GCC(1)
✨ NAME ✨
gcc — GNU project C and C++ compiler. (GNU manual, section 1. Hiii bestie, you found the compiler page 💖)
🦋 SYNOPSIS 🦋
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
💖 DESCRIPTION 💖
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. the whole glow-up pipeline ✨ The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. yes, she has a whole alter ego 💅
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. do NOT try to combine them, bestie, it will not eat.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. so if you don't see the no- version, that's why 🫶
Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes.
🌈 OPTIONS 🌈
🎀 Option Summary 🎀
Here is a summary of all the options, grouped by type. Explanations are in the following sections. it's giving encyclopedia but make it cute ✨
👑 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-fcanon-prefix-map-fplugin=file-fplugin-arg-name=arg-fdump-ada-spec[-slim]-fada-spec-parent=unit-fdump-go-spec=file
🌸 C Language Options 🌸
-ansi-std=standard-aux-info filename-fno-asm-fno-builtin-fno-builtin-function-fcond-mismatch-ffreestanding-fgimple-fgnu-tm-fgnu89-inline-fhosted-flax-vector-conversions-fms-extensions-foffload=arg-foffload-options=arg-fopenacc-fopenacc-dim=geom-fopenmp-fopenmp-simd-fopenmp-target-simd-clone[=device-type]-fpermitted-flt-eval-methods=standard-fplan9-extensions-fsigned-bitfields-funsigned-bitfields-fsigned-char-funsigned-char-fstrict-flex-arrays[=n]-fsso-struct=endianness
🦋 C++ 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-immediate-escalation-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-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-Wdangling-reference-Wno-delete-incomplete-Wdelete-non-virtual-dtor-Wno-deprecated-array-compare-Wdeprecated-copy-Wdeprecated-copy-dtor-Wno-deprecated-enum-enum-conversion-Wno-deprecated-enum-float-conversion-Weffc++-Wno-elaborated-enum-base-Wno-exceptions-Wextra-semi-Wno-global-module-Wno-inaccessible-base-Wno-inherited-variadic-ctor-Wno-init-list-lifetime-Winvalid-constexpr-Winvalid-imported-macros-Wno-invalid-offsetof-Wno-literal-suffix-Wmismatched-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-Wself-move-Wsign-promo-Wsized-deallocation-Wsuggest-final-methods-Wsuggest-final-types-Wsuggest-override-Wno-template-id-cdtor-Wno-terminate-Wno-vexing-parse-Wvirtual-inheritance-Wno-virtual-move-assign-Wvolatile-Wzero-as-null-pointer-constant
🍓 Objective-C and Objective-C++ Language Options 🍓
-fconstant-string-class=class-name-fgnu-runtime-fnext-runtime-fno-nil-receivers-fobjc-abi-version=n-fobjc-call-cxx-cdtors-fobjc-direct-dispatch-fobjc-exceptions-fobjc-gc-fobjc-nilcheck-fobjc-std=objc1-fno-local-ivars-fivar-visibility=[public|protected|private|package]-freplace-objc-classes-fzero-link-gen-decls-Wassign-intercept-Wno-property-assign-default-Wno-protocol-Wobjc-root-class-Wselector-Wstrict-selector-match-Wundeclared-selector
💅 Diagnostic 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|sarif-stderr|sarif-file|json|json-stderr|json-file]-fno-diagnostics-json-formatting-fno-diagnostics-show-option-fno-diagnostics-show-caret-fno-diagnostics-show-labels-fno-diagnostics-show-line-numbers-fno-diagnostics-show-cwe-fno-diagnostics-show-rule-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-fdiagnostics-escape-format=[unicode|bytes]-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]
💅 Warning Options 💅
okay bestie, here's the big roster — every warning knob GCC ships, in the exact order the docs keep them. no skipping, no "etc." energy, we're listing all of them because you deserve the whole menu and not just the highlight reel. 🫶 hydrate first.
-fsyntax-only,-fmax-errors=n,-Wpedantic,-pedantic-errors,-fpermissive,-w,-Wextra,-Wall,-Wabi=n,-Waddress,-Wno-address-of-packed-member,-Waggregate-return,-Walloc-size-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,-Warray-compare,-Warray-parameter,-Warray-parameter=n,-Wno-attributes,-Wattribute-alias=n,-Wno-attribute-alias,-Wno-attribute-warning-Wbidi-chars=[none|unpaired|any|ucn],-Wbool-compare,-Wbool-operation,-Wno-builtin-declaration-mismatch,-Wno-builtin-macro-redefined,-Wc90-c99-compat,-Wc99-c11-compat,-Wc11-c23-compat,-Wc++-compat,-Wc++11-compat,-Wc++14-compat,-Wc++17-compat,-Wc++20-compat-Wno-c++11-extensions,-Wno-c++14-extensions,-Wno-c++17-extensions,-Wno-c++20-extensions,-Wno-c++23-extensions,-Wcalloc-transposed-args,-Wcast-align,-Wcast-align=strict,-Wcast-function-type,-Wcast-qual,-Wchar-subscripts,-Wclobbered,-Wcomment,-Wcompare-distinct-pointer-types,-Wno-complain-wrong-lang,-Wconversion,-Wno-coverage-mismatch,-Wno-cpp
the way this list just keeps going 😭 we're literally only on the c's. stay with me, we're doing the whole thing.
-Wdangling-else,-Wdangling-pointer,-Wdangling-pointer=n,-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,-Wenum-int-mismatch,-Werror,-Werror=*,-Wexpansion-to-defined-Wfatal-errors,-Wflex-array-member-not-at-end,-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,-Whardened,-Wimplicit,-Wimplicit-fallthrough,-Wimplicit-fallthrough=n,-Wno-implicit-function-declaration,-Wno-implicit-int,-Winfinite-recursion,-Winit-self,-Winline,-Wno-int-conversion,-Wint-in-bool-context,-Wno-int-to-pointer-cast,-Wno-invalid-memory-model,-Winvalid-pch,-Winvalid-utf8,-Wno-unicode
ngl the -Wformat crew alone could be its own group chat. obsessed with their consistency. ✨
-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,-Wopenacc-parallelism,-Wopenmp,-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,-Wstrict-flex-arrays,-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,-Wtrivial-auto-var-init,-Wno-tsan,-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,-Wuse-after-free,-Wuse-after-free=n,-Wuseless-cast,-Wno-varargs,-Wvariadic-macros,-Wvector-operation-performance,-Wvla,-Wvla-larger-than=byte-size,-Wno-vla-larger-than,-Wvolatile-register-var,-Wwrite-strings,-Wno-xor-used-as-pow,-Wzero-length-bounds
that's the whole roster, start to finish, not one name dropped. 🎀
🧚 Static Analyzer Options 🧚
and then there's the analyzer side of the family — the ones that dig through your code like they're reading your old posts. no cap, these are all here too. 🔍
-fanalyzer,-fanalyzer-call-summaries,-fanalyzer-checker=name,-fno-analyzer-feasibility,-fanalyzer-fine-grained,-fanalyzer-show-events-in-system-headers,-fno-analyzer-state-merge,-fno-analyzer-state-purge,-fno-analyzer-suppress-followups,-fanalyzer-transitivity,-fno-analyzer-undo-inlining,-fanalyzer-verbose-edges,-fanalyzer-verbose-state-changes,-fanalyzer-verbosity=level-fdump-analyzer,-fdump-analyzer-callgraph,-fdump-analyzer-exploded-graph,-fdump-analyzer-exploded-nodes,-fdump-analyzer-exploded-nodes-2,-fdump-analyzer-exploded-nodes-3,-fdump-analyzer-exploded-paths,-fdump-analyzer-feasibility,-fdump-analyzer-infinite-loop,-fdump-analyzer-json,-fdump-analyzer-state-purge,-fdump-analyzer-stderr,-fdump-analyzer-supergraph,-fdump-analyzer-untracked
the -fdump-analyzer-* squad is giving "I made a whole spreadsheet about your code" and honestly? iconic. 📊
-Wno-analyzer-double-fclose,-Wno-analyzer-double-free,-Wno-analyzer-exposure-through-output-file,-Wno-analyzer-exposure-through-uninit-copy,-Wno-analyzer-fd-access-mode-mismatch,-Wno-analyzer-fd-double-close,-Wno-analyzer-fd-leak,-Wno-analyzer-fd-phase-mismatch,-Wno-analyzer-fd-type-mismatch,-Wno-analyzer-fd-use-after-close,-Wno-analyzer-fd-use-without-check,-Wno-analyzer-file-leak,-Wno-analyzer-free-of-non-heap,-Wno-analyzer-imprecise-fp-arithmetic,-Wno-analyzer-infinite-loop,-Wno-analyzer-infinite-recursion,-Wno-analyzer-jump-through-null,-Wno-analyzer-malloc-leak,-Wno-analyzer-mismatching-deallocation,-Wno-analyzer-null-argument,-Wno-analyzer-null-dereference,-Wno-analyzer-out-of-bounds,-Wno-analyzer-overlapping-buffers,-Wno-analyzer-possible-null-argument,-Wno-analyzer-possible-null-dereference,-Wno-analyzer-putenv-of-auto-var,-Wno-analyzer-shift-count-negative,-Wno-analyzer-shift-count-overflow,-Wno-analyzer-stale-setjmp-buffer-Wno-analyzer-tainted-allocation-size,-Wno-analyzer-tainted-assertion,-Wno-analyzer-tainted-array-index,-Wno-analyzer-tainted-divisor,-Wno-analyzer-tainted-offset,-Wno-analyzer-tainted-size,-Wanalyzer-symbol-too-complex,-Wanalyzer-too-complex,-Wno-analyzer-undefined-behavior-strtok,-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-va-arg-type-mismatch,-Wno-analyzer-va-list-exhausted,-Wno-analyzer-va-list-leak,-Wno-analyzer-va-list-use-after-va-end,-Wno-analyzer-write-to-const,-Wno-analyzer-write-to-string-literal
the -Wno-analyzer-* wall of text ate and left no crumbs, fr fr. 💅
🎀 C and Objective-C-only Warning Options 🎀
these ones are specifically for the C and Objective-C girlies — if that's not your language, you can vibe past this bit. but we're still listing every single one. 🍓
-Wbad-function-cast,-Wmissing-declarations,-Wmissing-parameter-type,-Wdeclaration-missing-parameter-type,-Wmissing-prototypes,-Wmissing-variable-declarations,-Wnested-externs-Wold-style-declaration,-Wold-style-definition,-Wstrict-prototypes,-Wtraditional,-Wtraditional-conversion,-Wdeclaration-after-statement,-Wpointer-sign
🦋 Debugging Options 🦋
last stretch bestie, we're so close. these are the debug-info knobs — the ones that decide what your debugger gets to see about you later. 👀
-g,-glevel,-gdwarf,-gdwarf-version,-gbtf,-gctf,-gctflevel,-ggdb,-grecord-gcc-switches,-gno-record-gcc-switches,-gstrict-dwarf,-gno-strict-dwarf,-gas-loc-support,-gno-as-loc-support,-gas-locview-support,-gno-as-locview-support,-gcodeview-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,-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
the way every single one of these is a -g or an -f with a whole personality… iconic behavior honestly 🌈
lowkey the-gno-*ones are the "actually, no" versions of the-g*ones — same name, one littleno, different vibe entirely. don't mix them up bestie. 💀
✨ Optimization Options ✨
okay bestie, this is the big one. this is the section everyone fights about in the group chat at 2am. it's giving "my compiler is smarter than yours" energy and honestly? valid. 💅
no descriptions here, just the full roster — names, exactly as they are, in the exact order the page lists them. no skipping, no summarising, we respect the lore:
-faggressive-loop-optimizations-falign-functions[=n[:m:[n2[:m2]]]]-falign-jumps[=n[:m:[n2[:m2]]]]-falign-labels[=n[:m:[n2[:m2]]]]-falign-loops[=n[:m:[n2[:m2]]]]-fmin-function-alignment=[n]-fno-allocation-dce-fallow-store-data-races-fassociative-math-fauto-profile-fauto-profile[=path]-fauto-inc-dec-fbranch-probabilities-fcaller-saves-fcombine-stack-adjustments-fconserve-stack-ffold-mem-offsets-fcompare-elim-fcprop-registers-fcrossjumping-fcse-follow-jumps-fcse-skip-blocks-fcx-fortran-rules-fcx-limited-range-fdata-sections-fdce-fdelayed-branch-fdelete-null-pointer-checks-fdevirtualize-fdevirtualize-speculatively-fdevirtualize-at-ltrans-fdse-fearly-inlining-fipa-sra-fexpensive-optimizations-ffat-lto-objects-ffast-math-ffinite-math-only-ffloat-store-fexcess-precision=style-ffinite-loops-fforward-propagate-ffp-contract=style-ffunction-sections-fgcse-fgcse-after-reload-fgcse-las-fgcse-lm-fgraphite-identity-fgcse-sm-fhoist-adjacent-loads-fif-conversion-fif-conversion2-findirect-inlining-finline-stringops[=fn]-finline-functions-finline-functions-called-once-finline-limit=n-finline-small-functions-fipa-modref-fipa-cp-fipa-cp-clone-fipa-bit-cp-fipa-vrp-fipa-pta-fipa-profile-fipa-pure-const-fipa-reference-fipa-reference-addressable-fipa-stack-alignment-fipa-icf-fira-algorithm=algorithm-flive-patching=level-fira-region=region-fira-hoist-pressure-fira-loop-pressure-fno-ira-share-save-slots-fno-ira-share-spill-slots-fisolate-erroneous-paths-dereference-fisolate-erroneous-paths-attribute-fivopts-fkeep-inline-functions-fkeep-static-functions-fkeep-static-consts-flimit-function-alignment-flive-range-shrinkage-floop-block-floop-interchange-floop-strip-mine-floop-unroll-and-jam-floop-nest-optimize-floop-parallelize-all-flra-remat-flto-flto-compression-level-flto-partition=alg-fmerge-all-constants-fmerge-constants-fmodulo-sched-fmodulo-sched-allow-regmoves-fmove-loop-invariants-fmove-loop-stores-fno-branch-count-reg-fno-defer-pop-fno-fp-int-builtin-inexact-fno-function-cse-fno-guess-branch-probability-fno-inline-fno-math-errno-fno-peephole-fno-peephole2-fno-printf-return-value-fno-sched-interblock-fno-sched-spec-fno-signed-zeros-fno-toplevel-reorder-fno-trapping-math-fno-zero-initialized-in-bss-fomit-frame-pointer-foptimize-sibling-calls-fpartial-inlining-fpeel-loops-fpredictive-commoning-fprefetch-loop-arrays-fprofile-correction-fprofile-use-fprofile-use=path-fprofile-partial-training-fprofile-values-fprofile-reorder-functions-freciprocal-math-free-frename-registers-freorder-blocks-freorder-blocks-algorithm=algorithm-freorder-blocks-and-partition-freorder-functions-frerun-cse-after-loop-freschedule-modulo-scheduled-loops-frounding-math-fsave-optimization-record-fsched2-use-superblocks-fsched-pressure-fsched-spec-load-fsched-spec-load-dangerous-fsched-stalled-insns-dep[=n]-fsched-stalled-insns[=n]-fsched-group-heuristic-fsched-critical-path-heuristic-fsched-spec-insn-heuristic-fsched-rank-heuristic-fsched-last-insn-heuristic-fsched-dep-count-heuristic-fschedule-fusion-fschedule-insns-fschedule-insns2-fsection-anchors-fselective-scheduling-fselective-scheduling2-fsel-sched-pipelining-fsel-sched-pipelining-outer-loops-fsemantic-interposition-fshrink-wrap-fshrink-wrap-separate-fsignaling-nans-fsingle-precision-constant-fsplit-ivs-in-unroller-fsplit-loops-fsplit-paths-fsplit-wide-types-fsplit-wide-types-early-fssa-backprop-fssa-phiopt-fstdarg-opt-fstore-merging-fstrict-aliasing-fipa-strict-aliasing-fthread-jumps-ftracer-ftree-bit-ccp-ftree-builtin-call-dce-ftree-ccp-ftree-ch-ftree-coalesce-vars-ftree-copy-prop-ftree-dce-ftree-dominator-opts-ftree-dse-ftree-forwprop-ftree-fre-fcode-hoisting-ftree-loop-if-convert-ftree-loop-im-ftree-phiprop-ftree-loop-distribution-ftree-loop-distribute-patterns-ftree-loop-ivcanon-ftree-loop-linear-ftree-loop-optimize-ftree-loop-vectorize-ftree-parallelize-loops=n-ftree-pre-ftree-partial-pre-ftree-pta-ftree-reassoc-ftree-scev-cprop-ftree-sink-ftree-slsr-ftree-sra-ftree-switch-conversion-ftree-tail-merge-ftree-ter-ftree-vectorize-ftree-vrp-ftrivial-auto-var-init-funconstrained-commons-funit-at-a-time-funroll-all-loops-funroll-loops-funsafe-math-optimizations-funswitch-loops-fipa-ra-fvariable-expansion-in-unroller-fvect-cost-model-fvpt-fweb-fwhole-program-fwpa-fuse-linker-plugin-fzero-call-used-regs--param name=value-O-O0-O1-O2-O3-Os-Ofast-Og-Oz
like… that's a whole personality trait. iconic behaviour honestly 🎀
✨ Program Instrumentation Options ✨
ok so this is the "let's spy on our own program" corner of the compiler 🕵️♀️ — profiling, coverage, sanitizers, all the little detectives. the whole guest list:
-p-pg-fprofile-arcs--coverage-ftest-coverage-fcondition-coverage-fprofile-abs-path-fprofile-dir=path-fprofile-generate-fprofile-generate=path-fprofile-info-section-fprofile-info-section=name-fprofile-note=path-fprofile-prefix-path=path-fprofile-update=method-fprofile-filter-files=regex-fprofile-exclude-files=regex-fprofile-reproducible=[multithreaded|parallel-runs|serial]-fsanitize=style-fsanitize-recover-fsanitize-recover=style-fsanitize-trap-fsanitize-trap=style-fasan-shadow-offset=number-fsanitize-sections=s1,s2,...-fsanitize-undefined-trap-on-error-fbounds-check-fcf-protection=[full|branch|return|none|check]-fharden-compares-fharden-conditional-branches-fhardened-fharden-control-flow-redundancy-fhardcfr-skip-leaf-fhardcfr-check-exceptions-fhardcfr-check-returning-calls-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]-fstack-protector-fstack-protector-all-fstack-protector-strong-fstack-protector-explicit-fstack-check-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit-fsplit-stack-fstrub=disable-fstrub=strict-fstrub=relaxed-fstrub=all-fstrub=at-calls-fstrub=internal-fvtable-verify=[std|preinit|none]-fvtv-counts-fvtv-debug-finstrument-functions-finstrument-functions-once-finstrument-functions-exclude-function-list=sym,sym,...-finstrument-functions-exclude-file-list=file,file,...-fprofile-prefix-map=old=new-fpatchable-function-entry=N[,M]
the way this list just keeps going… respectfully, slay 🦋
✨ Preprocessor Options ✨
this is the part that runs before anything else even happens. the pre-show. the getting-ready-with-me. 💖 all of these, verbatim, in order:
-Aquestion=answer-A-question[=answer]-C-CC-Dmacro[=defn]-dD-dI-dM-dN-dU-fdebug-cpp-fdirectives-only-fdollars-in-identifiers-fexec-charset=charset-fextended-identifiers-finput-charset=charset-flarge-source-files-fmacro-prefix-map=old=new-fmax-include-depth=depth-fno-canonical-system-headers-fpch-deps-fpch-preprocess-fpreprocessed-ftabstop=width-ftrack-macro-expansion-fwide-exec-charset=charset-fworking-directory-H-imacros file-include file-M-MD-MF-MG-MM-MMD-MP-MQ-MT-Mno-modules-no-integrated-cpp-P-pthread-remap-traditional-traditional-cpp-trigraphs-Umacro-undef-Wp,option-Xpreprocessor option
note how the single-letter ones hit different — -M, -MD, -MF, -MG, -MM, -MMD, -MP, -MQ, -MT… a whole alphabet of drama 😭
✨ Assembler Options ✨
short and sweet, two options, no notes:
-Wa,option-Xassembler option
✨ Linker Options ✨
the part where everything finally gets stitched together 🧵✨ here's the lineup:
object-file-name-fuse-ld=linker-llibrary-nostartfiles-nodefaultlibs-nolibc-nostdlib-nostdlib++-e entry--entry=entry-pie-pthread-r-rdynamic-s-static-static-pie-static-libgcc-static-libstdc++-static-libasan-static-libtsan-static-liblsan-static-libubsan-shared-shared-libgcc-symbolic-T script-Wl,option-Xlinker option-u symbol-z keyword
object-file-name sitting right there at the top with no dash like it owns the place 💀 main character energy fr fr
✨ Directory Options ✨
where do we look for things?? great question 🌸 here's every path-related option, exactly as listed:
-Bprefix-Idir-I--idirafter dir-imacros file-imultilib dir-iplugindir=dir-iprefix file-iquote dir-isysroot dir-isystem dir-iwithprefix dir-iwithprefixbefore dir-Ldir-no-canonical-prefixes--no-sysroot-suffix-nostdinc-nostdinc++--sysroot=dir
✨ Code Generation Options ✨
this is the "how should the actual machine code look" section 🛠️ — the full list, untouched:
-fcall-saved-reg-fcall-used-reg-ffixed-reg-fexceptions-fnon-call-exceptions-fdelete-dead-exceptions-funwind-tables-fasynchronous-unwind-tables-fno-gnu-unique-finhibit-size-directive-fcommon-fno-ident-fpcc-struct-return-fpic-fPIC-fpie-fPIE-fno-plt-fno-jump-tables-fno-bit-tests-frecord-gcc-switches-freg-struct-return-fshort-enums-fshort-wchar-fverbose-asm-fpack-struct[=n]-fleading-underscore-ftls-model=model-fstack-reuse=reuse_level-ftrampolines-ftrampoline-impl=[stack|heap]-ftrapv-fwrapv-fvisibility=[default|internal|hidden|protected]-fstrict-volatile-bitfields-fsync-libcalls
-fpic and -fPIC right next to each other… they're cousins and the case matters, don't @ me 🙈
✨ Developer Options ✨
and FINALLY, the backstage pass 🎟️ — the debugging, dumping, reporting, "I am a compiler engineer and I need to see everything" section. the whole thing:
-dletters-dumpspecs-dumpmachine-dumpversion-dumpfullversion-fcallgraph-info[=su,da]-fchecking-fchecking=n-fdbg-cnt-list-fdbg-cnt=counter-value-list-fdisable-ipa-pass_name-fdisable-rtl-pass_name-fdisable-rtl-pass-name=range-list-fdisable-tree-pass_name-fdisable-tree-pass-name=range-list-fdump-debug-fdump-earlydebug-fdump-noaddr-fdump-unnumbered-fdump-unnumbered-links-fdump-final-insns[=file]-fdump-ipa-all-fdump-ipa-cgraph-fdump-ipa-inline-fdump-lang-all-fdump-lang-switch-fdump-lang-switch-options-fdump-lang-switch-options=filename-fdump-passes-fdump-rtl-pass-fdump-rtl-pass=filename-fdump-statistics-fdump-tree-all-fdump-tree-switch-fdump-tree-switch-options-fdump-tree-switch-options=filename-fcompare-debug[=opts]-fcompare-debug-second-fenable-kind-pass-fenable-kind-pass=range-list-fira-verbose=n-flto-report-flto-report-wpa-fmem-report-wpa-fmem-report-fpre-ipa-mem-report-fpost-ipa-mem-report-fopt-info-fopt-info-options[=file]-fmultiflags-fprofile-report-frandom-seed=string-fsched-verbose=n-fsel-sched-verbose-fsel-sched-dump-cfg-fsel-sched-pipelining-verbose-fstats-fstack-usage-ftime-report-ftime-report-details-fvar-tracking-assignments-toggle-gtoggle-print-file-name=library-print-libgcc-file-name-print-multi-directory-print-multi-lib-print-multi-os-directory-print-prog-name=program-print-search-dirs-Q-print-sysroot-print-sysroot-headers-suffix-save-temps-save-temps=cwd-save-temps=obj-time[=file]
heads up bestie,-fdisable-...and-fenable-...names carry apass_name/range-listshape and the manpage wraps them across lines — I've kept every character, just un-wrapped so your eyes don't cry. 💖
and that's where this chunk hands off 🫶 keep scrolling, there's more where that came from 💅
🦋 Machine-Dependent Options 🦋
okay so THIS is where GCC stops being one thing and becomes, like, every thing?? bestie each chip family gets its own lil menu of flags and we are going through ALL of them. no skipping. no summarizing. every single one makes it in. 💅
✨ AArch64 Options ✨
the 64-bit ARM girlies, she's serving:
-mabi=name-mbig-endian-mlittle-endian-mgeneral-regs-only-mcmodel=tiny-mcmodel=small-mcmodel=large-mstrict-align-mno-strict-align-momit-leaf-frame-pointer-mtls-dialect=desc-mtls-dialect=traditional-mtls-size=size-mfix-cortex-a53-835769-mfix-cortex-a53-843419-mlow-precision-recip-sqrt-mlow-precision-sqrt-mlow-precision-div-mpc-relative-literal-loads-msign-return-address=scope-mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti-mharden-sls=opts-march=name-mcpu=name-mtune=name-moverride=string-mverbose-cost-dump-mstack-protector-guard=guard-mstack-protector-guard-reg=sysreg-mstack-protector-guard-offset=offset-mtrack-speculation-moutline-atomics-mearly-ldp-fusion-mlate-ldp-fusion
🌸 Adapteva Epiphany Options 🌸
yes she's real, yes she's niche, and she's got receipts:
-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 💜
short, sweet, GPU-coded:
-march=gpu-mtune=gpu-mstack-size=bytes
🎀 ARC Options 🎀
she's LONG. she's THOROUGH. she ate:
-mbarrel-shifter-mjli-always-mcpu=cpu-mA6-mARC600-mA7-mARC700-mdpfp-mdpfp-compact-mdpfp-fast-mno-dpfp-lrsr-mea-mno-mpy-mmul32x16-mmul64-matomic-mnorm-mspfp-mspfp-compact-mspfp-fast-msimd-msoft-float-mswap-mcrc-mdsp-packa-mdvbf-mlock-mmac-d16-mmac-24-mrtsc-mswape-mtelephony-mxy-misize-mannotate-align-marclinux-marclinux_prof-mlong-calls-mmedium-calls-msdata-mirq-ctrl-saved-mrgf-banked-regs-mlpc-width=width-G num-mvolatile-cache-mtp-regno=regno-malign-call-mauto-modify-reg-mbbit-peephole-mno-brcc-mcase-vector-pcrel-mcompact-casesi-mno-cond-exec-mearly-cbranchsi-mexpand-adddi-mindexed-loads-mlra-mlra-priority-none-mlra-priority-compact-mlra-priority-noncompact-mmillicode-mmixed-code-mq-class-mRcq-mRcw-msize-level=level-mtune=cpu-mmultcost=num-mcode-density-frame-munalign-prob-threshold=probability-mmpy-option=multo-mdiv-rem-mcode-density-mll64-mfpu=fpu-mrf16-mbranch-index
🫶 ARM Options 🫶
the 32-bit legend herself. iconic behaviour only:
-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-mfix-cortex-a57-aes-1742098-mfix-cortex-a72-aes-1655431-munaligned-access-mneon-for-64bits-mslow-flash-data-masm-syntax-unified-mrestrict-it-mverbose-cost-dump-mpure-code-mcmse-mfix-cmse-cve-2021-35465-mstack-protector-guard=guard-mstack-protector-guard-offset=offset-mfdpic-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]
🍓 AVR Options 🍓
microcontroller bestie, we see you:
-mmcu=mcu-mabsdata-maccumulate-args-mbranch-cost=cost-mfuse-add=level-mcall-prologues-mgas-isr-prologues-mint8-mflmap-mdouble=bits-mlong-double=bits-mn_flash=size-mno-interrupts-mmain-is-OS_task-mrelax-mrmw-mstrict-X-mtiny-stack-mrodata-in-ram-mfract-convert-truncate-mshort-calls-mskip-bug-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
💖 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 🦋
macOS-coded, and yes she's giving the energy of someone who repeats herself for emphasis. lowkey a couple of these show up twice in the source and we are keeping BOTH, no cap:
-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-nodefaultrpaths-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 🔥
kernel-adjacent baddie:
-mbig-endian-mlittle-endian-mframe-limit=bytes-mxbpf-mco-re-mno-co-re-mjmpext-mjmp32-malu32-mv3-atomics-mbswap-msdiv-msmov-mcpu=version-masm=dialect-minline-memops-threshold=bytes
💀 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-matomic-libcalls-mbig-switch-mcaller-copies-mdisable-fpregs-mdisable-indexing-mordered-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mcoherent-ldcw-mjump-in-delay-mlinker-opt-mlong-calls-mlong-load-store-mno-atomic-libcalls-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-msoft-mult-msio-mwsio-munix=unix-std-nolibdld-static-threads
⭐ IA-64 Options ⭐
big list energy, she's committed:
-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
and that's the chip-family tour so far bestie — there's more where this came from, so don't go anywhere 💅✨
💜 LoongArch Options
okay bestie we're in the architecture-specific corner of the options now and it's giving ✨menu✨. LoongArch has so many knobs and we are listing every single one of them, no cap, zero skips, not a crumb left behind 🍓
-march=cpu-type -mtune=cpu-type -mabi=base-abi-type -mfpu=fpu-type
-msimd=simd-type -msoft-float -msingle-float -mdouble-float -mlsx -mno-lsx
-mlasx -mno-lasx -mbranch-cost=n -mcheck-zero-division
-mno-check-zero-division -mcond-move-int -mno-cond-move-int
-mcond-move-float -mno-cond-move-float -memcpy -mno-memcpy -mstrict-align
-mno-strict-align -mmax-inline-memcpy-size=n -mexplicit-relocs=style
-mexplicit-relocs -mno-explicit-relocs -mdirect-extern-access
-mno-direct-extern-access -mcmodel=code-model -mrelax -mpass-mrelax-to-as
-mrecip -mrecip=opt -mfrecipe -mno-frecipe -mdiv32 -mno-div32 -mlam-bh
-mno-lam-bh -mlamcas -mno-lamcas -mld-seq-sa -mno-ld-seq-sa
-mtls-dialect=opt🌸 M32R/D Options
the M32R/D lineup, served fresh 🎀
-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
smol list, big energy 💅
-mcpu=cpu -msim -memregs=number🌈 M680x0 Options
the 680x0 family reunion, and yes every cousin showed up 🫶
-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
MCore said "here's my whole personality" and honestly? iconic 👑
-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✨ MicroBlaze Options
micro in name, mighty in flag count 🌟
-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 this is THE list. MIPS went absolutely feral with options and I respect it so much 😭 every single one is here, do not come for me, nothing got cut fr fr
-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 -mexplicit-relocs=release
-mcheck-zero-division -mno-check-zero-division -mdivide-traps
-mdivide-breaks -mload-store-pairs -mno-load-store-pairs -mstrict-align
-mno-strict-align -mno-unaligned-access -munaligned-access -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-optngl that one was a marathon and we ate it 💅
🧚 MMIX Options
MMIX said "keep it cute, keep it simple" 🌸
-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 list, immaculate vibes ✨
-meb -mel -mmul.x -mno-crt0👑 MSP430 Options
notice how several of these end in = with nothing after — that's not me being sloppy, that's literally how they're written, so they stay exactly like that, periodt 💅
-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
three little icons, standing in a row 🫶
-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 -mcmodel=code-model✨ PDP-11 Options
vintage bestie energy, the PDP-11 said "retro is forever" 🎀
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16
-mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra🦋 PowerPC Options
literally just a pointer to the next list, no notes 💀
See RS/6000 and PowerPC Options.
🌈 PRU Options
-mmcu=mcu -minrt -mno-relax -mloop -mabi=variant🔥 RISC-V Options
RISC-V really said "here's my whole wardrobe" and we're here for it 💖
-mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv
-mno-fdiv -mdiv -mno-div -misa-spec=ISA-spec-string -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 -mno-riscv-attribute -malign-data=type
-mbig-endian -mlittle-endian -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
-mcsr-check -mno-csr-check -mmovcc -mno-movcc -minline-atomics
-mno-inline-atomics -minline-strlen -mno-inline-strlen -minline-strcmp
-mno-inline-strcmp -minline-strncmp -mno-inline-strncmp -mtls-dialect=desc
-mtls-dialect=trad🎀 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
okay this is the big one, the RS/6000 + PowerPC list, and yes it's massive and yes we're doing all of it because leaving options out is not the vibe 😤 every flag, exactly as written, no shortcuts 🫶
-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 -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-privilegedwe did it. that's the whole PowerPC shelf. take a breath bestie, more parts incoming 🌈💖
-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
🌸 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
🦋 V850 Options 🦋
-mlong-calls,-mno-long-calls,-mep,-mno-ep,-mprolog-function,-mno-prolog-function,-mspace,-mtda=n,-msda=n,-mzda=n,-mapp-regs,-mno-app-regs,-mdisable-callt,-mno-disable-callt,-mv850e2v3,-mv850e2,-mv850e1,-mv850es,-mv850e,-mv850,-mv850e3v5,-mloop,-mrelax,-mlong-jumps,-msoft-float,-mhard-float,-mgcc-abi,-mrh850-abi,-mbig-switch
💖 VAX Options 💖
-mg,-mgnu,-munix,-mlra
🫶 Visium Options 🫶
-mdebug,-msim,-mfpu,-mno-fpu,-mhard-float,-msoft-float,-mcpu=cpu-type,-mtune=cpu-type,-msv-mode,-muser-mode
🍓 VMS Options 🍓
-mvms-return-codes,-mdebug-main=prefix,-mmalloc64,-mpointer-size=size
🧚 VxWorks Options 🧚
-mrtp,-msmp,-non-static,-Bstatic,-Bdynamic,-Xbind-lazy,-Xbind-now
🔥 x86 Options 🔥
-mtune=cpu-type,-march=cpu-type,-mtune-ctrl=feature-list,-mdump-tune-features,-mno-default,-mfpmath=unit,-masm=dialect,-mno-fancy-math-387,-mno-fp-ret-in-387,-m80387,-mhard-float,-msoft-float,-mno-wide-multiply,-mrtd,-malign-double,-mpreferred-stack-boundary=num,-mincoming-stack-boundary=num,-mcld,-mcx16,-msahf,-mmovbe,-mcrc32,-mmwait,-mrecip,-mrecip=opt,-mvzeroupper,-mprefer-avx128,-mprefer-vector-width=opt,-mpartial-vector-fp-math,-mmove-max=bits,-mstore-max=bits,-mnoreturn-no-callee-saved-registers,-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,-mcet-switch,-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,-mavx512fp16,-mavxifma,-mavxvnniint8,-mavxneconvert,-mcmpccxadd,-mamx-fp16,-mprefetchi,-mraoint,-mamx-complex,-mavxvnniint16,-msm3,-msha512,-msm4,-mapxf,-musermsr,-mavx10.1,-mavx10.1-256,-mavx10.1-512,-mevex512,-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,-mrelax-cmpxchg-loop,-mindirect-branch=choice,-mfunction-return=choice,-mindirect-branch-register,-mharden-sls=choice,-mindirect-branch-cs-prefix,-mneeded,-mno-direct-extern-access,-munroll-only-small-loops,-mlam=choice
💅 x86 Windows Options 💅
-mconsole,-mcrtdll=library,-mdll,-mnop-fun-dllimport,-mthread,-municode,-mwin32,-mwindows,-fno-set-stack-executable
🌟 Xstormy16 Options 🌟
-msim
🎀 Xtensa Options 🎀
-mconst16,-mno-const16,-mfused-madd,-mno-fused-madd,-mforce-no-pic,-mserialize-volatile,-mno-serialize-volatile,-mtext-section-literals,-mno-text-section-literals,-mauto-litpools,-mno-auto-litpools,-mtarget-align,-mno-target-align,-mlongcalls,-mno-longcalls,-mabi=abi-type,-mextra-l32r-costs=cycles,-mstrict-align,-mno-strict-align
👑 zSeries Options 👑
zSeries bestie, you don't need a whole new list — see S/390 and zSeries Options up above. Same options, same vibe. 💖
✨ Options Controlling the Kind of Output ✨
okay so compilation is a whole little journey, and it can have up to four stages: preprocessing, compilation proper, assembly and linking — always in that order, no queue-jumping 💅
GCC can preprocess and compile several files, and it'll either make several assembler input files or one assembler input file. Then each assembler input file produces an object file, and linking combines all the object files — the ones that just got compiled and the ones you passed in as input — into one executable file. iconic behaviour fr fr 🫶
For any given input file, the file name suffix decides what kind of compilation happens. Vibe check on the suffixes:
file.c— C source code that must be preprocessed.file.i— C source code that should not be preprocessed.file.ii— C++ source code that should not be preprocessed.file.m— Objective-C source code. Heads up bestie: you must link with thelibobjclibrary to make an Objective-C program work.file.mi— Objective-C source code that should not be preprocessed.file.mm,file.M— Objective-C++ source code. Note that you must link with thelibobjclibrary to make an Objective-C++ program work. Also note that.Mmeans a literal capital M, okay? 👀file.mii— Objective-C++ source code that should not be preprocessed.file.h— C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or C, C++ header file to be 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 code that must be preprocessed. Note that in.cxx, the last two letters must both be literallyx. Likewise,.Crefers to a literal capital C. yes the case matters, it's giving ✨file.mm,file.M— Objective-C++ source code that must be preprocessed.file.mii— Objective-C++ source code that should not be preprocessed.file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— C++ header file to be turned into a precompiled header or Ada spec.file.f,file.for,file.ftn,file.fi— Fixed form Fortran source code that should not be preprocessed.file.F,file.FOR,file.fpp,file.FPP,file.FTN— Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).file.f90,file.f95,file.f03,file.f08,file.fii— Free form Fortran source code that should not be preprocessed.file.F90,file.F95,file.F03,file.F08— Free form Fortran source code that must be preprocessed (with the traditional preprocessor).file.go— Go source code.file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These files are also called specs.file.adb— Ada source code file containing a library unit body (a subprogram or package body). These files are also called bodies.file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed.other— An object file to be fed straight into linking. Any file name with no recognized suffix gets treated this way.
And if the suffix guessing isn't hitting, you can specify the input language explicitly with the -x option:
-x language— okay so bestie, this one is literally you telling gcc exactly what language the next input files are in, instead of letting it guess from the file name suffix. It's giving "I know better than the compiler" energy, and honestly, sometimes you do. 💅 This option applies to all following input files until the next-xoption. 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-x none— this turns off any language specification, so subsequent files are handled according to their file name suffixes again, just like if you never used-xat all. It's the "never mind, go back to default" option. 🦋 No cap.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. It's giving "why am I here?" 😭 Literally nothing happens. Periodt.
-c— compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix.c,.i,.s, etc., with.o. Unrecognized input files, not requiring compilation or assembly, are ignored. So like, it's the "I just want the object files, not the whole executable" vibe. 🎀 Slay.-S— stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix.c,.i, etc., with.s. Input files that don't require compilation are ignored. It's the "give me the assembly, bestie" energy. ✨-E— stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files that don't require preprocessing are ignored. So it's like, "just preprocess and dump it to stdout" — very main character. 👑-o file— place the primary output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. It's like, the ultimate "put it here" button. 📁
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.
gcc -c foo.c ...will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.
-dumpbase dumpbase— this option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:
gcc -save-temps -S foo.csaves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.cpreprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. -save-temps, -gsplit-dwarf, -fcallgraph-info) without it.
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and 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 ...creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.
When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values:
gcc -c foo.c -o dir/foobar.o -dumpbase " ...will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.
-dumpbase-ext auxdropsuf— when forming the name of an auxiliary (but not a dump) output file, drop trailingauxdropsuffromdumpbasebefore appending any suffixes. If not specified, this option defaults to the suffix of a defaultdumpbase, i.e., the suffix of the input file when-dumpbaseis not present in the command line, ordumpbaseis combined withdumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.
This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdir dumppfx— when forming the name of an auxiliary or dump output file, usedumppfxas a prefix:
gcc -dumpdir pfx- -c foo.c ...creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.
It defaults to the location of the output file, unless the output file is a special file like /dev/null. Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.
So like, when you're compiling from multiple input files and you actually specified -dumpbase? The dumpbase — minus any auxdropsuf suffix — plus a lil dash gets appended to (or straight-up overrides, if it's got any directory components in it) an explicit or defaulted dumppfx. That way each of your multiple compilations gets its very own differently-named aux and dump outputs. no cap, each one's unique 💅
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...That outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. The dump outputs keep the input file suffix, bestie: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. ✨
Now contrast that with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...Which, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs just go in dir/pfx-main.*. simple as that 🎀
When you're compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also goes through that dumpbase- transformation above (like, say, the compilation of foo.c and bar.c above, but without -c). And if neither -dumpdir nor -dumpbase are given, the linker output base name — minus auxdropsuf if specified, or the executable suffix otherwise — plus a dash gets appended to the default dumppfx instead.
But heads up, bestie: unlike earlier cases of linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...does not append the output name main to dumppfx, because -dumpdir was explicitly specified. The whole goal here is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if that's what you want. Only an explicitly-specified -dumpbase would get combined with it, so we don't just yeet a meaningful option into the void. 😭
When compiling and then linking from a single input file, the linker output base name only gets appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in the single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination — even if it gets overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd. 🦋
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name gets broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overridden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase — as given or as derived from the linker output name but not from inputs, even in cases where this combination wouldn't otherwise be used as such — is passed down with a trailing period replacing the compiler-added dash (if any), as a -dumpdir option to lto-wrapper. And since it's involved in linking, this program doesn't normally get any -dumpbase and -dumpbase-ext, and it just ignores them. 🙈
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, makes sure it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers. periodt.
-v— Print (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program and of the preprocessor and the compiler proper. it's giving receipts 📜-###— Like-vexcept the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or"./-_". This is useful for shell scripts to capture the driver-generated command lines. lowkey a cheat code for scripting 🫶--help— Print (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified then--helpis also passed on to the various processes invoked bygcc, so that they can display the command-line options they accept. And if the-Wextraoption has also been specified (prior to the--helpoption), then command-line options that have no documentation associated with them are also displayed. the way it just tells you everything ✨--target-help— Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. 👑--help={class|[^]qualifier}[,...]— Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. these are the supported classes:optimizers— Display all of the optimization options supported by the compiler.warnings— Display all of the options controlling warning messages produced by the compiler.target— Display target-specific options. Unlike the--target-helpoption however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended--help=syntax.params— Display the values recognized by the--paramoption.language— Display 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, one needs to selectcommonclass.common— Display the options that are common to all languages.
and these are the supported qualifiers:
undocumented— Display only those options that are undocumented.joined— Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target.separate— Display options taking an argument that appears as a separate word following the original option, such as:-o output-file.
So like, for example, to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option.
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]The output is sensitive to the effects of previous command-line options, so for example it's possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizersAlternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabledit's literally a whole detective arc 🔍
--version— Display the version number and copyrights of the invoked GCC. flex the flex 💅-pass-exit-codes— Normally thegccprogram exits with the code of1if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, thegccprogram instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if an internal compiler error is encountered. good to know fr fr 😭-pipe— Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. slay 🌸-specs=file— Process file after the compiler reads in the standard specs file, in order to override the defaults which thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. More than one-specs=filecan be specified on the command line, and they are processed in order, from left to right. order matters bestie 🎀-wrapper— Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... main character energy for your debugger ✨
-ffile-prefix-map=old=new— when you're compiling files that live in the directory old, this makes the compiler write down every reference to them as if they lived in new instead. Giving this one option is literally the same as giving all the individual-f*-prefix-mapoptions at once. It's the move for making reproducible builds that don't care where they were built. Heads up tho: directories that get referenced by directives are not touched by these options. Peep-fmacro-prefix-map,-fdebug-prefix-map,-fprofile-prefix-mapand-fcanon-prefix-maptoo. 🫶-fcanon-prefix-map— so for all the-f*-prefix-mapoptions, the normal vibe is that old gets compared against the filename that would normally show up in the compilation result, and that comparison is textual — or, on case-insensitive filesystems, it ignores character case, and on DOS-based filesystems it treats slashes and backslashes as equal. This option says nope, do those comparisons on the canonicalized paths of old and the referenced filename instead. ✨-fplugin=name.so— loads the plugin code sitting in the filename.so, which is assumed to be a shared object that the compilerdlopens. The base name of that shared object file is what identifies the plugin when it comes to parsing arguments (see-fplugin-arg-name-key=valuebelow, bestie). Every plugin is supposed to define the callback functions spelled out in the Plugins API. 👑-fplugin-arg-name-key=value— defines an argument called key with a value of value, for the plugin named name. Simple as. 💅-fdump-ada-spec[-slim]— for C and C++ source and include files, this generates the corresponding Ada specs. yes, Ada. she's still here. 🎀-fada-spec-parent=unit— teams up with-fdump-ada-spec[-slim]up there to generate Ada specs as child units of the parent unit.-fdump-go-spec=file— for input files in any language, this generates the corresponding Go declarations into file. It spits out Goconst,type,var, andfuncdeclarations, which lowkey slaps as a starting point if you're writing a Go interface to code that was written in some other language. 🌈@file— reads command-line options out of file. The options it reads get inserted right in place of the original@fileoption. If file doesn't exist, or can't be read, then the option is treated literally — and it is not removed. 🙈
Options inside file are separated by whitespace. Wanna sneak a whitespace character into an option? Just wrap the whole option in single or double quotes. Wanna include any character (backslash included)? Prefix the character you want with a backslash. And plot twist: file can itself contain more @file options — those get processed recursively. it's giving inception fr fr. 🌀
🦋 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 exact same way you'd call it for compiling C programs (usually under the name gcc). 💜
But here's the tea tho: using gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files — unless -x is used. This program is also super handy when you're precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed under the name c++. ⭐
When you compile C++ programs, you may specify a lot of the same command-line options you'd use for compiling programs in any language; or command-line options that are meaningful for C and related languages; or options that are meaningful only for C++ programs. slay, options. 🎀
✨ Options Controlling C Dialect ✨
The following 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 equivalent to-std=c90. In C++ mode, it's equivalent to-std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code) — stuff like the asm and typeof keywords, and predefined macros like unix and vax that identify the type of system you're on. It also enables the undesirable and rarely used ISO trigraph feature. (yes bestie, undesirable, I said what I said. 💀) For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.
The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, obviously, but it's useful to put them in header files that might get included in compilations done with -ansi. Alternate predefined macros like __unix__ and __vax__ are also available, with or without -ansi. 💖
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, you need -Wpedantic in addition to -ansi.
The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and hold back from declaring certain functions or defining certain macros that the ISO standard doesn't call for — this is to avoid interfering with any programs that might use those names for other things.
Functions that are normally built in but don't have semantics defined by ISO C (like alloca and ffs) are not built-in functions when -ansi is used.
-std=— determines the language standard. This option is currently only supported when compiling C or C++. 📚
The compiler can accept several base standards, like c90 or c++98, and GNU dialects of those standards, like gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that don't contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, like the asm and typeof keywords — but not other GNU extensions that have no meaning in ISO C90, like omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may get rejected. 😭 The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are:
c90,c89,iso9899:1990— 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. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly, but not entirely, relating to optional C99 features from Annexes F and G). Peephttps://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. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated.c17,c18,iso9899:2017,iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is 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__, and so is supported to the same extent as C11.c23,c2x,iso9899:2024— ISO C23, the 2023 revision of the ISO C standard (expected to be published in 2024). Support for this version is experimental and incomplete. The namec2xis deprecated.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. 👑gnu23,gnu2x— the next version of the ISO C standard, still under development, plus GNU extensions. Support for this version is experimental and incomplete. The namegnu2xis deprecated.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. no cap, bestie — don't build your whole thesis on it.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.c++2c,c++26— the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.gnu++2c,gnu++26— GNU dialect of-std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-aux-infofilename — output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fno-asm— do not recognize"asm","inline"or"typeof"as a keyword, so that code can use these words as identifiers. You can use the keywords"__asm__","__inline__"and"__typeof__"instead. In C,-ansiimplies-fno-asm.
In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23.
-fno-builtin,-fno-builtin-function— don't recognize built-in functions that do not begin with__builtin_as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fcond-mismatch— allow 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++. soz, C++ girlies 💅-ffreestanding— assert that compilation targets a freestanding environment. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at"main". The most obvious example is an OS kernel. This is equivalent to-fno-hosted.-fgimple— enable parsing of function definitions marked with"__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes. tiny lab vibes, bestie 🧪✨-fgnu-tm— when the option-fgnu-tmis specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline— the option-fgnu89-inlinetells GCC to use the traditional GNU semantics for"inline"functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.
-fhosted— assert that compilation targets a hosted environment. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in which"main"has a return type of"int". Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-flax-vector-conversions— allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. 🚩 red flag, bestie — leave it in 2010 where it belongs.-fms-extensions— accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi.
-foffload=disable,-foffload=default,-foffload=target-list— specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to-foffload=default, is to generate code for all supported offload targets. The-foffload=disableform generates code only for the host fallback, while-foffload=target-listgenerates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under "OFFLOAD_TARGET_NAMES".
-foffload-options=options,-foffload-options=target-triplet-list=options— with-foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the-foffload-options=target-list=optionsform. The target-list is a comma-separated list in the same format as for the-foffload=option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906-fopenacc— enable handling of OpenACC directives#pragma accin C/C++ and!$accin free-form Fortran and!$acc,c$accand*$accin fixed-form Fortran. When-fopenaccis specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenacc-dim=geom— specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of':'-separated sizes, in order'gang','worker'and,'vector'. A size can be omitted, to use a target-specific default value.-fopenmp— enable handling of OpenMP directives#pragma omp,[[omp::directive(...)]],[[omp::sequence(...)]]and[[omp::decl(...)]]in C/C++ and!$ompin Fortran. It additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. When-fopenmpis specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenmpimplies-fopenmp-simd.-fopenmp-simd— enable handling of OpenMP's"simd","declare simd","declare reduction","assume","ordered","scan"and"loop"directive, and of combined or composite directives with"simd"as constituent with"#pragma omp","[[omp::directive(...)]]","[[omp::sequence(...)]]"and"[[omp::decl(...)]]"in C/C++ and"!$omp"in Fortran. It additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. Other OpenMP directives are ignored. Unless-fopenmpis additionally specified, the"loop"region binds to the current task region, independent of the specified"bind"clause.-fopenmp-target-simd-clone-fopenmp-target-simd-clone=device-type— okay so besides making SIMD clones for functions marked with thedeclare simddirective, GCC will also make clones for functions marked with the OpenMPdeclare targetdirective that are good for vectorization, whenever this option is on. device-type can benone,host,nohost, orany— they line up with the keywords of thedevice_typeclause ofdeclare target, and clones get generated for the intersection of the devices you named.-fopenmp-target-simd-cloneis exactly the same as-fopenmp-target-simd-clone=any, and-fno-openmp-target-simd-cloneis exactly the same as-fopenmp-target-simd-clone=none.
At -O2 and above — but not -Os or -Og — this optimization defaults to -fopenmp-target-simd-clone=nohost. Anywhere else it's off by default. 💅
-fpermitted-flt-eval-methods=style— ISO/IEC TS 18661-3 defines some brand-new allowed values forFLT_EVAL_METHODthat say: operations and constants whose semantic type is an interchange or extended format should be evaluated at the precision and range of that type. These new values are a superset of what C99/C11 allows, and C99/C11 doesn't even define what other positiveFLT_EVAL_METHODvalues mean. So yeah — code that conforms to C11 might not have been written expecting these new values to ever show up. 😭
-fpermitted-flt-eval-methods picks whether the compiler allows only the FLT_EVAL_METHOD values from C99/C11, or the extended set from ISO/IEC TS 18661-3.
style is either c11 or ts-18661-3, whichever fits.
Default in a standards-compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. Default in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. slay, consistency.
The -fdeps-* options pull out structured dependency information for a source. That means working out which resources other source files provide that'll be needed to compile this source, plus which resources this source provides. Why tho? So build tools can wire up required dependencies between compilation rules of dependent sources based on their contents, instead of you having to hand-feed that info into the build system yourself. iconic behavior honestly.
-fdeps-file=file— where to write the structured dependency information. 📝-fdeps-format=format— which format to use for the structured dependency info.p1689r5is the only supported format right now, periodt. Heads up though: when you pass this argument, the output of-MFgets stripped of some information (specifically C++ modules) so it doesn't end up using extended makefile syntax that most tools just don't understand. 🫶-fdeps-target=file— the same energy as-MT, but for structured dependency information. It says which target will ultimately need any required resources and provide any resources extracted from the source that other sources might need. 🎀-fplan9-extensions— accepts some non-standard constructs used in Plan 9 code. It enables-fms-extensions, lets you pass pointers to structures with anonymous fields to functions that expect pointers to elements of the field's type, and lets you refer to anonymous fields declared using atypedef. Only supported for C — not C++.-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 say eithersignedorunsigned. Default? Signed, bestie, because that's the consistent thing: basic integer types likeintare signed types. ✨-fsigned-char— makes the typecharsigned, just likesigned char.
Ngl this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. And the same way, -fno-signed-char is equivalent to -funsigned-char. 🔁
-funsigned-char— makes the typecharunsigned, just likeunsigned char.
Every kind of machine has its own default for what char should be. It's either like unsigned char by default or like signed char by default. That's just how it is. 🦋
Ideally a portable program should always use signed char or unsigned char whenever it depends on the signedness of an object. But like... so many programs were written using plain char and expecting it to be signed, or expecting it to be unsigned, depending on the machines they were written for. This option and its inverse let you make a program like that work with the opposite default. character development, honestly.
The type char is always a distinct type from both signed char and unsigned char, even though its behavior is always just like one of those two. no cap.
-fstrict-flex-arrays(C and C++ only)-fstrict-flex-arrays=level(C and C++ only) — controls when the trailing array of a structure is treated as a flexible array member for the purpose of accessing the elements of that array. level sets how strict we're being.
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, the strictest of all: all trailing arrays of structures get treated as flexible array members. 💅
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, the least strict. In that case a trailing array is treated as a flexible array member only when it's declared as a flexible array member per the C99 standard onwards.
The possible values of level are the same as for the strict_flex_array attribute.
You can control this for one specific trailing array field of a structure using the variable attribute strict_flex_array attribute.
Also the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. they're besties, basically. 🫶
-fsso-struct=endianness— sets the default scalar storage order of structures and unions to the endianness you name. Accepted values arebig-endian,little-endian, andnativefor the target's native endianness (that's the default). Not supported for C++.
⚠️ Warning: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it, if the endianness you specified isn't the native endianness of the target. like. be careful bestie.
✨ Options Controlling C++ Dialect ✨
This section is all about the command-line options that are only meaningful for C++ programs. That said, 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, like -std, are relevant for C++ programs too.
Here's the list of options that are only for compiling C++ programs: 💖
-fabi-version=n— use version n of the C++ ABI. Default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Which 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 of the C++ ABI that first appeared in G++ 3.4, and was the default all the way 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_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, like ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name 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, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. 😭
Version 13, which first appeared in G++ 8.2, fixes that accidental change in version 12. redemption arc. ✨
Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G++ 10.3, corrects the G++ 10 ABI tag regression.
Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.
Version 17, which first appeared in G++ 12, fixes the layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags.
See also -Wabi. 👑
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name whenever it defines a symbol with an incorrect mangled name. This switch says which ABI version to use for the alias.
With -fabi-version=0 (the default, bestie), this defaults to 13 — that's GCC 8.2 compatibility. If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2. periodt. 💅
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. no cap.
-fno-access-control— turn off all access checking. This switch is mainly useful for working around bugs in the access control code. lowkey a lifesaver when the compiler is being dramatic. 🙈-faligned-new— enable support for C++17newof types that require more alignment thanvoid* ::operator new(std::size_t)provides. A numeric argument like-faligned-new=32can specify how much alignment (in bytes) is provided by that function, but few users will need to override the default ofalignof(std::max_align_t). This flag is enabled by default for-std=c++17. slay. ✨-fchar8_t,-fno-char8_t— enable support forchar8_tas adopted for C++20. This includes the addition of a 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. iconic. 🦋
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified. 😭
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested-fcheck-new— check that the pointer returned byoperator newis non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies thatoperator newonly returns0if it is declaredthrow(), in which case the compiler always checks the return value even without this option. In all other cases, whenoperator newhas a non-empty exception specification, memory exhaustion is signalled by throwingstd::bad_alloc. See alsonew (nothrow). iykyk. 💀-fconcepts,-fconcepts-ts— enable support for the C++ Concepts feature for constraining template arguments. With-std=c++20and above, Concepts are part of the language standard, so-fconceptsdefaults to on. 🌈
Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, can additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and may be removed in GCC 15; users are expected to convert their code to C++20 concepts. periodt.
-fconstexpr-depth=n— set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is512. vibe check. 🌀-fconstexpr-cache-depth=n— set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. bestie, you got this. ✨-fconstexpr-fp-except— Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined. obsessed. 👑
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except-fconstexpr-loop-limit=n— set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is262144(1<<18). 🔁-fconstexpr-ops-limit=n— set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is33554432(1<<25). 😮💨-fcontracts— enable experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard. experimental. 🧪
On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining:
void
handle_contract_violation (const std::experimental::contract_violation&);There are different sets of additional flags that can be used together to specify which contracts will be checked and how, for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together. cannot be used together. 🚫
-fcontract-mode=[on|off]— control whether any contracts have any semantics at all. Defaults toon.-fcontract-assumption-mode=[on|off]— [N4820] control whether contracts with level axiom should have the assume semantic. Defaults toon.-fcontract-build-level=[off|default|audit]— [N4820] specify which level of contracts to generate checks for. Defaults todefault.-fcontract-continuation-mode=[on|off]— [N4820] control whether to allow the program to continue executing after a contract violation. That is, do checked contracts have the maybe semantic described below rather than the never semantic. Defaults tooff.-fcontract-role=<name>:<default>,<audit>,<axiom>— [P1332] specify the concrete semantics for each contract level of a particular contract role.-fcontract-semantic=[default|audit|axiom]:<semantic>— [P1429] specify the concrete semantic for a particular contract level.-fcontract-strict-declarations=[on|off]— control whether to reject adding contracts to a function after its first declaration. Defaults tooff.
The possible concrete semantics that can be specified with -fcontract-role or -fcontract-semantic are:
"ignore"— this contract has no effect."assume"— this contract is treated like C++23[[assume]]."check_never_continue","never","abort"— this contract is checked. If it fails, the violation handler is called. If the handler returns,std::terminateis called."check_maybe_continue","maybe"— this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally.
-fcoroutines— enable support for the C++ coroutines extension (experimental). 🌀-fdiagnostics-all-candidates— permit the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. noted. 👀-fno-elide-constructors— the C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline. 💅
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
-fno-enforce-eh-specs— don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like definingNDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. undefined behavior at run time. 💀-fextern-tls-init,-fno-extern-tls-init— okay so bestie, C++11 and OpenMP are out here allowingthread_localandthreadprivatevariables to have dynamic (runtime) initialization. ✨ To make that happen, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the-fno-extern-tls-initoption. On targets that support symbol aliases, the default is-fextern-tls-init. On targets that do not support symbol aliases, the default is-fno-extern-tls-init. 💅-ffold-simple-inlines,-fno-fold-simple-inlines— permit the C++ frontend to fold calls tostd::move,std::forward,std::addressofandstd::as_const. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless-fno-inlineis active. 🫶-fno-gnu-keywords— do not recognizetypeofas a keyword, so that code can use this word as an identifier. You can use the keyword__typeof__instead. This option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc. 🎀-fno-immediate-escalation— do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified in P2564R3. For example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}compiles in C++20: f is an immediate-escalating function (due to the auto it is a function template and is declared constexpr) and id(t) is an immediate-escalating expression, so f is promoted to consteval. Consequently, the call to id(t) is in an immediate context, so doesn't have to produce a constant (that is the mechanism allowing consteval function composition). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) is not a constant expression, the compiler rejects the code. This option is turned on by default; it is only effective in C++20 mode or later. 😭
-fimplicit-constexpr— make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations. ⭐-fno-implicit-templates— never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. 💀-fno-implicit-inline-templates— don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. 🧚-fno-implement-inlines— to save space, do not emit out-of-line copies of inline functions controlled by#pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. 🔥-fmodules-ts,-fno-modules-ts— enable support for C++20 modules. The-fno-modules-tsis usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. 🌈-fmodule-header,-fmodule-header=user,-fmodule-header=system— compile a header file to create an importable header unit. 🎀-fmodule-implicit-inline— member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.) 💅-fno-module-lazy— disable lazy module importing and module mapper creation. 🦋-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]— an oracle to query for module name to filename mappings. If unspecified theCXX_MODULE_MAPPERenvironment variable is used, and if that is unset, an in-process default is provided. ✨-fmodule-only— only emit the Compiled Module Interface, inhibiting any object file. 🌸-fms-extensions— disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. 💜-fnew-inheriting-ctors— enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless-fabi-version=10or lower is specified. 🫶-fnew-ttp-matching— enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for-std=c++17. ⭐-fno-nonansi-builtins— disable built-in declarations of functions that are not mandated by ANSI/ISO C. These includeffs,alloca,_exit,index,bzero,conjf, and other related functions. 🍓-fnothrow-opt— treat athrow()exception specification as if it were anoexceptspecification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call toterminaterather thanunexpected. 😭-fno-operator-names— do not treat the operator name keywordsand,bitand,bitor,compl,not,orandxoras synonyms as keywords. 💅-fno-optional-diags— disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. 🎀-fno-pretty-templates— when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.void f(T) [with T = int]rather thanvoid f(int)) so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use-fno-pretty-templatesto disable them. 🌈-fno-rtti— disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_castandtypeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. Thedynamic_castoperator can still be used for casts that do not require run-time type information, i.e. casts tovoid *or to unambiguous base classes. Mixing code compiled with-frttiwith that 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— enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition. 🔥
-fstrict-enums— allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type. 🦋-fstrong-eval-order— evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with-std=c++17.-fstrong-eval-order=someenables just the ordering of member access and shift expressions, and is the default without-std=c++17. ✨-ftemplate-backtrace-limit=n— set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. 💖-ftemplate-depth=n— bestie, this caps how deep template class instantiation can go atn. Why do we even need a limit? because otherwise endless recursion during template class instantiation would just… never stop, and we'd be stuck there forever 😭 ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (this got changed to 1024 in C++11, glow up ✨). The default value is900, because apparently the compiler can run out of stack space before it even hits 1024 in some situations. iconic behavior tbh.-fno-threadsafe-statics— do not emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. Use this if you wanna reduce code size slightly in code that doesn't need to be thread-safe. If it does need to be thread-safe… bestie, no 💀-fuse-cxa-atexit— registers destructors for objects with static storage duration with the__cxa_atexitfunction instead of theatexitfunction. This one's required for fully standards-compliant handling of static destructors, but it only works if your C library supports__cxa_atexit. Conditional queen 👑-fno-use-cxa-get-exception-ptr— don't use the__cxa_get_exception_ptrruntime routine. This makesstd::uncaught_exceptionincorrect, but it's necessary if the runtime routine is not available. Pick your struggle 🫠-fvisibility-inlines-hidden— this switch declares that you, the user, do not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't appear in the export table of a DSO and don't require a PLT indirection when used within the DSO. Enabling this can have a dramatic effect on load and link times of a DSO, because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. the way it just speeds everything up?? obsessed 💅
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. subtle but important, okurrr.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect — not on this, bestie, don't even try.
Explicitly instantiated inline methods are unaffected by this option, as 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 that of Microsoft Visual Studio. bridge-building fr fr 🫶
The flag 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 are permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it's possible for code to rely, perhaps accidentally, on the Visual Studio behavior. (not on purpose… or was it 🙈)
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. so don't do that 💀
-fno-weak— do not use weak symbol support, even if it's provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. It may be removed in a future release of G++. so like… it's giving deprecated energy, stay away ✋-fext-numeric-literals(C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, those suffixes are treated as C++11 user-defined literal numeric suffixes. It's on by default for all pre-C++11 dialects and all GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. It's off by default for ISO C++11 onwards (-std=c++11, ...). the switch-up is real 🌸-nostdinc++— don't search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) half the search, all the vibe.-flang-info-include-translate-flang-info-include-translate-not-flang-info-include-translate=header— these three inform you of include translation events. The first will note accepted include translations, the second will note declined include translations. Theheaderform will inform of include translations relating to that specific header. Ifheaderis of the form"user"or<system>, it will be resolved to a specific user or system header using the include path. tea ☕ but for headers.-flang-info-module-cmi-flang-info-module-cmi=module— inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. Themoduleform will not reading a specific module's CMI. (yes that's how it's worded, don't look at me 😭)modulemay be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in<>or"").-stdlib=libstdc++,libc++— when G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: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). The option switches G++ to use the headers from the specified library and to emit-lstdc++or-lc++respectively, when a C++ runtime is required for linking. choosing your runtime like choosing an outfit 🎀
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag(C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. mismatched tags?? we don't know her 💅-Wcomma-subscript(C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in( )is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and with -std=c++23 regardless of -Wno-deprecated.
This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. bestie, tread carefully 😭
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning points out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;the "incomplete parameter type" trick is so sneaky, iykyk 🧚
-Wctor-dtor-privacy(C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. it's giving locked out of your own house 🚪-Wdangling-reference(C++ and Objective-C++ only) — warn when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is danglingIn the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference r is dangling. This warning also detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is danglingCertain functions are safe in this respect, for example std::use_facet: they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a #pragma:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic popThe #pragma can also surround the class; in that case, the warning will be disabled for all the member functions. group-hug exemption 🫶
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);and yeah bestie, that one deserves the side-eye 🙈
- ...where
std::minmaxreturnsstd::pair<const int&, const int&>, and both references go dangling after the end of the full expression that contains the call tostd::minmax. 💀
The warning does not yell at std::span-like classes, bestie. We're considering classes shaped like this:
template<typename T>
struct Span {
T* data_;
std::size len_;
};as std::span-like, meaning the class is a non-union class that has a pointer data member and a trivial destructor. 🦋
You can turn the warning off with the gnu::no_dangling attribute.
This warning is enabled by -Wall. ⭐
-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 is unsafe todeletean instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. 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 also deprecates if the class has a user-provided destructor. 💅-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disables the warning about the case when the usual arithmetic conversions get applied on operands where one is of enumeration type and the other is of a different enumeration type. That conversion was deprecated in C++20. Like, 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, this warning can be enabled by -Wenum-conversion. ✨
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disables the warning about the case when the usual arithmetic conversions get applied on operands where one is of enumeration type and the other is of a floating-point type. Deprecated in C++20. Example time:
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, this warning can be enabled by -Wenum-conversion. 🌸
-Wno-elaborated-enum-base— for C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with a given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning gets upgraded to an error with-pedantic-errors. 😭-Wno-init-list-lifetime(C++ and Objective-C++ only) — do not warn about uses ofstd::initializer_listthat are likely to result in dangling pointers. Since the underlying array for aninitializer_listis handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:- If a function returns a temporary
initializer_list, or a 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.
- If a function returns a temporary
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior- When a list constructor stores the
beginpointer from theinitializer_listargument, this 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. 🫠
-Winvalid-constexpr— warns when a function never produces a constant expression. In C++20 and earlier, for everyconstexprfunction and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template markedconstexprfor which no invocation satisfies the requirements of a core constant expression.
This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it will be an ordinary warning. Like:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}-Winvalid-imported-macros— verify all imported macro definitions are valid at the end of compilation. Not enabled by default, because it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros. 🧚-Wno-literal-suffix(C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from<inttypes.h>. For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}In this case, PRId64 is treated as a separate preprocessing token. 🎀
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.
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 required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but-Wno-narrowingsuppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 💅
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat. ✨
-Wnoexcept(C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e.throw()ornoexcept) but is known by the compiler to never throw an exception. 👑-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature makingnoexceptpart of a function type changes the mangled name of a symbol relative to C++14. Enabled by-Wabiand-Wc++17-compat.
Example, bestie:
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) — warns when the destination of a call to a raw memory function such asmemsetormemcpyis an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call tomemsetbelow is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning. 🫶
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if-Weffc++is specified. The-Wdelete-non-virtual-dtoroption (enabled by-Wall) should be preferred because it warns about the unsafe cases without false positives. 💖-Wregister(C++ and Objective-C++ only) — warns on uses of theregisterstorage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of theregisterkeyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17. 💅-Wreorder(C++ and Objective-C++ only) — bestie, this one clocks you when the order of your member initializers in the code does NOT match the order they actually get executed in. The compiler is gonna quietly rearrange them to match declaration order and then be like "hey, heads up, I did a thing." Example of the crime:
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 about it. This warning is enabled by -Wall. 💅
-Wno-pessimizing-move(C++ and Objective-C++ only) — this warning warns when a call tostd::moveprevents copy elision. Not me sabotaging my own optimization 😭 A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type. Like:
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, the std::move call prevents copy elision. 🫠 This warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — this warning warns about redundant calls tostd::move; that is, when a move operation would have been performed even without thestd::movecall. It's giving extra for no reason 💀 This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}Here, the std::move call is redundant. Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue. This warning is enabled by -Wextra.
-Wrange-loop-construct(C++ and Objective-C++ only) — warns when a C++ range-based for-loop is creating an unnecessary copy. the way thatconst auto xis just out here duplicating everything 🙈 This can happen when the range declaration is not a reference, but 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 warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, in every iteration of the loop a temporary value of type double is created and destroyed, to which the reference const double & is bound. 🫶 This warning is enabled by -Wall.
-Wredundant-tags(C++ and Objective-C++ only) — warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. Extra words, no purpose, we love to see it 🌸 For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedOn the other hand, in this example there is no warning:
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) — do not 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. Here's the lore: if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. that's a whole crime, bestie If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main.Cfile, as those are unlikely to have multiple definitions.-Wsubobject-linkageis enabled by default.-Weffc++(C++ and Objective-C++ only) — warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books 📚 (yes, the sacred texts):- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have
operator=return a reference to*this. - Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload
&&,||, or,.
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. iykyk 🧚
-Wno-exceptions(C++ and Objective-C++ only) — disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. your catch blocks are out of order, babe 💅-Wstrict-null-sentinel(C++ and Objective-C++ only) — warn about the use of an uncastedNULLas sentinel. When compiling only with GCC this is a valid sentinel, asNULLis defined to__null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. ⚠️-Wno-non-template-friend(C++ and Objective-C++ only) — disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such 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 compatibility problems, and is enabled by default. ancient compiler lore 🏺-Wold-style-cast(C++ and Objective-C++ only) — warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast,static_cast,reinterpret_cast, andconst_cast) are less vulnerable to unintended effects and much easier to search for. glow-up your casts, bestie ✨-Woverloaded-virtual,-Woverloaded-virtual=n(C++ and Objective-C++ only) — warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int); // does not override
};the A class version of f is hidden in B, and code like:
B* b;
b->f();fails to compile. oof 😭
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add using A::f; to B.
The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall. 💖
-Wno-pmf-conversions(C++ and Objective-C++ only) — disable the diagnostic for converting a bound pointer to member function to a plain pointer. 🙈-Wsign-promo(C++ and Objective-C++ only) — warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. the standard said what it said 👑-Wtemplates(C++ and Objective-C++ only) — warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. no templates in this house, but the STL is exempt, we're not monsters 🌈-Wmismatched-new-delete(C++ and Objective-C++ only) — warn for mismatches between calls tooperator neworoperator deleteand the corresponding call to the allocation or deallocation function. This includes invocations of C++operator deletewith pointers returned from either mismatched forms ofoperator new, or from other functions that allocate objects for which theoperator deleteisn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned fromoperator newfor which the deallocation function isn't suitable. new/delete pairings must MATCH, that's the whole vibe check 🔥
For example, the delete expression in the function below is diagnosed because it doesn't match the array form of the new expression the pointer argument was returned from. Similarly, the call to free is also diagnosed.
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
}okay so</p> the sibling warning here is -Wmismatched-dealloc, and it's the one that catches mismatches between other allocation/deallocation pairs — like, everything that is not operator new and operator delete. It's giving "we do the same thing but for the non-special ones" energy. 💅
-Wmismatched-new-delete is included in -Wall, bestie. No cap.
-Wmismatched-tags (C++ and Objective-C++ only)
Yell at you when you declare structs, classes, class templates and their specializations with a class-key that doesn't match either the definition or the first declaration (when there's no definition at all). 👀
Like, look at this. The declaration of struct Object in the argument list of draw triggers the warning. To dodge it, either yeet the redundant class-key struct or swap it for class so it matches its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);Important</p>: it's not wrong to declare a class with the class-key struct — the example above literally shows that. -Wmismatched-tags is just here to help you get a consistent style of class declarations. And for code meant to be portable to Windows-based compilers, the warning helps prevent unresolved references caused by the difference in the mangling of symbols declared with different class-keys. You can use the option on its own or alongside -Wredundant-tags. ✨
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes. Some coding rules are like "no multiple inheritance" and this is how you enforce that rule. 🚫
The warning is inactive inside a system header file, such as the STL, so you can still use the STL, no problem. You may also define classes that indirectly use multiple inheritance. 👑
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Same deal — some coding rules disallow multiple inheritance and this enforces that rule. The warning is inactive inside a system header file, such as the STL, so STL stays usable. You may also define classes that indirectly use virtual inheritance. 🦋
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. 🫣
This is dangerous. If the virtual base is reachable 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, this warning can be disabled.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules disallow namespaces and this is how you enforce it. The warning is inactive inside a system header file, such as the STL, so you can still use the STL. You may also use using directives and qualified names. 💫
-Wno-template-id-cdtor (C++ and Objective-C++ only)
Disable the warning about using simple-template-id as the declarator-id of a constructor or destructor — which became invalid in C++20 via DR 2237. 😭 Like, for example:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat. ⭐
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately result in a call to terminate. 💀
-Wno-vexing-parse (C++ and Objective-C++ only)
Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted 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 will suggest options for how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. 💡
This warning is enabled by default.
