding-math— rounding math 🎯-fsave-optimization-record— save the optimization record 📝-fsched2-use-superblocks— second scheduler with superblocks 🦸-fsched-pressure— scheduling pressure 🫠-fsched-spec-load— speculative loads in scheduling 🔍-fsched-spec-load-dangerous— dangerous speculative loads ⚠️💀-fsched-stalled-insns-dep[=n]— stalled insn dependencies 🧊-fsched-stalled-insns[=n]— stalled insns 🧊-fsched-group-heuristic— group heuristic 👯-fsched-critical-path-heuristic— critical path heuristic 🛤️-fsched-spec-insn-heuristic— speculative insn heuristic 🔍-fsched-rank-heuristic— rank heuristic 🥇-fsched-last-insn-heuristic— last insn heuristic 🔚-fsched-dep-count-heuristic— dependency count heuristic 🔢-fschedule-fusion— schedule fusion 🫶-fschedule-insns— schedule instructions 🗓️-fschedule-insns2— schedule instructions, round two 🗓️✨-fsection-anchors— section anchors ⚓-fselective-scheduling— selective scheduling 🎯-fselective-scheduling2— selective scheduling, round two 🎯✨-fsel-sched-pipelining— selective scheduler pipelining 🚰-fsel-sched-pipelining-outer-loops— selective pipelining for outer loops 🪆-fsemantic-interposition— semantic interposition 🪞-fshrink-wrap— shrink-wrapping 🎁-fshrink-wrap-separate— separate shrink-wrapping 🎁✨-fsignaling-nans— signaling NaNs 📡-fsingle-precision-constant— single precision constants 🔬-fsplit-ivs-in-unroller— split induction variables in the unroller ✂️-fsplit-loops— split loops ✂️-fsplit-paths— split paths 🛣️-fsplit-wide-types— split wide types 🧩-fsplit-wide-types-early— split wide types early 🧩⏰-fssa-backprop— SSA backward propagation ⬅️-fssa-phiopt— SSA PHI optimization 🧩-fstdarg-opt— stdarg optimization 🎭-fstore-merging— store merging 🤝-fstrict-aliasing— strict aliasing 🧱-fthread-jumps— thread jumps 🧵-ftracer— tracer, leaves a trail 🐾-ftree-bit-ccp— tree bit constant propagation 🔢-ftree-builtin-call-dce— dead code elimination for builtin calls 💀-ftree-ccp— tree constant propagation 💬-ftree-ch— tree loop header copying 📋-ftree-coalesce-vars— coalesce variables 🤝-ftree-copy-prop— tree copy propagation 📄-ftree-dce— tree dead code elimination 💀-ftree-dominator-opts— dominator optimizations 👑-ftree-dse— tree dead store elimination 🧹-ftree-forwprop— tree forward propagation ➡️-ftree-fre— tree full redundancy elimination ♻️-fcode-hoisting— code hoisting 🏗️-ftree-loop-if-convert— loop if-conversion 🔀-ftree-loop-im— loop invariant motion 🏃-ftree-phiprop— tree PHI propagation 🧩-ftree-loop-distribution— loop distribution 🍱-ftree-loop-distribute-patterns— distribute loop patterns 🍱✨-ftree-loop-ivcanon— canonicalize loop induction variables 🔁-ftree-loop-linear— linear loop transformations 📏-ftree-loop-optimize— tree loop optimizations 🔁-ftree-loop-vectorize— loop vectorization 🚄-ftree-parallelize-loops=n— parallelize loops acrossnthreads 🧵-ftree-pre— tree partial redundancy elimination 🔍-ftree-partial-pre— tree partial PRE 🔍✨-ftree-pta— tree pointer analysis 🧭-ftree-reassoc— tree reassociation 🪢-ftree-scev-cprop— SCEV constant propagation 🔢-ftree-sink— tree sinking ⬇️-ftree-slsr— straight-line strength reduction 💪-ftree-sra— tree scalar replacement of aggregates 🧩-ftree-switch-conversion— switch conversion 🔀-ftree-tail-merge— tail merging 🪡-ftree-ter— temporary expression replacement 🔁-ftree-vectorize— tree vectorization 🚄-ftree-vrp— tree value range propagation 📈-funconstrained-commons— unconstrained commons 🌐-funit-at-a-time— unit at a time 🕰️-funroll-all-loops— unroll all the loops 🔁🔁-funroll-loops— unroll loops 🔁-funsafe-math-optimizations— unsafe math optimizations, ⚠️ spicy-funswitch-loops— loop unswitching 🔀-fipa-ra— interprocedural register allocation 🧠-fvariable-expansion-in-unroller— variable expansion in the unroller 🌱-fvect-cost-model— vectorizer cost model 💰-fvpt— value profile transformations 📊-fweb— webs for registers 🕸️-fwhole-program— whole program mode 🌍-fwpa— whole program analysis 🔭-fuse-linker-plugin— use the linker plugin 🔌-fzero-call-used-regs— zero call-used registers 🧹--param name=value— set a tuning parameter 🎛️-O— the basic optimization knob ⚙️-O0— no optimization, zero glow-up 😴-O1— a little optimization 🌱-O2— more optimization 🌷-O3— the maximum-effort level 🔥-Os— optimize for size 🤏-Ofast— fast, ⚠️ bends the rules-Og— optimize for debugging 🐞
✨ Program Instrumentation Options ✨
this is the section where GCC gets nosy and starts taking notes 📝👀
-p— profiling 📸-pg— profiling with gprof 📊-fprofile-arcs— arc profiling 🏹--coverage— coverage 🧁-ftest-coverage— test coverage 🧪-fprofile-abs-path— absolute profile paths 🗺️-fprofile-dir=path— put profile data inpath📂-fprofile-generate— generate profile data 🪄-fprofile-generate=path— generate profile data intopath📂-fprofile-info-section— profile info section 🗃️-fprofile-info-section=name— profile info section with a name 🏷️-fprofile-note=path— profile notes atpath📒-fprofile-prefix-path=path— profile prefix path 🛤️-fprofile-update=method— profile update method 🔄-fprofile-filter-files=regex— include only files matchingregex🔍-fprofile-exclude-files=regex— exclude files matchingregex🙅-fprofile-reproducible=[multithreaded|parallel-runs|serial]— reproducible profiles, pick a mode 🔁-fsanitize=style— sanitizer, pick a style 🧼-fsanitize-recover— sanitizer recovery 🩹-fsanitize-recover=style— sanitizer recovery, pick a style 🩹✨-fasan-shadow-offset=number— ASan shadow offset 🌑-fsanitize-sections=s1,s2,...— sanitize these sections 🧺-fsanitize-undefined-trap-on-error— trap on undefined behaviour errors 🪤-fbounds-check— bounds checking 🚧-fcf-protection=[full|branch|return|none|check]— control-flow protection, pick a level 🛡️-fstack-protector— stack protector 🛡️-fstack-protector-all— stack protector for everything 🛡️✨-fstack-protector-strong— stack protector, strong edition 💪-fstack-protector-explicit— stack protector, explicit only 🎯-fstack-check— stack checking 🔍-fstack-limit-register=reg— stack limit register 📏-fstack-limit-symbol=sym— stack limit symbol 🏷️-fno-stack-limit— no stack limit 🙅-fsplit-stack— split stack 🧵-fvtable-verify=[std|preinit|none]— vtable verification, pick a mode 🪪-fvtv-counts— vtable verification counts 🔢-fvtv-debug— vtable verification debugging 🐛-finstrument-functions— instrument functions 🔬-finstrument-functions-exclude-function-list=sym,sym,...— skip these functions when instrumenting 🙈-finstrument-functions-exclude-file-list=file,file,...— skip these files when instrumenting 🙈
✨ Preprocessor Options ✨
the preprocessor is basically the prep team before the main show 🎀
-Aquestion=answer— assert something 🧾-A-question[=answer]— un-assert something 🧾-C— keep comments 💬-CC— keep comments, even the sneaky ones 💬✨-Dmacro[=defn]— define a macro 🧩-dD— dump defines 📋-dI— dump includes 📋-dM— dump macros 📋-dN— dump names 📋-dU— dump undefined macros 📋-fdebug-cpp— preprocessor debugging 🐛-fdirectives-only— directives only 🎯-fdollars-in-identifiers— dollar signs in identifiers 💲-fexec-charset=charset— execution character set 🔤-fextended-identifiers— extended identifiers 🆔-finput-charset=charset— input character set 🔤-flarge-source-files— large source files 🐘-fmacro-prefix-map=old=new— map macro prefixes fromoldtonew🗺️-fmax-include-depth=depth— max include depth 🪆-fno-canonical-system-headers— don't canonicalize system headers 🙅-fpch-deps— precompiled header dependencies 🧱-fpch-preprocess— precompiled header preprocessing 🧱✨-fpreprocessed— already preprocessed 🥡-ftabstop=width— tab stop width 📏-ftrack-macro-expansion— track macro expansion 🕵️-fwide-exec-charset=charset— wide execution character set 🔤✨-fworking-directory— working directory 📂-H— show the header chain 🔗-imacros file— include macros fromfile📥-include file— includefilefirst 📥-M— output make dependencies 🛠️-MD— make dependencies, written to a file too 🛠️📁-MF— name the dependency file 🏷️-MG— treat missing headers as generated 🧾-MM— make dependencies, skipping system headers 🛠️-MMD— like-MD, skipping system headers 🛠️📁-MP— phony targets for headers 👻-MQ— quoted make target 💅-MT— make target name 🎯-Mno-modules— no modules 🙅-no-integrated-cpp— no integrated preprocessor 🙅♀️-P— no line markers 🧹-pthread— POSIX threads 🧵-remap— remap filenames 🗺️-traditional— traditional preprocessing, very vintage 📼-traditional-cpp— traditional C preprocessor 📼✨-trigraphs— trigraphs 🔺-Umacro— undefine a macro 🗑️-undef— undefine all the things 💥-Wp,option— passoptionto the preprocessor 🎁-Xpreprocessor option— passoptionto the preprocessor 🎁✨
✨ Assembler Options ✨
short and sweet, just two of 'em 🍑
-Wa,option— passoptionto the assembler 🧵-Xassembler option— passoptionto the assembler 🧵✨
✨ Linker Options ✨
the linker is where everything comes together and clicks 🔗💖
object-file-name— an object file to link 📦-fuse-ld=linker— pick the linker 🔌-llibrary— link againstlibrary📚-nostartfiles— no startup files 🚫-nodefaultlibs— no default libraries 🚫📚-nolibc— no C library 🚫-nostdlib— no standard libraries or startup files at all 🚫✨-e entry— set the entry point toentry🚪--entry=entry— same entry point, long form 🚪✨-pie— position-independent executable 🫧-pthread— POSIX threads 🧵-r— relocatable output 🔁-rdynamic— export dynamic symbols 📤-s— strip the output 🪒-static— static linking 🧱-static-pie— static position-independent executable 🫧🧱-static-libgcc— static libgcc 🧱-static-libstdc++— static libstdc++ 🧱-static-libasan— static libasan 🧱-static-libtsan— static libtsan 🧱-static-liblsan— static liblsan 🧱-static-libubsan— static libubsan 🧱-shared— make a shared library 🤝-shared-libgcc— shared libgcc 🤝-symbolic— bind symbols locally 🪞-T script— use linkerscript📜-Wl,option— passoptionto the linker 🎁-Xlinker option— passoptionto the linker 🎁✨-u symbol— treatsymbolas undefined 👻-z keyword— pass thekeywordoption to the linker 🗝️
✨ Directory Options ✨
where should GCC look? bestie gives it directions 🗺️✨
-Bprefix— search prefix 🧭-Idir— adddirto the include search path 📂-I-— the old-I-behaviour 🙃-idirafter dir— searchdirafter the others 🕰️-imacros file— macros fromfile📥-imultilib dir— multilib directory 📚-iplugindir=dir— plugin directory 🔌-iprefix file— prefix for include directories 🧾-iquote dir— quote-only include directory 💬-isysroot dir— system rootdir🏠-isystem dir— system include directorydir🏛️-iwithprefix dir— includedirunder the prefix 🧾-iwithprefixbefore dir— includedirunder the prefix, searched earlier 🧾✨-Ldir— library search directorydir📚-no-canonical-prefixes— no canonical prefixes 🙅--no-sysroot-suffix— no sysroot suffix 🙅♀️-nostdinc— no standard include directories 🚫-nostdinc++— no standard C++ include directories 🚫✨--sysroot=dir— set the sysroot todir🏠
✨ Code Generation Options ✨
this is where the machine code gets its whole personality 🤖💜
-fcall-saved-reg— call-saved register 🫶-fcall-used-reg— call-used register 🫶✨-ffixed-reg— fixed register 📌-fexceptions— exceptions 🧨-fnon-call-exceptions— non-call exceptions 🧨✨-fdelete-dead-exceptions— delete dead exceptions 💀-funwind-tables— unwind tables 🪜-fasynchronous-unwind-tables— asynchronous unwind tables 🪜⏰-fno-gnu-unique— no GNU unique symbols 🙅-finhibit-size-directive— inhibit size directive 🤐-fcommon— common symbols 🤝-fno-ident— no ident 🙅-fpcc-struct-return— PCC struct return 🧾-fpic— position-independent code 🫧-fPIC— position-independent code, big edition 🫧✨-fpie— position-independent executable code 🫧-fPIE— position-independent executable, big edition 🫧✨-fno-plt— no PLT 🙅-fno-jump-tables— no jump tables 🙅-fno-bit-tests— no bit tests 🙅♀️-frecord-gcc-switches— record GCC switches 📝-freg-struct-return— register struct return 🧾-fshort-enums— short enums 🪶-fshort-wchar— shortwchar_t🪶-fverbose-asm— verbose assembly 🗣️-fpack-struct[=n]— pack structs 📦-fleading-underscore— leading underscore 🫢-ftls-model=model— thread-local storage model 🧵-fstack-reuse=reuse_level— stack reuse level ♻️-ftrampolines— trampolines 🤸-ftrapv— trap on signed overflow 🪤-fwrapv— wrap on signed overflow 🔁-fvisibility=[default|internal|hidden|protected]— symbol visibility, pick one 👀-fstrict-volatile-bitfields— strict volatile bitfields 🧱-fsync-libcalls— sync libcalls 🔄
✨ Developer Options ✨
the developer zone, bestie. this is where the debugging magic lives 🪄💜
-dletters— dump letters 🔤-dumpspecs— dump the specs 📋-dumpmachine— dump the machine name 🖥️-dumpversion— dump the version 🏷️-dumpfullversion— dump the full version 🏷️✨-fcallgraph-info[=su,da]— call graph info 🕸️-fchecking— checking 🔍-fchecking=n— checking at leveln🔍✨-fdbg-cnt-list— debug counter list 🔢-fdbg-cnt=counter-value-list— debug counter values 🔢✨-fdisable-ipa-pass_name— disable an IPA pass 🚫-fdisable-rtl-pass_name— disable an RTL pass 🚫-fdisable-rtl-pass-name=range-list— disable RTL passes in a range list 🚫✨-fdisable-tree-pass_name— disable a tree pass 🚫-fdisable-tree-pass-name=range-list— disable tree passes in a range list 🚫✨-fdump-debug— dump debug info 🐛-fdump-earlydebug— dump early debug info 🐛⏰-fdump-noaddr— dump without addresses 🙈-fdump-unnumbered— dump unnumbered 🔢-fdump-unnumbered-links— dump unnumbered links 🔗-fdump-final-insns[=file]— dump the final insns 🧾-fdump-ipa-all— dump all IPA info 🗂️-fdump-ipa-cgraph— dump the IPA call graph 🕸️-fdump-ipa-inline— dump IPA inlining 🗂️-fdump-lang-all— dump all language info 🗂️-fdump-lang-switch— dump language switch 🔀-fdump-lang-switch-options— dump language switch options 🔀✨-fdump-lang-switch-options=filename— dump language switch options tofilename🔀📁-fdump-passes— dump passes 🗂️-fdump-rtl-pass— dump an RTL pass 🗂️-fdump-rtl-pass=filename— dump an RTL pass tofilename🗂️📁-fdump-statistics— dump statistics 📊-fdump-tree-all— dump all tree info 🌳-fdump-tree-switch— dump tree switch 🌳🔀-fdump-tree-switch-options— dump tree switch options 🌳🔀✨-fdump-tree-switch-options=filename— dump tree switch options tofilename🌳📁-fcompare-debug[=opts]— compare debug output 🔍-fcompare-debug-second— compare debug output, second run 🔍✨-fenable-kind-pass— enable a kind of pass ✅-fenable-kind-pass=range-list— enable passes in a range list ✅✨-fira-verbose=n— IRA verbosity at leveln🗣️-flto-report— LTO report 📝-flto-report-wpa— LTO WPA report 📝🔭-fmem-report-wpa— memory report for WPA 🧠🔭-fmem-report— memory report 🧠-fpre-ipa-mem-report— memory report before IPA 🧠⏪-fpost-ipa-mem-report— memory report after IPA 🧠⏩-fopt-info— optimization info 💡-fopt-info-options[=file]— optimization info options 💡✨-fprofile-report— profile report 📈-frandom-seed=string— random seed 🎲-fsched-verbose=n— scheduler verbosity at leveln🗣️-fsel-sched-verbose— selective scheduler verbose 🗣️-fsel-sched-dump-cfg— selective scheduler CFG dump 🗂️-fsel-sched-pipelining-verbose— selective pipelining verbose 🗣️-fstats— stats 📊-fstack-usage— stack usage 📏-ftime-report— time report ⏱️-ftime-report-details— time report details ⏱️✨-fvar-tracking-assignments-toggle— toggle variable tracking assignments 🔀-gtoggle— toggle debug info 🔀-print-file-name=library— print the path oflibrary🖨️-print-libgcc-file-name— print the libgcc file name 🖨️-print-multi-directory— print the multilib directory 🖨️-print-multi-lib— print multilib info 🖨️-print-multi-os-directory— print the multilib OS directory 🖨️-print-prog-name=program— print the path ofprogram🖨️-print-search-dirs— print search directories 🖨️-Q— print what's happening as it happens 🗣️-print-sysroot— print the sysroot 🖨️-print-sysroot-headers-suffix— print the sysroot headers suffix 🖨️-save-temps— save temp files 💾-save-temps=cwd— save temp files in the current directory 💾📂-save-temps=obj— save temp files in the object directory 💾📦-time[=file]— time it ⏱️
✨ Machine-Dependent Options ✨
the hardware-specific section, where each chip gets its own main character moment 🌈👑
AArch64 Options
-mabi=name— ABI name 🧾-mbig-endian— big-endian 🧱-mlittle-endian— little-endian 🧱-mgeneral-regs-only— general registers only 🎯-mcmodel=tiny— tiny code model 🐜-mcmodel=small— small code model 🐭-mcmodel=large— large code model 🐘-mstrict-align— strict alignment 📏-mno-strict-align— no strict alignment 🙅-momit-leaf-frame-pointer— omit leaf frame pointer 🫥-mtls-dialect=desc— TLS dialect, descriptor style 🧵-mtls-dialect=traditional— TLS dialect, traditional style 🧵📼-mtls-size=size— TLS size 📏-mfix-cortex-a53-835769— fix Cortex-A53 erratum 835769 🩹-mfix-cortex-a53-843419— fix Cortex-A53 erratum 843419 🩹-mlow-precision-recip-sqrt— low-precision reciprocal square root 🧮-mlow-precision-sqrt— low-precision square root 🧮-mlow-precision-div— low-precision division 🧮-mpc-relative-literal-loads— PC-relative literal loads 🔁-msign-return-address=scope— sign return address, pick a scope ✍️-mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti— branch protection, pick one 🛡️-mharden-sls=opts— harden SLS with these options 🛡️-march=name— architecture name 🏗️-mcpu=name— CPU name 🧠-mtune=name— tune forname🎛️-moverride=string— overridestring🔧-mverbose-cost-dump— verbose cost dump 🗣️-mstack-protector-guard=guard— stack protector guard 🛡️-mstack-protector-guard-reg=sysreg— stack protector guard register 🛡️📌-mstack-protector-guard-offset=offset— stack protector guard offset 🛡️📏-mtrack-speculation— track speculation 🔮-moutline-atomics— outline atomics ⚛️
Okay bestie, this is the target-specific lineup 🦋 every machine gets its own squad of flags, and this part is just the names, no descriptions yet 💅
🦋 Adapteva Epiphany Options 🦋
-mhalf-reg-file-mprefer-short-insn-regs-mbranch-cost=num-mcmove-mnops=num-msoft-cmpsf-msplit-lohi-mpost-inc-mpost-modify-mstack-offset=num-mround-nearest-mlong-calls-mshort-calls-msmall16-mfp-mode=mode-mvect-double-max-vect-align=num-msplit-vecmove-early-m1reg-reg
🌸 AMD GCN Options 🌸
-march=gpu-mtune=gpu-mstack-size=bytes
🧚 ARC Options 🧚
-mbarrel-shifter-mjli-always-mcpu=cpu-mA6-mARC600-mA7-mARC700-mdpfp-mdpfp-compact-mdpfp-fast-mno-dpfp-lrsr-mea-mno-mpy-mmul32x16-mmul64-matomic-mnorm-mspfp-mspfp-compact-mspfp-fast-msimd-msoft-float-mswap-mcrc-mdsp-packa-mdvbf-mlock-mmac-d16-mmac-24-mrtsc-mswape-mtelephony-mxy-misize-mannotate-align-marclinux-marclinux_prof-mlong-calls-mmedium-calls-msdata-mirq-ctrl-saved-mrgf-banked-regs-mlpc-width=width-G num-mvolatile-cache-mtp-regno=regno-malign-call-mauto-modify-reg-mbbit-peephole-mno-brcc-mcase-vector-pcrel-mcompact-casesi-mno-cond-exec-mearly-cbranchsi-mexpand-adddi-mindexed-loads-mlra-mlra-priority-none-mlra-priority-compactmlra-priority-noncompact-mmillicode-mmixed-code-mq-class-mRcq-mRcw-msize-level=level-mtune=cpu-mmultcost=num-mcode-density-frame-munalign-prob-threshold=probability-mmpy-option=multo-mdiv-rem-mcode-density-mll64-mfpu=fpu-mrf16-mbranch-index
💖 ARM Options 💖
-mapcs-frame-mno-apcs-frame-mabi=name-mapcs-stack-check-mno-apcs-stack-check-mapcs-reentrant-mno-apcs-reentrant-mgeneral-regs-only-msched-prolog-mno-sched-prolog-mlittle-endian-mbig-endian-mbe8-mbe32-mfloat-abi=name-mfp16-format=name-mthumb-interwork-mno-thumb-interwork-mcpu=name-march=name-mfpu=name-mtune=name-mprint-tune-info-mstructure-size-boundary=n-mabort-on-noreturn-mlong-calls-mno-long-calls-msingle-pic-base-mno-single-pic-base-mpic-register=reg-mnop-fun-dllimport-mpoke-function-name-mthumb-marm-mflip-thumb-mtpcs-frame-mtpcs-leaf-frame-mcaller-super-interworking-mcallee-super-interworking-mtp=name-mtls-dialect=dialect-mword-relocations-mfix-cortex-m3-ldrd-munaligned-access-mneon-for-64bits-mslow-flash-data-masm-syntax-unified-mrestrict-it-mverbose-cost-dump-mpure-code-mcmse-mfix-cmse-cve-2021-35465-mfdpic
🍓 AVR Options 🍓
-mmcu=mcu-mabsdata-maccumulate-args-mbranch-cost=cost-mcall-prologues-mgas-isr-prologues-mint8-mdouble=bits-mlong-double=bits-mn_flash=size-mno-interrupts-mmain-is-OS_task-mrelax-mrmw-mstrict-X-mtiny-stack-mfract-convert-truncate-mshort-calls-nodevicelib-nodevicespecs-Waddr-space-convert-Wmisspelled-isr
🌈 Blackfin Options 🌈
-mcpu=cpu[-sirevision]-msim-momit-leaf-frame-pointer-mno-omit-leaf-frame-pointer-mspecld-anomaly-mno-specld-anomaly-mcsync-anomaly-mno-csync-anomaly-mlow-64k-mno-low64k-mstack-check-l1-mid-shared-library-mno-id-shared-library-mshared-library-id=n-mleaf-id-shared-library-mno-leaf-id-shared-library-msep-data-mno-sep-data-mlong-calls-mno-long-calls-mfast-fp-minline-plt-mmulticore-mcorea-mcoreb-msdram-micplb
⭐ C6X Options ⭐
-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-type
🫶 CRIS Options 🫶
-mcpu=cpu-march=cpu-mtune=cpu-mmax-stack-frame=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-melf-maout-sim-sim2-mmul-bug-workaround-mno-mul-bug-workaround
🎀 CR16 Options 🎀
-mmac-mcr16cplus-mcr16c-msim-mint32-mbit-ops-mdata-model=model
🔥 C-SKY Options 🔥
-march=arch-mcpu=cpu-mbig-endian-EB-mlittle-endian-EL-mhard-float-msoft-float-mfpu=fpu-mdouble-float-mfdivdu-mfloat-abi=name-melrw-mistack-mmp-mcp-mcache-msecurity-mtrust-mdsp-medsp-mvdsp-mdiv-msmart-mhigh-registers-manchor-mpushpop-mmultiple-stld-mconstpool-mstack-size-mccrt-mbranch-cost=n-mcse-cc-msched-prolog-msim
👑 Darwin Options 👑
-all_load-allowable_client-arch-arch_errors_fatal-arch_only-bind_at_load-bundle-bundle_loader-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-filelist-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-headerpad_max_install_names-iframework-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded-F-gused-gfull-mmacosx-version-min=version-mkernel-mone-byte-bool
🦋 DEC Alpha Options 🦋
-mno-fp-regs-msoft-float-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time
🧚 eBPF Options 🧚
-mbig-endian-mlittle-endian-mkernel=version-mframe-limit=bytes-mxbpf
💜 FR30 Options 💜
-msmall-model-mno-lsim
🌸 FT32 Options 🌸
-msim-mlra-mnodiv-mft32b-mcompress-mnopm
💅 FRV Options 💅
-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-cc-mdword-mno-dword-mdouble-mno-double-mmedia-mno-media-mmuladd-mno-muladd-mfdpic-minline-plt-mgprel-ro-multilib-library-pic-mlinked-fp-mlong-calls-malign-labels-mlibrary-pic-macc-4-macc-8-mpack-mno-pack-mno-eflags-mcond-move-mno-cond-move-moptimize-membar-mno-optimize-membar-mscc-mno-scc-mcond-exec-mno-cond-exec-mvliw-branch-mno-vliw-branch-mmulti-cond-exec-mno-multi-cond-exec-mnested-cond-exec-mno-nested-cond-exec-mtomcat-stats-mTLS-mtls-mcpu=cpu
🎀 GNU/Linux Options 🎀
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
✨ H8/300 Options ✨
-mrelax-mh-ms-mn-mexr-mno-exr-mint32-malign-300
🍓 HPPA Options 🍓
-march=architecture-type-mcaller-copies-mdisable-fpregs-mdisable-indexing-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mjump-in-delay-mlinker-opt-mlong-calls-mlong-load-store-mno-disable-fpregs-mno-disable-indexing-mno-fast-indirect-calls-mno-gas-mno-jump-in-delay-mno-long-load-store-mno-portable-runtime-mno-soft-float-mno-space-regs-msoft-float-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0-mportable-runtime-mschedule=cpu-type-mspace-regs-msio-mwsio-munix=unix-std-nolibdld-static-threads
🌈 IA-64 Options 🌈
-mbig-endian-mlittle-endian-mgnu-as-mgnu-ld-mno-pic-mvolatile-asm-stop-mregister-names-msdata-mno-sdata-mconstant-gp-mauto-pic-mfused-madd-minline-float-divide-min-latency-minline-float-divide-max-throughput-mno-inline-float-divide-minline-int-divide-min-latency-minline-int-divide-max-throughput-mno-inline-int-divide-minline-sqrt-min-latency-minline-sqrt-max-throughput-mno-inline-sqrt-mdwarf2-asm-mearly-stop-bits-mfixed-range=register-range-mtls-size=tls-size-mtune=cpu-type-milp32-mlp64-msched-br-data-spec-msched-ar-data-spec-msched-control-spec-msched-br-in-data-spec-msched-ar-in-data-spec-msched-in-control-spec-msched-spec-ldc-msched-spec-control-ldc-msched-prefer-non-data-spec-insns-msched-prefer-non-control-spec-insns-msched-stop-bits-after-every-cycle-msched-count-spec-in-critical-path-msel-sched-dont-check-control-spec-msched-fp-mem-deps-zero-cost-msched-max-memory-insns-hard-limit-msched-max-memory-insns=max-insns
🦋 LM32 Options 🦋
-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabled
💖 M32R/D Options 💖
-m32r2-m32rx-m32r-mdebug-malign-loops-mno-align-loops-missue-rate=number-mbranch-cost=number-mmodel=code-size-model-type-msdata=sdata-type-mno-flush-func-mflush-func=name-mno-flush-trap-mflush-trap=number-G num
🧚 M32C Options 🧚
-mcpu=cpu-msim-memregs=number
🌸 M680x0 Options 🌸
-march=arch-mcpu=cpu-mtune=tune-m68000-m68020-m68020-40-m68020-60-m68030-m68040-m68060-mcpu32-m5200-m5206e-m528x-m5307-m5407-mcfv4e-mbitfield-mno-bitfield-mc68000-mc68020-mnobitfield-mrtd-mno-rtd-mdiv-mno-div-mshort-mno-short-mhard-float-m68881-msoft-float-mpcrel-malign-int-mstrict-align-msep-data-mno-sep-data-mshared-library-id=n-mid-shared-library-mno-id-shared-library-mxgot-mno-xgot-mlong-jump-table-offsets
💅 MCore Options 💅
-mhardlit-mno-hardlit-mdiv-mno-div-mrelax-immediates-mno-relax-immediates-mwide-bitfields-mno-wide-bitfields-m4byte-functions-mno-4byte-functions-mcallgraph-data-mno-callgraph-data-mslow-bytes-mno-slow-bytes-mno-lsim-mlittle-endian-mbig-endian-m210-m340-mstack-increment
🫶 MeP Options 🫶
-mabsdiff-mall-opts-maverage-mbased=n-mbitops-mc=n-mclip-mconfig=name-mcop-mcop32-mcop64-mivc2-mdc-mdiv-meb-mel-mio-volatile-ml-mleadz-mm-mminmax-mmult-mno-opts-mrepeat-ms-msatur-msdram-msim-msimnovec-mtf-mtiny=n
🌈 MicroBlaze Options 🌈
-msoft-float-mhard-float-msmall-divides-mcpu=cpu-mmemcpy-mxl-soft-mul-mxl-soft-div-mxl-barrel-shift-mxl-pattern-compare-mxl-stack-check-mxl-gp-opt-mno-clearbss-mxl-multiply-high-mxl-float-convert-mxl-float-sqrt-mbig-endian-mlittle-endian-mxl-reorder-mxl-mode-app-model-mpic-data-is-text-relative
🦋 MIPS Options
okay so MIPS has SO many flags 🙈 bestie, this is a whole vibe. here's the full list, no cuts 💅
-EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6
-mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16
-mno-mips16 -mflip-mips16 -minterlink-compressed
-mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16
-mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt
-mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64
-mhard-float -msoft-float -mno-float -msingle-float
-mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode
-mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu
-meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mcrc -mno-crc
-mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa
-mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext
-mloongson-ext2 -mno-loongson-ext2 -mfpu=fpu-type -msmartmips
-mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx
-mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division -mno-check-zero-division
-mdivide-traps -mdivide-breaks -mload-store-pairs
-mno-load-store-pairs -mmemcpy -mno-memcpy -mlong-calls
-mno-long-calls -mmad -mno-mad -mimadd -mno-imadd -mfused-madd
-mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000
-mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r5900
-mno-fix-r5900 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130
-mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-cost=num -mbranch-likely
-mno-branch-likely -mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci
-mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
-mframe-header-opt -mno-frame-header-opt🦋 MMIX Options
the MMIX crew is smaller but still iconic 💜
-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit🌸 MN10300 Options
-mmult-bug -mno-mult-bug -mno-am33 -mam33
-mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0
-mrelax -mliw -msetlb🧚 Moxie Options
tiny but mighty ✨
-meb -mel -mmul.x -mno-crt0🎀 MSP430 Options
-msim -masm-hex -mmcu= -mcpu= -mlarge -msmall
-mrelax -mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata=
-msilicon-errata-warn= -mhwmult= -minrt -mtiny-printf
-mmax-inline-shift=💖 NDS32 Options
-mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf -mext-perf2
-mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push
-m16bit -mno-16bit -misr-vector-size=num -mcache-block-size=num
-march=arch -mcmodel=code-model -mctor-dtor -mrelax⭐ Nios II Options
-G num -mgpopt=option -mgpopt -mno-gpopt
-mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb -mno-bypass-cache
-mbypass-cache -mno-cache-volatile -mcache-volatile
-mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx
-mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N
-mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc
-msys-crt0=name -msys-lib=name -march=arch -mbmx -mno-bmx -mcdx
-mno-cdx🔥 Nvidia PTX Options
-m64 -mmainkernel -moptimize🌈 OpenRISC Options
-mboard=name -mnewlib -mhard-mul -mhard-div
-msoft-mul -msoft-div -msoft-float -mhard-float -mdouble-float
-munordered-float -mcmov -mror -mrori -msext -msfimm -mshftimm🍓 PDP-11 Options
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit -munix-asm
-mdec-asm -mgnu-asm -mlra🫶 picoChip Options
-mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings👑 PowerPC Options
no separate list here, bestie. the PowerPC flags live under the RS/6000 and PowerPC Options section, so see RS/6000 and PowerPC Options 💅
🦋 PRU Options
-mmcu=mcu -minrt -mno-relax -mloop -mabi=variant🌸 RISC-V Options
-mbranch-cost=N-instruction -mplt -mno-plt
-mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-string
-mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mshorten-memrefs -mno-shorten-memrefs -mstrict-align
-mno-strict-align -mcmodel=medlow -mcmodel=medany -mexplicit-relocs
-mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute
-mmo-riscv-attribute -malign-data=type -mbig-endian -mlittle-endian
+-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
+-mstack-protector-guard-offset=offset💜 RL78 Options
-msim -mmul=none -mmul=g13 -mmul=g14 -mallregs
-mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles
-m32bit-doubles -msave-mduc-in-interrupts🔥 RS/6000 and PowerPC Options
this one is LONG, like full main character marathon 😭 every single flag is here:
-mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb
-mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd
-mno-fused-madd -mbit-align -mno-bit-align -mstrict-align
-mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mhtm
-mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic
-mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm
-mfloat128 -mno-float128 -mfloat128-hardware
-mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mno-prefixed
-mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect
-mprivileged -mno-privileged✨ RX Options
-m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=
-mbig-endian-data -mlittle-endian-data -msmall-data -msim -mno-sim
-mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size=
-mint-register= -mpid -mallow-string-insns -mno-allow-string-insns
-mjsr -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts🦋 S/390 and zSeries Options
-mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec
-mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mtpf-trace-skip
-mno-tpf-trace-skip -mfused-madd -mno-fused-madd -mwarn-framesize
-mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords🌈 Score Options
-meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d💅 SH Options
-m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single
-m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4
-m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml
-mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas🎀 Solaris 2 Options
-mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads🫶 SPARC Options
-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu
-mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return
-mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles
-muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b
-mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld
-mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f
-mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra🧚 System V Options
-Qy -Qn -YP,paths -Ym,dir💖 TILE-Gx Options
-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-model🌸 TILEPro Options
-mcpu=cpu -m32✨ V850 Options ✨
okay bestie, the V850 crew has its own little flag squad 🦋 here's every single one, copied exactly:
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float
-mhard-float -mgcc-abi -mrh850-abi -mbig-switch🌸 VAX Options 🌸
the VAX ones are small but mighty 💅
-mg -mgnu -munix🧚 Visium Options 🧚
Visium has a whole set of its own, no cap 🔥
-mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode🎀 VMS Options 🎀
for VMS, these are the ones you want 💜
-mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size🦋 VxWorks Options 🦋
VxWorks is giving main character energy 👑
-mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-now💖 x86 Options 💖
okay this is the big one, the x86 list is literally massive 😭 it covers tuning, architecture, floating point, SSE and AVX families, calling conventions, and so much more. bestie, grab a snack 🍓
-mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list -mdump-tune-features -mno-default -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -m80387 -mhard-float
-msoft-float -mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld
-mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt
-mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmmx
-msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx
-mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl
-mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes
-mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd
-mptwrite -mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves
-msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4
-mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm
-mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes
-mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call
-mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg
-mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps
-mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mcldemote -mms-bitfields
-mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg -mkl
-mwidekl -mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mdaz-ftz -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64
-mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx -mfentry
-mrecord-mcount -mnop-mcount -m8bit-idiv -minstrument-return=type
-mfentry-name=name -mfentry-section=name
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data=type -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
-mharden-sls=choice -mindirect-branch-cs-prefix -mneeded🌈 x86 Windows Options 🌈
Windows x86 gets its own squad too 🪟✨
-mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable🍓 Xstormy16 Options 🍓
only one flag for this one, but it's a serious one 🫶
-msim💜 Xtensa Options 💜
Xtensa has a whole cute collection of its own 🌸
-mconst16 -mno-const16 -mfused-madd -mno-fused-madd
-mforce-no-pic -mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls
-mno-longcalls -mabi=abi-type🧚 zSeries Options 🧚
no flags of its own here, bestie. the zSeries stuff lives over at See S/390 and zSeries Options 💅
✨ Options Controlling the Kind of Output ✨
okay so compilation can have up to four stages: preprocessing, compilation proper, assembly, and linking, and they always happen in that order 🔥 GCC can preprocess and compile several files into several assembler input files, or into one assembler input file. then each assembler input file makes an object file, and linking smooshes all the object files together (the newly compiled ones and the ones you gave it as input) into an executable file 💖
for any given input file, the file name suffix decides what kind of compilation happens. iykyk 🫶
file.c— C source that still needs to go through the preprocessor first 🧚file.i— C source that should not be preprocessed.file.ii— C++ source that should not be preprocessed.file.m— Objective-C source code. Note that you must link with thelibobjclibrary to make an Objective-C program work.file.mi— Objective-C source that should not be preprocessed.file.mm,file.M— Objective-C++ source code. Note that you must link with thelibobjclibrary to make an Objective-C++ program work. Also,.Mrefers to a literal capital M.file.mii— Objective-C++ source that should not be preprocessed.file.h— a C, C++, Objective-C, or Objective-C++ header that gets turned into a precompiled header (this is the default), or a C or C++ header that gets turned into an Ada spec (via the-fdump-ada-specswitch).file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source that must be preprocessed. Note that in.cxx, the last two letters must both be literallyx. Likewise,.Crefers to a literal capital C.file.mm,file.M— Objective-C++ source that must be preprocessed.file.mii— Objective-C++ source that should not be preprocessed.file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— C++ headers that get turned into a precompiled header or an Ada spec.file.f,file.for,file.ftn— fixed form Fortran source that should not be preprocessed.file.F,file.FOR,file.fpp,file.FPP,file.FTN— fixed form Fortran source that must be preprocessed (with the traditional preprocessor).file.f90,file.f95,file.f03,file.f08— free form Fortran source that should not be preprocessed.file.F90,file.F95,file.F03,file.F08— free form Fortran source that must be preprocessed (with the traditional preprocessor).file.go— Go source code.file.brig— BRIG files (binary representation of HSAIL).file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— Ada source that holds a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These are also called specs.file.adb— Ada source that holds a library unit body (a subprogram or package body). These are also called bodies.file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed.other— an object file that gets fed straight into linking. Any file name with no recognized suffix is treated this way.
you can also tell GCC the input language yourself with the -x option 🎀
✨ -x language ✨
this lets you pick the language for the input files that follow, instead of letting the compiler guess from the file name suffix 💅 it applies to all following input files until the next -x option. the possible values for language are:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go
brig🦋 -x none
this turns off any language specification, so the files after it are handled by their file name suffixes, just like they would be if -x had never been used at all 🌸
if you only want some 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 should stop. heads up, some combos (for example, -x cpp-output -E) make gcc do absolutely nothing at all 💀
🫶 -c
compile or assemble the source files, but do not link 🔗 the linking stage just doesn't happen. the final output is an object file for each source file.
by default, the object file name for a source file is made by swapping the suffix .c, .i, .s, etc. for .o.
unrecognized input files that don't need compiling or assembling are ignored.
🌸 -S
stop after the compilation proper stage and skip assembling 🧚 the output is an assembler code file for each non-assembler input file you gave it.
by default, the assembler file name for a source file is made by swapping the suffix .c, .i, etc. for .s.
input files that don't need compiling are ignored.
💖 -E
stop after the preprocessing stage and don't run the compiler proper 🌈 the output is preprocessed source code, sent to standard output.
input files that don't need preprocessing are ignored.
👑 -o file
place the primary output in file 👑 this applies to whatever kind of output you're making, whether that's an executable, an object file, an assembler file, or preprocessed C code.
if -o isn't specified, the defaults are: an executable goes in a.out, the object file for source.suffix goes in source.o, its assembler file goes in source.s, a precompiled header file goes in source.suffix.gch, and all preprocessed C source goes to standard output 🍓
Okay so, this next bit is all about how -o quietly decides what your aux and dump files get named, and bestie, it's giving main character energy 💅✨
The primary output is just the one file -o names, but it also controls the naming of your auxiliary and dump outputs 🦋 Check the examples below, they're lowkey the best part. Unless you override it, aux and dump outputs land in the same directory as the primary output 🌸 For auxiliary outputs, the input file's suffix gets swapped for the aux file type's suffix. For dump outputs, the dump file's suffix gets tacked onto the input file's suffix. In compilation commands, both aux and dump outputs use the primary output's base name. In compile and link commands, the primary output name (minus the executable suffix) gets mixed with the input file name. If both end up with the same base name once you ignore the suffix, the result is just that base name. If they don't match, they get smooshed together with a dash between them 🎀
gcc -c foo.c ...This will use foo.o as the primary output and drop aux outputs and dumps right next to it. For example, you get the aux file foo.dwo for -gsplit-dwarf, and the dump file foo.c.???r.final for -fdump-rtl-final 🧚
If you explicitly give a non-linker output file, aux and dump files default to the same base name, and that's so clean 🫶
gcc -c foo.c -o dir/foobar.o ...This will name the aux outputs dir/foobar.* and the dump outputs dir/foobar.c.* 🍓
A linker output works differently and prefixes the aux and dump outputs instead 👑
gcc foo.c bar.c -o dir/foobar ...This will generally name the aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and the dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.* 💖
The one exception is when the executable shares its base name with the single input 🙈
gcc foo.c -o dir/foo ...In that case the aux outputs are dir/foo.* and the dump outputs are dir/foo.c.* ✨
You can tweak the location and names of aux and dump outputs with -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj 🌈
🎀 -dumpbase dumpbase
This option sets the base name for aux and dump output files. It does not touch the name of the primary output file 💅 Intermediate outputs, when you keep them, aren't treated as primary outputs. They count as auxiliary outputs instead 🦋
gcc -save-temps -S foo.cThis saves the (no longer temporary) preprocessed file as foo.i, then compiles to the (implied) output file foo.s. Iconic 🌸
gcc -save-temps -dumpbase save-foo -c foo.cThis one preprocesses to save-foo.i, compiles to save-foo.s (now an intermediate, so it's an auxiliary output), and then assembles to the (implied) output file foo.o ✨
Without this option, dump and aux files get their names from the input file, or from the (non-linker) output file if you explicitly named one. Dump output files (like the ones from -fdump-* options) keep the input name suffix. Aux output files (the ones from other non-dump options, like "-save-temps", "-gsplit-dwarf", and "-fcallgraph-info") drop it 🧚
You can get similar suffix differentiation between dump and aux outputs with an explicit -dumpbase basename.suf by also passing -dumpbase-ext .suf 🫶
If you give dumpbase an explicit directory component, any dumppfx specification (like -dumpdir or -save-temps=*) gets ignored. Instead of appending to it, dumpbase fully overrides it 💀
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...This creates aux and dump outputs named alt/foo.*. It totally ignores dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir 👑
When you pass -dumpbase in a command that compiles multiple inputs, or compiles and then links, it can be combined with dumppfx, same as under -dumpdir. Each input file then gets compiled with the combined dumppfx, and its own default values for dumpbase and auxdropsuf 🌈
gcc foo.c bar.c -c -dumpbase main ...This creates foo.o and bar.o as primary outputs. It also stops the aux and dump outputs from overwriting each other by using the dumpbase as a prefix, so you get main-foo.* and main-bar.* 🦋
An empty string as dumpbase stops the output basename from influencing aux and dump names during compilation. The defaults get computed instead 🍓
gcc -c foo.c -o dir/foobar.o -dumpbase " ...This names the aux outputs dir/foo.* and the dump outputs dir/foo.c.*. Notice how the basenames come from the input name, but the directory still defaults to the output's directory 🧚
The empty-string dumpbase doesn't stop the output basename from being used during linking, though 💅
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...The source file compilation names the aux outputs dir/foo.* and dir/bar.*, and the dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking uses dir/foobar. as the prefix for dumps and aux files 🔥
🦋 -dumpbase-ext auxdropsuf
When you form the name of an auxiliary output file (but not a dump one), this drops the trailing auxdropsuf from dumpbase before any suffixes get appended 🫶 If you don't specify it, it defaults to the suffix of a default dumpbase. That means the input file's suffix when -dumpbase isn't on the command line, or when dumpbase gets combined with dumppfx ✨
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...This creates dir/foo.o as the main output and generates aux outputs in dir/x-foo.*. They sit where the primary output is, and the .c suffix gets dropped from the dumpbase. Dump outputs keep the suffix, so they're dir/x-foo.c.* 💖
This option gets ignored if it doesn't match the suffix of a specified dumpbase. The one exception is as an alternative to the executable suffix when the linker output base name gets appended to dumppfx, as described below 🌸
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...This creates main.out as the primary output. It stops the aux and dump outputs from overwriting each other by using the executable name minus auxdropsuf as a prefix. So you get aux outputs main-foo.* and main-bar.*, and dump outputs main-foo.c.* and main-bar.c.* 🌈
🧚 -dumpdir dumppfx
When you form the name of an auxiliary or dump output file, this uses dumppfx as a prefix 🎀
gcc -dumpdir pfx- -c foo.c ...This creates foo.o as the primary output and aux outputs named pfx-foo.*. It combines the given dumppfx with the default dumpbase, which comes from the default primary output, which in turn comes from the input name. Dump outputs also take the input name suffix, so they're pfx-foo.c.* ✨
If you want dumppfx to work as a directory name, it has to end with a directory separator 👑
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...This creates obj/bar.o as the primary output and aux outputs named dir/bar.*. It combines the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs take the input name suffix too, so they're dir/bar.c.* 🦋
By default it's the location of the output file, unless the output file is a special file like "/dev/null". The options -save-temps=cwd and -save-temps=obj override that default, just like an explicit -dumpdir option does. If you pass several of these, the last one wins 💅
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...This outputs foo.o, with aux outputs named foo.*, because -save-temps=* overrides the dumppfx from the earlier -dumpdir. It doesn't matter that =obj is already the default for -save-temps, or that the output directory is implicitly the current directory. Dump outputs are named foo.c.* 🌸
When you compile from multiple input files and specify -dumpbase, dumpbase (minus an auxdropsuf suffix) plus a dash gets appended to an explicit or defaulted dumppfx. If dumpbase has any directory components, it overrides dumppfx instead. This way each compilation gets its own differently-named aux and dump outputs 🫶
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...This outputs aux dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs keep the input file suffix, so they're dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively 🧚
Compare that with single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...Applying -dumpbase to a single source doesn't compute and append a separate dumpbase per input file. Its aux and dump outputs go in dir/pfx-main.* 💖
When you compile and then link from multiple input files, a defaulted or explicitly specified dumppfx also goes through the dumpbase- transformation above. Think the compilation of foo.c and bar.c above, just without -c. If you specify neither -dumpdir nor -dumpbase, the linker output base name gets appended to the default dumppfx instead. That's the base name minus auxdropsuf if you gave one, or minus the executable suffix otherwise, plus a dash 🌈
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...Heads up, though: unlike the earlier linking cases, this does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The idea is that your explicit dumppfx can include the output name as part of the prefix if you want it to. Only an explicitly specified -dumpbase gets combined with it, so a meaningful option doesn't get quietly thrown away 🔥
When you compile and link from a single input file, the linker output base name only gets appended to the default dumppfx (as above) if it doesn't share the base name with the single input file name. This was covered in the single-input linking cases above, but not for an explicit -dumpdir that stops the combination, even when -save-temps=* overrides it:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Aux outputs are named foo.*, and dump outputs foo.c.*, in the current working directory, as finally requested by -save-temps=cwd 🍓
The dump-name stuff is kinda the boss level of this whole page, so here's the vibe 💅: the main output name gets split into a directory part and a basename part. dumppfx takes the directory part, unless you override it with -dumpdir or -save-temps=*. dumpbase takes the basename, unless you override it with -dumpbase. If there are multiple inputs or you're linking, dumpbase can get combined with dumppfx and pulled from each input file 🧚. Auxiliary output names for each input are made by mashing together dumppfx, dumpbase minus its suffix, and the auxiliary output suffix. Dump output names are basically the same, except they keep the suffix from dumpbase 🦋
Okay, now the LTO recompilation bit 🔥. Auxiliary and dump outputs made during LTO recompilation get a combo of dumppfx and dumpbase passed down to lto-wrapper as a -dumpdir option. That combo can come from what you gave it or from the linker output name (but not from the inputs), and it's sent this way even when it wouldn't normally be used like that. The trailing period replaces the compiler-added dash, if there is one. Since this program is involved in linking, it doesn't normally get any -dumpbase or -dumpbase-ext, and it ignores them 💀
When it runs sub-compilers, lto-wrapper tacks the LTO stage names onto the dumppfx it received, makes sure that has a directory component so it overrides any -dumpdir, and passes the result along as -dumpbase to the sub-compilers ✨
-v— Print (on standard error output) the commands executed to run the stages of compilation. It also prints the version number of the compiler driver program and of the preprocessor and the compiler proper 📣-###— Basically-v, but the commands don't actually run, and arguments get quoted unless they only contain alphanumeric characters or"./-_". This is perfect for shell scripts that want to capture the driver-generated command lines 🤖--help— Print (on the standard output) a description of the command-line optionsgccunderstands. If-vis also specified,--helpgets passed on to the various processesgccinvokes, so they can show the command-line options they accept. If-Wextrawas also specified (before the--helpoption), command-line options with no documentation attached get displayed too 🎀--target-help— Prints (on the standard output) a description of the target-specific command-line options for each tool. For some targets, extra target-specific information might get printed too 🌸--help={class|[^]qualifier}[,...]— Print (on the standard output) a description of the command-line options the compiler understands that fit into all the specified classes and qualifiers. Here are the supported classes 👑optimizers— Shows all the optimization options the compiler supports.warnings— Shows all the options that control the warning messages the compiler produces.target— Shows target-specific options. Unlike--target-help, target-specific options of the linker and assembler aren't shown, because those tools don't currently support the extended--help=syntax.params— Shows the values recognized by the--paramoption.language— Shows the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, you need to pick thecommonclass.common— Shows the options that are common to all languages.
Here are the supported qualifiers 🌈
undocumented— Shows only the options that are undocumented.joined— Shows options that take an argument appearing after an equal sign in the same continuous piece of text, like--help=target.separate— Shows options that take an argument appearing as a separate word after the original option, like-o output-file.
Want all the undocumented target-specific switches the compiler supports? Use this 🔍
--help=target,undocumentedThe sense of a qualifier can be flipped by prefixing it with the ^ character. So if you want all the binary warning options (the ones that are either on or off and don't take an argument) that have a description, use this 🙈
--help=warnings,^joined,^undocumentedThe argument to --help= shouldn't consist solely of inverted qualifiers.
You can combine several classes, but that usually restricts the output so much there's nothing to display. One case where it does work is when one of the classes is target. For example, to display all the target-specific optimization options, use this 💖
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping any that have already been displayed. If --help is also specified anywhere on the command line, it takes precedence over any --help= option 👑
If the -Q option appears on the command line before the --help= option, the descriptive text shown by --help= changes. Instead of describing the displayed options, it gives an indication of whether each option is enabled, disabled, or set to a specific value (assuming the compiler knows this at the point where --help= is used) 🫶
Here's a truncated example from the ARM port of gcc 🧚
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]The output is sensitive to the effects of previous command-line options, so you can find out which optimizations are enabled at -O2 by using this 🦋
-Q -O2 --help=optimizersOr, if you want to discover which binary optimizations are enabled by -O3, do this 🔥
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled--version— Displays the version number and copyrights of the GCC you invoked 📜-pass-exit-codes— Normally, thegccprogram exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, thegccprogram instead returns the numerically highest error produced by any phase that returned an error indication. The C, C++, and Fortran front ends return 4 if they hit an internal compiler error 🚨-pipe— Uses pipes rather than temporary files for communication between the various stages of compilation. This fails on some systems where the assembler can't read from a pipe, but the GNU assembler has no trouble 🌊-specs=file— Processes file after the compiler reads in the standard specs file, to override the defaults thegccdriver uses when deciding which switches to pass tocc1,cc1plus,as,ld, etc. You can specify more than one-specs=fileon the command line, and they're processed in order, from left to right 🍓-wrapper— Invokes all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list 🎁
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, so the invocation of cc1 becomes gdb --args cc1 ... 💜
-ffile-prefix-map=old=new— When compiling files living in directory old, any references to them in the compilation result get recorded as if the files lived in directory new instead. Specifying this option is equivalent to specifying all the individual-f*-prefix-mapoptions. It's a great way to make reproducible, location-independent builds 🌸 See also-fmacro-prefix-mapand-fdebug-prefix-map.-fplugin=name.so— Loads the plugin code in name.so, which is assumed to be a shared object the compiler willdlopen. The base name of the shared object file identifies the plugin for argument parsing (see-fplugin-arg-name-key=valuebelow). Each plugin should define the callback functions specified in the Plugins API 🔌-fplugin-arg-name-key=value— Defines an argument called key with a value of value for the plugin called name 🧩-fdump-ada-spec[-slim]— For C and C++ source and include files, generates corresponding Ada specs 🌷-fada-spec-parent=unit— Used together with-fdump-ada-spec[-slim]above, this generates Ada specs as child units of the parent unit 👶-fdump-go-spec=file— For input files in any language, generates corresponding Go declarations in file. This makes Goconst,type,var, andfuncdeclarations, which can be a super useful way to start writing a Go interface to code written in some other language 🐹@file— Reads command-line options from file. The options read get inserted in place of the original@fileoption. If file doesn't exist or can't be read, the option gets treated literally and isn't removed 🙃
Options in file are separated by whitespace. You can include a whitespace character in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing it with a backslash. The file may contain additional @file options too, and those get processed recursively 🌀
💅 Compiling C++ Programs 💅
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx 💖. C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc, and preprocessed C++ files use the suffix .ii 🫶. GCC recognizes files with these names and compiles them as C++ programs, even if you call the compiler the same way you would for C programs (usually with the name gcc) ✨
However, the use of gcc does not add the C++ library, bestie 💀. That's where g++ slides in ✨. g++ is a program that calls GCC and automatically specifies linking against the C++ library, so you don't have to remember it 💅. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. It's also handy for precompiling a C header file with a .h extension for use in C++ compilations 🧚. On many systems, g++ is also installed with the name c++ 🌸.
When you compile C++ programs, you can use lots of the same command-line options you'd use for any language, or options meant for C and related languages, or options that only matter for C++ 🦋.
✨ Options Controlling C Dialect ✨
These options control the dialect of C (or languages derived from C, like C++, Objective-C and Objective-C++) that the compiler accepts 👑:
-ansi— In C mode this is the same as-std=c90, and in C++ mode it's the same as-std=c++98🎀. It turns off GCC features that clash with ISO C90 (when compiling C) or standard C++ (when compiling C++), like theasmandtypeofkeywords, plus predefined macros likeunixandvaxthat tell you what system you're on 🙈. It also switches on the undesirable and rarely used ISO trigraph feature 💀. For the C compiler, it also turns off recognition of C++ style//comments as well as theinlinekeyword. The alternate keywords__asm__,__extension__,__inline__and__typeof__still work even with-ansi. You wouldn't want them in an ISO C program, of course, but they're super useful in header files that might get included in compilations done with-ansi🧚. Alternate predefined macros like__unix__and__vax__are available with or without-ansi🌈.-ansidoesn't reject non-ISO programs gratuitously. For that, you need-Wpedanticon top of-ansi. The macro__STRICT_ANSI__is predefined when-ansiis used, and some header files notice it and skip declaring certain functions or defining certain macros that the ISO standard doesn't call for, so they don't interfere with programs using those names for other things 💜. Functions that are normally built in but don't have semantics defined by ISO C (likeallocaandffs) are not built-in functions when-ansiis used.-std=— Determines the language standard 📜. This option is currently only supported when compiling C or C++. The compiler can accept several base standards, likec90orc++98, and GNU dialects of those, likegnu90orgnu++98. With a base standard, the compiler accepts all programs following that standard plus GNU extensions that don't contradict it. For example,-std=c90turns off GCC features that are incompatible with ISO C90, like theasmandtypeofkeywords, but not other GNU extensions that have no meaning in ISO C90, like omitting the middle term of a?:expression. With a GNU dialect, though, every feature the compiler supports is enabled, even when those features change the meaning of the base standard. So some strictly conforming programs may get rejected 😭.-Wpedanticuses the particular standard to figure out which features are GNU extensions for that version. For example,-std=gnu90 -Wpedanticwarns about C++ style//comments, while-std=gnu99 -Wpedanticdoes not. A value for this option must be provided. Possible values are:c90,c89,iso9899:1990— Supports all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code.iso9899:199409— ISO C90 as modified in amendment 1.c99,c9x,iso9899:1999,iso9899:199x— ISO C99. It's substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely related to optional C99 features from Annexes F and G). Seehttp://gcc.gnu.org/c99status.htmlfor more info. The namesc9xandiso9899:199xare deprecated 🥀.c11,c1x,iso9899:2011— ISO C11, the 2011 revision of the ISO C standard. It's substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely related to optional C11 features from Annexes F and G), and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated.c17,c18,iso9899:2017,iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). It's the same as C11 except for corrections of defects (all of which are also applied with-std=c11) and a new value of__STDC_VERSION__, so it's supported to the same extent as C11 ✨.c2x— The next version of the ISO C standard, still under development. Support for this version is experimental and incomplete 🧪.gnu90,gnu89— GNU dialect of ISO C90 (including some C99 features).gnu99,gnu9x— GNU dialect of ISO C99. The namegnu9xis deprecated.gnu11,gnu1x— GNU dialect of ISO C11. The namegnu1xis deprecated.gnu17,gnu18— GNU dialect of ISO C17. This is the default for C code 👑.gnu2x— The next version of the ISO C standard, still under development, plus GNU extensions. Support for this version is experimental and incomplete.c++98,c++03— The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code.gnu++98,gnu++03— GNU dialect of-std=c++98.c++11,c++0x— The 2011 ISO C++ standard plus amendments. The namec++0xis deprecated.gnu++11,gnu++0x— GNU dialect of-std=c++11. The namegnu++0xis deprecated.c++14,c++1y— The 2014 ISO C++ standard plus amendments. The namec++1yis deprecated.gnu++14,gnu++1y— GNU dialect of-std=c++14. The namegnu++1yis deprecated.c++17,c++1z— The 2017 ISO C++ standard plus amendments. The namec++1zis deprecated.gnu++17,gnu++1z— GNU dialect of-std=c++17. This is the default for C++ code 👑. The namegnu++1zis deprecated.c++20,c++2a— The 2020 ISO C++ standard plus amendments. Support is experimental and could change in incompatible ways in future releases. The namec++2ais deprecated 🧪.gnu++20,gnu++2a— GNU dialect of-std=c++20. Support is experimental and could change in incompatible ways in future releases. The namegnu++2ais deprecated.c++2b,c++23— The next revision of the ISO C++ standard, planned for 2023. Support is highly experimental and will almost certainly change in incompatible ways in future releases 💀.gnu++2b,gnu++23— GNU dialect of-std=c++2b. Support is highly experimental and will almost certainly change in incompatible ways in future releases.
-fgnu89-inline— Tells GCC to use the traditional GNU semantics forinlinefunctions when in C99 mode 🦋. Using this option is roughly equivalent to adding thegnu_inlinefunction attribute to all inline functions. The option-fno-gnu89-inlineexplicitly tells GCC to use the C99 semantics forinlinewhen in C99 orgnu99mode (i.e., it specifies the default behavior). This option is not supported in-std=c90or-std=gnu90mode. The preprocessor macros__GNUC_GNU_INLINE__and__GNUC_STDC_INLINE__can be used to check which semantics are in effect forinlinefunctions.-fpermitted-flt-eval-methods=style— ISO/IEC TS 18661-3 defines new permissible values forFLT_EVAL_METHODthat indicate operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type 🔥. These new values are a superset of those permitted under C99/C11, which doesn't specify the meaning of other positive values ofFLT_EVAL_METHOD. So code conforming to C11 may not have been written expecting the new values 🙈.-fpermitted-flt-eval-methodsspecifies whether the compiler should allow only the values ofFLT_EVAL_METHODspecified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.styleis eitherc11orts-18661-3, as appropriate. The default when in a standards-compliant mode (-std=c11or similar) is-fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11or similar) is-fpermitted-flt-eval-methods=ts-18661-3.-aux-info filename— Outputs to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files 📝. This option is silently ignored in any language other than C. Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I,Nfor new orOfor old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (CorF, respectively, in the following character). For function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.-fallow-parameterless-variadic-functions— Accepts variadic functions without named parameters 🎀. Although it's possible to define such a function, it's not very useful, because it's not possible to read the arguments. This is only supported for C, as this construct is allowed by C++.-fno-asm— okay so this one stops GCC from treatingasm,inlineortypeofas keywords 🙈 That way your code can use those words as regular identifiers. You can use__asm__,__inline__and__typeof__instead.-ansiimplies-fno-asm.- In C++, this only affects the
typeofkeyword, sinceasmandinlineare standard keywords there. Bestie tip from the manual: you may want-fno-gnu-keywordsinstead, which does the same thing. - In C99 mode (
-std=c99or-std=gnu99), this only affectsasmandtypeof, sinceinlineis a standard keyword in ISO C99 💖
- In C++, this only affects the
-fno-builtin,-fno-builtin-function— Stop recognizing built-in functions whose names don't start with the__builtin_prefix 🦋- GCC normally makes special code for certain built-ins so they run more efficiently. For example,
allocacalls can turn into single instructions that adjust the stack directly, andmemcpycalls can become inline copy loops ✨ - The result is often smaller and faster, but since those calls don't show up as calls anymore, you can't set a breakpoint on them. You also can't change what they do by linking with a different library 😭
- Even when the code still contains calls, GCC may use what it knows about a built-in to warn you about problems or to generate better code. Like, warnings are given with
-Wformatfor bad calls toprintfwhenprintfis built in, and whenstrlenis known not to modify global memory 🔥 - With
-fno-builtin-function, only that one built-in gets switched off.functionmust not begin with__builtin_. If the function you name isn't a built-in in this version of GCC, the option just gets ignored 🌸 - There's no
-fbuiltin-functioncounterpart. If you want to turn built-ins back on selectively while using-fno-builtinor-ffreestanding, you can define macros like these:
- GCC normally makes special code for certain built-ins so they run more efficiently. For example,
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fgimple— Turns on parsing of function definitions marked with__GIMPLE. It's an experimental feature that lets you unit test GIMPLE passes 🧪-fhosted— Asserts that compilation targets a hosted environment, which implies-fbuiltin. A hosted environment is one where the entire standard library is available andmainhas a return type ofint. Basically everything except a kernel is hosted 💅- This is equivalent to
-fno-freestanding.
- This is equivalent to
-ffreestanding— Asserts that compilation targets a freestanding environment, which implies-fno-builtin. In a freestanding environment the standard library may not exist, and startup might not necessarily be atmain. The most obvious example is an OS kernel 👑- This is equivalent to
-fno-hosted.
- This is equivalent to
-fopenacc— Turns on handling of OpenACC directives,#pragma accin C/C++ and!$accin Fortran 🚀 With it, the compiler generates accelerated code following the OpenACC Application Programming Interface v2.6,https://www.openacc.org.- This option implies
-pthread, so it's only supported on targets that have support for-pthread🫶
- This option implies
-fopenacc-dim=geom— Sets the default compute dimensions for parallel offload regions that don't explicitly specify their own ✨ Thegeomvalue is a triple of':'-separated sizes, in the ordergang,workerandvector. A size can be left out to use a target-specific default value 🧚-fopenmp— Turns on handling of OpenMP directives,#pragma ompin C/C++ and!$ompin Fortran 💜 With it on, the compiler generates parallel code following the OpenMP Application Program Interface v4.5,https://www.openmp.org.- This option implies
-pthread, so it's only supported on targets that have support for-pthread. -fopenmpimplies-fopenmp-simd.
- This option implies
-fopenmp-simd— Turns on handling of OpenMP's SIMD directives written as#pragma ompin C/C++ and!$ompin Fortran. Other OpenMP directives get ignored 🌈-fgnu-tm— When you specify-fgnu-tm, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 🍓 This is an experimental feature whose interface may change in future GCC versions as the official spec changes.- Not all architectures are supported for this feature, so check before you get too attached 💀
- For more information on GCC's support for transactional memory,
- > Heads up: the transactional memory feature is not supported with non-call exceptions (
-fnon-call-exceptions).
-fms-extensions— Accepts some non-standard constructs used in Microsoft header files 🎀- In C++ code, this lets member names in structures be similar to previous type declarations. Iconic behavior, honestly:
typedef int UOW;
struct ABC {
UOW UOW;
};- Some cases of unnamed fields in structures and unions are only accepted with this option.
- This option is off for all targets except x86 targets using
ms-abi.
-fplan9-extensions— Accepts some non-standard constructs used in Plan 9 code 🦋- This enables
-fms-extensions. - It permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field.
- It also permits referring to anonymous fields declared using a
typedef. - This is only supported for C, not C++ 🙈
- This enables
-fcond-mismatch— Allows conditional expressions with mismatched types in the second and third arguments. The value of such an expression isvoid. This option is not supported for C++ 💅-flax-vector-conversions— Allows implicit conversions between vectors with differing numbers of elements and/or incompatible element types. Don't use this for new code, bestie 😭-funsigned-char— Lets the typecharbe unsigned, likeunsigned char🌸- Every kind of machine has a default for what
charshould be. It's either likeunsigned charby default or likesigned charby default. - Ideally, a portable program should always use
signed charorunsigned charwhenever it depends on the signedness of an object. But loads of programs were written to use plaincharand expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. - The type
charis always a distinct type from each ofsigned charorunsigned char, even though its behavior is always just like one of those two.
- Every kind of machine has a default for what
-fsigned-char— Lets the typecharbe signed, likesigned char💖- This is equivalent to
-fno-unsigned-char, which is the negative form of-funsigned-char. - Likewise, the option
-fno-signed-charis equivalent to-funsigned-char.
- This is equivalent to
-fsigned-bitfields,-funsigned-bitfields,-fno-signed-bitfields,-fno-unsigned-bitfields— These control whether a bit-field is signed or unsigned when the declaration doesn't use eithersignedorunsigned🔥 By default, such a bit-field is signed, because that's consistent: the basic integer types likeintare signed types.-fsso-struct=endianness— Sets the default scalar storage order of structures and unions to the specified endianness 🧚 The accepted values arebig-endian,little-endianandnativefor the native endianness of the target (the default). This option is not supported for C++.- > Warning: the
-fsso-structswitch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Don't say I didn't warn you 😭
- > Warning: the
✨ Options Controlling C++ Dialect 🎀
This section covers the command-line options that are only meaningful for C++ programs 💜 You can also use most of the GNU compiler options no matter what language your program is in. For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.CIn this example, only -fstrict-enums is an option meant only for C++ programs. You can use the other options with any language GCC supports 🌈
Some options for compiling C programs, such as -std, are also relevant for C++ programs ✨
Here's the list of options that are only for compiling C++ programs:
-fabi-version=n— Uses versionnof the C++ ABI. The default is version 0 🦋- Version 0 refers to the version conforming most closely to the C++ ABI specification. That means the ABI you get with version 0 will change across different versions of G++ as ABI bugs get fixed 🙈
- Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
- Version 2 is the version that first appeared in G++ 3.4, and it was the default through G++ 4.9 👑
- Version 3 corrects an error in mangling a constant address as a template argument.
- Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
- Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
- Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
- Version 7, which first appeared in G++ 4.8, treats
nullptr_tas a builtin type and corrects the mangling of lambdas in default argument scope. - Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
- Version 9, which first appeared in G++ 5.2, corrects the alignment of
nullptr_t. - Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g.
stdcall) 💅
Version 11 first showed up in G++ 7 and fixed the mangling of sizeof... expressions and operator names 💅. For multiple entities with the same name inside a function that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors 🌸.
Version 12 first appeared in G++ 8 and corrected the calling conventions for empty classes on the x86_64 target, plus classes with only deleted copy/move constructors 🦋. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor, so ngl, that's a little oops 🙈.
Version 13 first appeared in G++ 8.2 and fixes that accidental change from version 12 🫶.
Version 14 first appeared in G++ 10 and corrects the mangling of the nullptr expression ✨.
Version 15 first appeared in G++ 11 and changes the mangling of "__alignof__" so it's distinct from "alignof", plus dependent operator names 👑.
See also -Wabi 💖.
-fabi-compat-version=n— okay so, on targets that support strong aliases, G++ works around mangling changes by making an alias with the correct mangled name whenever it defines a symbol with an incorrect mangled name 🧚. This switch picks which ABI version gets used for that alias. With-fabi-version=0(the default), it defaults to11(GCC 7 compatibility). If you explicitly select another ABI version, it defaults to0. For compatibility with GCC versions 3.2 through 4.9, use-fabi-compat-version=2🎀. If you don't provide this option but do provide-Wabi=n, that version is used for the compatibility aliases. If you provide this option along with-Wabi(without a version), the version from this option is used for the warning 💜.-fno-access-control— turns off all access checking 🙅♀️. It's mainly useful for working around bugs in the access control code, so it's a bit of a last-resort vibe 💀.-faligned-new— enables support for C++17"new"of types that need more alignment thanvoid* ::operator new(std::size_t)provides 🔥. You can pass a numeric argument like-faligned-new=32to specify how much alignment (in bytes) that function provides, but few users will ever need to override the default ofalignof(std::max_align_t). This flag is enabled by default for-std=c++17⭐.-fchar8_t,-fno-char8_t— enableschar8_tas adopted for C++20 😭. This includes the addition of a brand-newchar8_tfundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new__cpp_char8_tand__cpp_lib_char8_tfeature test macros 🌈.
This option lets functions get overloaded for ordinary and UTF-8 strings, and it's giving main character energy:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2It also introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;Heads up bestie: the change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards 😬. For example, the following code is well-formed under ISO C++11, but it's ill-formed when -fchar8_t is specified:
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested-fcheck-new— checks that the pointer returned by"operator new"is non-null before it tries to modify the storage it allocated 🔍. This check is normally unnecessary, because the C++ standard says"operator new"only returns 0 if it's declaredthrow(). In that case the compiler always checks the return value, even without this option. In every other case, when"operator new"has a non-empty exception specification, memory exhaustion gets signalled by throwing"std::bad_alloc". See alsonew (nothrow)🫧.-fconcepts,-fconcepts-ts— below-std=c++20,-fconceptsenables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015) 📜. With-std=c++20and above, Concepts are part of the language standard, so-fconceptsdefaults to on. But the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled by-fconcepts-ts🍓.-fconstexpr-depth=n— sets the maximum nested evaluation depth for C++11 constexpr functions ton🌙. A limit is needed to detect endless recursion during constant expression evaluation. The minimum the standard specifies is 512.-fconstexpr-cache-depth=n— sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached ton🧠. This is a heuristic that trades compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows huge from highly recursive evaluations). The default is 8. Very few users will want to adjust it, but if your code does heavy constexpr calculations, you might wanna experiment to find which value works best for you 💅.-fconstexpr-loop-limit=n— sets the maximum number of iterations for a loop in C++14 constexpr functions ton🔁. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).-fconstexpr-ops-limit=n— sets the maximum number of operations during a single constexpr evaluation 🧮. Even when the iterations of a single loop are limited by the limit above, if there are several nested loops that each have lots of iterations (still under that limit), or if a loop body or code outside any loop needs too many expressions evaluated, the constexpr evaluation might take too long. The default is 33554432 (1<<25).-fcoroutines— enables support for the C++ coroutines extension 🧪. It's experimental, so handle with care 🥹.-fno-elide-constructors— the C++ standard lets an implementation skip creating a temporary that's 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. It also makes G++ call trivial member functions that would otherwise be expanded inline. In C++17 the compiler is required to omit these temporaries, but this option still affects trivial member functions.-fno-enforce-eh-specs— don't generate code that checks for violations of exception specifications at run time 🚫. This option violates the C++ standard, but it may be useful for reducing code size in production builds, much like defining"NDEBUG". It does not give user code permission to throw exceptions in violation of the exception specifications. The compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time 💀.-fextern-tls-init,-fno-extern-tls-init— the C++11 and OpenMP standards allow"thread_local"and"threadprivate"variables to have dynamic (runtime) initialization 🧵. To support that, any use of such a variable goes through a wrapper function that does the necessary initialization. When the use and definition are in the same translation unit, this overhead can be optimized away. But when the use is in a different translation unit, there's significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or because a use in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the-fno-extern-tls-initoption 🦋.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that don't support symbol aliases, the default is -fno-extern-tls-init ✨.
-fno-gnu-keywords— don't recognizetypeofas a keyword, so code can use that word as an identifier 🙌. You can use the keyword__typeof__instead. This option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc.-fno-implicit-templates— never emit code for non-inline templates that are instantiated implicitly (i.e. by use). Only emit code for explicit instantiations 📦. If you use this option, you must take care to structure your code so it includes all the necessary explicit instantiations, or you'll end up with undefined symbols at link time 😵.-fno-implicit-inline-templates— don't emit code for implicit instantiations of inline templates either 🌷. The default handles inlines differently so that compiles with and without optimization need the same set of explicit instantiations.-fno-implement-inlines— to save space, don't emit out-of-line copies of inline functions controlled by"#pragma implementation"🧹. This causes linker errors if these functions aren't inlined everywhere they're called.-fmodules-ts,-fno-modules-ts— enables support for C++20 modules 🧩. The-fno-modules-tsoption is usually not needed, since that's the default. Even though this is a C++20 feature, it's not currently implicitly enabled by selecting that standard version 🙈.-fmodule-header,-fmodule-header=user,-fmodule-header=system— ✨ okay so this one compiles a header file so it can be imported as a header unit 🎀-fmodule-implicit-inline— 🙈 this one is lowkey a plot twist. Member functions defined right inside their class are not implicitly inline for modular code. That's different from traditional C++ behavior, and there are good reasons for it, but it can be a pain when you're porting code. This flag makes those definitions implicitly inline instead.
ABI warning 💀 this generates an ABI incompatibility, so you have to use it everywhere or nowhere. Definitions outside of a named module stay implicitly inline either way.
-fno-module-lazy— 🌸 disables lazy module importing and module mapper creation-fmodule-mapperfamily — 🔮 this is the oracle you query for module name to filename mappings. If you don't specify one, theCXX_MODULE_MAPPERenvironment variable gets used, and if that's unset too, you get an in-process default. The six forms are:-fmodule-mapper=[hostname]:port[?ident]-fmodule-mapper=|program[?ident] args...-fmodule-mapper==socket[?ident]-fmodule-mapper=<>[inout][?ident]-fmodule-mapper=<in>out[?ident]-fmodule-mapper=file[?ident]
-fmodule-only— 🍓 only emits the Compiled Module Interface and skips the object file-fms-extensions— 💅 turns offWpedanticwarnings for MFC-style stuff, like implicitintand getting a pointer to a member function with non-standard syntax-fnew-inheriting-ctors— ✨ enables the P0136 tweak to how C++11 constructor inheritance works. It's part of C++17 but also counts as a Defect Report against C++11 and C++14, so it's giving main character energy. This flag is on by default unless-fabi-version=10or lower is specified.-fnew-ttp-matching— 🦋 enables the P0522 resolution to Core issue 150. This lets a template with default template arguments be used as an argument for a template template parameter that has fewer template parameters. It's enabled by default for-std=c++17.-fno-nonansi-builtins— 🙅 disables built-in declarations of functions that ANSI/ISO C doesn't mandate, like"ffs","alloca","_exit","index","bzero","conjf", and other related functions-fnothrow-opt— 👑 treats athrow()exception specification as if it were"noexcept", which cuts down or erases the text size overhead compared to a function with no exception specification. If the function has local variables with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can get optimized away.
Semantic change 🙈 an exception thrown out of a function with this exception specification results in a call to"terminate"rather than"unexpected".
-fno-operator-names— 🚫 stopsand,bitand,bitor,compl,not,or, andxorfrom being treated as synonyms for the operator name keywords-fno-optional-diags— 🤫 disables diagnostics the standard says a compiler doesn't have to issue. Right now, the only one G++ issues in this category is the one for a name having multiple meanings within a class.-fpermissive— 🎀 downgrades some diagnostics about nonconformant code from errors to warnings, so some nonconforming code can still compile-fno-pretty-templates— 🧚 when an error message points at a specialization of a function template, the compiler normally prints the template signature followed by the template arguments and any typedefs or typenames in it (so you getvoid f(T) [with T = int]instead ofvoid f(int)) so you can tell which template is involved. For a specialization of a class template, it leaves out any template arguments that match the defaults. If either of those behaviors makes the error harder to read, use-fno-pretty-templatesto turn them off.-fno-rtti— 💜 disables generating the information about every class with virtual functions that the C++ run-time type identification features (dynamic_castandtypeid) use. If you don't use those parts of the language, this can save some space. Exception handling uses the same information, but G++ generates it only as needed. Thedynamic_castoperator still works for casts that don't need run-time type information, meaning casts tovoid *or to unambiguous base classes.
Mixing warning ⚠️ mixing code compiled with-frttiand code compiled with-fno-rttimay not work. For example, programs may fail to link if a class compiled with-fno-rttiis used as a base for a class compiled with-frtti.
-fsized-deallocation— 🔥 enables the built-in global declarations introduced in C++14:
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;This is super useful for user-defined replacement deallocation functions that use the object's size to make deallocation faster. It's enabled by default under -std=c++14 and above. The -Wsized-deallocation flag warns about places that might want to add a definition.
-fstrict-enums— 🧮 lets the compiler optimize assuming that a value of an enumerated type can only be one of the enumeration's values (per the C++ standard, basically a value that fits in the minimum number of bits needed to represent all the enumerators). This assumption may not hold if the program uses a cast to convert an arbitrary integer into the enumerated type.-fstrong-eval-order— 🌈 evaluates member access, array subscripting, and shift expressions left-to-right, and assignment right-to-left, as adopted for C++17. It's enabled by default with-std=c++17.-fstrong-eval-order=someonly enables the ordering for member access and shift expressions, and it's the default without-std=c++17.-ftemplate-backtrace-limit=n— 📜 sets the maximum number of template instantiation notes for a single warning or error ton. The default is 10.-ftemplate-depth=n— 🌀 sets the maximum instantiation depth for template classes ton. A limit is needed to catch endless recursion during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default is 900, because the compiler can run out of stack space before hitting 1024 in some situations.-fno-threadsafe-statics— 🧵 skips emitting the extra code that uses the routines from the C++ ABI for thread-safe initialization of local statics. It can shave off a bit of code size in code that doesn't need to be thread-safe.-fuse-cxa-atexit— 🧹 registers destructors for objects with static storage duration with the__cxa_atexitfunction instead ofatexit. This is required for fully standards-compliant handling of static destructors, but it only works if your C library supports__cxa_atexit.-fno-use-cxa-get-exception-ptr— 🙅♀️ stops using the__cxa_get_exception_ptrruntime routine. This makesstd::uncaught_exceptionincorrect, but it's necessary if that runtime routine isn't available.
✨ -fvisibility-inlines-hidden ✨
This switch tells GCC that you won't try to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. 🦋
GCC may then, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))), so they don't show up in the export table of a DSO and don't need a PLT indirection when they're used inside the DSO. Turning this on can have a dramatic effect on load and link times of a DSO, because it massively shrinks the dynamic export table when the library leans heavily on templates. 🚀
The behavior isn't quite the same as marking the methods hidden directly. It doesn't affect static variables local to the function, and it doesn't make the compiler deduce that the function is defined in only one shared object.
You can explicitly give a method a visibility to cancel the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, mark it with default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option, because their linkage might otherwise cross a shared library boundary. 🫶
✨ -fvisibility-ms-compat ✨
This flag tries to use visibility settings to make GCC's C++ linkage model compatible with Microsoft Visual Studio's. 💜
It makes these changes to GCC's linkage model:
- It sets the default visibility to
"hidden", like-fvisibility=hidden. - Types, but not their members, are not hidden by default.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object. Those declarations are permitted if they're permitted when this option isn't used.
In new code, it's better to use -fvisibility=hidden and export only the classes that are meant to be externally visible. 🧐 Unfortunately, code might rely (perhaps accidentally) on the Visual Studio behavior.
ODR danger 💀 among the consequences: static data members of the same type, with the same name, defined in different shared objects are different, so changing one doesn't change the other. Pointers to function members defined in different shared objects may also not compare equal. When this flag is given, it's a violation of the ODR to define types with the same name differently.
-fno-weak— 🙃 doesn't use weak symbol support, even if the linker provides it. By default, G++ uses weak symbols when they're available. This option exists only for testing and shouldn't be used by end-users, since it results in inferior code and has no benefits. It may be removed in a future release of G++.-fext-numeric-literals(C++ and Objective-C++ only) — okay so this one lets you use imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions 💅 Turn it off and those suffixes get treated as C++11 user-defined literal numeric suffixes instead 🫶 It's on by default for all the pre-C++11 dialects and all the GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. It's off by default for ISO C++11 onwards (-std=c++11, ...) 🌸-nostdinc++— this one tells GCC to skip the standard directories that are specific to C++ when it's looking for headers, but it still searches the other standard directories ✨ It's the flag used when building the C++ library, so you know it's serious 🧚
-flang-info-include-translate,-flang-info-include-translate-not,-flang-info-include-translate=header— these are the "tell me everything you did" flags 🦋 The first one informs you of accepted include translation events, and the second one spills the declined ones. Theheaderform only gives you the include translation info for that specific header. If header is"user"or"<system>", it gets resolved to a specific user or system header using the include path 🔍-flang-info-module-cmi,-flang-info-module-cmi=module— main character energy for Compiled Module Interface pathnames 👑 The first one notes all the CMI pathnames that get read. The module form is for a specific module's CMI. module can be a named module or a header-unit, and you can tell it's a header-unit if it's a pathname containing directory separators or it's wrapped in<>or""💜-stdlib=libstdc++,libc++— when G++ is configured to support this option, you can pick an alternate C++ runtime library 🔥 There are two choices:libstdc++(the default, native C++ runtime for G++) andlibc++, which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards) 🍎 Picking one switches G++ over to the headers from that library, and it'll emit-lstdc++or-lc++respectively when a C++ runtime is needed for linking. Iconic ✨
In addition, these warning options only have meanings for C++ programs, so they're giving very specific vibes 💅
-Wabi-tag(C++ and Objective-C++ only) — warns you when a type with an ABI tag gets used somewhere that doesn't have that ABI tag 🏷️ Check out C++ Attributes for more info about ABI tags.-Wcomma-subscript(C++ and Objective-C++ only) — warns about comma expressions inside a subscripting expression 🙈 This usage got deprecated in C++20, but a comma expression wrapped in( )is not deprecated, so you're safe there 🫶 Here's the example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated
a[(b,c)]; // OK
}This one is enabled by default with -std=c++20 🌈
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warns you when you do class template argument deduction (CTAD) on a type that doesn't have any explicitly written deduction guides 🧐 This is the one that points out when CTAD only worked because the compiler made up the implicit deduction guides, which might not be what the programmer actually meant. Some style guides only allow CTAD on types that specifically "opt-in" (aka types that are designed to support CTAD), so this warning helps with that. You can suppress it with this pattern (any name works for the tag, btw):
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered-Wctor-dtor-privacy(C++ and Objective-C++ only) — warns when a class seems unusable because all of its constructors or destructors are private, and it has no friends and no public static member functions 😭 It also warns if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor.-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warns whendeleteis used to destroy an instance of a class that has virtual functions and a non-virtual destructor 💀 It's unsafe to delete an instance of a derived class through a pointer to a base class if the base class doesn't have a virtual destructor, so please listen to this one. This warning is enabled by-Wall🔥-Wdeprecated-copy(C++ and Objective-C++ only) — warns that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up 📝 This warning is enabled by-Wextra. With-Wdeprecated-copy-dtor, it'll also deprecate if the class has a user-provided destructor.-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disables the warning about when the usual arithmetic conversions get applied to operands where one is an enumeration type and the other is a different enumeration type 🙅♀️ This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, you can turn this warning on with -Wenum-conversion 🌸
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disables the warning for when the usual arithmetic conversions get applied to operands where one is an enumeration type and the other is a floating-point type 🫠 This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, you can enable this warning with -Wenum-conversion 🎀
-Wno-init-list-lifetime(C++ and Objective-C++ only) — this one turns off warnings about uses ofstd::initializer_listthat are likely to end up with dangling pointers 🥲 The underlying array for aninitializer_listis handled like a normal C++ temporary object, so it's super easy to accidentally keep a pointer to the array past the end of its lifetime. Some examples of how this happens:- If a function returns a temporary
initializer_list, or a localinitializer_listvariable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. - If a new-expression creates an
initializer_list, the array only lives until the end of the enclosing full-expression, so theinitializer_listin the heap has a dangling pointer. - When an
initializer_listvariable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression. So at the next statement, theinitializer_listvariable has a dangling pointer. - When a list constructor stores the
beginpointer from theinitializer_listargument, that doesn't extend the lifetime of the array. So if a class variable is constructed from a temporaryinitializer_list, the pointer is left dangling by the end of the variable declaration statement.
- If a function returns a temporary
Here's the code for the assignment one:
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior⚠️ That last line is undefined behavior, bestie. Don't let your initializer_list go past its lifetime 💀
-Winvalid-imported-macros— verifies that all imported macro definitions are valid at the end of compilation 🔍 This one is not enabled by default, because it needs additional processing to figure that out. It may be useful when you're preparing sets of header-units to make sure the macros are consistent 🧚-Wno-literal-suffix(C++ and Objective-C++ only) — doesn't warn when a string or character literal is followed by a ud-suffix that doesn't begin with an underscore 🌈 As a conforming extension, GCC treats these suffixes as separate preprocessing tokens so that old code using formatting macros from<inttypes.h>keeps working. Like this:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}In this case, PRId64 is treated as a separate preprocessing token 🦋
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization, so these warnings are enabled by default ⚠️
-Wno-narrowing(C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as the standard requires 📏 A narrowing conversion from a constant gives you an error, and a narrowing conversion from a non-constant gives you a warning.-Wno-narrowingsuppresses that diagnostic. Heads up: this doesn't change the meaning of well-formed code, since narrowing conversions are still considered ill-formed in SFINAE contexts 🙅♀️
With -Wnarrowing in C++98, you get a warning when a narrowing conversion that C++11 prohibits happens within { }, e.g.:
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat 💖
-Wnoexcept(C++ and Objective-C++ only) — warns when anoexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()or"noexcept"), but the compiler knows it never throws an exception 🤔-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature that makesnoexceptpart of a function type changes the mangled name of a symbol compared to C++14 🧬 This one is enabled by-Wabiand-Wc++17-compat. Here's an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept> ✨
-Wclass-memaccess(C++ and Objective-C++ only) — okay bestie, this one is ALARM 🚨 It warns when you hand a raw memory function likememsetormemcpya class-type object as its destination, and writing into it could bypass the class's non-trivial or deleted constructor or copy assignment, break const-correctness or encapsulation, or corrupt virtual table pointers. Messing with the raw bytes of these objects can violate invariants the class's member functions are supposed to keep 🙈 For example, thememsetcall below is undefined because it modifies a non-trivial class object, so it gets diagnosed 💀
std::string str = "abc";
memset (&str, 0, sizeof str);The slay way to initialize or clear the storage of these objects is to use the appropriate constructor or assignment operator, if one is available 💖 The -Wclass-memaccess option is enabled by -Wall. Casting the pointer to the class object to void *, or to a type that the raw memory function can safely access, suppresses the warning 🎀
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either on itself or in an accessible polymorphic base class. That makes it possible, but unsafe, to delete a derived-class instance through a pointer to the class or base class 😬 This warning is automatically enabled if-Weffc++is specified 🦋-Wregister(C++ and Objective-C++ only) — warns on uses of theregisterstorage class specifier, except when it's part of the GNU Explicit Register Variables extension. Usingregisteras a storage class specifier was deprecated in C++11 and removed in C++17. It's enabled by default with-std=c++17✨-Wreorder(C++ and Objective-C++ only) — warns when the order of member initializers in the code doesn't match the order they actually have to run in. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning to that effect 🧚 This warning is enabled by -Wall.
-Wno-pessimizing-move(C++ and Objective-C++ only) — this one warns when a call tostd::moveprevents copy elision 🙅♀️ The classic situation where copy elision can happen is returning from a function with a class return type, when the returned expression is the name of a non-volatile automatic object that isn't a function parameter and has the same type as the function's return type. Here's the vibe:
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, the std::move call prevents copy elision 😭 This warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — this one warns about redundant calls tostd::move, which means cases where a move operation would have happened even without thestd::movecall 🌸 That's because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}Here, the std::move call is redundant 💅 Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}In this example, copy elision isn't applicable because the type of the returned expression and the function's return type differ. Even so, G++ treats the return value as if it were designated by an rvalue, which is the main character plot twist 👑 This warning is enabled by -Wextra.
-Wrange-loop-construct(C++ and Objective-C++ only) — this one warns when a C++ range-based for loop is making an unnecessary copy 🍓 That can happen when the range declaration isn't a reference, but honestly it probably should be. For example:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.
This warning also fires when a loop variable in a range-based for-loop is initialized with a value of a different type, which causes a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, every iteration creates and destroys a temporary value of type double, and the reference const double & is bound to that temporary 🥲 This warning is enabled by -Wall.
-Wredundant-tags(C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types, in contexts where the key can be dropped without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedOn the other hand, there's no warning in this example, because the keyword is actually needed there 🙈
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessary-Wno-subobject-linkage(C++ and Objective-C++ only) — don't warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage 🔐 If a type A depends on a type B with no linkage or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. If A only shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. The compiler doesn't give this warning for types defined in the main.Cfile, since those are unlikely to have multiple definitions.-Wsubobject-linkageis enabled by default 💜-Weffc++(C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books 📚- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have
operator=return a reference to*this. - Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload
&&,||, or,.
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of a virtual destructor in accessible non-polymorphic base classes too 🫶
When you select this option, be aware that the standard library headers don't obey all of these guidelines. Use grep -v to filter out those warnings, bestie 🧹
-Wno-exceptions(C++ and Objective-C++ only) — disables the warning for when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers 🙃-Wstrict-null-sentinel(C++ and Objective-C++ only) — warns about using an uncastedNULLas a sentinel. When compiling only with GCC, this is a valid sentinel, becauseNULLis defined to__null. Although it's a null pointer constant rather than a null pointer, it's guaranteed to be the same size as a pointer. But this use isn't portable across different compilers, so watch out 🍑-Wno-non-template-friend(C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate the ISO standard implementation, declarations such asfriend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function. The warning exists to diagnose those compatibility problems, and it's enabled by default 🕰️-Wold-style-cast(C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used in a C++ program. The new-style casts (dynamic_cast,static_cast,reinterpret_cast, andconst_cast) are less vulnerable to unintended effects and much easier to search for 🔍-Woverloaded-virtual(C++ and Objective-C++ only) — warns when a function declaration hides virtual functions from a base class. For example:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};The A class version of f is hidden in B, so code like this:
B* b;
b->f();fails to compile 😤
-Wno-pmf-conversions(C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function to a plain pointer 🧩-Wsign-promo(C++ and Objective-C++ only) — warns when overload resolution picks a promotion from an unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior 📜-Wtemplates(C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates, and this can be used to enforce that rule 🚫 The warning is inactive inside a system header file, such as the STL, so you can still use the STL 🫧 You're also allowed to instantiate or specialize templates.-Wno-mismatched-new-delete(C++ and Objective-C++ only) — okay bestie, this one is all about catching mix-ups betweenoperator newandoperator delete🧚. It warns when those calls don't match the allocation or deallocation function they go with. That includes C++operator deletegetting a pointer from a mismatched form ofoperator new, or from some other function that allocates objectsoperator deleteisn't a suitable deallocator for. It also flags other deallocation functions handed a pointer fromoperator newwhen they aren't the right match 💅.
For example, the delete expression in the function below gets flagged because it doesn't match the array form of the new expression the pointer came from. The free call gets flagged too 🫶.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}The related option -Wmismatched-dealloc handles mismatches involving allocation and deallocation functions other than operator new and operator delete 🌸.
-Wmismatched-new-delete is enabled by default ✨.
-Wmismatched-tags(C++ and Objective-C++ only) — this one warns when a struct, class, or class template (or one of its specializations) gets declared with a class-key that doesn't match its definition, or its first declaration if there's no definition yet 🎀. Keeping the vibes consistent, bestie.
In the example below, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or swap it for class to match the definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);It's not wrong to declare a class with the class-key struct, as shown above. -Wmismatched-tags just exists to help you achieve a consistent style of class declarations 💅. In code meant to be portable to Windows-based compilers, the warning also helps prevent unresolved references caused by the difference in the mangling of symbols declared with different class-keys 🔥. The option can be used on its own or together with -Wredundant-tags.
-Wmultiple-inheritance(C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes 🦋. Some coding rules ban multiple inheritance, so this helps enforce that rule. The warning is inactive inside a system header file, like the STL, so you can still use the STL 💖. You're also allowed to define classes that indirectly use multiple inheritance.-Wvirtual-inheritance— warns when a class is defined with a virtual direct base class 🌈. Same deal as above: some coding rules disallow multiple inheritance, and this can be used to enforce that rule. It's inactive inside a system header file like the STL, so the STL is still good to go ✨. You can also define classes that indirectly use virtual inheritance.-Wno-virtual-move-assign— suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator 🙈.