-Wno-class-conversion (C++ and Objective-C++ only)
Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void — because such a conversion function will never be called. 🚫
-Wvolatile (C++ and Objective-C++ only)
Warn about deprecated uses of the volatile qualifier. This includes:
- postfix and prefix
++and--expressions ofvolatile-qualified types - using simple assignments where the left operand is a
volatile-qualified non-class type for their value - compound assignments where the left operand is a
volatile-qualified non-class type volatile-qualified function return typevolatile-qualified parameter type- structured bindings of a
volatile-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)
Warn when a literal 0 is used as a null pointer constant. This can be useful to make the conversion to nullptr in C++11 easier. 🎀
-Waligned-new
Warn about a new-expression of a type that requires greater alignment than alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall. 🔥
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 💅
-Wno-placement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, like constructing an object in a buffer that is smaller than the type of the object. 😱 For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large:
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— This is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the followingnewexpression is not diagnosed at this level even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array:
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();-Wplacement-new=2— At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2:
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that don't catch via reference. 👀
- With
-Wcatch-value=1(or-Wcatch-valuefor short) warn about polymorphic class types that are caught by value. - With
-Wcatch-value=2warn about all class types that are caught by value. - With
-Wcatch-value=3warn about all types that are not caught by reference.
-Wcatch-value is enabled by -Wall. ✨
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs. 📎
-Wno-delete-incomplete (C++ and Objective-C++ only)
Do not warn when deleting a pointer to an incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. 💀
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolons after in-class function definitions. That stray ; is not the moment, bestie. 🙈
-Wno-global-module (C++ and Objective-C++ only)
Disable the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives. 🧚
-Wno-inaccessible-base (C++, Objective-C++ only)
This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option. 💜
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 😭
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they're writing nonportable code and who have deliberately chosen to ignore the warning about it. 🫶
The restrictions on offsetof may be relaxed in a future version of the C++ standard. ⭐
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;or vice versa. Enabled by -Wextra along with -fsized-deallocation. 💅
-Wsuggest-final-types— bestie, this one's a lil nudge for your C++ code. It warns about types that have virtual methods where your code quality would be way better if you'd just slapped the C++11finalspecifier on that type, or — if you can — declared it in an anonymous namespace instead. Why do we care? Because that lets GCC go way more feral about devirtualizing polymorphic calls. 🔥 Heads up tho: this warning hits different with link-time optimization, because then the info about the class hierarchy graph is actually complete.-Wsuggest-final-methods— same energy, but for virtual methods. Warns when a method would be better off marked with the C++11finalspecifier, or (if possible) when its type should live in an anonymous namespace or get thefinalspecifier itself. Again, link-time optimization makes this way more effective since the class hierarchy graph info is more complete. 💅 And a pro tip from the original: consider the-Wsuggest-final-typessuggestions first, then rebuild with the new annotations.-Wsuggest-override— warns you about overriding virtual functions that aren't marked with theoverridekeyword. Like, girl, tag your overrides. ✨-Wno-conversion-null(C++ and Objective-C++ only) — turns off the warning for conversions betweenNULLand non-pointer types. FYI-Wconversion-nullis enabled by default, so this is you actively saying "no thank you."
🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
Okay so this whole section is the command-line options that only make sense for Objective-C and Objective-C++ programs. But bestie, you can still use most of the language-independent GNU compiler options too. Like, say you wanna compile a file some_class.m, you'd do:
gcc -g -fgnu-runtime -O -c some_class.mIn that example, -fgnu-runtime is an option that's meant only for Objective-C and Objective-C++ programs — but the other options? Use 'em with any language GCC supports, no cap.
Also note: since Objective-C is literally an extension of the C language, your Objective-C compilations can also use C-front-end-specific options (e.g. -Wtraditional). Same deal the other way: Objective-C++ compilations can use C++-specific options (e.g. -Wabi). 🌈
Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name— use class-name as the name of the class to instantiate for each literal string specified with the syntax"@"..."". The default class name is"NXConstantString"if you're on the GNU runtime, and"NSConstantString"if you're on the NeXT runtime (more on that below). And on Darwin / macOS platforms, if-fconstant-cfstringsis also present, it overrides the-fconstant-string-classsetting and makes those"@"...""literals get laid out as constant CoreFoundation strings. Side note:-fconstant-cfstringsis just an alias for the target-specific-mconstant-cfstringsequivalent. 💫-fgnu-runtime— generates object code that's compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. 🫶-fnext-runtime— generates output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS. The macro__NEXT_RUNTIME__is predefined if (and only if) this option is used. 👑-fno-nil-receivers— assumes that all Objective-C message dispatches ([receiver message:arg]) in this translation unit guarantee the receiver is notnil. That lets the runtime use more efficient entry points. Heads up: this option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.-fobjc-abi-version=n— use version n of the Objective-C ABI for the selected runtime. Right now this option is only supported for the NeXT runtime. In that case:- Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
- Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
- Version 2 is the modern (64-bit) ABI.
And if you specify nothing at all? 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, it checks if 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 any such instance variables, in order, and then returnsself. Similarly, it checks if any instance variable is a C++ object with a non-trivial destructor, and if so it synthesizes a special- (void) .cxx_destructmethod that runs all those default destructors, in reverse order. 😤
Those generated - (id) .cxx_construct and - (void) .cxx_destruct methods only operate on instance variables declared in the current Objective-C class — not the ones inherited from superclasses. It's the Objective-C runtime's job to invoke all such 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; 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 actually supports invoking those - (id) .cxx_construct and - (void) .cxx_destruct methods. 💀
-fobjc-direct-dispatch— allows fast jumps to the message dispatcher. On Darwin this gets done via the comm page. ⚡-fobjc-exceptions— enables syntactic support for structured exception handling in Objective-C, kinda like what C++ offers. You need this option to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. It's available with both the GNU runtime and the NeXT runtime — but not in conjunction 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. Only available with the NeXT runtime, bestie — the GNU runtime has a totally different garbage collection implementation that doesn't need special compiler flags.-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, checks for a nil receiver in method invocations before doing the actual method call. This is the default, and you can turn it off with-fno-objc-nilcheck. Important caveat: class methods and super calls are never checked for nil this way, no matter what this flag is set to. And currently this flag does absolutely nothing when the GNU runtime is used, or with an older version of the NeXT runtime ABI. 🤷-fobjc-std=objc1— conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language — it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When you use this with the Objective-C or Objective-C++ compiler, any Objective-C syntax that GCC 4.0 doesn't recognize gets rejected. Super useful if you need to make sure your Objective-C code can compile with older versions of GCC. 🧚-freplace-objc-classes— emits a special marker instructingld(1)not to statically link in the resulting object file, and letsdyld(1)load it in at run time instead. This is used together with the Fix-and-Continue debugging mode, where the object file in question can be recompiled and dynamically reloaded while your program is running — no need to restart the program itself. Currently, Fix-and-Continue functionality is only available with the NeXT runtime on Mac OS X 10.3 and later.-fzero-link— when compiling for the NeXT runtime, the compiler normally replaces calls toobjc_getClass("...")(when the class name is known at compile time) with static class references that get initialized at load time, which makes run-time performance better. Specifying-fzero-linksuppresses that behavior and makes calls toobjc_getClass("...")stick around. This is useful in Zero-Link debugging mode, since it lets individual class implementations be modified during program execution. And the GNU runtime? It currently always retains calls toobjc_get_class("...")regardless of command-line options. 🌸-fno-local-ivars— by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. That can cause shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying-fno-local-ivarsdisables that behavior, so you dodge all the variable shadowing drama. 🫧-fivar-visibility=[public|protected|private|package]— sets the default instance variable visibility to the option you pick, so that instance variables declared outside the scope of any access modifier directives default to that visibility.-gen-decls— dumps 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 is being intercepted by the garbage collector.-Wno-property-assign-default(Objective-C and Objective-C++ only) — don't warn if a property for an Objective-C object has no assign semantics specified. 🤐-Wno-protocol(Objective-C and Objective-C++ only) — so if a class is declared to implement a protocol, a warning gets issued for every method in the protocol that the class doesn't implement. The default behavior is to warn for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. But if you use-Wno-protocol, then methods inherited from the superclass count as implemented, and no warning gets issued for them. ✨-Wobjc-root-class(Objective-C and Objective-C++ only) — warns if a class interface lacks a superclass. Most classes will inherit fromNSObject(orObject) for example. When you're declaring classes that are meant to be root classes, you can suppress the warning by marking their interfaces with__attribute__((objc_root_class)). 💖-Wselector(Objective-C and Objective-C++ only, bestie 💅) — yells at you if multiple methods with different types for the same selector show up during compilation. The check runs over the list of methods way out at the final stage of compilation. On top of that, a check happens for every selector hanging out in a@selector(...)expression, as long as a matching method for that selector was found during compilation. And here's the tea ☕: because these checks only scan the method table at the very end of compilation, you get no warnings at all if the final stage never gets reached — say an error popped up mid-compile, or you're using-fsyntax-only.-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types exist for a given selector, and you're trying to send a message with that selector to a receiver of typeidorClass. When this flag is off (and off is the default, lowkey the chill mode), the compiler skips those warnings if all the differences it found are confined to types that share the same size and alignment.-Wundeclared-selector(Objective-C and Objective-C++ only) — warns if 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. Timing matters here, bestie: this option always runs its checks the moment a@selector(...)expression is found, whereas-Wselectoronly checks at the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before they're used. aesthetic AND correct ✨-print-objc-runtime-info— generates a C header describing the largest structure that's passed by value, if any exists.
✨ Options to Control Diagnostic Messages Formatting ✨
Okay so back in the day, diagnostic messages were formatted completely ignoring the output device's aspect — like, its width, whatever, no thoughts, head empty. 💀 The options below let you take the wheel on the formatting algorithm for diagnostics: how many characters per line, how often source location info gets reported, all of it. Heads up though: some language front ends may simply not honor these options.
-fmessage-length=n— tries to format error messages so they fit on lines of about n characters. If n is zero, then no line-wrapping happens at all and each error message lands on a single line. This is the default for all front ends.
Note: this option also affects the display of the #error and #warning pre-processor directives, plus the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas.
-fdiagnostics-plain-output— asks for diagnostic output to look as plain as humanly possible. Super useful when you're runningdejagnuor other utilities that need to parse diagnostics output and prefer it stay more stable over time. Right now,-fdiagnostics-plain-outputis equivalent to this exact set of options:
-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers
-fdiagnostics-color=never -fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events
-fdiagnostics-text-art-charset=noneFuture tea 🍵: if GCC ever changes the default appearance of its diagnostics, the corresponding option to disable that new behavior gets added to this list.
-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once; that is, if the message is too long to fit on a single physical line and has to be wrapped, the source location won't get emitted (as a prefix) again and again and again on the continuation lines. This is the default behavior, periodt.-fdiagnostics-show-location=every-line— only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit that same source location info (as a prefix) for every physical line that results from breaking a message that's too long for one line. every line gets the lore 🔥-fdiagnostics-color[=WHEN]-fno-diagnostics-color— use color in diagnostics. WHEN isnever,always, orauto. The default depends on how the compiler was configured; it can be any of those WHEN options, or alsoneverif theGCC_COLORSenvironment variable isn't present in the environment, andautootherwise.automakes GCC use color only when standard error is a terminal, and when it's not executing 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. (Peep the section in your text terminal's documentation for the permitted values and what they mean as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=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 entirely. The supported capabilities are as follows, bestie — buckle up:
"error="— SGR substring forerror:markers."warning="— SGR substring forwarning:markers."note="— SGR substring fornote:markers."path="— SGR substring for colorizing paths of control-flow events as printed via-fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns."range1="— SGR substring for first additional range."range2="— SGR substring for second additional range."locus="— SGR substring for location information,file:lineorfile:line:columnetc."quote="— SGR substring for information printed within quotes."fnname="— SGR substring for names of C++ functions."targs="— SGR substring for C++ function template parameter bindings."fixit-insert="— SGR substring for fix-it hints suggesting text to be inserted or replaced."fixit-delete="— SGR substring for fix-it hints suggesting text to be deleted."diff-filename="— SGR substring for filename headers within generated patches."diff-hunk="— SGR substring for the starts of hunks within generated patches."diff-delete="— SGR substring for deleted lines within generated patches."diff-insert="— SGR substring for inserted lines within generated patches."type-diff="— SGR substring for highlighting mismatching types within template arguments in the C++ frontend."valid="— SGR substring for highlighting valid elements within text art diagrams."invalid="— SGR substring for highlighting invalid elements within text art diagrams.
-fdiagnostics-urls[=WHEN]— uses escape sequences to embed URLs inside diagnostics. For example, when-fdiagnostics-show-optionemits text showing the command-line option controlling a diagnostic, it embeds a URL for the documentation of that option. link in your error message, iconic behavior 🔗✨
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 executing in an emacs shell or any graphical terminal known to be incompatible with this feature — see below, bestie.
The default depends on how the compiler has been 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 that the default is affected by environment variables. Under such a configuration, GCC defaults to using 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 of them are.
However, even with -fdiagnostics-urls=always the behavior still depends on those environment variables: if GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS 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. yes, the beep. that one. 🙈
At this time GCC also tries to detect a few terminals that are known to not implement the URL feature, and that have bugs — or at least had bugs in some versions still in use — where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known-to-be-buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with -fdiagnostics-urls=always — so, y'know, at your own risk. 💀
-fno-diagnostics-show-option— by default, each diagnostic emitted includes text indicating the command-line option that directly controls that diagnostic (if such an option is known to the diagnostic machinery). Specifying the-fno-diagnostics-show-optionflag suppresses that whole behavior.-fno-diagnostics-show-caret— okay so by default every diagnostic comes with the original source line and a little caret^pointing at the column, like it's pointing a manicured nail at the exact problem. This option says "no thank you, keep it minimal" and suppresses that whole thing. Heads up though: the source line gets truncated to n characters if you gave it-fmessage-length=n. And when the output is going to the terminal, the width is limited to whatever theCOLUMNSenvironment variable says, or if that's not set, to the terminal width. Periodt.-fno-diagnostics-show-labels— so when source code gets printed (the whole-fdiagnostics-show-caretsituation), diagnostics can label ranges of source code with relevant info, like the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intThis option suppresses those labels (so in the example above, the vertical bars and the char * and long int text just... don't show up). ✨
-fno-diagnostics-show-cwe— diagnostic messages can optionally have an associated CWE ("https://cwe.mitre.org/index.html") identifier. GCC itself only provides that metadata for some of the-fanalyzerdiagnostics. GCC plugins may also hand over diagnostics with that metadata. By default, if the info is there, it gets printed with the diagnostic. This option suppresses printing of that metadata. 🫶-fno-diagnostics-show-rules— diagnostics can optionally have rules attached to them, like from a coding standard or a specification. GCC itself doesn't do this for any of its diagnostics, but plugins might. By default, if that info is present, it gets printed with the diagnostic. This option suppresses printing of that metadata.-fno-diagnostics-show-line-numbers— by default, when source code is printed (via-fdiagnostics-show-caret), there's a left margin showing line numbers. This option suppresses that left margin. no more numbers, we're free 💅-fdiagnostics-minimum-margin-width=width— this controls the minimum width of that left margin printed by-fdiagnostics-show-line-numbers. It defaults to6.-fdiagnostics-parseable-fixits— emits fix-it hints in a machine-parseable format, suitable for IDEs to consume. 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 expressed as a half-open range, counted in bytes, starting at byte 1 for the initial column. In the example above, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_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 is to be removed. An empty range (e.g. "45:3-45:3") means the string is to be inserted at the given position. 🎀
-fdiagnostics-generate-patch— prints fix-it hints tostderrin unified diff format, after any diagnostics are printed. Like:
--- 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 (peep -fdiagnostics-color). 🌈
-fdiagnostics-show-template-tree— in the C++ frontend, when diagnostics print mismatching template types, like:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, like:
map<
[...],
vector<
[double != float]>>The parts that differ get highlighted with color (double and float in this case). it's giving family tree but for types 🌳
-fno-elide-type— by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types get printed as[...]to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. show me everything, no secrets 💀
-fdiagnostics-path-format=KIND— specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
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 attempts 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— this one gives you extra info when printing control-flow paths associated with a diagnostic.
If this option is provided then the stack depth gets printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration get appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. iykyk 🔥
-fno-show-column— do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such asdejagnu. 🙈-fdiagnostics-column-unit=UNIT— select the units for the column number. This affects traditional diagnostics (in the absence of-fno-show-column), as well as JSON format diagnostics if requested.
The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character — like with tab characters — or it may be smaller, like with multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes. math girlies stay winning 💜
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0. 💾
-fdiagnostics-column-origin=ORIGIN— bestie, this sets the origin for column numbers, like which column gets to be number one. The default1matches old-school GCC behavior and the GNU style guide. Some utilities may perform better with an origin of0; any non-negative value may be specified. 🎯-fdiagnostics-escape-format=FORMAT— so when GCC prints pertinent source lines for a diagnostic, it normally tries to print the source bytes directly. But some diagnostics are about encoding issues in the source file, like malformed UTF-8 or Unicode normalization drama. Those diagnostics are flagged so GCC will escape bytes that aren't printable ASCII when printing their pertinent source lines. This option controls how those bytes get escaped. The defaultFORMAT,unicode, displays Unicode characters that aren't printable ASCII as<U+XXXX>, and bytes that don't correspond to a Unicode character validly encoded in UTF-8 will be displayed as hexadecimal in the form<XX>. For example, a source line containing the stringbeforefollowed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding0xCF 0x80) followed by the byte0xBF(a stray UTF-8 trailing byte), followed by the stringafterwill be printed for such a diagnostic as:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-text-art-charset=CHARSET— some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font. This option selects which characters should be used for printing such diagrams, if any.CHARSETisnone,ascii,unicode, oremoji. Thenonevalue suppresses the printing of such diagrams. Theasciivalue will ensure that such diagrams are pure ASCII ("ASCII art"). Theunicodevalue will allow for conservative use of unicode drawing characters (such as box-drawing characters). Theemojivalue further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character). The default isemoji, except when the environment variableLANGis set toC, in which case the default isascii. 🎨-fdiagnostics-format=FORMAT— select a different format for printing diagnostics.FORMATistext,sarif-stderr,sarif-file,json,json-stderr, orjson-file. The default istext. Thesarif-stderrandsarif-fileformats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or to a file namedsource.sarif, respectively. Thejsonformat is a synonym forjson-stderr. Thejson-stderrandjson-fileformats are identical, apart from where the JSON is emitted to — with the former, the JSON is emitted to stderr, whereas withjson-fileit is written tosource.gcc.json. 📄
The emitted JSON consists of a top-level JSON array containing JSON objects representing the diagnostics.
Diagnostics can have child diagnostics. For example, this error and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"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
}
}
],
"escape-source": false,
"message": "...this statement, but the latter is ..."
}
]
"escape-source": false,
"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 within 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 omitted 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, which have labels. It might be printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"escape-source": false,
"message": "invalid operands to binary + ..."
}If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| 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
}
}
],
"escape-source": false,
"message": "\u2018struct s\u2019 has no member named ..."
}okay so for the fix-it hint bestie: the "string" value is literally what you're supposed to swap in for the text that runs from "start" up to but not including "next". 💅 And the spicy edge cases:
- Deletions? They show up as an empty value for "string". 🫥
- Insertions? You get "start" equal to "next". ✨
If the diagnostic has a whole path of control-flow events riding along with it, then it gets a "path" array of objects, each one representing an event. Every event object carries a "description" string, a "location" object, plus a "function" string and a "depth" number, and those last two are there so we can represent interprocedural paths. The "function" is the current function at that event, and the "depth" is the stack depth relative to some baseline — the higher, the more frames are within the stack. 📚
So like, the intraprocedural example that got 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
}
}
]Diagnostics also have a boolean attribute called "escape-source", and it's hinting whether non-ASCII bytes should be escaped when printing the pertinent lines of source code — it's "true" for diagnostics involving source encoding issues. 🔤
✨ Options to Request or Suppress Warnings ✨
-fno-diagnostics-json-formatting— bestie, listen. By default, when JSON is emitted for diagnostics (via-fdiagnostics-format=sarif-stderr,-fdiagnostics-format=sarif-file,-fdiagnostics-format=json,-fdiagnostics-format=json-stderr,-fdiagnostics-format=json-file), GCC is gonna add newlines and indentation to visually emphasize the hierarchical structure of the JSON. 🌈
Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect. ⚠️
This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases. 👵
💖 Options to Request or Suppress Warnings 💖
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. like, they're not wrong wrong, but they're giving "hmm bestie what are you doing" energy 🙈
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC. 👑
-fsyntax-only— Check the code for syntax errors, but don't do anything beyond that. That's it, that's the whole vibe. 🛑-fmax-errors=n— Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. 🏃♀️💨 If n is0(the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option. 👑-w— Inhibit all warning messages. silence, bestie. 🤫-Werror— Make all warnings into errors. every little warning is now a whole crime. no cap. 🚨-Werror=— Make the specified warning into an error. The specifier for a warning is appended; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. This switch takes a negative form, to be used to negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings not be errors, even when-Werroris in effect. 🙅♀️
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.) 🔍
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. asymmetry, iconic 💅
-Wfatal-errors— This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. one strike and it's over. periodt. ⚰️
You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. 📖 For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 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 in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command-line. 🧠✨
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. 🕵️♀️
The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and some instances of other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of control and data flow sensitive warnings, in some cases it may also cause false positives. 😭
🌟 -Wpedantic / -pedantic 🌟
-Wpedantic-pedantic— Issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or 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 version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors). 🚫
-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 warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign. 🎯
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— ok so bestie, this one is strict. It makes the compiler throw a whole error whenever the base standard (peep-Wpedanticfor the deets) says a diagnostic is required — and that includes some cases where there's undefined behavior at compile-time, plus some other cases that don't stop valid-by-the-standard programs from compiling. 🫶
🚨 Important vibe check: this is not the same as-Werror=pedantic. That latter one is lowkey useless — it only turns into errors the diagnostics controlled by-Wpedantic, whereas this option also hits required diagnostics that are always on, or controlled by options other than-Wpedantic.
So if you want the required diagnostics that are warnings by default to become errors, but you do not also wanna enable the -Wpedantic diagnostics? You can pass -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic if you want them on but only as warnings). 💅
Some required diagnostics are errors by default, but you can soften them into warnings using -fpermissive or their own specific warning option, like -Wno-error=narrowing.
And some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic — but -pedantic-errors still turns them into errors too. For instance, bestie, we got:
-Wattributes(for standard attributes)-Wchanges-meaning(C++)-Wcomma-subscript(C++23 or later)-Wdeclaration-after-statement(C90 or earlier)-Welaborated-enum-base(C++11 or later)-Wimplicit-int(C99 or later)-Wimplicit-function-declaration(C99 or later)-Wincompatible-pointer-types-Wint-conversion-Wlong-long(C90 or earlier)-Wmain-Wnarrowing(C++11 or later)-Wpointer-arith-Wpointer-sign-Wincompatible-pointer-types-Wregister(C++17 or later)-Wvla(C90 or earlier)-Wwrite-strings(C++11 or later)
-fpermissive— this one downgrades some required diagnostics about nonconformant code from errors down to warnings. So yeah, using-fpermissivelegit lets some nonconforming code compile. 🙈 Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:-Wdeclaration-missing-parameter-type(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Wincompatible-pointer-types(C and Objective-C only)-Wint-conversion(C and Objective-C only)-Wnarrowing(C++ and Objective-C++ only)-Wreturn-mismatch(C and Objective-C only)
Fun fact: -fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90). ✨
-Wall— the iconic one, no cap. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even when macros are in the mix. It also enables some language-specific warnings described over in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 💖
-Wall turns on the following warning flags:
-Waddress-Waligned-new(C++ and Objective-C++ only)-Warray-bounds=1(only with-O2)-Warray-compare-Warray-parameter=2-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wc++17compat-Wc++20compat-Wcatch-value(C++ and Objective-C++ only)-Wchar-subscripts-Wclass-memaccess(C++ and Objective-C++ only)-Wcomment-Wdangling-else-Wdangling-pointer=2-Wdelete-non-virtual-dtor(C++ and Objective-C++ only)-Wduplicate-decl-specifier(C and Objective-C only)-Wenum-compare(in C/ObjC; this is on by default in C++)-Wenum-int-mismatch(C and Objective-C only)-Wformat=1-Wformat-contains-nul-Wformat-diag-Wformat-extra-args-Wformat-overflow=1-Wformat-truncation=1-Wformat-zero-length-Wframe-address-Wimplicit(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Winfinite-recursion-Winit-self(C++ and Objective-C++ only)-Wint-in-bool-context-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++)-Wmismatched-dealloc-Wmismatched-new-delete(C++ and Objective-C++ only)-Wmissing-attributes-Wmissing-braces(only for C/ObjC)-Wmultistatement-macros-Wnarrowing(C++ and Objective-C++ only)-Wnonnull-Wnonnull-compare-Wopenmp-simd(C and C++ only)-Woverloaded-virtual=1(C++ and Objective-C++ only)-Wpacked-not-aligned-Wparentheses-Wpessimizing-move(C++ and Objective-C++ only)-Wpointer-sign(only for C/ObjC)-Wrange-loop-construct(C++ and Objective-C++ only)-Wreorder(C++ and Objective-C++ only)-Wrestrict-Wreturn-type-Wself-move(C++ and Objective-C++ only)-Wsequence-point-Wsign-compare(C++ and Objective-C++ only)-Wsizeof-array-div-Wsizeof-pointer-div-Wsizeof-pointer-memaccess-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtautological-compare-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-Wunused-but-set-variable-Wunused-const-variable=1(only for C/ObjC)-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-value-Wunused-variable-Wuse-after-free=2-Wvla-parameter-Wvolatile-register-var-Wzero-length-bounds
⚠️ Heads up though: some warning flags are not implied by -Wall. Some of them warn about constructions that users generally don't consider questionable, but which occasionally you might wanna check for. Others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra, but many of them must be enabled individually. 😤
-Wextra— this one enables some extra warning flags that-Walldoesn't cover. (This option used to be called-W. The older name is still supported, but the newer name is more descriptive.) 🎀-Wabsolute-value(only for C/ObjC)-Walloc-size-Wcalloc-transposed-args-Wcast-function-type-Wclobbered-Wdeprecated-copy(C++ and Objective-C++ only)-Wempty-body-Wenum-conversion(only for C/ObjC)-Wexpansion-to-defined-Wignored-qualifiers(only for C/C++)-Wimplicit-fallthrough=3-Wmaybe-uninitialized-Wmissing-field-initializers-Wmissing-parameter-type(C/ObjC only)-Wold-style-declaration(C/ObjC only)-Woverride-init(C/ObjC only)-Wredundant-move(C++ and Objective-C++ only)-Wshift-negative-value(in C++11 to C++17 and in C99 and newer)-Wsign-compare(C++ and Objective-C++ only)-Wsized-deallocation(C++ and Objective-C++ only)-Wstring-compare-Wtype-limits-Wuninitialized-Wunused-parameter(only with-Wunusedor-Wall)-Wunused-but-set-parameter(only with-Wunusedor-Wall)
The option -Wextra also prints warning messages for the following cases, bestie:
- 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) — warns about code affected by ABI changes. That includes code that may not be compatible with the vendor-neutral C++ ABI, as well as the psABI for the 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 discovered since the initial release. But -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n). 🔥
-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. 😳 And there may also be cases where warnings are emitted even though the code that's generated is compatible.
You should rewrite your code to avoid these warnings if you're concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers. periodt. 💅
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This was fixed in -fabi-version=3.
- SIMD vector types declared using
"__attribute ((vector_size))"were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
"__attribute ((const))"and"noreturn"were mangled as type qualifiers, and"decltype"of a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
"va_arg"to complain. On most targets this does not actually affect the parameter passing ABI, as there's no way to pass an argument smaller than"int".
Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
- When it was mangling a function type that had function-cv-qualifiers, the un-qualified function type got incorrectly treated as a substitution candidate. Fixed in
-fabi-version=8, which is the default for GCC 5.1. ✨ decltype(nullptr)lowkey had an alignment of 1, which led to unaligned accesses. Heads up though: this did not affect the ABI of a function that takes anullptr_tparameter, because parameters get a minimum alignment anyway. Fixed in-fabi-version=9, the default for GCC 5.2. 🎀- Target-specific attributes that change the identity of a type — think ia32 calling conventions on a function type (
stdcall,regparm, etc.) — did not affect the mangled name, which caused name collisions when function pointers were used as template arguments. 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 that have
long doublemembers get passed in memory as specified in psABI. Before GCC 4.4, that was not the case. Like, for example:
union U {
long double ld;
int i;
};union U is now always passed in memory. 🦋
🎀 More of the -W girlies 🎀
-Wno-changes-meaning(C++ and Objective-C++ only) — C++ is strict about this: an unqualified use of a name inside a class has to mean the same thing across the whole scope of the class, so declaring the name after you've used it is ill-formed. Look:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaningBy default the B1 case is only a warning, because both declarations have the same type, while the B2 case is an error. Both diagnostics can be turned off with -Wno-changes-meaning. Or, if you'd rather, the error case can be downgraded to a warning with -Wno-error=changes-meaning or -fpermissive. Both diagnostics are also suppressed by -fms-extensions. ✨
-Wchar-subscripts— Warns you if an array subscript has typechar. This is a super common oopsie, because programmers keep forgetting thatcharis signed on some machines. 👀 This warning is enabled by-Wall.-Wno-coverage-mismatch— Warns you if feedback profiles don't match when you're using the-fprofile-useoption. If a source file changes between compiling with-fprofile-generateand compiling with-fprofile-use, the files with the profile feedback can fail to match the source file, and GCC just cannot use that profile feedback info. By default this warning is enabled and is treated as an error.-Wno-coverage-mismatchdisables the warning, and-Wno-error=coverage-mismatchdisables the error. Disabling the error for this warning can give you poorly optimized code, and it's only really useful for very minor changes like bug fixes to an existing codebase.
Completely disabling the warning is not recommended. 💀
-Wno-coverage-too-many-conditions— Warns you if-fcondition-coverageis used and an expression has too many terms, so GCC gives up on coverage. Coverage gets given up when there are more terms in the conditional than there are bits in agcov_type_unsigned. This warning is enabled by default.-Wno-coverage-invalid-line-number— Warns you in case a function ends earlier than it begins, because of invalid linenum macros. The warning is only emitted with--coverageenabled. By default, this warning is enabled and is treated as an error.-Wno-coverage-invalid-line-numberdisables the warning, or-Wno-error=coverage-invalid-line-numberdisables the error. 🦋-Wno-cpp(C, Objective-C, C++, Objective-C++ and Fortran only) — Suppresses the warning messages that#warningdirectives emit.-Wdouble-promotion(C, C++, Objective-C and Objective-C++ only) — Gives you a warning when a value of typefloatgets implicitly promoted todouble. CPUs with a 32-bit "single-precision" floating-point unit implementfloatin hardware, but emulatedoublein software. On a machine like that, doing computations withdoublevalues is much more expensive because of all the overhead that software emulation needs. And it's easy to accidentally do computations withdouble, because floating-point literals are implicitly of typedouble. For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler does the entire computation with double, because the floating-point literal is a double. 😭
-Wduplicate-decl-specifier(C and Objective-C only) — Warns you if a declaration has a duplicateconst,volatile,restrictor_Atomicspecifier. This warning is enabled by-Wall.-Wformat-Wformat=n— Checks calls toprintfandscanf, etc., to make sure the arguments you supplied have types that fit the format string you specified, and that the conversions in the format string actually make sense. This covers standard functions, plus others specified by format attributes, in theprintf,scanf,strftimeandstrfmonfamilies —strfmonis an X/Open extension, not in the C standard — or other target-specific families. Which functions get checked without format attributes having been specified depends on the standard version selected, and those checks of functions without the attribute specified are disabled by-ffreestandingor-fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version — but not for strfmon formats, since those aren't in any version of the C standard. 💅
-Wformat=1-Wformat— Option-Wformatis equivalent to-Wformat=1, and-Wno-formatis equivalent to-Wformat=0. Since-Wformatalso checks for null format arguments for several functions,-Wformatalso implies-Wnonnull. Some aspects of this level of format checking can be disabled by the options-Wno-format-contains-nul,-Wno-format-extra-args, and-Wno-format-zero-length.-Wformatis enabled by-Wall.-Wformat=2— Enables-Wformatplus additional format checks. Currently equivalent to-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. ✨
-Wno-format-contains-nul— If-Wformatis specified, don't warn about format strings that contain NUL bytes.-Wno-format-extra-args— If-Wformatis specified, don't warn about excess arguments to aprintforscanfformat function. The C standard says such arguments are just ignored. Where the unused arguments sit between used arguments that are specified with$operand number specifications, warnings are normally still given, since the implementation couldn't know what type to pass tova_argto skip the unused arguments. However, in the case ofscanfformats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says such unused arguments are allowed.-Wformat-overflow-Wformat-overflow=level— Warns about calls to formatted input/output functions likesprintfandvsprintfthat might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it's estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. 👀-Wformat-overflow-Wformat-overflow=1— Level 1 of-Wformat-overflow, enabled by-Wformat, takes a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call tosprintfbelow is diagnosed because even with bothaandbequal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length can't be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. 🔥
okay so bestie, level 2 is where it gets spicy 🌶️ — the call in the example up there gets diagnosed again, but this time it's because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits past the end of the destination buffer 😭. To make the call safe no matter what values those two variables hold, the size of the destination buffer has to be bumped up to at least 34 bytes. And GCC is a real one — it drops the minimum size of the buffer right into an informational note that follows the warning. 🫶
An alternative to making the destination buffer bigger is to constrain the range of formatted values. The max length of string arguments can be bounded by specifying the precision in the format directive. And when numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier will shrink the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type, then using either the %hi format directive or casting the argument to short drops the maximum required size of the buffer to 24 bytes. slay 💅
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 specified, don't warn about zero-length formats. The C standard specifies that zero-length formats are allowed, so like… they're legal, bestie. 🎀-Wformat-nonliteral— if-Wformatis specified, also warn if the format string isn't a string literal and therefore can't be checked — unless the format function takes its format arguments as ava_list. 🕵️-Wformat-security— if-Wformatis specified, also warn about uses of format functions that represent possible security problems. Right now this warns about calls toprintfandscanffunctions where the format string isn't a string literal and there are no format arguments, like inprintf (foo);. That can be a straight-up security hole if the format string came from untrusted input and contains%n. (This is currently a subset of what-Wformat-nonliteralwarns about, but in the future warnings may be added to-Wformat-securitythat aren't included in-Wformat-nonliteral.) danger danger 🚨-Wformat-signedness— if-Wformatis specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. ✨-Wformat-truncation-Wformat-truncation=level— warn about calls to formatted input/output functions likesnprintfandvsnprintfthat might result in output truncation. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on thelevelargument and on optimization. Turning on optimization will in most cases improve the accuracy of the warning, but it may also cause false positives — it's giving tradeoffs 😅. Except as noted otherwise, the option uses the same logic-Wformat-overflow.-Wformat-truncation-Wformat-truncation=1— Level 1 of-Wformat-truncation, enabled by-Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation. cautious queen 👑-Wformat-truncation=2— Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude. 😈
-Wformat-y2k— if-Wformatis specified, also warn aboutstrftimeformats that may yield only a two-digit year. not the Y2K flashbacks 💀-Wnonnull— warn about passing a null pointer for arguments marked as requiring a non-null value by thenonnullfunction attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option. 🚫
-Wnonnull-compare— warn when comparing an argument marked with thenonnullfunction attribute against null inside the function. like bestie it literally cannot be null, why are we checking 😭
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference— warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when-fdelete-null-pointer-checksis active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. 🔍-Wnrvo(C++ and Objective-C++ only) — warn if the compiler doesn't elide the copy from a local variable to the return value of a function in a context where it's allowed by[class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler can't elide copies from bothv1andv2, so it elides neither. rude honestly 🙄
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}-Winfinite-recursion— warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions.-Winfinite-recursionis included in-Wall. 🔁
Compare with -Wanalyzer-infinite-recursion, which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer).
-Winit-self(C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the-Wuninitializedoption. 🌀
For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++.
-Wno-implicit-int(C and Objective-C only) — this option controls warnings when a declaration doesn't specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone, with-Wno-error=implicit-int. 💅
This warning is upgraded to an error by -pedantic-errors.
-Wno-implicit-function-declaration(C and Objective-C only) — this option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone, with-Wno-error=implicit-function-declaration. 👀
This warning is upgraded to an error by -pedantic-errors.
-Wimplicit(C and Objective-C only) — same as-Wimplicit-intand-Wimplicit-function-declaration. This warning is enabled by-Wall. ✨-Whardened— warn when-fhardeneddidn't enable an option from its set (for which see-fhardened). For instance, using-fhardenedand-fstack-protectorat the same time on the command line causes-Whardenedto warn, because-fstack-protector-strongisn't enabled by-fhardened. 🛡️
This warning is enabled by default and has effect only when -fhardened is enabled.
-Wimplicit-fallthrough—-Wimplicit-fallthroughis the same as-Wimplicit-fallthrough=3and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0. 🔢-Wimplicit-fallthrough=n— warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}This warning doesn't warn when the last statement of a case can't fall through, e.g. when there's a return statement or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g. 🧠
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur: 🙈
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it's also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:
-Wimplicit-fallthrough=0disables the warning altogether. 🚫-Wimplicit-fallthrough=1matches".*"regular expression, any comment is used as fallthrough comment.-Wimplicit-fallthrough=2case insensitively matches".*falls?[ \t-]*thr(ough|u).*"regular expression.-Wimplicit-fallthrough=3case sensitively matches one of the following regular expressions:
So the patterns it's matching against for that level are these, literally, character for character, no edits:
"-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]*)?"And then -Wimplicit-fallthrough=4 is the picky one — it case sensitively matches one of these regular expressions:
"-fallthrough"
"@fallthrough@"
"lint -fallthrough[ \t]*"
"[ \t]*FALLTHR(OUGH|U)[ \t]*"And -Wimplicit-fallthrough=5? It's giving absolutely nothing bestie — it doesn't recognize any comments as fallthrough comments, only attributes disable the warning. Periodt. 💅
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:
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) 💖
Controls whether warnings triggered by the warn_if_not_aligned attribute get issued. These warnings are enabled by default, so this is your off switch if you're feeling spicy.
-Wignored-qualifiers (C and C++ only) ✨
Warns if the return type of a function has a type qualifier like const. Why do we care? For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. And ISO C prohibits qualified void return types on function definitions — so those return types always get a warning even without this option. Not me sneaking in an unconditional warning 😭
This warning is also enabled by -Wextra.
-Wno-ignored-attributes (C and C++ only) 🌸
This one controls warnings when an attribute is ignored. And it's lowkey different from the -Wattributes option: this warns whenever the compiler decides to drop an attribute, not when the attribute is unknown, used in a wrong place, etc. Enabled by default. ✨
-Wmain 👑
Warns if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. Like, main character energy only, bestie — no weird variants. 💅
This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.
This warning is upgraded to an error by -pedantic-errors. So fr fr, be careful.
-Wmisleading-indentation (C and C++ only) 😭
Warns when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.
In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */Gotcha indeed. 🙈
In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic, like this:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);And it's not issued after a #line directive, since that typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references. Respect the autogenerated files, they've been through enough.
This warning is enabled by -Wall in C and C++. 🎀
-Wmissing-attributes 🦋
Warns when a declaration of a function is missing one or more attributes that a related function is declared with, and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. 😬
Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.
In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning..
You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types. 💖
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size, the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes:
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);The way that alloc_size just vanished — not slay. 💀
-Wmissing-braces 🌸
Warns if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed:
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, Objective-C++ and Fortran only) 🎀
Warns if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only. Only those two, bestie, don't get greedy.
-Wno-missing-profile 💜
This option controls warnings if feedback profiles are missing when using the -fprofile-use option. It diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that 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 this is not recommended and should be done only when non-existent profile data is justified.
Like, the compiler is literally telling you to regenerate, listen to her.
-Wmismatched-dealloc 🔥
Warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc. Unless disabled by the -fno-builtin option, the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete, are implicitly associated as matching allocators and deallocators.
In the following example, mydealloc is the deallocator for pointers returned from myalloc:
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
}See that? free (p) is a mismatch — wrong deallocator, bestie. 💅
In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.
Option -Wmismatched-dealloc is included in -Wall.
-Wmultistatement-macros 🦋
Warns about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, in which only the first statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;will increment y unconditionally, not just when c holds. 😭 The can usually be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;Wrap it, protect it, love it. This warning is enabled by -Wall in C and C++. ✨
-Wparentheses 💖
Warns if parentheses are omitted in certain contexts — like 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. 👑
Also warns if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. Math girlies stay losing to C semantics, ngl.
Also warns for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. 😬
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}Not the accidental function-call vibe. This warning is enabled by -Wall. 🎀
-Wno-self-move(C++ and Objective-C++ only) — bestie, this one clocks you when a value gets moved into itself withstd::move. And like… such astd::movetypically has no effect at all. It's giving "I did nothing and I'm all out of ideas" 💀
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}This warning is enabled by -Wall. ✨
-Wsequence-point— warns about code that may have undefined semantics because it breaks the sequence point rules in the C and C++ standards. Okay so let's talk about sequence points, because this is genuinely the drama 🎀
The C and C++ standards define the order in which expressions get evaluated in terms of sequence points, which are basically a partial ordering between the execution of parts of the program: the stuff executed before the sequence point, and the stuff executed after it. These happen after the evaluation of a full expression (one that isn't part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than what the sequence point rules say, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, because like, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. Periodt.
It is also not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior 😭 the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable. No cap.
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs. Slay, honestly.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. 🫶
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>. 👑
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr— do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. (Yeah, that's the classic "here's a pointer to my dead local, have fun" move 🙈)-Wreturn-mismatch— warns aboutreturnstatements without an expression in functions that do not returnvoid. Also warns about areturnstatement with an expression in a function whose return type isvoid, unless the expression type is alsovoid. As a GNU extension, the latter case is accepted without a warning unless-Wpedanticis used.
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. 💀
This warning is specific to C and enabled by default. In C99 and later language dialects, it is treated as an error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch.
-Wreturn-type— warns whenever a function is defined with a return type that defaults toint(unless-Wimplicit-intis active, which takes precedence). Also warns if execution may reach the end of the function body, or if the function does not contain anyreturnstatement at all.
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. ⚠️
Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. Yes bestie, even then.
This warning is enabled by default in C++ and by -Wall otherwise.
-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-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap. 🌀
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
-Wswitch— 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 provoke warnings when this option is used (even if there is adefaultlabel). This warning is enabled by-Wall. ✨-Wswitch-default— warns whenever aswitchstatement does not have adefaultcase. Like, where's your safety net bestie 😭-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 provoke warnings when this option is used. The only difference between-Wswitchand this option is that this option gives a warning about an omitted enumeration code even if there is adefaultlabel. So it's-Wswitchbut with zero chill 💅-Wno-switch-bool— do not warn when aswitchstatement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range— this option controls warnings when aswitchcase has a value that is outside of its respective type range. This warning is enabled by default for C and C++ programs.-Wno-switch-unreachable— do not warn when aswitchstatement contains statements between the controlling expression and the first case label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs. 🦋
-Wsync-nand(C and C++ only) — warns when__sync_fetch_and_nandand__sync_nand_and_fetchbuilt-in functions are used. These functions changed semantics in GCC 4.4. So like, heads up, the vibe shifted 🍓-Wtrivial-auto-var-init— warns when-ftrivial-auto-var-initcannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of aswitchstatement. 🙈-Wunused-but-set-parameter— warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable— warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by-Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall. 💖
-Wunused-function— 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 this warning use the unused attribute.
-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function is not used. This warning is enabled by-Wall.-Wunused-parameter— warns whenever a function parameter is unused aside from its declaration. This option is not enabled by-Wunusedunless-Wextrais also specified. 🎀
To suppress this warning use the unused attribute.
-Wno-unused-result— do not warn if a caller of a function marked with attributewarn_unused_resultdoes not use its return value. The default is-Wunused-result. 👑-Wunused-variable— bestie, this one yells whenever a local or static variable is just… sitting there unused aside from its declaration. It also implies-Wunused-const-variable=1for C, but not for C++. And it's enabled by-Wall.
Wanna silence it? Slap the "unused" attribute on the thing.
-Wunused-const-variable-Wunused-const-variable=n— warns whenever a constant static variable is unused aside from its declaration. Same deal, use the"unused"attribute to hush it.-Wunused-const-variable=1— only cares about unused static const variables defined in the main compilation unit, and lowkey ignores static const variables declared in any header that got included. This level is enabled by either-Wunused-variableor-Wunusedfor C, but not for C++. In C this declares variable storage, but in C++ it's not an error since const variables take the place of"#define"s.-Wunused-const-variable=2— this level also warns for unused constant static variables in headers (excluding system headers). It's equivalent to the short form-Wunused-const-variable. You gotta explicitly ask for this level in both C and C++ because it might be hard to clean up all headers included. 🫠
-Wunused-value— warns whenever a statement computes a result that is explicitly not used. To suppress it, cast the unused expression to"void". This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. So like,"x[i,j]"triggers a warning, while"x[(void)i,j]"does not. Enabled by-Wall.-Wunused— all the above-Wunusedoptions combined, except the ones documented as needing to be specified explicitly. 💅
Want a warning about an unused function parameter? You must either specify -Wextra -Wunused (heads up, -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.
Also important: -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.
-Wuse-after-free(C, Objective-C, C++ and Objective-C++ only)-Wuse-after-free=n— warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. It's enabled at all optimization levels but may give you different results with optimization than without. 😭-Wuse-after-free=1— at level 1 the warning only tries to diagnose unconditional uses of pointers made indeterminate by a deallocation call or a successful call to"realloc", regardless of whether or not the call actually reallocated memory. This includes double-"free"calls as well as uses in arithmetic and relational expressions. Even though it's undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.-Wuse-after-free=2— at level 2, on top of unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to"free"in the following function is diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}-Wuse-after-free=3— at level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined, but equality tests sometimes show up after calls to"realloc"as an attempt to figure out whether the call relocated the object to a different address. They get diagnosed at a separate level to help gradually transition legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}To dodge the warning at this level, store offsets into allocated memory instead of pointers. That approach means you don't need to adjust the stored pointers after reallocation. ✨
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast(C, Objective-C, C++ and Objective-C++ only) — warns when an expression is cast to its own type. This warning does not fire when a class object is converted to a non-reference type, because that's a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}-Wuninitialized— warns if an object with automatic or allocated storage duration is used without having been initialized. In C++, it also warns if a non-static reference or non-static"const"member appears in a class without constructors.
On top of that, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)
Wanna warn about code that uses the uninitialized value of the variable in its own initializer? Use the -Winit-self option.
These warnings occur for individual uninitialized 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 version of GCC you're using. 🙈
Also note there may be no warning about a variable that's used only to compute a value that itself is never used, because such computations may get deleted by data flow analysis before the warnings are printed.
In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in this snippet:
struct A {
int a;
int b;
A() : a(b) { }
};-Wno-invalid-memory-model— this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that's either invalid for the operation or outside the range of values of the"memory_order"enumeration. For example, since the"__atomic_store"and"__atomic_store_n"built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized— for an object with automatic or allocated storage duration, if there's a path from the function entry to a use of the object that's initialized, but there exist some other paths where the object isn't initialized, the compiler emits a warning if it can't prove the uninitialized paths aren't executed at run time.
Plus, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) Annotating the function with attribute "access (none)" indicates that the argument isn't used to access the object and 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 determine when the code is correct in spite of appearing to have 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 the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". The compiler sees only the calls to "setjmp". It can't know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there's in fact no problem, because "longjmp" can't actually be called at the place that would cause a problem. 😮💨
Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— warns when a"#pragma"directive is encountered that GCC doesn't understand. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. That's not the case if the warnings are only enabled by the-Wallcommand-line option.-Wno-pragmas— don't warn about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas. See also-Wunknown-pragmas. 🎀-Wno-prio-ctor-dtor— bestie, this one turns OFF the warning that fires when you slap a priority between0and100on a constructor or destructor. Like, theconstructoranddestructorattributes let you assign a priority so you can control the order stuff runs in — beforemaingets called, or after it comes back. But the priority values must be greater than 100, because the compiler literally reserves0--100for the implementation itself. So if you're in that range, that's a you problem, and this flag just... stops the compiler from telling you. 🙈-Wstrict-aliasing— okay so this only does anything when-fstrict-aliasingis active, got it? It warns about code that might break the strict aliasing rules the compiler uses when it optimizes. It does not catch every single case, but it tries to catch the more common pitfalls. It's included in-Wall. And it's exactly equivalent to-Wstrict-aliasing=3.-Wstrict-aliasing=n— same deal, only active when-fstrict-aliasingis active, warns about code that might break the strict aliasing rules the compiler relies on for optimization. Higher levels = higher accuracy (fewer false positives). But higher levels also = more effort, kinda like how-Oworks. And again,-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.
🦋 -Wstrict-aliasing levels, in order, no skips
- Level 1 — most aggressive, quick, least accurate. Possibly useful when higher levels stay quiet but
-fstrict-aliasingstill breaks your code, because it has very few false negatives. However — and this is the tradeoff — it has many false positives. It warns for all pointer conversions between possibly incompatible types, even if they're never dereferenced. Runs in the front end only. - Level 2 — aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. It also warns about incomplete types. Runs in the front end only.
- Level 3 (the default for
-Wstrict-aliasing) — should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end:"*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
-Wstrict-overflow,-Wstrict-overflow=n— this one is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note: it does not warn about all cases where code might overflow — it only warns about cases where the compiler actually implements some optimization. So yeah, this warning depends on your optimization level. 😭
An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually happens. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on the important stuff, several warning levels are defined. And no warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires — in particular when determining whether a loop will be executed at all.
🔥 -Wstrict-overflow levels
-Wstrict-overflow=1— warn about cases that are both questionable and easy to avoid. For example, the compiler simplifies"x + 1 > x"to 1. This level of-Wstrict-overflowis enabled by-Wall; higher levels are not, so you have to explicitly ask for them.-Wstrict-overflow=2— also warn about other cases where a comparison is simplified to a constant. For example:"abs (x) >= 0". This can only be simplified when signed integer overflow is undefined, because"abs (INT_MIN)"overflows to"INT_MIN", which is less than zero.-Wstrict-overflow(with no level) is the same as-Wstrict-overflow=2.-Wstrict-overflow=3— also warn about other cases where a comparison is simplified. For example:"x + 1 > 1"is simplified to"x > 0".-Wstrict-overflow=4— also warn about other simplifications not covered by the above cases. For example:"(x * 10) / 5"is simplified to"x * 2".-Wstrict-overflow=5— also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example:"x + 2 > y"is simplified to"x + 1 >= y". This is reported only at the highest warning level, because this simplification applies to many comparisons, so this level gives a very large number of false positives. 💀
-Wstring-compare— warns for calls to"strcmp"and"strncmp"whose result is determined to be either zero or non-zero in tests for such equality, owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound, in the case of"strncmp"). Such calls could be mistakes, bestie. Like, look at this — the call to"strcmp"below is diagnosed 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— warns for calls to string manipulation functions like"memcpy"and"strcpy"that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. That argument is meaningful only for functions that operate on character arrays — but not for raw memory functions like"memcpy", which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object, or at most"SIZE_MAX / 2"bytes.
The option produces the best results with optimization enabled, but it can detect a small subset of simple buffer overflows even without optimization, in calls to the GCC built-in functions like "__builtin_memcpy" that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, it issues a warning for the "strcpy" call below because it copies at least 5 characters (the string "blue", including the terminating NUL) into the buffer of size 4:
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}Option -Wstringop-overflow=2 is enabled by default.
🌈 -Wstringop-overflow levels
-Wstringop-overflow,-Wstringop-overflow=1— uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers, unless the size of the largest surrounding object is known. When the destination may be one of several objects, it's assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting, the option warns for the same code as when the"_FORTIFY_SOURCE"macro is defined to a non-zero value.-Wstringop-overflow=2— uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers, since they may point to arrays containing unknown numbers of elements. This is the default setting of the option.-Wstringop-overflow=3— uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option, and it may result in warnings for safe code. 🙈-Wstringop-overflow=4— uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to-Wstringop-overflow=3, this setting may result in warnings for benign code.
-Wno-stringop-overread— warns for calls to string manipulation functions like"memchr", or"strcpy", that are determined to read past the end of the source sequence.
Option -Wstringop-overread is enabled by default.
-Wno-stringop-truncation— do not warn for calls to bounded string manipulation functions like"strncat","strncpy", and"stpncpy"that may either truncate the copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning, use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}Okay so like, more strncpy tea bestie, because GCC is not letting this slide. 🍓
Another example, and this one is giving chaos: this call to strncpy copies into d only the characters before the terminating NUL — and it doesn't even append the NUL at the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake (like, everyone does it, no shade 🌸), so the call gets diagnosed. If you know your result isn't supposed to be NUL-terminated, just call memcpy instead, bestie, problem solved. 💅
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}Next one, and this is also a no:
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}Here the call to strncpy uses the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the copy will not be NUL-terminated. So yeah, diagnosed too. 😭 To dodge the warning, pass sizeof buf - 1 as the bound and set the last element of the buffer to NUL.
Now, in situations where a character array is meant to hold a sequence of bytes with no terminating NUL — like, that's the whole vibe on purpose — you can annotate such an array with the attribute nonstring to skip this warning. But (big but 🫶) such arrays are not suitable arguments to 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. It's protective, not petty. 👑
✨ -Wstrict-flex-arrays (C and C++ only) ✨
Warn about improper usages of flexible array members according to the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure — if that attribute's available, that is. Otherwise it goes by the level of the option -fstrict-flex-arrays=level. Heads up: -Wstrict-flex-arrays is only effective when level is greater than 0. Zero = no vibes here. 💀
- When level=1, warnings get issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
- When level=2, on top of level=1, extra warnings get issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
- When level=3, on top of level=2, even more warnings get issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.
This option hits different when -ftree-vrp is active (which is the default for -O2 and above), but some warnings may still be diagnosed even without optimization. It's just doing the most when it can. ✨
✨ -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]returns_nonnull| ✨
Warn for cases where adding an attribute might be beneficial. The attributes currently supported are listed below, bestie:
-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc-Wsuggest-attribute=returns_nonnull-Wno-suggest-attribute=returns_nonnull
— These warn about functions that might be candidates for attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it can't prove the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis needs the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. Basically the smarter GCC gets, the better it clocks you. 🧚
-Wsuggest-attribute=format-Wmissing-format-attribute
— Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones — GCC is guessing, not reading your mind. GCC guesses that function pointers with format attributes used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again — only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. It's giving ✨heuristic✨, and we respect the hustle. 💅
-Wsuggest-attribute=cold
— Warn about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions which always lead to a call to another cold function, such as wrappers of C++ throw or fatal error reporting functions leading to abort. Cold as in never-warm, not cold as in the attitude. 😌
✨ -Walloc-size ✨
Warn about calls to allocation functions decorated with attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Too small = caught. 🔍
✨ -Walloc-zero ✨
Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Why do we care? Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc it's been deprecated 😬), so relying on it may result in subtle portability bugs and should be avoided. fr fr, just don't. 🚫
✨ -Wcalloc-transposed-args ✨
Warn about calls to allocation functions decorated with attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning — a lil fashion police moment, no cap. 👑
The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element. So calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is intentional, you can suppress the warning by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4) or by using sizeof in the later argument as well. Chaotic workarounds? Maybe. Valid? Yes. 💅
✨ -Walloc-size-larger-than=byte-size ✨
Warn about calls to functions decorated with attribute alloc_size that attempt to allocate objects larger than the specified number of bytes — or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than.
✨ -Wno-alloc-size-larger-than ✨
Disable -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. Turning it all the way off, periodt. 🙈
✨ -Walloca ✨
This option warns on all uses of alloca in the source. Every single one. It's watching. 👀
✨ -Walloca-larger-than=byte-size ✨
This option warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. And — plot twist — arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. Rude, but okay. 😭
For example, a bounded case of alloca could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}In the above example, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning. Boundaries, bestie. ✨
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}If -Walloca-larger-than=500 were passed, the above would trigger a warning — but this time because of the lack of bounds checking. No leash, no chill. 🦋
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. Sneaky!! 😱
This option also warns when alloca is used in a loop. (Loops + unbounded stack = not the vibe.)
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size.
✨ -Wno-alloca-larger-than ✨
Disable -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. Off means off. 💤
✨ -Warith-conversion ✨
Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with B<-Wconversion>
c = c + 1; // only warns with B<-Warith-conversion>
}See how the second line is lowkey fine until you flip this on? That's the whole point. 🔥
✨ -Warray-bounds ✨
✨ -Warray-bounds=n ✨
Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It's more effective when -ftree-vrp is active (the default for -O2 and above), but a subset of instances are issued even without optimization. It catches what it can, whenever it can. 🫶
By default, the trailing array of a structure gets treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as any of these 💅
- a flexible array member per the C99 standard onwards (
[]), - a GCC zero-length array extension (
[0]), - or a one-element array (
[1]).
Which means, bestie, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned about by default. No cap.
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats the trailing array of a structure as a flexible array member:
- when
level<=1, no change to the default behavior. 💤 - when
level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays; - when
level=3, in addition tolevel=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays. -Warray-bounds=1— this is the default warning level of-Warray-boundsand is enabled by-Wall; higher levels are not, and must be explicitly requested. 🫶-Warray-bounds=2— this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. Heads up though: this warning level may give a larger number of false positives and is deactivated by default. 🙈
💖 -Warray-compare 💖
Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;-Warray-compare is enabled by -Wall. ✨
🎀 -Warray-parameter 🎀
🎀 -Warray-parameter=n 🎀
Warn about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.
If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.
For example, the warning triggers for the second declaration of f because the first one with the keyword static specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to f. 👀
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
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 denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not 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.
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0. 💅
🌈 -Wattribute-alias=n 🌈
🌈 -Wno-attribute-alias 🌈
Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias.
-Wattribute-alias=1— the default warning level of the-Wattribute-aliasoption diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. 🐛-Wattribute-alias=2— at this level-Wattribute-aliasalso diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the-Wmissing-attributesoption, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0. ✨
💀 -Wbidi-chars=[none|unpaired|any|ucn] 💀
Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified. 🧚
🍓 -Wbool-compare 🍓
Warn about boolean expression compared with an integer value different from true/false. For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall. 👑
🎀 -Wbool-operation 🎀
Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall. ✨
💜 -Wduplicated-branches 💜
Warn when an if-else has identical branches. This warning detects cases like
if (p != NULL)
return 0;
else
return 0;It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:
int i = x ? *p : *p;🌸 -Wduplicated-cond 🌸
Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }🦋 -Wframe-address 🦋
Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall. 💀
🌈 -Wno-discarded-qualifiers (C and Objective-C only) 🌈
Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning. 🫶
🌈 -Wno-discarded-array-qualifiers (C and Objective-C only) 🌈
Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning. 💅
💖 -Wno-incompatible-pointer-types (C and Objective-C only) 💖
Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types.
This warning is upgraded to an error by -pedantic-errors. 😤
💖 -Wno-int-conversion (C and Objective-C only) 💖
Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=int-conversion.
This warning is upgraded to an error by -pedantic-errors. 😤
⭐ -Wzero-length-bounds ⭐
Warn 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 accesses to them are undefined. 🚨
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 is diagnosed by -Warray-bounds because it is beyond 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
}and btw bestie, the option -Wzero-length-bounds gets switched on for you by -Warray-bounds. cute little tag-team situation 💅
-Wno-div-by-zero— ok so this one says don't warn about integer division by zero at compile time. Floating-point division by zero? Not warned about, because it can be a totally legit way of getting infinities and NaNs. it's giving "intentional math, not a typo" ✨-Wsystem-headers— print warning messages for stuff found in system header files. Normally warnings from system headers are suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However — and this is the part everyone misses — using-Wallin conjunction with this option does not warn about unknown pragmas in system headers. For that,-Wunknown-pragmasmust also be used. two options, one vibe check 🫶-Wtautological-compare— warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }It also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }which will always be false. like bestie, that's never gonna hit 💀
This warning is enabled by -Wall.
-Wtrampolines— warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. (stack made executable = the drama of it all, keep an eye on that one 👀)-Wfloat-equal— warn if floating-point values are used in equality comparisons. The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. fr fr, floats are not your normal besties, treat them with the range-overlap respect they deserve 🦋-Wtraditional(C and Objective-C only) — warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
here's the whole list, no skips, we're thorough in this house 💅
- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the
#appeared in column 1 on the line. Therefore-Wtraditionalwarns about directives that traditional C understands but ignores because the#does not appear as the first character on the line. It also suggests you hide directives like#pragmanot understood by traditional C by indenting them. Some traditional implementations do not recognize#elif, so this option suggests avoiding it altogether. - A function-like macro that appears without arguments.
- The unary plus operator.
- The
Uinteger constant suffix, or theForLfloating-point constant suffixes. (Traditional C does support theLsuffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the_MIN/_MAXmacros in<limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. - A function declared external in one block and then used after the end of the block.
- A
switchstatement has an operand of typelong. - A non-
staticfunction declaration follows astaticone. This construct is not accepted by some traditional C compilers. - The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
- Usage of ISO string concatenation is detected.
- Initialization of automatic aggregates.
- Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
- Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g.
__STDC__to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. - Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros,
PARAMSandVPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion(C and Objective-C only) — warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.-Wdeclaration-after-statement(C and Objective-C only) — warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. This warning is upgraded to an error by-pedantic-errors. so if you're feeling brave, that's the pipeline 😭-Wshadow— warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis used. Same as-Wshadow=global.-Wno-shadow-ivar(Objective-C only) — do not warn whenever a local variable shadows an instance variable in an Objective-C method. the chill mode, we love her 🌸-Wshadow=global— warn for any shadowing. Same as-Wshadow. no exceptions, everything gets called out, periodt ✨-Wshadow=local— warn when a local variable shadows another local variable or parameter.-Wshadow=compatible-local— warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to-Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since the two variable i in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing const char *i by char *i does not emit a warning.
This warning is also enabled by -Wshadow=local. look at them, a little enabling friendship 🫶
-Wlarger-than=byte-size— warn whenever an object is defined whose size exceeds byte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size ofSIZE_MAXor more or by-Wno-larger-than.
Also warn for calls to bounded functions such as 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— disable-Wlarger-than=warnings. The option is equivalent to-Wlarger-than=SIZE_MAXor larger. same energy, different spelling, iykyk 💅-Wframe-larger-than=byte-size— bestie, this one yells at you if a function's stack frame gets bigger than byte-size. 🫶 BUT. And this is important. The math it does to figure out the frame size is approximate and not conservative — meaning the real requirements might be somewhat greater than byte-size even when you get no warning at all. Sneaky! Also, anything you allocate withalloca, variable-length arrays, or related constructs is not counted by the compiler when it decides whether to warn.-Wframe-larger-than=PTRDIFF_MAXis enabled by default, and you can shut these warnings off either by giving a byte-size ofSIZE_MAXor more, or just by saying-Wno-frame-larger-than. 💅-Wno-frame-larger-than— turns off the-Wframe-larger-than=warnings. It's literally the same thing as-Wframe-larger-than=SIZE_MAXor larger. Same energy, different spelling. ✨-Wfree-nonheap-object— warns you when you try to deallocate something that was never allocated on the heap, or when you use a pointer that did not come back from a prior call to the matching allocation function. 👑 For example: becausestpcpyreturns a pointer to the terminating nul character and not to the start of the object, thefreecall below gets diagnosed. Not me catching that:
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall. 🎀
-Wstack-usage=byte-size— warns you if a function's stack usage might blow past byte-size. The computation here is conservative (unlike its frame-larger-than cousin 👀), and any space allocated viaalloca, variable-length arrays, or related constructs is included when the compiler decides whether to warn.
The message matches the output of -fstack-usage. So:
- If the stack usage is fully static but exceeds the amount you specified, it's:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is enabled by default. You can kill these warnings either by giving a byte-size of SIZE_MAX or more, or with -Wno-stack-usage. 🦋
-Wno-stack-usage— disables the-Wstack-usage=warnings. Equivalent to-Wstack-usage=SIZE_MAXor larger. Same deal as above, just the off switch. 🎀-Wunsafe-loop-optimizations— warns when a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And if you pair it with-funsafe-loop-optimizations, it warns when the compiler does go ahead and make those assumptions. 👀-Wno-pedantic-ms-format(MinGW targets only) — when you use this together with-Wformatand-pedanticwithout GNU extensions, it disables the warnings about non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets — the ones that depend on the MS runtime. 💅-Wpointer-arith— warns about anything that depends on the "size of" a function type or ofvoid. GNU C hands these types a size of 1, for convenience when doing calculations withvoid *pointers and pointers to functions. In C++, it also warns when an arithmetic operation involvesNULL. This warning is also enabled by-Wpedantic.
Heads up bestie: this warning gets upgraded to an error by -pedantic-errors. 😭
-Wno-pointer-compare— stops warning when a pointer is compared with a zero character constant (which usually means somebody meant to dereference it). For example:
const char *p = foo ();
if (p == '\0')
return 42;Note that the code above is invalid in C++11. ⚠️ This warning is enabled by default.
-Wno-tsan— disables warnings about unsupported features in ThreadSanitizer. ThreadSanitizer does not supportstd::atomic_thread_fenceand can report false positives. 🙈-Wtype-limits— warns if a comparison is always true or always false because of the limited range of the data type — but it does not warn for constant expressions. So like, if an unsigned variable is compared against zero with<or>=, that's a warning. This one's also enabled by-Wextra. ✨-Wabsolute-value(C and Objective-C only) — warns on calls to standard functions that compute the absolute value of an argument when a better-suited standard function exists. For example, callingabs(3.14)triggers it, because the right function for the absolute value of adoubleisfabs. It also fires when the argument to such a function has an unsigned type. You can silence it with an explicit type cast, and it's enabled by-Wextratoo. 💜-Wcomment,-Wcomments— warns whenever a comment-start sequence/*shows up inside a/*comment, or whenever a backslash-newline shows up inside a//comment. Enabled by-Wall. 🎀-Wtrigraphs— warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about — except the ones that would form escaped newlines.
This option is implied by -Wall. If -Wall isn't given, it's still enabled unless trigraphs are enabled. And if you want trigraph conversion without the warnings, but still want the other -Wall warnings, you say:
-trigraphs -Wall -Wno-trigraphs-Wundef— warns if an undefined identifier is evaluated in an#ifdirective. Those identifiers get replaced with zero. 🫶-Wexpansion-to-defined— warns wheneverdefinedis encountered in the expansion of a macro (including when the macro is expanded by an#ifdirective). That usage is not portable. Also enabled 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 are not warned about. 💅
Note bestie: if a macro is actually used, but only inside skipped conditional blocks, the preprocessor will report it as unused. To dodge the warning in that case, you could tighten the scope of the macro's definition — like moving it into the first skipped block. Or you could just give it a dummy use, something like:
#if defined the_macro_causing_the_warning
#endif-Wno-endif-labels— stops warning whenever an#elseor an#endifare followed by text. This sometimes happens in older programs with code like:
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO should be in comments. 🙈 This warning is on by default.
-Wbad-function-cast(C and Objective-C only) — warns when a function call is cast to a non-matching type. Like, warns if a call to a function returning an integer type gets cast to a pointer type. 💀-Wc90-c99-compat(C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. For instance: variable length arrays, thelong longtype, thebooltype, compound literals, designated initializers, and so on. This option is independent of the standards mode, and warnings are disabled in the expression that follows__extension__. ✨-Wc99-c11-compat(C and Objective-C only) — warns about features that aren't in ISO C99 but are in ISO C11. For instance: anonymous structures and unions, the_Atomictype qualifier, the_Thread_localstorage-class specifier, the_Alignasspecifier, theAlignofoperator, the_Generickeyword, and so on. Also independent of the standards mode, and warnings are disabled in the expression that follows__extension__. 🦋-Wc11-c23-compat,-Wc11-c2x-compat(C and Objective-C only) — warns about features that aren't in ISO C11 but are in ISO C23. For instance: omitting the string in_Static_assert, use of[[]]syntax for attributes, use of decimal floating-point types, and so on. Independent of the standards mode, and warnings are disabled in the expression that follows__extension__. The name-Wc11-c2x-compatis deprecated. 😭
And when you're not compiling in C23 mode, these warnings get upgraded to errors by -pedantic-errors. ⚠️
-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that fall outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion fromvoid *to a pointer to a non-voidtype. 👑-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. 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. 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. 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. Enabled by-Wall. ✨-Wno-c++11-extensions(C++ and Objective-C++ only) — stops warning about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if-Wpedanticis used. 🫶-Wno-c++14-extensions(C++ and Objective-C++ only) — stops warning about C++14 constructs in code being compiled with an older C++ standard. Even without this option, some C++14 constructs only get diagnosed if-Wpedanticis used. 💜-Wno-c++17-extensions(C++ and Objective-C++ only) — stops warning about C++17 constructs in code being compiled with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed if-Wpedanticis used. 🦋-Wno-c++20-extensions(C++ and Objective-C++ only) — bestie, this one shuts off the nagging about C++20 constructs when you're compiling with an older C++ standard. And like, plot twist: even if you don't pass this, some C++20 stuff will still get diagnosed — but only if you're using-Wpedantic. So yeah, it's giving "conditional silence". 🎀-Wno-c++23-extensions(C++ and Objective-C++ only) — same vibe, newer year ✨. Don't warn about C++23 constructs when the code is being compiled against an older C++ standard. And again — even without this option, some C++23 constructs are only diagnosed if-Wpedanticis in the mix. Periodt.-Wno-c++26-extensions(C++ and Objective-C++ only) — the future is now, and this suppresses warnings about C++26 constructs in code compiled with an older C++ standard. And yes, same little caveat as its siblings: some C++26 constructs only get diagnosed when-Wpedanticis used. 💅-Wcast-qual— warn whenever a pointer gets cast in a way that strips a type qualifier off the target type. Like, if aconst char *gets cast to a plain oldchar *? Yeah, it's gonna call that out. 😭
It also warns when a cast introduces a type qualifier in an unsafe way — and the classic example is casting char ** to const char **, which is unsafe. Behold the chaos:
/* 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';Lowkey a horror story, no cap.
-Wcast-align— warns whenever a pointer cast increases the required alignment of the target. Example: achar *cast to anint *on machines where integers can only be accessed at two- or four-byte boundaries. The machine said "I can't even". 🫶-Wcast-align=strict— same energy as above, but it does not care about the target machine. Warns whenever a pointer cast bumps up the required alignment of the target — like castingchar *toint *regardless of what machine you're on. Strict is strict. 👑-Wcast-function-type— warns when a function pointer gets cast to an incompatible function pointer. Here's the tea ☕:- In a cast involving function types with a variable argument list, only the types of the initial arguments that are provided are considered.
- Any parameter of pointer-type matches any other pointer-type.
- Any benign differences in integral types are ignored — like
intvs.longon ILP32 targets. - Likewise, type qualifiers are ignored.
- The function type
void (*) (void)is special — it matches everything, which you can use to suppress this warning. - In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type.
This warning is enabled by -Wextra. 💖
-Wwrite-strings— okay so when compiling C, this gives string constants the typeconst char[length], so that copying the address of one into a non-constchar *pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant — but only if you've been very careful about usingconstin declarations and prototypes. Otherwise, it's just a nuisance. That's literally why-Walldoes not request these warnings. 🙈
When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
And heads up: this warning is upgraded to an error by -pedantic-errors in C++11 mode or later. Not a drill. 💀
-Wclobbered— warns for variables that might get changed bylongjmporvfork. This warning is also enabled by-Wextra. 🦋-Wno-complain-wrong-lang— by default, language front ends complain when a command-line option is valid but not applicable to that front end. This can be disabled with-Wno-complain-wrong-lang, which is mostly useful when you're invoking a single compiler driver for multiple source files written in different languages. Like so:
$ g++ -fno-rtti a.cc b.f90The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang. So basically it's the "stop being dramatic about mismatched languages" button. 🌈
-Wcompare-distinct-pointer-types(C and Objective-C only) — warns if pointers of distinct types are compared without a cast. This warning is enabled by default. 💜-Wconversion— warns for implicit conversions that may alter a value. The list is long, so buckle up:- conversions between real and integer — like
abs (x)whenxisdouble - conversions between signed and unsigned — like
unsigned ui = -1 - conversions to smaller types — like
sqrtf (M_PI)
- conversions between real and integer — like
It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value isn't changed by the conversion, like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion. ✨
For C++, it also warns for confusing overload resolution for user-defined conversions, and conversions that never use a type conversion operator: conversions to void, the same type, a base class, or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.
And one more: warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. 🎀
-Wdangling-else— warns about constructions where there may be confusion about 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). This is often not what the programmer expected, as illustrated above by the indentation the programmer chose. When there's the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there'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. 💅
-Wdangling-pointer-Wdangling-pointer=n— warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. It also warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without. 😭-Wdangling-pointer=1— at level 1, the warning diagnoses only unconditional uses of dangling pointers.-Wdangling-pointer=2— at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers.