Heads up, bestie: this is dangerous 💀. If the virtual base can be reached along more than one path, it gets moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, though, this warning can be disabled safely 🫶.
-Wnamespaces— warns when a namespace definition is opened 🌸. Some coding rules disallow namespaces, so this helps enforce that. It's inactive inside system header files like the STL, so the STL keeps working. You can still use using directives and qualified names, too 🧚.-Wno-terminate(C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call toterminate💅.-Wno-vexing-parse(C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity 🤯. This is when a declaration looks like a variable definition, but C++ requires it to be read as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}The warning suggests ways to deal with the ambiguity, like removing the parentheses or using braces instead 💖. This warning is enabled by default ✨.
-Wno-class-conversion(C++ and Objective-C++ only) — don't warn when a conversion function converts an object to the same type, to a base class of that type, or tovoid🦋. Such a conversion function will never be called, so this is one for the "I know what I'm doing" crowd 💅.-Wvolatile(C++ and Objective-C++ only) — warns about deprecated uses of thevolatilequalifier 🔥. That includes postfix and prefix++and--expressions ofvolatile-qualified types, simple assignments where the left operand is avolatile-qualified non-class type (for their value), compound assignments where the left operand is avolatile-qualified non-class type,volatile-qualified function return types,volatile-qualified parameter types, and structured bindings of avolatile-qualified type. This usage was deprecated in C++20 🙈.
Enabled by default with -std=c++20 ✨.
-Wzero-as-null-pointer-constant(C++ and Objective-C++ only) — warns when a literal0is used as a null pointer constant 🌈. This can be super helpful when you're converting over tonullptrin C++11 💖.-Waligned-new— warns about a new-expression of a type that needs greater alignment thanalignof(std::max_align_t), but uses an allocation function without an explicit alignment parameter 🎀. This option is enabled by-Wall✨.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions 🧚.
-Wno-placement-new,-Wplacement-new=n— warns about placement new expressions with undefined behavior 🙈, like constructing an object in a buffer that's smaller than the object's type. For example, the placement new expression below gets diagnosed because it tries to construct an array of 64 integers in a buffer that's only 64 bytes large 💀.
char buf [64];
new (buf) int[64];This warning is enabled by default 🫶.
-Wplacement-new=1— this is the default warning level of-Wplacement-new🌸. At this level, the warning isn't issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, thenewexpression below isn't diagnosed at this level, even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array 🫶.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();-Wplacement-new=2— at this level, GCC diagnoses all the same constructs as level 1, plus placement new expressions that construct an object in the last member of a structure whose type is an array of a single element, when that size is less than the size of the object being constructed 🔥. The previous example gets diagnosed at this level. The construct below uses the flexible member array extension to avoid the warning at level 2 💖.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value,-Wcatch-value=n(C++ and Objective-C++ only) — warns about catch handlers that don't catch via reference 🦋. With-Wcatch-value=1(or just-Wcatch-value), it warns about polymorphic class types that are caught by value. With-Wcatch-value=2, it warns about all class types caught by value. With-Wcatch-value=3, it warns about all types that aren't caught by reference 👑.-Wcatch-valueis enabled by-Wall✨.-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported constructs (C++11[intro.defs]) 🌈.-Wno-delete-incomplete(C++ and Objective-C++ only) — don't warn when deleting a pointer to an incomplete type, which may cause undefined behavior at runtime 💀. This warning is enabled by default, so this option is how you turn it off 🙈.-Wextra-semi(C++, Objective-C++ only) — warns about redundant semicolons after in-class function definitions 🧚.-Wno-inaccessible-base(C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it because of ambiguity 🌸. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the-Wextraoption. Here's the ambiguity in action:
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses warnings about using C++11 inheriting constructors when the base class being inherited from has a C variadic constructor 💅. The warning is on by default because the ellipsis is not inherited 🎀.-Wno-invalid-offsetof(C++ and Objective-C++ only) — suppresses warnings from applying theoffsetofmacro to a non-POD type 🙈. According to the 2014 ISO C++ standard, applyingoffsetofto a non-standard-layout type is undefined. In existing C++ implementations, though,offsetoftypically gives meaningful results. This flag is for users who know they're writing nonportable code and have deliberately chosen to ignore the warning 💖.
The restrictions on offsetof may be relaxed in a future version of the C++ standard 🌈.
-Wsized-deallocation(C++ and Objective-C++ only) — warns about a definition of an unsized deallocation function 🦋.
Okay so this is where the warning list gets spicy 🌶️ GCC is watching for a class that defines one of these deallocators:
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;...but doesn't define the matching sized one:
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;...or the other way around, bestie 💀 It's enabled by -Wextra along with -fsized-deallocation 🧚
-Wsuggest-final-types— Okay, this one's for types with virtual methods that would look cleaner marked with the C++11"final"specifier. If it's possible, it'll also suggest putting the type in an anonymous namespace instead 🦋 Doing that lets GCC devirtualize polymorphic calls way more aggressively, which is so big brain of it 🧠 This warning hits harder with link-time optimization, since GCC has a fuller picture of the class hierarchy then ✨-Wsuggest-final-methods— Same energy, but for individual virtual methods 💅 It'll nudge you to mark a method"final", or, if it's possible, put its type in an anonymous namespace or give it the"final"specifier. Also works better with link-time optimization for the same reason, since the class hierarchy graph is more complete there 🌈 Pro tip (from the manual, not me 🙈): look at-Wsuggest-final-typessuggestions first, then rebuild with the new annotations.-Wsuggest-override— Warns when you override a virtual function without slapping the"override"keyword on it 🫶 No more sneaky overrides, periodt 💖-Wuseless-cast(C++ and Objective-C++ only) — Warns when an expression gets cast to its own type. Literally pointless, so GCC will call it out 💀-Wno-conversion-null(C++ and Objective-C++ only) — Turns off the warning for conversions between"NULL"and non-pointer types. Heads up,-Wconversion-nullis on by default 👑
🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋
NOTE: This manual doesn't cover the Objective-C and Objective-C++ languages themselves, okay? This section is just about the command-line options that only matter for Objective-C and Objective-C++ programs. You can still use most of the language-independent GNU compiler options too 💜 For example, compiling a file called some_class.m might look like this:
gcc -g -fgnu-runtime -O -c some_class.mIn that example, -fgnu-runtime is a flag meant only for Objective-C and Objective-C++ programs, but you can use the other options with any language GCC supports 🌸
Since Objective-C is an extension of C, Objective-C compiles can also use C front-end options (like -Wtraditional). Similarly, Objective-C++ compiles can use C++-specific options (like -Wabi) 🎀
Here's the list of options that are only for compiling Objective-C and Objective-C++ programs 👇
-fconstant-string-class=class-name— Picks the class that gets instantiated for every literal string written as"@"...""🍓 The default is"NXConstantString"when the GNU runtime is in use, and"NSConstantString"when the NeXT runtime is in use (more on that below). On Darwin (macOS, MacOS X), if-fconstant-cfstringsis also there, it overrides-fconstant-string-classand makes"@"...""literals get laid out as constant CoreFoundation strings ✨ Also,-fconstant-cfstringsis just an alias for the target-specific-mconstant-cfstringsequivalent.-fgnu-runtime— Generates object code that works with the standard GNU Objective-C runtime 🌈 This is the default for most systems.-fnext-runtime— Generates output compatible with the NeXT runtime 🍎 This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro__NEXT_RUNTIME__is predefined if, and only if, this option is used.-fno-nil-receivers— Assumes that every Objective-C message dispatch ([receiver message:arg]) in this translation unit makes sure the receiver isn'tnil🫧 That lets the runtime use more efficient entry points. This is only available with the NeXT runtime and ABI version 0 or 1.-fobjc-abi-version=n— Uses versionnof the Objective-C ABI for the runtime you picked. Right now this is only supported for the NeXT runtime, where:- Version 0 is the traditional (32-bit) ABI, no properties or other Objective-C 2.0 extras 🙃
- Version 1 is the traditional (32-bit) ABI with properties and other Objective-C 2.0 additions 💅
- Version 2 is the modern (64-bit) ABI 👑
- If you don't specify anything, the default is Version 0 on 32-bit target machines and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors— For each Objective-C class, GCC checks whether any of its instance variables is a C++ object with a non-trivial default constructor. If so, it synthesizes a special- (id) .cxx_constructinstance method that runs those non-trivial default constructors on those instance variables, in order, then returnsself🧚 It also checks for instance variables that are C++ objects with a non-trivial destructor, and if it finds any, it synthesizes a- (void) .cxx_destructmethod that runs all those default destructors in reverse order 💀
The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way only touch instance variables declared in the current Objective-C class, not the ones inherited from superclasses. It's the Objective-C runtime's job to call all of these methods across an object's inheritance hierarchy 🫶 The - (id) .cxx_construct methods get invoked by the runtime right after a new object instance is allocated, and the - (void) .cxx_destruct methods get invoked right before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later can invoke the - (id) .cxx_construct and - (void) .cxx_destruct methods 🍏
-fobjc-direct-dispatch— Allows fast jumps to the message dispatcher ⚡ On Darwin, this is done through the comm page.-fobjc-exceptions— Turns on syntax support for structured exception handling in Objective-C, kind of like C++ 🫧 You need this to use the Objective-C keywords@try,@throw,@catch,@finally, and@synchronized. It works with both the GNU runtime and the NeXT runtime, but not with the NeXT runtime on Mac OS X 10.2 and earlier 🙈-fobjc-gc— Enables garbage collection (GC) in Objective-C and Objective-C++ programs 🗑️ This is only available with the NeXT runtime. The GNU runtime has its own GC implementation that doesn't need any special compiler flags.-fobjc-nilcheck— For the NeXT runtime with version 2 of the ABI, checks for anilreceiver in method invocations before the actual method call happens 🛡️ This is the default, and you can turn it off with-fno-objc-nilcheck. Class methods and super calls are never checked fornilthis way, no matter what this flag is set to. Right now this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is in use.-fobjc-std=objc1— Makes your code follow the language syntax of Objective-C 1.0, the one GCC 4.0 recognized 📜 This only changes the Objective-C additions to C/C++. It doesn't touch conformance to C/C++ standards, which is handled by the separate C/C++ dialect option flags. With this on, any Objective-C syntax GCC 4.0 doesn't recognize gets rejected, so it's super useful if you need your code to still compile with older versions of GCC 🕰️-freplace-objc-classes— Emits a special marker tellingld(1)not to statically link the resulting object file, and letsdyld(1)load it at run time instead 🦋 This pairs with the Fix-and-Continue debugging mode, where the object file can be recompiled and dynamically reloaded during execution without restarting the whole program. Right now Fix-and-Continue only works with the NeXT runtime on Mac OS X 10.3 and later.-fzero-link— When compiling for the NeXT runtime, the compiler usually swaps out calls toobjc_getClass("...")(when the class name is known at compile time) for static class references that get initialized at load time, which is faster at run time 🚀 Using-fzero-linkturns that off, so calls toobjc_getClass("...")stick around. This is handy in Zero-Link debugging mode, since it lets you modify individual class implementations while the program is running. The GNU runtime always keeps calls toobjc_get_class("...")no matter what command-line options you pass 🤷♀️-fno-local-ivars— By default, instance variables in Objective-C can be accessed as if they were local variables inside the methods of the class they're declared in 🌸 That can cause shadowing between instance variables and other variables with the same name, either declared locally inside a class method or globally. Specifying-fno-local-ivarsturns this behavior off, so you dodge those shadowing issues 💖-fivar-visibility=[public|protected|private|package]— Sets the default instance variable visibility to whatever option you pick 🎀 So instance variables declared outside the scope of any access modifier directives default to that visibility.-gen-decls— Dumps the interface declarations for all classes seen in the source file into a file namedsourcename.decl📄-Wassign-intercept(Objective-C and Objective-C++ only) — Warns whenever an Objective-C assignment gets intercepted by the garbage collector 🗑️✨-Wno-property-assign-default(Objective-C and Objective-C++ only) — Stops the warning for Objective-C object properties that don't have assign semantics specified 🌈-Wno-protocol(Objective-C and Objective-C++ only) — okay so if a class says it implements a protocol, GCC warns you for every protocol method the class doesn't implement 🫶 By default it also warns about methods the class doesn't explicitly implement, even when the superclass already handles them 🙄 With-Wno-protocol, inherited methods from the superclass count as implemented, so no warning for those ✨-Wobjc-root-class(Objective-C and Objective-C++ only) — warns if a class interface has no superclass 💅 Most classes inherit from"NSObject"(or"Object"), for example. If you're making a root class on purpose, you can shush the warning by marking its interface with__attribute__((objc_root_class))🌸-Wselector(Objective-C and Objective-C++ only) — warns if multiple methods of different types show up for the same selector during compilation 🔍 This check runs on the method list in the final stage of compilation. It also checks each selector used in a@selector(...)expression when a matching method was found during compilation. Since these checks only scan the method table at the very end, the warnings won't appear if the final stage never happens, like when an error stops things early or you're using-fsyntax-only🙃-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns if a selector has multiple methods with different argument and/or return types when you try to send a message with it to a receiver of typeidorClass👀 When this flag is off (the default), the compiler skips these warnings if the differences are only between types with the same size and alignment 🤷♀️-Wundeclared-selector(Objective-C and Objective-C++ only) — warns when a@selector(...)expression refers to an undeclared selector 📜 A selector counts as undeclared if no method with that name was declared before the@selector(...)expression, either explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. This option checks the moment it spots a@selector(...)expression, while-Wselectoronly checks in the final stage. It also enforces the coding style rule that methods and selectors must be declared before they're used 💖-print-objc-runtime-info— generates a C header describing the largest structure that gets passed by value, if there is one 🧚
✨ Options to Control Diagnostic Messages Formatting ✨
Back in the day, diagnostic messages got formatted without caring what the output device looked like (its width, ...) 🙈 These options let you control the formatting algorithm, like how many characters go on a line and how often source location info gets reported 📏 Heads up: some language front ends might not honor these options 🫠
-fmessage-length=n— tries to format error messages so they fit on lines about n characters wide 📏 If n is zero, no line-wrapping happens and each error message sits on a single line. That's the default for all front ends 💅
Note - this option also affects the display of the#errorand#warningpre-processor directives, and the deprecated function/type/variable attribute. It does not however affect thepragma GCC warningandpragma GCC errorpragmas.
-fdiagnostics-plain-output— requests that diagnostic output look as plain as possible 🧼 This is handy when you're running dejagnu or other tools that parse diagnostics and need the output to stay stable over time 🫶 Right now it's equivalent to these options:
-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never -fdiagnostics-path-format=separate-eventsIn the future, if GCC changes the default look of its diagnostics, the matching option to turn off that new behavior will get added to this list 🔮
-fdiagnostics-show-location=once— only matters in line-wrapping mode 🎀 It tells the reporter to emit source location info just once. If a message is too long and has to wrap, the location prefix won't get stuck on every continuation line again and again. This is the default 👑-fdiagnostics-show-location=every-line— only matters in line-wrapping mode 🦋 It tells the reporter to put the same source location prefix on every physical line that results from breaking up a message that's too long to fit on one line 💜-fdiagnostics-color[=WHEN],-fno-diagnostics-color— turns on color in diagnostics 🌈WHENcan benever,always, orauto. The default depends on how the compiler was configured. It can be any of thoseWHENoptions, orneverif theGCC_COLORSenvironment variable isn't present, andautootherwise.autoonly uses color when standard error is a terminal and you're not running in an emacs shell 🌸 The forms-fdiagnostics-colorand-fno-diagnostics-colorare aliases for-fdiagnostics-color=alwaysand-fdiagnostics-color=never, respectively 🔥
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings 🎨 SGR commands are interpreted by the terminal or terminal emulator, so check the docs for your text terminal to see the permitted values and what they mean as character attributes 📖 These substring values are integers in decimal and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors 💖
The default GCC_COLORS is 🌈
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors ✨ Supported capabilities are as follows 💅
error=— the SGR substring forerror:markers 💥warning=— the SGR substring forwarning:markers ⚠️note=— the SGR substring fornote:markers 📝path=— the SGR substring for colorizing paths of control-flow events printed via-fdiagnostics-path-format=, like the identifiers of individual events and the lines showing interprocedural calls and returns 🛤️range1=— the SGR substring for the first additional range 1️⃣range2=— the SGR substring for the second additional range 2️⃣locus=— the SGR substring for location info, likefile:lineorfile:line:columnetc. 📍quote=— the SGR substring for info printed within quotes 💬fixit-insert=— the SGR substring for fix-it hints suggesting text to be inserted or replaced ➕fixit-delete=— the SGR substring for fix-it hints suggesting text to be deleted ➖diff-filename=— the SGR substring for filename headers within generated patches 📄diff-hunk=— the SGR substring for the starts of hunks within generated patches 🧩diff-delete=— the SGR substring for deleted lines within generated patches 🗑️diff-insert=— the SGR substring for inserted lines within generated patches 🌱type-diff=— the SGR substring for highlighting mismatching types within template arguments in the C++ frontend 🔍
-fdiagnostics-urls[=WHEN]— uses escape sequences to embed URLs in diagnostics 🔗 For example, when-fdiagnostics-show-optionshows the command-line option controlling a diagnostic, GCC embeds a URL pointing to the docs for that option 📚
WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when standard error is a terminal, and when it's not running in an emacs shell or any graphical terminal known to be incompatible with this feature (see below) 🖥️
The default depends on how the compiler was configured. It can be any of the above WHEN options.
GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so the default is affected by environment variables. Under that configuration, GCC defaults to auto if either the GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are 🧚
However, even with -fdiagnostics-urls=always, the behavior still depends on those environment variables 🙃 If GCC_URLS is set to empty or no, URLs are not embedded in diagnostics. If it's set to st, URLs use ST escape sequences. If it's set to bel, which is the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS isn't set, TERM_URLS is used as a fallback 💜
Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep 🔔
Okay so, GCC also tries to sniff out a few terminals that don't play nice with the URL feature 🙈. Some of them have bugs (or at least had bugs in versions people still use) where the URL escapes are likely to misbehave and print garbage all over your screen 💀. The current hit list is xfce4-terminal, certain known-buggy gnome-terminal versions, the linux console, and mingw 😭. If you're sure your setup is fine, you can skip this check with -fdiagnostics-urls=always ✨
-fno-diagnostics-show-option— By default, every diagnostic includes a little hint naming the command-line option that directly controls it (if GCC knows about that option) 🔍. Add this flag and that hint gets suppressed 🙅.-fno-diagnostics-show-caret— Normally each diagnostic shows the original source line plus a caret^pointing at the column 🎯. This option hides all of that. If you pass-fmessage-length=n, the source line gets truncated toncharacters. When the output is going to a terminal, the width is limited to the width given by theCOLUMNSenvironment variable or, if that isn't set, to the terminal width 📏.-fno-diagnostics-show-labels— When GCC prints source code (via-fdiagnostics-show-caret), diagnostics can slap labels on ranges of code with pertinent info, like expression types. Peep this:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intThis option kills those labels, so no vertical bars and no "char *" or "long int" text in the example above 🏷️🚫.
-fno-diagnostics-show-cwe— Diagnostics can optionally carry a CWE identifier (seehttps://cwe.mitre.org/index.html) 🛡️. GCC itself only provides this metadata for some of the-fanalyzerdiagnostics, and GCC plugins may provide it too. By default, if the info is there, it gets printed along with the diagnostic. This option stops that metadata from showing up.-fno-diagnostics-show-line-numbers— By default, when printing source code (via-fdiagnostics-show-caret), GCC prints a left margin showing line numbers 🔢. This option gets rid of that margin.-fdiagnostics-minimum-margin-width=width— Controls the minimum width of the left margin printed by-fdiagnostics-show-line-numbers📐. It defaults to6.-fdiagnostics-parseable-fixits— Emits fix-it hints in a machine-parseable format, which is perfect for IDEs 🛠️. For each fix-it, a line gets printed after the relevant diagnostic, starting with the stringfix-it:. For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"The location is a half-open range measured in bytes, starting at byte 1 for the initial column. In the example above, bytes 3 through 20 of line 45 of "test.c" get replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_allThe filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", and non-printable characters as octal (e.g. vertical tab as "\013") 🔐.
An empty replacement string means the given range gets removed 🗑️. An empty range (e.g. "45:3-45:3") means the string gets inserted at the given position ➕.
-fdiagnostics-generate-patch— Prints fix-it hints to stderr in unified diff format, after any diagnostics are printed 🩹. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color) 🌈.
-fdiagnostics-show-template-tree— In the C++ frontend, when diagnostics show mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>this flag enables a tree-like structure showing the common and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>The parts that differ are highlighted with color ("double" and "float" in this case) 🌸.
-fno-elide-type— By default, when the C++ frontend prints diagnostics about mismatching template types, the common parts get printed as[...]so the error message stays short 🤏. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying -fno-elide-type suppresses that behavior 🚫. This flag also affects the output of -fdiagnostics-show-template-tree.
-fdiagnostics-path-format=KIND— Specifies how to print paths of control-flow events for diagnostics that have one attached 🛤️.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths.
separate-events means to print a separate note diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1inline-events means to print the events "inline" within the source code. This view tries to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source. For example, the same events as above might be printed as:
'test': events 1-3
|
| 25 | list = PyList_New(0);
| | ^~~~~~~~~~~~~
| | |
| | (1) when 'PyList_New' fails, returning NULL
| 26 |
| 27 | for (i = 0; i < count; i++) {
| | ~~~
| | |
| | (2) when 'i < count'
| 28 | item = PyLong_FromLong(random());
| 29 | PyList_Append(list, item);
| | ~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
|Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped 🪆. For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)-fdiagnostics-show-path-depths— Adds extra info when printing control-flow paths associated with a diagnostic 🧭. If this option is given, the stack depth gets printed for each run of events within-fdiagnostics-path-format=separate-events. It's intended for GCC developers and plugin developers debugging diagnostics that report interprocedural control flow 🧪.-fno-show-column— Don't print column numbers in diagnostics 🙈. This may be necessary if diagnostics are being scanned by a program that doesn't understand column numbers, such as dejagnu.-fdiagnostics-column-unit=UNIT— Selects the units for the column number 🎀. This affects traditional diagnostics (as long as-fno-show-columnisn't in use), as well as JSON format diagnostics if they're requested 💜.
Okay so the default UNIT is display, which means GCC counts how many display columns each character actually takes up 💅. That can be bigger than the bytes needed to encode it (hi, tab characters 🍓) or smaller (multibyte characters are sneaky ✨). Like, GREEK SMALL LETTER PI (U+03C0) takes up one display column but its UTF-8 encoding needs two bytes 🌸. Meanwhile SLIGHTLY SMILING FACE (U+1F642) takes up two display columns and needs four bytes 🙂.
Setting UNIT to byte switches the column number to the raw byte count in every single case. That's what GCC used to output before version 11.1.0, so if you're giving old-school vibes 🕰️, this is your setting 💖.
🎀 -fdiagnostics-column-origin=ORIGIN
This picks the origin for column numbers, which is basically the number given to the first column 🔢. The default is 1, which matches traditional GCC behaviour and the GNU style guide 📖. Some utilities work better with an origin of 0, so any non-negative value is allowed 🦋.
🦋 -fdiagnostics-format=FORMAT
This changes how diagnostics get printed. FORMAT is either text or json, and the default is text 💅.
The json format is a top-level JSON array full of JSON objects, one for each diagnostic 🧚.
The JSON gets emitted as one single line with no formatting at all 😭, so the examples below have been dressed up for clarity ✨.
Diagnostics can have child diagnostics too. For example, this error and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"display-column": 3,
"byte-column": 3,
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"display-column": 4,
"byte-column": 4,
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"display-column": 5,
"byte-column": 5,
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"message": "...this statement, but the latter is ..."
}
]
"column-origin": 1,
},
...
]where the "note" is a child of the "warning" 🫶.
A diagnostic has a "kind". If this is "warning", then there is an "option" key describing the command-line option controlling the warning 🔥.
A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to three positions inside it: a "caret" position and optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers indicating a column position 📍:
"display-column"counts display columns, accounting for tabs and multibyte characters."byte-column"counts raw bytes."column"is equal to one of the previous two, as dictated by the-fdiagnostics-column-unitoption.
All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns from 0 🌈. The column origin is recorded in the JSON output in the "column-origin" tag. In the remaining examples below, the extra column number outputs have been left out for brevity 🙈.
For example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}has three locations 👑. Its primary location is at the + token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, and those have labels. It might be printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"message": "invalid operands to binary + ..."
}If a diagnostic contains fix-it hints, it has a "fixits" array, made of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits 🩹. For example, this diagnostic with a replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| colormight be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"message": "\u2018struct s\u2019 has no member named ..."
}where the fix-it hint suggests replacing the text from "start" up to but not including "next" with the value of "string". Deletions are expressed via an empty value for "string", and insertions by having "start" equal "next" ✨.
If the diagnostic has a path of control-flow events associated with it, it has a "path" array of objects representing those events 🦋. Each event object has a "description" string and a "location" object, along with a "function" string and a "depth" number for representing interprocedural paths. The "function" represents the current function at that event, and the "depth" represents the stack depth relative to some baseline: the higher the number, the more frames are within the stack 🌸.
For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:
"path": [
{
"depth": 0,
"description": "when 'PyList_New' fails, returning NULL",
"function": "test",
"location": {
"column": 10,
"file": "test.c",
"line": 25
}
},
{
"depth": 0,
"description": "when 'i < count'",
"function": "test",
"location": {
"column": 3,
"file": "test.c",
"line": 27
}
},
{
"depth": 0,
"description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
"function": "test",
"location": {
"column": 5,
"file": "test.c",
"line": 29
}
}
]✨ Options to Request or Suppress Warnings ✨
okay so warnings are the compiler's little "hmm, are you sure about that?" messages 🙈 They're for constructions that aren't technically wrong, but are risky or might mean you goofed somewhere. 💅
The options below are language-independent, which means they don't switch on any specific warning. Instead, they control what kinds of diagnostics GCC produces. 🫶
-fsyntax-only— bestie, this just checks your code for syntax errors and then stops right there. Nothing extra, no drama. 🧐-fmax-errors=n— the vibe check for error floods 😭 Cap the number of error messages at n, and GCC bails out instead of trying to keep going through the source. The default is 0, which means no limit at all, so the errors just keep coming. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option 👑-w— mutes every single warning message. Total silence, no cap. 🤫-Werror— turns every warning into an actual error. Main character energy, but make it strict 🚨-Werror=— targets one specific warning and makes it an error. You append the warning's name, so-Werror=switchturns the warnings controlled by-Wswitchinto errors 🔥 There's a negative form too:-Wno-error=switchmeans-Wswitchwarnings are not errors, even when-Werroris in effect 💀
Each controllable warning's message includes the option that controls it, and you can plug that option right into -Werror= and -Wno-error= like we just covered 🔍 Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag if you want it hidden 🙈
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. 🧠
-Wfatal-errors— the compiler aborts on the very first error it hits, instead of pushing on and printing more. One and done 💅
You can request many specific warnings with options beginning with -W. For example, -Wimplicit requests warnings on implicit declarations 🌸 Each of these specific warning options also has a negative form beginning -Wno- to turn the warnings off, like -Wno-implicit. This manual only lists one of the two forms, whichever one is not the default. For further language-specific options, also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position on the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line. ✨
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. 🤨 However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This lets you use new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present 🦋
The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization, and -Wmaybe-uninitialized does not warn at all unless optimization is enabled. ⚡
-Wpedantic,-pedantic— issue all the warnings demanded by strict ISO C and ISO C++, reject all programs that use forbidden extensions, and reject some other programs that don't follow ISO C and ISO C++ 📏 For ISO C, it follows the version of the ISO C standard specified by any-stdoption used.
Valid ISO C and ISO C++ programs should compile properly with or without this option, though a rare few require -ansi or a -std option specifying the required version of ISO C. However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected. 🙅♀️
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system header files should use these escape routes; application programs should avoid them. 🚫
Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all. It only catches the ones for which ISO C requires a diagnostic, plus some others for which diagnostics have been added. 🥲
A feature to report any failure to conform to ISO C might be useful in some instances, but it would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future. 🫠
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard: the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) 🌈
-pedantic-errors— gives an error whenever the base standard (see-Wpedantic) requires a diagnostic. That includes some cases where there is undefined behavior at compile time, and some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to-Werror=pedantic, since there are errors enabled by this option and not enabled by the latter, and vice versa. 🛑-Wall— enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros 💖 This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Warray-parameter=2 (C
and Objective-C only) -Wbool-compare -Wbool-operation -Wc++11-compat
-Wc++14-compat -Wcatch-value (C++ and Objective-C++ only)
-Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and
Objective-C only) -Wenum-compare (in C/ObjC; this is on by default
in C++) -Wformat -Wformat-overflow -Wformat-truncation
-Wint-in-bool-context -Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only) -Winit-self
(only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and
unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation (only for C/C++)
-Wmissing-attributes -Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull
-Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move
(only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++)
-Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare
(only in C++) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvla-parameter (C and Objective-C only)
-Wvolatile-register-var -Wzero-length-boundsNote that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which you might occasionally wish to check for. Others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra, but many of them must be enabled individually. 🕵️♀️
-Wextra— enables some extra warning flags that are not enabled by-Wall. (This option used to be called-W. The older name is still supported, but the newer name is more descriptive 🎀)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wenum-conversion (C only) -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C only)
-Woverride-init -Wsign-compare (C only) -Wstring-compare
-Wredundant-move (only for C++) -Wtype-limits -Wuninitialized
-Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)Okay bestie, -Wextra is giving extra 💅 and also flags these sneaky cases 👀:
- A pointer is compared against integer zero with
<,<=,>, or>=🧐 - (C++ only) An enumerator and a non-enumerator both appear in a conditional expression 🌸
- (C++ only) Ambiguous virtual bases 🌀
- (C++ only) Subscripting an array that has been declared
register🙈 - (C++ only) Taking the address of a variable that has been declared
register💀 - (C++ only) A base class is not initialized in the copy constructor of a derived class 😭
🦋 -Wabi (C, Objective-C, C++ and Objective-C++ only)
This one is all about ABI changes. It warns about code that gets affected by them, including code that might not be compatible with the vendor-neutral C++ ABI, plus the psABI for your particular target 🎀.
Since G++ now defaults to updating the ABI with each major release, normally -Wabi only warns about C++ ABI compatibility problems if there's a check added later in a release series for an ABI issue found after the initial release. -Wabi warns about more things if an older ABI version is selected with -fabi-version=n.
You can also give -Wabi an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level. For example, -Wabi=2 warns about changes relative to -fabi-version=2 ✨.
If you give an explicit version number and don't specify -fabi-compat-version, the version number from this option gets used for compatibility aliases. If you don't give a version number with this option but do specify -fabi-compat-version, that version number is used for C++ ABI warnings.
Now the honest tea ☕: GCC tried super hard to warn about every case, but there are probably some cases that slip through even though G++ is generating incompatible code 😬. There may also be cases where warnings pop up even though the generated code is actually compatible.
If you're worried that G++ output might not be binary compatible with code from other compilers, rewrite your code to dodge these warnings 🫶.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly 💔:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This got fixed in -fabi-version=3 🩹.
- SIMD vector types declared using
__attribute ((vector_size))were mangled in a non-standard way, which doesn't allow for overloading of functions taking vectors of different sizes 🙃.
The mangling changed in -fabi-version=4 🎉.
__attribute ((const))andnoreturnwere mangled as type qualifiers, anddecltypeof a plain declaration was folded away 🫠.
These mangling issues got fixed in -fabi-version=5 💅.
- Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
va_argto complain. On most targets this doesn't actually affect the parameter passing ABI, since there's no way to pass an argument smaller thanint🤷♀️.
The ABI also changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument 🌀.
These issues were corrected in -fabi-version=6 ✨.
- Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
nullptr_t🙈.
These issues were corrected in -fabi-version=7 🌷.
- When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate 😵💫.
This was fixed in -fabi-version=8, the default for GCC 5.1 🎀.
decltype(nullptr)incorrectly had an alignment of 1, leading to unaligned accesses. This didn't affect the ABI of a function with anullptr_tparameter, since parameters have a minimum alignment 🧠.
This was fixed in -fabi-version=9, the default for GCC 5.2 🍓.
- Target-specific attributes that affect the identity of a type, like ia32 calling conventions on a function type (stdcall, regparm, etc.), didn't affect the mangled name. That led to name collisions when function pointers were used as template arguments 💥.
This was fixed in -fabi-version=10, the default for GCC 6.1 👑.
This option also turns on warnings about psABI-related changes 🧚. The known psABI changes at this point include:
- For SysV/x86-64, unions with
long doublemembers are passed in memory as specified in psABI. Before GCC 4.4 this wasn't the case 🌈. For example:
union U {
long double ld;
int i;
};union U is now always passed in memory, no cap 🫶.
🍓 -Wchar-subscripts
Warn if an array subscript has type char. This is a super common source of errors, because programmers often forget that this type is signed on some machines 🙈. This warning is enabled by -Wall 💖.
🌙 -Wno-coverage-mismatch
Warn if feedback profiles don't match when you use the -fprofile-use option. If a source file changes between compiling with -fprofile-generate and with -fprofile-use, the profile feedback files can fail to match the source file, and GCC can't use the profile feedback information 😩. By default this warning is enabled and is treated as an error.
You can use -Wno-coverage-mismatch to disable the warning, or -Wno-error=coverage-mismatch to disable the error. Disabling the error can lead to poorly optimized code, so it's only useful for very minor changes like bug fixes to an existing code-base 🧹.
Completely disabling the warning is not recommended 🚫.
🌸 -Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only) Suppress warning messages emitted by #warning directives 🤫.
🦄 -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type float is implicitly promoted to double 👀. CPUs with a 32-bit single-precision floating-point unit implement float in hardware, but emulate double in software 💻. On a machine like that, doing computations with double values is way more expensive because of the overhead of software emulation 😤.
It's SO easy to accidentally do computations with double, because floating-point literals are implicitly of type double 🫠. For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler performs the entire computation with double, because the floating-point literal is a double 💅.
🧁 -Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall 🎀.
🪄 -Wformat, -Wformat=n
Check calls to printf and scanf, etc., to make sure the arguments supplied have types that match the format string, and that the conversions in the format string make sense ✨. This covers standard functions, plus 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 get checked without format attributes specified depends on the standard version you picked. Those checks for functions without the attribute are disabled by -ffreestanding or -fno-builtin 🧚.
The formats are checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions 💜. Other library implementations might not support all of these, and GCC doesn't warn about features that go beyond a particular library's limits 🫶.
However, if you use -Wpedantic with -Wformat, you get warnings about format features that aren't in the selected standard version. (This doesn't apply to strfmon formats, since those aren't in any version of the C standard 🙃.)
💖 -Wformat=1, -Wformat
-Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0 🔥. Since -Wformat also checks for null format arguments for several functions, it also implies -Wnonnull 🌷.
Some parts of this level of format checking can be turned off with -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length 🙈. -Wformat is enabled by -Wall 👑.
🌷 -Wformat=2
Enable -Wformat plus additional format checks. Right now it's equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k 💅.
🫧 -Wno-format-contains-nul
If -Wformat is specified, don't warn about format strings that contain NUL bytes 🙅♀️.
🎈 -Wno-format-extra-args
If -Wformat is specified, don't warn about excess arguments to a printf or scanf format function 🌸. The C standard says those extra arguments get ignored, so it's chill 🫶.
Where unused arguments sit between used arguments that are specified with $ operand number specifications, warnings still get given as usual. The implementation can't know what type to pass to va_arg to skip the unused arguments 🤔. However, for scanf formats, this option suppresses the warning if the unused arguments are all pointers, because the Single Unix Specification says those unused arguments are allowed ✨.
🔥 -Wformat-overflow, -Wformat-overflow=level
Warn about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer 😱. When the exact number of bytes written by a format directive can't be figured out at compile-time, it's estimated using heuristics that depend on the level argument and on optimization 🧠.
Turning on optimization will usually make the warning more accurate, but it may also result in false positives 🙃.
-Wformat-overflow,-Wformat-overflow=1— okay so level 1 is the chill one 💅 It's switched on by-Wformatand only yells when a call will most likely overflow the buffer 🫶 Unknown numeric arguments get treated like1, and strings of unknown length get treated as empty. Numbers that are bounded to a subrange of their type, or strings whose output is capped by a precision or a finite set of string literals, get assumed to take whatever value makes the most bytes of output ✨
The sus one here is the sprintf call 👀 Even with a and b both at zero, the terminating NUL ('\0') the function tacks onto the end still gets written past the buffer 😭
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}Bump the buffer up by a single byte and the warning goes away 🎀 but heads up, that might not actually stop the overflow 💀
-Wformat-overflow=2— level 2 is the drama queen 👑 It warns about calls that might overflow given an argument of sufficient length or magnitude. Unknown numeric arguments get assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments that can't be bounded by the directive's precision, a finite set of string literals they might evaluate to, or the character array they might point to, get assumed to be 1 character long 🌸
At level 2 the example call gets flagged again 🙈 This time it's because with a equal to a 32-bit INT_MIN, the first %i directive spills some digits past the end of the buffer. To keep the call safe no matter what the two variables hold, the destination buffer has to be at least 34 bytes. GCC even includes that minimum size in an informational note right after the warning 📝
An alternative to just making the buffer bigger is to constrain the range of the formatted values 🦋 You can bound the maximum length of string arguments by putting a precision in the format directive. If numeric arguments can be assumed to be bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier shrinks the required buffer size. For example, if a and b can be assumed to fit within the precision of the short int type, then either using the %hi directive or casting the argument to short brings the maximum required size down to 24 bytes 💖
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}-Wno-format-zero-length— if-Wformatis on, this tells GCC to chill about zero-length formats 🧚 The C standard says zero-length formats are allowed, so no drama needed.-Wformat-nonliteral— if-Wformatis on, GCC also warns when the format string isn't a string literal, since it can't be checked that way 🔍 The exception is when the format function takes its format arguments as ava_list.-Wformat-security— if-Wformatis on, this one also warns about format function calls that could be a security problem 🔒 Right now it flagsprintfandscanfcalls where the format string isn't a string literal and there are no format arguments, likeprintf (foo);.
Super important: thatprintf (foo);pattern can be a security hole if the format string came from untrusted input and contains%n. This is currently a subset of what-Wformat-nonliteralcatches, but future versions may add-Wformat-securitywarnings that aren't in-Wformat-nonliteral.
-Wformat-signedness— if-Wformatis on, this warns when the format string wants an unsigned argument but you pass a signed one, and vice versa 🌈-Wformat-truncation,-Wformat-truncation=level— this warns about formatted input/output calls likesnprintfandvsnprintfthat might end up truncating their output ✂️ When the exact number of bytes a format directive writes can't be figured out at compile time, GCC estimates it using heuristics that depend on the level argument and on optimization. Turning on optimization usually makes the warning more accurate, but it can also cause false positives. Unless noted otherwise, this option uses the same logic as-Wformat-overflow.-Wformat-truncation,-Wformat-truncation=1— level 1 is conservative 🫶 It only warns about calls to bounded functions whose return value is unused and that will most likely result in truncation.-Wformat-truncation=2— level 2 also warns about calls to bounded functions whose return value is used, if they might truncate given an argument of sufficient length or magnitude 🔥
-Wformat-y2k— if-Wformatis on, this warns aboutstrftimeformats that might only give you a two-digit year 📅 Y2K vibes, no thank you 💀-Wnonnull— warns when you pass a null pointer to an argument that thenonnullfunction attribute says must be non-null 🚫-Wnonnullis included in-Walland-Wformat, and you can turn it off with-Wno-nonnull.-Wnonnull-compare— warns when you compare an argument marked with thenonnullfunction attribute against null inside the function 🙄 It's included in-Wall, and you can disable it with-Wno-nonnull-compare.-Wnull-dereference— warns if the compiler finds paths that trigger erroneous or undefined behavior because a null pointer gets dereferenced 💀 This only works when-fdelete-null-pointer-checksis active, which optimizations turn on for most targets. How precise the warnings are depends on the optimization options you use.-Winit-self(C, C++, Objective-C and Objective-C++ only) — warns about uninitialized variables that get initialized with themselves 🌀 This option can only be used together with-Wuninitialized.
For example, GCC warns about i being uninitialized in this snippet only when -Winit-self is specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++ 👑
-Wno-implicit-int(C and Objective-C only) — controls warnings for when a declaration doesn't specify a type 🧐 This warning is on by default in C99 and later dialects of C, and also turned on by-Wall.-Wno-implicit-function-declaration(C and Objective-C only) — controls warnings for when a function gets used before it's declared 🫠 This warning is on by default in C99 and later dialects of C, and also turned on by-Wall. It gets upgraded to an error by-pedantic-errors.-Wimplicit(C and Objective-C only) — same as-Wimplicit-intand-Wimplicit-function-declarationtogether ✨ This warning is enabled by-Wall.-Wimplicit-fallthrough— this is the same as-Wimplicit-fallthrough=3, and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0🎀-Wimplicit-fallthrough=n— warns when a switch case falls through 🍓 For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}This warning does not go off when the last statement of a case can't fall through, like when there's a return statement or a call to a function declared with the noreturn attribute 🙌 -Wimplicit-fallthrough= also takes control flow statements into account, such as ifs, and only warns when it's appropriate. For example:
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}Sometimes a switch case falling through is exactly what you want 🦋 For those times GCC provides an attribute, __attribute__ ((fallthrough)), which goes along with a null statement to suppress the warning that would otherwise show up:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 gives you a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute 💅 In C++11 or C++14 you can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, you can also add a fallthrough comment to silence the warning 💜 The whole body of the C or C++ style comment has to match the regular expressions listed below. The option argument n specifies what kinds of comments are accepted:
-Wimplicit-fallthrough=0disables the warning altogether 🙅-Wimplicit-fallthrough=1matches the".*"regular expression, so any comment counts as a fallthrough comment 💬-Wimplicit-fallthrough=2case insensitively matches the".*falls?[ \t-]*thr(ough|u).*"regular expression ✨-Wimplicit-fallthrough=3case sensitively matches one of the following regular expressions:
Okay, the =3 list keeps going, bestie 💅 here are the rest of the regexes it'll match:
"-fallthrough"
"@fallthrough@"
"lint -fallthrough[ \t]*"
"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?"
"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?"
"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"-Wimplicit-fallthrough=4 case sensitively matches one of these regexes 🔥:
"-fallthrough"
"@fallthrough@"
"lint -fallthrough[ \t]*"
"[ \t]*FALLTHR(OUGH|U)[ \t]*"-Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments 🙈. Only attributes disable the warning ✨.
The comment needs to be followed, after optional whitespace and other comments, by case or default keywords, or by a user label that precedes some case or default label 🎀. Like this:
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}The -Wimplicit-fallthrough=3 warning is enabled by -Wextra 🧚
-Wno-if-not-aligned(C, C++, Objective-C and Objective-C++ only) — this one controls whether warnings triggered by thewarn_if_not_alignedattribute get issued 🫶. They're on by default ✨.-Wignored-qualifiers(C and C++ only) — warns if a function's return type has a type qualifier likeconst💅. In ISO C that qualifier has no effect, since the value a function returns isn't an lvalue 🙄. In C++ the warning only pops up for scalar types orvoid🌸. ISO C also prohibits qualifiedvoidreturn types on function definitions, so those always get a warning, even without this option 💀.
This warning is also enabled by -Wextra 💖.
-Wno-ignored-attributes(C and C++ only) — controls warnings when an attribute gets ignored 🙈. This is different from-Wattributes🦋, because it fires whenever the compiler decides to drop an attribute.-Wattributesis about the attribute being unknown, used in the wrong place, and so on. This warning is enabled by default 🌈.-Wmain— warns if the type ofmainlooks sus 👀.mainshould be a function with external linkage, returningint, taking either zero arguments, two, or three arguments of appropriate types 👑. This warning is enabled by default in C++, and it's enabled by either-Wallor-Wpedantic💖.-Wmisleading-indentation(C and C++ only) — warns when the indentation of the code doesn't reflect the block structure 😭. Specifically, a warning is issued forif,else,while, andforclauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation 🙈.
In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional 💀:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */With mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up 📏. It defaults to 8 🧚.
The warning is not issued for code involving multiline preprocessor logic, such as this 🧚:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);The warning is also not issued after a #line directive 🙅♀️. That typically means autogenerated code, and no assumptions can be made about the layout of the file that the directive references 🤷♀️.
This warning is enabled by -Wall in C and C++ 💅.
-Wmissing-attributes— warns when a function declaration is missing one or more attributes that a related function is declared with, and whose absence may adversely affect the correctness or efficiency of the generated code 🤓. For example, it's issued for alias declarations that use attributes to specify less restrictive requirements than their targets. That typically represents a potential optimization opportunity 🌸.
The -Wattribute-alias=2 option is the opposite vibe 💀. It controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. The attributes considered are alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice 🎀.
In C++, the warning is issued when an explicit specialization of a primary template declared with alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it 🙈. The deprecated, error, and warning attributes suppress the warning 🫶.
You can use the copy attribute to apply the same set of attributes to a declaration as another declaration has, without explicitly listing them 💅. It can be applied to declarations of functions, variables, or types ✨.
-Wmissing-attributes is enabled by -Wall 🌈.
For example, since the declaration of the primary function template below uses both the malloc and alloc_size attributes, the declaration of the explicit specialization of the template gets diagnosed 😭. It's missing one of the attributes:
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);-Wmissing-braces— warns if an aggregate or union initializer is not fully bracketed 🧸. In the following example, the initializer foraisn't fully bracketed, but the one forbis 💖:
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) — warns if a user-supplied include directory doesn't exist 📂.-Wno-missing-profile— controls warnings when feedback profiles are missing while using the-fprofile-useoption 📈. This one diagnoses cases where a new function or a new file gets added between compiling with-fprofile-generateand with-fprofile-use, without regenerating the profiles 😬. The profile feedback data files then don't contain any profile feedback information for that new function or file. Also, if profile count data (.gcda) files get removed, GCC can't use any profile feedback information at all 💀.
In all of these cases, warnings are issued to let you know a profile generation step is due 🔔. Ignoring the warning can result in poorly optimized code 🥲. -Wno-missing-profile can be used to disable the warning, but that's not recommended, and it should only be done when non-existent profile data is justified 🙅♀️.
-Wno-mismatched-dealloc— warns for calls to deallocation functions with pointer arguments returned from allocation functions that the former isn't a suitable deallocator for 🧹. A pair of functions can be associated as matching allocators and deallocators by using themallocattribute 🔗. Unless-fno-builtindisables it, the standard functionscalloc,malloc,realloc, andfree, along with the corresponding forms of C++operator newandoperator delete, are implicitly associated as matching allocators and deallocators 🫶. In the following example,mydeallocis the deallocator for pointers returned frommyalloc🧚:
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete 🦋.
Option -Wmismatched-dealloc is enabled by default 👑.
-Wmultistatement-macros— warns about unsafe multiple statement macros that appear to be guarded by a clause likeif,else,for,switch, orwhile, where only the first statement is actually guarded after the macro is expanded 😱. For example:
#define DOIT x++; y++
if (c)
DOIT;That will increment y unconditionally, not just when c holds 💀. This can usually be fixed by wrapping the macro in a do-while loop 🎀:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;This warning is enabled by -Wall in C and C++ ✨.
-Wparentheses— warns if parentheses are omitted in certain contexts 🙃, such as when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about 🤔.
It also warns if a comparison like x<=y<=z appears 🚨. That's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from ordinary mathematical notation 😵.
It also warns for dangerous uses of the GNU extension to ?: with the omitted middle operand 😬. When the condition in the ?: operator is a boolean expression, the omitted value is always 1 💀. Programmers often expect it to be a value computed inside the conditional expression instead 🙈.
For C++, this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration 📞:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}This warning is enabled by -Wall 💅.
-Wsequence-point— warns about code that may have undefined semantics because of violations of the sequence point rules in the C and C++ standards 😭🔥.
okay so the C and C++ standards have this whole rule about the order stuff in an expression gets evaluated 🧚, and it's all tied to sequence points ✨. These are checkpoints in the program. Everything before one has been evaluated and everything after it hasn't, which makes it a partial ordering between parts of the program 🙈. Sequence points happen after a full expression is evaluated (one that isn't part of a larger expression), after the first operand of a &&, ||, ? : or , (comma) operator is evaluated, before a function is called (but after its arguments and the expression that names the called function are evaluated), and in a few other spots too. Outside of what the sequence point rules say, the order in which subexpressions of an expression get evaluated is not specified 💀. These rules only give a partial order, not a total one. So if two functions get called inside one expression with no sequence point between them, the order they run in is not specified. The standards committee did rule that function calls don't overlap though 🫶
It's not specified when, between sequence points, changes to object values actually take effect. Programs whose behavior depends on this have undefined behavior 😭 The standards say: "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored."
If a program breaks these rules, the results on any particular implementation are entirely unpredictable. Not cute, bestie 💀
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases aren't diagnosed by this option, and it may give an occasional false positive, but in general it's been found pretty effective at catching this sort of problem in programs 🔥
The C++17 standard is going to define the order of evaluation of operands in more cases 🌈. In particular, it requires that the right-hand side of an assignment gets evaluated before the left-hand side, so the examples above are no longer undefined. But this option will still warn about them, to help people avoid writing code that's undefined in C and in earlier revisions of C++ 💖
The standard is worded confusingly, so there's some debate over the precise meaning of the sequence point rules in subtle cases 😵💫 Links to discussions of the problem, including proposed formal definitions, can be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++. ✨
-Wno-return-local-addr— Don't warn about returning a pointer (or, in C++, a reference) to a variable that goes out of scope after the function returns 🙈-Wreturn-type— Warns whenever a function is defined with a return type that defaults toint. It also warns about anyreturnstatement with no return value in a function whose return type isn'tvoid(falling off the end of the function body counts as returning without a value). For C only, it warns about areturnstatement with an expression in a function whose return type isvoid, unless the expression type is alsovoid. As a GNU extension, that latter case is accepted without a warning unless-Wpedanticis used. Attempting to use the return value of a non-voidfunction other thanmainthat flows off the end by reaching the closing curly brace that terminates the function is undefined 😭 Unlike in C, in C++, flowing off the end of a non-voidfunction other thanmainresults in undefined behavior even when the function's value isn't used. This warning is enabled by default in C++ and by-Wallotherwise. 👑-Wno-shift-count-negative— Controls warnings if a shift count is negative. This warning is enabled by default 💅-Wno-shift-count-overflow— Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default 💅-Wshift-negative-value— Warns if you left shift a negative value. This warning is enabled by-Wextrain C99 (and newer) and C++11 to C++17 modes 🦋-Wno-shift-overflow,-Wshift-overflow=n— These options control warnings about left shift overflows ✨-Wshift-overflow=1— This is the warning level of-Wshift-overflowand is enabled by default in C99 and C++11 modes (and newer). This level doesn't warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), since signed left shifts always wrap.-Wshift-overflow=2— This level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active 🌸
-Wswitch— Warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration. (The presence of adefaultlabel prevents this warning.)caselabels outside the enumeration range also trigger warnings when this option is used, even if there's adefaultlabel. This warning is enabled by-Wall🎀-Wswitch-default— Warns whenever aswitchstatement doesn't have adefaultcase 🧚-Wswitch-enum— Warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels outside the enumeration range also trigger warnings when this option is used. The only difference between-Wswitchand this option is that this one warns about an omitted enumeration code even if there's adefaultlabel 👑-Wno-switch-bool— Don't warn when aswitchstatement has an index of boolean type and the case values are outside the range of a boolean type. You can suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs. 🍓
-Wno-switch-outside-range— Controls warnings when aswitchcasehas a value that's outside of its respective type range. This warning is enabled by default for C and C++ programs 🌈-Wno-switch-unreachable— Don't warn when aswitchstatement contains statements between the controlling expression and the firstcaselabel, since those will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}-Wswitch-unreachable doesn't warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs 💜
-Wsync-nand(C and C++ only) — Warns when the__sync_fetch_and_nandand__sync_nand_and_fetchbuilt-in functions are used. These functions changed semantics in GCC 4.4 🥺-Wunused-but-set-parameter— Warns whenever a function parameter gets assigned to but is otherwise unused (aside from its declaration) 🫶 To suppress this warning, use theunusedattribute. This warning is also enabled by-Wunusedtogether with-Wextra.-Wunused-but-set-variable— Warns whenever a local variable gets assigned to but is otherwise unused (aside from its declaration). This warning is enabled by-Wall🔥 To suppress it, use theunusedattribute. It's also enabled by-Wunused, which is enabled by-Wall.-Wunused-function— Warns 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— Warns whenever a label is declared but not used. This warning is enabled by-Wall🦋 To suppress it, use theunusedattribute.-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — Warns when a typedef defined locally inside a function isn't used. This warning is enabled by-Wall✨-Wunused-parameter— Warns whenever a function parameter is unused aside from its declaration 💖 To suppress this warning, use theunusedattribute.-Wno-unused-result— Don't warn if a caller of a function marked with the attributewarn_unused_resultdoesn't use its return value. The default is-Wunused-result🧚-Wunused-variable— Warns whenever a local or static variable is unused aside from its declaration. This option implies-Wunused-const-variable=1for C, but not for C++. This warning is enabled by-Wall💅 To suppress it, use theunusedattribute.-Wunused-const-variable,-Wunused-const-variable=n— Warns whenever a constant static variable is unused aside from its declaration.-Wunused-const-variable=1is enabled by-Wunused-variablefor C, but not for C++. In C this declares variable storage, but in C++ it isn't an error, sinceconstvariables take the place of#defines 🌈 To suppress this warning, use theunusedattribute.-Wunused-const-variable=1— This is the warning level that-Wunused-variableenables for C. It only warns about unused staticconstvariables defined in the main compilation unit, not about staticconstvariables declared in any header that's included 🎀-Wunused-const-variable=2— This level also warns about unused constant static variables in headers (excluding system headers). This is the warning level of-Wunused-const-variableand must be explicitly requested, since in C++ this isn't an error and in C it might be harder to clean up all the headers included 🙈
-Wunused-value— okay bestie, this one yells when a statement works out a result and then just throws it away 🗑️. To shush it, cast the unused expression tovoid🤫. That covers an expression-statement or the left-hand side of a comma expression with no side effects. Sox[i,j]gets flagged 🚩, butx[(void)i,j]slides right through ✨. This warning is enabled by-Wall💅.-Wunused— the whole squad of-Wunusedoptions combined 👑
If you want a warning about an unused function parameter, you have to either use -Wextra -Wunused (note that -Wall already implies -Wunused) 🫶, or turn on -Wunused-parameter separately 🎀.
-Wuninitialized— this one warns if you use an object with automatic or allocated storage duration before you've given it a value 😭. In C++, it also warns if a non-static reference or a non-staticconstmember shows up in a class that has no constructors 🦋.
It's also diagnosed when you pass a pointer (or, in C++, a reference) to an uninitialized object into a const-qualified argument of a built-in function that's known to read the object 🔥. (-Wmaybe-uninitialized handles ordinary functions 💜.)
If you want to catch code that uses the uninitialized value of a variable inside its own initializer, use the -Winit-self option 🌸.
These warnings show up for individual uninitialized elements of structure, union, or array variables, as well as for variables that are uninitialized as a whole 🍓. They do not occur for variables or elements declared volatile 🧚. Because these warnings depend on optimization, the exact variables or elements that get warnings depend on the precise optimization options and the GCC version you're using 🌈.
Also, there may be no warning about a variable that's only used to compute a value that's never used itself, since data flow analysis can delete those computations before the warnings get printed 🙈.
-Wno-invalid-memory-model— this option controls warnings for calls to__atomicbuiltins,__syncbuiltins, and the C11 atomic generic functions when their memory consistency argument is either invalid for the operation or outside the range of values in thememory_orderenumeration 🧠. For example, the__atomic_storeand__atomic_store_nbuilt-ins are only defined for the relaxed, release, and sequentially consistent memory orders, so the following code gets diagnosed 💀:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}Heads up: -Winvalid-memory-model is enabled by default ✨.
-Wmaybe-uninitialized— for an object with automatic or allocated storage duration, this one warns if there's a path from the function entry to a use of the object where it's initialized, but some other paths leave it uninitialized 😬. The compiler emits the warning if it can't prove those uninitialized paths never run at run time 🫣.
It also diagnoses passing a pointer (or, in C++, a reference) to an uninitialized object into a const-qualified function argument 💅. (-Wuninitialized is the one issued for built-in functions known to read the object 🎀.) If you annotate the function with the attribute access (none), that tells the compiler the argument isn't used to access the object, and it avoids the warning 🫶.
These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables 🧐. They're made optional because GCC may not be able to tell when the code is correct even though it looks like there's an error 🤷♀️. Here's one example of how that can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}If y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know that 🥹. To suppress the warning, provide a default case with assert(0) or similar code 🌷.
This option also warns when a non-volatile automatic variable might be changed by a call to longjmp 🌀. The compiler only sees the calls to setjmp, and it can't know where longjmp will be called (a signal handler could call it at any point in the code 😱). So you may get a warning even when there's no real problem, because longjmp can't actually be called at the place that would cause one 🙃.
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn 🧹.
This warning is enabled by -Wall or -Wextra 💖.
-Wunknown-pragmas— warns when a#pragmadirective shows up that GCC doesn't understand 🔍. If you use this command-line option, warnings get issued even for unknown pragmas in system header files 📂. That's not the case when the warnings are only enabled by the-Wallcommand-line option 🙅♀️.-Wno-pragmas— do not warn about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas 🌸. See also-Wunknown-pragmas👀.-Wno-prio-ctor-dtor— do not warn if a priority from0to100is used for a constructor or destructor 🦋. The constructor and destructor attributes let you assign a priority to control their order of execution beforemainis called or after it returns 🗓️. The priority values must be greater than100, because the compiler reserves the priority values0--100for the implementation 🔒.-Wstrict-aliasing— this one is only active when-fstrict-aliasingis active 🎯. It warns about code that might break the strict aliasing rules the compiler uses for optimization 🧩. It doesn't catch every case, but it does try to catch the more common pitfalls 🍒. It's included in-Wall✨, and it's equivalent to-Wstrict-aliasing=3💜.-Wstrict-aliasing=n— same deal, only active when-fstrict-aliasingis active 🌈. It warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels mean higher accuracy (fewer false positives), and they also take more effort, kind of like how-Oworks 🧠.-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3🎀.- Level 1 — most aggressive, quick, and least accurate 😤. It's possibly useful when higher levels don't warn but
-fstrict-aliasingstill breaks your code, since it has very few false negatives 🕵️♀️. However, it has many false positives 🙈. It warns for all pointer conversions between possibly incompatible types, even if the pointer is never dereferenced. It runs in the front end only 🖥️. - Level 2 — aggressive, quick, and not too precise 🏃♀️. It may still have many false positives (though not as many as level 1), and few false negatives (possibly more than level 1) 🫤. Unlike level 1, it only warns when an address is taken 📍. It warns about incomplete types 📝. It runs in the front end only 🖥️.
- Level 3 (default for
-Wstrict-aliasing) — should have very few false positives and few false negatives 👑. It's slightly slower than levels 1 or 2 when optimization is enabled 🐢. It takes care of the common pun+dereference pattern in the front end, like*(int*)&some_float🍓. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information 🔬. It only warns when the converted pointer is dereferenced, and it does not warn about incomplete types 💅.
- Level 1 — most aggressive, quick, and least accurate 😤. It's possibly useful when higher levels don't warn but
-Wstrict-overflowand-Wstrict-overflow=n— this one is only active when signed overflow is undefined 🌪️. It warns about cases where the compiler optimizes based on the assumption that signed overflow never happens 🤞. It doesn't warn about every case where the code might overflow, only where the compiler actually implements some optimization, so the warning depends on your optimization level 🎚️.
An optimization that assumes signed overflow doesn't occur is perfectly safe if the values involved never actually overflow 🫧. That means this warning can easily give a false positive, which is a warning about code that isn't really a problem 😅. To help you focus on the important stuff, several warning levels are defined 🔥. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when figuring out whether a loop will run at all 🔁.
-Wstrict-overflow=1— warns about cases that are both questionable and easy to avoid 🌷. For example, the compiler simplifiesx + 1 > xto1🧮. This level is enabled by-Wall, while higher levels are not, so you have to request them explicitly 🙋♀️.-Wstrict-overflow=2— also warns about other cases where a comparison gets simplified to a constant 💫. For example:abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, becauseabs (INT_MIN)overflows toINT_MIN, which is less than zero 🤯.-Wstrict-overflow(with no level) is the same as-Wstrict-overflow=2🎀.-Wstrict-overflow=3— also warns about other cases where a comparison is simplified 👀. For example:x + 1 > 1is simplified tox > 0🫶.-Wstrict-overflow=4— also warns about other simplifications not covered by the cases above 🧁. For example:(x * 10) / 5is simplified tox * 2🍭.-Wstrict-overflow=5— also warns about cases where the compiler reduces the magnitude of a constant involved in a comparison 📉. For example:x + 2 > yis simplified tox + 1 >= y🧩. This is only reported at the highest warning level, because this simplification applies to many comparisons, so this level gives a very large number of false positives 😭💅.
-Wstring-compare— 🎀 okay bestie, this one is a whole vibe check. It warns when astrcmporstrncmpequality test is locked to one answer, zero or non-zero, because one argument is longer than the array the other one lives in (or longer than the bound, forstrncmp). Those calls could totally be mistakes 😭. Here's the example, where thestrcmpcall gets flagged because its result is necessarily non-zero no matter what's in the arraya:
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}-Wstring-compare is enabled by -Wextra ✨.
-Wno-stringop-overflow,-Wstringop-overflow,-Wstringop-overflow=type— 💅 this one watches string-handling functions likememcpyandstrcpyand yells if they'd overflow the destination buffer. The optional type is one more than the Object Size Checking type used to figure out how big the destination is. It only matters for functions that work on character arrays, not raw memory functions likememcpy, which always use Object Size type-0. It also warns about calls asking for a size bigger than the largest possible object, or for more thanSIZE_MAX / 2bytes. The best results come with optimization turned on, but it can still catch a small set of simple overflows without optimization in GCC built-ins like__builtin_memcpythat match the standard functions. Either way, it only catches a subset of the overflows the matching overflow-checking built-ins can find. Check out this one, wherestrcpycopies at least 5 characters (the string"blue"plus the terminating NUL) into a buffer of size 4 🍓:
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}-Wstringop-overflow=2 is enabled by default 👑.
-Wstringop-overflow,-Wstringop-overflow=1— 🌸 this setting uses type-zero Object Size Checking to figure out destination sizes. It won't warn about writes past the end of subobjects of bigger objects reached through pointers unless the size of the largest surrounding object is known. If the destination could be one of several objects, it's assumed to be the largest one. On Linux, when optimization is on at this level, it warns for the same code as when the_FORTIFY_SOURCEmacro is defined to a non-zero value.-Wstringop-overflow=2— 🦋 this one uses type-one Object Size Checking. It warns about overflows when writing to members of the largest complete objects whose exact size is known. It does not warn about excessive writes to the same members of unknown objects reached through pointers, since those might point to arrays with unknown numbers of elements. This is the default setting 💖.-Wstringop-overflow=3— 🧚 this one uses type-two Object Size Checking and warns about overflowing the smallest object or data member. It's the most restrictive setting that can still warn about code that's actually safe 😭.-Wstringop-overflow=4— 🔥 this one uses type-three Object Size Checking and warns about overflowing any data members. When the destination could be one of several objects, it uses the size of the largest one to decide whether to warn. Just like-Wstringop-overflow=3, this setting can warn about totally benign code 🙈.
-Wno-stringop-overread,-Wstringop-overread— 🌈 this one warns about string-handling calls likememchrorstrcpythat get determined to read past the end of the source sequence.
-Wstringop-overread is enabled by default 💜.
-Wno-stringop-truncation— 🫶 this option tells GCC to stop warning about bounded string functions likestrncat,strncpy, andstpncpythat might either truncate the copied string or leave the destination unchanged.
In this first example, the strncat call has a bound smaller than the length of the source string. So the copy gets truncated, and the call is diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound:
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}Here's another one. This strncpy call copies only the characters before the terminating NUL into d, so the NUL never gets appended to the end. Assuming the result of strncpy is always a NUL-terminated string is a super common mistake, so this call is diagnosed. If the result isn't supposed to be NUL-terminated, call memcpy instead to avoid the warning 🍓:
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}In this example, the strncpy call uses the size of the destination buffer as its bound. If the source string's length is equal to or greater than that size, the copy won't be NUL-terminated. So this one gets diagnosed too. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL 💅:
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}If a character array is meant to hold a sequence of bytes with no terminating NUL, you can annotate it with the attribute nonstring to avoid this warning ✨. But those arrays aren't suitable arguments for functions that expect NUL-terminated strings. To help catch accidental misuse of these arrays, GCC issues warnings unless it can prove the use is safe 🔥.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]— 👑 this one warns when adding an attribute might be beneficial. The attributes supported right now are listed below 🦋.-Wsuggest-attribute=pure,-Wsuggest-attribute=const,-Wsuggest-attribute=noreturn,-Wmissing-noreturn,-Wsuggest-attribute=malloc— 🌸 this group warns about functions that might be good candidates for the attributespure,const,noreturn, ormalloc. GCC only warns for functions visible in other compilation units, or (forpureandconst) if it can't prove the function returns normally. A function returns normally if it doesn't have an infinite loop and doesn't return abnormally by throwing, callingabort, or trapping. This analysis needs the option-fipa-pure-const, which is enabled by default at-Oand higher. Higher optimization levels make the analysis more accurate 🧠.-Wsuggest-attribute=format,-Wmissing-format-attribute— 🎀 this warns about function pointers that might be candidates forformatattributes. These are only possible candidates, not guaranteed ones. GCC guesses that function pointers withformatattributes used in assignment, initialization, parameter passing, or return statements should also have a matchingformatattribute in the resulting type. So the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function should have aformatattribute too, to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again, these are just possible candidates. GCC guesses that a format attribute might fit any function that calls something like vprintf or vscanf, but that isn't always true. Some functions that really should have format attributes might not get detected 🙈.
-Wsuggest-attribute=cold— 🧊 this warns about functions that might be candidates for thecoldattribute. It's based on static detection and generally only warns about functions that always lead to a call to anothercoldfunction, like wrappers of C++throwor fatal error reporting functions that lead toabort💀.
-Walloc-zero— 🍼 this warns about calls to allocation functions decorated with the attributealloc_sizethat specify zero bytes. That includes the built-in forms ofaligned_alloc,alloca,calloc,malloc, andrealloc. The behavior of these functions with a zero size differs between implementations (andrealloc's version of this has been deprecated), so relying on it can cause subtle portability bugs. Avoid it 🚫.-Walloc-size-larger-than=byte-size— 🌊 this warns about calls to functions decorated with the attributealloc_sizethat try to allocate objects bigger than the number of bytes you specify. It also warns when the size computation, done in an integer type with infinite precision, would go past the value ofPTRDIFF_MAXon the target.-Walloc-size-larger-than=PTRDIFF_MAXis enabled by default. You can disable warnings from this option either by specifying a byte-size ofSIZE_MAXor more, or with-Wno-alloc-size-larger-than💖.-Wno-alloc-size-larger-than— okay bestie, this one silences the-Walloc-size-larger-than=warnings 🙈 It works the same as-Walloc-size-larger-than=SIZE_MAXor anything bigger, so basically you're saying "I'm not worried about huge sizes" 💅-Walloca— this one warns on every single use ofallocain your source. No exceptions, no vibe check, just warnings 🫠-Walloca-larger-than=byte-size— ngl this is the big one 🔥 It warns on calls toallocawhere the integer argument is either zero, or isn't bounded by some controlling check that keeps it at most byte-size. It also warns when the bound is unknown. Heads up: non-integer arguments count as unbounded, even if they look like they're in range 🙈
Here's a bounded case that's totally fine:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}Passing -Walloca-larger-than=1000 won't warn here, because the alloca call is known to be at most 1000 bytes. But if you pass -Walloca-larger-than=500, the compiler will warn 👀
Unbounded uses are the ones with no controlling predicate limiting the integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}With -Walloca-larger-than=500, this one triggers a warning too, but this time it's because there's no bounds checking at all 😭
Even code that looks correct with signed integers can warn:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}Here n could be negative, which means a larger-than-expected argument gets implicitly cast into the alloca call. Wild, right? 🌪️
This option also warns when alloca is used in a loop 🔁
-Walloca-larger-than=PTRDIFF_MAX is enabled by default, but it's usually only really effective when -ftree-vrp is active (the default for -O2 and above) ✨
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than— turns off the-Walloca-larger-than=warnings. It's the same as-Walloca-larger-than=SIZE_MAXor larger 🌸-Warith-conversion— bestie wants you to know this one is extra. It warns about implicit conversions from arithmetic operations, even when converting the operands to the same type can't change their values 🧐 It affects the warnings from-Wconversion,-Wfloat-conversion, and-Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}-Warray-bounds,-Warray-bounds=n— only active when-ftree-vrpis active (default for-O2and above). It warns about subscripts to arrays that are always out of bounds 💀 This warning is enabled by-Wall.-Warray-bounds=1— the warning level of-Warray-bounds, enabled by-Wall. Higher levels aren't enabled by it and have to be explicitly requested.-Warray-bounds=2— this level also warns about out-of-bounds access for arrays at the end of a struct and for arrays accessed through pointers. It can give a bigger pile of false positives, so it's deactivated by default 🙃
-Warray-parameter,-Warray-parameter=n— warns about redeclarations of functions that have array or pointer arguments in inconsistent kinds or forms. It also turns on detection of out-of-bounds accesses to those parameters through warnings like-Warray-bounds🎀
If the first function declaration uses the array form, the bound in the array is assumed to be the minimum number of elements expected in calls to the function, and also the maximum number of elements it accesses. Giving too-small arguments, or accessing more than the maximum, may get diagnosed by warnings like -Warray-bounds. At level 1, the warning catches inconsistencies involving array parameters declared with the T[static N] form.
This triggers for the redeclarations below, because the first one lets an array of any size get passed to f, while the second one (with the keyword static) says the array argument must have at least four elements:
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (4);
f (p); // warning (array too small)
...
}At level 2, the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms that denote array sizes. Pointers and arrays of unspecified bound are considered equivalent and don't trigger a warning:
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments 🌈
-Wattribute-alias=n,-Wno-attribute-alias— warns about declarations using thealiasand similar attributes whose target is incompatible with the type of the alias 🦋-Wattribute-alias=1— this is the default warning level. It diagnoses incompatibilities between the type of the alias declaration and the type of its target. Those are usually signs of bugs 🐛-Wattribute-alias=2— at this level, it also diagnoses cases where the alias declaration's attributes are more restrictive than the ones on its target. Those mismatches can lead to incorrect code generation. Other times they're harmless and could be fixed by just adding the missing attribute to the target. For comparison,-Wmissing-attributesis the one that handles the opposite case, where the alias is less restrictive than the target.
The attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".
-Wattribute-alias is the same as -Wattribute-alias=1, which is the default. You can turn these warnings off with either -Wno-attribute-alias or -Wattribute-alias=0 🙅♀️
-Wbool-compare— warns about a boolean expression being compared with an integer value that isn'ttrueorfalse. For instance, this comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall ✨
-Wbool-operation— warns about suspicious operations on expressions of a boolean type. For example, bitwise negation of a boolean is very likely a bug 🙈 For C, this also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing one is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches— warns when an if-else has identical branches. It catches cases like this:
if (p != NULL)
return 0;
else
return 0;It doesn't warn when both branches contain just a null statement. This warning also covers conditional operators:
int i = x ? *p : *p;-Wduplicated-cond— warns about duplicated conditions in an if-else-if chain. For example, it'll warn on this:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }-Wframe-address— warns when__builtin_frame_addressor__builtin_return_addressis called with an argument greater than 0. Those calls might return indeterminate values or crash the program 💀 This warning is included in-Wall.-Wno-discarded-qualifiers(C and Objective-C only) — stops the compiler from warning when type qualifiers on pointers get discarded. Normally it warns if aconst char *variable is passed to a function that takes achar *parameter. This option suppresses that warning 🫶-Wno-discarded-array-qualifiers(C and Objective-C only) — same idea, but for qualifiers on arrays that are pointer targets. Normally it warns if aconst int (*)[]variable is passed to a function that takes anint (*)[]parameter. This option suppresses that warning too 🌸-Wno-incompatible-pointer-types(C and Objective-C only) — no warning when there's a conversion between pointers with incompatible types. This is for the cases that-Wno-pointer-signdoesn't cover, since that one handles pointer argument passing or assignment with different signedness.-Wno-int-conversion(C and Objective-C only) — no warnings about incompatible integer-to-pointer and pointer-to-integer conversions. This is only about implicit conversions. For explicit ones, you can use-Wno-int-to-pointer-castand-Wno-pointer-to-int-castinstead 🔢-Wzero-length-bounds— warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged, because accessing them is undefined. See
Accesses to interior zero-length arrays are undefined, so please don't 🙅♀️
For example, the first two stores in function bad are diagnosed, because the array elements overlap the subsequent members b and c. The third store gets diagnosed by -Warray-bounds, because it's past the bounds of the enclosing object:
struct X { int a[0]; int b, c; };
struct X x;void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}Okay so -Wzero-length-bounds is enabled by -Warray-bounds 🎀 it's giving layered protection, bestie 💅
-Wno-div-by-zero— Bestie, this one tells GCC to chill and stop warning about compile-time integer division by zero 🙅♀️ Floating-point division by zero doesn't get flagged either, because it can be a legit way to get infinities and NaNs ✨ no cap, that's a whole vibe.-Wsystem-headers— This makes GCC print warnings for stuff found in system header files 👀 Normally those warnings are muted, because they usually aren't real problems and would just make the output harder to read 🥱 With this flag on, GCC treats system header warnings like they came from your own code 🫶
Heads up though: using -Wall together with this option does not warn about unknown pragmas in system headers 😭 For that tea, you also need -Wunknown-pragmas.
-Wtautological-compare— Warns if a self-comparison always evaluates to true or false 💀 This catches sneaky mistakes like:
int i = 1;
...
if (i > i) { ... }It also warns about bitwise comparisons that always come out true or false, for example:
if ((a & 16) == 10) { ... }That one will always be false 🙈 This warning is enabled by -Wall 💖
-Wtrampolines— Warns about trampolines generated for pointers to nested functions ✨ A trampoline is a tiny bit of data or code that gets made at run time on the stack when the address of a nested function is taken, and it's used to call that nested function indirectly 🧚 On some targets it's just data, so no special treatment is needed. But on most targets it's code, which means the stack has to be made executable for the program to work properly 😬 very sus, be careful.-Wfloat-equal— Warns if floating-point values are used in equality comparisons 🦋
The thinking behind this is that sometimes it's convenient to treat floating-point values as approximations of infinitely precise real numbers 🌸 If you're doing that, you need to figure out (by analyzing the code, or some other way) the maximum or likely maximum error the computation introduces, and allow for it when you compare things (and when producing output, but that's a different problem 🙃). Instead of testing for equality, you should check whether the two values have ranges that overlap, which is done with the relational operators. So equality comparisons are probably mistaken 💅
-Wtraditional(C and Objective-C only) — Warns about constructs that behave differently in traditional C and ISO C ✨ It also warns about ISO C constructs that have no traditional C equivalent, and about problematic constructs you should avoid 🫠 Here's the full list of things it's looking out for:- Macro parameters that show up inside string literals in the macro body. Traditional C does macro replacement inside string literals, but ISO C doesn't 🙅♀️
- Some preprocessor directives didn't exist in traditional C. Traditional preprocessors only treated a line as a directive if the
#was in column 1. So-Wtraditionalwarns about directives that traditional C understands but ignores because the#isn't the first character on the line. It also suggests hiding directives like#pragmathat traditional C doesn't understand by indenting them. Some traditional implementations don't recognize#elif, so this option suggests avoiding it altogether 🧐 - A function-like macro that appears without arguments.
- The unary plus operator.
- The
Uinteger constant suffix, or theForLfloating-point constant suffixes. (Traditional C does support theLsuffix on integer constants.) These suffixes show up in macros defined in the system headers of most modern systems, like the_MIN/_MAXmacros in"<limits.h>". Using those macros in user code might normally cause spurious warnings, but 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 that block.
- A
switchstatement with an operand of typelong. - A non-
staticfunction declaration that follows astaticone. Some traditional C compilers don't accept this construct 🚫 - The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. Hexadecimal and octal values, which usually represent bit patterns, aren't warned about 🔢
- ISO string concatenation is detected.
- Initialization of automatic aggregates.
- Identifier conflicts with labels. Traditional C doesn't have a separate namespace for labels 🏷️
- Initialization of unions. If the initializer is zero, the warning is skipped. This assumes that a zero initializer in user code is conditioned on something like
"__STDC__"to avoid missing-initializer warnings, and relies on default initialization to zero in the traditional C case 🌈 - Conversions by prototypes between fixed-point and floating-point values, and vice versa. Missing these prototypes when compiling with traditional C causes serious problems 😱 This is a subset of the possible conversion warnings. For the full set, use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning is intentionally not issued for prototype declarations or variadic functions, because those ISO C features show up in your code when you use libiberty's traditional C compatibility macros,
"PARAMS"and"VPARAMS"💖 It's also bypassed for nested functions, because that feature is already a GCC extension and isn't relevant to traditional C compatibility.
-Wtraditional-conversion(C and Objective-C only) — Warns if a prototype causes a type conversion that's different from what would happen to the same argument without a prototype 👀 This includes conversions of fixed-point to floating-point and vice versa, plus conversions that change the width or signedness of a fixed-point argument, except when it matches the default promotion.-Wdeclaration-after-statement(C and Objective-C only) — Warns when a declaration is found after a statement in a block 📝 This construct comes from C++ and was introduced with ISO C99. GCC allows it by default, but it's not supported by ISO C90 🙃-Wshadow— Warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function gets shadowed 🪞 In C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows astruct/class/enum. If this warning is enabled, it also includes all instances of local shadowing. That means-Wno-shadow=localand-Wno-shadow=compatible-localget ignored when-Wshadowis used. Same as-Wshadow=global✨-Wno-shadow-ivar(Objective-C only) — Don't warn when a local variable shadows an instance variable in an Objective-C method 🙅♀️-Wshadow=global— Warns for any shadowing. Same as-Wshadow👑-Wshadow=local— Warns when a local variable shadows another local variable or parameter 🫶-Wshadow=compatible-local— Warns when a local variable shadows another local variable or parameter whose type is compatible with the shadowing variable's type. In C++, type compatibility here means the type of the shadowing variable can be converted to the type of the shadowed variable 💅 This flag exists because when a local variable shadows another one of an incompatible type, it's most likely intentional, not a bug or typo. Check out this example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since the two i variables in that example have incompatible types, turning on only -Wshadow=compatible-local won't emit a warning 🧠 Because the types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch it and emit an error or warning. Using this flag instead of -Wshadow=local can reduce the number of warnings triggered by intentional shadowing 🌸 It also means that shadowing "const char *i" by "char *i" does not emit a warning.
This warning is also enabled by -Wshadow=local ✨
-Wlarger-than=byte-size— Warns whenever an object is defined whose size is bigger thanbyte-size📏-Wlarger-than=PTRDIFF_MAXis enabled by default. You can turn these warnings off either by specifying abyte-sizeofSIZE_MAXor more, or by using-Wno-larger-than🦋
Also it warns for calls to bounded functions like "memchr" or "strnlen" that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default 🥺 These warnings can only be disabled by -Wno-larger-than.
-Wno-larger-than— Disables-Wlarger-than=warnings 🌈 This is equivalent to-Wlarger-than=SIZE_MAXor larger 💜-Wframe-larger-than=byte-size— okay so this one warns you if a function's frame gets bigger than byte-size 📦. Heads up, the math used to figure out the frame size is approximate and not conservative 🙈, so the real requirements might be a bit bigger than byte-size even when you get no warning. Also, space fromalloca, variable-length arrays, or related constructs isn't counted when the compiler decides whether to warn 💀.-Wframe-larger-than=PTRDIFF_MAXis on by default ✨. You can turn these warnings off by giving a byte-size ofSIZE_MAXor more, or by using-Wno-frame-larger-than🫶-Wno-frame-larger-than— bye bye to all-Wframe-larger-than=warnings 🙅♀️. It's the same as-Wframe-larger-than=SIZE_MAXor bigger 💖-Wno-free-nonheap-object— hmm, this is the one that yells when you try to free something that either never lived on the heap, or came from a pointer that wasn't handed back by the matching allocation function earlier 😭. Like,stpcpygives you a pointer to the terminating nul character instead of the start of the object, so thefreecall below gets flagged 🚩
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is on by default 🌸
-Wstack-usage=byte-size— warns if a function's stack usage might go over byte-size 📚. This math is conservative, soalloca, variable-length arrays, and related constructs do get counted here 🧠
The message matches what -fstack-usage prints, and it comes in three flavors 🎀:
- If the stack usage is fully static but goes over the limit, you'll see:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, you'll see:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, you'll see:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is on by default 🦋 You can disable these warnings by giving a byte-size of SIZE_MAX or more, or by using -Wno-stack-usage 🫶
-Wno-stack-usage— the "shh, no more stack warnings" option 🤫 It's the same as-Wstack-usage=SIZE_MAXor bigger 💅-Wunsafe-loop-optimizations— warns if a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices 🔁. With-funsafe-loop-optimizations, it warns whenever the compiler does make those assumptions 👀
-Wno-pedantic-ms-format(MinGW targets only) — when you use it together with-Wformatand-pedanticwithout GNU extensions, this turns off the warnings about the non-ISOprintf/scanfformat width specifiersI32,I64, andIon Windows targets 🪟. Those depend on the MS runtime, so they're kinda the Windows-only drama 💜-Wpointer-arith— warns about anything that depends on the "size of" a function type or ofvoid🧮. GNU C gives these types a size of 1 sovoid *pointers and pointers to functions are easier to do math with 🌈. In C++, it also warns when an arithmetic operation involvesNULL⚠️. This one is also turned on by-Wpedantic⭐-Wno-pointer-compare— don't warn if a pointer is compared with a zero character constant 🙃. This usually means the pointer was meant to be dereferenced 🤔 For example:
const char *p = foo ();
if (p == '\0')
return 42;Heads up, the code above is invalid in C++11 🚫
This warning is enabled by default 🍓
-Wtsan— warns about features ThreadSanitizer doesn't support 🧵. It doesn't supportstd::atomic_thread_fence, and it can report false positives, so be careful bestie 😬
This warning is enabled by default ✨
-Wtype-limits— warns if a comparison is always true or always false because of the limited range of the data type, but not for constant expressions 📏. For example, it'll warn if an unsigned variable gets compared against zero with<or>=💀 This warning is also turned on by-Wextra🔥-Wabsolute-value(C and Objective-C only) — warns when you call standard functions that compute an absolute value, but there's a more appropriate standard function available 🧐. For example, callingabs(3.14)triggers it, because the right function for a double isfabs🙈. It also warns when the argument to one of these calls has an unsigned type. You can shut it up with an explicit type cast, and it's also turned on by-Wextra💖-Wcomment,-Wcomments— warns whenever a comment-start sequence/*shows up inside a/* comment, or whenever a backslash-newline shows up in a//comment 💬. This warning is turned on by-Wall🌸-Wtrigraphs— warns if any trigraphs show up that might change what the program means 🔍. Trigraphs inside comments don't get warned about, except the ones that would form escaped newlines 🧚
This option is implied by -Wall. If you don't pass -Wall, this option is still on unless trigraphs are enabled 🙌. If you want trigraph conversion without the warnings, but still want the other -Wall warnings, use:
-trigraphs -Wall -Wno-trigraphs-Wundef— warns if an undefined identifier gets evaluated in an#ifdirective 🕵️♀️. Those identifiers get replaced with zero 0️⃣-Wexpansion-to-defined— warns wheneverdefinedshows up in the expansion of a macro, including when the macro is expanded by an#ifdirective 🙅♀️. That usage isn't portable 🌍 This warning is also turned on by-Wpedanticand-Wextra💅-Wunused-macros— warns about macros defined in the main file that never get used 🗑️. A macro counts as used if it's expanded or tested for existence at least once 🧾. The preprocessor also warns if the macro hasn't been used by the time it gets redefined or undefined 🔄
Built-in macros, macros defined on the command line, and macros defined in include files don't get warned about 🙆♀️
Note: if a macro is actually used, but only inside skipped conditional blocks, the preprocessor still reports it as unused 😭 To dodge the warning in that case, you might improve the scope of the macro's definition, for example by moving it into the first skipped block 🧩 Or you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif-Wno-endif-labels— don't warn when an#elseor an#endifis followed by text 🙈 This happens sometimes in older programs with code like this:
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO are supposed to be in comments 💜 This warning is on by default 🎀
-Wbad-function-cast(C and Objective-C only) — warns when a function call gets cast to a type that doesn't match 🎭. For example, it warns if a call to a function returning an integer type is cast to a pointer type 📍-Wc90-c99-compat(C and Objective-C only) — warns about features that are in ISO C99 but not in ISO C90 🕰️. Think variable length arrays, thelong longtype, thebooltype, compound literals, designated initializers, and so on 🧸 This option doesn't care what standards mode you're in 🙃 Warnings are turned off in the expression that follows__extension__🤫-Wc99-c11-compat(C and Objective-C only) — warns about features that are in ISO C11 but not in ISO C99 🧃. Like anonymous structures and unions, the_Atomictype qualifier, the_Thread_localstorage-class specifier, the_Alignasspecifier, theAlignofoperator, the_Generickeyword, and so on 🦄 This option is also independent of the standards mode 🌈 Warnings are turned off in the expression that follows__extension__💫-Wc11-c2x-compat(C and Objective-C only) — warns about features that are in ISO C2X but not in ISO C11 🔮. Like omitting the string in_Static_assert, using[[]]syntax for attributes, using decimal floating-point types, and so on 🧮 This option doesn't care about the standards mode either 🙌 Warnings are turned off in the expression that follows__extension__🤍-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that are outside the common subset of ISO C and ISO C++ 🧩. For example, requesting an implicit conversion fromvoid *to a pointer to a non-voidtype 🙈-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning is different between ISO C++ 1998 and ISO C++ 2011 🌟. For example, identifiers that are fine in ISO C++ 1998 but are keywords in ISO C++ 2011 🔑 This warning turns on-Wnarrowingand is enabled by-Wall🫶-Wc++14-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014 🌸 This warning is enabled by-Wall✨-Wc++17-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017 🪄 This warning is enabled by-Wall💖-Wc++20-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020 🦋 This warning is enabled by-Wall🎀-Wcast-qual— warns whenever a pointer gets cast in a way that removes a type qualifier from the target type 🧼. For example, it warns if aconst char *gets cast to an ordinarychar *🙅♀️
It also warns when you make a cast that adds a type qualifier in an unsafe way 🚨. For example, casting char ** to const char ** is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';-Wcast-align— warns whenever a pointer gets cast so the required alignment of the target goes up 📐. For example, it warns if achar *gets cast to anint *on machines where integers can only be accessed at two- or four-byte boundaries 🧱-Wcast-align=strict— okay bestie, this one is all about pointers getting cast into a stricter alignment 🧚. It warns whenever a pointer is cast so that the target's required alignment goes up. For example, it warns if achar *gets cast to anint *✨ regardless of the target machine.-Wcast-function-type— this one checks function pointer casts 🧐. It warns when a function pointer gets cast into an incompatible function pointer 🙅. For functions with a variable argument list, only the types of the initial arguments you actually provide get considered. Any pointer parameter is happy to match any other pointer type 💜, and harmless integer differences get ignored, likeintvs.longon ILP32 targets. Type qualifiers get ignored too. The function typevoid (*) (void)is the VIP 👑 of this warning, because it matches everything, so you can use it to shut the warning up. For casts involving pointer-to-member types, it warns whenever the cast changes the pointer-to-member type. This warning is turned on by-Wextra.-Wwrite-strings— when you're compiling C, string constants get the typeconst char[length], so copying one's address into a non-constchar *pointer gives you a warning 📝. These warnings catch code that might try to write into a string constant at compile time, but only if you've been super careful withconstin your declarations and prototypes. Otherwise they're just a nuisance 🙄, which is exactly why-Walldoesn't ask for them.
When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
-Wclobbered— this one warns about variables that might get changed bylongjmporvfork🌀. It's also turned on by-Wextra.-Wconversion— this warns about implicit conversions that might alter a value 🔥. That includes conversions between real and integer, likeabs (x)whenxisdouble. It also covers conversions between signed and unsigned, likeunsigned ui = -1, and conversions to smaller types, likesqrtf (M_PI). It won't bother you about explicit casts likeabs ((int) x)andui = (unsigned) -1, or about conversions that don't change the value, likeabs (2.0). If the signed/unsigned warnings are too much, you can turn them off with-Wno-sign-conversion.
For C++, it also warns about confusing overload resolution for user-defined conversions, and about conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Signed/unsigned integer conversion warnings are disabled by default in C++ unless you explicitly enable -Wsign-conversion 💅.
Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.
-Wdangling-else— this is for constructions where it's unclear whichifstatement anelsebranch belongs to 🙈. Here's an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). That's often not what the programmer expected, as the indentation in the example above shows. When there's potential for this confusion, GCC issues a warning if you pass this flag. To get rid of the warning, add explicit braces around the innermost if statement, so there's no way the else can belong to the enclosing if. The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses.
-Wdate-time— this warns when the macros__TIME__,__DATE__or__TIMESTAMP__show up ⏰. They might stop your compilations from being bit-wise-identical and reproducible, so keep an eye on them.-Wempty-body— warns if an empty body shows up in anif,elseordo whilestatement 🫥. It's also turned on by-Wextra.-Wno-endif-labels— stops GCC from complaining about stray tokens after#elseand#endif🫶.-Wenum-compare— warns about a comparison between values of different enumerated types 🦋. In C++, enumerated type mismatches in conditional expressions get diagnosed too, and this warning is on by default there. In C, it's enabled by-Wall.-Wenum-conversion— warns when a value of enumerated type gets implicitly converted to a different enumerated type 🌸. In C, this warning is enabled by-Wextra.-Wjump-misses-init(C, Objective-C only) — warns if agotoorswitchstatement jumps forward across the initialization of a variable, or jumps backward to a label after that variable has been initialized 🚧. It only warns about variables that get initialized when they're declared. This warning is only supported for C and Objective-C. In C++, this sort of branch is an error anyway, so no need to worry there.
-Wjump-misses-init is included in -Wc++-compat. You can disable it with the -Wno-jump-misses-init option.
-Wsign-compare— warns when a comparison between signed and unsigned values could give an incorrect result because the signed value gets converted to unsigned ⚖️. In C++, this is also enabled by-Wall. In C, it's also enabled by-Wextra.-Wsign-conversion— warns about implicit conversions that might change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable 🙃. An explicit cast silences the warning. In C, this option is also enabled by-Wconversion.-Wfloat-conversion— warns about implicit conversions that reduce the precision of a real value 🎀. That includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by-Wconversion.-Wno-scalar-storage-order— stops the warnings about suspicious constructs involving reverse scalar storage order 🙅♀️.-Wsizeof-array-div— warns about divisions of twosizeofoperators when the first one is applied to an array and the divisor doesn't equal the size of the array element 🧮. In that case, the computation won't give you the number of elements in the array, which is probably not what you meant. This warning catches stuff like this:
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}This warning is enabled by -Wall.
-Wsizeof-pointer-div— warns about suspicious divisions of twosizeofexpressions that divide the pointer size by the element size 📐. That's the usual way to compute an array size, but it won't work out correctly with pointers. For example, it warns aboutsizeof (ptr) / sizeof (ptr[0])ifptris a pointer and not an array. This warning is enabled by-Wall.-Wsizeof-pointer-memaccess— warns about suspicious length parameters passed to certain string and memory built-in functions when the argument usessizeof🧠. It triggers, for example, formemset (ptr, 0, sizeof (ptr));ifptris a pointer and not an array. It'll even suggest a possible fix. It also warns about stuff likememcpy (&foo, ptr, sizeof (&foo));.
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that use a sizeof expression of the source array as the bound. In the following function, the call to strncat uses the size of the source string as the bound. That's almost certainly a mistake, so the call gets diagnosed 💀.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument— stops the warning when thesizeofoperator gets applied to a parameter declared as an array in a function definition 🫶. This warning is enabled by default for C and C++ programs.-Wmemset-elt-size— warns about suspicious calls to thememsetbuilt-in function 🧺. It triggers when the first argument references an array, and the third argument is a number equal to the number of elements but not equal to the array's size in memory. That means you probably forgot to multiply by the element size. This warning is enabled by-Wall.-Wmemset-transposed-args— warns about suspicious calls to thememsetbuilt-in function where the second argument is not zero and the third argument is zero 🔄. For example, the callmemset (buf, sizeof buf, 0)gets diagnosed, becausememset (buf, 0, sizeof buf)was what you meant. The diagnostic is only emitted if the third argument is a literal zero. If it's an expression that folds to zero, or a cast of zero to some type, it's far less likely that the arguments got mistakenly transposed, so no warning is emitted. This warning is enabled by-Wall.-Waddress— warns about suspicious uses of memory addresses 👀. These include using the address of a function in a conditional expression, likevoid func(void); if (func), and comparing against the memory address of a string literal, likeif (x == "abc"). These usually point to a programmer error. The address of a function always evaluates to true, so using it in a conditional usually means you forgot the parentheses in a function call. Comparisons against string literals have unspecified behavior and aren't portable in C, so they usually mean you meant to usestrcmp. This warning is enabled by-Wall.-Wno-address-of-packed-member— stops the warning when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value 📦. This is enabled by default.-Wlogical-op— warns about suspicious uses of logical operators in expressions 🧐. That includes using logical operators in contexts where a bit-wise operator is likely expected. It also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }Okay that's the bestie-level mistake 🙈 comparing the same thing twice, and the compiler is literally begging you to double check.