The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall. 🔥
This example triggers the warning at level 1 — the address of the unnamed temporary is unconditionally referenced outside of its scope:
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}In the following function, the store of the address of the local variable x in the escaped pointer *p triggers the warning at level 1:
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}In this example, the array a is out of scope when the pointer s is used. Since the code that sets s is conditional, the warning triggers at level 2:
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}-Wdate-time— warns when macros__TIME__,__DATE__or__TIMESTAMP__are encountered, because they might prevent bit-wise-identical reproducible compilations. Reproducibility girlies stay winning. 🧚-Wempty-body— warns if an empty body occurs in anif,elseordo whilestatement. This warning is also enabled by-Wextra. 🍓-Wno-endif-labels— do not warn about stray tokens after#elseand#endif. That's it, that's the whole thing. ⭐-Wenum-compare— warns about a comparison between values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C, this warning is enabled by-Wall. 🌸-Wenum-conversion— warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by-Wextrain C. 🦋-Wenum-int-mismatch(C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. For example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat. 💖
-Wjump-misses-init(C, Objective-C only) — okay bestie, this one's for when agotostatement or aswitchstatement hops forward right over a variable's initialization, or jumps backward to a label sitting after that variable got initialized. 🚨 It only cares about variables initialized at the point of declaration, so don't come for it expecting more. And listen — this warning is literally only a thing in C and Objective-C, because in C++ that kind of branch is an error anyway. No cap.
-Wjump-misses-init is included in -Wc++-compat, and you can turn it off with -Wno-jump-misses-init. 💅
-Wsign-compare— warns when you compare a signed value to an unsigned one and the result could be straight-up wrong because the signed value gets converted to unsigned. 😭 In C++, this warning is also enabled by-Wall. In C, it's also enabled by-Wextra.-Wsign-conversion— warns about implicit conversions that might flip the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast shuts it up. ✨ In C, this option is also enabled by-Wconversion.-Wflex-array-member-not-at-end(C and C++ only) — warns when a structure that has a C99 flexible array member as its last field is not at the end of another structure. It's giving messy. 🙈 It warns about stuff like:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };-Wfloat-conversion— warns about implicit conversions that lose precision on a real value. That covers real → integer, and higher-precision real → lower-precision real. This option is also enabled by-Wconversion. 🍓-Wno-scalar-storage-order— do not warn about suspicious constructs involving reverse scalar storage order. That's it, that's the whole vibe.-Wsizeof-array-div— warns about dividing twosizeofoperators when the first one is applied to an array and the divisor doesn't match the size of the array element. In that case the math does not give you the number of elements in the array, which is almost definitely what you wanted. 😭 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 classic way to compute array size, except it does not work out when you're holding a pointer. 💀 It warns about e.g.sizeof (ptr) / sizeof (ptr[0])whenptrisn't an array but a pointer. This warning is enabled by-Wall.-Wsizeof-pointer-memaccess— warns about suspicious length parameters handed to certain string and memory built-in functions when the argument usessizeof. It fires for example onmemset (ptr, 0, sizeof (ptr));whenptris a pointer and not an array, and it even suggests a possible fix — or onmemcpy (&foo, ptr, sizeof (&foo));. 👑
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that pass a sizeof expression of the source array as the bound. Check this out — in the function below, 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— do not warn when thesizeofoperator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs, so this is your off switch.-Wmemset-elt-size— warns about suspicious calls to thememsetbuilt-in 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 size of the array in memory. That's the tell that you forgot to multiply by the element size. 🫠 This warning is enabled by-Wall.-Wmemset-transposed-args— warns about suspiciousmemsetbuilt-in calls where the second argument isn't zero and the third argument is zero. For example,memset (buf, sizeof buf, 0)gets diagnosed, because you clearly meantmemset (buf, 0, sizeof buf). The diagnostic only shows up if that third argument is a literal zero — if it's an expression folded to zero, or a cast of zero to some type, then transposed args are way less likely and no warning is emitted. This warning is enabled by-Wall.-Waddress— warns about suspicious uses of address expressions. 🌈 That includes comparing the address of a function or a declared object to the null pointer constant, like:
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, like:
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}and tests of the results of pointer addition or subtraction for equality to null, like:
void f (const int *p, int i)
{
return p + i == NULL;
}Uses like these typically mean the programmer goofed: the address of most functions and objects necessarily evaluates to true (the exception being weak symbols), so using one in a conditional might mean you forgot parentheses in a function call or forgot to dereference in an array expression. 👀 The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. Comparisons against string literals give unspecified behavior and aren't portable, and they suggest you meant to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.
-Wno-address-of-packed-member— do not warn 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 was probably expected. It also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }-Wlogical-not-parentheses— warns about logical not used on the left-hand side operand of a comparison. This option does not warn if the right operand is considered a boolean expression. Its whole purpose is catching suspicious code like this:
int a;
...
if (!a > 1) { ... }You can suppress the warning by wrapping 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. (And in languages where you can return an array, that also earns you a warning.) 😭-Wno-aggressive-loop-optimizations— warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of those iterations.-Wno-attributes— do not warn if an unexpected__attribute__shows up, like unrecognized attributes or function attributes applied to variables, etc. Heads up though: this does not stop errors for incorrect use of supported attributes. ⚠️
Warnings about ill-formed uses of standard attributes get upgraded to errors by -pedantic-errors.
Also, by using -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables the warning about this declaration:
[[vendor::attr]] void f();You can also kill warnings for all attributes in a namespace using -Wno-attributes=vendor::, which prevents warnings about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();Note that -Wno-attributes= does not imply -Wno-attributes. 👑
-Wno-builtin-declaration-mismatch— warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype gets called with arguments whose promoted types don't match what the function expects. When-Wextrais specified, it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions are in scope. 🎀
For example, the call to memset below gets diagnosed because the function expects a value of type size_t as its argument, but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed too.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of__TIMESTAMP__,__TIME__,__DATE__,__FILE__, and__BASE_FILE__. 🦋-Wstrict-prototypes(C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types.) 🌸-Wold-style-declaration(C and Objective-C only) — ok bestie, this one is a vibe check on your declarations. It yells at you for obsolescent usages per the C Standard in a declaration — like, for example, it warns if storage-class specifiers likestaticare not the first things in a declaration. Rude but fair. Also, heads up: this warning is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warns you if an old-style function definition is used. Yes, even if there's a previous prototype, you still get the warning, no cap. Little nuance tho: a definition using()is not considered an old-style definition in C23 mode, because it's equivalent to(void)in that case — but it is considered an old-style definition for older standards. Timeline matters, bestie. 🕰️-Wmissing-parameter-type(C and Objective-C only) — fires when a function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }This warning is also enabled by -Wextra. 💅
-Wno-declaration-missing-parameter-type(C and Objective-C only) — don't warn if a function declaration contains a parameter name without a type. Why care? Because such function declarations do not provide a function prototype and prevent most type checking in function calls. kinda huge tbh.
This warning is enabled by default. In C99 and later dialects of C, it is treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes(C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. And yes, it's issued even if the definition itself provides a prototype — so use this option to detect global functions that don't have a matching prototype declaration in a header file. 📋
Not valid for C++ tho, because all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflict with an earlier declaration. For C++ you want -Wmissing-declarations to detect missing declarations instead. iykyk.
-Wmissing-variable-declarations(C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use it to detect global variables that do not have a matchingexterndeclaration in a header file. 🕵️♀️-Wmissing-declarations— warns if a global function is defined without a previous declaration. Does so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.- In C, no warnings are issued for functions with previous non-prototype declarations — use
-Wmissing-prototypesto detect missing prototypes. - In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
- In C, no warnings are issued for functions with previous non-prototype declarations — use
-Wmissing-field-initializers— warns if a structure's initializer has some fields missing. Like, look at this:
struct s { int f, g, h; };
struct s x = { 3, 4 };That causes such a warning, because x.h is implicitly zero. sneaky zero!!
In C this option does not warn about designated initializers, so this modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Likewise, in C++ this option does not warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. 🎀
-Wno-missing-requires— by default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write:
bool satisfied = requires { requires C<T> };so satisfied is only true if concept C is satisfied for type T.
This warning can be disabled with -Wno-missing-requires. ✨
-Wno-missing-template-keyword— the member access tokens.,->and::must be followed by thetemplatekeyword if the parent object is dependent and the member being named is a template.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}This warning can be disabled with -Wno-missing-template-keyword.
-Wno-multichar— do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code. implementation-defined = chaos, bestie. 😭-Wnormalized=[none|id|nfc|nfkc]— okay so in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. But here's the tea: sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which, when applied, ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you're using identifiers that have not been normalized — this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default. 🙏
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 👀
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🌈
-Wno-attribute-warning— do not warn about usage of functions declared withwarningattribute. By default, this warning is enabled.-Wno-attribute-warningcan be used to disable the warning, or-Wno-error=attribute-warningcan be used to disable the error when compiled with-Werrorflag.-Wno-deprecated— do not warn about usage of deprecated features. she's retired, leave her alone 💅-Wno-deprecated-declarations— do not warn about uses of functions, variables, and types marked as deprecated by using thedeprecatedattribute.-Wno-overflow— do not warn about compile-time overflow in constant expressions.-Wno-odr— warn about One Definition Rule violations during link-time optimization. Enabled by default. 🔥-Wopenacc-parallelism— warn about potentially suboptimal choices related to OpenACC parallelism.-Wno-openmp— warn about suspicious OpenMP code. 🕵️♀️-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model.-Woverride-init(C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.
-Wno-override-init-side-effects(C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.-Wpacked— warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. little benefit!! the disrespect!! For instance, in this code, the variablef.xinstruct baris misaligned even thoughstruct bardoes not 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— the 4.1, 4.2 and 4.3 series of GCC ignore thepackedattribute on bit-fields of typechar. This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between fieldaandbin this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));-Wpacked-bitfield-compat— this one's on by default, bestie. If you want it gone, hit it with-Wno-packed-bitfield-compatand it'll stop. 💅
-Wpacked-not-aligned— (C, C++, Objective-C and Objective-C++ only, so basically the whole squad 🫶)
Warns you if a struct field with an explicitly specified alignment inside a packed struct or union ends up misaligned. So like, for struct S, you'd get a warning that goes:
warning: alignment 1 of 'struct S' is less than 8
in this code right here:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall. ✨
-Wpadded— warns you when padding sneaks into a structure, whether it's there to align one lil element or to align the whole entire structure. And ngl, sometimes when that happens you can literally just rearrange the fields, cut down the padding, and make the whole structure smaller. It's giving glow-up. 💖-Wredundant-decls— warns you if anything gets declared more than once in the same scope. Even in cases where declaring it multiple times is technically valid and changes absolutely nothing. Yes bestie, even then. 🙈-Wrestrict— warns when an object referenced by arestrict-qualified parameter (or in C++, a__restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap.
Okay so example time: the call to the strcpy function below is trying to truncate the string by replacing its first characters with the last four. But the way it writes the terminating NUL into a[4]? The copies overlap, so the call gets diagnosed. Caught in 4K. 💀
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option catches some instances of simple overlap even with no optimization at all, but fr fr it works best at -O2 and above. It's included in -Wall. 🔥
-Wnested-externs— (C and Objective-C only)
Warns if an extern declaration shows up inside a function. That's it, that's the whole vibe check.
-Winline— warns if a function declared as inline cannot actually be inlined. Even with this option on, though, the compiler does not warn about failures to inline functions declared in system headers.
The compiler uses a whole variety of heuristics to figure out whether or not to inline a function. For example, it takes into account the size of the function being inlined and how much inlining has already happened in the current function. So like... seemingly insignificant changes in your source program can make the warnings from -Winline appear or disappear. Chaos. Absolute chaos. 😭
-Winterference-size— warns about use of C++17std::hardware_destructive_interference_sizewithout specifying its value with--param destructive-interference-size. It also warns about questionable values for that option. 🚩
So this variable is intended to be used for controlling class layout, to avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};Here one and two are meant to be far enough apart that stores to one won't require accesses to the other to reload the cache line. Personal space matters. 🫶
By default, --param destructive-interference-size and --param constructive-interference-size are set based on the current -mtune option — typically to the L1 cache line size for that particular target CPU, and sometimes to a range if you're tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu). No mixing, bestie, I'm serious.
If ABI stability is important to you — like if the use is in a header for a library — you should probably just not use the hardware interference size variables at all. Alternatively, you can force a particular value with --param.
And if you're confident that your use of the variable does not affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size. ✨
-Wint-in-bool-context— warns about suspicious use of integer values where boolean values are expected. Like conditional expressions (?:) using non-boolean integer constants in boolean context, likeif (a <= b ? 2 : 3). Or left shifting signed integers in boolean context, likefor (a = 0; 1 << a; a++);. Same energy for all kinds of multiplications regardless of the data type. This warning is enabled by-Wall. 👑-Wno-int-to-pointer-cast— suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error.Wint-to-pointer-castis enabled by default.-Wno-pointer-to-int-cast— (C and Objective-C only)
Suppresses warnings from 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 cannot be used. So close yet so far. 💔-Winvalid-utf8— warns if an invalid UTF-8 character is found. This warning is on by default for C++23 if-finput-charset=UTF-8is used, and it gets turned into an error with-pedantic-errors.-Wno-unicode— don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens.Wunicodeis enabled by default. 🌈-Wlong-long— warns if thelong longtype is used. This is enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. To inhibit the warning messages, use-Wno-long-long.
This warning is upgraded to an error by -pedantic-errors.
-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either-Wpedanticor-Wtraditional. To inhibit the warning messages, use-Wno-variadic-macros.-Wno-varargs— do not warn upon questionable usage of the macros used to handle variable arguments likeva_start. These warnings are enabled by default. 🎀-Wvector-operation-performance— warns if a vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning girlies. ✨
A vector operation can be implemented a few ways:
- "piecewise" — meaning the scalar operation is performed on every vector element.
- "in parallel" — meaning the vector operation is implemented using scalars of wider type, which normally is more performance efficient.
- "as a single scalar" — meaning the vector fits into a scalar type.
-Wvla— warns if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array.
This warning is upgraded to an error by -pedantic-errors.
- *
-Wvla-larger-than=byte-size* — if this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. It's similar to how-Walloca-larger-than=byte-size works, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. 🙈
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than— disables-Wvla-larger-than=warnings. The option is equivalent to-Wvla-larger-than=SIZE_MAXor larger. 💅-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds.
If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed. 👀
For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one specifies that the array argument must have at least n elements. In addition, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments. 🦋
-Wvolatile-register-var— warns if a register variable is declaredvolatile. Thevolatilemodifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by-Wall.-Wno-xor-used-as-pow— (C, C++, Objective-C and Objective-C++ only)
Disables warnings about uses of ^, the exclusive or operator, where it appears the code meant exponentiation. Specifically, the warning occurs when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant.
In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. Different language, different lore, iykyk. 💅
This warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with -Wno-xor-used-as-pow.
-Wdisabled-optimization— warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often the problem is that your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 😮💨-Wpointer-sign(C and Objective-C only) — bestie, this one clocks you when you pass a pointer argument or assign a pointer with a different signedness 😭. Only C and Objective-C get this energy. It's implied by-Walland by-Wpedantic, and you can turn it off with-Wno-pointer-sign.
Heads up: this warning gets upgraded to an error by -pedantic-errors. No takebacks. 💀
-Wstack-protector— only awake when-fstack-protectoris active, okay? It warns about functions that aren't protected against stack smashing. It's giving security guard energy 🫶.-Woverlength-strings— warns about string constants that are longer than the "minimum maximum" length the C standard says. Modern compilers are lowkey chill and generally allow string constants way longer than the standard's minimum limit, but if your program needs to be very portable, bestie, avoid the long strings ✨.
The limit applies after string constant concatenation, and it does not count the trailing NUL. In C90 the limit was 509 characters; in C99 it got raised to 4095. C++98 doesn't specify a normative minimum maximum, so we do not diagnose overlength strings in C++ at all 🙈.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants(C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix, periodt. When you use it together with-Wsystem-headersit also warns about such constants in system header files 👀. This can be useful when you're preparing code to use with the"FLOAT_CONST_DECIMAL64"pragma from the decimal floating-point extension to C99.-Wno-lto-type-mismatch— during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires-fltoto be enabled. Enabled by default 💅.-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the"designated_init"attribute. Quiet mode: activated 🎀.
✨ Options That Control Static Analysis ✨
-fanalyzer— okay so this one enables a static analysis of program flow, and it goes looking for "interesting" interprocedural paths through your code, then issues warnings for problems found on them. It's giving detective energy 🔥.
Lowkey important: this analysis is much more expensive than other GCC warnings. Your compile time is gonna feel it 😭.
In technical terms, it performs coverage-guided symbolic execution of the code being compiled. It is neither sound nor complete: it can have false positives and false negatives. It is a bug-finding tool, not a tool for proving program correctness. Fr fr, don't treat it like a proof.
The analyzer is only suitable for use on C code in this release.
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalThis option is only available if GCC was configured with analyzer support enabled.
-Wanalyzer-symbol-too-complex— if-fanalyzeris enabled, the analyzer uses various heuristics to attempt to track the state of memory, but bestie, sufficiently complicated code can defeat them 💀.
By default, the analysis silently stops tracking values of expressions if they exceed the threshold defined by --param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for such expressions. The -Wanalyzer-symbol-too-complex option warns you if this occurs.
-Wanalyzer-too-complex— if-fanalyzeris enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but again, sufficiently complicated code can defeat them 🙈.
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns you if this occurs.
-Wno-analyzer-allocation-size— this warning requires-fanalyzer, which enables it; to disable it, use-Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of "sizeof (*pointer)".
See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html). 🧚
-Wno-analyzer-deref-before-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-deref-before-checkto disable it.
This diagnostic warns for paths through the code in which a pointer is checked for "NULL" after it has already been dereferenced, suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced. It's giving "you closed the door after the cat already left" energy 😭.
This diagnostic also considers values passed to a function argument marked with "__attribute__((nonnull))" as requiring a non-NULL value, and thus will complain if such values are checked for "NULL" after returning from such a function call.
This diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. Periodt.
-Wno-analyzer-double-fclose— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-fcloseto disable it.
This diagnostic warns for paths through the code in which a "FILE *" can have fclose called on it more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html). 💅
-Wno-analyzer-double-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-freeto disable it.
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc.
See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html). 🍓
-Wno-analyzer-exposure-through-output-file— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-exposure-through-output-fileto disable it.
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Bestie that's a whole leak 🫶.
See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).
-Wanalyzer-exposure-through-uninit-copy— this warning requires both-fanalyzerand the use of a plugin to specify a function that copies across a"trust boundary". Use-Wno-analyzer-exposure-through-uninit-copyto disable it.
This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 💀
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).
-Wno-analyzer-fd-access-mode-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-access-mode-mismatchto disable it.
This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. Sir, that door only swings one way 🙈.
This diagnostic also warns for code paths in a which a function with attribute "fd_arg_read (N)" is called with a file descriptor opened with "O_WRONLY" at referenced argument "N" or a function with attribute "fd_arg_write (N)" is called with a file descriptor opened with "O_RDONLY" at referenced argument N.
-Wno-analyzer-fd-double-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-double-closeto disable it.
This diagnostic warns for paths through code in which a file descriptor can be closed more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html). ✨
-Wno-analyzer-fd-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-leakto disable it.
This diagnostic warns for paths through code in which an open file descriptor is leaked. It's giving "where did you leave that" 😭.
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-fd-phase-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-phase-mismatchto disable it.
This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call "accept" on a stream socket that has not yet had "listen" successfully called on it. Iconic behaviour, honestly — you gotta listen before you accept 💅.
See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).
-Wno-analyzer-fd-type-mismatch— bestie this one NEEDS-fanalyzer, it literally can't exist without it (turning it on enables it), and if you wanna shut it up you use-Wno-analyzer-fd-type-mismatchto disable it. 💅
What it does: it snitches on code paths where you're doing an operation on the wrong type of file descriptor. Like, trying to do socket stuff on an fd you got from open, or trying to do a stream socket operation on a datagram socket. Wrong vibe entirely, and the analyzer will call it out. 🙈
-Wno-analyzer-fd-use-after-close— again, requires-fanalyzer(which is what turns it on), and-Wno-analyzer-fd-use-after-closeturns it back off.
This one catches paths where a read or write gets called on a file descriptor that's already closed. Bestie, the fd is gone, why are we still calling it. 💀
It also flags paths where a function carrying the attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) gets called with a closed file descriptor sitting at referenced argument N. 👑
-Wno-analyzer-fd-use-without-check— yep,-fanalyzerrequired (it enables it), and-Wno-analyzer-fd-use-without-checkdisables it.
This diagnostic warns about paths through code where a file descriptor gets used without ever being checked for validity. No vibe check on the fd, no service. 🚫
Same deal as above but for unchecked-ness: it also warns when a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor at referenced argument N without being checked for validity.
-Wno-analyzer-file-leak—-fanalyzerneeded (it enables it),-Wno-analyzer-file-leakdisables it.
This one warns about paths through the code where a <stdio.h> FILE * stream object gets leaked. The stream said bye and you never said it back. 😭
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-free-of-non-heap—-fanalyzerrequired (it enables it),-Wno-analyzer-free-of-non-heapdisables it.
Warns about paths where free is called on a non-heap pointer — like an on-stack buffer, or a global. You can't just free the stack, bestie, that's not its lore. 💀
See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).
-Wno-analyzer-imprecise-fp-arithmetic—-fanalyzerrequired (it enables it),-Wno-analyzer-imprecise-fp-arithmeticdisables it.
Warns for paths through the code where floating-point arithmetic is used in spots where precise computation is needed. At the moment, it only warns on floating-point operands used inside the calculation of an allocation size. That's the whole current scope, no more no less. 🎯
-Wno-analyzer-infinite-loop—-fanalyzerrequired (it enables it),-Wno-analyzer-infinite-loopdisables it.
This diagnostic warns for paths through the code that look like they lead to an infinite loop. 🔁
Specifically, the analyzer fires this warning when it "sees" a loop where:
- no externally-visible work could be being done within the loop
- there is no way to escape from the loop
- the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true
One way this warning gets emitted is when there's an execution path through a loop where taking the path on one iteration implies that the same path will be taken on all subsequent iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}The analyzer will complain for the above case because if opcode ever matches none of the cases, the switch follows the implicit default case, making the body of the loop a no-op with cpu_state.pc unchanged — so the same value of opcode gets used on all subsequent iterations, and boom, infinite loop. 🌀
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).
-Wno-analyzer-infinite-recursion—-fanalyzerrequired (it enables it),-Wno-analyzer-infinite-recursiondisables it.
This diagnostic warns for paths through the code that appear to lead to infinite recursion. 🪆🪆🪆
Specifically, when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with that at the entry to the previous frame of that function on the stack. The warning is issued if nothing in memory appears to be changing; any changes observed to parameters or globals are assumed to lead to termination of the recursion and thus suppress the warning.
Heads up: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're trying to trigger this diagnostic. 🚨
Compare with -Winfinite-recursion, which gives a similar diagnostic but is implemented in a different way.
See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).
-Wno-analyzer-jump-through-null—-fanalyzerrequired (it enables it),-Wno-analyzer-jump-through-nulldisables it.
Warns for paths through the code where a NULL function pointer gets called. Nothing there, bestie. Empty. Void. 💀
-Wno-analyzer-malloc-leak—-fanalyzerrequired (it enables it),-Wno-analyzer-malloc-leakdisables it.
Warns for paths through the code where a pointer allocated via an allocator is leaked — either malloc, or a function marked with attribute malloc. You allocated it and then just… walked away. Rude. 😭
See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html).
-Wno-analyzer-mismatching-deallocation—-fanalyzerrequired (it enables it),-Wno-analyzer-mismatching-deallocationdisables it.
Warns for paths through the code where the wrong deallocation function is called on a pointer value, based on which function was used to allocate it. The diagnostic warns about mismatches between free, scalar delete and vector delete[], plus those marked as allocator/deallocator pairs using attribute malloc. ✨
See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).
-Wno-analyzer-out-of-bounds—-fanalyzerrequired (it enables it),-Wno-analyzer-out-of-boundsdisables it.
Warns for paths through the code where a buffer is definitely read or written out-of-bounds. The diagnostic applies where the analyzer can determine a constant offset, and for accesses past the end of a buffer (also a constant capacity). Further, it does limited checking for accesses past the end when both the offset and the capacity are symbolic. 🎀
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).
When the analyzer can, it emits a text art diagram visualizing the spatial relationship between the memory region it predicts would be accessed versus the range of memory that's valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. 🌈
That diagram can be suppressed using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers—-fanalyzerrequired (it enables it),-Wno-analyzer-overlapping-buffersdisables it.
Warns for paths through the code where overlapping buffers get passed to an API whose behavior on such buffers is undefined. 👀
Specifically, the diagnostic occurs on calls to the following functions:
memcpystrcatstrcpy
for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument—-fanalyzerrequired (it enables it),-Wno-analyzer-possible-null-argumentdisables it.
Warns for paths through the code where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. The attribute said "please, no NULLs", and here we are. 🙈
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-possible-null-dereference—-fanalyzerrequired (it enables it),-Wno-analyzer-possible-null-dereferencedisables it.
Warns for paths through the code where a possibly-NULL value is dereferenced. 💥
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-null-argument—-fanalyzerrequired (it enables it),-Wno-analyzer-null-argumentdisables it.
Warns for paths through the code where a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. Not "maybe NULL" — we know. 🔥
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-null-dereference—-fanalyzerrequired (it enables it),-Wno-analyzer-null-dereferencedisables it.
Warns for paths through the code where a value known to be NULL is dereferenced. Straight to the void, no notes. 💀
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-putenv-of-auto-var—-fanalyzerrequired (it enables it),-Wno-analyzer-putenv-of-auto-vardisables it.
okay so continuing the analyzer-warning tour, bestie — grab your strawberry matcha 🍓, we're still in the -fanalyzer cinematic universe 💅
This diagnostic warns for paths through the code in which a call to putenv is passed a pointer to an automatic variable or an on-stack buffer. bestie no 😭 that stack memory is gone the second the function returns.
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).
-Wno-analyzer-shift-count-negative— so this one is enabled by-fanalyzer, and if you want it gone you hit it with-Wno-analyzer-shift-count-negative. It flags paths through your code where somebody tries a shift with a negative count 🙈. It's giving the same energy as the-Wshift-count-negativediagnostic in the C/C++ front ends, except this one is built on analyzing interprocedural paths instead of just parsing the syntax tree. but — the analyzer doesn't prioritize hunting these down, so false negatives are more likely here than with the other warnings. lowkey tragic.-Wno-analyzer-shift-count-overflow— also requires-fanalyzer, which turns it on; kill it with-Wno-analyzer-shift-count-overflow. This one warns about paths where a shift's count is greater than or equal to the precision of the operand's type 💀. Same deal as-Wshift-count-overflowin the C/C++ front ends, but again it's doing interprocedural path analysis instead of just reading the syntax tree. And again, the analyzer doesn't prioritize these paths, so false negatives are more likely relative to other warnings. fr fr.-Wno-analyzer-stale-setjmp-buffer— needs-fanalyzer, which enables it; disable with-Wno-analyzer-stale-setjmp-buffer. It warns on paths wherelongjmpis called to rewind to ajmp_buftied to asetjmpcall in a function that has already returned 😭.
When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. Trying to rewind to it via longjmp would reference a stack frame that no longer exists — and likely lead to a crash (or worse). or worse?? yeah, or worse. 🙈
-Wno-analyzer-tainted-allocation-size— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-allocation-size. Warns for paths where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized first — so an attacker could inject an excessively large allocation and potentially cause a denial of service attack. not the vibe. not the vibe at all.
See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).
-Wno-analyzer-tainted-assertion— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-assertion. Warns for paths where an attacker-controllable value is used in a condition without being sanitized first, and that condition guards a call to a function marked with attributenoreturn(like__builtin_unreachable). Those functions usually mean the program is terminating abnormally — assertion failure handlers, that kind of thing. For example:
assert (some_tainted_value < SOME_LIMIT);In such cases:
- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure 😭
- when assertion-checking is disabled, like by defining
NDEBUG: an attacker could inject data that subverts the process, since it presumably violates a precondition that the code is assuming. oh that's bad bad
heads up bestie: when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even has a chance to "see" them — so this diagnostic can only generate warnings on builds where assertion-checking is enabled. 💅
For the purpose of this warning, any function marked with attribute noreturn counts as a possible assertion failure handler, including __builtin_unreachable. And note that these functions are sometimes removed by the optimizer before the analyzer "sees" them — so optimization should be disabled when you're trying to trigger this diagnostic. fyi.
See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}despite the above not being an assertion failure, strictly speaking. the analyzer is being extra and honestly? respect.
-Wno-analyzer-tainted-array-index— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-array-index. Warns for paths where an attacker-controllable value is used as the index of an array access without being sanitized — so an attacker could inject an out-of-bounds access. out of bounds, out of pocket, out of my good graces 💀
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-tainted-divisor— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-divisor. Warns for paths where an attacker-controllable value is used as the divisor in a division or modulus operation without being sanitized — so an attacker could inject a division-by-zero. divide by zero my beloved nemesis 🙈
See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).
-Wno-analyzer-tainted-offset— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-offset. Warns for paths where an attacker-controllable value is used as a pointer offset without being sanitized — so an attacker could inject an out-of-bounds access. ✨ pointer math but make it a crime ✨
See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).
-Wno-analyzer-tainted-size— requires-fanalyzerwhich enables it; disable with-Wno-analyzer-tainted-size. Warns for paths where an attacker-controllable value is used as the size of an operation such asmemsetwithout being sanitized — so an attacker could inject an out-of-bounds access. 🫶 stay safe out there.
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-undefined-behavior-strtok— requires-fanalyzer, which enables it; disable with-Wno-analyzer-undefined-behavior-strtok. Warns for paths where a call is made tostrtokwith undefined behavior. undefined behavior is not a personality trait bestie 😭
Specifically, passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior.
-Wno-analyzer-unsafe-call-within-signal-handler— requires-fanalyzer, which enables it; disable with-Wno-analyzer-unsafe-call-within-signal-handler. Warns for paths where a function known to be async-signal-unsafe (such asfprintf) is called from a signal handler. signal handlers are the club bathroom, you cannot do everything in there 🚫
See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).
-Wno-analyzer-use-after-free— requires-fanalyzer, which enables it; disable with-Wno-analyzer-use-after-free. Warns for paths where a pointer is used after a deallocator is called on it — eitherfree, or a deallocator referenced by attributemalloc. the audacity of a freed pointer 💅
See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).
-Wno-analyzer-use-of-pointer-in-stale-stack-frame— requires-fanalyzer, which enables it; disable with-Wno-analyzer-use-of-pointer-in-stale-stack-frame. Warns for paths where a pointer is dereferenced that points to a variable in a stale stack frame. 🌸 that variable has left the building, she does not work here anymore.-Wno-analyzer-va-arg-type-mismatch— requires-fanalyzer, which enables it; disable with-Wno-analyzer-va-arg-type-mismatch. Warns for interprocedural paths where the analyzer catches an attempt to useva_argto extract a value passed to a variadic call — but with a type that doesn't match the type of the expression passed to the call. wrong type, wrong era, wrong everything 🙈
See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).
-Wno-analyzer-va-list-exhausted— requires-fanalyzer, which enables it; disable with-Wno-analyzer-va-list-exhausted. Warns for interprocedural paths where the analyzer catches an attempt to useva_argto grab the next value passed to a variadic call — except all the values in theva_listhave already been consumed. there is no next value bestie. she's empty. the well is dry. 💀
See CWE-685: Function Call With Incorrect Number of Arguments (https://cwe.mitre.org/data/definitions/685.html).
-Wno-analyzer-va-list-leak— requires-fanalyzer, which enables it; disable with-Wno-analyzer-va-list-leak. Warns for interprocedural paths where the analyzer catches thatva_startorva_copyhas been called on ava_listwith no corresponding call tova_end. 🎀 you started her, now you have to end her, that's the rule.-Wno-analyzer-va-list-use-after-va-end— requires-fanalyzer, which enables it; disable with-Wno-analyzer-va-list-use-after-va-end. Warns for interprocedural paths where the analyzer catches an attempt to use ava_listafterva_endhas been called on it.va_list. (yes that's genuinely how the sentence ends, iconic tbh 💅)-Wno-analyzer-write-to-const— requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable it.
This diagnostic warns for paths through the code where the analyzer catches an attempt to write through a pointer to a "const" object. But heads up bestie — the analyzer does not prioritize hunting down those paths, so false negatives are way more likely here compared to its other warnings. It's giving "we'll get to it when we get to it" energy 💅
-Wno-analyzer-write-to-string-literal— this warning needs-fanalyzerto even exist (that's what turns it on); hit it with-Wno-analyzer-write-to-string-literalto turn it off.
What it flags: paths through the code where the analyzer spots an attempt to write through a pointer to a string literal. And same deal as before — the analyzer doesn't prioritize these paths, so false negatives are more likely relative to other warnings. ngl, it's a little shy about reporting 🙈
-Wno-analyzer-use-of-uninitialized-value— this warning needs-fanalyzer, which enables it; use-Wno-analyzer-use-of-uninitialized-valueto disable it.
What it flags: paths through the code where an uninitialized value gets used. Trash behavior, honestly 💀
See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).
The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:
"alloca"- The built-in functions
"__builtin_alloc","__builtin_alloc_with_align","__builtin_calloc","__builtin_free","__builtin_malloc","__builtin_memcpy","__builtin_memcpy_chk","__builtin_memset","__builtin_memset_chk","__builtin_realloc","__builtin_stack_restore", and"__builtin_stack_save" "calloc""free""malloc""memset""operator delete""operator delete []""operator new""operator new []""realloc""strdup""strndup"
of the following functions for working with file descriptors:
"open""close""creat""dup","dup2"and"dup3""isatty""pipe", and"pipe2""read""write""socket","bind","listen","accept", and"connect"
of the following functions for working with "<stdio.h>" streams:
- The built-in functions
"__builtin_fprintf","__builtin_fprintf_unlocked","__builtin_fputc","__builtin_fputc_unlocked","__builtin_fputs","__builtin_fputs_unlocked","__builtin_fwrite","__builtin_fwrite_unlocked","__builtin_printf","__builtin_printf_unlocked","__builtin_putc","__builtin_putchar","__builtin_putchar_unlocked","__builtin_putc_unlocked","__builtin_puts","__builtin_puts_unlocked","__builtin_vfprintf", and"__builtin_vprintf" "fopen""fclose""ferror""fgets""fgets_unlocked""fileno""fread""getc""getchar""fprintf""printf""fwrite"
and of the following functions:
- The built-in functions
"__builtin_expect","__builtin_expect_with_probability","__builtin_strchr","__builtin_strcpy","__builtin_strcpy_chk","__builtin_strlen","__builtin_va_copy", and"__builtin_va_start" - The GNU extensions
"error"and"error_at_line" "getpass""longjmp""putenv""setjmp""siglongjmp""signal""sigsetjmp""strcat""strchr""strlen"
Oh and also ✨ — various functions with an "__analyzer_" prefix have special meaning to the analyzer, described in the GCC Internals manual. iykyk 📖
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--param analyzer-min-snodes-for-call-summary=value
The following options control the analyzer. Buckle up bestie, there's a bunch 🎀
-fanalyzer-call-summaries— simplifies interprocedural analysis by computing the effect of certain calls, instead of exploring every single path through the function from callsite to each possible return.
If it's enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value). So like, only the messy ones get the shortcut 💅
-fanalyzer-checker=name — restrict the analyzer to run just the named checker, and enable it. One checker, all the focus, main character energy 👑-fanalyzer-debug-text-art-headings— this option is for analyzer developers. If enabled, the analyzer adds extra annotations to any diagrams it generates. Little labels on the art, cute 🎨-fno-analyzer-feasibility— this option is for analyzer developers.
By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found are rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code. Turning off the "is this even possible" check, fr
-fanalyzer-fine-grained— this option is for analyzer developers.
Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. Every statement, its own little moment ✨
-fanalyzer-show-duplicate-count— this option is for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. "and 12 others like it" energy 😭-fanalyzer-show-events-in-system-headers— by default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header. With-fanalyzer-show-events-in-system-headerssuch events are no longer suppressed. No more hiding in the headers 🦋-fno-analyzer-state-merge— this option is for analyzer developers.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge— this option is for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant, e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues. Keeping the old receipts around, just in case 🧾
-fno-analyzer-suppress-followups— this option is for analyzer developers.
By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics.
The diagnostics that terminate analysis along a path are:
-Wanalyzer-null-argument-Wanalyzer-null-dereference-Wanalyzer-use-after-free-Wanalyzer-use-of-pointer-in-stale-stack-frame-Wanalyzer-use-of-uninitialized-value
With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. Chaos mode: keep walking even after the explosion 💀
-fanalyzer-transitivity— this option enables transitivity of constraints within the analyzer. If A constrains B and B constrains C… you get the vibe ✨-fno-analyzer-undo-inlining— this option is for analyzer developers.
-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. So the stack frames be lying a little 😭
By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer.
-fanalyzer-verbose-edges— this option is for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. More words, more edges, more everything 🗣️-fanalyzer-verbose-state-changes— this option is for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.-fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.
The level can be one of:
0— at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-"free"diagnostic, both calls to"free"will be shown.1— as per the previous level, but also show events for the entry to each function.2— as per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g."true path taken"at a conditional).
This level is the default. Iconic default behavior 💖
3— as per the previous level, but show all control flow events, not just significant ones. Every single one, no filter 🔥4— this level is for analyzer developers; it adds various other events intended for debugging the analyzer.
-fdump-analyzer— dump internal details about what the analyzer is doing tofile.analyzer.txt.-fdump-analyzer-stderroverrides this option. Receipts go in a file 🧾-fdump-analyzer-stderr— dump internal details about what the analyzer is doing to stderr. This option overrides-fdump-analyzer. Straight to your terminal instead 💅-fdump-analyzer-callgraph— dump a representation of the call graph suitable for viewing with GraphViz tofile.callgraph.dot. Pretty graph time 🌈-fdump-analyzer-exploded-graph— dump a representation of the"exploded graph"suitable for viewing with GraphViz tofile.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. Color-coded!! 🎨✨-fdump-analyzer-exploded-nodes— emit diagnostics showing where nodes in the"exploded graph"are in relation to the program source. Pointing at the source like "this one right here" 👉👀-fdump-analyzer-exploded-nodes-2— dump a textual representation of the"exploded graph"tofile.eg.txt. Text version, for the readers 📖-fdump-analyzer-exploded-nodes-3— dumps a text version of the "exploded graph" into one dump file per node, namedfile.eg-id.txt. Bestie. That is a lot of files. Like, a genuinely large number of dump files. You have been warned 💀-fdump-analyzer-exploded-paths— dumps a text version of the "exploded path" for every single diagnostic, into files namedfile.idx.kind.epath.txt. Each diagnostic gets its own lil path receipt ✨-fdump-analyzer-feasibility— spills the internal tea about the analyzer's search for feasible paths. It's written in a form you can actually look at with GraphViz, going to filenames shaped likefile.*.fg.dot,file.*.tg.dot, andfile.*.fpath.txt🦋-fdump-analyzer-infinite-loop— dumps the internal deets about the analyzer's hunt for infinite loops. Also GraphViz-friendly, written to filenames of the formfile.*.infinite-loop.dot🔁-fdump-analyzer-json— dumps a compressed JSON representation of the analyzer's internals tofile.analyzer.json.gz. Heads up: the precise format is subject to change, so don't go building your whole personality on it, iykyk 🙈-fdump-analyzer-state-purge— same deal as-fdump-analyzer-supergraph(dumps a "supergraph" you can open in GraphViz), but it annotates the graph with info about what state gets purged at each node. Written tofile.state-purge.dot. 🧹-fdump-analyzer-supergraph— dumps representations of the "supergraph" for GraphViz viewing intofile.supergraph.dotandfile.supergraph-eg.dot. These show, like, every control flow graph in the program, with interprocedural edges for calls and returns. The second dump has annotations showing nodes in the "exploded graph" plus the diagnostics attached to them. Chef's kiss 🤌-fdump-analyzer-untracked— emits custom warnings with internal details, and these are specifically for analyzer developers. Not you, bestie, unless you're literally building the analyzer 🫶
✨ Options for Debugging Your Program ✨
okay so to make GCC spill extra info for a debugger, in almost every case you literally just add -g to your other options. Periodt. Some debug formats can co-exist — like DWARF with CTF — but only when you explicitly enable each one by adding its respective command-line option to your other options. No automatic mixing, you gotta ask 💅
GCC does let you use -g together with -O. But the shortcuts that optimized code takes can lowkey be wild:
- some variables you declared might not exist at all 😭
- flow of control may briefly wander somewhere you did not expect
- some statements may never execute, because they compute constant results or their values are already on hand
- some statements may execute in different places, because they got moved out of loops
Nevertheless — and this is the important bit — it is possible to debug optimized output. Which is exactly why it's reasonable to use the optimizer on programs that might have bugs. Slay.
If you're not using some other optimization option, consider -Og together with -g. Why? Because with no -O option at all, some compiler passes that collect information useful for debugging don't run at all. So -Og may straight up 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 probably makes other debuggers crash or outright refuse to read the program. If you want to control for certain whether to generate the extra information, use -gvms (see below). ⚠️
-ggdb— produces debugging information for use by GDB. Translation: use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), and include GDB extensions if at all possible. 👑-gdwarf/-gdwarf-version— produces debugging information in DWARF format (if that's supported).versioncan be2,3,4or5. The default version for most targets is5— with the exceptions of VxWorks, TPF and Darwin / macOS, which default to version2, and AIX, which defaults to version4. 🦋
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. (Yes the naming is a mess, we don't talk about it 💀)
-gbtf— requests BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated alongside DWARF debug information when both debug formats are enabled explicitly via their respective command line options ✨-gctf/-gctflevel— requests CTF debug information and useslevelto say how much CTF debug information should be produced. If-gctfis specified with no value forlevel, the default level of CTF debug information is2. 🎀
CTF debug information can be generated alongside DWARF debug information when both debug formats are enabled explicitly via their respective command line options.
- Level
0produces no CTF debug information at all. So-gctf0negates-gctf. - Level
1produces CTF information for tracebacks only. That includes callsite information, but does not include type information. - Level
2produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gvms— produces debugging information in Alpha/VMS debug format (if that's supported). This is the format used byDEBUGon Alpha/VMS systems.-gcodeview— produces debugging information in CodeView debug format (if that's supported). This is the format used by Microsoft Visual C++ on Windows. 💻-glevel/-ggdblevel/-gvmslevel— request debugging information and uselevelto say how much information. The default level is2. 📊- Level
0produces no debug information at all. So-g0negates-g. - Level
1produces 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, plus line number tables, but no information about local variables. - Level
3includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use-g3. 🎉
- Level
If you use multiple -g options, with or without level numbers, the last such option is the one that is effective. Last one wins, no takebacks 💅
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead, use an additional -glevel option to change the debug level for DWARF. Two separate flags, bestie, don't smoosh them together 🫶
-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— instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. > Only use this with debuggers that can't handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two. That's not a vibe, that's a bill 💀-fno-merge-debug-strings— tells the linker to not merge together strings in the debugging information that are identical across different object files. Merging isn't supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file, at the cost of increasing link processing time. Merging is enabled by default. ✨-fdebug-prefix-map=old=new— when compiling files that live in directoryold, record debugging information describing them as if the files lived in directorynewinstead. You can use this to swap a build-time path for an install-time path in the debug info. You can also use it to turn an absolute path into a relative path by using.fornew. This can give you more reproducible builds, which are location independent — but it may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-mapand-fcanon-prefix-map. 🗺️-fvar-tracking— runs the variable tracking pass. It computes where variables are stored at each position in the code. Better debugging information then gets generated (if the debugging information format supports this information). 🔍
It's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments— annotates assignments to user variables early in the compilation and attempts to carry those annotations all the way through to the end, trying to improve debug information while optimizing. Use of-gdwarf-4is recommended alongside it. 🎀
It can be enabled even if var-tracking is disabled — in which case annotations are created and maintained, but discarded at the end. By default this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
-gsplit-dwarf— if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension.dwo. This option lets the build system avoid linking files with debug information. To be useful, this option requires a debugger capable of reading.dwofiles. 🧚-gdwarf32/-gdwarf64— if DWARF debugging information is enabled,-gdwarf32selects the 32-bit DWARF format and-gdwarf64selects the 64-bit DWARF format. The default is target specific; on most targets it's-gdwarf32though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. ⚠️-gdescribe-dies— adds description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. 🏷️-gpubnames— generates DWARF".debug_pubnames"and".debug_pubtypes"sections. 📇-ggnu-pubnames— bestie, this one makes.debug_pubnamesand.debug_pubtypessections, formatted so they can be converted into a GDB index 💅. It's only useful with a linker that can produce GDB index version 7, no cap.-fdebug-types-section— when you're on DWARF Version 4 or higher, type DIEs can go live in their own.debug_typessection instead of being part of.debug_info✨. Putting them in a separate comdat section is more efficient since the linker can then yeet the duplicates. But — and this is important — not all DWARF consumers support.debug_typessections yet, and on some objects.debug_typesproduces larger instead of smaller debugging information 😭. It's giving "sometimes it ate, sometimes it flopped".-grecord-gcc-switches-gno-record-gcc-switches— this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to theDW_AT_producerattribute in DWARF debugging information 🌸. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also-frecord-gcc-switchesfor another way of storing compiler options into the object file 👑-gstrict-dwarf— disallow using extensions of later DWARF standard version than selected with-gdwarf-version🚫. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed.-gno-strict-dwarf— allow using extensions of later DWARF standard version than selected with-gdwarf-version✅.-gas-loc-support— this tells the compiler that the assembler supports.locdirectives 🫶. It may then use them for the assembler to generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself 💖.
This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.
-gno-as-loc-support— force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated 🔥.-gas-locview-support— informs the compiler that the assembler supports "view" assignment and reset assertion checking in.locdirectives ✨.
This option will be enabled by default if, at GCC configure time, the assembler was found to support them.
-gno-as-locview-support— force GCC to assign view numbers internally, if-gvariable-location-viewsare explicitly requested 💅.-gcolumn-info-gno-column-info— emit location column information into DWARF debugging information, rather than just file and line 📍. This option is enabled by default.-gstatement-frontiers-gno-statement-frontiers— this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output ofis_stmtmarkers in the line number table 🎀. This is enabled by default when compiling with optimization (-Os,-O1,-O2, ...), and outputting DWARF 2 debug information at the normal level.-gvariable-location-views-gvariable-location-views=incompat5-gno-variable-location-views— augment variable location lists with progressive view numbers implied from the line number table 🦋. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact 😬. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views 💀.
There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation 🙈. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it. So like… heads up.
-ginternal-reset-location-views-gno-internal-reset-location-views— attempt to determine location views that can be omitted from location view lists 👑. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists 😭. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function.-ginline-points-gno-inline-points— generate extended debug information for inlined functions ✨. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled 🌸.-gz[=type]— produce compressed debug sections in DWARF format, if that is supported 💖. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one ofnone(don't compress debug sections), orzlib(use zlib compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected 🚫. Otherwise, if the assembler does not support them,-gzis silently ignored when producing object files 🙈.-femit-struct-debug-baseonly— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined 🔥.
This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger 💀. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header 🌈.
This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger 🫶. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]— specify the struct-like types for which the compiler generates debug information 👑. The intent is to reduce duplicate struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs ✨.
A specification has the syntax
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs 💅. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain 😭. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug information 🌸. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files 🎀. The value sys means those types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application 🧪.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm— emit DWARF unwind info as compiler generated.eh_framesection instead of using GAS.cfi_*directives 🦋.-fno-eliminate-unused-debug-types— normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled 💅. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared) 👑. More often, however, this results in a significant amount of wasted space 💀.
⚡ Options That Control Optimization ⚡
These options control various sorts of optimizations 🚀.
okay bestie, we're deep in the optimization corner of this manual and it is SO much, but I got you 💖
okay so without any optimization option, your compiler is in full chill mode and its entire personality is: keep compilation cheap, and make debugging do exactly what you'd expect. no plot twists, no surprises. 💅
and the way that works is that statements are independent — if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. fr fr. iconic behavior. 🎀
but the second you turn on optimization flags? bestie, the compiler starts trying to improve the performance and/or code size — and it pays for that with compilation time and possibly with your ability to debug the program. 😭 tradeoffs, always.
the compiler performs optimization based on the knowledge it has of the program. compiling multiple files at once to a single output file mode lets the compiler use information gained from all of the files when compiling each of them. teamwork makes the dream work, lowkey. 🫶
and not all optimizations are controlled directly by a flag — only the optimizations that have a flag are listed in this section. the rest are just vibing backstage. 🙈
⚠️ okay this part matters: most optimizations are completely disabled at-O0or if an-Olevel is not set on the command line, even if individual optimization flags are specified. and similarly,-Ogsuppresses many optimization passes. so yeah, setting one tiny flag does not un-chill the whole compiler. not me learning that the hard way 💀
depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. so if you wanna know the exact set of optimizations that are enabled at each level, invoke GCC with:
-Q --help=optimizersand it will literally tell you. iykyk ✨
-O,-O1— Optimize. optimizing compilation takes somewhat more time, and a lot more memory for a large function.
with -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fmerge-constants -fmove-loop-invariants
-fmove-loop-stores -fomit-frame-pointer -freorder-blocks
-fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types
-fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-ter -funit-at-a-time-O2— Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. as compared to-O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O1. 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-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap👀 please note the warning under-fgcseabout invoking-O2on programs that use computed gotos. that one's not a vibe, that's a warning.
💜 NOTE: In Ubuntu 8.10 and later versions,-D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions,-D_FORTIFY_SOURCE=3, is set by default, and is activated when-Ois set to 2 or higher. this enables additional compile-time and run-time checks for several libc functions. to disable, specify either-U_FORTIFY_SOURCEor-D_FORTIFY_SOURCE=0.
🦋 NOTE: In Debian 13 and Ubuntu 24.04 and later versions,-D_TIME_BITS=64together with-D_FILE_OFFSET_BITS=64is set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4.
-O3— Optimize yet more.-O3turns on all optimizations specified by-O2and 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-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— Reduce compilation time and make debugging produce the expected results. this one is the default 👑-Os— Optimize for size.-Osenables all-O2optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcit 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. smol but mighty 🍓
-Ofast— Disregard strict standards compliance. yes really.-Ofastenables all-O3optimizations. it also enables optimizations that are not valid for all standard-compliant programs. it turns on-ffast-math,-fallow-store-data-racesand the Fortran-specific-fstack-arrays, unless-fmax-stack-var-sizeis specified, and-fno-protect-parens. it turns off-fsemantic-interposition.
💀 bestie. standards compliance? gone. iconic in a reckless way. know what you're signing up for.
-Og— Optimize debugging experience.-Ogshould be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. it is a better choice than-O0for producing debuggable code because some compiler passes that collect debug information are disabled at-O0.
like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra-Oz— Optimize aggressively for size rather than speed. this may increase the number of instructions executed if those instructions require fewer bytes to encode.-Ozbehaves similarly to-Osincluding enabling most-O2optimizations.
🌸 and listen: if you use multiple -O options, with or without level numbers, the last such option is the one that is effective. last one wins, periodt.
options of the form -fflag specify machine-independent flags. most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. in the table below, only one of the forms is listed — the one you typically use. you can figure out the other form by either removing no- or adding it. so like, you already have the cheat code. 🎀
the following options control specific optimizations. they are either activated by -O options or are related to ones that are. you can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired. 💅
-fno-defer-pop— For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels-O1and higher,-fdefer-popis the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once.-fforward-propagate— Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.
-ffp-contract=style—-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them.-ffp-contract=onenables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.
The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise. 🧚
-fomit-frame-pointer— Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.
🫶 careful bestie: note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-sibling-calls— Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen— Optimize various standard C string functions (e.g.strlen,strchrorstrcpy) and their_FORTIFY_SOURCEcounterparts into faster alternatives.
Enabled at levels -O2, -O3.
-finline-stringops[=fn]— Expand memory and string operations (for now, onlymemset) inline, even when the length is variable or big enough as to require looping. This is most useful along with-ffreestandingand-fno-builtin.
In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches. This option is disabled by default. 💀
and that's where we're at bestie, the optimization lore keeps going... 🌈
-fno-inline— bestie, this one says do not expand any functions inline, except the ones tagged with thealways_inlineattribute. That's the default when you're not optimizing, btw. And if you want to exempt a single function from being inlined? Just mark it with thenoinlineattribute. iconic.-finline-small-functions— this one glues functions into their callers when the function's body is smaller than the expected function call code (so the whole program gets smaller, slay). The compiler heuristically decides which functions are simple enough to be worth integrating like this. And the way this inlining applies to all functions, even the ones not declared inline? obsessed. 💅
Enabled at levels -O2, -O3, -Os.
-findirect-inlining— inlines indirect calls too, the ones discovered to be known at compile time thanks to previous inlining. Lowkey important caveat: this option only does anything when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-finline-functions— consider all functions for inlining, even if they're not declared inline. The compiler heuristically decides which functions are worth integrating this way.
And if all calls to a given function get integrated, and the function is declared static, then normally it is not output as assembler code in its own right. no cap.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once— consider allstaticfunctions called once for inlining into their caller, even if they're not markedinline. If a call to a given function gets integrated, then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og. 🫶
-fearly-inlining— inline functions marked byalways_inline, plus functions whose body seems smaller than the function call overhead, early — before doing-fprofile-generateinstrumentation and the real inlining pass. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. main character energy fr fr.
Enabled by default.
-fipa-sra— perform interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference by parameters passed by value. ✨
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag lets you coarsely control that limit.nis the size of functions that can be inlined, measured in number of pseudo instructions.
Inlining is actually controlled by a bunch of parameters, which you can specify individually with --param name=value. The -finline-limit=n option sets some of these parameters like this:
max-inline-insns-single— is set ton/2.max-inline-insns-auto— is set ton/2.
See below for docs on the individual parameters controlling inlining and their defaults.
Note: there may be no value of -finline-limit that results in default behavior. 💀
Note: pseudo instruction here means an abstract measurement of a function's size. It in no way represents a count of assembly instructions, and its exact meaning might change from one release to another.
-fno-keep-inline-dllexport— a more fine-grained version of-fkeep-inline-functions, which applies only to functions declared using thedllexportattribute or declspec.-fkeep-inline-functions— in C, emitstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, emit any and all inline functions into the object file.-fkeep-static-functions— emitstaticfunctions into the object file, even if the function is never used. 🎀-fkeep-static-consts— emit variables declaredstatic constwhen optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check whether a variable is referenced, regardless of whether optimization is on or off, use the -fno-keep-static-consts option.
-fmerge-constants— attempt to merge identical constants (string constants and floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants— attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. On top of -fmerge-constants, this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable — including multiple instances of the same variable in recursive calls — to have distinct locations, so using this option results in non-conforming behavior. not it bestie, be careful. 😭
-fmodulo-sched— perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. 🔥-fmodulo-sched-allow-regmoves— perform more aggressive SMS-based modulo scheduling with register moves allowed. Setting this flag deletes certain anti-dependences edges, which triggers the generation of reg-moves based on the life-range analysis. Heads up: this option is effective only with-fmodulo-schedenabled.-fno-branch-count-reg— disable the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based on the result. This option 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 from the generated instruction stream that were introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse— do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may get confused by the optimizations performed when this option is not used. 🙈
The default is -ffunction-cse
-fno-zero-initialized-in-bss— if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code. 🌸
This option turns off that behavior, because some programs explicitly rely on variables going to the data section — e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps— perform optimizations that check 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 -O1, -O2, -O3, -Os.
-fsplit-wide-types— when you're using a type that occupies multiple registers, likelong longon a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but it may make debugging more difficult. 🫠
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early— fully split wide types early, instead of very late. This option has no effect unless-fsplit-wide-typesis turned on.
This is the default on some targets.
-fcse-follow-jumps— in common subexpression elimination (CSE), scan through jump instructions when the target of the jump isn't reached by any other path. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— similar to-fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blockscauses CSE to follow the jump around the body of theif. ⭐
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse— perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. 💜
Note: when compiling a program using computed gotos — a GCC extension — you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm— when-fgcse-lmis enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. ate. 💅
Enabled by default when -fgcse is enabled.
-fgcse-sm— when-fgcse-smis enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with-fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level. 😔
-fgcse-las— when-fgcse-lasis enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload— turn this on and GCC runs a whole extra redundant load elimination pass after reload, bestie. Its whole job? Cleaning up redundant spilling. That's it, that's the gig. 🧹
It gets switched on by -O3, -fprofile-use, and -fauto-profile. Slay.
-faggressive-loop-optimizations— this one tells the loop optimizer to get feral 😤 and use language constraints to figure out how many times a loop runs. Heads up though: this assumes your loop code does not invoke undefined behavior — like no signed integer overflows, no out-of-bound array accesses, none of that. Those iteration bounds then guide loop unrolling, peeling, and loop exit test optimizations. Enabled by default, no cap.-funconstrained-commons— tells the compiler that variables declared in common blocks (think Fortran) might later get overridden with longer trailing arrays. So it stops doing certain optimizations that need to know the array bounds. It's giving "don't assume, bestie."-fcrossjumping— performs the cross-jumping transformation, which unifies equivalent code and saves code size. 💅 But lowkey? The resulting code may or may not actually perform better than without cross-jumping. No promises.
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. Enabled by default at-O1and higher on architectures that do support it. ✨-fdce— performs dead code elimination (DCE) on RTL. Enabled by default at-O1and higher. Bye-bye dead code 👋-fdse— performs dead store elimination (DSE) on RTL. Enabled by default at-O1and higher. Dead stores? Not on my watch. 💀-fif-conversion— attempts to transform conditional jumps into branch-less equivalents. That includes conditional moves,min,max, set flags andabsinstructions, plus some tricks doable by standard arithmetics. Whether conditional execution gets used on chips that have it is controlled by-fif-conversion2.
Enabled at levels -O1, -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 -O1, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor— okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards. For a hierarchy with virtual bases, the base and complete variants are clones — meaning two copies of the function. 😭 With this option, the base and complete variants become thunks that call a common implementation. Deduplication era.
Enabled by -Os.
-fdelete-null-pointer-checks— assumes that programs cannot safely dereference null pointers, and that no code or data element lives at address zero. This enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless null-pointer checks — those assume a memory access to address zero always traps, so if a pointer is checked after it's already been dereferenced, it can't be null.
Ngl though, in some environments this assumption is just not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
Enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR 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 the analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into original form. It's giving "I tried, bestie, but I'm reverting." 🙈-fdevirtualize-at-ltrans— streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This enables more devirtualization but significantly increases the size of streamed data. For this reason it's disabled by default. 🫶-fexpensive-optimizations— performs a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free— attempts to remove redundant extension instructions. Especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse— in C++, the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination takes advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable that optimization. To preserve stores before the constructor starts (e.g. because youroperator newclears the object storage) but still treat the object as dead after the destructor, you can use-flifetime-dse=1. The default behavior can be explicitly selected with-flifetime-dse=2. And-flifetime-dse=0is equivalent to-fno-lifetime-dse. Got all that? 💖-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— use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be:priority— specifies Chow's priority coloringCB— specifies Chaitin-Briggs coloring
Chaitin-Briggs coloring isn't implemented for all architectures, but for those targets that do support it, it's the default because it generates better code. Periodt.
-fira-region=region— use specified regions for the integrated register allocator. The region argument should be one of these:all— use all loops as register allocation regions. Can give the best results for machines with a small and/or irregular register set.mixed— use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O,-O2, ...).one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-O0.
-fira-hoist-pressure— use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. Usually results in smaller code, but it can slow the compiler down. 😮💨
Enabled at level -Os for all targets.
-fira-loop-pressure— use IRA to evaluate register pressure in loops for decisions to move loop invariants. Usually generates faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.
Enabled at level -O3 for some targets.
-fno-ira-share-save-slots— disables sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger. 📦-fno-ira-share-spill-slots— disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that doesn't get a hard register gets a separate stack slot, and as a result function stack frames are larger. Big frames energy.-flra-remat— enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. 🔁
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch— if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions.
Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.
-fschedule-insns— if supported for the target machine, attempts to reorder instructions to eliminate execution stalls caused by required data being unavailable. Helps machines with slow floating point or memory load instructions by letting other instructions be issued until the result of the load or floating-point instruction is actually required. ⏳
Enabled at levels -O2, -O3.
-fschedule-insns2— similar to-fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. Especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. 🌸
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock— disables instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation — i.e. with-fschedule-insnsor at-O2or higher.-fno-sched-spec— disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation — i.e. with-fschedule-insnsor at-O2or higher.-fsched-pressure— turns on register pressure sensitive insn scheduling before register allocation</p> 💅. Bestie, this only makes sense when scheduling before register allocation is actually a thing, i.e. with-fschedule-insnsor at-O2or higher. Turning it on can improve the generated code and decrease its size by stopping register pressure from climbing above the number of available hard registers — and therefore stopping the subsequent spills in register allocation. No spills, no tears 😭✨-fsched-spec-load— lets the compiler do speculative motion of some load instructions. Again: only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher 🦋-fsched-spec-load-dangerous— the bold one 👑. Allows speculative motion of more load instructions. Same vibe check as above: only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-stalled-insns-fsched-stalled-insns=n— defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. So:-fno-sched-stalled-insnsmeans that no insns are moved prematurely, and-fsched-stalled-insns=0means there is no limit on how many queued insns can be moved prematurely. And-fsched-stalled-insnswithout a value is equivalent to-fsched-stalled-insns=1🎀-fsched-stalled-insns-dep-fsched-stalled-insns-dep=n— defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. Heads up: this only has an effect during the second scheduling pass, and only if-fsched-stalled-insnsis used 🫶.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0, and-fsched-stalled-insns-depwithout a value is equivalent to-fsched-stalled-insns-dep=1.-fsched2-use-superblocks— when scheduling after register allocation, use superblock scheduling ✨. This allows motion across basic block boundaries, which gives you faster schedules. It's experimental, though — not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm 🙈. Also it 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. It favors the instruction that belongs to a schedule group 🫶. On by default when 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 on the critical path 👑. On by default when 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. It favors speculative instructions with greater dependency weakness 💅. On 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. It favors the instruction belonging to a basic block with greater size or frequency 📈. On 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. It favors the instruction that is less dependent on the last instruction scheduled 🌸. On 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. It favors the instruction that has more instructions depending on it 🦋. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-freschedule-modulo-scheduled-loops— okay so modulo scheduling happens before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule 😭. Use this option to control that behavior.-fselective-scheduling— schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass ✨-fselective-scheduling2— schedule instructions using the selective scheduling algorithm, but this time it runs instead of the second scheduler pass 💅-fsel-sched-pipelining— enables software pipelining of innermost loops during selective scheduling. Ngl this option has no effect unless one of-fselective-schedulingor-fselective-scheduling2is turned on 🫶-fsel-sched-pipelining-outer-loops— when pipelining loops during selective scheduling, also pipeline outer loops 🌈. Also has no effect unless-fsel-sched-pipeliningis turned on.-fsemantic-interposition— okay bestie, some object formats, like ELF, allow interposing of symbols by the dynamic linker. What that means: for symbols exported from the DSO, the compiler can't perform interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change 😭. While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it is expensive in the terms of code quality.
With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same 🎀.
The flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak ✨
-fshrink-wrap— emit function prologues only before the parts of the function that actually need it, instead of dumping them at the top of the function like it's 1997 🙈. Enabled by default at-Oand higher 👑-fshrink-wrap-separate— shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed 💅. On by default, but 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 the registers around such calls 🌸. Such allocation is done only when it seems to result in better code. This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead. Enabled at levels-O2,-O3,-Os✨-fcombine-stack-adjustments— tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them 🫶. Enabled by default at-O1and higher.-fipa-ra— use caller save registers for allocation if those registers are not used by any called function 💅. In that case it's not necessary to save and restore them around calls. This is only possible if called functions are part of the same compilation unit as the current function and they are compiled before it 👑.
Enabled at levels -O2, -O3, -Os — however, the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage cannot be known exactly (this happens on targets that do not expose prologues and epilogues in RTL) 😭
-fconserve-stack— attempt to minimize stack usage. The compiler tries to use 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— perform reassociation on trees ✨. Enabled by default at-O1and higher.-fcode-hoisting— perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible 🌈. This is especially useful as a code size optimization, but it often helps for code speed as well 💅. Enabled by default at-O2and higher.-ftree-pre— perform partial redundancy elimination (PRE) on trees. Enabled by default at-O2and-O3🫶-ftree-partial-pre— make partial redundancy elimination (PRE) more aggressive 🔥. Enabled by default at-O3.-ftree-forwprop— perform forward propagation on trees. Enabled by default at-O1and higher ✨-ftree-fre— perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation 👑. This analysis is faster than PRE, though it exposes fewer redundancies. Enabled by default at-O1and higher.-ftree-phiprop— perform hoisting of loads from conditional pointers on trees 🌸. Enabled by default at-O1and higher.-fhoist-adjacent-loads— speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction 💅. Enabled by default at-O2and higher.-ftree-copy-prop— perform copy propagation on trees. This pass eliminates unnecessary copy operations ✨. Enabled by default at-O1and higher.-fipa-pure-const— discover which functions are pure or constant 🫶. Enabled by default at-O1and higher.-fipa-reference— discover which static variables do not escape the compilation unit 🦋. Enabled by default at-O1and higher.-fipa-reference-addressable— discover read-only, write-only and non-addressable static variables. Enabled by default at-O1and higher ✨-fipa-stack-alignment— reduce stack alignment on call sites if possible 🎀. Enabled by default.-fipa-pta— perform interprocedural pointer analysis and interprocedural modification and reference analysis 👑. Heads up though: this option can cause excessive memory and compile-time usage on large compilation units 💀. It is not enabled by default at any optimization level.-fipa-profile— bestie this one does interprocedural profile propagation ✨ Functions that are only called from cold functions get marked cold too. It also sniffs out functions that only run once (thinkcold,noreturn, static constructors, static destructors, the whole one-time-only squad 💅). Then cold functions — and the loop-less chunks of those run-once functions — get optimized for size. Enabled by default at-O1and higher, no cap 🫶-fipa-modref— interprocedural mod/ref analysis, periodt. This one analyzes the side effects of functions (aka which memory locations get modified or referenced) and that unlocks better optimization across the function call boundary 🌸 Enabled by default at-O1and higher.-fipa-cp— interprocedural constant propagation 💖 It reads the whole program to figure out when the values passed to functions are constants, then optimizes accordingly. If your app is passing constants into functions, this can substantially boost performance. Enabled by default at-O2,-Osand-O3. It's also enabled by-fprofile-useand-fauto-profile⭐-fipa-cp-clone— function cloning to make interprocedural constant propagation stronger 💪 When it's on, IPA constant propagation clones a function when an externally visible function can be called with constant arguments. But listen — because this can spawn multiple copies of functions, it may significantly increase code size (peep--param ipa-cp-unit-growth=value). Enabled by default at-O3. Also enabled by-fprofile-useand-fauto-profile🎀-fipa-bit-cp— when enabled, does interprocedural bitwise constant propagation ✨ Enabled by default at-O2and by-fprofile-useand-fauto-profile. Heads up tho: it requires that-fipa-cpis enabled 🙈-fipa-vrp— when enabled, does interprocedural propagation of value ranges 🌈 Enabled by default at-O2. It requires that-fipa-cpis enabled too.-fipa-icf— Identical Code Folding for functions and read-only variables 👑 It reduces code size, but ngl it may disturb unwind stacks by replacing a function with an equivalent one that has a different name 😭 And it works more effectively with link-time optimization enabled.
Okay so even though the vibe is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold. Different besties, different finds 💅
Enabled by default at -O2 and -Os.
-flive-patching=level— controls GCC's optimizations so the output is suitable for live-patching 🩹
Here's the lore: if the compiler's optimization uses a function's body (or info extracted from its body) to optimize/change another function, that latter one is called an impacted function of the former. So if you patch a function, its impacted functions should be patched too — it's giving package deal fr fr.
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc. 😌
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. To control that number and make the list of impacted functions easier to compute, IPA optimizations can be partially enabled at two different levels ✨
The level argument should be one of these:
inline-clone— only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. So when you patch a function, all its callers and its clones' callers are impacted, and therefore need to be patched as well 🫠
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enables inlining of static functions. So when you patch a static function, all its callers are impacted and thus need to be patched as well 💀
On top of all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables these additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone 👑
This flag is disabled by default.
⚠️ Note that-flive-patchingis not supported with link-time optimization (-flto).
-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 erroneous or undefined behavior into a trap. Enabled 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 from a null value being used in a way forbidden by areturns_nonnullornonnullattribute 😳 Same deal: isolate those paths from the main control flow and turn the erroneous/undefined statement into a trap. This one is not currently enabled, but may be enabled by-O2in the future.-ftree-sink— performs forward store motion on trees 🌳 Enabled by default at-O1and higher.-ftree-bit-ccp— sparse conditional bit constant propagation on trees, plus it propagates pointer alignment information ✨ This pass only operates on local scalar variables and is enabled by default at-O1and higher — except for-Og. It requires that-ftree-ccpis enabled.-ftree-ccp— sparse conditional constant propagation (CCP) on trees 💫 Only operates on local scalar variables, enabled by default at-O1and higher.-fssa-backprop— propagates info about uses of a value up the definition chain so the definitions get simpler 🧚 For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-O1and higher.-fssa-phiopt— pattern matching on SSA PHI nodes to optimize conditional code 🌸 Enabled by default at-O1and higher, except for-Og.-ftree-switch-conversion— converts simple initializations in aswitchinto initializations from a scalar array 🔥 Enabled by default at-O2and higher.-ftree-tail-merge— hunts 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. Enabled by default at-O2and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter.-ftree-dce— dead code elimination (DCE) on trees 🧹 Enabled by default at-O1and 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 💅 Enabled by default at-O2and higher if-Osis not also specified.-ffinite-loops— assumes that a loop with an exit will eventually take that exit and not loop forever ♾️ This lets the compiler remove loops that otherwise have no side-effects, since it doesn't count eventual endless looping as a side effect. Enabled by default at-O2for C++ with-std=c++11or higher.-ftree-dominator-opts— a whole lil variety show of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 🌈 It also does jump threading (to reduce jumps to jumps). Enabled by default at-O1and higher.-ftree-dse— dead store elimination (DSE) on trees 🗑️ A dead store is a store into a memory location that's later overwritten by another store without any intervening loads — in that case the earlier store can just be deleted. Enabled by default at-O1and higher.-ftree-ch— loop header copying on trees 🎀 Beneficial because it increases the effectiveness of code motion optimizations, and it also saves one jump. Enabled by default at-O1and higher. It is not enabled for-Os, since it usually increases code size.-ftree-loop-optimize— loop optimizations on trees 🔁 Enabled by default at-O1and higher.-ftree-loop-linear,-floop-strip-mine,-floop-block— loop nest optimizations ✨ Same as-floop-nest-optimize. To use this code transformation, GCC has to be configured with--with-islto enable the Graphite loop transformation infrastructure.-fgraphite-identity— enables the identity transformation for graphite 💎 For every SCoP we generate the polyhedral representation and transform it back to gimple. Using-fgraphite-identitywe can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops 🦋-floop-nest-optimize— enables the isl based loop nest optimizer 👑 This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. Heads up: this option is experimental 🙈-floop-parallelize-all— uses the Graphite data dependence analysis to identify loops that can be parallelized 🏃♀️ It parallelizes all the loops that can be analyzed to not contain loop carried dependences, without checking that it's profitable to parallelize the loops. Not me just going for it 💅-ftree-coalesce-vars— bestie, when GCC is transforming your program out of the SSA representation, this flag says try to cut down on copying by coalescing versions of different user-defined variables, not just the compiler's lil temporary ones 💅. Heads up tho: this can severely limit your ability to debug an optimized program that you compiled with-fno-var-tracking-assignments😭. In its negated form, this flag prevents SSA coalescing of user variables. It's enabled by default if optimization is enabled, and ngl it does very little otherwise 🫶.-ftree-loop-if-convert— attempt to transform conditional jumps in the innermost loops into branch-less equivalents ✨. The whole point is removing control-flow from the innermost loops so the vectorization pass can handle those loops better 🔥. Enabled by default if vectorization is enabled.-ftree-loop-distribution— perform loop distribution. This one can improve cache performance on big loop bodies and lets further loop optimizations happen, like parallelization or vectorization 👑. For example, this loop:
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOis transformed into:
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's also enabled by -fprofile-use and -fauto-profile 💖.
-ftree-loop-distribute-patterns— perform loop distribution of patterns that can be code generated with calls to a library 📚. 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 into:
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop is transformed into a call to memset zero. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile ✨.
-floop-interchange— perform loop interchange outside of graphite 💅. This flag can improve cache performance on loop nests and lets further loop optimizations, like vectorization, take place 🔥. For example, this loop:
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];is transformed into:
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 💖.
-floop-unroll-and-jam— apply unroll and jam transformations on feasible loops 🎀. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops together. Enabled by default at-O3. It is also enabled by-fprofile-useand-fauto-profile✨.-ftree-loop-im— perform loop invariant motion on trees 🌸. This pass moves only invariants that are hard to handle at the RTL level — think function calls, operations that expand to nontrivial sequences of insns. With-funswitch-loopsit also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching 🫶. The pass also includes store motion.-ftree-loop-ivcanon— create a canonical counter for the number of iterations in loops where figuring out the iteration count requires complicated analysis 😭. Later optimizations can then determine the number easily. Useful especially in connection with unrolling 💅.-ftree-scev-cprop— perform final value replacement ✨. If a variable gets modified in a loop in a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation — as long as it's sufficiently cheap 💸. This reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher.-fivopts— perform induction variable optimizations on trees 🌳 — that's strength reduction, induction variable merging, and induction variable elimination, bestie 👑.-ftree-parallelize-loops=n— parallelize loops, i.e., split their iteration space to run in n threads 🔥. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive rather than constrained, e.g., by memory bandwidth. This option implies-pthread, and thus is only supported on targets that have support for-pthread🫶.-ftree-pta— perform function-local points-to analysis on trees 🎀. Enabled by default at-O1and higher, except for-Og.-ftree-sra— perform scalar replacement of aggregates ✨. This pass replaces structure references with scalars to prevent committing structures to memory too early 💅. Enabled by default at-O1and higher, except for-Og.-fstore-merging— perform merging of narrow stores to consecutive memory addresses 🧚. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions 👑. Enabled by default at-O2and higher as well as-Os.-ftree-ter— perform temporary expression replacement during the SSA->normal phase 🌸. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but it gives the expanders much more complex trees to work on, resulting in better RTL generation 🔥. Enabled by default at-O1and higher.-ftree-slsr— perform straight-line strength reduction on trees ✨. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible 💸. Enabled by default at-O1and higher.-ftree-vectorize— perform vectorization on trees 🌈. This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif not explicitly specified 👑.-ftree-loop-vectorize— perform loop vectorization on trees 🌀. Enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftree-slp-vectorize— perform basic block vectorization on trees 💅. Enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftrivial-auto-var-init=choice— initialize automatic variables with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use 🛡️. Important tho, bestie: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized —-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized 😭. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the firstcaseof a"switch"statement ⚠️. Use-Wtrivial-auto-var-initto report all such cases.
The three values of choice are:
uninitialized— doesn't initialize any automatic variables. This is C and C++'s default 🫶.pattern— initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and aren't values that programmers can rely on for useful program semantics 💀. The current value is a byte-repeatable pattern with byte"0xFE". The values used for pattern initialization might be changed in the future 👀.zero— initialize automatic variables with zeroes ✨.
The default is uninitialized.
Note that the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), rather than to their interpretation as numerical values 🧠. This distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the "range 160..400" will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range — and even if they do, the interpretation of the value held by the variable will depend on the bias 😵. A hardbool variable that uses say "0X5A" and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type ⚠️. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested 🎀.
You can control this behavior for a specific variable by using the variable attribute "uninitialized" 💅.
-fvect-cost-model=model— bestie, this one changes up the cost model that vectorization runs on 💅. The model you pass has to be one ofunlimited,dynamic,cheaporvery-cheap. So like:unlimited— the vectorized code-path is just assumed to be profitable, no questions asked ✨dynamic— a runtime check guards the vectorized code-path, so it only kicks in for iteration counts that'll probably run faster than the original scalar loop would've 🌸cheap— this disables vectorization of loops where doing it would be cost prohibitive, like when you'd need runtime checks for data dependence or alignment — but otherwise it's literally equal to thedynamicmodelvery-cheap— only allows vectorization if the vector code would entirely replace the scalar code being vectorized. Example: if each iteration of a vectorized loop can only handle exactly four iterations of the scalar loop, thenvery-cheaponly allows vectorization when 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 vectorizing loops marked with the OpenMPsimddirective 👑. The model argument should be one ofunlimited,dynamic,cheap. All values of model mean the same thing as described in-fvect-cost-model, and by default a cost model defined with-fvect-cost-modelgets used.-ftree-vrp— does Value Range Propagation on trees 🦋. It's similar to the constant propagation pass, except instead of values, ranges of values get propagated. Which means the optimizers can yeet unnecessary range checks like array bound checks and null pointer checks. Enabled by default at-O2and higher. Lowkey important: null pointer check elimination only happens if-fdelete-null-pointer-checksis enabled.-fsplit-paths— splits paths leading to loop backedges 🔥. This can improve dead code elimination and common subexpression elimination. Enabled by default at-O3and above.-fsplit-ivs-in-unroller— lets values of induction variables in later iterations of the unrolled loop be expressed using the value from the first iteration ⭐. This breaks long dependency chains, which improves the efficiency of the scheduling passes.
A combo of -fweb and CSE is often enough to get the same effect. But that's not reliable when the loop body is more complicated than a single basic block 🙈. And 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— with this one, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code 💖.
Enabled by default for PowerPC targets, disabled by default everywhere else.
-fpartial-inlining— inline parts of functions 🌈. Heads up: this option only has any effect when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning— performs predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) that were performed in previous iterations of loops 💜.
Enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays— if the target machine supports it, generates instructions to prefetch memory, improving performance of loops that access large arrays ✨.
Ngl this option may generate better or worse code; results are highly dependent on the structure of the loops inside your source code 😭.
Disabled at level -Os.
-fno-printf-return-value— do not substitute constants for the known return value of formatted output functions such assprintf,snprintf,vsprintf, andvsnprintf(but notprintfoffprintf) 🙈. This transformation lets GCC optimize or even eliminate branches based on the known return value of these functions when they're called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when-fprintf-return-valueis in effect, both the branch and the body of theifstatement (but not the call tosnprint) can be optimized away wheniis a 32-bit or smaller integer, because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above 🎀. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. And -fprintf-return-value is enabled by default.
-fno-peephole-fno-peephole2— disable any machine-specific peephole optimizations 💅. The difference between-fno-peepholeand-fno-peephole2is in how they're implemented in the compiler; 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— don't guess branch probabilities using heuristics 🦋.
GCC uses heuristics to guess branch probabilities when they're not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can get complex, and in some cases it may be useful to disable the heuristics so the effects of __builtin_expect are easier to understand.
It's also possible to specify expected probability of the expression with the __builtin_expect_with_probability built-in function ⭐.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks— reorder basic blocks in the compiled function to reduce the number of taken branches and improve code locality 🔥.
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm— use the specified algorithm for basic block reordering 💖. The algorithm argument can besimple, which does not increase code size (except sometimes due to secondary effects like alignment), orstc, the "software trace cache" algorithm, which tries to put all often-executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition— on top of reordering basic blocks in the compiled function to reduce the number of 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 is not 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— reorder functions in the object file to improve code locality 🌸. This is implemented by using special subsections.text.hotfor most frequently executed functions and.text.unlikelyfor unlikely executed functions. The reordering is done by the linker, so the object file format has to support named sections and the linker has to 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— lets the compiler assume the strictest aliasing rules applicable to the language being compiled 💅. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, 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;
}The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So the code above works as expected ✨. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}⚠️ Bestie that last one is undefined behavior fr fr, no cap. Do not do that.
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fipa-strict-aliasing— controls whether the rules of-fstrict-aliasingget applied across function boundaries 🦋. Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary.
-fipa-strict-aliasing is enabled by default and is effective only in combination with -fstrict-aliasing.
-falign-functions,-falign-functions=n,-falign-functions=n:m,-falign-functions=n:m:n2,-falign-functions=n:m:n2:m2— ok so this one lines up the start of your functions to the next power-of-two boundary that's greater than or equal to n, and it's allowed to skip up to m-1 bytes to get there 🎯. The whole point? Making sure that at least the first m bytes of the function can get fetched by the CPU without crossing an n-byte alignment boundary. It's a code-performance glow-up, and ngl the alignment gets totally ignored for functions that count as cold. If you want alignment on every single function, use-fmin-function-alignmentinstead.
If m isn't specified, it just defaults to n. Periodt.
Examples, because examples slay: -falign-functions=32 aligns functions to the next 32-byte boundary ✨. -falign-functions=24 aligns to the next 32-byte boundary only if that can be done by skipping 23 bytes or less. And -falign-functions=32:7 aligns to the next 32-byte boundary only if that can be done by skipping 6 bytes or less.
That second pair of n2:m2 values lets you set a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.
Heads up bestie: 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 — they both mean functions are not aligned.
If n isn't specified or is zero, you get a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment— with this one on, the compiler tries to avoid unnecessarily overaligning functions 💅. It attempts to tell the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. Cute, controlled, no wasted bytes.-falign-labels,-falign-labels=n,-falign-labels=n:m,-falign-labels=n:m:n2,-falign-labels=n:m:n2:m2— this one aligns all branch targets to a power-of-two boundary ✨.
Its parameters are analogous to the -falign-functions option, same energy fr fr. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.
If -falign-loops or -falign-jumps apply and their values are greater than this one, then their values get used instead 👑
If n isn't specified or is zero, you get a machine-dependent default which is very likely to be 1, meaning no alignment at all. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops,-falign-loops=n,-falign-loops=n:m,-falign-loops=n:m:n2,-falign-loops=n:m:n2:m2— aligns loops to a power-of-two boundary 🌀. If your loops run many times, this makes up for any execution of the dummy padding instructions. It's a performance optimization, and the alignment gets ignored for loops considered cold.
If -falign-labels is greater than this value, then its value gets used instead.
Parameters are analogous to -falign-functions, same vibe. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n option value is 65536.
If n isn't specified or is zero, you get a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps,-falign-jumps=n,-falign-jumps=n:m,-falign-jumps=n:m:n2,-falign-jumps=n:m:n2:m2— aligns branch targets to a power-of-two boundary, specifically for branch targets where the targets can only be reached by jumping 🦘. In this case, no dummy operations need to be executed, so it's lowkey free real estate. It's a performance optimization, and alignment gets ignored for jumps considered cold.
If -falign-labels is greater than this value, then its value gets used instead.
Parameters are analogous to -falign-functions, as always. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n isn't specified or is zero, you get a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment— sets the minimal alignment of functions to the next power-of-two boundary greater than or equal to n 💖. Unlike-falign-functions, this alignment gets applied to all functions, even the ones considered cold. And the alignment is also not affected by-flimit-function-alignment. No exceptions, bestie.-fno-allocation-dce— do not remove unused C++ allocations in dead code elimination 💀. They stay. Even if they're doing nothing.-fallow-store-data-races— lets the compiler perform optimizations that might introduce new data races on stores, without proving the variable can't be concurrently accessed by other threads 😭. It does not affect optimization of local data. It's safe to use this option if you already know that global data will not be accessed by multiple threads.
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single-threaded context but may be unsafe in a multi-threaded context 🚨. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast.
-funit-at-a-time— this option is left for compatibility reasons.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder— do not reorder top-level functions, variables, andasmstatements 🙅. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it's better to use attributes when possible — no cap.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally, -fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps— with this option, the compiler turns calls to__builtin_unreachableinto traps, instead of using them for optimization 🪤. This also affects any such calls implicitly generated by the compiler.
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but it does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one 👑.
This option is enabled by default at -O0 and -Og.
-fweb— constructs webs as commonly used for register allocation purposes and assigns each web its own individual pseudo register 🕸️✨. This lets the register allocation pass operate on pseudos directly, and it also strengthens several other optimization passes, like CSE, loop optimizer, and trivial dead code remover. It can, however, make debugging impossible 😭, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program— assumes that the current compilation unit represents the whole program being compiled 👑. All public functions and variables, with the exception ofmainand those merged by attributeexternally_visible, become static functions and in effect get optimized more aggressively by interprocedural optimizers. Main character energy fr fr.
With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known from the resolution info passed to the link-time optimizer by the linker plugin. It's still useful if no linker plugin is used, or during the incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).
-flto[=n]— this option runs the standard link-time optimizer 🌈🔥. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files get linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. It's giving "one big happy file".
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It's recommended that you compile all the files participating in the same link with the same options, and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oThe first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual ✨. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. Besties sharing everything 💖.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cThe above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation, and optimizes them as usual to produce myprog.
The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step 👑. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command 🙅.
Okay so whole-program optimization? It only works if the compiler can make certain whole program assumptions, bestie. It needs to know which functions and variables can be reached by libraries and runtime stuff outside of the link-time optimized unit. When the linker supports it, the linker plugin (peep -fuse-linker-plugin) hands the compiler the tea about which symbols are used and which are externally visible. And when the linker plugin isn't available? You gotta use -fwhole-program so the compiler can still make those assumptions — and that leads to way more aggressive optimization decisions. Slay. ✨
When a file gets compiled with -flto but without -fuse-linker-plugin, the object file it spits out is bigger than a regular object file, because it's carrying GIMPLE bytecodes and the usual final code (check -ffat-lto-objects). Which means — plot twist — object files with LTO info can be linked as normal object files! And if -fno-lto gets passed to the linker, no interprocedural optimizations get applied at all. Just know that when -fno-fat-lto-objects is enabled, the compile stage is faster, but 💀 you cannot do a regular, non-LTO link on them. Not possible. Periodt.
When it's time to produce the final binary, GCC only applies link-time optimizations to the files that actually contain bytecode. So yes, you can absolutely mix and match object files and libraries with GIMPLE bytecodes and final object code — GCC picks for itself which files to optimize in LTO mode and which ones to link with no further processing. Iconic behavior, honestly. 🎀
Generally, options you specify at link time override the ones from compile time, though in some cases GCC tries to infer link-time options from whatever settings were used to compile the input files. And 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.
⚠️ But ngl, it's generally ineffective to specify an optimization level option only at link time and not at compile time. Two reasons, bestie: 1. Compiling without optimization suppresses compiler passes that gather the information needed for effective optimization at link time. 2. Some early optimization passes can only happen at compile time, never at link time.
There are some code generation flags GCC preserves when it generates bytecodes, because they're needed during the final link. Currently, 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 — get combined based on this little scheme right here:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpieCertain ABI-changing flags must match across all compilation units, and if you try to override that at link time with a conflicting value, it just gets ignored. 🙈 That includes options like -freg-struct-return and -fpcc-struct-return.
Other options — think -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 though. 💅
Diagnostic options like -Wstringop-overflow also get passed through to the link stage, and their setting matches the compile-step at function granularity. Note that this only matters for diagnostics emitted during optimization. Also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time. 🌸
When you need to pass options to the assembler via -Wa or -Xassembler, make sure you either compile those translation units with -fno-lto, or consistently use the same assembler options on all translation units. Alternatively you can just specify assembler options at LTO link time. Whatever vibes. 🦋
To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. Don't do it. 🫶
If LTO runs into objects with C linkage declared with incompatible types in separate translation units that are being linked together (that's undefined behavior according to ISO C99 6.2.7, btw), a non-fatal diagnostic may be issued. The behavior is still undefined at run time, bestie. Similar diagnostics may be raised for other languages too.
Another feature of LTO that is lowkey iconic: it's possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. 💜
If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; and to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. Teamwork, fr fr. ✨
If you're not using a linker with plugin support and/or you don't enable the linker plugin, then the objects inside libfoo.a get extracted and linked as usual — but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. 🌈
Link-time optimizations do not require the presence of the whole program to operate. If the program doesn't require any symbols to be exported, it's possible to combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions, which may lead to improved optimization opportunities. And heads up: use of -fwhole-program is not needed when the linker plugin is active (see -fuse-linker-plugin). 🎀
The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. Sorry bestie, that's just the rules. 💀
Also: link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 👑
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. And even without the option value, GCC tries to automatically detect a running GNU make's job server. 🧚
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. Easy.
-flto-partition=alg— specifies the partitioning algorithm the link-time optimizer uses. The value is either1to1, to specify a partitioning mirroring the original source files, orbalanced, to specify partitioning into equally sized chunks (whenever possible), ormax, to create a new partition for every symbol where possible. Specifyingnoneas an algorithm disables partitioning and streaming completely. The default value isbalanced. While1to1can be used as a workaround for various code ordering issues, themaxpartitioning is intended for internal testing only (do not come for me, I'm just the messenger 🙈). The valueonespecifies that exactly one partition should be used, while the valuenonebypasses partitioning and executes the link-time optimization step directly from the WPA phase. 🍓-flto-compression-level=n— this option specifies the level of compression used for intermediate language written to LTO object files, and it's only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. Forzstd, valid values are0(no compression) to19(maximum compression), whilezlibsupports values from0to9. Values outside this range are 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 option relies on plugin support in the linker, which is available ingoldor in GNUld2.21 or newer. 🫶
Okay so real talk, this option pulls object files that have GIMPLE bytecode out of library archives. 📦✨ Why do we even care? Because it makes the optimization quality go up — the link-time optimizer gets to see a whole lot more code. And the info in there says which symbols can be accessed externally (by a non-LTO object, or during dynamic linking, iykyk). The code-quality glow-up you get on binaries (and on shared libraries that use hidden visibility) is basically the same as what -fwhole-program gives you. Peep -flto for the actual description of what this flag does and how to use it. 🫶
This one is on by default whenever LTO support is built into GCC and GCC was configured for use with a linker that supports plugins (GNU ld 2.21 or newer, or gold). 👑
✨ MORE OPTIONS (pt. 53) ✨
-ffat-lto-objects— Fat LTO objects are object files that carry both the intermediate language and the object code. That double life means they work for LTO linking and for normal linking, no drama. 🦋 Heads up though: this one only does anything when you're compiling with-flto, and it's straight-up ignored at link time.
-fno-fat-lto-objects compiles faster than plain LTO, but it needs your entire toolchain to know what LTO even is. It requires a linker with linker plugin support just for the basic functionality. On top of that, nm, ar and ranlib all need linker plugin support if you want a full-featured build environment (like, one capable of building static libraries and stuff). GCC ships the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to those tools. And with non-fat LTO, your makefiles need editing so they actually use them. 😭
Fun fact: modern binutils have a plugin auto-load mechanism. Dropping the linker plugin into $libdir/bfd-plugins does the exact same thing as using the command wrappers (gcc-ar, gcc-nm and gcc-ranlib). 💅
The default is -fno-fat-lto-objects on targets that have linker plugin support.
-fcompare-elim— After register allocation and post-register allocation instruction splitting, this one hunts for arithmetic instructions that compute processor flags in a way that's similar to a comparison operation based on that arithmetic. If it can, it eliminates the explicit comparison operation. 🔍
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O1, -O2, -O3, -Os. ✨
-ffold-mem-offsets-fno-fold-mem-offsets— Tries to eliminateaddinstructions by folding them into memory loads/stores. 🧚
Enabled at levels -O2, -O3.
-fcprop-registers— After register allocation and post-register allocation instruction splitting, it runs a copy-propagation pass to try to reduce scheduling dependencies and occasionally delete the copy entirely. ✂️
Enabled at levels -O1, -O2, -O3, -Os.
-fprofile-correction— Profiles gathered from an instrumented binary for multi-threaded programs can be messy and inconsistent, because counter updates get missed. When you pass this, GCC uses heuristics to correct or smooth out those inconsistencies. By default, GCC just throws an error message when it spots an inconsistent profile. 💀
This option is enabled by -fauto-profile.
-fprofile-partial-training— With-fprofile-use, every part of the program that didn't run during the training run gets optimized aggressively for size instead of speed. But sometimes it's just not practical to train all the possible hot paths in a program. (Like, the program might have functions specific to a certain piece of hardware, and the training might not cover every hardware configuration it's run on. 🙈) With-fprofile-partial-training, profile feedback gets ignored for all functions that weren't executed during the training run, so they get optimized as if they were compiled with no profile feedback at all. Result: better performance when the training run isn't representative, but also significantly bigger code. 😬-fprofile-use-fprofile-use=path— Turns on profile feedback-directed optimizations, plus these specific optimizations, most of which are only really profitable when you've got profile feedback to work with:-fbranch-probabilities-fprofile-values-funroll-loops-fpeel-loops-ftracer-fvpt-finline-functions-fipa-cp-fipa-cp-clone-fipa-bit-cp-fpredictive-commoning-fsplit-loops-funswitch-loops-fgcse-after-reload-ftree-loop-vectorize-ftree-slp-vectorize-fvect-cost-model=dynamic-ftree-loop-distribute-patterns-fprofile-reorder-functions
Before you can use this, bestie, you have to generate the profiling information first. ✨
By default, GCC spits out an error message if the feedback profiles don't match the source code. You can turn that error into a warning with -Wno-error=coverage-mismatch — but ngl, that can leave you with poorly optimized code. Also, by default, GCC emits a warning if the feedback profiles don't exist at all (see -Wmissing-profile). 🫠
If you give it path, GCC looks there to find the profile feedback data files. See -fprofile-dir.
-fauto-profile-fauto-profile=path— Turns on sampling-based feedback-directed optimizations, plus these specific optimizations, most of which are only really 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-correction
path is the name of a file containing AutoFDO profile information. Leave it out and it defaults to fbdata.afdo in the current directory. 📁
To produce an AutoFDO profile data file you need to run your program with the perf utility on a supported GNU/Linux target system. More info lives at https://perf.wiki.kernel.org/. 🌐
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen use the create_gcov tool to convert that raw profile data into a format GCC can actually use. You also have to hand this tool the unstripped binary for your program. See https://github.com/google/autofdo. 🎀
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo💖 FLOATING-POINT OPTIONS 💖
The next batch of options controls how the compiler behaves around floating-point arithmetic. They're all about trading off speed against correctness. And listen — every single one of these has to be specifically enabled. No freebies. 💅
-ffloat-store— Don't store floating-point variables in registers, and block other options that might change whether a floating-point value gets pulled from a register or from memory. 🚫
This one saves you from undesirable excess precision on machines like the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same situation on the x86 architecture. For most programs, that extra precision does nothing but good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for those, after you've modified them to store all pertinent intermediate computations into variables. 🧮
-fexcess-precision=style— This gives you finer control over excess precision on machines where floating-point operations 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 — meaning operations might get carried out in a wider precision than the types specified in the source if that means faster code, and it's genuinely unpredictable when rounding back to the source-specified types takes place. 😵💫 When you're compiling C or C++, if you specify-fexcess-precision=standardthen excess precision follows the rules in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas-ffloat-storeonly affects assignments). This option is enabled by default for C or C++ when a strict conformance option like-std=c99or-std=c++17is used. And-ffast-mathenables-fexcess-precision=fastby default no matter whether a strict conformance option is used or not. If you specify-fexcess-precision=16, then constants and the results of expressions with types_Float16and__bf16are computed without excess precision. ✨
Careful bestie:-fexcess-precision=standardis not implemented for languages other than C or C++. On the x86, it does nothing if you specify-mfpmath=sseor-mfpmath=sse+387— in the first case, IEEE semantics apply with no excess precision, and in the second, the rounding is unpredictable. 🫠
-ffast-math— Sets the options-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fast. 🎉
It also causes the preprocessor macro __FAST_MATH__ to be defined.
⚠️ This one is not turned on by any-Ooption other than-Ofast, because it can produce incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. That said, it can yield faster code for programs that don't need those guarantees. 🔥
-fno-math-errno— Don't seterrnoafter calling math functions that execute as a single instruction, e.g.sqrt. A program that relies on IEEE exceptions for math error handling might want this flag for the speed while keeping IEEE arithmetic compatibility. 🏎️
⚠️ This one is not turned on by any -O option, because it can produce incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, give you faster code for programs that don't require those guarantees.
The default is -fmath-errno.
On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to even consider the possibility that it might, and -fno-math-errno is the default there. 🍎
-funsafe-math-optimizations— Allow optimizations for floating-point arithmetic that (a) assume arguments and results are valid, and (b) may violate IEEE or ANSI standards. When it's used at link time, it may pull in libraries or startup files that change the default FPU control word, or do other similar optimizations. 😳
This option is not turned on by any -O option since it can give you wrong 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 specs. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math— bestie, this one lets the compiler re-associate operands in series of floating-point operations 🔥. That violates the ISO C and C++ language standard by possibly changing the computation result 💀. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — and thus cannot be used on code that relies on rounding behavior like(x + 2**52) - 2**52. It may also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both-fno-signed-zerosand-fno-trapping-mathbe in effect. Moreover, 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.
-freciprocal-math— allows the reciprocal of a value to be used instead of dividing by the value if this enables optimizations ✨. For examplex / ycan be replaced withx * (1/y), which is useful if(1/y)is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value 🙈.
The default is -fno-reciprocal-math.
-ffinite-math-only— allows optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs 👀.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros— allows optimizations for floating-point arithmetic that ignore the signedness of zero 🌸. IEEE arithmetic specifies the behavior of distinct+0.0and-0.0values, which then prohibits simplification of expressions such asx+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.
-fno-trapping-math— compiles code assuming that floating-point operations cannot generate user-visible traps 🚨. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that-fno-signaling-nansbe in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💅.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS.
-frounding-math— disables transformations and optimizations that assume default floating-point rounding behavior 🎀. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99'sFENV_ACCESSpragma. This command-line option will be used along with-ftrapping-mathto specify the default state forFENV_ACCESS.
-fsignaling-nans— compiles code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations 😳. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies-ftrapping-math.
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined ✨.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact— do not allow the built-in functionsceil,floor,roundandtrunc, and theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments 💖. ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661 😭.
-fsingle-precision-constant— treats floating-point constants as single precision instead of implicitly converting them to double-precision constants 🌈.-fcx-limited-range— when enabled, this option states that a range reduction step is not needed when performing complex division 🦋. Also, there is no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case. The default is-fno-cx-limited-range, but is enabled by-ffast-math.
This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages 👑.
-fcx-fortran-rules— complex multiplication and division follow Fortran rules 🍓. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options 👀. This section includes experimental options that may produce broken code 💀.
-fbranch-probabilities— after running a program compiled with-fprofile-arcs, you can compile it a second time using-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken 🔥. When a program compiled with-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. See details about the file naming in-fprofile-arcs.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often ✨.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— if combined with-fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered 📊.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions— function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order 🌸.
Enabled with -fprofile-use.
-fvpt— if combined with-fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions 💜.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers— attempts to avoid false dependencies in scheduled code by making use of registers left over after register allocation 🎀. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register" 😭.
Enabled by default with -funroll-loops.
-fschedule-fusion— performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow ⚡.
Enabled at levels -O2, -O3, -Os.
-ftracer— performs tail duplication to enlarge superblock size 🦋. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— bestie, this one unrolls loops whose iteration count we can figure out at compile time, or right when we hit the loop's entry point. 💫 And the way it's a package deal:-funroll-loopsimplies-frerun-cse-after-loop,-fweband-frename-registers. It also flips on complete loop peeling, aka completely yeeting loops that have a small constant number of iterations. Heads up tho — this makes your code bigger, and it may or may not make it run faster. It's giving "we tried" energy. 🙈
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops— unrolls every single loop, even when we have zero clue how many iterations there'll be at loop entry. And like… this usually makes programs run more slowly. 🐌 It implies the same options as-funroll-loops. Not the slay you were hoping for, bestie.-fpeel-loops— peels loops where we have enough intel that they don't roll much (intel coming from profile feedback or static analysis). It also turns on complete loop peeling (yes, again: complete removal of loops with a small constant number of iterations). 🍌
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level-O1and higher, except for-Og. ✨-fmove-loop-stores— enables the loop store motion pass in the GIMPLE loop optimizer. This one moves invariant stores to after the end of the loop, and in exchange it carries the stored value in a register across the iteration. 🫶 Note bestie: for this option to actually do anything,-ftree-loop-imhas to be enabled too. Enabled at level-O1and higher, except for-Og.-fsplit-loops— splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. Split it, diva. 💅
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops— moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the result of the condition). Out of the loop, out of your mind. 🌈
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one. Obsessed, honestly. So like:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🧚
-ffunction-sections-fdata-sections— these two put each function or data item into its own section in the output file, if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. 📦
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies. vibes: unpredictable.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping). 🗑️
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.
⚠️ Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.
-fstdarg-opt— optimizes the prologue of variadic argument functions with respect to usage of those arguments. ✨-fsection-anchors— tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. ⚓
For example, the implementation of the following function foo:
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option. 🫠
-fzero-call-used-regs=choice— zeroes call-used registers at function return to increase program security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers. 🔒
The possible values of choice are the same as for the zero_call_used_regs attribute. The default is skip.
You can control this behavior for a specific function by using the function attribute zero_call_used_regs.
--param name=value— okay so in some places, GCC uses various constants to control how much optimization gets done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the--paramoption. 🎛️
🚨 The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options.
In each case, the value is an integer. The following choices of name are recognized for all targets:
predictable-branch-outcome— when a branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. 🔮max-rtl-if-conversion-insns— RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.max-rtl-if-conversion-predictable-cost— RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internalseq_costmetric. The compiler will try to provide a reasonable default for this parameter using theBRANCH_COSTtarget macro. 💸max-crossjump-edges— the maximum number of incoming edges to consider for cross-jumping. The algorithm used by-fcrossjumpingis O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. ⏳min-crossjump-insns— the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.max-grow-copy-bb-insns— the maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.max-goto-duplication-insns— the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more thanmax-goto-duplication-insnsare unfactored. 🌀max-delay-slot-insn-search— the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.max-delay-slot-live-search— when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. 📝max-gcse-memory— the approximate maximum amount of memory inkBthat can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.max-gcse-insertion-ratio— if the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.
max-pending-list-length— the max number of pending dependencies scheduling is allowed to pile up before it flushes the current state and starts over. Large functions with few branches or calls can create excessively large lists that just gobble memory and resources for no reason. 💀max-modulo-backtrack-attempts— the max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. so like, be careful bestie. 🫠max-inline-functions-called-once-loop-depth— the maximal loop depth of a call that inline heuristics consider when trying to inline all functions called once. 🌀max-inline-functions-called-once-insns— maximal estimated size of functions produced while inlining functions called once. 📏max-inline-insns-single— several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declaredinlineand methods implemented in a class declaration (C++). 💅max-inline-insns-auto— when you use-finline-functions(included in-O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declaredinlinecan be applied (--param max-inline-insns-auto). ✨max-inline-insns-small— this is bound applied to calls which are considered relevant with-finline-small-functions. 🎀max-inline-insns-size— this is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization opportunities exposed by inlining. 🦋uninlined-function-insns— number of instructions accounted by inliner for function overhead such as function prologue and epilogue. 📋uninlined-function-time— extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. ⏱️inline-heuristics-hint-percent— the scale (in percents) applied toinline-insns-single,inline-insns-single-O2,inline-insns-autowhen inline heuristics hints that inlining is very profitable (will enable later optimizations). 💡uninlined-thunk-insns,uninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-timebut applied to function thunks. 🧩inline-min-speedup— when estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on--param max-inline-insns-singleand--param max-inline-insns-auto. 🚀large-function-insns— the limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by--param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. 🐘large-function-growth— specifies maximal growth of large function caused by inlining in percents. For example, parameter value100limits large function growth to 2.0 times the original size. 📈large-unit-insns— the limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by--param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to--param large-unit-insnsbefore applying--param inline-unit-growth. 🏗️lazy-modules— maximum number of concurrently open C++ module files when lazy loading. 🐢inline-unit-growth— specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value20limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. ❄️ipa-cp-unit-growth— specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value10limits unit growth to 1.1 times the original size. 🌱ipa-cp-large-unit-insns— the size of translation unit that IPA-CP pass considers large. 🧠large-stack-frame— the limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. 🥞large-stack-frame-growth— specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value1000limits large stack frame growth to 11 times the original size. 🗻max-inline-insns-recursive,max-inline-insns-recursive-auto— specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. 🔁
--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead.
max-inline-recursive-depth,max-inline-recursive-depth-auto— specifies the maximum recursion depth used for recursive inlining. 🕳️
--param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead.
min-inline-recursive-probability— recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers. 😵💫
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents).
early-inlining-insns— specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty. 🌅max-early-inliner-iterations— limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. 🔗comdat-sharing-probability— probability (in percent) that C++ inline function with comdat visibility are shared across multiple compilation units. 🤝modref-max-bases,modref-max-refs,modref-max-accesses— specifies the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis. 🗂️modref-max-tests— specifies the maximal number of tests alias oracle can perform to disambiguate memory locations using the mod/ref information. This parameter ought to be bigger than--param modref-max-basesand--param modref-max-refs. 🔍modref-max-depth— specifies the maximum depth of DFS walk used by modref escape analysis. Setting to0disables the analysis completely. 🚫modref-max-escape-points— specifies the maximum number of escape points tracked by modref per SSA-name. 🎈modref-max-adjustments— specifies the maximum number the access range is enlarged during modref dataflow analysis. 📐profile-func-internal-id— a parameter to control whether to use function internal id in profile database lookup. If the value is0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. 🆔min-vect-loop-bound— the minimum number of iterations under which loops are not vectorized when-ftree-vectorizeis used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. 🔄gcse-cost-distance-ratio— scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less thangcse-unrestricted-cost. Specifying0disables hoisting of simple expressions. ⚖️gcse-unrestricted-cost— cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying0allows all expressions to travel unrestricted distances. 🛤️max-hoist-depth— the depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of0does not limit on the search, but may slow down compilation of huge functions. 🌳max-tail-merge-comparisons— the maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. 🧬max-tail-merge-iterations— how many times the pass is allowed to run over a function, max. It's literally just there to stop tree tail merging from eating your whole afternoon in compile time. 💅store-merging-allow-unaligned— gives the store merging pass the green light to introduce unaligned stores if it's legal to do so. Only if it's legal bestie, we are not committing crimes.max-stores-to-merge— the ceiling on how many stores the store merging pass will try to squish together into wider stores.max-store-chains-to-track— how many store chains get tracked at the same time while the pass tries to merge them into wider stores. The multitasking limit, basically.max-stores-to-track— the max number of stores tracked simultaneously while trying to merge them into wider stores in the store merging pass. Yes it's a different knob from the one above, they're siblings, they're both iconic. 🫶max-unrolled-insns— the most instructions a loop can have and still be unrolled. And if it does get unrolled, this param also decides how many times the loop code gets unrolled.max-average-unrolled-insns— same deal as above, except the instruction count is biased by the probabilities of their execution. Still also controls how many times the loop code is unrolled when unrolling happens.max-unroll-times— max number of unrollings for a single loop.max-peeled-insns— the most instructions a loop can have and still be peeled. And if it's peeled, this also sets how many times the loop code gets peeled.max-peel-times— max number of peelings of a single loop.max-peel-branches— max number of branches hanging out on the hot path through the peeled sequence.max-completely-peeled-insns— max insns of a completely peeled loop.max-completely-peel-times— max iterations of a loop for it to even be suitable for complete peeling.max-completely-peel-loop-nest-depth— max depth of a loop nest that's suitable for complete peeling.max-unswitch-insns— max insns of an unswitched loop.max-unswitch-depth— max depth of a loop nest that's getting unswitched.lim-expensive— the minimum cost an expression has to hit to count as "expensive" in loop invariant motion. 💸min-loop-cond-split-prob— when FDO profile info is available, this one sets the minimum probability threshold for a semi-invariant condition statement to trigger a loop split.iv-consider-all-candidates-bound— the bound on the number of induction variable candidates. Below it, every candidate gets considered for each use in induction variable optimizations. Above it? Only the most relevant ones get looked at, because otherwise we'd be doing quadratic time complexity and nobody has time for that. 😭iv-max-considered-uses— the induction variable optimizations straight up give up on loops that contain more induction variable uses than this.iv-always-prune-cand-set-bound— if the candidate set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. Tidy set, happy set. ✨avg-loop-niter— average number of iterations of a loop.dse-max-object-size— max size in bytes of objects tracked bytewise by dead store elimination. Heads up: larger values may result in larger compilation times.dse-max-alias-queries-per-store— max number of queries into the alias oracle per store. Larger values mean larger compilation times, but they may also mean more removed dead stores. Trade-offs, bestie.scev-max-expr-size— bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer down, periodt.scev-max-expr-complexity— bound on the complexity of expressions in the scalar evolutions analyzer. Same vibe: complex expressions slow the analyzer.max-tree-if-conversion-phi-args— max number of arguments in a PHI that TREE if conversion supports, unless the loop is marked with asimdpragma.vect-max-layout-candidates— max number of possible vector layouts (permutations and friends) to consider when optimizing code that's about to be vectorized.vect-max-version-for-alignment-checks— max number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.vect-max-version-for-alias-checks— max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.vect-max-peeling-for-alignment— max number of loop peels used to enhance access alignment for the vectorizer. A value of-1means no limit.max-iterations-to-track— max number of iterations of a loop that the brute-force algorithm for analyzing loop iteration counts will even attempt to evaluate.hot-bb-count-fraction— this is the denominator n of the fraction1/nof the maximal execution count of a basic block in the entire program, and a basic block needs to at least have that much to count as hot. Default is10000, meaning a basic block is hot if its execution count is greater than1/10000of the maximal execution count.0means it's never considered hot. Used in non-LTO mode.hot-bb-count-ws-permille— the number of most executed permilles, ranging from0to1000, of the profiled execution of the whole program that a basic block's execution count has to be part of to count as hot. Default is990, so a basic block is hot if its execution count contributes to the upper 990 permilles — that's 99.0% — of the profiled execution of the entire program.0means never hot. Used in LTO mode.hot-bb-frequency-fraction— the denominator n of the fraction1/nof the execution frequency of a function's entry block that a basic block of that function needs to at least have to be considered hot. Default is1000, meaning a basic block is hot within a function if it's executed more frequently than1/1000of the frequency of the function's entry block.0means never considered hot.unlikely-bb-count-fraction— the denominator n of the fraction1/nof the number of profiled runs of the entire program, below which a basic block's execution count has to fall for the block to be considered unlikely executed. Default is20, meaning a basic block counts as unlikely executed if it runs in fewer than1/20, or 5%, of the program's runs.0means it's always considered unlikely executed.max-predicted-iterations— the max number of loop iterations we predict statically. Super useful when a function has one loop with a known bound and another with an unknown bound: the known iteration count gets predicted correctly, while the unknown one averages out to roughly 10. So the loop without bounds ends up looking artificially cold next to the other one. 🙈builtin-expect-probability— controls the probability of the expression having the specified value. Takes a percentage as input, so0 ... 100.builtin-string-cmp-inline-length— the max length of a constant string for a builtin string cmp call to be eligible for inlining.align-threshold— pick the fraction of the maximal frequency of executions of a basic block in a function at which to align that basic block.align-loop-iterations— a loop that's expected to iterate at least the selected number of iterations gets aligned. 📐tracer-dynamic-coveragetracer-dynamic-coverage-feedback— this value limits superblock formation once the given percentage of executed instructions is covered. It's there to stop unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter only kicks in when profile feedback is available. Real profiles (as opposed to statically estimated ones) are much less balanced, which lets the threshold be a larger value.
tracer-max-code-growth— stop tail duplication once code growth hits the given percentage. Kinda an artificial limit tbh, since most duplicates get eliminated later in cross jumping, so you can set it way higher than the code growth you actually want.tracer-min-branch-ratio— stop reverse growth when the reverse probability of the best edge drops below this threshold, in percent.tracer-min-branch-probabilitytracer-min-branch-probability-feedback— stop forward growth if the best edge has a probability lower than this threshold.
Just like tracer-dynamic-coverage, there are two parameters here. tracer-min-branch-probability-feedback is for compiling with profile feedback, tracer-min-branch-probability is for compiling without it. The value for profile-feedback compilation needs to be more conservative (higher) to make the tracer effective.
stack-clash-protection-guard-size— sets the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 💀stack-clash-protection-probe-interval— stack clash protection works by probing stack space as it gets allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but again — a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.max-cse-path-length— the max number of basic blocks chilling on a path that CSE will even look at. 💅max-cse-insns— the max number of instructions CSE processes before it flushes. like a snack break but for the compiler.ggc-min-expand— okay so GCC uses a garbage collector to manage its own memory allocation, main character energy fr. This parameter sets the minimum percentage by which the GC's heap is allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC can't calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging. 🐛
ggc-min-heapsize— minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands byggc-min-expand%beyondggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns— the max number of instruction reload should look backward for an equivalent register. Bigger values = more aggressive optimization, so compile time goes up but performance is probably slightly better. bestie it's a trade-off. 🎀max-cselib-memory-locations— the max number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance.max-sched-ready-insns— the max number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. 💀max-sched-region-blocks— the max number of blocks in a region to be considered for interblock scheduling.max-pipeline-region-blocks— the max number of blocks in a region to be considered for pipelining in the selective scheduler.max-sched-region-insns— the max number of insns in a region to be considered for interblock scheduling.max-pipeline-region-insns— the max number of insns in a region to be considered for pipelining in the selective scheduler.min-spec-prob— the minimum probability (in percents) of reaching a source block for interblock speculative scheduling.max-sched-extend-regions-iters— the max number of iterations through CFG to extend regions. A value of0disables region extensions.max-sched-insn-conflict-delay— the max conflict delay for an insn to be considered for speculative motion.sched-spec-prob-cutoff— the minimal probability of speculation success (in percents), so that speculative insns are scheduled.sched-state-edge-prob-cutoff— the minimum probability an edge must have for the scheduler to save its state across it.sched-mem-true-dep-cost— minimal distance (in CPU cycles) between store and load targeting same memory locations.selsched-max-lookahead— the max size of the lookahead window of selective scheduling. It is a depth of search for available instructions.selsched-max-sched-times— the max number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.selsched-insns-to-rename— the max number of best instructions in the ready list that are considered for renaming in the selective scheduler.sms-min-sc— the minimum value of stage count that swing modulo scheduler generates.max-last-value-rtl— the max size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register.max-combine-insns— the max number of instructions the RTL combiner tries to combine.integer-share-limit— small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed ✨. This sets the max value of a shared integer constant.ssp-buffer-size— the minimum size of buffers (i.e. arrays) that receive stack smashing protection when-fstack-protectoris used.
This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of functions protected by the stack protector. 🛡️
min-size-for-stack-sharing— the minimum size of variables taking part in stack slot sharing when not optimizing.max-jump-thread-duplication-stmts— max number of statements allowed in a block that needs to be duplicated when threading jumps.max-jump-thread-paths— the max number of paths to consider when searching for jump threading opportunities. When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider.max-fields-for-field-sensitive— max number of fields in a structure treated in a field sensitive manner during pointer analysis.prefetch-latency— estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (seesimultaneous-prefetches).simultaneous-prefetches— max number of prefetches that can run at the same time.l1-cache-line-size— the size of cache line in L1 data cache, in bytes.l1-cache-size— the size of L1 data cache, in kilobytes.l2-cache-size— the size of L2 data cache, in kilobytes.prefetch-dynamic-strides— whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride.
prefetch-minimum-stride— minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, prefetch hints will not be issued.
This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers.
A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and constant.
destructive-interference-sizeconstructive-interference-size
The values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end. 🦋
The destructive interference size is intended to be used for layout, and thus has ABI impact. The default value is not expected to be stable, and on some targets varies with -mtune, so use of this variable in a context where ABI stability is important, such as the public interface of a library, is strongly discouraged; if it is used in that context, users can stabilize the value using this option.
The constructive interference size is less sensitive, as it is typically only used in a static_assert to make sure that a type fits within a cache line.
See also -Winterference-size. 👑
loop-interchange-max-num-stmts— the max number of stmts in a loop to be interchanged.loop-interchange-stride-ratio— the minimum ratio between stride of two loops for interchange to be profitable.min-insn-to-prefetch-ratio— the minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.prefetch-min-insn-to-mem-ratio— the minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop. 🌸use-canonical-types— okay so this one decides whether the compiler uses the "canonical" type system. bestie, it should always be1, because that's the more efficient internal mechanism for comparing types in C++ and Objective-C++ ✨. BUT — if bugs in the canonical type system are making your compilations fail, flip this to0to disable canonical types. dramatic rescue move 💅switch-conversion-max-branch-ratio— switch initialization conversion is out here refusing to create arrays bigger than this value times the number of branches in the switch. it's giving ✋ "no, we're not doing that"max-partial-antic-length— the maximum length of the partial antic set computed during tree partial redundancy elimination optimization (-ftree-pre) when optimizing at-O3and above. so like, for some kinds of source code the enhanced partial redundancy elimination optimization can just run away and eat ALL the memory on your host machine 😭. this parameter puts a limit on the length of the sets computed, which stops the runaway behavior. setting it to0= unlimited set length (unhinged behavior, but okay).rpo-vn-max-loop-depth— the maximum 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 get value-numbered optimistically, and the remaining ones do not.sccvn-max-alias-queries-per-access— the max number of alias-oracle queries we do when hunting for redundancies in loads and stores 🔍. if this limit gets hit, the search is aborted and the load or store is just not considered redundant. fun fact: the number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. slay.ira-max-loops-num— IRA uses regional register allocation by default 🎀. if a function has more loops than the number given here, only at most the given number of the most frequently-executed loops form regions for regional register allocation.ira-max-conflict-table-size— so IRA uses a fancy algorithm to compress the conflict table, but the table can still need excessive amounts of memory for huge functions 💀. if the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that doesn't need to build a pseudo-register conflict table.ira-loop-reserved-regs— IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see-O3). the number of available registers reserved for some other purposes is given by this parameter. default = the best found from numerous experiments 🔬.ira-consider-dup-in-all-alts— makes IRA consider matching constraint (duplicated operand number) heavily in all available alternatives for preferred register class. if set to zero, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. otherwise, IRA will check all available alternatives for preferred register class even if it already found some choice with an appropriate register class, and respect the found qualified matching constraint 👑ira-simple-lra-insn-threshold— approximate function insn number in 1K units that triggers simple local RA.lra-inheritance-ebb-probability-cutoff— LRA tries to reuse values reloaded in registers in subsequent insns, and this optimization is called inheritance ✨. EBB is used as a region to do this optimization. the parameter defines a minimal fall-through edge probability (in percentage) used to add BB to inheritance EBB in LRA. the default value was chosen from numerous runs of SPEC2000 on x86-64.loop-invariant-max-bbs-in-loop— loop invariant motion can be very expensive, both in compilation time and in the amount of needed compile-time memory, with very large loops 😮💨. loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them.loop-max-datarefs-for-datadeps— building data dependencies is expensive for very large loops. this parameter limits the number of data references in loops that are considered for data dependence analysis. these large loops are not handled by the optimizations using loop data dependencies.max-vartrack-size— sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. if this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. if the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. setting the parameter to zero makes it unlimited 💫.max-vartrack-expr-depth— sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. this trades compilation time for more complete debug information. if it's set too low, value expressions that ARE available and could be represented in debug information may end up not being used 😭; setting it higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow.max-debug-marker-count— sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL 💥. if a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion.min-nondebug-insn-uid— use uids starting at this parameter for nondebug insns. the range below the parameter is reserved exclusively for debug insns created by-fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted 🫶.ipa-sra-deref-prob-threshold— IPA-SRA replaces a pointer which is known not to be NULL with one or more new parameters only when the probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter.ipa-sra-ptr-growth-factor— IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less than or equal toipa-sra-ptr-growth-factortimes the size of the original pointer parameter.ipa-sra-ptrwrap-growth-factor— additional maximum allowed growth of total size of new parameters that ipa-sra replaces a pointer to an aggregate with, if it points to a local variable that the caller only writes to and passes it as an argument to other functions.ipa-sra-max-replacements— maximum pieces of an aggregate that IPA-SRA tracks. as a consequence, it's also the maximum 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 replace scalar parts of aggregates with uses of independent scalar variables. these parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. it's giving dual-wield energy ⚔️sra-max-propagations— the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation.tm-max-aggregate-size— when making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. this option only applies when using-fgnu-tm.graphite-max-nb-scop-params— to avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded 📏. a value of zero can be used to lift the bound. a variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.hardcfr-max-blocks— disable-fharden-control-flow-redundancyfor functions with a larger number of blocks than the specified value. zero removes any limit 🌈.hardcfr-max-inline-blocks— force-fharden-control-flow-redundancyto use out-of-line checking for functions with a larger number of basic blocks than the specified value.loop-block-tile-size— loop blocking or strip mining transforms, enabled with-floop-blockor-floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. the strip length can be changed using theloop-block-tile-sizeparameter 🎚️.ipa-jump-function-lookups— specifies the number of statements visited during jump function offset discovery.ipa-cp-value-list-size— IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization.ipa-cp-value-list-sizeis the maximum number of values and types it stores per one formal parameter of a function 👑.ipa-cp-eval-threshold— IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceedipa-cp-eval-threshold.ipa-cp-max-recursive-depth— maximum depth of recursive cloning for self-recursive function 🌀.ipa-cp-min-recursive-probability— recursive cloning only when the probability of the call being executed exceeds the parameter.ipa-cp-profile-count-base— bestie, when you're using-fprofile-use, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation. it's giving data-driven vibes ✨ipa-cp-recursive-freq-factor— the number of times interprocedural copy propagation expects recursive functions to call themselves. like, how many times does it think the function is gonna loop back on itself? 🔁ipa-cp-recursion-penalty— percentage penalty the recursive functions will receive when they are evaluated for cloning. so if it's recursive, it gets a lil demerit 💀ipa-cp-single-call-penalty— percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. one call? penalty. harsh but ok 🫠ipa-max-agg-items— IPA-CP is also capable to propagate a number of scalar values passed in an aggregate.ipa-max-agg-itemscontrols the maximum number of such values per one parameter. so like, how many lil scalar values can ride along in one aggregate? 🎀ipa-cp-loop-hint-bonus— when IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus ofipa-cp-loop-hint-bonusto the profitability score of the candidate. knowing loop iterations? that's a slay, here's bonus points 💅ipa-max-loop-predicates— the maximum number of different predicates IPA will use to describe when loops in a function have known properties. like, how many different ways can it say "this loop is predictable"? 🔮ipa-max-aa-steps— during its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters. In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examiningipa-max-aa-stepsstatements modifying memory. so it's like, after this many memory-modifying statements, it's like "aight imma head out, everything's clobbered" 😭ipa-max-switch-predicate-bounds— maximal number of boundary endpoints of case ranges of switch statement. For switch exceeding this limit, IPA-CP will not construct cloning cost predicate, which is used to estimate cloning benefit, for default case of the switch statement. so if your switch is too extra, no predicate for the default case 🚫ipa-max-param-expr-ops— IPA-CP will analyze conditional statement that references some function parameter to estimate benefit for cloning upon certain constant value. But if number of operations in a parameter expression exceedsipa-max-param-expr-ops, the expression is treated as complicated one, and is not handled by IPA analysis. too many ops? it's complicated, not touching it 🙅♀️lto-partitions— specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. so like, more partitions than CPUs, got it? 👑lto-min-partition— size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions. don't wanna split tiny programs into a million pieces, that's wasteful 💸lto-max-partition— size of max partition for WHOPR (in estimated instructions). to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning. upper bound, only for balanced partitioning, periodt ⚖️lto-max-streaming-parallelism— maximal number of parallel processes used for LTO streaming. how many processes can stream in parallel? this many 🚀cxx-max-namespaces-for-diagnostic-help— the maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. so when you typo a name, it'll look through this many namespaces to help a bestie out 🔍sink-frequency-threshold— the maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink. so like, how far can a statement sink? higher number = more aggressive sinking. memory operands get a lil boost because they're extra profitable 💧max-stores-to-sink— the maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. if you turn off vectorization or if-conversion, set this to 0, ok? 🛑case-values-threshold— the smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. so like, when do we switch to a jump table? when there's at least this many cases. 0 means let the machine decide 🎰jump-table-max-growth-ratio-for-size— the maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when optimizing for size. optimizing for size? here's the max growth ratio for jump tables 📏jump-table-max-growth-ratio-for-speed— the maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when optimizing for speed. optimizing for speed? here's the max growth ratio for jump tables 🏎️tree-reassoc-width— set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value. so if it's non-zero, it overrides the target's default heuristics. parallel instructions go brrr 🌀sched-pressure-algorithm— choose between the two available implementations of-fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. Seehaifa-sched.ccin the GCC sources for more details. The default choice depends on the target. so algorithm 1 = original, conservative, prevents reordering. algorithm 2 = compromise, needs regular register file and accurate pressure classes. default depends on target. checkhaifa-sched.ccfor tea ☕max-slsr-cand-scan— set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. how many candidates do we scan? this many 🔎asan-globals— enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using-fsanitize=addressoption. To disable global objects protection use--param asan-globals=0. global objects get buffer overflow detection by default with-fsanitize=address. turn it off with--param asan-globals=0🛡️asan-stack— enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using-fsanitize=address. To disable stack protection use--param asan-stack=0option. stack objects get protection by default with-fsanitize=address. disable with--param asan-stack=0📚asan-instrument-reads— enable buffer overflow detection for memory reads. This kind of protection is enabled by default when using-fsanitize=address. To disable memory reads protection use--param asan-instrument-reads=0. memory reads get checked by default with-fsanitize=address. disable with--param asan-instrument-reads=0👀asan-instrument-writes— enable buffer overflow detection for memory writes. This kind of protection is enabled by default when using-fsanitize=address. To disable memory writes protection use--param asan-instrument-writes=0option. memory writes get checked by default with-fsanitize=address. disable with--param asan-instrument-writes=0✍️asan-memintrin— enable detection for built-in functions. This kind of protection is enabled by default when using-fsanitize=address. To disable built-in functions protection use--param asan-memintrin=0. built-in functions get detection by default with-fsanitize=address. disable with--param asan-memintrin=0🧠asan-use-after-return— enable detection of use-after-return. This kind of protection is enabled by default when using the-fsanitize=addressoption. To disable it use--param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable it, add"detect_stack_use_after_return=1"to the environment variableASAN_OPTIONS.
use-after-return detection is on by default with -fsanitize=address. disable with --param asan-use-after-return=0. but heads up: the check is disabled at run time by default. to enable it, add "detect_stack_use_after_return=1" to ASAN_OPTIONS env var. ⚠️
asan-instrumentation-with-call-threshold— if number of memory accesses in function being instrumented is greater or equal to this number, use callbacks instead of inline checks. E.g. to disable inline code use--param asan-instrumentation-with-call-threshold=0. if memory accesses >= this number, use callbacks instead of inline checks. to disable inline code, set it to 0. 📞asan-kernel-mem-intrinsic-prefix— if nonzero, prefix calls to"memcpy","memset"and"memmove"with__asan_or__hwasan_for-fsanitize=kernel-addressor-fsanitize=kernel-hwaddress, respectively. nonzero? then prefix those mem calls with__asan_or__hwasan_depending on the sanitizer. 🏷️hwasan-instrument-stack— enable hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is enabled by default when using-fsanitize=hwaddressand disabled by default when using-fsanitize=kernel-hwaddress. To disable stack instrumentation use--param hwasan-instrument-stack=0, and to enable it use--param hwasan-instrument-stack=1. statically sized stack vars get hwasan instrumentation by default with-fsanitize=hwaddress, but disabled by default with-fsanitize=kernel-hwaddress. disable with--param hwasan-instrument-stack=0, enable with--param hwasan-instrument-stack=1. 📦hwasan-random-frame-tag— when using stack instrumentation, decide tags for stack variables using a deterministic sequence beginning at a random tag for each frame. With this parameter unset tags are chosen using the same sequence but beginning from 1. This is enabled by default for-fsanitize=hwaddressand unavailable for-fsanitize=kernel-hwaddress. To disable it use--param hwasan-random-frame-tag=0. stack instrumentation? tags for stack vars use a deterministic sequence starting at a random tag per frame. unset? same sequence but from 1. enabled by default for-fsanitize=hwaddress, unavailable for-fsanitize=kernel-hwaddress. disable with--param hwasan-random-frame-tag=0. 🎲hwasan-instrument-allocas— enable hwasan instrumentation of dynamically sized stack-allocated variables. This kind of instrumentation is enabled by default when using-fsanitize=hwaddressand disabled by default when using-fsanitize=kernel-hwaddress. To disable instrumentation of such variables use--param hwasan-instrument-allocas=0, and to enable it use--param hwasan-instrument-allocas=1. dynamically sized stack vars get hwasan instrumentation by default with-fsanitize=hwaddress, disabled by default with-fsanitize=kernel-hwaddress. disable with--param hwasan-instrument-allocas=0, enable with--param hwasan-instrument-allocas=1. 📦hwasan-instrument-reads— enable hwasan checks on memory reads. Instrumentation of reads is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable checking memory reads use--param hwasan-instrument-reads=0. hwasan checks on memory reads are on by default for both sanitizers. disable with--param hwasan-instrument-reads=0. 👀hwasan-instrument-writes— enable hwasan checks on memory writes. Instrumentation of writes is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable checking memory writes use--param hwasan-instrument-writes=0. hwasan checks on memory writes are on by default for both sanitizers. disable with--param hwasan-instrument-writes=0. ✍️hwasan-instrument-mem-intrinsics— enable hwasan instrumentation of builtin functions. Instrumentation of these builtin functions is enabled by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable instrumentation of builtin functions use--param hwasan-instrument-mem-intrinsics=0. hwasan instrumentation of builtin functions is on by default for both sanitizers. disable with--param hwasan-instrument-mem-intrinsics=0. 🧠use-after-scope-direct-emission-threshold— if the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. local variable size <= this number? directly poison/unpoison shadow memory instead of runtime callbacks. ☠️tsan-distinguish-volatile— emit special instrumentation for accesses to volatiles. special instrumentation for volatile accesses? yes please. ⚡tsan-instrument-func-entry-exit— emit instrumentation calls to__tsan_func_entry()and__tsan_func_exit(). instrumentation calls to__tsan_func_entry()and__tsan_func_exit(). 📞max-fsm-thread-path-insns— the max number of instructions to copy when duplicating blocks on a finite state automaton jump thread path. literally just a cap so things don’t go feral. 🧵threader-debug— set it asthreader-debug=[none|all]and boom, you get verbose dumping of the threader solver. very “let me see the receipts” energy. 👀parloops-chunk-size— chunk size of omp schedule for loops parallelized by parloops. chunk it up, bestie. 🍰parloops-schedule— schedule type of omp schedule for loops parallelized by parloops. your choices arestatic,dynamic,guided,auto,runtime. pick your vibe. ✨parloops-min-per-thread— the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. note that for a parallelized loop nest the minimum number of iterations of the outermost loop per thread is two. so basically: two, no cap. 2️⃣max-ssa-name-query-depth— max depth of recursion when querying properties of SSA names in things like fold routines. one level of recursion corresponds to following a use-def chain. don’t go too deep or you’ll get lost. 🌀max-speculative-devirt-maydefs— the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. say that five times fast, i dare you. 💀ranger-debug— specifies the type of debug output to be issued for ranges. ranger, debug, slay. 🏹unroll-jam-min-percent— the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. gotta hit the percentage or it’s not worth it. 💯unroll-jam-max-unroll— the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. cap it. 🔁max-rtl-if-conversion-unpredictable-cost— maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. unpredictable branch? not on this budget. 💸max-variable-expansions-in-unroller— if-fvariable-expansion-in-unrolleris used, this is the maximum number of times that an individual variable will be expanded during loop unrolling. know the limit. 📈partial-inlining-entry-probability— maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen. percentages, percentages. 🎲max-tracked-strlens— maximum number of strings for which strlen optimization pass will track string lengths. string girlies, this one’s for you. 🧶gcse-after-reload-partial-fraction— the threshold ratio for performing partial redundancy elimination after reload. fraction energy. 🧮gcse-after-reload-critical-fraction— the threshold ratio of critical edges execution count that permit performing redundancy elimination after reload. critical edges, critical decisions. ⚠️max-loop-header-insns— the maximum number of insns in loop header duplicated by the copy loop headers pass. don’t duplicate too much, okay? 📋vect-epilogues-nomask— enable loop epilogue vectorization using smaller vector size. smaller vector, still cute. 🐣vect-partial-vector-usage— controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code.0stops the vectorizer from ever using partial vector loads and stores.1allows partial vector loads and stores if vectorization removes the need for the code to iterate.2allows partial vector loads and stores in all loops. the parameter only has an effect on targets that support partial vector loads and stores. so0,1, or2— choose your fighter. 🥊vect-inner-loop-cost-factor— the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized. the factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. the default value of this parameter is50. fifty, iconic. 5️⃣0️⃣vect-induction-float— enable loop vectorization of floating point inductions. float like a butterfly. 🦋vrp-sparse-threshold— maximum number of basic blocks before VRP uses a sparse bitmap cache. sparse but make it fashion. ✨vrp-switch-limit— maximum number of outgoing edges in a switch before VRP will not process it. switch it up, but only so much. 🔀vrp-vector-threshold— maximum number of basic blocks for VRP to use a basic cache vector. vector cache, okay. 🧊avoid-fma-max-bits— maximum number of bits for which we avoid creating FMAs. no FMAs for you. 🙅fully-pipelined-fma— whether the target fully pipelines FMA instructions. if non-zero, reassociation considers the benefit of parallelizing FMA’s multiplication part and addition part, assuming FMUL and FMA use the same units that can also do FADD. pipeline it, periodt. 🚰sms-loop-average-count-threshold— a threshold on the average loop count considered by the swing modulo scheduler. average loop count, but make it a threshold. 📊sms-dfa-history— the number of cycles the swing modulo scheduler considers when checking conflicts using DFA. history is important, bestie. 📜graphite-allow-codegen-errors— whether codegen errors should be ICEs when-fchecking. so, should it crash hard? this decides. 🧊sms-max-ii-factor— a factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop. tune it. 🎛️lra-max-considered-reload-pseudos— the max number of reload pseudos which are considered during spilling a non-reload pseudo. pseudo-spilling drama. 💅max-pow-sqrt-depth— maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. square roots, deep. √max-dse-active-local-stores— maximum number of active local stores in RTL dead store elimination. dead stores? eliminated. 🗑️asan-instrument-allocas— enable asan allocas/VLAs protection. asan said protect the allocas and VLAs. 🛡️max-iterations-computation-cost— bound on the cost of an expression to compute the number of iterations. cost bound, okay. 💰max-isl-operations— maximum number of isl operations,0means unlimited. zero means go wild. 🎢graphite-max-arrays-per-scop— maximum number of arrays per scop. arrays, but per scop. 📦max-vartrack-reverse-op-size— max. size of loc list for which reverse ops should be added. max size, reverse ops. 🔄fsm-scale-path-stmts— scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing to--param=max-jump-thread-duplication-stmts. scaling and comparing, very demure. ⚖️uninit-control-dep-attempts— maximum number of nested calls to search for control dependencies during uninitialized variable analysis. uninitialized variable girlies, this is your limit. 🕵️uninit-max-chain-len— maximum number of predicates anded for each predicate ored in the normalized predicate chain. predicates anded, predicates ored. logic core. 🧠uninit-max-num-chains— maximum number of predicates ored in the normalized predicate chain. chain limit. ⛓️sched-autopref-queue-depth— hardware autoprefetcher scheduler model control flag. number of lookahead cycles the model looks into; at' 'only enable instruction sorting heuristic. note that' 'is literally a space in quotes, bestie. 👀loop-versioning-max-inner-insns— the maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy. too big? not copying. 🚫loop-versioning-max-outer-insns— the maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning. outer loop limit, with a discount. 🏷️ssa-name-def-chain-limit— the maximum number of SSA_NAME assignments to follow in determining a property of a variable such as its value. this limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics. so it’s a chain limit for SSA_NAME assignments. ⛓️store-merging-max-size— maximum size of a single store merging region in bytes. bytes, bestie. 📏hash-table-verification-limit— the number of elements for which hash table verification is done for each searched element. verify, but only this many. ✅max-find-base-term-values— maximum number of VALUEs handled during a single find_base_term call. VALUEs, capitalized, period. 💎analyzer-max-enodes-per-program-point— the maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point. exploded nodes? yes, that’s the term. 💥analyzer-max-constraints— the maximum number of constraints per state. constraints per state, okay. 🚧analyzer-min-snodes-for-call-summary— the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites. supernodes! 🦸analyzer-max-enodes-for-full-dump— the maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose format. dot dump, but not too much. 📉analyzer-max-recursion-depth— the maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper. recursion depth limit, no infinite vibes. 🌀analyzer-max-svalue-depth— the maximum depth of a symbolic value, before approximating the value as unknown. symbolic value depth, then unknown. ❓analyzer-max-infeasible-edges— the maximum number of infeasible edges to reject before declaring a diagnostic as infeasible. reject infeasible edges, then declare infeasible. 🤷gimple-fe-computed-hot-bb-threshold— the number of executions of a basic block which is considered hot. the parameter is used only in GIMPLE FE. hot bb threshold, only in GIMPLE FE. 🔥analyzer-bb-explosion-factor— the maximum number of ‘after supernode’ exploded nodes within the analyzer per supernode, before terminating analysis. ‘after supernode’ in quotes, keep it. 💥analyzer-text-art-string-ellipsis-threshold— the number of bytes at which to ellipsize string literals in analyzer text art diagrams. text art, but make it ellipsized. ✂️analyzer-text-art-ideal-canvas-width— the ideal width in characters of text art diagrams generated by the analyzer. ideal width, in characters. 🎨analyzer-text-art-string-ellipsis-head-len— the number of literal bytes to show at the head of a string literal in text art when ellipsizing it. head bytes. 👑analyzer-text-art-string-ellipsis-tail-len— the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it. tail bytes. 🐒ranger-logical-depth— maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges. logical depth, ranger edition. 🧙ranger-recompute-depth— maximum depth of instruction chains to consider for recomputation in the outgoing range calculator. recompute depth. 🔁relation-block-limit— maximum number of relations the oracle will register in a basic block. the oracle registers relations, and this is the limit. 🔮
min-pagesize
Minimum page size for warning purposes. 🎀
openacc-kernels
Specify mode of OpenACC kernels constructs handling. With --param=openacc-kernels=decompose, OpenACC kernels constructs are decomposed into parts, a sequence of compute constructs, each then handled individually. This is work in progress. With --param=openacc-kernels=parloops, OpenACC kernels constructs are handled by the parloops pass, en bloc. This is the current default. 💅
openacc-privatization
Control whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options emit OpenACC privatization diagnostics. With --param=openacc-privatization=quiet, don't diagnose. This is the current default. With --param=openacc-privatization=noisy, do diagnose. 👑
The following choices of name are available on AArch64 targets:
aarch64-vect-compare-costs
When vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. This includes:
- Trying both SVE and Advanced SIMD, when SVE is available.
- Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
- Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.
aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1. 🧚
aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal for double type. The precision of division is propotional to this param when division approximation is enabled. The default value is 2. (yes bestie, "propotional" is literally what the page says, I'm not fixing it, that's a documentation moment fr fr 💀)
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4, inclusive. ✨
0— Use the default heuristics.1— Use only Advanced SIMD for auto-vectorization.2— Use only SVE for auto-vectorization.3— Use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.4— Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
The default value is 0.
aarch64-ldp-policy
Fine-grained policy for load pairs. With --param=aarch64-ldp-policy=default, use the policy of the tuning structure. This is the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if the source pointer is aligned to at least double the alignment of the type. 💅
aarch64-stp-policy
Fine-grained policy for store pairs. With --param=aarch64-stp-policy=default, use the policy of the tuning structure. This is the current default. With --param=aarch64-stp-policy=always, emit stp regardless of alignment. With --param=aarch64-stp-policy=never, do not emit stp. With --param=aarch64-stp-policy=aligned, emit stp only if the source pointer is aligned to at least double the alignment of the type. 🫶
aarch64-ldp-alias-check-limit
Limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to form an ldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, at the expense of increased compile time. ⏳
aarch64-ldp-writeback
Param to control which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass. A value of zero disables writeback handling. One means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of two means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair.
aarch64-loop-vect-issue-rate-niters
The tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations. 🌈
aarch64-vect-unroll-limit
The vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter set's the upper bound of how much the vectorizer will unroll the main loop. The default value is four. 🔥
The following choices of name are available on GCN targets:
gcn-preferred-vectorization-factor
Preferred vectorization factor: default, 32, 64. 🦋
The following choices of name are available on i386 and x86_64 targets:
x86-stlf-window-ninsns
Instructions number above which STFL stall penalty can be compensated.
x86-stv-max-visits
The maximum number of use and def visits when discovering a STV chain before the discovery is aborted.
🌸 Program Instrumentation Options 🌸
GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. 🕵️♀️
-p-pg— Generate extra code to write profile information suitable for the analysis 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— Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable.
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. 😭 (this naming thing gets people every single time, bookmark it)
-fcondition-coverage— Add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read withgcov --conditions. ⭐--coverage— This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for-fprofile-arcs-ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details.- Compile the source files with
-fprofile-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.
- Compile the source files with
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.
- 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 thegcovdocumentation for further information. 👑
No cap: that last bullet is the whole payoff — you run it, then you read it with gcov, and suddenly you know exactly which lines of your code never got touched. Iconic. 💖
okay bestie we are still deep in -fprofile-arcs territory and honestly? it's giving detective energy 🔍
With -fprofile-arcs, for every function of your program, GCC creates a program flow graph, and then it finds a spanning tree for that graph. Here's the tea: only the arcs that are not on the spanning tree have to be instrumented — the compiler adds code to count the number of times those arcs get executed. And when an arc is the only exit or only entrance to a block, the instrumentation code can just be added to that block. Otherwise? A whole new basic block must be created to hold the instrumentation code. no cap, that's just how it goes 💅
And with -fcondition-coverage, for each conditional in your program, GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression. cute, right? 🎀
-ftest-coverage— this one produces a notes file that thegcovcode-coverage utility can use to show program coverage. Each source file's note file is calledauxname.gcno. Scroll back up to the-fprofile-arcsoption for the whole explanation ofauxnameplus the instructions on how to generate test coverage data. Also, real talk: coverage data matches the source files more closely if you do not optimize. ✨-fprofile-abs-path— automatically converts relative source file names into absolute path names in the.gcnofiles. Why do you care? Because it letsgcovfind the correct sources in projects where compilations happen with different working directories. absolute queen behaviour 👑-fprofile-dir=path— sets the directory to search for the profile data files to path. This option only affects the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-arcs, and used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used, we're flexible like that. By default, GCC uses the current directory as path, so the profile data file ends up in the same directory as the object file. Now, to prevent the file name clashing: if the object file name is not an absolute path, we mangle the absolute path of thesourcename.gcdafile and use it as the file name of a.gcdafile. See details about the file naming in-fprofile-arcs. See similar option-fprofile-note. 🦋
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p— process ID.%q{VAR}— value of environment variableVAR.
-fprofile-generate,-fprofile-generate=path— enables the 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. both, bestie, don't skip steps 🙈
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
-fprofile-info-section,-fprofile-info-section=name— registers the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to".gcov_info". A pointer to the profile information generated by-fprofile-arcsis 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. Heads up: this option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O. 🌸
The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}The program could dump the profiling information registered in this linker set for example like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the "__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge the profile information into .gcda files on the host filesystem. 🧚
-fprofile-note=path— if path is specified, GCC saves.gcnofile into path location. ⚠️ But if you combine the option with multiple source files, the.gcnofile will be overwritten. you've been warned.-fprofile-prefix-path=path— this option can be used in combination withprofile-generate=profile_dirandprofile-use=profile_dirto inform GCC where the base directory of the built source tree is. By default,profile_dirwill contain files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because the profile data will not be found during the optimized build. In such setups,-fprofile-prefix-path=pathwith path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. ✨-fprofile-prefix-map=old=new— when compiling files residing in directory old, record profiling information (with--coverage) describing them as if the files resided in directory new instead. See also-ffile-prefix-mapand-fcanon-prefix-map.-fprofile-update=method— alters the update method for an application instrumented for profile feedback based optimization. The method argument should be one ofsingle,atomicorprefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. 💅
Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.
If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications. 😭
For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state. fr fr, that's the fine print.
-fprofile-filter-files=regex— instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example,-fprofile-filter-files=main\.c;module.*\.cwill instrument onlymain.cand all C files starting with'module'. 🎀-fprofile-exclude-files=regex— instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons). For example,-fprofile-exclude-files=/usr/.*will prevent instrumentation of all files that are located in the/usr/folder. bye bye/usr/👋-fprofile-reproducible=[multithreaded|parallel-runs|serial]— controls the level of reproducibility of the profile gathered by"-fprofile-generate". This makes it possible to rebuild a program with the same outcome, which is useful for, say, distribution packages. 📦
With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run but also before "fork" function is invoked.
okay so real talk: execution counts for some parts of your program can literally depend on stuff like how long your temp file names are, or memory space randomization (which can mess with hash-table collision rates, bestie 💀). that means that part of your program is just... not reproducible. rude of it. you can tag it with the "no_instrument_function" function attribute to opt out. and gcov-dump with -l can dump the gathered data so you can go verify that yeah, they really are reproducible. iykyk 🔍
with -fprofile-reproducible=parallel-runs, the collected profile stays reproducible no matter the order the data gets streamed into the gcda files. which means you can run multiple instances of your instrumented program in parallel — like, say, with "make -j". slay for build times 🎀 but heads up: this reduces the quality of the gathered data, especially indirect call profiling. tradeoffs, we love them.
💖 -fsanitize=address
turn on AddressSanitizer, a fast memory error detector. memory access instructions get instrumented so it can catch out-of-bounds and use-after-free bugs. this option also enables -fsanitize-address-use-after-scope. more deets at <https://github.com/google/sanitizers/wiki/AddressSanitizer>. ✨
the run-time behavior can be influenced with the ASAN_OPTIONS environment variable. set it to "help=1" and the available options get shown at startup of the instrumented program. the full list of supported options lives at <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>. 💅
ngl, you cannot combine this option with-fsanitize=threador-fsanitize=hwaddress. also, the only target-fsanitize=hwaddressis currently supported on is AArch64.
want more accurate stack traces? try options like -O0, -O1, or -Og (which, for instance, prevent most function inlining), or -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls — and this one is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). and since multiple runs of the program may give you backtraces with different addresses thanks to ASLR (Address Space Layout Randomization), you might wanna turn ASLR off. on Linux you do that with setarch \uname -m` -R ./prog`. 🐧
💖 -fsanitize=kernel-address
turns on AddressSanitizer for the Linux kernel. see <https://github.com/google/kernel-sanitizers> for more. 🧚
💖 -fsanitize=hwaddress
turns on Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. memory access instructions are instrumented to catch out-of-bounds and use-after-free bugs. this option enables -fsanitize-address-use-after-scope. details at <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>. ✨
run-time behavior can be influenced with the HWASAN_OPTIONS environment variable. set it to "help=1" and the available options are shown at startup of the instrumented program.
cannot be combined with-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64. periodt.