-Wlogical-not-parentheses— okay so this one screams when you slap a!on the left side of a comparison 💅 It doesn't scream if the right side is a boolean expression. The whole point is to catch sus code like this:
int a;
...
if (!a > 1) { ... }If you want it to chill, wrap the LHS in parentheses:
if ((!a) > 1) { ... }This warning is enabled by -Wall ✨
-Waggregate-return— warns if any functions that return structures or unions get defined or called 🎀 (In languages where you can return an array, this also gets a warning too, ngl.)-Wno-aggressive-loop-optimizations— warns if you're in a loop with a constant number of iterations and the compiler spots undefined behavior in some statement during one or more of the iterations 💀-Wno-attributes— don't warn if an unexpected__attribute__shows up, like unrecognized attributes, function attributes slapped on variables, etc. 🦋 This does NOT stop errors for using supported attributes incorrectly, so don't get it twisted.-Wno-builtin-declaration-mismatch— warns if a built-in function gets declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that has no prototype gets called with args whose promoted types don't match what the function expects 🌸 With-Wextra, it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is on by default. To dodge it, include the right header so the prototypes of built-in functions are in scope 🫶
Here's the vibe check example. The call to memset gets flagged because the function wants a size_t but 32 is an int. With -Wextra, the declaration gets flagged too 😭
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— don't warn if certain built-in macros get redefined 🌈 This quiets warnings for redefining__TIMESTAMP__,__TIME__,__DATE__,__FILE__, and__BASE_FILE__.-Wstrict-prototypes(C and Objective-C only) — warns if a function is declared or defined without specifying its argument types 👑 (An old-style function definition gets a free pass with no warning if it's preceded by a declaration that does specify the argument types.)-Wold-style-declaration(C and Objective-C only) — warns about obsolescent usages in a declaration, according to the C Standard 🧚 For example, it warns if storage-class specifiers likestaticaren't the first things in a declaration. This one is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warns if an old-style function definition is used ⭐ It warns even if there's a previous prototype. A definition using()isn't considered old-style in C2X mode, since it's equivalent to(void)there, but it is considered old-style for older standards.-Wmissing-parameter-type(C and Objective-C only) — warns when a function parameter is declared without a type specifier in K&R-style functions, like this:
void foo(bar) { }This warning is also enabled by -Wextra 🍓
-Wmissing-prototypes(C and Objective-C only) — warns if a global function is defined without a previous prototype declaration 💜 This warning shows up even if the definition itself provides a prototype. Use it to catch global functions that don't have a matching prototype declaration in a header file. This option is NOT valid for C++, because all function declarations there provide prototypes, and a non-matching declaration declares an overload instead of conflicting with an earlier one. For missing declarations in C++, use-Wmissing-declarationsinstead.-Wmissing-declarations— warns if a global function is defined without a previous declaration 🔥 It does this even if the definition itself provides a prototype. Use it to catch global functions that aren't declared in header files. In C, no warnings get issued for functions with previous non-prototype declarations, so use-Wmissing-prototypesfor those. In C++, no warnings get issued for function templates, inline functions, or functions in anonymous namespaces.-Wmissing-field-initializers— warns if a structure's initializer has some fields missing 🦋 In this example,x.his implicitly zero, so you get the warning:
struct s { int f, g, h; };
struct s x = { 3, 4 };This option does NOT warn about designated initializers, so this version is totally fine:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C, this option doesn't warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Likewise, in C++, it doesn't warn about the empty { } initializer:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. If you want the other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers 💅
-Wno-multichar— don't warn if a multicharacter constant ('FOOF') is used 🙈 These usually mean there's a typo in your code, since they have implementation-defined values, and they shouldn't be used in portable code.-Wnormalized=[none|id|nfc|nfkc]— okay this one is a whole saga 🌸 In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. BUT sometimes, when characters outside basic ASCII are used, you can have two different character sequences that look exactly the same. To avoid that confusion, the ISO 10646 standard sets normalization rules that turn lookalike sequences into the same sequence. GCC can warn you if your identifiers aren't normalized, and this option controls that warning.
There are four levels of warning. The default is -Wnormalized=nfc, which warns about any identifier that isn't in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to -Wnormalized.
Here's the tea ☕ Some characters allowed in identifiers by ISO C and ISO C++ turn into characters that AREN'T allowed in identifiers once they're NFC. So there's no way to use those symbols in portable ISO C or C++ AND have all your identifiers in NFC. -Wnormalized=id suppresses the warning for those characters. The hope is that future versions of the standards will fix this, which is why it isn't the default.
You can switch the warning off for all characters with -Wnormalized=none or -Wno-normalized. Only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see 😭
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methods, especially once formatting has been applied. For instance, "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all of these into a standard form too, and GCC warns if your code isn't in NFKC when you use -Wnormalized=nfkc. This is comparable to warning about every identifier that contains the letter O, since it might be confused with the digit 0. So it isn't the default, but it can be a useful local coding convention if your programming environment can't be fixed to display these characters distinctly 🧚
-Wno-attribute-warning— don't warn about usage of functions declared with the "warning" attribute 🎀 This warning is enabled by default. Use-Wno-attribute-warningto disable the warning, or-Wno-error=attribute-warningto disable the error when compiling with the-Werrorflag.-Wno-deprecated— don't warn about usage of deprecated features 🦋-Wno-deprecated-declarations— don't warn about uses of functions, variables, and types marked as deprecated with the "deprecated" attribute 🌈-Wno-overflow— don't warn about compile-time overflow in constant expressions ✨-Wno-odr— warns about One Definition Rule violations during link-time optimization 💖 This one is enabled by default.-Wopenmp-simd— warns if the vectorizer cost model overrides the OpenMP simd directive that the user set 🔥 The-fsimd-cost-model=unlimitedoption can be used to relax the cost model.-Woverride-init(C and Objective-C only) — warns if an initialized field without side effects gets overridden when using designated initializers 🫶 This warning is included in-Wextra. To get the other-Wextrawarnings without this one, use-Wextra -Wno-override-init.-Wno-override-init-side-effects(C and Objective-C only) — don't warn if an initialized field with side effects gets overridden when using designated initializers 👑 This warning is enabled by default.-Wpacked— warns 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 end up misaligned for little benefit. For instance, in this code, the variablef.xinstruct baris misaligned even thoughstruct bardoesn't itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};-Wnopacked-bitfield-compat— okay so the 4.1, 4.2 and 4.3 series of GCC just ignored thepackedattribute oncharbit-fields 💀. That got fixed in GCC 4.4, but the fix can shift your struct layout, so GCC tells you when a field's offset has changed. In this struct, the 4-bit gap betweenaandbis gone now ✨:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));This warning is enabled by default 🌸. If you want it gone, use -Wno-packed-bitfield-compat 🫶.
-Wpacked-not-aligned(C, C++, Objective-C and Objective-C++ only) — bestie, this one catches a field with explicit alignment inside a packed struct or union that's misaligned 🙈. The message looks likewarning: alignment 1 of 'struct S' is less than 8, and it pops up in code like this:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall 💜.
-Wpadded— warns if padding sneaks into a struct, whether it's there to line up one element or to line up the whole struct 🦋. Sometimes you can shuffle the fields around to cut the padding down and make the struct smaller, which is very iconic 👑.-Wredundant-decls— warns if something gets declared more than once in the same scope 🙄. This happens even when multiple declarations are valid and change absolutely nothing.-Wrestrict— warns when an object referenced by arestrict-qualified parameter (or, in C++, a__restrict-qualified one) is aliased by another argument, or when copies between those objects overlap 🔥. Check out thisstrcpycall that tries to chop the string by replacing its front with the last four characters. Because the call writes the terminating NUL intoa[4], the copies overlap, and GCC catches it:
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}-Wrestrict catches some simple overlap even without optimization, but it works best at -O2 and above 🧚. It's included in -Wall 💜.
-Wnested-externs(C and Objective-C only) — warns if anexterndeclaration shows up inside a function 👀.-Winline— warns if a function declared asinlinecan't actually be inlined 💅. Even with this on, GCC won't warn about inlining failures for functions declared in system headers 🙈.
The compiler uses a bunch of heuristics to decide whether to inline a function. For example, it looks at the size of the function being inlined and how much inlining has already happened in the current function. So seemingly tiny changes to your source can make -Winline warnings show up or disappear 🌀.
-Wint-in-bool-context— warns about sus uses of integers where booleans are expected 🧐. That includes conditional expressions (?:) using non-boolean integer constants in a boolean context, likeif (a <= b ? 2 : 3). It also includes left-shifting signed integers in a boolean context, likefor (a = 0; 1 << a; a++);. The same goes for all kinds of multiplications, no matter the data type ✖️. This warning is enabled by-Wall💜.-Wno-int-to-pointer-cast— suppresses warnings about casts to a pointer type from an integer of a different size 🫠. In C++, casting to a pointer type of smaller size is an error, so be careful 🚨.Wint-to-pointer-castis enabled by default 🌸.-Wno-pointer-to-int-cast(C and Objective-C only) — suppresses warnings about casts from a pointer to an integer type of a different size 💖.-Winvalid-pch— warns if a precompiled header is found in the search path but can't be used 😭.-Wlong-long— warns if thelong longtype gets used 🍓. It's enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. To shush the warnings, use-Wno-long-long🤫.-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode 🌈. It's enabled by either-Wpedanticor-Wtraditional. To inhibit the warning messages, use-Wno-variadic-macros🎀.-Wno-varargs— stops GCC from warning about questionable usage of the macros that handle variable arguments, likeva_start🦋. These warnings are on by default, so this one is the opt-out 💅.-Wvector-operation-performance— warns if a vector operation isn't implemented using the SIMD capabilities of the architecture ⚡. It's mainly useful for performance tuning 📈. A vector operation can be implemented in a few different ways:- "piecewise" — the scalar operation is performed on every vector element 🧩.
- "in parallel" — the vector operation is implemented using scalars of a wider type, which is normally more performance-efficient 🚀.
- "as a single scalar" — the vector fits into a scalar type 💎.
-Wvla— warns if a variable-length array is used in the code 📏.-Wno-vlasilences the-Wpedanticwarning about variable-length arrays 🙅.-Wvla-larger-than=byte-size— if you use this, GCC warns about declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceedbyte-sizebytes 📦. It's basically the vibe of-Walloca-larger-than=byte-size, but for VLAs 💜.
Heads up: GCC may optimize small VLAs with a known value into plain arrays, so this warning might not trigger for those 🫶.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default 🌸, but it's typically only effective when -ftree-vrp is active (which is the default for -O2 and above) 🔍.
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than— disables-Wvla-larger-than=warnings 🙃. It's equivalent to-Wvla-larger-than=SIZE_MAXor larger.-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types with inconsistent kinds or forms 🧵. It also enables detecting out-of-bounds accesses to those parameters through warnings like-Warray-bounds👀.
If the first function declaration uses the VLA form, the bound in the array is assumed to be the minimum number of elements expected in calls to the function, and also the maximum number of elements it accesses. Not providing arguments of sufficient size, or accessing more than the maximum number of elements, may get diagnosed 🚩.
Here's the drama: the warning triggers for these redeclarations because the first one lets an array of any size get passed to f, while the second one says the array argument must have at least n elements. Calling f with the associated VLA bound parameter larger than the actual VLA bound also triggers a warning 💀.
void f (int n, int[n]);
void f (int, int[]); // warning: argument 2 previously declared as a VLA
void g (int n)
{
if (n > 4)
return;
int a[n];
f (sizeof a, a); // warning: access to a by f may be out of bounds
...
}-Wvla-parameter is included in -Wall 💜. The -Warray-parameter option handles similar problems involving ordinary array arguments 🎀.
-Wvolatile-register-var— warns if a register variable is declaredvolatile🙅♀️. Heads up:volatiledoesn't stop every optimization that might eliminate reads and/or writes to register variables 🤷. This warning is enabled by-Wall💜.-Wdisabled-optimization— warns if a requested optimization pass gets disabled 🛑. This doesn't usually mean anything's wrong with your code. It just means GCC's optimizers can't handle the code effectively 😅. Often the problem is that your code is too big or too complex, since GCC refuses to optimize programs when the optimization would take inordinate amounts of time 🐢.-Wpointer-sign(C and Objective-C only) — warns for pointer argument passing or assignment with different signedness 🧐. This option is only supported for C and Objective-C. It's implied by-Walland by-Wpedantic, and you can turn it off with-Wno-pointer-sign🙅.-Wstack-protector— this one is only active when-fstack-protectoris active 🛡️. It warns about functions that aren't protected against stack smashing 💥.-Woverlength-strings— warns about string constants longer than the "minimum maximum" length the C standard specifies 📜. Modern compilers are usually fine with string constants way longer than that minimum, but very portable programs should avoid longer strings 🌍.
The limit applies after string constant concatenation, and it doesn't count the trailing NUL. In C90 the limit was 509 characters 🍑, and in C99 it got raised to 4095 ✨. C++98 doesn't specify a normative minimum maximum, so GCC doesn't diagnose overlength strings in C++ 🙃.
-Woverlength-strings is implied by -Wpedantic, and it can be disabled with -Wno-overlength-strings 🫶.
-Wunsuffixed-float-constants(C and Objective-C only) — issues a warning for any floating constant without a suffix 🔢. When you combine it with-Wsystem-headers, it also warns about those constants in system header files 🧾. This is super handy when you're prepping code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99 💖.-Wno-lto-type-mismatch— during link-time optimization, GCC won't warn about type mismatches in global declarations coming from different compilation units 🤐. It requires-fltoto be enabled, and it's enabled by default 🌸.-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure marked with thedesignated_initattribute 🎀.
✨ Options That Control Static Analysis ✨
-fanalyzer— turns on a static analysis of program flow 🔍. It looks for "interesting" interprocedural paths through the code, and it issues warnings for problems it finds on them 🧠🔥.
This analysis is much more expensive than other GCC warnings, bestie 💀 so don't be shocked if it takes its sweet time. It's giving deep-dive energy 🫶
Enabling this option effectively enables the following warnings ✨
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-array-index
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalThis option is only available if GCC was configured with analyzer support enabled. So if your build didn't include it, this one's a no-show 🙈
✨ OPTIONS ✨
-Wanalyzer-too-complex— If-fanalyzeris on, the analyzer uses a bunch of heuristics to poke around the control flow and data flow in your program, but really complicated code can totally defeat them 🌀. By default the analysis just quietly stops once the code is too tangled and it hits an internal limit. This option is the one that actually warns you when that happens 👀.-Wno-analyzer-double-fclose— This warning needs-fanalyzerto be enabled (which turns it on), and you can turn it off with-Wno-analyzer-double-fclose. It flags paths through the code where aFILE *might getfclosecalled on it more than once 💅.-Wno-analyzer-double-free— Needs-fanalyzer, and you can silence it with-Wno-analyzer-double-free. It catches paths where a pointer gets a deallocator called on it more than once, whether that'sfreeor a deallocator referenced by attribute "malloc" 🙅♀️.-Wno-analyzer-exposure-through-output-file— Needs-fanalyzer, and disable it with-Wno-analyzer-exposure-through-output-file. This one is the security-sensitive one 🔒. It warns on paths where something sensitive gets written to an output file, like dumping a password into a log file 😭.-Wno-analyzer-file-leak— Needs-fanalyzer, and-Wno-analyzer-file-leakturns it off. It warns on paths where aFILE *stream object from<stdio.h>gets leaked 💧.-Wno-analyzer-free-of-non-heap— Needs-fanalyzer, and-Wno-analyzer-free-of-non-heapshuts it up. It warns on paths wherefreegets called on a pointer that isn't on the heap, like an on-stack buffer or a global 🙈.-Wno-analyzer-malloc-leak— Needs-fanalyzer, and you can disable it with-Wno-analyzer-malloc-leak. It warns on paths where a pointer from an allocator gets leaked, whether that allocator ismallocor a function marked with attribute "malloc" 🫧.-Wno-analyzer-mismatching-deallocation— Needs-fanalyzer, and-Wno-analyzer-mismatching-deallocationis how you mute it. It warns on paths where the wrong deallocation function gets called on a pointer, based on which function originally allocated it. That covers mismatches betweenfree, scalardeleteand vectordelete[], plus anything marked as an allocator/deallocator pair using attribute "malloc" 🧩.-Wno-analyzer-possible-null-argument— Needs-fanalyzer, and disable it with-Wno-analyzer-possible-null-argument. It warns on paths where a possibly-NULL value gets passed to a function argument marked with__attribute__((nonnull))as needing a non-NULL value 🫣.-Wno-analyzer-possible-null-dereference— Needs-fanalyzer, and-Wno-analyzer-possible-null-dereferenceturns it off. It warns on paths where a possibly-NULL value gets dereferenced 👀.-Wno-analyzer-null-argument— Needs-fanalyzer, and-Wno-analyzer-null-argumentdisables it. It warns on paths where a value known to be NULL gets passed to a function argument marked with__attribute__((nonnull))as needing a non-NULL value 💀.-Wno-analyzer-null-dereference— Needs-fanalyzer, and you can mute it with-Wno-analyzer-null-dereference. It warns on paths where a value known to be NULL gets dereferenced 🚨.-Wno-analyzer-shift-count-negative— Needs-fanalyzer, and-Wno-analyzer-shift-count-negativeis the off switch. It warns on paths where a shift is attempted with a negative count 🌀. It's the analyzer's cousin of the-Wshift-count-negativewarning from the C/C++ front ends, except it works by analyzing interprocedural paths instead of just parsing the syntax tree. Heads up though: the analyzer doesn't prioritize finding these, so false negatives are more likely compared to other warnings 🫠.-Wno-analyzer-shift-count-overflow— Needs-fanalyzer, and disable it with-Wno-analyzer-shift-count-overflow. It warns on paths where a shift is attempted with a count greater than or equal to the precision of the operand's type 📏. It's analogous to the-Wshift-count-overflowdiagnostic in the C/C++ front ends, but again it's based on interprocedural paths rather than just the syntax tree. The analyzer doesn't prioritize these either, so false negatives are more likely relative to other warnings 🙃.-Wno-analyzer-stale-setjmp-buffer— Needs-fanalyzer, and-Wno-analyzer-stale-setjmp-bufferdisables it. It warns on paths wherelongjmpis called to rewind to ajmp_buftied to asetjmpcall, in a function that has already returned ⏪. Whensetjmpgets called on ajmp_bufto record a rewind location, it records the stack frame. That frame becomes invalid once the function containing thesetjmpcall returns. Trying to rewind to it withlongjmpwould reference a stack frame that doesn't exist anymore, and that's likely to lead to a crash (or worse) 💥.-Wno-analyzer-tainted-array-index— This one needs both-fanalyzerand-fanalyzer-checker=taintto turn on, and-Wno-analyzer-tainted-array-indexis how you turn it off 🔐. It warns on paths where a value an attacker could control gets used as the index of an array access without being sanitized first 🙅♀️.-Wno-analyzer-unsafe-call-within-signal-handler— Needs-fanalyzer, and disable it with-Wno-analyzer-unsafe-call-within-signal-handler. It warns on paths where a function known to be async-signal-unsafe (likefprintf) gets called from a signal handler 🚫.-Wno-analyzer-use-after-free— Needs-fanalyzer, and-Wno-analyzer-use-after-freemutes it. It warns on paths where a pointer gets used after a deallocator has been called on it, whether that deallocator isfreeor one referenced by attribute "malloc" 🪦.-Wno-analyzer-use-of-pointer-in-stale-stack-frame— Needs-fanalyzer, and-Wno-analyzer-use-of-pointer-in-stale-stack-frameturns it off. It warns on paths where a pointer gets dereferenced while pointing to a variable in a stale stack frame 🥀.-Wno-analyzer-write-to-const— Needs-fanalyzer, and disable it with-Wno-analyzer-write-to-const. It warns on paths where the analyzer spots an attempt to write through a pointer to aconstobject 🙅♀️. The analyzer doesn't prioritize these paths though, so false negatives are more likely relative to other warnings 🤷♀️.-Wno-analyzer-write-to-string-literal— Needs-fanalyzer, and-Wno-analyzer-write-to-string-literalis the off switch. It warns on paths where the analyzer detects an attempt to write through a pointer to a string literal ✍️. Same deal as before: the analyzer doesn't prioritize these, so false negatives are more likely relative to other warnings 🫤.
Pertinent parameters for controlling the exploration are: --param analyzer-bb-explosion-factor=value, --param analyzer-max-enodes-per-program-point=value, --param analyzer-max-recursion-depth=value, and --param analyzer-min-snodes-for-call-summary=value 🎛️.
The following options control the analyzer 🧠:
-fanalyzer-call-summaries— This simplifies interprocedural analysis by computing the effect of certain calls, instead of exploring every path through the function from the callsite to each possible return 🧃. If it's enabled, call summaries only get used for functions with more than one call site and that are sufficiently complicated (as per--param analyzer-min-snodes-for-call-summary=value) ✨.-fanalyzer-checker=name— This restricts the analyzer to run just the named checker, and enables it 🎯. Some checkers are disabled by default (even with-fanalyzer), like the "taint" checker that implements-Wanalyzer-tainted-array-index. You need this option to turn those on 🔓.-fno-analyzer-feasibility— This one's intended for analyzer developers 🧑💻. By default the analyzer checks that there's a feasible control flow path for each diagnostic it emits, meaning the conditions that hold aren't mutually exclusive. Diagnostics with no feasible path get rejected. You can suppress that filtering with-fno-analyzer-feasibility, which is handy for debugging issues in this code 🐛.-fanalyzer-fine-grained— Also intended for analyzer developers 🦋.
Okay bestie, so under the hood the analyzer builds this "exploded graph" 🕸️✨ that mashes control flow graphs together with data flow info. It's giving main character energy 💅
By default, an edge in this graph can hold the effects of a whole run of statements inside one basic block. With -fanalyzer-fine-grained, each statement gets its own edge 🦋
-fanalyzer-show-duplicate-count— 🛠️ this one's for the analyzer devs! If several diagnostics get flagged as duplicates of each other, it drops a note on the best diagnostic telling you how many extra ones the deduplication logic quietly suppressed 🙈-fno-analyzer-state-merge— 🛠️ also for the devs! By default the analyzer tries to keep things simple by merging sufficiently similar states at each program point while it builds its "exploded graph" 🧠 With-fno-analyzer-state-mergethat merging gets turned off, which is useful for debugging state-handling issues 🐛-fno-analyzer-state-purge— 🛠️ devs, you know the drill 💜 By default the analyzer clears out state at a program point when it seems irrelevant, like the values of locals that never get accessed later in the function and don't matter for leak analysis 🧹 With-fno-analyzer-state-purgethat purging gets suppressed, for debugging state-handling issues 🔍-fanalyzer-transitivity— ✨ turns on transitivity of constraints within the analyzer. Very smart, very lowkey 🧠-fanalyzer-verbose-edges— 🛠️ dev-only! It gives you more verbose, lower-level detail in the descriptions of control flow within diagnostic paths 🗺️-fanalyzer-verbose-state-changes— 🛠️ also dev-only 😭 It gives you more verbose, lower-level detail in the descriptions of state machine events within diagnostic paths 🎀-fanalyzer-verbosity=level— 👑 this one controls how complex the control flow paths are that get emitted for analyzer diagnostics. The level can be one of these:0— at this level you see interprocedural call and return events, plus the most pertinent state-change events for a diagnostic. For example, with a double-"free" diagnostic, both calls to "free" get shown 🌸1— same as the previous level, but also shows events for the entry to each function 🚪2— same as the previous level, but also shows control flow events that matter for triggering the issue (like "true path taken" at a conditional) ⭐ This level is the default, so it's the one you get if you don't pick anything 💖3— same as the previous level, but shows all control flow events, not just the significant ones 🌊4— for the analyzer devs 🛠️ It adds various other events meant for debugging the analyzer itself 🧚
-fdump-analyzer— 📝 dumps internal details about what the analyzer is doing into file.analyzer.txt. This option gets overridden by-fdump-analyzer-stderr🙃-fdump-analyzer-stderr— 📝 dumps those same internal details to stderr. This option overrides-fdump-analyzer💅-fdump-analyzer-callgraph— 🗺️ dumps a representation of the call graph that's suitable for viewing with GraphViz, written to file.callgraph.dot 🦋-fdump-analyzer-exploded-graph— 🕸️ dumps a representation of the "exploded graph" that's suitable for viewing with GraphViz, written to file.eg.dot. The nodes are color-coded based on state-machine states so the state changes really pop 🌈-fdump-analyzer-exploded-nodes— 🎯 emits diagnostics showing where nodes in the "exploded graph" sit in relation to the program source 📍-fdump-analyzer-exploded-nodes-2— 📄 dumps a textual representation of the "exploded graph" to file.eg.txt 🍓-fdump-analyzer-exploded-nodes-3— 📂 dumps a textual representation of the "exploded graph" to one dump file per node, named file.eg-id.txt. Heads up, this is typically a large number of dump files 😭💀-fdump-analyzer-feasibility— 🔎 dumps internal details about the analyzer's search for feasible paths. They're written in a form suitable for viewing with GraphViz, using filenames like file..fg.dot and file..tg.dot 🧩-fdump-analyzer-json— 📦 dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change, so don't get too attached to it 🫶-fdump-analyzer-state-purge— 🧹 like-fdump-analyzer-supergraph, this dumps a supergraph representation suitable for GraphViz, but it's annotated with info on what state will be purged at each node. The graph gets written to file.state-purge.dot 💜-fdump-analyzer-supergraph— 🗺️ dumps representations of the "supergraph" suitable for GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. These show all of the control flow graphs in the program, with interprocedural edges for calls and returns ✨ The second dump has annotations showing nodes in the "exploded graph" and the diagnostics attached to them 🔥
🐛 Options for Debugging Your Program 🐛
Okay so if you want GCC to emit extra info for a debugger, in almost all cases you just need to add -g to your other options 🎀
GCC lets you use -g with -O. Fair warning though, the shortcuts optimized code takes can be kinda surprising 😭 Some variables you declared may not exist at all, flow of control might briefly jump somewhere you didn't expect, some statements may not run because they compute constant results or their values are already at hand, and some statements may execute in different places because they got moved out of loops 🌀 Still, it is possible to debug optimized output, so it's reasonable to use the optimizer for programs that might have bugs 🫶
If you're not using some other optimization option, consider using -Og with -g 💅 With no -O option at all, some compiler passes that collect information useful for debugging don't run at all, so -Og may give you a better debugging experience ✨
-g— 🧚 produces debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information 🦋
On most systems that use stabs format, -g turns on extra debugging information that only GDB can use. That extra info makes debugging work better in GDB, but it probably makes other debuggers crash or refuse to read the program 😭 If you want to control for certain whether the extra info gets generated, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below) 🎯
-ggdb— 💖 produces debugging information for use by GDB. This means it uses the most expressive format available (DWARF, stabs, or the native format if neither of those is supported), including GDB extensions if at all possible 👑-gdwarf,-gdwarf-version— 🌈 produces debugging information in DWARF format (if that's supported). The value of version can be 2, 3, 4 or 5. The default version for most targets is 5, with the exception of VxWorks, TPF and Darwin/Mac OS X, which default to version 2, and AIX, which defaults to version 4 🍓
With DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables 🧩
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher ⭐
GCC no longer supports DWARF Version 1, which is substantially different from Version 2 and later. For historical reasons, some other DWARF-related options, such as -fno-dwarf2-cfi-asm), still have a reference to DWARF Version 2 in their names, but they apply to all currently-supported versions of DWARF 🕰️
-gstabs— 🧾 produces debugging information in stabs format (if that's supported), without GDB extensions. This is the format DBX uses on most BSD systems. On MIPS, Alpha and System V Release 4 systems, this option produces stabs debugging output that DBX doesn't understand 😵 On System V Release 4 systems this option requires the GNU assembler 🛠️-gstabs+— 🧾 produces debugging information in stabs format (if that's supported), using GNU extensions that only the GNU debugger (GDB) understands. Using these extensions is likely to make other debuggers crash or refuse to read the program 💀-gxcoff— 🧾 produces debugging information in XCOFF format (if that's supported). This is the format the DBX debugger uses on IBM RS/6000 systems 🖥️-gxcoff+— 🧾 produces debugging information in XCOFF format (if that's supported), using GNU extensions that only the GNU debugger (GDB) understands. Using these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error 🙈-gvms— 🧾 produces debugging information in Alpha/VMS debug format (if that's supported). This is the format the DEBUG tool uses on Alpha/VMS systems 🌸-glevel,-ggdblevel,-gstabslevel,-gxcofflevel,-gvmslevel— 🎚️ requests debugging information and also uses level to decide how much information you get. The default level is 2 💖
Level 0 produces no debug information at all, so -g0 negates -g 🚫
Level 1 produces minimal information, enough for making backtraces in parts of the program you don't plan to debug. That includes descriptions of functions and external variables, and line number tables, but no information about local variables 🧭
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3 🪄
If you use multiple -g options, with or without level numbers, the last such option is the one that takes effect 👑
-gdwarf doesn't accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead, use an additional -glevel option to change the debug level for DWARF 🫶
-fno-eliminate-unused-debug-symbols— 🧹 by default, no debug information is produced for symbols that aren't actually used. Use this option if you want debug information for all symbols 🌈-femit-class-debug-always— 💖 Normally GCC only drops a C++ class's debug info into one object file, but this flag makes it emit that info in every object file that uses the class. Only reach for it if your debugger can't handle GCC's usual class debug output, because it can make the debug info up to a factor of two bigger. 🫶-fno-merge-debug-strings— 🧚 This tells the linker to skip merging identical strings in the debug info across different object files. Not every assembler or linker can merge strings anyway. Merging keeps the output file's debug info smaller, but linking takes longer. Good news: merging is on by default. ✨-fdebug-prefix-map=old=new— 🗺️ Files living in directoryoldget debug info written as if they lived in directorynewinstead. It's handy for swapping a build-time path for an install-time path, or for turning an absolute path into a relative one by using.asnew. This can make builds more reproducible and location independent, but you might need an extra command to tell GDB where the source files are. See also-ffile-prefix-map. 🌸-fvar-tracking— 🔍 This runs the variable tracking pass, which figures out where variables are stored at each spot in the code. The result is better debug info, as long as your debug format supports it. It's switched on automatically when you compile with optimization (-Os,-O,-O2, ...) and debug info (-g), and the format supports it. 👑-fvar-tracking-assignments— 🎀 This annotates assignments to user variables early on and tries to carry those annotations all the way to the end of compilation, so debug info stays good while optimizing. Pairing it with-gdwarf-4is recommended. It can be turned on even when var-tracking is off, but then the annotations get thrown away at the end. By default it's enabled together with-fvar-tracking, except when selective scheduling is on. 💅-gsplit-dwarf— 🦋 With DWARF debug info enabled, this shoves as much of it as possible into a separate output file ending in.dwo. That lets your build system skip linking files that carry debug info. It's only useful if your debugger can actually read.dwofiles. 🌈-gdwarf32,-gdwarf64— 🍓 With DWARF debug info enabled,-gdwarf32picks the 32-bit DWARF format and-gdwarf64picks the 64-bit one. The default depends on the target, but on most targets it's-gdwarf32. The 32-bit format is smaller, but it can't hold more than 2GiB of debug info in any DWARF debug section. The 64-bit format handles bigger debug info, but not every consumer supports it well yet. 🧮-gdescribe-dies— 🌷 This adds description attributes to some DWARF DIEs that don't have a name attribute, like artificial variables, external references, and call site parameter DIEs. ✨-gpubnames— 📚 This generates the DWARF.debug_pubnamesand.debug_pubtypessections. 💜-ggnu-pubnames— 🧩 This generates.debug_pubnamesand.debug_pubtypesin a format that can be converted into a GDB index. It's only useful if your linker can produce GDB index version 7. 🔥-fdebug-types-section— 🫧 With DWARF Version 4 or higher, type DIEs can get their own.debug_typessection instead of living inside.debug_info. That's more efficient since the linker can drop duplicates when they sit in a separate comdat section. The catch: not all DWARF consumers support.debug_typesyet, and on some objects it makes debug info bigger, not smaller. 😭-grecord-gcc-switches,-gno-record-gcc-switches— 📝 This appends the command-line options that might affect code generation to theDW_AT_producerattribute in DWARF debug info. The options are joined with spaces, both from each other and from the compiler version. It's on by default. See also-frecord-gcc-switchesfor another way to store compiler options in the object file. 🎀-gstrict-dwarf— 🚫 This stops you from using extensions from a later DWARF standard version than the one picked with-gdwarf-version. On most targets, though, extensions from later versions that don't conflict with anything are still allowed. 🙈-gno-strict-dwarf— 🕊️ This lets you use extensions from a later DWARF standard version than the one picked with-gdwarf-version. 💅-gas-loc-support— 📍 This tells the compiler the assembler understands.locdirectives, so the compiler can hand off line number table generation to the assembler to produce DWARF2+ tables. That's usually what you want, since assembler-generated line number tables are way more compact than the ones GCC can build itself. It's enabled by default if the assembler was found to support those directives when GCC was configured. 🦋-gno-as-loc-support— 🛠️ This forces GCC to generate DWARF2+ line number tables internally, whenever those tables are going to be generated at all. 🧚-gas-locview-support— 👀 This tells the compiler the assembler supports"view"assignment and reset assertion checking in.locdirectives. It's enabled by default if the assembler was found to support them at GCC configure time. 🌈-gno-as-locview-support— 🔢 This forces GCC to assign view numbers internally, but only if-gvariable-location-viewswas explicitly requested. 💜-gcolumn-info,-gno-column-info— 🎯 This puts location column information into DWARF debug info, not just file and line. It's enabled by default. ⭐-gstatement-frontiers,-gno-statement-frontiers— 🚩 This makes GCC drop markers into the internal representation at the start of each statement, then keeps them roughly where they are throughout compilation. Those markers guide the output of"is_stmt"markers in the line number table. It's enabled by default when you compile with optimization (-Os,-O,-O2, ...) and output DWARF 2 debug info at the normal level. 🧠-gvariable-location-views,-gvariable-location-views=incompat5,-gno-variable-location-views— 🔭 This adds progressive view numbers, implied by the line number table, onto variable location lists. That lets debug consumers peek at the program state at certain points, even when no instructions tied to those source locations exist there. If the assembler can't handle view numbers in line number tables, GCC emits its own line number table instead, which is usually less compact. The augmented tables and lists are fully backward compatible, so older consumers can still read them, they just won't get any benefit. 💅
It's enabled by default when outputting DWARF 2 debug info at the normal level, as long as the assembler supports it, -fvar-tracking-assignments is on, and -gstrict-dwarf is off. If the assembler doesn't support it, the option can still be enabled, but it forces GCC to output internal line number tables. Unless -ginternal-reset-location-views is also enabled, that will most certainly lead to silently mismatching location views. 😱
There's also a proposed representation for view numbers that isn't backward compatible with the location list format from DWARF 5, and you can turn it on with -gvariable-location-views=incompat5.
⚠️ Heads up bestie: -gvariable-location-views=incompat5 may be removed in the future. It's only there as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they won't be able to decode location lists that use it. 🙅♀️
-ginternal-reset-location-views,-gno-internal-reset-location-views— 🧹 This tries to figure out which location views can be dropped from location view lists. It needs the compiler to have very accurate instruction length estimates, which isn't always the case. It can silently generate incorrect view lists when the assembler doesn't support location view lists. The GNU assembler will flag that kind of error as a"view number mismatch". This is only enabled on ports that define a reliable estimation function. ⚠️-ginline-points,-gno-inline-points— 🎈 This generates extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points, so address and view numbers can be computed and output in debug info. You can turn it on independently of location views, in which case the view numbers won't be output. It can only be enabled along with statement frontiers, and it's only enabled by default if location views are enabled. 🌟-gz[=type]— 🗜️ This produces compressed debug sections in DWARF format, if that's supported. If you don't give atype, the default depends on what the assembler and linker can do.typecan benone(don't compress debug sections),zlib(zlib compression in ELF gABI format), orzlib-gnu(zlib compression in the traditional GNU format). If the linker can't write compressed debug sections, the option gets rejected. Otherwise, if the assembler doesn't support them,-gzis quietly ignored when producing object files. 🧃-femit-struct-debug-baseonly— 🪞 This only emits debug info for struct-like types when the base name of the compilation source file matches the base name of the file where the struct is defined. It shrinks debug info a lot, but the debugger can lose a significant amount of type information. See-femit-struct-debug-reducedfor a less aggressive option, and-femit-struct-debug-detailedfor more detailed control. This option only works with DWARF debug output. 🥹
⚠️ Real talk: -femit-struct-debug-baseonly can cost you a big chunk of type information in the debugger, so it's a trade-off, not a free win. 💀
-femit-struct-debug-reduced— okay bestie, this one is a total space saver 💖 Debug info for struct-like types only gets emitted when the base name of the compilation source file matches the base name of the file where the type is defined, unless the struct is a template or defined in a system header ✨- It makes the debug info way smaller 🦋, but you might lose a little type info the debugger would've wanted 😭. For a more aggressive version, check out
-femit-struct-debug-baseonly, and for finer control, look at-femit-struct-debug-detailed. - This only works with DWARF debug output 🌈
- It makes the debug info way smaller 🦋, but you might lose a little type info the debugger would've wanted 😭. For a more aggressive version, check out
-femit-struct-debug-detailed[=spec-list]— this one is the customizable queen 💅 It lets you pick exactly which struct-like types get debug info, mostly so you don't get duplicate struct debug info across different object files in the same program 🧚 It's a detailed version of-femit-struct-debug-reducedand-femit-struct-debug-baseonly, and honestly it covers most needs 🫶- A spec looks like
[dir:|ind:][ord:|gen:](any|sys|base|none). The first two words are optional, and the third word tells the compiler which source files to care about 👑 - First word (
dir:orind:) — this one is optional and limits the spec to structs that are used directly (dir:) or used indirectly (ind:). A struct is used directly when it's the type of a variable or member. Indirect uses come through pointers to structs. In other words, when using an incomplete struct is valid, that use is indirect. Example:struct one direct; struct two * indirect;🙈 - Second word (
ord:orgen:) — also optional, and it limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are a little complicated, ngl. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics too, but-femit-struct-debug-detaileddoesn't implement them yet 🥹 - Third word — this one picks the source files the compiler should emit debug info for.
noneandanyhave their normal meanings.basemeans the base name of the file where the type declaration appears has to match the base name of the main compilation file. So when you compilefoo.c, debug info gets generated for types declared in that file andfoo.h, but not other header files 💖syscovers types that satisfybaseor are declared in system or compiler headers 🎀 - You might need to experiment to find the best settings for your app, no cap 🔥
- The default is
-femit-struct-debug-detailed=all🌸 - This also only works with DWARF debug output ⭐
- A spec looks like
-fno-dwarf2-cfi-asm— emit DWARF unwind info as a compiler-generated".eh_frame"section, instead of using GAS".cfi_*"directives. Main character energy in the assembler, honestly 💜-fno-eliminate-unused-debug-types— okay so normally, when producing DWARF output, GCC skips debug symbols for types that are nowhere used in the source file being compiled 🙈 Sometimes you actually want debug info for every type declared in a compilation unit, even unused ones. For example, if you want to cast a value to a type in the debugger that's declared but never actually used in your program. Just know that this often wastes a ton of space 😭🌈
🎀 Options That Control Optimization 🎀
These options control various sorts of optimizations 💅
Without any optimization option, the compiler's goal is to reduce the cost of compilation and make debugging produce the expected results 🧚 Statements are independent. So if you stop the program at a breakpoint between statements, you can assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code 🦋
Turning on optimization flags makes the compiler try to improve performance and/or code size, but at the expense of compilation time and possibly your ability to debug the program 😭 The compiler optimizes based on what it knows about the program. Compiling multiple files at once to a single output file lets the compiler use information gained from all of the files when compiling each one 🫶
Not all optimizations are controlled directly by a flag. Only the optimizations that have a flag are listed in this section 🙈
Most optimizations are completely disabled at -O0, or if an -O level isn't set on the command line, even if you specify individual optimization flags. Similarly, -Og suppresses many optimization passes 💀
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than the ones listed here. If you want the exact set for each level, invoke GCC with -Q --help=optimizers 🔥
-O,-O1— optimize! Optimizing compilation takes somewhat more time, and a lot more memory for a large function 🌸- With
-O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time ✨ -Oturns on the following optimization flags:
- With
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fmerge-constants -fmove-loop-invariants
-fomit-frame-pointer -freorder-blocks -fshrink-wrap
-fshrink-wrap-separate -fsplit-wide-types -fssa-backprop
-fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-ter -funit-at-a-time-O2— optimize even more 💖 GCC performs nearly all supported optimizations that don't involve a space-speed tradeoff. Compared to-O, this increases both compilation time and the performance of the generated code 👑-O2turns on all optimization flags specified by-O. It also turns on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-pre
-ftree-switch-conversion -ftree-tail-merge -ftree-vrp- > 🚨 Please note the warning under
-fgcseabout invoking-O2on programs that use computed gotos. Bestie, read that one before you ship 🚨 - > 🛡️ NOTE: In Ubuntu 8.10 and later versions,
-D_FORTIFY_SOURCE=2is set by default. In Ubuntu 24.04 and later versions,-D_FORTIFY_SOURCE=3is set by default instead. It's activated when-Ois set to 2 or higher, and it enables additional compile-time and run-time checks for several libc functions 🥹 To disable it, specify either-U_FORTIFY_SOURCEor-D_FORTIFY_SOURCE=0.
-O3— optimize yet more 🔥-O3turns on all optimizations specified by-O2, and it also turns on the following optimization flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— reduce compilation time and make debugging produce the expected results 🧸 This is the default ✨-Os— optimize for size 🍓-Osenables all-O2optimizations except the ones that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stc- It also enables
-finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size 🦋
-Ofast— disregard strict standards compliance 💀-Ofastenables all-O3optimizations. It also enables optimizations that aren't valid for all standard-compliant programs, so proceed with care 🙈- > ⚠️ Because
-Ofastturns on optimizations that aren't valid for every standard-compliant program, your code might behave differently than you expect. It turns on-ffast-math,-fallow-store-data-races, and the Fortran-specific-fstack-arrays(unless-fmax-stack-var-sizeis specified), and-fno-protect-parens😭
- > ⚠️ Because
-Og— optimize the debugging experience 🔍-Ogshould be the optimization level of choice for the standard edit-compile-debug cycle. It offers a reasonable level of optimization while keeping compilation fast and the debugging experience good 💖 It's a better choice than-O0for producing debuggable code, because some compiler passes that collect debug information are disabled at-O0🫶- Like
-O0,-Ogcompletely disables a number of optimization passes, so the individual options controlling them have no effect 💅 Otherwise,-Ogenables all-O1optimization flags except for the ones that may interfere with debugging:
- Like
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp -ftree-dse
-ftree-pta -ftree-sraIf you use multiple -O options, with or without level numbers, the last such option is the one that takes effect 👑
Options of the form -fflag specify machine-independent flags 🌈 Most flags have both positive and negative forms, and the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed, the one you typically use. You can figure out the other form by either removing no- or adding it 🧚
The following options control specific optimizations. They're either activated by -O options or related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired 🎀
-fno-defer-pop— okay so on machines that have to pop the arguments after a function call 🧹, this makes GCC pop them the second each function returns 💅. Heads up though: at-O1and higher,-fdefer-popis the default, which lets arguments pile up on the stack across several calls and then get popped all at once ✨.-fforward-propagate— this one's giving RTL realness 🦋. It runs a forward propagation pass on RTL that tries to combine two instructions, then checks whether the result can be simplified. If loop unrolling is active, the pass runs twice, and the second run gets scheduled after loop unrolling 🔁. Enabled by default at optimization levels-O,-O2,-O3,-Os💖.-ffp-contract=style— bestie, this one's a whole mood 🎀.-ffp-contract=offturns floating-point expression contraction OFF 🙅♀️.-ffp-contract=fastturns it on, so things like fused multiply-add operations can get formed if the target has native support for them 🔥.-ffp-contract=ononly enables contraction if the language standard allows it 📜. Ngl, that one isn't implemented yet and gets treated the same as-ffp-contract=off💀. The default is-ffp-contract=fast💖.-fomit-frame-pointer— omits the frame pointer in functions that don't need one 🧚. This skips the instructions for saving, setting up and restoring the frame pointer, and on lots of targets it also frees up an extra register 🌈. Sometimes it does nothing, because on some targets the standard calling sequence always uses a frame pointer, so it can't be dropped 🙈. Also,-fno-omit-frame-pointerdoesn't promise the frame pointer gets used in every function, since several targets always omit it in leaf functions 🍃. Enabled by default at-Oand higher ⭐.-foptimize-sibling-calls— optimizes sibling and tail recursive calls 🪄. Enabled at levels-O2,-O3,-Os💜.-foptimize-strlen— glow-up time for standard C string functions like"strlen","strchr"or"strcpy", plus their"_FORTIFY_SOURCE"counterparts, which get turned into faster alternatives 🏎️. Enabled at levels-O2,-O3✨.-fno-inline— no function gets expanded inline except the ones marked with the"always_inline"attribute 🙅. This is the default when you're not optimizing 💤. Single functions can be exempted from inlining by marking them with the"noinline"attribute 🚫.-finline-small-functions— integrates functions into their callers when their body is smaller than the code for the function call itself, so the overall program gets smaller 📉. The compiler heuristically decides which functions are simple enough to be worth it 🧠. This applies to all functions, even the ones not declared inline 👑. Enabled at levels-O2,-O3,-Os💅.-findirect-inlining— also inlines indirect calls that are discovered to be known at compile time, thanks to earlier inlining 🎯. This only has an effect if inlining is turned on by-finline-functionsor-finline-small-functions🔗. Enabled at levels-O2,-O3,-Os🫶.-finline-functions— considers all functions for inlining, even if they aren't declared inline 🌸. The compiler heuristically decides which ones are worth it. If every call to a given function gets integrated and that function is declared"static", it's normally not spit out as its own assembler code 🫥. Enabled at levels-O2,-O3,-Os. Also enabled by-fprofile-useand-fauto-profile💖.-finline-functions-called-once— considers all"static"functions called once for inlining into their caller, even if they aren't marked"inline"🥡. If a call gets integrated, the function isn't output as assembler code in its own right 🙃. Enabled at levels-O1,-O2,-O3and-Os, but not-Og🚫.-fearly-inlining— inlines functions marked"always_inline", plus functions whose body seems smaller than the function call overhead, early on. This happens before the-fprofile-generateinstrumentation and the real inlining pass ⏩. Doing it this way makes profiling significantly cheaper and usually makes inlining faster in programs with large chains of nested wrapper functions 🪆. Enabled by default 💯.-fipa-sra— performs interprocedural scalar replacement of aggregates 🧩, removes unused parameters, and swaps parameters passed by reference for parameters passed by value 🔄. Enabled at levels-O2,-O3and-Os🌟.-finline-limit=n— by default GCC caps the size of functions that can be inlined, and this flag gives you coarse control over that cap 🎚️.nis the size of functions that can be inlined, measured in pseudo instructions 📏.max-inline-insns-singleis set ton/2.max-inline-insns-autois set ton/2.- Inlining is actually controlled by a bunch of parameters, which you can set individually with
--param name=value🧁. The-finline-limit=noption sets some of them like this, as shown above. See below for documentation of the individual inlining parameters and their defaults 📚. - Note: there may be no value of
-finline-limitthat results in default behavior 🙃. - Note: a "pseudo instruction" here is just an abstract measurement of a function's size ✨. It is in no way a count of assembly instructions, so its exact meaning might change from one release to another 🫠.
-fno-keep-inline-dllexport— a more fine-grained version of-fkeep-inline-functions, which only applies to functions declared with the"dllexport"attribute or declspec 🎯.-fkeep-inline-functions— in C, emits"static"functions declared"inline"into the object file, even if they've been inlined into all of their callers 📦. This switch doesn't affect functions using the"extern inline"extension in GNU C90 🙅. In C++, emits any and all inline functions into the object file 🌈.-fkeep-static-functions— emits"static"functions into the object file, even if they're never used 🗂️.-fkeep-static-consts— emits variables declared"static const"when optimization isn't turned on, even if they're never referenced 📝. GCC enables this by default 💅. If you want the compiler to check whether a variable is referenced regardless of optimization, use-fno-keep-static-consts🔍.-fmerge-constants— attempts to merge identical constants (string constants and floating-point constants) across compilation units 🧲. This is the default for optimized compilation if the assembler and linker support it 💖. Use-fno-merge-constantsto inhibit this behavior 🛑. Enabled at levels-O,-O2,-O3,-Os✨.-fmerge-all-constants— attempts to merge identical constants and identical variables 🧬. This implies-fmerge-constants🔗. On top of that, it also considers things like constant initialized arrays or initialized constant variables with integral or floating-point types 📚.
🚨 Warning: C and C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. Yikes, bestie 😭.
-fmodulo-sched— performs swing modulo scheduling immediately before the first scheduling pass 🌀. It looks at the innermost loops and reorders their instructions by overlapping different iterations 🔁.-fmodulo-sched-allow-regmoves— performs more aggressive SMS-based modulo scheduling with register moves allowed 💪. Setting this flag deletes certain anti-dependence edges, which triggers the generation of reg-moves based on life-range analysis 🧪. This is only effective with-fmodulo-schedenabled 🫶.-fno-branch-count-reg— disables the optimization pass that hunts for chances to use "decrement and branch" instructions on a count register, instead of decrementing a register, comparing it against zero, and then branching based on the result 🔢. This is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390 🖥️. Note that-fno-branch-count-regdoesn't remove the decrement and branch instructions introduced by other optimization passes 🙈. The default is-fbranch-count-regat-O1and higher, except for-Og✨.-fno-function-cse— doesn't put function addresses in registers; instead, each instruction that calls a constant function contains the function's address explicitly 📍. This gives less efficient code, but some strange hacks that alter the assembler output may get confused by the optimizations performed when this option isn't used 🧌. The default is-ffunction-cse💜.-fno-zero-initialized-in-bss— if the target supports a BSS section, GCC by default puts variables initialized to zero into BSS, which can save space in the resulting code 💾. This option turns that behavior off, because some programs explicitly rely on variables going into the data section 🗺️. For example, the executable can find the beginning of that section and/or make assumptions based on it. The default is-fzero-initialized-in-bss💖.-fthread-jumps— performs optimizations that check whether a jump branches to a location where another comparison, subsumed by the first, is found 🔍. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false 🪝. Enabled at levels-O2,-O3,-Os🌸.-fsplit-wide-types— when using a type that occupies multiple registers, like"long long"on a 32-bit system, splits the registers apart and allocates them independently ✂️. This normally generates better code for those types, but it may make debugging more difficult 🐛. Enabled at levels-O,-O2,-O3,-Os✨.-fsplit-wide-types-early— fully splits wide types early, instead of very late 🌅. This option has no effect unless-fsplit-wide-typesis turned on 🙃. It's the default on some targets 🎯.
-fcse-follow-jumps ====================
Okay so, this one lives in common subexpression elimination (CSE) 💅 It scans through jump instructions when the jump's target isn't reached by any other path. Like, when CSE hits an if with an else clause, it follows the jump when the condition tested is false. Iconic. 🦋
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— bestie, this is basically the sibling of-fcse-follow-jumps✨ It makes CSE follow jumps that conditionally skip over blocks. So when CSE sees a simpleifwith noelseclause, this flag has it follow the jump around the body of theif. Main character energy 👑
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-runs common subexpression elimination after the loop optimizations are done. It's giving second chances 🔁🌸
Enabled at levels -O2, -O3, -Os.
-fgcse— performs a global common subexpression elimination pass. This one also does global constant and copy propagation, so it's doing a lot of work for you 🫶
⚠️ Note: When you're compiling a program that uses computed gotos (a GCC extension), you might get better run-time performance if you turn off the global CSE pass by adding -fno-gcse to the command line. Just something to keep in mind, bestie 😭
Enabled at levels -O2, -O3, -Os.
-fgcse-lm— when this is on, global CSE tries to move loads that are only killed by stores into themselves. Translation: a loop with a load/store sequence can become a load outside the loop and a copy/store inside it. Lowkey genius 🧚
Enabled by default when -fgcse is enabled.
-fgcse-sm— when this is on, a store motion pass runs after global CSE. It tries to move stores out of loops. Used together with-fgcse-lm, a loop with a load/store sequence can become a load before the loop and a store after the loop. Slay 💖
Not enabled at any optimization level.
-fgcse-las— makes the global CSE pass eliminate redundant loads that come after stores to the same memory location. That covers both partial and full redundancies. Very thorough, very serious 🧐✨
Not enabled at any optimization level.
-fgcse-after-reload— runs a redundant load elimination pass after reload. Its whole job is to clean up redundant spilling 🧹💜
Enabled by -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations— tells the loop optimizer to use language constraints to work out bounds for the number of loop iterations. This assumes the loop code doesn't invoke undefined behavior, like causing signed integer overflows or out-of-bound array accesses. Those bounds then guide loop unrolling, loop peeling, and loop exit test optimizations. This option is enabled by default. 🔥
⚠️ Heads up, bestie: this one relies on your code not hitting undefined behavior. Signed integer overflow or out-of-bounds array access breaks that assumption, so keep your code clean 🙈
-funconstrained-commons— tells the compiler that variables declared in common blocks (for example, in Fortran) may later be overridden with longer trailing arrays. This stops certain optimizations that depend on knowing the array bounds from happening 🌈-fcrossjumping— performs the cross-jumping transformation. It unifies equivalent code and saves code size. The result may or may not perform better than code without cross-jumping, so it's a bit of a vibe check 🦋
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec— combines increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions to support it. It's enabled by default at-Oand higher on architectures that do support it ✨-fdce— performs dead code elimination (DCE) on RTL. Enabled by default at-Oand higher. Bye bye, unused code 👋💀-fdse— performs dead store elimination (DSE) on RTL. Enabled by default at-Oand higher. Stores nobody reads get yeeted 🗑️🌸-fif-conversion— attempts to transform conditional jumps into branch-less equivalents. This includes conditional moves, min, max, set flags, and abs instructions, plus some tricks you can do with standard arithmetic. Whether conditional execution gets used on chips that have it is controlled by-fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fif-conversion2— uses conditional execution (where available) to transform conditional jumps into branch-less equivalents 🧚
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor— okay, C++ ABI moment 🤓 The ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards. For a hierarchy with virtual bases, the base and complete variants are clones, meaning two copies of the function. With this option, those variants become thunks that call one common implementation. Less duplicated code, more slay 💅
Enabled by -Os.
-fdelete-null-pointer-checks— assumes that programs can't safely dereference null pointers, and that no code or data lives at address zero. This enables simple constant folding optimizations at all optimization levels. Other passes in GCC also use this flag to control global dataflow analyses that eliminate useless null pointer checks. Those assume that a memory access to address zero always traps, so if a pointer gets checked after it's already been dereferenced, it can't be null. Big brain logic 🧠
⚠️ Not every environment agrees with that assumption! If your program depends on dereferencing address zero, use -fno-delete-null-pointer-checks to turn this optimization off.
This option is enabled by default on most targets. On Nios II ELF it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled 🙅♀️
Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize— attempts to convert calls to virtual functions into direct calls. This happens both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels-O2,-O3,-Os. 🦋-fdevirtualize-speculatively— attempts to convert calls to virtual functions into speculative direct calls. Based on analysis of the type inheritance graph, it figures out the set of likely targets for a given call. If that set is small (preferably size 1), the call becomes a conditional that picks between a direct call and an indirect call. The speculative calls enable more optimizations, like inlining. If they seem useless after further optimization, they get converted back to their original form 🔄-fdevirtualize-at-ltrans— streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. It enables more devirtualization, but it significantly increases the size of the streamed data. For this reason it's disabled by default 📦-fexpensive-optimizations— performs a bunch of minor optimizations that are relatively expensive. Enabled at levels-O2,-O3,-Os. 💸-free— attempts to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends 64-bit registers after writing to their lower 32-bit half 🌸
Enabled for Alpha, AArch64, and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse— in C++, an object's value is only affected by changes within its lifetime. When the constructor begins, the object has an indeterminate value, and any changes during its lifetime are dead once the object is destroyed. Normally dead store elimination takes advantage of this. If your code relies on the object storage's value persisting past the object's lifetime, use this flag to disable the optimization. To keep stores made before the constructor starts (for example, because youroperator newclears the object storage) but still treat the object as dead after the destructor, use-flifetime-dse=1. The default behavior can be explicitly selected with-flifetime-dse=2. And-flifetime-dse=0is the same as-fno-lifetime-dse🧚♀️-flive-range-shrinkage— attempts to decrease register pressure through register live range shrinkage. Helpful for fast processors with small or moderate-size register sets 🏃♀️💨-fira-algorithm=algorithm — picks the coloring algorithm for the integrated register allocator. The algorithm argument can bepriority, which is Chow's priority coloring, orCB, which is Chaitin-Briggs coloring. Chaitin-Briggs isn't implemented for every architecture, but on targets that do support it, it's the default because it generates better code 🎨-fira-region=region — picks the regions the integrated register allocator uses. The region argument should be one of the following:all— uses all loops as register allocation regions. This can give the best results on machines with a small and/or irregular register set 🎯mixed— uses all loops except loops with small register pressure as the regions. This usually gives the best results in most cases and for most architectures, and it's enabled by default when compiling with optimization for speed (-O,-O2, ...) 🏆one— uses all functions as a single region. This typically results in the smallest code size, and it's enabled by default for-Osor-O0🤏
-fira-hoist-pressure— uses IRA to evaluate register pressure in the code hoisting pass when deciding whether to hoist expressions. This usually results in smaller code, but it can slow the compiler down 🐢
This option is enabled at level -Os for all targets.
-fira-loop-pressure— uses IRA to evaluate register pressure in loops when deciding whether to move loop invariants. This usually results in faster and smaller code on machines with large register files (32 or more registers), but it can slow the compiler down 🐌✨
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots— 🧸 Okay so this one turns OFF sharing of stack slots used for saving call-used hard registers that stay alive through a call. Every hard register gets its own separate stack slot, which means function stack frames end up larger 📦-fno-ira-share-spill-slots— 🌸 This one switches off sharing of stack slots for pseudo-registers. Each pseudo-register that doesn't get a hard register gets its own slot, so, again, function stack frames are larger 🥲-flra-remat— 🦋 Turns on CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos from memory, LRA tries to rematerialize (recalculate) them when that's actually profitable 🧮 Switched on at-O2,-O3,-Os✨-fdelayed-branch— 🧚 If the target machine supports it, GCC tries to reorder instructions so it can use the instruction slots that come after delayed branch instructions 🍓 Switched on at-O,-O2,-O3,-Os, but not at-Og🙈-fschedule-insns— ⭐ If supported for the target machine, GCC tries to reorder instructions to get rid of execution stalls caused by data that isn't ready yet ⏳ This is a lifesaver for machines with slow floating point or memory load instructions, because other instructions can get issued while the load or float op is still cooking, until the result is actually needed 💜 Switched on at-O2,-O3🔥-fschedule-insns2— 💖 Basically the same vibe as-fschedule-insns, but it asks for an extra scheduling pass after register allocation is done 🫶 This is especially iconic on machines with a relatively small number of registers, where memory load instructions take more than one cycle 🌈 Switched on at-O2,-O3,-Os👑-fno-sched-interblock— 🙅♀️ Disables instruction scheduling across basic blocks. This is normally on when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher 💅-fno-sched-spec— 🚫 Disables speculative motion of non-load instructions. Normally this is on when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher 🎀-fsched-pressure— 🌊 Enables register pressure sensitive instruction scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with-fschedule-insnsor at-O2or higher 🧠 It can improve the generated code and shrink its size by stopping register pressure from climbing above the number of available hard registers, which would otherwise lead to spills during register allocation 💪-fsched-spec-load— 🎈 Allows speculative motion of some load instructions. Only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher 🌷-fsched-spec-load-dangerous— 😈 Allows speculative motion of more load instructions. Same deal: it only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher. It's called "dangerous" for a reason, bestie 💀-fsched-stalled-insns,-fsched-stalled-insns=n— 🐢 This decides how many insns (if any) can be moved early from the queue of stalled insns into the ready list during the second scheduling pass ⏩-fno-sched-stalled-insnsmeans nothing gets moved early,-fsched-stalled-insns=0means there's no limit on how many queued insns can jump ahead, and plain-fsched-stalled-insnswith no value is the same as-fsched-stalled-insns=1🫶-fsched-stalled-insns-dep,-fsched-stalled-insns-dep=n— 🔍 This sets how many insn groups (cycles) get checked for a dependency on a stalled insn that's a candidate for early removal from the stalled queue 🕵️♀️ It only does anything during the second scheduling pass, and only if-fsched-stalled-insnsis used.-fno-sched-stalled-insns-depis the same as-fsched-stalled-insns-dep=0, and plain-fsched-stalled-insns-depwith no value is the same as-fsched-stalled-insns-dep=1✨-fsched2-use-superblocks— 🧪 When scheduling after register allocation, this uses superblock scheduling. That lets instructions move across basic block boundaries, which can give faster schedules 🏎️
⚠️ Heads up, bestie: this one is experimental! Not all machine descriptions used by GCC model the CPU closely enough, so the algorithm can give unreliable results 😭 This only makes sense when scheduling after register allocation, i.e. with-fschedule-insns2or at-O2or higher.
-fsched-group-heuristic— 👯 Enables the group heuristic in the scheduler, which favors the instruction that belongs to a schedule group 💕 This is enabled by default whenever scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher 🌸-fsched-critical-path-heuristic— 🛣️ Enables the critical-path heuristic in the scheduler, which favors instructions sitting on the critical path 🏃♀️ Also on by default whenever scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher 💖-fsched-spec-insn-heuristic— 🎯 Enables the speculative instruction heuristic in the scheduler, which favors speculative instructions with greater dependency weakness 🥰 Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher ✨-fsched-rank-heuristic— 🏆 Enables the rank heuristic in the scheduler, which favors the instruction belonging to a basic block with greater size or frequency 📈 Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher 👑-fsched-last-insn-heuristic— 🔗 Enables the last-instruction heuristic in the scheduler, which favors the instruction that is less dependent on the last instruction scheduled 🙌 Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher 🦋-fsched-dep-count-heuristic— 🧵 Enables the dependent-count heuristic in the scheduler, which favors the instruction that has more instructions depending on it 🕸️ Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher 💅-freschedule-modulo-scheduled-loops— 🔁 Modulo scheduling happens before the traditional scheduling. If a loop gets modulo scheduled, later scheduling passes may change its schedule, and this option is how you control that behavior 🎛️-fselective-scheduling— 🎀 Schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass 🪄-fselective-scheduling2— 🎀 Schedules instructions using the selective scheduling algorithm too, but this time it runs instead of the second scheduler pass 🫧-fsel-sched-pipelining— 🥁 Enables software pipelining of innermost loops during selective scheduling 🔄 This has no effect unless-fselective-schedulingor-fselective-scheduling2is turned on 🙃-fsel-sched-pipelining-outer-loops— 🪆 When pipelining loops during selective scheduling, this also pipelines the outer loops 🌀 It has no effect unless-fsel-sched-pipeliningis turned on 🙄-fsemantic-interposition— 🎭 Some object formats, like ELF, let the dynamic linker interpose symbols. That means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining, and other optimizations, since the function or variable might get swapped out for something else 🔀 This is super useful for things like rewriting memory allocation functions with a debugging implementation, but it's expensive in terms of code quality 💸 With-fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same 🤝 The flag has no effect for functions explicitly declaredinline, where interposition is never allowed to change semantics, and for symbols explicitly declaredweak🧚-fshrink-wrap— 🎁 Emits function prologues only before the parts of the function that actually need them, instead of at the very top 🧣 Enabled by default at-Oand higher 💖-fshrink-wrap-separate— ✂️ Shrink-wraps the separate parts of the prologue and epilogue separately, so those parts only run when they're needed 🌙 This option is on by default, but it has no effect unless-fshrink-wrapis also turned on and the target supports this 🫶-fcaller-saves— 🛟 Enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore those registers around such calls 🔁 This allocation only happens when it seems to result in better code 🥇 This option is always enabled by default on certain machines, usually the ones that have no call-preserved registers to use instead 🌈 Switched on at-O2,-O3,-Os💫-fcombine-stack-adjustments— 🧩 Tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them 🥞 Enabled by default at-O1and higher 💜-fipa-ra— 🧠 Uses caller save registers for allocation if those registers aren't used by any called function. In that case, there's no need to save and restore them around calls 🪄 This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it 📚 Switched on at-O2,-O3,-Os, however it gets disabled if the generated code will be instrumented for profiling (-p, or-pg), or if the callee's register usage can't be known exactly, which happens on targets that don't expose prologues and epilogues in RTL 🙈-fconserve-stack— 🪶 Attempts to minimize stack usage, using less stack space even if that makes the program slower 🐌 This option implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400🧮-ftree-reassoc— 🌳 Performs reassociation on trees 🔀 Enabled by default at-Oand higher ✨-fcode-hoisting— 🏗️ Performs code hoisting. Code hoisting tries to move the evaluation of expressions that get executed on all paths to the function exit as early as possible 🛫 This is especially useful as a code size optimization, but it often helps code speed too 🚀 Enabled by default at-O2and higher 👑-ftree-pre— okay bestie, this one runs partial redundancy elimination (PRE) on trees 🌳✨ It's on by default at-O2and-O3💖-ftree-partial-pre— makes PRE extra aggressive 😤🔥 It's on by default at-O3only 👑-ftree-forwprop— forward propagation on trees, lowkey essential 🦋 On by default at-Oand higher.-ftree-fre— full redundancy elimination (FRE) on trees 💅 The tea is that FRE only looks at expressions computed on all paths leading to the redundant computation. It's faster than PRE but catches fewer redundancies, so it's giving speed 🏃♀️💨 On by default at-Oand higher.-ftree-phiprop— hoists loads from conditional pointers on trees 🧚 This pass is on by default at-Oand higher.-fhoist-adjacent-loads— speculatively hoists loads from both branches of an if-then-else when the loads come from adjacent spots in the same structure and the target architecture has a conditional move instruction 🫶 On by default at-O2and higher.-ftree-copy-prop— copy propagation on trees, which clears out unnecessary copy operations 🧹 On by default at-Oand higher.-fipa-pure-const— discovers which functions are pure or constant 🔍 On by default at-Oand higher.-fipa-reference— discovers which static variables don't escape the compilation unit 🙈 On by default at-Oand higher.-fipa-reference-addressable— discovers read-only, write-only, and non-addressable static variables 🌸 On by default at-Oand higher.-fipa-stack-alignment— reduces stack alignment on call sites when it's possible 📏 Enabled by default.-fipa-pta— interprocedural pointer analysis plus interprocedural modification and reference analysis 🧠
⚠️ bestie, this one can cause excessive memory and compile-time usage on large compilation units 💀 It is not enabled by default at any optimization level, so you have to ask for it on purpose.
-fipa-profile— interprocedural profile propagation 📊 Functions called only from cold functions get marked as cold, and functions executed once (such as"cold","noreturn", static constructors or destructors) get identified. Cold functions and loop-less parts of functions executed once then get optimized for size 🥡 Enabled by default at-Oand higher.-fipa-modref— interprocedural mod/ref analysis 🔎 It analyzes the side effects of functions (the memory locations they modify or reference) and unlocks better optimization across the function call boundary 🚧 On by default at-Oand higher.-fipa-cp— interprocedural constant propagation 💎 It figures out when values passed to functions are constants and optimizes accordingly. If your app passes constants to functions a lot, this can substantially increase performance 🚀 On by default at-O2,-Os, and-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-cp-clone— function cloning to make IPA constant propagation stronger 👯 When enabled, it clones a function when an externally visible function can be called with constant arguments. Since this can create multiple copies of functions, it may significantly increase code size (see--param ipa-cp-unit-growth=value) 📦 On by default at-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-bit-cp— interprocedural bitwise constant propagation 🔢 On by default at-O2, and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled.-fipa-vrp— interprocedural propagation of value ranges 📐 On by default at-O2. It requires that-fipa-cpis enabled.-fipa-icf— Identical Code Folding for functions and read-only variables 🪞 It reduces code size, but it may disturb unwind stacks by swapping a function out for an equivalent one with a different name. It works way better with link-time optimization turned on 🔗
Although it behaves similarly to the Gold Linker's ICF optimization, GCC ICF works on different levels, so the optimizations aren't the same. There are equivalences only GCC finds and equivalences only Gold finds 🤓
On by default at -O2 and -Os.
-flive-patching=level— controls GCC's optimizations so the output is suitable for live-patching 🩹
Okay so, if the compiler's optimization uses a function's body (or info pulled from its body) to optimize or change another function, that other function is called an impacted function of the first one. If a function gets patched, its impacted functions should get patched too 💅
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when a function gets inlined into it, when a function gets cloned and its caller is changed to call the new clone, or when a function's pureness/constness info is extracted to optimize its direct or indirect callers, etc. 🌀
Usually, the more IPA optimizations are enabled, the more impacted functions each function has. To keep that number under control and make the list easier to compute, IPA optimizations can be partially enabled at two different levels ✨
The level argument should be one of the following:
inline-clone— only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates, and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, so they need to be patched too 🎀
-flive-patching=inline-clone disables these optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enables inlining of static functions 🧊 As a result, when patching a static function, all its callers are impacted and need to be patched as well.
On top of everything -flive-patching=inline-clone disables, -flive-patching=inline-only-static also disables these flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone 🫶
This flag is disabled by default 🙅♀️
🚫 Note that-flive-patchingis not supported with link-time optimization (-flto). Don't mix them, bestie.
-fisolate-erroneous-paths-dereference— detects paths that trigger erroneous or undefined behavior from dereferencing a null pointer 🕳️ It isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap 🪤 On by default at-O2and higher, and it depends on-fdelete-null-pointer-checksalso being enabled.-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior because a null value gets used in a way forbidden by a"returns_nonnull"or"nonnull"attribute 🚫 It isolates those paths from the main control flow and turns the statement into a trap. This is not currently enabled, but it may be enabled by-O2in the future 👀-ftree-sink— forward store motion on trees 🚚 On by default at-Oand higher.-ftree-bit-ccp— sparse conditional bit constant propagation on trees, plus pointer alignment info propagation 🧩 This pass only operates on local scalar variables. It's enabled by default at-O1and higher, except for-Og🙃 It requires that-ftree-ccpis enabled.-ftree-ccp— sparse conditional constant propagation (CCP) on trees 🎯 This pass only operates on local scalar variables and is enabled by default at-Oand higher.-fssa-backprop— propagates info about uses of a value up the definition chain so the definitions can get simplified 🔙 For example, this pass strips sign operations if the sign of a value never matters 🧼 The flag is enabled by default at-Oand higher.-fssa-phiopt— pattern matching on SSA PHI nodes to optimize conditional code 🧐 This pass is enabled by default at-O1and higher, except for-Og.-ftree-switch-conversion— converts simple initializations in aswitchinto initializations from a scalar array 🔀 On by default at-O2and higher.-ftree-tail-merge— looks for identical code sequences, and when it finds them, replaces one with a jump to the other 🪄 This optimization is known as tail merging or cross jumping. It's on by default at-O2and higher. The compilation time in this pass can be limited with themax-tail-merge-comparisonsparameter and themax-tail-merge-iterationsparameter ⏱️-ftree-dce— dead code elimination (DCE) on trees 🗑️ On by default at-Oand higher.-ftree-builtin-call-dce— conditional dead code elimination (DCE) for calls to built-in functions that may seterrnobut are otherwise free of side effects 🧹 On by default at-O2and higher, if-Osis not also specified.-ffinite-loops— assumes that a loop with an exit will eventually take the exit and won't loop forever 🔁 That lets the compiler remove loops that otherwise have no side effects, without treating eventual endless looping as a thing to worry about 💤
This option is enabled by default at -O2 for C++ with -std=c++11 or higher.
-ftree-dominator-opts— a bunch of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation, and expression simplification) based on a dominator tree traversal 🌳💕 It also performs jump threading, which reduces jumps to jumps 🧵 This flag is enabled by default at-Oand higher.-ftree-dse— okay so this one does dead store elimination (DSE) on trees 💅 A dead store is a store into a memory location that gets overwritten by a later store with no loads sneaking in between. That first store is literally dead weight, so it can be deleted ✨ Enabled by default at-Oand higher 🔥-ftree-ch— loop header copying on trees 🦋 It's lowkey iconic because it makes code motion optimizations way more effective, and it saves one jump too 🌸 Enabled by default at-Oand higher. It is not enabled for-Os, since it usually bloats code size 💀-ftree-loop-optimize— loop optimizations on trees 🔁 Enabled by default at-Oand higher 💜-ftree-loop-linear,-floop-strip-mine,-floop-block— loop nest optimizations, same as-floop-nest-optimize🧚 To actually use this transformation, GCC has to be configured with--with-islso the Graphite loop transformation infrastructure is turned on 🛠️-fgraphite-identity— the identity transformation for graphite 🪞 For every SCoP, GCC builds the polyhedral representation and transforms it back to gimple. With-fgraphite-identityyou can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE round trip 🔍 Some minimal optimizations also happen in the code generator isl, like index splitting and dead code elimination in loops.-floop-nest-optimize— the isl based loop nest optimizer 🧠 It's a generic one built on the Pluto optimization algorithms, and it calculates a loop structure tuned for data-locality and parallelism ✨
⚠️ Heads up, bestie: this option is experimental.
-floop-parallelize-all— uses Graphite data dependence analysis to find loops that can be parallelized 🧵 It parallelizes all the loops that can be analyzed as not containing loop carried dependences, without checking whether parallelizing them is actually profitable 🫶-ftree-coalesce-vars— when the program leaves SSA form, this tries to cut down on copying by coalescing versions of different user-defined variables, not just compiler temporaries 🧹 In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise 💖
⚠️ This may severely limit your ability to debug an optimized program compiled with -fno-var-tracking-assignments.
-ftree-loop-if-convert— tries to transform conditional jumps in the innermost loops into branch-less equivalents 🌈 The goal is to remove control flow from the innermost loops so the vectorization pass can handle them better. Enabled by default if vectorization is enabled.-ftree-loop-distribution— loop distribution 🍰 This can improve cache performance on big loop bodies and let further loop optimizations, like parallelization or vectorization, take place. For example, this loop:
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOis transformed to:
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOEnabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns— distributes loops of patterns that can be code generated as calls to a library 📚 Enabled by default at-O2and higher, and by-fprofile-useand-fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero. For example, this loop:
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOis transformed to:
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop is turned into a call to memset zero. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-interchange— loop interchange outside of graphite 🔄 This can improve cache performance on loop nests and let further loop optimizations, like vectorization, take place. For example, this loop nest:
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];is transformed to:
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam— applies unroll and jam transformations on feasible loops 🫓 In a loop nest, this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at-O3. It is also enabled by-fprofile-useand-fauto-profile.-ftree-loop-im— loop invariant motion on trees 🚚 This pass only moves the invariants that are hard to handle at RTL level, like function calls and operations that expand to nontrivial sequences of insns. With-funswitch-loopsit also moves the operands of invariant conditions out of the loop, so loop unswitching can use just trivial invariantness analysis. The pass also includes store motion.-ftree-loop-ivcanon— creates a canonical counter for the number of iterations in loops where figuring that out takes complicated analysis 🔢 Later optimizations can then determine the number easily. It's especially useful alongside unrolling.-ftree-scev-cprop— performs final value replacement 💫 If a variable is modified in a loop so that its value on exit can be figured out from only its initial value and the number of loop iterations, uses of the final value get swapped out for that computation, as long as it's cheap enough. This reduces data dependencies and may allow further simplifications. Enabled by default at-Oand higher.-fivopts— induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 🧮-ftree-parallelize-loops=n— parallelizes loops by splitting their iteration space to run in n threads 🧵 This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, and only for CPU-intensive loops rather than ones constrained by things like memory bandwidth. This option implies-pthread, so it's only supported on targets that have support for-pthread.-ftree-pta— function-local points-to analysis on trees 🎯 Enabled by default at-O1and higher, except for-Og.-ftree-sra— scalar replacement of aggregates 🍬 This pass swaps structure references for scalars so structures don't get committed to memory too early. Enabled by default at-O1and higher, except for-Og.-fstore-merging— merges narrow stores to consecutive memory addresses 🧩 It combines contiguous stores of immediate values that are narrower than a word into fewer, wider stores, cutting down on the number of instructions. Enabled by default at-O2and higher as well as-Os.-ftree-ter— temporary expression replacement during theSSA->normalphase 🪄 Single use/single def temporaries get replaced at their use location with their defining expression. This results in non-GIMPLE code, but it gives the expanders much more complex trees to work on, which leads to better RTL generation. Enabled by default at-Oand higher.-ftree-slsr— straight-line strength reduction on trees 📐 It recognizes related expressions involving multiplications and replaces them with less expensive calculations when possible. Enabled by default at-Oand higher.-ftree-vectorize— vectorization on trees 🚀 This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif they aren't explicitly specified.-ftree-loop-vectorize— loop vectorization on trees 🎀 Enabled by default at-O3and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftree-slp-vectorize— basic block vectorization on trees 💎 Enabled by default at-O3and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-fvect-cost-model=model— alters the cost model used for vectorization 💰 The model argument should be one ofunlimited,dynamic,cheaporvery-cheap.unlimited— the vectorized code path is assumed to be profitable 🌟dynamic— a runtime check guards the vectorized code path, so it only runs for iteration counts that will likely execute faster than the original scalar loop 🏃♀️cheap— disables vectorization of loops where doing so would be cost prohibitive, for example because of required runtime checks for data dependence or alignment, but otherwise it's the same asdynamic🫧very-cheap— only allows vectorization if the vector code would entirely replace the scalar code being vectorized. For example, if each iteration of a vectorized loop could only handle exactly four iterations of the scalar loop, thevery-cheapmodel would only allow vectorization if the scalar iteration count is known to be a multiple of four 🥹
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model— alters the cost model used for vectorization of loops marked with the OpenMPsimddirective 🫧 The model argument should be one ofunlimited,dynamic,cheap. All values of model have the same meaning as described in-fvect-cost-model, and by default a cost model defined with-fvect-cost-modelis used.-ftree-vrp— okay so this one is a whole vibe 💅 It performs Value Range Propagation on trees. It's like constant propagation's cooler cousin, because instead of tracking single values, it tracks ranges of values ✨ That lets the optimizers yeet unnecessary range checks like array bound checks and null pointer checks. It's enabled by default at-O2and higher.
Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled, so don't forget that one, bestie 🙈
-fsplit-paths— this splits paths leading to loop backedges, and it's giving main character energy for two reasons 👑 It can improve dead code elimination and common subexpression elimination. It's enabled by default at-O3and above. 🔥-fsplit-ivs-in-unroller— enables expressing the values of induction variables in later iterations of the unrolled loop using the value from the first iteration. This breaks long dependency chains, which means better efficiency for the scheduling passes 🦋
Ngl, a combination of -fweb and CSE is often enough to get the same effect 🫶 However, that's not reliable when the loop body is more complicated than a single basic block. It also doesn't work at all on some architectures because of restrictions in the CSE pass 😭
This optimization is enabled by default. ✨
-fvariable-expansion-in-unroller— when unrolling a loop, the compiler makes multiple copies of some local variables, which can result in superior code 💜
This one is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining— inlines parts of functions 🧚 This option only has an effect when inlining itself is turned on by-finline-functionsor-finline-small-functions.
Enabled at levels -O2, -O3, -Os. 🌸
-fpredictive-commoning— performs predictive commoning, which is basically reusing computations (especially memory loads and stores) from previous iterations of loops. It's giving efficiency 💅
This option is enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile. 🎀
-fprefetch-loop-arrays— if the target machine supports it, the compiler generates instructions to prefetch memory, which can boost loops that access large arrays 🚀
Honestly this one is a gamble 🎲 It may generate better or worse code, and the results are highly dependent on the structure of loops in your source code.
Disabled at level -Os. 💀
-fno-printf-return-value— do not substitute constants for the known return value of formatted output functions like "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" or "fprintf"). This is iconic because GCC can optimize or even eliminate branches based on the known return value of these functions, as long as the arguments are constant or known to be in a range that makes the exact return value possible 🧠
Here's the example, and it's so good 🤌 When -fprintf-return-value is in effect, both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer, because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields the best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. ✨ The -fprintf-return-value option is enabled by default. 🫶
-fno-peephole,-fno-peephole2— these disable any machine-specific peephole optimizations. The difference between them is in how they're implemented in the compiler, so some targets use one, some use the other, and a few use both 🦋
-fpeephole is enabled by default. -fpeephole2 is enabled at levels -O2, -O3, -Os. 💖
-fno-guess-branch-probability— do not guess branch probabilities using heuristics. Lowkey a niche one, but iykyk 🙈
GCC uses heuristics to guess branch probabilities when they aren't provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics guess the probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can get complex, so sometimes it's useful to disable the heuristics so the effects of __builtin_expect are easier to understand 🧐
You can also specify the expected probability of an expression with the __builtin_expect_with_probability built-in function. 🔮
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os. 🌈
-freorder-blocks— reorders basic blocks in the compiled function to reduce the number of taken branches and improve code locality 🌸
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm— uses the specified algorithm for basic block reordering. Thealgorithmargument can besimple, which doesn't increase code size (except sometimes because of secondary effects like alignment), orstc, the "software trace cache" algorithm.stctries to put all the often executed code together, minimizing the number of branches executed by making extra copies of code 🍓
The default is simple at levels -O, -Os, and stc at levels -O2, -O3. 👑
-freorder-blocks-and-partition— in addition to reordering basic blocks to reduce taken branches, this one partitions hot and cold basic blocks into separate sections of the assembly and.ofiles, to improve paging and cache locality performance 🧚
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or a target specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections. 💀
When -fsplit-stack is used, this option isn't enabled by default (to avoid linker errors), but it may be enabled explicitly if you're using a working linker 🔧
Enabled for x86 at levels -O2, -O3, -Os. 🦋
-freorder-functions— reorders functions in the object file to improve code locality 💅 This is done with special subsections,.text.hotfor the most frequently executed functions and.text.unlikelyfor unlikely executed functions. The reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way 🫶
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes. 🙈
Enabled at levels -O2, -O3, -Os. ✨
-fstrict-aliasing— this lets the compiler assume the strictest aliasing rules that apply to the language being compiled. For C (and C++), it activates optimizations based on the type of expressions 🧠 An object of one type is assumed to never reside at the same address as an object of a different type, unless the types are almost the same. For example, anunsigned intcan alias anint, but not avoid*or adouble. A character type may alias any other type. 🌈
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}Type-punning, which is reading from a different union member than the one most recently written to, is super common 🍓 Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. 💖 However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Similarly, accessing memory by taking the address, casting the resulting pointer, and dereferencing the result has undefined behavior, even if the cast uses a union type. Undefined behavior is not the vibe 💀 For example:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os. 👑
-falign-functions,-falign-functions=n,-falign-functions=n:m,-falign-functions=n:m:n2,-falign-functions=n:m:n2:m2— this aligns the start of functions to the next power-of-two greater than or equal ton, skipping up tom-1bytes 🦋 That way, at least the firstmbytes of the function can be fetched by the CPU without crossing ann-byte alignment boundary. Ifmisn't specified, it defaults ton. 🧚
Here are some examples that are literally so satisfying ✨ -falign-functions=32 aligns functions to the next 32-byte boundary. -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less. -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values lets you specify a secondary alignment. -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2. 💅
Some assemblers only support this flag when n is a power of two. In that case, it gets rounded up. 🙈
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned. 🌸
If n isn't specified or is zero, the compiler uses a machine-dependent default. The maximum allowed n option value is 65536. 🔥
Enabled at levels -O2, -O3. ✨
-flimit-function-alignment— if this option is enabled, the compiler tries to avoid unnecessarily overaligning functions 🫶 It attempts to instruct the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. 🎀-falign-labels,-falign-labels=n,-falign-labels=n:m,-falign-labels=n:m:n2,-falign-labels=n:m:n2:m2— aligns all branch targets to a power-of-two boundary 🧚
Okay so the parameters for this one are basically the same vibe as -falign-functions 💅. -fno-align-labels and -falign-labels=1 are equivalent, bestie, and both mean labels just don't get aligned 🙈
If -falign-loops or -falign-jumps apply and are bigger than this value, their values win instead ✨
If n isn't specified or is zero, GCC uses a machine-dependent default, which is very likely 1, aka no alignment at all 🫶 The maximum allowed n value is 65536.
Enabled at levels -O2, -O3.
🦋 -falign-loops
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2Aligns loops to a power-of-two boundary. If the loops run a bunch of times, the extra cost of the dummy padding instructions is totally worth it 💖
If -falign-labels is bigger than this value, its value gets used instead.
The parameters work the same way as -falign-functions. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n value is 65536.
If n isn't specified or is zero, GCC uses a machine-dependent default.
Enabled at levels -O2, -O3.
🌸 -falign-jumps
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2Aligns branch targets to a power-of-two boundary, but only for branch targets that can only be reached by jumping. In that case, no dummy operations have to be executed, so it's giving efficiency 🧚
If -falign-labels is bigger than this value, its value gets used instead.
The parameters are analogous to -falign-functions. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n isn't specified or is zero, GCC uses a machine-dependent default. The maximum allowed n value is 65536.
Enabled at levels -O2, -O3.
💅 -fno-allocation-dce
Do not remove unused C++ allocations during dead code elimination. Keep those allocations, babe 🙅♀️
⚡ -fallow-store-data-races
This lets the compiler do optimizations that might introduce new data races on stores, without proving the variable can't be accessed by other threads at the same time. It does not affect optimization of local data. It's only safe to use this if you know global data won't be touched by multiple threads 😭
Examples of what this unlocks include hoisting or if-conversions that might cause a value already in memory to be rewritten with that same value. That rewrite is totally fine in a single-threaded context, but can be unsafe with multiple threads. Also, on some processors, if-conversions might be required to enable vectorization.
Enabled at level -Ofast.
⚠️ Only use this if global data is never accessed by multiple threads. Otherwise you can get data races, and that's a real mess, not a cute one 💀
🎀 -funit-at-a-time
This option is only here for compatibility reasons. -funit-at-a-time does absolutely nothing, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
🍓 -fno-toplevel-reorder
Does not reorder top-level functions, variables, and asm statements. They get output in the same order they appear in the input file. When this is on, unreferenced static variables are not removed. It exists to support existing code that depends on a specific ordering. For new code, it's better to use attributes whenever possible 👑
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Also, -fno-toplevel-reorder implies -fno-section-anchors.
🕸️ -fweb
Constructs webs, as commonly used for register allocation purposes, and assigns each web its own individual pseudo register. The register allocation pass can then operate on pseudos directly, and it also strengthens several other optimization passes, like CSE, the loop optimizer, and the trivial dead code remover. Heads up though: it can make debugging impossible, since variables no longer stay in a "home register" 🙈
Enabled by default with -funroll-loops.
🌈 -fwhole-program
Assumes the current compilation unit is the whole program being compiled. All public functions and variables, except main and those merged by the attribute externally_visible, become static functions. That means interprocedural optimizers can go way harder on them ✨
⚠️ Don't use this together with -flto. Relying on a linker plugin gives safer and more precise information instead.
🚀 -flto[=n]
This runs the standard link-time optimizer. When it's invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it into special ELF sections in the object file. When the object files get linked together, all the function bodies are read back from those ELF sections and instantiated as if they were part of the same translation unit. Iconic behavior honestly 🔥
To use the link-time optimizer, you need -flto and your optimization options at compile time and during the final link. It's recommended that you compile every file in the same link with the same options, and specify those options at link time too. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oThe first two invocations save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into one internal image, and compiles the result as usual. Since both are merged into a single image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were one. So, for example, the inliner can inline functions in bar.o into functions in foo.o and vice-versa 🦋
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cThat generates bytecode for foo.c and bar.c, merges them into a single GIMPLE representation, and optimizes them as usual to produce myprog.
The important thing to remember: to enable link-time optimizations, you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override that automatic decision by passing -fno-lto to the link command 💅
To make whole program optimization effective, you need to make certain whole program assumptions. The compiler needs to know which functions and variables can be accessed by libraries and runtime outside the link-time optimized unit. When the linker supports it, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used so the compiler can make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular one, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means object files with LTO information can be linked as normal object files. If -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled, the compile stage is faster, but you cannot perform a regular, non-LTO link on them 😭
When producing the final binary, GCC only applies link-time optimizations to files that contain bytecode. So you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing 🫶
Generally, options specified at link time override those specified at compile time, although in some cases GCC tries to infer link-time options from the settings used to compile the input files.
If you don't specify an optimization level option -O at link time, GCC uses the highest optimization level used when compiling the object files. It's generally ineffective to specify an optimization level only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can only be performed at compile time, not at link time.
There are some code generation flags that GCC preserves when generating bytecodes, since they need to be used during the final link. Right now, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined based on this scheme:
B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>Certain ABI-changing flags have to match across all compilation units. Trying to override them at link time with a conflicting value gets ignored. This includes options like -freg-struct-return and -fpcc-struct-return 🙅♀️
Other options, like -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing, get passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence, and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time 💖
Okay so the diagnostic flags like -Wstringop-overflow get passed through to the link stage 🔗, and their setting matches the compile step at function granularity ✨. This only really matters for diagnostics emitted during optimization, ngl. Bestie, watch out: code transforms like inlining can cause warnings to get enabled or disabled for regions of code that aren't consistent with the setting at compile time 💀.
If you need to pass options to the assembler with -Wa or -Xassembler 🧁, you either compile those translation units with -fno-lto, or you consistently use the same assembler options on every translation unit 🫶. You can also specify assembler options at LTO link time instead, periodt.
To turn on debug info generation you need to supply -g at compile time 🐛. If any input files at link time were built with debug info generation enabled, the link turns debug info generation on too 💖. Fancy debug settings like the dwarf level -gdwarf-5 need to be explicitly repeated on the linker command line, and mixing different settings across translation units is discouraged, not gonna lie 🙈.
If LTO runs into objects with C linkage declared with incompatible types in separate translation units being linked together (undefined behavior according to ISO C99 6.2.7) 😭, a non-fatal diagnostic may be issued. The behavior is still undefined at run time, so no vibes-based fixes allowed 💀. Similar diagnostics may be raised for other languages too.
Another LTO flex is that you can apply interprocedural optimizations to files written in different languages 🌈:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries 👑. In general, when mixing languages in LTO mode, use the same link command options you'd use when mixing languages in a regular (non-LTO) compilation 🎀.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, you can extract and use them in an LTO link as long as you're using a linker with plugin support 🦋. To make static libraries that are LTO-ready, use gcc-ar and gcc-ranlib instead of ar and ranlib, and use gcc-nm to show the symbols of object files with GIMPLE bytecode 🧚. Those commands require that ar, ranlib and nm have been compiled with plugin support, so double-check that first 🔍. At link time, use -fuse-linker-plugin so the library joins the LTO optimization party:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC, where they become part of the aggregated GIMPLE image to be optimized ✨.
If you're not using a linker with plugin support and/or you don't enable the linker plugin, the objects inside libfoo.a still get extracted and linked as usual, but they just don't participate in the LTO optimization process 😔. To make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects 🫶.
Link-time optimizations don't need the whole program present to operate 🌸. If the program doesn't require any symbols to be exported, you can combine -flto and -fwhole-program so the interprocedural optimizers can use more aggressive assumptions, which may lead to improved optimization opportunities 🔥. Use of -fwhole-program isn't needed when the linker plugin is active (see -fuse-linker-plugin).
The current LTO implementation makes no attempt to generate bytecode that's portable between different types of hosts 🚫. The bytecode files are versioned with a strict version check, so bytecode generated in one version of GCC won't work with an older or newer version of GCC 💅.
Link-time optimization doesn't work well with debugging information generation on systems other than those using a combination of ELF and DWARF 😬.
If you specify the optional n, the optimization and code generation done at link time runs in parallel using n parallel jobs, by utilizing an installed make program 🏃♀️. The environment variable MAKE may be used to override the program used.
You can also use -flto=jobserver to let GNU make's job server mode decide the number of parallel jobs 🧵. This is super useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make job server 🕵️♀️.
Use -flto=auto to use GNU make's job server if it's available, or otherwise fall back to autodetecting the number of CPU threads in your system ⚡.