💖 -fsanitize=kernel-hwaddress
turns on Hardware-assisted AddressSanitizer for compiling the Linux kernel. similar to -fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences that are necessary for compiling the Linux 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 than-fsanitize=hwaddress. instrumenting the stack and alloca calls are not on by default, but you can still get them by specifying the command-line options--param hwasan-instrument-stack=1and--param hwasan-instrument-allocas=1respectively. and using a random frame tag is not implemented for kernel instrumentation. 🙈
💖 -fsanitize=pointer-compare
instruments comparison operations (<, <=, >, >=) that have pointer operands. this option must be combined with either -fsanitize=kernel-address or -fsanitize=address. and it cannot be combined with -fsanitize=thread.
note: by default the check is disabled at run time. to enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. using"detect_invalid_pointer_pairs=1"detects invalid operation only when both pointers are non-null. 💅
💖 -fsanitize=pointer-subtract
instruments subtraction with pointer operands. this option must be combined with either -fsanitize=kernel-address or -fsanitize=address. and it cannot be combined with -fsanitize=thread.
note: by default the check is disabled at run time. to enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. using"detect_invalid_pointer_pairs=1"detects invalid operation only when both pointers are non-null. 💅
💖 -fsanitize=shadow-call-stack
turns on ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (like stack buffer overflows, bestie 😭). it works by saving a function's return address to a separately allocated shadow call stack in the function prologue, then restoring the return address from the shadow call stack in the function epilogue. instrumentation only happens in functions that actually need to save the return address to the stack. 👑
currently it only supports the aarch64 platform. it's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. for user space programs, runtime support is not currently provided in libc and libgcc. users who want this feature in user space need to provide their own runtime support. and it should be noted that this may cause the ABI rules to be broken. 💀
on aarch64, the instrumentation makes use of the platform register "x18". this generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18 — otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer. 🙈
also, because there's no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. use -fno-exceptions to turn off exceptions.
see <https://clang.llvm.org/docs/ShadowCallStack.html> for more details. 🌸
💖 -fsanitize=thread
turns on ThreadSanitizer, a fast data race detector. memory access instructions get instrumented to detect data race bugs. more at <https://github.com/google/sanitizers/wiki#threadsanitizer>. run-time behavior can be influenced with the TSAN_OPTIONS environment variable; supported options listed at <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>.
cannot be combined with-fsanitize=address,-fsanitize=leak. nope, not happening.
note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions). 😭
💖 -fsanitize=leak
turns on LeakSanitizer, a memory leak detector. this option only matters for linking of executables — the executable gets linked against a library that overrides "malloc" and other allocator functions. more at <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>. run-time behavior can be influenced with the LSAN_OPTIONS environment variable.
cannot be combined with -fsanitize=thread. 🚫
💖 -fsanitize=undefined
turns on UndefinedBehaviorSanitizer, a fast undefined behavior detector. various computations get instrumented to detect undefined behavior at runtime. more at <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html>. run-time behavior can be influenced with the UBSAN_OPTIONS environment variable. current suboptions are, bestie, a whole little family: 👑
🌈 -fsanitize=shift
this one enables checking that the result of a shift operation isn't undefined. note that what exactly counts as undefined differs slightly between C and C++, and also between ISO C90 and C99, etc. this option has two suboptions of its own: -fsanitize=shift-base and -fsanitize=shift-exponent. ✨
🌈 -fsanitize=shift-exponent
enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
🌈 -fsanitize=shift-base
if the second argument of a shift operation is within range, check that the result of the shift operation isn't undefined. note again: what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. 💅
🌈 -fsanitize=integer-divide-by-zero
detect integer division by zero. yes, that one. the classic. 💀
🌈 -fsanitize=unreachable
with this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. when you reach the "__builtin_unreachable" call, the behavior is undefined. 🙈
🌈 -fsanitize=vla-bound
this option instructs the compiler to check that the size of a variable length array is positive. 🎀
🌈 -fsanitize=null
this one enables pointer checking. specifically, an application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed to by a NULL pointer. all the null shenanigans, caught. 👑
🌈 -fsanitize=return
enables return statement checking. programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. this option works in C++ only.
🌈 -fsanitize=signed-integer-overflow
enables signed integer overflow checking. we check that the result of "+", "*", and both unary and binary "-" does not overflow in signed arithmetics. this also detects "INT_MIN / -1" signed division. note: integer promotion rules must be taken into account. that is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;and that's the vibe check on that one, more coming bestie 🦋
-fsanitize=bounds— bestie, this turns on instrumentation of array bounds, so like, various out-of-bounds accesses get caught in the act 👀. Lowkey important caveat: flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with the one exception of flexible array member-like arrays where-fstrict-flex-arraysor-fstrict-flex-arrays=options, orstrict_flex_arrayattributes, say they shouldn't be treated like flexible array member-like arrays. Slay, specificity 💅-fsanitize=bounds-strict— the ✨strict✨ version of the above. Enables strict instrumentation of array bounds, so most out-of-bounds accesses get detected, flexible array member-like arrays included. Initializers of variables with static storage are still not instrumented tho.-fsanitize=alignment— this one checks the alignment of pointers when they're dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. It's giving "your pointer is not standing up straight" energy 🙈-fsanitize=object-size— enables instrumentation of memory references using the__builtin_dynamic_object_sizefunction. Various out-of-bounds pointer accesses get detected. Periodt.-fsanitize=float-divide-by-zero— detects floating-point division by zero. And here's the tea ☕: unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, because floating-point division by zero can totally be a legitimate way of obtaining infinities and NaNs. She's valid, leave her alone 💖-fsanitize=float-cast-overflow— enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. And again, unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. Heads up: this option does not work well withFE_INVALIDexceptions enabled 😭-fsanitize=nonnull-attribute— enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by thenonnullfunction attribute. No nulls allowed, we're exclusive 🚫-fsanitize=returns-nonnull-attribute— enables instrumentation of return statements in functions marked with thereturns_nonnullfunction attribute, to detect returning of null values from such functions. If you promised non-null, you better deliver 💅-fsanitize=bool— enables instrumentation of loads frombool. If a value other than 0/1 is loaded, a run-time error is issued. It's binary bestie, pick a lane 🎀-fsanitize=enum— enables instrumentation of loads from anenumtype. If a value outside the range of values for the enum type is loaded, a run-time error is issued. Stay in your enum era, king 🌈-fsanitize=vptr— enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. C++ girlies, this one's for you 🫶-fsanitize=pointer-overflow— enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math class but make it pointers 🔥-fsanitize=builtin— enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior and is diagnosed by this option. Like babe, don't do that 🙈
Note that sanitizers tend to increase the rate of false positive warnings, most notably those around-Wmaybe-uninitialized. We recommend against combining-Werrorand [the use of] sanitizers. Not me warning you about the chaos you're about to cause 😭
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.
-fno-sanitize=all— this option disables all previously enabled sanitizers. And bestie, careful:-fsanitize=allis not allowed, as some sanitizers cannot be used together. They don't play nice, okay? 💀-fasan-shadow-offset=number— this forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. Science experiment era 🧚-fsanitize-sections=s1,s2,...— sanitize global variables in selected user-defined sections. si may contain wildcards. Very "choose your fighter" of you ✨-fsanitize-recover[=opts]— so-fsanitize-recover=controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. That means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 😳. The-fno-sanitize-recover=option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enable sanitization of local variables to detect use-after-scope bugs. The option sets-fstack-reusetonone. Playing detective 🔍-fsanitize-trap[=opts]— the-fsanitize-trap=option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using__builtin_traprather than alibubsanlibrary routine. If this option is enabled for a certain sanitizer, it takes precedence over the-fsanitizer-recover=for that sanitizer —__builtin_trapwill be emitted and be fatal regardless of whether recovery is enabled or disabled using-fsanitize-recover=.
The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. Iconic, self-sufficient behavior 👑
Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can also be specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support — -fsanitize-trap=vptr is not allowed 🚫
-fsanitize-undefined-trap-on-error— this option is the deprecated equivalent of-fsanitize-trap=all. Old news bestie, we moved on 💅-fsanitize-coverage=trace-pc— enable coverage-guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_pcinto every basic block. Fuzzing era unlocked 🎀-fsanitize-coverage=trace-cmp— enable dataflow guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_cmp4or__sanitizer_cov_trace_cmp8for integral comparison with both operands variable, or__sanitizer_cov_trace_const_cmp1,__sanitizer_cov_trace_const_cmp2,__sanitizer_cov_trace_const_cmp4or__sanitizer_cov_trace_const_cmp8for integral comparison with one operand constant,__sanitizer_cov_trace_cmpfor__sanitizer_cov_trace_cmpdfor float or double comparisons, and__sanitizer_cov_trace_switchfor switch statements. That's a whole lotta trace functions and honestly? She ate 🔥-fcf-protection=[full|branch|return|none|check]— enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Security girlie 💖
The value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value return implements checking of validity at the point of returning from a function. The value full is an alias for specifying both branch and return. The value none turns off instrumentation.
To override -fcf-protection, -fcf-protection=none needs to be added and then with -fcf-protection=xxx.
The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value check is ignored at the 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 the return.
You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. Very "not today" of you ✨
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor 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.
-fharden-compares— okay so for every logical test that survives gimple optimizations and isn't the condition in a conditional branch (think: conditions tested for conditional moves, or conditions getting stored into boolean variables), GCC emits extra code to compute and verify the reversed condition, and to call__builtin_trapif the results don't match. Pair it with-fharden-conditional-branchesto cover all the conditionals. 🫶-fharden-conditional-branches— same energy, different target: for every non-vectorized conditional branch that survives gimple optimizations, GCC emits extra code to compute and verify the reversed condition, and to call__builtin_trapif the result is unexpected. Use with-fharden-comparesto cover all conditionals. ✨-fharden-control-flow-redundancy— emit extra code that sets booleans when entering basic blocks, and then verifies and traps, at function exits, when those booleans don't form an execution path compatible with the control flow graph. it's giving "vibe check for your control flow" fr fr.
Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls. Tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are available. 🎀
Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls, would also have verification issued before the call — but bestie, these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. 💀
-fhardcfr-skip-leaf— disable-fharden-control-flow-redundancyin leaf functions. simple as. 🍓-fhardcfr-check-exceptions— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. This option is enabled by default; use-fno-hardcfr-check-exceptionsto disable it.-fhardcfr-check-returning-calls— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any — so as to not prevent tail-call optimization, whether or not it's ultimately optimized to a tail call. 💅
This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations. iconic tbh.
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph beforenoreturncalls — either all of them (always), those that aren't expected to return control to the caller through an exception (no-xthrow, the default), those that may not return control to the caller through an exception either (nothrow), or none of them (never).
Checking before a noreturn function that may return control to the caller through an exception may cause checking to be performed more than once, if the exception is caught in the caller, whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since it would be redundant with the check performed before the call — but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint may even be another noreturn call, so checking may end up performed multiple times. 😭
Various optimizers may cause calls to be marked as noreturn and/or nothrow, even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, have made no-xthrow the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations. ✨
-fhardened— enables a whole set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but currently they are:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)The list of options enabled by -fhardened can be generated using the --help=hardened option. 👑
When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.
This option is intended to be used in production builds, not merely in debug builds.
Currently, -fhardened is only supported on GNU/Linux targets.
-fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. iykyk 🧚
-fstack-protector— emit extra code to check for buffer overflows, like stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. That includes functions that callalloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count. 💅-fstack-protector-all— like-fstack-protectorexcept that all functions are protected. no crumbs. 🦋-fstack-protector-strong— like-fstack-protectorbut includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count.
💖 NOTE: In Ubuntu 14.10 and later versions,-fstack-protector-strongis enabled by default for C, C++, ObjC, ObjC++, if none of-fno-stack-protector,-nostdlib, nor-ffreestandingare found.
-fstack-protector-explicit— like-fstack-protectorbut only protects those functions which have thestack_protectattribute. picky but make it fashion. 🎀-fstack-check— generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there's only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended. 🫶
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
- Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
- Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. 💀
Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection.
-fstack-clash-protection— generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. slay.
Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.
💖 NOTE: In Ubuntu 19.10 and later versions,-fstack-clash-protectionis enabled by default for C, C++, ObjC, ObjC++, unless-fno-stack-clash-protectionis found.
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit— generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to catch the signal without taking special precautions. 🔥
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Heads up bestie: this may only work with the GNU linker. 💅
And you can locally override stack limit checking by using the "no_stack_limit" function attribute. Iconic.
-fsplit-stack— generate code that automatically splits the stack before it overflows. 🫶 The resulting program has a discontiguous stack, which can only overflow if the program is literally unable to allocate any more memory. This is most useful when you're running threaded programs, because it's no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux.
Okay so here's the tea 🍵: when code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for that latter code to run. If compiling all code — including library code — with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. ✨
-fstrub=disable— disable stack scrubbing entirely, ignoring any"strub"attributes. See-fstrub=strict— functions default to"strub"mode"disabled", and apply strictly the restriction that only functions associated with"strub"-"callable"modes ("at-calls","callable"and"always_inline""internal") are"callable"by functions with"strub"-enabled modes ("at-calls"and"internal").-fstrub=relaxed— restore the default stack scrub ("strub") setting, namely,"strub"is only enabled as required by"strub"attributes associated with function and data types."Relaxed"means that strub contexts are only prevented from calling functions explicitly associated with"strub"mode"disabled". This option is only useful to override other-fstrub=*options that precede it in the command line.-fstrub=at-calls— enable"at-calls""strub"mode where viable. The primary use of this option is for testing. 🧪 It exercises the"strub"machinery in scenarios strictly local to a translation unit. This"strub"mode modifies function interfaces, so any function that is visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the"strub"attribute documentation for details on viability and eligibility requirements.-fstrub=internal— enable"internal""strub"mode where viable. The primary use of this option is for testing. This option is intended to exercise thoroughly parts of the"strub"machinery that implement the less efficient, but interface-preserving"strub"mode. Functions that would not be affected by this option are quite uncommon. 🙈-fstrub=all— enable some"strub"mode where viable. When both strub modes are viable,"at-calls"is preferred.-fdump-ipa-strubmadds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly the"strub"machinery. 💖-fvtable-verify=[std|preinit|none]— this option is only available when compiling C++ code. It turns on (or off, if using-fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. 💀
This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches "main". Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std.
-fvtv-debug— when used in conjunction with-fvtable-verify=stdor-fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. 📝 This information is written to a file namedvtv_set_ptr_data.login the directory named by the environment variableVTV_LOGS_DIRif that is defined or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. 🫠
-fvtv-counts— this is a debugging flag. When used in conjunction with-fvtable-verify=stdor-fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file namedvtv_count_data.login the directory named by the environment variableVTV_LOGS_DIRif that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information tovtv_class_set_sizes.login the same directory. 📊
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 😭
-finstrument-functions— generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms,"__builtin_return_address"does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 🔍
This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use "extern inline" in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 🍀
A function may be given the attribute "no_instrument_function", in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). ✨
-finstrument-functions-once— this is similar to-finstrument-functions, but the profiling functions are called only once per instrumented function, i.e. the first profiling function is called after the first entry into the instrumented function and the second profiling function is called before the exit corresponding to this first entry.
The definition of "once" for the purpose of this option is a little vague, bestie, because the implementation is not protected against data races. 🙃 As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread, but the calls are always paired — that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.
-finstrument-functions-exclude-file-list=file,file,...— set the list of functions that are excluded from instrumentation (see the description of-finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match. 🎯
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysexcludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...— this is similar to-finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as"vector<int> blah(const vector<int> &)", not the internal mangled name (e.g.,"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🌈-fpatchable-function-entry=N[,M]— bestie, this one drops N NOPs right at the very start of every function, and the function entry point sits before the Mth NOP 💅 If you ghost M, it defaults to0, so the function entry points at the address just at the first NOP. Those NOPs are basically reserving extra space so you can patch in whatever instrumentation you want at run time — provided the code segment is writable, no cap. The amount of space is controllable indirectly via the number of NOPs; the actual NOP instruction used corresponds to whatever the internal GCC back-end interfacegen_nopemits. And heads up: this behavior is target-specific and may also depend on the architecture variant and/or other compilation options 🦋
For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — 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. You can use that to increase the area size or to remove it completely on a single function. And 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. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address 👑
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14. Slay 💜
🎀 Options Controlling the Preprocessor 🎀
These options control the C preprocessor, which gets run on every C source file before the actual compilation even happens, periodt ✨
If you use the -E option, nothing is 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 controlling search paths for include files, documented over in Directory Options. Options that control preprocessor diagnostics live in Warning Options 🌸
-D name— predefine name as a macro, with definition1. Easy peasy.-D name=definition— the contents of definition get tokenized and processed as if they showed up during translation phase three in a#definedirective. In particular, the definition is truncated by embedded newline characters 😭
If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works 🫶
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. That ordering matters, iykyk 💅
-U name— cancel any previous definition of name, whether it was built in or provided with a-Doption. Not me undefining everything 🙈-include file— process file as if#include "file"appeared as the first line of the primary source file. BUT — plot twist — the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it gets searched for in the remainder of the#include "..."search chain as normal 🦋
If multiple -include options are given, the files are included in the order they appear on the command line.
-imacros file— exactly like-include, except that any output produced by scanning file gets thrown away 🗑️ Macros it defines remain defined, though. This lets you acquire all the macros from a header without also processing its declarations. Iconic.
All files specified by -imacros are processed before all files specified by -include.
-undef— do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined, bestie 💖-pthread— define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets ✨-M— instead of outputting the result of preprocessing, output a rule suitable formakedescribing 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 specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix, and with any leading directory parts removed. If there are many included files, the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules, you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal, no cap.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w 💀
-MM— like-Mbut do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. Sneaky but true 🧚
-MF file— when used with-Mor-MM, specifies a file to write the dependencies to. If no-MFswitch is given, the preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout 🌈
-MG— in conjunction with an option such as-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 renders it useless 🙈
This feature is used in automatic updating of makefiles.
-Mno-modules— disable dependency generation for compiled module interfaces.-MP— this option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around the 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— change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give:
$(objpfx)foo.o: foo.c-MQ target— same as-MT, but it quotes any characters which are special to Make. So-MQ '$(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. Lowkey genius 🔥
-MMD— like-MDexcept it mentions only user header files, not system header files.-fpreprocessed— tell the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so you can pass a file preprocessed with-Cto the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends 🦋
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.
-fdirectives-only— when preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options, bestie — listen up 💅
With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the -dD option is implicitly enabled ✨
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only 🌸
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only. Periodt.
-fdollars-in-identifiers— accept$in identifiers. That's it, that's the whole vibe 💜-fextended-identifiers— okay bestie, this one says accept universal character names and extended characters in identifiers. 💅 It's enabled by default for C99 (and later C standard versions) and C++. So if you're on C99+ or C++, it's already doing its thing, no cap.-fno-canonical-system-headers— when preprocessing, do not shorten system header paths with canonicalization. that's it, that's the vibe. ✨-fmax-include-depth=depth— sets the maximum depth of the nested#include. The default is200. 💀-ftabstop=width— sets the distance between tab stops. It helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than1or greater than100, the option is ignored. The default is8. 🎀-ftrack-macro-expansion[=level]— tracks locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. Thelevelparameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value0oflevelde-activates this option. Value1tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value2tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is2.
Note that -ftrack-macro-expansion=2 is activated by default. 👑
-fmacro-prefix-map=old=new— when preprocessing files residing in directoryold, expand the__FILE__and__BASE_FILE__macros as if the files resided in directorynewinstead. This can be used to change an absolute path to a relative path by using.fornewwhich can result in more reproducible builds that are location independent. This option also affects__builtin_FILE()during compilation. See also-ffile-prefix-mapand-fcanon-prefix-map. 🦋-fexec-charset=charset— sets the execution character set, used for string and character constants. The default isUTF-8.charsetcan be any encoding supported by the system'siconvlibrary routine. 💖-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 corresponds to the width ofwchar_tand the big-endian or little-endian byte order being used for code generation. As with-fexec-charset,charsetcan be any encoding supported by the system'siconvlibrary routine; however, you will have problems with encodings that do not fit exactly inwchar_t. 😭-finput-charset=charset— sets the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default isUTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict.charsetcan be any encoding supported by the system'siconvlibrary routine. 🌈-fpch-deps— when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 🫶-fpch-preprocess— this option allows use of a precompiled header together with-E. It inserts a special#pragma,#pragma GCC pch_preprocess "filename"in the output to mark the place where the precompiled header was found, and its filename. When-fpreprocessedis in use, GCC recognizes this#pragmaand loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.
You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. ✨
-fworking-directory— enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form-fno-working-directory. If the-Pflag is present in the command line, this option has no effect, since no#linedirectives are emitted whatsoever. 💜-A predicate=answer— make an assertion with the predicatepredicateand answeranswer. This form is preferred to the older form-A predicate(answer), which is still supported, because it does not use shell special characters. 🔥-A -predicate=answer— cancel an assertion with the predicatepredicateand answeranswer. 💅-C— do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. 😭
-CC— do not discard comments, including during macro expansion. This is like-C, except that comments contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments. 🎀
-P— inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. 🦋-traditional,-traditional-cpp— try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly. 👑
-trigraphs— support ISO C trigraphs. These are three-character sequences, all starting with??, that are defined by ISO C to stand for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. 💀
-remap— enable special code to work around file systems which only permit very short file names, such as MS-DOS. 🎀-H— print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the#includestack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with...xand a valid one with...!. 💅-dletters— says to make debugging dumps during compilation as specified byletters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined.-dM— instead of the normal output, generate a list of#definedirectives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no filefoo.h, the command
touch foo.h; cpp -dM foo.hshows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
-dD— like-dMexcept that it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.-dN— like-dD, but emit only the macro names, not their expansions.-dI— output#includedirectives in addition to the result of preprocessing.-dU— like-dDexcept that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and#undefdirectives are also output for macros tested but undefined at the time. 😭
-fdebug-cpp— this option is only useful for debugging GCC. When used from CPP or with-E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no effect. ✨
-Wp,option— bestie, this one is the chaos shortcut 🚨 it lets you completely bypass the compiler driver and shoveoptionstraight through to the preprocessor. Ifoptionhas commas in it, it gets split into multiple options at the commas. BUT (and this is a big but) a ton of options get modified, translated or interpreted by the compiler driver before they ever reach the preprocessor, and-Wpforcibly skips that whole phase. The preprocessor's direct interface is undocumented and subject to change, so whenever you possibly can, avoid-Wpand just let the driver handle the options instead. iykyk 💅-Xpreprocessor option— passoptionas an option to the preprocessor. Use this when you've got system-specific preprocessor options that GCC doesn't recognize. 🧚 Pro tip: if you want to pass an option that takes an argument, you have to use-Xpreprocessortwice — once for the option and once for the argument. Yes, twice. It's giving needy, but that's the deal.-no-integrated-cpp— do the preprocessing as a separate pass before compilation. By default GCC does preprocessing as an integrated part of input tokenization and parsing (all cozy and fused together ✨). If you pass this option, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) gets invoked twice instead — once for preprocessing only, and once for the actual compilation of the preprocessed input. This can be useful alongside-Bor-wrapperto specify an alternate preprocessor or do extra processing of the program source between normal preprocessing and compilation. lowkey a niche queen but she serves 👑-flarge-source-files— tells GCC to expect big ol' source files, and the price is slower compilation and higher memory usage. 😭 Specifically: GCC normally tracks both column numbers and line numbers within source files, and it normally prints both of those numbers in diagnostics. But once it's processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. That means diagnostics for later lines don't include column numbers. It also means options like-Wmisleading-indentationcease to work at that point, although the compiler prints a note if that happens. Passing-flarge-source-filessignificantly increases the number of source lines GCC can process before it stops tracking columns. big file energy 🫶
✨ Passing Options to the Assembler ✨
You can pass options to the assembler, bestie. Here's how 🎀
-Wa,option— passoptionas an option to the assembler. Ifoptioncontains commas, it is split into multiple options at the commas. simple as that 💖-Xassembler option— passoptionas an option to the assembler. Use this for system-specific assembler options that GCC doesn't recognize. And same as before: if you want to pass an option that takes an argument, you must 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 are meaningless if the compiler isn't doing a link step. no link, no vibes 💀
object-file-name— a file name that doesn't end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents — the linker sniffs them out fr fr.) If linking is done, these object files are used as input to the linker.-c,-S,-E— if any of these options is used, then the linker is not run, and object file names should not be used as arguments. periodt.-flinker-output=type— this option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin (she decides for herself ✨). But for debugging the compiler, and if incremental linking with a non-LTO object file is what you want, it may be useful to control the type manually. Here's the type breakdown, bestie, buckle up:- If
typeisexec, code generation produces a static binary. In this case-fpicand-fpieare both disabled. - If
typeisdyn, code generation produces a shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This allows you to build shared libraries without position-independent code on architectures where this is possible, i.e. on x86. 👑 - If
typeispie, code generation produces an-fpieexecutable. This results in similar optimizations asexecexcept that-fpieis not disabled if specified at compilation time. - If
typeisrel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if-ffat-lto-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. 🏎️ - Finally,
nolto-relconfigures 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. 💔
- If
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. fancy 💅
-fuse-ld=bfd— use thebfdlinker instead of the default linker.-fuse-ld=gold— use thegoldlinker instead of the default linker.-fuse-ld=lld— use the LLVMlldlinker instead of the default linker.-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker. mold my beloved 🦋-llibrary,-l library— search the library named library when linking. (The second alternative, with the library as a separate argument, is only for POSIX compliance and is not recommended — we don't do that here.) The-loption is passed directly to the linker by GCC. 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; the linker searches and processes libraries and object files in the order they are specified. So like, 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. order matters, bestie, it's giving lineup drama 😭
-lobjc— you need this special case of the-loption in order to link an Objective-C or Objective-C++ program. she's the exception 🎀-nostartfiles— do not use the standard system startup files when linking. The standard system libraries are used normally, unless-nostdlib,-nolibc, or-nodefaultlibsis used. 🫶-nodefaultlibs— do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-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. These entry points should be supplied through some other mechanism when this option is specified. you've been warned 💀
-nolibc— do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, libgcc or toolchain provided language support libraries such aslibgnat,libgfortranorlibstdc++unless options preventing their inclusion are used as well. This typically removes-lcfrom the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example-lpthreador-lmin some configurations. This is intended for bare-board targets when there is indeed no C library available. bare metal moment 🔥-nostdlib— do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is "__main", used to ensure C++ constructors are called.) 💖
-nostdlib++— do not implicitly link with standard C++ libraries.-e entry,--entry=entry— specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. main character energy, literally 👑-pie— bestie this makes a dynamically linked position independent executable on targets that actually support it 💅 but listen: for results that aren't chaotic, you ALSO have to pass the same set of options you used for compilation (-fpie,-fPIE, or the model suboptions) when you give this linker option. Same energy in, same energy out ✨-no-pie— don't make a dynamically linked position independent executable. that's it, that's the whole vibe 🙈-static-pie— makes a static position independent executable on targets that support it 🎀 a static PIE is lowkey like a static executable, except it can be loaded at any address with no dynamic linker involved, iconic. and yes, for predictable results you must ALSO specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you pass 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, no cap 💀-r— produces a relocatable object as output. it's 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 just the used ones 👑 you need this for some uses ofdlopen, or to let a program get backtraces from within itself. main character symbol table energy fr fr-s— removes ALL symbol table and relocation information from the executable. gone, deleted, no trace 💀-static— on systems that support dynamic linking, this overrides-pieand prevents linking with the shared libraries. on other systems? this option has literally no effect 🌸-shared— produces a shared object that can then be linked with other objects to make an executable. not all systems support this option, sadly 😭 and for predictable results, you must ALSO specify the same set of options used for compilation (-fpic,-fPIC, or model suboptions) when you specify this linker option.[1]-shared-libgcc,-static-libgcc— on systems that providelibgccas a shared library, these options force the use of either the shared or the static version, respectively. if no shared version oflibgccwas built when the compiler was configured, these options have no effect 🤷
there are several situations where an app should use the shared libgcc instead of the static one. the most common: when the app wants to throw and catch exceptions across different shared libraries 🎀 in that case, every one of those 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, because C++ programs typically use exceptions, so this is simply the right thing to do 💅
if instead you use the GCC driver to create shared libraries, you might find they're not always linked with the shared libgcc. if GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that doesn't support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. otherwise it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. this lets exceptions propagate through such shared libraries, without paying 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 sharedlibgcc. no shortcuts bestie 😤
-static-libasan— when-fsanitize=addressis used to link a program, the GCC driver automatically links againstlibasan. iflibasanis available as a shared library, and-staticis not used, then it links against the shared version oflibasan. the-static-libasanoption directs the GCC driver to linklibasanstatically, without necessarily linking other libraries statically 🦋-static-libtsan— when-fsanitize=threadis used to link a program, the GCC driver automatically links againstlibtsan. iflibtsanis available as a shared library, and-staticis not used, then it links against the shared version oflibtsan. the-static-libtsanoption directs the GCC driver to linklibtsanstatically, without necessarily linking other libraries statically 🧵-static-liblsan— when-fsanitize=leakis used to link a program, the GCC driver automatically links againstliblsan. ifliblsanis available as a shared library, and-staticis not used, then it links against the shared version ofliblsan. the-static-liblsanoption directs the GCC driver to linkliblsanstatically, without necessarily linking other libraries statically 💧-static-libubsan— when-fsanitize=undefinedis used to link a program, the GCC driver automatically links againstlibubsan. iflibubsanis available as a shared library, and-staticis not used, then it links against the shared version oflibubsan. the-static-libubsanoption directs the GCC driver to linklibubsanstatically, without necessarily linking other libraries statically 🔥-static-libstdc++— when theg++program is used to link a C++ program, it normally automatically links againstlibstdc++. iflibstdc++is available as a shared library, and-staticis not used, then it links against the shared version oflibstdc++. that's normally fine, honestly. BUT it's sometimes useful to freeze the version oflibstdc++your program uses without going all the way to a fully static link. the-static-libstdc++option directs theg++driver to linklibstdc++statically, without necessarily linking other libraries statically 🧊-symbolic— binds references to global symbols when building a shared object. warns about any unresolved references (unless overridden by the link editor option-Xlinker -z -Xlinker defs). only a few systems support this option, so check before you commit bestie 💅-Tscript — uses script as the linker script. this option is supported by most systems using the GNU linker. on some targets, like bare-board targets without an operating system, the-Toption may be required when linking, to avoid references to undefined symbols 👑-Xlinkeroption — passes option as an option to the linker. use this to supply system-specific linker options that GCC doesn't recognize 🎀
if you want to pass an option that takes a separate argument, you must use -Xlinker twice — once for the option and once for the argument. for example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. it does NOT work to write -Xlinker "-assert definitions", because that passes the entire string as a single argument, which is not what the linker expects 😭 read it again, it's important.
when using the GNU linker, it's usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. for example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. other linkers may not support this syntax for command-line options 🙈
-Wl,option — passes option as an option to the linker. if option contains commas, it's split into multiple options at the 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 using the GNU linker, you can also get the same effect with-Wl,-Map=output.map✨
NOTE: in Ubuntu 8.10 and later versions, forLDFLAGS, the option-Wl,-z,relrois used. to disable, use-Wl,-z,norelro. noted, bestie 📝
-usymbol — pretends the symbol symbol is undefined, to force linking of library modules to define it. you can use-umultiple times with different symbols to force loading of additional library modules 👑-zkeyword —-zis passed directly on to the linker along with the keyword keyword. see the section in your linker's documentation for the permitted values and what they mean 🦋
🌈 Options for Directory Search 🌈
these options specify directories to search for header files, for libraries, and for parts of the compiler 💖 iconic behaviour honestly.
-Idir-iquotedir-isystemdir-idirafterdir
adds the directory dir to the list of directories searched for header files during preprocessing 🎀 if dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
directories specified with -iquote apply ONLY to the quote form of the directive, #include "file". directories specified with -I, -isystem, or -idirafter apply to lookup for BOTH the #include "file" and #include <file> directives ✨
you can specify any number or combination of these options on the command line to search for header files in several directories. the lookup order is as follows:
- 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 directories specified by
-iquoteoptions are searched in left-to-right order, as they appear on the command line. - directories specified with
-Ioptions are scanned in left-to-right order. - directories specified with
-isystemoptions are scanned in left-to-right order. - standard system directories are scanned.
- directories specified with
-idirafteroptions are scanned in left-to-right order.
you can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories 👑 BUT you should not use this option to add directories that contain vendor-supplied system header files — use -isystem for that, bestie, this is the rule 📏
the -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that's applied to the standard system directories 💜
okay so if a standard system include directory, or a directory you specified with -isystem, ALSO gets specified with -I… bestie, that -I is just gonna be ignored. 🙈 The directory still gets searched, but as a system directory at its normal spot in the system include chain. Why? Because we are protecting GCC's whole procedure for fixing buggy system headers, plus the ordering for the "#include_next" directive, from getting accidentally scrambled. No chaos allowed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options instead. 💅
-I-— Split the include path. This option has been deprecated. Please use-iquoteinstead for the-Idirectories before the-I-, and just remove the-I-option entirely. 🫶
Any directories specified with -I options before -I- are searched only for headers requested with "#include "file"" — they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.
On top of that, -I- also inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". And there is no way to override this effect of -I-. It's giving "no take-backs." 😭
-iprefix prefix— Specifyprefixas the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include the final/. 🌸-iwithprefix dir,-iwithprefixbefore dir— Appenddirto the prefix specified previously with-iprefix, and add the resulting directory to the include search path.-iwithprefixbeforeputs it in the same place-Iwould;-iwithprefixputs it where-idirafterwould. same destination, different seat assignments. ✨-isysroot dir— This option is like the--sysrootoption, but it applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the--sysrootoption for more info. 🦋-imultilib dir— Usediras a subdirectory of the directory containing target-specific C++ headers. 👑-nostdinc— Do not search the standard system directories for header files. Only the directories explicitly specified with-I,-iquote,-isystem, and/or-idirafteroptions (and the directory of the current file, if appropriate) are searched. you're on your own, but at least you chose it. 💀-nostdinc++— Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 💜-iplugindir=dir— Set the directory to search for plugins that are passed by-fplugin=nameinstead of-fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🚫-Ldir— Add directorydirto the list of directories to be searched for-l. 🍓-Bprefix— This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. the whole backstage pass. 🎀
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. 😬
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. iykyk. ✨
-no-canonical-prefixes— Do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix. 🦋--sysroot=dir— 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 instead searchesdir/usr/includeanddir/usr/lib. relocation, but make it elegant.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 💀
--no-sysroot-suffix— For some targets, a suffix is added to the root directory specified with--sysroot, depending on the other options used, so that headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include. This option disables the addition of such a suffix. 🌸
🌈 Options for Code Generation Conventions 🌈
These machine-independent options control the interface conventions used in code generation. the social rules of the generated code, basically. 💅
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one that is not the default. You can figure out the other form by either removing no- or adding it. 🫶
-fstack-reuse=reuse-level— This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.reuse_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. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 😭
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. respect the old code, or suffer the runtime consequences. 💀
-ftrapv— This option generates traps for signed overflow on addition, subtraction, 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. last active one standing wins. ✨-fwrapv— This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options-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-pointer— This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. 🦋-fstrict-overflow— This option implies-fno-wrapv -fno-wrapv-pointerand when negated implies-fwrapv -fwrapv-pointer. 👑-fexceptions— bestie, this one turns ON exception handling ✨ it makes GCC generate the extra code needed to propagate exceptions. Heads up though: for some targets this means GCC generates frame unwind info for all functions, which can add significant data size overhead — but it does NOT affect execution, so breathe 💖. If you don't specify it, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that don't normally need it. That said, you might need to turn it on when compiling C code that needs to interoperate properly with exception handlers written in C++. And you might wanna disable it if you're compiling older C++ programs that don't use exception handling. iykyk.-fnon-call-exceptions— this generates code that lets trapping instructions throw exceptions 🫶. But ngl, it requires platform-specific runtime support that does not exist everywhere — so vibe check your target first. Also it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does NOT allow exceptions to be thrown from arbitrary signal handlers such as"SIGALRM". This enables-fexceptions.-fdelete-dead-exceptions— so basically, instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away 🎀. This does not affect calls to functions except those with the"pure"or"const"attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. And the optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. periodt.-funwind-tables— similar to-fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way 💅. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. so like, not your problem bestie.-fasynchronous-unwind-tables— generates unwind table in DWARF format, if supported by the target machine 🌸. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). iconic.-fno-gnu-unique— okay so on systems with recent GNU assembler and C library, the C++ compiler uses the"STB_GNU_UNIQUE"binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of"RTLD_LOCAL". This is necessary to avoid problems with a library used by two different"RTLD_LOCAL"plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol 🙈. But this causes"dlclose"to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via"dlclose"and"dlopen", you can use-fno-gnu-unique.-fpcc-struct-return— returns"short""struct"and"union"values in memory like longer ones, rather than in registers 👑. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). slay for compatibility honestly.
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the-fpcc-struct-returnswitch is not binary compatible with code compiled with the-freg-struct-returnswitch. Use it to conform to a non-default application binary interface. 💀
-freg-struct-return— returns"struct"and"union"values in registers when possible ✨. This is more efficient for small structures than-fpcc-struct-return. main character energy fr.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative 😌.
Warning: code compiled with the-freg-struct-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface. 💀
-fshort-enums— allocates to an"enum"type only as many bytes as it needs for the declared range of possible values 🎀. Specifically, the"enum"type is equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type. ngl that's kinda efficient of it.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 💀
-fshort-wchar— overrides the underlying type for"wchar_t"to be"short unsigned int"instead of the default for the target 💜. This option is useful for building programs to run under WINE. obsessed, honestly.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 💀
-fcommon— in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard 🌈. Tentative definitions are distinct from declarations of a variable with the"extern"keyword, which do not allocate storage. ok bestie stay with me here.
The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. rude but ok.
The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition 🫶. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors. so like, nostalgia mode.
-fno-ident— ignore the"#ident"directive 🙈. that's it. that's the whole thing.-finhibit-size-directive— don't output a".size"assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory 💅. This option is used when compilingcrtstuff.c; you should not need to use it for anything else. lowkey leave this one alone bestie.-fverbose-asm— puts extra commentary information in the generated assembly code to make it more readable ✨. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). very niche, very demure.
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.
The added comments include:
- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
- hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbitsThe comments are intended for humans rather than machines and hence the precise format of the comments is subject to change 👀. so don't go parsing them bestie.
-frecord-gcc-switches— this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created 📝. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the-fverbose-asmswitch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also-grecord-gcc-switchesfor another way of storing compiler options into the object file. 💖-fpic— bestie, this one generates position-independent code (PIC), which is what you want if you're building a shared library, if the target machine supports it. PIC code reaches all its constant addresses through a global offset table (GOT). The dynamic loader fills in those GOT entries when the program starts up — and heads up, the dynamic loader is not part of GCC, it's part of your operating system, so that's not our department 💅. Now, if the GOT for your linked executable gets bigger than some machine-specific maximum, the linker will hit you with an error saying-fpicdoesn't work, and the move there is to recompile with-fPICinstead. Those maxes? 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. The x86 has no such limit, iconic behaviour honestly.
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. And code generated for the IBM RS/6000 is always position-independent, no cap.
When this flag is set, the macros "__pic__" and "__PIC__" get defined to 1.
-fPIC— if the target machine supports it, this emits position-independent code suitable for dynamic linking, and it avoids any limit on the size of the global offset table. This option actually makes a difference on AArch64, m68k, PowerPC and SPARC. Same deal as before: PIC requires special support, so it only works on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" get defined to 2.
-fpie,-fPIE— these are giving cousins of-fpicand-fPIC, except the position-independent code they generate can only be linked into executables. Usually you use these to compile code that's gonna be linked with the-pieGCC option.
Both -fpie and -fPIE define the macros "__pie__" and "__PIE__". The value is 1 for -fpie and 2 for -fPIE. Slay 🎀
-fno-plt— do not use the PLT for external function calls in position-independent code. Instead, load the callee address at the call sites straight from the GOT and branch to it. This makes for 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 to be sitting in a specific register, this gives the compiler more register allocation freedom. Heads up though: lazy binding requires use of 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 to use the GOT instead.
-fno-jump-tables— do not use jump tables forswitchstatements, even where that would be more efficient than other code generation strategies. This one's useful alongside-fpicor-fPICfor 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 require a GOT and this option isn't needed at all.-fno-bit-tests— do not use bit tests forswitchstatements, even where that would be more efficient than other code generation strategies.-ffixed-reg— treat the register named reg as a fixed register; 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. Which names are accepted is machine-specific, and they're defined in the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice. No flip-flopping, bestie 💀
-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. And using it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Like, catastrophic. Do not.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
Same warning, same energy: it is an error to use this flag with the frame pointer or stack pointer. Using it for other registers with fixed pervasive roles in the machine's execution model produces disastrous results.
And there's a whole different flavour of disaster that results from using this flag for a register in which function values may be returned. 😭
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]— with no value given, this packs all structure members together with no holes. When a value is given (and it has to be a small power of two), structure members get packed according to that value, representing the maximum alignment — meaning objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Plus it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-fleading-underscore— this option and its counterpart,-fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. And ngl, not all targets provide complete support for this switch.
-ftls-model=model— change up the thread-local storage model being used. The model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. Note that the choice is subject to optimization: the compiler may 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 -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like, say, HP-PA or IA-64 — do nothing.
So what even is a trampoline? It's a small piece of code 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. Which means it requires the stack to be made executable for the program to work properly. Yes, that's the vibe 🫠
-fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with-ftrampolinesand code compiled with-fno-trampolinesare not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care.
For languages other than Ada, the "-ftrampolines" and "-fno-trampolines" options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
-ftrampoline-impl=[stack|heap]— by default, trampolines are generated on the stack. But certain platforms (like the Apple M1) do not permit an executable stack. Compiling with-ftrampoline-impl=heapgenerates calls to"__gcc_nested_func_ptr_created"and"__gcc_nested_func_ptr_deleted"in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.
PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you"setjmp", instantiate nested functions, and then"longjmp"back to a state prior to having allocated those nested functions. It's giving chaos, honestly.
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the option you pick — all symbols get marked with this unless they're overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Like, please. Do it 💜
Despite the nomenclature, default always means public — i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) — however, a superior solution made possible by this option, to marking things hidden when the default is public, is to make the default hidden and mark things public. This is the norm with DLLs on Windows, and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 👑
okay so bestie, if you're adding visibility support to code that's already out in the wild, you're gonna want #pragma GCC visibility in your life. The way it works is you wrap the declarations you wanna set visibility for, like this:
#pragma GCC visibility push(hidden)
...
#pragma GCC visibility popThat's it. That's the whole trick. 🎀
But listen — symbol visibility is part of the API interface contract, okay? It's not a lil vibe you sprinkle on at the end. So all new code should always specify visibility whenever it isn't the default. Like, declarations that are only for use inside the local DSO should always be marked explicitly as hidden, so you dodge the PLT indirection overheads — and honestly making it abundantly clear also helps readability and lets the code self-document, which is giving main character energy fr fr. ✨
Heads up: because of ISO C++ specification requirements,operator newandoperator deletemust always be of default visibility. No exceptions, no negotiating. 💅
Also be aware that headers from outside your project — especially system headers, and headers from any other library you use — might not be expecting to be compiled with visibility other than the default. They're just out here living their lives, not knowing. You may need to explicitly say:
#pragma GCC visibility push(default)before including any such headers. 🫶
Now, the cute part: extern declarations are not affected by -fvisibility, so a whole lot of code can be recompiled with -fvisibility=hidden with zero modifications. Slay. But — and this is the catch — that means 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. 👑
And ngl, -fvisibility does affect C++ vague linkage entities. Which means, for instance, an exception class that gets thrown between DSOs has to be explicitly marked with default visibility, so that the type_info nodes get unified between the DSOs. Otherwise it's giving chaos. 😭
There's an overview of all these techniques, their benefits, and how to use them right here: <https://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields— turn this on if accesses to volatile bit-fields (or other structure fields, though tbh the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. Real talk: targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide, and with this flag you can declare all peripheral bit-fields asunsigned short(assuming short is 16 bits on these targets) to force GCC into 16-bit accesses instead of, maybe, a more efficient 32-bit access. ✨
If this option is disabled, the compiler just uses the most efficient instruction. In that same example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers totally unrelated to the one being updated. 💀
In some cases, like when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that's correctly aligned for the target machine. In that case GCC falls back to generating multiple accesses rather than code that'll fault or truncate the result at run time. Which is the responsible thing to do, honestly.
Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended that you 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 one controls whether any out-of-line instance of the__syncfamily of functions may be used to implement the C++11__atomicfamily of functions. The default value is enabled, so the only useful form of the option is-fno-sync-libcalls. It's used in the implementation of the libatomic runtime library. 🌸
🛠️ GCC Developer Options 🛠️
This section is all about the command-line options that are primarily of interest to GCC developers — including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. That means options that produce debug dumps at various points in the 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. Bestie, you should rarely need any of these for ordinary compilation and linking tasks. This is the backstage pass, not the main entrance. 🎟️
Many 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 constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. Let's break that down, because it's giving lore:
- The base dump file name is the name of the output file produced by the compiler, if it's explicitly specified and not 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. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late get numbered higher than the pass named
final— even if they're executed earlier. Iconic chaos. 💅 - The phase letter is one of
i(inter-procedural analysis),l(language-specific),r(RTL), ort(tree). - The files are created in the directory of the output file.
-fcallgraph-info-fcallgraph-info=MARKERS— makes the compiler output callgraph information for the program, on a per-object-file basis. The info is generated in the common VCG format. It can be decorated with additional per-node and/or per-edge information if you also specify a list of comma-separated markers. When thesumarker is specified, the callgraph is decorated with stack usage information — it's equivalent to-fstack-usage. When thedamarker is specified, the callgraph gets decorated with information about dynamically allocated objects. ✨
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.
-dletters-fdump-rtl-pass-fdump-rtl-pass=filename— says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. 🔥
Some -dletters switches have different meaning when -E is used for preprocessing. (iykyk)
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments— dump after branch alignments have been computed.-fdump-rtl-asmcons— dump after fixing rtl statements that have unsatisfied in/out constraints.-fdump-rtl-auto_inc_dec— dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.-fdump-rtl-barriers— dump after cleaning up the barrier instructions.-fdump-rtl-bbpart— dump after partitioning hot and cold basic blocks.-fdump-rtl-bbro— dump after block reordering.-fdump-rtl-btl1-fdump-rtl-btl2—-fdump-rtl-btl1and-fdump-rtl-btl2enable dumping after the two branch target load optimization passes.-fdump-rtl-bypass— dump after jump bypassing and control flow optimizations.-fdump-rtl-combine— dump after the RTL instruction combination pass.-fdump-rtl-compgotos— dump after duplicating the computed gotos.-fdump-rtl-ce1-fdump-rtl-ce2-fdump-rtl-ce3—-fdump-rtl-ce1,-fdump-rtl-ce2, and-fdump-rtl-ce3enable dumping after the three if conversion passes.-fdump-rtl-cprop_hardreg— dump after hard register copy propagation.-fdump-rtl-csa— dump after combining stack adjustments.-fdump-rtl-cse1-fdump-rtl-cse2—-fdump-rtl-cse1and-fdump-rtl-cse2enable dumping after the two common subexpression elimination passes.-fdump-rtl-dce— dump after the standalone dead code elimination passes.-fdump-rtl-dbr— dump after delayed branch scheduling.-fdump-rtl-dce1-fdump-rtl-dce2—-fdump-rtl-dce1and-fdump-rtl-dce2enable dumping after the two dead store elimination passes.-fdump-rtl-eh— dump after finalization of EH handling code.-fdump-rtl-eh_ranges— dump after conversion of EH handling range regions.-fdump-rtl-expand— dump after RTL generation.-fdump-rtl-fwprop1-fdump-rtl-fwprop2—-fdump-rtl-fwprop1and-fdump-rtl-fwprop2enable dumping after the two forward propagation passes.-fdump-rtl-gcse1-fdump-rtl-gcse2—-fdump-rtl-gcse1and-fdump-rtl-gcse2enable dumping after global common subexpression elimination.-fdump-rtl-init-regs— dump after the initialization of the registers.-fdump-rtl-initvals— dump after the computation of the initial value sets.-fdump-rtl-into_cfglayout— dump after converting to cfglayout mode.-fdump-rtl-ira— dump after iterated register allocation.-fdump-rtl-jump— dump after the second jump optimization.-fdump-rtl-loop2—-fdump-rtl-loop2enables dumping after the rtl loop optimization passes.-fdump-rtl-mach— dump after performing the machine dependent reorganization pass, if that pass exists.-fdump-rtl-mode_sw— dump after removing redundant mode switches.-fdump-rtl-rnreg— dump after register renumbering.-fdump-rtl-outof_cfglayout— dump after converting from cfglayout mode.-fdump-rtl-peephole2— dump after the peephole pass.-fdump-rtl-postreload— dump after post-reload optimizations.-fdump-rtl-pro_and_epilogue— dump after generating the function prologues and epilogues.-fdump-rtl-sched1-fdump-rtl-sched2—-fdump-rtl-sched1and-fdump-rtl-sched2enable dumping after the basic block scheduling passes.
-fdump-rtl-ree— dump right after sign/zero extension elimination ✨-fdump-rtl-seqabstr— dump right after common sequence discovery 🔍-fdump-rtl-shorten— dump right after shortening branches ✂️-fdump-rtl-sibling— dump right after sibling call optimizations 🫶-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— these options enable dumping after five rounds of instruction splitting 🌸-fdump-rtl-sms— dump right after modulo scheduling. Heads up bestie, this pass only runs on some architectures 👀-fdump-rtl-stack— dump right after the conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants 👑-fdump-rtl-subreg1,-fdump-rtl-subreg2—-fdump-rtl-subreg1and-fdump-rtl-subreg2enable dumping after the two subreg expansion passes 💖-fdump-rtl-unshare— dump right after all rtl has been unshared 🧚-fdump-rtl-vartrack— dump right after variable tracking 🦋-fdump-rtl-vregs— dump right after converting virtual registers to hard registers ✨-fdump-rtl-web— dump right 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— okay so these dumps are defined but they always produce empty files 😭 not a single crumb of content, it's giving ghost town 👻-da,-fdump-rtl-all— produce all the dumps listed above 🌈-dA— annotate the assembler output with miscellaneous debugging information 📝-dD— dump all macro definitions, at the end of preprocessing, in addition to normal output 💖-dH— produce a core dump whenever an error occurs 💀-dp— annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed 🔍-dP— dump the RTL in the assembler output as a comment before each instruction. Also turns on-dpannotation ✨-dx— just generate RTL for a function instead of compiling it. Usually used with-fdump-rtl-expand🫶
-fdump-debug— dump the debugging information generated during the debug generation phase 🐛-fdump-earlydebug— dump the debugging information generated during the early debug generation phase 🌱-fdump-noaddr— when doing debugging dumps, suppress address output. This makes it way more feasible to usediffon debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations 💅-freport-bug— collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs 🧊-fdump-unnumbered— when doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g✨-fdump-unnumbered-links— when doing debugging dumps (see the-doption above), suppress instruction numbers for the links to the previous and next instructions in a sequence 🔗-fdump-ipa-switch,-fdump-ipa-switch-options— control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:all— enables all inter-procedural analysis dumps 👑cgraph— dumps information about call-graph optimization, unused function removal, and inlining decisions 🕸️inline— dump right after function inlining 🌸strubm— dump right after selecting "strub" modes, and recording the selections as function attributes 🎀strub— dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking 🧼
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— dump 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— control the dumping of language-specific information. The options and filename portions behave as described in the-fdump-treeoption. The following switch values are accepted:all— enable all language-specific dumps 👑class— dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. This option is applicable to C++ only 🎀module— dump module information. Optionslineno(locations),graph(reachability),blocks(clusters),uid(serialization),alias(mergeable),asmname(Elrond),eh(mapper) &vops(macros) may provide additional information. This option is applicable to C++ only 🧚raw— dump the raw internal tree data. This option is applicable to C++ only 🌱
-fdump-passes— print on stderr the list of optimization passes that are turned on and off by the current command-line options 📋-fdump-statistics-option— enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in.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— 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 which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available:address— print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment 📍asmname— ifDECL_ASSEMBLER_NAMEhas been set for a given decl, use that in the dump instead ofDECL_NAME. Its primary use is ease of use working backward from mangled names in the assembly file 🔤slim— when dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path. When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures. When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation 💅raw— print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation 🌱details— enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes 🔍stats— enable dumping various statistics about the pass (not honored by every dump option) 📊blocks— enable showing basic block boundaries (disabled in raw dumps) 🧱graph— for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot. This option currently only works for RTL dumps, and the RTL is always dumped in slim form 🕸️vops— enable showing virtual operands for every statement 👻lineno— enable showing line numbers for statements 🔢uid— enable showing the unique ID (DECL_UID) for each variable 🆔verbose— enable showing the tree dump for each statement 📢eh— enable showing the EH region number holding each statement 🌸scev— enable showing scalar evolution analysis details 📈optimized— enable showing optimization information (only available in certain passes) ✨missed— enable showing missed optimization information (only available in certain passes) 😭note— enable other detailed optimization information (only available in certain passes) 📝all— turn on all options, exceptraw,slim,verboseandlineno👑optall— turn on all optimization options, i.e.,optimized,missed, andnote🌈
To figure out what tree dumps are available or find the dump for a pass of interest, follow the steps below 👇
- Invoke GCC with
-fdump-passesand in the stderr output look for a code that corresponds to the pass you are interested in. For example, the codestree-evrp,tree-vrp1, andtree-vrp2correspond 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 the-fdump-tree-evrpoption. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below 💖
- The pass dump is gonna be in a file whose name is three components glued together with periods: the name of the source file GCC was invoked to compile, a numeric suffix showing the pass number with a lil
tafter it for tree passes (andrfor RTL passes, bestie), and then finally the pass code. So like, the Early VRP pass dump could be chillin' in a file calledmyfile.c.038t.evrpin the current working directory. 🫶
Heads up though: those numeric codes are not stable and can absolutely change from one version of GCC to another. Don't @ me. 😭
-fopt-info-fopt-info-options-fopt-info-options=filename— controls optimization dumps from the various optimization passes. If you use the-optionsform, then options is a list of-separated option keywords that pick which dump details and optimizations you wanna see. 💅
So the options break down into three groups, and this is the vibe check:
- options describing what kinds of messages should be emitted,
- options describing the verbosity of the dump, and
- options describing which optimizations should be included.
The options from each group can be freely mixed together, no drama, because they're non-overlapping. But — if there's any conflict, the later options override the earlier options on the command line. Last one wins, periodt. ✨
The "what messages tho" group 🎀
optimized— prints info when an optimization is successfully applied. It's up to a pass to decide which information is relevant. Like, the vectorizer passes print the source location of loops that got successfully vectorized. Slay.missed— prints info about missed optimizations. Individual passes control what info goes into the output. (We love a transparent king. 🙈)note— prints verbose info about optimizations, like certain transformations, more detailed messages about decisions, etc.all— prints detailed optimization information. This is literallyoptimized,missed, andnoteall bundled up. The full combo meal. 🍓
The verbosity group 🌸
internals— by default only "high-level" messages get emitted. This option turns on extra, more detailed messages, which are probably only gonna be interesting to GCC developers. Ngl, it's giving iykyk. 💀
The "which optimizations" group 🦋
One or more of these keywords can be used to describe a group of optimizations:
ipa— enable dumps from all interprocedural optimizations.loop— enable dumps from all loop optimizations.inline— enable dumps from all inlining optimizations.omp— enable dumps from all OMP (Offloading and Multi Processing) optimizations.vec— enable dumps from all vectorization optimizations.optall— enable dumps from all optimizations. This is a superset of all the optimization groups listed above. Main character energy fr fr. 👑
So if options is omitted, it defaults to optimized-optall, which means dump messages about successful optimizations from all the passes, while omitting messages that are treated as "internals". Easy.
If the filename is provided, then the dumps from all the applicable optimizations get concatenated into that filename. Otherwise the dump just goes straight onto stderr. And listen — even though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided, then all but the first such option are ignored. 😤
Also important: the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, you should use stderr instead. No cap.
In this example, the optimization info goes to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs a missed optimization report from all the passes into missed.all. And this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints information about missed optimization opportunities from vectorization passes on stderr. 💖
Fun fact bestie:-fopt-info-vec-missedis equivalent to-fopt-info-missed-vec. The order of the optimization group names and message types listed after-fopt-infodoes not matter. Iconic.
As another example:
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt. 🌈
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. So only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities. The second one? Ghosted. 💀
-fsave-optimization-record— writes aSRCFILE.opt-record.json.gzfile detailing what optimizations were performed, for those optimizations that support-fopt-info. 🎀
Warning bestie: this option is experimental and the format of the data within the compressed JSON file is subject to change. Don't build your whole personality on it.
It's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, plus these extra goodies for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted.
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. Nested messages, it's giving inception. 🌀
-fsched-verbose=n— on targets that use instruction scheduling, this option controls how much debugging output the scheduler prints to the dump files. ✨
So like, for n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. The numbers just keep unlocking stuff, it's a whole ladder. 💅
-fenable-kind-pass-fdisable-kind-pass=range-list— this is a set of options used to explicitly disable/enable optimization passes.
Okay but real talk: these options are intended for use for debugging GCC. Regular compiler users should use the normal options for enabling/disabling passes instead. This is dev-only territory, iykyk. 🙈
The -fdisable crew
-fdisable-ipa-pass— disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.-fdisable-rtl-pass-fdisable-rtl-pass=range-list— disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option-fdump-passes.-fdisable-tree-pass-fdisable-tree-pass=range-list— disable tree pass pass. See-fdisable-rtlfor the description of option arguments.
The -fenable crew 🌸
-fenable-ipa-pass— enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.-fenable-rtl-pass-fenable-rtl-pass=range-list— enable RTL pass pass. See-fdisable-rtlfor option argument description and examples.-fenable-tree-pass-fenable-tree-pass=range-list— enable tree pass pass. See-fdisable-rtlfor the description of option arguments.
Here are some examples showing uses of these options, bestie. 👇
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking-fchecking=n— enable internal consistency checking. The default depends on the compiler configuration.-fchecking=2enables further internal consistency checking that might affect code generation. 🧚
-frandom-seed=string— this option provides a seed that GCC uses in place of random numbers when generating 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 the object files that produce them. You can use the-frandom-seedoption to produce reproducibly identical object files. 🔥
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32).
The string should be different for every file you compile. fr fr bestie, don't be reusing it 💅
-save-temps— okay so this one keeps the "temporary" intermediate files forever instead of yeeting them, and names them as auxiliary output files, exactly as described under-dumpbaseand-dumpdir. ✨ When you pair it with the-xcommand-line option,-save-tempsis lowkey smart enough to not overwrite an input source file that happens to have the same extension as an intermediate file. If you want that corresponding intermediate file anyway, just rename the source file before you use-save-temps. 🎀-save-temps=cwd— same energy as-save-temps -dumpdir ./. periodt.-save-temps=obj— equivalent to-save-temps -dumpdir outdir/, whereoutdir/is the directory of the output file you specified after the-ooption, including any directory separators. If you didn't use-oat all, then-save-temps=objjust behaves like-save-temps=cwd. slay.-time[=file]— this reports the CPU time each subprocess in the compilation sequence took. For C source files, that's the compiler proper and the assembler (plus the linker if linking is happening). 🦋
Without an output file specified, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01First number on each line = the "user time", aka the time spent executing the program itself. Second number = "system time", aka time spent executing operating system routines on behalf of the program. Both are in seconds. ⭐
With an output file specified, the output gets appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>The "user time" and "system time" move before the program name, and the options passed to the program get displayed too, so later you can tell exactly what file was being compiled and with which options. obsessed tbh
-fdump-final-insns[=file]— dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is.), the dump file's name is determined by appending.gkdto the dump base name, see-dumpbase. 👑-fcompare-debug[=opts]— if no error occurs during compilation, it runs the compiler a second time, adding opts and-fcompare-debug-secondto the arguments passed to that second compilation. It dumps the final internal representation in both compilations, and prints an error if they differ. 🫶
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's used for opts; otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 💀
-fcompare-debug-second— this option is implicitly passed to the compiler for that second compilation requested by-fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed so they contain the.gkadditional extension during the second compilation, to avoid overwriting those generated by the first. 🌸
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. ngl that's a very specific little niche
-gtoggle— turns off generation of debug info, if leaving out this option generates it; or turns it on at level 2 otherwise. Where this argument sits in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times you give it. This is mainly intended to be used with-fcompare-debug. ✨-fvar-tracking-assignments-toggle— toggles-fvar-tracking-assignments, in the same way that-gtoggletoggles-g. 🎀-Q— makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. 🔥-ftime-report— makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes.
If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr then the -ftime-report information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. iykyk
-ftime-report-details— records the time consumed by infrastructure parts separately for each pass. ⭐-fira-verbose=n— controls the verbosity of the dump file for the integrated register allocator. Default value is5. If the value n is greater than or equal to10, the dump output is sent to stderr using the same format as n minus10. 👑-flto-report— prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via-flto).
Disabled by default.
-flto-report-wpa— like-flto-report, but only 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 before or after interprocedural optimization. 💅-fmultiflags— this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time.
Like "TFLAGS", this allows the target libraries to be built for portable baseline environments, while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if the defaults are not ideal for their target environment, whereas rebuilding the runtime libraries is usually not as easy or desirable. 🌈
Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.
This option is discarded by the driver once it's done processing driver self spec.
It's also useful to check that "TFLAGS" are being used to build all target libraries, by configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries. ✨
-fprofile-report— makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. 🎀-fstack-usage— makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending.suto the auxname. auxname is generated from the name of the output file, if explicitly specified and it isn't an executable; otherwise it's the basename of the source file. An entry is made up of three fields:- The name of the function.
- A number of bytes.
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier dynamic means the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it isn't present, the amount of these adjustments is not 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. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. 🙈-fdbg-cnt-list— 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 bound. counter-value-list is a comma-separated list ofname:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples, which sets the name of the counter and list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with-fdbg-cnt=dce:2-4:10-11,tail_call:10,dbg_cnt(dce)returns true only for second, third, fourth, tenth and eleventh invocation. Fordbg_cnt(tail_call)true is returned for first 10 invocations. 🔥-print-file-name=library— prints the full absolute name of the library file library that would be used when linking---and does nothing else. With this option, GCC does not compile or link anything; it just prints the file name. 👑-print-multi-directory— prints the directory name that matches the multilib selected by all the other switches hanging out in your command line. This directory is supposed to exist inGCC_EXEC_PREFIX. no cap, it's just vibes-checking your multilib setup 🎀-print-multi-lib— prints the mapping from multilib directory names → the compiler switches that enable them. The directory name gets separated from the switches by;, and every switch starts with an@instead of the-, with no spaces between multiple switches. It's giving "please make shell processing easier" energy 🐚-print-multi-os-directory— prints the path to OS libraries for the multilib you selected, relative to somelibsubdirectory. If OS libraries are present in thelibsubdirectory and no multilibs are used, this is usually just.; if OS libraries are present inlibsuffixsibling directories it prints e.g.../lib64,../libor../lib32; or if OS libraries are present inlib/subdirsubdirectories it prints e.g.amd64,sparcv9orev6. so many flavors of path, bestie, and it knows exactly which one is yours ✨-print-multiarch— prints the path to OS libraries for the selected multiarch, relative to somelibsubdirectory. that's it, that's the whole thing 🦋-print-prog-name=program— same energy as-print-file-name, but it searches for a program such ascppinstead. 🫶-print-libgcc-file-name— same as-print-file-name=libgcc.a. This one's your bestie when you're using-nostdlibor-nodefaultlibsbut you still want to link withlibgcc.a. You can literally do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`-print-search-dirs— prints the name of the configured installation directory plus a list of program and library directoriesgccsearches — and doesn't do anything else. This one is iconic for whengcchits you with the error messageinstallation problem, cannot exec cpp0: No such file or directory. To fix it you either need to putcpp0and the other compiler components wheregccexpects to find them, or you can set the environment variableGCC_EXEC_PREFIXto the directory where you installed them. Don't forget the trailing/! 😭-print-sysroot— prints the target sysroot directory used during compilation. This is the target sysroot specified either at configure time or with the--sysrootoption, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing — silence, bestie, pure silence 💀-print-sysroot-headers-suffix— prints the suffix that gets added to the target sysroot when searching for headers, or it throws an error if the compiler isn't configured with such a suffix — and doesn't do anything else. ✨-dumpmachine— prints the compiler's target machine (for example,i686-pc-linux-gnu) — and doesn't do anything else. 🎀-dumpversion— prints the compiler version (for example,3.0,6.3.0or7) — and doesn't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). mystery number reveal, iykyk 🔢-dumpfullversion— prints the full compiler version — and doesn't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version. 👑-dumpspecs— prints the compiler's built-in specs — and doesn't do anything else. (This is used when GCC itself is being built.) 🔧
🛠️ Machine-Dependent Options 🛠️
Every target machine supported by GCC can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with -m. cute little naming ritual, honestly 🎀
Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.
🌸 AArch64 Options 🌸
These options are defined for AArch64 implementations:
-mabi=name— generate code for the specified data model. Permissible values areilp32for a SysV-like data model whereint,long intand pointers are 32 bits, andlp64for a SysV-like data model whereintis 32 bits, butlong intand pointers are 64 bits. The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. no mixing, bestie, pick a lane 🚧-mbig-endian— generate big-endian code. This is the default when GCC is configured for anaarch64_be-*-*target. 🐘-mgeneral-regs-only— generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. regs on a diet ✨-mlittle-endian— generate little-endian code. This is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target. 🐭-mcmodel=tiny— generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked. smol but mighty 🧚-mcmodel=small— generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. 👑-mcmodel=large— generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The-mcmodel=largeoption is incompatible with-mabi=ilp32,-fpicand-fPIC. big model, big restrictions, fr fr 💀-mtp=name— specify the system register to use as a thread pointer. The valid values aretpidr_el0,tpidrro_el0,tpidr_el1,tpidr_el2,tpidr_el3. For backwards compatibility the aliasesel0,el1,el2,el3are also accepted. The default setting istpidr_el0. It is recommended to compile all code intended to interoperate with the same value of this option to avoid accessing a different thread pointer from the wrong exception level. keep it consistent or the exception levels get messy 🌀-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. 🎯-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— omit or keep the frame pointer in leaf functions. The former behavior is the default. 🍃-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. Supported locations areglobalfor a global canary orsysregfor a canary in an appropriate system register. With the latter choice the options-mstack-protector-guard-reg=regand-mstack-protector-guard-offset=offsetfurthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. 🐧-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. 💅-mtls-dialect=traditional— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. 🏛️-mtls-size=size— specify bit size of immediate TLS offsets. Valid values are12,24,32,48. This option requires binutils 2.26 or newer. ⚙️-mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. patch that erratum, bestie 🩹-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker. 🔗-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— enable or disable the reciprocal square root approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. approximation = less precise, don't forget it 😭-mlow-precision-sqrt,-mno-low-precision-sqrt— enable or disable the square root approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies-mlow-precision-recip-sqrt. 🧮-mlow-precision-div,-mno-low-precision-div— enable or disable the division approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision. ➗-mtrack-speculation,-mno-track-speculation— enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to__builtin_speculation_safe_copyto permit a more efficient code sequence to be generated. 🕵️-moutline-atomics,-mno-outline-atomics— enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g.-march=armv8.1-aor-march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using-mcpu=when the selected cpu supports thelsefeature. This option is on by default. ⚛️-march=name— specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form-march=arch{+[no]feature}*. The table below summarizes the permissible values for arch and the features that they enable by default: architectures, assemble ✨🏗️✨
okay bestie, here's the arch lineup — arch value, architecture, and the features it includes by default, in that exact order:
armv8-a— Armv8-A —+fp,+simdarmv8.1-a— Armv8.1-A —armv8-a,+crc,+lse,+rdmaarmv8.2-a— Armv8.2-A —armv8.1-aarmv8.3-a— Armv8.3-A —armv8.2-a,+pautharmv8.4-a— Armv8.4-A —armv8.3-a,+flagm,+fp16fml,+dotprodarmv8.5-a— Armv8.5-A —armv8.4-a,+sb,+ssbs,+predresarmv8.6-a— Armv8.6-A —armv8.5-a,+bf16,+i8mmarmv8.7-a— Armv8.7-A —armv8.6-a,+ls64armv8.8-a— Armv8.8-a —armv8.7-a,+mopsarmv8.9-a— Armv8.9-a —armv8.8-aarmv9-a— Armv9-A —armv8.5-a,+sve,+sve2armv9.1-a— Armv9.1-A —armv9-a,+bf16,+i8mmarmv9.2-a— Armv9.2-A —armv9.1-a,+ls64armv9.3-a— Armv9.3-A —armv9.2-a,+mopsarmv9.4-a— Armv9.4-A —armv9.3-aarmv8-r— Armv8-R —armv8-r
The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. 🫶
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.
💅 -mtune=name
Specify the name of the target processor for which GCC should tune the performance of the code. The permissible values for this option 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, 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, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a710, cortex-a715, cortex-a720, ampere1, ampere1a, ampere1b, cobalt-100 and native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC should tune for a big.LITTLE system. 💖
The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.
Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
fr fr, this option cannot be suffixed by feature modifiers. periodt. 💀
🌸 -mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.
-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.
🦋 -moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.
This option is only intended to be useful when developing GCC. iykyk, it's giving compiler-dev-only energy 🧚
⭐ -mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
💜 -mpc-relative-literal-loads / -mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.
🎀 -msign-return-address=scope
Select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none.
This option has been deprecated by -mbranch-protection. rip bestie 🙈
👑 -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti
Select the branch protection features to use. none is the default and turns off all types of branch protection. standard turns on all types of branch protection features. If a feature has additional tuning options, then standard sets it to its standard level. pac-ret[+leaf] turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument leaf can be used to extend the signing to include leaf functions. The optional argument b-key can be used to sign the functions with the B-key instead of the A-key. bti turns on branch target identification mechanism.
🔥 -mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:
retbrblr
In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening.
🌈 -mearly-ra=scope
Determine when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1.
The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.
-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.
✨ -mearly-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above.
💖 -mlate-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above.
🫶 -msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.
🧚 -march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
crc— Enable CRC extension. This is on by default for-march=armv8.1-a.crypto— Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.fp— Enable floating-point instructions. This is on by default for all possible values for options-marchand-mcpu.simd— Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options-marchand-mcpu.sve— Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.lse— Enable Large System Extension instructions. This is on by default for-march=armv8.1-a.rdma— Enable Round Double Multiply Accumulate instructions. This is on by default for-march=armv8.1-a.fp16— turns on the FP16 extension. It also turns on floating-point instructions, so you're getting both, no extra charge 💖fp16fml— turns on the FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. It's on by default for-march=armv8.4-a. Using it with architectures older than Armv8.2-A? Not supported, bestie, don't even try it 🙈rcpc— turns on the RCpc extension. This unlocks theLDAPRinstructions for load-acquire atomic semantics, and it passes that along to the assembler, so inline asm statements can use instructions from the RCpc extension. Iconic teamwork ✨dotprod— turns on the Dot Product extension. Also enables Advanced SIMD instructions.aes— turns on the Armv8-a aes and pmull crypto extension. Also enables Advanced SIMD instructions.sha2— turns on the Armv8-a sha2 crypto extension. Also enables Advanced SIMD instructions.sha3— turns on the sha512 and sha3 crypto extension. Also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supported.sm4— turns on the sm3 and sm4 crypto extension. Also enables Advanced SIMD instructions. Using this with architectures prior to Armv8.2-A is not supported.profile— turns on the Statistical Profiling extension. Heads up: this one only enables the extension at the assembler level and does not affect code generation.rng— turns on the Armv8.5-a Random Number instructions. Again, this is assembler-level only and does not affect code generation.memtag— turns on the Armv8.5-a Memory Tagging Extensions. Using this with architectures prior to Armv8.5-A is not supported.sb— turns on the Armv8-a Speculation Barrier instruction. Assembler level only, does not affect code generation. It's on by default for-march=armv8.5-a.ssbs— turns on the Armv8-a Speculative Store Bypass Safe instruction. Assembler level only, does not affect code generation. On by default for-march=armv8.5-a.predres— turns on the Armv8-a Execution and Data Prediction Restriction instructions. Assembler level only, does not affect code generation. On by default for-march=armv8.5-a.sve2— turns on the Armv8-a Scalable Vector Extension 2. Also enables SVE instructions.sve2-bitperm— turns on SVE2 bitperm instructions. Also enables SVE2 instructions.sve2-sm4— turns on SVE2 sm4 instructions. Also enables SVE2 instructions.sve2-aes— turns on SVE2 aes instructions. Also enables SVE2 instructions.sve2-sha3— turns on SVE2 sha3 instructions. Also enables SVE2 instructions.tme— turns on the Transactional Memory Extension.i8mm— turns on 8-bit Integer Matrix Multiply instructions. Also enables Advanced SIMD and floating-point instructions. On by default for-march=armv8.6-a. Using it with architectures prior to Armv8.2-A is not supported.f32mm— turns on 32-bit Floating point Matrix Multiply instructions. Also enables SVE instructions. Using it with architectures prior to Armv8.2-A is not supported.f64mm— turns on 64-bit Floating point Matrix Multiply instructions. Also enables SVE instructions. Using it with architectures prior to Armv8.2-A is not supported.bf16— turns on brain half-precision floating-point instructions. Also enables Advanced SIMD and floating-point instructions. On by default for-march=armv8.6-a. Using it with architectures prior to Armv8.2-A is not supported.ls64— turns on the 64-byte atomic load and store instructions for accelerators. On by default for-march=armv8.7-a.mops— turns on the instructions that accelerate memory operations likememcpy,memmove,memset. On by default for-march=armv8.8-aflagm— turns on the Flag Manipulation instructions Extension.pauth— turns on the Pointer Authentication Extension.cssc— turns on the Common Short Sequence Compression instructions.sme— turns on the Scalable Matrix Extension.sme-i16i64— turns on the FEAT_SME_I16I64 extension to SME.sme-f64f64— turns on the FEAT_SME_F64F64 extension to SME.+@item sme2turns on the Scalable Matrix Extension 2. This also enables SME instructions. (yeah the raw manpage literally has that+@itemartifact wedged in there — not me editing it, I'm just being faithful to the source, fr fr 💀)lse128— turns on the LSE128 128-bit atomic instructions extension. Also enables LSE instructions.d128— turns on support for 128-bit system register read/write instructions. Also enables the LSE128 extension.gcs— turns on support for Armv9.4-a Guarded Control Stack extension.the— turns on support for Armv8.9-a/9.4-a translation hardening extension.rcpc3— turns on the RCpc3 (Release Consistency) extension.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. The lore is lore 🧚
🌈 Adapteva Epiphany Options 🌈
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file— don't allocate any register in the ranger32...r63. That lets your code run on hardware variants that just don't have those registers. Adapting, as one does 💅-mprefer-short-insn-regs— preferentially allocate registers that allow short instruction generation. This can bump up the instruction count, so it may either reduce or increase your overall code size. It's giving chaos, honestly 🦋-mbranch-cost=num— set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.-mcmove— enable the generation of conditional moves.-mnops=num— emit num NOPs before every other generated instruction.-mno-soft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. This is faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is-msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.-mstack-offset=num— set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the rangesp+0...sp+7can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Also note that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate--with-stack-offset=numoption.