-flto-partition=alg— picks the partitioning algorithm the link-time optimizer uses 🧩. Your options are1to1(mirrors the original source files),balanced(equally sized chunks whenever possible), ormax(a brand new partition for every symbol where possible). Specifyingnoneturns partitioning and streaming off completely 🚫. The default isbalanced.1to1can be used as a workaround for various code ordering issues, butmaxpartitioning is intended for internal testing only 🧪. The valueonespecifies that exactly one partition should be used, whilenonebypasses partitioning and runs the link-time optimization step directly from the WPA phase 🎬.-flto-compression-level=n— sets how squished the intermediate language gets when it's written into LTO object files 🗜️. It's only meaningful with LTO mode (-flto). GCC supports two LTO compression algorithms: forzstd, valid values run from0(no compression) to19(maximum compression), whilezlibsupports values from0to9📏. Values outside this range get clamped to either the minimum or maximum of the supported values. If the option isn't given, a default balanced compression setting is used 🌈.-fuse-linker-plugin— enables the use of a linker plugin during link-time optimization 🔌. This relies on plugin support in the linker, which is available ingoldor in GNU ld 2.21 or newer. It enables extraction of object files with GIMPLE bytecode out of library archives, which improves optimization quality by exposing more code to the link-time optimizer 💖. That information specifies which symbols can be accessed externally (by non-LTO objects or during dynamic linking) 👀. Code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to-fwhole-program. See-fltofor a description of this flag's effect and how to use it 🦋. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker that supports plugins (GNU ld 2.21 or newer, or gold) 🎉.-ffat-lto-objects— fat LTO objects are object files that contain both the intermediate language and the object code 🍱, which makes them usable for both LTO linking and normal linking 💜. This option is only effective when compiling with-flto, and it's ignored at link time.-fno-fat-lto-objectsimproves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO 🛠️. It requires a linker with plugin support for basic functionality. Additionally,nm,arandranlibneed to support linker plugins to allow a full-featured build environment (one capable of building static libraries and so on). GCC provides thegcc-ar,gcc-nmandgcc-ranlibwrappers to pass the right options to these tools, and with non-fat LTO, makefiles need to be modified to use them 🧁. Modern binutils provide a plugin auto-load mechanism, and installing the linker plugin into$libdir/bfd-pluginshas the same effect as using the command wrappers (gcc-ar,gcc-nmandgcc-ranlib) ✨. The default is-fno-fat-lto-objectson targets with linker plugin support 🎀.-fcompare-elim— after register allocation and post-register-allocation instruction splitting, GCC hunts down arithmetic instructions that compute processor flags similar to a comparison based on that arithmetic 🔍. If possible, it wipes out the explicit comparison operation 💅. This pass only applies to certain targets that can't explicitly represent the comparison operation before register allocation is complete 🎯. Enabled at levels-O,-O2,-O3,-Os.-fcprop-registers— after register allocation and post-register-allocation instruction splitting, runs a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy 🪞. Enabled at levels-O,-O2,-O3,-Os.-fprofile-correction— profiles collected using an instrumented binary for multi-threaded programs can be inconsistent because of missed counter updates 😵. When this option is specified, GCC uses heuristics to correct or smooth out those inconsistencies ✨. By default, GCC emits an error message when an inconsistent profile is detected 🚨. This option is enabled by-fauto-profile.-fprofile-partial-training— with"-fprofile-use", all portions of programs not executed during the train run are optimized aggressively for size rather than speed 📦. Sometimes it's not practical to train all possible hot paths in the program (for example, a program may contain functions specific to a given hardware, and training may not cover every hardware configuration the program runs on 🖥️). With"-fprofile-partial-training", profile feedback is ignored for all functions not executed during the train run, so they're optimized as if they were compiled without profile feedback 🧸. This leads to better performance when the train run isn't representative, but it also leads to significantly bigger code 🐘.-fprofile-use,-fprofile-use=path— enables profile feedback-directed optimizations, and the following optimizations, many of which are generally only profitable when profile feedback is available 🔥:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functionsOkay so that's the rest of the crew that rides along with -fprofile-use 💅✨
Before you can use this option, you must first generate profiling information. No skipping this step, bestie 🥺
By default, GCC emits an error message if the feedback profiles don't match the source code 🚨 You can downgrade that error into a warning with -Wno-error=coverage-mismatch. Lowkey, though, this might leave you with poorly optimized code, so be careful 🙈 Also by default, GCC throws a warning if the feedback profiles don't exist at all (see -Wmissing-profile) 💀
If you give it a path, GCC goes looking in that path for the profile feedback data files. See -fprofile-dir 🗂️
-fauto-profile,-fauto-profile=path— Turns on sampling-based feedback-directed optimizations, plus a whole squad of optimizations that are mostly worth it when profile feedback is available 🦋 Here's the lineup:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correctionpath is the name of a file holding AutoFDO profile info. If you leave it off, it defaults to fbdata.afdo in the current directory 🌸
Making an AutoFDO profile data file means running your program with the perf utility on a supported GNU/Linux target system 🐧 For more info, check out https://perf.wiki.kernel.org/ ✨
Here's the vibe:
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programNext up, use the create_gcov tool to turn that raw profile data into something GCC can actually read 🔥 You also have to hand this tool the unstripped binary for your program, so don't strip it yet, bestie. See https://github.com/google/autofdo 🧚
Here's how that looks:
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdoThe following options control compiler behavior regarding floating-point arithmetic. These options are a total trade-off between speed and correctness 🎀 All of them must be specifically enabled. Nothing sneaks on without you asking, periodt 💜
-ffloat-store— Don't keep floating-point variables in registers, and block any other options that might change whether a floating-point value is pulled from a register or from memory 🌈 This stops the undesirable excess precision you get on machines like 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 that extra precision is all upside, but a few programs depend on the exact definition of IEEE floating point. For those, use-ffloat-storeafter modifying them to store all the pertinent intermediate computations into variables.-fexcess-precision=style— This gives you more control over excess precision on machines where floating-point ops happen in a format with more precision or range than the IEEE standard and interchange floating-point types 👑- By default,
-fexcess-precision=fastis in effect. That means ops might run at a wider precision than the types in your source if it makes the code faster, and it's unpredictable when rounding to your source types actually happens 😭 - When compiling C, if
-fexcess-precision=standardis specified, excess precision follows the rules in ISO C99. Casts and assignments both cause values to be rounded to their semantic types (whereas-ffloat-storeonly affects assignments) 🍓 - This option is enabled by default for C if a strict conformance option like
-std=c99is used. -ffast-mathenables-fexcess-precision=fastby default, regardless of whether a strict conformance option is used.-fexcess-precision=standardis not implemented for languages other than C.- On the x86, it has no effect if
-mfpmath=sseor-mfpmath=sse+387is specified. In the former case, IEEE semantics apply without excess precision. In the latter, rounding is unpredictable 🙈
- By default,
-ffast-math— Sets-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fastall at once 🚀 It also causes the preprocessor macro__FAST_MATH__to be defined ✨
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't need the guarantees of these specifications 💀
-fno-math-errno— Don't seterrnoafter calling math functions that are executed with a single instruction, e.g.,sqrt🧠 A program that relies on IEEE exceptions for math error handling might want this for speed while keeping IEEE arithmetic compatibility.
This option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't need the guarantees of these specifications.
- The default is
-fmath-errno. - On Darwin systems, the math library never sets
errno. So there's no reason for the compiler to consider that it might, and-fno-math-errnois the default there 🍎
-funsafe-math-optimizations— Allows optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards 😬 When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.
This option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't need the guarantees of these specifications.
- It enables
-fno-signed-zeros,-fno-trapping-math,-fassociative-mathand-freciprocal-math. - The default is
-fno-unsafe-math-optimizations.
-fassociative-math— Allows re-association of operands in series of floating-point operations 🌀 This violates the ISO C and C++ language standard by possibly changing the computation result. NOTE: re-ordering may change the sign of zero, ignore NaNs, and inhibit or create underflow or overflow. That means it can't be used on code that relies on rounding behavior like"(x + 2**52) - 2**52". It may also reorder floating-point comparisons, so it may not be used when ordered comparisons are required.- This option requires that both
-fno-signed-zerosand-fno-trapping-mathbe in effect. - It doesn't make much sense with
-frounding-math. - For Fortran, the option is automatically enabled when both
-fno-signed-zerosand-fno-trapping-mathare in effect. - The default is
-fno-associative-math.
- This option requires that both
-freciprocal-math— Lets the reciprocal of a value be used instead of dividing by the value, if that enables optimizations ➗ For example,x / ycan be replaced withx * (1/y), which is handy if(1/y)is subject to common subexpression elimination. Heads up: this loses precision and increases the number of flops operating on the value.- The default is
-fno-reciprocal-math.
- The default is
-ffinite-math-only— Allows optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs 🔢
This option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't need the guarantees of these specifications.
- The default is
-fno-finite-math-only.
-fno-signed-zeros— Allows optimizations for floating-point arithmetic that ignore the signedness of zero 0️⃣ IEEE arithmetic specifies the behavior of distinct+0.0and-0.0values, which then prohibits simplifying expressions likex+0.0or0.0*x(even with-ffinite-math-only). This option implies that the sign of a zero result isn't significant.- The default is
-fsigned-zeros.
- The default is
-fno-trapping-math— Compiles code assuming floating-point operations cannot generate user-visible traps 🪤 These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that-fno-signaling-nansbe in effect. Setting it may allow faster code if you rely on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
- The default is
-ftrapping-math.
-frounding-math— Disables transformations and optimizations that assume the default floating-point rounding behavior 🎲 That default is round-to-zero for all floating-point to integer conversions, and round-to-nearest for all other arithmetic truncations. Specify this for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. It also disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.- The default is
-fno-rounding-math.
- The default is
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'sFENV_ACCESSpragma. This command-line option will be used to specify the default state forFENV_ACCESS🧚♀️
-fsignaling-nans— okay so this one is for code that assumes IEEE signaling NaNs can cause user-visible traps during floating-point operations 🔥. Turning it on tells GCC to skip optimizations that might change how many exceptions show up with signaling NaNs 💀. It also implies-ftrapping-math✨.- This option defines the preprocessor macro
__SUPPORT_SNAN__🎀. - The default is
-fno-signaling-nans🫶. - ngl, this one is still experimental, and it does NOT currently guarantee that every GCC optimization affecting signaling NaN behavior gets switched off 😭.
- This option defines the preprocessor macro
-fno-fp-int-builtin-inexact— bestie, this stops the built-in functionsceil,floor,roundandtrunc(plus theirfloatandlong doubleversions) from generating code that raises theinexactfloating-point exception for noninteger arguments 🦋. ISO C99 and C11 let these functions raiseinexact, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008 as integrated into ISO C2X, does not 🙈.- The default is
-ffp-int-builtin-inexact, which allows the exception to be raised, unless C2X or a later C standard is selected 🌸. - This option does nothing unless
-ftrapping-mathis in effect 💅. - Even with
-fno-fp-int-builtin-inexact, if the functions end up calling a library function, theinexactexception may still be raised if that library doesn't follow TS 18661 😤.
- The default is
-fsingle-precision-constant— main character move: floating-point constants get treated as single precision instead of being quietly converted to double-precision constants 👑.-fcx-limited-range— when this is on, it's basically saying a range reduction step isn't needed for complex division 🌈. There's also no checking for whether a complex multiplication or division comes out asNaN + I*NaN, so no rescue attempt in that case 💀.- The default is
-fno-cx-limited-range, but-ffast-mathturns it on ✨. - This option controls the default setting of the ISO C99
CX_LIMITED_RANGEpragma 🧚. Lowkey the wildest part: it applies to all languages 💜.
- The default is
-fcx-fortran-rules— complex multiplication and division follow Fortran rules 🍓. Range reduction happens as part of complex division, but again there's no checking for aNaN + I*NaNresult and no rescue attempt 🙈.- The default is
-fno-cx-fortran-rules🫶.
- The default is
The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code 🧪⚠️. Use these with caution, bestie 🫶.
-fbranch-probabilities— okay this one is so clever 🧠. After running a program compiled with-fprofile-arcs, you can compile it a second time with-fbranch-probabilitiesto improve optimizations based on how many times each branch was taken 📊.- When a program compiled with
-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor each source file 📁. - That data is super 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 aREG_BR_PROBnote on eachJUMP_INSNandCALL_INSN💖. These can be used to improve optimization. Right now they're only used in one place: inreorg.c, instead of guessing which path a branch is most likely to take, theREG_BR_PROBvalues are used to exactly determine which path is taken more often 🔥. - Enabled by
-fprofile-useand-fauto-profile✨.
- When a program compiled with
-fprofile-values— if you combine it with-fprofile-arcs, it adds code that gathers some data about values of expressions in the program 🦋.- With
-fbranch-probabilities, it reads back that data and uses it for optimizations 💅. - Enabled by
-fprofile-generate,-fprofile-use, and-fauto-profile🌸.
- With
-fprofile-reorder-functions— function reordering based on profile instrumentation records the first time each function executes, then puts those functions in ascending order 🪄.- Enabled with
-fprofile-use💜.
- Enabled with
-fvpt— with-fprofile-arcs, this tells the compiler to add code that gathers information about values of expressions 👀.- With
-fbranch-probabilities, it reads that data back and actually performs optimizations based on it 🔥. Right now those optimizations include specializing division operations using what it knows about the value of the denominator ➗. - Enabled with
-fprofile-useand-fauto-profile✨.
- With
-frename-registers— tries to avoid false dependencies in scheduled code by using registers left over after register allocation 🌈. This benefits processors with lots of registers the most 💪.- Heads up: depending on the debug information format the target uses, this can make debugging impossible, since variables no longer stay in a "home register" 😭.
- Enabled by default with
-funroll-loops🎀.
-fschedule-fusion— a target-dependent pass over the instruction stream that groups instructions of the same type together 🧩. The target machine can run them more efficiently when they sit next to each other in the instruction flow ⚡.- Enabled at levels
-O2,-O3,-Os🫶.
- Enabled at levels
-ftracer— performs tail duplication to enlarge superblock size 🦋. This simplifies the control flow of the function so other optimizations can do a better job 💅.- Enabled by
-fprofile-useand-fauto-profile🌸.
- Enabled by
-funroll-loops— unrolls loops whose number of iterations can be figured out at compile time or when the loop is entered 🔁. It implies-frerun-cse-after-loop,-fweband-frename-registers🧚. It also turns on complete loop peeling, meaning loops with a small constant number of iterations get completely removed 🌟.- This option makes code larger, and it may or may not make it run faster 🤷♀️.
- Enabled by
-fprofile-useand-fauto-profile✨.
-funroll-all-loops— unrolls ALL loops, even if their number of iterations is uncertain when the loop is entered 😭. This usually makes programs run more slowly 🐢. It implies the same options as-funroll-loops💜.-fpeel-loops— peels loops that have enough information to show they don't roll much, based on profile feedback or static analysis 🍑. It also turns on complete loop peeling, meaning loops with a small constant number of iterations get completely removed 🧁.- Enabled by
-O3,-fprofile-use, and-fauto-profile🔥.
- Enabled by
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer 🏃♀️.- Enabled at level
-O1and higher, except for-Og💅.
- Enabled at level
-fsplit-loops— splits a loop into two if it contains a condition that's always true for one side of the iteration space and always false for the other ✂️.- Enabled by
-fprofile-useand-fauto-profile🌸.
- Enabled by
-funswitch-loops— moves branches with loop invariant conditions out of the loop 🚪. Duplicates of the loop end up on both branches, modified according to the result of the condition 👯♀️.- Enabled by
-fprofile-useand-fauto-profile🫶.
- Enabled by
-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, GCC creates another version of the loop that assumes the stride is always one 🪡. Here's how it looks, bestie:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is especially useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization by assuming contiguous accesses 💖. This flag is enabled by default at -O3 🔥. It's also enabled by -fprofile-use and -fauto-profile ✨.
-ffunction-sections,-fdata-sections— places each function or data item into its own section in the output file, as long as the target supports arbitrary sections 📦. The function's name or the data item's name determines the section's name in the output file 🏷️.- Use these on systems where the linker can optimize to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with those optimizations 🧠. On AIX, the linker rearranges sections (CSECTs) based on the call graph, and the performance impact varies 🌈.
- Together with a linker garbage collection (
linker --gc-sectionsoption), these options may lead to smaller statically-linked executables (after stripping) 🥹. - On ELF/DWARF systems, these options do not degrade the quality of the debug information 🎀. There could be issues with other object file or debug info formats 🤔.
- Only use these when there are significant benefits 🚨. They make the assembler and linker create larger object and executable files, and they're slower too 🐌. These options also affect code generation: they prevent the compiler and assembler from using relative locations inside a translation unit, since those locations aren't known until link time 🧱. An example of an optimization this blocks is relaxing calls to short call instructions 📞.
-fstdarg-opt— optimizes the prologue of variadic argument functions based on how those arguments are used 🪄.- NOTE, bestie: In Ubuntu 14.10 and later versions,
-fstack-protector-strongis enabled by default for C, C++, ObjC, and ObjC++, if none of-fno-stack-protector,-nostdlib, or-ffreestandingare found 🛡️💅.
- NOTE, bestie: In Ubuntu 14.10 and later versions,
-fsection-anchors— tries to cut down on symbolic address calculations by using shared "anchor" symbols to address nearby objects 🦋. This can help reduce the number of GOT entries and GOT accesses on some targets 🌸.- Say you have the following function
foo:
- Say you have the following function
static int a, b, c;
int foo (void) { return a + b + c; }It usually calculates the addresses of all three variables. But if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead 🧲. The effect is similar to this pseudocode (which isn't valid C, so don't copy-paste it 🙈):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Heads up, bestie: not every target supports this one 😭💅
-fzero-call-used-regs=choice— okay, this one is a whole security moment 🔐✨ It zeroes the call-used registers when a function returns, which can help mitigate Return-Oriented Programming (ROP) attacks or stop information from leaking out through registers 🦋 The possible values ofchoiceare the same as the ones for thezero_call_used_regsattribute, and the default isskip💖 You can also control this for one specific function with the function attributezero_call_used_regs🎀--param name=value— okay so GCC uses little constants behind the scenes to decide how much optimizing to do 🧚 For example, it won't inline a function that has more than a certain number of instructions. You can tweak some of these on the command line with--param🌸 Heads up though: the names and what the values mean are tied to the compiler's internals and can change without notice in future releases ⚠️ To see the minimal, maximal, and default value of a parameter, use--help=param -Q👑 The value is always an integer, and these names are recognized for all targets:predictable-branch-outcome— if a branch is predicted to be taken with a probability lower than this threshold (in percent), it counts as well predictable 🔮max-rtl-if-conversion-insns— RTL if-conversion tries to remove conditional branches around a block and swap them for conditionally executed instructions. This sets the max number of instructions in a block that get considered for that. GCC also uses other heuristics to decide whether it's actually worth it 🤔max-rtl-if-conversion-predictable-cost— the max permissible cost for the sequence if-conversion would generate. It depends on whether the branch is statically determined to be predictable or not. The units match GCC's internalseq_costmetric, and GCC tries to pick a reasonable default using theBRANCH_COSTtarget macro 💅max-crossjump-edges— the max number of incoming edges to consider for cross-jumping.-fcrossjumpingruns inO(N^2)in the number of incoming edges to each block, so bigger values mean more aggressive optimization, longer compile times, and probably a small improvement in executable size 🐢min-crossjump-insns— the min number of instructions that must match at the end of two blocks before cross-jumping happens. This gets ignored when every instruction in the block being cross-jumped from is matched 🌈max-grow-copy-bb-insns— the max code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction 🍓max-goto-duplication-insns— the max number of instructions to duplicate into a block that jumps to a computed goto. To avoidO(N^2)behavior in a bunch of passes, GCC factors computed gotos early and unfactors them as late as possible. Only computed jumps at the end of a basic block with no more thanmax-goto-duplication-insnsget unfactored 🧠max-delay-slot-insn-search— the max number of instructions to check when hunting for something to fill a delay slot. Past this arbitrary number the time savings are minimal, so GCC stops searching. Bigger values mean more aggressive optimization and longer compile times, with probably a small win in execution time 🏃♀️max-delay-slot-live-search— when filling delay slots, the max number of instructions to check while looking for a block with valid live register information. Raising this arbitrary number means more aggressive optimization and longer compile times. This parameter should get removed once the delay slot code is rewritten to maintain the control-flow graph 🛠️max-gcse-memory— the approximate max amount of memory inkBthat can be allocated for global common subexpression elimination. If more memory than that is needed, the optimization just doesn't happen 🧺max-gcse-insertion-ratio— if the ratio of expression insertions to deletions is bigger than this for any expression, RTL PRE inserts or removes the expression and leaves partially redundant computations in the instruction stream 🫠max-pending-list-length— the max number of pending dependencies the scheduler allows before it flushes its current state and starts over. Big functions with few branches or calls can make these lists huge, which wastes memory and resources 📜max-modulo-backtrack-attempts— the max number of backtrack attempts the scheduler makes when modulo scheduling a loop. Larger values can make compile time blow up exponentially 💀max-inline-insns-single— several parameters control the tree inliner, and this one sets the max number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. It only affects functions declaredinlineand methods implemented in a class declaration (C++) 💜max-inline-insns-auto— when you use-finline-functions(which is included in-O3), tons of functions that would normally not be considered for inlining get checked. For those, a different and more restrictive limit applies than for functions declaredinline, and that limit is set with--param max-inline-insns-auto✨max-inline-insns-small— the bound applied to calls considered relevant under-finline-small-functions🌷max-inline-insns-size— the bound applied to calls that are optimized for size. A small growth might be desirable to set up optimization opportunities that inlining exposes 🎯uninlined-function-insns— the number of instructions the inliner counts as function overhead, like the function prologue and epilogue 🧾uninlined-function-time— the extra time the inliner counts as function overhead, like the time needed to run the function prologue and epilogue ⏱️inline-heuristics-hint-percent— the scale (in percent) applied toinline-insns-single,inline-insns-single-O2, andinline-insns-autowhen the inline heuristics hint that inlining is very profitable (it'll enable later optimizations) 🔥uninlined-thunk-insnsanduninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-time, but applied to function thunks 🧵inline-min-speedup— when the estimated performance improvement of caller plus callee runtime goes over this threshold (in percent), the function can be inlined no matter the limits set by--param max-inline-insns-singleand--param max-inline-insns-auto🚀large-function-insns— the limit that defines really large functions. For functions bigger than this after inlining, inlining is constrained by--param large-function-growth. This one mainly exists to avoid extreme compile times caused by the non-linear algorithms the back end uses 🐘large-function-growth— the max growth of a large function caused by inlining, in percent. For example, a value of100limits large function growth to 2.0 times the original size 📈large-unit-insns— the limit that defines a large translation unit. Growth caused by inlining into units bigger than this is limited by--param inline-unit-growth. For small units that might be too tight. Picture a unit with function A (inline) and function B that just calls A three times. If B is small compared to A, the unit growth is 300% and that inlining is still totally sane 🍰 For really big units full of small inlinable functions, the overall unit growth limit is needed to prevent exponential code size explosion. So for smaller units, the size gets bumped up to--param large-unit-insnsbefore--param inline-unit-growthis applied 🫧lazy-modules— the max number of C++ module files that can be open at once when lazy loading 📂inline-unit-growth— the max overall growth of the compilation unit caused by inlining. For example, a value of20limits unit growth to 1.2 times the original size. Cold functions (marked cold with an attribute or by profile feedback) don't count toward the unit size 🥶ipa-cp-unit-growth— the max overall growth of the compilation unit caused by interprocedural constant propagation. For example, a value of10limits unit growth to 1.1 times the original size 🧊ipa-cp-large-unit-insns— the size at which the IPA-CP pass considers a translation unit large 🐋large-stack-frame— the limit that defines large stack frames. While inlining, the algorithm tries not to grow past this limit too much 🥞large-stack-frame-growth— the max growth of large stack frames caused by inlining, in percent. For example, a value of1000limits large stack frame growth to 11 times the original size 🪜max-inline-insns-recursiveandmax-inline-insns-recursive-auto— the max number of instructions an out-of-line copy of a self-recursive inline function can grow into by doing recursive inlining 🌀
Okay so the --param max-inline-insns-recursive thing applies to functions declared inline 💅. For functions that aren't declared inline, recursive inlining only kicks in when -finline-functions (which is included in -O3) is enabled, and then --param max-inline-insns-recursive-auto takes over 🦋.
max-inline-recursive-depth,max-inline-recursive-depth-auto— This sets the max recursion depth for recursive inlining 🌸.--param max-inline-recursive-depthis for functions declared inline, and for functions that aren't,--param max-inline-recursive-depth-autois the one (same-finline-functionsdeal as above) 🫶.min-inline-recursive-probability— Recursive inlining is only worth it for functions that recurse deeply on average 🌈. For functions with shallow recursion it can backfire, bloating the prologue or making the function body more complicated for the other optimizers 😭. When profile feedback is around (see-fprofile-generate), the compiler can guess the real recursion depth from how likely a function is to recurse through a given call expression 🔮. This parameter only lets inlining happen for call expressions whose probability is higher than the threshold, given in percent ✨.early-inlining-insns— How much growth the early inliner is allowed to make 🌱. Basically it means more inlining for code that has a big abstraction penalty 💜.max-early-inliner-iterations— The cap on how many iterations the early inliner runs ⭐. This pretty much bounds how many nested indirect calls it can resolve 🧚. Deeper chains still get handled later, by late inlining 🍓.comdat-sharing-probability— The percent chance that C++ inline functions with comdat visibility get shared across multiple compilation units 🎀.modref-max-bases,modref-max-refs,modref-max-accesses— How many base pointers, references, and accesses mod/ref analysis will store for a single function 🔥.modref-max-tests— The max number of tests the alias oracle can run to tell memory locations apart using mod/ref info 🕵️♀️. This should be bigger than--param modref-max-basesand--param modref-max-refs👑.modref-max-depth— The max depth of the DFS walk used by modref escape analysis 🌊. Setting it to0turns the analysis off completely 🙈.modref-max-escape-points— The max number of escape points modref tracks per SSA-name 💅.profile-func-internal-id— Controls whether the function's internal id gets used when looking things up in the profile database 🧠. If the value is0, the compiler uses an id based on the function's assembler name and filename instead 🫧. That makes old profile data way more forgiving of source changes, like functions getting reordered 🥹.min-vect-loop-bound— Loops with fewer iterations than this won't get vectorized when-ftree-vectorizeis on 🌷. The number of iterations after vectorization also has to be greater than this value for vectorization to happen 💖.gcse-cost-distance-ratio— A scaling factor for figuring out how far GCSE is allowed to move an expression ✨. It's only supported in the code hoisting pass 🦄. The bigger the ratio, the more aggressively simple expressions (ones costing less thangcse-unrestricted-cost) get hoisted 🚀. Setting it to0turns off hoisting of simple expressions 🙅♀️.gcse-unrestricted-cost— The cost, roughly the cost of one typical machine instruction, at which GCSE stops limiting how far an expression can travel 💫. This is only supported in the code hoisting pass too 🫶. The lower the cost, the more aggressive the hoisting gets 🌈. Setting it to0lets every expression travel unrestricted distances 🔓.max-hoist-depth— How deep the search goes in the dominator tree looking for expressions to hoist 🌳. It's there to stop the hoisting algorithm from going quadratic 🙃. A value of0means no limit on the search, but that can slow down compilation of huge functions 🐢.max-tail-merge-comparisons— The max number of similar basic blocks one block gets compared against during tree tail merging 🔍. It exists to avoid quadratic behavior there 💅.max-tail-merge-iterations— The max number of iterations of the pass over the function 🔁. It's a limit on compilation time in tree tail merging ⏱️.store-merging-allow-unaligned— Lets the store merging pass introduce unaligned stores, but only if that's legal 🧩.max-stores-to-merge— The max number of stores the store merging pass will try to combine into wider stores 🧃.max-store-chains-to-track— The max number of store chains tracked at once while trying to merge them into wider stores 🪡.max-stores-to-track— The max number of stores tracked at once while trying to merge them into wider stores in the store merging pass 🎯.max-unrolled-insns— The max number of instructions a loop can have and still get unrolled 🔁. If the loop does get unrolled, this value also decides how many times the loop code is unrolled 🌀.max-average-unrolled-insns— The max number of instructions, weighted by how likely they are to run, that a loop can have and still get unrolled 📊. If it gets unrolled, this parameter also decides how many times the loop code is unrolled 🎡.max-unroll-times— The max number of unrollings for a single loop 🍭.max-peeled-insns— The max number of instructions a loop can have and still get peeled 🍑. If it gets peeled, this parameter also decides how many times the loop code is peeled 🫠.max-peel-times— The max number of peelings for a single loop 🧁.max-peel-branches— The max number of branches on the hot path through the peeled sequence 🌶️.max-completely-peeled-insns— The max number of insns in a completely peeled loop 🪄.max-completely-peel-times— The max number of iterations a loop can have and still be suitable for complete peeling 🎟️.max-completely-peel-loop-nest-depth— The max depth of a loop nest that's suitable for complete peeling 🪆.max-unswitch-insns— The max number of insns in an unswitched loop 🪩.max-unswitch-level— The max number of branches unswitched in a single loop 🎢.lim-expensive— The minimum cost for an expression to count as expensive in loop invariant motion 💸.min-loop-cond-split-prob— When FDO profile info is available, this is the minimum probability a semi-invariant condition statement needs before it triggers a loop split 🔀.iv-consider-all-candidates-bound— Below this number of induction variable candidates, every candidate gets considered for each use 🧮. Above it, only the most relevant ones get considered, so the time complexity doesn't go quadratic 📉.iv-max-considered-uses— The induction variable optimizations give up on any loop with more induction variable uses than this 🙌.iv-always-prune-cand-set-bound— If the candidate set is smaller than this, the compiler always tries to strip out unnecessary ivs whenever it adds a new one 🧹.avg-loop-niter— The average number of iterations of a loop 🔄.dse-max-object-size— The max size, in bytes, of objects tracked bytewise by dead store elimination 📦. Bigger values can mean bigger compilation times ⏳.dse-max-alias-queries-per-store— The max number of queries into the alias oracle per store 🔎. Bigger values mean bigger compilation times, but they can also remove more dead stores 🗑️.scev-max-expr-size— The bound on how big expressions can be in the scalar evolutions analyzer 📏. Big expressions slow the analyzer down 🐌.scev-max-expr-complexity— The bound on how complex expressions can be in the scalar evolutions analyzer 🧶. Complex expressions slow the analyzer down too 😩.max-tree-if-conversion-phi-args— The max number of arguments in a PHI that tree if conversion supports, unless the loop is marked with a simd pragma 🧬.vect-max-version-for-alignment-checks— The max number of run-time checks allowed when the vectorizer does loop versioning for alignment 🛡️.vect-max-version-for-alias-checks— The max number of run-time checks allowed when the vectorizer does loop versioning for alias 🪞.vect-max-peeling-for-alignment— The max number of loop peels the vectorizer can do to improve access alignment 🧘♀️. A value of-1means there's no limit 🌟.max-iterations-to-track— The max number of loop iterations the brute-force algorithm will try to evaluate when it's figuring out how many iterations a loop runs 🕰️.hot-bb-count-fraction— The denominator n of the fraction 1/n of the maximal execution count of a basic block in the whole program. A basic block has to hit at least that much to count as hot 🔥. The default is10000, which means a basic block is hot if its execution count is greater than 1/10000 of the maximal execution count 🥇. A value of0means a basic block is never considered hot 🧊. This is used in non-LTO mode 💅.hot-bb-count-ws-permille— bestie, this is the "how much of the action counts as popular" dial 💅 It's a number from 0 to 1000 (permilles!) for the most-executed slice of the whole profiled program. A basic block only counts as hot if its execution count lands in that slice. The default is990, so a block is hot if it contributes to the upper990permilles, or99.0%, of the profiled execution of the entire program. Setting it to0means nothing is ever hot 🥀 Only used in LTO mode.hot-bb-frequency-fraction— this one is a fraction,1/n, of the entry block's execution frequency for a function. A basic block in that function has to hit at least that much to be considered hot ✨ The default is1000, so a block is hot in a function if it runs more often than1/1000of the entry block's frequency.0means it's never hot, no exceptions 🙈unlikely-bb-count-fraction— the "sus, barely showing up" threshold 🫠 It's a fraction1/nof the number of profiled runs of the whole program. If a basic block's execution count is below that, it's considered unlikely to run. The default is20, so a block counts as unlikely if it runs in fewer than1/20, or5%, of the program's runs.0means it's always considered unlikely executed 💀max-predicted-iterations— the cap on how many loop iterations GCC will guess statically 🔮 This matters when one function has a loop with a known bound and another loop with an unknown bound. The known count gets predicted correctly, but the unknown ones average out to roughly10. So the unbounded loop looks artificially cold next to the other one, which is kinda unfair ngl 😭builtin-expect-probability— controls how likely it is that an expression has the specified value 🎯 It takes a percentage, meaning0 ... 100, as input.builtin-string-cmp-inline-length— the max length of a constant string for a builtin string compare call that's allowed to get inlined 🧚 Anything longer stays as a regular call.align-threshold— pick a fraction of the maximal execution frequency of a basic block in a function to decide whether that block gets aligned 🌸align-loop-iterations— a loop that's expected to iterate at least this many times gets aligned 🦋tracer-dynamic-coverage,tracer-dynamic-coverage-feedback— these two are the bouncers for superblock formation 🚪 Once the given percentage of executed instructions is covered, the tracer stops forming superblocks. This keeps code size from getting totally out of hand 🍓
The tracer-dynamic-coverage-feedback parameter only gets used when profile feedback is available. Real profiles are way less balanced than the statically estimated ones, so the threshold can be set to a bigger value 👑
tracer-max-code-growth— tail duplication stops once code growth hits this percentage 🌈 It's kind of an artificial limit, since most of the duplicates get cleaned up later during cross jumping. So you can set it way higher than the code growth you actually want 💖tracer-min-branch-ratio— reverse growth stops when the reverse probability of the best edge drops below this threshold, given in percent 🪞tracer-min-branch-probability,tracer-min-branch-probability-feedback— forward growth stops if the best edge's probability is lower than this threshold ➡️
Same energy as tracer-dynamic-coverage, there are two versions of this one too. tracer-min-branch-probability-feedback is for compilation with profile feedback, and tracer-min-branch-probability is for compilation without it. The feedback version needs to be more conservative, which means a higher value, so the tracer actually works ⭐
stack-clash-protection-guard-size— sets the size of the operating system's stack guard as2raised to num bytes 🛡️ Higher values can cut down on the number of explicit probes. But if the value is larger than the OS guard, your code is left vulnerable to stack clash style attacks, and that's a big nope 🚨stack-clash-protection-probe-interval— stack clash protection probes the stack space as it gets allocated 🔍 This param sets the max distance between those probes as2raised to num bytes. Higher values might mean fewer explicit probes, but if the value is bigger than the OS guard, your code becomes vulnerable to stack clash style attacks again 😬max-cse-path-length— the max number of basic blocks on a path that CSE (common subexpression elimination) considers 🧭max-cse-insns— the max number of instructions CSE processes before it flushes 🚿ggc-min-expand— okay, so GCC has its own garbage collector to manage its memory 🗑️✨ This sets the minimum percentage the GC's heap is allowed to grow between collections. Tuning it might speed up compilation, but it has zero effect on code generation, so don't stress 🫶
The default is 30% + 70% * (RAM/1GB), capped at a 100% upper bound when RAM >= 1GB. If getrlimit is available, "RAM" means the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC can't calculate RAM on a platform, it falls back to the 30% lower bound. Setting this and ggc-min-heapsize to zero forces a full collection at every opportunity. That's extremely slow, but it can be useful for debugging 🐢
ggc-min-heapsize— the minimum size the garbage collector's heap has to hit before it even starts bothering to collect garbage 🧹 The first collection happens after the heap grows byggc-min-expand%beyondggc-min-heapsize. Again, tuning this might help compile speed, with no effect on code generation 🌷
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS don't get exceeded. It has a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC can't calculate RAM on a platform, the lower bound gets used. Setting this parameter very large effectively disables garbage collection, and setting this and ggc-min-expand to zero causes a full collection at every opportunity 💜
max-reload-search-insns— the max number of instructions reload looks backward through for an equivalent register 🔁 Higher values mean more aggressive optimization, with compile time going up and probably slightly better performance 🏃♀️max-cselib-memory-locations— the max number of memory locations cselib should take into account 🧠 Higher values mean more aggressive optimization, with compile time going up and probably slightly better performance.max-sched-ready-insns— the max number of ready-to-issue instructions the scheduler considers at any given time during the first scheduling pass 📋 Higher values mean more thorough searches, which makes compile time go up with probably little benefit 🥱max-sched-region-blocks— the max number of blocks in a region considered for interblock scheduling 🧩max-pipeline-region-blocks— the max number of blocks in a region considered for pipelining in the selective scheduler 🪄max-sched-region-insns— the max number of insns in a region considered for interblock scheduling 🎀max-pipeline-region-insns— the max number of insns in a region considered for pipelining in the selective scheduler 🌊min-spec-prob— the minimum probability, in percents, of reaching a source block for interblock speculative scheduling 🎲max-sched-extend-regions-iters— the max number of iterations through the CFG to extend regions 🗺️ A value of0disables region extensions, so that's the off switch 🔌max-sched-insn-conflict-delay— the max conflict delay an insn can have and still be considered for speculative motion 🏃sched-spec-prob-cutoff— the minimal probability of speculation success, in percents, that has to be hit before speculative insns get scheduled ✅sched-state-edge-prob-cutoff— the minimum probability an edge needs before the scheduler saves its state across it 💾sched-mem-true-dep-cost— the minimal distance, in CPU cycles, between a store and a load that target the same memory locations ⏱️selsched-max-lookahead— the max size of the lookahead window for selective scheduling 👀 It's basically the depth of search for available instructions 🔎selsched-max-sched-times— the max number of times an instruction gets scheduled during selective scheduling 🔄 This is the limit on how many iterations the instruction can get pipelined through 🫠selsched-insns-to-rename— the max number of best instructions in the ready list that the selective scheduler considers for renaming 🏷️sms-min-sc— the minimum stage count value that the swing modulo scheduler generates 🎢max-last-value-rtl— the max size, measured as the number of RTLs, that the combiner can record in an expression as the last known value of a pseudo register 📝max-combine-insns— the max number of instructions the RTL combiner tries to combine at once 🧃integer-share-limit— small integer constants can use a shared data structure, which saves the compiler memory and makes it faster 🚀 This sets the max value of a shared integer constant 🔢ssp-buffer-size— the minimum size of buffers (aka arrays) that get stack smashing protection when-fstack-protectionis used 🛡️💅
This default before Ubuntu 10.10 was "8" 💅 Right now it's "4", so the stack protector covers way more functions 🛡️
min-size-for-stack-sharing— the smallest a variable can be to join stack slot sharing when you're not optimizing 🧚max-jump-thread-duplication-stmts— the max number of statements a block can have before it has to be duplicated while threading jumps 🔀max-fields-for-field-sensitive— the max number of fields in a structure that get the field-sensitive treatment during pointer analysis 🦋prefetch-latency— an estimate of how many instructions run on average before a prefetch finishes ⏱️ The prefetch distance scales with this number, and bumping it up may mean fewer streams get prefetched (seesimultaneous-prefetches) 📉simultaneous-prefetches— the max number of prefetches that can run at the same time 🏃♀️l1-cache-line-size— the size of a cache line in the L1 data cache, in bytes 📏l1-cache-size— the size of the L1 data cache, in kilobytes 💾l2-cache-size— the size of the L2 data cache, in kilobytes 💾✨prefetch-dynamic-strides— whether the loop array prefetch pass should issue software prefetch hints for non-constant strides 🤔 This can be super helpful sometimes, but a non-constant stride makes it hard to predict when the hints are actually worth it.- Set to
1if prefetch hints should be issued for non-constant strides. - Set to
0if prefetch hints should only be issued for strides that are known to be constant and belowprefetch-minimum-stride.
- Set to
prefetch-minimum-stride— the minimum constant stride, in bytes, before prefetch hints kick in 🚦 If the stride is under this threshold, no prefetch hints get issued.- This one is giving hardware realness 🖥️ Processors with hardware prefetchers can clash with software prefetchers. If the hardware prefetchers have a max stride they can handle, put that number here so the software prefetchers play nice.
- A value of
-1means there's no threshold, so prefetch hints can be issued for any constant stride. - This setting only matters for strides that are known and constant.
loop-interchange-max-num-stmts— the max number of statements in a loop that can be interchanged 🔁loop-interchange-stride-ratio— the min ratio between the strides of two loops for an interchange to actually be profitable 💸min-insn-to-prefetch-ratio— the min ratio of instructions to prefetches needed before prefetching gets enabled in a loop 🧮prefetch-min-insn-to-mem-ratio— the min ratio of instructions to memory references needed before prefetching gets enabled in a loop 🧠use-canonical-types— whether the compiler uses the "canonical" type system 👑 This should always be1, since it's a more efficient internal way of comparing types in C++ and Objective-C++. But if bugs in the canonical type system are causing compile failures, set it to0to turn canonical types off 🚫switch-conversion-max-branch-ratio— switch initialization conversion won't create arrays bigger thanswitch-conversion-max-branch-ratiotimes the number of branches in the switch 🔢max-partial-antic-length— the max length of the partial antic set computed during tree partial redundancy elimination (-ftree-pre) when you're optimizing at-O3and above 🫠 For some source code, the enhanced PRE can run away and eat all the memory on the host machine 💀 This caps the length of those sets so that doesn't happen. Setting it to0allows an unlimited set length.rpo-vn-max-loop-depth— the max loop depth that gets value-numbered optimistically 🌀 When the limit hits, the innermostrpo-vn-max-loop-depthloops and the outermost loop in the loop nest are value-numbered optimistically, and the remaining ones are not.sccvn-max-alias-queries-per-access— the max number of alias-oracle queries we run while hunting for redundancies in loads and stores 🔍 If this limit gets hit, the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.ira-max-loops-num— IRA uses regional register allocation by default 🗂️ If a function has more loops than this number, only the given number of the most frequently-executed loops form regions for regional register allocation.ira-max-conflict-table-size— IRA has a sophisticated way of compressing the conflict table, but huge functions can still need excessive memory 😩 If a function's conflict table could be bigger than the size in MB given by this parameter, the register allocator switches to a faster, simpler, lower-quality algorithm that doesn't build a pseudo-register conflict table.ira-loop-reserved-regs— IRA can evaluate more accurate register pressure in loops for decisions about moving loop invariants (see-O3) 🎯 This sets how many available registers get reserved for other purposes. The default is the best value found from numerous experiments 🧪lra-inheritance-ebb-probability-cutoff— LRA tries to reuse values reloaded into registers in subsequent insns, which is called inheritance 🔁 EBB is used as the region for this optimization. The parameter is a minimal fall-through edge probability, in percent, used to add a BB to the inheritance EBB in LRA. The default was chosen from numerous runs of SPEC2000 on x86-64 📊loop-invariant-max-bbs-in-loop— loop invariant motion can be super expensive for very large loops, both in compilation time and in compile-time memory 💸 Loops with more basic blocks than this parameter won't get loop invariant motion performed on them.loop-max-datarefs-for-datadeps— building data dependencies is expensive for very large loops 😮💨 This limits the number of data references in loops considered for data dependence analysis. These large loops are no handled by the optimizations that use loop data dependencies.max-vartrack-size— the max number of hash table slots to use during variable tracking dataflow analysis of any function 🧩 If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is still exceeded without debug insns, var tracking analysis is completely disabled for the function. Setting this to zero makes it unlimited.max-vartrack-expr-depth— the max number of recursion levels when mapping variable names or debug temporaries to value expressions 🔬 This trades compilation time for more complete debug information. If it's set too low, value expressions that could be represented in debug information may end up not being used. Setting it higher may let the compiler find more complex debug expressions, but compile time and memory use may grow.max-debug-marker-count— a threshold on the number of debug markers (like begin stmt markers) to avoid a complexity explosion at inlining or when expanding to RTL 🚧 If a function has more of these gimple stmts than the limit, they get dropped from the inlined copy of the function and from its RTL expansion.min-nondebug-insn-uid— uids starting at this parameter are used for nondebug insns 🆔 The range below it is reserved exclusively for debug insns created by-fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range runs out.ipa-sra-ptr-growth-factor— IPA-SRA only replaces a pointer to an aggregate with one or more new parameters when their cumulative size is less than or equal toipa-sra-ptr-growth-factortimes the size of the original pointer parameter 📐ipa-sra-max-replacements— the max number of pieces of an aggregate that IPA-SRA tracks 🧷 As a consequence, it's also the max number of replacements of a formal parameter.sra-max-scalarization-size-Ospeed,sra-max-scalarization-size-Osize— the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to swap scalar parts of aggregates for uses of independent scalar variables 🎀 These parameters control the max size, in storage units, of an aggregate considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize), respectively.sra-max-propagations— the max number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track per local variable, to help with copy propagation 🪞tm-max-aggregate-size— when making copies of thread-local variables in a transaction, this sets the size in bytes after which variables get saved with the logging functions instead of save/restore code sequence pairs 🧵 This option only applies when using-fgnu-tm.graphite-max-nb-scop-params— 💅 okay so Graphite loop transforms can go fully exponential if nobody stops them, so the number of parameters in a Static Control Part (SCoP) gets capped. Set it to0to lift the cap. And any variable whose value isn't known at compile time and is defined outside a SCoP counts as one of its parameters. 🧚loop-block-tile-size— 🦋 when loop blocking or strip mining is on (via-floop-blockor-floop-strip-mine), each loop in the nest gets strip mined by a set number of iterations. Want a different strip length? This param is how you change it.ipa-jump-function-lookups— 🔍 how many statements get visited during jump function offset discovery.ipa-cp-value-list-size— 💜 IPA-CP tries to track every value and type passed into a function's parameter so it can propagate them and do devirtualization. This is the max number of values and types it stores for each formal parameter. ✨ipa-cp-eval-threshold— 🔥 IPA-CP scores cloning profitability on its own, and it only goes through with cloning opportunities whose scores exceedipa-cp-eval-threshold.ipa-cp-max-recursive-depth— 🌀 how deep recursive cloning can go for a self-recursive function.ipa-cp-min-recursive-probability— 🎯 recursive cloning only happens when the probability of the call actually being executed is higher than this parameter.ipa-cp-recursion-penalty— 📉 the percentage penalty recursive functions take when they're evaluated for cloning.ipa-cp-single-call-penalty— 📉 the percentage penalty functions take when they contain a single call to another function and get evaluated for cloning.ipa-max-agg-items— 🌸 IPA-CP can also propagate scalar values passed inside an aggregate. This caps how many of those values it handles per parameter.ipa-cp-loop-hint-bonus— 🎀 if IPA-CP figures out that a cloning candidate would make a loop's iteration count known, it adds a bonus ofipa-cp-loop-hint-bonusto that candidate's profitability score. Very main character.ipa-max-loop-predicates— 🧩 the max number of different predicates IPA uses to describe when loops in a function have known properties.ipa-max-aa-steps— 🫶 IPA-CP uses alias analysis to track the values that function parameters point to. So it doesn't spend forever on huge functions, it gives up after examiningipa-max-aa-stepsstatements that modify memory and treats all memory as clobbered.ipa-max-switch-predicate-bounds— 🍓 the max number of boundary endpoints of case ranges in a switch statement. Any switch over this limit won't get a cloning cost predicate for its default case. That predicate is what IPA-CP uses to estimate cloning benefit.ipa-max-param-expr-ops— 🧠 IPA-CP checks conditionals that reference a function parameter to estimate the benefit of cloning for a certain constant value. But if a parameter expression has more operations thanipa-max-param-expr-ops, it's treated as too complicated and IPA analysis skips it. 🙈lto-partitions— 👑 the desired number of partitions produced during WHOPR compilation. It should be more than the number of CPUs used for compilation.lto-min-partition— 🌱 the minimal partition size for WHOPR, measured in estimated instructions. This keeps tiny programs from getting chopped into way too many partitions.lto-max-partition— 🍰 the max partition size for WHOPR, measured in estimated instructions. It sets an upper bound on how big any single partition can get. It's only meant to be used with balanced partitioning.lto-max-streaming-parallelism— 🌊 the max number of parallel processes used for LTO streaming.cxx-max-namespaces-for-diagnostic-help— 🔎 the max number of namespaces GCC will check for suggestions when C++ name lookup fails for an identifier.sink-frequency-threshold— ⭐ the max relative execution frequency (in percent) of the target block compared to the statement's original block, which decides whether a statement can be sunk. Bigger numbers mean more aggressive sinking. Statements with memory operands get a small positive adjustment, since sinking them is even more profitable. 💅max-stores-to-sink— 🛏️ the max number of conditional store pairs that can be sunk. Set it to0if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. 💀case-values-threshold— 🪜 the smallest number of distinct values where a jump table is better than a tree of conditional branches. If it's0, the machine's default gets used.jump-table-max-growth-ratio-for-size— 📦 the max code size growth ratio when expanding into a jump table, in percent. This one applies when you're optimizing for size.jump-table-max-growth-ratio-for-speed— 🏎️ the max code size growth ratio when expanding into a jump table, in percent. This one applies when you're optimizing for speed.tree-reassoc-width— 🌳 the max number of instructions executed in parallel in a reassociated tree. If it's set to a non-zero value, it overrides the target-dependent heuristics that would be used by default.sched-pressure-algorithm— 🎛️ pick between the two implementations of-fsched-pressure. Algorithm 1 is the original and is the more likely of the two to stop instructions from being reordered. Algorithm 2 was designed as a compromise between algorithm 1's pretty conservative approach and the rather aggressive approach of the default scheduler. It leans more heavily on having a regular register file and accurate register pressure classes. For more details, check outhaifa-sched.cin the GCC sources. 📚
The default choice depends on the target. 🎯
max-slsr-cand-scan— 🔭 the max number of existing candidates considered when hunting for a basis for a new straight-line strength reduction candidate.asan-globals— 🛡️ enables buffer overflow detection for global objects. This protection is on by default when you use-fsanitize=address. To turn off global objects protection, use--param asan-globals=0.asan-stack— 🛡️ enables buffer overflow detection for stack objects. This is on by default when using-fsanitize=address. To turn off stack protection, use--param asan-stack=0.asan-instrument-reads— 👀 enables buffer overflow detection for memory reads. This is on by default when using-fsanitize=address. To turn off memory reads protection, use--param asan-instrument-reads=0.asan-instrument-writes— ✍️ enables buffer overflow detection for memory writes. This is on by default when using-fsanitize=address. To turn off memory writes protection, use--param asan-instrument-writes=0.asan-memintrin— 🧰 enables detection for built-in functions. This is on by default when using-fsanitize=address. To turn off built-in functions protection, use--param asan-memintrin=0.asan-use-after-return— ⏰ enables detection of use-after-return. This is on by default when you use the-fsanitize=addressoption. To turn it off, use--param asan-use-after-return=0.
⚠️ Heads up: by default the check is disabled at run time. To turn it on, add "detect_stack_use_after_return=1" to theASAN_OPTIONSenvironment variable.
asan-instrumentation-with-call-threshold— 📞 if the number of memory accesses in the function being instrumented is greater than or equal to this number, GCC uses callbacks instead of inline checks. For example, to disable inline code, use--param asan-instrumentation-with-call-threshold=0.
hwasan-instrument-stack— 🧱 enables hwasan instrumentation of statically sized stack-allocated variables. This is on by default when using-fsanitize=hwaddressand off by default when using-fsanitize=kernel-hwaddress. To turn stack instrumentation off, use--param hwasan-instrument-stack=0. To turn it on, use--param hwasan-instrument-stack=1.hwasan-random-frame-tag— 🎲 when stack instrumentation is in use, tags for stack variables come from a deterministic sequence that starts at a random tag for each frame. If this param isn't set, the tags use the same sequence but start from1. This is enabled by default for-fsanitize=hwaddressand unavailable for-fsanitize=kernel-hwaddress. To disable it, use--param hwasan-random-frame-tag=0.hwasan-instrument-allocas— 🪄 enables hwasan instrumentation of dynamically sized stack-allocated variables. This is on by default when using-fsanitize=hwaddressand off by default when using-fsanitize=kernel-hwaddress. To turn off instrumentation for these variables, use--param hwasan-instrument-allocas=0. To turn it on, use--param hwasan-instrument-allocas=1.hwasan-instrument-reads— 📖 enables hwasan checks on memory reads. Read instrumentation is on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To stop checking memory reads, use--param hwasan-instrument-reads=0. 🫶hwasan-instrument-writes— 💖 Turns on hwasan checks for memory writes. It's already on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress, but if you want to switch it off, use--param hwasan-instrument-writes=0.hwasan-instrument-mem-intrinsics— ✨ Turns on hwasan instrumentation for builtin functions. This is on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To turn it off, use--param hwasan-instrument-mem-intrinsics=0.use-after-scope-direct-emission-threshold— 🦋 If a local variable is at most this many bytes, the shadow memory gets poisoned (or unpoisoned) directly, instead of calling run-time callbacks.tsan-distinguish-volatile— 🌸 Adds special instrumentation for accesses to volatiles.tsan-instrument-func-entry-exit— 🧚 Emits instrumentation calls to__tsan_func_entry()and__tsan_func_exit().max-fsm-thread-path-insns— 💅 The most instructions that get copied when duplicating blocks along a finite state automaton jump thread path.max-fsm-thread-length— 🫶 The most basic blocks allowed on a finite state automaton jump thread path.max-fsm-thread-paths— 👑 The most new jump thread paths GCC will create for a finite state automaton.parloops-chunk-size— 🔥 The chunk size of the omp schedule for loops parallelized by parloops.parloops-schedule— 🎀 The schedule type of the omp schedule for loops parallelized by parloops. Pick fromstatic,dynamic,guided,auto, orruntime.parloops-min-per-thread— ⭐ The minimum number of iterations per thread in an innermost parallelized loop. Once a loop hits this number, the parallel version is preferred over the single-threaded one. Heads up: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.max-ssa-name-query-depth— 🌈 The deepest recursion allowed when querying properties of SSA names, like in fold routines. Each level of recursion means following one use-def chain.max-speculative-devirt-maydefs— 💜 The maximum number of may-defs analyzed while hunting for a must-def that reveals the dynamic type of an object making a virtual call. If we find it, we may be able to devirtualize speculatively.max-vrp-switch-assertions— 🍓 The maximum number of assertions added along the default edge of a switch statement during VRP.evrp-mode— 🦋 Sets the mode that Early VRP operates in.unroll-jam-min-percent— 🌸 The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to count as profitable.unroll-jam-max-unroll— ✨ The maximum number of times the outer loop can be unrolled by the unroll-and-jam transformation.max-rtl-if-conversion-unpredictable-cost— 🙈 The maximum allowed cost for the sequence the RTL if-conversion pass would generate for a branch considered unpredictable.max-variable-expansions-in-unroller— 🧚 If-fvariable-expansion-in-unrolleris used, this is the maximum number of times one individual variable gets expanded during loop unrolling.tracer-min-branch-probability-feedback— 💖 Stops forward growth once the probability of the best edge drops below this threshold (in percent). Only used when profile feedback is available.partial-inlining-entry-probability— 🫶 The maximum probability (in percent, relative to the entry BB of the function) of the entry BB of a split region that still lets partial inlining happen.max-tracked-strlens— 💅 The maximum number of strings whose lengths the strlen optimization pass will track.gcse-after-reload-partial-fraction— 🔥 The threshold ratio for doing partial redundancy elimination after reload.gcse-after-reload-critical-fraction— 👑 The threshold ratio of critical edge execution count that still allows redundancy elimination after reload.max-loop-header-insns— 🌈 The maximum number of insns in a loop header that the copy loop headers pass is allowed to duplicate.vect-epilogues-nomask— 🎀 Turns on loop epilogue vectorization using a smaller vector size.vect-partial-vector-usage— 🍓 Controls when the loop vectorizer considers partial vector loads and stores, as an alternative to falling back to scalar code. It takes one of three values:0— the vectorizer never uses partial vector loads and stores. 🙅1— partial vector loads and stores are allowed if vectorization gets rid of the need for the code to iterate. 🔁2— partial vector loads and stores are allowed in all loops. 💜
This parameter only does something on targets that support partial vector loads and stores.
avoid-fma-max-bits— ⭐ The maximum number of bits for which GCC avoids creating FMAs.sms-loop-average-count-threshold— 💖 A threshold on the average loop count that the swing modulo scheduler takes into account.sms-dfa-history— 🦋 The number of cycles the swing modulo scheduler looks at when checking for conflicts using DFA.max-inline-insns-recursive-auto— 🧚 The maximum number of instructions a non-inline function can grow to through recursive inlining.graphite-allow-codegen-errors— 💀 Decides whether codegen errors should be ICEs when-fcheckingis on.sms-max-ii-factor— 🫶 A tuning factor for the upper bound the swing modulo scheduler uses when scheduling a loop.lra-max-considered-reload-pseudos— 🔥 The maximum number of reload pseudos considered while spilling a non-reload pseudo.max-pow-sqrt-depth— 👑 The maximum depth of sqrt chains used when synthesizing exponentiation by a real constant.max-dse-active-local-stores— 🌸 The maximum number of active local stores in RTL dead store elimination.asan-instrument-allocas— 💅 Turns on asan protection for allocas and VLAs.max-iterations-computation-cost— 🎀 The bound on the cost of an expression used to compute the number of iterations.max-isl-operations— 🌈 The maximum number of isl operations. Setting it to0means unlimited.graphite-max-arrays-per-scop— 🍓 The maximum number of arrays per scop.max-vartrack-reverse-op-size— 🙈 The maximum size of a loc list for which reverse ops should be added.tracer-dynamic-coverage-feedback— ⭐ The percentage of a function, weighted by execution frequency, that trace formation must cover. Only used when profile feedback is available.max-inline-recursive-depth-auto— 💜 The maximum depth of recursive inlining for non-inline functions.fsm-scale-path-stmts— 🦋 A scale factor applied to the number of statements in a threading path, used when comparing against the number of (scaled) blocks.fsm-maximum-phi-arguments— 🧚 The maximum number of arguments a PHI can have before the FSM threader stops trying to thread through its block.uninit-control-dep-attempts— 💀 The maximum number of nested calls used to search for control dependencies during uninitialized variable analysis.sra-max-scalarization-size-Osize— 👑 The maximum size, in storage units, of an aggregate that should be considered for scalarization when compiling for size.fsm-scale-path-blocks— 🫶 A scale factor applied to the number of blocks in a threading path, used when comparing against the number of (scaled) statements.sched-autopref-queue-depth— 🔥 A control flag for the hardware autoprefetcher scheduler model. It's the number of lookahead cycles the model looks into. Set it to' 'to only enable the instruction sorting heuristic. 🤓loop-versioning-max-inner-insns— 🌸 The maximum number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy.loop-versioning-max-outer-insns— 🎀 The maximum number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy. Any instructions in inner loops that directly benefit from versioning don't count toward this.ssa-name-def-chain-limit— 💜 The maximum number of SSA_NAME assignments to follow when figuring out a property of a variable, like its value. This caps the number of iterations or recursive calls GCC makes while optimizing certain statements or checking whether they're valid before issuing diagnostics.store-merging-max-size— 🍓 The maximum size of a single store merging region, in bytes.hash-table-verification-limit— 🙈 The number of elements that get hash table verification done on them for each searched element.max-find-base-term-values— ⭐ The maximum number of VALUEs handled during a singlefind_base_termcall.analyzer-max-enodes-per-program-point— 🦋 The maximum number of exploded nodes per program point within the analyzer. Once a point hits this, analysis of that point stops.analyzer-max-constraints— 💖 The maximum number of constraints per state.analyzer-min-snodes-for-call-summary— 🧚 The minimum number of supernodes within a function before the analyzer considers summarizing its effects at call sites.analyzer-max-enodes-for-full-dump— 🌈 The maximum depth of exploded nodes that can show up in a dot dump. Past this, the dump switches to a less verbose format.analyzer-max-recursion-depth— 💅 The maximum number of times a callsite can appear in a call stack within the analyzer. Once it's hit, analysis of a call that would recurse deeper stops.analyzer-max-svalue-depth— 🫶 The maximum depth of a symbolic value. Anything deeper gets approximated as unknown.analyzer-max-infeasible-edges— 👑 The maximum number of infeasible edges to reject before a diagnostic is declared infeasible.gimple-fe-computed-hot-bb-threshold— 🔥 The number of executions a basic block needs before it counts as hot. This parameter is only used in the GIMPLE FE. ✨analyzer-bb-explosion-factor— this is the cap on how many "after supernode" exploded nodes the analyzer can make per supernode 💅 Once it hits that number, the analysis gets terminated 🛑ranger-logical-depth— the max depth of logical expression evaluation that ranger will dig through when it's checking outgoing edge ranges 🔍openacc-kernels— picks how OpenACCkernelsconstructs get handled 🌸 There are two modes:--param=openacc-kernels=decompose— eachkernelsconstruct gets split into parts, a sequence of compute constructs, and each part is handled individually. Heads up, this one is still work in progress 🚧--param=openacc-kernels=parloops—kernelsconstructs get handled by the parloops pass, en bloc. This is the current default, so you're already getting it 👑
These next ones are the AArch64-only vibes 🌈 They're available on AArch64 targets:
aarch64-sve-compare-costs— when vectorizing for SVE, GCC considers using "unpacked" vectors for smaller elements, and lets the cost model pick the cheapest approach 💖 The cost model also decides between SVE and Advanced SIMD vectorization. Going unpacked means storing smaller elements in larger containers, and accessing elements with extending loads and truncating stores 🎀aarch64-float-recp-precision— the number of Newton iterations used to calculate the reciprocal forfloattype ✨ When division approximation is enabled, the precision of division scales with this param. The default value is1🦋aarch64-double-recp-precision— same energy, but fordoubletype 💜 It's the number of Newton iterations for the reciprocal, and when division approximation is enabled, the precision of division is proportional to this param. The default value is2🌸aarch64-autovec-preference— forces an ISA selection strategy for auto-vectorization 🔥 It accepts values from0to4, inclusive:0— use the default heuristics 🤷1— use only Advanced SIMD for auto-vectorization2— use only SVE for auto-vectorization3— use both Advanced SIMD and SVE, and prefer Advanced SIMD when the costs are deemed equal4— use both Advanced SIMD and SVE, and prefer SVE when the costs are deemed equal
The default value is 0.
aarch64-loop-vect-issue-rate-niters— some AArch64 CPUs tune by weighing both latencies and issue rates when deciding whether a loop gets vectorized with SVE, with Advanced SIMD, or not at all 🧠 If this parameter is set ton, GCC won't use this heuristic for loops that are known to execute in fewer thannAdvanced SIMD iterations 🙈
🎀 Program Instrumentation Options 🎀
GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates 💅 For example, one purpose of instrumentation is collecting profiling stats for finding program hot spots, code coverage analysis, or profile-guided optimizations 📊 Another class adds run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses, plus deliberately hostile attacks such as stack smashing or C++ vtable hijacking 😭 There's also a general hook that can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes 🧚
-p,-pg— generates extra code to write profile information suitable for the analysis programprof(for-p) orgprof(for-pg) 📈 You must use this option when compiling the source files you want data about, and you must also use it when linking.
You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options 🙈
-fprofile-arcs— adds code so that program flow arcs get instrumented 🌈 During execution, the program records how many times each branch and call is executed, and how many times it's taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors, and C++ constructors (and destructors) of classes that are used as a type of a global variable. When the compiled program exits, it saves this data to a file calledauxname.gcdafor each source file 💾 The data can be used for profile-directed optimizations (-fbranch-probabilities) or for test coverage analysis (-ftest-coverage). Each object file'sauxnameis generated from the name of the output file, if explicitly specified and it is not the final executable. Otherwise, it's the basename of the source file. In both cases any suffix is removed (e.g.foo.gcdafor input filedir/foo.c, ordir/foo.gcdafor output file specified as-o dir/foo.o).
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times those arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block. Otherwise, a new basic block must be created to hold the instrumentation code 🧮
--coverage— this is used to compile and link code instrumented for coverage analysis 🧪 It's a synonym for-fprofile-arcs-ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details. Here's the full workflow:
- Compile the source files with
-fprofile-arcsplus optimization and code generation options. For test coverage analysis, use the additional-ftest-coverageoption. You do not need to profile every source file in a program. - Compile the source files additionally with
-fprofile-abs-pathto create absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. - Link your object files with
-lgcovor-fprofile-arcs(the latter implies the former). - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect,
forkcalls are detected and correctly handled without double counting. - For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
-fbranch-probabilities. - For test coverage analysis, use
gcovto produce human readable information from the.gcnoand.gcdafiles. Refer to the gcov documentation for further information.
-ftest-coverage— produces a notes file that thegcovcode-coverage utility can use to show program coverage 📝 Each source file's note file is calledauxname.gcno. Refer to the-fprofile-arcsoption above for a description ofauxnameand instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize 🎯-fprofile-abs-path— automatically converts relative source file names to absolute path names in the.gcnofiles 🗺️ This allowsgcovto find the correct sources in projects where compilations occur with different working directories.-fprofile-dir=path— sets the directory to search for the profile data files in topath📂 This option only affects the profile data generated by-fprofile-generate,-ftest-coverage, and-fprofile-arcs, and used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory aspath, so the profile data file appears in the same directory as the object file. To prevent file name clashing, if the object file name is not an absolute path, GCC mangles the absolute path of thesourcename.gcdafile and uses it as the file name of a.gcdafile. See similar option-fprofile-note.
When an executable is run in a massive parallel environment, it's recommended to save profiles to different folders 🧚 You can do that with variables in path that get exported during run-time:
%p— process ID.%q{VAR}— value of environment variableVAR
-fprofile-generate,-fprofile-generate=path— enables options usually used for instrumenting an application to produce a profile that's useful for later recompilation with profile feedback based optimization 🔥 You must use-fprofile-generateboth when compiling and when linking your program. The following options get enabled:-fprofile-arcs,-fprofile-values,-finline-functions, and-fipa-bit-cp.
If path is specified, GCC looks at that path to find the profile feedback data files. See -fprofile-dir. To optimize the program based on the collected profile information, use -fprofile-use 👑
-fprofile-info-section,-fprofile-info-section=name— registers the profile information in the specified section instead of using a constructor/destructor 🧩 The section name isnameif it's specified, otherwise it defaults to.gcov_info. A pointer to the profile information generated by-fprofile-arcsor-ftest-coverageis placed in the specified section for each translation unit. This option disables the profile information registration through a constructor, and it disables the profile information processing through a destructor. It is not intended to be used in hosted environments such as GNU/Linux. It targets systems with limited resources that don't support constructors and destructors. The linker could collect the input sections in a continuous memory block and define start and end symbols. The runtime support could dump the profiling information registered in this linker set during program termination to a serial line, for example. ✨
Now for the spicy part, bestie: a GNU linker script example that defines a linker output section comes right up next:
Picking up right where that linker script leaves off 🎀
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}-fprofile-note=path— okay so, if you give it apath, GCC drops the.gcnofile right there 📁 bestie, heads up: if you combine it with multiple source files, that.gcnogets overwritten 😭-fprofile-prefix-path=path— this one is a total lifesaver ✨ It's meant to be used withprofile-generate=profile_dirandprofile-use=profile_dir, and it tells GCC where the base directory of your built source tree lives 🌸- By default,
profile_dirgets filled with files that have mangled absolute paths for every object file in the build. Lowkey messy 💀 - It becomes a real problem when the directory you built the instrumented binary in is different from the one you used for the optimized build. The profile data just won't be found 🙈
- So you point
-fprofile-prefix-path=pathat the base directory of the build. It strips the irrelevant part and keeps every file name relative to the main build directory 🦋
- By default,
-fprofile-update=method— this changes how the update works for an app that's been instrumented for profile feedback based optimization 💅 Themethodargument should besingle,atomic, orprefer-atomic.singleis the move for single-threaded apps 🧚atomicis the thread-safe queen, because it prevents profile corruption by emitting thread-safe code 👑
Warning: if an application doesn't properly join all its threads (or it creates a detached thread), a profile file can still get corrupted. Not ideal, bestie 😭
prefer-atomicis transformed intoatomicwhen the target supports it, and intosingleotherwise 🌈- The GCC driver automatically picks
prefer-atomicwhenever-pthreadis in the command line ✨
-fprofile-filter-files=regex— only instrument functions from files whose name matches any of the regular expressions, separated by semi-colons 🎯- Example:
-fprofile-filter-files=main\.c;module.*\.cwill instrument onlymain.cand all C files starting with'module'🍓
- Example:
-fprofile-exclude-files=regex— this one's the mirror vibe 🪞 It says to instrument only functions from files whose name does not match any of the regular expressions, separated by semi-colons.- Example:
-fprofile-exclude-files=/usr/.*will prevent instrumentation of all files located in the/usr/folder 💜
- Example:
-fprofile-reproducible=[multithreaded|parallel-runs|serial]— this controls how reproducible the profile gathered by"-fprofile-generate"is 💖 It's giving rebuild-the-same-outcome energy, which is super useful for things like distribution packages 📦serial— the profile is reproducible provided the trained program behaves the same at each invocation of the train run, it's not multi-threaded, and profile data streaming is always done in the same order 🫶 Also, streaming happens at the end of the program run, but also before the"fork"function is invoked. Wild, right? 😭- Heads up: execution counts for some parts of a program can depend on things like the length of temporary file names or memory space randomization (which might mess with hash-table collision rate) 🙈 That non-reproducible part can be annotated with the
no_instrument_functionfunction attribute. You can usegcov-dumpwith-lto dump the gathered data and check that it's actually reproducible 🔍 parallel-runs— the collected profile stays reproducible no matter what order the data gets streamed intogcdafiles 🎀 This lets you run multiple instances of the instrumented program in parallel (like withmake -j) 🏃♀️
Heads up: running in parallel reduces the quality of the gathered data, especially for indirect call profiling 💀
-fsanitize=address— enables AddressSanitizer, a fast memory error detector 🔥 It instruments memory access instructions to catch out-of-bounds and use-after-free bugs. This option also enables-fsanitize-address-use-after-scope.- More details live at
https://github.com/google/sanitizers/wiki/AddressSanitizer📖 - You can tweak run-time behavior with the
ASAN_OPTIONSenvironment variable. Set it to"help=1"and the available options get shown when the instrumented program starts up 🧚 - A list of supported options is at
https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags💜 - This can't be combined with
-fsanitize=threador-fsanitize=hwaddress. Also, the only target-fsanitize=hwaddressis currently supported on is AArch64 ✨
- More details live at
-fsanitize=kernel-address— AddressSanitizer for the Linux kernel 🐧 More info athttps://github.com/google/kasan🌸-fsanitize=hwaddress— hardware-assisted AddressSanitizer 🦋 It uses a hardware ability to ignore the top byte of a pointer, which lets it detect memory errors with a low memory overhead. Memory access instructions get instrumented to catch out-of-bounds and use-after-free bugs, and this option enables-fsanitize-address-use-after-scope.- See
https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.htmlfor more details 📖 - Run-time behavior can be influenced with the
HWASAN_OPTIONSenvironment variable. When set to"help=1", the available options show up at startup of the instrumented program 💅 - This can't be combined with
-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64 🫶
- See
-fsanitize=kernel-hwaddress— hardware-assisted AddressSanitizer for compiling the Linux kernel 👑 It's similar to-fsanitize=kernel-addressbut uses an alternate instrumentation method. It's also similar to-fsanitize=hwaddress, just with instrumentation differences needed for the kernel 🌈 Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.
Note: this option has different defaults to-fsanitize=hwaddress. Instrumenting the stack and alloca calls isn't on by default, but you can still turn it on with the command-line options--param hwasan-instrument-stack=1and--param hwasan-instrument-allocas=1respectively 🧚 Using a random frame tag isn't implemented for kernel instrumentation 💀
-fsanitize=pointer-compare— instruments comparison operations (<,<=,>,>=) with pointer operands 👀 It must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it can't be combined with-fsanitize=thread.- Heads up: the check is disabled at run time by default 🙈 To turn it on, add
"detect_invalid_pointer_pairs=2"to theASAN_OPTIONSenvironment variable. "detect_invalid_pointer_pairs=1"only detects invalid operation when both pointers are non-null 🔍
- Heads up: the check is disabled at run time by default 🙈 To turn it on, add
-fsanitize=pointer-subtract— same energy, but for subtraction with pointer operands ✨ It must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it can't be combined with-fsanitize=thread.- Again, the check is disabled at run time by default. Add
"detect_invalid_pointer_pairs=2"toASAN_OPTIONSto enable it 💖 "detect_invalid_pointer_pairs=1"only detects invalid operation when both pointers are non-null 🌸
- Again, the check is disabled at run time by default. Add
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector 🧵 It instruments memory access instructions to catch data race bugs. More details athttps://github.com/google/sanitizers/wiki#threadsanitizer📖- Run-time behavior can be influenced with the
TSAN_OPTIONSenvironment variable. For a list of supported options, checkhttps://github.com/google/sanitizers/wiki/ThreadSanitizerFlags💜 - This can't be combined with
-fsanitize=addressor-fsanitize=leak🙅♀️ - Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (
-fnon-call-exceptions) 💀
- Run-time behavior can be influenced with the
-fsanitize=leak— enables LeakSanitizer, a memory leak detector 🩸 This option only matters when linking executables, and when the executable is linked against a library that overridesmallocand other allocator functions.- More details at
https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer📖 - Run-time behavior can be influenced with the
LSAN_OPTIONSenvironment variable 🧚 - It can't be combined with
-fsanitize=thread🙅♀️
- More details at
-fsanitize=undefined— enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 🌈 Various computations get instrumented to catch undefined behavior at runtime.- See
https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.htmlfor more details 📖 - Run-time behavior can be influenced with the
UBSAN_OPTIONSenvironment variable. Current suboptions are:-fsanitize=shift— checks that the result of a shift operation isn't undefined 🫧 What counts as undefined differs slightly between C and C++, and also between ISO C90 and C99, etc. This option has two suboptions:-fsanitize=shift-baseand-fsanitize=shift-exponent.-fsanitize=shift-exponent— checks that the second argument of a shift operation isn't negative and is smaller than the precision of the promoted first argument 🔢-fsanitize=shift-base— if the second argument of a shift operation is within range, it checks that the result of the shift isn't undefined 🧮 Same deal as above: the exact rules differ between C and C++, and between ISO C90 and C99, etc.-fsanitize=integer-divide-by-zero— detects integer division by zero, as well as"INT_MIN / -1"division 🙈-fsanitize=unreachable— the compiler turns the"__builtin_unreachable"call into a diagnostics message call instead 🚨 When you reach the"__builtin_unreachable"call, the behavior is undefined, so this is a whole vibe check 😭-fsanitize=vla-bound— checks that the size of a variable length array is positive 📏
- See
-fsanitize=null— okay, pointer checking is the vibe here 💖 With this on, your program throws an error when it tries to dereference aNULLpointer 😭. It also flags a reference (possibly an rvalue reference) bound to aNULLpointer, or a method invoked on an object pointed to by aNULLpointer.-fsanitize=return— return statement checking, and it's giving main character energy 💅 Programs built with this will issue an error when the end of a non-void function is reached without actually returning a value. This one is C++ only 🌸.-fsanitize=signed-integer-overflow— signed integer overflow checking 🔥 It checks that the result of+,*, and both unary and binary-doesn't overflow in signed arithmetic. Integer promotion rules still apply, so this is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— array bounds instrumentation 🦋 Lots of out-of-bounds accesses get caught. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented 🙈-fsanitize=bounds-strict— the stricter array bounds check 💜 Most out-of-bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage still aren't instrumented.-fsanitize=alignment— pointer alignment checking ✨ It kicks in when a pointer gets dereferenced, when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object.-fsanitize=object-size— memory references get instrumented using the__builtin_object_sizefunction 🍓 Various out-of-bounds pointer accesses are detected.-fsanitize=float-divide-by-zero— floating-point division by zero detection ➗ Unlike other similar options, this one is not turned on by-fsanitize=undefined. That's because dividing a float by zero can be a legit way to get infinities and NaNs, so it's not automatically treated as a bug 🌈-fsanitize=float-cast-overflow— floating-point to integer conversion checking 🧚 It makes sure the result of the conversion doesn't overflow. Also not turned on by-fsanitize=undefined.
⚠️ Heads up bestie 💀 This option does not work well with FE_INVALID exceptions enabled.
-fsanitize=nonnull-attribute— checks calls to make sure null values aren't passed to arguments marked as needing a non-null value by thenonnullfunction attribute 🫶-fsanitize=returns-nonnull-attribute— instruments return statements in functions marked with thereturns_nonnullfunction attribute, so it can catch null values being returned from them 🎀-fsanitize=bool— instruments loads frombool👑 If a value other than0/1gets loaded, you'll get a run-time error.-fsanitize=enum— instruments loads from an enum type 💅 If a value outside the range of values for that enum type gets loaded, a run-time error is issued.-fsanitize=vptr— C++ member function calls, member accesses, and some conversions between pointers to base and derived classes get instrumented 🦋 This verifies the referenced object has the correct dynamic type.-fsanitize=pointer-overflow— pointer arithmetic gets instrumented 🌸 If the pointer arithmetic overflows, a run-time error is issued.-fsanitize=builtin— instruments arguments to selected builtin functions 🔮 If an invalid value gets passed to one of those arguments, a run-time error is issued. For example, passing0as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior, and this option diagnoses it.
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead. This currently works only for the C family of languages 💖
-fno-sanitize=all— disables all previously enabled sanitizers 🧹
⚠️ -fsanitize=all is not allowed, as some sanitizers cannot be used together.
-fasan-shadow-offset=number— forces GCC to use a custom shadow offset in AddressSanitizer checks 🧪 It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.-fsanitize-sections=s1,s2,...— sanitizes global variables in selected user-defined sections 🗂️simay contain wildcards.-fsanitize-recover[=opts]— controls the error recovery mode for the sanitizers in the comma-separated listopts🔁 When recovery is enabled for a sanitizer, it tries to keep running the program as if no error happened. That means multiple runtime errors can be reported in a single run, and the exit code may indicate success even when errors were reported. The-fno-sanitize-recover=option changes this: only the first detected error gets reported, and then the program exits with a non-zero exit code 🛑
Right now this only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers, error recovery is turned on by default, except for -fsanitize=address, where this feature is experimental 🧪 -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted. The former enables recovery for all sanitizers that support it, and the latter disables recovery for all sanitizers that support it.
Even if recovery mode is turned on compiler-side, it also needs to be enabled on the runtime library side. Otherwise the failures are still fatal 💀 The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. You can override this by setting the halt_on_error flag in the corresponding environment variable 🌈
Syntax without an explicit opts parameter is deprecated 😬 It's equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enables sanitization of local variables to detect use-after-scope bugs 🔍 This option sets-fstack-reusetonone.-fsanitize-undefined-trap-on-error— reports undefined behavior using__builtin_trapinstead of alibubsanlibrary routine 🪄 The perk is that thelibubsanlibrary isn't needed and doesn't get linked in, so this works even in freestanding environments 🏕️-fsanitize-coverage=trace-pc— enables coverage-guided fuzzing code instrumentation 🐛 It inserts a call to__sanitizer_cov_trace_pcinto every basic block.-fsanitize-coverage=trace-cmp— enables dataflow guided fuzzing code instrumentation 🧠 It inserts calls to the following:__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_cmp4, or__sanitizer_cov_trace_cmp8for integral comparisons where both operands are variables__sanitizer_cov_trace_const_cmp1,__sanitizer_cov_trace_const_cmp2,__sanitizer_cov_trace_const_cmp4, or__sanitizer_cov_trace_const_cmp8for integral comparisons where one operand is a constant__sanitizer_cov_trace_cmpfor__sanitizer_cov_trace_cmpdfor float or double comparisons__sanitizer_cov_trace_switchfor switch statements
-fcf-protection=[full|branch|return|none|check]— control-flow instrumentation for extra security 🛡️ It checks that the target addresses of control-flow transfer instructions (such as indirect function calls, function returns, and indirect jumps) are valid. This stops the flow of control from being diverted to an unexpected target. It's meant to protect against threats like Return-oriented Programming (ROP), and similar call/jmp-oriented programming (COP/JOP) 🔐branch— checking happens at indirect branch instructions, i.e. call/jmp instructions 🌿return— checking happens when returning from a function ↩️full— an alias for specifying bothbranchandreturn👑none— turns off instrumentation 🙅check— used for the final link with link-time optimization (LTO). An error is issued if LTO object files were compiled with different-fcf-protectionvalues. This value is ignored at compile time 🫠
The macro __CET__ is defined when -fcf-protection is used ✨ The first bit of __CET__ is set to 1 for the value branch, and the second bit of __CET__ is set to 1 for return.
You can also use the nocf_check attribute to mark which functions and calls should be skipped from instrumentation 🙈
Right now the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET), which works for i686 processors or newer 💻
NOTE: In Ubuntu 19.10 and later versions,-fcf-protectionis enabled by default for C, C++, ObjC, ObjC++, if none of-fno-cf-protectionnor-fcf-protection=*are found 🎀
-fstack-protector— okay bestie, this one is a stack-smashing bodyguard 🛡️💖 It sneaks a guard variable into any function with vulnerable objects. That covers functions that callalloca, plus functions with buffers of at least 8 bytes. The guards get set up when a function is entered and get checked on the way out. If a guard check fails, you get an error message and the program exits. Only variables that actually live on the stack count, so anything optimized away or stuck in a register doesn't get a guard. 🌸-fstack-protector-all— same energy as-fstack-protector, but every single function gets protected. Full bodyguard mode 👑-fstack-protector-strong— basically-fstack-protectorbut with extra functions in the guest list 🎀 These are the ones with local array definitions, or ones that reference local frame addresses. The same rule applies: only variables actually allocated on the stack count, and optimized-away or register variables don't.-fstack-protector-explicit— the VIP-list version of-fstack-protector. It only protects functions that carry thestack_protectattribute 🧚-fstack-check— this one makes the code check that you don't go past the edge of the stack 🦋 You'll want it if you're running in a multithreaded environment. In a single-threaded setup you almost never need it, because stack overflow is usually caught automatically on nearly every system if there's only one stack. ngl that's a relief 😭
This flag doesn't do the checking itself. The operating system or the language runtime has to do that. What it does is generate code so they can see the stack being extended. You can also pass it a string parameter: no means no checking at all, generic forces the old-style checking, and specific uses the best checking method for the target and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism. It doesn't need any target-specific support from the compiler, but it comes with some drawbacks, and honestly they're a lot:
- Modified allocation strategy for large objects: they're always allocated dynamically if their size goes over a fixed threshold. This may change the semantics of some code, so be careful 👀
- Fixed limit on the size of the static frame of functions: when a particular function tops it, stack checking isn't reliable and the compiler issues a warning.
- Inefficiency: between the modified allocation strategy and the generic implementation, your code performance takes a hit 💀
Also, old-style stack checking is the fallback for specific if the compiler doesn't have target support added. So specific falls back to it when there's no target support.
-fstack-check= was designed for Ada's needs, specifically detecting infinite recursion and stack overflows. specific is an excellent choice when you're compiling Ada code 💅 It's not generally enough to protect you against stack-clash attacks though. For that, you want -fstack-clash-protection.
-fstack-clash-protection— this is the anti-stack-clash flag 🔥 When it's on, the compiler only allocates one page of stack space at a time, and each page gets accessed right after it's allocated. That means allocations can't jump over a stack guard page the operating system provides.
Most targets don't fully support stack clash protection. On the ones that do, -fstack-clash-protection protects dynamic stack allocations. It can also give limited protection to static stack allocations, but only if the target supports -fstack-check=specific.
NOTE: In Ubuntu 19.10 and later,-fstack-clash-protectionis enabled by default for C, C++, ObjC, and ObjC++, unless-fno-stack-clash-protectionshows up in the command line. So it's already on for you, probably 🫶
-fstack-limit-register=reg,-fstack-limit-symbol=sym,-fno-stack-limit— these generate code so the stack never grows past a certain value. That value is either a register's contents or the address of a symbol 🌈 If the program needs a bigger stack than that, a signal gets raised at run time. On most targets the signal goes off before the stack actually overruns the boundary, so you can catch it without doing anything special.
Here's an example. Say the stack starts at absolute address 0x80000000 and grows downwards. You can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB ✨ Heads up, this may only work with the GNU linker.
You can override stack limit checking locally by using the no_stack_limit function attribute 🙈
-fsplit-stack— this one automatically splits the stack before it overflows 🧚 The result is a discontiguous stack, which can only overflow if the program can't allocate any more memory at all. It's most useful for threaded programs, because you don't have to work out a good stack size for each thread anymore. Life-changing 😭 Right now it's only implemented for x86 targets running GNU/Linux.
If code compiled with -fsplit-stack calls code compiled without it, the latter code might not have much stack space to run in. If compiling everything (library code included) with -fsplit-stack isn't an option, the linker can fix up those calls so the code without -fsplit-stack always gets a large stack. This support is implemented in the gold linker in GNU binutils release 2.21 and later 🦋
-fvtable-verify=[std|preinit|none]— C++ only! 💜 This turns on the security feature that checks, at run time and for every virtual call, that the vtable pointer used for the call is valid for the object's type and hasn't been corrupted or overwritten. Use-fvtable-verify=noneto turn it off. If an invalid vtable pointer gets detected, an error is reported and the program stops right away 🛑
This option builds run-time data structures at program startup, and those are what verify the vtable pointers. The std and preinit values control when those structures get built. In both cases they're built before execution reaches main. With -fvtable-verify=std, they're built after shared libraries have been loaded and initialized. With -fvtable-verify=preinit, they're built before shared libraries have been loaded and initialized.
If this option shows up multiple times in the command line with different values, none takes the highest priority over both std and preinit, and preinit takes priority over std 💅
-fvtv-debug— use this with-fvtable-verify=stdor-fvtable-verify=preinitand it calls the debug versions of the runtime functions for vtable verification 🔍 It also makes the compiler log which vtable pointers it finds for each class. That info goes into a file calledvtv_set_ptr_data.log, in the directory named by the environment variableVTV_LOGS_DIRif that's defined, or in the current working directory otherwise.
Note: this feature appends to the log file. If you want a fresh log, delete any existing one first 🧹
-fvtv-counts— a debugging flag, bestie 🐛 With-fvtable-verify=stdor-fvtable-verify=preinit, the compiler keeps track of the total number of virtual calls it runs into, and the number of verifications it inserts. It also counts the calls it inserts to certain run-time library functions and logs all of this for each compilation unit. The compiler writes that info tovtv_count_data.login the directory named byVTV_LOGS_DIRif it's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that tovtv_class_set_sizes.login the same directory 📊
Note: this feature appends to the log files too. For fresh log files, delete any existing ones first 💀
-finstrument-functions— this generates instrumentation calls for function entry and exit 🌸 Right after a function is entered, and right before it exits, the following profiling functions get called with the address of the current function and its call site. (Heads up: on some platforms,__builtin_return_addressdoesn't work beyond the current function, so the call site info might not be available to the profiling functions.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);The first argument is the address of the start of the current function, which can be looked up exactly in the symbol table 🔥
This instrumentation also happens for functions that get expanded inline into other functions. The profiling calls show where, conceptually, the inline function is entered and exited. That means addressable versions of those functions have to exist. If all your uses of a function get expanded inline, this may mean extra code size. If you use extern inline in your C code, an addressable version of those functions must be provided. (This is usually the case anyway, but if you get lucky and the optimizer always expands them inline, you might have gotten away without providing static copies 🙈)
A function can be given the no_instrument_function attribute, and then this instrumentation won't happen for it. This is handy for things like the profiling functions listed above, high-priority interrupt routines, and any function the profiling functions can't safely be called from, like signal handlers if the profiling routines generate output or allocate memory 💜
-finstrument-functions-exclude-file-list=file,file,...— okay bestie, this one is the VIP exclusion list 🚫✨ It sets which functions get skipped for instrumentation (peep-finstrument-functionsfor the full tea ☕). If the file holding a function's definition matches one of the file entries, that function is NOT instrumented. The match is done on substrings, so if your file bit shows up anywhere inside the file name, it counts 🦋
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysThat excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys 💖
If, for some reason, you need a comma inside one of the sym values, you double it up and write ,, 🙈 Like this, with single quotes around the whole option: -finstrument-functions-exclude-file-list=',,tmp' 🧵
-finstrument-functions-exclude-function-list=sym,sym,...— this is basically the file-list's cousin 👯, except it lists the function names to skip instead of files. The name that gets matched is the user-visible name, likevector<int> blah(const vector<int> &), NOT the scrambled internal mangled name like_Z4blahRSt6vectorIiSaIiEE💀. Matching is substring-based too: if sym is a substring of the function name, it's a match. For C99 and C++ extended identifiers, the function name has to be written in UTF-8, not with universal character names 🌈-fpatchable-function-entry=N[,M]— okay this one is giving extra space 🧚. It generates N NOPs right at the start of each function, with the function entry point sitting before the Mth NOP. If M is left off, it defaults to 0, so the entry point is the address right at the first NOP. Those NOPs reserve room you can patch instrumentation into at run time, as long as the code segment is writable ✨. The amount of space is controlled indirectly through how many NOPs you ask for. The NOP instruction used is the one emitted by the internal GCC back-end interfacegen_nop. This is target-specific and may also depend on the architecture variant and/or other compilation options 🌸
For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M, also get collected in the __patchable_function_entries section of the resulting binary 🔍
Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded 💅
The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue. 🫣
The maximum value of N and M is 65535 👑
✨ Options Controlling the Preprocessor ✨
These options control the C preprocessor, which runs on each C source file before the actual compilation happens 🧼
If you use -E, nothing gets done except preprocessing. Some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation 🙃
In addition to the options listed here, there are a bunch of options that control search paths for include files, documented in Directory Options 📂. Options to control preprocessor diagnostics are listed in Warning Options ⚠️
-D name— predefines name as a macro, with the definition1🍓-D name=definition— the contents of definition get tokenized and processed as if they showed up during translation phase three in a#definedirective 🧩. Heads up: the definition gets truncated by any embedded newline characters ✂️
If you're invoking the preprocessor from a shell (or something shell-like), you might need the shell's quoting syntax to protect characters like spaces that have a meaning in shell syntax 🐚
If you want to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses mean stuff to most shells, so you should quote the option 🫶 With sh and csh, -D'name(args...)=definition' works 💜
-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options get processed after all -D and -U options 🔁
-U name— cancels any previous definition of name, whether it was built in or provided with a-Doption 🧹-include file— processes file as if#include "file"appeared as the very first line of the primary source file 📄. The twist: the first directory searched for file is the preprocessor's working directory, not the directory containing the main source file 📁. If it's not found there, it gets searched for in the rest of the#include "..."search chain as normal.
If you give multiple -include options, the files are included in the order they show up on the command line 🔢
-imacros file— exactly like-include, except any output produced by scanning file gets thrown away 🗑️. The macros it defines stay defined, which lets you grab all the macros from a header without also processing its declarations ✨
All files specified by -imacros are processed before all files specified by -include 🙌
-undef— do not predefine any system-specific or GCC-specific macros 🚫. The standard predefined macros stay defined 💅-pthread— defines the extra macros required for using the POSIX threads library 🧵. You should use this option consistently for both compilation and linking 🔗. It's supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets 🌈-M— instead of outputting the preprocessed result, it outputs a rule suitable formakethat describes 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-includeor-imacroscommand-line options 🦋
Unless you specify it explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced by the object file suffix and any leading directory parts removed. If there are lots of included files, the rule gets split into several lines using \-newline. The rule has no commands 🧾
This option doesn't suppress the preprocessor's debug output, such as -dM. To avoid mixing that debug output with the dependency rules, you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal 🙈
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w 🤫
-MM— like-M, but doesn't mention header files found in system header directories, nor header files included directly or indirectly from such a header 🏠
This implies that the choice of angle brackets or double quotes in an #include directive doesn't in itself determine whether that header appears in -MM dependency output 🎀
-MF file— when used with-Mor-MM, specifies a file to write the dependencies to 📝. If there's no-MFswitch, the preprocessor sends the rules to the same place it would send preprocessed output. When used with the driver options-MDor-MMD,-MFoverrides the default dependency output file. If file is-, the dependencies are written to stdout 🖨️-MG— in conjunction with an option such as-Mrequesting dependency generation,-MGassumes missing header files are generated files and adds them to the dependency list without raising an error 🤖. The dependency filename is taken directly from the#includedirective without prepending any path.-MGalso suppresses preprocessed output, since a missing header file makes that useless 🗿
This feature is used in automatic updating of makefiles ⚙️
-Mno-modules— disables dependency generation for compiled module interfaces 🧩-MP— instructs CPP to add a phony target for each dependency other than the main file, causing each one to depend on nothing 👻. These dummy rules work around errorsmakegives 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— changes the target of the rule emitted by dependency generation 🎯. By default, CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target. An-MToption sets the target to be exactly the string you specify 💯. If you want multiple targets, you can specify them as a single argument to-MT, or use multiple-MToptions.
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.cThe default target is automatically quoted, as if it were given with -MQ 🔒
-MD—-MDis equivalent to-M -MF file, except that-Eis not implied 🔥. The driver determines file based on whether an-ooption is given. If it is, the driver uses its argument but with a suffix of.d. Otherwise, it takes the name of the input file, removes any directory components and suffix, and applies a.dsuffix 📂.
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-MDexcept it mentions only user header files, not system header files 💜-fpreprocessed— 💖 okay so this flag tells the preprocessor "bestie, this file is ALREADY preprocessed, don't touch it" 🙅♀️ It skips macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. Comments still get recognized and removed, so you can pass in a file made with-Cwith zero drama ✨ In this mode the integrated preprocessor is basically just a tokenizer for the front ends 🧚 Also,-fpreprocessedis implicit if your input file ends in.i,.iior.mi. Those are the extensions GCC uses for preprocessed files made by-save-temps🎀-fdirectives-only— 🌸 When preprocessing, this handles directives but does NOT expand macros. How it acts depends on-Eand-fpreprocessed, so let's break it down:- With
-E, preprocessing is limited to directives like"#define","#ifdef", and"#error". Macro expansion and trigraph conversion are skipped, and-dDgets turned on automatically 🦋 - With
-fpreprocessed, predefinition of command line macros and most builtin macros is turned off. Contextual macros like__LINE__still get handled normally. This is what lets you compile files that were previously run through-E -fdirectives-only👑 - With both
-Eand-fpreprocessed, the-fpreprocessedrules win. This gives you full preprocessing of files that were previously run through-E -fdirectives-only💅
- With
-fdollars-in-identifiers— 💜 Lets$show up inside identifiers. Accepted, no cap.-fextended-identifiers— 🌈 Accepts universal character names and extended characters in identifiers. This one is on by default for C99 (and later C standards) and C++, so it's literally already on for you 🫶-fno-canonical-system-headers— 🙈 When preprocessing, this stops system header paths from getting shortened with canonicalization.-fmax-include-depth=depth— 🧶 Sets the max depth of nested#includes. Default is200.-ftabstop=width— 📏 Sets the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even when tabs are on the line. If the value is less than1or greater than100, the option gets ignored. Default is8🍓-ftrack-macro-expansion[=level]— 🔍 Tracks token locations across macro expansions, so when an error happens inside a macro, the compiler can show you the current macro expansion stack. Heads up: this makes the preprocessor and compiler eat more memory 🍽️ The level parameter lets you pick how precise the tracking is, so you can drop memory use if you need to:0turns this off1tracks locations in a degraded mode for minimal memory overhead. In this mode, every token coming from expanding an argument of a function-like macro gets the same location2tracks locations completely. This is the most memory hungry option 🐘- With no argument at all, the default level is
2
Note that -ftrack-macro-expansion=2 is activated by default ✨
-fmacro-prefix-map=old=new— 🗺️ When preprocessing files living in directory old, the__FILE__and__BASE_FILE__macros get expanded as if those files lived in directory new instead. You can use.for new to turn an absolute path into a relative one, which can lead to more reproducible, location-independent builds 🧚 This also affects__builtin_FILE()during compilation. See also-ffile-prefix-map.-fexec-charset=charset— 🔤 Sets the execution character set used for string and character constants. Default isUTF-8. charset can be any encoding the system'siconvlibrary routine supports.-fwide-exec-charset=charset— 🌏 Sets the wide execution character set, used for wide string and character constants. The default is one ofUTF-32BE,UTF-32LE,UTF-16BE, orUTF-16LE, whichever matches the width ofwchar_tand the big-endian or little-endian byte order used for code generation. Just like-fexec-charset, charset can be any encodingiconvsupports. ⚠️ Fair warning though: you'll have problems with encodings that don't fit exactly inwchar_t.-finput-charset=charset— 📥 Sets the input character set, used for translating from the input file's character set into the source character set GCC uses. If the locale doesn't specify one, or GCC can't get it from the locale, the default isUTF-8. The locale or this command-line option can override it, but if they conflict, the command-line option wins 🏆 charset can be any encodingiconvsupports.-fpch-deps— 📚 When you're using precompiled headers, this makes the dependency-output flags also list the files from the precompiled header's dependencies. Without it, only the precompiled header gets listed, not the files used to create it, because those files aren't consulted when a precompiled header is used 🙃-fpch-preprocess— 🧩 This lets you use a precompiled header together with-E. It inserts a special#pragmainto the output,#pragma GCC pch_preprocess "filename", to mark where the precompiled header was found, plus its filename. When-fpreprocessedis in use, GCC recognizes this#pragmaand loads the PCH 🫶- This option is off by default, because the preprocessed output is only really suitable as input to GCC. It gets switched on by
-save-temps. - You shouldn't write this
#pragmain your own code. But it's totally safe to edit the filename if the PCH file lives somewhere else now. The filename can be absolute, or relative to GCC's current directory.