ABI change alert!! 🚨 Don't just sprinkle -mstack-offset around like confetti, bestie.
-mno-round-nearest— make the scheduler assume that the rounding mode has been set to truncating. The default is-mround-nearest.-mlong-calls— if not otherwise specified by an attribute, assume all calls might be beyond the offset range of theb/blinstructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.-mshort-calls— if not otherwise specified by an attribute, assume all direct calls are in the range of theb/blinstructions, so use these instructions for direct calls. The default is-mlong-calls.-msmall16— assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which-mlong-callssemantics are in effect.-mfp-mode=mode— set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. 🌸
mode can be set to one of the following values:
caller— any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.truncate— this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.round-nearest— this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.int— this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mno-split-lohi,-mno-postinc,-mno-postmodify— code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults aremsplit-lohi,-mpost-inc, and-mpost-modify.-mnovect-double— change the preferred SIMD mode to SImode. The default is-mvect-double, which uses DImode as preferred SIMD mode.-max-vect-align=num— the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.-msplit-vecmove-early— split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 💀-m1reg-reg— bestie, this one lets you pick a whole register whose entire personality is holding the constant-1💅. Why?? Because it makes loading tiny negative constants and certain bitmasks way faster, slay. What can reg be? Onlyr43andr63(both meaning "yeah use that register as a fixed register"), ornone, which is exactly what it sounds like — no register gets this job. Default is-m1reg-none. Periodt.
✨ AMD GCN Options ✨
These ones exist specifically for the AMD GCN port, no cap 🎀.
-march=gpu-mtune=gpu— sets the architecture type or the tuning for gpu. Here's every value gpu is allowed to be, babes:fiji— compile for GCN3 Fiji devices (gfx803). Support is deprecated; whether you even get it depends on how GCC was configured, so peep--with-archand--with-multilib-list.gfx900— compile for GCN5 Vega 10 devices (gfx900).gfx906— compile for GCN5 Vega 20 devices (gfx906).gfx908— compile for CDNA1 Instinct MI100 series devices (gfx908).gfx90a— compile for CDNA2 Instinct MI200 series devices (gfx90a).gfx1030— compile for RDNA2gfx1030devices (GFX10 series).gfx1036— compile for RDNA2gfx1036devices (GFX10 series).gfx1100— compile for RDNA3gfx1100devices (GFX11 series).gfx1103— compile for RDNA3gfx1103devices (GFX11 series).
-msram-ecc=on-msram-ecc=off-msram-ecc=any— compiles binaries that are suited for devices with the SRAM-ECC feature turned on, turned off, or either mode 🦋. On some devices this feature can be flipped per-process, and your compiled code has to match whatever mode the device is in. Default isany, for devices that support it.-mstack-size=bytes— tells it how many bytes of stack space each GPU thread (a wave-front, bestie) gets to ask for. ⚠️ Be careful: there might be so many threads and only limited memory to go around. The size of the stack allocation can also mess with run-time performance, highkey. Default is32KBwhen you're using OpenACC or OpenMP, and1MBotherwise.-mxnack=on-mxnack=off-mxnack=any— compiles binaries suited for devices with the XNACK feature on, off, or either mode 💖. Some devices always require XNACK, and some let the user configure it. The compiled code must match the device mode, period. Default is-mxnack=anyon devices that support Unified Shared Memory, and-mxnack=nootherwise.
🌸 ARC Options 🌸
The following options control the architecture variant for which code is being compiled (yes, that's the whole vibe of this lil group):
-mbarrel-shifter— generate instructions that the barrel shifter supports ✨. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect.-mjli-always— forces calls to a function to use thejli_sinstruction. Only valid for the ARCv2 architecture, iykyk.-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: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.hs4x— compile for ARC HS4x CPU.hs4xd— compile for ARC HS4xD CPU.hs4x_rel31— compile for ARC HS4x CPU release 3.10a.arc600_norm— compile for ARC 600 CPU with "norm" instructions enabled.arc600_mul32x16— compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.arc600_mul64— compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.arc601_norm— compile for ARC 601 CPU with "norm" instructions enabled.arc601_mul32x16— compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.arc601_mul64— compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.nps400— compile for ARC 700 on NPS400 chip.em_mini— compile for ARC EM minimalist configuration featuring a reduced register set.
-mdpfp-mdpfp-compact— generate double-precision FPX instructions, tuned for the compact implementation 🎀.-mdpfp-fast— generate double-precision FPX instructions, tuned for the fast implementation 🔥.-mno-dpfp-lrsr— disables thelrandsrinstructions from using FPX extension aux registers.-mea— generate extended arithmetic instructions. Right now onlydivaw,adds,subs, andsat16are supported. Only valid for-mcpu=ARC700.-mno-mpy— don't generate "mpy"-family instructions for ARC700. Heads up: this option is deprecated.-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions.-mmul64— generatemul64andmulu64instructions. Only valid for-mcpu=ARC600.-mnorm— generate "norm" instructions. This is the default if-mcpu=ARC700is in effect.-mspfp-mspfp-compact— generate single-precision FPX instructions, tuned for the compact implementation.-mspfp-fast— generate single-precision FPX instructions, tuned for the fast implementation ⚡.-msimd— enables generation of ARC SIMD instructions via target-specific builtins. Only valid for-mcpu=ARC700.-msoft-float— bestie, this option is ignored 💀. It's only here for compatibility purposes. Software floating-point code is emitted by default, and that default can be overridden by the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision.-mswap— generate "swap" instructions.-matomic— enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores 🙈.-mdiv-rem— enablesdivandreminstructions for ARCv2 cores.-mcode-density— enables code density instructions for ARC EM. This option is on by default for ARC HS.-mll64— enables double load/store operations for ARC HS cores.-mtp-regno=regno— specify the thread pointer register number.-mmpy-option=multo— compile ARCv2 code with a multiplier design option. You can specify it with either a string or a numeric value for multo.wlh1is the default value. The recognized values are:0,none— no multiplier available.1,w— 16x16 multiplier, fully pipelined. The following instructions are enabled:mpywandmpyuw.2,wlh1— 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled:mpy,mpyu,mpym,mpymu, andmpy_s.3,wlh2— 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled:mpy,mpyu,mpym,mpymu, andmpy_s.4,wlh3— two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled:mpy,mpyu,mpym,mpymu, andmpy_s.5,wlh4— one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled:mpy,mpyu,mpym,mpymu, andmpy_s.6,wlh5— one 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled:mpy,mpyu,mpym,mpymu, andmpy_s.7,plus_dmpy— ARC HS SIMD support.8,plus_macd— ARC HS SIMD support.9,plus_qmacw— ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu— enables support for specific floating-point hardware extensions for ARCv2 cores 🧚. Supported values for fpu are:fpus— enables support for single-precision floating-point hardware extensions.fpud— enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.fpuda— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.fpuda_div— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.fpuda_fma— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.fpuda_all— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.fpus_div— enables support for single-precision floating-point, square-root and divide hardware extensions.
fpud_div— turns on support for the double-precision floating-point, square-root and divide hardware extensions. Bestie, this one includes optionfpus_div. Not available for ARC EM. 🙈fpus_fma— turns on support for single-precision floating-point plus the fused multiply-and-add hardware extensions. ✨fpud_fma— turns on support for double-precision floating-point plus fused multiply-and-add hardware extensions. And yeah, this one includes optionfpus_fma. Not available for ARC EM. 💀fpus_all— turns on support for all the single-precision floating-point hardware extensions. the whole menu, no crumbs. 🍓fpud_all— turns on support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM. 😭-mirq-ctrl-saved=register-range, blink, lp_count — tells the compiler which general-purpose registers your processor automatically saves and restores when it enters and leaves an interrupt.register-rangeis written as two registers with a dash between them, and the range always starts with"r0"— the upper limit is the"fp"register.blinkandlp_countare optional. This option is only valid for ARC EM and ARC HS cores, so if you're on something else, it's giving ✨not for you✨.-mrgf-banked-regs=number — says how many registers get replicated into the second register bank when a fast interrupt hits. Fast interrupts are the ones with the highest priority level P0, and they only save the PC and STATUS32 registers, so the interrupt entry and exit sequences don't do memory transactions. Reach for this one when you're running fast interrupts on an ARC V2 family processor. Permitted values are4,8,16, and32. pick your fighter 💅-mlpc-width=width — sets the width of the"lp_count"register. Valid values for width are8,16,20,24,28and32bits, and the default width is locked to32bits. If the width is less than 32, the compiler won't try to transform loops in your program to use the zero-delay loop mechanism unless it knows the"lp_count"register can hold the loop-counter value it needs. Depending on the width you pick, the compiler and run-time library might still keep using the loop mechanism for various needs. Also, this option defines the macro"__ARC_LPC_WIDTH__"with the value of width. 🎀-mrf16— tells the compiler to generate code for a 16-entry register file. This option defines the"__ARC_RF16__"preprocessor macro. no cap 👑-mbranch-index— enables the use of"bi"or"bih"instructions to implement jump tables. 🔥
The following options are passed straight 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. Also sets the preprocessor symbol"__Xdsp_packa". > Heads up: this option is deprecated. 😭-mdvbf— passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol"__Xdvbf". > This option is deprecated. 💀-mlock— passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol"__Xlock". 🫶-mmac-d16— passed down to the assembler. Also sets the preprocessor symbol"__Xxmac_d16". > This option is deprecated. 🙈-mmac-24— passed down to the assembler. Also sets the preprocessor symbol"__Xxmac_24". > This option is deprecated. 💅-mrtsc— passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol"__Xrtsc". > This option is deprecated. ⭐-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol"__Xswape". 🌸-mtelephony— passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol"__Xtelephony". > This option is deprecated. 😭-mxy— passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol"__Xxy". 🦋
The following options control how the assembly code is annotated: 📝
-misize— annotate assembler instructions with estimated addresses.-mannotate-align— does nothing. Preserved for backward compatibility. (she's just vibing, contributing nothing 💀)
The following options are passed through to the linker: 🔗
-marclinux— passed through to the linker, to specify use of the"arclinux"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is not requested.-marclinux_prof— passed through to the linker, to specify use of the"arclinux_prof"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is requested.
The following options control the semantics of generated code: 🧚
-mlong-calls— generate calls as register indirect calls, which gives you access to the full 32-bit address range. iconic behavior honestly 💜-mmedium-calls— don't use anything less than a 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls gets suppressed so the 25-bit range can be used, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets. 🌈-Gnum — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is4for any ARC configuration, or8when we have double load/store operations. 🍓-mno-sdata— do not generate sdata references. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-mvolatile-cache— use ordinarily cached memory accesses for volatile references. This is the default. 💅-mno-volatile-cache— enable cache bypass for volatile references.
The following options fine tune code generation: ✨
-malign-call— does nothing. Preserved for backward compatibility. 💀-mauto-modify-reg— enable the use of pre/post modify with register displacement. 🔥-mbbit-peephole— does nothing. Preserved for backward compatibility. 🙈-mno-brcc— this option disables a target-specific pass inarc_reorgthat generates compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions when they're driven by the combiner pass. 😭-mcase-vector-pcrel— use PC-relative switch case tables to enable case table shortening. This is the default for-Os. 🎀-mcompact-casesi— enable the compact"casesi"pattern. This is the default for-Os, and it's only available for ARCv1 cores. > This option is deprecated. 💅-mno-cond-exec— disable the ARCompact-specific pass that generates conditional execution instructions.
Okay so here's the tea, bestie: because of delay slot scheduling and the interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lacking. That's why the ARC port has kept a special pass around that hunts for more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have already been done. This pass generally — but not always — improves performance and code size, at the cost of extra compilation time, which is exactly why there's an option to switch it off. And if you're having a problem with call instructions exceeding their allowable offset range because they got conditionalized, you should consider using -mmedium-calls instead. 🫶
-mearly-cbranchsi— enable pre-reload use of the"cbranchsi"pattern. ⭐-mexpand-adddi— expand"adddi3"and"subdi3"at RTL generation time into"add.f","adc"etc. > This option is deprecated. 💀-mindexed-loads— enable the use of indexed loads. Careful though, this can be problematic, because some optimizers then assume that indexed stores exist, which is not the case. 😭-mlra— enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e.-mno-lra). 🙈-mlra-priority-none— don't indicate any priority for target registers.-mlra-priority-compact— indicate target register priority forr0..r3/r12..r15. 💅-mlra-priority-noncompact— reduce 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 are often shortened by calling a special function inlibgcc; this is called a millicode call. Since these calls can pose performance issues, and/or cause linking issues when you link in a nonstandard way, this option exists so you can turn millicode call generation on or off. 🌸-mcode-density-frame— this option enables the compiler to emit"enter"and"leave"instructions. These instructions are only valid for CPUs with the code-density feature. 🦋-mmixed-code— does nothing. Preserved for backward compatibility. 💀-mq-class— this option is deprecated. Enableqinstruction alternatives. This is the default for-Os. 🎀-mRcq— does nothing. Preserved for backward compatibility. 🙈-mRcw— does nothing. Preserved for backward compatibility. 😭-msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:0— no size optimization. > This level is deprecated and treated like1. 💀1— short instructions are used opportunistically.2— in addition, alignment of loops and of code after barriers are dropped.3— in addition, optional data alignment is dropped, and the optionOsis enabled. 🔥
This defaults to 3 when -Os is in effect. Otherwise, if this isn't set, the behavior is equivalent to level 1. ⭐
-mtune=cpu — set instruction scheduling parameters for cpu, overriding anything implied by-mcpu=. 👑
Supported values for cpu are:
ARC600— tune for ARC600 CPU.ARC601— tune for ARC601 CPU.ARC700— tune for ARC700 CPU with standard multiplier block.ARC700-xmac— tune for ARC700 CPU with XMAC block.ARC725D— tune for ARC725D CPU.
ARC750D— tune for the ARC750D CPU 💅core3— tune for an ARCv2 core3 type CPU. This option enable usage of the"dbnz"instruction. (yes, it enables, we said what we said ✨)release31a— tune for ARC4x release 3.10a.-mmultcost=num — the cost to assume for a multiply instruction, with4being equal to a normal instruction.-munalign-prob-threshold=probability — does nothing. Preserved for backward compatibility. it's giving decorative 🎀
The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release. ngl bestie, they're on their way out, don't get attached 😭
-margonaut— obsolete FPX.-mbig-endian,-EB— compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build"arceb-elf32"and"arceb-linux-uclibc"targets, for which big endian is the default.-mlittle-endian,-EL— compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build"arc-elf32"and"arc-linux-uclibc"targets, for which little endian is the default.-mbarrel_shifter— replaced by-mbarrel-shifter.-mdpfp_compact— replaced by-mdpfp-compact.-mdpfp_fast— replaced by-mdpfp-fast.-mdsp_packa— replaced by-mdsp-packa.-mEA— replaced by-mea.-mmac_24— replaced by-mmac-24.-mmac_d16— replaced by-mmac-d16.-mspfp_compact— replaced by-mspfp-compact.-mspfp_fast— replaced by-mspfp-fast.-mtune=cpu — valuesarc600,arc601,arc700andarc700-xmacfor cpu are replaced byARC600,ARC601,ARC700andARC700-xmacrespectively.-multcost=num — replaced by-mmultcost.
🦋 ARM Options 🦋
These -m options are defined for the ARM port. it's giving very specific, very demure 💅
-mabi=name — generate code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcs,aapcs-linuxandiwmmxt.-mapcs-frame— generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying-fomit-frame-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— this is a synonym for-mapcs-frameand is deprecated.-mthumb-interwork— generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is-mno-thumb-interwork, since slightly larger code is generated when-mthumb-interworkis specified. In AAPCS configurations this option is meaningless.-mno-sched-prolog— prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is-msched-prolog.-mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are:soft,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. do not mix them, bestie, that's a heartbreak waiting to happen 😭
-mgeneral-regs-only— generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.-mlittle-endian— generate code for a processor running in little-endian mode. This is the default for all standard configurations.-mbig-endian— generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.-mbe8,-mbe32— when linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀-march=name[+extension...]— this specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the-mcpu=option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, armv9-a, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.
Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it. the way that order matters here… 🫶
Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions. (rip to those architectures, they got nothing 🎀)
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
- The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-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
- 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, bestie, but with 32 double-precision registers and the half-precision floating-point conversion operations. two things at once, she's multitasking 💅+vfpv4-d16— VFPv4 floating-point instructions, this time rocking 16 double-precision registers.+vfpv4— VFPv4 floating-point instructions with the full 32 double-precision registers. glow up ✨+neon— the Advanced SIMD (Neon) v1 instructions plus the VFPv3 floating-point instructions. oh and psst —+neon-vfpv3is literally just an alias for this one, same energy, different name 🦋+neon-fp16— Advanced SIMD (Neon) v1 and VFPv3 floating-point instructions, and yes she comes with the half-precision floating-point conversion operations. we love a complete package 💖+nosimd— turns the Advanced SIMD instructions off. note that this does not disable floating point, so don't come for me. 🚫+nofp— turns off the floating-point and Advanced SIMD instructions. both. gone. 😭
armv8-a 👑
+crc— the Cyclic Redundancy Check (CRC) instructions. she checks. she verifies. ✅+simd— the ARMv8-A Advanced SIMD and floating-point instructions. the whole squad 🫶+crypto— the cryptographic instructions 🔐+nocrypto— disables the cryptographic instructions. not today, crypto.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. absolute wipeout 💀+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. heads up: this also enables the Advanced SIMD and floating-point instructions. two for one, iconic 💅+nocrypto— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. all of them, bestie. 😭+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.2-a & armv8.3-a 🌸
ok so these two share the exact same extension list, so I'm giving you the tea once and you can apply it to both, fr fr.
+fp16— the half-precision floating-point data processing instructions. and yes, 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 the Advanced SIMD and floating-point instructions. stacking extensions like it's nothing 🔥+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. also enables the Advanced SIMD and floating-point instructions. she brings friends 🫶+dotprod— enables the Dot Product extension. this also enables Advanced SIMD instructions.+nocrypto— disables the cryptographic extension.+nofp— disables 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. matrix math main character energy 👑+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. the whole friend group shows up 💖+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— disables the cryptographic extension.+nofp— disables 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— 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— disables the cryptographic extension.+nofp— disables 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.
armv8.6-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— disables the cryptographic extension.+nofp— disables 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. fun fact:+vfpv3xdis an alias for this exact extension, iykyk 🧚+fp— the VFPv3 floating-point instructions with 16 double-precision registers. and yep,+vfpv3-d16works as an alias for this one too.+vfpv3xd-d16-fp16— the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. say that five times fast, bestie 😭+vfpv3-d16-fp16— the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.+nofp— disables the floating-point extension.+idiv— the ARM-state integer division instructions.+noidiv— disables 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. both flavors, periodt 💅+nofp— disables the floating-point extensions. plural. all of them.
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. she does both 🔥+fp— the single-precision floating-point instructions.+fp.dp— the single- and double-precision floating-point instructions.+nofp— disables the floating-point extension.+cdecp0,+cdecp1, ... ,+cdecp7— enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7. pick your coprocessor, babe 🎀+pacbti— enables the Pointer Authentication and Branch Target Identification Extension. that name is a whole mouthful but she's important 👑
armv8-m.main 🫶
+dsp— the DSP instructions.+nodsp— disables the DSP extension.+fp— the single-precision floating-point instructions.+fp.dp— the single- and double-precision floating-point instructions.+nofp— disables the floating-point extension.+cdecp0,+cdecp1, ... ,+cdecp7— enables 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— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. the full shutdown, bestie 💀
-march=native makes the compiler go detective mode and auto-detect the architecture of the build computer. Okay but fr, at present this feature is only supported on GNU/Linux, and not all architectures are recognized. And if the auto-detect is unsuccessful, the option just... does nothing. No effect. Iconic of it honestly.
-mtune=name 🎯
This one specifies the name of the target ARM processor that GCC should tune the performance of the code for. For some ARM implementations you can get better performance by using this option, so it's lowkey worth knowing.
Permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, 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, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c, 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, star-mc1, xgene1.
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system ✨. Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that runs well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. Heads up tho — the effects of this option may change in future GCC versions as CPU models come and go. fr fr, it's a moving target.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. Same deal as before bestie 💅
-mcpu=name[+extension...] 👑
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). It's giving two-for-one, we love to see it.
But listen — where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. So -mcpu doesn't just steamroll them, okay?
Many of the supported CPUs implement optional architectural extensions. Where this is so, the architectural extensions are normally enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted.
For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered:
- floating-point and Advanced SIMD instructions will only be used if
-mfloat-abiis not set tosoft - any setting of
-mfpuother thanautowill override the available floating-point and SIMD extension instructions
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune. (Yeah, the whole giant list up there. Go look. I'll wait. 🫶)
The following extension options are common to the listed CPUs:
+nodsp— Disable the DSP instructions oncortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nopacbti— Disable the Pointer Authentication and Branch Target Identification Extension oncortex-m52andcortex-m85.+nomve— Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nomve.fp— Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+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 oncortex-m52andcortex-m55.+nofp— Disables the floating-point instructions onarm9e,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,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Disables the floating-point and SIMD instructions ongeneric-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-a53andcortex-a55.+nofp.dp— Disables the double-precision component of the floating-point instructions oncortex-r5,cortex-r7,cortex-r8,cortex-r52,cortex-r52plusandcortex-m7.+nosimd— Disables the SIMD (but not floating-point) instructions ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— Enables the cryptographic instructions oncortex-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-a53andcortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a. 🧚
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mfpu=name 🌊
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8.
Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. Sneaky little shortcuts, we stan. 💖
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g.-mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless-funsafe-math-optimizationsis also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. 😭 Not the precision bestie!!
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
-mfp16-format=name 🍓
Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. So if you're wondering why it's missing, it's giving none. Mystery solved.
-mstructure-size-boundary=n 📏
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. And here's the plot twist — different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. 😬
This option is deprecated. She's retired, respectfully.
-mabort-on-noreturn 💀
Generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return. Like, you said you weren't coming back, so... bestie, stay gone.
-mlong-calls
-mno-long-calls 📞
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction. So if you're reaching waaaay too far away, this is the move. ✨
...and like, even when you turn this bad boy on, bestie, it does not turn every function call into a long call. There's a whole vibe-check heuristic happening. Calls get left alone if the function is static, if it's got the "short_call" attribute, if it's chillin' inside the scope of a "#pragma no_long_calls" directive, or if its definition was already compiled inside the current compilation unit. Slay, right? But the exceptions to the exception? Weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions hanging out inside the scope of a "#pragma long_calls" directive always get turned into long calls. No exceptions. Periodt.
This feature is not enabled by default, so don't come crying to me when it's off. Specifying -mno-long-calls puts the default behavior right back, and so does wrapping your function calls in the scope of a "#pragma long_calls_off" directive. 🎀
Heads up bestie: these switches have no effect on how the compiler generates code to handle function calls via function pointers. Like, zero. None. Don't @ me.
-msingle-pic-base— Treats the register used for PIC addressing as read-only instead of reloading it in the prologue for every single function. The runtime system is the one responsible for initializing that register with a proper value before execution starts. It's giving "you handle the setup, I'll handle the rest."-mpic-register=reg— You pick which register does the PIC addressing. For the standard PIC base case, the default is whatever suitable register the compiler decides on. For the single PIC base case, the default isR9if the target is EABI based or stack-checking is enabled — otherwise it'sR10. 👑-mpic-data-is-text-relative— Assumes the displacement between the text and data segments is fixed at static link time, which lets you use PC-relative addressing operations to reach data you know lives in the data segment. For non-VxWorks RTP targets this option is on by default, and when you disable it on those targets it turns on-msingle-pic-baseby default. Chain reaction energy.-mpoke-function-name— 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 inspect the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know there's a function name embedded immediately preceding this location, and it has length "((pc[-3]) & 0xff000000)". Detective work, honestly. 🕵️
-mthumb,-marm— Pick your fighter: code that executes in ARM state or code that executes in Thumb state. The default for most configurations is ARM state, but that default can be changed by configuring GCC with the--with-mode=stateconfigure option. You can also override ARM/Thumb mode per-function using thetarget("thumb")andtarget("arm")function attributes or pragmas. Mix and match, it's your world.-mflip-thumb— Switches ARM/Thumb modes on alternating functions. This one exists purely for regression testing of mixed Thumb/ARM code generation and is not intended for ordinary use in compiling code. Not for you, bestie. It's a compiler-dev thing.-mtpcs-frame— Generates a stack frame that's compliant with 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— Generates a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that calls no other functions.) The default is-mno-apcs-leaf-frame. Yes, the default really is theapcsone. I don't make the rules. 😭-mcallee-super-interworking— Gives all externally visible functions in the file being compiled an ARM instruction set header that switches to Thumb mode before running the rest of the function. This lets those functions get called from non-interworking code. Not valid in AAPCS configurations because interworking is already enabled by default there.-mcaller-super-interworking— Lets calls via function pointers (including virtual functions) execute correctly whether or not the target code was compiled for interworking. There's a small overhead cost to executing a function pointer when this is on. Also not valid in AAPCS configurations, same reason — interworking's already on by default.-mtp=name— Sets the access model for the thread local storage pointer. The modelsoftgenerates calls to"__aeabi_read_tp". Other accepted models aretpidrurw,tpidruroandtpidrprw, which fetch the thread pointer straight from the corresponding system register (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface, and the argumentcp15is also accepted as a convenience alias oftpidruro. The argumentautouses the best available method for the selected processor. The default setting isauto. ✨-mtls-dialect=dialect— Sets the dialect for accessing thread local storage. Two dialects are supported —gnuandgnu2. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect selects the GNU descriptor scheme, which gives better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. Set in your ways? Respect.-mword-relocations— Only generates absolute relocations on word-sized values (i.e.R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when-fpicor-fPICis specified. This option conflicts with-mslow-flash-data. ⚔️-mfix-cortex-m3-ldrd— Some Cortex-M3 cores can cause data corruption when"ldrd"instructions with overlapping destination and base registers get used. This option avoids generating those instructions. It's enabled by default when-mcpu=cortex-m3is specified. 🛡️-mfix-cortex-a57-aes-1742098,-mno-fix-cortex-a57-aes-1742098,-mfix-cortex-a72-aes-1655431,-mno-fix-cortex-a72-aes-1655431— Enable (or disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. Enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified. 🔐-munaligned-access,-mno-unaligned-access— Enables (or disables) reading and writing of 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 for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access isn't enabled, then words in packed data structures get accessed a byte at a time. Slow and steady, I guess. The ARM attribute"Tag_CPU_unaligned_access"gets set in the generated object file to either true or false, depending on this option's setting. If unaligned access is enabled, then the preprocessor symbol"__ARM_FEATURE_UNALIGNED"is also defined.-mneon-for-64bits— This option is deprecated and has no effect. That's it. That's the whole thing. 💀-mslow-flash-data— Assumes loading data from flash is slower than fetching instructions, so literal loads get minimized for better performance. Only supported when compiling for ARMv7 M-profile, and it's off by default. It conflicts with-mword-relocations.-masm-syntax-unified— Assumes inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2 — however, that may change in future releases of GCC. Divided syntax should be considered deprecated. 👀-mrestrict-it— Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.-mprint-tune-info— Prints CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler, and it's not intended for ordinary use in compiling code. Disabled by default. Compiler-dev business only.-mverbose-cost-dump— Enables verbose cost model dumping in the debug dump files. This one's provided for use in debugging the compiler. 🔍-mpure-code— Doesn't allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, it gives all text sections the ELF processor-specific section attribute"SHF_ARM_PURECODE". 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", which can be found onhttps://developer.arm.com/documentation/ecm0359818/latest/. 🎀-mfix-cmse-cve-2021-35465— Mitigates against a potential security issue with the"VLLDM"instruction in some M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option-mcpu=is used with"cortex-m33","cortex-m35p","cortex-m52","cortex-m55","cortex-m85"or"star-mc1". The option-mno-fix-cmse-cve-2021-35465can be used to disable the mitigation. 🛡️✨-mstack-protector-guard=guard,-mstack-protector-guard-offset=offset— Generates stack protection code using a canary at guard. Supported locations areglobalfor a global canary, ortlsfor a canary accessible via the TLS register. The-mstack-protector-guard-offset=option is for use with-fstack-protector-guard=tlsand not for use in user-land code. 👑
okay that was a lot of ARM lore, drink some water, more incoming 💖
-mfdpic-mno-fdpic
okay so these two pick the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions ✨. When your compiler is configured for arm-*-uclinuxfdpiceabi targets, -mfdpic is on by default and it implies -fPIE if you didn't hand it any of the PIC/PIE-related options. On other targets it only switches on the FDPIC-specific code generation features, so you have to explicitly provide the PIC/PIE-related options yourself as needed. no cap.
heads up bestie: static linking is not supported 💀 because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use-staticand-Wl,-dynamic-linkeroptions.
The opposite -mno-fdpic option is useful (and honestly required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs 🫶.
-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]— enables branch protection features (armv8.1-m.main only, periodt).
Let's break the values down because it's giving ✨menu✨:
none— generate code without branch protection or return address signing. literally just vibes.standard[+leaf]— generate code with all branch protection features enabled at their standard level 👑.pac-ret[+leaf]— generate code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.leaf— when return address signing is enabled, also sign leaf functions even if they do not write the return address to memory 🍓.+bti— add landing-pad instructions at the permitted targets of indirect branch instructions.
If the +pacbti architecture extension is not enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code will remain backwards-compatible with earlier versions of the architecture, but the additional security can be enabled at run time on processors that support the PACBTI extension. iykyk.
Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.
Any setting other than none is supported only on armv8-m.main or later. 💅
The default is to generate code without branch protection or return address signing.
🌸 AVR Options 🌸
These options are defined for AVR implementations, bestie 🦋:
- *
-mmcu=mcu* — specify the AVR instruction set architecture (ISA) or device type. The default for this option isavr2.
The following AVR devices and ISAs are supported ✨. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) file specs-mcu. Only the latter is provided by the compiler according to the supported mcus below. The rest is supported by AVR-LibC (https://www.nongnu.org/avr-libc/), or by means of atpack (https://gcc.gnu.org/wiki/avr-gcc#atpack) files from the hardware manufacturer 👑.
💜 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, ata5795, 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, atmega324pb, 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, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, 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, avr64da28, avr64da32, avr64da48, avr64da64, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48.
💜 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, attiny416auto, attiny417, attiny424, attiny426, attiny427, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny824, attiny826, attiny827, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny1624, attiny1626, attiny1627, attiny3214, attiny3216, attiny3217, attiny3224, attiny3226, attiny3227, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr16dd14, avr16dd20, avr16dd28, avr16dd32, avr16ea28, avr16ea32, avr16ea48, avr16eb14, avr16eb20, avr16eb28, avr16eb32, avr32da28, avr32da32, avr32da48, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32ea28, avr32ea32, avr32ea48.
💜 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, avr128da28, avr128da32, avr128da48, avr128da64, avr128db28, avr128db32, avr128db48, avr128db64.
💜 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
"Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.
💜 avr1
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.
-mabsdata— bestie, this one says "assume every single bit of data in static storage can be reached byLDS/STSinstructions". It only does anything on the reduced Tiny devices, like theATtiny40. Also peep theabsdataAVR Variable Attributes thing, that variable attribute. iykyk 💅-maccumulate-args— accumulate all the outgoing function arguments and grab/release the stack space they need just once, in the function prologue/epilogue. Without this option, the outgoing args get pushed before the call and popped after. 😤
Here's the tea: popping args after a function call is expensive on AVR, so accumulating that stack space can lowkey give you smaller executables, because you don't have to pull the arguments back off the stack after the call. ✨
This option can shrink your code size for functions that make several calls to functions that take their arguments on the stack — think calls to printf-like functions. 🎀
-mbranch-cost=cost— set the branch cost for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is0. no cap 🌸-mcall-prologues— function prologues/epilogues get expanded as calls to the appropriate subroutines. Code size is smaller. slay 💖-mfuse-add,-mno-fuse-add,-mfuse-add=level— optimize indirect memory accesses on reduced Tiny devices. The default useslevel=1for optimizations-Ogand-O1, andlevel=2for higher optimizations. Valid values for level are0,1and2. 👑-mdouble=bits,-mlong-double=bits— set the size (in bits) of thedoubleorlong doubletype, respectively. Possible values for bits are32and64. Whether or not a specific value for bits is even allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and the exact same thing applies to the default values of these options. fr fr 🦋-mgas-isr-prologues— interrupt service routines (ISRs) may use the__gcc_isrpseudo instruction that GNU Binutils supports. If this option is on, you can still turn the feature off for individual ISRs using the AVR Function Attributesno_gccisrfunction attribute. This feature is activated by default if optimization is on (but not with-Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683). 💅-mint8— assumeintis an 8-bit integer. This messes with the sizes of literally every type: acharis 1 byte, anintis 1 byte, alongis 2 bytes, and along longis 4 bytes. Please note: this option does not conform to the C standards, but it gives you smaller code size. so pick your poison 😭-mmain-is-OS_task— do not save registers inmain. The effect is exactly the same as attaching the AVR Function AttributesOS_taskattribute tomain. It's activated by default if optimization is on. 🫶-mno-interrupts— the generated code is not compatible with hardware interrupts. Code size is smaller. that's the tradeoff bestie 💀-mrelax— try to replaceCALL(respectivelyJMP) instructions with the shorterRCALL(respectivelyRJMP) instruction, whenever that's applicable. 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 before the code is actually located. So the assembler code the compiler generates is the same, but the instructions in the executable may differ from the instructions in the assembler code. 👀
Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below. 🔥
-mrodata-in-ram,-mno-rodata-in-ram— locate the.rodatasections for read-only data in RAM (respectively, in program memory). For most devices there's no choice here, and this option acts kinda like an assertion. 😅
Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory by default, provided the required GNU Binutils support (PR31124, https://sourceware.org/PR31124) is available. In that case, -mrodata-in-ram can be used to go back to the old layout with .rodata in RAM. 🎀
-mstrict-X— use the address registerXthe way the hardware actually proposes. Which meansXis only used in indirect, post-increment or pre-decrement addressing. 💅
Without this option, the X register may 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 is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-mtiny-stack— only change the lower 8 bits of the stack pointer. 🧚-mfract-convert-truncate— allow using 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. The user takes responsibility for supplying the sub-processes — like compiler proper, assembler and linker — with the appropriate command line options. Which means the user has to supply her private device specs file by means of-specs=path-to-specs-file. There's no more need for the-mmcu=mcuoption. 😌
This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point to a directory containing a folder named device-specs, which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. 👑
-Waddr-space-convert— warn about conversions between address spaces when the resulting address space is not contained in the incoming address space. ⚠️-Wmisspelled-isr— warn if the ISR is misspelled, i.e. without the__vectorprefix. Enabled by default. 🚨
🌈 EIND and Devices with More Than 128 Ki Bytes of Flash 🌈
Pointers in this implementation are 16 bits wide. The address of a function or label is represented as a word address, so that indirect jumps and calls can target any code address in the range of 64 Ki words. 💖
To facilitate indirect jumps on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that 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 as follows by the compiler, and they're subject to some limitations: 👇
- The compiler never sets
EIND. 🚫 - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order 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 a 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 jumps to such a stub. The stub contains a direct jump to the desired address. 🦋
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option
-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 code is supposed to work for a setup withEIND != 0, a custom linker script has to be used in order to place the sections whose name starts with.trampolinesinto the segment whereEINDpoints 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 (https://www.nongnu.org/avr-libc/user-manual/). 📖 - It is legitimate for user-specific startup code to set up
EINDearly, for example by means of initialization code located in section.init3. Such code runs prior to the general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC 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 address of a function or code label.>
- <Computed goto.>
- <If prologue-save function is used, see
-mcall-prologues> command-line option. - <Switch/case dispatch tables. If you do not want such dispatch> tables you can specify the
-fno-jump-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();
}...and then the app gets linked with -Wl,--defsym,func_4=0x4. Alternative route, bestie: you can just define func_4 inside the linker script instead. Either way, you got options 💅
🌈 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers
Okay so some AVR devices have memories bigger than that 64 KiB range your lil 16-bit pointers can reach. And to get at memory outside that 64 KiB bubble, the contents of a RAMP register get used as the high part of the address: your X, Y, Z address register gets concatenated with the RAMPX, RAMPY, RAMPZ special function register (respectively, obvi) to build a wide address. And RAMPD? That one hangs out with direct addressing. 🫶
- The startup code initializes the
RAMPspecial function registers with zero. - If a AVR Named Address Spaces, named address space other than generic or
__flashis used, thenRAMPZis set as needed before the operation. - If the device supports RAM larger than 64 KiB and the compiler needs to change
RAMPZto accomplish an operation,RAMPZis reset to zero after the operation. - If the device comes with a specific
RAMPregister, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. - RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the
RAMPregisters, you must reset it to zero after the access. No cap, do it. 💀
✨ AVR Built-in Macros ✨
GCC defines a whole pile of built-in macros so your user code can vibe-check for the presence or absence of features. Almost every one of the built-in macros below is deduced from device capabilities, which means they're triggered by the -mmcu= command-line option. 🎀
For even more AVR-specific built-in macros, go peek at AVR Named Address Spaces and AVR Built-in Functions.
__AVR_ARCH__— A build-in macro that resolves to a decimal number identifying the architecture, and it depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6", respectively, and
100, 102, 103, 104, 105, 106, 107for 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__,-mmcu=attiny261adefines__AVR_ATtiny261A__, etc.
The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase. ✨
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_DEVICE_NAME__— Setting-mmcu=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined toatmega8.
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_XMEGA__— The device / architecture belongs to the XMEGA family of devices.__AVR_HAVE_ADIW__— The device has theADIWandSBIWinstructions.__AVR_HAVE_ELPM__— The device has theELPMinstruction.__AVR_HAVE_ELPMX__— The device has theELPM Rn,ZandELPM Rn,Z+instructions.__AVR_HAVE_LPMX__— The device has theLPM Rn,ZandLPM Rn,Z+instructions.__AVR_HAVE_MOVW__— The device has theMOVWinstruction to perform 16-bit register-register moves.__AVR_HAVE_MUL__— The device has a hardware multiplier. slay, math go brrr__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 that the program counter (PC) is 3 bytes wide.__AVR_2_BYTE_PC__— The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.__AVR_HAVE_8BIT_SP__,__AVR_HAVE_16BIT_SP__— The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by-mtiny-stack.__AVR_HAVE_SPH__,__AVR_SP8__— The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by-mmcu=and in the cases of-mmcu=avr2and-mmcu=avr25also 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. 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, LAS and LAT).__AVR_SFR_OFFSET__=offset— Instructions that can address I/O special function registers directly likeIN,OUT,SBI, etc. may use a different address as if addressed by an instruction to access RAM likeLDorSTS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.__AVR_SHORT_CALLS__— The-mshort-callscommand line option is set.__AVR_PM_BASE_ADDRESS__=addr— Some devices support reading from flash memory by means ofLD*instructions. The flash memory is seen in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put.rodatainto RAM. This is handled by the default linker description file, and is currently available foravrtinyandavrxmega3. Even more convenient, there is no need to use address spaces like__flashor features like attributeprogmemandpgm_read_*. 🦋__AVR_HAVE_FLMAP__— This macro is defined provided the following conditions are met:- The device has the
NVMCTRL_CTRLB.FLMAPbitfield. This applies to the AVR64* and AVR128* devices. - It's not known at assembler-time which emulation will be used.
- The device has the
This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).
__AVR_RODATA_IN_RAM__— This macro is undefined when the code is compiled for a core architecture.
When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.
__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 resp. 64-bit double. 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 to2or3.__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).
💅 AVR Internal Options 💅
These next ones? The compiler uses them internally, and they're how device specs files and the compiler proper talk to each other. You do not need to set these by hand — and lowkey, they are not optimization options. Using these wrong can hand you sub-optimal or just plain wrong code. 🙈 They're documented for completeness, and so you can get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files.
-mn-flash=num— Assume that the flash memory has a size of num times 64 KiB. This determines which__flashNaddress spaces are available.-mflmap— The device has theFLMAPbit field located in special function registerNVMCTRL_CTRLB.-mrmw— Assume that the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT.-mshort-calls— Assume thatRJMPandRCALLcan target the whole program memory. This option is used for multilib generation and selection for the devices from architectureavrxmega3.-mskip-bug— Generate code without skips (CPSE,SBRS,SBRC,SBIS,SBIC) over 32-bit instructions.-msp8— Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architecturesavr2andavr25. These architectures mix devices with and withoutSPH.
🌊 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.
sirevision is the optional little extra here, and it says which silicon revision of the target Blackfin processor we're dealing with. Whatever workarounds exist for that silicon revision? Enabled, bestie. If sirevision is none, literally no workarounds are enabled, zero, none. If sirevision is any, every single workaround for the targeted processor gets enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits — the major and minor numbers in the silicon revision. If sirevision is none, __SILICON_REVISION__ isn't defined at all. If sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. And if you just skip this optional sirevision entirely? GCC assumes the latest known silicon revision of the targeted Blackfin processor. lowkey generous of it.
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. 💅
okay so heads up: support forbf561is incomplete. Forbf561, only the preprocessor macro is defined. that's it. that's the whole thing. 😭
-msim— says the program is gonna run on the simulator, which causes the simulator BSP provided by libgloss to be linked in. This option only does anything for thebfin-elftoolchain. Also, some other options —-mid-shared-libraryand-mfdpic— imply-msimall on their own.-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This dodges the instructions to save, set up and restore frame pointers, and frees up an extra register in leaf functions. ✨-mspecld-anomaly— when enabled, the compiler makes sure the generated code doesn't 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 makes sure the generated code doesn't 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 the program is arbitrarily large. This is the default.-mstack-check-l1— do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.-mid-shared-library— generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies-fPIC. With abfin-elftarget, this option implies-msim.-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used. This is the default.-mleaf-id-shared-library— generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. slay.-mno-leaf-id-shared-library— do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns. 💀-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.-msep-data— generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.-mno-sep-data— generate code that assumes the data segment follows the text segment. This is the default.-mlong-calls,-mno-long-calls— tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. iykyk.
-mfast-fp— link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. 🔥-minline-plt— enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without-mfdpic.-mmulticore— build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro__BFIN_MULTICORE. It can only be used with-mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main.
If this option is not used, the single-core application programming model is used.
-mcorea— build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro__BFIN_COREAis defined. This option can only be used in conjunction with-mmulticore.-mcoreb— build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro__BFIN_COREBis defined. When this option is used,coreb_mainshould be used instead ofmain. This option can only be used in conjunction with-mmulticore.-msdram— build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro__BFIN_SDRAMis defined. The loader should initialize SDRAM before loading the application.-micplb— assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
🌈 C6X Options 🌈
-march=name— this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are:c62x,c64x,c64x+,c67x,c67x+,c674x.-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default. 💅-msim— choose startup files and linker script suitable for the simulator.-msdata=default— put small global and static data in the.neardatasection, which is pointed to by registerB14. Put small uninitialized global and static data in the.bsssection, which is adjacent to the.neardatasection. Put small read-only data into the.rodatasection. The corresponding sections used for large pieces of data are.fardata,.farand.const.-msdata=all— put all data, not just small objects, into the sections reserved for small data, and use addressing relative to theB14register to access them.-msdata=none— make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the.fardatasection, and all uninitialized data in the.farsection. Put all constant data into the.constsection.
🦋 CRIS Options 🦋
These options are defined specifically for the CRIS ports. niche but iconic.
-march=architecture-type,-mcpu=architecture-type— generate code for the specified architecture. The choices for architecture-type arev3,v8andv10for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0.-mtune=architecture-type— tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for-march=architecture-type.-mmax-stack-frame=n— warn when the stack frame of a function exceeds n bytes.-metrax4,-metrax100— the options-metrax4and-metrax100are synonyms for-march=v3and-march=v8respectively. cute lil shortcuts 🫶-mmul-bug-workaround,-mno-mul-bug-workaround— work around a bug in themulsandmuluinstructions for CPU models where it applies. This option is disabled by default.-mpdebug— enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the#NO_APPformatted-code indicator to the assembler at the beginning of the assembly file.-mcc-init— do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.-mno-side-effects— do not emit instructions with side effects in addressing modes other than post-increment.-mstack-align,-mno-stack-align,-mdata-align,-mno-data-align,-mconst-align,-mno-const-align— these options (theno-options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. 🌸-m32-bit,-m16-bit,-m8-bit— bestie these are the cousins of the stack-, data- and const-align options from right above 👆 they make your stack frame, writable data and constants all line up as 32-bit, 16-bit or 8-bit aligned respectively. The default? 32-bit alignment, always has been, no cap.-mno-prologue-epilogue,-mprologue-epilogue— okay so with-mno-prologue-epiloguethe normal function prologue and epilogue (the lil setup routine for the stack frame) get omitted entirely, and no return instructions or return sequences get generated in the code either. 🫢
⚠️ Use this option only together with visual inspection of the compiled code, fr fr. No warnings, no errors, nothing — GCC just quietly does not tell you when call-saved registers must be saved, or when storage for local variables needs to be allocated. It's giving silent treatment. 💀
-melf— legacy no-op option. That's it. That's the whole thing. It exists, it does nothing, it slays in its own way. 💅-sim— this one arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data all get allocated consecutively. Cozy.-sim2— like-sim, but it passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Big addresses energy. ✨
🌸 C-SKY Options 🌸
GCC supports these options when compiling for C-SKY V2 processors. Let's go.
-march=arch— specify the C-SKY target architecture. Valid values for arch are:ck801,ck802,ck803,ck807, andck810. The default isck810. 👑-mcpu=cpu— specify the C-SKY target processor. Valid values for cpu are:ck801,ck801t,ck802,ck802t,ck802j,ck803,ck803h,ck803t,ck803ht,ck803f,ck803fh,ck803e,ck803eh,ck803et,ck803eht,ck803ef,ck803efh,ck803ft,ck803eft,ck803efht,ck803r1,ck803hr1,ck803tr1,ck803htr1,ck803fr1,ck803fhr1,ck803er1,ck803ehr1,ck803etr1,ck803ehtr1,ck803efr1,ck803efhr1,ck803ftr1,ck803eftr1,ck803efhtr1,ck803s,ck803st,ck803se,ck803sf,ck803sef,ck803seft,ck807e,ck807ef,ck807,ck807f,ck810e,ck810et,ck810ef,ck810eft,ck810,ck810v,ck810f,ck810t,ck810fv,ck810tv,ck810ft, andck810ftv. Yes it's a long list. It ate. 😌-mbig-endian,-EB,-mlittle-endian,-EL— select big- or little-endian code. The default is little-endian.-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard.- Specifying
softcauses GCC to generate output containing library calls for floating-point operations. softfpallows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
- Specifying
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. Don't mix and match, it will not go well. 🙈
-mhard-float,-msoft-float— select hardware or software floating-point implementations. The default is soft float. 🧊-mdouble-float,-mno-double-float— when-mhard-floatis in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, enable generation of"frecipd","fsqrtd", and"fdivd"instructions. This is the default except when compiling for CK803.-mfpu=fpu— select the floating-point processor. This option can only be used with-mhard-float. Values for fpu arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_divd(-mdouble-float -mdivdu). ✨-melrw,-mno-elrw— enable the extended"lrw"instruction. This option defaults to on for CK801 and off otherwise.-mistack,-mno-istack— enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the "interrupt" and "isr" function attributes.
-mmp— enable multiprocessor instructions; the default is off.-mcp— enable coprocessor instructions; the default is off.-mcache— enable coprocessor instructions; the default is off.-msecurity— enable C-SKY security instructions; the default is off.-mtrust— enable C-SKY trust instructions; the default is off.-mdsp,-medsp,-mvdsp— enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off.-mdiv,-mno-div— generate divide instructions. Default is off.-msmart,-mno-smart— generate code for Smart Mode, using only registers numbered0-7to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off.-mhigh-registers,-mno-high-registers— generate code using the high registers numbered16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors.-manchor,-mno-anchor— generate code using global anchor symbol addresses.-mpushpop,-mno-pushpop— generate code using"push"and"pop"instructions. This option defaults to on.-mmultiple-stld,-mstm,-mno-multiple-stld,-mno-stm— generate code using"stm"and"ldm"instructions. This option isn't supported on CK801 but is enabled by default on other processors.-mconstpool,-mno-constpool— create constant pools in the compiler instead of deferring it to the assembler. This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors.-mstack-size,-mno-stack-size— emit".stack_size"directives for each function in the assembly output. This option defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.-mbranch-cost=n— set the branch costs to roughly"n"instructions. The default is1.-msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences.
🚨 Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. Like, just leave it off bestie. 😭
-msim— links the librarylibsemi.awhich is in compatible with simulator. Applicable to ELF compiler only. 🍓
🌈 Darwin Options 🌈
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 💅
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. Sneaky. 👀
-Fdir— add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by-Ioptions and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like:
#include <Framework/header.h>where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory. 🎀
-iframeworkdir— like-Fexcept the directory is a treated as a system directory. The main difference between this-iframeworkand-Fis that with-iframeworkthe compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.-gused— emit debugging information for symbols that are used. For stabs debugging format, this enables-feliminate-unused-debug-symbols. This is by default ON.-gfull— emit debugging information for all symbols and types. All of them. Every single one. 🌟-fconstant-cfstrings— the-fconstant-cfstringsis an alias for-mconstant-cfstrings. Twins fr fr. 👯-mconstant-cfstrings— when the NeXT runtime is being used (the default on these systems), override any-fconstant-string-classsetting and cause@"..."literals to be laid out as constant CoreFoundation strings.-mmacosx-version-min=version— the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include12,10.12, 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 on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible. Adaptable icon behavior honestly. ✨
-mkernel— turns on kernel dev mode, bestie 👑 it's giving "building the actual OS". It sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weakand-fno-rttiwherever those even apply. And for PowerPC targets it also sets-mno-altivec,-msoft-float,-fno-builtinand-mlong-branch.-mone-byte-bool— overrides the defaults for"bool"so thatsizeof(bool)==1. By defaultsizeof(bool)is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so on x86 this option literally does nothing, no cap.
⚠️ Warning bestie, this one's serious: the -mone-byte-bool switch makes GCC spit out code that is not binary compatible with code generated without it. Using this switch may require recompiling all the other modules in a program, system libraries included 😭. You use it to conform to a non-default data model, that's the whole point.
-mfix-and-continue-ffix-and-continue-findirect-data— these three generate code suitable for fast turnaround development, like letting GDB dynamically load.ofiles into programs that are already running 🏃♀️✨. Just so you know,-findirect-dataand-ffix-and-continueare only there for backwards compatibility.-all_load— loads all members of static archive libraries. Go see manld(1)for more info.-arch_errors_fatal— makes errors about files having the wrong architecture fatal 💀.-bind_at_load— marks the output file so that the dynamic linker will bind all undefined references the moment the file is loaded or launched.-bundle— produces a Mach-o bundle format file. Again, manld(1)has the tea.-bundle_loaderexecutable — this one specifies the executable that will load the build output file being linked. Manld(1), bestie.-dynamiclib— when you pass this, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand.-force_cpusubtype_ALL— forces GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.-nodefaultrpaths— do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless you add-nodefaultrpathsto the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. It's giving "mandatory", fr fr.
The big Darwin linker pass-through list — these all get handed off to the Darwin linker, and the Darwin linker man page describes each one in detail. Not me listing all of them, but we're listing all of them 💅
-allowable_clientclient_name-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-headerpad_max_install_names-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded
DEC Alpha Options 🦋
These -m options are the ones defined for the DEC Alpha implementations. Retro era, iconic era ✨:
-mno-soft-float-msoft-float— use (or, with theno-version, don't use) the hardware floating-point instructions for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. Unless those routines get replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines issue floating-point operations. If you're compiling for an Alpha without floating-point operations, you must make sure the library is built so as not to call them.
Heads up: Alpha implementations without floating-point operations are required to have floating-point registers. Yes, really 🙈.
-mfp-reg-mno-fp-regs— generate code that uses (or doesn't use) the floating-point register set.-mno-fp-regsimplies-msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers, and floating-point results are passed in$0instead of$f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value that gets called by code compiled with-mno-fp-regsmust also be compiled with that option. Periodt.
A typical use of this option is building a kernel that doesn't use, and therefore doesn't need to save and restore, any floating-point registers.
-mieee— the Alpha architecture implements floating-point hardware optimized for maximum performance. It's mostly compliant with the IEEE floating-point standard, but for full compliance, software assistance is required. This option generates code that's fully IEEE-compliant, except that the inexact-flag is not maintained (see below 👇). If this option is turned on, the preprocessor macro"_IEEE_FP"is defined during compilation. The resulting code is less efficient but can correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option-ieee_with_no_inexact.
DEBIAN SPECIFIC: this option is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of the -ffast-math set) is specified — because the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs, and all of those will cause a program to SIGFPE when it tries to use the results without -mieee 💀😭.
-mieee-with-inexact— this is like-mieeeexcept the generated code also maintains the IEEE inexact-flag. Turning this on makes the generated code implement fully-compliant IEEE math. On top of"_IEEE_FP","_IEEE_FP_EXACT"also gets defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option-ieee_with_inexact.- *
-mfp-trap-mode=trap-mode* — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option-fptmtrap-mode. The trap mode can be set to one of four values:n— this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).u— in addition to the traps enabled byn, underflow traps are enabled as well.su— likeu, but the instructions are marked to be safe for software completion (see 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-fprmrounding-mode. The rounding-mode can be one of:n— normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number, or towards the even machine number in case of a tie.m— round towards minus infinity.c— chopped rounding mode. Floating-point numbers are rounded towards zero.d— dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode 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 😭. This means that without software assistance it is impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on what an application needs, you can pick different levels of precision:p— program precision. This is the default and means a trap handler can only identify which program caused a floating-point exception.f— function precision. The trap handler can determine the function that caused a floating-point exception.i— instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant— this option marks the generated code as IEEE conformant. You must not use this option unless you also specify-mtrap-precision=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. That's it, that's the whole job 💅.-mbuild-constants— normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it 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 construct all integer constants using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. Self-relocating king behavior 👑✨.
-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— bestie, these four pairs are the yes/no switches for the optional instruction sets: BWX, CIX, FIX and MAX. They tell GCC whether it's allowed to actually generate code that uses them. And the default? It's giving "whatever your CPU already said", fr fr — GCC goes with the instruction sets supported by the CPU type you passed to the-mcpu=option, or, if you didn't pass one, the CPU that GCC was built on. ✨-mfloat-vax,-mfloat-ieee— picks whether the generated code uses VAX F and G floating-point arithmetic, or doesn't (aka uses IEEE single and double precision instead). 🎀-mexplicit-relocs,-mno-explicit-relocs— okay so story time 💅: the older Alpha assemblers had no way to generate symbol relocations except through assembler macros. And using those macros means you can't get optimal instruction scheduling. Sad. But GNU binutils as of version 2.12 supports a shiny new syntax that lets the compiler explicitly mark which relocations apply to which instructions. ✨ Honestly though, this option is mostly a debugging vibe, because GCC detects what the assembler can do when it's built and sets the default accordingly. No cap.-msmall-data,-mlarge-data— when-mexplicit-relocsis in effect, static data gets accessed via gp-relative relocations. With-msmall-data, objects 8 bytes long or smaller get parked in a small data area (the.sdataand.sbsssections) and accessed via 16-bit relocations off of the$gpregister. That caps the small data area at 64KB, but it lets variables be directly accessed in a single instruction. 🦋
The default is -mlarge-data. With that one, the data area is limited to just below 2GB. And programs that need more than 2GB of data have to use malloc or mmap to allocate it in the heap instead of in the program's data segment. 🫶
Oh and when you're generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data. Iykyk.
-msmall-text,-mlarge-text— with-msmall-text, the compiler assumes the code of the entire program (or shared library) fits in 4MB, so it's reachable with a branch instruction. And with-msmall-data, the compiler can assume all local symbols share the same$gpvalue, which drops the number of instructions needed for a function call from 4 down to 1. The way that's a glow-up. 💅
The default is -mlarge-text.
-mcpu=cpu_type— sets the instruction set and the instruction scheduling parameters for machine type cpu_type. You can give it either the EV style name or the matching chip number, whichever you vibe with. GCC supports scheduling parameters for the EV4, EV5 and EV6 families of processors, and it picks default instruction-set values from the processor you name. If you don't specify a processor type at all, GCC defaults to the processor the compiler was built on. 🧚
Supported values for cpu_type are:
ev4,ev45,21064— schedules as an EV4 and has no instruction set extensions.ev5,21164— schedules as an EV5 and has no instruction set extensions.ev56,21164a— schedules as an EV5 and supports the BWX extension.pca56,21164pc,21164PC— schedules as an EV5 and supports the BWX and MAX extensions.ev6,21264— schedules as an EV6 and supports the BWX, FIX, and MAX extensions.ev67,21264a— schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor. Just so you know: -mcpu=native has no effect if GCC doesn't recognize the processor. 🙈
-mtune=cpu_type— sets only the instruction scheduling parameters for machine type cpu_type. The instruction set stays exactly where it was, untouched. 💜
Native toolchains also support the value native here, selecting the best architecture option for the host processor. And same deal: -mtune=native has no effect if GCC doesn't recognize the processor.
-mmemory-latency=time— tells the scheduler what latency to assume for typical memory references as seen by the application. Fair warning bestie, this number is highly dependent on the memory access patterns your application uses and the size of the external cache on the machine. It's giving "it depends". 😭
Valid options for time are:
- number — a decimal number representing clock cycles.
L1,L2,L3,main— the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note thatL3is only valid for EV5. ⭐
🌈 eBPF Options 🌈
-mframe-limit=bytes— this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify should be less than or equal to 32767. It defaults to whatever limit is imposed by the version of the Linux kernel you're targeting. 👑-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default. 💖-mjmpext,-mno-jmpext— enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.-mjmp32,-mno-jmp32— enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.-malu32,-mno-alu32— enable or disable generation of 32-bit ALU instructions. Enabled for CPU v3 and above.-mv3-atomics,-mno-v3-atomics— enable or disable instructions for general atomic operations introduced in CPU v3. Enabled for CPU v3 and above.-mbswap,-mno-bswap— enable or disable byte swap instructions. Enabled for CPU v4 and above.-msdiv,-mno-sdiv— enable or disable signed division and modulus instructions. Enabled for CPU v4 and above.-msmov,-mno-smov— enable or disable sign-extending move and memory load instructions. Enabled for CPU v4 and above.-mcpu=version— this specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is v4. 🦋
Supported values for version are:
v1— the first stable eBPF ISA with no special features or extensions.v2— supports the jump extensions, as in-mjmpext.v3— all features ofv2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32 - general atomic operations, as in
-mv3-atomics
- 32-bit jump operations, as in
v4— all features ofv3, plus:- byte swap instructions, as in
-mbswap - signed division and modulus instructions, as in
-msdiv - sign-extending move and memory load instructions, as in
-msmov
- byte swap instructions, as in
-mco-re— enable BPF Compile Once - Run Everywhere (CO-RE) support. It requires-gbtfand is implied by it, so they're basically a package deal. 🫶-mno-co-re— disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target. ✨-mxbpf— generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:- Save and restore callee-saved registers at function entry and exit, respectively.
-masm=dialect— outputs assembly instructions using the eBPF selected dialect. The default isnormal.
Supported values for dialect are:
normal— outputs normal assembly dialect.pseudoc— outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes— specifies a size threshold in bytes at or below whichmemmove,memcpyandmemsetshall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline — but since BPF doesn't allow libcalls, exceeding this threshold results in a compile-time error. 😭 The default is 1024 bytes.
🌸 FR30 Options 🌸
These options are defined specifically for the FR30 port.
-msmall-model— use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. 💅-mno-lsim— assume that runtime support has been provided, and so there'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— specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked.
Bestie, 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 are needed. 🚨
-mlra— enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload.-mnodiv— do not usedivandmodinstructions.-mft32b— enable use of the extended instructions of the FT32B processor. 🔥-mcompress— compress all code using the Ft32B code compression scheme.-mnopm— do not generate code that reads program memory.
💜 FRV Options 💜
-mgpr-32— only use the first 32 general-purpose registers.-mgpr-64— use all 64 general-purpose registers.-mfpr-32— use only the first 32 floating-point registers.-mfpr-64— use all 64 floating-point registers.-mhard-float— use hardware instructions for floating-point operations.-msoft-float— use library routines for floating-point operations.-malloc-cc— dynamically allocate condition code registers.-mfixed-cc— do not try to dynamically allocate condition code registers, only useicc0andfcc0. 🎯-mdword— change ABI to use double word insns.-mno-dword— do not use double word instructions.-mdouble— use floating-point double instructions.-mno-double— do not use floating-point double instructions.-mmedia— use media instructions.-mno-media— do not use media instructions.-mmuladd— use multiply and add/subtract instructions.-mno-muladd— do not use multiply and add/subtract instructions.-mfdpic— picks the FDPIC ABI, which uses function descriptors to represent pointers to functions. With no PIC/PIE-related options hanging around, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data live within a 12-bit range from the GOT base address; with-fPICor-fPIE, GOT offsets get computed with 32 bits. And with abfin-elftarget, this option implies-msim. ✨-minline-plt— enables inlining of PLT entries in function calls to functions that are not known to bind locally. Bestie, it does literally nothing without-mfdpic. It's on by default if you're optimizing for speed and compiling for shared libraries (so,-fPICor-fpic), or when an optimization option like-O3or above is chillin' in the command line. 💅-mTLS— assume a large TLS segment when generating thread-local code.-mtls— do not assume a large TLS segment when generating thread-local code.-mgprel-ro— enables the use of"GPREL"relocations in the FDPIC ABI for data that's known to be in read-only sections. It's enabled by default, except for-fpicor-fpie: even though it might help make the global offset table smaller, it trades 1 instruction for 4. With-fPICor-fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids needing a GOT entry for the referenced symbol, so it's way more likely to be a win. If it isn't,-mno-gprel-rocan disable it. 🎀-multilib-library-pic— link with the (library, not FD) pic libraries. It's implied by-mlibrary-pic, and also by-fPICand-fpicwithout-mfdpic. You should literally never have to use it explicitly.-mlinked-fp— follows the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This one is enabled by default and can be disabled with-mno-linked-fp.-mlong-calls— use indirect addressing to call functions outside the current compilation unit. This lets those 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 option only does anything when VLIW packing is enabled. It doesn't create new packets; it just adds NOPs to the ones already there.-mlibrary-pic— generate position-independent EABI code.-macc-4— use only the first four media accumulator registers.-macc-8— use all eight media accumulator registers.-mpack— pack VLIW instructions.-mno-pack— do not pack VLIW instructions.-mno-eflags— do not mark ABI switches ine_flags.-mcond-move— enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move— disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc— enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc— disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec— enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec— disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch— run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch— do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec— enable optimization of"&&"and"||"in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec— disable optimization of"&&"and"||"in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec— enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec— disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar— this switch removes redundant"membar"instructions from the compiler-generated code. It's enabled by default.-mno-optimize-membar— this switch disables the automatic removal of redundant"membar"instructions from the generated code.-mtomcat-stats— causesgasto print out tomcat statistics. 🐱 (not an actual cat bestie, it's a chip name, iykyk)- *
-mcpu=cpu* — select the processor type to generate code for. Possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple. 🔥
🐧 GNU/Linux Options 🐧
These -m options are defined for GNU/Linux targets, babes:
-mglibc— use the GNU C library. This is the default except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets.-muclibc— use uClibc C library. This is the default on*-*-linux-*uclibc*targets.-mmusl— use the musl C library. This is the default on*-*-linux-*musl*targets.-mbionic— use Bionic C library. This is the default on*-*-linux-*android*targets.-mandroid— compile code compatible with the Android platform. This is the default on*-*-linux-*android*targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. And finally, this option causes the preprocessor macro "__ANDROID__" to be defined. 🤖
-tno-android-cc— disable compilation effects of-mandroid, i.e., do not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— disable linking effects of-mandroid, i.e., pass standard Linux linking options to the linker.
🌸 H8/300 Options 🌸
These -m options are defined for the H8/300 implementations:
-mrelax— shorten some address references at link time, when possible; uses the linker option-relax.-mh— generate code for the H8/300H.-ms— generate code for the H8S.-mn— generate code for the H8S and H8/300H in the normal mode. This switch must be used either with-mhor-ms.-ms2600— generate code for the H8S/2600. This switch must be used with-ms.-mexr— extended registers are stored on the stack before execution of a function with monitor attribute. Default option is-mexr. This option is valid only for H8S targets.-mno-exr— extended registers are not stored on the stack before execution of a function with monitor attribute. Default option is-mno-exr. This option is valid only for H8S targets.-mint32— make"int"data 32 bits by default.-malign-300— on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries.-malign-300causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300. 💫
👑 HPPA Options 👑
These -m options are defined for the HPPA family of computers:
-march=architecture-type — generate code for the specified architecture. The choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. Refer to/usr/lib/sched.modelson an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but **not the other way around. 🫶-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-matomic-libcalls— generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations provided they are implemented as a single access.
The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. So all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.
This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets.
-mbig-switch— does nothing. Preserved for backward compatibility. (iconic behaviour honestly, it's just vibing 💀)-mcaller-copies— the caller copies function arguments passed by hidden reference. Use this option with care, bestie, because it is not compatible with the default 32-bit runtime. That said, only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP.-mcoherent-ldcw— useldcw/ldcdcoherent cache-control hint.-mdisable-fpregs— disable floating-point registers. Equivalent to"-msoft-float".-mdisable-indexing— prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.-mfast-indirect-calls— generate code that assumes calls never cross space boundaries. This lets GCC emit code that performs faster indirect calls.
⚠️ Heads up: this option does not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range— tell the compiler to treat register-range as fixed registers, aka ones the register allocator is literally not allowed to touch. So useful for kernel code, bestie. A register range is two registers with a dash between them, and yes you can stack multiple ranges, just separate them with commas. 💅-mgas— turn on assembler directives that only GAS understands. iykyk.-mgnu-ld— use options that are specific to GNU ld. When you're building a shared library, this passes-sharedto ld. It's the default when GCC is configured, explicitly or implicitly, with the GNU linker. Important: this option does not change which ld gets called — it only changes what parameters get handed to that ld. The ld that's called is decided by the--with-ldconfigure option, GCC's program search path, and finally the user'sPATH. Wanna know which linker GCC is using? Runwhich \gcc -print-prog-name=ld`. And heads up: this option is only available on the 64-bit HP-UX GCC, aka configured withhppa*64*--hpux`.-mhp-ld— use options specific to HP ld. It 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 deal as before: this option does not affect which ld is called, only what parameters get passed to it. Which ld gets called is determined by the--with-ldconfigure option, GCC's program search path, and finally the user'sPATH. Check it withwhich \gcc -print-prog-name=ld`. Only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*--hpux`. 🫶-mlinker-opt— enables the optimization pass in the HP-UX linker. ⚠️ Note that 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. So like, be aware.-mlong-calls— generate code that uses long call sequences. This guarantees a call can always reach linker generated stubs. The default behavior is to generate long calls only when the distance from the call site to the beginning of the function or translation unit (whichever applies) exceeds a predefined limit set by the branch type being used. Those 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 you use the -ffunction-sections option, or when you use the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
Normally you do not want this option because it degrades performance. But it may be useful in large applications, especially when partial linking is used to build the application.
The types of long calls used depend on what the assembler and linker can do, and on 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, on 32-bit ELF systems in pic code an indirect call is used, and it is quite long. 😭
-mlong-load-store— generate 3-instruction load and store sequences, as sometimes required by the HP-UX 10 linker. This is equivalent to the+koption to the HP compilers.-mjump-in-delay— this option is ignored and provided for compatibility purposes only. She's just here for the vibes. 💀-mno-space-regs— generate 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.
-mordered— assume memory references are ordered and barriers are not needed.-mportable-runtime— use the portable calling conventions proposed by HP for ELF systems.-mschedule=cpu-type— schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are7007100,7100LC,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.-msio— generate the predefine,_SIO, for server IO. The default is-mwsio. That one generates the predefines,__hp9000s700,__hp9000s700__and_WSIO, for workstation IO. These options are available under HP-UX and HI-UX.-msoft-float— generate output containing library calls for floating point. ⚠️ Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
Heads up, -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
-msoft-mult— use software integer multiplication.
This disables the use of the "xmpyu" instruction.
-munix=unix-std— generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are93,95and98.93is supported on all HP-UX versions.95is available on HP-UX 10.10 and later.98is available on HP-UX 11.11 and later. The default values are93for HP-UX 10.00,95for HP-UX 10.10 though to 11.00, and98for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED, and the startfile unix95.o. -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability.
-nolibdld— suppress the generation of link options to searchlibdld.slwhen the-staticoption is specified on HP-UX 10 and later.-static— okay so the HP-UX implementation ofsetlocalein libc has a dependency onlibdld.sl. There isn't an archive version oflibdld.sl. Thus, when the-staticoption is specified, special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.
-threads— add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. ✨
👑 IA-64 Options 👑
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian— generate code for a big-endian target. This is the default for HP-UX.-mlittle-endian— generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.-mgnu-as,-mno-gnu-as— generate (or don't) code for the GNU assembler. This is the default.-mgnu-ld,-mno-gnu-ld— generate (or don't) code for the GNU linker. This is the default.-mno-pic— generate code that does not use a global pointer register. The result is not position independent code, and it violates the IA-64 ABI. 🚨-mvolatile-asm-stop,-mno-volatile-asm-stop— generate (or don't) a stop bit immediately before and after volatile asm statements.-mregister-names,-mno-register-names— generate (or don't)in,loc, andoutregister names for the stacked registers. This may make assembler output more readable. 💅-mno-sdata,-msdata— disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.-mconstant-gp— generate code that uses a single constant global pointer value. This is useful when compiling kernel code.-mauto-pic— generate code that is self-relocatable. This implies-mconstant-gp. This is useful when compiling firmware code.-minline-float-divide-min-latency— generate code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generate code for inline divides of floating-point values using the maximum throughput algorithm.-mno-inline-float-divide— do not generate inline code for divides of floating-point values.-minline-int-divide-min-latency— generate code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generate code for inline divides of integer values using the maximum throughput algorithm.-mno-inline-int-divide— do not generate inline code for divides of integer values.-minline-sqrt-min-latency— generate code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generate code for inline square roots using the maximum throughput algorithm.-mno-inline-sqrt— do not generate inline code for"sqrt".-mfused-madd,-mno-fused-madd— do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.-mno-dwarf2-asm,-mdwarf2-asm— don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.-mearly-stop-bits,-mno-early-stop-bits— allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but it does not always do so. 🙈-mfixed-range=register-range— tells GCC to treat that register range as fixed registers, aka ones the register allocator is just not allowed to touch 💅. Super handy when you're compiling kernel code, bestie. A register range is two registers with a dash between them, and you can stack multiple ranges by separating them with a comma. Slay.-mtls-size=tls-size— sets the bit size of immediate TLS offsets. Valid values are14,22, and64. That's it, that's the whole vibe ✨.-mtune=cpu-type— tunes instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley. Periodt.-milp32-mlp64— generate 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, iykyk.-mno-sched-br-data-spec-msched-br-data-spec— (dis/en)able data speculative scheduling before reload. This makes GCC generateld.ainstructions and the matching check instructions (ld.c/chk.a). Default setting is disabled.-msched-ar-data-spec-mno-sched-ar-data-spec— (en/dis)able data speculative scheduling after reload. Also generatesld.ainstructions and the matching check instructions (ld.c/chk.a). Default setting is enabled.-mno-sched-control-spec-msched-control-spec— (dis/en)able control speculative scheduling 🌸. This feature only exists during region scheduling (so, before reload). It generatesld.sinstructions and the matching check instructionschk.s. Default setting is disabled.-msched-br-in-data-spec-mno-sched-br-in-data-spec— (en/dis)able speculative scheduling of the instructions that depend on the data speculative loads before reload. Only actually does anything with-msched-br-data-specenabled, bestie. Default setting is enabled.-msched-ar-in-data-spec-mno-sched-ar-in-data-spec— (en/dis)able speculative scheduling of the instructions that depend on the data speculative loads after reload. Only effective with-msched-ar-data-specenabled. Default setting is enabled.-msched-in-control-spec-mno-sched-in-control-spec— (en/dis)able speculative scheduling of the instructions that depend on the control speculative loads 👑. Only effective with-msched-control-specenabled. Default setting is enabled.-mno-sched-prefer-non-data-spec-insns-msched-prefer-non-data-spec-insns— if enabled, data-speculative instructions only get picked for the schedule when there are literally no other choices left at that moment. Makes data speculation way more conservative 🎀. Default setting is disabled.-mno-sched-prefer-non-control-spec-insns-msched-prefer-non-control-spec-insns— if enabled, control-speculative instructions only get chosen for the schedule when there's nothing else available right then. Makes control speculation much more conservative. Default setting is disabled.-mno-sched-count-spec-in-critical-path-msched-count-spec-in-critical-path— if enabled, speculative dependencies get counted when computing instruction priorities. Makes the speculation a lil bit more conservative. Default setting is disabled.-msched-spec-ldc— use a simple data speculation check ✨. This option is on by default.-msched-control-spec-ldc— use a simple check for control speculation. This option is on by default.-msched-stop-bits-after-every-cycle— drop a stop bit after every cycle when scheduling. This option is on by default.-msched-fp-mem-deps-zero-cost— assume floating-point stores and loads probably won't conflict when they land in the same instruction group. This option is disabled by default.-msel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. This flag is disabled by default.-msched-max-memory-insns=max-insns— caps how many memory insns can sit in one instruction group, giving lower priority to subsequent memory insns trying to schedule into that same group. Genuinely useful for dodging cache bank conflicts 🔥. Default value is1.-msched-max-memory-insns-hard-limit— makes the limit frommsched-max-memory-insnsa hard limit, so more than that number simply cannot be in an instruction group. Otherwise the limit is "soft", meaning non-memory operations are preferred once the limit is hit, but memory operations can still get scheduled in. 🙈
🌈 LM32 Options 🌈
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled— enable barrel-shift instructions 💫.-mdivide-enabled— enable divide and modulus instructions.-mmultiply-enabled— enable multiply instructions.-msign-extend-enabled— enable sign extend instructions.-muser-enabled— enable user-defined instructions. Main character energy fr.