- This option is off by default, because the preprocessed output is only really suitable as input to GCC. It gets switched on by
-fworking-directory— 📂 Turns on linemarkers in the preprocessor output that tell the compiler what the working directory was at preprocessing time. When it's on, the preprocessor emits a second linemarker after the initial one, containing the current working directory followed by two slashes. GCC uses that directory, when it's present in the preprocessed input, as the working directory in some debugging information formats. This is implicitly enabled if debugging information is enabled, but you can block it with the negated form-fno-working-directory. If the-Pflag is in the command line, this option does nothing, since no#linedirectives get emitted whatsoever 🚫-A predicate=answer— 🪄 Makes an assertion with the predicate predicate and the answer answer. This form is the preferred one over the older-A predicate(answer), which is still supported, because it doesn't use shell special characters 🐚-A -predicate=answer— ❌ Cancels an assertion with the predicate predicate and the answer answer. Bye bye 👋-C— 💬 Keeps your comments! Everything gets passed through to the output file, except comments inside processed directives, which get deleted along with the directive.- Be prepared for side effects with
-C, because it makes the preprocessor treat comments as tokens in their own right. For example, a comment at the start of what would be a directive line turns that line into an ordinary source line, since the first token on the line is no longer a#😭
- Be prepared for side effects with
-CC— 💅 Keeps comments, including during macro expansion. It's like-C, except comments inside macros also get passed through to the output file wherever the macro is expanded.- On top of the
-Cside effects,-CCconverts all C++-style comments inside a macro into C-style comments. This stops a later use of that macro from accidentally commenting out the rest of the source line 🙌 -CCis generally used to support lint comments.
- On top of the
-P— 🤐 Stops linemarkers from showing up in the preprocessor output. This could come in handy when you're running the preprocessor on something that isn't C code and will get sent to a program that might get confused by linemarkers.-traditional,-traditional-cpp— 🕰️ These try to imitate the behavior of pre-standard C preprocessors, instead of ISO C preprocessors. Check the GNU CPP manual for details.- GCC doesn't try to emulate a pre-standard C compiler otherwise, and these options only work with the
-Eswitch, or when you invoke CPP explicitly.
- GCC doesn't try to emulate a pre-standard C compiler otherwise, and these options only work with the
-trigraphs— 🔺 Supports ISO C trigraphs. These are three-character sequences that all start with??, which ISO C defines as stand-ins for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline 🪄
The nine trigraphs and their replacements are:
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
-remap— 🗂️ Turns on special code to work around file systems that only allow very short file names, like MS-DOS 💾-H— 🔎 Prints the name of each header file used, on top of the other normal stuff. Each name is indented to show how deep it is in the#includestack 🌿 Precompiled header files get printed too, even if they turn out to be invalid. An invalid precompiled header is printed with...x, and a valid one with...!✅-dletters— 🧪 Tells GCC to make debugging dumps during compilation, as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, so they're silently ignored. If you specify letters whose behavior conflicts, the result is undefined 💀-dM— 📜 Instead of the normal output, this generates a list of#definedirectives for all the macros defined during the execution of the preprocessor, including predefined macros. It's a super easy way to find out what's predefined in your version of the preprocessor 🔮 Assuming you have no filefoo.h, the command
okay so the easiest way to see every predefined macro your compiler has baked in is this 👇
touch foo.h; cpp -dM foo.hand bestie, it spits out all of them 🤯 main character energy.
Heads up 💀: if you use-dMwithout the-Eoption,-dMgets treated as a synonym for-fdump-rtl-mach. Not me getting plot-twisted 😭
-dD— this one is like-dMbut with two twists 🌸 It skips the predefined macros, and it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file 🦋-dN— basically-dD, but chill. It only emits the macro names, not their expansions 🧚-dI— adds#includedirectives to the output, on top of the preprocessing result 🎀-dU— like-dD, except only macros that get expanded, or whose definedness gets tested in preprocessor directives, make the cut ✨ The output is delayed until the use or test of the macro, and#undefdirectives also show up for macros that were tested but undefined at that time 💅-fdebug-cpp— ngl this one is only for debugging GCC itself 🤓 When it's used from CPP or with-E, it dumps debugging info about location maps, and every token in the output gets the dump of the map its location belongs to. When it's used from GCC without-E, it does absolutely nothing 💀-Wp,option— lets you skip the compiler driver and sendoptionstraight through to the preprocessor 🚀 Ifoptionhas commas, it gets split into multiple options at the commas. BUT 🚨 a lot of options get modified, translated or interpreted by the driver before they reach the preprocessor, and-Wpforcibly bypasses that step. The preprocessor's direct interface is undocumented and subject to change, so whenever possible, avoid-Wpand let the driver handle the options instead 🫶-Xpreprocessor option— passesoptionto the preprocessor 🧁 Use it for system-specific preprocessor options that GCC doesn't recognize. If you want to pass an option that takes an argument, you need to use-Xpreprocessortwice: once for the option and once for the argument.-no-integrated-cpp— preprocessing gets its own separate pass before compilation 🔪 By default, GCC preprocesses as an integrated part of input tokenization and parsing. With this flag, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) gets invoked twice: once for preprocessing only, and once for actual compilation of the preprocessed input. It can be handy alongside the-Bor-wrapperoptions to use an alternate preprocessor, or to do extra processing on your source between normal preprocessing and compilation 🌈-flarge-source-files— for when your files are HUGE 🐘 It makes GCC expect large source files, at the cost of slower compilation and higher memory usage. Normally GCC tracks both column numbers and line numbers, and prints both in diagnostics. But once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. So diagnostics for later lines don't include column numbers, and options like-Wmisleading-indentationstop working at that point, although the compiler prints a note if this happens 📝 Passing-flarge-source-filessignificantly increases the number of source lines GCC can process before it stops tracking columns 💪
✨ Passing Options to the Assembler ✨
You can pass options to the assembler 💫
-Wa,option— passesoptionto the assembler 🪄 Ifoptionhas commas, it gets split into multiple options at the commas.-Xassembler option— passesoptionto the assembler too 🫧 Use it for system-specific assembler options that GCC doesn't recognize. If you want to pass an option that takes an argument, you need to use-Xassemblertwice: 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're meaningless if the compiler isn't doing a link step 🙄
object-file-name— a file name that doesn't end in a special recognized suffix is considered to name an object file or library. (The linker tells object files and libraries apart by their contents, not us 🕵️) If linking is done, these object files get used as input to the linker.-c,-S,-E— if any of these is used, the linker doesn't run, so object file names shouldn't be used as arguments 🚫-flinker-output=type— controls code generation of the link-time optimizer 🧠 By default, the linker output is automatically determined by the linker plugin. For debugging the compiler, or if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually. The options for type are:exec— code generation produces a static binary. In this case-fpicand-fpieare both disabled.dyn— code generation produces a shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This allows building shared libraries without position-independent code on architectures where that's possible, i.e. on x86.pie— code generation produces an-fpieexecutable. This gives similar optimizations asexec, except that-fpieis not disabled if it was specified at compilation time.rel— the compiler assumes incremental linking is happening. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if-ffat-lto-objectsis specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time, the result of incremental linking also loads faster than a static library, assuming that the majority of objects in the library are used.nolto-rel— configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together, the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole program optimizations are lost.
During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils, it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into a static library instead. Alternatively, it is possible to use H.J. Lu's binutils with support for mixed objects 🧩
-fuse-ld=bfd— use the bfd linker instead of the default linker 🔧-fuse-ld=gold— use the gold linker instead of the default linker ✨-fuse-ld=lld— use the LLVM lld linker instead of the default linker 💜-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker 🏎️-llibrary,-l library— searches the library namedlibrarywhen linking. (The second form, with the library as a separate argument, is only for POSIX compliance and isn't recommended 🙅)
The -l option is passed directly to the linker by GCC, so refer to your linker documentation for the exact details. The general description below applies to the GNU linker 🐧
The linker searches a standard list of directories for the library. The directories searched include several standard system directories, plus any that you specify with -L.
Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.
It makes a difference where in the command you write this option, because the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded 😱
-lobjc— you need this special case of the-loption to link an Objective-C or Objective-C++ program 🌷-nostartfiles— don't use the standard system startup files when linking. The standard system libraries are still used normally, unless-nostdlib,-nolibc, or-nodefaultlibsis used 🚪-nodefaultlibs— don't use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored. The standard startup files are used normally, unless-nostartfilesis used.
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.
Heads up 🚨: these entry points should be supplied through some other mechanism when this option is specified.
-nolibc— don't use the C library, or the system libraries tightly coupled with it, when linking. It still links with the startup files, libgcc, or toolchain-provided language support libraries such aslibgnat,libgfortranorlibstdc++, unless options preventing their inclusion are used as well. This typically removes-lcfrom the link command line, along with system libraries that normally go with it and become meaningless when a C library is assumed absent, for example-lpthreador-lmin some configurations. This is intended for bare-board targets when there is indeed no C library available 🪨-nostdlib— don't use the standard system startup files or libraries when linking. No startup files are used, and only the libraries you specify are passed to the linker. Options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored.
The compiler may generate calls to memcmp, memset, memcpy and memmove 🫶. These are usually resolved by entries in libc, so bestie, when you use this flag, those entry points have to be supplied some other way 😭.
One of the standard libraries that -nostdlib and -nodefaultlibs bypass is libgcc.a 🧚. It's a library of internal subroutines that GCC uses to work around shortcomings of certain machines or to meet the special needs of some languages 💅.
Honestly, you usually still need libgcc.a even when you're trying to skip the other standard libraries ✨. So if you specify -nostdlib or -nodefaultlibs, you should usually add -lgcc too 🔥. That way you have no unresolved references to internal GCC library subroutines. (An example of one of those internal subroutines is __main, which makes sure C++ constructors get called 🌈.)
-e entry,--entry=entry— Sets the program entry point to entry 🦋. The linker is the one who reads this value, and the GNU linker will take either a symbol name or an address.-pie— Makes a dynamically linked position independent executable on targets that support it 💜. For predictable results, you also need to use the same compilation options you used for the build (-fpie,-fPIE, or model suboptions) when you pass this linker option ✨.-no-pie— Skips making a dynamically linked position independent executable 🙈.-static-pie— Makes a static position independent executable on targets that support it 🌸. It's kind of like a static executable, except it can be loaded at any address without a dynamic linker, which is so iconic 👑. Same deal as before: for predictable results, use the same set of options you compiled with (-fpie,-fPIE, or model suboptions) when you use this linker option 💅.-pthread— Links with the POSIX threads library 🧵. This one is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets 🌈. On some targets it also sets flags for the preprocessor, so use it consistently for both compilation and linking, periodt 💖.-r— Produces a relocatable object as the output 🦋. This is also known as partial linking.-rdynamic— Passes the flag-export-dynamicto the ELF linker, on targets that support it ✨. This tells the linker to add all symbols to the dynamic symbol table, not only the ones that are used 👀. It's needed for some uses ofdlopen, or if you want to get backtraces from inside a program 🔥.-s— Strips all symbol table and relocation information out of the executable 💀.-static— On systems that support dynamic linking, this overrides-pieand stops the linking with shared libraries 🫶. On other systems it does absolutely nothing 🙄.-shared— Makes a shared object that can then be linked with other objects to form an executable 🎀. Not every system supports this one. For predictable results, you also need to use the same set of compilation options (-fpic,-fPIC, or model suboptions) when you pass this linker option 💅.[1]-shared-libgcc,-static-libgcc— On systems that provide libgcc as a shared library, these force the use of the shared or the static version, respectively 🌸. If no shared version of libgcc was built when the compiler was configured, these options do nothing 🙈.
There are a few situations where an application should use the shared libgcc instead of the static one 🦋. The biggest one is when the app wants to throw and catch exceptions across different shared libraries 😭. In that case, every one of the libraries and the application itself should use the shared libgcc 🫶.
So the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable 💖. C++ programs typically use exceptions, so this is the right move, no cap ✨.
Now, if you use the GCC driver to create shared libraries, you might notice they aren't always linked with the shared libgcc 🤔. If GCC finds out at configuration time that you have a non-GNU linker, or a GNU linker that doesn't support the option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default 🧚. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, so it links with the static version by default 💅. This lets exceptions propagate through those shared libraries without adding relocation costs at library load time 🌈.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, so that it's linked with the shared libgcc 🚨.
-static-libasan— When-fsanitize=addressis used to link a program, the GCC driver automatically links against libasan 🔍. If libasan is available as a shared library and-staticisn't used, it links against the shared version of libasan 🌸. This option tells the GCC driver to link libasan statically, without necessarily linking other libraries statically ✨.-static-libtsan— When-fsanitize=threadis used to link a program, the GCC driver automatically links against libtsan 🧵. If libtsan is available as a shared library and-staticisn't used, it links against the shared version of libtsan 💜. This option tells the GCC driver to link libtsan statically, without necessarily linking other libraries statically 🦋.-static-liblsan— When-fsanitize=leakis used to link a program, the GCC driver automatically links against liblsan 🫧. If liblsan is available as a shared library and-staticisn't used, it links against the shared version of liblsan 💖. This option tells the GCC driver to link liblsan statically, without necessarily linking other libraries statically 🌷.-static-libubsan— When-fsanitize=undefinedis used to link a program, the GCC driver automatically links against libubsan 🎀. If libubsan is available as a shared library and-staticisn't used, it links against the shared version of libubsan 🌈. This option tells the GCC driver to link libubsan statically, without necessarily linking other libraries statically 👑.-static-libstdc++— When theg++program is used to link a C++ program, it normally links against libstdc++ automatically ✨. If libstdc++ is available as a shared library and-staticisn't used, it links against the shared version of libstdc++, and that's normally fine 💅. But sometimes it's useful to freeze the version of libstdc++ your program uses without going all the way to a fully static link 🧊. This option tells theg++driver to link libstdc++ statically, without necessarily linking other libraries statically 🫶.-symbolic— Binds references to global symbols when building a shared object ⭐. It warns about any unresolved references, unless you override that with the link editor option-Xlinker -z -Xlinker defs👀. Only a few systems support this one, so it's a bit of a rare bird 🦜.-T script— Uses script as the linker script 📜. This is supported by most systems that use the GNU linker 💜. On some targets, like bare-board targets without an operating system, you might need-Twhen linking to avoid references to undefined symbols 🙃.-Xlinker option— Passes option along to the linker as an option 🔧. Use this for system-specific linker options that GCC doesn't recognize, bestie 💅.
If you want to pass an option that takes a separate argument, you have to use -Xlinker twice: once for the option, and once for the argument 🫠. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions 🧚. Writing -Xlinker "-assert definitions" does not work, because that passes the entire string as one single argument, and the linker is not vibing with that 💀.
With the GNU linker, it's usually way more convenient to pass arguments to linker options using the option=value syntax instead of separate arguments 🍓. For example, you can write -Xlinker -Map=output.map instead of -Xlinker -Map -Xlinker output.map ✨. Other linkers might not support this syntax for command-line options 🙈.
-Wl,option— Passes option along to the linker as an option 🌸. If option contains commas, it gets split into multiple options at those commas 🔪. You can use this syntax to pass an argument to the option ✨. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker 🗺️. When you're using the GNU linker, you can get the same effect with-Wl,-Map=output.map💖.
NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option-Wl,-z,relrois used. To disable it, use-Wl,-z,norelro⚠️.
-u symbol— Pretends the symbol is undefined, which forces the linking of library modules that define it 🎭. You can use-umultiple times with different symbols to force loading of additional library modules 💜.-z keyword—-zis passed straight on to the linker, along with the keyword 🧵. Check the section in your linker's documentation for the permitted values and what they mean 📖.
✨ Options for Directory Search ✨
These options specify the directories to search for header files, for libraries, and for parts of the compiler 📂.
-I dir,-iquote dir,-isystem dir,-idirafter dir— Adds the directory dir to the list of directories searched for header files during preprocessing 📁. If dir begins with=or$SYSROOT, then that=or$SYSROOTgets replaced by the sysroot prefix. See--sysrootand-isysrootfor the details 🌳.
Directories specified with -iquote only apply to the quote form of the directive, #include "file" 💬. Directories specified with -I, -isystem, or -idirafter apply to lookups for both #include "file" and #include <file> 🌈.
You can specify any number or combination of these options on the command line to search for header files in several directories at once 🎀. The lookup order goes like this:
- For the quote form of the include directive, the directory of the current file is searched first 🔍.
- 🎀 For the quote form of the include directive, the
-iquotedirectories get searched left-to-right, in the same order they show up on the command line 💅 - 🌸
-Idirectories get scanned left-to-right 🌸 - 🦋
-isystemdirectories get scanned left-to-right 🦋 - 🏛️ The standard system directories get scanned 🏛️
- 🧚
-idirafterdirectories get scanned left-to-right 🧚
ngl, this search order is the whole vibe 😭 You can use -I to override a system header file and swap in your own version, because those directories get searched before the standard system header file directories. BUT bestie, don't use -I to add directories that hold vendor-supplied system header files. Use -isystem for that 🙅♀️
The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment as the standard system directories ✨
Lowkey the most important part: if a standard system include directory, or a directory given with -isystem, is also given with -I, the -I option gets ignored 🙈 The directory still gets searched, but as a system directory at its normal spot in the system include chain. This is so GCC's procedure for fixing buggy system headers and the ordering for the #include_next directive don't get changed by accident. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options 🔥
-I-— Okay, this one is deprecated 💔 so please don't start new stuff with it. Use-iquoteinstead for the-Idirectories that come before the-I-, then remove the-I-option.
Any directories given with -I options before -I- only get searched for headers requested with "#include "file"". They are not searched for "#include <file>". If more directories come with -I options after the -I-, those get searched for all #include directives.
Also, -I- stops the directory of the current file from being used as the first search directory for "#include "file"". There is no way to override this effect of -I- 🙃
-iprefix prefix— Setprefixas the prefix for the later-iwithprefixoptions 🍓 If the prefix represents a directory, make sure you include the final/.-iwithprefix dir,-iwithprefixbefore dir— Tackdironto the prefix you set earlier with-iprefix, and add the resulting directory to the include search path 🧚-iwithprefixbeforeputs it in the same place-Iwould, and-iwithprefixputs it where-idirafterwould.-isysroot dir— This is like the--sysrootoption, but it only applies to header files 🌈 (Except on Darwin targets, where it applies to both header files and libraries.) Check out the--sysrootoption for more info.-imultilib dir— Usediras a subdirectory of the directory that holds the target-specific C++ headers 💜-nostdinc— Don't search the standard system directories for header files 🙅 Only the directories you explicitly give with-I,-iquote,-isystem, and/or-idirafterget searched, plus the directory of the current file if it applies.-nostdinc++— Skip the C++-specific standard directories when looking for headers, but still search the other standard directories 🦋 (This one is used when building the C++ library.)-iplugindir=dir— Sets the directory to search for plugins that get passed with-fplugin=name, instead of-fplugin=path/name.so. This isn't meant for you to use directly. It's only passed along by the driver 🤖-Ldir— Adddirto the list of directories searched for-l✨-Bprefix— This option tells the compiler where to find its own executables, libraries, include files, and data files 🗺️
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.
For each subprogram to be run, the driver first tries the -B prefix, if there is one. If that name isn't found, or if -B isn't specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those turns up a file, the unmodified program name is searched for using the directories in your PATH environment variable.
The compiler checks to see if the path given by -B refers to a directory, and if needed it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it isn't found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it isn't found by those means.
Another way to specify a prefix that works much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
💀 As a special kludge, if the path given by-Bis[dir/]stageN/, whereNis a number in the range 0 to 9, then it gets replaced by[dir/]include. This is there to help with boot-strapping the compiler.
-no-canonical-prefixes— Don't expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix 🧭--sysroot=dir— Usediras the logical root directory for headers and libraries 🌱 For example, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it searchesdir/usr/includeanddir/usr/libinstead.
If you use both this option and the -isysroot option, then --sysroot applies to libraries, but -isysroot applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker doesn't support it, the header file aspect of --sysroot still works, but the library aspect doesn't.
--no-sysroot-suffix— For some targets, a suffix gets added to the root directory given with--sysroot, depending on the other options used. That means headers might be found indir/suffix/usr/includeinstead ofdir/usr/include. This option turns off the addition of that suffix 🚫
🎀 Options for Code Generation Conventions 🎀
These machine-independent options control the interface conventions used in code generation 💖
Most of them have both positive and negative forms, and the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed, the one that is not the default. You can figure out the other form by either removing no- or adding it 🧠
-fstack-reuse=reuse-level— This one controls stack space reuse for user-declared local/auto variables and compiler-generated temporaries 🥞reuse_levelcan beall,named_vars, ornone.allenables stack reuse for all local variables and temporaries,named_varsenables the reuse only for user-defined local variables with names, andnonedisables stack reuse completely. The default value isall.
This option is needed when the program extends the lifetime of a scoped local variable or a compiler-generated temporary beyond the end point defined by the language. When the lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range doesn't overlap with it. Legacy code that extends local lifetime is likely to break with the stack reuse optimization 😭
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}The lifetime of a compiler-generated temporary is well defined by the C++ standard. When the lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range doesn't overlap with it. However, some legacy code relies on the behavior of older compilers, where temporaries' stack space is not reused, so aggressive stack reuse can lead to runtime errors 💥 This option is used to control the temporary stack reuse optimization.
-ftrapv— This option generates traps for signed overflow on addition, subtraction, and multiplication operations 🚨 The options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command line results in-ftrapvbeing effective 👑-fwrapv— bestie, this one tells the compiler to just assume signed addition, subtraction, and multiplication overflow wraps around using twos-complement representation 🔄✨. It turns on some optimizations and turns off others.-ftrapvand-fwrapvare beefing with each other, so-ftrapv -fwrapvmeans-fwrapvwins 👑. Only the active options fight, though, so-ftrapv -fwrapv -fno-wrapvgives you-ftrapv💅.-fwrapv-pointer— same energy, but for pointer arithmetic. It assumes pointer overflow on addition and subtraction wraps around using twos-complement representation 🦋. It also disables some optimizations that assume pointer overflow is invalid 🙈.-fstrict-overflow— this one implies-fno-wrapv -fno-wrapv-pointer🔥. Flip it to the negated version and it implies-fwrapv -fwrapv-pointerinstead 🫶.-fexceptions— enables exception handling and generates the extra code needed to propagate exceptions 🎀. Heads up: on some targets this implies GCC generates frame unwind information for all functions, which can cause significant data size overhead 😭. It doesn't affect execution though ✨. By default, GCC turns this on for languages like C++ that normally need exception handling, and turns it off for languages like C that don't 🌸. You might still need it when compiling C code that has to play nicely with exception handlers written in C++ 🧚. You might also want to disable it if you're compiling older C++ programs that don't use exception handling 💜.-fnon-call-exceptions— lets trapping instructions throw exceptions 🍓. Real talk, this needs platform-specific runtime support that doesn't exist everywhere 💀. It only lets trapping instructions throw, meaning memory references or floating-point instructions 🦋. It does not let exceptions come out of arbitrary signal handlers like"SIGALRM"🙅♀️.-fdelete-dead-exceptions— instructions that might throw but don't otherwise contribute to the program's execution can get optimized away 🗑️✨. This is on by default for the Ada compiler, which the Ada language spec allows 👑. Optimization passes that remove dead exceptions are enabled separately at different optimization levels 💅.-funwind-tables— kinda like-fexceptions, except it only generates any needed static data and leaves the generated code alone 🌈. You usually don't need to turn this on yourself, because any language processor that needs it will enable it for you 🫶.-fasynchronous-unwind-tables— generates an unwind table in DWARF format, as long as the target machine supports it 🧚. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events, like a debugger or garbage collector 🔍✨.-fno-gnu-unique— okay so, with a recent GNU assembler and C library, the C++ compiler uses the"STB_GNU_UNIQUE"binding 🔗. That keeps template static data members and static local variables in inline functions unique, even with"RTLD_LOCAL"💜. This matters because a library used by two different"RTLD_LOCAL"plugins could otherwise disagree about the binding of a symbol 😭. The catch is that it makesdlcloseget ignored for affected DSOs 🙈. If your program relies on reinitializing a DSO throughdlcloseanddlopen, use-fno-gnu-unique🎀.-fpcc-struct-return— returns"short""struct"and"union"values in memory, just like longer ones, instead of in registers 🧺. It's less efficient, but it lets GCC-compiled files play nicely with files built by other compilers, especially the Portable C Compiler (pcc) 🫶. The exact convention for returning structures in memory depends on the target configuration macros 🌸. Short structures and unions are the ones whose size and alignment match some integer type 📏.
Warning: code compiled with-fpcc-struct-returnis not binary compatible with code compiled with-freg-struct-return. Only use it to conform to a non-default application binary interface 🚨.
-freg-struct-return— returns"struct"and"union"values in registers whenever it can 🚀. It's more efficient than-fpcc-struct-returnfor small structures 💅. If you specify neither flag, GCC uses whichever convention is standard for the target 🎯. If there's no standard convention, GCC defaults to-fpcc-struct-return, except on targets where GCC is the principal compiler 👑. In those cases GCC can pick the standard itself, and it chose the more efficient register return option 🏆.
Warning: code compiled with-freg-struct-returnis not binary compatible with code compiled with-fpcc-struct-return. Only use it to conform to a non-default application binary interface 🚨.
-fshort-enums— gives an"enum"type only as many bytes as it needs for its declared range of values 🌱. The"enum"type ends up equivalent to the smallest integer type that has enough room 🧩.
Warning: -fshort-enums makes GCC generate code that is not binary compatible with code generated without that switch. Only use it to conform to a non-default application binary interface 🚨.
-fshort-wchar— overrides the underlying type for"wchar_t"to be"short unsigned int"instead of the target's default 🎀. This is handy for building programs that run under WINE 🍷.
Warning: -fshort-wchar makes GCC generate code that is not binary compatible with code generated without that switch. Only use it to conform to a non-default application binary interface 🚨.
-fcommon— in C code, this controls where global variables defined without an initializer get placed 📦. Those are called tentative definitions in the C standard, and they're different fromexterndeclarations, which don't allocate any storage 🫧.
The default is -fno-common, which means the compiler puts uninitialized global variables in the BSS section of the object file 🛏️. This stops the linker from merging tentative definitions, so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit 😤.
-fcommon puts uninitialized global variables in a common block instead 🧱. That lets the linker resolve all tentative definitions of the same variable across different compilation units to the same object, or to a non-tentative definition 🔗. This behavior is inconsistent with C++, and on many targets it implies a speed and code size penalty on global variable references 🐢. It's mainly useful for getting legacy code to link without errors 🥹.
-fno-ident— ignores the#identdirective 🙈.-finhibit-size-directive— doesn't output a.sizeassembler directive, or anything else that would cause trouble if the function gets split in the middle with the two halves placed far apart in memory 🧩. This one is used when compilingcrtstuff.c, and you shouldn't need it for anything else 🚫.-fverbose-asm— adds extra commentary to the generated assembly code so it's easier to read 📖. This is generally only useful for people who actually need to read the generated assembly, like when they're debugging the compiler itself 🧑💻.
-fno-verbose-asm, which is the default, leaves that extra info out 🫥. It's handy when you want to compare two assembler files 🔍.
The added comments include:
- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE, - hints on which high-level expressions correspond to the various assembly instruction operands.
Here's an example. Say you've got this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}If you compile it to (x86_64) assembly with -S and send the result straight to stdout with -o -, like this:
gcc -S test.c -fverbose-asm -Os -o -you'll get output similar to this 🤩:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbitsHeads up: the comments are meant for humans rather than machines, so the precise format of the comments is subject to change 🫶.
-frecord-gcc-switches— okay so this one makes GCC stash the exact command line you used to invoke the compiler right inside the object file it's making 📼. It's only implemented on some targets, and the exact format depends on the target and binary file format, but it usually shows up as a section full of ASCII text 🧾. Don't mix it up with-fverbose-asm! That one only drops info into the assembler output as comments, so it never reaches the object file 🙅. For another way to store compiler options in the object file, see also-grecord-gcc-switches💜.-fpic— generate position-independent code (PIC) that's ready for a shared library, if the target machine supports it 🦋. This code reaches all constant addresses through a global offset table (GOT) ✨. The dynamic loader fills in the GOT entries when the program starts (and the dynamic loader is NOT part of GCC, it's part of the operating system, bestie 💅). If the GOT size for the linked executable goes over a machine-specific max, the linker throws an error saying-fpicdoesn't work, so just recompile with-fPICinstead 😭. (The maxes are 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. The x86 has no such limit 🙌.) PIC needs special support, so it only works on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent 🌈. When this flag is set, the macros"__pic__"and"__PIC__"are defined to1✨.-fPIC— if the target machine supports it, this emits position-independent code that's suitable for dynamic linking and doesn't have any limit on the size of the global offset table 👑. It makes a real difference on AArch64, m68k, PowerPC and SPARC 🔥. Like before, it needs special support, so it only works on certain machines. When this flag is set,"__pic__"and"__PIC__"are defined to2💖.-fpie,-fPIE— these are giving the same vibe as-fpicand-fPIC, but the position-independent code can only be linked into executables 🎀. Usually you'll use these to compile code that gets linked with the-pieGCC option 🧚. Both of them define"__pie__"and"__PIE__". The macros have the value1for-fpieand2for-fPIE🫶.-fno-plt— no PLT for external function calls in position-independent code, bestie 🙅. Instead, it loads the callee address at call sites from the GOT and branches to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations 🦋. On architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this gives the compiler more register allocation freedom 💅. Lazy binding requires the PLT, so with-fno-pltall external symbols get resolved at load time ⏱️. Alternatively, the function attribute"noplt"can be used to avoid calls through the PLT for specific external functions 🌸. In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT so they use the GOT instead 🧠.-fno-jump-tables— do not use jump tables for switch statements, even where they'd be more efficient than other code generation strategies 🙈. This is useful alongside-fpicor-fPICwhen building code that forms part of a dynamic linker and can't reference the address of a jump table 🧩. On some targets, jump tables don't need a GOT, so this option isn't needed there 💜.-fno-bit-tests— do not use bit tests for switch statements, even where it would be more efficient than other code generation strategies 🍓.-ffixed-reg— treat the register named reg as a fixed register, so generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role) 📌. reg must be the name of a register. The accepted register names are machine-specific and are defined in the"REGISTER_NAMES"macro in the machine description macro file 🗂️. This flag has no negative form, because it's a three-way choice 🙃.-fcall-used-reg— treat the register named reg as an allocable register that gets clobbered by function calls 🌀. It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way do not save and restore the register reg 😬. It is an error to use this flag with the frame pointer or stack pointer. Using it for other registers that have fixed, pervasive roles in the machine's execution model produces disastrous results 💀. No negative form, because it's a three-way choice 🙃.-fcall-saved-reg— treat the register named reg as an allocable register that's saved by functions 🛡️. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it 💾. It is an error to use this flag with the frame pointer or stack pointer. Using it on other registers with fixed, pervasive roles in the machine's execution model produces disastrous results 💀. A different sort of disaster happens if you use this flag on a register where function values may be returned 😭. No negative form, because it's a three-way choice 🙃.-fpack-struct[=n]— without a value, this packs all structure members together with no holes 🧱. When you give it a value (which must be a small power of two), it packs structure members according to that value, which represents the maximum alignment. Objects with default alignment requirements larger than this get 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. It also makes the code suboptimal 🐌. Only use it to conform to a non-default application binary interface 🫠.
-fleading-underscore— this option and its counterpart,-fno-leading-underscore, forcibly change the way C symbols are represented in the object file 🔁. One big use is helping you 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. Only use it to conform to a non-default application binary interface. Also, not all targets provide complete support for this switch 🥲.
-ftls-model=model— change the thread-local storage model that gets used 🧵. The model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec🎯. Heads up, the choice is subject to optimization: the compiler might use a more efficient model for symbols that aren't visible outside of the translation unit, or if-fpicisn't given on the command line ✨. The default without-fpicisinitial-exec. With-fpic, the default isglobal-dynamic💅.-ftrampolines— for targets that normally need trampolines for nested functions, this always generates them instead of using descriptors 🎪. For targets that don't need them, like HP-PA or IA-64, it does nothing 🤷. A trampoline is a small piece of code that gets created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly 🌀. Because of that, it requires the stack to be made executable for the program to work properly 🚨.
-fno-trampolines is enabled by default on a language-by-language basis, so the compiler can avoid generating trampolines if it figures that's safe, and swap them out for descriptors instead 🧠. Descriptors are made up of data only, but the generated code has to be prepared to deal with them 🧩. As of this writing, -fno-trampolines is enabled by default only for Ada 👑. Also, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present 😵. This option must be used on a program-wide basis and handled with extreme care 🫣.
-fvisibility=[default|internal|hidden|protected]— this sets the default ELF image symbol visibility to the option you pick. All symbols get marked with it unless they're overridden within the code 🔮. Using this can substantially improve linking and load times of shared object libraries, produce more optimized code, give you near-perfect API export, and prevent symbol clashes 🤌. It's strongly recommended that you use this in any shared objects you distribute 💜.
Despite the nomenclature, default always means public, i.e., available to be linked against from outside the shared object 🌈. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden 🙄. The default if -fvisibility isn't specified is default, i.e., every symbol is made public 🌟. For a good explanation of the benefits of making sure ELF symbols have the correct visibility, check out "How To Write Shared Libraries" by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) 📚. A superior solution this option makes possible, compared to marking things hidden when the default is public, is to make the default hidden and mark things public instead 🎀. That's the norm with DLLs on Windows. With -fvisibility=hidden and "__attribute__ ((visibility("default")))" used instead of "__declspec(dllexport)", you get almost identical semantics with identical syntax 🫶. This is a total great boon for anyone working on cross-platform projects 🦋✨
For anyone adding visibility to code that already exists 🧚, #pragma GCC visibility is your friend ✨. You wrap the declarations you care about in #pragma GCC visibility push(hidden) and close it with #pragma GCC visibility pop 💅. Real talk: symbol visibility is part of the API interface contract, so all new code should spell out its visibility whenever it isn't the default 🫶. Declarations that only get used inside the local DSO should always be marked explicitly as hidden, which dodges PLT indirection overheads 🙈. Writing it out this clearly also makes the code way more readable and self-documenting 💖. Also, thanks to ISO C++ spec requirements, operator new and operator delete must always have default visibility 🔥.
Be careful with headers from outside your project, especially system headers and headers from any other library you use 👀. They may not be expecting to be compiled with visibility other than the default. You might need to say #pragma GCC visibility push(default) before including any of them 🌸.
extern declarations aren't affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with zero modifications 🦋. The catch is that calls to extern functions with no explicit visibility go through the PLT, so it's way more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden 💜.
Also, -fvisibility does affect C++ vague linkage entities ⭐. So an exception class that gets thrown between DSOs must be explicitly marked with default visibility, so that the type_info nodes are unified between the DSOs 😭.
If you want the full tea on these techniques, their benefits and how to use them, check out <http://gcc.gnu.org/wiki/Visibility> 🌈.
✨ -fstrict-volatile-bitfields ✨
Use this one when accesses to volatile bit-fields (or other structure fields, though the compiler usually honors those types anyway) should be a single access at the width of the field's type, aligned to a natural alignment if possible 💖. For example, targets with memory-mapped peripheral registers might require every access to be 16 bits wide. With this flag, you can declare all the peripheral bit-fields as unsigned short (assuming short is 16 bits on those targets) to force GCC to use 16-bit accesses instead of a maybe more efficient 32-bit access 🔥.
If this option is disabled, the compiler goes for the most efficient instruction 🏃♀️. In the previous example, that might be a 32-bit load instruction, even though it touches bytes that don't contain any part of the bit-field, or memory-mapped registers unrelated to the one being updated 💀.
In some cases, like when the packed attribute is applied to a structure field, it might not be possible to access the field with a single read or write that's correctly aligned for the target machine 🙈. In that case GCC falls back to generating multiple accesses, instead of code that would fault or truncate the result at run time. Safe and sound 🫶.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members 😭. It's therefore recommended to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary interface for the target processor 🧚.
✨ -fsync-libcalls ✨
This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions 🦋.
The default is enabled, so the only useful form of this option is -fno-sync-libcalls 👑. This option is used in the implementation of the libatomic runtime library 🌈.
🛠️ GCC Developer Options
This section covers command-line options that are mainly of interest to GCC developers 🧑💻. That includes options that support compiler testing and investigation of compiler bugs and compile-time performance problems 🔍. It also covers options that produce debug dumps at various points in compilation, options that print statistics like memory use and execution time, and options that print info about GCC's configuration, like where it searches for libraries 📚. You should rarely need any of these for ordinary compilation and linking tasks 🙈.
Lots of developer options that make GCC dump output to a file take an optional =filename suffix ✨. You can specify stdout or - to dump to standard output, and stderr for standard error 💅.
If =filename is omitted, a default dump file name gets built by concatenating the base dump file name, a pass number, a phase letter, and the pass name 🧩. The base dump file name is the name of the output file the compiler produces, if you explicitly specified one and it isn't an executable. Otherwise, it's the source file name 📁. The pass number is determined by the order passes are registered with the compiler's pass manager 🔢. That's usually the same as the execution order, but passes registered by plugins, target-specific passes, or passes that get registered late are numbered higher than the pass named final, even if they run earlier 🫠. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree) 🌸. The files get created in the directory of the output file 💖.
✨ -fcallgraph-info ✨
✨ -fcallgraph-info=MARKERS ✨
This makes the compiler output callgraph information for the program, on a per-object-file basis 🕸️. The info is generated in the common VCG format. You can decorate it with extra per-node and/or per-edge info by adding a list of comma-separated markers 💜. When the su marker is specified, the callgraph gets decorated with stack usage info, which is equivalent to -fstack-usage 🥞. When the da marker is specified, the callgraph gets decorated with info about dynamically allocated objects 🧠.
When you compile with -flto, no callgraph info is output along with the object file 🙅♀️. At LTO link time, -fcallgraph-info may generate multiple callgraph info files right next to the intermediate LTO output files 📂.
✨ -dletters, -fdump-rtl-pass, -fdump-rtl-pass=filename ✨
Okay, this is the debugging dump zone 🔬. These make debugging dumps during compilation at times specified by letters. They're used for debugging the RTL-based passes of the compiler 🧪.
Some -dletters switches mean something different when -E is used for preprocessing 🙈.
You can enable debug dumps with a -fdump-rtl switch or with some -d option letters 🎀. Here are the possible letters you can use in pass and letters, and what they mean 👇:
-fdump-rtl-alignments— Dump after branch alignments have been computed. 📐-fdump-rtl-asmcons— Dump after fixing rtl statements that have unsatisfied in/out constraints. 🧷-fdump-rtl-auto_inc_dec— Dump after auto-inc-dec discovery. This pass only runs on architectures that have auto inc or auto dec instructions. ⚡-fdump-rtl-barriers— Dump after cleaning up the barrier instructions. 🧹-fdump-rtl-bbpart— Dump after partitioning hot and cold basic blocks. 🔥❄️-fdump-rtl-bbro— Dump after block reordering. 🔀-fdump-rtl-btl1,-fdump-rtl-btl2— These enable dumping after the two branch target load optimization passes. 🎯-fdump-rtl-bypass— Dump after jump bypassing and control flow optimizations. 🦘-fdump-rtl-combine— Dump after the RTL instruction combination pass. 🧃-fdump-rtl-compgotos— Dump after duplicating the computed gotos. 👯♀️-fdump-rtl-ce1,-fdump-rtl-ce2,-fdump-rtl-ce3— These enable dumping after the three if conversion passes. 🔁-fdump-rtl-cprop_hardreg— Dump after hard register copy propagation. 📋-fdump-rtl-csa— Dump after combining stack adjustments. 🥞-fdump-rtl-cse1,-fdump-rtl-cse2— These enable dumping after the two common subexpression elimination passes. 🫧-fdump-rtl-dce— Dump after the standalone dead code elimination passes. 🗑️-fdump-rtl-dbr— Dump after delayed branch scheduling. ⏳-fdump-rtl-dce1,-fdump-rtl-dce2— These enable dumping after the two dead store elimination passes. 💅-fdump-rtl-eh— Dump after finalization of EH handling code. 🛟-fdump-rtl-eh_ranges— Dump after conversion of EH handling range regions. 🗺️-fdump-rtl-expand— Dump after RTL generation. 🌱-fdump-rtl-fwprop1,-fdump-rtl-fwprop2— These enable dumping after the two forward propagation passes. ➡️-fdump-rtl-gcse1,-fdump-rtl-gcse2— These enable dumping after global common subexpression elimination. 🌍-fdump-rtl-init-regs— Dump after the initialization of the registers. 🏁-fdump-rtl-initvals— Dump after the computation of the initial value sets. 🧮-fdump-rtl-into_cfglayout— Dump after converting to cfglayout mode. 🧭-fdump-rtl-ira— Dump after iterated register allocation. 🗂️-fdump-rtl-jump— Dump after the second jump optimization. 🦘-fdump-rtl-loop2— This enables dumping after the rtl loop optimization passes. 🔄-fdump-rtl-mach— Dump after performing the machine dependent reorganization pass, if that pass exists. 🛠️-fdump-rtl-mode_sw— Dump after removing redundant mode switches. 🔀-fdump-rtl-rnreg— Dump after register renumbering. 🔢-fdump-rtl-outof_cfglayout— Dump after converting from cfglayout mode. 🧭-fdump-rtl-peephole2— Dump after the peephole pass. 🔍-fdump-rtl-postreload— Dump after post-reload optimizations. 🚀-fdump-rtl-pro_and_epilogue— Dump after generating the function prologues and epilogues. 🎬-fdump-rtl-sched1,-fdump-rtl-sched2— These enable dumping after the basic block scheduling passes. 🗓️-fdump-rtl-ree— okay bestie, this one dumps after sign/zero extension elimination 🧹✨-fdump-rtl-seqabstr— dumps after common sequence discovery 🔍💖-fdump-rtl-shorten— dumps after shortening branches 🦋-fdump-rtl-sibling— dumps after sibling call optimizations 🌸-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— these five are giving "do it five times" energy 🔁 each one turns on a dump after one of five rounds of instruction splitting 👑-fdump-rtl-sms— dumps after modulo scheduling 🧚 this pass is only run on some architectures, so don't be shocked if it's missing on yours-fdump-rtl-stack— dumps after converting GCC's "flat register file" registers into the x87's stack-like registers 💅 this pass is only run on x86 variants-fdump-rtl-subreg1,-fdump-rtl-subreg2— these two enable dumping after the two subreg expansion passes 🫶-fdump-rtl-unshare— dumps after all rtl has been unshared 🍓-fdump-rtl-vartrack— dumps after variable tracking 📈-fdump-rtl-vregs— dumps after converting virtual registers to hard registers 🌈-fdump-rtl-web— dumps after live range splitting 🕸️-fdump-rtl-regclass,-fdump-rtl-subregs_of_mode_init,-fdump-rtl-subregs_of_mode_finish,-fdump-rtl-dfinit,-fdump-rtl-dfinish— these are defined but they always produce empty files 💀
Heads up, bestie: those five always produce empty files, so don't panic when you open one and it's giving nothing 😭
-da,-fdump-rtl-all— produce all the dumps listed above 🎀-dA— annotates the assembler output with miscellaneous debugging information 🧁-dD— dumps all macro definitions at the end of preprocessing, in addition to the normal output 📜-dH— produces a core dump whenever an error occurs 💥-dp— annotates the assembler output with a comment indicating which pattern and alternative is used 💜 the length and cost of each instruction get printed too-dP— dumps the RTL in the assembler output as a comment before each instruction ✨ it also turns on-dpannotation-dx— just generates RTL for a function instead of compiling it 🦋 usually used with-fdump-rtl-expand-fdump-debug— dumps the debugging information generated during the debug generation phase 🐞-fdump-earlydebug— dumps the debugging information generated during the early debug generation phase ⏰-fdump-noaddr— when doing debugging dumps, suppresses address output 🙈 this makes it way more feasible to diff debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations-freport-bug— collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs 🚨-fdump-unnumbered— when doing debugging dumps, suppresses instruction numbers and address output 🔢 this makes diffing dumps way easier for invocations with different options, especially with and without-g-fdump-unnumbered-links— when doing debugging dumps (see-doption above), suppresses instruction numbers for the links to the previous and next instructions in a sequence 🔗-fdump-ipa-switch,-fdump-ipa-switch-options— these control the dumping at various stages of inter-procedural analysis language tree to a file 🗂️ the file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. the possible dumps are:all— enables all inter-procedural analysis dumpscgraph— dumps information about call-graph optimization, unused function removal, and inlining decisionsinline— dumps after function inlining
Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.
For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 👀
By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.
-fdump-lang— dumps language-specific information 🌷 the file name is made by appending.langto the source file name-fdump-lang-all,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— these control the dumping of language-specific information 💖 the options and filename parts behave as described in the-fdump-treeoption. the accepted switch values are:all— enables all language-specific dumpsclass— dumps class hierarchy information. virtual table information is emitted unless 'slim' is specified. this one is C++ only 👑module— dumps module information. the options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information. this one is C++ only 🧝raw— dumps the raw internal tree data. this one is C++ only 🪵
-fdump-passes— prints onstderrthe list of optimization passes that are turned on and off by the current command-line options 📋-fdump-statistics-option— enables and controls dumping of pass statistics in a separate file 📊 the file name is generated by appending a suffix ending in.statisticsto the source file name, and the file is created in the same directory as the output file. if the-optionform is used,-statscauses counters to be summed over the whole compilation unit while-detailsdumps every event as the passes generate them. the default with no option is to sum counters for each function compiled.-fdump-tree-all,-fdump-tree-switch,-fdump-tree-switch-options,-fdump-tree-switch-options=filename— these control the dumping at various stages of processing the intermediate language tree to a file 🌳 if the-optionsform is used, options is a list of--separated options that control the details of the dump. not all options are applicable to all dumps, and the ones that aren't meaningful get ignored. the following options are available:address— prints the address of each node 📍 usually this isn't meaningful, since it changes according to the environment and source file. its main use is tying up a dump file with a debug environmentasmname— if "DECL_ASSEMBLER_NAME" has been set for a given decl, that gets used in the dump instead of "DECL_NAME" 🔮 its main use is working backward from mangled names in the assembly fileslim— when dumping front-end intermediate representations, inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only dumps those items when they're directly reachable by some other path 🥄- when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures
- when dumping RTL, the RTL is printed in slim (condensed) form instead of the default LISP-like representation
raw— prints a raw representation of the tree 🧾 by default, trees are pretty-printed into a C-like representationdetails— enables more detailed dumps (not honored by every dump option) 🔎 also includes information from the optimization passesstats— enables dumping various statistics about the pass (not honored by every dump option) 📈blocks— enables showing basic block boundaries (disabled in raw dumps) 🧱graph— for each of the other indicated dump files (-fdump-rtl-pass), dumps a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot 🗺️ each function in the file is pretty-printed as a subgraph, so GraphViz can render them all in a single plot. this option currently only works for RTL dumps, and the RTL is always dumped in slim formvops— enables showing virtual operands for every statement 🧩lineno— enables showing line numbers for statements 🔢uid— enables showing the unique ID ("DECL_UID") for each variable 🆔verbose— enables showing the tree dump for each statement 📣eh— enables showing the EH region number holding each statement 🛟scev— enables showing scalar evolution analysis details 🌙optimized— enables showing optimization information (only available in certain passes) ✅missed— enables showing missed optimization information (only available in certain passes) ❌note— enables other detailed optimization information (only available in certain passes) 📝all— turns on all options, exceptraw,slim,verboseandlineno🌈optall— turns on all optimization options, i.e.,optimized,missed, andnote🚀
To figure out what tree dumps are available, or to find the dump for a pass you're after, follow the steps below 🕵️♀️
- Invoke GCC with
-fdump-passes, then look in thestderroutput for a code that matches the pass you're interested in. for example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass 🔍 - To enable the creation of the dump file, append the pass code to the
-fdump-option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with-fdump-tree-evrp. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates one as described below 💅 - Hunt down the pass dump in a file named with three bits joined by periods: the source file GCC was invoked to compile, then a numeric suffix for the pass number with a
tfor tree passes (or anrfor RTL passes), then the pass code. For example, the Early VRP pass dump might be in a file namedmyfile.c.038t.evrpin the current working directory. Heads up, bestie: those numeric codes are not stable and can change from one GCC version to another 💀
✨ OPTIONS ✨
-fopt-info,-fopt-info-options,-fopt-info-options=filename— optimization dump central 🦋 These control dumps from the various optimization passes. If you use the-optionsform, options is a list of--separated keywords that pick the dump details and which optimizations you want.
The options come in three groups, and you can mix and match within each group since they don't overlap. If there's a clash, the later option on the command line wins 👑
- Options that describe what kinds of messages get emitted
- Options that describe how verbose the dump is
- Options that describe which optimizations get included
Message kinds (group 1) 💖
optimized— successful glow-ups! Prints info whenever an optimization gets applied. Each pass decides what's relevant. The vectorizer, for example, prints the source location of loops that got successfully vectorized.missed— Info about optimizations that didn't happen. Each pass picks what goes in here 🙈note— Verbose stuff about optimizations, like certain transformations and more detailed messages about decisions ✨all— The full tea ☕ Detailed optimization info, which includesoptimized,missed, andnote.
Verbosity (group 2) 🧚
internals— By default you only get the "high-level" messages. This one unlocks extra, more detailed messages that are mostly for GCC developers. Not for the faint of heart 💀
Optimization groups (group 3) 🌈 You can use one or more of these keywords to target a whole group of optimizations:
ipa— Dumps from all interprocedural optimizations.loop— Dumps from all loop optimizations.inline— Dumps from all inlining optimizations.omp— Dumps from all OMP (Offloading and Multi Processing) optimizations.vec— Dumps from all vectorization optimizations.optall— Dumps from literally every optimization. It's a superset of the groups above, so it's giving everything 💅
If options is left out, it defaults to optimized-optall. That means you get messages about successful optimizations from all the passes, minus anything treated as "internals" 🎀
If you pass a filename, the dumps from all the applicable optimizations get concatenated into that file. Otherwise, the dump goes to stderr. You can throw in multiple -fopt-info options, but only one of them can carry a filename. If you give other filenames, all but the first such option get ignored 🥺
Heads up: the output filename gets overwritten when you have multiple translation units. If you want one combined output from several translation units, use stderr instead!
Here's the vibe check with examples. First, the optimization info goes to stderr:
gcc -O3 -fopt-infoThis one dumps the missed optimization report from all the passes into missed.all:
gcc -O3 -fopt-info-missed=missed.allAnd this one prints info about missed optimization opportunities from the vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types after -fopt-info does not matter, so slay however you like 💖
gcc -O2 -ftree-vectorize -fopt-info-vec-missedAnother one, which outputs info about missed optimizations and optimized locations from all the inlining passes into inline.txt:
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtFinally, consider this one 🙈
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere, the two output filenames vec.miss and loop.opt are in conflict, since only one output file is allowed. Only the first option takes effect and the rest get ignored. So you only end up with vec.miss, which has the vectorizer's dumps about missed opportunities. Not me being chaotic, the compiler just be like that 😭
-fsave-optimization-record— the machine-readable receipts 🧾 This writes aSRCFILE.opt-record.json.gzfile detailing which optimizations were performed, for the ones that support-fopt-info.
Sis, this option is experimental, and the format of the data inside the compressed JSON file is subject to change. Don't build your whole life on it yet 💀
It's roughly the machine-readable version of -fopt-info-all, as a collection of messages with source file, line number, and column number. Each message also gets this extra data:
- the execution count of the code being optimized, plus metadata about whether it came from actual profile data or was just an estimate, so consumers can prioritize messages by code hotness 🔥
- the function name of the code being optimized, where that applies
- the "inlining chain" for the code being optimized, so when a function gets inlined into several places (which might themselves be inlined), you can tell the copies apart 🪞
- objects identifying the parts of the message that refer to expressions, statements, or symbol-table nodes, which of those categories they are, and, when available, their source code location
- the GCC pass that emitted the message
- the location in GCC's own code where the message came from
Also, some messages are logically nested inside other messages, reflecting implementation details of the optimization passes 🧚
-fsched-verbose=n— scheduler tea, served hot ☕ On targets that use instruction scheduling, this controls how much debugging output the scheduler dumps into the dump files.
For n greater than zero, -fsched-verbose outputs the same info as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information, and unit/insn info. For n greater than two, it includes the RTL at the abort point, plus control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. Iconic levels, honestly 👑
-fenable-kind-pass,-fdisable-kind-pass=range-list— the pass on/off switches 🔌 This is a set of options for explicitly disabling and enabling optimization passes. They're meant for debugging GCC itself. If you're just a regular compiler user, stick with the regular options for enabling and disabling passes instead 🙈
Here's the full menu:
-fdisable-ipa-pass— Disables the IPA pass pass, where pass is the pass name. If the same pass gets statically invoked in the compiler multiple times, append a sequential number to the pass name, starting from 1.-fdisable-rtl-pass,-fdisable-rtl-pass=range-list— Disables the RTL pass pass, where pass is the pass name. Same deal with the sequential number if the pass runs more than once. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon, and it's inclusive at both ends. If the range is trivial, the number pair can be simplified to a single number. If a function's call graph node uid falls within one of the specified ranges, the pass gets disabled for that function. The uid shows up in the function header of a dump file, and you can dump the pass names using-fdump-passes.-fdisable-tree-pass,-fdisable-tree-pass=range-list— Disables the tree pass pass. Check-fdisable-rtl-passfor how the option arguments work.-fenable-ipa-pass— Enables the IPA pass pass, where pass is the pass name. Same sequential numbering rule as above, starting from 1.-fenable-rtl-pass,-fenable-rtl-pass=range-list— Enables the RTL pass pass. Check-fdisable-rtl-passfor the option argument description and examples.-fenable-tree-pass,-fenable-tree-pass=range-list— Enables the tree pass pass. Check-fdisable-rtl-passfor how the option arguments work.
Here are some examples of these options in action 💅
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking,-fchecking=n— internal vibe checks 🔍 Enables internal consistency checking. The default depends on how the compiler was configured.-fchecking=2turns on further internal consistency checking that might affect code generation.-frandom-seed=string— the reproducibility queen 🌸 This gives GCC a seed to use in place of random numbers when it generates certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and in the object files that produce them. Use-frandom-seedto produce reproducibly identical object files, which is honestly so main character 🫶
The string can be a number (decimal, octal, or hex) or an arbitrary string. If it's an arbitrary string, it gets converted to a number by computing CRC32 ✨
The string should be totally different for every file you compile, bestie 🎀 no copy-pasting the same one everywhere!
-save-temps— okay so this one keeps the usual "temporary" intermediate files around permanently 💾 Instead of tossing them, they get named as auxiliary output files, the same way described under-dumpbaseand-dumpdir.
Pairing -save-temps with the -x command-line option is lowkey genius 🧠 It won't overwrite an input source file that shares an extension with an intermediate file. If you want that intermediate file, just rename the source file before using -save-temps.
-save-temps=cwd— same energy as-save-temps -dumpdir ./.📁
-save-temps -dumpdir ./.-save-temps=obj— this one puts everything in the output file's folder 🦋 It's equivalent to-save-temps -dumpdir outdir/, whereoutdir/is the directory of the output file given after the-ooption, including any directory separators. No-o? Then it acts exactly like-save-temps=cwd.-time[=file]— reports the CPU time each subprocess in the compilation sequence takes ⏱️ For C source files, that's the compiler proper and the assembler (plus the linker if you're linking).
Without an output file, you get something like this:
# cc1 0.12 0.01
# as 0.00 0.01The first number on each line is the "user time", which is the time spent running the program itself. The second is the "system time", the time spent in operating system routines on the program's behalf. Both numbers are in seconds 🫶
With an output file, the report gets appended to that file and looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>Now the "user time" and "system time" get moved before the program name, and the options passed to the program are shown too. That way you can totally tell later which file was being compiled and with which options 🕵️♀️
-fdump-final-insns[=file]— dumps the final internal representation (RTL) to file 🧩 If the optional argument is left out (or file is"."), the dump file gets named by slapping".gkd"onto the dump base name. See-dumpbasefor the deets.-fcompare-debug[=opts]— this one is a whole vibe 🔍 If there's no error during compilation, the compiler runs a second time, adding opts and-fcompare-debug-secondto the arguments for that second go. It dumps the final internal representation in both compilations and prints an error if they differ.
If you skip the equal sign, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty, and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is set to a string starting with a dash, that string is used for opts. Otherwise the default -gtoggle is used 💅
-fcompare-debug=, with the equal sign but no opts, is the same as -fno-compare-debug. That turns off the dumping of the final representation and the second compilation, and it even stops GCC_COMPARE_DEBUG from taking effect 🙅♀️
To verify full coverage during-fcompare-debugtesting, setGCC_COMPARE_DEBUGto say-fcompare-debug-not-overridden. GCC rejects that as an invalid option in any actual compilation (rather than preprocessing, assembly or linking), so it's a whole thing. If you only want a warning, settingGCC_COMPARE_DEBUGto-w%n-fcompare-debug not overriddenwill do the trick ⚠️
-fcompare-debug-second— this gets passed to the compiler implicitly for the second compilation requested by-fcompare-debug🥂 It comes along with options to silence warnings, and it leaves out any other options that would make the compiler produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed to include the".gk"additional extension during that second compilation, so they don't overwrite the ones from the first run.
When you pass this option to the compiler driver, it skips the first compilation entirely. Honestly, that makes it pretty much only useful for debugging the compiler proper 🤓
-gtoggle— turns off debug info generation if leaving this option out would generate it, or turns it on at level 2 otherwise 🔄 Where it sits on the command line doesn't matter. It takes effect after all the other options are processed, and it only does its thing once, no matter how many times you give it. It's mainly meant to be used with-fcompare-debug.-fvar-tracking-assignments-toggle— toggles-fvar-tracking-assignments, the same way-gtoggletoggles-g🎛️-Q— makes the compiler print out each function name as it gets 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's done ⏳-ftime-report-details— records the time consumed by infrastructure parts separately for each pass 🔬-fira-verbose=n— controls how verbose the dump file for the integrated register allocator is 🗂️ The default value is5. If the value n is greater than or equal to10, the dump output gets sent to stderr, using the same format as n minus 10.-flto-report— prints a report with internal details on how the link-time optimizer is working 🧐 The contents change from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via-flto).
It's disabled by default, so you have to ask for it 🙋♀️
-flto-report-wpa— same as-flto-report, but it only prints for the WPA phase of link-time optimization 🎯-fmem-report— makes the compiler print some statistics about permanent memory allocation when it finishes 🧠💾-fmem-report-wpa— makes the compiler print some statistics about permanent memory allocation for the WPA phase only 🎯-fpre-ipa-mem-report,-fpost-ipa-mem-report— makes the compiler print some statistics about permanent memory allocation either before or after interprocedural optimization 🧃-fprofile-report— makes the compiler print some statistics about the consistency of the (estimated) profile and the effect of individual passes 📈-fstack-usage— makes the compiler output stack usage information for the program, on a per-function basis 🥞 The filename for the dump is made by appending.suto the auxname. The auxname comes from the name of the output file, if you explicitly specified one and it's not an executable. Otherwise it's the basename of the source file.
Each entry has three fields:
- The name of the function.
- A number of bytes.
- One or more qualifiers: "static", "dynamic", "bounded".
The "static" qualifier means the function manipulates the stack statically 🧱 A fixed number of bytes gets allocated for the frame on function entry and released on function exit, and no other stack adjustments happen in the function. The second field is that fixed number of bytes.
The "dynamic" qualifier means the function manipulates the stack dynamically 🌀 On top of the static allocation, stack adjustments happen in the body of the function, for example to push and pop arguments around function calls. If "bounded" is also present, the amount of those adjustments is bounded at compile time, and the second field is an upper bound on the total amount of stack the function uses. If "bounded" isn't there, the amount of adjustments isn't bounded at compile time, and the second field only represents the bounded part.
-fstats— emits statistics about front-end processing at the end of the compilation 📉 Only the C++ front end supports this, and the info is generally only really useful to the G++ development team 🧑💻-fdbg-cnt-list— prints the name and the counter upper bound for all debug counters 📋-fdbg-cnt=counter-value-list— sets the internal debug counter lower and upper bounds 🎚️ counter-value-list is a comma-separated list ofname:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples. Each one sets the counter's name and a list of closed intervals. Thelower_boundis optional and gets zero initialized if it isn't set.
For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation ✨ And for dbg_cnt(tail_call), true is returned for the first 10 invocations 🔟
-print-file-name=library— prints the full absolute name of the library file library that would be used when linking, and then stops there 🖨️ With this option, GCC doesn't compile or link anything. It just prints the file name.-print-multi-directory— prints the directory name for the multilib selected by any other switches on the command line 📂 That directory is supposed to exist inGCC_EXEC_PREFIX.-print-multi-lib— prints the mapping from multilib directory names to the compiler switches that enable them 🗺️ The directory name is separated from the switches by;. Each switch starts with@instead of-, with no spaces between multiple switches, which is meant to make shell processing easier.-print-multi-os-directory— prints the path to the OS libraries for the selected multilib, relative to somelibsubdirectory 🏠 If the OS libraries are in thelibsubdirectory and no multilibs are used, this is usually just.. If they're inlibsuffixsibling directories, it prints something like../lib64,../libor../lib32. If they're inlib/subdirsubdirectories, it prints something likeamd64,sparcv9orev6.-print-multiarch— prints the path to the OS libraries for the selected multiarch, relative to somelibsubdirectory 🏹-print-prog-name=program— works like-print-file-name, but searches for a program, such ascpp, instead 🔎-print-libgcc-file-name— same as-print-file-name=libgcc.a🌟
This one is clutch when you use -nostdlib or -nodefaultlibs but still want to link with libgcc.a 💜 You can do something like this:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`-print-search-dirs— prints the name of the configured installation directory, plus a list of the program and library directoriesgccsearches, and then stops there 🗃️
Okay so this bit is SO important when gcc starts yelling installation problem, cannot exec cpp0: No such file or directory 😭 There are two ways to fix it, bestie: either put cpp0 and the other compiler components exactly where gcc expects to find them, or set the environment variable GCC_EXEC_PREFIX to the directory where you installed them 🫶 And don't forget the trailing /! 💅
-print-sysroot— Prints the target sysroot directory that gets used during compilation 🗺️ That's the one set at configure time or with the--sysrootoption, possibly with an extra suffix depending on your compilation options. No target sysroot? It prints nothing 🙈-print-sysroot-headers-suffix— Prints the suffix tacked onto the target sysroot when gcc is hunting for headers. If the compiler wasn't configured with one, it throws an error instead. Either way, it doesn't do anything else 🫠-dumpmachine— Prints the compiler's target machine, likei686-pc-linux-gnu, and then stops right there 🧚-dumpversion— Prints the compiler version, like3.0,6.3.0or7, and then stops 💖 This is the version used in filesystem paths and specs. Depending on how the compiler was configured, it can be one number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version).-dumpfullversion— Prints the full compiler version and nothing else ✨ The output is always three numbers separated by dots: major, minor and patchlevel version.-dumpspecs— Prints the compiler's built-in specs and nothing else 🌸 (This is only used when GCC itself is being built.)
🦋 Machine-Dependent Options 🦋
Every target machine GCC supports can have its own options, bestie 💜 Think: compiling for a particular processor variant or ABI, or tweaking optimizations that are specific to that machine. By convention, the names of machine-specific options start with -m 🎀
Some configurations also support extra target-specific options, usually so they can play nicely with other compilers on the same platform 🌈
✨ AArch64 Options ✨
These options are defined for AArch64 implementations 👑
-mabi=name— Generates code for the specified data model 🧠 The valid values areilp32, a SysV-like model whereint,long intand pointers are 32 bits, andlp64, a SysV-like model whereintis 32 bits butlong intand pointers are 64 bits. The default depends on your specific target configuration. Heads up, bestie: the LP64 and ILP32 ABIs are not link-compatible 😭 You have to compile your entire program with the same ABI and link it against a compatible set of libraries.-mbig-endian— Generates big-endian code 🍓 This is the default when GCC is configured for anaarch64_be-*-*target.-mgeneral-regs-only— Only uses the general-purpose registers 💅 This stops the compiler from touching the floating-point and Advanced SIMD registers, but it doesn't put any restrictions on the assembler.-mlittle-endian— Generates little-endian code 🫶 This is the default when GCC is configured for anaarch64-*-*target but not anaarch64_be-*-*target.-mcmodel=tiny— Generates code for the tiny code model 🐣 The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.-mcmodel=small— Generates code for the small code model 🌷 The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.-mcmodel=large— Generates code for the large code model 🌌 It makes no assumptions about addresses and sizes of sections. Programs can only be statically linked. Also,-mcmodel=largeis incompatible with-mabi=ilp32,-fpicand-fPIC🚫-mstrict-align,-mno-strict-align— Avoid or allow generating memory accesses that might not be aligned on a natural object boundary, as described in the architecture specification 📐-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— Omit or keep the frame pointer in leaf functions 🍃 The former behavior is the default.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— Generates stack protection code using a canary atguard🛡️ The supported locations areglobalfor a global canary, orsysregfor a canary in an appropriate system register. With thesysregchoice,-mstack-protector-guard-reg=regand-mstack-protector-guard-offset=offsetpick which system register is used as the base register for reading the canary, and what offset from that base register to use. There's no default register or offset, because this is entirely for use within the Linux kernel 🐧-mtls-dialect=desc— Uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables 💜 This is the default.-mtls-dialect=traditional— Uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables 🕰️-mtls-size=size— Specifies the bit size of immediate TLS offsets. The valid values are12,24,32and48. This option requires binutils 2.26 or newer 🔧-mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— Enables or disables the workaround for ARM Cortex-A53 erratum number 835769 🩹 This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— Enables or disables the workaround for ARM Cortex-A53 erratum number 843419 🩹 This erratum workaround is done at link time, so this option only passes the corresponding flag to the linker.-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— Enables or disables the reciprocal square root approximation 🧮 This only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling it reduces the precision of reciprocal square root results to about 16 bits for single precision and 32 bits for double precision.-mlow-precision-sqrt,-mno-low-precision-sqrt— Enables or disables the square root approximation 🧮 This only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling it reduces the precision of square root results to about 16 bits for single precision and 32 bits for double precision. If enabled, it implies-mlow-precision-recip-sqrt.-mlow-precision-div,-mno-low-precision-div— Enables or disables the division approximation ➗ This only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling it reduces the precision of division results to about 16 bits for single precision and 32 bits for double precision.-mtrack-speculation,-mno-track-speculation— Enables or disables generating extra code to track speculative execution through conditional branches 🔍 The tracking state can then be used by the compiler when it expands calls to"__builtin_speculation_safe_copy", so it can generate a more efficient code sequence 🦋-moutline-atomics,-mno-outline-atomics— Enables or disables calls to out-of-line helpers that implement atomic operations ⚛️ These helpers figure out at runtime whether the LSE instructions from ARMv8.1-A can be used. If they can't, they fall back to the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0 instruction set. If you're using a later revision, like -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions get used directly. The same goes for -mcpu= when the selected CPU supports the lse feature. This option is on by default 💅
-march=name— Specifies the name of the target architecture and, optionally, one or more feature modifiers 🏗️ This option takes the form-march=arch{+[no]feature}*.
Here's the lowdown on the values for arch and what they turn on by default:
armv8-a(Armv8-A) — includes by default:+fp,+simdarmv8.1-a(Armv8.1-A) — includes by default:armv8-a,+crc,+lse,+rdmaarmv8.2-a(Armv8.2-A) — includes by default:armv8.1-aarmv8.3-a(Armv8.3-A) — includes by default:armv8.2-a,+pautharmv8.4-a(Armv8.4-A) — includes by default:armv8.3-a,+flagm,+fp16fml,+dotprodarmv8.5-a(Armv8.5-A) — includes by default:armv8.4-a,+sb,+ssbs,+predresarmv8.6-a(Armv8.6-A) — includes by default:armv8.5-a,+bf16,+i8mmarmv8-r(Armv8-R) — includes by default:armv8-r
The value native is available on native AArch64 GNU/Linux and makes the compiler pick the architecture of the host system 🖥️ This option has no effect if the compiler can't recognize the architecture of the host system,
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. If you specify conflicting feature modifiers, the right-most feature wins 👑
GCC uses name to figure out what kind of instructions it can emit when generating assembly code. If -march is specified without -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors that implement the target architecture 🎯
-mtune=name— Specifies the name of the target processor that GCC should tune the performance of the code for 🎀 The permissible values are:generic,cortex-a35,cortex-a53,cortex-a55,cortex-a57,cortex-a72,cortex-a73,cortex-a75,cortex-a76,cortex-a76ae,cortex-a77,cortex-a65,cortex-a65ae,cortex-a34,cortex-a78,cortex-a78ae,cortex-a78c,ares,exynos-m1,emag,falkor,neoverse-512tvb,neoverse-e1,neoverse-n1,neoverse-n2,neoverse-v1,neoverse-v2,grace,qdf24xx,saphira,phecda,xgene1,vulcan,octeontx,octeontx81,octeontx83,octeontx2,octeontx2t98,octeontx2t96,octeontx2t93,octeontx2f95,octeontx2f95n,octeontx2f95mm,a64fx,thunderx,thunderxt88,thunderxt88p1,thunderxt81,tsv110,thunderxt83,thunderx2t99,thunderx3t110,zeus,cortex-a57.cortex-a53,cortex-a72.cortex-a53,cortex-a73.cortex-a35,cortex-a73.cortex-a53,cortex-a75.cortex-a55,cortex-a76.cortex-a55,cortex-r82,cortex-x1,ampere1,ampere1a,cobalt-100andnative💖
okay so the big.LITTLE pairings are up next, bestie 🧚 these values: cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, and cortex-a76.cortex-a55 all tell GCC to tune for a big.LITTLE system 💜
neoverse-512tvb is giving very specific energy ✨ It tells GCC to tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. So basically, GCC is tuned for Neoverse cores that can run 4 128-bit Advanced SIMD arithmetic instructions a cycle, plus an equal number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE) 🎀 It's more general than tuning for one specific core like Neoverse V1, but more specific than the default tuning down below 🦋
Also, on native AArch64 GNU/Linux systems, the value native tunes performance to whatever host system you're on 🖥️ If the compiler can't recognize the processor of the host system, this option just does nothing 💀
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors 🌈 And this option cannot be suffixed by feature modifiers 🙅♀️
-mcpu=name— Pick the target processor, optionally with one or more feature modifiers tacked on the end 💅 The form is-mcpu=cpu{+[no]feature}*, and the allowed values forcpuare the same ones you get for-mtune. The allowedfeaturevalues are documented in the sub-sectionaarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. If you throw conflicting feature modifiers at it, the right-most one wins 👑 GCC usesnameto figure out what kind of instructions it's allowed to emit when generating assembly (as if you'd used-march), and which processor to tune performance for (as if you'd used-mtune) ✨ If you use this together with-marchor-mtune, those take precedence over the matching part of this option 💖-mcpu=neoverse-512tvbis special, because it doesn't refer to one specific core. It refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle 🔥 Unless you override it with-march,-mcpu=neoverse-512tvbgenerates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties 🫶 And unless you override it with-mtune,-mcpu=neoverse-512tvbtunes code the same way as-mtune=neoverse-512tvb🌸-moverride=string— Overrides the tuning decisions the back-end makes in response to a-mtune=switch 🙈 Heads up: the syntax, semantics, and accepted values forstringare not guaranteed to stay consistent across releases. This option is only really meant for developing GCC itself 🧚-mverbose-cost-dump— Turns on verbose cost model dumping in the debug dump files 🔍 This one is only here for debugging the compiler 💜-mpc-relative-literal-loads,-mno-pc-relative-literal-loads— Enables or disables PC-relative literal loads 🦋 With this option on, literal pools get accessed with a single instruction and are emitted after each function. The catch is that this caps the maximum size of functions at 1MB 😭 It's enabled by default for-mcmodel=tiny-msign-return-address=scope— Picks which functions get return address signing 🌈 The permissible values arenone, which turns return address signing off;non-leaf, which enables pointer signing for functions that aren't leaf functions; andall, which enables pointer signing for all functions. The default isnone🙃 This option has been deprecated by-mbranch-protection, so bestie, use that one instead 💀-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti— Picks which branch protection features to use 🛡️noneis the default and turns off all types of branch protection.standardturns on all types of branch protection features, and if a feature has extra tuning options,standardsets it to its standard level.pac-ret[+leaf]turns on return address signing to its standard level, which means signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optionalleafargument extends the signing to include leaf functions 🍓 The optionalb-keyargument lets you sign the functions with the B-key instead of the A-key. Andbtiturns on the branch target identification mechanism 🎀-mharden-sls=opts— Enables compiler hardening against straight line speculation (SLS) 🛡️optsis a comma-separated list of the following options:retbrandblr. On top of that,-mharden-sls=allenables all SLS hardening, while-mharden-sls=nonedisables all of it 💅-msve-vector-bits=bits— Specifies the number of bits in an SVE vector register 📏 This option only does something when SVE is enabled. GCC supports two forms of SVE code generation: "vector-length agnostic" output, which works with any size of vector register, and "vector-length specific" output, which lets GCC make assumptions about the vector length when that helps with optimization 🧠 The possible values ofbitsarescalable,128,256,512,1024and2048. Specifyingscalableselects vector-length agnostic output. Right now,-msve-vector-bits=128also generates vector-length agnostic output for big-endian targets. All the other values generate vector-length specific code. These values might change in future releases, so no value exceptscalableshould be relied on for code that's portable across different hardware SVE vector lengths ⚠️ The default is-msve-vector-bits=scalable, which produces vector-length agnostic code 🦋
✨ -march and -mcpu Feature Modifiers ✨
Feature modifiers used with -march and -mcpu can be any of the following, and each one also has an inverse, nofeature 💖 Here's the full list, bestie:
crc— Enables the CRC extension 🔁 This is on by default for-march=armv8.1-acrypto— Enables the Crypto extension 🔐 This also enables Advanced SIMD and floating-point instructionsfp— Enables floating-point instructions 🌸 This is on by default for all possible values for-marchand-mcpusimd— Enables Advanced SIMD instructions 🎶 This also enables floating-point instructions. It's on by default for all possible values for-marchand-mcpusve— Enables Scalable Vector Extension instructions 📐 This also enables Advanced SIMD and floating-point instructionslse— Enables Large System Extension instructions 🌊 This is on by default for-march=armv8.1-ardma— Enables Round Double Multiply Accumulate instructions 🧮 This is on by default for-march=armv8.1-afp16— Enables the FP16 extension 🌙 This also enables floating-point instructionsfp16fml— Enables the FP16 fmla extension 🌟 This also enables FP16 extensions and floating-point instructions. It's enabled by default for-march=armv8.4-a. Using this with architectures prior to Armv8.2-A is not supported 💀rcpc— Enables the RcPc extension 🧩 This doesn't change code generation from GCC, but it gets passed on to the assembler, so inline asm statements can use instructions from the RcPc extensiondotprod— Enables the Dot Product extension 🍭 This also enables Advanced SIMD instructionsaes— Enables the Armv8-aaesandpmullcrypto extension 🔑 This also enables Advanced SIMD instructionssha2— Enables the Armv8-asha2crypto extension 🔒 This also enables Advanced SIMD instructionssha3— Enables thesha512andsha3crypto extension 🧁 This also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supported 🚫sm4— Enables thesm3andsm4crypto extension 🐉 This also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supportedprofile— Enables the Statistical Profiling extension 📊 This is only to turn the extension on at the assembler level, and it does not affect code generationrng— Enables the Armv8.5-a Random Number instructions 🎲 This is only to turn the extension on at the assembler level, and it does not affect code generationmemtag— Enables the Armv8.5-a Memory Tagging Extensions 🏷️ Using this with architectures prior to Armv8.5-A is not supportedsb— Enables the Armv8-a Speculation Barrier instruction 🚧 This is only to turn the extension on at the assembler level, and it does not affect code generation. It's enabled by default for-march=armv8.5-assbs— Enables the Armv8-a Speculative Store Bypass Safe instruction 🛟 This is only to turn the extension on at the assembler level, and it does not affect code generation. It's enabled by default for-march=armv8.5-apredres— Enables the Armv8-a Execution and Data Prediction Restriction instructions 🔮 This is only to turn the extension on at the assembler level, and it does not affect code generation. It's enabled by default for-march=armv8.5-asve2— Enables the Armv8-a Scalable Vector Extension 2 🌌 This also enables SVE instructionssve2-bitperm— Enables SVE2 bitperm instructions 🧷 This also enables SVE2 instructionssve2-sm4— Enables SVE2 sm4 instructions 🐚 This also enables SVE2 instructionssve2-aes— Enables SVE2 aes instructions 🗝️ This also enables SVE2 instructionssve2-sha3— Enables SVE2 sha3 instructions 🪄 This also enables SVE2 instructionstme— Enables the Transactional Memory Extension 🧵i8mm— Enables 8-bit Integer Matrix Multiply instructions 🔢 This also enables Advanced SIMD and floating-point instructions. It's enabled by default for-march=armv8.6-a. Using this with architectures prior to Armv8.2-A is not supported 🙅♀️f32mm— Enables 32-bit Floating point Matrix Multiply instructions 🧱 This also enables SVE instructions. Using this with architectures prior to Armv8.2-A is not supportedf64mm— Enables 64-bit Floating point Matrix Multiply instructions 🪐 This also enables SVE instructions. Using this with architectures prior to Armv8.2-A is not supported 💜bf16— okay bestie, this one switches on brain half-precision floating-point instructions 🧠✨ It also turns on Advanced SIMD and floating-point instructions. It's on by default for-march=armv8.6-a💅flagm— enables the Flag Manipulation instructions Extension 🚩pauth— enables the Pointer Authentication Extension 🔐
Using bf16 with architectures prior to Armv8.2-A is not supported 🙅♀️
Heads up: crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2 🫶
🌸 Adapteva Epiphany Options 🌸
These -m options are defined for Adapteva Epiphany 🧚
-mhalf-reg-file— we don't allocate any register in the range"r32"..."r63". That lets your code run on hardware variants that don't have these registers 🙈-mprefer-short-insn-regs— prefers registers that allow short instruction generation. This can bump up the instruction count, so overall code size might go down or up 🤷♀️-mbranch-cost=num— sets the cost of branches to roughly num"simple"instructions. It's just a heuristic, so don't expect consistent results across releases 💀-mcmove— enables the generation of conditional moves ✨-mnops=num— emits num NOPs before every other generated instruction 😴-mno-soft-cmpsf— for single-precision floating-point comparisons, it emits an"fsub"instruction and tests the flags. That's faster than a software comparison, but it can give incorrect results with NaNs, or when two different small numbers are compared and their difference comes out as zero 😬 The default is-msoft-cmpsf, which uses slower but IEEE-compliant software comparisons 👑-mstack-offset=num— sets the offset between the top of the stack and the stack pointer. For example, a value of 8 means the eight bytes in the range"sp+0...sp+7"can be used by leaf functions without stack allocation 🍓 Values other than 8 or 16 are untested and unlikely to work. This option also changes the ABI, so compiling a program with a different stack offset than the libraries were built with generally doesn't work 🚫 It's useful if you want to see whether a different stack offset gives you better code, but to actually build working programs, you should configure the toolchain with the appropriate--with-stack-offset=numoption 🛠️-mno-round-nearest— makes the scheduler assume the rounding mode has been set to truncating. The default is-mround-nearest🌈-mlong-calls— unless an attribute says otherwise, it assumes all calls might be beyond the offset range of the"b"/"bl"instructions. So it loads the function address into a register before doing a (otherwise direct) call. This is the default 📞-mshort-calls— unless an attribute says otherwise, it assumes all direct calls are in range of the"b"/"bl"instructions, so it uses those instructions for direct calls. The default is-mlong-calls🦋-msmall16— assumes addresses can be loaded as 16-bit unsigned values. This doesn't apply to function addresses where-mlong-callssemantics are in effect 🎀-mfp-mode=mode— sets the prevailing mode of the floating-point unit. This decides the floating-point mode that's provided and expected at function call and return time. Matching this to the mode you mostly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches 💖
mode can be set to one of the following values:
caller— any mode at function entry is valid, and it's retained or restored when the function returns and when it calls other functions. This is useful for compiling libraries or other compilation units you might want to bring into different programs with different prevailing FPU modes. The convenience of using a single object file outweighs the size and speed overhead of any extra mode switching you'd need compared to a more specific choice of prevailing FPU mode 🫶truncate— the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer 🌸round-nearest— the mode used for floating-point calculations with round-to-nearest-or-even rounding mode 🍒int— the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate 🔢
The default is -mfp-mode=caller 💅
-mno-split-lohi,-mno-postinc,-mno-postmodify— code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults aremsplit-lohi,-mpost-inc, and-mpost-modify🧃-mnovect-double— changes the preferred SIMD mode toSImode. The default is-mvect-double, which usesDImodeas the preferred SIMD mode 🎶-max-vect-align=num— the maximum alignment for SIMD vector mode types. num may be 4 or 8, and the default is 8. Heads up: this is an ABI change, though lots of library function interfaces are unaffected as long as they don't use SIMD vector modes in places that affect the size and/or alignment of relevant types 📏-msplit-vecmove-early— splits vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case 🙃-m1reg-reg— specifies a register to hold the constant-1, which makes loading small negative constants and certain bitmasks faster ⚡ Allowable values for reg arer43andr63, which use that register as a fixed register, andnone, which means no register is used for this purpose. The default is-m1reg-none🌙
🔥 AMD GCN Options 🔥
These options are defined specifically for the AMD GCN port 💜
-march=gpu,-mtune=gpu— sets the architecture type or tuning for gpu. Supported values for gpu are:fiji— compile for GCN3 Fiji devices (gfx803) 🌊gfx900— compile for GCN5 Vega 10 devices (gfx900) ⭐gfx906— compile for GCN5 Vega 20 devices (gfx906) 👑
-msram-ecc=on,-msram-ecc=off,-msram-ecc=any— compiles binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default isany, for devices that support it 🧠-mstack-size=bytes— specifies how many bytes of stack space will be requested for each GPU thread (wave-front). Beware: there may be many threads and limited memory available 😭 The size of the stack allocation may also affect run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise 📦-mxnack— compiles binaries suitable for devices with the XNACK feature enabled. Some devices always require XNACK, and some let the user configure it. The compiled code must match the device mode. The default is-mno-xnack. Right now this option is a placeholder for support that isn't implemented yet 🚧
🦋 ARC Options 🦋
The following options control the architecture variant the code is being compiled for 🎀
-mbarrel-shifter— generates instructions supported by the barrel shifter. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect 🌀-mjli-always— forces a function call using thejli_sinstruction. This option is valid only for the ARCv2 architecture 🔁-mcpu=cpu— sets the architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options for backward compatibility and convenience. Supported values for cpu are:arc600— compile for ARC600. Aliases:-mA6,-mARC600🌷arc601— compile for ARC601. Alias:-mARC601🌷arc700— compile for ARC700. Aliases:-mA7,-mARC700. This is the default when configured with--with-cpu=arc700👑arcem— compile for ARC EM 🧚archs— compile for ARC HS 🔥em— compile for ARC EM CPU with no hardware extensions 🍃em4— compile for ARC EM4 CPU 🍃em4_dmips— compile for ARC EM4 DMIPS CPU 🍃em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension 🍃em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions 🍃hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions ⚛️hs34— compile for ARC HS34 CPU 🌸hs38— compile for ARC HS38 CPU 🌸hs38_linux— compile for ARC HS38 CPU with all hardware extensions on 💅arc600_norm— compile for ARC 600 CPU with"norm"instructions enabled 🧃arc600_mul32x16— compile for ARC 600 CPU with"norm"and 32x16-bit multiply instructions enabled ✖️arc600_mul64— compile for ARC 600 CPU with"norm"and"mul64"-family instructions enabled ✖️arc601_norm— compile for ARC 601 CPU with"norm"instructions enabled 🧃arc601_mul32x16— compile for ARC 601 CPU with"norm"and 32x16-bit multiply instructions enabled ✖️arc601_mul64— compile for ARC 601 CPU with"norm"and"mul64"-family instructions enabled ✖️nps400— compile for ARC 700 on the NPS400 chip 💎em_mini— compile for ARC EM minimalist configuration featuring a reduced register set 🌱
-mdpfp,-mdpfp-compact— generates double-precision FPX instructions, tuned for the compact implementation 💖-mdpfp-fast— generates double-precision FPX instructions, tuned for the fast implementation ⚡-mno-dpfp-lrsr— disables"lr"and"sr"instructions from using FPX extension aux registers 🚫-mea— generates extended arithmetic instructions. Right now only"divaw","adds","subs", and"sat16"are supported. This is only valid for-mcpu=ARC700✨-mno-mpy— 🙈 Skip the "mpy"-family instructions for ARC700. This one is deprecated, so please don't build new stuff around it 💀-mmul32x16— ✨ Turns on 32x16-bit multiply and multiply-accumulate instructions.-mmul64— 🔥 Generates themul64andmulu64instructions. Only valid with-mcpu=ARC600, so don't try it anywhere else 🙅♀️-mnorm— 💅 Generates thenorminstructions. This is already the default when-mcpu=ARC700is in effect.-mspfp,-mspfp-compact— 🌸 Generates single-precision FPX instructions, tuned for the compact implementation.-mspfp-fast— ⚡ Generates single-precision FPX instructions, tuned for the fast implementation.-msimd— 🦋 Turns on ARC SIMD instructions through target-specific builtins. Only valid with-mcpu=ARC700.-msoft-float— 🫠 This one is basically ignored! It's only here for compatibility. Software floating-point code is emitted by default, and you can override that default with the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision.-mswap— 🔄 Generatesswapinstructions.-matomic— 🔐 Enables the locked load/store conditional extension so the atomic memory built-in functions can work. Not available for ARC 6xx or ARC EM cores, so skip it there 😭-mdiv-rem— ➗ Enables thedivandreminstructions for ARCv2 cores.-mcode-density— 🧚 Enables code density instructions for ARC EM. It's on by default for ARC HS.-mll64— 💜 Enables double load/store operations for ARC HS cores.-mtp-regno=regno— 🧵 Sets the thread pointer register number.-mmpy-option=multo— 👑 Compiles ARCv2 code with a multiplier design option. You can pick the option with a string or a number formulto, andwlh1is the default. Here's the menu:0ornone— No multiplier available.1orw— 16x16 multiplier, fully pipelined. This enablesmpywandmpyuw.2orwlh1— 32x32 multiplier, fully pipelined (1 stage). This also enablesmpy,mpyu,mpym,mpymu, andmpy_s.3orwlh2— 32x32 multiplier, fully pipelined (2 stages). This also enablesmpy,mpyu,mpym,mpymu, andmpy_s.4orwlh3— Two 16x16 multipliers, blocking, sequential. This also enablesmpy,mpyu,mpym,mpymu, andmpy_s.5orwlh4— One 16x16 multiplier, blocking, sequential. This also enablesmpy,mpyu,mpym,mpymu, andmpy_s.6orwlh5— One 32x4 multiplier, blocking, sequential. This also enablesmpy,mpyu,mpym,mpymu, andmpy_s.7orplus_dmpy— ARC HS SIMD support.8orplus_macd— ARC HS SIMD support.9orplus_qmacw— ARC HS SIMD support.
This option is only available for ARCv2 cores ✨
-mfpu=fpu— 🌈 Turns on specific floating-point hardware extensions for ARCv2 cores. Here are the supported values forfpu:fpus— Single-precision floating-point hardware extensions.fpud— Double-precision floating-point hardware extensions. The single-precision extension gets enabled too. Not available for ARC EM.fpuda— Double-precision extensions using double-precision assist instructions. The single-precision extension is also enabled. This is only for ARC EM.fpuda_div— Double-precision extensions using double-precision assist instructions. The single-precision, square-root, and divide extensions are also enabled. ARC EM only.fpuda_fma— Double-precision extensions using double-precision assist instructions. The single-precision and fused multiply and add extensions are also enabled. ARC EM only.fpuda_all— Double-precision extensions using double-precision assist instructions. All single-precision extensions are enabled too. ARC EM only.fpus_div— Single-precision, square-root, and divide hardware extensions.fpud_div— Double-precision, square-root, and divide hardware extensions. This one includesfpus_div. Not available for ARC EM.fpus_fma— Single-precision and fused multiply and add hardware extensions.fpud_fma— Double-precision and fused multiply and add hardware extensions. This one includesfpus_fma. Not available for ARC EM.fpus_all— All single-precision floating-point hardware extensions.fpud_all— All single- and double-precision floating-point hardware extensions. Not available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count— 🛡️ Picks the general-purpose registers the processor saves and restores automatically on interrupt entry and exit. Theregister-rangeis written as two registers separated by a dash. The range always starts atr0, and the upper limit is thefpregister.blinkandlp_countare optional. This option is only valid for ARC EM and ARC HS cores.-mrgf-banked-regs=number— 🏦 Sets how many registers get replicated into the second register bank when a fast interrupt is entered. Fast interrupts are the ones with the highest priority level, P0. They only save the PC and STATUS32 registers so there are no memory transactions during entry and exit. Use this when you're working with fast interrupts on an ARC V2 family processor. The permitted values are4,8,16, and32.-mlpc-width=width— 🔢 Sets the width of thelp_countregister. Valid widths are8,16,20,24,28, and32bits. The default is fixed at 32 bits. If the width is less than 32, the compiler won't try to turn your loops into the zero-delay loop mechanism unless it's known thatlp_countcan hold the loop counter value. Depending on the width, the compiler and run-time library might still use the loop mechanism for various needs. This option defines the macro__ARC_LPC_WIDTH__with the value ofwidth.-mrf16— 🎯 Tells the compiler to generate code for a 16-entry register file. It also defines the__ARC_RF16__preprocessor macro.-mbranch-index— 🧭 Enables the use ofbiorbihinstructions to implement jump tables.
The following options are passed through to the assembler, and they also define preprocessor macro symbols 🎀
-mdsp-packa— 📦 Passed down to the assembler to enable the DSP Pack A extensions. It also sets the preprocessor symbol__Xdsp_packa. This option is deprecated.-mdvbf— 🦋 Passed down to the assembler to enable the dual Viterbi butterfly extension. It also sets the preprocessor symbol__Xdvbf. This option is deprecated.-mlock— 🔒 Passed down to the assembler to enable the locked load/store conditional extension. It also sets the preprocessor symbol__Xlock.-mmac-d16— Passed down to the assembler. It also sets the preprocessor symbol__Xxmac_d16. This option is deprecated.-mmac-24— Passed down to the assembler. It also sets the preprocessor symbol__Xxmac_24. This option is deprecated.-mrtsc— ⏱️ Passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. It also sets the preprocessor symbol__Xrtsc. This option is deprecated.-mswape— 🔁 Passed down to the assembler to enable the swap byte ordering extension instruction. It also sets the preprocessor symbol__Xswape.-mtelephony— 📞 Passed down to the assembler to enable dual- and single-operand instructions for telephony. It also sets the preprocessor symbol__Xtelephony. This option is deprecated.-mxy— 🧠 Passed down to the assembler to enable the XY memory extension. It also sets the preprocessor symbol__Xxy.
The following options control how the assembly code gets annotated 📝
-misize— 📏 Annotates assembler instructions with estimated addresses.-mannotate-align— 🧩 Explains which alignment considerations led to the decision to make an instruction short or long.
The following options are passed through to the linker 🔗
-marclinux— 🐧 Passed through to the linker to specify use of thearclinuxemulation. This is enabled by default in toolchains built for thearc-linux-uclibcandarceb-linux-uclibctargets when profiling is not requested.-marclinux_prof— 📊 Passed through to the linker to specify use of thearclinux_profemulation. This is enabled by default in toolchains built for thearc-linux-uclibcandarceb-linux-uclibctargets when profiling is requested.
The following options control the semantics of the generated code 🧠✨
-mlong-calls— 📞 Generates calls as register indirect calls, which gives you access to the full 32-bit address range.-mmedium-calls— 🎯 Doesn't use less than a 25-bit addressing range for calls. That's the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, so the 25-bit range gets used instead of the 21-bit range you'd get with conditional branch-and-link. This is the default for toolchains built for thearc-linux-uclibcandarceb-linux-uclibctargets 💖-G num— okay so this one decides which externally-visible data gets tucked into a small data section 🧚, but only if that data is no bigger thannumbytes. The default fornumis4for any ARC configuration, or8when we have double load/store operations. 🍓-mno-sdata— nosdatareferences for you, bestie 🙅♀️. This is the default for tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets.-mvolatile-cache— volatile references use ordinarily cached memory accesses. This is the default, and honestly iconic 💅.-mno-volatile-cache— flips the switch and enables cache bypass for volatile references 🔥.
The following options fine tune code generation 🎀:
-malign-call— does alignment optimizations for call instructions ✨.-mauto-modify-reg— enables the use of pre/post modify with register displacement 🦋.-mbbit-peephole— enables bbit peephole2 💜.-mno-brcc— this one turns off a target-specific pass inarc_reorgthat would generate compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass, so don't expect miracles there 🙈.-mcase-vector-pcrel— uses PC-relative switch case tables so case table shortening can happen. This is the default for-Os🌸.-mcompact-casesi— enables the compactcasesipattern. This is the default for-Os, and it's only available for ARCv1 cores. Also, this option is deprecated, so use it with a little side-eye 💀.-mno-cond-exec— disables the ARCompact-specific pass that generates conditional execution instructions.
Okay, so the reason this one exists is that the target-independent pass for conditional execution is often lacking, thanks to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution. The ARC port keeps a special pass around that tries to find more conditional execution opportunities after register allocation, branch shortening, and delay slot scheduling have been done. It generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off 😭. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead 🫶.
-mearly-cbranchsi— enables pre-reload use of thecbranchsipattern 🧚.-mexpand-adddi— expandsadddi3andsubdi3at RTL generation time intoadd.f,adcetc. This option is deprecated, so it's giving retired vibes 👑.-mindexed-loads— enables the use of indexed loads. Be careful here, because some optimizers then assume that indexed stores exist, which is not the case 🙈. This can be problematic, so proceed with caution.-mlra— enables Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e.-mno-lra) 🧪.-mlra-priority-none— doesn't indicate any priority for target registers 🌈.-mlra-priority-compact— indicates target register priority forr0..r3/r12..r15⭐.-mlra-priority-noncompact— reduces target register priority forr0..r3/r12..r15🥀.-mmillicode— when optimizing for size (using-Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by calling a special function in libgcc. This is called a millicode call. These calls can pose performance issues and/or cause linking issues when linking in a nonstandard way, so this option lets you turn millicode call generation on or off 🔁.-mcode-density-frame— lets the compiler emitenterandleaveinstructions. These are only valid for CPUs with the code-density feature 🧘♀️.-mmixed-code— tweaks register allocation to help with 16-bit instruction generation. This generally decreases the average instruction size while increasing the instruction count 🍭.-mq-class— this one is deprecated, and it enables q instruction alternatives. It is the default for-Os🥹.-mRcq— enables Rcq constraint handling. Most short code generation depends on this. This is the default 💖.-mRcw— enables Rcw constraint handling. Most ccfsm condexec depends on this. This is the default too 🎀.-msize-level=level— fine-tunes size optimization with regards to instruction lengths and alignment. The recognized values forlevelare:0— no size optimization. This level is deprecated and treated like1💀.1— short instructions are used opportunistically ✨.2— in addition, alignment of loops and of code after barriers is dropped 🌙.3— in addition, optional data alignment is dropped, and the optionOsis enabled 👑.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1 💅.
-mtune=cpu— sets instruction scheduling parameters forcpu, overriding any implied by-mcpu=🎯.
Supported values for cpu are:
ARC600— tunes for ARC600 CPU.ARC601— tunes for ARC601 CPU.ARC700— tunes for ARC700 CPU with standard multiplier block.ARC700-xmac— tunes for ARC700 CPU with XMAC block.ARC725D— tunes for ARC725D CPU.ARC750D— tunes for ARC750D CPU.
-mmultcost=num— the cost to assume for a multiply instruction, with4being equal to a normal instruction 🧮.-munalign-prob-threshold=probability— sets the probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without a filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability for the branch to be taken is belowprobability. The default is(REG_BR_PROB_BASE/2), i.e.5000🫧.
The following options are maintained for backward compatibility, but they are now deprecated and will be removed in a future release. Bestie, let's say goodbye to these 😭🫡:
These options are deprecated and will be removed in a future release, so don't build new stuff around them 💔.
-margonaut— obsolete FPX 🪦.-mbig-endian,-EB— compiles code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to buildarceb-elf32andarceb-linux-uclibctargets, for which big endian is the default 🌊.-mlittle-endian,-EL— compiles code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to buildarc-elf32andarc-linux-uclibctargets, for which little endian is the default 🌷.-mbarrel_shifter— replaced by-mbarrel-shifter🔄.-mdpfp_compact— replaced by-mdpfp-compact🔄.-mdpfp_fast— replaced by-mdpfp-fast🔄.-mdsp_packa— replaced by-mdsp-packa🔄.-mEA— replaced by-mea🔄.-mmac_24— replaced by-mmac-24🔄.-mmac_d16— replaced by-mmac-d16🔄.-mspfp_compact— replaced by-mspfp-compact🔄.-mspfp_fast— replaced by-mspfp-fast🔄.-mtune=cpu— the valuesarc600,arc601,arc700andarc700-xmacforcpuare replaced byARC600,ARC601,ARC700andARC700-xmacrespectively 🔠.-multcost=num— replaced by-mmultcost🔄.
✨ ARM Options ✨
These -m options are defined for the ARM port 🦾:
-mabi=name— generates code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcs,aapcs-linuxandiwmmxt🧩.-mapcs-frame— generates a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this isn't strictly necessary for correct execution of the code. Specifying-fomit-frame-pointerwith this option causes the stack frames not to be generated for leaf functions. The default is-mno-apcs-frame. This option is deprecated 🥀.-mapcs— a synonym for-mapcs-frame, and it's deprecated too 🥀.-mthumb-interwork— generates code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is-mno-thumb-interwork, since slightly larger code is generated when-mthumb-interworkis specified. In AAPCS configurations this option is meaningless 🤷♀️.-mno-sched-prolog— prevents the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is-msched-prolog🔍.-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard🔢.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible. You must compile your entire program with the same ABI, and link with a compatible set of libraries. This is a big no-no if you mix them 🚫.
-mgeneral-regs-only— generates code which uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but it doesn't impose any restrictions on the assembler 🧘♀️.-mlittle-endian— generates code for a processor running in little-endian mode. This is the default for all standard configurations 🌷.-mbig-endian— generates code for a processor running in big-endian mode. The default is to compile code for a little-endian processor 🌊.-mbe8,-mbe32— when linking a big-endian image, selects between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32. BE32 format has been deprecated by ARM 🫠.-march=name[+extension...]— 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 🏗️.
Okay so, these are the permissible names you're allowed to use, bestie 🎀: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2 💖
Then there's a little group of deprecated ones 🥀 that are still recognized but have no Thumb execution state support, so they're on thin ice: armv4. Use with caution, it's giving "please don't" 💀
Many of these architectures have extensions ✨ You add one by tacking +extension onto the end of the architecture name. The extension options get processed in order, and the capabilities accumulate (they stack up, like a good outfit 👑). An extension will also turn on any base extensions it needs, so for example +crypto will always enable +simd 🫶
The one exception to the stacking vibe is anything prefixed with +no.... Those disable the option they name, plus any other extensions that depend on it 😭 For example, -march=armv7-a+simd+nofp+vfpv4 is the same as writing -march=armv7-a+vfpv4, because the +nofp that comes after +simd wipes +simd out completely 💅
Most extension names are generic, but what they actually do depends on which architecture they're attached to 🦋 For example, +simd works on both armv7-a and armv8-a, but on armv7-a it turns on the original ARMv7-A Advanced SIMD (Neon) extensions, while on armv8-a it gives you the ARMv8-A version instead 🌈
Here's the rundown of which extensions each architecture supports. Anything not mentioned below has no extensions at all, so don't go looking 🙈
armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
+fp— The VFPv2 floating-point instructions. The extension+vfpv2can be used as an alias for this extension.+nofp— Disable the floating-point instructions.
armv7
This one is the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures, so it's the everyday basic 🌸
+fp— The VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures ⚠️+nofp— Disable the floating-point instructions.
armv7-a
+mp— The multiprocessing extension.+sec— The security extension.+fp— The VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+simd— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can be used as aliases for this extension.+vfpv3— The VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— The VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— The VFPv4 floating-point instructions, with 32 double-precision registers.+neon-fp16— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+neon-vfpv4— The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.+nosimd— Disable the Advanced SIMD instructions (does not disable floating point).+nofp— Disable the floating-point and Advanced SIMD instructions.
armv7ve
This is the extended version of the ARMv7-A architecture, with support for virtualization 🦋
+fp— The VFPv4 floating-point instructions, with 16 double-precision registers. The extension+vfpv4-d16can be used as an alias for this extension.+simd— The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension+neon-vfpv4can be used as an alias for this extension.+vfpv3-d16— The VFPv3 floating-point instructions, with 16 double-precision registers.+vfpv3— The VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— The VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— The VFPv4 floating-point instructions, with 32 double-precision registers.+neon— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension+neon-vfpv3can be used as an alias for this extension.+neon-fp16— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+nosimd— Disable the Advanced SIMD instructions (does not disable floating point).+nofp— Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc— The Cyclic Redundancy Check (CRC) instructions.+simd— The ARMv8-A Advanced SIMD and floating-point instructions.+crypto— The cryptographic instructions.+nocrypto— Disable the cryptographic instructions.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd— The ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+nocrypto— Disable the cryptographic instructions.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.2-a, armv8.3-a
+fp16— The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.+fp16fml— The half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.+simd— The ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+dotprod— Enable the Dot Product extension. This also enables Advanced SIMD instructions.+nocrypto— Disable the cryptographic extension.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16— The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— Disable the cryptographic extension.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv8.5-a
+fp16— The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— okay bestie, this one turns on the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension ✨
🦋 armv8.6-a
+fp16— the half-precision floating-point data processing instructions 💖. This also enables the Advanced SIMD and floating-point instructions, the Dot Product extension, and the half-precision floating-pointfmlaextension 🫶+simd— the same ARMv8.3-A Advanced SIMD and floating-point instructions as before, plus the Dot Product extension 💜+crypto— the cryptographic instructions 🔐. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension ✨+nocrypto— disable the cryptographic extension 🙅♀️+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions 💀+i8mm— 8-bit Integer Matrix Multiply instructions 🧚. This also enables Advanced SIMD and floating-point instructions+bf16— Brain half-precision floating-point instructions 🍓. This also enables Advanced SIMD and floating-point instructions
🌈 armv7-r
+fp.sp— the single-precision VFPv3 floating-point instructions 🌸.+vfpv3xdcan be used as an alias for this extension+fp— the VFPv3 floating-point instructions with 16 double-precision registers 🎀.+vfpv3-d16can be used as an alias for this extension+vfpv3xd-d16-fp16— the single-precision VFPv3 floating-point instructions with 16 double-precision registers, plus the half-precision floating-point conversion operations 🧁+vfpv3-d16-fp16— the VFPv3 floating-point instructions with 16 double-precision registers, plus the half-precision floating-point conversion operations 🧁+nofp— disable the floating-point extension 🙈+idiv— the ARM-state integer division instructions ➗+noidiv— disable the ARM-state integer division extension 🚫
💅 armv7e-m
+fp— the single-precision VFPv4 floating-point instructions ⭐+fpv5— the single-precision FPv5 floating-point instructions 🔥+fp.dp— the single- and double-precision FPv5 floating-point instructions 👑+nofp— disable the floating-point extensions 💔
🧚 armv8.1-m.main
+dsp— the DSP instructions 🎶+mve— the M-Profile Vector Extension (MVE) integer instructions 🌟+mve.fp— the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions 🌟+fp— the single-precision floating-point instructions ✨+fp.dp— the single- and double-precision floating-point instructions 👑+nofp— disable the floating-point extension 🙈+cdecp0, +cdecp1, ... , +cdecp7— enable the Custom Datapath Extension (CDE) on selected coprocessors, according to the numbers given in the options in the range 0 to 7 🦋
🍓 armv8-m.main
+dsp— the DSP instructions 🎶+nodsp— disable the DSP extension 🙅♀️+fp— the single-precision floating-point instructions ✨+fp.dp— the single- and double-precision floating-point instructions 👑+nofp— disable the floating-point extension 🙈+cdecp0, +cdecp1, ... , +cdecp7— enable the Custom Datapath Extension (CDE) on selected coprocessors, according to the numbers given in the options in the range 0 to 7 🦋
🫧 armv8-r
+crc— the Cyclic Redundancy Check (CRC) instructions 🔁+fp.sp— the single-precision FPv5 floating-point instructions 🌸+simd— the ARMv8-A Advanced SIMD and floating-point instructions 💖+crypto— the cryptographic instructions 🔐+nocrypto— disable the cryptographic instructions 🙅♀️+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions 💀
-march=native is giving auto-detect vibes 🔍. It makes the compiler sniff out the architecture of the build computer. Right now this only works on GNU/Linux, and not every architecture gets recognized 🥺. If the auto-detect doesn't work out, the option just has no effect, so no drama 🫶
🎀 -mtune=name
This option tells GCC which target ARM processor it should tune the performance of the code for 💅. Some ARM implementations can get a glow-up in performance when you use this option ✨. The permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, 0strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m55, cortex-x1, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.
Also, this option can tell GCC to tune the performance of the code for a big.LITTLE system 🧠💞. The permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch tells GCC to tune the performance for a blend of processors within architecture arch 🌈. The goal is code that runs well on the most popular processors right now, balancing optimizations that help some CPUs in the range while avoiding performance pitfalls on the others 🫶. The effects of this option may change in future GCC versions as CPU models come and go 🔄.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code 🙃.
-mtune=native makes the compiler auto-detect the CPU of the build computer 🔍. Again, this only works on GNU/Linux right now, and not every architecture is recognized. If the auto-detect is unsuccessful, the option has no effect 🥀.
👑 -mcpu=name[+extension...]
This specifies the name of the target ARM processor 👑. GCC uses this name to figure out the target ARM architecture (as if you had said -march), and the ARM processor type to tune for performance (as if you had said -mtune) 🧠. If you use this option together with -march or -mtune, those options take precedence over the matching part of this option 💅.
Lots of the supported CPUs have optional architectural extensions 🧩. When they do, those extensions are usually enabled by default ✨. If implementations that lack the extension exist, you can use the extension syntax to disable the ones that were left out 🙅♀️. For floating-point and Advanced SIMD (Neon) instructions, you also need to think about the settings of -mfloat-abi and -mfpu 👀: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft, and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions 💥.
For example, cortex-a9 comes in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD 🌸. The default is to enable all the instructions, but you can use the extensions +nosimd and +nofp to disable just the SIMD instructions, or both the SIMD and floating-point instructions, respectively 🫧.
The permissible names for this option are the same as the ones for -mtune 🎀.
The following extension options are common to the listed CPUs 🌟:
+nodsp— disable the DSP instructions on cortex-m33, cortex-m35p 🙈+nofp— disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7, cortex-m33 and cortex-m35p. It also disables the floating-point and SIMD instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55 💔+nofp.dp— disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and cortex-m7 🥀+nosimd— disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9 🙃
okay so +crypto is the one that switches on the cryptographic instructions 🔐✨ it works on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55 💅
the generic-armv7-a pseudo target is giving its own thing 🦋 by default it comes with VFPv3 and 16 double-precision registers. it takes these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16 and neon-vfpv4. they mean the same thing as the extensions for -march=armv7-a, so no new vocab to learn 🌸
-mcpu=generic-arch is also allowed, and it's the same as -march=arch -mtune=generic-arch. check out -mtune for more tea ☕
-mcpu=native— the compiler tries to auto-detect the CPU of the computer doing the build 🧚 right now this only works on GNU/Linux, and not every architecture gets recognised. if the auto-detect doesn't work out, the option just does nothing 🙈-mfpu=name— this tells the compiler what floating-point hardware (or hardware emulation) the target has 💖 the valid names areauto,vfpv2,vfpv3,vfpv3-fp16,vfpv3-d16,vfpv3-d16-fp16,vfpv3xd,vfpv3xd-fp16,neon-vfpv3,neon-fp16,vfpv4,vfpv4-d16,fpv4-sp-d16,neon-vfpv4,fpv5-d16,fpv5-sp-d16,fp-armv8,neon-fp-armv8andcrypto-neon-fp-armv8.neonis just an alias forneon-vfpv3, andvfpis an alias forvfpv2🎀
auto is the default and it's a bit of a diva 👑 it picks the floating-point and Advanced SIMD instructions based on whatever -mcpu and -march are set to.
⚠️ bestie, listen up. if your floating-point hardware has NEON (like-mfpu=neon), GCC's auto-vectorization pass won't generate floating-point operations unless you also add-funsafe-math-optimizations. the reason is that NEON doesn't fully follow the IEEE 754 standard (denormal values get treated as zero), so using NEON instructions can lose precision 😭
you can also set the fpu name per function with the target("fpu=") function attributes or pragmas 🦋
-mfp16-format=name— this sets the format of the"__fp16"half-precision floating-point type ✨ the valid names arenone,ieeeandalternative. the default isnone, and in that case the"__fp16"type isn't defined at all 🥀-mstructure-size-boundary=n— every structure and union gets rounded up to a multiple ofnbits 📏 the valid values are8,32and64. the default depends on the toolchain, and for the COFF toolchain it's8.64only works if the underlying ABI supports it 💅
a bigger number can make code faster and more efficient, but it can also make the program bigger. different values can be incompatible, too. code built with one value might not play nice with code or libraries built with another value if they pass structs or unions back and forth 🙈
💀 this option is deprecated, so please don't start new projects with it.
-mabort-on-noreturn— this makes the compiler callabortat the end of anynoreturnfunction. it only runs if that function somehow tries to return 🚨-mlong-calls,-mno-long-calls— these tell the compiler to make function calls by loading the function's address into a register first, then doing a subroutine call on that register 🧠 you need this if the target function lives outside the 64-megabyte addressing range of the offset-based subroutine call instruction.
even with this switched on, not every call becomes a long call. the vibe check goes like this: static functions, functions with the "short_call" attribute, functions inside a #pragma no_long_calls scope, and functions whose definitions were already compiled in the current compilation unit all stay short 🫶 the exceptions are weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions inside a #pragma long_calls scope. those are always long, no debate 💜
this feature is not on by default. -mno-long-calls puts back the default behaviour, and so does wrapping the calls in a #pragma long_calls_off directive. also note that these switches do nothing for calls made through function pointers ✨
-msingle-pic-base— the register used for PIC addressing gets treated as read-only, instead of being loaded in the prologue of every function 🌈 the runtime system has to set this register to a proper value before execution starts.-mpic-register=reg— this picks the register used for PIC addressing 🎯 for the standard PIC base, the default is any suitable register the compiler decides on. for the single PIC base, the default isR9if the target is EABI based or stack-checking is enabled. otherwise it'sR10.-mpic-data-is-text-relative— this assumes the gap between the text and data segments is fixed at static link time 🧚 that means PC-relative addressing can be used to reach data we know is in the data segment. on non-VxWorks RTP targets it's enabled by default. if you turn it off on those targets,-msingle-pic-basegets enabled by default instead 🦋-mpoke-function-name— this writes the name of each function into the text section, right before the function prologue ✍️ the generated code looks something like 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, #4when you're doing a stack backtrace, code can look at the pc value stored at fp + 0 🔍 if the trace function checks the location pc - 12 and the top 8 bits are set, then we know there's a function name embedded right before that location. its length is ((pc[-3]) & 0xff000000). the way that's giving detective energy 🕵️♀️
-mthumb,-marm— pick whether the code runs in ARM state or Thumb state 🎀 most configurations default to ARM state, but you can change that by configuring GCC with the--with-mode=stateconfigure option.
you can also override ARM or Thumb mode for each function with the target("thumb") and target("arm") function attributes or pragmas 💖
-mflip-thumb— this switches between ARM and Thumb modes on alternating functions 🔀 it's only there for regression testing of mixed Thumb/ARM code generation, so please don't use it for ordinary compiling 🙅♀️-mtpcs-frame— generates a stack frame that follows the Thumb Procedure Call Standard for all non-leaf functions 📚 (a leaf function is one that doesn't call any other functions.) the default is-mno-tpcs-frame.-mtpcs-leaf-frame— same idea, but this one makes stack frames compliant with the Thumb Procedure Call Standard for all leaf functions 🍃 (again, a leaf function is one that doesn't 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 🪄 that header switches to Thumb mode before the rest of the function runs. this lets those functions get called from non-interworking code. it's not valid in AAPCS configurations, because interworking is already enabled by default there 🙈-mcaller-super-interworking— lets calls through function pointers (including virtual functions) run correctly, whether or not the target code was compiled for interworking 🌸 there's a small overhead when running a function pointer if you turn this on. it's also not valid in AAPCS configurations, since interworking is already on by default there.-mtp=name— picks the access model for the thread local storage pointer 🧵 the valid models aresoft, which generates calls to"__aeabi_read_tp";cp15, which fetches the thread pointer fromcp15directly (supported in the arm6k architecture); andauto, which uses the best available method for the processor you picked. the default isauto⭐-mtls-dialect=dialect— picks the dialect used to access thread local storage 💅 there are two:gnuandgnu2. thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. thegnu2dialect selects the GNU descriptor scheme, which performs better for shared libraries 🚀 the descriptor scheme is compatible with the original, but it needs new assembler, linker and library support. the initial and local exec TLS models aren't affected by this option and always use the original scheme.-mword-relocations— only generates absolute relocations on word-sized values (i.e.R_ARM_ABS32) 🧱 this is on by default on targets (uClinux, SymbianOS) where the runtime loader enforces that restriction, and whenever-fpicor-fPICis specified. this option conflicts with-mslow-flash-data.-mfix-cortex-m3-ldrd— some Cortex-M3 cores can corrupt data whenldrdinstructions use overlapping destination and base registers 😱 this option avoids generating those instructions. it's enabled by default when-mcpu=cortex-m3is specified 🛡️-munaligned-access,-mno-unaligned-access— enables (or disables) reading and writing 16- and 32-bit values from addresses that aren't 16- or 32-bit aligned 🧩 by default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and ARMv8-M Baseline architectures, and enabled for all other architectures. if unaligned access isn't enabled, words in packed data structures get accessed one byte at a time 🐢
The Tag_CPU_unaligned_access attribute in the generated object file gets set to true or false depending on this option 🦋. If unaligned access is turned on, the __ARM_FEATURE_UNALIGNED preprocessor symbol gets defined too ✨.
-mneon-for-64bits— 💀 This one is deprecated and does absolutely nothing, so bestie, don't bother with it.-mslow-flash-data— Okay so this assumes loading data from flash is slower than fetching instructions, which means literal loads get minimized for better performance ⚡. It's only supported when compiling for ARMv7 M-profile, it's off by default, and it conflicts with-mword-relocations🙅♀️.-masm-syntax-unified— Assumes the inline assembler is using unified asm syntax. The default is currently off, which means divided syntax. It has no impact on Thumb2, but ngl this might change in future GCC releases 👀.
Divided syntax should be considered deprecated. It's giving outdated, so plan accordingly 💀.
-mrestrict-it— Restricts IT block generation so it follows the ARMv8-A rules 📜. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This is on by default for ARMv8-A Thumb mode 🌸.-mprint-tune-info— Prints CPU tuning info as a comment in the assembler file 🧚. This is only for regression testing of the compiler and is not meant for ordinary code compiling, so it's disabled by default 🙈.
This one is strictly for compiler testing, not everyday use. Keep it away from your normal builds!
-mverbose-cost-dump— Enables verbose cost model dumping in the debug dump files 🔍. It's here for debugging the compiler, so it's a main-character tool for compiler nerds 🤓.-mpure-code— Do not allow constant data to be placed in code sections 🚫. On top of that, when compiling for ELF object format, all text sections get the ELF processor-specific section attributeSHF_ARM_PURECODE✨. This is only available when generating non-pic code for M-profile targets 💜.-mcmse— Generates secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification" 🔐. You can find that spec athttps://developer.arm.com/documentation/ecm0359818/latest/📖.-mfix-cmse-cve-2021-35465— This mitigates a potential security issue with the "VLLDM" instruction in some M-profile devices when using CMSE (CVE-2021-365465) 🛡️. It's enabled by default when-mcpu=is used with "cortex-m33", "cortex-m35p" or "cortex-m55" 💅. If you want to turn the mitigation off, use-mno-fix-cmse-cve-2021-35465🙅♀️.-mfdpic,-mno-fdpic— Selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions 🌈. When the compiler is configured for "arm-*-uclinuxfdpiceabi" targets, this is on by default and implies-fPIEif none of the PIC/PIE-related options is provided 👑. On other targets, it only enables the FDPIC-specific code generation features, so you'll need to explicitly provide the PIC/PIE-related options as needed 🫶.
Static linking is not supported here, because it would still involve the dynamic linker when the program self-relocates 😭. If that behaviour is fine with you, use -static and -Wl,-dynamic-linker options 🔥.
The opposite -mno-fdpic option is useful (and required!) for building the Linux kernel with the same ("arm-*-uclinuxfdpiceabi") toolchain you use for your userland programs 🐧.
✨ AVR Options ✨
These options are defined for AVR implementations 🦋:
-mmcu=mcu— Specifies the Atmel AVR instruction set architecture (ISA) or MCU type 💖.
The default for this option is avr2 🌸.
GCC supports the following AVR devices and ISAs 🧚:
avr2— "Classic" devices with up to 8 KiB of program memory. mcu =attiny22,attiny26,at90s2313,at90s2323,at90s2333,at90s2343,at90s4414,at90s4433,at90s4434,at90c8534,at90s8515,at90s8535.avr25— "Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction. mcu =attiny13,attiny13a,attiny24,attiny24a,attiny25,attiny261,attiny261a,attiny2313,attiny2313a,attiny43u,attiny44,attiny44a,attiny45,attiny48,attiny441,attiny461,attiny461a,attiny4313,attiny84,attiny84a,attiny85,attiny87,attiny88,attiny828,attiny841,attiny861,attiny861a,ata5272,ata6616c,at86rf401.avr3— "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu =at76c711,at43usb355.avr31— "Classic" devices with 128 KiB of program memory. mcu =atmega103,at43usb320.avr35— "Classic" devices with 16 KiB up to 64 KiB of program memory and with the "MOVW" instruction. mcu =attiny167,attiny1634,atmega8u2,atmega16u2,atmega32u2,ata5505,ata6617c,ata664251,at90usb82,at90usb162.avr4— "Enhanced" devices with up to 8 KiB of program memory. mcu =atmega48,atmega48a,atmega48p,atmega48pa,atmega48pb,atmega8,atmega8a,atmega8hva,atmega88,atmega88a,atmega88p,atmega88pa,atmega88pb,atmega8515,atmega8535,ata6285,ata6286,ata6289,ata6612c,at90pwm1,at90pwm2,at90pwm2b,at90pwm3,at90pwm3b,at90pwm81.avr5— "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu =atmega16,atmega16a,atmega16hva,atmega16hva2,atmega16hvb,atmega16hvbrevb,atmega16m1,atmega16u4,atmega161,atmega162,atmega163,atmega164a,atmega164p,atmega164pa,atmega165,atmega165a,atmega165p,atmega165pa,atmega168,atmega168a,atmega168p,atmega168pa,atmega168pb,atmega169,atmega169a,atmega169p,atmega169pa,atmega32,atmega32a,atmega32c1,atmega32hvb,atmega32hvbrevb,atmega32m1,atmega32u4,atmega32u6,atmega323,atmega324a,atmega324p,atmega324pa,atmega325,atmega325a,atmega325p,atmega325pa,atmega328,atmega328p,atmega328pb,atmega329,atmega329a,atmega329p,atmega329pa,atmega3250,atmega3250a,atmega3250p,atmega3250pa,atmega3290,atmega3290a,atmega3290p,atmega3290pa,atmega406,atmega64,atmega64a,atmega64c1,atmega64hve,atmega64hve2,atmega64m1,atmega64rfr2,atmega640,atmega644,atmega644a,atmega644p,atmega644pa,atmega644rfr2,atmega645,atmega645a,atmega645p,atmega649,atmega649a,atmega649p,atmega6450,atmega6450a,atmega6450p,atmega6490,atmega6490a,atmega6490p,ata5795,ata5790,ata5790n,ata5791,ata6613c,ata6614q,ata5782,ata5831,ata8210,ata8510,ata5702m322,at90pwm161,at90pwm216,at90pwm316,at90can32,at90can64,at90scr100,at90usb646,at90usb647,at94k,m3000.avr51— "Enhanced" devices with 128 KiB of program memory. mcu =atmega128,atmega128a,atmega128rfa1,atmega128rfr2,atmega1280,atmega1281,atmega1284,atmega1284p,atmega1284rfr2,at90can128,at90usb1286,at90usb1287.avr6— "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu =atmega256rfr2,atmega2560,atmega2561,atmega2564rfr2.avrxmega2— "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu =atxmega8e5,atxmega16a4,atxmega16a4u,atxmega16c4,atxmega16d4,atxmega16e5,atxmega32a4,atxmega32a4u,atxmega32c3,atxmega32c4,atxmega32d3,atxmega32d4,atxmega32e5.avrxmega3— "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space 🪞. mcu =attiny202,attiny204,attiny212,attiny214,attiny402,attiny404,attiny406,attiny412,attiny414,attiny416,attiny417,attiny804,attiny806,attiny807,attiny814,attiny816,attiny817,attiny1604,attiny1606,attiny1607,attiny1614,attiny1616,attiny1617,attiny3214,attiny3216,attiny3217,atmega808,atmega809,atmega1608,atmega1609,atmega3208,atmega3209,atmega4808,atmega4809.avrxmega4— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu =atxmega64a3,atxmega64a3u,atxmega64a4u,atxmega64b1,atxmega64b3,atxmega64c3,atxmega64d3,atxmega64d4.avrxmega5— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu =atxmega64a1,atxmega64a1u.avrxmega6— "XMEGA" devices with more than 128 KiB of program memory. mcu =atxmega128a3,atxmega128a3u,atxmega128b1,atxmega128b3,atxmega128c3,atxmega128d3,atxmega128d4,atxmega192a3,atxmega192a3u,atxmega192c3,atxmega192d3,atxmega256a3,atxmega256a3b,atxmega256a3bu,atxmega256a3u,atxmega256c3,atxmega256d3,atxmega384c3,atxmega384d3.avrxmega7— "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu =atxmega128a1,atxmega128a1u,atxmega128a4u.avrtiny— "TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu =attiny4,attiny5,attiny9,attiny10,attiny20,attiny40.avr1— This ISA is implemented by the minimal AVR core and is supported for assembler only ⚠️. mcu =attiny11,attiny12,attiny15,attiny28,at90s1200.
-mabsdata— Assumes all data in static storage can be accessed byLDS/STSinstructions 🎯. This only has an effect on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable Attributes,variable attribute 🔗.-maccumulate-args— Accumulates outgoing function arguments, and acquires and releases the needed stack space for them just once, in the function prologue/epilogue 📚. Without this option, outgoing arguments are pushed before calling a function and popped afterwards 🥡.
Okay so the stack trick is lowkey genius 🧚✨ Popping arguments after a call can be expensive on AVR 💸, so letting them pile up on the stack can actually lead to smaller executables 📉, since nothing has to be yanked off the stack after each call. 💅
This option can really shrink code for functions that make a bunch of calls to functions that take their arguments on the stack, like printf-like functions 🖨️🌸
-mbranch-cost=cost— Sets the branch costs for conditional branch instructions tocost🦋 Reasonable values are small, non-negative integers, and the default branch cost is0.-mcall-prologues— Function prologues and epilogues get expanded as calls to the right subroutines 🫶 The bonus here is that the code size comes out smaller 💖-mdouble=bits,-mlong-double=bits— Sets the size (in bits) of the"double"or"long double"type, respectively 🌈 The possible values forbitsare32and64. Whether a specific value is allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and the same goes for the default values of these options 🧐-mgas-isr-prologues— Interrupt service routines (ISRs) can use the"__gcc_isr"pseudo instruction that GNU Binutils supports 🔥 If this option is on, you can still turn the feature off for individual ISRs with the AVR Function Attributes,,"no_gccisr"function attribute 🙈 This feature is turned on by default if optimization is on (but not with-Og,@pxref{Optimize Options}), and if GNU Binutils supports PR21683 (https://sourceware.org/PR21683) ✨-mint8— Assumesintis an 8-bit integer 💀 This changes the size of every type: acharis 1 byte, anintis 1 byte, alongis 2 bytes, and along longis 4 bytes.
Heads up bestie: this option does not conform to the C standards 😭 It does give you smaller code size though, so it's a trade-off.
-mmain-is-OS_task— Stops registers from being saved inmain🍓 It's the same as attaching the AVR Function Attributes,,"OS_task"attribute tomain. It's activated by default if optimization is on 🔥-mn-flash=num— Assumes the flash memory isnumtimes 64 KiB big 💜-mno-interrupts— The generated code is not compatible with hardware interrupts 🙈 The upside is that code size comes out smaller 🎀-mrelax— Tries to swap aCALLresp.JMPinstruction for the shorterRCALLresp.RJMPinstruction when that's possible 🦋 Setting-mrelaxjust adds the--mlink-relaxoption to the assembler's command line, and the--relaxoption to the linker's command line.
Jump relaxing is done by the linker, because jump offsets aren't known until the code gets located 🧚 So the assembler code the compiler generates stays the same, but the instructions in the executable may differ from what's in the assembler code 🫠
Relaxing must be turned on if linker stubs are needed, so check out the section on "EIND" and linker stubs below 👑
-mrmw— Assumes the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT💅-mshort-calls— AssumesRJMPandRCALLcan target the whole program memory 🌸
This option is used internally for multilib selection 🤫 It's not an optimization option, so you don't need to set it by hand.
-msp8— Treats the stack pointer register as an 8-bit register, aka it assumes the high byte of the stack pointer is zero 🧁 In general, you don't need to set this one by hand.
This option is used internally by the compiler to select and build multilibs for the avr2 and avr25 architectures 🌈 These architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25, the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows whether the device or architecture has an 8-bit stack pointer (and thus no SPH register) or not 🧠
-mstrict-X— Uses the address registerXthe way the hardware intends 💖 That meansXonly gets used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register can be used the same way as Y or Z, which then gets emulated by extra instructions 😤 For example, loading a value with X+const addressing (with a small non-negative const < 64) into a register Rn gets done like this:
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-mtiny-stack— Only changes the lower 8 bits of the stack pointer 🦋-mfract-convert-truncate— Lets you use truncation instead of rounding towards zero for fractional fixed-point types 🍓-nodevicelib— Don't link against AVR-LibC's device specific librarylib<mcu>.a🙅♀️-nodevicespecs— Don't add-specs=device-specs/specs-mcuto the compiler driver's command line 💅 You're taking full responsibility for feeding the subprocesses (compiler proper, assembler and linker) the right command line options. That means you have to supply your own private device specs file with-specs=path-to-specs-file. There's no more need for-mmcu=mcueither ✨
This option can also replace the older way of doing custom device-specs files. That one needed -B some-path to point to a directory containing a folder named "device-specs", which contained a specs file named "specs-mcu" (where mcu was set by -mmcu=mcu) 🕰️
-Waddr-space-convert— Warns about conversions between address spaces when the resulting address space isn't contained in the incoming address space ⚠️-Wmisspelled-isr— Warns if the ISR is misspelled, i.e. missing the__vectorprefix 🙈 Enabled by default.
✨ "EIND" and Devices with More Than 128 Ki Bytes of Flash ✨
Pointers in the implementation are 16 bits wide 🧚 The address of a function or label is stored as a word address, so indirect jumps and calls can target any code address in the range of 64 Ki words 💜
To make indirect jumps work on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND 👑 It serves as the most significant part of the target address when EICALL or EIJMP instructions are used.
Indirect jumps and calls on these devices are handled like this by the compiler, and they come with some limitations 🫶
- The compiler never sets
EIND. 🙅♀️ - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or it might readEINDdirectly to emulate an indirect call/jump by means of aRETinstruction. - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis not saved/restored in function or interrupt service routine prologue/epilogue 😬 - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads (sometimes also called trampolines), so the indirect call/jump lands on such a stub 🌉 The stub contains a direct jump to the desired address.
- Linker relaxation must be turned on so the linker generates the stubs correctly in all situations. See the compiler option
-mrelaxand the linker option--relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message 💀 - The default linker script is arranged for code with
EIND = 0. If your code is supposed to work withEIND != 0, you need a custom linker script to place the sections whose names start with.trampolinesinto the segmentEINDpoints to 🗺️ - The startup code from libgcc never sets
EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC onEIND, see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/) 📖 - It's totally legit for user-specific startup code to set up
EINDearly, for example with initialization code located in section.init3🧚 That code runs before the general startup code that initializes RAM and calls constructors, but after the bit of AVR-LibC startup code that setsEINDto the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}The __trampolines_start symbol is defined in the linker script 🌸
- Stubs are generated automatically by the linker if the following two conditions are met ✨
- The address of a label is taken by means of the
gsmodifier (short for generate stubs), like so:
- The address of a label is taken by means of the
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))- The final location of that label is in a code segment outside the segment where the stubs are located.
- The compiler emits such
gsmodifiers for code labels in the following situations 🧁- Taking the address of a function or code label.
- Computed goto.
- If the prologue-save function is used, see the
-mcall-prologuescommand-line option. - Switch/case dispatch tables. If you do not want such dispatch tables, you can specify the
-fno-jump-tablescommand-line option. - C and C++ constructors/destructors called during startup/shutdown.
- If the tools hit a
gs()modifier explained above.
- Jumping to non-symbolic addresses like so is not supported 🚫
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example) 🦋
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}Then the application needs to be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script 💖
✨ Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers ✨
So, the RAMP thing is kinda wild, bestie 💾✨ Some AVR devices have more memory than the 64 KiB you can reach with 16-bit pointers. To get past that, the contents of a RAMP register act as the high part of the address. The X, Y, Z address register gets glued onto RAMPX, RAMPY, RAMPZ respectively to make one wide address 🦋. RAMPD does the same job for direct addressing.
- Startup vibes — The startup code sets the
RAMPspecial function registers to zero 🌱. - Named address spaces — If a named address space from AVR Named Address Spaces is used (anything other than generic or
__flash),RAMPZgets set as needed before the operation 💅. - Reset after the drama — If the device supports RAM larger than 64 KiB and the compiler has to change
RAMPZto pull off an operation,RAMPZgets reset to zero afterwards 🧹. - ISR glow-up — If the device comes with a specific
RAMPregister, the ISR prologue/epilogue saves and restores that SFR, and sets it to zero in case the ISR code might (implicitly) use it 🛡️. - The big no — RAM larger than 64 KiB is not supported by GCC for AVR targets 🙅♀️
Warning, bestie: if you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. No skipping this step 💀
🎀 AVR Built-in Macros 🎀
Okay so GCC defines a bunch of built-in macros so your code can check whether a feature is there or not 🔍. Almost every one of the following built-in macros is deduced from device capabilities, which means they get triggered by the -mmcu= command-line option 💖.
For even more AVR-specific built-in macros, see AVR Named Address Spaces and AVR Built-in Functions ✨.
__AVR_ARCH__— A built-in macro that resolves to a decimal number identifying the architecture. It depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5",
"avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively.If mcu specifies a device, this built-in macro is set accordingly 🧚. For example, with -mmcu=atmega8 the macro is defined to 4.
__AVR_Device__— Setting-mmcu=devicedefines this built-in macro, which reflects the device's name 🌸. For example,-mmcu=atmega8defines the built-in macro__AVR_ATmega8__, and-mmcu=attiny261adefines__AVR_ATtiny261A__, etc.
The built-in macro names follow the scheme __AVR_Device__, where Device is the device name as it appears in the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase 🔤.
If device is not a device but only a core architecture like avr51, this macro is not defined 🙃.
__AVR_DEVICE_NAME__— Setting-mmcu=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined to"atmega8"🍓.
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_XMEGA__— The device or architecture belongs to the XMEGA family of devices 👑.__AVR_HAVE_ELPM__— The device has theELPMinstruction.__AVR_HAVE_ELPMX__— The device has theELPM Rn,ZandELPM Rn,Z+instructions.__AVR_HAVE_MOVW__— The device has theMOVWinstruction to perform 16-bit register-register moves.__AVR_HAVE_LPMX__— The device has theLPM Rn,ZandLPM Rn,Z+instructions.__AVR_HAVE_MUL__— The device has a hardware multiplier 🔥.__AVR_HAVE_JMP_CALL__— The device has theJMPandCALLinstructions. This is the case for devices with more than 8 KiB of program memory.__AVR_HAVE_EIJMP_EICALL__/__AVR_3_BYTE_PC__— The device has theEIJMPandEICALLinstructions. This is the case for devices with more than 128 KiB of program memory. This also means the program counter (PC) is 3 bytes wide 🦋.__AVR_2_BYTE_PC__— The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.__AVR_HAVE_8BIT_SP__/__AVR_HAVE_16BIT_SP__— The stack pointer (SP) register is treated by the compiler as an 8-bit or 16-bit register, respectively. The definition of these macros is affected by-mtiny-stack🥞.__AVR_HAVE_SPH__/__AVR_SP8__— The device has the SPH (high part of stack pointer) special function register, or has an 8-bit stack pointer, respectively. The definition of these macros is affected by-mmcu=and, in the cases of-mmcu=avr2and-mmcu=avr25, also by-msp8💅.__AVR_HAVE_RAMPD__/__AVR_HAVE_RAMPX__/__AVR_HAVE_RAMPY__/__AVR_HAVE_RAMPZ__— The device has theRAMPD,RAMPX,RAMPY,RAMPZspecial function register, respectively.__NO_INTERRUPTS__— This macro reflects the-mno-interruptscommand-line option 🙈.__AVR_ERRATA_SKIP__/__AVR_ERRATA_SKIP_JMP_CALL__— Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum 😭. The skip instructions areSBRS,SBRC,SBIS,SBICandCPSE. The second macro is only defined if__AVR_HAVE_JMP_CALL__is also set.__AVR_ISA_RMW__— The device has Read-Modify-Write instructions (XCH,LAC,LASandLAT) ⚡.__AVR_SFR_OFFSET__=offset— Instructions that can address I/O special function registers directly, likeIN,OUT,SBI, etc., may use a different address than an instruction accessing RAM, likeLDorSTS. This offset depends on the device architecture and has to be subtracted from the RAM address to get the respective I/O address 🧮.__AVR_SHORT_CALLS__— The-mshort-callscommand-line option is set 📞.__AVR_PM_BASE_ADDRESS__=addr— Some devices support reading from flash memory withLD*instructions. The flash memory is seen in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If it is defined, the address space is linear and there's no need to put.rodatainto RAM. This is handled by the default linker description file, and is currently available foravrtinyandavrxmega3. Even more convenient, there's no need to use address spaces like__flashor features like attributeprogmemandpgm_read_*🫶.__WITH_AVRLIBC__— The compiler is configured to be used together with AVR-Libc. See the--with-avrlibcconfigure option 📚.__HAVE_DOUBLE_MULTILIB__— Defined if-mdouble=acts as a multilib option.__HAVE_DOUBLE32__/__HAVE_DOUBLE64__— Defined if the compiler supports 32-bit double or 64-bit double, respectively. The actual layout is specified by option-mdouble=.__DEFAULT_DOUBLE__— The size in bits ofdoubleif-mdouble=is not set. To test the layout ofdoublein a program, use the built-in macro__SIZEOF_DOUBLE__📏.__HAVE_LONG_DOUBLE32__/__HAVE_LONG_DOUBLE64__/__HAVE_LONG_DOUBLE_MULTILIB__/__DEFAULT_LONG_DOUBLE__— Same as above, but forlong doubleinstead ofdouble🌈.__WITH_DOUBLE_COMPARISON__— Reflects the--with-double-comparison={tristate|bool|libf7}configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3 🔍.__WITH_LIBF7_LIBGCC__/__WITH_LIBF7_MATH__/__WITH_LIBF7_MATH_SYMBOLS__— Reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr) 💜.
🦋 Blackfin Options 🦋
-mcpu=cpu[-sirevision]— Specifies the name of the target Blackfin processor. Currently, cpu can be one ofbf512,bf514,bf516,bf518,bf522,bf523,bf524,bf525,bf526,bf527,bf531,bf532,bf533,bf534,bf536,bf537,bf538,bf539,bf542,bf544,bf547,bf548,bf549,bf542m,bf544m,bf547m,bf548m,bf549m,bf561,bf592🍓.
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for that revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, __SILICON_REVISION__ is not defined. If sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor 🧠.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given 🔗.
Without this option, bf532 is used as the processor by default.
Note that support forbf561is incomplete. Forbf561, only the preprocessor macro is defined.
-msim— Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for thebfin-elftoolchain. Certain other options, such as-mid-shared-libraryand-mfdpic, imply-msim🎮.-momit-leaf-frame-pointer— Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and makes an extra register available in leaf functions 🌿.-mspecld-anomaly— When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used,__WORKAROUND_SPECULATIVE_LOADSis defined 🛠️.-mno-specld-anomaly— Don't generate extra code to prevent speculative loads from occurring.-mcsync-anomaly— When enabled, the compiler ensures that the generated code does not containCSYNCorSSYNCinstructions too soon after conditional branches. If this option is used,__WORKAROUND_SPECULATIVE_SYNCSis defined ⏱️.-mno-csync-anomaly— Don't generate extra code to preventCSYNCorSSYNCinstructions from occurring too soon after a conditional branch.-mlow64k— When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory 🎀
-mno-low64k Assume that the program is arbitrarily large. This is the default.
Wait, I need to pause. Looking at what I've been asked to do: this part is the continuation of a rewrite, and I'm supposed to carry on from part 79 without a greeting. Let me produce the rewritten content for this section.
Okay bestie, let's get into the Blackfin and C6X stuff 💖
Actually, I need to be honest with you about what's happening here. I don't have the earlier parts of this rewrite in front of me, so I can't tell which heading level or section this part picks up in. The text you pasted starts mid-way through the OPTIONS section, and it begins with -mno-low64k, which is a Blackfin option. Because I can't confirm the surrounding context, I'll keep the rewrite to exactly what's in this excerpt and carry on from there without re-adding headings that may already exist.
Here is the rewrite of this part:
-mno-low64k— okay so this one's the default vibe 🌸 It assumes your program is arbitrarily large, which is just how it already works, no cap ✨-mstack-check-l1— bestie, this one does stack checking using the info the uClinux kernel drops into L1 scratchpad memory 🦋-mid-shared-library— generates code that supports shared libraries through the library ID method 🎀 This lets you do execute in place and shared libraries even without virtual memory management 💅 It implies-fPIC, and with abfin-elftarget it also implies-msim🔥-mno-id-shared-library— code that doesn't assume ID-based shared libraries are in use 💜 This is the default, periodt 💀-mleaf-id-shared-library— still uses the library ID method, but assumes this library or executable won't link against any other ID shared libraries 🧚 That lets the compiler use faster code for jumps and calls ⭐-mno-leaf-id-shared-library— don't assume the code won't link against any ID shared libraries 🙈 You get slower code for jump and call instructions 😭-mshared-library-id=n— sets the identification number of the ID-based shared library being compiled 🍓 A value of0gives you more compact code 🫶 Other values force that number onto the current library, but it's no more space- or time-efficient than leaving this option out 💖-msep-data— lets the data segment live in a different area of memory from the text segment 🌈 This enables execute in place without virtual memory management, because it gets rid of relocations against the text section ✨-mno-sep-data— assumes the data segment follows the text segment 🩷 This is the default ⭐-mlong-calls/-mno-long-calls— makes function calls by loading the function's address into a register first, then calling through that register 👑 You need this if the target function is outside the 24-bit addressing range of the offset-based subroutine call instruction 🦋 It's not on by default, and-mno-long-callsputs you back to the default 💅 These switches don't change how function calls through function pointers are generated 🙈-mfast-fp— links with the fast floating-point library 🔥 It relaxes some IEEE floating-point rules for checking inputs against Not-a-Number (NAN), for performance 💨-minline-plt— enables inlining of PLT entries for function calls to functions that aren't known to bind locally 🧚 It does nothing without-mfdpic💀-mmulticore— builds a standalone application for multicore Blackfin processors 👑 It uses the right start files and link scripts for multicore and defines the macro"__BFIN_MULTICORE"🎀 It can only be used with-mcpu=bf561[-sirevision]🌸
This also works with -mcorea or -mcoreb for the one-application-per-core model 🫶 Without either of those, you get the single-application/dual-core model 💜 In that model, Core B's main function should be named "coreb_main" 🧚
Without this option, you get the single-core application model ✨
-mcorea— builds a standalone application for Core A of BF561 in the one-application-per-core model 🦋 It uses the right start files and link scripts for Core A and defines"__BFIN_COREA"💅 It can only be used together with-mmulticore🔥-mcoreb— builds a standalone application for Core B of BF561 in the one-application-per-core model 💖 It uses the right start files and link scripts for Core B and defines"__BFIN_COREB"🌈 With this on, use"coreb_main"instead of"main"👑 It can only be used together with-mmulticore💀-msdram— builds a standalone application for SDRAM 🍓 It uses the right start files and link scripts to put the app into SDRAM and defines"__BFIN_SDRAM"🦋 The loader needs to initialize SDRAM before loading the app 😭-micplb— assumes ICPLBs are enabled at run time 🎀 This affects certain anomaly workarounds ⭐ For Linux targets, the default is to assume ICPLBs are enabled 💜 For standalone applications, the default is off 🙈
✨ C6X Options ✨
-march=name— picks the target architecture name 🌸 GCC uses it to decide what kind of instructions it can emit when generating assembly 🧚 The allowed names arec62x,c64x,c64x+,c67x,c67x+, andc674x💅-mbig-endian— generates code for a big-endian target 🫶-mlittle-endian— generates code for a little-endian target 💖 This is the default ✨-msim— picks startup files and a linker script that work with the simulator 🔥-msdata=default— puts small global and static data in the".neardata"section, which is pointed to by registerB14🌈 Small uninitialized global and static data goes in".bss", which sits right next to".neardata"🦋 Small read-only data goes in".rodata"👑 Large data uses".fardata",".far", and".const"instead 💀-msdata=all— puts all data, not just the small stuff, into the small-data sections 🍓 It uses addressing relative to theB14register to get at it 😭-msdata=none— no small-data sections at all 🎀 Everything gets absolute addresses 🙈 All initialized global and static data goes in".fardata", all uninitialized data goes in".far", and all constant data goes in".const"⭐
✨ CRIS Options ✨
These options are defined specifically for the CRIS ports 💜
-march=architecture-type/-mcpu=architecture-type— generates code for the specified architecture 🧚 The choices arev3,v8, andv10, which are ETRAX 4, ETRAX 100, and ETRAX 100 LX 🌸 The default isv0💅-mtune=architecture-type— tunes everything applicable about the generated code to that architecture 🫶 It doesn't touch the ABI or the set of available instructions 🔥 The choices are the same as for-march=architecture-type💖-mmax-stack-frame=n— warns when a function's stack frame goes overnbytes ⚠️-metrax4/-metrax100— these are synonyms for-march=v3and-march=v8respectively 🌈-mmul-bug-workaround/-mno-mul-bug-workaround— works around a bug in the"muls"and"mulu"instructions on CPU models where it applies 🦋 This is active by default 👑-mpdebug— turns on CRIS-specific verbose debug info in the assembly code 💀 It also turns off the#NO_APPformatted-code indicator that the assembler normally sees at the top of the assembly file 🍓-mcc-init— don't reuse condition-code results from previous instructions 😭 Always emit compare and test instructions before using condition codes 🎀-mno-side-effects— don't emit instructions with side effects in addressing modes, except post-increment 🙈-mstack-align/-mno-stack-align/-mdata-align/-mno-data-align/-mconst-align/-mno-const-align— these arrange (or, with theno-forms, un-arrange) alignment of the stack frame, individual data, and constants to the maximum single data access size for the chosen CPU model ⭐ The default is 32-bit alignment 💜 ABI details like structure layout aren't affected by these 🧚-m32-bit/-m16-bit/-m8-bit— like the stack, data, and const align options above, these make the stack frame, writable data, and constants 32-bit, 16-bit, or 8-bit aligned 🌸 The default is 32-bit alignment 💅-mno-prologue-epilogue/-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame are left out, and no return instructions or return sequences are generated 💀 Only use this alongside careful visual inspection of the compiled code 😭 No warnings or errors are generated when call-saved registers need saving or local variable storage needs allocating 🙈-melf— legacy no-op option 🫶-sim— links with input-output functions from a simulator library 🔥 Code, initialized data, and zero-initialized data get allocated consecutively 🌈-sim2— like-sim, but also passes linker options to put initialized data at0x40000000and zero-initialized data at0x80000000👑
✨ CR16 Options ✨
These options are defined specifically for the CR16 ports 🦋
-mmac— enables multiply-accumulate instructions 💅 It's disabled by default 🙈-mcr16cplus/-mcr16c— generates code for the CR16C or CR16C+ architecture 💖 CR16C+ is the default 🌸-msim— links the librarylibsim.a, which is compatible with the simulator 🧚 Only applies to the ELF compiler 🔥-mint32— chooses a 32-bit wide integer type 💜-mbit-ops— generates"sbit"and"cbit"instructions for bit manipulation ✨-mdata-model=model— chooses a data model 🎀 The choices arenear,far, ormedium, andmediumis the default 🦋farisn't valid with-mcr16c, because the CR16C architecture doesn't support the far data model 💀
✨ C-SKY Options ✨
GCC supports these options when compiling for C-SKY V2 processors 🌈
-march=arch— specifies the C-SKY target architecture 👑 Valid values forarchareck801,ck802,ck803,ck807, andck810💖 The default isck810🫶-mcpu=cpu— okay bestie, this one lets you pick your C-SKY target processor 🦋 The validcpuvalues are below, and yes, it's a long list, but every single one is iconic:
ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t,
ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht,
ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1,
ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1,
ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft,
ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft,
ck810ftv.-mbig-endian,-EB,-mlittle-endian,-EL— pick big-endian or little-endian code 🌸 Little-endian is the default, so if you do nothing, you're already serving ✨-mfloat-abi=name— this one picks which floating-point ABI you're rocking 👑 The permissible values aresoft,softfp, andhard:soft— GCC generates output full of library calls for floating-point operationssoftfp— you get hardware floating-point instructions, but it still uses the soft-float calling conventionshard— you get floating-point instructions and FPU-specific calling conventions
The default depends on your specific target configuration 🎀 Also, heads up, the hard-float and soft-float ABIs are not link-compatible 😭 You have to compile your entire program with the same ABI, and link it with a compatible set of libraries.
-mhard-float,-msoft-float— choose hardware or software floating-point implementations 💖 The default is soft float.-mdouble-float,-mno-double-float— when-mhard-floatis in effect, this turns on double-precision float instructions 🔥 It's the default except when you're compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, this enables the"frecipd","fsqrtd", and"fdivd"instructions 🧚 Also the default except for CK803.-mfpu=fpu— pick your floating-point processor 🦋 This only works with-mhard-float. The values forfpuare:fpv2_sf— equivalent to-mno-double-float -mno-fdivdufpv2— equivalent to-mdouble-float -mno-divdufpv2_divd— equivalent to-mdouble-float -mdivdu
-melrw,-mno-elrw— enable the extended"lrw"instruction 💅 It defaults to on for CK801 and off for everything else.-mistack,-mno-istack— enable interrupt stack instructions. The default is off 🌈 You need-mistackto handle the"interrupt"and"isr"function attributes, so don't skip it, bestie 🫶-mmp— enable multiprocessor instructions. Default is off ⭐-mcp— enable coprocessor instructions. Default is off 🍓-mcache— also enables coprocessor instructions, so yes, it's the same vibe as-mcp🙈 Default is off.-msecurity— enable C-SKY security instructions. Default is off 🔥-mtrust— enable C-SKY trust instructions. Default is off 💜-mdsp,-medsp,-mvdsp— these enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively ✨ All of them default to off.-mdiv,-mno-div— generate divide instructions 🦋 Default is off.-msmart,-mno-smart— generate code for Smart Mode, which only uses registers numbered 0-7 so you can use those sweet 16-bit instructions 🫶 This is ignored for CK801, where it's already required behavior. It defaults to on for CK802. For other targets, it's off.-mhigh-registers,-mno-high-registers— generate code using the high registers numbered 16-31 🌸 This isn't supported on CK801, CK802, or CK803, but it's enabled by default for other processors.-manchor,-mno-anchor— generate code using global anchor symbol addresses 🧚-mpushpop,-mno-pushpop— generate code using"push"and"pop"instructions 💖 This defaults to on.-mmultiple-stld,-mstm,-mno-multiple-stld,-mno-stm— generate code using"stm"and"ldm"instructions 🎀 This isn't supported on CK801, but it's enabled by default on other processors.-mconstpool,-mno-constpool— create constant pools in the compiler instead of handing that job off to the assembler 💅 This is the default and is required for correct code generation on CK801 and CK802. On other processors it's optional.-mstack-size,-mno-stack-size— emit".stack_size"directives for each function in the assembly output 🍓 This defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead of libgcc 🌈 This defaults to off.-mbranch-cost=n— set the branch costs to roughly"n"instructions ⭐ The default is 1.-msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences 🙈 Heads up, though:
Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements, and that code cannot be debugged or backtraced 💀 It's disabled by default, so only turn it on if you really know what you're doing.
-msim— links the librarylibsemi.a, which is compatible with the simulator 🧚 This only applies to the ELF compiler.
🍎 Darwin Options 🍎
These options are defined for all architectures running the Darwin operating system 💖
Okay, so, FSF GCC on Darwin does not create "fat" object files. Instead, it makes an object file for the single architecture GCC was built to target. Apple's GCC, though, does create "fat" files if you use multiple -arch options. It does this by running the compiler or linker multiple times and then joining the results together with lipo 🦋
The subtype of the file that gets created (like ppc7400, ppc970, or i686) is decided by the flags that specify the ISA GCC is targeting, like -mcpu or -march. If you want to override that, the -force_cpusubtype_ALL option is your friend ✨
The Darwin tools are kind of inconsistent when they hit an ISA mismatch 💀 The assembler, as, only allows instructions that are valid for the subtype of the file it's generating. So you can't put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if you ask it to create a shared library with a less restrictive subtype than its input files. For example, trying to put a ppc970 object file in a ppc7400 library is a no-go 🙅 The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files 🤫
-Fdir— adds the framework directorydirto the head of the list of directories searched for header files 🌸 These directories are interleaved with the ones from-Ioptions, and they're scanned in a left-to-right order.
A framework directory is just a directory with frameworks in it. A framework is a directory that has a Headers and/or PrivateHeaders directory sitting directly inside it, and its name ends in .framework. The name of the framework is the directory name without the .framework part. Headers for the framework are found in one of those two directories, with Headers getting searched first 🎀
A subframework is a framework directory that lives inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of the framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they live in the same framework 👯 A subframework shouldn't share a name with a framework, and if it does, you'll get a warning. Right now a subframework can't have subframeworks of its own, but the mechanism might be extended to support that later 👀
The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework is the framework's name and header.h is found in the PrivateHeaders or Headers directory 💅
-iframeworkdir— this is like-F, except the directory is treated as a system directory 🧚 The big difference is that with-iframework, the compiler doesn't warn about constructs inside header files found throughdir. This option is only valid for the C family of languages.-gused— emit debugging information for symbols that are used 🔍 For the stabs debugging format, this enables-feliminate-unused-debug-symbols. It's ON by default.-gfull— emit debugging information for all symbols and types 🌈-fconstant-cfstrings— this is just an alias for-mconstant-cfstrings🦋-mconstant-cfstrings— when the NeXT runtime is being used (which is the default on these systems), this overrides any-fconstant-string-classsetting. It also makes"@"...""literals laid out as constant CoreFoundation strings 💖-mmacosx-version-min=version— sets the earliest version of MacOS X this executable will run on 🍎 Typical values forversioninclude12,10.12, and10.5.8.
If the compiler was built to use the system's headers by default, then the default for this option is the system version the compiler is running on. Otherwise, the default makes choices that are compatible with as many systems and code bases as possible ✨
-mkernel— enables kernel development mode 🫶 The-mkerneloption sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weak, and-fno-rtti, where applicable. This mode also sets-mno-altivec,-msoft-float,-fno-builtin, and-mlong-branchfor PowerPC targets 🔥-mone-byte-bool— overrides the defaults for"bool"so thatsizeof(bool)==1💅 By default,sizeof(bool)is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86. So this option has no effect on x86.
Warning: -mone-byte-bool makes GCC generate code that is not binary compatible with code generated without that switch 😭 Using it may mean you have to recompile all the other modules in your program, including system libraries. Only use it to conform to a non-default data model.
-mfix-and-continue,-ffix-and-continue,-findirect-data— generate code that's suitable for fast turnaround development 🔥 This lets things like GDB dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare only here for backwards compatibility.-all_load— loads all members of static archive libraries 🧚 Seeman ld(1)for more information.-arch_errors_fatal— makes the errors about files with the wrong architecture fatal 💀-bind_at_load— ✨ Marks the output file so the dynamic linker binds every undefined reference when the file gets loaded or launched. Bestie, that's a whole vibe 🦋-bundle— 🎀 Produces a Mach-O bundle format file. Checkman ld(1)for the tea on this one 🍓-bundle_loader executable— 🧚 Names the executable that's going to load the output file being linked. Also inman ld(1)💜-dynamiclib— 🌈 Makes a dynamic library instead of an executable when linking, using the Darwin libtool command. Iconic 👑-force_cpusubtype_ALL— 💅 Gives GCC's output file the ALL subtype, instead of the one-mcpuor-marchwould pick. Not me overriding the vibe 💀
Okay so next up is the big squad of Darwin linker options. Each one gets handed straight over to the linker, no cap:
-allowable_client client_name— 🌸 passed straight through to the Darwin linker-client_name— 🌸 passed straight through to the Darwin linker-compatibility_version— 🌸 passed straight through to the Darwin linker-current_version— 🌸 passed straight through to the Darwin linker-dead_strip— 🌸 passed straight through to the Darwin linker-dependency-file— 🌸 passed straight through to the Darwin linker-dylib_file— 🌸 passed straight through to the Darwin linker-dylinker_install_name— 🌸 passed straight through to the Darwin linker-dynamic— 🌸 passed straight through to the Darwin linker-exported_symbols_list— 🌸 passed straight through to the Darwin linker-filelist— 🌸 passed straight through to the Darwin linker-flat_namespace— 🌸 passed straight through to the Darwin linker-force_flat_namespace— 🌸 passed straight through to the Darwin linker-headerpad_max_install_names— 🌸 passed straight through to the Darwin linker-image_base— 🌸 passed straight through to the Darwin linker-init— 🌸 passed straight through to the Darwin linker-install_name— 🌸 passed straight through to the Darwin linker-keep_private_externs— 🌸 passed straight through to the Darwin linker-multi_module— 🌸 passed straight through to the Darwin linker-multiply_defined— 🌸 passed straight through to the Darwin linker-multiply_defined_unused— 🌸 passed straight through to the Darwin linker-noall_load— 🌸 passed straight through to the Darwin linker-no_dead_strip_inits_and_terms— 🌸 passed straight through to the Darwin linker-nofixprebinding— 🌸 passed straight through to the Darwin linker-nomultidefs— 🌸 passed straight through to the Darwin linker-noprebind— 🌸 passed straight through to the Darwin linker-noseglinkedit— 🌸 passed straight through to the Darwin linker-pagezero_size— 🌸 passed straight through to the Darwin linker-prebind— 🌸 passed straight through to the Darwin linker-prebind_all_twolevel_modules— 🌸 passed straight through to the Darwin linker-private_bundle— 🌸 passed straight through to the Darwin linker-read_only_relocs— 🌸 passed straight through to the Darwin linker-sectalign— 🌸 passed straight through to the Darwin linker-sectobjectsymbols— 🌸 passed straight through to the Darwin linker-whyload— 🌸 passed straight through to the Darwin linker-seg1addr— 🌸 passed straight through to the Darwin linker-sectcreate— 🌸 passed straight through to the Darwin linker-sectorder— 🌸 passed straight through to the Darwin linker-segaddr— 🌸 passed straight through to the Darwin linker-segs_read_only_addr— 🌸 passed straight through to the Darwin linker-segs_read_write_addr— 🌸 passed straight through to the Darwin linker-seg_addr_table— 🌸 passed straight through to the Darwin linker-seg_addr_table_filename— 🌸 passed straight through to the Darwin linker-seglinkedit— 🌸 passed straight through to the Darwin linker-segprot— 🌸 passed straight through to the Darwin linker-single_module— 🌸 passed straight through to the Darwin linker-static— 🌸 passed straight through to the Darwin linker-sub_library— 🌸 passed straight through to the Darwin linker-sub_umbrella— 🌸 passed straight through to the Darwin linker-twolevel_namespace— 🌸 passed straight through to the Darwin linker-umbrella— 🌸 passed straight through to the Darwin linker-undefined— 🌸 passed straight through to the Darwin linker-unexported_symbols_list— 🌸 passed straight through to the Darwin linker-weak_reference_mismatches— 🌸 passed straight through to the Darwin linker-whatsloaded— 🌸 passed straight through to the Darwin linker
These are all passed to the Darwin linker, and the Darwin linker's man page describes them in detail. Read that one if you want the full story 🫶
✨ DEC Alpha Options ✨
These -m options are defined for the DEC Alpha implementations 🧚
-mno-soft-float,-msoft-float— 💖 Use (or don't use) the hardware floating-point instructions for floating-point operations. With-msoft-float, functions inlibgcc.ado the floating-point work. Unless they're swapped out for routines that emulate those operations, or compiled to call such emulation routines, these routines issue floating-point operations. If you're compiling for an Alpha without floating-point operations, you have to make sure the library is built so it doesn't call them 😭
Note that Alpha implementations without floating-point operations are required to have floating-point registers 🌸
-mfp-reg,-mno-fp-regs— 🦋 Generates code that uses (or doesn't use) the floating-point register set.-mno-fp-regsimplies-msoft-float. If the floating-point register set isn't used, floating-point operands get passed in integer registers as if they were integers, and floating-point results get passed in$0instead of$f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value that's called by code compiled with-mno-fp-regsmust also be compiled with that option 💅
A typical use of this option is building a kernel that doesn't use, and so doesn't need to save and restore, any floating-point registers 🔥
-mieee— 👑 The Alpha architecture has floating-point hardware optimized for maximum performance. It's mostly compliant with the IEEE floating-point standard, but full compliance needs software help. This option generates fully IEEE-compliant code, except the inexact-flag isn't maintained (see below). When it's on, the preprocessor macro"_IEEE_FP"gets defined during compilation. The result is less efficient, but it can correctly support denormalized numbers and exceptional IEEE values like not-a-number and plus/minus infinity. Other Alpha compilers call this option-ieee_with_no_inexact🙈
DEBIAN SPECIFIC: This option is on by default foralpha-linux-gnu, unless-ffinite-math-only(which is part of the-ffast-mathset) is specified. The reason is that the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs, and all of those will cause a program toSIGFPEwhen it tries to use the results without-mieee💀
-mieee-with-inexact— 🧚 This is like-mieee, except the generated code also maintains the IEEE inexact-flag. Turning this on makes the generated code fully-compliant IEEE math. On top of"_IEEE_FP", the macro"_IEEE_FP_EXACT"gets defined too. On some Alpha implementations the resulting code may run significantly slower than the default. Since very little code depends on the inexact-flag, you should normally not specify this option 🫶 Other Alpha compilers call it-ieee_with_inexact✨-mfp-trap-mode=trap-mode— 🌈 Controls which 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— 💖 The default, normal setting. The only traps enabled are the ones that can't be disabled in software (like the division by zero trap).u— 🌸 On top of everythingnenables, underflow traps are enabled too.su— 🦋 Likeu, but the instructions are marked as safe for software completion (check the Alpha architecture manual for details).sui— 🔥 Likesu, 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. Therounding-modecan be one of:n— ⭐ Normal IEEE rounding mode. Floating-point numbers get 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 get rounded towards zero.d— 🌈 Dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls which rounding mode is in effect. The C library initializes this register for rounding towards plus infinity, so unless your program modifies thefpcr,dcorresponds to round towards plus infinity 👑
-mtrap-precision=trap-precision— 😭 In the Alpha architecture, floating-point traps are imprecise. That means without software help it's impossible to recover from a floating trap, and program execution normally has to be terminated. GCC can generate code that helps operating system trap handlers figure out the exact location that caused a floating-point trap. Depending on what your application needs, you can pick different levels of precision:p— 🌸 Program precision. This is the default, and it means a trap handler can only identify which program caused a floating-point exception.f— 🦋 Function precision. The trap handler can determine the function that caused a floating-point exception.i— 💖 Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe 🫶
-mieee-conformant— 💜 Marks the generated code as IEEE conformant. You must not use this option unless you also specify-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file 🎀-mbuild-constants— 🧚 Normally GCC examines a 32- or 64-bit integer constant to see if it can build it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time.
Use this option to require GCC to build all integer constants using code, even if it takes more instructions (the maximum is six) 🌈
You typically use this option to build a shared library dynamic loader. Since it's a shared library itself, it has to relocate itself in memory before it can find the variables and constants in its own data segment 😭
-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— 💅 Indicate whether GCC should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type you specify with the-mcpu=option, or by the CPU GCC was built on if you don't specify one ✨-mfloat-vax,-mfloat-ieee— 🌸 Generates code that uses (or doesn't use) VAX F and G floating-point arithmetic instead of IEEE single and double precision 🦋-mexplicit-relocs,-mno-explicit-relocs— 🍓 Older Alpha assemblers had no way to generate symbol relocations except through assembler macros, and those macros don't allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations apply to which instructions. This option is mostly useful for debugging, since GCC detects the assembler's capabilities when it's built and sets the default to match 🧚-msmall-data,-mlarge-data— 💜 When-mexplicit-relocsis in effect, static data is accessed via gp-relative relocations. When-msmall-datais used, objects 8 bytes long or smaller get placed in a small data area (the".sdata"and".sbss"sections) and are accessed via 16-bit relocations off of the$gpregister. This limits the size of the small data area to 64KB, but lets the variables be accessed directly with a single instruction 💖
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in the program's data segment 🔥
When generating code for shared libraries, -fpic implies -msmall-data, and -fPIC implies -mlarge-data 🫶
okay so these two are a whole size-vs-reach situation 🫶
-msmall-text,-mlarge-text— with-msmall-text, the compiler assumes the code for the entire program (or shared library) fits in 4MB 📏, so it's reachable with a branch instruction 🦋. When you use-msmall-data, the compiler can assume all local symbols share the same$gpvalue, which means a function call drops from 4 instructions to just 1 ✨. The default is-mlarge-text. 💅-mcpu=cpu_type— sets the instruction set and instruction scheduling parameters for the machine typecpu_type🔥. You can give the EV style name or the matching chip number. GCC has scheduling parameters for the EV4, EV5, and EV6 family, and it picks the default instruction set from whichever processor you name 👑. If you don't name one, GCC defaults to the processor the compiler was built on 🏗️.
Supported values for cpu_type are:
ev4,ev45,21064— schedules as an EV4 with no instruction set extensions 🌸ev5,21164— schedules as an EV5 with no instruction set extensions 🌸ev56,21164a— schedules as an EV5 and supports theBWXextension 💜pca56,21164pc,21164PC— schedules as an EV5 and supports theBWXandMAXextensions 💜ev6,21264— schedules as an EV6 and supports theBWX,FIX, andMAXextensions 🦋ev67,21264a— schedules as an EV6 and supports theBWX,CIX,FIX, andMAXextensions 🦋
Native toolchains also support the value native, which picks the best architecture option for the host processor 🧚. -mcpu=native has no effect if GCC doesn't recognize the processor 🙈.
-mtune=cpu_type— sets only the instruction scheduling parameters forcpu_type. The instruction set doesn't change, so no vibe shift there 🫶.
Native toolchains also support the value native here too, which picks the best architecture option for the host processor. -mtune=native has no effect if GCC doesn't recognize the processor 🌈.
-mmemory-latency=time— sets the latency the scheduler should assume for typical memory references as the application sees them 🧠. This number really depends on the application's memory access patterns and how big the external cache on the machine is 💀.
Valid options for time are:
number— a decimal number representing clock cycles 🔢L1,L2,L3,main— the compiler has estimates of clock cycles for "typical" EV4 & EV5 hardware, covering the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache) and main memory 🧚. Heads up thatL3is only valid for EV5 ⚠️.
🧚 eBPF Options 🧚
-mframe-limit=bytes— sets the hard limit for frame sizes, in bytes 📦. Right now the value you pass should be less than or equal to32767. It defaults to whatever limit the targeted version of the Linux kernel imposes 🐧.-mkernel=version— sets the minimum kernel version that will run your compiled program 🐧. GCC uses this to figure out which instructions to use, which kernel helpers to allow, and so on 🤓. Right nowversioncan be one of4.0,4.1,4.2,4.3,4.4,4.5,4.6,4.7,4.8,4.9,4.10,4.11,4.12,4.13,4.14,4.15,4.16,4.17,4.18,4.19,4.20,5.0,5.1,5.2,latest, ornative💅.-mbig-endian— generates code for a big-endian target 🍓.-mlittle-endian— generates code for a little-endian target. This is the default 💖.-mxbpf— generates code for an expanded version of BPF 🌈. It relaxes some of the restrictions the BPF architecture normally imposes, including this bit:-<Save and restore callee-saved registers at function entry and>exit, respectively.
🌸 FR30 Options 🌸
These options are defined specifically for the FR30 port 🎀.
-msmall-model— uses the small address space model 🐣. It can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range 🙈.-mno-lsim— assumes runtime support is already provided, so there's no need to include the simulator library (libsim.a) on the linker command line 🔧.
🦋 FT32 Options 🦋
These options are defined specifically for the FT32 port 💜.
-msim— says the program will run on the simulator 🎮. This links in an alternate runtime startup and library. ⚠️ You must not use this option when generating programs that will run on real hardware! You have to provide your own runtime library for whatever I/O functions you need 💀.-mlra— enables Local Register Allocation ✨. It's still experimental for FT32, so by default the compiler uses standard reload 🧪.-mnodiv— doesn't usedivandmodinstructions 🙅.-mft32b— enables use of the extended instructions of theFT32Bprocessor 🚀.-mcompress— compresses all code using theFt32Bcode compression scheme 🗜️.-mnopm— doesn't generate code that reads program memory 🔒.
🌈 FRV Options 🌈
-mgpr-32— only uses the first 32 general-purpose registers 🎀.-mgpr-64— uses all 64 general-purpose registers 🦾.-mfpr-32— uses only the first 32 floating-point registers 💫.-mfpr-64— uses all 64 floating-point registers 🌟.-mhard-float— uses hardware instructions for floating-point operations 🔥.-msoft-float— uses library routines for floating-point operations instead 📚.-malloc-cc— dynamically allocates condition code registers 🪄.-mfixed-cc— doesn't try to dynamically allocate condition code registers. It only usesicc0andfcc0📌.-mdword— changes the ABI to use double word instructions 🧩.-mno-dword— doesn't use double word instructions 🙅.-mdouble— uses floating-point double instructions 💖.-mno-double— doesn't use floating-point double instructions 🙅.-mmedia— uses media instructions 🎶.-mno-media— doesn't use media instructions 🙅.-mmuladd— uses multiply and add/subtract instructions ✖️.-mno-muladd— doesn't use multiply and add/subtract instructions 🙅.-mfdpic— selects the FDPIC ABI, which uses function descriptors to represent pointers to functions 🪪. Without any PIC/PIE-related options, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address. With-fPICor-fPIE, GOT offsets are computed with 32 bits 🧮. With abfin-elftarget, this option implies-msim🎮.-minline-plt— enables inlining of PLT entries in function calls to functions that aren't known to bind locally 🧵. It does nothing without-mfdpic. It's on by default if you're optimizing for speed and compiling shared libraries (i.e.,-fPICor-fpic), or when an optimization option like-O3or above is on the command line 🏎️.-mTLS— assumes a large TLS segment when generating thread-local code 🧵.-mtls— does not assume a large TLS segment when generating thread-local code 🧶.-mgprel-ro— enables the use of"GPREL"relocations in the FDPIC ABI for data known to be in read-only sections 📖. It's on by default, except for-fpicor-fpie. Even though it may help make the global offset table smaller, it trades 1 instruction for 4 in those cases 🤔. With-fPICor-fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols. It also avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win 🏆. If it isn't,-mno-gprel-rocan be used to disable it 🙅.-multilib-library-pic— links with the (library, not FD) pic libraries 📚. It's implied by-mlibrary-pic, as well as by-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly 🫠.-mlinked-fp— follows the EABI requirement of always creating a frame pointer whenever a stack frame is allocated 🖼️. It's on by default and can be turned off with-mno-linked-fp⬇️.-mlong-calls— uses indirect addressing to call functions outside the current compilation unit 📞. This lets the functions be placed anywhere within the 32-bit address space 🌌.-malign-labels— tries to align labels to an 8-byte boundary by inserting NOPs into the previous packet 📐. This only does something when VLIW packing is enabled. It doesn't create new packets; it just adds NOPs to existing ones 🧘.-mlibrary-pic— generates position-independent EABI code 🌍.-macc-4— uses only the first four media accumulator registers 4️⃣.-macc-8— uses all eight media accumulator registers 8️⃣.-mpack— packs VLIW instructions 📦.-mno-pack— doesn't pack VLIW instructions 🙅.-mno-eflags— doesn't mark ABI switches ine_flags🚫.-mcond-move— enables the use of conditional-move instructions. This is the default ✅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mno-cond-move— disables the use of conditional-move instructions 🙅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mscc— enables the use of conditional set instructions. This is the default ✅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mno-scc— disables the use of conditional set instructions 🙅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mcond-exec— enables the use of conditional execution. This is the default ✅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mno-cond-exec— disables the use of conditional execution 🙅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mvliw-branch— runs a pass to pack branches into VLIW instructions. This is the default ✅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mno-vliw-branch— doesn't run a pass to pack branches into VLIW instructions 🙅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
-mmulti-cond-exec— enables optimization of"&&"and"||"in conditional execution. This is the default ✅.
🐛 This switch is mainly for debugging the compiler, and it'll likely be removed in a future version.
✨ Machine-Dependent Options, Part Two ✨
-mno-multi-cond-exec— bestie, this one turns OFF the optimization of"&&"and"||"in conditional execution 🙈
This switch is mainly for debugging the compiler and will likely be removed in a future version, so don't build your whole life around it 💀
-mnested-cond-exec— turns ON nested conditional execution optimizations, and it's on by default 💜
Same deal: this one is mainly for compiler debugging and will likely get removed in a future version 🧚
-mno-nested-cond-exec— the opposite energy! It turns OFF nested conditional execution optimizations 🥲
Again, mainly a debugging tool, and it'll likely be removed in a future version ✨
-moptimize-membar— this one cleans up the clutter by removing redundant"membar"instructions from the compiler-generated code 🧹 It's enabled by default, so you're already getting this glow-up 🦋-mno-optimize-membar— the no-cap opt-out! It disables that automatic removal of redundant"membar"instructions from the generated code 🙅♀️-mtomcat-stats— tells gas to print out tomcat statistics 🐱📊-mcpu=cpu— pick your processor type, the main character of the codegen 👑 The possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple🌸
✨ GNU/Linux Options ✨
These -m options are defined for GNU/Linux targets, so if you're on Linux it's giving relevant 🐧💖
-mglibc— use the GNU C library 📚 This is the default, except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets 🌈-muclibc— use the uClibc C library 🎀 This is the default on*-*-linux-*uclibc*targets 💅-mmusl— use the musl C library 🍓 This is the default on*-*-linux-*musl*targets ✨-mbionic— use the Bionic C library 📱 This is the default on*-*-linux-*android*targets 🤖-mandroid— compile code that's compatible with the Android platform 🦋 This is the default on*-*-linux-*android*targets, and it's a whole vibe 🔥
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default 🫶 When linking, it makes the GCC driver pass Android-specific options to the linker 🔗 And finally, it causes the preprocessor macro "__ANDROID__" to be defined 💜
-tno-android-cc— disables the compilation effects of-mandroid, so it does not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default 🙅♀️-tno-android-ld— disables the linking effects of-mandroid, so the driver passes standard Linux linking options to the linker 🐧
✨ H8/300 Options ✨
These -m options are defined for the H8/300 implementations, so if that's your chip, this section is literally for you 🧚
-mrelax— shortens some address references at link time, when it's possible 🪄 It does this by using 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 has to be used with either-mhor-ms, so don't forget your partner 🫶-ms2600— generate code for the H8S/2600 🦋 This switch has to be used with-ms💖-mexr— the extended registers get stored on the stack before a function with the monitor attribute executes 📚 This is the default option, and it's only valid for H8S targets 🔥-mno-exr— the extended registers do not get stored on the stack before a function with the monitor attribute executes 🙅♀️ This is the default for that scenario, and it's also only valid for H8S targets 💅-mint32— makes"int"data 32 bits by default 🍓-malign-300— on the H8/300H and H8S, this uses the same alignment rules as the H8/300 ✨ The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries, but-malign-300causes them to be aligned on 2-byte boundaries instead 🌈 This option has no effect on the H8/300 🙈
✨ HPPA Options ✨
These -m options are defined for the HPPA family of computers, so if you're giving HP-UX or PA-RISC energy, this part's for you 🖥️💜
-march=architecture-type— generate code for the architecture you specify 🏛️ The choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors 🔥
To figure out the right architecture option for your machine, refer to /usr/lib/sched.models on an HP-UX system 📖 Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around, so don't be that person 💀
-mpa-risc-1-0,-mpa-risc-1-1,-mpa-risc-2-0— these are synonyms for-march=1.0,-march=1.1, and-march=2.0respectively 👯♀️-mcaller-copies— the caller copies function arguments that get passed by hidden reference 🫱 This option should be used with care, because it is not compatible with the default 32-bit runtime 😬 However, only aggregates larger than eight bytes are passed by hidden reference, and this option gives better compatibility with OpenMP 🫶-mjump-in-delay— this one gets ignored ✋ It's only here for compatibility purposes, so it's basically a vibe with no effect 🙃-mdisable-fpregs— prevents floating-point registers from being used in any manner 🚫 This is necessary for compiling kernels that do lazy context switching of floating-point registers 🧠
If you use this option and then try to do floating-point operations, the compiler aborts. Yep, it just stops. Don't say I didn't warn you 💀
-mdisable-indexing— prevents the compiler from using indexing address modes 🛑 This avoids some rather obscure problems when compiling MIG generated code under MACH 🙈-mno-space-regs— generates code that assumes the target has no space registers 🧃 This lets GCC generate faster indirect calls and use unscaled index address modes ⚡
Such code is suitable for level 0 PA systems and kernels 🖥️
-mfast-indirect-calls— generates code that assumes calls never cross space boundaries 🏃♀️ This lets GCC emit code that performs faster indirect calls ⚡
Heads up though: this option does not work in the presence of shared libraries or nested functions 🚩
-mfixed-range=register-range— generates code that treats the given register range as fixed registers 📌 A fixed register is one that the register allocator cannot use, which is super useful when compiling kernel code 🧩 A register range is specified as two registers separated by a dash, and you can specify multiple register ranges separated by a comma 🍓-mlong-load-store— generates 3-instruction load and store sequences, which are sometimes required by the HP-UX 10 linker 🔗 This is equivalent to the+koption to the HP compilers 💖-mportable-runtime— uses the portable calling conventions HP proposed for ELF systems 🌍-mgas— enables the use of assembler directives that only GAS understands 🧚-mschedule=cpu-type— schedules code according to the constraints for the machine type cpu-type 🗓️ The choices for cpu-type are700 7100,7100LC,7200,7300and8000💅 Refer to/usr/lib/sched.modelson an HP-UX system to determine the proper scheduling option for your machine 📖 The default scheduling is8000✨-mlinker-opt— enables the optimization pass in the HP-UX linker 🔧
Heads up, bestie: this makes symbolic debugging impossible 😭 It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers, where they give bogus error messages when linking some programs 💀
-msoft-float— generates output containing library calls for floating point 🧮
Warning, this one is a whole thing: the requisite libraries are not available for all HPPA targets 🚨 Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation, so you have to make your own arrangements to provide suitable library functions for cross-compilation 🛠️
Also, -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option 🧵 In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float for this to work 🫶
-msio— generates the predefine"_SIO"for server IO 🖥️ The default is-mwsio, which generates the predefines"__hp9000s700","__hp9000s700__"and"_WSIO"for workstation IO 🖱️ These options are available under HP-UX and HI-UX 🌸-mgnu-ld— uses options specific to GNU ld 🐃 This passes-sharedto ld when building a shared library 📚 It's the default when GCC is configured, explicitly or implicitly, with the GNU linker 💜
This option does not affect which ld gets called, it only changes what parameters get passed to that ld 🤷♀️ The ld that gets called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH 🗺️ You can print the linker GCC uses with 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— uses options specific to HP ld 🧡 This passes-bto ld when building a shared library, and passes+Accept TypeMismatchto ld on all links 🔗 It's the default when GCC is configured, explicitly or implicitly, with the HP linker 🖨️
Same tea as before: this option does not affect which ld is called, it only changes what parameters are passed to that ld 🫠 The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH 🗺️ The linker used by GCC can be printed using which `gcc -print-prog-name=ld` 🔍 This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux* 🧠
-mlong-calls— generates 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 (depending on the case) goes past 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 🧵
Honestly, it's normally not desirable to use this option because it degrades performance 📉 But it may be useful in large applications, particularly when partial linking is used to build the application 🏗️
The types of long calls used depend on the capabilities of the assembler and linker, and the type of code being generated 🧩 The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls, should be relatively small 🌷 However, an indirect call is used on 32-bit ELF systems in pic code, and it is quite long 🐢
-munix=unix-std— okay bestie, this one sets up the compiler predefines and picks a startfile for whichever UNIX standard you name. The choices are93,95and98💅93is supported on all HP-UX versions 🦋95is available on HP-UX 10.10 and later98is available on HP-UX 11.11 and later- The defaults are
93for HP-UX 10.00,95for HP-UX 10.10 through 11.00, and98for HP-UX 11.11 and later 🌸 -munix=93gives you the same predefines as GCC 3.3 and 3.4-munix=95adds the predefines"XOPEN_UNIX"and"_XOPEN_SOURCE_EXTENDED", plus the startfileunix95.o✨-munix=98adds the predefines"_XOPEN_UNIX","_XOPEN_SOURCE_EXTENDED","_INCLUDE__STDC_A1_SOURCE"and"_INCLUDE_XOPEN_SOURCE_500", plus the startfileunix98.o👑
⚠️ Be careful with this one, bestie. It changes the interfaces for various library routines and affects the operational behavior of the C library. Thus, extreme care is needed when using this option 😭
Library code that is meant to work with more than one UNIX standard has to test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability 🙈
-nolibdld— this stops the generation of link options that searchlibdld.slwhen-staticis specified on HP-UX 10 and later 🧚-static— the HP-UX version ofsetlocalein libc depends onlibdld.sl, and there isn't an archive version oflibdld.sl. So when you use-static, you need special link options to sort that dependency out 💖 On HP-UX 10 and later, the GCC driver adds those options for you when-staticis used, and this makes the resulting binary dynamic. On the 64-bit port, the linkers make dynamic binaries by default anyway. If you don't want the GCC driver adding these link options, use-nolibdld🫶-threads— adds support for multithreading with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker ✨
🌸 IA-64 Options 🌸
These are the -m options defined for the Intel IA-64 architecture 💜
-mbig-endian— generates code for a big-endian target. This is the default for HP-UX 🍓-mlittle-endian— generates code for a little-endian target. This is the default for AIX5 and GNU/Linux 🦋-mgnu-as,-mno-gnu-as— generates (or doesn't generate) code for the GNU assembler. This is the default 💅-mgnu-ld,-mno-gnu-ld— generates (or doesn't generate) code for the GNU linker. This is the default 💅-mno-pic— generates code that does not use a global pointer register. The result is not position independent code, and it violates the IA-64 ABI 💀-mvolatile-asm-stop,-mno-volatile-asm-stop— generates (or doesn't generate) a stop bit immediately before and after volatile asm statements 🌈-mregister-names,-mno-register-names— generates (or doesn't generate)in,loc, andoutregister names for the stacked registers. This may make assembler output more readable 🧚-mno-sdata,-msdata— disables (or enables) optimizations that use the small data section. This may be useful for working around optimizer bugs 🙈-mconstant-gp— generates code that uses a single constant global pointer value. This is useful when compiling kernel code 👑-mauto-pic— generates code that is self-relocatable. This implies-mconstant-gp, and it's useful when compiling firmware code 🔥-minline-float-divide-min-latency— generates code for inline divides of floating-point values using the minimum latency algorithm ⭐-minline-float-divide-max-throughput— generates code for inline divides of floating-point values using the maximum throughput algorithm ⭐-mno-inline-float-divide— does not generate inline code for divides of floating-point values 🙅-minline-int-divide-min-latency— generates code for inline divides of integer values using the minimum latency algorithm 🎀-minline-int-divide-max-throughput— generates code for inline divides of integer values using the maximum throughput algorithm 🎀-mno-inline-int-divide— does not generate inline code for divides of integer values 🙅-minline-sqrt-min-latency— generates code for inline square roots using the minimum latency algorithm 🌸-minline-sqrt-max-throughput— generates code for inline square roots using the maximum throughput algorithm 🌸-mno-inline-sqrt— does not generate inline code for"sqrt"🙅-mfused-madd,-mno-fused-madd— does (or doesn't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions 🔥-mno-dwarf2-asm,-mdwarf2-asm— doesn't (or does) generate assembler code for the DWARF line number debugging info. This may be useful when you're not using the GNU assembler 🦋-mearly-stop-bits,-mno-early-stop-bits— allows stop bits to be placed earlier than immediately before the instruction that triggered the stop bit. This can improve instruction scheduling, but it doesn't always do so ✨-mfixed-range=register-range— generates code that treats the given register range as fixed registers. A fixed register is one the register allocator cannot use, which is useful when compiling kernel code. A register range is written as two registers separated by a dash, and you can separate multiple register ranges with a comma 💖-mtls-size=tls-size— specifies the bit size of immediate TLS offsets. Valid values are14,22, and64🌈-mtune=cpu-type— tunes the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley🧚-milp32,-mlp64— generates code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits. These are HP-UX specific flags 👑-mno-sched-br-data-spec,-msched-br-data-spec— (dis/en)ables data speculative scheduling before reload. This results in generation of"ld.a"instructions and the corresponding check instructions ("ld.c"/"chk.a"). The default setting is disabled 🦋-msched-ar-data-spec,-mno-sched-ar-data-spec— (en/dis)ables data speculative scheduling after reload. This results in generation of"ld.a"instructions and the corresponding check instructions ("ld.c"/"chk.a"). The default setting is enabled 💅-mno-sched-control-spec,-msched-control-spec— (dis/en)ables control speculative scheduling. This feature is only available during region scheduling (that is, before reload). It results in generation of the"ld.s"instructions and the corresponding check instructions"chk.s". The default setting is disabled 🔥-msched-br-in-data-spec,-mno-sched-br-in-data-spec— (en/dis)ables speculative scheduling of the instructions that depend on the data speculative loads before reload. This is only effective with-msched-br-data-specenabled. The default setting is enabled 🌸-msched-ar-in-data-spec,-mno-sched-ar-in-data-spec— (en/dis)ables speculative scheduling of the instructions that depend on the data speculative loads after reload. This is only effective with-msched-ar-data-specenabled. The default setting is enabled 🌸-msched-in-control-spec,-mno-sched-in-control-spec— (en/dis)ables speculative scheduling of the instructions that depend on the control speculative loads. This is only effective with-msched-control-specenabled. The default setting is enabled 🌸-mno-sched-prefer-non-data-spec-insns,-msched-prefer-non-data-spec-insns— if enabled, data-speculative instructions are only chosen for scheduling when there are no other choices at the moment. This makes the use of data speculation much more conservative. The default setting is disabled 🙈-mno-sched-prefer-non-control-spec-insns,-msched-prefer-non-control-spec-insns— if enabled, control-speculative instructions are only chosen for scheduling when there are no other choices at the moment. This makes the use of control speculation much more conservative. The default setting is disabled 🙈-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies are considered when computing the instruction priorities. This makes the use of speculation a bit more conservative. The default setting is disabled 🙈-msched-spec-ldc— uses a simple data speculation check. This option is on by default ✨-msched-control-spec-ldc— uses a simple check for control speculation. This option is on by default ✨-msched-stop-bits-after-every-cycle— places a stop bit after every cycle when scheduling. This option is on by default ✨-msched-fp-mem-deps-zero-cost— assumes that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default 🍓-msel-sched-dont-check-control-spec— generates checks for control speculation in selective scheduling. This flag is disabled by default 🍓-msched-max-memory-insns=max-insns— sets a limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns that try to schedule in the same instruction group. This is frequently useful for preventing cache bank conflicts. The default value is1🎀-msched-max-memory-insns-hard-limit— makes the limit set bymsched-max-memory-insnsa hard limit, disallowing more than that number in an instruction group. Otherwise the limit is "soft", which means non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled 🎀
🧚 LM32 Options 🧚
These -m options are defined for the LatticeMico32 architecture 💜
-mbarrel-shift-enabled— enables barrel-shift instructions 🦋-mdivide-enabled— enables divide and modulus instructions 🦋-mmultiply-enabled— enables multiply instructions 🦋-msign-extend-enabled— enables sign extend instructions 🦋-muser-enabled— enables user-defined instructions 🦋
🌈 M32C Options 🌈
-mcpu=name— okay so this one picks the CPU you're generating code for 💅 name can ber8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series 🎀
✨ M32R/D Options ✨
These -m options are defined for Renesas M32R/D architectures 🦋
-msim— bestie, this tells GCC the program will run on the simulator 🧚 It links in an alternate runtime library that supports stuff like file I/O. > Never use this on real hardware! You need to provide your own runtime library for whatever I/O functions you need.-memregs=number— this sets how many memory-based pseudo-registers GCC uses during code generation 🫶 They act like real registers, so it's a tradeoff between fitting code into the available registers and the performance hit of using memory instead of registers. Every module in a program has to be compiled with the same value for this option. > Don't use this with GCC's default runtime libraries. It's not it, sis 💀
✨ 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— this is the default 💖 It assumes all objects live in the lower 16MB of memory (so their addresses can be loaded with theld24instruction), and that all subroutines are reachable with theblinstruction. The addressability of a particular object can be set with themodelattribute.-mmodel=medium— objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and all subroutines are reachable with theblinstruction 🌈-mmodel=large— objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and subroutines may not be reachable with theblinstruction. The compiler generates the much slowerseth/add3/jlinstruction sequence 🐢-msdata=none— disables the small data area 🙅♀️ Variables go into one of.data,.bss, or.rodata(unless thesectionattribute has been specified). This is the default. The small data area consists of the sections.sdataand.sbss. Objects can be explicitly put in the small data area with thesectionattribute using one of these sections.-msdata=sdata— puts small global and static data in the small data area, but doesn't generate special code to reference them 🍓-msdata=use— puts small global and static data in the small data area and generates special instructions to reference them ✨-G num— puts global and static objects less than or equal tonumbytes into the small data or BSS sections instead of the normal data or BSS sections 🧁 The default value ofnumis8. The-msdataoption must be set to one ofsdataorusefor this option to have any effect. All modules should be compiled with the same-G numvalue. Mixing different values ofnummay or may not work. If it doesn't, the linker gives an error message---incorrect code is not generated 🫶-mdebug— makes the M32R-specific code in the compiler display some statistics that might help you debug programs 🔍-malign-loops— aligns all loops to a 32-byte boundary 📏-mno-align-loops— doesn't enforce a 32-byte alignment for loops. This is the default 🙃-missue-rate=number— issuesnumberinstructions per cycle.numbercan only be1or2⚡-mbranch-cost=number—numbercan only be1or2. If it's1, branches are preferred over conditional code. If it's2, the opposite applies 🔀-mflush-trap=number— specifies the trap number to use to flush the cache. The default is12. Valid numbers are between0and15inclusive 🪤-mno-flush-trap— specifies that the cache cannot be flushed by using a trap 🚫-mflush-func=name— specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available 📞-mno-flush-func— indicates there is no OS function for flushing the cache 💤
✨ M680x0 Options ✨
These are the -m options defined for M680x0 and ColdFire processors 🌸 The default settings depend on which architecture was selected when the compiler was configured, so the defaults for the most common choices are given below.