🦋 LoongArch Options 🦋
These command-line options are defined for LoongArch targets:
-march=cpu-type— generate instructions for the machine type cpu-type. Unlike-mtune=cpu-type, which merely tunes the generated code for that cpu-type,-march=cpu-typelets GCC generate code that might not run at all on processors other than the one you named 😭. Specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise.
The choices for cpu-type are:
native— picks the CPU to generate code for at compilation time by figuring out the processor type of the machine doing the compiling. Using-march=nativeenables every instruction subset the local machine supports (so yeah, the result might not run on different machines 💀). Using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set.loongarch64— a generic CPU with 64-bit extensions.la464— LoongArch LA464 CPU with LBT, LSX, LASX, LVZ. Iconic.
-mtune=cpu-type— optimize the output for the given processor, specified by microarchitecture name ✨.-mabi=base-abi-type— generate code for the specified calling convention. base-abi-type can be one of:lp64d— uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64f— uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64s— uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.
-mfpu=fpu-type— generate code for the specified FPU type, which can be one of:64— allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.32— allow the use of hardware floating-point instructions for 32-bit operations.none,0— prevent the use of hardware floating-point instructions 🚫.
-msimd=simd-type— enable generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:lasx— enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).lsx— enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).none— no LoongArch SIMD instruction may be generated.
-msoft-float— forces-mfpu=noneand blocks the use of floating-point registers for parameter passing. Heads up: this option may change the target ABI 👀.-msingle-float— forces-mfpu=32and allows 32-bit floating-point registers for parameter passing. This option may change the target ABI.-mdouble-float— forces-mfpu=64and allows 32/64-bit floating-point registers for parameter passing. This option may change the target ABI.-mlasx-mno-lasx-mlsx-mno-lsx— incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation 🌸. Enabling LASX withmlasxautomatically enables LSX, and diabling LSX withmno-lsxautomatically disables LASX. These are driver-only options: they act on the finalmsimdconfiguration state and make incremental chagnes in the order they appear on the GCC driver's command line, deriving the final / canonicalizedmsimdoption that gets passed to the compiler proper.-mbranch-cost=n— set the cost of branches to roughly n instructions.-mcheck-zero-division-mno-check-zero-divison— trap (do not trap) on integer division by zero 🫶. The default is-mcheck-zero-divisionfor-O0or-Og, and-mno-check-zero-divisionfor other optimization levels.-mcond-move-int-mno-cond-move-int— conditional moves for integral data in general-purpose registers are enabled (disabled). The default is-mcond-move-int.-mcond-move-float-mno-cond-move-float— conditional moves for floating-point registers are enabled (disabled). The default is-mcond-move-float.-mmemcpy-mno-memcpy— force (do not force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which lets GCC inline most constant-sized copies ✨. Setting the optimization level to-Osalso forces the use ofmemcpy, but-mno-memcpymay override this behavior if explicitly specified, regardless of the order these options appear on the command line.-mstrict-align-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-align.-msmall-data-limit=number— put global and static data smaller than number bytes into a special section (on some targets). The default value is0.-mmax-inline-memcpy-size=n— okay so this one inlines all the block moves that arenbytes or less, like calls tomemcpyor structure copies. The default value ofnis1024. slay 💅-mcmodel=code-model— pick your code model, bestie. it's one of these:tiny-static— (Not implemented yet) 🙈tiny— (Not implemented yet) 🙈normal— the text segment has to live within 128MB addressing space, and the data segment has to live within 2GB addressing space.medium— both the text segment and the data segment have to be within 2GB addressing space.large— (Not implemented yet) 🙈extreme— this mode does not limit the size of the code segment or the data segment. but lowkey important:-mcmodel=extremeis incompatible with-fpltand/or-mexplicit-relocs=none.
The default code model is normal. ✨
-mexplicit-relocs=style— this sets when to use assembler relocation operators for symbolic addresses. the alternative is using assembler macros instead, which can limit instruction scheduling but allows linker relaxation. so:- with
-mexplicit-relocs=nonethe assembler macros are always used - with
-mexplicit-relocs=alwaysthe assembler relocation operators are always used - with
-mexplicit-relocs=autothe compiler uses the relocation operators where linker relaxation is impossible (to improve code quality), and macros everywhere else
- with
The default value is decided by the assembler capability detected at GCC build-time plus the setting of -mrelax: -mexplicit-relocs=none if the assembler doesn't support relocation operators at all, -mexplicit-relocs=always if it supports them but -mrelax isn't enabled, and -mexplicit-relocs=auto if it supports them and -mrelax is enabled. 🧚
-mexplicit-relocs— an alias of-mexplicit-relocs=always, kept around for backward compatibility. 🫶-mno-explicit-relocs— an alias of-mexplicit-relocs=none, also for backward compatibility.-mdirect-extern-access,-mno-direct-extern-access— use / don't use GOT to access external symbols. the default is-mno-direct-extern-access, which means GOT is used for external symbols with default visibility, but not used for other external symbols.
With -mdirect-extern-access, GOT isn't used at all and all external symbols are PC-relatively addressed. it's only suitable for environments where no dynamic link happens — like firmwares, OS kernels, executables linked with -static or -static-pie. also, -mdirect-extern-access is not compatible with -fPIC or -fpic. 💀
-mrelax,-mno-relax— take (or don't take) advantage of linker relaxations. if-mpass-mrelax-to-asis enabled, this option also gets passed to the assembler. The default is figured out at GCC build-time by detecting the matching assembler support:-mrelaxif the assembler supports both the-mrelaxoption and conditional branch relaxation (and that's required, or the.aligndirectives and conditional branch instructions in the assembly GCC outputs might get rejected by the assembler because of a relocation overflow), and-mno-relaxotherwise. 😭-mpass-mrelax-to-as,-mno-pass-mrelax-to-as— pass (or don't pass) the-mrelaxor-mno-relaxoption to the assembler. The default is determined at GCC build-time by detecting the matching assembler support:-mpass-mrelax-to-asif the assembler supports the-mrelaxoption,-mno-pass-mrelax-to-asotherwise. this one is mostly useful for debugging, or for interop with assemblers that aren't the build-time one. 🔍-mrecip— this enables the reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to crank up precision, instead of doing a divide (or square root and divide) for floating-point arguments. these instructions only get generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. it's off by default. before you can use this, you must make sure the target CPU supportsfrecipeandfrsqrteinstructions. and ngl: while the throughput of the sequence is higher than the non-reciprocal instruction, the precision can drop by up to 2 ulp (i.e. the inverse of1.0equals0.99999994). 😬-mrecip=opt— controls which reciprocal estimate instructions may be used.optis a comma-separated list of options, and any of them can be preceded by a!to invert it:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.rsqrt— enable the approximation for scalar reciprocal square root.vec-rsqrt— enable the approximation for vectorized reciprocal square root.
so like, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations except scalar square root. ✨
-mfrecipe,-mno-frecipe— use (or don't use)frecipe.{s/d}andfrsqrte.{s/d}instructions. when built with-march=la664, it's enabled by default. the default is-mno-frecipe. 🌸-mdiv32,-mno-div32— use (or don't use)div.w[u]andmod.w[u]instructions with input that isn't sign-extended. when built with-march=la664, it's enabled by default. the default is-mno-div32.-mlam-bh,-mno-lam-bh— use (or don't use)am{swap/add}[_db].{b/h}instructions. when built with-march=la664, it's enabled by default. the default is-mno-lam-bh.-mlamcas,-mno-lamcas— use (or don't use)amcas[_db].{b/h/w/d}instructions. when built with-march=la664, it's enabled by default. the default is-mno-lamcas.-mld-seq-sa,-mno-ld-seq-sa— whether a load-load barrier (dbar 0x700) is needed. when built with-march=la664, it's enabled by default. the default is-mno-ld-seq-sa, meaning the load-load barrier is needed. 💅-mtls-dialect=opt— controls which TLS dialect may be used for the general dynamic and local dynamic TLS models:trad— use traditional TLS. this is the default.desc— use TLS descriptors.
loongarch-vect-unroll-limit— the vectorizer uses whatever tuning info it has to decide whether unrolling the main vectorized loop would be worth it, and by how much. this parameter sets the upper bound on how much the vectorizer will unroll the main loop. the default value is six. 🎀
🍓 M32C Options 🍓
-mcpu=name— pick the CPU you're generating code for.namecan ber8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series. 💜-msim— says the program is gonna run on the simulator. this makes an alternate runtime library get linked in, one that supports stuff like file I/O. you must not use this option when you're generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions you need. ⚠️-memregs=number— sets how many memory-based pseudo-registers GCC uses during code generation. these pseudo-registers get used like real registers, so there's a tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. note that all modules in a program must be compiled with the same value for this option. because of that, you must not use this option with GCC's default runtime libraries. 😭
🦋 M32R/D Options 🦋
these -m options are defined for Renesas M32R/D architectures:
-m32r2— generate code for the M32R/2.-m32rx— generate code for the M32R/X.-m32r— generate code for the M32R. this is the default. 👑-mmodel=small— assume all objects live in the lower 16MB of memory (so their addresses can be loaded with theld24instruction), and assume all subroutines are reachable with theblinstruction. this is the default.
the addressability of a particular object can be set with the model attribute.
-mmodel=medium— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume all subroutines are reachable with theblinstruction.-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume subroutines may not be reachable with theblinstruction (the compiler generates the much slowerseth/add3/jlinstruction sequence). 💀-msdata=none— disable use of the small data area. variables get put into one of.data,.bss, or.rodata(unless thesectionattribute has been specified). this is the default.
the small data area consists of the sections .sdata and .sbss. objects may be explicitly put in the small data area with the section attribute using one of these sections.
-msdata=sdata— put small global and static data in the small data area, but don't generate special code to reference them.-msdata=use— put small global and static data in the small data area, and generate special instructions to reference them.-G num— put global and static objects that are 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 num value. compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated. 🫶
-mdebug— makes the M32R-specific code in the compiler display some statistics that might help when debugging programs. 🔍-malign-loops— align all loops to a 32-byte boundary. ✨-mno-align-loops— bestie, do not enforce a 32-byte alignment for loops. This is the default. 💅-missue-rate=number— Issue number instructions per cycle. number can only be1or2. ✨-mbranch-cost=number— number can only be1or2. If it is1then branches are preferred over conditional code, if it is2, then 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 that there is no OS function for flushing the cache. 🙈
🌸 M680x0 Options 🌸
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below. 💖
-march=arch— Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are:isaa,isaaplus,isabandisac. GCC defines a macro__mcfarch__whenever it is generating code for a ColdFire target. The arch in this macro is one of the-marcharguments given above. When used together,-marchand-mtuneselect code that runs on a family of similar processors but that is optimized for a particular microarchitecture. ✨-mcpu=cpu— Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above. 🦋
-mtune=tune— Tune the code for a particular microarchitecture within the constraints set by-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=68020-40for code that needs to run relatively well on68020,68030and68040targets.-mtune=68020-60is similar but includes68060targets as well. These two options select the same tuning decisions as-m68020-40and-m68020-60respectively. GCC defines the macros__mcarchand__mcarch__when tuning for 680x0 architecture arch. It also definesmcarchunless either-ansior a non-GNU-stdoption is used. If GCC is tuning for a range of architectures, as selected by-mtune=68020-40or-mtune=68020-60, it defines the macros for every architecture in the range. GCC also defines the macro__muarch__when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above. 💖-m68000,-mc68000— Generate output for a68000. This is the default when the compiler is configured for68000-based systems. It is equivalent to-march=68000. Use this option for microcontrollers with a68000orEC000core, including the68008,68302,68306,68307,68322,68328and68356. 🔥-m68010— Generate output for a68010. This is the default when the compiler is configured for68010-based systems. It is equivalent to-march=68010. 🎀-m68020,-mc68020— Generate output for a68020. This is the default when the compiler is configured for68020-based systems. It is equivalent to-march=68020. ✨-m68030— Generate output for a68030. This is the default when the compiler is configured for68030-based systems. It is equivalent to-march=68030. 🌸-m68040— Generate output for a68040. This is the default when the compiler is configured for68040-based systems. It is equivalent to-march=68040. This option inhibits the use of68881/68882instructions that have to be emulated by software on the68040. Use this option if your68040does not have code to emulate those instructions. 🦋-m68060— Generate output for a68060. This is the default when the compiler is configured for68060-based systems. It is equivalent to-march=68060. This option inhibits the use of68020and68881/68882instructions that have to be emulated by software on the68060. Use this option if your68060does not have code to emulate those instructions. 💀-mcpu32— Generate output for aCPU32. This is the default when the compiler is configured forCPU32-based systems. It is equivalent to-march=cpu32. Use this option for microcontrollers with aCPU32orCPU32+core, including the68330,68331,68332,68333,68334,68336,68340,68341,68349and68360. 👑-m5200— Generate output for a520XColdFire CPU. This is the default when the compiler is configured for520X-based systems. It is equivalent to-mcpu=5206, and is now deprecated in favor of that option. Use this option for microcontroller with a5200core, including theMCF5202,MCF5203,MCF5204andMCF5206. 🙈-m5206e— Generate output for a5206eColdFire CPU. The option is now deprecated in favor of the equivalent-mcpu=5206e. 🍓-m528x— Generate output for a member of the ColdFire528Xfamily. The option is now deprecated in favor of the equivalent-mcpu=528x. 🌈-m5307— Generate output for a ColdFire5307CPU. The option is now deprecated in favor of the equivalent-mcpu=5307. ⭐-m5407— Generate output for a ColdFire5407CPU. The option is now deprecated in favor of the equivalent-mcpu=5407. 💜-mcfv4e— Generate output for a ColdFireV4efamily CPU (e.g.547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to-mcpu=547x, and is now deprecated in favor of that option. 🧚-m68020-40— Generate output for a68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a68020/68881or a68030or a68040. The generated code does use the68881instructions that are emulated on the68040. The option is equivalent to-march=68020 -mtune=68020-40. ✨-m68020-60— Generate output for a68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a68020/68881or a68030or a68040. The generated code does use the68881instructions that are emulated on the68060. The option is equivalent to-march=68020 -mtune=68020-60. 💖-mhard-float,-m68881— Generate floating-point instructions. This is the default for68020and above, and for ColdFire devices that have an FPU. It defines the macro__HAVE_68881__on M680x0 targets and__mcffpu__on ColdFire targets. 🔥-msoft-float— Do not generate floating-point instructions; use library calls instead. This is the default for68000,68010, and68832targets. It is also the default for ColdFire devices that have no FPU. 🌸-mdiv,-mno-div— Generate (do not generate) ColdFire hardware divide and remainder instructions. If-marchis used without-mcpu, the default is"on"for ColdFire architectures and"off"for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by-mcpu). For example, the default is"off"for-mcpu=5206and"on"for-mcpu=5206e. GCC defines the macro__mcfhwdiv__when this option is enabled. 🦋-mshort— Consider typeintto be 16 bits wide, likeshort int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. 🎀-mno-short— Do not consider typeintto be 16 bits wide. This is the default. ✨-mnobitfield,-mno-bitfield— Do not use the bit-field instructions. The-m68000,-mcpu32and-m5200options imply-mnobitfield. 💀-mbitfield— Do use the bit-field instructions. The-m68020option implies-mbitfield. This is the default if you use a configuration designed for a68020. 👑-mrtd— Use a different function-calling convention, in which functions that take a fixed number of arguments return with thertdinstruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (includingprintf); otherwise incorrect code is generated for calls to those functions. In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) Thertdinstruction is supported by the68010,68020,68030,68040,68060and CPU32 processors, but not by the68000or5200. 💅
The default is -mno-rtd.
-malign-int,-mno-align-int— okay so this is the alignment vibe check 🫶 Do you want GCC to line upint,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 bestie, it costs you memory. Pick your fighter.
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. That's a real compatibility situation, not a cute one 💀
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align,-mstrict-align— do not (do) assume that unaligned memory references are handled by the system. Simple as that ✨-msep-data— generate code that lets the data segment live in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies-fPIC. main character energy fr fr 👑-mno-sep-data— generate code that assumes the data segment follows the text segment. This is the default.-mid-shared-library— generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies-fPIC💅-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used. This is the default.- *
-mshared-library-id=n* — specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. iykyk 🎀 -mxgot,-mno-xgot— when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;-fPICsuffices. 🌸
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobarIf this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code. No cap 🧚
-mlong-jump-table-offsets— use 32-bit offsets inswitchtables. The default is to use 16-bit offsets.
✨ MCore Options ✨
These are the -m options defined for the Motorola M*Core processors. Let's go 💖
-mhardlit,-mno-hardlit— inline constants into the code stream if it can be done in two instructions or less.-mdiv,-mno-div— use the divide instruction. (Enabled by default).-mrelax-immediate,-mno-relax-immediate— allow arbitrary-sized immediates in bit operations.-mwide-bitfields,-mno-wide-bitfields— always treat bit-fields asint-sized.-m4byte-functions,-mno-4byte-functions— force all functions to be aligned to a 4-byte boundary.-mcallgraph-data,-mno-callgraph-data— emit callgraph information.-mslow-bytes,-mno-slow-bytes— prefer word access when reading byte quantities.-mlittle-endian,-mbig-endian— generate code for a little-endian target.-m210,-m340— generate code for the 210 processor.-mno-lsim— assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.- *
-mstack-increment=size* — set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is0x1000. Be careful with this one bestie 😭
🌈 MicroBlaze Options 🌈
-msoft-float— use software emulation for floating point (default).-mhard-float— use hardware floating-point instructions.-mmemcpy— do not optimize block moves, usememcpy.-mno-clearbss— this option is deprecated. Use-fno-zero-initialized-in-bssinstead.- *
-mcpu=cpu-type* — use features of, and schedule 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— use software multiply emulation (default).-mxl-soft-div— use software emulation for divides (default).-mxl-barrel-shift— use the hardware barrel shifter.-mxl-pattern-compare— use pattern compare instructions.-msmall-divides— use table lookup optimization for small signed integer divisions.-mxl-stack-check— this option is deprecated. Use-fstack-checkinstead.-mxl-gp-opt— use GP-relative.sdata/.sbsssections.-mxl-multiply-high— use multiply high instructions for high part of 32x32 multiply.-mxl-float-convert— use hardware floating-point conversion instructions.-mxl-float-sqrt— use hardware floating-point square root instruction.-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target.-mxl-reorder— use reorder instructions (swap and byte reversed load/store).-mxl-mode-app-model— select application model app-model. Valid models are:executable— normal executable (default), uses startup codecrt0.o. 🫶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.owhich 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.
-mpic-data-is-text-relative— assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not supported.
💜 MIPS Options 💜
-EB— generate big-endian code.-EL— generate little-endian code. This is the default formips*el-*-*configurations.- *
-march=arch* — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are:mips1,mips2,mips3,mips4,mips32,mips32r2,mips32r3,mips32r5,mips32r6,mips64,mips64r2,mips64r3,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 *nf*1_1.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.
-mtune=arch— optimize for arch, bestie. Among other things, this controls the way instructions get scheduled and the perceived cost of arithmetic operations (yes, it's giving ✨vibes-based accounting✨). The list of arch values is the same as for-march. 🎀
When this option isn't used, GCC optimizes for the processor specified by -march. And here's the slay: by using -march and -mtune together, you can generate code that runs on a whole family of processors but optimizes for one particular member of that family. Main character of the family, if you will. 👑
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.
-mips1— equivalent to-march=mips1. 💅-mips2— equivalent to-march=mips2.-mips3— equivalent to-march=mips3.-mips4— equivalent to-march=mips4.-mips32— equivalent to-march=mips32.-mips32r3— equivalent to-march=mips32r3.-mips32r5— equivalent to-march=mips32r5.-mips32r6— equivalent to-march=mips32r6.-mips64— equivalent to-march=mips64.-mips64r2— equivalent to-march=mips64r2.-mips64r3— equivalent to-march=mips64r3.-mips64r5— equivalent to-march=mips64r5.-mips64r6— equivalent to-march=mips64r6.-mips16,-mno-mips16— generate (or, like, do NOT generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🌸
MIPS16 code generation can also be controlled on a per-function basis by means of the mips16 and nomips16 attributes. So yes, you can do it function-by-function, no cap.
-mmips16e2,-mno-mips16e2— use (or don't use) the MIPS16e2 ASE. This option modifies the behavior of the-mips16option such that it targets the MIPS16e2 ASE. It's giving glow-up energy for your MIPS16. ✨-mflip-mips16— generate MIPS16 code on alternating functions. Okay so this one is literally just for regression testing of mixed MIPS16/non-MIPS16 code generation, and it is not intended for ordinary use in compiling user code. It's a test-harness girlie, not a daily-driver girlie. 🙈-minterlink-compressed,-mno-interlink-compressed— require (or don't 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. So -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. Vintage icon behavior, we respect it. 💜-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generate code for the given ABI. Pick your fighter. 💅
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 <https://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. 🧚
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 🫶
-mabicalls,-mno-abicalls— generate (or don't generate) code that is suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems.-mshared,-mno-shared— generate (or don't generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects-mabicalls. ✨
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. ⚡
-mshared is the default.
-mplt,-mno-plt— assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects-mno-shared -mabicalls. For the n64 ABI, this option has no effect without-msym32. 😭
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.
-mxgot,-mno-xgot— lift (or don't lift) the usual restrictions on the size of the global offset table. 👑
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobarIf this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32— assume that general-purpose registers are 32 bits wide.-mgp64— assume that general-purpose registers are 64 bits wide.-mfp32— assume that floating-point registers are 32 bits wide.-mfp64— assume that floating-point registers are 64 bits wide.-mfpxx— do not assume the width of floating-point registers. Mysterious. Elusive. She contains multitudes. 🌈-mhard-float— use floating-point coprocessor instructions.-msoft-float— do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.-mno-float— equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-pointprintfformats).
If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. 💀
-msingle-float— assume that the floating-point coprocessor only supports single-precision operations.-mdouble-float— assume that the floating-point coprocessor supports double-precision operations. This is the default. ✨-modd-spreg,-mno-odd-spreg— enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI,-mno-odd-spregis set by default.-mabs=2008,-mabs=legacy— these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with theabs.fmtandneg.fmtmachine instructions. 🎀
By default or when -mabs=legacy is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. Periodt. 💅
-mnan=2008,-mnan=legacy— these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. (Yes, they literally swapped the bits. The way that the IEEE just flipped a whole convention. 😭)
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc,-mno-llsc— use (or don't use)ll,sc, andsyncinstructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them. 🔥-mllsc/-mno-llsc—-mllschits different when your runtime environment can emulate the instructions, and-mno-llscis the moment when you're compiling for nonstandard ISAs. You can make either one the default by configuring GCC with--with-llscor--without-llscrespectively 💅.--with-llscis the default for some configurations; bestie, go check the installation documentation for the deets.-mdsp/-mno-dsp— Use (or don't use, no pressure) revision 1 of the MIPS DSP ASE ✨. This option defines the preprocessor macro__mips_dsp. It also defines__mips_dsp_revto 1.-mdspr2/-mno-dspr2— Use (do not use) revision 2 of the MIPS DSP ASE 🌸. This option defines the preprocessor macros__mips_dspand__mips_dspr2. It also defines__mips_dsp_revto 2.-msmartmips/-mno-smartmips— Use (do not use) the MIPS SmartMIPS ASE. Periodt.-mpaired-single/-mno-paired-single— Use (do not use) paired-single floating-point instructions. Heads up though: this option requires hardware floating-point support to be enabled 👑-mdmx/-mno-mdmx— Use (do not use) MIPS Digital Media Extension instructions. Real talk, this option can only be used when generating 64-bit code and it requires hardware floating-point support to be enabled.-mips3d/-mno-mips3d— Use (do not use) the MIPS-3D ASE. The option-mips3dimplies-mpaired-single— it's giving package deal 🎀.-mmicromips/-mno-micromips— Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes.
-mmt/-mno-mt— Use (do not use) MT Multithreading instructions.-mmcu/-mno-mcu— Use (do not use) the MIPS MCU ASE instructions.-meva/-mno-eva— Use (do not use) the MIPS Enhanced Virtual Addressing instructions.-mvirt/-mno-virt— Use (do not use) the MIPS Virtualization (VZ) instructions.-mxpa/-mno-xpa— Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.-mcrc/-mno-crc— Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv/-mno-ginv— Use (do not use) the MIPS Global INValidate (GINV) instructions.-mloongson-mmi/-mno-loongson-mmi— Use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext/-mno-loongson-ext— Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2/-mno-loongson-ext2— Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.-mlong64— Forcelongtypes to be 64 bits wide. See-mlong32for an explanation of the default and the way that the pointer size is determined.-mlong32— Forcelong,int, and pointer types to be 32 bits wide.
The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller. That's just how it is 💅
-msym32/-mno-sym32— Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses 🔥.-Gnum — Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see-mgpoptfor details.
The default -G option depends on the configuration.
-mlocal-sdata/-mno-local-sdata— Extend (do not extend) the-Gbehavior to local data too, such as to static variables in C.-mlocal-sdatais the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. Iconic problem-solving tbh 🫶
-mextern-sdata/-mno-extern-sdata— Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the-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 is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a section attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data.
-mgpopt/-mno-gpopt— Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-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 such situations, the boot monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data/-mno-embedded-data— Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems 🍓.-muninit-const-in-rodata/-mno-uninit-const-in-rodata— Put uninitializedconstvariables in the read-only data section. This option is only meaningful in conjunction with-membedded-data.-mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings:-mcode-readable=yes— Instructions may freely access executable sections. This is the default setting.-mcode-readable=pcrel— MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.-mcode-readable=no— Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses/-mno-split-addresses— Enable (disable) use of the%hi()and%lo()assembler relocation operators. This option has been superseded by-mexplicit-relocsbut is retained for backwards compatibility. It's giving legacy, and we respect it 🙈-mexplicit-relocs=none,-mexplicit-relocs=base,-mexplicit-relocs=pcrel,-mexplicit-relocs,-mno-explicit-relocs— These options control whether explicit relocs (such as%gp_rel) are used. The default value depends on the version of GAS when GCC itself was built.
The base explicit-relocs support introdunced into GAS in 2001. The pcrel explicit-relocs support introdunced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo.
-mcheck-zero-division/-mno-check-zero-division— Trap (do not trap) on integer division by zero. The default is-mcheck-zero-division.-mdivide-traps/-mdivide-breaks— MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (SIGFPE). Use-mdivide-trapsto allow conditional traps on architectures that support them and-mdivide-breaksto force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mload-store-pairs/-mno-load-store-pairs— Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding 💅-mstrict-align,-mno-strict-align,-munaligned-access,-mno-unaligned-access— Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So-mstrict-alignmay be needed by kernel. The options-munaligned-accessand-mno-unaligned-accessare obsoleted, and only for backward-compatible. Not them being old news 😭-mmemcpy/-mno-memcpy— Force (do not force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies.-mlong-calls/-mno-long-calls— Disable (do not disable) use of thejalinstruction. Calling functions usingjalis more efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad/-mno-mad— Enable (disable) use of themad,maduandmulinstructions, as provided by the R4650 ISA.-mimadd,-mno-imadd— turns on (or off) themaddandmsubinteger instructions. Default is-mimaddon any architecture that actually supportsmaddandmsub… except the 74k, bestie, because on that one they found it made code slower. 💀 so it's off there.-mfused-madd,-mno-fused-madd— lets you use (or not use) the floating-point multiply-accumulate instructions when they're available. Default is-mfused-madd.
Heads up on the R8000 CPU though: when multiply-accumulate instructions get used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. Which may be, like, not the vibe in some circumstances. On other processors the result is numerically identical to just doing separate multiply, add, subtract and negate instructions. 🌸
-nocpp— tells the MIPS assembler to not run its preprocessor over your user assembler files (the ones with a.ssuffix) when it assembles them. no preprocessor party, we're being simple today. 🙈-mfix-24k,-mno-fix-24k— works around the 24K E48 errata (lost data on stores during refill 😭). These workarounds are implemented by the assembler, not 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 it's executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if it's executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if it's 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 it's executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— works around certain R10000 errata:ll/scsequences may not behave atomically on revisions prior to 3.0. On revisions 2.6 and earlier they may deadlock. 💀
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— does 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 instead always schedules anopinstruction there. Why? Because of a hardware bug in the R5900 chip: the short loop bug, under certain conditions, makes loops execute only once or twice. 😭 The workaround is implemented by the assembler, not by GCC.-mfix-rm7000,-mno-fix-rm7000— works around the RM7000dmult/dmultuerrata. These workarounds are implemented by the assembler, not by GCC.-mfix-vr4120,-mno-fix-vr4120— works around certain VR4120 errata:dmultudoes not always produce the correct result.divandddivdo 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 VR4130mflo/mfhierrata. The workarounds are implemented by the assembler, not by GCC — although GCC does avoid usingmfloandmfhiif the VR4130macc,macchi,dmaccanddmacchiinstructions are available instead. 👑-mfix-sb1,-mno-fix-sb1— works around certain SB-1 CPU core errata. (Right now this flag works around the SB-1 revision 2F1andF2floating-point errata.)-mr10k-cache-barrier=setting— tells GCC whether it should insert cache barriers to dodge the side effects of speculation on R10K processors. okay this one is a whole saga, sit down. 🍓
Like lots of processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts those instructions if the prediction was wrong. But — on the R10K, even the aborted instructions can have side effects. not me getting haunted by instructions that literally didn't happen. 😭
This problem only affects kernel stores and, depending on the system, kernel loads. For example: a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. And if a DMA operation writes to that same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. 💀 See the R10K processor manual for the full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions do not have side effects:
- the memory occupied by the current function's stack frame;
- the memory occupied by an incoming stack argument;
- the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. 🫶
If the input program contains a function declaration such as:
void foo (void);then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (like hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store— inserts a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=store— inserts a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=none— disables insertion of cache barriers. This is the default setting.
-mflush-func=func,-mno-flush-func— specifies the function to call to flush the I and D caches, or says don't call any such function at all. If it is called, the function must take the same arguments as the common_flush_func, that is: the address of the memory range for which the cache is being flushed, the size of the memory range, and the 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 roughly num "simple" instructions. This cost is only a heuristic, bestie — it is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the-mtunesetting.-mbranch-likely,-mno-branch-likely— enables or disables use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if the selected architecture supports them. Exception: for the MIPS32 and MIPS64 architectures (and the processors implementing them), Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. 🚫-mcompact-branches=never,-mcompact-branches=optimal,-mcompact-branches=always— these control which form of branches get generated. The default is-mcompact-branches=optimal.-mcompact-branches=neverensures compact branch instructions will never be generated.-mcompact-branches=alwaysensures a compact branch instruction will be generated if it's available for MIPS Release 6 onwards. If a compact branch instruction is not available (or it's pre-R6), a delay slot form of the branch is used instead.- If it's used for MIPS16/microMIPS targets, it will just be ignored right now. The behaviour for MIPS16/microMIPS may change in future, since they do have some compact branch instructions.
-mcompact-branches=optimalwill cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available. ✨
-mfp-exceptions,-mno-fp-exceptions— specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, then we can use both FP pipes. Otherwise we can only use one FP pipe. one pipe is giving sad girl hours. 😭
-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 that it thinks should execute in parallel. 🧚
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3.
-msynci,-mno-synci— enables (disables) generation ofsynciinstructions on architectures that support it. Thesynciinstructions (if enabled) are generated when__builtin___clear_cacheis compiled.
This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it's generally safe to use synci. However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. 💀 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 is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. 💅
-mrelax-pic-calls is the default when GCC was configured to use an assembler and a linker that both support the .reloc assembly directive, and -mexplicit-relocs is in effect. If you're on -mno-explicit-relocs tho, no worries — the assembler and the linker can pull this optimization off all by themselves, no compiler help needed. 💅
-mmcount-ra-address,-mno-mcount-ra-address— emit (or don't emit, respectively) code that lets_mcountmess with the calling function's return address. When it's enabled, the usual_mcountinterface gets a glow-up with a brand new ra-address parameter, typeintptr_t *, passed in register$12. Then_mcountcan change the return address by doing both of these things:- Returning the new address in register
$31. ✨ - Storing the new address into
*ra-address— but only if ra-address isn't null, bestie. 🙈
- Returning the new address in register
Default is -mno-mcount-ra-address.
-mframe-header-opt,-mno-frame-header-opt— turn frame header optimization on (or off) for the o32 ABI. In the o32 ABI, calling functions allocate 16 bytes on the stack so the called function has somewhere to write out register arguments. With this on, GCC suppresses that frame header allocation whenever it can tell nobody's using it. Lowkey free stack savings. 🎀
Heads up: this optimization is off by default at all optimization levels. Every single one. So don't come for me when it's not doing anything.
-mlxc1-sxc1,-mno-lxc1-sxc1— when applicable, enable (or disable) generating thelwxc1,swxc1,ldxc1andsdxc1instructions. Enabled by default.-mmadd4,-mno-madd4— when applicable, enable (or disable) generating 4-operandmadd.s,madd.dand their related instructions. Enabled by default.
👑 MMIX Options 👑
These ones are defined for the MMIX, and honestly, iconic architecture behavior:
-mlibfuncs,-mno-libfuncs— specify that intrinsic library functions are being compiled, which means all values go in registers, no matter the size. Size? Never heard of her. 💅-mepsilon,-mno-epsilon— generate floating-point comparison instructions that compare with respect to therEepsilon register.-mabi=mmixware,-mabi=gnu— generate code that passes function parameters and return values that (inside the called function) show up as registers$0and up — as opposed to the GNU ABI, which uses the global registers$231and up. Two completely different vibes, pick your fighter.-mzero-extend,-mno-zero-extend— when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, instead of the sign-extending ones.-mknuthdiv,-mno-knuthdiv— make the result of a division that yields a remainder carry the same sign as the divisor. With the default,-mno-knuthdiv, the sign of the remainder follows the sign of the dividend instead. Both are arithmetically valid, fr fr — but the latter is almost exclusively the one people actually use.-mtoplevel-symbols,-mno-toplevel-symbols— prepend (or don't prepend) a:to all global symbols, so the assembly code can be used with thePREFIXassembly directive.-melf— generate an executable in the ELF format, rather than the default mmo format that the mmix simulator uses.-mbranch-predict,-mno-branch-predict— use (or don't use) the probable-branch instructions whenever static branch prediction says a branch is probable.-mbase-addresses,-mno-base-addresses— generate (or don'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 then gets 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 — and this is the catch — the number of different data items you can address is limited. Which means a program rocking lots of static data may need-mno-base-addresses. 😭-msingle-exit,-mno-single-exit— force (or don't force) the generated code to have a single exit point in each function.
🌈 MN10300 Options 🌈
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug— generate code to dodge bugs in the multiply instructions for the MN10300 processors. This is the default.-mno-mult-bug— do not generate code to avoid those multiply-instruction bugs for the MN10300 processors.-mam33— generate code using features specific to the AM33 processor.-mno-am33— do not generate code using AM33-specific features. This is the default.-mam33-2— generate code using features specific to the AM33/2.0 processor.-mam34— generate code using features specific to the AM34 processor.-mtune=cpu-type — use the timing characteristics of the indicated CPU type when scheduling instructions. Note that this does **not change the targeted processor type. The CPU type has to be one ofmn10300,am33,am33-2oram34.-mreturn-pointer-on-d0— when generating a function that returns a pointer, return that pointer in botha0andd0. Otherwise the pointer comes back only ina0, and any attempt to call such functions without a prototype results in errors. Heads up: this option is on by default — use-mno-return-pointer-on-d0to turn it off.-mno-crt0— don't link in the C run-time initialization object file.-mrelax— tell the linker it should run a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only does anything when you use it on the command line for the final link step. 🫶
⚠️ This option makes symbolic debugging impossible. Not "harder". Impossible. Choose wisely, bestie.
-mliw— let the compiler generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro__LIW__.-mno-liw— don't let the compiler generate Long Instruction Word instructions. This option defines the preprocessor macro__NO_LIW__.-msetlb— let the compiler generate theSETLBandLccinstructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro__SETLB__.-mno-setlb— don't let the compiler generateSETLBorLccinstructions. This option defines the preprocessor macro__NO_SETLB__.
🦋 Moxie Options 🦋
-meb— generate big-endian code. This is the default formoxie-*-*configurations.-mel— generate 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:
-masm-hex— force assembly output to always use hex constants. Normally those constants would be signed decimals, but this option exists for testsuite and/or aesthetic purposes. It's giving vibes over function. 💅-mmcu=— select the MCU to target. This gets used to create a C preprocessor symbol based on the MCU name, converted to UPPER CASE and pre- and post-fixed with__. That symbol is in turn used by themsp430.hheader file to pick an MCU-specific supplementary header file.
The option also sets the ISA to use. If the MCU name is one known to only support the 430 ISA, then that's what gets selected — otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly, the generic msp430x MCU name selects the 430X ISA.
On top of all that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and makes the linker go hunting for a script called xxx.ld.
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond the hard-coded ones. 👀
GCC searches for the devices.csv file using the following methods, in this exact precedence order — first beats second, second beats third:
- Include path specified with
-Iand-L—devices.csvwill be searched for in each of the directories specified by include paths and linker library search paths. - Path specified by the environment variable
MSP430_GCC_INCLUDE_DIR— set the value of the global environment variableMSP430_GCC_INCLUDE_DIRto the full path of the directory containingdevices.csv, and GCC will search that directory fordevices.csv. If it's found there, that directory also gets registered as an include path and a linker library path. Which means header files and linker scripts living in that directory can be used without manually specifying-Iand-Lon the command line. Convenient queen behavior. 👑 - The
msp430-elf{,bare}/include/devicesdirectory — finally, GCC will examinemsp430-elf{,bare}/include/devicesfrom the toolchain root directory. This directory does not exist in a default installation, but if you created it and copieddevices.csvin there, the MCU data will be read. And same as above, this directory also gets registered as an include path and linker library path.
If none of the above search methods find devices.csv, then the hard-coded MCU data is what gets used. Periodt.
-mwarn-mcu,-mno-warn-mcu— enables or disables warnings about conflicts between the MCU name you gave to-mmcuand the ISA set by-mcpuand/or the hardware multiply support set by-mhwmult. It also toggles warnings about unrecognized MCU names. This option is on by default.-mcpu=— specifies the ISA to use. Accepted values aremsp430,msp430xandmsp430xv2. 🚨 This option is deprecated. The-mmcu=option is the one you should be using to select the ISA.-msim— link to the simulator runtime libraries and linker script. Overrides any scripts that-mmcu=would have selected.-mlarge— use large-model addressing (20-bit pointers, 20-bitsize_t).-msmall— use small-model addressing (16-bit pointers, 16-bitsize_t).-mrelax— this option gets passed to the assembler and the linker, and it lets the linker perform certain optimizations that can't be done until the final link. ✨mhwmult=— ok so this one tells everyone what kind of hardware multiply your target can do 💪 accepted values arenone(no hardware multiply at all),16bit(the og 16-bit-only multiply that the early MCUs had),32bit(the 16/32-bit multiply the later MCUs support), andf5series(the 16/32-bit multiply that F5-series MCUs rock). You can also just sayautoand let GCC deduce the hardware multiply support from the MCU name you handed it via-mmcu— and bestie, if there's no-mmcuat all, or the MCU name is unrecognized, then it just assumes no hardware multiply support."auto"is the default setting, periodt.
Hardware multiplies are normally done by calling a library routine, which saves space in the generated code 🫶 but when you compile at -O3 or higher, the hardware multiplier gets invoked inline instead — bigger code, but faster, no cap.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish, so they're safe to use inside interrupt handlers as well as in normal code ✨
-minrt— turns on a minimum runtime environment: no static initializers, no constructors. It's giving memory-constrained-device energy 🙈 the compiler drops special symbols into some objects that tell the linker and runtime which code fragments are actually required.-mtiny-printf— enables the reduced-code-size"printf"and"puts"library functions 💅 tiny versions of these are not reentrant, so you gotta use them with caution in multi-threaded applications, okay?
Support for streams has been removed, and the string to be printed will always be sent to stdout via the "write" syscall. The string is not buffered before it is 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 between 0 and 64 inclusive, and sets the maximum number of inline shift instructions that should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, anmspabihelper function is used instead. The default value is4.
This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value 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 corresponding attribute. The fourth possible value —"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 the memory regions.-msilicon-errata=— passes on a request to the assembler to enable the fixes for the named silicon errata 🛠️-msilicon-errata-warn=— passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.-mwarn-devices-csv-mno-warn-devices-csv— warn ifdevices.csvis not found or there are problems parsing it (default: on) 💖
🦋 NDS32 Options 🦋
These options are defined for NDS32 implementations:
-mbig-endian— generate code in big-endian mode.-mlittle-endian— generate code in little-endian mode.-mreduced-regs— use reduced-set registers for register allocation.-mfull-regs— use full-set registers for register allocation.-mcmov— generate conditional move instructions.-mno-cmov— do not generate conditional move instructions.-mext-perf— generate performance extension instructions.-mno-ext-perf— do not generate performance extension instructions.-mext-perf2— generate performance extension 2 instructions.-mno-ext-perf2— do not generate performance extension 2 instructions.-mext-string— generate string extension instructions.-mno-ext-string— do not generate string extension instructions.-mv3push— generate v3push25/pop25instructions.-mno-v3push— do not generate v3push25/pop25instructions.-m16-bit— generate 16-bit instructions.-mno-16-bit— do not generate 16-bit instructions.- *
-misr-vector-size=num* — specify the size of each interrupt vector, which must be4or16. - *
-mcache-block-size=num* — specify the size of each cache block, which must be a power of 2 between4and512. - *
-march=arch* — specify the name of the target architecture. - *
-mcmodel=code-model* — set the code model to one of:small— all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.medium— the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.large— all the text and data segments can be within 4GB addressing space.
-mctor-dtor— enable constructor/destructor feature.-mrelax— guide linker to relax instructions 🔥
🌸 Nios II Options 🌸
These are the options defined for the Altera Nios II processor, bestie 👑
-Gnum — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is8.-mgpopt=option-mgpopt-mno-gpopt— generate (do not generate) GP-relative accesses. The following option names are recognized:none— do not generate GP-relative accesses.local— generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a"section"attribute.global— as forlocal, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same-Gsetting.data— generate GP-relative accesses for all data objects in the program. If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.all— generate GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries.
You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section 😭
- *
-mgprel-sec=regexp* — this option specifies additional section names that can be accessed via GP-relative addressing. It is most useful in conjunction with"section"attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.
This option does not affect the behavior of the -G option, and the specified sections are in addition to the standard ".sdata" and ".sbss" small-data sections that are recognized by -mgpopt.
- *
-mr0rel-sec=regexp* — this option specifies names of sections that can be accessed via a 16-bit offset from"r0"; that is, in the low 32K or high 32K of the 32-bit address space. It is most useful in conjunction with"section"attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.
In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory.
-mel-meb— generate little-endian (default) or big-endian (experimental) code, respectively.- *
-march=arch* — this specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are:r1,r2.
The preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the targeted ISA level 💫
-mbypass-cache-mno-bypass-cache— force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache.-mno-cache-volatile-mcache-volatile— volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.-mno-fast-sw-div-mfast-sw-div— do not use table-based fast divide for small numbers. The default is to use the fast divide at-O3and above.-mno-hw-mul-mhw-mul-mno-hw-mulx-mhw-mulx-mno-hw-div-mhw-div— enable or disable emitting"mul","mulx"and"div"family of instructions by the compiler. The default is to emit"mul"and not emit"div"and"mulx".-mbmx-mno-bmx-mcdx-mno-cdx— enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Enabling these instructions also requires-march=r2. Since these instructions are optional extensions to the R2 architecture, the default is not to emit them 🌈-mcustom-insn=N,-mno-custom-insn— okay so each-mcustom-insn=Nturns on a custom instruction with encodingNwhenever the compiler generates code that uses insn. Like,-mcustom-fadds=253makes it emit custom instruction253for single-precision floating-point add ops, instead of the default vibe which is just calling a library. 💅
The values of insn that are supported are listed below. Unless it says otherwise, floating-point ops are expected to be implemented with normal IEEE 754 semantics, and they line up directly with the C operators or the equivalent GCC built-in functions. no cap.
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 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 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 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_lroundf"function when-fno-math-errnois used.floatis,floatus,floatid,floatud— conversion from signed or unsigned integer types to floating-point types.
Also, bestie, listen: all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or 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. it's giving automatic, honestly 🫶
fwrx— write src1 into the least significant half of X and src2 into the most significant half of X.fwry— write src1 into Y.frdxhi,frdxlo— read the most or least (respectively) significant half of X and store it in dest.frdy— read the value of Y and store it into dest.
Heads up 💡: you can gain 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— this one enables a predefined, named set of custom instruction encodings (peep-mcustom-insnabove). Currently the following sets are defined:-mcustom-fpu-cfg=60-1is equivalent to:-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant-mcustom-fpu-cfg=60-2is equivalent to:-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant-mcustom-fpu-cfg=72-3is 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-mcustom-fpu-cfg=fph2is 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
Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. so specific > preset, always. 👑
Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma.
The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. Please note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234 are only generated if -ffinite-math-only is specified. The custom instruction enabled by -mcustom-round=248 is only generated if -fno-math-errno is specified. And in contrast to the other configurations, -fsingle-precision-constant is not set. 💀
These additional -m options are available for the Altera Nios II ELF (bare-metal) target:
-mhal— link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with-msys-crt0=to specify the location of the alternate startup code provided by the HAL BSP.-msmallc— link with a limited version of the C library,-lsmallc, rather than Newlib.-msys-crt0=startfile— startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with-mhal.-msys-lib=systemlib— systemlib is the library name of the library that provides low-level system calls required by the C library, e.g."read"and"write". This option is typically used to link with a library provided by a HAL BSP.
✨ Nvidia PTX Options ✨
These options are defined for Nvidia PTX:
-m64— ignored, but preserved for backward compatibility. Only 64-bit ABI is supported. so like, it's there, it does nothing, we love a decorative icon. 🎀-march=architecture-string— generate code for the specified PTX ISA target architecture (e.g.sm_35). Valid architecture strings aresm_30,sm_35,sm_53,sm_70,sm_75andsm_80. The default depends on how the compiler has been configured, see--with-arch.
This option sets the value of the preprocessor macro "__PTX_SM__"; for instance, for sm_35, it has the value 350.
-misa=architecture-string— alias of-march=.-march-map=architecture-string— select the closest available-march=value that is not more capable. For instance, for-march-map=sm_50select-march=sm_35, and for-march-map=sm_53select-march=sm_53.-mptx=version-string— generate code for the specified PTX ISA version (e.g.7.0). Valid version strings include3.1,6.0,6.3, and7.0. The default PTX ISA version is6.0, unless a higher version is required for specified PTX ISA target architecture via option-march=.
This option sets the values of the preprocessor macros "__PTX_ISA_VERSION_MAJOR__" and "__PTX_ISA_VERSION_MINOR__"; for instance, for 3.1 the macros have the values 3 and 1, respectively.
-mmainkernel— link in code for a__mainkernel. This is for stand-alone instead of offloading execution.-moptimize— apply partitioned execution optimizations. This is the default when any level of optimization is selected.-msoft-stack— generate code that does not use".local"memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or"alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option-mgomp.-muniform-simt— switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc,free,vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array"int __nvptx_uni[]"stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0outside of SIMD regions). Each thread can bitwise-and the bitmask at position"tid.y"with current lane index to compute the master lane index.-mgomp— generate code for use in OpenMP offloading: enables-msoft-stackand-muniform-simtoptions, and selects corresponding multilib variant.
🌈 OpenRISC Options 🌈
These options are defined for OpenRISC:
-mboard=name— configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is"or1ksim".-mnewlib— bestie, this one does literally nothing. It's ✨ignored✨, kept around for compatibility only. Back in the day it picked linker and preprocessor options for use with newlib, and now it's just vibing in the option list. 🫶-msoft-div,-mhard-div— pick whether divide ops use software or hardware divide (l.div,l.divu) instructions. The default is hardware divide. 🍓-msoft-mul,-mhard-mul— same energy, but for multiply (l.mul,l.muli). Default here is hardware multiply.-msoft-float,-mhard-float— choose software or hardware for floating point operations. Default is software.-mdouble-float— when-mhard-floatis selected, this turns on generation of double-precision floating point instructions. By default, functions fromlibgccare used to perform double-precision floating point operations. 💅-munordered-float— when-mhard-floatis selected, this enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default, functions fromlibgccdo the unordered floating point compare and set flag operations instead.-mcmov— enable generation of conditional move (l.cmov) instructions. By default the equivalent gets generated using set and branch. no cap.-mror— enable generation of rotate right (l.ror) instructions. By default, functions fromlibgccperform rotate right operations.-mrori— enable generation of rotate right with immediate (l.rori) instructions. By default, functions fromlibgcchandle rotate right with immediate operations.-msext— enable generation of sign extension (l.ext*) instructions. By default, memory loads are used to perform 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.-mcmodel=small— generate OpenRISC code for the small model: the GOT is limited to 64k. This is the default model. 👑-mcmodel=large— generate OpenRISC code for the large model: the GOT may grow up to 4G in size. big GOT energy.
🌈 PDP-11 Options 🌈
These options are defined for the PDP-11:
-mfpu— use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies-m45.-msoft-float— do not use hardware floating point.-mac0— return floating-point results in ac0 (fr0 in Unix assembler syntax).-mno-ac0— return floating-point results in memory. This is the default.-m40— generate code for a PDP-11/40. Implies-msoft-float-mno-split.-m45— generate code for a PDP-11/45. This is the default.-m10— generate code for a PDP-11/10. Implies-msoft-float-mno-split.-mint16,-mno-int32— use 16-bitint. This is the default.-mint32,-mno-int16— use 32-bitint.-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 oldreloadallocator is used.
🦋 PowerPC Options 🦋
These are listed under
🌸 PRU Options 🌸
These command-line options are defined for PRU target:
-minrt— link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features are removed. 😭
This option disables support for static initializers and constructors. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features.
bestie, hear me: the minimal startup code would not passargcandargvarguments tomain, so the latter must be declared asint main (void). This is already the norm for most firmware projects. 💖
-mmcu=mcu— specify the PRU hardware variant to use. A correspondingly named spec file would be loaded, passing the memory region sizes to the linker and defining hardware-specific C macros.
Newlib provides only the sim spec, intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package.
-mno-relax— make GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption.-mloop— allow (or do not allow) GCC to use the LOOP instruction.-mabi=variant— specify the ABI variant to output code for.-mabi=tiselects the unmodified TI ABI while-mabi=gnuselects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences:
Function Pointer Size
TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.
Optional Return Value Pointer
Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.
The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti.
Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions. 💀
✨ RISC-V Options ✨
These command-line options are defined for RISC-V targets:
-mbranch-cost=n— set the cost of branches to roughly n instructions.-mplt,-mno-plt— when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is-mplt.-mabi=ABI-string— specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example-march=rv64ifd -mabi=lp64dmeans that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with-march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or-march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.
The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well specified at present, and are subject to change. 🙈
-mfdiv,-mno-fdiv— do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.-mdiv,-mno-div— do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.-misa-spec=ISA-spec-string— specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are:2.2— produce code conforming to version 2.2.20190608— produce code conforming to version 20190608.20191213— produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.
-march=ISA-string— generate code for given RISC-V ISA (e.g.rv64im). ISA strings must be lower-case. Examples includerv64i,rv32g,rv32e, andrv32imaf. Additionally, a special value help (-march=help) is accepted to list all supported extensions. 👑
The syntax of the ISA string is defined as follows:
- The string must start with
rv32orrv64, followed byi,e, org, referred to as the base ISA. - The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g.
zba) and single-letter (e.g.v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores. - An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
- Extension names may include an optional version number, following the syntax
<major>p<minor>or<major>, (e.g.m2p1orm2).
Supported extension are listed below:
i— supported version2.0,2.1— base integer extension.e— supported version2.0— reduced base integer extension.g— supported version-— general-purpose computing base extension,gwill expand toi,m,a,f,d,zicsrandzifencei.m— supported version2.0— integer multiplication and division extension.a— supported version2.0,2.1— atomic extension.f— supported version2.0,2.2— single-precision floating-point extension.d— supported version2.0,2.2— double-precision floating-point extension.c— supported version2.0— compressed extension.h— supported version1.0— hypervisor extension.
v— version1.0— Vector extension. 💅zicsr— version2.0— Control and status register access extension.zifencei— version2.0— Instruction-fetch fence extension.zicond— version1.0— Integer conditional operations extension.za64rs— version1.0— Reservation set size of 64 bytes.za128rs— version1.0— Reservation set size of 128 bytes.zawrs— version1.0— Wait-on-reservation-set extension.zba— version1.0— Address calculation extension.zbb— version1.0— Basic bit manipulation extension.zbc— version1.0— Carry-less multiplication extension.zbs— version1.0— Single-bit operation extension.zfinx— version1.0— Single-precision floating-point in integer registers extension.zdinx— version1.0— Double-precision floating-point in integer registers extension.zhinx— version1.0— Half-precision floating-point in integer registers extension.zhinxmin— version1.0— Minimal half-precision floating-point in integer registers extension.zbkb— version1.0— Cryptography bit-manipulation extension.zbkc— version1.0— Cryptography carry-less multiply extension.zbkx— version1.0— Cryptography crossbar permutation extension.zkne— version1.0— AES Encryption extension.zknd— version1.0— AES Decryption extension.zknh— version1.0— Hash function extension.zkr— version1.0— Entropy source extension.zksed— version1.0— SM4 block cipher extension.zksh— version1.0— SM3 hash function extension.zkt— version1.0— Data independent execution latency extension.zk— version1.0— Standard scalar cryptography extension.zkn— version1.0— NIST algorithm suite extension.zks— version1.0— ShangMi algorithm suite extension.zihintntl— version1.0— Non-temporal locality hints extension.zihintpause— version1.0— Pause hint extension.zicboz— version1.0— Cache-block zero extension.zicbom— version1.0— Cache-block management extension.zicbop— version1.0— Cache-block prefetch extension.zic64b— version1.0— Cache block size isf 64 bytes.ziccamoa— version1.0— Main memory supports all atomics in A.ziccif— version1.0— Main memory supports instruction fetch with atomicity requirement.zicclsm— version1.0— Main memory supports misaligned loads/stores.ziccrse— version1.0— Main memory supports forward progress on LR/SC sequences.zicntr— version2.0— Standard extension for base counters and timers.zihpm— version2.0— Standard extension for hardware performance counters.ztso— version1.0— Total store ordering extension.zve32x— version1.0— Vector extensions for embedded processors.zve32f— version1.0— Vector extensions for embedded processors.zve64x— version1.0— Vector extensions for embedded processors.zve64f— version1.0— Vector extensions for embedded processors.zve64d— version1.0— Vector extensions for embedded processors.zvl32b— version1.0— Minimum vector length standard extensionszvl64b— version1.0— Minimum vector length standard extensionszvl128b— version1.0— Minimum vector length standard extensionszvl256b— version1.0— Minimum vector length standard extensionszvl512b— version1.0— Minimum vector length standard extensionszvl1024b— version1.0— Minimum vector length standard extensionszvl2048b— version1.0— Minimum vector length standard extensionszvl4096b— version1.0— Minimum vector length standard extensionszvbb— version1.0— Vector basic bit-manipulation extension.zvbc— version1.0— Vector carryless multiplication extension.zvkb— version1.0— Vector cryptography bit-manipulation extension.zvkg— version1.0— Vector GCM/GMAC extension.zvkned— version1.0— Vector AES block cipher extension.zvknha— version1.0— Vector SHA-2 secure hash extension.zvknhb— version1.0— Vector SHA-2 secure hash extension.zvksed— version1.0— Vector SM4 Block Cipher extension.zvksh— version1.0— Vector SM3 Secure Hash extension.zvkn— version1.0— Vector NIST Algorithm Suite extension,zvknwill expand tozvkned,zvknhb,zvkbandzvkt.zvknc— version1.0— Vector NIST Algorithm Suite with carryless multiply extension,zvkncwill expand tozvknandzvbc.zvkng— version1.0— Vector NIST Algorithm Suite with GCM extension,zvkngwill expand tozvknandzvkg.zvks— version1.0— Vector ShangMi algorithm suite extension,zvkswill expand tozvksed,zvksh,zvkbandzvkt.zvksc— version1.0— Vector ShangMi algorithm suite with carryless multiplication extension,zvkscwill expand tozvksandzvbc.zvksg— version1.0— Vector ShangMi algorithm suite with GCM extension,zvksgwill expand tozvksandzvkg.zvkt— version1.0— Vector data independent execution latency extension.zfh— version1.0— Half-precision floating-point extension.zfhmin— version1.0— Minimal half-precision floating-point extension.zvfh— version1.0— Vector half-precision floating-point extension.zvfhmin— version1.0— Vector minimal half-precision floating-point extension.zvfbfmin— version1.0— Vector BF16 converts extension.zfa— version1.0— Additional floating-point extension.zmmul— version1.0— Integer multiplication extension.zca— version1.0— Integer compressed instruction extension.zcf— version1.0— Compressed single-precision floating point loads and stores extension.zcd— version1.0— Compressed double-precision floating point loads and stores extension.zcb— version1.0— Simple compressed instruction extension.zce— version1.0— Compressed instruction extensions for embedded processors.zcmp— version1.0— Compressed push pop extension.zcmt— version1.0— Table jump instruction extension.smaia— version1.0— Advanced interrupt architecture extension.smepmp— version1.0— PMP Enhancements for memory access and execution prevention on Machine mode.smstateen— version1.0— State enable extension.ssaia— version1.0— Advanced interrupt architecture extension for supervisor-mode.sscofpmf— version1.0— Count overflow & filtering extension.ssstateen— version1.0— State-enable extension for supervisor-mode.sstc— version1.0— Supervisor-mode timer interrupts extension.svinval— version1.0— Fine-grained address-translation cache invalidation extension.svnapot— version1.0— NAPOT translation contiguity extension.svpbmt— version1.0— Page-based memory types extension.xcvmac— version1.0— Core-V multiply-accumulate extension.xcvalu— version1.0— Core-V miscellaneous ALU extension.xcvelw— version1.0— Core-V event load word extension.xtheadba— version1.0— T-head address calculation extension.xtheadbb— version1.0— T-head basic bit-manipulation extension.xtheadbs— version1.0— T-head single-bit instructions extension.xtheadcmo— version1.0— T-head cache management operations extension.xtheadcondmov— version1.0— T-head conditional move extension.xtheadfmemidx— version1.0— T-head indexed memory operations for floating-point registers extension.xtheadfmv— version1.0— T-head double floating-point high-bit data transmission extension.xtheadint— version1.0— T-head acceleration interruption extension.xtheadmac— version1.0— T-head multiply-accumulate extension.xtheadmemidx— version1.0— T-head indexed memory operation extension.xtheadmempair— version1.0— T-head two-GPR memory operation extension.xtheadsync— version1.0— T-head multi-core synchronization extension.xventanacondops— version1.0— Ventana integer conditional operations extension.
okay bestie, breathe 💖 that was a LOT of extensions and you absolutely ate that list, no notes.
When -march= is not specified, use the setting from -mcpu.
If both -march and -mcpu= are not specified, the default for this argument is system dependent, and users who want a specific architecture extensions should specify one explicitly. like, don't just vibe and hope, specify it, fr fr.
✨ the CPU-picking era ✨
-mcpu=processor-string— Use architecture of and optimize the output for the given processor, specified by particular CPU name. 💅
Permissible values for this option are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34, sifive-e76, sifive-s21, sifive-s51, sifive-s54, sifive-s76, sifive-u54, sifive-u74, sifive-x280, sifive-xp450, sifive-x670.
Note that-mcpudoes not override-marchor-mtune. it's giving "I mind my own business" energy 🙈
-mtune=processor-string— Optimize the output for the given processor, specified by microarchitecture or particular CPU name.
Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, thead-c906, size, sifive-p400-series, sifive-p600-series, and all valid options for -mcpu=.
When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not specified. 🌸
The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance. 🎀
-mpreferred-stack-boundary=num— okay so this one tries to keep your stack boundary aligned to a 2 raised to the num byte boundary. If you don't pass it, the default is4, which is16bytes, which is128-bits, math girlie 💅
⚠️ WARNING bestie listen to me: if you use this switch you must build all your modules with the same value. All of them. Including any libraries. Including the system libraries and the startup modules. Do not play with this one 🙈
-msmall-data-limit=n— shoves global and static data that's smaller than n bytes into a special section (this only happens on some targets, so vibe check your target first ✨).-msave-restore,-mno-save-restore— do (or don't) use smaller-but-slower prologue and epilogue code that goes through library function calls. The default is the fast inline prologues and epilogues, because speed is the moment 💖-mmovcc,-mno-movcc— do (or don't) produce branchless conditional-move code sequences, even on targets that don't have dedicated instructions for conditional operations. If you enable it, sequences of ALU operations get produced using base integer ISA instructions where that's profitable 🫶-minline-atomics,-mno-inline-atomics— do (or don't) use the smaller-but-slower subword atomic emulation code that callslibatomicfunctions. The default is fast inline subword atomics that don't needlibatomicat all 🔥-minline-strlen,-mno-inline-strlen— do (or don't) attempt to inlinestrlencalls if possible. Inlining only happens if the string is properly aligned and instructions for accelerated processing are available. The default is to not inlinestrlencalls.-minline-strcmp,-mno-inline-strcmp— do (or don't) attempt to inlinestrcmpcalls if possible. Inlining only happens if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inlinestrcmpcalls.
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.
-minline-strncmp,-mno-inline-strncmp— do (or don't) attempt to inlinestrncmpcalls if possible. Inlining only happens if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inlinestrncmpcalls.
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.
-mshorten-memrefs,-mno-shorten-memrefs— do (or don't) try to get more mileage out of compressed load/store instructions by swapping a load/store of'base register + large offset'for a new load/store of'new base + small offset'. If that new base gets stashed in a compressed register, then the new load/store can be compressed too, slay 🎀 Currently this only targets 32-bit integer load/stores.-mstrict-align,-mno-strict-align— don't (or do) generate unaligned memory accesses. The default depends on whether the processor you're optimizing for supports fast unaligned access or not ✨-mcmodel=medlow— generate code for the medium-low code model. The program and its statically defined symbols have to lie inside a single2 GiBaddress range and must sit between absolute addresses-2 GiBand+2 GiB. Programs can be statically or dynamically linked. This is the default code model 👑-mcmodel=medany— generate code for the medium-any code model. The program and its statically defined symbols must be within any single2 GiBaddress range. Programs can be statically or dynamically linked.
The code generated by the medium-any code model is position-independent, but it is not guaranteed to function correctly when linked into position-independent executables or libraries. Not me warning you twice but like, I'm warning you 🙈
-mexplicit-relocs,-mno-exlicit-relocs— use (or don't use) assembler relocation operators when dealing with symbolic addresses. (Yes bestie, the-mno-one is spelled exactly like that in the manual. No cap. Don't @ me 💀) The alternative is to use assembler macros instead, which may limit optimization.-mrelax,-mno-relax— take (or don't take) advantage of linker relaxations to reduce the number of instructions needed to materialize symbol addresses. The default is to take advantage of linker relaxations 🌈-mriscv-attribute,-mno-riscv-attribute— emit (or don't emit) the RISC-V attribute so extra info gets recorded into ELF objects. This feature needs at least binutils2.32🧚-mcsr-check,-mno-csr-check— enables or disables the CSR checking.-malign-data=type— controls how GCC aligns variables and constants of array, structure, or union types. The supported values for type arexlen, which uses x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default 💜-mbig-endian— generate big-endian code. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.-mlittle-endian— generate little-endian code. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*but not ariscv64be-*-*orriscv32be-*-*target.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block.
With that second choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which register to use as the base register for reading the canary, and what offset from that base register to read it from. There's no default register or offset, because this is entirely for use within the Linux kernel 🔥
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.-mtls-dialect=trad— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
🌸 RL78 Options 🌸
-msim— links in extra target libraries to support operation inside a simulator ✨-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— picks what kind of hardware multiplication and division support you want. 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 found on the RL78/G13 (S2 core) targets. The"g14"value selects 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"👑
On top of that, a C preprocessor macro 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— specifies which RL78 core to target. 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 💅
If this option is set, it also selects the type of hardware multiply support to use, unless that gets overridden by an explicit -mmul=none option on the command line. So specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables the use of hardware multiplications altogether.
Note bestie: even though the RL78/G14 core is the default target, specifying-mcpu=g14or-mcpu=rl78on the command line does change the behavior of the toolchain, since it also enables G14 hardware multiply support. If these options are not specified on the command line, then software multiplication routines get used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains that didn't have hardware multiply and divide support. The way that 😭
On top of that, a C preprocessor macro gets defined based on the setting of this option. The possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10,-mg13,-mg14,-mrl78— these are aliases for the corresponding-mcpu=option. They're provided for backwards compatibility 🫶-mallregs— lets the compiler use all of the available registers. By default, registers"r24..r31"are reserved for use in interrupt handlers. With this option enabled, those registers can be used in ordinary functions as well 🔥-m64bit-doubles,-m32bit-doubles— makes the"double"data type be64bits (-m64bit-doubles) or32bits (-m32bit-doubles) in size. The default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— says that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers, since that makes the interrupt handlers faster. The target option-mg13needs to be passed for this to work, because this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation, or if it calls another function 💜
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt,-mno-powerpc-gpopt,-mpowerpc-gfxopt,-mno-powerpc-gfxopt,-mpowerpc64,-mno-powerpc64,-mmfcrf,-mno-mfcrf,-mpopcntb,-mno-popcntb,-mpopcntd,-mno-popcntd,-mfprnd,-mno-fprnd,-mcmpb,-mno-cmpb,-mhard-dfp,-mno-hard-dfp— these are the whole squad of "which instructions does your processor actually have" switches 👇
You use these options to specify which instructions are available on the processor you're using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above 💅
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select 🌈
-mmfcrf lets GCC spit out that move-from-condition-register-field instruction — the one that lives on the POWER4 processor and all the other chips running the PowerPC V2.01 architecture. -mpopcntb unlocks the popcount and double-precision FP reciprocal estimate instruction, which is a POWER5-and-PowerPC-V2.02-architecture kinda thing. -mpopcntd gets you the popcount instruction from the POWER7 processor and the other PowerPC V2.06 architecture processors. -mfprnd brings the FP round-to-integer instructions, found on the POWER5+ processor and other PowerPC V2.03 architecture friends. -mcmpb generates the compare bytes instruction, chillin' on the POWER6 processor and other PowerPC V2.05 architecture processors. And -mhard-dfp generates the decimal floating-point instructions that some POWER processors have. 💅
The -mpowerpc64 option lets GCC generate those extra 64-bit instructions that exist in the full PowerPC64 architecture, and it treats GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64, bestie. ✨
🎀 -mcpu=cpu_type 🎀
Set the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. The supported values for cpu_type are:
401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, power10, powerpc,
powerpc64, powerpc64le, rs64, and native.-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le are giving pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types respectively — and an appropriate, generic processor model gets assumed for scheduling purposes. 🌸
Specifying native as the cpu type detects and selects the architecture option that corresponds to the host processor of the system doing the compilation. Heads up though: -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
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protectThe particular options set for any particular CPU varies between compiler versions — it depends on what setting seems to produce optimal code for that CPU. It doesn't necessarily reflect the actual hardware's capabilities, iykyk. 👑
If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like:
-mcpu=970 -mno-altivecOn AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment. 🫶
✨ -mtune=cpu_type ✨
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type or register usage, the way -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. Slay, division of labour. 💅
🦋 -mcmodel=small 🦋
Generate PowerPC64 code for the small model: the TOC is limited to 64k. Cute and compact.
🦋 -mcmodel=medium 🦋
Generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux. 💖
🦋 -mcmodel=large 🦋
Generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space. Main character energy, honestly.
🌈 -maltivec / -mno-altivec 🌈
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics such as vec_splat, vec_extract, and vec_insert match array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. Endianness really said "pick a side" 😭
🌈 -mvrsave / -mno-vrsave 🌈
Generate VRSAVE instructions when generating AltiVec code.
🌈 -msecure-plt 🌈
Generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option.
🌈 -mbss-plt 🌈
Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
🌈 -misel / -mno-isel 🌈
This switch enables or disables the generation of ISEL instructions.
🌈 -mvsx / -mno-vsx 🌈
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set.
🌈 -mcrypto / -mno-crypto 🌈
Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔐
🌈 -mhtm / -mno-htm 🌈
Enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
🌈 -mpower8-fusion / -mno-power8-fusion 🌈
Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. (Yes bestie, the manpage literally says "does not keeps" — we're keeping it fr fr. 💀)
🌈 -mquad-memory / -mno-quad-memory 🌈
Generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode.
🌈 -mquad-memory-atomic / -mno-quad-memory-atomic 🌈
Generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode.
🌈 -mfloat128 / -mno-float128 🌈
Enable/disable the __float128 keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation. 🧚
🌈 -mfloat128-hardware / -mno-float128-hardware 🌈
Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.
🌈 -m32 / -m64 🌈
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
🌈 -mfull-toc / -mno-fp-in-toc / -mno-sum-in-toc / -mminimal-toc 🌈
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 😭
If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code.
🫶 TOC space is giving "closet with way too many outfits" — sometimes you just gotta declutter.
🌈 -maix64 / -maix32 🌈
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 💅
-mxl-compat,-mno-xl-compat— okay so this one makes GCC spit out code that plays nicer with IBM XL compiler semantics when you're using the AIX-compatible ABI 💅. It passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, on top of the argument FPRs. It also does not assume that the most significant double in a 128-bitlong doublevalue is properly rounded when comparing values and converting todouble. And it uses XL symbol names forlong doublesupport routines.
The tea here 🫖: the AIX calling convention got extended but was never initially documented, all to handle some obscure K&R C case where you call a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers reach for floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — it's only necessary when you're calling subroutines compiled by IBM XL compilers without optimization. iykyk 😭
-mpe— supports 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 actually run. For this to work the system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or else the specs file has to be overridden with the-specs=option to point at the right directory location. And heads up bestie ⚠️: the Parallel Environment does not support threads, so-mpeand-pthreadare straight up incompatible. periodt.-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of larger types (think floating-point doubles) onto their natural size-based boundary 🦋. Meanwhile-malign-powertells GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.
But on 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. no cap 💀
-msoft-float,-mhard-float— generate code that does not use (uses) the floating-point register set ✨. Software floating-point emulation is provided if you use-msoft-float, and you also gotta pass the option to GCC when linking. don't forget that part fr fr.-mmultiple,-mno-multiple— generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions 👑. These get generated by default on POWER systems, and not generated on PowerPC systems. Do not use-mmultipleon little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode. The exceptions arePPC740andPPC750, which do permit these instructions in little-endian mode. iconic little guys 🫶-mupdate,-mno-update— generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location 🌸. These instructions are generated by default. But if you use-mno-update, there's a small window between the time the stack pointer is updated and the address of the previous frame is stored — which means code that walks the stack frame across interrupts or signals may get corrupted data 😭. lowkey scary ngl.-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generate code that tries to avoid (not avoid) the use of indexed load or store instructions 💅. These instructions can incur a performance penalty on Power6 processors in certain situations, like when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise.-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions ✨. These are generated by default if hardware floating point is used. Plot twist: the machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. the way that things get remapped… 🦋-mmulhw,-mno-mulhw— generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors 💖. These instructions are generated by default when targeting those processors.-mdlmzb,-mno-dlmzb— generate code that uses (does not use) the string-searchdlmzbinstruction on the IBM 405, 440, 464 and 476 processors 🌈. This instruction is generated by default when targeting those processors.-mno-bit-align,-mbit-align— on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field 🎀.