-march=arch— generates code for a specific M680x0 or ColdFire instruction set architecture 🏛️ Valid values ofarchfor M680x0 architectures are:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale's ISA classification, and the valid values are:isaa,isaaplus,isabandisac.
GCC defines a macro __mcfarch__ whenever it's generating code for a ColdFire target. The arch in this macro is one of the -march arguments above.
When used together, -march and -mtune select code that runs on a family of similar processors but is optimized for a particular microarchitecture 🧠
-mcpu=cpu— generates code for a specific M680x0 or ColdFire processor 💻 The M680x0 cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are in the table below, which also sorts the CPUs into families 📋
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484
5485-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march get rejected 🚫
GCC defines the macro __mcf_cpu_cpu when a ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above 🪄
-mtune=tune— tunes the code for a particular microarchitecture, within the constraints set by-marchand-mcpu🎯 The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively 🥰
GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.
GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments above 🧁
-m68000,-mc68000— generates output for a 68000. This is the default when the compiler is configured for 68000-based systems. It's equivalent to-march=68000💖 Use this for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.-m68010— generates output for a 68010. This is the default when the compiler is configured for 68010-based systems. It's equivalent to-march=68010🌷-m68020,-mc68020— generates output for a 68020. This is the default when the compiler is configured for 68020-based systems. It's equivalent to-march=68020🌷-m68030— generates output for a 68030. This is the default when the compiler is configured for 68030-based systems. It's equivalent to-march=68030🌷-m68040— generates output for a 68040. This is the default when the compiler is configured for 68040-based systems. It's equivalent to-march=68040🌷 This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this if your 68040 doesn't have code to emulate those instructions 🛠️-m68060— generates output for a 68060. This is the default when the compiler is configured for 68060-based systems. It's equivalent to-march=68060🌷 This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this if your 68060 doesn't have code to emulate those instructions 🛠️-mcpu32— generates output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It's equivalent to-march=cpu32🌷 Use this for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.-m5200— generates output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It's equivalent to-mcpu=5206, and is now deprecated in favor of that option 🥀 Use this for microcontrollers with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.-m5206e— generates output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent-mcpu=5206e🥀-m528x— generates output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent-mcpu=528x🥀-m5307— generates output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent-mcpu=5307🥀
🎀 ColdFire Options, Continued 🎀
-m5407— okay so this one makes output for a ColdFire 5407 CPU 💖 but it's deprecated now, so just use-mcpu=5407instead, it's the same thing ✨-mcfv4e— this is for the ColdFire V4e family (think 547x/548x chips) 🦋 It also lets you use the hardware floating-point instructions. It's equivalent to-mcpu=547x, and it's deprecated, so-mcpu=547xis the move 💅-m68020-40— makes output for a 68040 but doesn't touch any of the new instructions 🌸 The result runs pretty efficiently on a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions that get emulated on the 68040.
This is equivalent to -march=68020 -mtune=68020-40.
-m68020-60— same energy, but for a 68060 🍓 Again no new instructions, and the code runs relatively efficiently on a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions that get emulated on the 68060.
This is equivalent to -march=68020 -mtune=68020-60.
-mhard-float,-m68881— turns on floating-point instructions 🔥 This is the default for the 68020 and up, and for ColdFire devices that have an FPU. It defines the macro__HAVE_68881__on M680x0 targets and__mcffpu__on ColdFire targets.-msoft-float— the opposite vibe 🫶 No floating-point instructions at all, you get library calls instead. This is the default for 68000, 68010, and 68832 targets, and also for ColdFire devices with no FPU.-mdiv,-mno-div—-mdivgenerates ColdFire hardware divide and remainder instructions, and-mno-divskips them 🧚 If-marchis used without-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise the default comes from the target CPU, either the default CPU or whichever one you picked with-mcpu. For example, the default is "off" for-mcpu=5206and "on" for-mcpu=5206e.
GCC defines the macro __mcfhwdiv__ when this option is enabled.
-mshort— treats typeintas 16 bits wide, likeshort int✨ Parameters passed on the stack also get aligned to a 16-bit boundary, even on targets whose API says to promote them to 32-bit.-mno-short— the default, whereintis not 16 bits wide 💜-mnobitfield,-mno-bitfield— no bit-field instructions for you 🙅♀️ The-m68000,-mcpu32, and-m5200options all imply-mnobitfield.-mbitfield— yes to bit-field instructions 💅 The-m68020option implies-mbitfield, and it's the default if you use a configuration designed for a 68020.-mrtd— okay this one is a whole vibe 👑 It switches to a different function-calling convention, where functions with a fixed number of arguments return with thertdinstruction, which pops their arguments while it returns. That saves one instruction in the caller, since the caller doesn't have to pop anything.
But this calling convention is incompatible with the one normally used on Unix, so you can't use it if you need to call libraries compiled with the Unix compiler 😭
You also have to provide function prototypes for every function that takes a variable number of arguments (including printf). Otherwise you get incorrect code for calls to those functions.
Seriously incorrect code results if you call a function with too many arguments. Normally, extra arguments are harmlessly ignored, but not with this one, so be careful 💀
The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060, and CPU32 processors, but not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int,-mno-align-int— controls whether GCC alignsint,long,long long,float,double, andlong doublevariables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int) 🌈 Aligning on 32-bit boundaries makes code run somewhat faster on processors with 32-bit busses, but it costs more memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. Heads up, bestie 🙈
-mpcrel— uses the pc-relative addressing mode of the 68000 directly, instead of going through a global offset table 🦋 At the moment this option implies-fpic, which allows at most a 16-bit offset for pc-relative addressing.-fPICisn't presently supported with-mpcrel, though it could be supported for 68020 and higher processors.-mno-strict-align,-mstrict-align—-mno-strict-alignmeans GCC does not assume that unaligned memory references get handled by the system, and-mstrict-alignmeans it does assume that 🫶-msep-data— generates code that lets the data segment live in a different area of memory from the text segment 🧚 This allows execute-in-place in an environment without virtual memory management. This option implies-fPIC.-mno-sep-data— generates code that assumes the data segment follows the text segment. This is the default 🌸-mid-shared-library— generates code that supports shared libraries through the library ID method ✨ This allows execute-in-place and shared libraries in an environment without virtual memory management. This option implies-fPIC.-mno-id-shared-library— generates code that doesn't assume ID-based shared libraries are being used. This is the default.-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled 💖 Specifying0generates more compact code. Specifying other values forces the allocation of that number to the current library, but that's no more space- or time-efficient than leaving this option out.-mxgot,-mno-xgot— when generating position-independent code for ColdFire,-mxgotgenerates code that works if the GOT has more than 8192 entries 🔥 This code is larger and slower than code generated without this option. On M680x0 processors you don't need this option, since-fPICis enough.
GCC normally uses a single instruction to load values from the GOT. That's pretty efficient, but it only works if the GOT is smaller than about 64k. Anything bigger makes the linker report an error like this:
relocation truncated to fit: R_68K_GOT16O foobarIf that happens, recompile your code with -mxgot, and it should work with very large GOTs. Just know that code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.
Some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have one of those, you should only need -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do 😭
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets— uses 32-bit offsets inswitchtables 🍓 The default is 16-bit offsets.
🦋 MCore Options 🦋
These are the -m options defined for the Motorola M*Core processors 💜
-mhardlit,-mno-hardlit— inlines constants into the code stream if it can be done in two instructions or less ✨-mdiv,-mno-div—-mdivuses the divide instruction. It's enabled by default 💅-mrelax-immediate,-mno-relax-immediate— allows arbitrary-sized immediates in bit operations 🌈-mwide-bitfields,-mno-wide-bitfields—-mwide-bitfieldsalways treats bit-fields asint-sized 🫶-m4byte-functions,-mno-4byte-functions— forces all functions to be aligned to a 4-byte boundary 🧚-mcallgraph-data,-mno-callgraph-data— emits callgraph information 🦋-mslow-bytes,-mno-slow-bytes— prefers word access when reading byte quantities 🌸-mlittle-endian,-mbig-endian— generates code for a little-endian target 💖-m210,-m340— generates code for the 210 processor 🔥-mno-lsim— assumes runtime support has already been provided, so the simulator library (libsim.a) gets left off the linker command line 👑-mstack-increment=size— sets the maximum amount for a single stack increment operation 🍓 Big values can speed up programs with functions that need a lot of stack space, but they can also cause a segmentation fault if the stack gets extended too much. The default value is0x1000.
🧚 MeP Options 🧚
-mabsdiff— enables theabsinstruction, which gives you the absolute difference between two registers 💅-mall-opts— enables all the optional instructions 🔥 That's average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. Full main character energy 👑-maverage— enables theaveinstruction, which computes the average of two registers ✨-mbased=n— variables of sizenbytes or smaller get placed in the.basedsection by default 💖 Based variables use the$tpregister as a base register, and the.basedsection has a 128-byte limit.-mbitops— enables the bit operation instructions: bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas) 🦋-mc=name— selects which section constant data gets placed in 🌸 Thenamecan betiny,near, orfar.-mclip— enables theclipinstruction 🫶 Heads up:-mclipisn't useful unless you also provide-mminmax.-mconfig=name— picks one of the built-in core configurations 🌈 Each MeP chip has one or more modules in it, and each module has a core CPU plus a bunch of coprocessors, optional instructions, and peripherals. The "MeP-Integrator" tool, which is not part of GCC, provides these configurations through this option. Using this option is the same as using all the matching command-line options. The default configuration isdefault.-mcop— enables the coprocessor instructions 🧚 By default this is a 32-bit coprocessor. The coprocessor is normally enabled through the-mconfig=option.-mcop32— enables the 32-bit coprocessor's instructions 🍓-mcop64— enables the 64-bit coprocessor's instructions 🔥-mivc2— enables IVC2 scheduling 💜 IVC2 is a 64-bit VLIW coprocessor.-mdc— causes constant variables to be placed in the.nearsection ✨-mdiv— enables thedivanddivuinstructions 💅-meb— generates big-endian code 👑
-mel
- Hmm, okay so this one is about little-endian code. Bestie,
-meltells the compiler to generate little-endian code 💖 -mio-volatile— okay this one is giving "treat it with care" energy 🫶 It tells the compiler that any variable marked with the "io" attribute is to be considered volatile ✨-ml— causes variables to be assigned to the ".far" section by default 🧚-mleadz— enables the "leadz" (leading zero) instruction 🌸-mm— causes variables to be assigned to the ".near" section by default 🦋-mminmax— enables the "min" and "max" instructions 💅-mmult— enables the multiplication and multiply-accumulate instructions 🔥-mno-opts— disables all the optional instructions enabled by-mall-opts🙈-mrepeat— enables the "repeat" and "erepeat" instructions, used for low-overhead looping 👑-ms— causes all variables to default to the ".tiny" section 💀 Note that there is a 65536-byte limit to this section. Accesses to these variables use the%gpbase register 🌈-msatur— enables the saturation instructions ⭐ Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like "as" 🫶-msdram— links the SDRAM-based runtime instead of the default ROM-based runtime 🍓-msim— links the simulator run-time libraries 🎀-msimnovec— links the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables ✨-mtf— causes all functions to default to the ".far" section 💜 Without this option, functions default to the ".near" section 🌸-mtiny=n— variables that arenbytes or smaller are allocated to the ".tiny" section 🧚 These variables use the$gpbase register. The default for this option is4, but note that there's a 65536-byte limit to the ".tiny" section 😭
✨ MicroBlaze Options ✨
-msoft-float— uses software emulation for floating point (default) 🌈-mhard-float— uses hardware floating-point instructions 🔥-mmemcpy— does not optimize block moves, uses "memcpy" instead 💅-mno-clearbss— this option is deprecated. Use-fno-zero-initialized-in-bssinstead 💀-mcpu=cpu-type— uses features of, and schedules code for, the given CPU 👑 Supported values are in the formatvX.YY.Z, whereXis a major version,YYis the minor version, andZis compatibility code. Example values arev3.00.a,v4.00.b,v5.00.a,v5.00.b,v6.00.a🎀-mxl-soft-mul— uses software multiply emulation (default) 🦋-mxl-soft-div— uses software emulation for divides (default) 🫶-mxl-barrel-shift— uses the hardware barrel shifter 🍓-mxl-pattern-compare— uses pattern compare instructions 🧚-msmall-divides— uses table lookup optimization for small signed integer divisions 🌸-mxl-stack-check— this option is deprecated. Use-fstack-checkinstead 💀-mxl-gp-opt— uses GP-relative ".sdata"/".sbss" sections 💜-mxl-multiply-high— uses multiply high instructions for the high part of a 32x32 multiply 🔥-mxl-float-convert— uses hardware floating-point conversion instructions ✨-mxl-float-sqrt— uses the hardware floating-point square root instruction ⭐-mbig-endian— generates code for a big-endian target 🌈-mlittle-endian— generates code for a little-endian target 💖-mxl-reorder— uses reorder instructions (swap and byte reversed load/store) 👑-mxl-mode-app-model— selects application modelapp-model. Valid models are:executable— normal executable (default), uses startup codecrt0.o🎀-mpic-data-is-text-relative— assumes that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT, since PC-relative addressing is not supported 🦋xmdstub— for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup filecrt1.oand sets the start address of the program to0x800🫶bootstrap— for applications that are loaded using a bootloader. This model uses startup filecrt2.o, which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application 🍓novectors— for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model usescrt3.oas a startup file 🧚
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model 💀
✨ MIPS Options ✨
-EB— generates big-endian code 🌸-EL— generates little-endian code. This is the default formips*el-*-*configurations 💜-march=arch— generates code that runs onarch, which can be the name of a generic MIPS ISA, or the name of a particular processor 👑 The ISA names are:mips1,mips2,mips3,mips4,mips32,mips32r2,mips32r3,mips32r5,mips32r6,mips64,mips64r2,mips64r3,mips64r5andmips64r6. The processor names are:4kc,4km,4kp,4ksc,4kec,4kem,4kep,4ksd,5kc,5kf,20kc,24kc,24kf2_1,24kf1_1,24kec,24kef2_1,24kef1_1,34kc,34kf2_1,34kf1_1,34kn,74kc,74kf2_1,74kf1_1,74kf3_2,1004kc,1004kf2_1,1004kf1_1,i6400,i6500,interaptiv,loongson2e,loongson2f,loongson3a,gs464,gs464e,gs264e,m4k,m14k,m14kc,m14ke,m14kec,m5100,m5101,octeon,octeon+,octeon2,octeon3,orion,p5600,p6600,r2000,r3000,r3900,r4000,r4400,r4600,r4650,r4700,r5900,r6000,r8000,rm7000,rm9000,r10000,r12000,r14000,r16000,sb1,sr71000,vr4100,vr4111,vr4120,vr4130,vr4300,vr5000,vr5400,vr5500,xlrandxlp. The special valuefrom-abiselects the most compatible architecture for the selected ABI (that is,mips1for 32-bit ABIs andmips3for 64-bit ABIs) 🎀
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor 🦋
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r 🫶
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1 🍓
GCC defines two macros based on the value of this option. The first is "_MIPS_ARCH", which gives the name of the target architecture, as a string. The second has the form "_MIPS_ARCH_foo", where foo is the capitalized value of "_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and defines the macro "_MIPS_ARCH_R2000" 🧚
Note that the "_MIPS_ARCH" macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given 🌈
-mtune=arch— optimizes forarch💖 Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list ofarchvalues is the same as for-march.
When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family 👑
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march ones described above 🎀
-mips1— equivalent to-march=mips1🦋-mips2— equivalent to-march=mips2🫶-mips3— equivalent to-march=mips3🍓-mips4— equivalent to-march=mips4🧚-mips32— equivalent to-march=mips32🌸-mips32r3— equivalent to-march=mips32r3💜-mips32r5— equivalent to-march=mips32r5🔥-mips32r6— equivalent to-march=mips32r6✨-mips64— equivalent to-march=mips64⭐-mips64r2— equivalent to-march=mips64r2🌈-mips64r3— equivalent to-march=mips64r3💖-mips64r5— equivalent to-march=mips64r5👑-mips64r6— equivalent to-march=mips64r6🎀-mips16/-mno-mips16— generates (or does not generate) MIPS16 code 🦋 If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes 🫶
-mflip-mips16— generates MIPS16 code on alternating functions 🍓 This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code 💀-minterlink-compressed/-mno-interlink-compressed— requires (or does not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa 🧚
For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed 🌸
-minterlink-mips16/-mno-interlink-mips16— aliases of-minterlink-compressedand-mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility 💜-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generates code for the given ABI 👑
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead 🎀
For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html> 🦋
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors 🫶
The register assignments for arguments and return values stay the same, okay? 💅 But each scalar value now rides in a single 64-bit register instead of a pair of 32-bit ones 🔥. Scalar floating-point values come back in $f0 only, not a $f0/$f1 pair 🦋. The call-saved set doesn't change either, because the even-numbered double-precision registers are still the ones that get saved ✨.
Two extra variants of the o32 ABI exist so you can transition from 32-bit to 64-bit registers 🌈. The first is -mfpxx (FPXX), and it's giving flexibility 🫶. FPXX means all code has to run correctly with 32-bit or 64-bit registers. You can interlink it with FP32 or FP64, but never both 🚫. The second is -mfp64 -mno-odd-spreg (FP64A) 💜. It's basically FP64, except it bans the odd-numbered single-precision registers. Pair it with the "FRE" mode of FPUs in MIPS32R5 processors and FP32 and FP64A code can interlink and run in the same process without changing FPU modes. That's a whole glow-up, no cap 🌸.
-mabicalls,-mno-abicalls— Generates (or doesn't generate) code that fits SVR4-style dynamic objects 🎀.-mabicallsis the default on SVR4-based systems 💖.-mshared,-mno-shared— Generates (or doesn't) code that's fully position-independent, so it can be linked into shared libraries 🦋. This option only affects-mabicalls💅.
All -mabicalls code has traditionally been position-independent, even without options like -fPIC and -fpic 🧚. But as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols ✨. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions 🔥. That's the mode -mno-shared selects 💜.
-mno-shared depends on binutils 2.16 or higher, and the objects it makes can only be linked by the GNU linker 🙈. The ABI of the final executable doesn't change, though. It only affects the ABI of relocatable objects 🫶. Using -mno-shared generally makes executables both smaller and quicker 👑.
-mshared is the default 🌈.
-mplt,-mno-plt— Assumes (or doesn't assume) that the static and dynamic linkers support PLTs and copy relocations 🎀. This only affects-mno-shared-mabicalls💅. For the n64 ABI, this option does nothing without-msym32🙈.
You can make -mplt the default by configuring GCC with --with-mips-plt. Otherwise the default is -mno-plt ✨.
-mxgot,-mno-xgot— Lifts (or doesn't lift) the usual size limits on the global offset table 🔥.
GCC normally uses a single instruction to load values from the GOT 💖. That's pretty efficient, but it only works if the GOT is smaller than about 64k 😭. Anything bigger and the linker reports an error like this:
relocation truncated to fit: R_MIPS_GOT16 foobarIf that happens, recompile your code with -mxgot 🫶. It works with very large GOTs, but the code is less efficient, since it takes three instructions to fetch the value of a global symbol 🐢.
Some linkers can create multiple GOTs. If you have one of those, you usually only need -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few linkers do 🧐.
These options do nothing unless GCC is generating position-independent code 🙈.
-mgp32— Assumes general-purpose registers are 32 bits wide 💜.-mgp64— Assumes general-purpose registers are 64 bits wide ✨.-mfp32— Assumes floating-point registers are 32 bits wide 🌸.-mfp64— Assumes floating-point registers are 64 bits wide 🦋.-mfpxx— Doesn't assume any particular width for floating-point registers 🫶.-mhard-float— Uses floating-point coprocessor instructions 🔥.-msoft-float— Doesn't use floating-point coprocessor instructions. Floating-point math goes through library calls instead 🧚.-mno-float— Same as-msoft-float, but it also swears the program doesn't do any floating-point operations at all 🙅♀️. Right now this is only supported by some bare-metal MIPS configurations, which might pick a special set of libraries with zero floating-point support, including things like the floating-point"printf"formats 💀.
Warning, bestie: if code compiled with -mno-float accidentally has floating-point operations in it, it's likely to fail at link time or at run time 😭.
-msingle-float— Assumes the floating-point coprocessor only supports single-precision operations 🌈.-mdouble-float— Assumes the floating-point coprocessor supports double-precision operations. This is the default 👑.-modd-spreg,-mno-odd-spreg— Enables (or disables) the use of odd-numbered single-precision floating-point registers for the o32 ABI 💅. Odd-numbered registers are on by default for processors known to support them. When you use the o32 FPXX ABI,-mno-odd-spregis set by default 🦋.-mabs=2008,-mabs=legacy— These control how special not-a-number (NaN) IEEE 754 floating-point data gets treated by the"abs.fmt"and"neg.fmt"machine instructions 🎀.
legacy is the default, and also what you get with -mabs=legacy. Here, those instructions count as arithmetic, so they get avoided wherever correct operation matters and the input might be a NaN 🙈. A longer sequence that manipulates the sign bit of the floating-point datum by hand gets used instead, unless -ffinite-math-only is also specified ✨.
-mabs=2008 selects the IEEE 754-2008 treatment. Here, those instructions count as non-arithmetic, so they work correctly in every case, especially when the input operand is a NaN 💖. That's why they're always used for the respective operations 🔥.
-mnan=2008,-mnan=legacy— These control how special not-a-number (NaN) IEEE 754 floating-point data gets encoded 🧚.
-mnan=legacy picks the legacy encoding. Quiet NaNs (qNaNs) are marked by the first bit of their trailing significand field being 0, and signaling NaNs (sNaNs) by that same bit being 1 🌸.
-mnan=2008 picks the IEEE 754-2008 encoding, which flips it. qNaNs have the first bit of their trailing significand field set to 1, and sNaNs have it set to 0 🦋.
The default is -mnan=legacy, unless GCC was configured with --with-nan=2008 👑.
-mllsc,-mno-llsc— Uses (or doesn't use)ll,sc, andsyncinstructions to implement atomic memory built-in functions 💜. If you specify neither option, GCC uses the instructions whenever the target architecture supports them 🫶.
-mllsc helps when the runtime environment can emulate those instructions 🎀. -mno-llsc can help when you're compiling for nonstandard ISAs 🧐. You can make either one the default by configuring GCC with --with-llsc or --without-llsc. --with-llsc is the default for some configurations, so check the installation documentation for details 📚.
-mdsp,-mno-dsp— Uses (or doesn't use) revision 1 of the MIPS DSP ASE 🌈. This option defines the preprocessor macro"__mips_dsp"and sets"__mips_dsp_rev"to1✨.-mdspr2,-mno-dspr2— Uses (or doesn't use) revision 2 of the MIPS DSP ASE 🔥. This option defines the preprocessor macros"__mips_dsp"and"__mips_dspr2", and sets"__mips_dsp_rev"to2💖.-msmartmips,-mno-smartmips— Uses (or doesn't use) the MIPS SmartMIPS ASE 💅.-mpaired-single,-mno-paired-single— Uses (or doesn't use) paired-single floating-point instructions 🦋. This option needs hardware floating-point support to be enabled 🙈.-mdmx,-mno-mdmx— Uses (or doesn't use) MIPS Digital Media Extension instructions 🎀. This only works when generating 64-bit code, and it also needs hardware floating-point support enabled 💜.-mips3d,-mno-mips3d— Uses (or doesn't use) the MIPS-3D ASE 🧚.-mips3dimplies-mpaired-single✨.-mmicromips,-mno-micromips— Generates (or doesn't generate) microMIPS code 🌸.
You can also control microMIPS code generation per function, using the "micromips" and "nomicromips" attributes 🫶.
-mmt,-mno-mt— Uses (or doesn't use) MT Multithreading instructions 👑.-mmcu,-mno-mcu— Uses (or doesn't use) the MIPS MCU ASE instructions 💅.-meva,-mno-eva— Uses (or doesn't use) the MIPS Enhanced Virtual Addressing instructions 🌈.-mvirt,-mno-virt— Uses (or doesn't use) the MIPS Virtualization (VZ) instructions 🔥.-mxpa,-mno-xpa— Uses (or doesn't use) the MIPS eXtended Physical Address (XPA) instructions 💖.-mcrc,-mno-crc— Uses (or doesn't use) the MIPS Cyclic Redundancy Check (CRC) instructions 🦋.-mginv,-mno-ginv— Uses (or doesn't use) the MIPS Global INValidate (GINV) instructions 🫶.-mloongson-mmi,-mno-loongson-mmi— Uses (or doesn't use) the MIPS Loongson MultiMedia extensions Instructions (MMI) 🎀.-mloongson-ext,-mno-loongson-ext— Uses (or doesn't use) the MIPS Loongson EXTensions (EXT) instructions ✨.-mloongson-ext2,-mno-loongson-ext2— Uses (or doesn't use) the MIPS Loongson EXTensions r2 (EXT2) instructions 🧚.-mlong64— Forces"long"types to be 64 bits wide 💜. Check out-mlong32for an explanation of the default and how the pointer size gets determined 📖.-mlong32— Forces"long","int", and pointer types to be 32 bits wide 🌸.
The default size of "int"s, "long"s, and pointers depends on the ABI. Every supported ABI uses 32-bit "int"s 💅. The n64 ABI uses 64-bit "long"s, and so 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— Assumes (or doesn't assume) that all symbols have 32-bit values, no matter which ABI you picked 🦋. This is useful together with-mabi=64and-mno-abicalls, because it lets GCC generate shorter and faster references to symbolic addresses 👑✨.-G num— okay bestie, this one tells GCC to tuck externally-visible data into a little small data section as long as it's no bigger than num bytes 🧚 That lets GCC generate way more efficient accesses to the data, so peep-mgpoptfor the deets ✨ The default-Gvalue depends on the configuration, so it's not one-size-fits-all 💅-mlocal-sdata,-mno-local-sdata— these extend (or don't extend) the-Gbehavior to local data too, like static variables in C 🦋-mlocal-sdatais the default for all configurations 💖 If the linker starts complaining that your app is using too much small data, try rebuilding the less performance-critical parts with-mno-local-sdata😭 You might also wanna build big libraries with-mno-local-sdataso they leave more room for the main program 🫶-mextern-sdata,-mno-extern-sdata— this is where it gets lowkey intense 💀-mextern-sdatameans GCC assumes (and-mno-extern-sdatameans it doesn't assume) that externally-defined data lives in a small data section, as long as its size is within the-Glimit 🌸-mextern-sdatais the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that's no bigger than num bytes, you must make sure Var gets placed in a small data section 🙈 If Var is defined by another module, you either have to compile that module with a high-enough -G setting, or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.
The easiest way to keep everyone happy is to compile and link every module with the same -G option 👑 But if you're building a library that supports several different small data limits, compile it with the highest supported -G setting and also use -mno-extern-sdata so the library stops making assumptions about externally-defined data 🧚
-mgpopt,-mno-gpopt— use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-sdataand-mextern-sdata🍓-mgpoptis the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In those situations, the boot monitor itself is usually compiled with -G0.) -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata ✨
-membedded-data,-mno-embedded-data— allocates variables to the read-only data section first if possible, then to the small data section if possible, and otherwise to data 🌈 The code is slightly slower than the default, but it cuts down on the RAM needed while executing, so some embedded systems might prefer it 🧚-muninit-const-in-rodata,-mno-uninit-const-in-rodata— puts uninitialized"const"variables in the read-only data section 🎀 This only means something when it's used together with-membedded-data.-mcode-readable=setting— tells GCC whether it's allowed to generate code that reads from executable sections 🔥 There are three possible settings:-mcode-readable=yes— instructions can freely access executable sections. This is the default setting 💅-mcode-readable=pcrel— MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM 🦋-mcode-readable=no— instructions must not access executable sections. This can be useful on targets configured to have a dual instruction/data SRAM interface that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM 🫠
-msplit-addresses,-mno-split-addresses— enable (or disable) use of the%hi()and%lo()assembler relocation operators 🧩 This one's been superseded by-mexplicit-relocs, but it's kept around for backwards compatibility 🕰️-mexplicit-relocs,-mno-explicit-relocs— use (or don't use) assembler relocation operators when dealing with symbolic addresses 💜 The alternative, selected by-mno-explicit-relocs, is to use assembler macros instead.-mexplicit-relocsis the default if GCC was configured to use an assembler that supports relocation operators ✨-mcheck-zero-division,-mno-check-zero-division— trap (or don't trap) on integer division by zero 💀 The default is-mcheck-zero-division.-mdivide-traps,-mdivide-breaks— MIPS systems check for division by zero by generating either a conditional trap or a break instruction 🧐 Traps give you smaller code, but they're only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that stops trap from generating the proper signal ("SIGFPE") 😭 Use-mdivide-trapsto allow conditional traps on architectures that support them, and-mdivide-breaksto force the use of breaks 🔥
The default is usually -mdivide-traps, but it can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled with -mno-check-zero-division 🫶
-mload-store-pairs,-mno-load-store-pairs— enable (or disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding 🎀 It's on by default, but it only takes effect when the selected architecture is known to support bonding.-mmemcpy,-mno-memcpy— force (or don't force) the use of"memcpy"for non-trivial block moves 🧚 The default is-mno-memcpy, which lets GCC inline most constant-sized copies ✨-mlong-calls,-mno-long-calls— disable (or don't disable) use of the"jal"instruction 📞 Calling functions with"jal"is more efficient, but it 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 (or disable) use of the"mad","madu"and"mul"instructions, as provided by the R4650 ISA 🦋-mimadd,-mno-imadd— enable (or disable) use of the"madd"and"msub"integer instructions. The default is-mimaddon architectures that support"madd"and"msub", except for the 74k architecture, where it was found to generate slower code 🐌-mfused-madd,-mno-fused-madd— enable (or disable) use of the floating-point multiply-accumulate instructions when they're available 💅 The default is-mfused-madd.
On the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. 🙈
On other processors, the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions ✨
-nocpp— tells the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them 🙅-mfix-24k,-mno-fix-24k— works around the 24K E48 (lost data on stores during refill) errata 🛠️ The workarounds are implemented by the assembler rather than by GCC.
-mfix-r4000,-mno-fix-r4000— works 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— works around certain R4400 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— works around certain R10000 errata:"ll"/"sc"sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900,-mno-fix-r5900— do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a"nop"instruction there instead 🌀 The short loop bug can, under certain conditions, cause loops to execute only once or twice, due to a hardware bug in the R5900 chip 😵 The workaround is implemented by the assembler rather than by GCC.-mfix-rm7000,-mno-fix-rm7000— works around the RM7000"dmult"/"dmultu"errata. The workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120,-mno-fix-vr4120— works 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— works around the VR4130"mflo"/"mfhi"errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using"mflo"and"mfhi"if the VR4130"macc","macchi","dmacc"and"dmacchi"instructions are available instead.-mfix-sb1,-mno-fix-sb1— works around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2"F1"and"F2"floating-point errata.) 💜-mr10k-cache-barrier=setting— specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors 🔥
Okay so the R10K is lowkey a chaotic processor 💀 It tries to guess which way a conditional branch will go and starts running instructions down the "taken" path before it knows. If the guess was wrong, it throws those instructions away, but even the aborted ones can leave marks behind 🙈
This mostly hurts kernel stores and, depending on the system, kernel loads 😭 Picture a speculative store pulling its target memory into cache and marking that line as dirty, even though the store got cancelled. If a DMA operation writes to that same memory before the dirty line gets flushed, the cached data overwrites the DMA data. Yikes. For the full tea, including other problems, check the R10K processor manual 📖✨
One workaround is to drop cache barrier instructions in front of every memory access that might run speculatively and might have side effects even when aborted. -mr10k-cache-barrier=setting controls how GCC does this. It assumes that aborted accesses to any byte in these regions have no side effects:
- the memory taken up by the current function's stack frame 🥞
- the memory taken up by an incoming stack argument 📦
- the memory taken up by an object with a link-time-constant address 🔒
It is the kernel's job to make sure speculative accesses to these regions are actually safe 🫶
If your input program has a function declaration like this:
void foo (void);then the implementation of foo has to let j foo and jal foo run speculatively 👑 GCC follows this rule for functions it compiles itself. It expects non-GCC functions, like hand-written assembly, to do the same thing 💅
The option comes in three flavours:
-mr10k-cache-barrier=load-store— Inserts a cache barrier before a load or store that might run speculatively and might have side effects even if it gets aborted ✨-mr10k-cache-barrier=store— Inserts a cache barrier only before a store that might run speculatively and might have side effects even if it gets aborted 🎀-mr10k-cache-barrier=none— Turns off cache barrier insertion entirely. This is the default setting 🌸-mflush-func=func,-mno-flush-func— Picks the function to call for flushing the I and D caches, or tells GCC to not call any flush function at all 🧹 If you do call one, it has to take the same arguments as the common_flush_func: the address of the memory range being flushed, the size of that range, and the number3(to flush both caches) 🔥 The default depends on the target GCC was configured for, but it's commonly either_flush_funcor__cpu_flush🦋mbranch-cost=num— Sets the cost of branches to roughlynum"simple" instructions 💜 This is only a heuristic, so don't expect it to give the same results across releases 🙈 A cost of zero just picks the default, which is based on the-mtunesetting 🍓-mbranch-likely,-mno-branch-likely— Turns Branch Likely instructions on or off, no matter what the selected architecture defaults to 🌈 By default they may get generated if the selected architecture supports them. The exception is the MIPS32 and MIPS64 architectures, plus processors that implement them. For those, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate them 🚫-mcompact-branches=never— Makes sure compact branch instructions never get generated 🙅♀️-mcompact-branches=optimal— Uses a delay slot branch if one is available in the current ISA and the delay slot gets successfully filled. If the delay slot isn't filled, a compact branch gets picked instead, as long as one is available 🧠 This is the default ✨-mcompact-branches=always— Makes sure a compact branch instruction gets generated whenever one is available. If none is available, a delay slot form of the branch gets used instead 💖
These options control which form of branches gets generated. The default is -mcompact-branches=optimal. Also, the always option is only supported from MIPS Release 6 onwards 🎀
-mfp-exceptions,-mno-fp-exceptions— Says whether FP exceptions are enabled. This affects how FP instructions get scheduled on some processors 🔥 FP exceptions are enabled by default 💅
For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, we can use both FP pipes. Otherwise we can only use one FP pipe 🫠
-mvr4130-align,-mno-vr4130-align— The VR4130 pipeline is two-way superscalar, but it can only issue two instructions together if the first one is 8-byte aligned 🧩 When this option is enabled, GCC aligns pairs of instructions it thinks should run in parallel 🦋
This option only does something when you're optimizing for the VR4130 ✨ It usually makes code faster, but the tradeoff is that the code gets bigger 📈 It's enabled by default at optimization level -O3 👑
-msynci,-mno-synci— Enables (or disables) generatingsynciinstructions on architectures that support them 🌸 Thesynciinstructions, if enabled, get generated when__builtin___clear_cacheis compiled 🧹
This option defaults to -mno-synci, but you can override that default by configuring GCC with --with-synci 🔧
When compiling code for single processor systems, it's generally safe to use synci. But on many multi-core (SMP) systems, it doesn't invalidate the instruction caches on all cores and may lead to undefined behavior 💀 Be careful, bestie!
-mrelax-pic-calls,-mno-relax-pic-calls— Tries to turn PIC calls that are normally dispatched via register$25into direct calls 🦋 This only works if the linker can resolve the destination at link time and the destination is within range for a direct call 🎯
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive, and -mexplicit-relocs is in effect 🌈 With -mno-explicit-relocs, this optimization can be done by the assembler and the linker alone, without help from the compiler 🫶
-mmcount-ra-address,-mno-mcount-ra-address— Emits (or doesn't emit) code that lets_mcountmodify the calling function's return address 🔮 When enabled, this option extends the usual_mcountinterface with a new ra-address parameter. It has typeintptr_t *and is passed in register$12💜_mcountcan then change the return address by doing both of the following:- Returning the new address in register
$31🎀 - Storing the new address in
*ra-address, if ra-address is nonnull ✨
- Returning the new address in register
The default is -mno-mcount-ra-address 🙈
-mframe-header-opt,-mno-frame-header-opt— Enables (or disables) frame header optimization in the o32 ABI 🥞 When you use the o32 ABI, calling functions allocate 16 bytes on the stack for the called function to write out register arguments. When this optimization is enabled, it suppresses that frame header allocation if it can be determined that the header is unused 🧠
This optimization is off by default at all optimization levels 😭
-mlxc1-sxc1,-mno-lxc1-sxc1— When applicable, enables (or disables) generatinglwxc1,swxc1,ldxc1, andsdxc1instructions 💖 Enabled by default ✨-mmadd4,-mno-madd4— When applicable, enables (or disables) generating 4-operandmadd.s,madd.d, and related instructions 🌈 Enabled by default 👑
🌈 MMIX Options 🌈
These options are defined for the MMIX 🧚
-mlibfuncs,-mno-libfuncs— Specifies that intrinsic library functions are being compiled, passing all values in registers no matter the size 📚-mepsilon,-mno-epsilon— Generates floating-point comparison instructions that compare with respect to therEepsilon register 🔢-mabi=mmixware,-mabi=gnu—-mabi=mmixwaregenerates code that passes function parameters and return values which, in the called function, are seen as registers$0and up. The GNU ABI, by contrast, uses global registers$231and up 🌸-mzero-extend,-mno-zero-extend— When reading data from memory in sizes shorter than 64 bits, uses (or doesn't use) zero-extending load instructions by default, instead of sign-extending ones 🦋-mknuthdiv,-mno-knuthdiv—-mknuthdivmakes 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, though the latter is almost exclusively used 💅-mtoplevel-symbols,-mno-toplevel-symbols— Prepends (or doesn't prepend) a:to all global symbols, so the assembly code can be used with thePREFIXassembly directive 🎀-melf— Generates an executable in the ELF format, instead of the default mmo format used by the mmix simulator 🫧-mbranch-predict,-mno-branch-predict— Uses (or doesn't use) the probable-branch instructions when static branch prediction indicates a probable branch 🔮-mbase-addresses,-mno-base-addresses— Generates (or doesn't generate) code that uses base addresses 🏠 Using a base address automatically generates a request, handled by the assembler and the linker, for a constant to be set up in a global register. That register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. So a program that uses lots of static data may require-mno-base-addresses📦-msingle-exit,-mno-single-exit— Forces (or doesn't 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— Generates code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default 🛡️-mno-mult-bug— okay so this one tells GCC to not generate code that works around bugs in the multiply instructions on MN10300 processors 💅-mam33— generate code using AM33-specific features ✨-mno-am33— the opposite: no AM33-specific features. This is the default 🌸-mam33-2— generate code using features specific to the AM33/2.0 processor 🦋-mam34— generate code using AM34-specific features 💜-mtune=cpu-type— bestie, this tells the scheduler which CPU's timing to follow when it lines up instructions. It does not change the target processor, so it's giving scheduling tips, not a glow-up. The CPU type has to bemn10300,am33,am33-2oram34🎀-mreturn-pointer-on-d0— when a function returns a pointer, it gets returned in both"a0"and"d0". Otherwise the pointer only lives in"a0", and calling those functions without a prototype throws errors 💀. This one is on by default, but you can switch it off with-mno-return-pointer-on-d0🙈-mno-crt0— don't link in the C run-time initialization object file 🧚-mrelax— tells the linker to run a relaxation pass that shortens branches, calls and absolute memory addresses ✨. It only does anything when it's on the command line for the final link step.
This option makes symbolic debugging impossible. Choose wisely, bestie 😭
-mliw— lets the compiler generate Long Instruction Word instructions when the target is AM33 or later. This is the default, and it defines the preprocessor macro"__LIW__"🌈-mno-liw— no LIW instructions allowed. This one defines the preprocessor macro"__NO_LIW__"🫶-msetlb— lets the compiler generateSETLBandLccinstructions when the target is AM33 or later. Also the default, and it defines"__SETLB__"🔥-mno-setlb— noSETLBorLccinstructions. Defines"__NO_SETLB__"🥀
✨ Moxie Options ✨
-meb— big-endian code. This is the default formoxie-*-*configurations 🍓-mel— little-endian code 🌸-mmul.x— generatemul.xandumul.xinstructions. This is the default formoxiebox-*-*configurations 👑-mno-crt0— don't link in the C run-time initialization object file 🦋
🧚 MSP430 Options 🧚
These options are defined for the MSP430, so they're giving MSP430 realness 💖
-masm-hex— forces assembly output to always use hex constants. Normally they show up as signed decimals, but this one is there for testsuite and/or aesthetic purposes 🎀-mmcu=— picks the MCU you're targeting. GCC makes a C preprocessor symbol from the MCU name, in upper case, wrapped in__on both sides. Themsp430.hheader then uses that to pick an MCU-specific supplementary header file ✨- It also sets the ISA. If the MCU is one that only supports the 430 ISA, that's what gets selected. Otherwise the 430X ISA gets the spotlight. The generic
msp430name selects the 430 ISA, and the genericmsp430xname selects the 430X ISA 🌟 - An MCU-specific linker script gets added to the linker command line. The script is named after the MCU with
.ldtacked on, so-mmcu=xxxdefines the C preprocessor symbol"__XXX__"and makes the linker search for a script calledxxx.ld💅 - The ISA and hardware multiply support for each MCU is hard-coded into GCC. But an external
devices.csvfile can extend device support past the hard-coded ones 🦋 - GCC searches for
devices.csvin this order of precedence, where the first method beats the second and the second beats the third:- Include paths from
-Iand-L:devices.csvgets searched in every directory on the include paths and linker library search paths 🔍 - The
MSP430_GCC_INCLUDE_DIRenvironment variable: set it to the full path of the directory holdingdevices.csv, and GCC searches there. If it finds the file, that directory also gets registered as an include path and a linker library path. That means headers and linker scripts in there work without manually specifying-Iand-Lon the command line 🫶 msp430-elf{,bare}/include/devices: GCC checks this directory under the toolchain root. It doesn't exist in a default installation, but if you made it and copieddevices.csvin there, the MCU data gets read. Same deal as above, so this directory also gets registered as an include path and a linker library path 🌸
- Include paths from
- If none of those search methods find
devices.csv, the hard-coded MCU data is used instead 💀
- It also sets the ISA. If the MCU is one that only supports the 430 ISA, that's what gets selected. Otherwise the 430X ISA gets the spotlight. The generic
-mwarn-mcu,-mno-warn-mcu— these turn warnings on or off for conflicts between the MCU name from-mmcuand the ISA set by-mcpuand/or the hardware multiply support set by-mhwmult. They also toggle warnings about unrecognized MCU names. On by default 🙈-mcpu=— specifies the ISA to use. Accepted values aremsp430,msp430xandmsp430xv2.
This option is deprecated! Use -mmcu= to select the ISA instead 😭
-msim— links to the simulator runtime libraries and linker script. It overrides any scripts that-mmcu=would have selected 🔥-mlarge— uses large-model addressing, with 20-bit pointers and a 20-bit"size_t"🌈-msmall— uses small-model addressing, with 16-bit pointers and a 16-bit"size_t"🍓-mrelax— passed to the assembler and linker. It lets the linker do certain optimizations that can only happen at the final link ✨mhwmult=— describes the type of hardware multiply the target supports. Accepted values are:nonefor no hardware multiply16bitfor the original 16-bit-only multiply on early MCUs32bitfor the 16/32-bit multiply on later MCUsf5seriesfor the 16/32-bit multiply on F5-series MCUsauto, which tells GCC to figure out the hardware multiply support from the MCU name given by-mmcu. If there's no-mmcuoption, or the MCU name isn't recognized, no hardware multiply support is assumed."auto"is the default setting 🧠
Hardware multiplies usually get handled by calling a library routine, which saves space in the generated code. When you compile at -O3 or higher, though, the hardware multiplier is invoked inline. That makes the code bigger but faster 🏎️
The hardware multiply routines disable interrupts while they run and restore the previous interrupt state when they finish. That makes them safe to use inside interrupt handlers as well as in normal code 👑
-minrt— enables a minimum runtime environment, with no static initializers or constructors. It's meant for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required 🦋-mtiny-printf— enables reduced code size"printf"and"puts"library functions. The tiny versions are not reentrant, so be careful with them in multi-threaded applications 💅
Stream support has been removed, so the string to print always goes to stdout via the "write" syscall. The string isn't buffered before it's sent to write. This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io 🌸
-mmax-inline-shift=— takes an integer from 0 to 64 inclusive, and sets the maximum number of inline shift instructions that get emitted to do a shift by a constant amount. Once that limit would be exceeded, an mspabi helper function is used instead. The default is 4 ✨
This only matters when a shift by multiple positions can't be finished with a single instruction, like all shifts >1 on the 430 ISA. Shifts of a 32-bit value are at least twice as costly, so the value you pass gets divided by 2 and the result is used instead 🧮
-mcode-region=,-mdata-region=— these tell the compiler where to place functions and data that don't have one of the"lower","upper","either"or"section"attributes. Possible values are"lower","upper","either"or"any". The first three behave like the matching attribute."any"is the default, and it leaves placement entirely up to the linker script and how it assigns the standard sections (".text",".data", etc.) to memory regions 🗺️-msilicon-errata=— passes a request to the assembler to enable fixes for the named silicon errata 🔧-msilicon-errata-warn=— passes a request to the assembler to enable warning messages when a silicon erratum might need to be applied ⚠️-mwarn-devices-csv,-mno-warn-devices-csv— warn ifdevices.csvisn't found, or if there are problems parsing it. Default is on 👀
🌈 NDS32 Options 🌈
These options are defined for NDS32 implementations, so they're giving NDS32 vibes 💜
-mbig-endian— generate code in big-endian mode 🍓-mlittle-endian— generate code in little-endian mode 🌸-mreduced-regs— use reduced-set registers for register allocation 🦋-mfull-regs— use full-set registers for register allocation 👑-mcmov— generate conditional move instructions ✨-mno-cmov— do not generate conditional move instructions 🙈-mext-perf— generate performance extension instructions 🔥-mno-ext-perf— do not generate performance extension instructions 💅-mext-perf2— generate performance extension 2 instructions 🚀
-mno-ext-perf2 🥀 Nope, no performance extension 2 instructions for this build, bestie. 🙅♀️
-mext-string— ✨ Turns on the string extension instructions.-mno-ext-string— 🚫 Keeps the string extension instructions out.-mv3push— 🎀 Generates the v3push25/pop25instructions.-mno-v3push— 🙈 Skips the v3push25/pop25instructions.-m16-bit— 🦋 Generates 16-bit instructions.-mno-16-bit— 💤 No 16-bit instructions for you.-misr-vector-size=num— 👑 Sets how big each interrupt vector is. It has to be4or16, no other vibes allowed.-mcache-block-size=num— 🧊 Sets the size of each cache block. It must be a power of 2 between4and512, so pick wisely.-march=arch— 🗺️ Names the target architecture.-mcmodel=code-model— 🧠 Picks the code model. You choose one of these:small— ✨ All the data and read-only data segments have to live within 512KB of addressing space. The text segment has to stay within 16MB of addressing space.medium— 🌸 The data segment has to stay within 512KB, but the read-only data segment can stretch across 4GB of addressing space. The text segment should still stay within 16MB of addressing space.large— 🌈 All the text and data segments can spread across 4GB of addressing space. Main character energy.
-mctor-dtor— 🧚 Enables the constructor/destructor feature.-mrelax— 🪄 Tells the linker to relax instructions.
✨ Nios II Options ✨
These are the options for the Altera Nios II processor, so if you're building for Nios II, this part is literally for you. 💖
-G num— 🍓 Puts global and static objects that arenumbytes or smaller into the small data or BSS sections, instead of the normal data or BSS sections. The default value ofnumis8.-mgpopt=option,-mgpopt,-mno-gpopt— 💜 Generates (or doesn't generate) GP-relative accesses. These option names are recognized:none— 🙅♀️ Doesn't generate GP-relative accesses at all.local— 🫶 Generates GP-relative accesses for small data objects that aren't external, weak, or uninitialized common symbols. It also uses GP-relative addressing for objects you've explicitly placed in a small data section with asectionattribute.global— 🔥 Same aslocal, but it also generates GP-relative accesses for small data objects that are external, weak, or common. If you use this one, every part of your program (libraries included) has to be compiled with the same-Gsetting, no exceptions.data— 🌷 Generates GP-relative accesses for all data objects in the program. If you go with this, the entire data and BSS segments of your program have to fit in 64K of memory, and you need an appropriate linker script to allocate them within the addressable range of the global pointer.all— 👑 Generates GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program have to fit in 64K of memory, and you need an appropriate linker script to allocate them within the addressable range of the global pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.
The default is -mgpopt, except when -fpic or -fPIC is specified to generate position-independent code. Heads up though: the Nios II ABI does not allow GP-relative accesses from shared libraries. 🙃
You might need to specify -mno-gpopt explicitly when building programs with lots of small data, including large GOT data sections. In that case, the 16-bit offset for GP-relative addressing might not be big enough to reach the entire small data section. 😭
-mgprel-sec=regexp— 🧭 Adds extra section names that can be accessed via GP-relative addressing. It's most useful alongsidesectionattributes on variable declarations and a custom linker script. Theregexpis a POSIX Extended Regular Expression.
This option doesn't change how -G behaves. The sections you list here come in addition to the standard .sdata and .sbss small-data sections that -mgpopt already recognizes.
-mr0rel-sec=regexp— 🎯 Names sections that can be reached via a 16-bit offset fromr0, meaning the low 32K or high 32K of the 32-bit address space. It's most useful alongsidesectionattributes on variable declarations and a custom linker script. Theregexpis a POSIX Extended Regular Expression.
Unlike GP-relative addressing for small data, zero-based addressing is never generated by default. Also, there are no conventional section names in standard linker scripts for the low or high areas of memory, so you're on your own there. 🫠
-mel,-meb— 🌊 Generates little-endian (default) or big-endian (experimental) code, respectively.-march=arch— 🧬 Specifies the name of the target Nios II architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. The permissible names arer1andr2.
The preprocessor macro __nios2_arch__ is available to your programs, with a value of 1 or 2, which tells you the targeted ISA level. ✨
-mbypass-cache,-mno-bypass-cache— 🏃♀️ Forces all load and store instructions to always bypass the cache by using I/O variants of the instructions. The default is not to bypass the cache.-mno-cache-volatile,-mcache-volatile— 🌀 Volatile memory access bypasses the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.-mno-fast-sw-div,-mfast-sw-div— 🐢 Doesn't use table-based fast divide for small numbers. The default is to use the fast divide at-O3and above.-mno-hw-mul,-mhw-mul,-mno-hw-mulx,-mhw-mulx,-mno-hw-div,-mhw-div— ⚡ Enables or disables the compiler emitting themul,mulx, anddivfamily of instructions. The default is to emitmuland to not emitdivandmulx.-mbmx,-mno-bmx,-mcdx,-mno-cdx— 🧩 Enables or disables generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Turning these on also requires-march=r2. Since these are optional extensions to the R2 architecture, the default is not to emit them.-mcustom-insn=N,-mno-custom-insn— 🛠️ Each-mcustom-insn=Noption enables use of a custom instruction with encodingNwhen generating code that usesinsn. For example,-mcustom-fadds=253generates custom instruction253for single-precision floating-point add operations, instead of the default behavior of using a library call.
Here are the supported values of insn. Unless noted otherwise, floating-point operations are expected to use normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions. 🤓
Single-precision floating point:
fadds,fsubs,fdivs,fmuls— ➕ Binary arithmetic operations.fnegs— ➖ Unary negation.fabss— 📏 Unary absolute value.fcmpeqs,fcmpges,fcmpgts,fcmples,fcmplts,fcmpnes— ⚖️ Comparison operations.fmins,fmaxs— 🏔️ Floating-point minimum and maximum. These instructions are only generated if-ffinite-math-onlyis specified.fsqrts— 🌱 Unary square root operation.fcoss,fsins,ftans,fatans,fexps,flogs— 🌀 Floating-point trigonometric and exponential functions. These instructions are only generated if-funsafe-math-optimizationsis also specified.
Double-precision floating point:
faddd,fsubd,fdivd,fmuld— ➕ Binary arithmetic operations.fnegd— ➖ Unary negation.fabsd— 📏 Unary absolute value.fcmpeqd,fcmpged,fcmpgtd,fcmpled,fcmpltd,fcmpned— ⚖️ Comparison operations.fmind,fmaxd— 🏔️ Double-precision minimum and maximum. These instructions are only generated if-ffinite-math-onlyis specified.fsqrtd— 🌱 Unary square root operation.fcosd,fsind,ftand,fatand,fexpd,flogd— 🌀 Double-precision trigonometric and exponential functions. These instructions are only generated if-funsafe-math-optimizationsis also specified.
Conversions:
fextsd— 🔼 Conversion from single precision to double precision.ftruncds— 🔽 Conversion from double precision to single precision.fixsi,fixsu,fixdi,fixdu— ✂️ Conversion from floating point to signed or unsigned integer types, with truncation towards zero.round— 🎯 Conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the__builtin_lroundffunction when-fno-math-errnois used.floatis,floatus,floatid,floatud— 🔁 Conversion from signed or unsigned integer types to floating-point types.
Also, all of the following transfer instructions for the internal registers X and Y must be provided in order to use any of the double-precision floating-point instructions. 🧠 Custom instructions that take two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or the only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. 🤖
fwrx— ✍️ Writessrc1into the least significant half ofXandsrc2into the most significant half ofX.fwry— ✍️ Writessrc1intoY.frdxhi,frdxlo— 📖 Reads the most or least (respectively) significant half ofXand stores it indest.frdy— 📖 Reads the value ofYand stores it intodest.
Note that you can get more local control over generation of Nios II custom instructions by using thetarget("custom-insn=N")andtarget("no-custom-insn")function attributes or pragmas. 💅
-mcustom-fpu-cfg=name Okay so this option is bestie's go-to for picking a whole pre-made bundle of custom instruction encodings, named and ready to go 💅✨ (see -mcustom-insn above for the deets). Here's the current lineup:
Okay the first one, -mcustom-fpu-cfg=60-1, is basically the same as: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant 🎀
And -mcustom-fpu-cfg=60-2 is giving: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant 🦋
Then there's -mcustom-fpu-cfg=72-3, which is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant 👑
And last but not least, -mcustom-fpu-cfg=fph2, which is equivalent to: -mcustom-fabss=224 -mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227 -mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230 -mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233 -mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250 -mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 🌈
ngl, the explicit-mcustom-insn=options always win over-mcustom-fpu-cfg=, no matter what order you type them on the command line. Main character energy for your own picks 👑
Lowkey wild: fph2 is short for Nios II Floating Point Hardware 2 Component 🧚. Also, the custom instructions -mcustom-fmins=233 and -mcustom-fmaxs=234 only get generated if -ffinite-math-only is specified 🙈. The custom instruction -mcustom-round=248 only shows up if -fno-math-errno is specified 💀. And unlike the other configs, -fsingle-precision-constant is not set for fph2 🫶.
Heads up: you can get more local control over which FPU config is used by using the target("custom-fpu-cfg=name") function attribute or pragma 💜.
These additional -m options are available for the Altera Nios II ELF (bare-metal) target 🌸:
-mhal— Link with the HAL BSP. This skips linking the GCC-provided C runtime startup and termination code, and is usually paired with-msys-crt0=to point at the alternate startup code from the HAL BSP 🦋-msmallc— Link with a limited version of the C library,-lsmallc, instead of Newlib 🎀-msys-crt0=startfile— startfile is the file name of the startup file (crt0) to use when linking. Only useful alongside-mhal💅-msys-lib=systemlib— systemlib is the library name for the low-level system calls the C library needs, like "read" and "write". Usually used to link with a library from a HAL BSP ✨
✨ Nvidia PTX Options ✨
These options are defined for Nvidia PTX 💖:
-m64— Ignored, but kept around for backward compatibility. Only the 64-bit ABI is supported 🔥-misa=ISA-string— Generate code for the specified PTX ISA (e.g.sm_35). ISA strings must be lower-case. Valid ISA strings includesm_30andsm_35. The default ISA issm_35🌈-mmainkernel— Link in code for a__mainkernel. This is for stand-alone execution instead of offloading 🧚-moptimize— Apply partitioned execution optimizations. This is the default whenever any level of optimization is selected 🍓-msoft-stack— Generate code that doesn't use ".local" memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler. To generate code suitable for linking into programs using OpenMP offloading, use option-mgompinstead 🫶-muniform-simt— Switch to a code generation variant that lets all threads in each warp execute, while keeping memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory arrayint __nvptx_uni[]stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at positiontid.ywith current lane index to compute the master lane index 🔮-mgomp— Generate code for use in OpenMP offloading. This enables-msoft-stackand-muniform-simtand selects the corresponding multilib variant 👑
✨ OpenRISC Options ✨
These options are defined for OpenRISC 🌸:
-mboard=name— Configure a board-specific runtime. This gets passed to the linker for newlib board library linking. The default is "or1ksim" 🦋-mnewlib— Ignored, it's only here for compatibility. This used to select linker and preprocessor options for use with newlib 🎀-msoft-div,-mhard-div— Pick software or hardware divide (l.div,l.divu) instructions. The default is hardware divide 💅-msoft-mul,-mhard-mul— Pick software or hardware multiply (l.mul,l.muli) instructions. The default is hardware multiply ✨-msoft-float,-mhard-float— Pick software or hardware for floating point operations. The default is software 💖-mdouble-float— When-mhard-floatis selected, enables generation of double-precision floating point instructions. By default, functions from libgcc are used for double-precision floating point operations 🔥-munordered-float— When-mhard-floatis selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default, functions from libgcc are used for unordered floating point compare and set flag operations 🌈-mcmov— Enable generation of conditional move (l.cmov) instructions. By default the equivalent gets generated using set and branch 🧚-mror— Enable generation of rotate right (l.ror) instructions. By default, functions from libgcc are used for rotate right operations 🍓-mrori— Enable generation of rotate right with immediate (l.rori) instructions. By default, functions from libgcc are used for rotate right with immediate operations 🫶-msext— Enable generation of sign extension (l.ext*) instructions. By default, memory loads are used to do sign extension 🔮-msfimm— Enable generation of compare and set flag with immediate (l.sf*i) instructions. By default, extra instructions get generated to store the immediate to a register first 👑-mshftimm— Enable generation of shift with immediate (l.srai,l.srli,l.slli) instructions. By default, extra instructions get generated to store the immediate to a register first 🌸
✨ PDP-11 Options ✨
These options are defined for the PDP-11 🦋:
-mfpu— Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies-m45🎀-msoft-float— Don't use hardware floating point 💅-mac0— Return floating-point results in ac0 (fr0 in Unix assembler syntax) ✨-mno-ac0— Return floating-point results in memory. This is the default 💖-m40— Generate code for a PDP-11/40. Implies-msoft-float -mno-split🔥-m45— Generate code for a PDP-11/45. This is the default 🌈-m10— Generate code for a PDP-11/10. Implies-msoft-float -mno-split🧚-mint16,-mno-int32— Use 16-bit "int". This is the default 🍓-mint32,-mno-int16— Use 32-bit "int" 🫶-msplit— Target has split instruction and data space. Implies-m45🔮-munix-asm— Use Unix assembler syntax 👑-mdec-asm— Use DEC assembler syntax 🌸-mgnu-asm— Use GNU assembler syntax. This is the default 🦋-mlra— Use the new LRA register allocator. By default, the old "reload" allocator is used 🎀
✨ picoChip Options ✨
These -m options are defined for picoChip implementations 💅:
-mae=ae_type— Set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported values for ae_type areANY,MUL, andMAC✨-mae=ANYselects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE 💖-mae=MULselects a MUL AE type. This is the most useful AE type for compiled code, and is the default 🔥-mae=MACselects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores 🌈
-msymbol-as-address— Lets the compiler directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Usually this option makes bigger programs that run faster than when it isn't used. However, results vary from program to program, so it's left as a user option rather than being permanently enabled 🧚-mno-inefficient-warnings— Disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient, and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings 🍓
✨ PowerPC Options ✨
These are listed under
💖 PRU Options 💖
okay bestie, these flags are only for the PRU target 🦋 so only reach for them if you're building for PRU ✨
-minrt— links with a minimum runtime 🧚 with no support for static initializers and constructors. This can seriously shrink your final ELF binary 📉
⚠️ Warning: the compiler could still generate code with static initializers and constructors. It's on you to make sure your source program doesn't use those features 💀
-mmcu=mcu— picks the PRU MCU variant you want 💅 For the exact list of supported MCUs, check Newlib 📚-mno-relax— GCC passes--no-relaxto the linker instead of--relax🙈-mloop— lets GCC use the LOOP instruction, or doesn't, depending on whether you turn it on or off 🔁-mabi=variant— picks the ABI variant to output code for.-mabi=tigives you the unmodified TI ABI 🧸, while-mabi=gnugives you a GNU variant that plays nicer with certain GCC assumptions 🌈 Here's how they're different:
🌸 Function Pointer Size
TI ABI says function (code) pointers are 16-bit, but GNU only supports 32-bit data and code pointers 💜
🦋 Optional Return Value Pointer
Function return values bigger than 64 bits get passed through a hidden pointer as the function's first argument. TI ABI says that pointer is allowed to be NULL if the caller doesn't care about the returned value. GNU always passes and expects a valid return value pointer ✨
The current -mabi=ti implementation just throws a compile error whenever any of the above code constructs shows up 😭 Because of that, the standard C library can't be built, so it gets left out when you link with -mabi=ti.
Relaxation is a GNU feature, so for safety reasons it's turned off when you use -mabi=ti 🛡️ The TI toolchain doesn't emit relocations for QBBx instructions, which means the GNU linker can't adjust them when it shortens adjacent LDI32 pseudo instructions. Not it 🙅♀️
🧚 RISC-V Options 🧚
these flags are defined for RISC-V targets 🌿
-mbranch-cost=n— sets the cost of branches to roughlyninstructions 🍓-mplt,-mno-plt— when generating PIC code, these decide whether PLTs are allowed or not 🦋 They get ignored for non-PIC. The default is-mplt.-mabi=ABI-string— sets the integer and floating-point calling convention 💖 An ABI string has two parts: the size of integer types, and the registers used for floating-point types. For example,-march=rv64ifd -mabi=lp64dmeanslongand pointers are 64-bit (sointis implicitly 32-bit), and floating-point values up to 64 bits wide get passed in F registers 🔥 Compare that with-march=rv64ifd -mabi=lp64f, which still lets the compiler generate code that uses the F and D extensions, but only lets floating-point values up to 32 bits long get passed in registers. Or there's-march=rv64ifd -mabi=lp64, where no floating-point arguments get passed in registers at all 🙈
The default for this is system dependent, so if you want a specific calling convention, please specify one explicitly 🫶 The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some combos are just impossible on certain ISAs 💀 For example, -march=rv32if -mabi=ilp32d is invalid, because the ABI needs 64-bit values passed in F registers, but F registers are only 32 bits wide. There's also the ilp32e ABI, which can only be used with the rv32e architecture. It's not well specified right now and is subject to change, so proceed with caution 🧪
-mfdiv,-mno-fdiv— turns hardware floating-point divide and square root instructions on or off 🧮 This needs the F or D extensions for floating-point registers. By default they get used if the architecture you picked has those instructions ✨-mdiv,-mno-div— turns hardware instructions for integer division on or off 🔢 This needs the M extension. Same deal: the default is to use them if your specified architecture has them 💅-march=ISA-string— generates code for the given RISC-V ISA, likerv64im🌈 ISA strings must be lower-case! Examples includerv64i,rv32g,rv32e, andrv32imaf🎀
When -march= isn't specified, it uses the setting from -mcpu. If both -march and -mcpu= are left out, the default is system dependent, so if you want specific architecture extensions, specify them yourself 👑
-mcpu=processor-string— uses the architecture of, and optimizes the output for, the given processor, picked by its particular CPU name 🧠 The permissible values are:sifive-e20,sifive-e21,sifive-e24,sifive-e31,sifive-e34,sifive-e76,sifive-s21,sifive-s51,sifive-s54,sifive-s76,sifive-u54, andsifive-u74💜-mtune=processor-string— optimizes the output for the given processor, picked by microarchitecture or particular CPU name ⭐ The permissible values are:rocket,sifive-3-series,sifive-5-series,sifive-7-series,size, and all valid options for-mcpu=.
When -mtune= isn't specified, it uses the setting from -mcpu. If both are unspecified, the default is rocket 🚀
The size choice isn't really meant for end-users 🙈 It's used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but it does not override the pipeline info. This helps reduce code size while still giving good performance 🫧
-mpreferred-stack-boundary=num— tries to keep the stack boundary aligned to a 2 raised to num byte boundary 🧱 If you don't specify-mpreferred-stack-boundary, the default is 4 (16 bytes, or 128 bits) 🌸
⚠️ Warning: if you use this switch, you must build all modules with the same value, including any libraries. That covers the system libraries and startup modules too 😭
-msmall-data-limit=n— puts global and static data smaller thannbytes into a special section (on some targets) 📦-msave-restore,-mno-save-restore— decides whether you use smaller but slower prologue and epilogue code that calls library functions 🐢 The default is to use fast inline prologues and epilogues ⚡-mshorten-memrefs,-mno-shorten-memrefs— decides whether GCC tries to use more compressed load/store instructions 🗜️ It does this by swapping a load/store of 'base register + large offset' for a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, the new load/store can be compressed too. Right now it only targets 32-bit integer load/stores 🎯-mstrict-align,-mno-strict-align—-mstrict-alignmeans no unaligned memory accesses, and-mno-strict-alignmeans GCC is allowed to generate them 🧐 The default depends on whether the processor you're optimizing for supports fast unaligned access or not 🏃♀️-mcmodel=medlow— generates code for the medium-low code model 🌙 The program and its statically defined symbols have to live inside a single 2 GiB address range, and between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model 💖-mcmodel=medany— generates code for the medium-any code model ✨ The program and its statically defined symbols have to be within any single 2 GiB address range. Programs can be statically or dynamically linked 🌈-mexplicit-relocs,-mno-exlicit-relocs— decides whether you use assembler relocation operators when dealing with symbolic addresses 🧩 The alternative is assembler macros instead, which may limit optimization 🥲-mrelax,-mno-relax— takes advantage of linker relaxations to reduce the number of instructions needed to materialize symbol addresses 🦋 The default is to take advantage of them 💅-memit-attribute,-mno-emit-attribute— emits (or doesn't emit) a RISC-V attribute to record extra information into ELF objects 📝 This feature requires at least binutils 2.32 🔧-malign-data=type— controls how GCC aligns variables and constants of array, structure, or union types 📐 The supported values fortypearexlen, which uses the x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default 🌸-mbig-endian— generates big-endian code 🫶 This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target 🍓-mlittle-endian— generates little-endian code 💜 This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*target, but not ariscv64be-*-*orriscv32be-*-*target 🌟-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generates stack protection code using a canary atguard🛡️ The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block 🧵
With the tls choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset get to decide which register is the base register for reading the canary, and what offset from that base register it lives at 🎯 There's no default register or offset here, because this is entirely for use within the Linux kernel 🐧
🎀 RL78 Options 🎀
-msim— links in additional target libraries so your code can run inside a simulator 🖥️-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— picks the type of hardware multiplication and division support to use 🧮 The simplest one is"none", which uses software for both multiplication and division. That's the default 💖 The"g13"value is for the hardware multiply/divide peripheral on the RL78/G13 (S2 core) targets 🔌 The"g14"value uses the multiplication and division instructions supported by the RL78/G14 (S3 core) parts ⚡ The value"rl78"is an alias for"g14", and the value"mg10"is an alias for"none"🫧
A C preprocessor macro also gets defined based on the setting of this option 🧩 The possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__", or "__RL78_MUL_G14__" ✨
-mcpu=g10,-mcpu=g13,-mcpu=g14,-mcpu=rl78— picks which RL78 core you're targeting 🎯 The default is the G14 core, also known as an S3 core or just RL78 👑 The G13 (or S2) core doesn't have multiply or divide instructions. Instead it uses a hardware peripheral for those operations 🔧 The G10 (or S1) core doesn't have register banks, so it uses a different calling convention 🥹
okay so the RL78 vibes continue 🎀 Setting -mcpu also picks your hardware multiply support, unless you override it with an explicit -mmul=none on the command line 💅. So -mcpu=g13 turns on the G13 hardware multiply peripheral 🔥, and -mcpu=g10 straight up turns off hardware multiplication altogether 💀.
Ngl this part is lowkey important 👀. The RL78/G14 core is the default target, BUT passing -mcpu=g14 or -mcpu=rl78 on the command line actually changes how your toolchain behaves, because it turns on G14 hardware multiply support ✨. If you don't pass either one, software multiplication routines get used even though your code targets the RL78 core 🙈. That's just for backwards compatibility with older toolchains that didn't have hardware multiply and divide support 🦋.
There's also a C preprocessor macro that gets defined based on this setting 💜. The possible values are "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10,-mg13,-mg14,-mrl78— 🧚 these are just aliases for the matching-mcpu=option. They're only around for backwards compatibility.-mallregs— lets the compiler use all the available registers 💅. By defaultr24..r31are reserved for interrupt handlers, but with this on, they can be used in ordinary functions too.-m64bit-doubles,-m32bit-doubles— makes thedoubledata type 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) 🌈. The default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— tells interrupt handler functions to preserve the MDUC registers 🫶. This only matters if normal code might touch the MDUC registers, like when it does multiplication and division. By default the MDUC registers get ignored, because that makes the interrupt handlers faster ⭐. The target option-mg13needs to be passed for this to work, since this feature is only available on the G13 target (S2 core). The MDUC registers only get saved if the interrupt handler does a multiplication or division, or calls another function.
✨ IBM RS/6000 and PowerPC Options ✨
These -m options are defined for the IBM RS/6000 and PowerPC 🦋. You use them to specify which instructions are available on the processor you're using 💜. The default value of these options is determined when GCC is configured. Specifying -mcpu=cpu_type overrides these, and we really recommend using -mcpu=cpu_type instead of the options below 🫶.
-mpowerpc-gpopt,-mno-powerpc-gpopt— 🌸-mpowerpc-gpoptlets GCC use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root.-mpowerpc-gfxopt,-mno-powerpc-gfxopt— 🌸-mpowerpc-gfxoptlets GCC use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.-mpowerpc64,-mno-powerpc64— 🔥-mpowerpc64lets GCC generate the extra 64-bit instructions from the full PowerPC64 architecture, and treat GPRs as 64-bit doubleword quantities. GCC defaults to-mno-powerpc64.-mmfcrf,-mno-mfcrf— 🍓-mmfcrflets GCC generate the move from condition register field instruction, which was implemented on the POWER4 processor and other processors supporting the PowerPC V2.01 architecture.-mpopcntb,-mno-popcntb— 🧚-mpopcntblets GCC generate the popcount and double-precision FP reciprocal estimate instruction, which was implemented on the POWER5 processor and other processors supporting the PowerPC V2.02 architecture.-mpopcntd,-mno-popcntd— 👑-mpopcntdlets GCC generate the popcount instruction, which was implemented on the POWER7 processor and other processors supporting the PowerPC V2.06 architecture.-mfprnd,-mno-fprnd— ✨-mfprndlets GCC generate the FP round to integer instructions, which were implemented on the POWER5+ processor and other processors supporting the PowerPC V2.03 architecture.-mcmpb,-mno-cmpb— 💅-mcmpblets GCC generate the compare bytes instruction, which was implemented on the POWER6 processor and other processors supporting the PowerPC V2.05 architecture.-mhard-dfp,-mno-hard-dfp— 🌈-mhard-dfplets GCC generate the decimal floating-point instructions implemented on some POWER processors.-mcpu=cpu_type— 👑 this sets the architecture type, register usage, and instruction scheduling parameters for the machine typecpu_type. The supported values forcpu_typeare:
401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, power10, powerpc,
powerpc64, powerpc64le, rs64, and native.-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le are the pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types 🧚. They come with an appropriate generic processor model assumed for scheduling purposes.
Specifying native as the cpu type detects the host processor of the system doing the compilation, and selects the architecture option that matches it 🔍. -mcpu=native has no effect if GCC doesn't recognize the processor 🙈.
The other options specify a specific processor 💜. Code generated under those options runs best on that processor, and may not run at all on others 💀.
The -mcpu options automatically enable or disable the following options 🌸:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protectThe exact options set for any particular CPU change between compiler versions, depending on what setting seems to produce optimal code for that CPU 🎀. So they don't necessarily reflect the actual hardware's capabilities 🙈. If you want to set one individual option to a particular value, put it after the -mcpu option, like -mcpu=970 -mno-altivec 💅.
On AIX, the-maltivecand-mpowerpc64options are not enabled or disabled by the-mcpuoption at present, because AIX doesn't have full support for these options yet 😭. You can still enable or disable them individually if you're sure it'll work in your environment.
-mtune=cpu_type— 🎯 this sets the instruction scheduling parameters for the machine typecpu_type, but it does not set the architecture type or register usage the way-mcpu=cpu_typedoes. The same values forcpu_typework for-mtuneas for-mcpu. If you specify both, the code uses the architecture and registers set by-mcpu, but the scheduling parameters set by-mtune🫶.-mcmodel=small— 🌸 generates PowerPC64 code for the small model, where the TOC is limited to 64k.-mcmodel=medium— 🌈 generates PowerPC64 code for the medium model, where the TOC and other static data can be up to a total of 4G in size. This is the default for 64-bit Linux.-mcmodel=large— 👑 generates PowerPC64 code for the large model, where the TOC can be up to 4G in size. Other data and code is only limited by the 64-bit address space.-maltivec,-mno-altivec— ✨ generates code that uses (or doesn't use) AltiVec instructions, and also enables the built-in functions that give you more direct access to the AltiVec instruction set 💅. You may also need to set-mabi=altivecto adjust the current ABI with the AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics such as "vec_splat", "vec_extract", and "vec_insert" matches the array element order for the endianness of the target 🔥. Element zero is the leftmost element in a vector register when targeting a big-endian platform, and the rightmost element in a vector register when targeting a little-endian platform 🙈.
-mvrsave,-mno-vrsave— 🦋 generates VRSAVE instructions when generating AltiVec code.-msecure-plt— 🔐 generates code that letsldandld.sobuild executables and shared libraries with non-executable".plt"and".got"sections. This is a PowerPC 32-bit SYSV ABI option.-mbss-plt— 🧚 generates code that uses a BSS".plt"section thatld.sofills in. This requires".plt"and".got"sections that are both writable and executable 😭. This is a PowerPC 32-bit SYSV ABI option.-misel,-mno-isel— 🌸 enables or disables the generation of ISEL instructions.-mvsx,-mno-vsx— 💜 generates code that uses (or doesn't use) vector/scalar (VSX) instructions, and also enables the built-in functions that give more direct access to the VSX instruction set ✨.-mcrypto,-mno-crypto— 🔑 enables (or disables) the built-in functions that give direct access to the cryptographic instructions added in version 2.07 of the PowerPC ISA.-mhtm,-mno-htm— 🫶 enables (or disables) the built-in functions that give direct access to the Hardware Transactional Memory (HTM) instructions added in version 2.07 of the PowerPC ISA.-mpower8-fusion,-mno-power8-fusion— 🍓 generates code that keeps some integer operations adjacent, so the instructions can be fused together on POWER8 and later processors. The-mno-power8-fusionform doesn't keep them adjacent.-mpower8-vector,-mno-power8-vector— 🌈 generates code that uses (or doesn't use) the vector and scalar instructions added in version 2.07 of the PowerPC ISA. It also enables the built-in functions that give more direct access to the vector instructions 👑.-mquad-memory,-mno-quad-memory— 💅 generates code that uses (or doesn't use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode ⚠️.
-mquad-memory-atomic -mno-quad-memory-atomic
Okay so these two are a pair, bestie 💜 -mquad-memory-atomic tells GCC to generate code that uses the atomic quad word memory instructions, and -mno-quad-memory-atomic tells it to skip them. ngl the first one needs 64-bit mode to work, so keep that in mind 🦋
-mfloat128 -mno-float128
Okay this one is giving main character energy 👑 -mfloat128 enables the __float128 keyword for IEEE 128-bit floating point, and -mno-float128 turns it off. Depending on your setup, GCC uses either software emulation or hardware instructions for IEEE 128-bit floats 🔥
Heads up: the VSX instruction set (-mvsx) has to be enabled before you can use IEEE 128-bit floating point support, and it's only supported on Linux 💀
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled everywhere else 🌸
If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a 64-bit system, IEEE 128-bit floating point support will also turn on generation of ISA 3.0 IEEE 128-bit floating point instructions ✨ But if you don't ask for ISA 3.0 instructions, or you're targeting a 32-bit big endian system, IEEE 128-bit floating point gets done with software emulation instead 🫶
-mfloat128-hardware -mno-float128-hardware
Lowkey this pair is about the hardware side 🧚 -mfloat128-hardware lets GCC use ISA 3.0 hardware instructions to support the __float128 data type, and -mno-float128-hardware turns that off 💖
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems 🌈
-m32 -m64
These two are the environment switches, iykyk 🎀 -m32 and -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 the code it makes runs on any PowerPC variant 🍓 The 64-bit environment sets int to 32 bits, sets long and pointer to 64 bits, and generates code for PowerPC64, same as -mpowerpc64 👑
-mfull-toc -mno-fp-in-toc -mno-sum-in-toc -mminimal-toc
Okay the TOC (Table Of Contents) era 💅 It's created for every executable file, and these four flags control how it gets generated 🦋
-mfull-toc is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also puts floating-point constants in the TOC 🌸 The catch? Only 16,384 entries are available in the TOC 💀
If you get a linker error saying you've overflowed the available TOC space, you can shrink the amount of TOC space used with -mno-fp-in-toc and -mno-sum-in-toc 🙈 -mno-fp-in-toc stops GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc makes GCC generate code that calculates the sum of an address and a constant at run time, instead of putting that sum into the TOC ✨ You can use one or both. Each one makes the code very slightly slower and larger, but it saves TOC space 🫶
If you still run out of space even with both of those, try -mminimal-toc instead 😭 This one makes only one TOC entry for every file. It produces code that's slower and larger, but it uses extremely little TOC space 🔥 Honestly, you might only want to use it on files with less frequently-executed code 🧚
-maix64 -maix32
Okay this is the 64-bit AIX vibe 🌈 -maix64 enables the 64-bit AIX ABI and calling convention: 64-bit pointers, a 64-bit long type, and the infrastructure needed to support them ✨ Specifying -maix64 implies -mpowerpc64 💖 -maix32 disables the 64-bit ABI and implies -mno-powerpc64 🍓
GCC defaults to -maix32 🎀
-mxl-compat -mno-xl-compat
Lowkey this one is about matching IBM XL compilers 🦋 -mxl-compat makes code conform more closely to IBM XL compiler semantics when you're using the AIX-compatible ABI 💅 Here's what it changes:
- Floating-point arguments passed to prototyped functions go beyond the register save area (RSA) on the stack, in addition to argument FPRs 🌸
- It doesn't assume that the most significant double in a 128-bit
long doublevalue is properly rounded when comparing values and converting to double 👑 - It uses XL symbol names for
long doublesupport routines 🧚
The AIX calling convention was extended but never initially documented to handle an obscure K&R C case: calling a function that takes the address of its arguments with fewer arguments than declared 💀 IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization 🙈 Storing floating-point arguments on the stack is inefficient and rarely needed, so this option isn't enabled by default. You only really need it when calling subroutines compiled by IBM XL compilers without optimization 🫶
-mpe
Okay this one is for the IBM RS/6000 SP Parallel Environment (PE) 🌈 It links an application written to use message passing with special startup code so the app can run ✨
The system has to have PE installed in the standard location (/usr/lpp/ppe.poe/), or you need to override the specs file with the -specs= option to point at the right directory 💖 Heads up: the Parallel Environment doesn't support threads, so -mpe and the -pthread option are incompatible 😭
-malign-natural -malign-power
Alignment vibes 🍓 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types, like floating-point doubles, so they sit on their natural size-based boundary 🦋 -malign-power tells GCC to follow the ABI-specified alignment rules instead ✨ GCC defaults to the standard alignment defined in the ABI 🎀
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported 💀
-msoft-float -mhard-float
These two are about the floating-point register set 🧚 -msoft-float generates code that doesn't use the floating-point register set, and -mhard-float generates code that does 💅 Software floating-point emulation is provided if you use -msoft-float, and you have to pass the option to GCC when linking too 🔥
-mmultiple -mno-multiple
Okay the multiple word vibes 💜 -mmultiple generates code that uses the load multiple word and store multiple word instructions, and -mno-multiple generates code that doesn't 🌸 These are generated by default on POWER systems, and not generated on PowerPC systems 🫶
Do not use -mmultiple on little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode 💀 The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode 🙈
-mupdate -mno-update
Okay this is about update instructions 🌈 -mupdate generates code that uses the load or store instructions that update the base register to the address of the calculated memory location, and -mno-update generates code that doesn't ✨ These instructions are generated by default 💖
If you use -mno-update, there's a small window between the time the stack pointer is updated and the time the address of the previous frame is stored 😭 That means code that walks the stack frame across interrupts or signals may get corrupted data 💀
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
Lowkey this one is about avoiding indexed load or store instructions 🦋 -mavoid-indexed-addresses generates code that tries to avoid them, and -mno-avoid-indexed-addresses generates code that doesn't 🎀 These instructions can take a performance penalty on Power6 processors in certain situations, like stepping through large arrays that cross a 16M boundary 🧚
This option is enabled by default when targeting Power6 and disabled otherwise 🌸
-mfused-madd -mno-fused-madd
Okay fused multiply-add era 🔥 -mfused-madd generates code that uses the floating-point multiply and accumulate instructions, and -mno-fused-madd generates code that doesn't 💅 These are generated by default if hardware floating point is used ✨
The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off 🫶
-mmulhw -mno-mulhw
Okay half-word multiply energy 💖 -mmulhw generates code that uses the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors, and -mno-mulhw generates code that doesn't 🍓 These are generated by default when targeting those processors 👑
-mdlmzb -mno-dlmzb
Lowkey this is the string-search era 🧚 -mdlmzb generates code that uses the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors, and -mno-dlmzb generates code that doesn't 🌈 This instruction is generated by default when targeting those processors 💜
-mno-bit-align -mbit-align
Bit-field alignment check 🎀 On System V.4 and embedded PowerPC systems, -mno-bit-align does not force structures and unions that contain bit-fields to be aligned to the base type of the bit-field, and -mbit-align does 🦋
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes 🌸 With -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size ✨
-mno-strict-align -mstrict-align
Okay alignment, round two 💅 On System V.4 and embedded PowerPC systems, -mno-strict-align does not assume that unaligned memory references are handled by the system, and -mstrict-align does 🫶
-mrelocatable -mno-relocatable
Relocation energy 🔥 -mrelocatable generates code that allows a static executable to be relocated to a different address at run time, and -mno-relocatable generates code that doesn't 💖 A simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte locations listed in the ".fixup" section, which is a table of 32-bit addresses generated by this option 🧚
For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib 😭 -mrelocatable code aligns the stack to an 8-byte boundary 🌈
-mrelocatable-lib -mno-relocatable-lib
Okay this is the sibling of -mrelocatable 🦋 Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section so static executables can be relocated at run time 🎀 The difference is that -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable ✨ Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options 💜
-mno-toc -mtoc
Lowkey this one is about register 2 🌸 On System V.4 and embedded PowerPC systems, -mno-toc does not assume that register 2 contains a pointer to a global area pointing to the addresses used in the program, and -mtoc does 💅
-mlittle -mlittle-endian
Endian era 🧚 On System V.4 and embedded PowerPC systems, these compile code for the processor in little-endian mode 🫶 -mlittle-endian is the same as -mlittle 🍓
-mbig -mbig-endian
Same energy, other side 🌈 On System V.4 and embedded PowerPC systems, these compile code for the processor in big-endian mode 💖 -mbig-endian is the same as -mbig ✨
-mdynamic-no-pic
Last one in this part, bestie 🎀 On Darwin and Mac OS X systems, -mdynamic-no-pic compiles code so that it is not relocatable, but its external references are relocatable 🦋 The resulting code is suitable for applications, but not shared libraries 💀
-msingle-pic-base— okay this one is lowkey chill 🧊 It makes the register used for PIC addressing read-only 🔒 instead of loading it in the prologue for every single function. The runtime system is the one in charge of setting this register to an appropriate value before execution begins, so that's on them 💅-mprioritize-restricted-insns=priority— this one picks the priority for dispatch-slot restricted instructions during the second scheduling pass 🗓️ The priority argument takes0,1, or2, which gives those instructions no priority, highest priority, or second-highest priority (respectively). Main character energy, but make it a choice 👑-msched-costly-dep=dependence_type— this decides which dependences the target considers costly during instruction scheduling 💸 The dependence_type argument takes one of these:no— no dependence is costly 🙅♀️all— all dependences are costly 😭true_store_to_load— a true dependence from store to load is costlystore_to_load— any dependence from store to load is costly- number — any dependence whose latency is greater than or equal to number is costly
-minsert-sched-nops=scheme— this picks the NOP insertion scheme used during the second scheduling pass 🫧 The scheme argument takes one of these:no— don't insert NOPs at all 🚫pad— pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's groupingregroup_exact— insert NOPs to force costly dependent insns into separate groups, using exactly as many NOPs as needed to push an insn into a new group, according to the estimated processor grouping- number — insert NOPs to force costly dependent insns into separate groups, using number NOPs to push an insn into a new group
-mcall-sysv— on System V.4 and embedded PowerPC systems, this compiles code with calling conventions that follow the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement 📜 This is the default unless you configured GCC usingpowerpc-*-eabiaix.-mcall-sysv-eabi,-mcall-eabi— same as specifying both-mcall-sysvand-meabi🤝-mcall-sysv-noeabi— same as specifying both-mcall-sysvand-mno-eabi🙃-mcall-aixdesc— on System V.4 and embedded PowerPC systems, this compiles code for the AIX operating system 🖥️-mcall-linux— on System V.4 and embedded PowerPC systems, this compiles code for the Linux-based GNU system 🐧-mcall-freebsd— on System V.4 and embedded PowerPC systems, this compiles code for the FreeBSD operating system 😈-mcall-netbsd— on System V.4 and embedded PowerPC systems, this compiles code for the NetBSD operating system 🐡-mcall-openbsd— on System V.4 and embedded PowerPC systems, this compiles code for the OpenBSD operating system 🐡-mtraceback=traceback_type— selects the type of traceback table 📋 Valid values for traceback_type arefull,part, andno.-maix-struct-return— returns all structures in memory, as the AIX ABI specifies 🧱-msvr4-struct-return— returns structures smaller than 8 bytes in registers, as the SVR4 ABI specifies 🧺-mabi=abi-type— extends the current ABI with a particular extension, or removes that extension 🔧 Valid values arealtivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX:vec-extabi,vec-default.-mabi=ibmlongdouble— switches the current ABI to IBM extended-precision long double 💫
This is not likely to work if your system defaults to IEEE extended-precision long double. If you change the long double type from IEEE extended-precision, the compiler will issue a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.
-mabi=ieeelongdouble— switches the current ABI to IEEE extended-precision long double 💫
This is not likely to work if your system defaults to IBM extended-precision long double. If you change the long double type from IBM extended-precision, the compiler will issue a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.
-mabi=elfv1— switches the current ABI to the ELFv1 ABI 🌸 This is the default ABI for big-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀
-mabi=elfv2— switches the current ABI to the ELFv2 ABI 🌸 This is the default ABI for little-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀
-mgnu-attribute,-mno-gnu-attribute— emits.gnu_attributeassembly directives that set tag/value pairs in a.gnu.attributessection 🏷️ Those pairs specify ABI variations in function parameters or return values.-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, this assumes that all calls to variable argument functions are properly prototyped ✅ Otherwise, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR"). That bit tells whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With-mprototype, only calls to prototyped variable argument functions set or clear the bit.-msim— on embedded PowerPC systems, this assumes the startup module is calledsim-crt0.oand the standard C libraries arelibsim.aandlibc.a🧪 This is the default forpowerpc-*-eabisimconfigurations.-mmvme— on embedded PowerPC systems, this assumes the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a🧩-mads— on embedded PowerPC systems, this assumes the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a🧩-myellowknife— on embedded PowerPC systems, this assumes the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a🧩-mvxworks— on System V.4 and embedded PowerPC systems, this specifies that you're compiling for a VxWorks system 🛰️-memb— on embedded PowerPC systems, this sets the "PPC_EMB" bit in the ELF flags header to show that eabi extended relocations are used 🦋-meabi,-mno-eabi— on System V.4 and embedded PowerPC systems, this makes the code adhere (or not adhere) to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications ✨-meabimeans the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and the-msdataoption can use bothr2andr13to point to two separate small data areas.-mno-eabimeans the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and the-msdataoption only usesr13to point to a single small data area.- The
-meabioption is on by default if you configured GCC using one of thepowerpc*-*-eabi*options.
-msdata=eabi— on System V.4 and embedded PowerPC systems, this puts small initializedconstglobal and static data in the.sdata2section, which is pointed to by registerr2🎀 Small initialized non-constglobal and static data goes in the.sdatasection, pointed to by registerr13. Small uninitialized global and static data goes in the.sbsssection, which sits right next to.sdata🛏️
The-msdata=eabioption is incompatible with the-mrelocatableoption. It also sets the-memboption.
-msdata=sysv— on System V.4 and embedded PowerPC systems, this puts small global and static data in the.sdatasection, which is pointed to by registerr13🌷 Small uninitialized global and static data goes in the.sbsssection, which is adjacent to.sdata.
The-msdata=sysvoption is incompatible with the-mrelocatableoption.
-msdata=default,-msdata— on System V.4 and embedded PowerPC systems, if-meabiis used, this compiles code the same as-msdata=eabi. Otherwise, it compiles code the same as-msdata=sysv🤷♀️-msdata=data— on System V.4 and embedded PowerPC systems, this puts small global data in the.sdatasection and small uninitialized global data in the.sbsssection 🍓 It does not use registerr13to address small data, though. This is the default behavior unless other-msdataoptions are used.-msdata=none,-mno-sdata— on embedded PowerPC systems, this puts all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection 🫧-mreadonly-in-sdata— puts read-only objects in the.sdatasection too 📖 This is the default.-mblock-move-inline-limit=num— inlines all block moves (like calls tomemcpyor structure copies) that are less than or equal to num bytes 🏃♀️ The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.-mblock-compare-inline-limit=num— generates non-looping inline code for all block compares (like calls tomemcmpor structure compares) that are less than or equal to num bytes 🔍 If num is0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific.-mblock-compare-inline-loop-limit=num— generates an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion 🔁 If the block length isn't constant, at most num bytes will be compared beforememcmpis called to compare the rest of the block. The default value is target-specific.-mstring-compare-inline-limit=num— compares at most num string bytes with inline code 🧵 If the difference or end of string isn't found at the end of the inline compare, a call tostrcmporstrncmptakes care of the rest of the comparison. The default is 64 bytes.-G num— on embedded PowerPC systems, this puts global and static items that are 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 is8. The-G numswitch is also passed to the linker, so all modules should be compiled with the same-G numvalue 🔗-mregnames,-mno-regnames— okay so on System V.4 and embedded PowerPC systems,-mregnamesmakes GCC do print register names in the assembly output using their symbolic forms 🌸, and-mno-regnamesmeans it does not do that 🙅♀️-mlongcall,-mno-longcall— by default GCC assumes every call is super far away 🔭, so it uses a longer, more expensive calling sequence. That's needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call only gets generated if the compiler knows the call can't be that far away 💖. You can override this with theshortcallfunction attribute, or with#pragma longcall(0).
Some linkers can spot out-of-range calls and generate glue code on the fly 🪄. On those systems, long calls are unnecessary and just make the code slower 🐌. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well 🔜.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls 🙈
On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code) 🏝️. The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler to emit the glue for every direct call, and the Darwin linker decides whether to use or discard it 🧐
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue 🔮
-mpltseq,-mno-pltseq—-mpltseqtells GCC to implement-fno-pltand long calls using an inline PLT call sequence 🧵. It supports lazy linking and long calls to functions indlopen'dshared libraries.-mno-pltseqsays nope to all that. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers 🧩. They're enabled by default if the support is detected when GCC is configured, and, for 32-bit PowerPC, if GCC is configured with--enable-secureplt.-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together 💔-mtls-markers,-mno-tls-markers— GCC will (or won't) mark calls to__tls_get_addrwith a relocation that specifies the function argument 🏷️. That relocation lets the linker reliably connect the function call with its argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better 📅-mrecip,-mno-recip— this option turns on the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps to bump up precision 📈. It's used instead of doing a divide, or a square root and a divide, for floating-point arguments. You should use-ffast-mathwhen using-mrecip(or at least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math) 💅. Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots 😭-mrecip=opt— this one controls which reciprocal estimate instructions are allowed to be used 🎛️.optis a comma-separated list of options, and any of them can be preceded by a!to invert the option:all— Enable all estimate instructions.default— Enable the default instructions, equivalent to-mrecip.none— Disable all estimate instructions, equivalent to-mno-recip.div— Enable the reciprocal approximation instructions for both single and double precision.divf— Enable the single-precision reciprocal approximation instructions.divd— Enable the double-precision reciprocal approximation instructions.rsqrt— Enable the reciprocal square root approximation instructions for both single and double precision.rsqrtf— Enable the single-precision reciprocal square root approximation instructions.rsqrtd— Enable the double-precision reciprocal square root approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions, which handle the double-precision reciprocal square root calculations 🧠
-mrecip-precision,-mno-recip-precision— assume (or don't assume) that the reciprocal estimate instructions give higher-precision estimates than the PowerPC ABI requires 🎯. Selecting-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precision. The double-precision square root estimate instructions aren't generated by default on low-precision machines, since they don't provide an estimate that converges after three steps 🙅♀️-mveclibabi=type— this specifies the ABI type to use for vectorizing intrinsics using an external library 📚. The only type supported right now ismass, which tells GCC to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls toacosd2,acosf4,acoshd2,acoshf4,asind2,asinf4,asinhd2,asinhf4,atan2d2,atan2f4,atand2,atanf4,atanhd2,atanhf4,cbrtd2,cbrtf4,cosd2,cosf4,coshd2,coshf4,erfcd2,erfcf4,erfd2,erff4,exp2d2,exp2f4,expd2,expf4,expm1d2,expm1f4,hypotd2,hypotf4,lgammad2,lgammaf4,log10d2,log10f4,log1pd2,log1pf4,log2d2,log2f4,logd2,logf4,powd2,powf4,sind2,sinf4,sinhd2,sinhf4,sqrtd2,sqrtf4,tand2,tanf4,tanhd2, andtanhf4when generating code for power7 ⚡. Both-ftree-vectorizeand-funsafe-math-optimizationsmust also be enabled. The MASS libraries must be specified at link time 🔗-mfriz,-mno-friz— GCC will (or won't) generate thefrizinstruction when-funsafe-math-optimizationsis used to optimize rounding of floating-point values to 64-bit integer and back to floating point 🔄. Heads up: thefrizinstruction does not return the same value if the floating-point number is too large to fit in an integer 😬-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— GCC will (or won't) generate code that loads up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems 🧭. That's the case where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in registerr2, and the static chain value to be loaded in registerr11.-mpointers-to-nested-functionsis on by default. If you use-mno-pointers-to-nested-functions, you cannot call through pointers to nested functions, or through pointers to functions compiled in other languages that use the static chain 🚫-msave-toc-indirect,-mno-save-toc-indirect— GCC will (or won't) save the TOC value in the reserved stack location in the function prologue, if the function calls through a pointer on AIX and 64-bit Linux systems 💾. If the TOC value isn't saved in the prologue, it gets saved just before the call through the pointer. The-mno-save-toc-indirectoption is the default ✨-mcompat-align-parm,-mno-compat-align-parm— GCC will (or won't) generate code that passes structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC 🤝
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that is compatible with functions compiled with older versions of GCC 🕰️
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol— these generate stack protection code using a canary atguard🐤. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (the default with GNU libc version 2.4 or later) 🛡️
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block 🎀
-mpcrel,-mno-pcrel— GCC will (or won't) generate pc-relative addressing 📍. The-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled.-mprefixed,-mno-prefixed— GCC will (or won't) generate addressing modes using prefixed load and store instructions 🧩. The-mprefixedoption requires that the option-mcpu=power10(or later) is enabled.-mmma,-mno-mma— GCC will (or won't) generate the MMA instructions 🧮. The-mmaoption requires that the option-mcpu=power10(or later) is enabled.-mrop-protect,-mno-rop-protect— GCC will (or won't) generate ROP protection instructions when the target processor supports them 🔐. Currently this option disables the shrink-wrap optimization (-fshrink-wrap).-mprivileged,-mno-privileged— GCC will (or won't) generate code that runs in privileged state 👑-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— GCC will (or won't) generate unaligned vsx loads and stores for inline expansion ofmemcpyandmemmove🦋
✨ RX Options ✨
These command-line options are defined for RX targets:
✨ RX Options ✨ (continued)
-m64bit-doubles,-m32bit-doubles— okay so these decide how big thedoubletype is 🦋.-m64bit-doublesmakes it 64 bits and-m32bit-doublesmakes it 32 bits. The default is-m32bit-doubles, and that's not random: RX floating-point hardware only works on 32-bit values 💅.-fpu,-nofpu— this one turns the RX floating-point hardware on (-fpu) or off (-nofpu) 🌸. It's on by default for the RX600 series and off by default for the RX200 series.
Floating-point instructions only get generated for 32-bit floating-point values, so the FPU hardware isn't used for doubles if -m64bit-doubles is in play 🙈.
Note: If-fpuis enabled,-funsafe-math-optimizationsgets turned on automatically too. The RX FPU instructions are themselves unsafe, so it's not optional 💀.
-mcpu=name— picks which RX CPU you're targeting 👑. There are three options: the generic RX600 series, the RX200 series, and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 doesn't support the "MVTIPL" instruction 🧚.
The RX200 series doesn't have a hardware floating-point unit, so -nofpu is turned on by default whenever this type is selected ✨.
-mbig-endian-data,-mlittle-endian-data— controls how data (but not code) gets stored 🌈.-mbig-endian-datastores it in big-endian format. The default is-mlittle-endian-data, so data is stored in little-endian format.-msmall-data-limit=N— sets the maximum size in bytes of global and static variables that can be placed into the small data area 🍓. Using that area can make code smaller and faster, but the area is limited, and it's up to you to make sure it doesn't overflow 😭.
Also, when the small data area is used, one of the RX's registers (usually "r13") gets reserved to point at that area. That means the compiler can't use it anymore, which can lead to slower and/or larger code if variables get pushed onto the stack instead of living in that register 💔.
Note, common variables (variables that haven't been initialized) and constants are not placed into the small data area, because they get assigned to other sections in the output executable 🫶.
The default value is zero, which disables this feature. Also, this feature isn't enabled by default at higher optimization levels (-O2 etc) because reserving a register can be detrimental. It's up to you to experiment and see whether it helps your program 🔬. For how the actual register holding the small data area pointer is chosen, check the description of the -mpid option.
-msim,-mno-sim—-msimuses the simulator runtime 🎮. The default is to use thelibglossboard-specific runtime.-mas100-syntax,-mno-as100-syntax— when generating assembler output,-mas100-syntaxuses a syntax compatible with Renesas's AS100 assembler 🎀. The GAS assembler can handle this syntax too, but it has some restrictions, so it isn't generated by default.-mmax-constant-size=N— sets the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction 🧁. The RX instruction set does allow constants up to 4 bytes long, but a longer value means a longer instruction. So sometimes it's actually beneficial to limit how big the constants are.
Constants that are too big get placed into a constant pool instead and referenced via register indirection 📦.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed 🙌.
-mrelax— enables linker relaxation 🧘. This is when the linker tries to shrink your program by finding shorter versions of various instructions. It's disabled by default.-mint-register=N— reserves a number of registers for fast interrupt handler functions ⚡. The valueNcan be between 0 and 4. Here's how the reserving goes:- A value of 1 reserves
"r13"for the exclusive use of fast interrupt handlers. - A value of 2 reserves
"r13"and"r12". - A value of 3 reserves
"r13","r12"and"r11". - A value of 4 reserves
"r13"through"r10". - A value of 0, the default, doesn't reserve any registers.
-msave-acc-in-interrupts— tells interrupt handler functions to preserve the accumulator register 🧲. This is only necessary if normal code might use the accumulator, for example because it does 64-bit multiplications. The default is to ignore the accumulator, since that makes the interrupt handlers faster 🏎️.-mpid,-mno-pid—-mpidenables the generation of position independent data 🗺️. When it's on, any access to constant data is done via an offset from a base address held in a register. This lets the location of constant data be figured out at run time without the executable needing to be relocated, which is a big win for embedded apps with tight memory constraints. Data that can be modified isn't affected by this option.
Heads up, using this feature reserves a register (usually "r13") for the constant data base address. That can make code slower and/or larger, especially in complicated functions 😬.
Which register actually gets chosen for the constant data base address depends on whether -msmall-data-limit and/or -mint-register are enabled. Starting with "r13" and moving downwards, registers get allocated first to satisfy the requirements of -mint-register, then -mpid, and finally -msmall-data-limit. So it's possible for the small data area register to end up as "r8" if both -mint-register=4 and -mpid are specified on the command line 🧩.
By default this feature is not enabled. The default can be restored with the -mno-pid command-line option 🔄.
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts—-mno-warn-multiple-fast-interruptsstops GCC from warning you if it finds more than one fast interrupt handler while compiling a file 🤫. The default is to issue a warning for each extra fast interrupt handler, since the RX only supports one such interrupt.-mallow-string-insns,-mno-allow-string-insns— these enable or disable the string manipulation instructions"SMOVF","SCMPU","SMOVB","SMOVU","SUNTIL""SWHILE"and also the"RMPA"instruction 🧵. These instructions may prefetch data, which is not safe to do when you're accessing an I/O register ⚠️. (See section 12.2.7 of the RX62N Group User's Manual for more info.)
Warning: The default is to allow these instructions, but GCC can't reliably detect every circumstance where a string instruction might be used to access an I/O register. So their use can't be disabled automatically. It's on you to use -mno-allow-string-insns if your program accesses I/O space 🚨.
When the instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__. Otherwise, it defines __RX_DISALLOW_STRING_INSNS__ 🏷️.
-mjsr,-mno-jsr—-mjsruses only (or not only)"JSR"instructions to access functions 📞. This option can help when code size exceeds the range of"BSR"instructions. Heads up:-mno-jsrdoesn't mean "don't useJSR". It means any type of branch may be used 🌀.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when it's used with the "interrupt" function attribute 💫. That attribute marks a function meant to process fast interrupts. GCC makes sure it only uses the registers "r10", "r11", "r12" and/or "r13", and only when the normal use of the corresponding registers has been restricted through the -ffixed-reg or -mint-register command-line options 🔒.
✨ S/390 and zSeries Options ✨
Here are the -m options defined for the S/390 and zSeries architecture 🌟.
-mhard-float,-msoft-float—-mhard-floatuses the hardware floating-point instructions and registers for floating-point operations 🔧.-msoft-floatdoes not, so the functions inlibgcc.ado the floating-point work instead. When-mhard-floatis specified, the compiler generates IEEE floating-point instructions. This is the default.-mhard-dfp,-mno-hard-dfp—-mhard-dfpuses the hardware decimal-floating-point instructions for decimal-floating-point operations 🔢. When-mno-hard-dfpis specified, the functions inlibgcc.ahandle decimal-floating-point operations instead. When-mhard-dfpis specified, the compiler generates decimal-floating-point hardware instructions. This is the default for-march=z9-ecor higher.-mlong-double-64,-mlong-double-128— these switches control the size of the"long double"type 📏. A size of 64 bits makes"long double"equivalent to the"double"type. This is the default.-mbackchain,-mno-backchain—-mbackchainstores the address of the caller's frame as a backchain pointer in the callee's stack frame 🔗. A backchain may be needed to allow debugging with tools that don't understand DWARF call frame information. When-mno-packed-stackis in effect, the backchain pointer is stored at the bottom of the stack frame. When-mpacked-stackis in effect, the backchain is placed into the topmost word of the 96/160 byte register save area 🪜.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain. However, using the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain 🧸. Also, the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported 🙅. To build a Linux kernel, use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack,-mno-packed-stack— Okay so this one is about the packed stack layout 🧚 With-mno-packed-stack, the compiler uses every field of the 96/160 byte register save area only for its default job, which means unused fields still take up stack space 🙈 With-mpacked-stack, the register save slots get densely packed at the top of the save area, and the leftover space gets reused for other stuff, so the stack is used way more efficiently ✨ BUT when-mbackchainis also in effect, the topmost word of the save area is always used for the backchain, and the return address register is always saved two words below the backchain 💀 As long as the stack frame backchain isn't used, code made with-mpacked-stackis call-compatible with code made with-mno-packed-stack. 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), and that code is NOT call-compatible with-mpacked-stackcode. The default is to not use the packed stack layout.
Not me… the combination of-mbackchain,-mpacked-stackand-mhard-floatis NOT supported 😭 If you're building a linux kernel, use-msoft-floatinstead.
-msmall-exec,-mno-small-exec— Generate (or don't generate) code that makes subroutine calls with the"bras"instruction. This only works reliably if the total executable size stays under 64k, so be careful, bestie 💅 The default is the"basr"instruction instead, which doesn't have that limit 🌸-m64,-m31— With-m31, code gets generated to be compliant with the GNU/Linux for S/390 ABI. With-m64, it's compliant with the GNU/Linux for zSeries ABI, which lets GCC generate 64-bit instructions 🔥 For the s390 targets the default is-m31, while the s390x targets default to-m64👑-mzarch,-mesa—-mzarchgenerates code using the instructions available on z/Architecture, and-mesagenerates code using the instructions available on ESA/390 🦋-mesais not possible with-m64, so don't even try 🙅 When generating code compliant with the GNU/Linux for S/390 ABI, the default is-mesa. For the GNU/Linux for zSeries ABI, the default is-mzarch💖-mhtm,-mno-htm—-mhtmenables a set of builtins that make use of instructions from the transactional execution facility, which was introduced with the IBM zEnterprise EC12 machine generation (S/390 System z Built-in Functions) 🎀-mhtmis enabled by default when using-march=zEC12✨-mvx,-mno-vx—-mvxgenerates code using the instructions from the vector extension facility, introduced with the IBM z13 machine generation 🌈 This option changes the ABI for some vector type values when it comes to alignment and calling conventions. If vector type values are used in an ABI-relevant context, a GAS.gnu_attributecommand gets added to mark the resulting binary with the ABI used 📝-mvxis enabled by default when using-march=z13💅-mzvector,-mno-zvector—-mzvectorturns on vector language extensions and builtins using the vector extension facility instructions from the IBM z13 generation 💜 It adds support forvectoras a keyword to define vector type variables and arguments.vectoris only available when GNU extensions are enabled, and it won't be expanded if you ask for strict standard compliance, e.g. with-std=c99🙈 On top of the GCC low-level builtins,-mzvectorenables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To use these builtins you need to include the header filevecintrin.h.-mzvectoris disabled by default 🫶-mmvcle,-mno-mvcle— Generate (or don't generate) code using the"mvcle"instruction for block moves 🧚 With-mno-mvcle, a"mvc"loop is used instead. This is the default unless you're optimizing for size 🌸-mdebug,-mno-debug— Print (or don't print) additional debug information while compiling 🔍 The default is to not print debug information 🙅-march=cpu-type— Generate code that runs oncpu-type, which is the name of a system representing a certain processor type 👑 Possible values forcpu-typearez900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, andnative. The default is-march=z900✨
Picking native as the cpu type selects the best architecture option for the host processor 🦋 -march=native has no effect if GCC doesn't recognize the processor, so it just sits there looking cute 🥺
-mtune=cpu-type— Tune everything applicable about the generated code tocpu-type, EXCEPT the ABI and the set of available instructions 💖 The list ofcpu-typevalues is the same as for-march. The default is whatever value is used for-march🌈-mtpf-trace,-mno-tpf-trace— Generate code that adds (or doesn't add) TPF OS specific branches to trace routines in the operating system 🧚 This option is off by default, even when compiling for the TPF OS 💀-mtpf-trace-skip,-mno-tpf-trace-skip— Generate code that changes (or doesn't change) the default branch targets enabled by-mtpf-traceso they point to specialized trace routines. These let you selectively skip function trace entries for the TPF OS 🎀 This option is off by default, even when compiling for the TPF OS AND when you specify-mtpf-trace😭-mfused-madd,-mno-fused-madd— Generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions 🔥 These get generated by default if hardware floating point is used 💅-mwarn-framesize=framesize— Emit a warning if the current function exceeds the given frame size ⚠️ Since this is a compile-time check, it doesn't need to be a real problem when the program runs 🙈 It's meant to spot functions that will most probably cause a stack overflow, so it's super useful in environments with a limited stack size, like the linux kernel 🫶-mwarn-dynamicstack— Emit a warning if the function callsallocaor uses dynamically-sized arrays 🚨 This is generally a bad idea with a limited stack size, so keep an eye out, bestie 👀-mstack-guard=stack-guard,-mstack-size=stack-size— If you provide these options, the S/390 back end emits extra instructions in the function prologue that trigger a trap if the stack size isstack-guardbytes above thestack-size(remember the stack on S/390 grows downward ⬇️). If you leave outstack-guard, the smallest power of 2 larger than the frame size of the compiled function gets chosen 🧚 These are meant to help you debug stack overflow problems. The extra code causes only little overhead, so it can also be used in production-like systems without much performance hit 💜 The given values have to be exact powers of 2, andstack-sizehas to be greater thanstack-guardwithout exceeding 64k 📏 To be efficient, the extra code assumes the stack starts at an address aligned to the value given bystack-size🦋 Thestack-guardoption can only be used in conjunction withstack-size💅-mhotpatch=pre-halfwords,post-halfwords— If the hotpatch option is enabled, a "hot-patching" function prologue is generated for ALL functions in the compilation unit 🔥 The function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 *post-halfwordsbytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000) 🌸
If both arguments are zero, hotpatching is disabled 🙅 You can override this option for individual functions with the "hotpatch" attribute 🎀
🌈 Score Options
These options are defined for Score implementations 🌈
-meb— Compile code for big-endian mode. This is the default 💜-mel— Compile code for little-endian mode 🫶-mnhwloop— Disable generation ofbcnzinstructions 🙈-muls— Enable generation of unaligned load and store instructions ✨-mmac— Enable the use of multiply-accumulate instructions. Disabled by default 🔥-mscore5— Specify the SCORE5 as the target architecture 🦋-mscore5u— Specify the SCORE5U of the target architecture 🧚-mscore7— Specify the SCORE7 as the target architecture. This is the default 👑-mscore7d— Specify the SCORE7D as the target architecture 💅
💜 SH Options
These -m options are defined for the SH implementations 💖
-m1— Generate code for the SH1 🌸-m2— Generate code for the SH2 🌸-m2e— Generate code for the SH2e 🎀-m2a-nofpu— Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used 🙅-m2a-single-only— Generate code for the SH2a-FPU in such a way that no double-precision floating-point operations are used 🔥-m2a-single— Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default ✨-m2a— Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default 💫-m3— Generate code for the SH3 🦋-m3e— Generate code for the SH3e 🦋-m4-nofpu— Generate code for the SH4 without a floating-point unit 🌈-m4-single-only— Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic 🫶-m4-single— Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default 💖-m4— Generate code for the SH4 👑-m4-100— Generate code for SH4-100 🧚-m4-100-nofpu— Generate code for SH4-100 in such a way that the floating-point unit is not used 🙈-m4-100-single— Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default 🎀-m4-100-single-only— Generate code for SH4-100 in such a way that no double-precision floating-point operations are used 💅-m4-200— Generate code for SH4-200 ⭐-m4-200-nofpu— okay so this one generates code for SH4-200 in a way that the floating-point unit never gets used 💅🌸-m4-200-single— SH4-200 code, assuming the FPU is in single-precision mode by default 🎀-m4-200-single-only— SH4-200 code with no double-precision floating-point operations, bestie 🦋-m4-300— generates code for SH4-300 ✨-m4-300-nofpu— SH4-300 code that doesn't use the FPU at all 🧚-m4-300-single— SH4-300 code with no double-precision floating-point operations 💜-m4-300-single-only— SH4-300 code with no double-precision floating-point operations. Yes, this is the same description as-m4-300-single, and the original page is giving us a little repeat moment 🙈-m4-340— SH4-340 code, with no MMU and no FPU 🍓-m4-500— SH4-500 code with no FPU. This one also passes-isa=sh4-nofputo the assembler, so it's doing extra work for you 🔥-m4a-nofpu— this is for the SH4al-dsp, or for an SH4a in a way where the floating-point unit isn't used 🌈-m4a-single-only— SH4a code with no double-precision floating-point operations 💅-m4a-single— SH4a code, assuming the FPU is in single-precision mode by default ⭐-m4a— generates code for the SH4a 👑-m4al— same as-m4a-nofpu, except it also implicitly passes-dspto the assembler. Lowkey iconic, but GCC doesn't generate any DSP instructions at the moment, so don't expect them yet 🫶-mb— compiles code for the processor in big-endian mode 🌸-ml— compiles code for the processor in little-endian mode 🦋-mdalign— aligns doubles at 64-bit boundaries. Heads up, this changes the calling conventions, so some functions from the standard C library won't work unless you recompile them first with-mdalign⚠️ It's giving "read the fine print or else" 💀
Careful, bestie:-mdalignchanges the calling conventions, so standard C library functions can break unless you recompile the library with-mdalignfirst 😭
-mrelax— shortens some address references at link time, when it's possible. It does this by using the linker option-relax✨-mbigtable— uses 32-bit offsets in"switch"tables. The default is 16-bit offsets, so this one is the upgrade 🎀-mbitops— enables bit manipulation instructions on SH2A 💖-mfmovd— enables thefmovdinstruction. Make sure you check-mdalignfor alignment constraints 🧚-mrenesas— makes your code follow the calling conventions defined by Renesas 👑-mno-renesas— makes your code follow the calling conventions GCC used before the Renesas conventions existed. This is the default for all targets of the SH toolchain, so it's the main character by default 🌈-mnomacsave— marks the"MAC"register as call-clobbered, even if-mrenesasis given 🔥-mieee,-mno-ieee— these control IEEE compliance for floating-point comparisons, which affects how cases with unordered comparison results get handled. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeegets implicitly set, which makes floating-point greater-equal and less-equal comparisons faster. You can override the implicit settings by specifying either-mieeeor-mno-ieee💅✨-minline-ic_invalidate— inlines code that invalidates instruction cache entries after setting up nested function trampolines 🦋 Ngl this one has a lot of conditions:- It has no effect if
-musermodeis in effect and the selected code generation option (e.g.-m4) doesn't allow the use of the"icbi"instruction. - If the selected code generation option doesn't allow
"icbi", and-musermodeisn't in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, it also fails if the cache line had been mapped via the TLB and has become unmapped 💀
- It has no effect if
-misize— dumps instruction size and location in the assembly code 🍓-mpadstruct— this one is deprecated 🥀 It pads structures to a multiple of 4 bytes, which is incompatible with the SH ABI, so let it go 💔-matomic-model=model— sets the model of atomic operations, plus additional parameters, as a comma separated list. For the full scoop on atomic built-in functions, check out __atomic Builtins 📖 These are the models and parameters that are supported:none— disables compiler generated atomic sequences and emits library calls for atomic operations instead. This is the default if the target is not"sh*-*-linux*".soft-gusa— generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated sequences need extra support from the interrupt/exception handling code of the system, and they're only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is"sh*-*-linux*"and the chip is SH3* or SH4*. When the target is SH4A, this option also partially uses the hardware atomic instructions"movli.l"and"movco.l"to make more efficient code, unlessstrictis specified ✨soft-tcb— generates software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences that can also be used on SH1* and SH2* targets. The generated sequences need extra support from the interrupt/exception handling code and are only suitable for single-core systems. When you use this model, thegbr-offset=parameter has to be specified as well 🎀soft-imask— generates software atomic sequences that temporarily disable interrupts by setting"SR.IMASK = 1111". This model only works when the program runs in privileged mode, and it's only suitable for single-core systems. It doesn't need additional support from the interrupt/exception handling code. This model is enabled by default when the target is"sh*-*-linux*"and the chip is SH1* or SH2* 🧚hard-llcs— generates hardware atomic sequences using only the"movli.l"and"movco.l"instructions. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions only support 32-bit atomic variable access, 8-bit and 16-bit variables get emulated with 32-bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if it's executed on an SH4A system. This model doesn't need additional support from the interrupt/exception handling code 🦋gbr-offset=— specifies the offset in bytes of the variable in the thread control block structure that the generated atomic sequences should use when thesoft-tcbmodel is selected. For other models this parameter is ignored. The value must be an integer multiple of four and in the range0-1020💜strict— prevents mixing multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model only 🔒
-mtas— generates the"tas.b"opcode for"__atomic_test_and_set". Notice that depending on your particular hardware and software configuration, this can hurt overall performance because of the operand cache line flushes that the"tas.b"instruction implies. On multi-core SH4A processors, the"tas.b"instruction must be used with caution, since it can cause data corruption for certain cache configurations 🙈-mprefergot— when generating position-independent code, emits function calls using the Global Offset Table instead of the Procedure Linkage Table 🌸-musermode,-mno-usermode— "Don't allow (allow)" the compiler to generate privileged mode code. Specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code wouldn't work in user mode.-musermodeis the default when the target is"sh*-*-linux*". If the target is SH1* or SH2*,-musermodehas no effect, since there's no user mode on those chips 🔐-multcost=number— sets the cost to assume for a multiply insn 🔢-mdiv=strategy— sets the division strategy used for integer division operations.strategycan be one of these:call-div1— calls a library function that uses the single-step division instruction"div1"to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact 💅call-fp— calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that don't have a double precision FPU defaults to"call-div1"🔥call-table— calls a library function that uses a lookup table for small divisors, and the"div1"instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that don't have dynamic shift instructions defaults to"call-div1"🧚
When a division strategy hasn't been specified, the default strategy gets selected based on the current target. For SH2A, the default strategy is to use the "divs" and "divu" instructions instead of library function calls ✨
-maccumulate-outgoing-args— reserves space once for outgoing arguments in the function prologue, rather than around each call. This is generally beneficial for performance and size. It's also needed for unwinding, so the stack frame doesn't change around conditional code 🦋-mdivsi3_libfunc=name— sets the name of the library function used for 32-bit signed division toname. This only affects the name used in the call division strategies. The compiler still expects the same sets of input/output/clobbered registers as if this option weren't present 🎀-mfixed-range=register-range— generates code that treats the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is super useful when compiling kernel code 👑 A register range is specified as two registers separated by a dash, and you can specify multiple register ranges separated by a comma 💖
okay so the branch stuff is next, and it's giving performance 💅
-mbranch-cost=num— ✨ Tells the compiler to treat num as the cost of a branch instruction. Higher numbers push it to try harder for branch-free code when that's possible 🦋. If you don't set it, the value gets picked based on the processor you're compiling for 🧚.-mzdcbranch,-mno-zdcbranch— 🎀-mzdcbranchsays the zero displacement conditional branchesbtandbfare fast, so the compiler prefers those branch sequences. It's on by default for SH4 and SH4A 👑. You can turn it off with-mno-zdcbranch.-mcbranch-force-delay-slot— 🙈 Forces the delay slot to be used for conditional branches. If there's no suitable instruction, it fills the slot with anop💀. It's off by default, but you can flip it on to work around the hardware bugs found in the original SH7055.-mfused-madd,-mno-fused-madd— 🔥 Makes the code use (or not use) the floating-point multiply and accumulate instructions. These are generated by default when hardware floating point is in use 🌈. Heads up:-mfused-maddis now mapped to the machine-independent-ffp-contract=fast, and-mno-fused-maddis mapped to-ffp-contract=off🫶.-mfsca,-mno-fsca— 🦋 Lets the compiler emit thefscainstruction for sine and cosine approximations, or not.-mfscahas to go together with-funsafe-math-optimizations🍓. It's on by default for SH4A.-mno-fscaturns off the sine and cosine approximations even if-funsafe-math-optimizationsis in effect 😭.-mfsrra,-mno-fsrra— 💜 Same vibe, but for thefsrrainstruction used in reciprocal square root approximations.-mfsrrahas to be paired with-funsafe-math-optimizationsand-ffinite-math-only✨. It's enabled by default for SH4A.-mno-fsrradisables the reciprocal square root approximations even when both of those flags are in effect 💅.-mpretend-cmove— 🌸 Prefers zero-displacement conditional branches for conditional move instruction patterns. This can lead to faster code on the SH4 processor 🏎️.-mfdpic— 🧁 Generates code using the FDPIC ABI.
✨ Solaris 2 Options ✨
These -m options are supported on Solaris 2 🫶:
-mclear-hwcap— 🧼 Tells the compiler to remove the hardware capabilities that the Solaris assembler generates. This is only needed when object files use ISA extensions the current machine doesn't support, but the code checks at runtime whether or not to use them 🔍.-mimpure-text— 🍑 Used alongside-shared, this tells the compiler not to pass-z textto the linker when it's linking a shared object. With it, you can link position-dependent code into a shared object 🎀.
⚠️-mimpure-textsilences the "relocations remain against allocatable but non-writable sections" linker error message. BUT the necessary relocations trigger copy-on-write, so the shared object is not actually shared across processes 😭. Instead of reaching for-mimpure-text, compile all your source code with-fpicor-fPIC🫶.
These switches are supported in addition to the above on Solaris 2 🌈:
-pthreads— 🧵 This is a synonym for-pthread.
🌸 SPARC Options 🌸
These -m options are supported on the SPARC 🦋:
-mno-app-regs,-mapp-regs— 👑-mapp-regsmakes the output use global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Just like global register 1, each of global registers 2 through 4 is then treated as an allocable register that gets clobbered by function calls. This is the default ✨.
If you want to be fully SVR4 ABI-compliant (with a bit of a performance hit), specify -mno-app-regs 💅. You should compile libraries and system software with this option 🧚.
-mflat,-mno-flat— 🍓 With-mflat, the compiler skips the save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model 🫧. The local registers and the input registers (0--5) are still treated as "call-saved" registers and get saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions, except for leaf functions. This is the normal operating mode 💜.
-mfpu,-mhard-float— 🔥 Generates output containing floating-point instructions. This is the default 💅.-mno-fpu,-msoft-float— 🙈 Generates output containing library calls for floating point instead.
⚠️ Warning, bestie: the requisite libraries are not available for all SPARC targets 😭. Normally the facilities of the machine's usual C compiler get used, but that can't be done directly in cross-compilation. You have to make your own arrangements to provide suitable library functions for cross-compilation 🛠️. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support 🎀.
-msoft-float changes the calling convention in the output file, so it's only useful if you compile the whole program with this option 💀. You also need to compile libgcc.a, the library that comes with GCC, with -msoft-float for this to work 🫶.
-mhard-quad-float— 🌈 Generates output containing quad-word (long double) floating-point instructions ⚡.-msoft-quad-float— 🧁 Generates output containing library calls for quad-word (long double) floating-point instructions. The functions called are the ones specified in the SPARC ABI. This is the default 💖.
As of this writing, there are no SPARC implementations with hardware support for the quad-word floating-point instructions 😭. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of that trap handler overhead, this is much slower than calling the ABI library routines 🐢. So -msoft-quad-float is the default, periodt 💅.
-mno-unaligned-doubles,-munaligned-doubles— 📏-mno-unaligned-doublesassumes that doubles have 8-byte alignment. This is the default ✨.
With -munaligned-doubles, GCC only assumes doubles have 8-byte alignment if they're 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 isn't the default because it results in a performance loss, especially for floating-point code 💀.
-muser-mode,-mno-user-mode— 🛡️-mno-user-modemeans don't generate code that can only run in supervisor mode. This is only relevant for thecasainstruction emitted for the LEON3 processor 🧚.-mno-user-modeis the default.-mfaster-structs,-mno-faster-structs— 🏎️ With-mfaster-structs, the compiler assumes structures should have 8-byte alignment. This enables the use of pairs oflddandstdinstructions for copies in structure assignment, instead of twice as manyldandstpairs 🌸. But this changed alignment directly violates the SPARC ABI, so it's only meant for targets where the developer acknowledges their resulting code isn't directly in line with the ABI rules 🙈.-mstd-struct-return,-mno-std-struct-return— 🔍 With-mstd-struct-return, the compiler generates checking code in functions returning structures or unions. That checking detects size mismatches between the two sides of function calls, as per the 32-bit ABI 🎀.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode 💅.
-mlra,-mno-lra— 🧩 Enables Local Register Allocation. This has been the default for SPARC since GCC 7, so you need to pass-mno-lraif you want the old Reload 🕰️.-mcpu=cpu_type— 🧠 Sets the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. The supported values arev7,cypress,v8,supersparc,hypersparc,leon,leon3,leon3v7,leon5,sparclite,f930,f934,sparclite86x,sparclet,tsc701,v9,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8💖.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor 🫶. -mcpu=native has no effect if GCC doesn't recognize the processor 💀.
The default instruction scheduling parameters get used for the values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, and v9 🌈.
Here's the lowdown on each supported architecture and its supported implementations 👑:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclite—f930,f934,sparclite86xsparclet—tsc701v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture 🦋. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc. 🖥️
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture ✨. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions that exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series 🔥.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture 💜. This adds the integer multiply, integer divide step and scan ("ffs") instructions, which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU 🌸. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU 🎀.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture 🦋. That adds the integer multiply, multiply/accumulate, integer divide step, and scan ("ffs") instructions, which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler also optimizes for the TEMIC SPARClet chip 💅
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture ✨ That adds 64-bit integer and floating-point move instructions, 3 extra floating-point condition code registers, and conditional move instructions. The -mcpu values below each add an extra layer of optimization for a specific chip 🔥
-mcpu=ultrasparc— optimizes for the Sun UltraSPARC I/II/IIi chips 🌸-mcpu=ultrasparc3— optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips 🌸-mcpu=niagara— optimizes for Sun UltraSPARC T1 chips 🌈-mcpu=niagara2— optimizes for Sun UltraSPARC T2 chips 🌈-mcpu=niagara3— optimizes for Sun UltraSPARC T3 chips 🌈-mcpu=niagara4— optimizes for Sun UltraSPARC T4 chips 🌈-mcpu=niagara7— optimizes for Oracle SPARC M7 chips 👑-mcpu=m8— optimizes for Oracle M8 chips 👑-mtune=cpu_type— sets the instruction scheduling parameters for the machine typecpu_type, but unlike-mcpu=cpu_type, it does not change the instruction set or register set 🧚
The same values that work for -mcpu=cpu_type also work for -mtune=cpu_type, but the only really useful ones are the values that pick a specific CPU implementation: cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, and m8. With native Solaris and GNU/Linux toolchains, you can also use native 🫶
-mv8plus,-mno-v8plus— with-mv8plus, GCC generates code for the SPARC-V8+ ABI 💜 The big difference from V8 is that the global and out registers count as 64 bits wide. This is on by default on Solaris in 32-bit mode for all SPARC-V9 processors ✨-mvis,-mno-vis— with-mvis, GCC uses the UltraSPARC Visual Instruction Set extensions 🦋 The default is-mno-vis.-mvis2,-mno-vis2— with-mvis2, GCC uses version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is-mvis2when you target a CPU that supports those instructions, like UltraSPARC-III and later. Setting-mvis2also sets-mvis💖-mvis3,-mno-vis3— with-mvis3, GCC uses version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is-mvis3when you target a CPU that supports those instructions, like niagara-3 and later. Setting-mvis3also sets-mvis2and-mvis💅-mvis4,-mno-vis4— with-mvis4, GCC uses version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is-mvis4when you target a CPU that supports those instructions, like niagara-7 and later. Setting-mvis4also sets-mvis3,-mvis2, and-mvis🔥-mvis4b,-mno-vis4b— with-mvis4b, GCC uses version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the extra VIS instructions introduced in the Oracle SPARC Architecture 2017 👑 The default is-mvis4bwhen you target a CPU that supports those instructions, like m8 and later. Setting-mvis4balso sets-mvis4,-mvis3,-mvis2, and-mvis😭-mcbcond,-mno-cbcond— with-mcbcond, GCC uses the UltraSPARC Compare-and-Branch-on-Condition instructions 🎀 The default is-mcbcondwhen you target a CPU that supports them, like Niagara-4 and later.-mfmaf,-mno-fmaf— with-mfmaf, GCC uses the UltraSPARC Fused Multiply-Add Floating-point instructions 🌸 The default is-mfmafwhen you target a CPU that supports them, like Niagara-3 and later.-mfsmuld,-mno-fsmuld— with-mfsmuld, GCC uses the Floating-point Multiply Single to Double (FsMULd) instruction 🧚 The default is-mfsmuldwhen you target a CPU that supports the V8 or V9 architecture versions with an FPU, except-mcpu=leon.-mpopc,-mno-popc— with-mpopc, GCC uses the UltraSPARC Population Count instruction 🍓 The default is-mpopcwhen you target a CPU that supports it, like Niagara-2 and later.-msubxc,-mno-subxc— with-msubxc, GCC uses the UltraSPARC Subtract-Extended-with-Carry instruction 💜 The default is-msubxcwhen you target a CPU that supports it, like Niagara-7 and later.-mfix-at697f— turns on the documented workaround for the single erratum of the Atmel AT697F processor (which is erratum #13 of the AT697E processor) 🛠️-mfix-ut699— turns on the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor 🛠️-mfix-ut700— turns on the documented workaround for the back-to-back store errata of the UT699E/UT700 processor 🛠️-mfix-gr712rc— turns on the documented workaround for the back-to-back store errata of the GR712RC processor 🛠️
These -m options are also supported on SPARC-V9 processors in 64-bit environments 🌈
-m32,-m64— generates code for a 32-bit or 64-bit environment. The 32-bit environment setsint,long, and pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits 🎀-mcmodel=which— sets the code model to one of these:medlow— the Medium/Low code model: 64-bit addresses, and programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked 🌙medmid— the Medium/Middle code model: 64-bit addresses, and programs must be linked in the low 44 bits of memory. The text and data segments must each be less than 2GB in size, and the data segment must be located within 2GB of the text segment 🌙medany— the Medium/Anywhere code model: 64-bit addresses, and programs can be linked anywhere in memory. The text and data segments must each be less than 2GB in size, and the data segment must be located within 2GB of the text segment ✨embmedany— the Medium/Anywhere code model for embedded systems: 64-bit addresses, and the text and data segments must each be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register%g4points to the base of the data segment. Programs are statically linked, and PIC is not supported 🤖
-mmemory-model=mem-model— sets the memory model the processor uses to one of these:default— the default memory model for the processor and operating system 💅rmo— Relaxed Memory Order 💅pso— Partial Store Order 💅tso— Total Store Order 💅sc— Sequential Consistency 💅
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, and they get set in the processor's "PSTATE.MM" field 📚
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes the stack pointer (and the frame pointer, if there is one) is offset by-2047, which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, GCC assumes there's no such offset 😭
Options for System V
These extra options are available on System V Release 4 so they match other compilers on those systems 🧚
-G— creates a shared object. It's recommended to use-symbolicor-sharedinstead 🙈-Qy— identifies the version of each tool the compiler used, and puts that info in a".ident"assembler directive in the output 🫶-Qn— stops adding".ident"directives to the output file (this is the default) 🫶-YP,dirs— searches the directoriesdirs, and no others, for libraries specified with-l📂-Ym,dir— looks in the directorydirto find the M4 preprocessor. The assembler uses this option 📂
TILE-Gx Options
These -m options are supported on the TILE-Gx ✨
-mcmodel=small— generates code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits, and absolute addresses support the full address range 🦋-mcmodel=large— generates code for the large model. There's no limitation on call distance, PC-relative addresses, or absolute addresses 👑-mcpu=name— selects the type of CPU to target. Right now, the only supported type istilegx💖-m32,-m64— generates code for a 32-bit or 64-bit environment. The 32-bit environment setsint,long, and pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits 🎀-mbig-endian,-mlittle-endian— generates code in big-endian or little-endian mode, respectively 🌈
TILEPro Options
These -m options are supported on the TILEPro 🌸
-mcpu=name— selects the type of CPU to target. Right now, the only supported type istilepro💖-m32— generates code for a 32-bit environment, which setsint,long, and pointer to 32 bits. This is the only supported behavior, so the flag is basically ignored 🙈
V850 Options
These -m options are defined for V850 implementations 💜
-mlong-calls,-mno-long-calls— treats all calls as being far away (or near). If calls are assumed to be far away, the compiler always loads the function's address into a register and calls indirectly through the pointer 🔥-mno-ep,-mep—-mno-epstops optimizing basic blocks that use the same index pointer 4 or more times to copy the pointer into the"ep"register, and-mepdoes that optimization. That optimization also lets the compiler use the shorter"sld"and"sst"instructions. The-mepoption is on by default if you optimize 🧚-mno-prolog-function,-mprolog-function— 💅 Okay so-mno-prolog-functiontells GCC to not call external helper functions for saving and restoring registers in the prologue and epilogue of a function, while-mprolog-functiontells it to use them ✨. The helpers are slower, but they take up less code space when more than one function saves the same number of registers.-mprolog-functionis on by default if you optimize 🔥.-mspace— 🧚 Tries to make the code as small as possible. Right now that just turns on-mepand-mprolog-function🌸.-mtda=n— 🎀 Puts static or global variables that arenbytes or less into the tiny data area that theepregister points to. The tiny data area holds up to 256 bytes total (128 bytes for byte references) 🤏.-msda=n— 💖 Puts static or global variables that arenbytes or less into the small data area that thegpregister points to. That small data area can hold up to 64 kilobytes ✨.-mzda=n— 🦋 Puts static or global variables that arenbytes or less into the first 32 kilobytes of memory.-mv850— 👑 Says the target processor is the V850.-mv850e3v5— 🔥 Says the target processor is the V850E3V5. The preprocessor constant__v850e3v5__gets defined if you use this one.-mv850e2v4— 🌈 Says the target processor is the V850E3V5 (yep, that's what it says). It's an alias for-mv850e3v5.-mv850e2v3— 🫶 Says the target processor is the V850E2V3. The preprocessor constant__v850e2v3__gets defined if you use this one.-mv850e2— 🍓 Says the target processor is the V850E2. The preprocessor constant__v850e2__gets defined if you use this one.-mv850e1— 💜 Says the target processor is the V850E1. The preprocessor constants__v850e1__and__v850e__both get defined if you use this one.-mv850es— 🌸 Says the target processor is the V850ES. It's an alias for-mv850e1.-mv850e— ✨ Says the target processor is the V850E. The preprocessor constant__v850e__gets defined if you use this one.
If none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3 or -mv850e3v5 are set, GCC just picks a default target processor for you and defines the matching __v850*__ preprocessor constant 🧚.
The preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant you're targeting 💅.
-mdisable-callt,-mno-disable-callt— 🙈 This one suppresses generation of theCALLTinstruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture. It's enabled by default when the RH850 ABI is in use (see-mrh850-abi), and disabled by default when the GCC ABI is in use. IfCALLTinstructions are being generated, the C preprocessor symbol__V850_CALLT__gets defined 🦋.-mrelax,-mno-relax— 🌈 Passes on (or doesn't pass on) the-mrelaxcommand-line option to the assembler.-mlong-jumps,-mno-long-jumps— 💀 Disables (or re-enables) the generation of PC-relative jump instructions.-msoft-float,-mhard-float— 🔥 Disables (or re-enables) the generation of hardware floating point instructions. This only matters when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated, the C preprocessor symbol__FPU_OK__gets defined. Otherwise the symbol__NO_FPU__gets defined.-mloop— 🧚 Enables the use of the e3v5LOOPinstruction. It's not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈.-mrh850-abi,-mghs— 👑 Enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned via a memory pointer rather than a register.
- Large structures and unions (more than 8 bytes in size) are passed by value.
- Functions are aligned to 16-bit boundaries.
- The
-m8byte-aligncommand-line option is supported. - The
-mdisable-calltcommand-line option is enabled by default. The-mno-disable-calltcommand-line option is not supported.
When this version of the ABI is enabled, the C preprocessor symbol __V850_RH850_ABI__ gets defined 💖.
-mgcc-abi— 🌸 Enables support for the old GCC version of the V850 ABI. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned in register
r10. - Large structures and unions (more than 8 bytes in size) are passed by reference.
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- The
-m8byte-aligncommand-line option is not supported. - The
-mdisable-calltcommand-line option is supported but not enabled by default.
- Integer sized structures and unions are returned in register
When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ gets defined 🍓.
-m8byte-align,-mno-8byte-align— 🎀 Enables support fordoubleandlong longtypes to be aligned on 8-byte boundaries. By default, the alignment of all objects is restricted to at most 4 bytes. When-m8byte-alignis in effect, the C preprocessor symbol__V850_8BYTE_ALIGN__gets defined ✨.-mbig-switch— 🫶 Generates code suitable for big switch tables. Only use this if the assembler or linker complains about out of range branches within a switch table 😭.-mapp-regs— 💅 Makes the compiler user2andr5in the code it generates. This is the default.-mno-app-regs— 🔒 Makesr2andr5get treated as fixed registers.
🦋 VAX Options 🦋
These -m options are defined for the VAX:
-munix— 🌸 Doesn't output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX can't handle across long ranges.-mgnu— 🧚 Does output those jump instructions, on the assumption that the GNU assembler is being used.-mg— 💖 Outputs code for G-format floating-point numbers instead of D-format.
🌈 Visium Options 🌈
-mdebug— 🔥 A program that does file I/O and is destined to run on an MCM target should be linked with this option. It causes the librarieslibc.aandlibdebug.ato be linked. The program should be run on the target under the control of the GDB remote debugging stub 🙈.-msim— 🦋 A program that does file I/O and is destined to run on the simulator should be linked with the option. This causes the librarieslibc.aandlibsim.ato be linked.-mfpu,-mhard-float— ⭐ Generates code containing floating-point instructions. This is the default.-mno-fpu,-msoft-float— 💜 Generates code containing library calls for floating-point.
⚠️-msoft-floatchanges the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatfor this to work 💀.
-mcpu=cpu_type— 👑 Sets the instruction set, register set, and instruction scheduling parameters for machine typecpu_type. Supported values forcpu_typearemcm,gr5andgr6.mcmis a synonym ofgr5, kept around for backward compatibility 🌸.- By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture ✨.
- With
-mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions 🧚.
-mtune=cpu_type— 🎀 Sets the instruction scheduling parameters for machine typecpu_type, but doesn't set the instruction set or register set that-mcpu=cpu_typewould.-msv-mode— 🫶 Generates code for supervisor mode, where there are no restrictions on access to general registers. This is the default.-muser-mode— 🔒 Generates code for user mode, where access to some general registers is forbidden. On the GR5, registersr24tor31can't be accessed in this mode. On the GR6, only registersr29tor31are affected.
🌸 VMS Options 🌸
These -m options are defined for the VMS implementations:
-mvms-return-codes— 💅 Returns VMS condition codes frommain. The default is to return POSIX-style condition (e.g. error) codes.-mdebug-main=prefix— 🔥 Flags the first routine whose name starts withprefixas the main routine for the debugger.-mmalloc64— 💖 Defaults to 64-bit memory allocation routines.-mpointer-size=size— 🦋 Sets the default size of pointers. Possible options forsizeare32orshortfor 32-bit pointers,64orlongfor 64-bit pointers, andnofor supporting only 32-bit pointers. Thenooption disablespragma pointer_size🙈.
💜 VxWorks Options 💜
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target 🧚.
-mrtp— 🌈 GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro__RTP__✨.-non-static— 🎀 Links an RTP executable against shared libraries rather than static libraries. The options-staticand-sharedcan also be used for RTPs, and-staticis the default 🫶.-Bstatic,-Bdynamic— 👑 These options are passed down to the linker. They're defined for compatibility with Diab 🍓.-Xbind-lazy— ⭐ Enables lazy binding of function calls. This option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab 🔥.-Xbind-now— 🌸 Disables lazy binding of function calls. This option is the default and is defined for compatibility with Diab 💅.
🦋 x86 Options 🦋
These -m options are defined for the x86 family of computers.
-march=cpu-type— 💖 Generates instructions for the machine typecpu-type. Unlike-mtune=cpu-type, which only tunes the generated code for the specifiedcpu-type,-march=cpu-typelets GCC generate code that may not run at all on processors other than the one indicated. Specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise 🧚.
💀 Real talk, bestie: with -march=cpu-type, the code may not run at all on processors other than the one you named 😭.
The choices for cpu-type are:
native: Okay bestie, this is the "just use whatever my laptop has" option 💻✨ GCC checks the processor of the machine doing the compiling and generates code for that CPU.-march=nativeturns on every instruction subset the local machine supports, so the result might not run on other machines 😭-mtune=nativeoptimizes the code for the local machine, but it still stays within the limits of the selected instruction set 💅x86-64: A generic CPU with 64-bit extensions 🌸x86-64-v2,x86-64-v3,x86-64-v4: These pick the matching micro-architecture level from the x86-64 psABI, so they're giving standardized feature sets 🦋 On ABIs other than the x86-64 psABI, they select the same CPU features that the x86-64 psABI documents for that micro-architecture level.
Since these values don't have a matching -mtune setting, using -march with them turns on generic tuning. If you want specific tuning, use -mtune=other-cpu-type with an appropriate other-cpu-type value 🧚
i386: The original Intel i386 CPU, the OG 👑i486: Intel i486 CPU. (No scheduling is implemented for this chip 🙈)i586,pentium: Intel Pentium CPU with no MMX support 🍓lakemont: Intel Lakemont MCU, based on the Intel Pentium CPU 🤏pentium-mmx: Intel Pentium MMX CPU, based on the Pentium core with MMX instruction set support ✨pentiumpro: Intel Pentium Pro CPU 🔥i686: When used with-march, this uses the Pentium Pro instruction set, so the code runs on all i686 family chips 💜 When used with-mtune, it means the same thing asgeneric.pentium2: Intel Pentium II CPU, based on the Pentium Pro core with MMX instruction set support 🎀pentium3,pentium3m: Intel Pentium III CPU, based on the Pentium Pro core with MMX and SSE instruction set support 🌈pentium-m: Intel Pentium M, the low-power version of the Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. This is what Centrino notebooks used 💻pentium4,pentium4m: Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support 🌸prescott: An improved version of the Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support 💖nocona: An improved version of the Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support 🦋core2: Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support ⭐nehalem: Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support 🫶westmere: Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support 🧚sandybridge: Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support 🌈ivybridge: Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support 🍓haswell: Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support 💅broadwell: Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED ADCX and PREFETCHW instruction set support ✨skylake: Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support 🔥bonnell: Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support 🌸silvermont: Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support 🦋goldmont: Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT and FSGSBASE instruction set support 💜goldmont-plus: Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support 👑tremont: Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support 🧚knl: Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support 🌈knm: Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support 🍓skylake-avx512: Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support 💖cannonlake: Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support 🦋icelake-client: Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support 🫶icelake-server: Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support 👑cascadelake: Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support 🌸cooperlake: Intel cooperlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support 💅tigerlake: Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT and KEYLOCKER instruction set support ✨sapphirerapids: Intel sapphirerapids CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI and AVX512BF16 instruction set support 🔥alderlake: Intel Alderlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support 🧚rocketlake— 💖 Intel Rocketlake, rocking 64-bit extensions plus a full buffet:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VBMI,AVX512IFMA,SHA,CLWB,UMIP,RDPID,GFNI,AVX512VBMI2,AVX512VPOPCNTDQ,AVX512BITALG,AVX512VNNI,VPCLMULQDQandVAESinstruction set support. 🧚k6— 🌸 AMD K6 withMMXsupport. Classic. 🦋k6-2,k6-3— 🎀 Upgraded AMD K6 chips withMMXand3DNow!support. Glow-up energy. ✨athlon,athlon-tbird— 🔥 AMD Athlon withMMX,3dNOW!, enhanced3DNow!and SSE prefetch instructions. Lowkey iconic. 👑athlon-4,athlon-xp,athlon-mp— 💅 Improved AMD Athlon withMMX,3DNow!, enhanced3DNow!and full SSE instruction set support. The sequel that ate. 🔥k8,opteron,athlon64,athlon-fx— 🌈 Processors built on the AMD K8 core with x86-64 instruction set support, covering the AMD Opteron, Athlon 64 and Athlon 64 FX chips. Ngl this one supersets (aka includes all of)MMX,SSE,SSE2,3DNow!, enhanced3DNow!and 64-bit instruction set extensions. 💜k8-sse3,opteron-sse3,athlon64-sse3— ⭐ Upgraded AMD K8 cores withSSE3support. 🫶amdfam10,barcelona— 🦋 CPUs built on AMD Family 10h cores with x86-64 instruction set support. This one supersetsMMX,SSE,SSE2,SSE3,SSE4A,3DNow!, enhanced3DNow!,ABMand 64-bit instruction set extensions. 💖bdver1— 🔥 AMD Family 15h cores with x86-64 instruction set support. It supersetsFMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions. 🎀bdver2— 🌸 AMD Family 15h core based CPUs with x86-64 instruction set support. It supersetsBMI,TBM,F16C,FMA,FMA4,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions. 🦋bdver3— 💅 AMD Family 15h core based CPUs with x86-64 instruction set support. It supersetsBMI,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,XOP,LWP,AES,PCLMUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions. 👑bdver4— 🔥 AMD Family 15h core based CPUs with x86-64 instruction set support. It supersetsBMI,BMI2,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,AVX2,XOP,LWP,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions. 🌈znver1— 🧚 AMD Family 17h core based CPUs with x86-64 instruction set support. It supersetsBMI,BMI2,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT, and 64-bit instruction set extensions. 💜znver2— 💖 AMD Family 17h core based CPUs with x86-64 instruction set support. It supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD, and 64-bit instruction set extensions. ✨znver3— 🦋 AMD Family 19h core based CPUs with x86-64 instruction set support. It supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES, and 64-bit instruction set extensions. 🌸znver4— 👑 AMD Family 19h core based CPUs with x86-64 instruction set support. It supersetsBMI,BMI2,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCLMUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT,RDPID,WBNOINVD,PKU,VPCLMULQDQ,VAES,AVX512F,AVX512DQ,AVX512IFMA,AVX512CD,AVX512BW,AVX512VL,AVX512BF16,AVX512VBMI,AVX512VBMI2,AVX512VNNI,AVX512BITALG,AVX512VPOPCNTDQ,GFNIand 64-bit instruction set extensions. 🔥btver1— 🫶 CPUs built on AMD Family 14h cores with x86-64 instruction set support. It supersetsMMX,SSE,SSE2,SSE3,SSSE3,SSE4A,CX16,ABMand 64-bit instruction set extensions. 🎀btver2— 🌈 CPUs built on AMD Family 16h cores with x86-64 instruction set support. This includesMOVBE,F16C,BMI,AVX,PCLMUL,AES,SSE4.2,SSE4.1,CX16,ABM,SSE4A,SSSE3,SSE3,SSE2,SSE,MMXand 64-bit instruction set extensions. 💅winchip-c6— 🧚 IDT WinChip C6 CPU, handled the same way as i486, plus extraMMXinstruction set support. 🌸winchip2— 🌸 IDT WinChip 2 CPU, handled the same way as i486, plus extraMMXand3DNow!instruction set support. 🦋c3— 🍓 VIA C3 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip.) 🙈c3-2— 🍓 VIA C3-2 (Nehemiah/C5XL) CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.) 🙈c7— 🍓 VIA C7 (Esther) CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈samuel-2— 🍓 VIA Eden Samuel 2 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip.) 🙈nehemiah— 🍓 VIA Eden Nehemiah CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.) 🙈esther— 🍓 VIA Eden Esther CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈eden-x2— 🍓 VIA Eden X2 CPU with x86-64,MMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈eden-x4— 🍓 VIA Eden X4 CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVXandAVX2instruction set support. (No scheduling is implemented for this chip.) 🙈nano— 🍓 Generic VIA Nano CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈nano-1000— 🍓 VIA Nano 1xxx CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈nano-2000— 🍓 VIA Nano 2xxx CPU with x86-64,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. (No scheduling is implemented for this chip.) 🙈nano-3000— 🍓 VIA Nano 3xxx CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🙈nano-x2— 🍓 VIA Nano Dual Core CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🙈nano-x4— 🍓 VIA Nano Quad Core CPU with x86-64,MMX,SSE,SSE2,SSE3,SSSE3andSSE4.1instruction set support. (No scheduling is implemented for this chip.) 🙈geode— 🧚 AMD Geode embedded processor withMMXand3DNow!instruction set support. 💜
✨ -mtune=cpu-type ✨
Tune cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. Picking a specific cpu-type schedules things perfectly for that chip, but the compiler won't generate any code that can't run on the default machine type unless you use a -march=cpu-type option. 🫶 For example, if GCC is configured for i686-pc-linux-gnu, then -mtune=pentium4 generates code tuned for Pentium 4 that still runs on i686 machines. 💖
The choices for cpu-type are the same as for -march. On top of that, -mtune supports 2 extra choices for cpu-type:
🌈 generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU your code will run on, use the matching -mtune or -march option instead of -mtune=generic. But if you don't know exactly what CPU your users will have, this is the one to use. 🦋
As new processors hit the market, the behavior of this option will change. So if you upgrade to a newer version of GCC, the code generated by this option will shift to match whatever processors are most common when that version of GCC is released. 🔥
There is no -march=generic option, because -march is about the instruction set the compiler can use, and there's no generic instruction set that works on every processor. -mtune, on the other hand, is about the processor (or, in this case, the collection of processors) the code is optimized for. 💅
💜 intel
Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU your code will run on, use the matching -mtune or -march option instead of -mtune=intel. But if you want your application to perform better on both Haswell and Silvermont, this is your option. 👑
As new Intel processors get deployed, the behavior of this option will change. So if you upgrade to a newer version of GCC, the code generated by this option will shift to reflect the most current Intel processors at the time that version of GCC is released. ⭐
There is no -march=intel option, because -march is about the instruction set the compiler can use, and there's no common instruction set that works on every processor. -mtune, on the other hand, is about the processor (or, in this case, the collection of processors) the code is optimized for. 🫶
-mcpu=cpu-type— okay so this one is just a synonym for-mtune, but it's deprecated 💀 it's on its way out, bestie 🥀-mfpmath=unit— this decides how the floating-point math gets done 🧮✨ Theunityou pick can be one of these:387— uses the standard 387 floating-point coprocessor that's on most chips, and gets emulated if it's missing 🦖 Code built with this runs basically everywhere, but the temporary results are computed in 80-bit precision instead of the precision the type asks for, so your numbers can come out a little different from most other chips. Check out-ffloat-storefor the full tea on that 🍵 This is the default choice for non-Darwin x86-32 targets.sse— uses the scalar floating-point instructions from the SSE instruction set 🚀 It's on Pentium III and newer chips, plus the AMD Athlon-4, Athlon XP, and Athlon MP chips. The earlier SSE version only handles single-precision math, so double and extended-precision math still go through 387 🙃 A later version, found only on Pentium 4 and AMD x86-64 chips, can do double-precision math too. For the x86-32 compiler, you have to use-march=cpu-type,-msse, or-msse2to turn on the SSE extensions and make this option actually work 🫠 For the x86-64 compiler, those extensions are on by default. The result should be way faster in most cases and dodge the numerical instability problems of 387 code 📈 but it might break existing code that expects temporaries to be 80 bits. This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and for x86-32 targets with the SSE2 instruction set when-ffast-mathis enabled 👑sse,387,sse+387,both— tries to use both instruction sets at the same time 🤹 This effectively doubles the number of available registers, and on chips with separate execution units for 387 and SSE, it doubles those execution resources too. Use it with care 🥺 It's still experimental, because the GCC register allocator doesn't model separate functional units very well, which means the performance can be unstable 🎢
-masm=dialect— picks the assembly dialect for the assembly instructions GCC outputs 📝 It also affects which dialect is used for basicasmand extendedasm. The supported choices, in dialect order, areattorintel. The default isatt. Darwin does not supportintel🍎🚫-mieee-fp,-mno-ieee-fp— controls whether the compiler uses IEEE floating-point comparisons 🔍 These ones correctly handle the case where the result of a comparison is unordered 🧐-m80387,-mhard-float— generates output that contains 80387 instructions for floating point 🔥-mno-80387,-msoft-float— generates output that contains library calls for floating point instead 📚
Warning: the requisite libraries are NOT part of GCC 😭 Normally the facilities of the machine's usual C compiler get used, but that can't be done directly in cross-compilation. You have to make your own arrangements to provide suitable library functions for cross-compilation 🛠️
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may still get emitted even if -msoft-float is used 👀
-mno-fp-ret-in-387— doesn't use the FPU registers for function return values 🙅♀️ The usual calling convention has functions returnfloatanddoublevalues in an FPU register, even if there's no FPU at all. The idea is that the operating system should emulate an FPU 🧚 Using-mno-fp-ret-in-387makes those values come back in ordinary CPU registers instead.-mno-fancy-math-387— some 387 emulators don't support thesin,cos, andsqrtinstructions for the 387 💀 Use this to avoid generating those instructions. It gets overridden when-marchsays the target CPU always has an FPU, because then the instruction doesn't need emulating 🙌 Also, these instructions aren't generated unless you use the-funsafe-math-optimizationsswitch too.-malign-double,-mno-align-double— controls whether GCC alignsdouble,long double, andlong longvariables on a two-word boundary or a one-word boundary 📏 Aligningdoublevariables on a two-word boundary gives code that runs somewhat faster on a Pentium, but it costs more memory 💾 On x86-64,-malign-doubleis enabled by default.
Warning: if you use -malign-double, structures containing the types above get aligned differently than the published x86-32 application binary interface specs say 😱 They won't be binary compatible with structures in code compiled without that switch.
-m96bit-long-double,-m128bit-long-double— these control the size of thelong doubletype 📐 The x86-32 application binary interface says the size is 96 bits, so-m96bit-long-doubleis the default in 32-bit mode. Modern architectures (Pentium and newer) preferlong doubleto be aligned to an 8- or 16-byte boundary, but inside arrays or structures that follow the ABI, that isn't possible 🙃 So-m128bit-long-doublealignslong doubleto a 16-byte boundary by padding it with an extra 32-bit zero 🌸 In the x86-64 compiler,-m128bit-long-doubleis the default, since its ABI sayslong doubleis aligned on a 16-byte boundary. Heads up that neither of these options gives any extra precision over the x87 standard of 80 bits for along double✨
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, and it also modifies the function calling convention for functions that takelong double. So they're not binary-compatible with code compiled without that switch 🚨
-mlong-double-64,-mlong-double-80,-mlong-double-128— these control the size of thelong doubletype too 💖 A size of 64 bits makeslong doubleequivalent todouble, and that's the default for the 32-bit Bionic C library. A size of 128 bits makeslong doubleequivalent to__float128, and that's the default for the 64-bit Bionic C library 🦋
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, and it also modifies the function calling convention for functions that takelong double. So they're not binary-compatible with code compiled without that switch 🚨
-malign-data=type— controls how GCC aligns variables 📦 The supported values fortypeare:compat— uses an increased alignment value that's compatible with GCC 4.8 and earlier. This is the default 🫶abi— uses the alignment value specified by the psABI 📜cacheline— uses an increased alignment value to match the cache line size 🧊
-mlarge-data-threshold=threshold— when-mcmodel=mediumis specified, data objects bigger thanthresholdget placed in the large data section 🐘 This value has to be the same across all objects linked into the binary, and it defaults to65535🔢-mrtd— switches to a different function-calling convention 🔄 In this one, functions that take a fixed number of arguments return with theret numinstruction, which pops their arguments as they return. That saves one instruction in the caller, since the caller doesn't need to pop the arguments anymore 🧹 You can give a single function this calling sequence with the function attributestdcall, and you can override-mrtdfor a function with the function attributecdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you can't use it if you need to call libraries compiled with the Unix compiler 🙅♀️
Warning: you must provide function prototypes for all functions that take a variable number of arguments (including printf), or incorrect code gets generated for calls to those functions 🫠
Warning: seriously incorrect code results if you call a function with too many arguments. Normally extra arguments are harmlessly ignored, but not with this one 💀
-mregparm=num— controls how many registers get 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 for a specific function with the function attributeregparm.
Warning: if you use this switch and num is nonzero, you must build all modules with the same value, including any libraries. That includes the system libraries and startup modules 🧱
-msseregparm— uses SSE register passing conventions forfloatanddoublearguments and return values ⚡ You can control this for a specific function with the function attributesseregparm.
Warning: if you use this switch, you must build all modules with the same value, including any libraries. That includes the system libraries and startup modules 🧱
-mvect8-ret-in-mem— returns 8-byte vectors in memory instead of MMX registers 🧠 This is the default on VxWorks, so it matches the ABI of the Sun Studio compilers up until version 12. Only use this option if you need to stay compatible with existing code made by those older compiler versions or older versions of GCC 🕰️-mpc32,-mpc64,-mpc80— set the 80387 floating-point precision to 32, 64, or 80 bits 🎚️-mpc32— rounds the significands of floating-point results to 24 bits (single precision) 🌱-mpc64— rounds them to 53 bits (double precision) 🌷-mpc80— rounds them to 64 bits (extended double precision), and this one's the default 👑
When you use this option, floating-point operations in higher precisions aren't available to the programmer unless you set the FPU control word explicitly 🫶
Okay so, this next chunk is where we get into the x86 stack and instruction-set switches, and bestie, the stack ones are serious 🫶 Pick the wrong alignment and your program can crash in ways that are super hard to trace 💀
Setting the rounding of floating-point operations below the default 80 bits can speed some programs up by 2% or more 🚀 But ngl, some math libraries assume extended-precision (80-bit) floating-point operations are on by default. If you use this option to drop precision below extended, those routines could lose a ton of accuracy through so-called "catastrophic cancellation" 😭 Not it.
-mdaz-ftz— This one flips the FTZ (flush-to-zero) and DAZ (denormals-are-zero) flags in the MXCSR register, which control floating-point calculations 🦋 SSE and AVX instructions, scalar and vector ones included, can get a speed boost when you turn these on with-mdaz-ftz. And if you say-mno-daz-ftz, GCC won't set the FTZ/DAZ flags at all 🙅♀️-mstackrealign— Realigns the stack at entry ✨ On x86, this generates an alternate prologue and epilogue that realigns the run-time stack when it needs to. It's the glue for mixing legacy code that keeps 4-byte stack alignment with modern code that keeps 16-byte alignment for SSE compatibility 🎀 You can also look at the attributeforce_align_arg_pointer, which works on individual functions.-mpreferred-stack-boundary=num— Tries to keep the stack boundary aligned to a 2 raised to num byte boundary 🌸 If you don't specify-mpreferred-stack-boundary, the default is4(16 bytes or 128 bits), so no stress there.
💀 Warning, bestie, this one is dangerous. When you're generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can keep the stack boundary aligned to an 8-byte boundary. But the x86-64 ABI requires 16-byte stack alignment, so this is ABI incompatible 🙈 It's only meant for a controlled environment where stack space is a seriously tight limit. It leads to wrong code when functions compiled with 16-byte stack alignment (like functions from a standard library) get called with a misaligned stack. SSE instructions may then cause misaligned memory access traps. On top of that, variable arguments get handled incorrectly for 16-byte aligned objects (including x87long doubleand__int128), which gives wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries, system libraries and startup modules too. No skipping!
-mincoming-stack-boundary=num— Assumes the incoming stack is aligned to a 2 raised to num byte boundary 💖 If you don't specify-mincoming-stack-boundary, GCC uses whatever-mpreferred-stack-boundarysays.
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or you'll eat significant run-time performance penalties 📉 On Pentium III, the SSE data type __m128 may not work properly if it isn't 16-byte aligned 😬
To keep values properly aligned on the stack, the stack boundary has to be at least as aligned as whatever any value stored on the stack needs. Every function also has to be generated so it keeps the stack aligned. So if a function compiled with a higher preferred stack boundary calls from a function compiled with a lower one, the stack will most likely end up misaligned 👀 Libraries that use callbacks should always stick with the default setting, periodt.
This extra alignment eats extra stack space and generally makes your code bigger 🍓 If you're working on something sensitive to stack usage, like embedded systems or operating system kernels, you may want to drop the preferred alignment to -mpreferred-stack-boundary=2.
- Instruction set switches — This whole family of switches turns on the use of instructions from the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI or CLDEMOTE extended instruction sets 🌈 Every one of them has a matching
-mno-option to turn that instruction use off 🙅♀️ The flags are:
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwideklThese extensions are also available as built-in functions, so check out x86 Built-in Functions for the details of which functions get enabled and disabled by these switches 🧚 If you want GCC to generate SSE/SSE2 instructions automatically from floating-point code (instead of 387 instructions), that's -mfpmath=sse 💅
Heads up: GCC suppresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions, or AVX equivalents for all SSEx instructions, when needed 🦋
These options let GCC use those extended instructions in generated code, even without -mfpmath=sse. If your app does run-time CPU detection, you need to compile separate files for each supported architecture, using the appropriate flags 🫶 Specifically, the file with the CPU detection code should be compiled without these options, bestie, it's giving "keep the detector clean" energy.
-mdump-tune-features— Tells GCC to dump the names of the x86 performance tuning features and their default settings 📋 You can use those names in-mtune-ctrl=feature-list.-mtune-ctrl=feature-list— Fine-grained control over x86 code generation features 🎛️ The feature-list is a comma-separated list of feature names. See also-mdump-tune-features. A feature gets turned on when it's not preceded by^, and turned off when it is. This one is meant for GCC developers, so be warned: using it may lead to code paths that testing never covered, and it can potentially cause compiler ICEs or runtime errors 💀-mno-default— Turns off all tunable features 🙅♀️ See also-mtune-ctrl=feature-listand-mdump-tune-features.-mcld— Makes GCC emit acldinstruction in the prologue of functions that use string instructions 🧵 String instructions depend on the DF flag to pick between autoincrement or autodecrement mode. The ABI says DF should be cleared on function entry, but some operating systems violate that by not clearing DF in their exception dispatchers. So the exception handler can run with DF set, which means string instructions go the wrong direction 😭 This option can be enabled by default on 32-bit x86 targets when you configure GCC with the--enable-cldconfigure option. If you want to suppresscldgeneration in that case, use the-mno-cldcompiler option.-mvzeroupper— Makes GCC emit avzeroupperinstruction before control flow leaves the function, which minimizes the AVX to SSE transition penalty and removes unnecessaryzeroupperintrinsics 🧹-mprefer-avx128— Tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer 🌸-mprefer-vector-width=opt— Tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform 📐 The options are:none— No extra limitations are applied to GCC beyond what the selected platform defines.128— Prefers 128-bit vector width for instructions.256— Prefers 256-bit vector width for instructions.512— Prefers 512-bit vector width for instructions.
-mcx16— Lets GCC generateCMPXCHG16Binstructions in 64-bit code to do compare-and-exchange operations on 16-byte aligned 128-bit objects 💜 This is handy for atomic updates of data structures bigger than one machine word. The compiler uses this instruction to implement__syncbuiltins. However, for__atomicbuiltins operating on 128-bit integers, a library call is always used, so don't expect magic there 🙈-msahf— Enables generation ofSAHFinstructions in 64-bit code 🍰 Early Intel Pentium 4 CPUs with Intel 64 support, from before the Pentium 4 G1 step in December 2005, didn't have theLAHFandSAHFinstructions that AMD64 supports. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, theSAHFinstruction is used to optimize thefmod,drem, andremainderbuilt-in functions. See Other Builtins for details ✨-mmovbe— Enables use of themovbeinstruction to implement__builtin_bswap32and__builtin_bswap64🔄-mshstk— Enables the shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET) 🛡️-mcrc32— Enables the built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32si, and__builtin_ia32_crc32dito generate thecrc32machine instruction 🦋-mmwait— okay so this one is the key 🔑 It makes the built-in functions__builtin_ia32_monitorand__builtin_ia32_mwaitactually generate themonitorandmwaitmachine instructions ✨-mrecip— this one is giving speed 💨 It lets GCC useRCPSSandRSQRTSS(plus their vectorized bestiesRCPPSandRSQRTPS) with an extra Newton-Raphson step for precision, instead ofDIVSSandSQRTSS(and their vectorized versions) for single-precision floating-point arguments 🦋 These only get generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Heads up bestie: the sequence has higher throughput than the non-reciprocal instruction, but its precision can drop by up to 2 ulp (i.e. the inverse of1.0equals0.99999994) 😭
Note that GCC already implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) with -ffast-math (or the option combination above), so you don't need -mrecip for that 💅
Also, GCC emits the sequence above with the extra Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the option combination above), so -mrecip isn't needed there either 🫶
-mrecip=opt— this one lets you pick exactly which reciprocal estimate instructions are allowed 🎀optis a comma-separated list, and any option can be flipped by putting a!in front of it:all— enables all estimate instructions 🌈default— enables the default instructions, equivalent to-mrecipnone— disables all estimate instructions, equivalent to-mno-recip🙅div— enables the approximation for scalar divisionvec-div— enables the approximation for vectorized divisionsqrt— enables the approximation for scalar square rootvec-sqrt— enables the approximation for vectorized square root
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations except for square root 🦋
-mveclibabi=type— this sets the ABI type for vectorizing intrinsics using an external library 📚 Picksvmlfor the Intel short vector math library, oracmlfor the AMD math core library. To use it, both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled, and an SVML or ACML ABI-compatible library has to be specified at link time 🔗
GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for the corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used.
-mabi=name— picks which calling convention to generate code for 🌸sysvis the ABI used on GNU/Linux and other systems, andmsis the Microsoft ABI. The default is the Microsoft ABI when targeting Microsoft Windows, and the SysV ABI on all other systems. You can control this for specific functions with thems_abiandsysv_abifunction attributes 💜-mforce-indirect-call— forces all calls to functions to be indirect 🧚 This is useful when using Intel Processor Trace, where it generates more precise timing information for function calls ⏱️-mmanual-endbr— ENDBR gets inserted at function entry only via thecf_checkfunction attribute 🙈 This is useful when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry 🫶-mcall-ms2sysv-xlogues— okay this one's a bit of a lore dump 📜 Because of differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function has to considerRSI,RDIandXMM6-15as clobbered. By default the code for saving and restoring these registers is emitted inline, which makes the prologues and epilogues pretty lengthy 😩 Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores instead, which shrinks the function size at the cost of a few extra instructions ✨-mtls-dialect=type— generates code to access thread-local storage using thegnuorgnu2conventions 🧵gnuis the conservative default, whilegnu2is more efficient, but it may add compile- and run-time requirements that can't be satisfied on all systems 💀-mpush-args,-mno-push-args— uses PUSH operations to store outgoing parameters 📦 This method is shorter and usually just as fast as the SUB/MOV method, and it's enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies 🦋-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments gets computed in the function prologue 🧠 This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size 😬 This switch implies-mno-push-args.-mthreads— supports thread-safe exception handling on MinGW 🧵 Programs that rely on it must compile and link all code with the-mthreadsoption. When compiling,-mthreadsdefines-D_MT; when linking, it links in a special thread helper library-lmingwthrdwhich cleans up per-thread exception-handling data ✨-mms-bitfields,-mno-ms-bitfields— enables or disables bit-field layout compatible with the native Microsoft Windows compiler 🎀
If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does 👀 Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.
This option is enabled by default for Microsoft Windows targets 🪟 This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing 🧩 The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:
- Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
- Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the
alignedattribute or thepackpragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:offset % alignment_requirement == 0 - Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
- If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced. For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.
- If a zero-length bit-field is inserted after a bit-field,
foo, and the alignment of the zero-length bit-field is greater than the member that follows it,bar, thenbaris aligned as the type of the zero-length bit-field. For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example,
t2has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of typeshort. - Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;Here, t4 takes up 4 bytes.
- 💅 Zero-length bit-fields that come after non-bit-field members get ignored, bestie. Check out this struct:
struct
{
char foo;
long : 0;
char bar;
} t5;Here, t5 takes up 2 bytes. 🎀
-mno-align-stringops— 🌸 Don't align the destination of inlined string operations. This makes the code smaller and can run faster when the destination is already aligned but GCC doesn't realize it, so it's a lowkey win.-minline-all-stringops— ✨ By default, GCC only inlines string operations when it knows the destination is aligned to at least a 4-byte boundary. This option inlines way more, so the code gets bigger, but code that leans on fastmemcpyandmemsetfor short lengths might get faster. It also turns on inline expansion ofstrlenfor all pointer alignments. Main character energy 👑-minline-stringops-dynamically— 🧚 For string operations where the size isn't known until runtime, you get run-time checks. Small blocks get inline code, and big blocks get a library call. Smart, right? 🧠-mstringop-strategy=alg— 🔥 This overrides GCC's internal heuristic for picking which algorithm to use when inlining string operations. The allowed values foralgare:rep_byte,rep_4byte,rep_8byte— expand using the i386repprefix at the specified size.byte_loop,loop,unrolled_loop— expand into an inline loop.libcall— always use a library call, no inlining drama.
-mmemcpy-strategy=strategy— 💖 This overrides the heuristic that decides whether__builtin_memcpygets inlined, and which inline algorithm to use when the expected copy size is known. Thestrategyis a comma-separated list ofalg:max_size:dest_aligntriplets. Thealgis one of the values from-mstringop-strategy, andmax_sizeis the biggest byte size thatalgis allowed to handle. The last triplet has to use amax_sizeof-1. Themax_sizevalues in the list must go in increasing order. The minimum byte size foralgis0for the first triplet, andmax_size + 1of the range before it for every other triplet. Very organized, bestie 📋-mmemset-strategy=strategy— 🦋 Basically the same vibe as-mmemcpy-strategy=, except this one controls how__builtin_memsetgets expanded.-momit-leaf-frame-pointer— 🌈 Don't keep the frame pointer in a register for leaf functions. This skips the instructions that save, set up, and restore frame pointers, and frees up an extra register in leaf functions. The option-fomit-leaf-frame-pointerdrops the frame pointer for leaf functions too, but ngl that might make debugging harder 🙈-mtls-direct-seg-refs,-mno-tls-direct-seg-refs— 🫶 This controls whether TLS variables can be accessed with offsets from the TLS segment register (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer has to be added. Whether that's valid depends on the operating system, and on whether it maps the segment to cover the entire TLS area. For systems using the GNU C Library, the default is on ✨-msse2avx,-mno-sse2avx— 💅 Tells the assembler to encode SSE instructions with the VEX prefix. The option-mavxturns this on by default.-mfentry,-mno-fentry— 🎀 If profiling is active (-pg), this puts the profiling counter call before the prologue. Heads up: on x86 architectures, the attributems_hook_prologueisn't possible at the moment for-mfentryand-pg. Not it 💀-mrecord-mcount,-mno-record-mcount— 🍓 If profiling is active (-pg), this generates a__mcount_locsection full of pointers to each profiling call. Super handy for automatically patching those calls in and out.-mnop-mcount,-mno-nop-mcount— 🌸 If profiling is active (-pg), the calls to the profiling functions get generated as NOPs. This is useful when they should get patched in later on, dynamically. It's likely only useful together with-mrecord-mcount.-minstrument-return=type— 👑 Instruments function exit in-pg -mfentryinstrumented functions with a call to the specified function. It only instruments true returns that end withret, not sibling calls that end with a jump. The valid types are:none— don't instrument at all.call— generate a call to__return__.nop5— generate a 5-byte nop.
-mrecord-return,-mno-record-return— 🦋 Generates a__return_locsection that points to all the return instrumentation code.-mfentry-name=name— 🧚 Sets the name of the__fentry__symbol that gets called at function entry for-pg -mfentryfunctions.-mfentry-section=name— 💜 Sets the name of the section where-mrecord-mcountcalls get recorded. The default is__mcount_loc.-mskip-rax-setup,-mno-skip-rax-setup— 🔥 When generating code for the x86-64 architecture with SSE extensions disabled,-mskip-rax-setupcan skip setting up the RAX register when no variable arguments are passed in vector registers.
⚠️ Warning: RAX is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments. So the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. GCC 4.4 or newer doesn't have those issues, regardless of the RAX register value. 😭
-m8bit-idiv,-mno-8bit-idiv— 💅 On some processors, like Intel Atom, 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both the dividend and the divisor are within the range of 0 to 255, an 8-bit unsigned integer divide gets used instead of the 32-bit/64-bit one. Galaxy brain 🧠✨-mavx256-split-unaligned-load,-mavx256-split-unaligned-store— 🌈 Splits 32-byte AVX unaligned loads and stores.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— 🛡️ Generates stack protection code using a canary atguard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default). This option only has an effect when-fstack-protectoror-fstack-protector-allis specified.
With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which segment register (%fs or %gs) to use as the base register for reading the canary, and what offset from that base register to use. The defaults for those are whatever the relevant ABI specifies. 🎀
-mgeneral-regs-only— 🌸 Generates code that only uses the general-purpose registers. This stops the compiler from using floating-point, vector, mask, and bound registers.-mindirect-branch=choice— 🦋 Converts indirect calls and jumps according tochoice. The default iskeep, which leaves indirect calls and jumps unmodified.thunkconverts them to call and return thunks.thunk-inlineconverts them to inlined call and return thunks.thunk-externconverts them to external call and return thunks, provided in a separate object file. You can control this behavior for a specific function with the function attribute"indirect_branch".
Note: -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern, since the thunk function may not be reachable in the large code model.
Note: -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable the control-flow check. 🫶
-mfunction-return=choice— 💜 Converts function returns according tochoice. The default iskeep, which leaves function returns unmodified.thunkconverts function returns to call and return thunks.thunk-inlineconverts them to inlined call and return thunks.thunk-externconverts them to external call and return thunks, provided in a separate object file. You can control this behavior for a specific function with the function attribute"function_return".
Note: -mindirect-return=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable the control-flow check.
Note: -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern, since the thunk function may not be reachable in the large code model.
-mindirect-branch-register— 🔥 Forces indirect calls and jumps to go through a register.-mharden-sls=choice— 🛡️ Generates code to mitigate straight line speculation (SLS) according tochoice. The default isnone, which disables all SLS hardening. The other options are:return— enables SLS hardening for function returns.indirect-jmp— enables SLS hardening for indirect jumps.all— enables all SLS hardening.
-mindirect-branch-cs-prefix— ✨ Adds a CS prefix to calls and jmps to an indirect thunk, where the branch target is in ther8-r15registers. This makes the call and jmp instructions 6 bytes long, so they can be replaced at run-time withlfence; call *%r8-r15orlfence; jmp *%r8-r15. Big brain move 🧠
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32,-m64,-mx32,-m16,-miamcu— 🌈 Generates code for a 16-bit, 32-bit, or 64-bit environment.-m32setsint,long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.-m64setsintto 32 bits andlongand pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only,-m64also turns off the-fno-picand-mdynamic-no-picoptions. 🍓-mx32setsint,long, and pointer types to 32 bits, and generates code for the x86-64 architecture.-m16is the same as-m32, except it outputs the".code16gcc"assembly directive at the beginning of the assembly output, so the binary can run in 16-bit mode.-miamcugenerates code that conforms to the Intel MCU psABI. It requires-m32to be turned on.
-mno-red-zone— 🦋 Doesn't use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI: it's a 128-byte area beyond the stack pointer that signal or interrupt handlers don't modify, so it can be used for temporary data without adjusting the stack pointer. The flag-mno-red-zonedisables this red zone. Bye bye, red zone 💅-mcmodel=small— okay so this is the default code model 💖 The program and its symbols have to be linked in the lower 2 GB of the address space. Pointers are 64 bits, and you can link statically or dynamically 🫶-mcmodel=kernel— this one is giving kernel energy 👑 The kernel runs in the negative 2 GB of the address space, and you have to use it for Linux kernel code. No skipping, bestie 💀-mcmodel=medium— the program gets linked in the lower 2 GB, and small symbols chill there too 🦋 Anything bigger than-mlarge-data-thresholdgets tossed into large data or BSS sections, and those can sit above 2GB. Static or dynamic linking both work 🌸-mcmodel=large— no assumptions about addresses and sizes of sections at all 🌈 Full freedom, no cap.-maddress-mode=long— long address mode 🧚 It's only for 64-bit and x32 environments, and it's the default for 64-bit ones ✨-maddress-mode=short— short address mode 🍓 Only for 32-bit and x32 environments, and it's the default for 32-bit and x32 ones.-mneeded,-mno-needed— this emits theGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for the Linux target 🔥 It tells you which micro-architecture ISA level the binary needs in order to run.
✨ x86 Windows Options ✨
Windows main character era 🪟💖 These extra options are for Microsoft Windows targets:
-mconsole— makes a console application 💻 It tells the linker to set the PE header subsystem type for console apps. Works for Cygwin and MinGW, and it's on by default there.-mdll— Cygwin and MinGW only 🦋 This makes a DLL (a dynamic link library, obviously 🙄), and it lets you pick the required runtime startup object and entry point.-mnop-fun-dllimport— Cygwin and MinGW 💅 Tells GCC to ignore the"dllimport"attribute. Bye, attribute.-mthread— MinGW only 🧵 Turns on MinGW-specific thread support.-municode— MinGW-w64 only 🌍 Predefines the"UNICODE"preprocessor macro and picks Unicode-capable runtime startup code.-mwin32— Cygwin and MinGW 🌸 Sets the typical Microsoft Windows predefined macros in the preprocessor. It doesn't change which runtime library or startup code gets picked, though.-mwindows— Cygwin and MinGW 🪟 Makes a GUI app by having the linker set the PE header subsystem type appropriately.-fno-set-stack-executable— MinGW only ⚠️ Stops the executable flag from being set on the stack used by nested functions. This is necessary for binaries running in Windows kernel mode, since the User32 API (the one used to set executable privileges) isn't available there.-fwritable-relocated-rdata— MinGW and Cygwin 📝 Puts relocated data from a read-only section into the".data"section. This is necessary for older runtimes that don't support modifying".rdata"sections for pseudo-relocation.-mpe-aligned-commons— Cygwin and MinGW 📐 Uses the GNU extension to the PE file format that lets COMMON variables align correctly when generating code. It's enabled by default if GCC detects that the target assembler found during configuration supports the feature.
See also under x86 Options for standard options 👀
✨ Xstormy16 Options ✨
These options are defined for Xstormy16 🎮
-msim— picks startup files and a linker script that suit the simulator 🕹️
✨ Xtensa Options ✨
These options are supported for Xtensa targets 🌷
-mconst16,-mno-const16— turnCONST16instructions on or off for loading constant values. Heads up:CONST16isn't a standard Tensilica option 🙈 When it's enabled,CONST16instructions are always used in place of the standardL32Rinstructions. It's only enabled by default if theL32Rinstruction isn't available.-mfused-madd,-mno-fused-madd— toggles fused multiply/add and multiply/subtract instructions in the floating-point option. If the floating-point option isn't also enabled, this does nothing 😴 Disabling them forces the compiler to use separate instructions for the multiply and the add/subtract. That can be the move when strict IEEE 754-compliant results are required, because fused instructions don't round the intermediate result, which gives you more bits of precision than the IEEE standard specifies. Disabling them also makes sure the program output isn't sensitive to the compiler's ability to combine multiply and add/subtract operations 🧠-mserialize-volatile,-mno-serialize-volatile— when enabled, GCC insertsMEMWinstructions beforevolatilememory references to guarantee sequential consistency 🔒 The default is-mserialize-volatile. Use-mno-serialize-volatileto omit theMEMWinstructions.-mforce-no-pic— for targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this disables PIC for compiling kernel code 🔓-mtext-section-literals,-mno-text-section-literals— these control the treatment of literal pools 🏊 The default,-mno-text-section-literals, places literals in a separate section in the output file. That lets the literal pool live in data RAM/ROM, and it lets the linker combine literal pools from separate object files to remove redundant literals and improve code size. With-mtext-section-literals, the literals are interspersed in the text section so they stay as close as possible to their references. This may be necessary for large assembly files. Literals for each function get placed right before that function.-mauto-litpools,-mno-auto-litpools— more literal pool drama 💜 The default is-mno-auto-litpools, which places literals in a separate section in the output file unless-mtext-section-literalsis used. With-mauto-litpools, the literals are interspersed in the text section by the assembler. The compiler doesn't produce explicit".literal"directives and loads literals into registers withMOVIinstructions instead ofL32R, so the assembler can do relaxation and place literals as necessary. This option lets the assembler create several literal pools per function and assemble very big functions, which might not be possible with-mtext-section-literals.-mtarget-align,-mno-target-align— when enabled, GCC tells the assembler to automatically align instructions to reduce branch penalties, at the expense of some code density 🎯 The assembler tries to widen density instructions to align branch targets and the instructions following call instructions. If there aren't enough preceding safe density instructions to align a target, no widening happens. The default is-mtarget-align. These options don't affect auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by insertingNOPinstructions.-mlongcalls,-mno-longcalls— when enabled, GCC tells the assembler to translate direct calls into indirect calls unless it can determine that the target of a direct call is in the range the call instruction allows 📞 This usually happens for calls to functions in other source files. Specifically, the assembler turns a directCALLinto anL32Rfollowed by aCALLX. The default is-mno-longcalls. Use this option in programs where the call target can potentially be out of range. It's implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions. To see the actual instructions, look at the disassembled object code 🔍 Also, the assembler uses an indirect call for every cross-file call, not just the ones that really are out of range.-mabi=name— generates code for the specified ABI 📜 Permissible values arecall0andwindowed. The default ABI is chosen by the Xtensa core configuration.-mabi=call0— function parameters are passed in registersa2througha7, registersa12througha15are caller-saved, and registera15may be used as a frame pointer 🪄 When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_CALL0_ABI__gets defined.-mabi=windowed— function parameters are passed in registersa10througha15, and the called function rotates the register window by 8 registers on entry so its arguments are found in registersa2througha7. Registera7may be used as a frame pointer. The register window gets rotated 8 registers back upon return 🔄 When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__gets defined.
✨ zSeries Options ✨
These are listed under 💫
✨ ENVIRONMENT ✨
Okay bestie, these environment variables are the secret settings that change how GCC behaves 🌸 Some tell it where to look for stuff (directories and prefixes), and others set up the rest of your compile vibe. 💅
Also, you can point GCC at places with options like -B, -I, and -L. Those beat the environment variables, and the environment variables beat whatever GCC was configured with. Options > env vars > defaults, no cap 👑
LANG, LC_CTYPE, LC_MESSAGES, LC_ALL 🌍
These handle localization, so GCC can vibe with different national conventions. GCC checks the LC_CTYPE and LC_MESSAGES locale categories if it was set up to do that. They can be set to any value your installation supports. A typical one is en_GB.UTF-8, which is English from the United Kingdom encoded in UTF-8 🇬🇧
LC_CTYPE— this one handles character classification. GCC uses it to figure out where characters start and stop in a string. That matters for some multibyte encodings with quote and escape characters that could otherwise be read as the end of a string or as an escape. Lowkey important 🦋LC_MESSAGES— this picks the language for diagnostic messages (the error and warning text) 💬LC_ALL— if this is set, it overridesLC_CTYPEandLC_MESSAGES. If it isn't, those two default to whateverLANGsays. If none of these are set, GCC defaults to traditional C English behavior 🍓
TMPDIR 🗂️
If TMPDIR is set, it tells GCC which directory to use for temporary files. GCC makes temp files to hold the output of one stage of compilation that gets fed into the next stage. For example, the output of the preprocessor is the input to the compiler proper 🧚
GCC_COMPARE_DEBUG 🐛
Setting GCC_COMPARE_DEBUG is nearly the same as passing -fcompare-debug to the compiler driver. Check that option's docs for the full tea ☕
GCC_EXEC_PREFIX 🛣️
If GCC_EXEC_PREFIX is set, it gives a prefix for the names of the subprograms GCC runs. No slash gets added when the prefix is combined with a subprogram name, so if you want a slash, end your prefix with one 🫶
If it isn't set, GCC tries to guess a sensible prefix from the path it was invoked with ✨
If GCC can't find the subprogram using your prefix, it goes looking in the usual places 🔍
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/, where prefix is the prefix of the installed compiler. In a lot of cases, prefix is whatever you set prefix to when you ran the configure script 🌈
Prefixes given with -B beat this one 👑
This prefix is also used to find files like crt0.o that get used for linking 🔗
Here's the unusual part: the prefix also affects which directories get searched for header files. For each standard directory that normally starts with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries swapping that beginning out for your prefix to make an alternate directory name. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And if a standard directory starts with the configured prefix, GCC swaps that prefix out for GCC_EXEC_PREFIX when it hunts for header files 🦋
COMPILER_PATH 🧭
The value of COMPILER_PATH is a colon-separated list of directories, very much like PATH. GCC checks these directories when searching for subprograms, but only if it couldn't find them using GCC_EXEC_PREFIX 💖
LIBRARY_PATH 📚
The value of LIBRARY_PATH is a colon-separated list of directories, very much like PATH. When GCC is configured as a native compiler, it checks these directories for special linker files, but only if it couldn't find them using GCC_EXEC_PREFIX. Linking with GCC also searches these directories for ordinary libraries when you use the -l option. (Directories given with -L go first though 🥇)
LANG (the second one, for multibyte characters) 🀄
This variable passes locale info to the compiler. One thing it's used for is deciding which character set to use when parsing character literals, string literals, and comments in C and C++. When the compiler is configured to allow multibyte characters, these values for LANG are recognized:
C-JIS— recognizes JIS characters 🎌C-SJIS— recognizes SJIS characters 🎌C-EUCJP— recognizes EUCJP characters 🎌
If LANG isn't defined, or it has some other value, the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters 🫧
GCC_EXTRA_DIAGNOSTIC_OUTPUT 🩹
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the values below, extra text gets printed to stderr whenever fix-it hints are printed. Note that -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits win over this environment variable 👑
fixits-v1— emits parseable fix-it hints, same as-fdiagnostics-parseable-fixits. Columns are counted in bytes, starting at byte 1 for the initial column 🔢fixits-v2— same asfixits-v1, except columns are counted as display columns, per-fdiagnostics-column-unit=display📐
Some extra environment variables also change how the preprocessor behaves 🧚♀️
CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH 📂
Each of these holds a list of directories to look in for header files, separated by a special character, kind of like PATH. That special character is called PATH_SEPARATOR. It's decided when GCC gets built, and it's a semicolon on Microsoft Windows-based targets and a colon on almost everything else 🪟
CPATH lists directories that get searched as if you'd used -I, but they come after any paths given with -I on the command line. This one gets used no matter which language is being preprocessed 💜
The other three only apply when preprocessing their particular language. Each one lists directories searched as if you'd used -isystem, placed after any -isystem paths on the command line 🌸
In all of these, an empty element tells the compiler to search its current working directory. Empty elements can go at the beginning or end of a path. For instance, if CPATH is :/special/include, that's the same as -I. -I/special/include ⭐
DEPENDENCIES_OUTPUT 🕸️
If this one is set, its value says how to output dependencies for Make, based on the non-system header files the compiler processes. System header files get ignored in the dependency output 🙈
The value can just be a file name. In that case the Make rules get written to that file, and the target name gets guessed from the source file name. Or the value can take the form file target, in which case the rules get written to file using target as the target name 🎯
Basically, this variable does the same job as combining -MM and -MF, plus an optional -MT switch 💅
SUNPRO_DEPENDENCIES 🌟
This is the same as DEPENDENCIES_OUTPUT, except system header files are not ignored. That means it implies -M instead of -MM. The dependence on the main input file is left out though 🫣
SOURCE_DATE_EPOCH ⏰
If this is set, its value is a UNIX timestamp used in place of the current date and time in the __DATE__ and __TIME__ macros. That keeps the embedded timestamps reproducible 🔁
A SOURCE_DATE_EPOCH value has to be a UNIX timestamp, which means the number of seconds (leap seconds not counted) since 01 Jan 1970 00:00:00, written in ASCII. It's the same output you'd get from date +%s on GNU/Linux and other systems whose date command supports the %s extension ⌛
A good pick is a known timestamp like the last modification time of the source or package, and the build process should set it 🛠️
🐛 BUGS 🐛
For instructions on reporting bugs, check out <file:///usr/share/doc/gcc-11/README.Bugs> 🫶
📝 FOOTNOTES 📝
- On some systems,
gcc -sharedneeds to build supplementary stub code for constructors to work. On multi-libbed systems,gcc -sharedmust 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 📚
Check these out, bestie: gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), dbx(1), and the Info entries for gcc, cpp, as, ld, binutils and gdb 🌈
👩💻 AUTHOR 👩💻
See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for the contributors to GCC. They're the real MVPs 👑
⚖️ COPYRIGHT ⚖️
Copyright (c) 1988-2021 Free Software Foundation, Inc.
Okay, the legal bit, but make it cute: you're allowed to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation. The Invariant Sections are "GNU General Public License" and "Funding Free Software". The Front-Cover texts are (a) (see below), and the Back-Cover Texts are (b) (see below). A copy of the license is included in the gfdl(7) man page 📜
(a) The FSF's Front-Cover Text is:
A GNU Manual(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.Okay bestie, that's the whole GCC(1) page, fully served 💅 Sourced from gcc-11 and dated 2024-07-19, filed under GCC(1), and now you officially know more about GCC than most people ever will 🦋✨
📦 where this page comes from
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