Example time, bestie: by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. shrink era 💅
-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems, do not (do) assume that unaligned memory references are handled by the system 🦋.-mrelocatable,-mno-relocatable— generate code that allows (does not allow) a static executable to be relocated to a different address at run time ✨. A simple embedded PowerPC system loader should relocate the entire contents of.got2and 4-byte locations listed in the.fixupsection, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with-mrelocatableor-mrelocatable-lib. And-mrelocatablecode aligns the stack to an 8-byte boundary. the way that alignment matters… 👑-mrelocatable-lib,-mno-relocatable-lib— like-mrelocatable,-mrelocatable-libgenerates a.fixupsection to allow static executables to be relocated at run time 💖. But-mrelocatable-libdoes not use the smaller stack alignment of-mrelocatable. Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. so flexible, so slay 🌸-mno-toc,-mtoc— on System V.4 and embedded PowerPC systems, do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program 🌈.-mlittle,-mlittle-endian— on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode 🦋. The-mlittle-endianoption is the same as-mlittle.-mbig,-mbig-endian— on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode 👑. The-mbig-endianoption is the same as-mbig.-mdynamic-no-pic— on Darwin / macOS systems, compile code so that it is not relocatable, but that its external references are relocatable 💅. The resulting code is suitable for applications, but not shared libraries. read that again bestie 😭-msingle-pic-base— treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function ✨. The runtime system is responsible for initializing this register with an appropriate value before execution begins. so like… you're on your own for that part 🫶- *
-mprioritize-restricted-insns=priority* — this one controls the priority that gets assigned to dispatch-slot restricted instructions during the second scheduling pass 🎀. The argument priority takes the value0,1, or2to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. - *
-msched-costly-dep=dependence_type* — controls which dependences are considered costly by the target during instruction scheduling 🦋. The argument dependence_type takes one of these values:no— no dependence is costly.all— all dependences are costly.true_store_to_load— a true dependence from store to load is costly.store_to_load— any dependence from store to load is costly.- number — any dependence for which the latency is greater than or equal to number is costly.
- *
-minsert-sched-nops=scheme* — controls which NOP insertion scheme is used during the second scheduling pass ✨. The argument scheme takes one of these values:no— don't insert NOPs.pad— pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.regroup_exact— insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.- number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
-mcall-sysv— on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement 💅. This is the default unless you configured GCC usingpowerpc-*-eabiaix.-mcall-sysv-eabi,-mcall-eabi— specify both-mcall-sysvand-meabioptions 🫶.-mcall-sysv-noeabi— specify both-mcall-sysvand-mno-eabioptions 🌸.-mcall-aixdesc— on System V.4 and embedded PowerPC systems, compile code for the AIX operating system 👑-mcall-linux— on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system 🐧✨-mcall-freebsd— on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system 💖-mcall-netbsd— on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system 🌈-mcall-openbsd— on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system 🦋- *
-mtraceback=traceback_type* — select the type of traceback table 🎀. Valid values for traceback_type arefull,part, andno. -maix-struct-return— return all structures in memory (as specified by the AIX ABI) 💅.-msvr4-struct-return— return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI) ✨.- *
-mabi=abi-type* — extend the current ABI with a particular extension, or remove such extension 🦋. Valid values are:altivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX:vec-extabi,vec-default. -mabi=ibmlongdouble— switches the current ABI over to IBM extended-precisionlong double. Bestie, real talk: this is not likely to work if your system already defaults to IEEE extended-precisionlong double🙈. And if you do flip thelong doubletype away from IEEE extended-precision, the compiler is gonna yell at you with a warning unless you slap on-Wno-psabi. Also, it requires-mlong-double-128to be enabled, no exceptions 💅.-mabi=ieeelongdouble— same deal but the other way: flips the current ABI to IEEE extended-precisionlong double. Also not likely to work if your system defaults to IBM extended-precisionlong double😭. Change thelong doubletype away from IBM extended-precision and boom, compiler warning — unless you use-Wno-psabi. Requires-mlong-double-128to be enabled too ✨.-mabi=elfv1— switches the current ABI to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux, so iconic of it 👑. Overriding the default ABI needs special system support and is, quote unquote, likely to fail in spectacular ways 💀.-mabi=elfv2— switches the current ABI to the ELFv2 ABI. This one is the default for little-endian PowerPC 64-bit Linux 🌸. And same warning as its sibling: overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀.-mgnu-attribute,-mno-gnu-attribute— emit.gnu_attributeassembly directives to set tag/value pairs in a.gnu.attributessection that specify ABI variations in function parameters or return values 🎀.-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With-mprototype, only calls to prototyped variable argument functions set or clear that bit 🦋.-msim— on embedded PowerPC systems, assume the startup module is calledsim-crt0.oand that the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations ✨.-mmvme— on embedded PowerPC systems, assume the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a💖.-mads— on embedded PowerPC systems, assume the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a🌈.-myellowknife— on embedded PowerPC systems, assume the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a. (Yes bestie, it's literally called yellowknife, we don't make the rules 🍓.)-mvxworks— on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system 🧚.-memb— on embedded PowerPC systems, set the"PPC_EMB"bit in the ELF flags header to indicate that eabi extended relocations are used 💅.-meabi,-mno-eabi— on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Picking-meabimeans the stack is aligned to an 8-byte boundary, a function"__eabi"is called from"main"to set up the EABI environment, and the-msdataoption can use both"r2"and"r13"to point to two separate small data areas. Picking-mno-eabimeans the stack is aligned to a 16-byte boundary, no EABI initialization function is called from"main", and the-msdataoption only uses"r13"to 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, put small initialized"const"global and static data in the.sdata2section, which is pointed to by register"r2". Put small initialized non-"const"global and static data in the.sdatasection, pointed to by register"r13". Put small uninitialized global and static data in the.sbsssection, which sits adjacent to.sdata. Heads up:-msdata=eabiis incompatible with the-mrelocatableoption 😭. It also sets the-memboption.-msdata=sysv— on System V.4 and embedded PowerPC systems, put small global and static data in the.sdatasection, pointed to by register"r13". Put small uninitialized global and static data in the.sbsssection, adjacent to.sdata. Also incompatible with-mrelocatable🙈.-msdata=default,-msdata— on System V.4 and embedded PowerPC systems: if-meabiis used, compile code exactly like-msdata=eabi; otherwise compile it like-msdata=sysv🦋.-msdata=data— on System V.4 and embedded PowerPC systems, put small global data in the.sdatasection. Put small uninitialized global data in the.sbsssection. Do not use register"r13"to address small data though. This is the default behavior unless other-msdataoptions are used ✨.-msdata=none,-mno-sdata— on embedded PowerPC systems, put all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection 💜.-mreadonly-in-sdata— put read-only objects in the.sdatasection as well. This is the default 🎀.-mblock-move-inline-limit=num— inline all block moves (like calls to"memcpy"or 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— generate non-looping inline code for all block compares (like calls to"memcmp"or structure compares) that are less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific 💅.-mblock-compare-inline-loop-limit=num— generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared before"memcmp"is called in to compare the rest of the block. The default value is target-specific 🔥.-mstring-compare-inline-limit=num— compare at most num string bytes with inline code. If the difference or the end of the string isn't found by the end of the inline compare, a call to"strcmp"or"strncmp"handles the rest of the comparison. The default is 64 bytes ⭐.-G num— on embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The-G numswitch also gets passed to the linker, so all modules should be compiled with the same-G numvalue — no cap, keep it consistent 🫶.-mregnames,-mno-regnames— on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms ✨.-mlongcall,-mno-longcall— by default, assume that all calls are far away, so a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call can't possibly be that far away. This setting can be overridden by the"shortcall"function attribute, or by#pragma longcall(0)💅.
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On those systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's 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 gets 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 it or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue 🔮.
-mpltseq,-mno-pltseq— implement (or do not implement)-fno-pltand long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with--enable-secureplt. Big warning bestie:-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together 💀.-mtls-markers,-mno-tls-markers— mark (or do not mark) calls to"__tls_get_addr"with a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better 🔥✨.
-mrecip,-mno-recip— okay so bestie, this one is giving "math but make it ✨vibes✨". It turns on the reciprocal estimate + reciprocal square root estimate instructions, paired with extra Newton-Raphson steps so the precision actually lands, instead of doing a whole divide (or a square root and a divide) for floating-point arguments. You should use-ffast-mathwhen you use-mrecip— or at least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math. Heads up though: the throughput of this sequence is generally higher than the non-reciprocal instruction, but precision can drop by up to 2 ulp — like, the inverse of1.0equals0.99999994— for reciprocal square roots. Not me getting approximate 💀-mrecip=opt— this is the one that decides which reciprocal estimate instructions are allowed out to play.optis a comma-separated list of options, and any of them can be preceded by a!to invert it:all— enable every estimate instruction. all of them. the whole squad. 👑default— enable the default instructions, which is 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 are the ones handling the double-precision reciprocal square root calculations. It's giving "everyone's invited except these three" 💅
-mrecip-precision,-mno-recip-precision— assume (or do not assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI mandates. Picking-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't provide an estimate that converges after three steps. no convergence, no invite 🫶-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics with an external library. Right now the only type supported ismass, which says 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. And the MASS libraries must be specified at link time — don't forget her, she'll be mad 😭-mfriz,-mno-friz— generate (or don't generate) thefrizinstruction when the-funsafe-math-optimizationsoption is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Important caveat bestie: thefrizinstruction does not return the same value if the floating-point number is too large to fit in an integer. she simply will not behave-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— generate (or don't generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, 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. And listen: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use-mno-pointers-to-nested-functions. blocked. denied. no entry.-msave-toc-indirect,-mno-save-toc-indirect— generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue, if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it's saved just before the call through the pointer. The-mno-save-toc-indirectoption is the default.-mcompat-align-parm,-mno-compat-align-parm— generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. story time: 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's compatible with functions compiled with older versions of GCC. The-mno-compat-align-parmoption is the default.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol— generate stack protection code using a canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (that's the default with GNU libc version 2.4 or later). With thetlschoice,-mstack-protector-guard-reg=regand-mstack-protector-guard-offset=offsetadditionally 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. And-mstack-protector-guard-symbol=symboloverrides the offset with a symbol reference to a canary in the TLS block. canary, but make it security 🐤-mpcrel,-mno-pcrel— generate (or don't generate) pc-relative addressing. The-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. prerequisites, bestie, she has needs-mprefixed,-mno-prefixed— generate (or don'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— generate (or don't generate) the MMA instructions. The-mmaoption requires that the option-mcpu=power10(or later) is enabled.-mrop-protect,-mno-rop-protect— generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). tradeoffs, always tradeoffs 😭-mprivileged,-mno-privileged— generate (or don't generate) code that will run in privileged state. main character energy, but make it kernel mode 👑-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— generate (or don't generate) unaligned vsx loads and stores for inline expansion ofmemcpyandmemmove.--param rs6000-vect-unroll-limit=— the vectorizer will check with target information to determine whether it'd be beneficial to unroll the main vectorized loop, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value isfour.
🌈 RX Options 🌈
These command-line options are defined for RX targets, bestie:
-m64bit-doubles,-m32bit-doubles— make thedoubledata type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is-m32bit-doubles. Note that RX floating-point hardware only works on 32-bit values, which is why the default is-m32bit-doubles. the hardware said 32 or nothing 💅-fpu,-nofpu— enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series. Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware isn't used for doubles if the-m64bit-doublesoption is used. Note: if the-fpuoption is enabled, then-funsafe-math-optimizationsis also enabled automatically — because the RX FPU instructions are themselves unsafe. yes literally, they said it, not me-mcpu=name— selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default isRX600. The only difference between RX600 and RX610 is that the RX610 does not support theMVTIPLinstruction. The RX200 series does not have a hardware floating-point unit, and so-nofpuis enabled by default when this type is selected.-mbig-endian-data,-mlittle-endian-data— store data (but not code) in the big-endian format. The default is-mlittle-endian-data, i.e. to store data in the little-endian format. data gets a whole different byte order than code, iconic honestly-msmall-data-limit=N— specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited, and it is up to the programmer to ensure that the area does not overflow. Also, when the small data area is used, one of the RX's registers (usuallyr13) is reserved for use pointing to this area, so it's no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register. Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable.
so yeah — the default value here is zero, and zero means this feature is off, periodt. and bestie, listen up: it does <em>not</em> get switched on by default even when you crank the optimization levels up high (-O2 and friends), because reserving a register can have genuinely detrimental effects. it's giving tradeoffs, fr fr. so it's literally on <em>you</em>, the programmer, to experiment and discover whether this feature is actually a benefit to your program or just a vibe. 🫶 and for the tea on how the actual register that holds the small data area pointer gets chosen? go read the description of the -mpid option, it's all spelled out there. 👑
-msim,-mno-sim— use the simulator runtime. the default is to use thelibglossboard-specific runtime instead. ✨-mas100-syntax,-mno-as100-syntax— when generating assembler output, use a syntax compatible with Renesas's AS100 assembler. the GAS assembler can also handle this syntax, but it's got some restrictions, so it is not 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. okay so the RX instruction set <em>does</em> allow constants up to 4 bytes long in instructions, but a longer value means a longer instruction — so sometimes it's lowkey beneficial to restrict how big the constants in your instructions get. constants that are too big get yeeted into a constant pool instead and referenced via register indirection. 🌸 the valueNcan be between0and4. a value of0(the default) or4means constants of any size are allowed.-mrelax— enable linker relaxation. linker relaxation is a whole process where the linker tries to shrink your program by finding shorter versions of various instructions. disabled by default. 🦋-mint-register=N— specify how many registers to reserve for fast interrupt handler functions. the valueNcan be between0and4. a value of1means register"r13"is reserved for the exclusive use of fast interrupt handlers. a value of2reserves"r13"and"r12". a value of3reserves"r13","r12"and"r11", and a value of4reserves"r13"through"r10". a value of0, which is the default, reserves no registers at all. 💜-msave-acc-in-interrupts— says that interrupt handler functions should preserve the accumulator register. this is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. the default is to ignore the accumulator, since that makes the interrupt handlers faster. 🔥-mpid,-mno-pid— enables generation of position independent data. when it's enabled, any access to constant data goes through an offset from a base address held in a register. this lets the location of constant data be figured out at run time without needing the executable to be relocated, which is a big benefit for embedded applications with tight memory constraints. data that can be modified is <em>not</em> affected by this option. 👑 note that using this feature reserves a register — usually"r13"— for the constant data base address. this can result in slower and/or larger code, especially in complicated functions. 😭 the actual register chosen to hold the constant data base address depends on whether the-msmall-data-limitand/or the-mint-registercommand-line options are enabled. starting with register"r13"and proceeding downwards, registers are allocated first to satisfy the requirements of-mint-register, then-mpidand finally-msmall-data-limit. so it's totally possible for the small data area register to end up as"r8"if both-mint-register=4and-mpidare specified on the command line. 🌈 by default this feature is not enabled. the default can be restored via the-mno-pidcommand-line option.-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— stops GCC from issuing a warning message 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 found, because the RX only supports one such interrupt. ⭐-mallow-string-insns,-mno-allow-string-insns— enables or disables the use of the string manipulation instructions"SMOVF","SCMPU","SMOVB","SMOVU","SUNTIL","SWHILE"and also the"RMPA"instruction. these instructions may prefetch data, which is not safe to do if you're accessing an I/O register. (see section 12.2.7 of the RX62N Group User's Manual for more info). 🙈 the default is to allow these instructions, but GCC can't reliably detect <em>every</em> circumstance where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. instead it's reliant upon the programmer to use the-mno-allow-string-insnsoption if their program accesses I/O space. when the instructions are enabled, GCC defines the C preprocessor symbol"__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol"__RX_DISALLOW_STRING_INSNS__". ✨-mjsr,-mno-jsr— use only (or not only)"JSR"instructions to access functions. this option can be used when code size exceeds the range of"BSR"instructions. note that-mno-jsrdoes <em>not</em> mean to not use"JSR"— it means that any type of branch may be used. 💅
note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when used with the"interrupt"function attribute. this attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers"r10","r11","r12"and/or"r13"— and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options. 🎀
🦋 S/390 and zSeries Options 🦋
these are the -m options defined for the S/390 and zSeries architecture. let's get into it, besties. 🌸
-mhard-float,-msoft-float— use (or do not use) the hardware floating-point instructions and registers for floating-point operations. when-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. when-mhard-floatis specified, the compiler generates IEEE floating-point instructions. this is the default. 🔥-mhard-dfp,-mno-hard-dfp— use (or do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. when-mno-hard-dfpis specified, functions inlibgcc.aare used to perform decimal-floating-point operations. 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 the"long double"type equivalent to the"double"type. this is the default. 💜-mbackchain,-mno-backchain— store (or do not store) the address of the caller's frame as a backchain pointer into the callee's stack frame. a backchain may be needed so you can debug 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 goes into the topmost word of the 96/160 byte register save area. 👑 in general, code compiled with-mbackchainis 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. note that the combination of-mbackchain,-mpacked-stackand-mhard-floatis not supported. in order to build a linux kernel, use-msoft-float. 😭 the default is to not maintain the backchain.-mpacked-stack,-mno-packed-stack— use (or do not use) the packed stack layout. when-mno-packed-stackis specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. when-mpacked-stackis specified, register save slots are densely packed at the top of the register save area; unused space gets reused for other purposes, allowing for more efficient use of the available stack space. <em>however</em>, when-mbackchainis also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 🫶 as long as the stack frame backchain isn't used, code generated with-mpacked-stackis call-compatible with code generated with-mno-packed-stack. note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. such code is not call-compatible with code compiled with-mpacked-stack. also, note that the combination of-mbackchain,-mpacked-stackand-mhard-floatis not supported. in order to build a linux kernel, use-msoft-float. 💀 the default is to not use the packed stack layout.-msmall-exec,-mno-small-exec— generate (or do not generate) code using the"bras"instruction to do subroutine calls. this only works reliably if the total executable size does not exceed 64k. the default is to use the"basr"instruction instead, which does not have this limitation. ✨-m64,-m31— when-m31is specified, generate code compliant to the GNU/Linux for S/390 ABI. when-m64is specified, generate code compliant to the GNU/Linux for zSeries ABI. this allows GCC in particular to generate 64-bit instructions. for the s390 targets, the default is-m31, while the s390x targets default to-m64. 🌈-mzarch,-mesa— when-mzarchis specified, generate code using the instructions available on z/Architecture. when-mesais specified, generate code using the instructions available on ESA/390. note that-mesais not possible with-m64. when generating code compliant to the GNU/Linux for S/390 ABI, the default is-mesa. when generating code compliant to the GNU/Linux for zSeries ABI, the default is-mzarch. 💅-mhtm,-mno-htm— the-mhtmoption enables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions.-mhtmis enabled by default when using-march=zEC12. ⭐-mvx,-mno-vx— bestie,-mvxsays "yes babe, use the instructions from the vector extension facility that IBM dropped with the z13 machine generation." And heads up — this one changes the ABI for some vector type values, like their alignment and calling conventions, no cap. So if vector type values show up in an ABI-relevant context, a GAS.gnu_attributecommand gets added to mark the resulting binary with the ABI it used. And-mvxis enabled by default when you're using-march=z13✨-mzvector,-mno-zvector—-mzvectorturns on vector language extensions and builtins using those same vector-extension-facility instructions from the IBM z13 generation 🦋 It adds support forvectoras a keyword to define vector type variables and arguments. But listen —vectoris only available when GNU extensions are enabled. It will not be expanded when you're requesting strict standard compliance, e.g. with-std=c99. And on top of the GCC low-level builtins,-mzvectoralso enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell 💜 To use those builtins, the header filevecintrin.hneeds to be included.-mzvectoris disabled by default.-mmvcle,-mno-mvcle— generate (or do not generate) code using the"mvcle"instruction to do block moves 💅 When-mno-mvcleis specified, it uses a"mvc"loop instead. That's the default unless you're optimizing for size.-mdebug,-mno-debug— print (or do not print) additional debug info while compiling. Default is to not print debug info, bestie.- *
-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type arez900/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.
Specifying native as cpu type can be used to select the best architecture option for the host processor. And lowkey important: -march=native has no effect if GCC does not recognize the processor 🙈
- *
-mtune=cpu-type* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for-march. The default is the value used for-march💫 -mtpf-trace,-mno-tpf-trace— generate code that adds (does not add) 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 (does not change) the default branch targets enabled by-mtpf-traceto point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default too, even when compiling for the TPF OS and specifying-mtpf-trace🌸-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used ✨- *
-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check, it doesn't need to be a real problem when the program runs. It's intended to identify functions that most probably cause a stack overflow 🔥 It's useful in an environment with limited stack size, e.g. the linux kernel. -mwarn-dynamicstack— emit a warning if the function calls"alloca"or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size, bestie 😭-mstack-guard=stack-guard , *-mstack-size=stack-size* — if these options are provided, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows *downward, okay). If the *stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems 🧚 The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. And the stack-guard option can only be used in conjunction with stack-size ⭐- *
-mhotpatch=pre-halfwords*,post-halfwords — if the hotpatch option is enabled, a"hot-patching"function prologue is generated for all functions in the compilation unit 👑 The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum1000000). After the label,2 *post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum1000000).
If both arguments are zero, hotpatching is disabled 💀
This option can be overridden for individual functions with the "hotpatch" attribute.
🌈 SH Options 🌈
These -m options are defined for the SH implementations:
-m1— generate code for the SH1.-m2— generate code for the SH2.-m2e— generate code for the SH2e.-m2a-nofpu— generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.-m2a-single-only— generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.-m2a-single— generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.-m2a— generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.-m3— generate code for the SH3.-m3e— generate code for the SH3e.-m4-nofpu— generate code for the SH4 without a floating-point unit.-m4-single-only— generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.-m4-single— generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.-m4— generate code for the SH4.-m4-100— generate code for SH4-100.-m4-100-nofpu— generate code for SH4-100 in such a way that the floating-point unit is not used.-m4-100-single— generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.-m4-100-single-only— generate code for SH4-100 in such a way that no double-precision floating-point operations are used.-m4-200— generate code for SH4-200.-m4-200-nofpu— generate code for SH4-200 without in such a way that the floating-point unit is not used.-m4-200-single— generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.-m4-200-single-only— generate code for SH4-200 in such a way that no double-precision floating-point operations are used.-m4-300— generate code for SH4-300.-m4-300-nofpu— generate code for SH4-300 without in such a way that the floating-point unit is not used.-m4-300-single— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-300-single-only— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-340— generate code for SH4-340 (no MMU, no FPU).-m4-500— generate code for SH4-500 (no FPU). Passes-isa=sh4-nofputo the assembler.-m4a-nofpu— generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.-m4a-single-only— generate code for the SH4a, in such a way that no double-precision floating-point operations are used.-m4a-single— generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.-m4a— generate code for the SH4a.-m4al— same as-m4a-nofpu, except that it implicitly passes-dspto the assembler. GCC doesn't generate any DSP instructions at the moment, bestie.-mb— compile code for the processor in big-endian mode.-ml— compile code for the processor in little-endian mode.-mdalign— align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with-mdalign😭-mrelax— shorten some address references at link time, when possible; uses the linker option-relax.-mbigtable— use 32-bit offsets in"switch"tables. The default is to use 16-bit offsets.-mbitops— enable the use of bit manipulation instructions on SH2A.-mfmovd— enable the use of the instruction"fmovd". Check-mdalignfor alignment constraints.-mrenesas— comply with the calling conventions defined by Renesas.-mno-renesas— comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain 💖-mnomacsave— mark the"MAC"register as call-clobbered, even if-mrenesasis given.-mieee,-mno-ieee— control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default-mieeeis implicitly enabled 🌸 If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either-mieeeor-mno-ieee⭐-minline-ic_invalidate— bestie, this one inlines the code that invalidates instruction cache entries after nested function trampolines get set up. ✨ But heads up: it does absolutely nothing if-musermodeis in effect and the selected code generation option (like-m4, say) doesn't allow theicbiinstruction. If the code generation option doesn't allowicbiand-musermodeisn't in effect, the inlined code just goes and manipulates the instruction cache address array directly with an associative write. 💅 And the way that this not only needs privileged mode at run time, but also fails if the cache line had been mapped via the TLB and has become unmapped? Yeah. It's giving risky.-misize— dumps instruction size and location straight into the assembly code. Obsessed. 🦋-mpadstruct— okay so this one is deprecated, no cap. It pads structures to multiples of 4 bytes, which is straight up incompatible with the SH ABI. ⚠️-matomic-model=model— sets the model of atomic operations plus any extra parameters, as a comma separated list. For the deets on the atomic built-in functions, go look at__atomic Builtins. Here's what's supported, bestie: 🌈none— disables compiler generated atomic sequences and just emits library calls for atomic operations. This is the default if the target is notsh*-*-linux*.soft-gusa— generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. These generated atomic 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 one is enabled by default when the target issh*-*-linux*and SH3* or SH4*. When the target is SH4A, it also partially utilizes the hardware atomic instructionsmovli.landmovco.lto create more efficient code — unlessstrictis specified.soft-tcb— generates software atomic sequences that use a variable in the thread control block. It's a variation of the gUSA sequences that can also be used on SH1* and SH2* targets. Same deal though: needs extra support from the interrupt/exception handling code, and is only suitable for single-core systems. When you're using this model, thegbr-offset=parameter has to be specified too. 💜soft-imask— generates software atomic sequences that temporarily disable interrupts by settingSR.IMASK = 1111. This model only works when the program runs in privileged mode and is only suitable for single-core systems. No extra support from the interrupt/exception handling code needed here. It's enabled by default when the target issh*-*-linux*and SH1* or SH2*.hard-llcs— generates hardware atomic sequences using themovli.landmovco.linstructions only. Only available on SH4A, and it's suitable for multi-core systems. Since the hardware instructions only support 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if it's executed on an SH4A system. For this model, no extra support from the interrupt/exception handling code is required. 👑gbr-offset=— this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when thesoft-tcbmodel has been selected. For other models, this parameter is ignored, periodt. The specified value must be an integer multiple of four and in the range0-1020. 🔢strict— prevents mixed usage of multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model only. No mixing allowed, bestie. 🚫
-mtas— generates thetas.bopcode for__atomic_test_and_set. But notice: depending on the particular hardware and software configuration, this can degrade overall performance because of the operand cache line flushes that are implied by thetas.binstruction. 😭 And on multi-core SH4A processors, thetas.binstruction must be used with caution since it can result in data corruption for certain cache configurations. Like, be careful.-mprefergot— when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. Slay. 🎀-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 issh*-*-linux*. If the target is SH1* or SH2*,-musermodehas no effect, since there's no user mode at all. 🙈-multcost=number— sets the cost to assume for a multiply insn. ✖️-mdiv=strategy— sets the division strategy to be used for integer division operations. strategy can be one of:call-div1— calls a library function that uses the single-step division instructiondiv1to 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 tocall-div1.call-table— calls a library function that uses a lookup table for small divisors and thediv1instruction 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 tocall-div1.
When a division strategy hasn't been specified, the default strategy is selected based on the current target. For SH2A, the default strategy is to use the divs and divu instructions instead of library function calls. 💅
-maccumulate-outgoing-args— reserves space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code. ✨-mdivsi3_libfunc=name— sets the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option weren't present. 🔤-mfixed-range=register-range— generates code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified, separated by a comma. 🛠️-mbranch-cost=num— assumes num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified, the value is selected depending on the processor type that's being compiled for. 🌿-mzdcbranch,-mno-zdcbranch— assume (do not assume) that zero displacement conditional branch instructionsbtandbfare fast. If-mzdcbranchis specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying-mno-zdcbranch. 🦋-mcbranch-force-delay-slot— forces the usage of delay slots for conditional branches, which stuffs the delay slot with anopif a suitable instruction can't be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055. 🐛-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. 💫-mfsca,-mno-fsca— allow or disallow the compiler to emit thefscainstruction for sine and cosine approximations. The option-mfscamust be used in combination with-funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using-mno-fscadisables sine and cosine approximations even if-funsafe-math-optimizationsis in effect. 🌊-mfsrra,-mno-fsrra— allow or disallow the compiler to emit thefsrrainstruction for reciprocal square root approximations. The option-mfsrramust be used in combination with-funsafe-math-optimizationsand-ffinite-math-only. It's enabled by default when generating code for SH4A. Using-mno-fsrradisables reciprocal square root approximations even if-funsafe-math-optimizationsand-ffinite-math-onlyare in effect. 🧮-mpretend-cmove— prefers zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor. 🔥-mfdpic— generates code using the FDPIC ABI. ⭐
☀️ Solaris 2 Options ☀️
These -m options are supported on Solaris 2, bestie: 🌸
-mclear-hwcap—-mclear-hwcaptells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them. 🧹-mimpure-text—-mimpure-text, used in addition to-shared, tells the compiler to not pass-z textto the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object. 💜-mimpure-text— this one suppresses that linker error message that goes "relocations remain against allocatable but non-writable sections". 💀 but bestie, listen: the relocations it needs trigger copy-on-write, so your shared object is not actually shared across processes anymore. Like it said "shared" on the label but it's lying. 🙈 So instead of reaching for-mimpure-text, just compile all your source code with-fpicor-fPIC. do it properly, periodt.
These switches are supported in addition to the above on Solaris 2:
-pthreads— this is literally just a synonym for-pthread. same vibe, different name. 💅
🌞 SPARC Options 🌞
These -m options are supported on the SPARC:
-mno-app-regs,-mapp-regs— say-mapp-regsand you get output using the 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 then gets treated as an allocable register that gets clobbered by function calls. This is the default. ✨
But if you wanna be fully SVR4 ABI-compliant and you're willing to eat some performance loss, say -mno-app-regs. You should compile libraries and system software with this option.
-mflat,-mno-flat— with-mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model instead. 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— generate output containing floating-point instructions. This is the default. ⭐-mno-fpu,-msoft-float— generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support. 💖
Heads up: -msoft-float changes the calling convention in the output file; therefore 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 in order for this to work. no half-measures bestie. 😭
-mhard-quad-float— generate output containing quad-word (long double) floating-point instructions. 🔥-msoft-quad-float— generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default. 🌸
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default. 💀
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. This is the default. ✨
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code. 📉
-muser-mode,-mno-user-mode— do not generate code that can only run in supervisor mode. This is relevant only for the "casa" instruction emitted for the LEON3 processor. This is the default. 👑-mfaster-structs,-mno-faster-structs— with-mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. 😳 it's giving "I know what I'm doing" energy, but fr fr only if you actually do.-mstd-struct-return,-mno-std-struct-return— with-mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI. 🦋
The default is -mno-std-struct-return. This option has no effect in 64-bit mode.
-mlra,-mno-lra— enable Local Register Allocation. This is the default for SPARC since GCC 7, so-mno-lraneeds to be passed to get the old Reload. 🙈- *
-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type 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 does not recognize the processor. 🧚
Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.
Here's a list of each supported architecture and their supported implementations, bestie: 👇
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 which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series. 💅
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🎀
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip. ⭐
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips. 🔥
- *
-mtune=cpu_type* — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option-mcpu=cpu_type does. 🌸
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used. 🦋
-mv8plus,-mno-v8plus— with-mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors. 👑-mvis,-mno-vis— with-mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is-mno-vis. 💖-mvis2,-mno-vis2— with-mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is-mvis2when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting-mvis2also sets-mvis. ✨-mvis3,-mno-vis3— with-mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is-mvis3when targeting a cpu that supports such instructions, such asniagara-3and later. Setting-mvis3also sets-mvis2and-mvis. 🎀-mvis4,-mno-vis4— okay so with-mvis4GCC writes code that eats up version 4.0 of the UltraSPARC Visual Instruction Set extensions 🔥 The default is-mvis4when you're targeting a cpu that supports those instructions, likeniagara-7and later. And bestie, turning on-mvis4also turns on-mvis3,-mvis2and-mvis. It's giving combo meal 💅-mvis4b,-mno-vis4b— with-mvis4bGCC goes off with version 4.0 of the UltraSPARC Visual Instruction Set extensions plus the extra VIS instructions that showed up in the Oracle SPARC Architecture 2017 ✨ Default is-mvis4bwhen the cpu supports that stuff, likem8and later. Setting-mvis4balso sets-mvis4,-mvis3,-mvis2and-mvis. Again, combo meal, no cap.-mcbcond,-mno-cbcond— with-mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions 🎀 Default is-mcbcondwhen targeting 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 🌸 Default is-mfmafwhen the CPU supports them, like Niagara-3 and later.-mfsmuld,-mno-fsmuld— with-mfsmuld, GCC takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction 💖 Default is-mfsmuldwhen targeting a CPU that supports the architecture versions V8 or V9 with FPU — except-mcpu=leon. Noted, bestie.-mpopc,-mno-popc— with-mpopc, GCC uses the UltraSPARC Population Count instruction 🌈 Default is-mpopcwhen targeting a CPU that supports such an instruction, like Niagara-2 and later.-msubxc,-mno-subxc— with-msubxc, GCC takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction ⭐ Default is-msubxcwhen targeting 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 corresponds to erratum #13 of the AT697E processor). Bugs? Handled 💅-mfix-ut699— turns on the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor. Iconic problem-solver behaviour.-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 supported in addition to the ones above on SPARC-V9 processors in 64-bit environments, bestie:
-m32,-m64— generate 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.-mcmodel=which — set the code model to one of these, okay?medlow— the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.medmid— the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.medany— the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.embmedany— the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported. 🫶
-mmemory-model=mem-model — set the memory model in force on the processor to one of:default— the default memory model for the processor and operating system.rmo— Relaxed Memory Orderpso— Partial Store Ordertso— Total Store Ordersc— Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field 📚
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by-2047which must be added back when making stack frame references 👑 This is the default in 64-bit mode. Otherwise, assume no such offset is present.
🌈 Options for System V 🌈
These additional options are available on System V Release 4 for compatibility with other compilers on those systems, bestie:
-G— Create a shared object. It is recommended that-symbolicor-sharedbe used instead. (Yeah, lowkey deprecated, use the other ones 💀)-Qy— Identify the versions of each tool used by the compiler, in a.identassembler directive in the output.-Qn— Refrain from adding.identdirectives to the output file (this is the default).-YP,dirs — Search the directories dirs, and no others, for libraries specified with-l.-Ym,dir — Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
🦋 V850 Options 🦋
These -m options are defined for V850 implementations, periodt:
-mlong-calls,-mno-long-calls— Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer ✨-mno-ep,-mep— Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into theepregister, and use the shortersldandsstinstructions. The-mepoption is on by default if you optimize 💅-mno-prolog-function,-mprolog-function— Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The-mprolog-functionoption is on by default if you optimize.-mspace— Try to make the code as small as possible 🎀 At present, this just turns on the-mepand-mprolog-functionoptions.-mtda=n — Put static or global variables whose size is n bytes or less into the tiny data area that registereppoints to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).-msda=n — Put static or global variables whose size is n bytes or less into the small data area that registergppoints to. The small data area can hold up to 64 kilobytes.-mzda=n — Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.-mv850— Specify that the target processor is the V850.-mv850e3v5— Specify that the target processor is the V850E3V5. The preprocessor constant__v850e3v5__is defined if this option is used.-mv850e2v4— Specify that the target processor is the V850E3V5. This is an alias for the-mv850e3v5option. (Yes bestie, it says V850E3V5. That's what the page says. Not a typo on my end 💖)-mv850e2v3— Specify that the target processor is the V850E2V3. The preprocessor constant__v850e2v3__is defined if this option is used.-mv850e2— Specify that the target processor is the V850E2. The preprocessor constant__v850e2__is defined if this option is used.-mv850e1— Specify that the target processor is the V850E1. The preprocessor constants__v850e1__and__v850e__are defined if this option is used.-mv850es— Specify that the target processor is the V850ES. This is an alias for the-mv850e1option.-mv850e— Specify that the target processor is the V850E. The preprocessor constant__v850e__is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target 👑
-mdisable-callt,-mno-disable-callt— This option suppresses generation of theCALLTinstruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined 🌸
-mrelax,-mno-relax— Pass on (or do not pass on) the-mrelaxcommand-line option to the assembler.-mlong-jumps,-mno-long-jumps— Disable (or re-enable) the generation of PC-relative jump instructions.-msoft-float,-mhard-float— Disable (or re-enable) the generation of hardware floating point instructions 🔥 This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol__FPU_OK__is defined, otherwise the symbol__NO_FPU__is defined.-mloop— Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental ⚠️ (so like, proceed with caution bestie)-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__ is defined 💜
-mgcc-abi— turns on support for the old GCC version of the V850 ABI, bestie. When this flavor of the ABI is active, these rules apply:- Integer sized structures and unions get 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 you're 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 get returned in register
When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ gets defined. periodt.
-m8byte-align,-mno-8byte-align— enables support fordoubleandlong longtypes to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When-m8byte-alignis in effect, the C preprocessor symbol__V850_8BYTE_ALIGN__is defined. ✨-mbig-switch— generates code suitable for big switch tables. Only use this option if the assembler/linker are complaining about out of range branches within a switch table. like, only when they start yelling at you. 😭-mapp-regs— this option causesr2andr5to be used in the code generated by the compiler. This setting is the default.-mno-app-regs— this option causesr2andr5to be treated as fixed registers.
🐢 VAX Options
These -m options are defined for the VAX:
-munix— do not output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX cannot handle across long ranges.-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used.-mg— output code for G-format floating-point numbers instead of D-format.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. 👀
🔭 Visium Options
-mdebug— a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the 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 which performs file I/O and is destined to run on the simulator should be linked with option. This causes librarieslibc.aandlibsim.ato be linked.-mfpu,-mhard-float— generate code containing floating-point instructions. This is the default. 🌸-mno-fpu,-msoft-float— generate code containing library calls for floating-point.
Heads up bestie,-msoft-floatchanges the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work. no cap. 💀
- *
-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type aremcm,gr5andgr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.
- *
-mtune=cpu_type* — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option-mcpu=cpu_type would. -msv-mode— generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.-muser-mode— generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registersr24tor31cannot 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— return VMS condition codes frommain. The default is to return POSIX-style condition (e.g. error) codes.- *
-mdebug-main=prefix* — flag the first routine whose name starts with prefix as the main routine for the debugger. -mmalloc64— default to 64-bit memory allocation routines.- *
-mpointer-size=size* — set the default size of pointers. Possible options for size are32orshortfor 32 bit pointers,64orlongfor 64 bit pointers, andnofor supporting only 32 bit pointers. The later option 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__.-msmp— select SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice. 🙈-non-static— link an RTP executable against shared libraries rather than static libraries. The options-staticand-sharedcan also be used for RTPs;-staticis the default.-Bstatic,-Bdynamic— these options are passed down to the linker. They are defined for compatibility with Diab.-Xbind-lazy— enable lazy binding of function calls. This option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab.-Xbind-now— disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
👑 x86 Options
These -m options are defined for the x86 family of computers. it's giving legendary lineage fr fr.
- *
-march=cpu-type* — generate instructions for the machine type cpu-type. In contrast to-mtune=cpu-type, which merely tunes the generated code for the specified cpu-type,-march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying-march=cpu-type implies-mtune=cpu-type, except where noted otherwise. ⚠️
The choices for cpu-type are:
native— this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using-march=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set.x86-64— a generic CPU with 64-bit extensions.x86-64-v2,x86-64-v3,x86-64-v4— these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.
Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.
i386— original Intel i386 CPU.i486— Intel i486 CPU. (No scheduling is implemented for this chip.)i586,pentium— Intel Pentium CPU with no MMX support.lakemont— Intel Lakemont MCU, based on Intel Pentium CPU.pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.pentiumpro— Intel Pentium Pro CPU.i686— when used with-march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with-mtune, it has the same meaning asgeneric.pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks.pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.prescott— improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.nocona— improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support.core2— Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.nehalem— Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.westmere— Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.sandybridge— Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.ivybridge— Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, 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, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.
broadwell— Intel Broadwell 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,ADCXandPREFETCHWinstruction set support. the mid-2010s workhorse, still serving looks ✨skylake— Intel Skylake 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,XSAVESandSGXinstruction set support. broadwell but make it <em>extra</em> — hiAES, hiSGX🫶bonnell— Intel Bonnell CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. tiny list, tiny chip, still 64-bit bestie 💖silvermont— Intel Silvermont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHWandRDRNDinstruction set support. the atom line said "we contain multitudes" and it was right 🦋goldmont— Intel Goldmont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPTandFSGSBASEinstruction set support. SHA and AES rolled in like they owned the place 💅goldmont-plus— Intel Goldmont Plus CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPIDandSGXinstruction set support. goldmont but with the "plus" upgrade, obsessed ✨tremont— Intel Tremont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPID,SGX,CLWB,GFNI-SSE,MOVDIRI,MOVDIR64B,CLDEMOTEandWAITPKGinstruction set support. she's giving "small core, big dreams" 🔥sierraforest— Intel Sierra Forest 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support. the e-core era arrived and it brought a WHOLE list 🌈grandridge— Intel Grand Ridge 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support. same roster assierraforest, different name tag, still iconic 💜clearwaterforest— Intel Clearwater Forest 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,ENQCMD,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3,SM4,USER_MSRandPREFETCHIinstruction set support. look at all those late additions — she collected them all no cap 🎀knl— Intel Knight's Landing 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,AVX512PF,AVX512ER,AVX512F,AVX512CDandPREFETCHWT1instruction set support. many-core era core, main character energy 👑knm— Intel Knights Mill 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,AVX512PF,AVX512ER,AVX512F,AVX512CDandPREFETCHWT1,AVX5124VNNIW,AVX5124FMAPSandAVX512VPOPCNTDQinstruction set support. knights mill walked so the server chips could run 🦋skylake-avx512— Intel Skylake Server 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,CLWB,AVX512VL,AVX512BW,AVX512DQandAVX512CDinstruction set support. server skylake, the one that introduced the wide-vector serve 💅cannonlake— Intel Cannonlake Server 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,AVX512IFMAandSHAinstruction set support. cannonlake walked so icelake could <em>run</em> ✨icelake-client— Intel Icelake Client 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,RDPIDandAVX512VPOPCNTDQinstruction set support. the laptop line, and yes the list really is that long, and yes she earned it 💜icelake-server— Intel Icelake Server 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,WBNOINVDandCLWBinstruction set support. client's older sibling who went to server school and came back withPCONFIG🫶cascadelake— Intel Cascadelake 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,CLWB,AVX512VL,AVX512BW,AVX512DQ,AVX512CDandAVX512VNNIinstruction set support. skylake-avx512 plusAVX512VNNIand honestly? glow up 💅cooperlake— Intel cooperlake 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,CLWB,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VNNIandAVX512BF16instruction set support. bfloat16 said "neural nets, bestie" 🧚tigerlake— Intel Tigerlake 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,AVX512CDPKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,MOVDIRI,MOVDIR64B,CLWB,AVX512VP2INTERSECTandKEYLOCKERinstruction set support. tiger lake, the one withKEYLOCKERand a truly enormous feature list, periodt 🔥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,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16andAVX512BF16instruction set support. AMX showed up with tiles and the whole list said "slay" 🎀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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKLandAVX-VNNIinstruction set support. hybrid cores, hybrid era, hybrid queen 👑rocketlake— Intel Rocketlake 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,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CDPKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPIDandAVX512VPOPCNTDQinstruction set support. the desktop chip that borrowed the server vector list, we love a crossover episode 💜graniterapids— Intel graniterapids 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,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16,AVX512BF16,AMX-FP16andPREFETCHIinstruction set support. the finale of the server roster,AMX-FP16andPREFETCHIsaid "we're done here" and ate 🌈graniterapids-d— Intel Granite Rapids D CPU, bestie. 64-bit extensions plusMOVBE,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,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512FP16,AVX512BF16,AMX-FP16,PREFETCHIandAMX-COMPLEXinstruction set support. (yes it saysMOVBEtwice, that's just how it is, don't @ me 💅)arrowlake— Intel Arrow Lake CPU, 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERTandCMPCCXADDinstruction set support. ✨arrowlake-s— Intel Arrow Lake S CPU, and it's basically its sibling up there but with extras. 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3andSM4instruction set support. the way it just keeps going 😭pantherlake— Intel Panther Lake CPU, 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,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3,SM4andPREFETCHIinstruction set support. 🐆k6— AMD K6 CPU withMMXinstruction set support.k6-2k6-3— improved versions of the AMD K6 CPU, withMMXand3DNow!instruction set support. glow up 💖athlonathlon-tbird— AMD Athlon CPU withMMX,3dNOW!, enhanced3DNow!andSSEprefetch instructions support. (note the lowercasedin3dNOW!here, that's not a typo, that's canon 🫶)athlon-4athlon-xpathlon-mp— improved AMD Athlon CPU withMMX,3DNow!, enhanced3DNow!and fullSSEinstruction set support.k8opteronathlon64athlon-fx— processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (this supersetsMMX,SSE,SSE2,3DNow!, enhanced3DNow!and 64-bit instruction set extensions.)k8-sse3opteron-sse3athlon64-sse3— improved versions of the AMD K8 cores withSSE3instruction set support.amdfam10barcelona— CPUs based on AMD Family 10h cores with x86-64 instruction set support. (this supersetsMMX,SSE,SSE2,SSE3,SSE4A,3DNow!, enhanced3DNow!,ABMand 64-bit instruction set extensions.)bdver1— CPUs based on AMD Family 15h cores with x86-64 instruction set support. (this 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. (this 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. (this 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. (this 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. (this 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.) Zen gang rise up 🧚znver2— AMD Family 17h core based CPUs with x86-64 instruction set support. (this 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. (this 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. (this 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.)znver5— AMD Family 1ah core based CPUs with x86-64 instruction set support. (this 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,GFNI,AVXVNNI,MOVDIRI,MOVDIR64B,AVX512VP2INTERSECT,PREFETCHIand 64-bit instruction set extensions.) 👑btver1— CPUs based on AMD Family 14h cores with x86-64 instruction set support. (this supersetsMMX,SSE,SSE2,SSE3,SSSE3,SSE4A,CX16,ABMand 64-bit instruction set extensions.)btver2— CPUs based 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, dealt in same way asi486with additionalMMXinstruction set support.winchip2— IDT WinChip 2 CPU, dealt in same way asi486with additionalMMXand3DNow!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.)lujiazui— ZHAOXIN lujiazui CPU with x86-64,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,POPCNT,AES,PCLMUL,RDRND,XSAVE,XSAVEOPT,FSGSBASE,CX16,ABM,BMI,BMI2,F16C,FXSR,RDSEEDinstruction set support. 🦋
yongfeng— ZHAOXIN yongfeng CPU, okurrr 💜 it's got x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support. That's a whole resume bestie.geode— AMD Geode embedded processor, rockin' MMX and 3DNow! instruction set support. retro queen behavior honestly 🎀
the -mtune situation 💅
-mtune=cpu-type— tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. So like… it changes the vibe of the codegen, not what instructions it's even allowed to use. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you also use a-march=cpu-typeoption. For example, if GCC is configured fori686-pc-linux-gnuthen-mtune=pentium4generates code that is tuned for Pentium 4 but still runs on i686 machines. Tuned for the diva, runs on the grandma's laptop. iconic ✨
The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:
generic— produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding-mtuneor-marchoption instead of-mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. bestie this is the "idk who's gonna run this" option and it's serving 🫶
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. so like, it's a moving target, no cap.
There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 💀 yeah there's just no universal instruction set, that's the whole tea.
intel— produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding-mtuneor-marchoption instead of-mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. two chips, one option, main character energy 👑
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. again: moving target, fr fr.
There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. same energy as generic, different flavor 🍓
-mcpu=cpu-type— a deprecated synonym for-mtune. it's giving "we don't do that anymore bestie" 😭 still works, still listed, but like… move on.-mfpmath=unit— generate floating-point arithmetic for selected unit unit. The choices for unit are:387— use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See-ffloat-storefor more detailed description. so it runs everywhere but the math is lowkey doing its own thing 🙈
This is the default choice for non-Darwin x86-32 targets.
sse— use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. so on x86-32 you gotta ask, on x86-64 it just comes free with the meal 💅
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. ⚠️ faster but it might snap some old code in half, sorry bestie.
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.
sse,387sse+387both— attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. 🫣 experimental means experimental, do not @ me.
-masm=dialect— output assembly instructions using selected dialect. Also affects which dialect is used for basicasmand extendedasm. Supported choices (in dialect order) areattorintel. The default isatt. Darwin does not supportintel. Darwin said no to intel dialect, and honestly, respect the boundary.-mieee-fp-mno-ieee-fp— control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. so like, the "neither of these is bigger, it's just chaos" case gets handled properly 🦋-m80387-mhard-float— generate output containing 80387 instructions for floating point.-mno-80387-msoft-float— generate output containing library calls for floating point.
⚠️ Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. bestie you're on your own for the libs, GCC is not your mom here 😭
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used. yeah, sneaky ones slip through, it's a whole thing.
-mno-fp-ret-in-387— do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. delulu? no, by design 💀
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387— some 387 emulators do not support thesin,cosandsqrtinstructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when-marchindicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the-funsafe-math-optimizationsswitch. so it's the "please don't do the fancy math, my emulator can't hang" option, and it gets overruled the second-marchsays the FPU is guaranteed 💅-malign-double-mno-align-double— control whether GCC alignsdouble,long double, andlong longvariables on a two-word boundary or a one-word boundary. Aligningdoublevariables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. speed for memory, the classic trade 🎀
On x86-64, -malign-double is enabled by default.
⚠️ Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch. this is a compatibility BREAKER bestie, flag it.
-m96bit-long-double-m128bit-long-double— these switches control the size oflong doubletype. The x86-32 application binary interface specifies the size to be 96 bits, so-m96bit-long-doubleis the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. padding queen, adding zeros for the alignment ✨
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double. no extra precision, just extra padding, don't get it twisted 🙈
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. read it again, it's important.
-mlong-double-64-mlong-double-80-mlong-double-128— these switches control the size oflong doubletype. A size of 64 bits makes thelong doubletype equivalent to thedoubletype. This is the default for 32-bit Bionic C library. A size of 128 bits makes thelong doubletype equivalent to the__float128type. This is the default for 64-bit Bionic C library.
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. same warning, same stakes, still not joking 😤
-malign-data=type— control how GCC aligns variables. Supported values for type arecompatuses increased alignment value compatible uses GCC 4.8 and earlier,abiuses alignment value as specified by the psABI, andcachelineuses increased alignment value to match the cache line size.compatis the default. pick your alignment lore: nostalgia, spec-compliant, or cache-line-matching 👑-mlarge-data-threshold=threshold— when-mcmodel=mediumor-mcmodel=largeis specified, data objects larger than threshold are placed in large data sections. The default is65535. big objects go in the big section, that's the whole rule 💜-mrtd— bestie, this one flips the whole function-calling convention 💅 and it's giving chaos if you're not ready.
With it, functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. That saves one instruction in the caller, since there's no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 🚨
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
-mregparm=num— 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 behavior for a specific function by using the function attributeregparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. no cap, every single one 🫠
-msseregparm— uses SSE register passing conventions forfloatanddoublearguments and return values. You can control this behavior for a specific function by using the function attributesseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. ✨
-mvect8-ret-in-mem— returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 👑-mpc32,-mpc64,-mpc80— set 80387 floating-point precision to 32, 64 or 80 bits respectively, iconic trio behavior 🎀
When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 💀
-mdaz-ftz— the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. SSE and AVX instructions including scalar and vector instructions could benefit from enabling the FTZ and DAZ flags when-mdaz-ftzis specified. Don't set FTZ/DAZ flags when-mno-daz-ftzor-sharedis specified,-mdaz-ftzwill set FTZ/DAZ flags even with-shared. 🌈-mstackrealign— realigns the stack at entry. On the x86, the-mstackrealignoption generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attributeforce_align_arg_pointer, applicable to individual functions. 🧚-mpreferred-stack-boundary=num— attempts to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is4(16 bytes or 128 bits).
Warning: when generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87long doubleand__int128), leading to wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 🫶
-mincoming-stack-boundary=num— assumes the incoming stack is aligned to a 2 raised to num byte boundary. If-mincoming-stack-boundaryis not specified, the one specified by-mpreferred-stack-boundaryis used.
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. 🌸
okay so this next one is a mega block of flags, and the way that it's just one enormous vibe check of x86 extensions — obsessed, honestly 💖 each one has a -mno- twin that turns it off, so if you see the flag, know the anti-flag exists too:
-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
-mavx512fp16
-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
-mwidekl
-mavxifma
-mavxvnniint8
-mavxneconvert
-mcmpccxadd
-mamx-fp16
-mprefetchi
-mraoint
-mamx-complex
-mavxvnniint16
-msm3
-msha512
-msm4
-mapxf
-musermsr
-mavx10.1
-mavx10.1-256
-mavx10.1-512These switches enable the use of instructions in the MMX, SSE, 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, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1 or CLDEMOTE extended instruction sets. Each has a corresponding -mno- option to disable use of these instructions. 🔥
These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. 👑
-mdump-tune-features— this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in-mtune-ctrl=feature-list. 🎀-mtune-ctrl=feature-list— the fine-grain dial for x86 code generation features, bestie.feature-listis a comma separated list of feature names. Peep-mdump-tune-featurestoo. When you specify it, the feature is turned on unless it's preceded with^, in which case it's turned off. This one is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 💀-mno-default— tells GCC to turn off all tunable features. See also-mtune-ctrl=feature-listand-mdump-tune-features. ✨-mcld— instructs GCC to emit acldinstruction in the prologue of functions that use string instructions. So here's the tea: string instructions depend on theDFflag to select between autoincrement or autodecrement mode. The ABI says theDFflag is supposed to be cleared on function entry, but some operating systems violate this specification by not clearing theDFflag in their exception dispatchers. The exception handler can be invoked with theDFflag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the--enable-cldconfigure option. Generation ofcldinstructions can be suppressed with the-mno-cldcompiler option in this case. 🫶-mvzeroupper— instructs GCC to emit avzeroupperinstruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty as well as remove unnecessaryzeroupperintrinsics. iconic behavior honestly 💅-mprefer-avx128— instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.-mprefer-vector-width=opt— instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.-mpartial-vector-fp-math— enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless-fno-trapping-mathis specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when-fno-trapping-mathis in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using-mdaz-ftzto set denormals-are-zero (DAZ) flag in the MXCSR register. 😭
This option is enabled by default.
-mmove-max=bits— instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are128,256and512. 🌈-mstore-max=bits— instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are128,256and512.none— No extra limitations applied to GCC other than defined by the selected platform.128— Prefer 128-bit vector width for instructions.256— Prefer 256-bit vector width for instructions.512— Prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers— optimizes functions withnoreturnattribute or_Noreturnspecifier by not saving in the function prologue callee-saved registers which are used in the function (except for theBPregister). Heads up tho: this option can interfere with debugging of the caller of thenoreturnfunction or any function further up in the call stack, so it is not enabled by default. 🚨-mcx16— enables GCC to generateCMPXCHG16Binstructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement__syncBuiltins. However, for__atomicBuiltins operating on 128-bit integers, a library call is always used. no cap 📌-msahf— enables generation ofSAHFinstructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked theLAHFandSAHFinstructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, theSAHFinstruction is used to optimizefmod,drem, andremainderbuilt-in functions; see Other Builtins for details. 🧚-mmovbe— enables use of themovbeinstruction to optimize byte swapping of four and eight byte entities.-mshstk— the-mshstkoption enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). 💜-mcrc32— enables built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32siand__builtin_ia32_crc32dito generate thecrc32machine instruction.-mmwait— enables built-in functions__builtin_ia32_monitor, and__builtin_ia32_mwaitto generate themonitorandmwaitmachine instructions.-mrecip— enables use ofRCPSSandRSQRTSSinstructions (and their vectorized variantsRCPPSandRSQRTPS) with an additional Newton-Raphson step to increase precision instead ofDIVSSandSQRTSS(and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals0.99999994). 😳
Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip.
-mrecip=opt— controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— Enable all estimate instructions.default— Enable the default instructions, equivalent to-mrecip.none— Disable all estimate instructions, equivalent to-mno-recip.div— Enable the approximation for scalar division.vec-div— Enable the approximation for vectorized division.sqrt— Enable the approximation for scalar square root.vec-sqrt— Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. slay 🎀
-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type aresvmlfor the Intel short vector math library andacmlfor the AMD math core library. To use this option, both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. ⭐
GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used.
-mabi=name— generate code for the specified calling convention. Permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributesms_abiandsysv_abi. 💖-mforce-indirect-call— force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates more precise timing information for function calls.-mmanual-endbr— insert ENDBR instruction at function entry only via thecf_checkfunction attribute. This is useful when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry.-mcet-switch— by default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables which leads to the jump targets reachable via any indirect jumps. 👑
-mcall-ms2sysv-xlogues— okay so bestie, 64-bit ABIs are not all besties 💀. 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, resulting in fairly lengthy prologues and epilogues. Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion oflibgccto perform these saves and restores, thus reducing function size at the cost of a few extra instructions. Lowkey iconic tradeoff ✨.-mtls-dialect=type— generate code to access thread-local storage using thegnuorgnu2conventions.gnuis the conservative default;gnu2is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. So like, read the room before you commit 💅.-mpush-args,-mno-push-args— usePUSHoperations to store outgoing parameters. This method is shorter and usually equally fast as the method usingSUB/MOVoperations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. No cap.-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies-mno-push-args. So it's giving "big brain but big binary" energy 😭.-mthreads— support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling 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. Slay.-mms-bitfields,-mno-ms-bitfields— enable/disable bit-field layout compatible with the native Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.
This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determine by these rules:
- 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
"aligned"attribute or the"pack"pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0- 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","bar"is aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4. For "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example,
"t2"has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. - 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 following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;Here, "t5" takes up 2 bytes.
-mno-align-stringops— do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. It's giving "trust me, it's already cute" energy 🎀.-minline-all-stringops— by default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast"memcpy"and"memset"for short lengths. The option enables inline expansion of"strlen"for all pointer alignments. Obsessed with the flexibility fr fr ✨.-minline-stringops-dynamically— for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. Bestie said "why not both?" 💅.-mstringop-strategy=alg— override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:rep_byterep_4byterep_8byte— expand using i386"rep"prefix of the specified size.byte_looploopunrolled_loop— expand into an inline loop.libcall— always use a library call.
-mmemcpy-strategy=strategy— override the internal decision heuristic to decide if"__builtin_memcpy"should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list ofalg:max_size:dest_aligntriplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be-1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and"max_size + 1"of the preceding range. The way that this is so specific… it's giving spreadsheet but make it compiler 😭.-mmemset-strategy=strategy— the option is similar to-mmemcpy-strategy=except that it is to control"__builtin_memset"expansion. Periodt.-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option-fomit-leaf-frame-pointerremoves the frame pointer for leaf functions, which might make debugging harder. So like, cute for speed but debugging said "not today" 🙈.-mtls-direct-seg-refs,-mno-tls-direct-seg-refs— controls whether TLS variables may be accessed with offsets from the TLS segment register (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on. No cap.
-msse2avx,-mno-sse2avx— specify that the assembler should encode SSE instructions with VEX prefix. The option-mavxturns this on by default. Iconic assembly glow-up ✨.-mfentry,-mno-fentry— if profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute"ms_hook_prologue"isn't possible at the moment for-mfentryand-pg. So like, don't try it bestie 💀.-mrecord-mcount,-mno-record-mcount— if profiling is active (-pg), generate a__mcount_locsection that contains pointers to each profiling call. This is useful for automatically patching and out calls. Lowkey main character energy for tooling 🫶.-mnop-mcount,-mno-nop-mcount— if profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with-mrecord-mcount. The way that they're a duo… 💅.-minstrument-return=type— instrument function exit in-pg-mfentryinstrumented functions with call to specified function. This only instruments true returns ending withret, but not sibling calls ending withjump. Valid types arenoneto not instrument,callto generate a call to__return__, ornop5to generate a 5 byte nop. Slay.-mrecord-return,-mno-record-return— generates a__return_locsection that points to all your return instrumentation code. lowkey niche, highkey useful if you're into that 🎀-mfentry-name=name— sets thenameof the__fentry__symbol that gets called at function entry for-pg -mfentryfunctions. name it whatever, bestie 💅-mfentry-section=name— sets the name of the section used to record-mrecord-mcountcalls. default is__mcount_locif you don't say anything 🌸-mskip-rax-setup,-mno-skip-rax-setup— when you're generating code for x86-64 with SSE extensions turned off,-mskip-rax-setuplets you skip setting up the RAX register when there are no variable arguments passed in vector registers. skipping the setup, iconic ✨
⚠️ Warning, bestie, listen up: since the RAX register is what avoids unnecessarily saving vector registers on the stack when passing variable arguments, 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. so if you're on anything modern you're chilling, but fr fr be careful 😭
-m8bit-idiv,-mno-8bit-idiv— on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. this option generates a run-time check: if both dividend and divisor are within range of 0 to 255, it uses 8-bit unsigned integer divide instead of the 32-bit/64-bit one. adaptive queen 👑-mavx256-split-unaligned-load,-mavx256-split-unaligned-store— splits 32-byte AVX unaligned load and store. that's it, that's the whole vibe 🦋-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 (that one's the default). this option only does anything when-fstack-protectoror-fstack-protector-allis specified 💖
with the tls 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 read it from. the defaults for those are as specified in the relevant ABI.
-mgeneral-regs-only— generates code that uses only the general-purpose registers. this stops the compiler from using floating-point, vector, mask and bound registers. minimalist era 🧚-mrelax-cmpxchg-loop— when it's emitting a compare-and-swap loop for__syncBuiltins and__atomicBuiltins that don't have a native instruction, this optimizes for the highly contended case by issuing an atomic load before the"CMPXCHG"instruction, and using the"PAUSE"instruction to save CPU power when restarting the loop. efficiency, we love to see it 🔥-mindirect-branch=choice— converts indirect call and jump withchoice. the default iskeep, which keeps indirect call and jump unmodified.thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts indirect call and jump to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attribute"indirect_branch"💅
note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. not compatible, don't try it 🙈
note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.
-mfunction-return=choice— converts function return withchoice. the default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts function return to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attribute"function_return"✨
note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.
note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model.
-mindirect-branch-register— forces indirect call and jump via register. do it through the register, no detours 💅-mharden-sls=choice— generates code to mitigate against straight line speculation (SLS) withchoice. the default isnone, which disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables all SLS hardening. pick your fighter 🌈-mindirect-branch-cs-prefix— adds a CS prefix tocallandjmpto indirect thunk with branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes to allow them to be replaced withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run-time. six bytes exactly, periodt 💯-mapx-inline-asm-use-gpr32— for inline asm support with APX, the EGPR feature was disabled by default to prevent potential illegal instruction with EGPR from happening. to invoke egpr usage in inline asm, use the new compiler option-mapx-inline-asm-use-gpr32, and you should make sure the instruction supports EGPR. your responsibility now bestie 🫶-mevex512,-mno-evex512— enables/disables 512-bit vector. it'll be default on if AVX512F is enabled. big vector energy 👑
these -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32,-m64,-mx32,-m16,-miamcu— generate code for a 16-bit, 32-bit or 64-bit environment. the details are all right here, don't skip ⬇️
the -m32 option sets "int", "long", and pointer types to 32 bits, and generates code that runs in 32-bit mode.
the -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits, and generates code for the x86-64 architecture. for Darwin only, the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.
the -mx32 option sets "int", "long", and pointer types to 32 bits, and generates code for the x86-64 architecture.
the -m16 option is the same as -m32, except that it outputs the ".code16gcc" assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.
the -miamcu option generates code which conforms to Intel MCU psABI. it requires the -m32 option to be turned on. no -m32, no deal 🎀
-mno-red-zone— don'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 location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. the flag-mno-red-zonedisables this red zone. bye bye red zone 🦋-mcmodel=small— generates code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. pointers are 64 bits. programs can be statically or dynamically linked. this is the default code model. the moment 💅-mcmodel=kernel— generates code for the kernel code model. the kernel runs in the negative 2 GB of the address space. this model has to be used for Linux kernel code. mandatory, no cap 🐧-mcmodel=medium— generates code for the medium model: the program is linked in the lower 2 GB of the address space. small symbols are also placed there. symbols with sizes larger than-mlarge-data-thresholdare put into large data or BSS sections and can be located above 2GB. programs can be statically or dynamically linked.-mcmodel=large— generates code for the large model. this model makes no assumptions about addresses and sizes of sections. no assumptions, free spirit 🌸-maddress-mode=long— generates code for long address mode. this is only supported for 64-bit and x32 environments. it's the default address mode for 64-bit environments.-maddress-mode=short— generates code for short address mode. this is only supported for 32-bit and x32 environments. it's the default address mode for 32-bit and x32 environments.-mneeded,-mno-needed— emits theGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. receipts for your CPU, iykyk 💯-mno-direct-extern-access— without-fpicor-fPIC, always use the GOT pointer to access external symbols. with-fpicor-fPIC, treat access to protected symbols as local symbols. the default is-mdirect-extern-access.
⚠️ Warning: shared libraries compiled with-mno-direct-extern-accessand an executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable. mismatched and messy, be careful bestie 😭
-munroll-only-small-loops— controls conservative small loop unrolling. it's default enabled by O2, and unrolls loops with less than 4 insns by 1 time. explicit-f[no-]unroll-[all-]loopswould disable this flag to avoid any unintended unrolling behavior that the user does not want. small loops only, respectfully 🎀-mlam=choice— LAM (linear-address masking) allows special bits in the pointer to be used for metadata. the default isnone. withu48, pointer bits in positions 62:48 can be used for metadata; withu57, pointer bits in positions 62:57 can be used for metadata. metadata in your pointers, fancy ✨
🪟 x86 Windows Options 🪟
these additional options are available for Microsoft Windows targets:
-mconsole— this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. this option is available for Cygwin and MinGW targets and is enabled by default on those targets. console app era 💻-mcrtdll=library— preprocess, compile or link with a specified C RunTime DLL library. This one is a whole little makeover: it adjusts the predefined macros__CRTDLL__,__MSVCRT__and__MSVCRT_VERSION__for the CRT library you picked, chooses the start file for that CRT library, and links against that CRT library. The recognized CRT library names for the preprocessor are:crtdll,msvcrt10,msvcrt20,msvcrt40,msvcrt-os,msvcr70,msvcr80,msvcr90,msvcr100,msvcr110,msvcr120anducrt. If you don't specify this option at all, then the default MinGW import librarymsvcrtgets used for linking and no other preprocessor adjustment happens. Heads up tho: MinGW import librarymsvcrtis literally just a symlink to (or a copy of) another MinGW CRT import library that got chosen during MinGW compilation. And MinGW import librarymsvcrt-osis for the Windows system CRT DLL librarymsvcrt.dll, and in most cases that's the default MinGW import library. Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT. 💀 So this option is for experimental and testing purposes only. Available for MinGW targets.-mdll— available for Cygwin and MinGW targets. It says a DLL — a dynamic link library — is to be generated, which enables the selection of the required runtime startup object and entry point. ✨-mnop-fun-dllimport— available for Cygwin and MinGW targets. It says thedllimportattribute should just be ignored, bestie, we're not doing that today.-mthreads— available for MinGW targets. It says MinGW-specific thread support is to be used. 🧵-municode— available for MinGW-w64 targets. It causes theUNICODEpreprocessor macro to be predefined, and it chooses Unicode-capable runtime startup code.-mwin32— available for Cygwin and MinGW targets. It says the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code. Lowkey important distinction, don't mix them up.-mwindows— available for Cygwin and MinGW targets. It says a GUI application is to be generated, by instructing the linker to set the PE header subsystem type appropriately. 🪟-fno-set-stack-executable— available for MinGW targets. It says the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, since there the User32 API — which is what's used to set executable privileges — isn't available. 👑-fwritable-relocated-rdata— available for MinGW and Cygwin targets. It says relocated-data in a read-only section is put into the.datasection. This is necessary for older runtimes that don't support modification of.rdatasections for pseudo-relocation.-mpe-aligned-commons— available for Cygwin and MinGW targets. It says the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used 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— choose startup files and linker script suitable for the simulator. 🧚
🌸 Xtensa Options
These options are supported for Xtensa targets:
-mconst16,-mno-const16— enable or disable use ofCONST16instructions for loading constant values. Real talk: theCONST16instruction is currently not a standard option from Tensilica. When it's enabled,CONST16instructions are always used in place of the standardL32Rinstructions. The use ofCONST16is enabled by default only if theL32Rinstruction is not available.-mfused-madd,-mno-fused-madd— enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option isn't also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strictly IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than the IEEE standard specifies. Disabling fused multiply add/subtract instructions also ensures that the program output isn't sensitive to the compiler's ability to combine multiply and add/subtract operations. ✨-mserialize-volatile,-mno-serialize-volatile— when this option is 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 option disables PIC for compiling kernel code. 💅-mtext-section-literals,-mno-text-section-literals— these options control the treatment of literal pools. The default is-mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also 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 in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function.-mauto-litpools,-mno-auto-litpools— these options control the treatment of literal pools. The default is-mno-auto-litpools, which places literals in a separate section in the output file unless-mtext-section-literalsis used. With-mauto-litpoolsthe literals are interspersed in the text section by the assembler. The compiler does not produce explicit.literaldirectives and loads literals into registers withMOVIinstructions instead ofL32R, to let the assembler do relaxation and place literals as necessary. This option allows the assembler to create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals.-mtarget-align,-mno-target-align— when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there aren't enough preceding safe density instructions to align a target, no widening is performed. The default is-mtarget-align. These options do not affect the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. 🎯-mlongcalls,-mno-longcalls— when this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a directCALLinstruction into anL32Rfollowed by aCALLXinstruction. The default is-mno-longcalls. This option should be used in programs where the call target can potentially be out of range. And here's the tea: this option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions — look at the disassembled object code to see the actual instructions. Also note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. 🙈-mabi=name— generate code for the specified ABI. Permissible values are:call0,windowed. The default ABI is chosen by the Xtensa core configuration.-mabi=call0— when this option is enabled, 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__is defined.-mabi=windowed— when this option is enabled, function parameters are passed in registersa10througha15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registersa2througha7. Registera7may be used as a frame pointer. The register window is rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__is defined. 🌀-mextra-l32r-costs=n — specify an extra cost of instruction RAM/ROM access forL32Rinstructions, in clock cycles. This affects, when optimizing for speed, whether loading a constant from a literal pool usingL32Ror synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is0.-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-alignfor cores that support both unaligned loads and stores in hardware, and-mstrict-alignfor all other cores. ⭐
💜 zSeries Options
These are listed under
💖 ENVIRONMENT 💖
okay so this section is literally the backstage pass of GCC — it's all the environment variables that change how the compiler behaves. some of them work by handing GCC directories or prefixes to use when it goes hunting for various kinds of files, and some just tweak other aspects of the compilation environment. 🎀
bestie, super important: you can also tell GCC where to look using options like-B,-Iand-L. those win over anything you set in the environment. and the environment in turn wins over whatever the GCC configuration says. so the order is: options beat env vars, env vars beat the config. 💅
🌍 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These environment variables control the way GCC uses localization information, which is how GCC manages to work with different national conventions. GCC looks at the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. Those locale categories can be set to any value your installation supports — a typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8. 🇬🇧✨
LC_CTYPEsets character classification. GCC uses it to figure out the character boundaries in a string; you need this for certain multibyte encodings that contain quote and escape characters which would otherwise get read as a string end or an escape. 😵💫LC_MESSAGESsets the language used in diagnostic messages.
If LC_ALL is set, it overrides the value of LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES default to the value of LANG. And if none of these are set at all, GCC defaults to traditional C English behavior. periodt. 💅
🗂️ TMPDIR
If TMPDIR is set, it specifies the directory GCC uses for temporary files. Why does it even need those? Because GCC uses temporary files to hold the output of one stage of compilation which is then used as input to the next stage — like, the output of the preprocessor becomes the input to the compiler proper. so it just needs a lil scratch space. 🍓
🔍 GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. Go read that option's documentation for the full tea. ✨
🧭 GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. Heads up: no slash is added when this prefix gets combined with the name of a subprogram — but you can absolutely specify a prefix that ends with a slash if that's your vibe.
- If
GCC_EXEC_PREFIXis not set, GCC attempts to figure out an appropriate prefix on its own, based on the pathname it is invoked with. 🔎 - If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram instead.
- The default value of
GCC_EXEC_PREFIXisprefix/lib/gcc/, where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran theconfigurescript. - Other prefixes specified with
-Btake precedence over this prefix. - This prefix is also used for finding files such as
crt0.othat are used for linking.
and here's the unusual bit: the prefix is also used in a weird way when finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc — more precisely, with the value of GCC_INCLUDE_DIR — GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And if a standard directory begins with the configured prefix, then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. 🤯
🛣️ COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you list here when searching for subprograms, if it cannot find them using GCC_EXEC_PREFIX. 💫
📚 LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When GCC is configured as a native compiler, it tries the directories you list here when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option — but directories specified with -L come first. 👑
🔁 LANG
yep, this one's back, because it does a second job. This variable is used to pass locale information to the compiler. One way that info gets used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, these are the LANG values it recognizes:
C-JIS— Recognize JIS characters.C-SJIS— Recognize SJIS characters.C-EUCJP— Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters. 🌈
🩺 GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. Watch out though: -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable. 💅
fixits-v1— Emit parseable fix-it hints, equivalent to-fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.fixits-v2— Asfixits-v1, but columns are expressed as display columns, as per-fdiagnostics-column-unit=display.
Some additional environment variables affect the behavior of the preprocessor. 🧚
📂 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. That special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon. 🌍
CPATHspecifies a list of directories to be searched as if specified with-I, but after any paths given with-Ioptions on the command line. This environment variable is used regardless of which language is being preprocessed.- The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with
-isystem, but after any paths given with-isystemoptions on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include. 🫶
🧾 DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name — in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name. 📝
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. slay.
☀️ SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted. 🙈
⏰ SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. 🔁✨
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command.
The value should be a known timestamp such as the last modification time of the source or package, and it should be set by the build process. 💖
🐛 BUGS 🐛
For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. (yes that's the whole section, it's giving brevity, we love that for it 💀)
📎 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.-sharedsuppresses the addition of startup code to alter the floating-point environment as done with-ffast-math,-Ofastor-funsafe-math-optimizationson some targets. ⚠️
👀 SEE ALSO 👀
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb. 💫
👑 AUTHOR 👑
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. 🫶
📜 COPYRIGHT 📜
Copyright (c) 1988-2024 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.and that's the whole page bestie!! 🎀 you made it to the very end of the entire GCC(1) man page, which is genuinely iconic of you. this edition is from the gcc-14 era, dated 2024-04-12, and it lives in section GCC(1) of your man pages. now go forth and compile something legendary, no cap. 💖✨🦋
📦 where this page comes from
- ubuntu noble: gcc-14-offload-amdgcn 14-20240412-0ubuntu1
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms