GCC(1)

🌈 NAME 🌈

gcc — GNU project C and C++ compiler 💖

okay bestie, this is the gcc(1) page from the GNU manual section, and it's the compiler page, so grab a snack 🍓 because she's long and she's iconic and she ate.

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✨ SYNOPSIS ✨

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

ngl, that list is only the most useful options — the rest live further down, so don't come for me. and pssst: g++ accepts mostly the same options as gcc. 🫶

---

💅 DESCRIPTION 💅

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. the "overall options" let you stop that whole glow-up at an intermediate stage — like, -c literally says do not run the linker, and then the output is just object files output by the assembler. periodt. ✨

Other options get passed on to one or more stages of processing. some of them control the preprocessor, some control the compiler itself. and then there's a whole other group controlling the assembler and the linker — most of those are not documented here, because honestly bestie, you rarely need any of them. 🙈

Most of the command-line options you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. so if the description for a particular option doesn't mention a source language, you can use that option with all supported languages. no cap. 💯

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're compiling C++ programs, you should invoke GCC as g++ instead. 👑

The gcc program accepts options and file names as operands. Many options have multi-letter names, so — and this is important — multiple single-letter options may not be grouped: -dv is very different from -d -v. they are not the same girl, do not treat them like the same girl. 💀

You can mix options and other arguments. for the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind — for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. 🎯

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. 🦋

Some options take one or more arguments, typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes, such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. 📏

---

👑 OPTIONS 👑

🎀 Option Summary 🎀

Here's a summary of all the options, grouped by type. the explanations are in the following sections — this bit is the vibe check, the rest is the lore. 🌸

Overall Options

• -c
• -S
• -E
• -o file
• -dumpbase dumpbase
• -dumpbase-ext auxdropsuf
• -dumpdir dumppfx
• -x language
• -v
• -###
• --help[=class[,...]]
• --target-help
• --version
• -pass-exit-codes
• -pipe
• -specs=file
• -wrapper
• @file
• -ffile-prefix-map=old=new
• -fplugin=file
• -fplugin-arg-name=arg
• -fdump-ada-spec[-slim]
• -fada-spec-parent=unit
• -fdump-go-spec=file

C Language Options

• -ansi
• -std=standard
• -aux-info filename
• -fallow-parameterless-variadic-functions
• -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
• -fpermitted-flt-eval-methods=standard
• -fplan9-extensions
• -fsigned-bitfields
• -funsigned-bitfields
• -fsigned-char
• -funsigned-char
• -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-implicit-templates
• -fno-implicit-inline-templates
• -fno-implement-inlines
• -fmodule-header[=kind]
• -fmodule-only
• -fmodules-ts
• -fmodule-implicit-inline
• -fno-module-lazy
• -fmodule-mapper=specification
• -fmodule-version-ignore
• -fms-extensions
• -fnew-inheriting-ctors
• -fnew-ttp-matching
• -fno-nonansi-builtins
• -fnothrow-opt
• -fno-operator-names
• -fno-optional-diags
• -fpermissive
• -fno-pretty-templates
• -fno-rtti
• -fsized-deallocation
• -ftemplate-backtrace-limit=n
• -ftemplate-depth=n
• -fno-threadsafe-statics
• -fuse-cxa-atexit
• -fno-weak
• -nostdinc++
• -fvisibility-inlines-hidden
• -fvisibility-ms-compat
• -fext-numeric-literals
• -flang-info-include-translate[=header]
• -flang-info-include-translate-not
• -flang-info-module-cmi[=module]
• -stdlib=libstdc++,libc++
• -Wabi-tag
• -Wcatch-value
• -Wcatch-value=n
• -Wno-class-conversion
• -Wclass-memaccess
• -Wcomma-subscript
• -Wconditionally-supported
• -Wno-conversion-null
• -Wctad-maybe-unsupported
• -Wctor-dtor-privacy
• -Wno-delete-incomplete
• -Wdelete-non-virtual-dtor
• -Wno-deprecated-array-compare
• -Wdeprecated-copy
• -Wdeprecated-copy-dtor
• -Wno-deprecated-enum-enum-conversion
• -Wno-deprecated-enum-float-conversion
• -Weffc++
• -Wno-exceptions
• -Wextra-semi
• -Wno-inaccessible-base
• -Wno-inherited-variadic-ctor
• -Wno-init-list-lifetime
• -Winvalid-imported-macros
• -Wno-invalid-offsetof
• -Wno-literal-suffix
• -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
• -Wsign-promo
• -Wsized-deallocation
• -Wsuggest-final-methods
• -Wsuggest-final-types
• -Wsuggest-override
• -Wno-terminate
• -Wuseless-cast
• -Wno-vexing-parse
• -Wvirtual-inheritance
• -Wno-virtual-move-assign
• -Wvolatile
• -Wzero-as-null-pointer-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|json]
• -fno-diagnostics-show-option
• -fno-diagnostics-show-caret
• -fno-diagnostics-show-labels
• -fno-diagnostics-show-line-numbers
• -fno-diagnostics-show-cwe
• -fdiagnostics-minimum-margin-width=width
• -fdiagnostics-parseable-fixits
• -fdiagnostics-generate-patch
• -fdiagnostics-show-template-tree
• -fno-elide-type
• -fdiagnostics-path-format=[none|separate-events|inline-events]
• -fdiagnostics-show-path-depths
• -fno-show-column
• -fdiagnostics-column-unit=[display|byte]
• -fdiagnostics-column-origin=origin
• -fdiagnostics-escape-format=[unicode|bytes]

✨ Warning Options ✨

okay bestie. deep breath. this is THE list. the whole roster of warning flags, right here, all of them, no skips, no "etc." — because gcc does not do "etc." 💅

think of these as your compiler's little alarm system. it's giving very much "I'm not mad, I'm just disappointed" energy, but in flag form. here's the full lineup:

-fsyntax-only  -fmax-errors=n  -Wpedantic -pedantic-errors -w
-Wextra  -Wall  -Wabi=n -Waddress  -Wno-address-of-packed-member

and it keeps going, because of course it does:

-Waggregate-return -Walloc-size-larger-than=byte-size  -Walloc-zero
-Walloca  -Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds
-Warray-bounds=n  -Warray-compare -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-c2x-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 -Wcast-align  -Wcast-align=strict
-Wcast-function-type  -Wcast-qual -Wchar-subscripts -Wclobbered
-Wcomment -Wconversion  -Wno-coverage-mismatch  -Wno-cpp

notice how many of these start with -Wno-? yeah. those are the "actually bestie, do NOT make a big deal out of this one" variants. it's the same energy as muting a group chat 🫶

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-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 -Werror  -Werror=*  -Wexpansion-to-defined
-Wfatal-errors -Wfloat-conversion  -Wfloat-equal  -Wformat
-Wformat=2 -Wno-format-contains-nul  -Wno-format-extra-args
-Wformat-nonliteral  -Wformat-overflow=n -Wformat-security
-Wformat-signedness  -Wformat-truncation=n -Wformat-y2k
-Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Wimplicit  -Wimplicit-fallthrough
-Wimplicit-fallthrough=n -Wno-implicit-function-declaration
-Wno-implicit-int -Winfinite-recursion -Winit-self  -Winline
-Wno-int-conversion  -Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch  -Wjump-misses-init

the =n and =byte-size and =[none|unpaired|any|ucn] ones? those are the ✨picky✨ flags, the ones where you get to pick a number or a mode. main character energy fr fr 👑

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-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-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

okay but the way -Wno-pragmas and -Wno-pragmas-adjacent stuff just sits there... iconic. and we are NOT done, obvi 💀

---

-Wsequence-point -Wshadow  -Wshadow=global  -Wshadow=local
-Wshadow=compatible-local -Wno-shadow-ivar -Wno-shift-count-negative
-Wno-shift-count-overflow  -Wshift-negative-value
-Wno-shift-overflow  -Wshift-overflow=n -Wsign-compare
-Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div  -Wsizeof-pointer-memaccess -Wstack-protector
-Wstack-usage=byte-size  -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow  -Wstrict-overflow=n -Wstring-compare
-Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation
-Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch
-Wno-switch-bool  -Wswitch-default  -Wswitch-enum
-Wno-switch-outside-range  -Wno-switch-unreachable  -Wsync-nand
-Wsystem-headers  -Wtautological-compare  -Wtrampolines  -Wtrigraphs
-Wtrivial-auto-var-init -Wtsan -Wtype-limits  -Wundef
-Wuninitialized  -Wunknown-pragmas -Wunsuffixed-float-constants
-Wunused -Wunused-but-set-parameter  -Wunused-but-set-variable
-Wunused-const-variable  -Wunused-const-variable=n -Wunused-function
-Wunused-label  -Wunused-local-typedefs -Wunused-macros
-Wunused-parameter  -Wno-unused-result -Wunused-value
-Wunused-variable -Wno-varargs  -Wvariadic-macros
-Wvector-operation-performance -Wvla  -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var  -Wwrite-strings
-Wzero-length-bounds

and THAT, bestie, is the whole warning roster. every single one. no crumbs left. 🍓

---

✨ Static Analyzer Options ✨

next up: the static analyzer. this is the friend who reads your texts before you send them and goes "bestie… you sure?" 🔍 the vibes are immaculate, the findings are brutal.

-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-state-merge -fanalyzer-state-purge
-fanalyzer-transitivity -fanalyzer-verbose-edges
-fanalyzer-verbose-state-changes -fanalyzer-verbosity=level
-fdump-analyzer -fdump-analyzer-stderr -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3
-fdump-analyzer-exploded-paths -fdump-analyzer-feasibility
-fdump-analyzer-json -fdump-analyzer-state-purge
-fdump-analyzer-supergraph -Wno-analyzer-double-fclose
-Wno-analyzer-double-free -Wno-analyzer-exposure-through-output-file
-Wno-analyzer-file-leak -Wno-analyzer-free-of-non-heap
-Wno-analyzer-malloc-leak -Wno-analyzer-mismatching-deallocation
-Wno-analyzer-null-argument -Wno-analyzer-null-dereference
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size
-Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor
-Wno-analyzer-tainted-offset -Wno-analyzer-tainted-size
-Wanalyzer-too-complex
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal

look at all those -fdump-analyzer-* flags. it's giving "I made a whole scrapbook of everything you did wrong and I'm showing you page by page" 📓✨

---

✨ C and Objective-C-only Warning Options ✨

these ones only show up for the C and Objective-C girlies. exclusive guest list, no plus-ones 🎀

-Wbad-function-cast  -Wmissing-declarations -Wmissing-parameter-type
-Wmissing-prototypes  -Wnested-externs -Wold-style-declaration
-Wold-style-definition -Wstrict-prototypes  -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement
-Wpointer-sign

short and sweet. ate and left no crumbs. 💅

---

✨ Debugging Options ✨

and finally for this stretch: the debugging flags. this is the section where you tell gcc "keep the receipts, bestie, I need to know everything that happened" 🧾💜

-g  -glevel  -gdwarf  -gdwarf-version -gbtf -gctf  -gctflevel -ggdb
-grecord-gcc-switches  -gno-record-gcc-switches -gstabs  -gstabs+
-gstrict-dwarf  -gno-strict-dwarf -gas-loc-support
-gno-as-loc-support -gas-locview-support  -gno-as-locview-support
-gcolumn-info  -gno-column-info  -gdwarf32  -gdwarf64
-gstatement-frontiers  -gno-statement-frontiers
-gvariable-location-views  -gno-variable-location-views
-ginternal-reset-location-views  -gno-internal-reset-location-views
-ginline-points  -gno-inline-points -gvms  -gxcoff  -gxcoff+
-gz[=type] -gsplit-dwarf  -gdescribe-dies  -gno-describe-dies
-fdebug-prefix-map=old=new  -fdebug-types-section
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list]
-fno-eliminate-unused-debug-symbols  -femit-class-debug-always
-fno-merge-debug-strings  -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments

okay notice -g is just… -g. one letter. no notes. a minimalist icon 🌸 and then right next to it, -ginternal-reset-location-views — the duality of girl 💀

that's the whole rundown for this chunk bestie, and there's SO much more coming 🦋

Okay so the option summary is basically a phonebook of flags, and I'm about to read you the entire phonebook. No skimming, no "you get the idea". Every single one. Sit down, hydrate, let's go. 💅

🌈 Optimization Options 🌈

These are the ones that make your code go fast (or make it go weird — your call, bestie). Every last one:

• -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]]]]
• -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
• -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-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

Periodt. That's the whole optimization menu, and yes, I typed every one. 😤

---

🎀 Program Instrumentation Options 🎀

Now we're getting into the watch-me-work flags — the ones that poke at your program and take notes. Every single one, no cap:

• -p
• -pg
• -fprofile-arcs
• --coverage
• -ftest-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
• -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
• -fstack-protector
• -fstack-protector-all
• -fstack-protector-strong
• -fstack-protector-explicit
• -fstack-check
• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit
• -fsplit-stack
• -fvtable-verify=[std|preinit|none]
• -fvtv-counts
• -fvtv-debug
• -finstrument-functions
• -finstrument-functions-exclude-function-list=sym,sym,...
• -finstrument-functions-exclude-file-list=file,file,...
• -fprofile-prefix-map=old=new

---

🦋 Preprocessor Options 🦋

The preprocessor is the little goblin that runs before the compiler even shows up, and these are its knobs:

• -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

---

🍓 Assembler Options 🍓

Tiny section, huge respect. Two flags, both icons:

• -Wa,option
• -Xassembler option

---

✨ Linker Options ✨

Where all your object files finally meet each other. Full list, bestie:

• object-file-name
• -fuse-ld=linker
• -llibrary
• -nostartfiles
• -nodefaultlibs
• -nolibc
• -nostdlib
• -e entry
• --entry=entry
• -pie
• -pthread
• -r
• -rdynamic
• -s
• -static
• -static-pie
• -static-libgcc
• -static-libstdc++
• -static-libasan
• -static-libtsan
• -static-liblsan
• -static-libubsan
• -shared
• -shared-libgcc
• -symbolic
• -T script
• -Wl,option
• -Xlinker option
• -u symbol
• -z keyword

---

🌸 Directory Options 🌸

Where does GCC go looking for things? These tell it. 💖

• -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 💜

The machine-code-shaping corner of the universe:

• -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
• -ftrapv
• -fwrapv
• -fvisibility=[default|internal|hidden|protected]
• -fstrict-volatile-bitfields
• -fsync-libcalls

---

🧚 Developer Options 🧚

This is the backstage pass section. Compiler-nerd main character energy, honestly. 👑

• -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]
• -fprofile-report
• -frandom-seed=string
• -fsched-verbose=n
• -fsel-sched-verbose
• -fsel-sched-dump-cfg
• -fsel-sched-pipelining-verbose
• -fstats
• -fstack-usage
• -ftime-report
• -ftime-report-details
• -fvar-tracking-assignments-toggle
• -gtoggle
• -print-file-name=library
• -print-libgcc-file-name
• -print-multi-directory
• -print-multi-lib
• -print-multi-os-directory
• -print-prog-name=program
• -print-search-dirs
• -Q
• -print-sysroot
• -print-sysroot-headers-suffix
• -save-temps
• -save-temps=cwd
• -save-temps=obj
• -time[=file]

---

🐚 Machine-Dependent Options 🐚

And now for the part where your hardware gets a say. 🌈

🌟 AArch64 Options

• -mabi=name
• -mbig-endian
• -mlittle-endian
• -mgeneral-regs-only
• -mcmodel=tiny
• -mcmodel=small
• -mcmodel=large
• -mstrict-align
• -mno-strict-align
• -momit-leaf-frame-pointer
• -mtls-dialect=desc
• -mtls-dialect=traditional
• -mtls-size=size
• -mfix-cortex-a53-835769
• -mfix-cortex-a53-843419
• -mlow-precision-recip-sqrt
• -mlow-precision-sqrt
• -mlow-precision-div
• -mpc-relative-literal-loads
• -msign-return-address=scope
• -mbranch-protection=none|standard|pac-ret[+leaf +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

---

And that's the whole flag phonebook, alphabetically deranged and fully preserved. We are not done though — there's so much more coming, so don't you dare close the terminal. 🫶

🧚 Adapteva Epiphany Options

For the Adapteva Epiphany — niche, gorgeous, lowkey a whole personality 💜. Here's the full flag roster:

• -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

GPU girlies rise up — this one's a tiny trio ✨

• -march=gpu
• -mtune=gpu
• -mstack-size=bytes

---

🦋 ARC Options

okay the ARC list is long bestie, grab a snack 🍓. Full rundown:

• -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 ARM section said "let me serve you an entire buffet" and honestly? she ate 💅 every single one:

• -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

---

🌸 AVR Options

tiny microcontroller energy, enormous option list 🫶

• -mmcu=mcu, -mabsdata, -maccumulate-args
• -mbranch-cost=cost, -mcall-prologues, -mgas-isr-prologues
• -mint8, -mdouble=bits, -mlong-double=bits
• -mn_flash=size, -mno-interrupts, -mmain-is-OS_task
• -mrelax, -mrmw, -mstrict-X, -mtiny-stack
• -mfract-convert-truncate, -mshort-calls
• -nodevicelib, -nodevicespecs
• -Waddr-space-convert, -Wmisspelled-isr

---

⭐ Blackfin Options

• -mcpu=cpu[-sirevision], -msim
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer
• -mspecld-anomaly, -mno-specld-anomaly
• -mcsync-anomaly, -mno-csync-anomaly
• -mlow-64k, -mno-low64k, -mstack-check-l1
• -mid-shared-library, -mno-id-shared-library, -mshared-library-id=n
• -mleaf-id-shared-library, -mno-leaf-id-shared-library
• -msep-data, -mno-sep-data
• -mlong-calls, -mno-long-calls
• -mfast-fp, -minline-plt
• -mmulticore, -mcorea, -mcoreb, -msdram, -micplb

---

💜 C6X Options

• -mbig-endian, -mlittle-endian, -march=cpu, -msim, -msdata=sdata-type

---

🦋 CRIS Options

• -mcpu=cpu, -march=cpu, -mtune=cpu
• -mmax-stack-frame=n, -melinux-stacksize=n
• -metrax4, -metrax100, -mpdebug, -mcc-init
• -mno-side-effects
• -mstack-align, -mdata-align, -mconst-align
• -m32-bit, -m16-bit, -m8-bit
• -mno-prologue-epilogue, -mno-gotplt
• -melf, -maout, -melinux, -mlinux, -sim, -sim2
• -mmul-bug-workaround, -mno-mul-bug-workaround

---

🎀 CR16 Options

• -mmac, -mcr16cplus, -mcr16c, -msim, -mint32, -mbit-ops, -mdata-model=model

---

🌈 C-SKY Options

• -march=arch, -mcpu=cpu
• -mbig-endian, -EB, -mlittle-endian, -EL
• -mhard-float, -msoft-float, -mfpu=fpu, -mdouble-float, -mfdivdu
• -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 besties this one's for you 💖. (yes, a couple of these show up twice in the original list — we're keeping it exactly as written, 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
• -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, mysterious, kind of a legend ngl 🔥

• -mbig-endian, -mlittle-endian, -mkernel=version
• -mframe-limit=bytes, -mxbpf
• -mco-re, -mno-co-re
• -mjmpext, -mjmp32, -malu32, -mcpu=version

---

🧚 FR30 Options

• -msmall-model, -mno-lsim

---

💅 FT32 Options

• -msim, -mlra, -mnodiv, -mft32b, -mcompress, -mnopm

---

🌸 FRV Options

another absolute unit of a list, she's giving encyclopedia 📚

• -mgpr-32, -mgpr-64, -mfpr-32, -mfpr-64
• -mhard-float, -msoft-float
• -malloc-cc, -mfixed-cc
• -mdword, -mno-dword, -mdouble, -mno-double
• -mmedia, -mno-media, -mmuladd, -mno-muladd
• -mfdpic, -minline-plt, -mgprel-ro, -multilib-library-pic
• -mlinked-fp, -mlong-calls, -malign-labels, -mlibrary-pic
• -macc-4, -macc-8, -mpack, -mno-pack, -mno-eflags
• -mcond-move, -mno-cond-move
• -moptimize-membar, -mno-optimize-membar
• -mscc, -mno-scc, -mcond-exec, -mno-cond-exec
• -mvliw-branch, -mno-vliw-branch
• -mmulti-cond-exec, -mno-multi-cond-exec
• -mnested-cond-exec, -mno-nested-cond-exec
• -mtomcat-stats, -mTLS, -mtls, -mcpu=cpu

---

🐧 GNU/Linux Options

• -mglibc, -muclibc, -mmusl, -mbionic, -mandroid
• -tno-android-cc, -tno-android-ld

---

🎀 H8/300 Options

• -mrelax, -mh, -ms, -mn, -mexr, -mno-exr, -mint32, -malign-300

---

🦋 HPPA Options

• -march=architecture-type, -mcaller-copies, -mdisable-fpregs
• -mdisable-indexing, -mfast-indirect-calls, -mgas, -mgnu-ld, -mhp-ld
• -mfixed-range=register-range, -mjump-in-delay, -mlinker-opt
• -mlong-calls, -mlong-load-store
• -mno-disable-fpregs, -mno-disable-indexing, -mno-fast-indirect-calls
• -mno-gas, -mno-jump-in-delay, -mno-long-load-store
• -mno-portable-runtime, -mno-soft-float, -mno-space-regs
• -msoft-float, -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0
• -mportable-runtime, -mschedule=cpu-type, -mspace-regs
• -msio, -mwsio, -munix=unix-std
• -nolibdld, -static, -threads

---

💜 IA-64 Options

• -mbig-endian, -mlittle-endian, -mgnu-as, -mgnu-ld, -mno-pic
• -mvolatile-asm-stop, -mregister-names, -msdata, -mno-sdata
• -mconstant-gp, -mauto-pic, -mfused-madd
• -minline-float-divide-min-latency, -minline-float-divide-max-throughput, -mno-inline-float-divide
• -minline-int-divide-min-latency, -minline-int-divide-max-throughput, -mno-inline-int-divide
• -minline-sqrt-min-latency, -minline-sqrt-max-throughput, -mno-inline-sqrt
• -mdwarf2-asm, -mearly-stop-bits
• -mfixed-range=register-range, -mtls-size=tls-size
• -mtune=cpu-type, -milp32, -mlp64
• -msched-br-data-spec, -msched-ar-data-spec, -msched-control-spec
• -msched-br-in-data-spec, -msched-ar-in-data-spec, -msched-in-control-spec
• -msched-spec-ldc, -msched-spec-control-ldc
• -msched-prefer-non-data-spec-insns, -msched-prefer-non-control-spec-insns
• -msched-stop-bits-after-every-cycle, -msched-count-spec-in-critical-path
• -msel-sched-dont-check-control-spec, -msched-fp-mem-deps-zero-cost
• -msched-max-memory-insns-hard-limit, -msched-max-memory-insns=max-insns

---

⭐ LM32 Options

• -mbarrel-shift-enabled, -mdivide-enabled, -mmultiply-enabled
• -msign-extend-enabled, -muser-enabled

---

🌈 M32R/D Options

• -m32r2, -m32rx, -m32r, -mdebug
• -malign-loops, -mno-align-loops
• -missue-rate=number, -mbranch-cost=number
• -mmodel=code-size-model-type, -msdata=sdata-type
• -mno-flush-func, -mflush-func=name
• -mno-flush-trap, -mflush-trap=number
• -G num

---

🍓 M32C Options

• -mcpu=cpu, -msim, -memregs=number

---

💖 M680x0 Options

the 68k section is chef's kiss — long live the classics ✨

• -march=arch, -mcpu=cpu, -mtune=tune
• -m68000, -m68020, -m68020-40, -m68020-60, -m68030, -m68040, -m68060
• -mcpu32, -m5200, -m5206e, -m528x, -m5307, -m5407
• -mcfv4e, -mbitfield, -mno-bitfield
• -mc68000, -mc68020, -mnobitfield
• -mrtd, -mno-rtd, -mdiv, -mno-div
• -mshort, -mno-short
• -mhard-float, -m68881, -msoft-float
• -mpcrel, -malign-int, -mstrict-align
• -msep-data, -mno-sep-data
• -mshared-library-id=n, -mid-shared-library, -mno-id-shared-library
• -mxgot, -mno-xgot, -mlong-jump-table-offsets

---

🎀 MCore Options

• -mhardlit, -mno-hardlit
• -mdiv, -mno-div
• -mrelax-immediates, -mno-relax-immediates
• -mwide-bitfields, -mno-wide-bitfields
• -m4byte-functions, -mno-4byte-functions
• -mcallgraph-data, -mno-callgraph-data
• -mslow-bytes, -mno-slow-bytes, -mno-lsim
• -mlittle-endian, -mbig-endian
• -m210, -m340, -mstack-increment

---

🦋 MeP Options

last one of this part and she's stacked, bestie 💅

• -mabsdiff, -mall-opts, -maverage, -mbased=n, -mbitops
• -mc=n, -mclip, -mconfig=name
• -mcop, -mcop32, -mcop64, -mivc2
• -mdc, -mdiv
• -meb, -mel
• -mio-volatile, -ml, -mleadz, -mm, -mminmax, -mmult
• -mno-opts, -mrepeat, -ms, -msatur, -msdram
• -msim, -msimnovec, -mtf, -mtiny=n

✨ MicroBlaze Options ✨

bestie, MicroBlaze said "here's my whole personality in flags" 💅 no skips, no crumbs:

• -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 💖

MIPS really said "I contain multitudes" and then dropped the longest menu ever. iconic behavior. Here's every single one, 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
• -mcheck-zero-division, -mno-check-zero-division
• -mdivide-traps, -mdivide-breaks
• -mload-store-pairs, -mno-load-store-pairs
• -munaligned-access, -mno-unaligned-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-opt

---

🦋 MMIX Options 🦋

• -mlibfuncs, -mno-libfuncs
• -mepsilon, -mno-epsilon
• -mabi=gnu, -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict, -mno-branch-predict
• -mbase-addresses, -mno-base-addresses
• -msingle-exit, -mno-single-exit

---

🌸 MN10300 Options 🌸

• -mmult-bug, -mno-mult-bug
• -mno-am33, -mam33, -mam33-2, -mam34
• -mtune=cpu-type
• -mreturn-pointer-on-d0
• -mno-crt0
• -mrelax, -mliw, -msetlb

---

🎀 Moxie Options 🎀

• -meb, -mel
• -mmul.x
• -mno-crt0

---

⭐ MSP430 Options ⭐

• -msim
• -masm-hex
• -mmcu=
• -mcpu=
• -mlarge, -msmall
• -mrelax
• -mwarn-mcu
• -mcode-region=, -mdata-region=
• -msilicon-errata=, -msilicon-errata-warn=
• -mhwmult=
• -minrt
• -mtiny-printf
• -mmax-inline-shift=

---

💅 NDS32 Options 💅

• -mbig-endian, -mlittle-endian
• -mreduced-regs, -mfull-regs
• -mcmov, -mno-cmov
• -mext-perf, -mno-ext-perf
• -mext-perf2, -mno-ext-perf2
• -mext-string, -mno-ext-string
• -mv3push, -mno-v3push
• -m16bit, -mno-16bit
• -misr-vector-size=num
• -mcache-block-size=num
• -march=arch
• -mcmodel=code-model
• -mctor-dtor
• -mrelax

---

🫶 Nios II Options 🫶

• -G num
• -mgpopt=option, -mgpopt, -mno-gpopt
• -mgprel-sec=regexp, -mr0rel-sec=regexp
• -mel, -meb
• -mno-bypass-cache, -mbypass-cache
• -mno-cache-volatile, -mcache-volatile
• -mno-fast-sw-div, -mfast-sw-div
• -mhw-mul, -mno-hw-mul
• -mhw-mulx, -mno-hw-mulx
• -mno-hw-div, -mhw-div
• -mcustom-insn=N, -mno-custom-insn
• -mcustom-fpu-cfg=name
• -mhal, -msmallc
• -msys-crt0=name, -msys-lib=name
• -march=arch
• -mbmx, -mno-bmx
• -mcdx, -mno-cdx

---

🔥 Nvidia PTX Options 🔥

• -m64
• -mmainkernel
• -moptimize

---

🌈 OpenRISC Options 🌈

• -mboard=name
• -mnewlib
• -mhard-mul, -mhard-div
• -msoft-mul, -msoft-div
• -msoft-float, -mhard-float, -mdouble-float
• -munordered-float
• -mcmov
• -mror, -mrori
• -msext
• -msfimm, -mshftimm
• -mcmodel=code-model

---

💜 PDP-11 Options 💜

• -mfpu, -msoft-float
• -mac0, -mno-ac0
• -m40, -m45, -m10
• -mint32, -mno-int16
• -mint16, -mno-int32
• -msplit
• -munix-asm, -mdec-asm, -mgnu-asm
• -mlra

---

🧚 picoChip Options 🧚

• -mae=ae_type
• -mvliw-lookahead=N
• -msymbol-as-address
• -mno-inefficient-warnings

---

👑 PowerPC Options 👑

See RS/6000 and PowerPC Options. 💫 (yes bestie, it's literally just a pointer, we love a redirect)

---

🍓 PRU Options 🍓

• -mmcu=mcu
• -minrt
• -mno-relax
• -mloop
• -mabi=variant

---

✨ RISC-V Options ✨

• -mbranch-cost=N-instruction
• -mplt, -mno-plt
• -mabi=ABI-string
• -mfdiv, -mno-fdiv
• -mdiv, -mno-div
• -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, -mmo-riscv-attribute
• -malign-data=type
• -mbig-endian, -mlittle-endian
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset

---

🌸 RL78 Options 🌸

• -msim
• -mmul=none, -mmul=g13, -mmul=g14
• -mallregs
• -mcpu=g10, -mcpu=g13, -mcpu=g14
• -mg10, -mg13, -mg14
• -m64bit-doubles, -m32bit-doubles
• -msave-mduc-in-interrupts

---

💖 RS/6000 and PowerPC Options 💖

okay this one is a whole vibe, buckle up bestie, we're going the full distance:

• -mcpu=cpu-type, -mtune=cpu-type
• -mcmodel=code-model
• -mpowerpc64
• -maltivec, -mno-altivec
• -mpowerpc-gpopt, -mno-powerpc-gpopt
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt
• -mmfcrf, -mno-mfcrf
• -mpopcntb, -mno-popcntb
• -mpopcntd, -mno-popcntd
• -mfprnd, -mno-fprnd
• -mcmpb, -mno-cmpb
• -mhard-dfp, -mno-hard-dfp
• -mfull-toc, -mminimal-toc, -mno-fp-in-toc, -mno-sum-in-toc
• -m64, -m32
• -mxl-compat, -mno-xl-compat
• -mpe
• -malign-power, -malign-natural
• -msoft-float, -mhard-float
• -mmultiple, -mno-multiple
• -mupdate, -mno-update
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses
• -mfused-madd, -mno-fused-madd
• -mbit-align, -mno-bit-align
• -mstrict-align, -mno-strict-align
• -mrelocatable, -mno-relocatable
• -mrelocatable-lib, -mno-relocatable-lib
• -mtoc, -mno-toc
• -mlittle, -mlittle-endian
• -mbig, -mbig-endian
• -mdynamic-no-pic
• -mswdiv
• -msingle-pic-base
• -mprioritize-restricted-insns=priority
• -msched-costly-dep=dependence_type
• -minsert-sched-nops=scheme
• -mcall-aixdesc, -mcall-eabi, -mcall-freebsd, -mcall-linux, -mcall-netbsd, -mcall-openbsd
• -mcall-sysv, -mcall-sysv-eabi, -mcall-sysv-noeabi
• -mtraceback=traceback_type
• -maix-struct-return, -msvr4-struct-return
• -mabi=abi-type
• -msecure-plt, -mbss-plt
• -mlongcall, -mno-longcall
• -mpltseq, -mno-pltseq
• -mblock-move-inline-limit=num
• -mblock-compare-inline-limit=num
• -mblock-compare-inline-loop-limit=num
• -mno-block-ops-unaligned-vsx
• -mstring-compare-inline-limit=num
• -misel, -mno-isel
• -mvrsave, -mno-vrsave
• -mmulhw, -mno-mulhw
• -mdlmzb, -mno-dlmzb
• -mprototype, -mno-prototype
• -msim, -mmvme, -mads, -myellowknife, -memb
• -msdata, -msdata=opt
• -mreadonly-in-sdata
• -mvxworks
• -G num
• -mrecip, -mrecip=opt, -mno-recip
• -mrecip-precision, -mno-recip-precision
• -mveclibabi=type
• -mfriz, -mno-friz
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions
• -msave-toc-indirect, -mno-save-toc-indirect
• -mpower8-fusion, -mno-mpower8-fusion
• -mpower8-vector, -mno-power8-vector
• -mcrypto, -mno-crypto
• -mhtm, -mno-htm
• -mquad-memory, -mno-quad-memory
• -mquad-memory-atomic, -mno-quad-memory-atomic
• -mcompat-align-parm, -mno-compat-align-parm
• -mfloat128, -mno-float128
• -mfloat128-hardware, -mno-float128-hardware
• -mgnu-attribute, -mno-gnu-attribute
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mprefixed, -mno-prefixed
• -mpcrel, -mno-pcrel
• -mmma, -mno-mmma
• -mrop-protect, -mno-rop-protect
• -mprivileged, -mno-privileged

---

🎀 RX Options 🎀

• -m64bit-doubles, -m32bit-doubles
• -fpu, -nofpu
• -mcpu=
• -mbig-endian-data, -mlittle-endian-data
• -msmall-data
• -msim, -mno-sim
• -mas100-syntax, -mno-as100-syntax
• -mrelax
• -mmax-constant-size=
• -mint-register=
• -mpid
• -mallow-string-insns, -mno-allow-string-insns
• -mjsr
• -mno-warn-multiple-fast-interrupts
• -msave-acc-in-interrupts

---

🦋 S/390 and zSeries Options 🦋

• -mtune=cpu-type, -march=cpu-type
• -mhard-float, -msoft-float
• -mhard-dfp, -mno-hard-dfp
• -mlong-double-64, -mlong-double-128
• -mbackchain, -mno-backchain
• -mpacked-stack, -mno-packed-stack
• -msmall-exec, -mno-small-exec
• -mmvcle, -mno-mvcle
• -m64, -m31
• -mdebug, -mno-debug
• -mesa, -mzarch
• -mhtm, -mvx, -mzvector
• -mtpf-trace, -mno-tpf-trace
• -mtpf-trace-skip, -mno-tpf-trace-skip
• -mfused-madd, -mno-fused-madd
• -mwarn-framesize
• -mwarn-dynamicstack
• -mstack-size
• -mstack-guard
• -mhotpatch=halfwords,halfwords

---

⭐ Score Options ⭐

• -meb, -mel
• -mnhwloop
• -muls
• -mmac
• -mscore5, -mscore5u
• -mscore7, -mscore7d

---

💅 SH Options 💅

• -m1, -m2, -m2e
• -m2a-nofpu, -m2a-single-only, -m2a-single, -m2a
• -m3, -m3e
• -m4-nofpu, -m4-single-only, -m4-single, -m4
• -m4a-nofpu, -m4a-single-only, -m4a-single, -m4a, -m4al
• -mb, -ml
• -mdalign
• -mrelax
• -mbigtable
• -mfmovd
• -mrenesas, -mno-renesas
• -mnomacsave
• -mieee, -mno-ieee
• -mbitops
• -misize
• -minline-ic_invalidate
• -mpadstruct
• -mprefergot
• -musermode
• -multcost=number
• -mdiv=strategy
• -mdivsi3_libfunc=name
• -mfixed-range=register-range
• -maccumulate-outgoing-args
• -matomic-model=atomic-model
• -mbranch-cost=num
• -mzdcbranch, -mno-zdcbranch
• -mcbranch-force-delay-slot
• -mfused-madd, -mno-fused-madd
• -mfsca, -mno-fsca
• -mfsrra, -mno-fsrra
• -mpretend-cmove
• -mtas

---

🌈 Solaris 2 Options 🌈

• -mclear-hwcap, -mno-clear-hwcap
• -mimpure-text, -mno-impure-text
• -pthreads

---

bestie we are so far from done and yet still going 💀 don't close the tab, more is coming 💖

SPARC Options 💫

SPARC besties, this is your whole lil' control panel — every single flag, no gatekeeping:

-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32  -m64  -mapp-regs  -mno-app-regs
-mfaster-structs  -mno-faster-structs
-mflat  -mno-flat -mfpu
-mno-fpu  -mhard-float  -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias  -mno-stack-bias -mstd-struct-return
-mno-std-struct-return -munaligned-doubles  -mno-unaligned-doubles
-muser-mode  -mno-user-mode -mv8plus  -mno-v8plus  -mvis  -mno-vis
-mvis2  -mno-vis2  -mvis3  -mno-vis3 -mvis4  -mno-vis4  -mvis4b
-mno-vis4b -mcbcond  -mno-cbcond  -mfmaf  -mno-fmaf  -mfsmuld
-mno-fsmuld -mpopc  -mno-popc  -msubxc  -mno-subxc -mfix-at697f
-mfix-ut699  -mfix-ut700  -mfix-gr712rc -mlra  -mno-lra

---

System V Options ✨

-Qy  -Qn  -YP,paths  -Ym,dir

---

TILE-Gx Options 🧚

-mcpu=CPU  -m32  -m64  -mbig-endian  -mlittle-endian -mcmodel=code-model

---

TILEPro Options 🦋

-mcpu=cpu  -m32

---

V850 Options 🌸

-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  -non-static  -Bstatic  -Bdynamic -Xbind-lazy
-Xbind-now

---

x86 Options 👑

okay this is THE list, the big one, the main character. x86 girlies we have so many options and here is every last one, periodt:

-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
-mmove-max=bits -mstore-max=bits -mmmx  -msse  -msse2  -msse3
-mssse3  -msse4.1  -msse4.2  -msse4  -mavx -mavx2  -mavx512f
-mavx512pf  -mavx512er  -mavx512cd  -mavx512vl -mavx512bw
-mavx512dq  -mavx512ifma  -mavx512vbmi  -msha  -maes -mpclmul
-mfsgsbase  -mrdrnd  -mf16c  -mfma  -mpconfig  -mwbnoinvd -mptwrite
-mprefetchwt1  -mclflushopt  -mclwb  -mxsavec  -mxsaves -msse4a
-m3dnow  -m3dnowa  -mpopcnt  -mabm  -mbmi  -mtbm  -mfma4  -mxop
-madx  -mlzcnt  -mbmi2  -mfxsr  -mxsave  -mxsaveopt  -mrtm  -mhle
-mlwp -mmwaitx  -mclzero  -mpku  -mthreads  -mgfni  -mvaes
-mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call
-mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq  -mavx512bitalg
-mmovdiri  -mmovdir64b  -mavx512vpopcntdq -mavx5124fmaps
-mavx512vnni  -mavx5124vnniw  -mprfchw  -mrdpid -mrdseed  -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile  -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mavx512fp16 -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  -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

---

x86 Windows Options 🫶

-mconsole  -mcygwin  -mno-cygwin  -mdll
-mnop-fun-dllimport  -mthread -municode  -mwin32  -mwindows
-fno-set-stack-executable

---

Xstormy16 Options ⭐

-msim

---

Xtensa Options 💅

-mconst16  -mno-const16 -mfused-madd  -mno-fused-madd
-mforce-no-pic -mserialize-volatile  -mno-serialize-volatile
-mtext-section-literals  -mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align  -mno-target-align -mlongcalls
-mno-longcalls -mabi=abi-type

---

zSeries Options 💖

See S/390 and zSeries Options. (yes bestie, that's literally all it says — go look over there 🫶)

---

Options Controlling the Kind of Output 🌟

okay so compiling is literally a four-stage pipeline and it always happens in this order, no skipping the queue:

1. preprocessing
2. compilation proper
3. assembly
4. linking

GCC can preprocess and compile a bunch of files either into several assembler input files, or into one single assembler input file. Then each assembler input file produces an object file, and linking combines all the object files — the ones it just compiled and the ones you handed it as input — into one executable file. That's the whole glow-up arc. ✨

For any given input file, the file name suffix is what decides which kind of compilation happens. So let's do a 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. Note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f, file.for, file.ftn — Fixed form Fortran source code that should not be preprocessed.
• file.F, file.FOR, file.fpp, file.FPP, file.FTN — Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90, file.f95, file.f03, file.f08 — Free form Fortran source code that should not be preprocessed.
• file.F90, file.F95, file.F03, file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.go — Go source code.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. (mystery file? object file. sorry bestie, them's the rules 😭)

---

You can also just tell gcc the language instead of letting the suffix do the talking, using -x:

• -x language — Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:

c  c-header  cpp-output
c++  c++-header  c++-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 — Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

---

So if you only want some of the stages, use -x (or filename suffixes) to tell gcc where to start, and then one of -c, -S, or -E to say where gcc is supposed to stop. And heads up: some combinations — for example, -x cpp-output -E — instruct gcc to do absolutely nothing at all. Not a single thing. It's giving "girl what are we even doing here" 💀

• -c — Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

• -S — Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

• -E — Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

• -o file — okay bestie, this one is literally the main character of output naming. It puts the primary output in file. And it applies to whatever sort of output is being produced: executable file, object file, assembler file, or preprocessed C code. No cap. 👑

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.c

saves 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.c

preprocesses to 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 trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when -dumpbase is not present in the command line, or dumpbase is combined with dumppfx.

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, use dumppfx as 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.*.

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.

gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...

outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

that, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.

When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that 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 is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option.

When compiling and then linking from a single input file, the linker output base name will only be 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 single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if 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 are named foo.c.*, and they land in the current working directory — as ultimately requested by -save-temps=cwd. 💅

Okay bestie, here's the intuitive-but-slightly-imprecise tea on the data flow, summing it all up: the primary output name gets broken into a directory part and a basename part. dumppfx gets set to that directory part, unless something overrides it with -dumpdir or -save-temps=*. And dumpbase gets set to the basename part, unless it's overridden by -dumpbase. Now if there are multiple inputs, or if we're linking, that dumpbase may get combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus its suffix, and the auxiliary output suffix. Dump output names are only different in that the suffix from dumpbase is retained. 🦋

Now for 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 in which this combination would not otherwise be used as such), gets passed down as a -dumpdir option to lto-wrapper, with a trailing period replacing the compiler-added dash, if any. And because it's involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them. Periodt.

When it's 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 the sub-compilers. Slay. ✨

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. 🫶
• -### — Like -v, except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines. Chef's kiss for automation.
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. And if the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. 🙈
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
• --help={class|[^]qualifier}[,...] — Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. Here are the supported classes, bestie:
  • optimizers — Display all of the optimization options supported by the compiler. 🔥
  • warnings — Display all of the options controlling warning messages produced by the compiler.
  • target — Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
  • params — Display the values recognized by the --param option.
  • language — Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class.
  • 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, for example, to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

You can flip the sense of a qualifier by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

Heads up: the argument to --help= should not consist solely of inverted qualifiers. That's a no-no. 💀

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. 👑

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=                                2
-mabort-on-noreturn                   [disabled]
-mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled

Iconic detective work, fr fr. 🕵️‍♀️

• --version — Display the version number and copyrights of the invoked GCC.
• -pass-exit-codes — Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.
• -pipe — Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. 🧚
• -specs=file — Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.
• -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 ....

• -ffile-prefix-map=old=new — When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. See also -fmacro-prefix-map, -fdebug-prefix-map and -fprofile-prefix-map. 🌈
• -fplugin=name.so — Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
• -fplugin-arg-name-key=value — Define an argument called key with a value of value for the plugin called name.
• -fdump-ada-spec[-slim] — For C and C++ source and include files, generate corresponding Ada specs.
• -fada-spec-parent=unit — In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
• -fdump-go-spec=file — For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language.
• @file — Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed. 📄

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.

---

✨ Compiling C++ Programs ✨

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx. C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc. And preprocessed C++ files use the suffix .ii. 🎀

GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc). Yes bestie, it just knows. 👑

So like, using gcc does NOT drag the C++ library in with it 💀 that's the whole tea. g++ is the program that calls GCC and automatically links against the C++ library for you. It also treats .c, .h and .i files as C++ source files instead of C source files — unless you use -x. And bestie, g++ is also the moment 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 many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. 🫶

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🌈 Options Controlling C Dialect 🌈

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts: 💅

• -ansi — In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. 😭 For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi. 👑

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. (yeah, it's a two-step thing, no shortcuts)

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used.

• -std= — Determine the language standard. This option is currently only supported when compiling C or C++. 🌸

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. 😬 The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

A value for this option must be provided; possible values are 💖

• c90
• c89
• iso9899:1990
  • Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409
  • ISO C90 as modified in amendment 1.
• c99
• c9x
• iso9899:1999
• iso9899:199x
  • ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated. 🚫
• c11
• c1x
• iso9899:2011
  • ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17
• c18
• iso9899:2017
• iso9899:2018
  • ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so is supported to the same extent as C11.
• c2x — The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🙈
• gnu90
• gnu89
  • GNU dialect of ISO C90 (including some C99 features).
• gnu99
• gnu9x
  • GNU dialect of ISO C99. The name gnu9x is deprecated.
• gnu11
• gnu1x
  • GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17
• gnu18
  • GNU dialect of ISO C17. This is the default for C code. 👑
• gnu2x
  • The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete.
• c++98
• c++03
  • The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
• gnu++98
• gnu++03
  • GNU dialect of -std=c++98.
• c++11
• c++0x
  • The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
• gnu++11
• gnu++0x
  • GNU dialect of -std=c++11. The name gnu++0x is deprecated.
• c++14
• c++1y
  • The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
• gnu++14
• gnu++1y
  • GNU dialect of -std=c++14. The name gnu++1y is deprecated.
• c++17
• c++1z
  • The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
• gnu++17
• gnu++1z
  • GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is 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 name c++2a is deprecated. ⚠️
• gnu++20
• gnu++2a
  • GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.
• c++2b
• c++23
  • The next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀
• gnu++2b
• gnu++23
  • GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀

• -aux-info filename — Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. 🎀

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration. ✨

• -fallow-parameterless-variadic-functions — Accept variadic functions without named parameters.

Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. 🙈 This is only supported for C as this construct is allowed by C++.

• -fno-asm — Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. In C, -ansi implies -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. 💅

• -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 — bestie this one lets you have conditional expressions with mismatched types in the second and third arguments 💅. The value of such an expression is void. Ngl this is not supported for C++ though, so like, C++ girlies stay home 🙈.
• -ffreestanding — this asserts that compilation targets a freestanding environment. It implies -fno-builtin. A freestanding environment is one where the standard library may not even 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 — turns on parsing of function definitions marked with __GIMPLE. Lowkey experimental, it's for unit testing of GIMPLE passes 🧪.
• -fgnu-tm — when you pass -fgnu-tm, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future 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 — this option tells GCC to use the traditional GNU semantics for inline functions when in C99 mode.

Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions.

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. 🎀

• -fhosted — asserts that compilation targets a hosted environment ✨. This implies -fbuiltin. A hosted environment is one where the entire standard library is available, and where main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
• -flax-vector-conversions — allows implicit conversions between vectors with differing numbers of elements and/or incompatible element types. 🚩 This option should not be used for new code, fr fr.
• -fms-extensions — accepts 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, like:

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. iykyk.

• -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=disable form generates code only for the host fallback, while -foffload=target-list generates 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=options form. The target-list is a comma-separated list in the same format as for the -foffload= option. 💖

Typical command lines are

-foffload-options=-lgfortran -foffload-options=-lm
-foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm

• -fopenacc — enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in Fortran 🔥. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
• -fopenacc-dim=geom — specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of :-separated sizes, in order gang, worker and, vector. A size can be omitted, to use a target-specific default value. 💫
• -fopenmp — enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran 👑. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.
• -fopenmp-simd — enable handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored 🙈.
• -fpermitted-flt-eval-methods=style — ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of FLT_EVAL_METHOD. As such, code conforming to C11 may not have been written expecting the possibility of the new values. 😵‍💫

-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of FLT_EVAL_METHOD specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.

style is either c11 or ts-18661-3 as appropriate.

The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.

• -fplan9-extensions — accept some non-standard constructs used in Plan 9 code.

This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++ 🫶.

• -fsigned-bitfields
• -funsigned-bitfields
• -fno-signed-bitfields
• -fno-unsigned-bitfields — these options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types. ✨
• -fsigned-char — let the type char be signed, like signed char.

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

• -funsigned-char — let the type char be unsigned, like unsigned char.

Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.

Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. 💅

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

• -fsso-struct=endianness — set the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. 🚨 like, be careful bestie.

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🦋 Options Controlling C++ Dialect 🦋

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -fstrict-enums -O -c firstClass.C

In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

Here is a list of options that are only for compiling C++ programs: 🌸

• -fabi-version=n — use version n of the C++ ABI. The default is version 0.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. 😭

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument. 🔥

• Version 4 — first appeared in G++ 4.5, and it implements a standard mangling for vector types. 🦋
• Version 5 — first appeared in G++ 4.6, and it corrects the mangling of const/volatile attributes on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter. ✨
• Version 6 — first appeared in G++ 4.7, and it 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 — first appeared in G++ 4.8; it treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope. 🎀
• Version 8 — first appeared in G++ 4.9, and it corrects the substitution behavior of function types with function-cv-qualifiers. 🌸
• Version 9 — first appeared in G++ 5.2, and it corrects the alignment of nullptr_t. 💖
• Version 10 — first appeared in G++ 6.1, and it adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall). 🔥
• Version 11 — first appeared in G++ 7, and it corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors. 👑
• Version 12 — first appeared in G++ 8, and it 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 — first appeared in G++ 8.2, and it fixes the accidental change in version 12. 🫶
• Version 14 — first appeared in G++ 10, and it corrects the mangling of the nullptr expression. ✨
• Version 15 — first appeared in G++ 11, and it changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names. 💅

See also -Wabi. 👀

---

• -fabi-compat-version=n — On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. 💅

• -fno-access-control — Turn off all access checking. This switch is mainly useful for working around bugs in the access control code. 🙈
• -faligned-new — Enable support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t).

This flag is enabled by default for -std=c++17. ✨

• -fchar8_t, -fno-char8_t — Enable support for char8_t as adopted for C++20. This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros.

This option enables functions to be overloaded for ordinary and UTF-8 strings:

int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f("text");     // Calls #1
int v2 = f(u8"text");   // Calls #2

and introduces new signatures for user-defined literals:

int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;

The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified.

char ca[] = u8"xx";     // error: char-array initialized from wide
                        //        string
const char *cp = u8"xx";// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx");     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8"xx"};  // error: no matching function for call to
                        //        `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s;            // error: conversion from
                        //        `basic_string<char8_t>' to non-scalar
                        //        type `basic_string<char>' requested

• -fcheck-new — Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow). 💀
• -fconcepts, -fconcepts-ts — Below -std=c++20, -fconcepts enables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015).

With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on. But the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled by -fconcepts-ts. 🌈

• -fconstexpr-depth=n — Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. 🔥
• -fconstexpr-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 is 8. 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. ✨
• -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.

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 is 262144 (1<<18). 🌸
• -fconstexpr-ops-limit=n — Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). 🦋
• -fcoroutines — Enable support for the C++ coroutines extension (experimental). 💅
• -fno-elide-constructors — The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. 😭

• -fno-enforce-eh-specs — Don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. 💀
• -fextern-tls-init
• -fno-extern-tls-init — okay so the C++11 and OpenMP standards literally let thread_local and threadprivate variables have dynamic (runtime) initialization ✨. To make that happen, every single use of such a variable has to go through a lil wrapper function that does whatever init is needed. When the use and the definition of the variable are chilling in the same translation unit, that overhead can just be optimized away 🎀. But when the use is in a different translation unit? Significant overhead, bestie — even if the variable doesn't actually need dynamic initialization 😭. So if you as the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or because a use of the variable in the defining TU runs before any uses in another TU), you can dodge that overhead with -fno-extern-tls-init 💅.

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 — lets the C++ frontend fold calls to std::move, std::forward, std::addressof and std::as_const 🫶. Unlike inlining, this means no debug information gets generated for those calls. And since nobody's out here debugging these functions, this flag is enabled by default unless -fno-inline is active ✨.
• -fno-gnu-keywords — do not recognize typeof as a keyword, so your code can use that word as an identifier 🙈. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
• -fimplicit-constexpr — makes inline functions implicitly constexpr if they satisfy the requirements for a constexpr function 👑. Can be used in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations. Iconic.
• -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 gotta take care to structure your code so it includes all the necessary explicit instantiations, or else you get undefined symbols at link time. No cap.
• -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, don't emit out-of-line copies of inline functions controlled by #pragma implementation 💾. Heads up though: this causes linker errors if those functions aren't inlined everywhere they're called. The way that consequence is just sitting right there.
• -fmodules-ts
• -fno-modules-ts — enable support for C++20 modules 🌈. The -fno-modules-ts is usually not needed, since that's 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 from traditional C++ behavior, for good reasons 😌. However, it may cause 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, the CXX_MODULE_MAPPER environment variable is used, and if that's 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, like 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 is also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified.
• -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters 👑. This flag is enabled by default for -std=c++17.
• -fno-nonansi-builtins — disable built-in declarations of functions that are not mandated by ANSI/ISO C 🙅. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification 🫶. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. The semantic effect: an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected 😳.
• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords 🚫.
• -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue 🙈. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
• -fpermissive — downgrade some diagnostics about nonconformant code from errors to warnings 😌. So using -fpermissive lets some nonconforming code actually compile.
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so 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 makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to 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_cast and typeid) 💀. If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

Mixing code compiled with -frtti with that compiled with -fno-rtti may not work 😭. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti.

• -fsized-deallocation — enable the built-in global declarations

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

as introduced in C++14 ✨. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.

• -fstrict-enums — allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators) 🔥. This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.
• -fstrong-eval-order — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17 👑. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17.
• -ftemplate-backtrace-limit=n — set the maximum number of template instantiation notes for a single warning or error to n 📝. The default value is 10.
• -ftemplate-depth=n — set the maximum instantiation depth for template classes to n 🌸. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, since the compiler can run out of stack space before hitting 1024 in some situations 😭.
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics 🧵. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function 💅. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit.
• -fno-use-cxa-get-exception-ptr — bestie, this one says don't use the "__cxa_get_exception_ptr" runtime routine. 💀 The tea is that this makes "std::uncaught_exception" lowkey lie to you (it goes incorrect), but it's genuinely necessary if that runtime routine just isn't available on your system. Trade-off city.
• -fvisibility-inlines-hidden — this switch declares that you, the user, are not out here trying to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. ✨

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. it's giving main character optimization energy 👑

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. 🙈

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.

• -fvisibility-ms-compat — this flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. 🌈

The flag makes these changes to GCC's linkage model:

1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 😭

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

• -fno-weak — do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. fr fr, do not touch this one bestie 🚫
• -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). ✨
• -nostdinc++ — do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🎀
• -flang-info-include-translate, -flang-info-include-translate-not, -flang-info-include-translate=header — inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path. 🔍
• -flang-info-module-cmi, -flang-info-module-cmi=module — inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will not reading a specific module's CMI. module may 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++) and libc++ 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. 💅

---

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. 🏷️
• -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. ✨

• -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 will point 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) { }
};
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered

• -Wctor-dtor-privacy (C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🚪
• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall. ⚠️
• -Wdeprecated-copy (C++ and Objective-C++ only) — warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.
• -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:

enum E1 { e };
enum E2 { f };
int k = f - e;

-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.

• -Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:

enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;

-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.

• -Wno-init-list-lifetime (C++ and Objective-C++ only) — do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example: 😭
  • If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
  • If a new-expression creates an "initializer_list", the array only lives until the end of the enclosing full-expression, so the "initializer_list" in the heap has a dangling pointer.
• okay so when an initializer_list variable gets assigned from a brace-enclosed initializer list, the temporary array built for the right side of that assignment only lives until the end of the full-expression — meaning at the very next statement, your initializer_list variable is sitting there holding a dangling pointer. 💀

// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior

• and when a list constructor stores the begin pointer from its initializer_list argument, that does not extend the lifetime of the array — so if a class variable gets constructed from a temporary initializer_list, that pointer is left dangling by the time the variable declaration statement finishes. no cap, it just ends. 🙈

• -Winvalid-imported-macros — this one makes the compiler verify that all imported macro definitions are valid at the end of compilation. it is not enabled by default, because figuring it out needs extra processing. lowkey slay when you're preparing sets of header-units and you wanna make sure the macros stay consistent. ✨
• -Wno-literal-suffix (C++ and Objective-C++ only) — turns off the warning you'd get when a string or character literal is followed by a ud-suffix that doesn't start with an underscore. as a conforming extension, GCC treats those suffixes as separate preprocessing tokens, and it does that purely to stay backwards-compatible with code using formatting macros from <inttypes.h>. like:

#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 the warnings you get when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — because 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 get diagnosed by default, because the standard literally says so. a narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning — but -Wno-narrowing suppresses that diagnostic. ⚠️ heads up though: this does not change the meaning of well-formed code, and narrowing conversions are still considered ill-formed in SFINAE contexts. it's giving "silence the messenger, not the problem."

with -Wnarrowing in C++98, you get a warning when a narrowing conversion prohibited by C++11 happens inside { }, e.g.

int i = { 2.2 }; // error: narrowing from double to int

this flag is included in -Wall and -Wc++11-compat. 🔥

• -Wnoexcept (C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but which the compiler knows never throws an exception. the compiler's out here knowing things the signature doesn't admit. 💅
• -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature that makes noexcept part of a function type changes the mangled name of a symbol relative to C++14. enabled by -Wabi and -Wc++17-compat. ✨

for example:

template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }

in C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>. the way that one keyword rewrites the whole symbol name. 😭

• -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like memset or memcpy is an object of class type, and when writing into such an object might bypass the class's 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 the class's member functions. for example, the call to memset below is undefined because it modifies a non-trivial class object, and it's therefore diagnosed. the safe way to initialize or clear the storage of objects of these 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, either itself or in an accessible polymorphic base class — because in that case it's possible (but unsafe) to delete an instance of a derived class through a pointer to the class itself or to the base class. this warning is automatically enabled if -Weffc++ is specified. 👑
• -Wregister (C++ and Objective-C++ only) — warns on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension. using the register keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. enabled by default with -std=c++17. it's giving "we don't do that anymore, bestie." 💅
• -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers written in the code doesn't match the order in which they must actually be executed. for instance:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

the compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning to that effect. this warning is enabled by -Wall. ⭐

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this warning fires when a call to std::move prevents copy elision. a typical scenario where copy elision can happen is when you return from a function with a class return type, the expression being returned is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function's return type.

struct T {
...
};
T fn()
{
  T t;
  ...
  return std::move (t);
}

but in this example, the std::move call prevents copy elision. 😭 not you sabotaging your own optimization.

this warning is enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — this warning flags redundant calls to std::move — i.e. when a move operation would've been performed anyway, even without the std::move call. this happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable, where copy elision isn't applicable. consider:

struct T {
...
};
T fn(T t)
{
  ...
  return std::move (t);
}

here, the std::move call is redundant. because G++ implements Core Issue 1579, another example is:

struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
  T t;
  ...
  return std::move (t);
}

in this example, copy elision isn't applicable because the type of the 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) — this warning warns when a C++ range-based for-loop is making an unnecessary copy. that can happen when the range declaration isn't a reference, but probably should be. for example:

struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

it does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. 📏

this warning also fires when a loop variable in a range-based for-loop is initialized with a value of a different type, 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, and the reference const double & is bound to it. 😭 every single lap.

this warning is enabled by -Wall.

• -Wredundant-tags (C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. for example:

struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

on 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

iykyk — sometimes the keyword is load-bearing. 💅

• -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. if a type A depends on a type B with no linkage or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. if A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. the compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. ✨
• -Weffc++ (C++ and Objective-C++ only) — warns about violations of the following style guidelines from Scott Meyers' Effective C++ series of books: 📚
• Define a copy constructor and an assignment operator for classes that have dynamically-allocated memory. like, if you're hand-managing memory, bestie, you need both. 💅
• Prefer initialization to assignment in constructors. init lists are the moment, fr fr. ✨
• Have operator= return a reference to *this. don't be weird about it. 🫶
• Don't try to return a reference when you must return an object. nope, can't fake it, the object has to be built. 💀
• Distinguish between prefix and postfix forms</b> of increment and decrement operators. they are NOT the same girl, they have different signatures. 🙈
• Never overload &&, ||, or , — just don't, the short-circuit vibe dies and everything is cursed. 😭

This option also switches on -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. but plot twist — the check is extended to also warn about the lack of a virtual destructor in accessible non-polymorphic base classes too. so it's that girl and then some. 👑

When you pick this option, be aware that the standard library headers do not obey all of these guidelines. so you'll get warnings from the STL and be like "why me". use grep -v to filter those ones out, bestie, it's the only way. 🎀

---

• -Wno-exceptions (C++ and Objective-C++ only) — turns off the warning about the case when an exception handler is shadowed by another handler. this one's actually useful because it can point out a wrong ordering of your exception handlers, so maybe think twice before silencing it. 👀
• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about using an uncasted NULL as a sentinel. when you're compiling with GCC alone this is a totally valid sentinel, because NULL is defined to __null. and even though it's a null pointer constant rather than a null pointer, it is guaranteed to be the same size as a pointer. but — and this is the tea — that use is not portable across different compilers. 🫠
• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared inside a template. back in very old versions of GCC, before the ISO standard got implemented, declarations like friend int foo(int), where the name of the friend is an unqualified-id, could be read as a particular specialization of a template function. the warning exists to diagnose compatibility problems, and it is enabled by default. 🧚
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type shows up inside a C++ program. the new-style casts — dynamic_cast, static_cast, reinterpret_cast, and const_cast — are way less vulnerable to unintended effects and much easier to search for. glow up your casts, periodt. ✨
• -Woverloaded-virtual (C++ and Objective-C++ only) — warns when a function declaration hides virtual functions from a base class. for example, in:

struct A {
  virtual void f();
};

struct B: public A {
  void f(int);
};

the A class version of f is hidden in B, and code like:

B* b;
b->f();

fails to compile. 💀 classic.

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function into a plain pointer. nope, not that. 🙅
• -Wsign-promo (C++ and Objective-C++ only) — warns 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. so it is what it is. 🤷
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. some coding rules straight up disallow templates, and this can be used to enforce that rule. the warning is inactive inside a system header file, such as the STL, so you can still use the STL. you may also instantiate or specialize templates. 🦋
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. this includes invocations of C++ operator delete with pointers returned from either mismatched forms of operator new, or from other functions that allocate objects for which the operator delete isn't a suitable deallocator — as well as calls to other deallocation functions with pointers returned from operator new for which the deallocation function isn't suitable. basically: don't pair up new and delete like they're a random couple, they have to match. 💔

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
}

the related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete. 🫶

-Wmismatched-new-delete is included in -Wall.

• -Wmismatched-tags (C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided. 🎯

for example, the declaration of struct Object in the argument list of draw triggers the warning. to avoid it, either remove the redundant class-key struct or replace it with class to match its definition.

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

it is not wrong to declare a class with the class-key struct as the example above shows. the -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. in code that is intended to be portable to Windows-based compilers, the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. the option can be used either on its own or in conjunction with -Wredundant-tags. 🌈

• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. some coding rules disallow multiple inheritance, and this may be used to enforce that rule. the warning is inactive inside a system header file, such as the STL, so one can still use the STL. one may also define classes that indirectly use multiple inheritance. 👑
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. some coding rules disallow multiple inheritance, and this may be used to enforce that rule. the warning is inactive inside a system header file, such as the STL, so one can still use the STL. one may also define classes that indirectly use virtual inheritance. 💜
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. this is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. if the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. so like... only if you know. 😭
• -Wnamespaces — warns when a namespace definition is opened. some coding rules disallow namespaces, and this may be used to enforce that rule. the warning is inactive inside a system header file, such as the STL, so one can still use the STL. one may also use using directives and qualified names. 🎀
• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. yikes, but okay. 💀
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns 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 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; such a conversion function will never be called. dead code behavior, honestly. 🙈
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. this includes postfix and prefix ++ and -- expressions of volatile-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 type, volatile-qualified parameter type, and 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) — warns when a literal 0 is used as null pointer constant. this can be useful to facilitate the conversion to nullptr in C++11. say it with me: nullptr, not 0. ✨
• -Waligned-new — warns about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. this option is enabled by -Wall.

normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 👑

• -Wno-placement-new, -Wplacement-new=n — warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. for example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. 😭

char buf [64];
new (buf) int[64];

this warning is enabled by default.

• -Wplacement-new=1 — this one is the default level for -Wplacement-new. At level 1, GCC keeps its mouth shut about some constructs that are strictly undefined but that it lets slide as extensions, for compatibility with legacy code. For example, the new expression below 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, on top of diagnosing all the same constructs as 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. So the example above would get diagnosed, but the construct below 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) — warns about catch handlers that don't catch via reference. Breakdown bestie:
  • with -Wcatch-value=1 (or just -Wcatch-value for short) it warns about polymorphic class types that are caught by value.
  • with -Wcatch-value=2 it warns about all class types that are caught by value.
  • with -Wcatch-value=3 it warns about all types that are not caught by reference.
  • -Wcatch-value is enabled by -Wall. 💅
• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs. iykyk
• -Wno-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. so yes, this one is the "please let me live dangerously" switch 🙈
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons after in-class function definitions. the little strays ; ; ;
• -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: 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 — suppresses 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. the ... doesn't come along for the ride, periodt
• -Wno-invalid-offsetof (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

The restrictions on offsetof may be relaxed in a future version of the C++ standard.

• -Wsized-deallocation (C++ and Objective-C++ only) — warns 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. matched set energy ✨

• -Wsuggest-final-types — warns about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete.
• -Wsuggest-final-methods — warns about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. Recommended order of operations: first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. 🫶
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. just say the word bestie
• -Wuse-after-free, -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. The warning is enabled at all optimization levels but may yield different results with optimization than without.
  • -Wuse-after-free=1 — at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-free calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
  • -Wuse-after-free=2 — at level 2, in addition to 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. 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 appear after calls to realloc as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid legacy code gradually transition to safe alternatives. 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 avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. 👑

• -Wuse-after-free=2 is included in -Wall.

• -Wuseless-cast (C++ and Objective-C++ only) — warns when an expression is casted to its own type. casting something to the thing it already is... not the vibe 💀
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default.

---

🌈 Options Controlling Objective-C and Objective-C++ Dialects 🌈

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi). 🦋

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. the chill default 🫶
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used. if and only if, no cap
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
• -fobjc-abi-version=n — picks version n of the Objective-C ABI for whichever runtime you're on, bestie. Heads up: this is only supported for the NeXT runtime rn. So, Version 0 = the traditional (32-bit) ABI, no properties, no Objective-C 2.0 extras, kinda vintage. Version 1 = traditional (32-bit) ABI but ✨with✨ properties and other Objective-C 2.0 additions. Version 2 = the modern (64-bit) ABI, the glow-up. If you say 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 — okay so for every Objective-C class, it checks whether any instance variable is a C++ object with a non-trivial default constructor. If yes, it synthesizes a special - (id) .cxx_construct instance method that runs those non-trivial default constructors on all such instance variables, in order, and then returns self. Same energy for destructors: it checks if any instance variable is a C++ object with a non-trivial destructor, and if so synthesizes a special - (void) .cxx_destruct method that runs all those default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way 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 has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. 🫶

• -fobjc-direct-dispatch — lets you do fast jumps to the message dispatcher. On Darwin this happens via the comm page. zoomy.
• -fobjc-exceptions — turns on syntactic support for structured exception handling in Objective-C, similar to what C++ offers. You need this option to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. Available with both the GNU runtime and the NeXT runtime (but not available together with the NeXT runtime on Mac OS X 10.2 and earlier, sorry).
• -fobjc-gc — enables garbage collection (GC) in Objective-C and Objective-C++ programs. Only available with the NeXT runtime, fr — the GNU runtime has a different garbage collection implementation that doesn't need special compiler flags.
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, it 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. Class methods and super calls are never checked for nil this way, no matter what this flag is set to. Currently this flag does literally nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language GCC 4.0 recognized. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards — those are controlled by the separate C/C++ dialect option flags, okay? When you use this with the Objective-C or Objective-C++ compiler, any Objective-C syntax GCC 4.0 doesn't recognize gets rejected. Useful if you need to make sure your Objective-C code compiles with older versions of GCC.
• -freplace-objc-classes — emits a special marker telling ld(1) not to statically link in the resulting object file, and letting dyld(1) load it at run time instead. Used together with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded while the program is running, without needing 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 ordinarily replaces calls to objc_getClass("...") (when the class name is known at compile time) with static class references initialized at load time, which improves run-time performance. Specifying -fzero-link suppresses that behavior and keeps calls to objc_getClass("...") around. Useful in Zero-Link debugging mode, since it lets individual class implementations be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. iykyk.
• -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from inside the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying -fno-local-ivars disables that behavior, thus avoiding variable shadowing issues. slay.
• -fivar-visibility=[public|protected|private|package] — sets the default instance variable visibility to the option you picked, so 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 to a file named sourcename.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) — do not warn if a property for an Objective-C object has no assign semantics specified.
• -Wno-protocol (Objective-C and Objective-C++ only) — 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. If you use -Wno-protocol, then methods inherited from the superclass count as implemented, and no warning is issued for them.
• -Wobjc-root-class (Objective-C and Objective-C++ only) — warns if a class interface lacks a superclass. Most classes will inherit from NSObject (or Object) for example. When declaring classes intended 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) — warns if multiple methods of different types for the same selector are found during compilation. The check happens on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation isn't reached — for example because an error is found during compilation, or because the -fsyntax-only option is being used.
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns if multiple methods with differing argument and/or return types are found for a given selector when you try to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns if a @selector(...) expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used. 👑
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if any.

---

✨ Options to Control Diagnostic Messages Formatting ✨

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location info should be reported. Note that some language front ends may not honor these options, smh.

• -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 is done; each error message appears on a single line. This is the default for all front ends.

Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas. 💅

• -fdiagnostics-plain-output — asks that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer it to remain more stable over time. -fdiagnostics-plain-output is currently equivalent to the following options:

-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers
-fdiagnostics-color=never -fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events

In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. 🌈

• -fdiagnostics-show-location=once — okay so this one only matters in line-wrapping mode, periodt. It tells the diagnostics reporter to drop the source location info once — like, if a message is too long to fit on one physical line and has to wrap, the source location will not get spammed again as a prefix on every continuation line after it. This is literally the default behavior bestie, it's the chill one. ✨
• -fdiagnostics-show-location=every-line — also only meaningful in line-wrapping mode. This one makes the reporter emit that same source location info (as a prefix) on every physical line that comes out of breaking a message that was too long to fit on one line. Every single one. No cap. 🫶
• -fdiagnostics-color[=WHEN] and -fno-diagnostics-color — COLOR IN YOUR DIAGNOSTICS, FINALLY 💅 WHEN is one of never, always, or auto. The default depends on how the compiler was configured, and it can be any of those WHEN options — or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when standard error is a terminal, and when it's not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are just aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. Iconic.

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 (go check the section in your text terminal's docs for the permitted values and what they mean as character attributes, bestie). 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

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. The supported capabilities are as follows, babes:

• error= — SGR substring for error: markers.
• warning= — SGR substring for warning: markers.
• note= — SGR substring for note: 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:line or file:line:column etc.
• quote= — SGR substring for information printed within quotes.
• fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced.
• fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
• diff-filename= — SGR substring for filename headers within generated patches.
• diff-hunk= — SGR substring for the starts of hunks within generated patches.
• diff-delete= — SGR substring for deleted lines within generated patches.
• diff-insert= — SGR substring for inserted lines within generated patches.
• type-diff= — SGR substring for highlighting mismatching types within template arguments in the C++ frontend.

• -fdiagnostics-urls[=WHEN] — bestie we're embedding URLs in your diagnostics via escape sequences 🔗. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, it'll embed a URL for the documentation of that option. Slay.

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 which is known to be incompatible with this feature — see below, iykyk.

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 GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

However, even with -fdiagnostics-urls=always the behavior is dependent 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 bestie: ST is an ANSI escape sequence, string terminator ESC , BEL is an ASCII character, CTRL-G that usually sounds like a beep. 😭

At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are 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 the -fdiagnostics-urls=always. 🙈

• -fno-diagnostics-show-option — by default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery, that is). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. Poof, gone. ✨
• -fno-diagnostics-show-caret — by default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this info. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width. 💅
• -fno-diagnostics-show-labels — by default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

This option suppresses the printing of these labels (in the example above, the vertical bars and the char * and long int text). Bye bye labels 👋

• -fno-diagnostics-show-cwe — diagnostic messages can optionally have an associated @url{https://cwe.mitre.org/index.html, CWE} identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata. 🎀
• -fno-diagnostics-show-line-numbers — by default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed showing line numbers. This option suppresses this left margin. No more numbers, bestie. 🔥
• -fdiagnostics-minimum-margin-width=width — this option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. 🦋
• -fdiagnostics-parseable-fixits — emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string fix-it:. For example:

fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:

00000000011111111112222222222
12345678901234567890123456789
  gtk_widget_showall (dlg);
  ^^^^^^^^^^^^^^^^^^
  gtk_widget_show_all

The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", non-printable characters as octal (e.g. vertical tab as "\013").

An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted at the given position. So clever fr fr 💖

• -fdiagnostics-generate-patch — print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:

--- test.c
+++ test.c
@ -42,5 +42,5 @

 void show_cb(GtkDialog *dlg)
 {
-  gtk_widget_showall(dlg);
+  gtk_widget_show_all(dlg);
 }

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). 🌈

• -fdiagnostics-show-template-tree — in the C++ frontend, when printing diagnostics showing mismatching template types, such as:

          could not convert 'std::map<int, std::vector<double> >()'
            from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:

          map<
            [...],
            vector<
              [double != float]>>

Tree structure, main character energy 👑

The parts that differ are highlighted with color ("double" and "float" in this case, bestie 👀).

---

• -fno-elide-type — okay so by default, when the C++ frontend prints diagnostics showing mismatching template types, the parts of the types that are all matchy-matchy get printed as "[...]" to keep the error message from being a whole novel. Like, 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 — no more shorthand, you get the whole thing. This flag also affects the output of the -fdiagnostics-show-template-tree flag. periodt 💅

• -fdiagnostics-path-format=KIND — tells GCC how to print paths of control-flow events for diagnostics that have such a path attached to them. 🛤️

KIND is none, separate-events, or inline-events, the default.

none means don't print diagnostic paths at all. Not me, not the path, we don't know her.

separate-events means 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 1

inline-events means 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. it's giving annotated source, honestly iconic ✨

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. stack frames getting their own little family tree 🫶

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 option gives you extra info when printing control-flow paths associated with a diagnostic. 🔍

If this is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=separate-events.

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. devs only, bestie — it's giving internal tools energy 🧚

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. poor dejagnu can't read columns, bless 😭
• -fdiagnostics-column-unit=UNIT — pick 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, in the case of tab characters, or it may be smaller, in the case of 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. one little smiley = four whole bytes, the math is mathing 🙂✨

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. nostalgia era, fr fr

• -fdiagnostics-column-origin=ORIGIN — choose the origin for column numbers, i.e. the column number assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified. 🔢
• -fdiagnostics-escape-format=FORMAT — when GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines. 🦋

This option controls how such bytes should be escaped.

The default FORMAT, unicode displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX>.

For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:

         before<U+03C0><BF>after

Setting 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

same vibe, raw bytes edition 💀

• -fdiagnostics-format=FORMAT — select a different format for printing diagnostics. FORMAT is text or json. The default is text.

The json format consists of a top-level JSON array containing JSON objects representing the diagnostics. structured, machine-readable, gorgeous 🌈

The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity. not one single newline of love, we're being efficient here

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 that's "warning", bestie, then there's an "option" key describing the command-line option that's 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 plus 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-unit option.

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;
              |               ^~~~~~
              |               color

might be printed in JSON form as:

               {
                   "children": [],
                   "fixits": [
                       {
                           "next": {
                               "column": 21,
                               "file": "demo.c",
                               "line": 8
                           },
                           "start": {
                               "column": 15,
                               "file": "demo.c",
                               "line": 8
                           },
                           "string": "color"
                       }
                   ],
                   "kind": "error",
                   "locations": [
                       {
                           "caret": {
                               "column": 15,
                               "file": "demo.c",
                               "line": 8
                           },
                           "finish": {
                               "column": 20,
                               "file": "demo.c",
                               "line": 8
                           }
                       }
                   ],
                   "escape-source": false,
                   "message": "\u2018struct s\u2019 has no member named ..."
               }

where the fix-it hint suggests replacing the text from "start" up to but not including "next" with "string"'s value. Deletions are expressed via an empty value for "string", insertions by having "start" equal "next". 🎀

If the diagnostic has a path of control-flow events associated with it, it has a "path" array of objects representing the events. Each event object has a "description" string, a "location" object, along with a "function" string and a "depth" number for representing interprocedural paths. The "function" represents the current function at that event, and the "depth" represents the stack depth relative to some baseline: the higher, the more frames are within the stack. 🧚

For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:

               "path": [
                   {
                       "depth": 0,
                       "description": "when 'PyList_New' fails, returning NULL",
                       "function": "test",
                       "location": {
                           "column": 10,
                           "file": "test.c",
                           "line": 25
                       }
                   },
                   {
                       "depth": 0,
                       "description": "when 'i < count'",
                       "function": "test",
                       "location": {
                           "column": 3,
                           "file": "test.c",
                           "line": 27
                       }
                   },
                   {
                       "depth": 0,
                       "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
                       "function": "test",
                       "location": {
                           "column": 5,
                           "file": "test.c",
                           "line": 29
                       }
                   }
               ]

Diagnostics have a boolean attribute "escape-source", hinting whether non-ASCII bytes should be escaped when printing the pertinent lines of source code ("true" for diagnostics involving source encoding issues). 🌈

---

✨ 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, not a crime, but the vibe is off, bestie. 👀

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 job. 🫶
• -fmax-errors=n — Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. She said what she said! 🔥
• -w — Inhibit all warning messages. Not me silencing the haters... 💀
• -Werror — Make all warnings into errors. Every little warning is a whole crime scene now, no cap. 🚨
• -Werror= — okay bestie, this one turns a specific warning into a full-on error. You slap the specifier on the end, so -Werror=switch makes the warnings that -Wswitch controls become errors. And it has a negative form too, for when you wanna cancel -Werror for one lil warning only — like -Wno-error=switch makes -Wswitch warnings not be errors, even while -Werror is out here running the whole show. 💅

Every warning message that's controllable shows you the option controlling it, and you can then feed that option straight into -Werror= and -Wno-error= like we just said. (Wanna hide that option from the message? -fno-diagnostics-show-option has your back.)

Heads up though, fr fr: writing -Werror=foo automatically implies -Wfoo. But -Wno-error=foo implies nothing at all. Not a thing. ✨

• -Wfatal-errors — the compiler just gives up. It aborts compilation on the first error instead of trying to keep going and printing more error messages. Dramatic? Yes. Effective? Also yes. 💀

---

So here's the vibe with warnings in general, bestie: you can request a whole bunch of specific warnings with options that start with -W, like -Wimplicit to ask for warnings on implicit declarations. And every one of these specific warning options has a negative form starting with -Wno- to turn them off — for example -Wno-implicit. This manual only lists one of the two forms, whichever one isn't the default. For language-specific options you also wanna check C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. And yes, you can get even more warnings by enabling the static analyzer. 👑

Some options, like -Wall and -Wextra, turn on other options, like -Wunused, which may turn on further options, like -Wunused-value. The combined effect of positive and negative forms is that more specific options win over less specific ones, no matter where they sit on the command line. For options of the same specificity, the last one takes effect. And options enabled or disabled via pragmas take effect as if they showed up at the very end of the command line. 🌈

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic saying the option isn't recognized. BUT — and this is the plot twist — if you use the -Wno- form, things get slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are already being produced. This is what lets you use new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. 🫶

The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization, and 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 — issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that don't follow ISO C and ISO C++. For ISO C, it follows the version of the ISO C standard specified by any -std option used. 🎀

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they get rejected. ✋

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. 🚫

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all — only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added. 😬

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future. Not happening, bestie. 💅

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there's a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they're required by the base standard. (It doesn't make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) ✨

• -pedantic-errors — give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there's undefined behavior at compile-time, and in some other cases that don't prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter, and vice versa. They're different, periodt. 🔥
• -Wall — this enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 👑

-Wall turns on the following warning flags:

-Waddress -Warray-bounds=1 (only with -O2) -Warray-compare
-Warray-parameter=2 (C and Objective-C only) -Wbool-compare
-Wbool-operation -Wc++11-compat  -Wc++14-compat -Wcatch-value (C++
and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wdangling-pointer=2 -Wduplicate-decl-specifier (C and Objective-C
only) -Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat -Wformat-overflow -Wformat-truncation -Wint-in-bool-context
-Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-
C only) -Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only
for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
(only for C/C++) -Wmismatched-dealloc -Wmismatched-new-delete (only
for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull
-Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move
(only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++)
-Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare
(only in C++) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wuse-after-free=3 -Wvla-parameter (C and
Objective-C only) -Wvolatile-register-var -Wzero-length-bounds

Note that 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 wish to 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 enables some extra warning flags that aren't enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive. glow-up 💅)

-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wenum-conversion (C only) -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C only)
-Woverride-init -Wsign-compare (C only) -Wstring-compare
-Wredundant-move (only for C++) -Wtype-limits -Wuninitialized
-Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the following cases:

• A pointer is compared against integer zero with "<", "<=", ">", or ">=".
• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array that has been declared "register".
• (C++ only) Taking the address of a variable that has been declared "register".
• (C++ only) A base class is not initialized in the copy constructor of a derived class.

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — warn about code affected by ABI changes. This 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 warns only 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. -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. There may also be cases where warnings are emitted even though the code that is generated is compatible. 😭

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers. bestie please just fix the code 💅

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include: 👇

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

• SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

• "__attribute ((const))" and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int. 🫠

Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t.

These issues were corrected in -fabi-version=7.

• When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.

This was fixed in -fabi-version=8, the default for GCC 5.1.

• decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment.

This was fixed in -fabi-version=9, the default for GCC 5.2.

• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

This was fixed in -fabi-version=10, the default for GCC 6.1.

This option also enables warnings about psABI-related changes. The known psABI changes at this point include: 👀

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. Prior to GCC 4.4, this was not the case. For example:

union U {
  long double ld;
  int i;
};

union U is now always passed in memory.

---

✨ -Wchar-subscripts ✨

Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall. 🎀

✨ -Wno-coverage-mismatch ✨

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 💀

✨ -Wno-coverage-invalid-line-number ✨

Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with --coverage enabled. 🦋

By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error.

✨ -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) ✨

Suppress warning messages emitted by "#warning" directives. 🤫

✨ -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) ✨

Give a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation. 😮‍💨

It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double because the floating-point literal is a double. 🍓

✨ -Wduplicate-decl-specifier (C and Objective-C only) ✨

Warn if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. 💅

✨ -Wformat ✨

✨ -Wformat=n ✨

Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin. 🔍

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). 🌸

-Wformat=1

-Wformat

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. ✨

-Wformat=2

Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. 🔥

✨ -Wno-format-contains-nul ✨

If -Wformat is specified, do not warn about format strings that contain NUL bytes.

✨ -Wno-format-extra-args ✨

If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored. 🙈

Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. 💜

✨ -Wformat-overflow ✨

✨ -Wformat-overflow=level ✨

Warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. 😬

-Wformat-overflow

-Wformat-overflow=1

Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 🌈

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

• -Wformat-overflow=2 — Level 2 is like, the overachiever — it warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. No cap, it's just being cautious.

At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. So helpful!

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

• -Wno-format-zero-length — If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. So chill.
• -Wformat-nonliteral — If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. Basically, "can't check it if it's not literal."
• -Wformat-security — If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) Security vibes, bestie!
• -Wformat-signedness — If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. Signed vs unsigned drama.
• -Wformat-truncation
• -Wformat-truncation=level — Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow. Truncation station! 🚂
  • -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. Conservative 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. Level 2: the sequel.
• -Wformat-y2k — If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. Y2K throwback! 🎉

---

• -Wnonnull — Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option. Null? Not on my watch. 💅

• -Wnonnull-compare — Warn when comparing an argument marked with the nonnull function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option. Comparing nonnull to null? Rude.

• -Wnull-dereference — Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. Dereferencing null is a no-no. 🚫
• -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-recursion is included in -Wall. Infinite loop? More like infinite oops. 🔁
• -Winit-self (C, C++, Objective-C and Objective-C++ only) — Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

This warning is enabled by -Wall in C++. Self-init? That's a self-own. 🙈

• -Wno-implicit-int (C and Objective-C only) — This option controls warnings when a declaration does not specify a type. This warning is enabled by default in C99 and later dialects of C, and also by -Wall. No type? That's a no from me.
• -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 in C99 and later dialects of C, and also by -Wall. The warning is made into an error by -pedantic-errors. Use before declare? Straight to jail. 🚔
• -Wimplicit (C and Objective-C only) — Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. All the implicit things!
• -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. Fallthrough, but make it default.
• -Wimplicit-fallthrough=n — Warn when a switch case falls through. For example:

switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

switch (cond)
  {
  case 1:
    if (i > 3) {
      bar (5);
      break;
    } else if (i < 1) {
      bar (0);
    } else
      return;
  default:
    ...
  }

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:

switch (cond)
  {
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    ...
  }

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches .*falls?[ \t-]*thr(ough|u).* regular expression.
• -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
• -Wimplicit-fallthrough=4 case sensitively matches one of the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t]*FALLTHR(OUGH|U)[ \t]*"
• -Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments by case or default keywords or by a user label that precedes some case or default label.

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    ...
  }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. Fallthrough realness! 🌈

-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)

okay so this one is the vibe check for warnings that pop off because of the warn_if_not_aligned attribute. It controls whether those warnings get issued. These warnings are enabled by default. So yeah, they're already on, bestie — no cap. 💅

---

-Wignored-qualifiers (C and C++ only)

Warn if the return type of a function has a type qualifier like const. For ISO C, that kind of qualifier literally has no effect, because the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 🫶

This warning is also enabled by -Wextra.

---

-Wno-ignored-attributes (C and C++ only)

This option controls warnings when an attribute is ignored. It's different from -Wattributes because it warns whenever the compiler decides to drop an attribute — not when the attribute is unknown, used in the wrong place, etc. This warning is enabled by default.

---

-Wmain

Warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic. ✨

---

-Wmisleading-indentation (C and C++ only)

Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation. 😭

In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional.

                     if (some_condition ())
                       foo ();
                       bar ();  /* Gotcha: this is not guarded by the "if".  */

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

                     if (flagA)
                       foo (0);
                   #if SOME_CONDITION_THAT_DOES_NOT_HOLD
                     if (flagB)
                   #endif
                       foo (1);

The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

This warning is enabled by -Wall in C and C++. 🔥

---

-Wmissing-attributes

Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. 👑

In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning.

You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.

                   template <class T>
                   T* __attribute__ ((malloc, alloc_size (1)))
                   allocate (size_t);

                   template <>
                   void* __attribute__ ((malloc))   // missing alloc_size
                   allocate<void> (size_t);

---

-Wmissing-braces

Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.

                   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)

Warn 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. 🦋

---

-Wno-missing-profile

This option controls warnings if feedback profiles are missing when using the -fprofile-use option. This option diagnoses those cases where a new function or a new file is added 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. 💀

---

-Wmismatched-dealloc

Warn 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
                   }

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

Warn about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, in which only the first statement is actually guarded after the macro is expanded. 🙈

For example:

                   #define DOIT x++; y++
                   if (c)
                     DOIT;

will increment y unconditionally, not just when c holds. The can usually be fixed by wrapping the macro in a do-while loop:

                   #define DOIT do { x++; y++; } while (0)
                   if (c)
                     DOIT;

This warning is enabled by -Wall in C and C++.

---

-Wparentheses

Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. 😵‍💫

Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.

Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.

For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:

                   {
                     // Declares a local variable called mymutex.
                     std::unique_lock<std::mutex> (mymutex);
                     // User meant std::unique_lock<std::mutex> lock (mymutex);
                   }

This warning is enabled by -Wall. 🌈

---

-Wsequence-point

Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. ⭐

okay so 💅 here's the thing that makes everyone's brain hurt: the C standard does not specify when, between sequence points, modifications to the values of objects actually take effect. So if your program's behavior depends on that timing, you've got undefined behavior on your hands and that is not a vibe. The C and C++ standards literally say:

"Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored."

And if a program breaks those rules? the results on any particular implementation are entirely unpredictable 😭. No take-backs, no do-overs, no apology tour.

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. So like, not perfect, but she's serving 💖.

Now for the glow-up era: 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++. Protective queen behavior, honestly 🫶.

The standard is worded confusingly (shocking, I know 🫠), 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> — go read it, it's giving academic drama 📚.

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 bestie, that dangling pointer is officially a you problem 💀.
• -Wreturn-type — warn whenever a function is defined with a return type that defaults to int. Also warn about any return statement with no return value in a function whose return type is not void (and yes, falling off the end of the function body counts as returning without a value 🫣).

For C only, warn about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used. And listen: 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. Yeah. Even if you never look at it. Rude 👑.

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 — warn if left shifting a negative value. This warning is enabled by -Wextra in 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 — warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used (even if there is a default label). This warning is enabled by -Wall.
• -Wswitch-default — warn whenever a switch statement does not have a default case.
• -Wswitch-enum — warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.
• -Wno-switch-bool — do not warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

switch ((int) (a == 4))
  {
  ...
  }

This warning is enabled by default for C and C++ programs.

• -Wno-switch-outside-range — this option controls warnings when a switch case 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 a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

switch (cond)
  {
   i = 15;
  ...
   case 5:
  ...
  }

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

switch (cond)
  {
   int i;
  ...
   case 5:
   i = 5;
  ...
  }

This warning is enabled by default for C and C++ programs.

• -Wsync-nand (C and C++ only) — warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4.
• -Wtrivial-auto-var-init — warn when -ftrivial-auto-var-init cannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of a switch statement.
• -Wunused-but-set-parameter — warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). To suppress this warning use the unused attribute. This warning is also enabled by -Wunused together with -Wextra.
• -Wunused-but-set-variable — warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall. To suppress this warning use the unused attribute. This warning is also enabled by -Wunused, which is enabled by -Wall.
• -Wunused-function — warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall.
• -Wunused-label — warn whenever a label is declared but not used. This warning is enabled by -Wall. To suppress this warning use the unused attribute.
• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
• -Wunused-parameter — warn whenever a function parameter is unused aside from its declaration. To suppress this warning use the unused attribute.
• -Wno-unused-result — do not warn if a caller of a function marked with attribute warn_unused_result does not use its return value. The default is -Wunused-result.
• -Wunused-variable — warn whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall. To suppress this warning use the unused attribute.
• -Wunused-const-variable, -Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration. -Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of #defines. To suppress this warning use the unused attribute.
  • -Wunused-const-variable=1 — this is the warning level that is enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
  • -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested since in C++ this isn't an error and in C it might be harder to clean up all headers included.
• -Wunused-value — warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not. This warning is enabled by -Wall.
• -Wunused — all the above -Wunused options combined 💫. In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
• -Wuninitialized — warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-static const member appears in a class without constructors.

Like, imagine handing a pointer (or in C++, a reference) to an object that's just vibing uninitialized over to a const-qualified argument of a built-in function that GCC knows reads the object? Yeah, that gets diagnosed too. (-Wmaybe-uninitialized is what fires up for ordinary functions, bestie.)

If you're specifically wanting a heads-up about code that uses a variable's own uninitialized value inside its own initializer, that's a whole different option — go grab -Winit-self for that one. 🫶

These warnings pop off for individual uninitialized elements of structure, union or array variables, and also for variables that are uninitialized as a whole. They do not show up for variables or elements declared volatile — no cap. And because these warnings literally depend on optimization, the exact variables or elements you get warned about depend on the precise optimization options and the exact version of GCC you're running. So like, don't be shocked when your friend's build yells at you and yours doesn't. 😭

Heads up: there may be no warning at all about a variable that's only used to compute a value that itself is never used, because those computations can get yeeted by data flow analysis before the warnings are even printed. Sneaky.

In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about b being uninitialized in this little 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 when they get a memory consistency argument that's either invalid for the operation or straight-up outside the range of values of the memory_order enumeration. Like, 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 gets diagnosed:

void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default. 💅

• -Wmaybe-uninitialized — okay so for an object with automatic or allocated storage duration: if there exists a path from the function entry to a use of the object that is initialized, but there are some other paths where the object isn't initialized, the compiler emits a warning if it cannot prove the uninitialized paths aren't executed at run time. Living on the edge fr.

On top of that, 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) tells GCC the argument isn't used to access the object, and that avoids the warning. ✨

These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables at all. Periodt.

These warnings are made optional because GCC may not be able to figure out when the code is actually correct despite looking like it has an error. Here's one example of how that goes down:

{
  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 only sees the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there's in fact no problem, because longjmp cannot in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warn when a #pragma directive is encountered that GCC doesn't understand. And bestie, if this command-line option is used, warnings are even issued for unknown pragmas in system header files. That is not the case if the warnings are only enabled by the -Wall command-line option. She's extra like that. 💅
• -Wno-pragmas — do not warn about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. 🦋
• -Wno-prio-ctor-dtor — do not warn if a priority from 0 to 100 is used for a constructor or destructor. The use of constructor and destructor attributes lets you assign a priority to the constructor/destructor to control its order of execution before main is called or after it returns. The priority values must be greater than 100, since the compiler reserves priority values between 0--100 for the implementation. 👑
• -Wstrict-aliasing — this option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. The warning does not catch all cases, but it does attempt to catch the more common pitfalls. It's included in -Wall. It is equivalent to -Wstrict-aliasing=3. 🌈
• -Wstrict-aliasing=n — same deal, only active when -fstrict-aliasing is active, and it warns about code that might break the strict aliasing rules the compiler is using for optimization. Higher levels = higher accuracy (fewer false positives). Higher levels also = more effort, kinda like how -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
  • Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, since it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
  • Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
  • Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
• -Wstrict-overflow
• -Wstrict-overflow=n — this option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. So this warning depends on the 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 does, in fact, occur. So this warning can easily give a false positive: a warning about code that's not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

• -Wstrict-overflow=1 — warn about cases that are both questionable and easy to avoid. For example the compiler simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Wstrict-overflow=2 — also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
• -Wstrict-overflow=4 — also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.
• -Wstrict-overflow=5 — also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 💀

• -Wstring-compare — warn 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. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array a.

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 — okay bestie this one is the buffer-overflow snitch 💅 It warns you about 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 it does to figure out the size of the destination. That argument only really matters for functions that operate on character arrays — raw memory functions like memcpy always use Object Size type-0, so the arg is meaningless there. This option also yells at you for calls that specify a size bigger than the largest possible object, or at most SIZE_MAX / 2 bytes. It gives its best results with optimization turned on, but it can still catch a small subset of simple buffer overflows even without optimization, in calls to GCC built-in functions like __builtin_memcpy that correspond to the standard functions. Either way, it only warns about a subset of the buffer overflows that the corresponding overflow-checking built-ins catch. For example, it warns on the strcpy call below because that call copies at least 5 characters (the string "blue", including the terminating NUL) into a buffer of size 4. 😭

enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
  static char buf [4];
  const char *str;
  switch (clr)
    {
      case blue: str = "blue"; break;
      case purple: str = "purple"; break;
      case yellow: str = "yellow"; break;
    }

  return strcpy (buf, str);   // warning here
}

Heads up bestie:</p purple> -Wstringop-overflow=2 is enabled by default. ✨

---

• -Wstringop-overflow, -Wstringop-overflow=1 — This one uses type-zero Object Size Checking to figure out the sizes of destination objects. At this setting it 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 could be one of several objects, it assumes it's the largest one of them. On Linux systems, with optimization enabled at this setting, it warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. 🫶
• -Wstringop-overflow=2 — This one uses type-one Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflows when writing to members of the largest complete objects whose exact size is known. But it does not warn for excessive writes to those same members of unknown objects referenced by pointers, since those pointers may point to arrays containing an unknown number of elements. This is the default setting of the option. 👑
• -Wstringop-overflow=3 — This one uses type-two Object Size Checking to figure out the sizes of destination objects. At this setting it 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 — This one uses type-three Object Size Checking to figure out the sizes of destination objects. At this setting it 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. Just like -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's less than the length of the source string. So the copy of the source gets truncated, and the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound. 🎀

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

As another example, this next call to strncpy copies to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, so the call is diagnosed. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. 🦋

void copy (char *d, const char *s)
{
  strncpy (d, s, strlen (s));
}

In this next one, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than that size, the result of the copy won't be NUL-terminated. So, yeah, this call is diagnosed too. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL. 🌸

void copy (const char *s)
{
  char buf[80];
  strncpy (buf, s, sizeof buf);
  ...
}

When a character array is meant to store a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to avoid this warning. But heads up: such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of those arrays, GCC issues warnings unless it can prove the use is safe. ⭐

---

• -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc] — Warns for cases where adding an attribute might be beneficial. The attributes currently supported are listed below. 💅
  • -Wsuggest-attribute=pure, -Wsuggest-attribute=const, -Wsuggest-attribute=noreturn, -Wmissing-noreturn, -Wsuggest-attribute=malloc — Warn about functions that might be candidates for the attributes pure, const, noreturn, or malloc. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it can't prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop and doesn't return abnormally by throwing, calling abort, or trapping. This analysis needs option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels make the analysis more accurate. 🔥
  • -Wsuggest-attribute=format, -Wmissing-format-attribute — Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes 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 that might not always be the case, and some functions for which format attributes are appropriate may not be detected. 🙈

• -Wsuggest-attribute=cold — Warn about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions that always lead to a call to another cold function — like wrappers of C++ throw, or fatal error reporting functions leading to abort. ❄️

---

• -Walloc-zero — Warns about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes, including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs between implementations (and in the case of realloc it's been deprecated), relying on it may cause subtle portability bugs and should be avoided. 💀

---

• -Walloc-size-larger-than=byte-size — Warns about calls to functions decorated with the attribute alloc_size that try 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 — Disables -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. 🫶

---

• -Walloca — This option warns on all uses of alloca in the source. 🚨

---

• -Walloca-larger-than=byte-size — This option warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. Arguments of non-integer types are considered unbounded, even if they appear to be constrained to the expected range. 😭

For example, a bounded case of alloca could be: 🌈

ok so bestie, picture this: you think you're being so safe with your bounds check, and the compiler is like "girl, no 💀". Here's the deal:

void func (size_t n)
{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
}

So in that lil scenario, if you passed -Walloca-larger-than=1000, no warning — because the call to alloca is known to be at most 1000 bytes. Cute, right? But if you'd passed -Walloca-larger-than=500 instead, the compiler would emit a warning. Caught. 🫶

Now. Unbounded uses are a whole different vibe: they're uses of alloca with no controlling predicate constraining its integer argument. Like:

void func ()
{
  void *p = alloca (n);
  f (p);
}

Pass -Walloca-larger-than=500 here and boom, warning — but this time it's because of the total lack of bounds checking. Not the size. The vibe check failed.

And ngl, even code that looks correct but involves signed integers can get flagged:

void func (signed int n)
{
  if (n < 500)
    {
      p = alloca (n);
      f (p);
    }
}

Because in that example, n could be negative 😭, which means a larger-than-expected argument gets implicitly cast into the alloca call. Sneaky.

Oh and also — this option warns when alloca is used in a loop. Yes bestie, loops. We see you.

-Walloca-larger-than=PTRDIFF_MAX is enabled by default, but it's usually only effective when -ftree-vrp is active (which is the 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.
• -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>
}

• -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.
  • -Warray-bounds=1 — This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.
  • -Warray-bounds=2 — This warning level also warns about out of bounds accesses to trailing struct members of one-element array types, and about the intermediate results of pointer arithmetic that may yield out of bounds values. Heads up: 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 arguments 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, 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. At level 1 the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.

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 with the keyword static specifies that the array argument must have at least four elements:

void f (int[static 4]);
void f (int[]);           // warning (inconsistent array form)

void g (void)
{
  int *p = (int *)malloc (4);
  f (p);                  // warning (array too small)
  ...
}

At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms 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. And the -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments. ✨

• -Wattribute-alias=n
• -Wno-attribute-alias — Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias.
  • -Wattribute-alias=1 — The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.
  • -Wattribute-alias=2 — At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation 😬. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.

Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.

-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.

• -Wbidi-chars=[none|unpaired|any|ucn] — Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. These characters can flip left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and the visual order. This is dangerous, fr: 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 warns for conditional operators:

  int i = x ? *p : *p;

• -Wduplicated-cond — Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

• -Wframe-address — Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall.
• -Wno-discarded-qualifiers (C and Objective-C only) — Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning.
• -Wno-discarded-array-qualifiers (C and Objective-C only) — Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning.
• -Wno-incompatible-pointer-types (C and Objective-C only) — Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.
• -Wno-int-conversion (C and Objective-C only) — ok so this one is like, "bestie, pls stop yelling about integer-to-pointer and pointer-to-integer conversions." Turning it on silences that warning. Important note though: this warning is about implicit conversions only. For the explicit ones you'd reach for -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast instead. Different vibes, different flags 💅
• -Wzero-length-bounds — warns you when you're accessing elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged, fr, because accessing them is undefined. See

Example time, and the first two stores in function bad get diagnosed because the array elements overlap the subsequent members b and c. The third store gets caught by -Warray-bounds because it's 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
}

Option -Wzero-length-bounds is enabled by -Warray-bounds.

• -Wno-div-by-zero — don't warn about compile-time integer division by zero. Floating-point division by zero? Not warned about at all, because it can be a totally legit way of getting infinities and NaNs. slay, math 🌈
• -Wsystem-headers — prints warning messages for constructs found in system header files. Normally warnings from system headers are suppressed, on the assumption that they usually don't indicate real problems and would just 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. But heads up though: using -Wall together with this option does not warn about unknown pragmas in system headers — for that, -Wunknown-pragmas must also be used. iykyk ✨
• -Wtautological-compare — warns 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) { ... }

will always be false. 💀

This warning is enabled by -Wall.

• -Wtrampolines — warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or 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. For some targets it's made up of data only and thus requires no special treatment. But for most targets it's made up of code, and thus requires the stack to be made executable for the program to work properly. lowkey a big deal 🫶
• -Wfloat-equal — warns if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing that, 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 whether the two values have ranges that overlap; and that's done with the relational operators, so equality comparisons are probably mistaken. not me writing == on floats 🙈

• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
  • Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — warns if a prototype causes a type conversion that's 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) — warns 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. sorry bestie, C90 said no 🎀
• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function 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=local and -Wno-shadow=compatible-local are ignored when -Wshadow is 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.
• -Wshadow=global — warns for any shadowing. Same as -Wshadow.
• -Wshadow=local — warns when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — warns when a local variable shadows another local variable or parameter whose type is compatible with 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's 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.

• *-Wlarger-than=byte-size* — warns whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-larger-than. 🌸
• -Wlarger-than= girlies: also warn for calls to bounded functions like memchr or strnlen when they specify a bound bigger than the largest possible object — which is PTRDIFF_MAX bytes by default, no cap. And these warnings? literally can only be silenced by -Wno-larger-than. That's the only way out bestie. 🙈
• -Wno-larger-than — turns OFF the -Wlarger-than= warnings. It's equivalent to -Wlarger-than=SIZE_MAX or bigger, so basically you're setting the bar so high nothing ever trips it. 💅
• -Wframe-larger-than=byte-size — warns you if a function frame gets bigger than byte-size. Heads up though: the math they do to figure out the stack frame size is approximate and not conservative, so the real requirements might be somewhat bigger than byte-size even if you got no warning at all. Also any space you allocate via alloca, variable-length arrays, or related constructs is not included by the compiler when it decides whether to warn you. -Wframe-larger-than=PTRDIFF_MAX is on by default. You can kill these warnings either by specifying byte-size of SIZE_MAX or more, or with -Wno-frame-larger-than.
• -Wno-frame-larger-than — disables the -Wframe-larger-than= warnings. Equivalent to -Wframe-larger-than=SIZE_MAX or larger. ✨
• -Wfree-nonheap-object — warns when you try to deallocate an object that was either not allocated on the heap, or that you're freeing through a pointer that did NOT come back from a prior call to the corresponding allocation function. Like, for example, stpcpy returns a pointer to the terminating nul character and not to the start of the object, so this free down here gets diagnosed:

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is included in -Wall. 🫶

• -Wstack-usage=byte-size — warns if a function's stack usage might exceed byte-size. Here the computation IS conservative (unlike frame-larger-than, we love consistency 💀). And any space allocated via alloca, variable-length arrays, or related constructs IS included by the compiler when deciding whether to warn.

The message matches the output of -fstack-usage:

• If the stack usage is fully static but goes over the specified amount, it's:

warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, it's:

warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and NOT bounded, it's:

warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. You can disable these warnings either by specifying byte-size of SIZE_MAX or more, or with -Wno-stack-usage.

• -Wno-stack-usage — disables -Wstack-usage= warnings. Equivalent to -Wstack-usage=SIZE_MAX or larger. 🌸
• -Wunsafe-loop-optimizations — warns if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And with -funsafe-loop-optimizations it warns if the compiler does make such assumptions. Either way somebody's getting a warning bestie 😭
• -Wno-pedantic-ms-format (MinGW targets only) — when you use this together with -Wformat and -pedantic without GNU extensions, it disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime. Windows things, iykyk. 🪟
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. GNU C assigns these types a size of 1, for convenience when doing calculations with void * pointers and pointers to functions. And in C++ it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic. 💅
• -Wno-pointer-compare — do NOT warn if a pointer is compared with a zero character constant. Which usually means the pointer was meant to be dereferenced. 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.

• -Wtsan — warns about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support std::atomic_thread_fence and can report false positives.

This warning is enabled by default. 💀

• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type, but it does NOT warn for constant expressions. So like, warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra. ✨
• -Wabsolute-value (C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function exists. For example, calling abs(3.14) triggers the warning because the appropriate function for a double argument is fabs. It also triggers warnings when the argument in a call to such a function has an unsigned type. You can suppress this with an explicit type cast, and it's also enabled by -Wextra. 🍓
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* appears inside a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall. 🎀
• -Wtrigraphs — warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. ✨

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Such identifiers get replaced with zero. 💜
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including when the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra. 🦋
• -Wunused-macros — warns about macros defined in the main file that are unused. 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's redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: if a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in that case, you might improve the scope of the macro's definition by, like, moving it into the first skipped block. Or alternatively you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — do NOT warn whenever an #else or an #endif is followed by text. This sometimes happens in older programs with code like:

#if FOO
...
#else FOO
...
#endif FOO

The second and third FOO should be in comments. This warning is on by default. 🙈

• -Wbad-function-cast (C and Objective-C only) — warns when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. Bestie that's giving "chaos" 💀
• -Wc90-c99-compat (C and Objective-C only) — warns about features not present in ISO C90 but present in ISO C99. Like, warns about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. ✨
• -Wc99-c11-compat (C and Objective-C only) — warns about features not present in ISO C99 but present in ISO C11. Like, warns about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. 💜
• -Wc11-c2x-compat (C and Objective-C only) — warns about features not present in ISO C11 but present in ISO C2X. Like, warns about omitting the string in _Static_assert, use of [[]] syntax for attributes, use of decimal floating-point types, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. 🦋
• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. a request for implicit conversion from void * to a pointer to non-void type. 🌈
• -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 in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall. 🔥
• -Wc++14-compat (C++ and Objective-C++ only) — 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) — do NOT warn about C++11 constructs in code being compiled using an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is used. 🫶
• -Wno-c++14-extensions (C++ and Objective-C++ only) — do NOT warn about C++14 constructs in code being compiled using an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if -Wpedantic is used. 🌸
• -Wno-c++17-extensions (C++ and Objective-C++ only) — bestie, this one says don't come for me about C++17 constructs when I'm literally compiling with an older C++ standard. And heads up: even if you don't pass this flag, some C++17 constructs still only get diagnosed when you use -Wpedantic. iykyk 💅
• -Wno-c++20-extensions (C++ and Objective-C++ only) — same energy as above but for C++20 constructs while you're compiling with an older C++ standard. And again, even without this option, a few C++20 constructs only get flagged under -Wpedantic. ✨
• -Wno-c++23-extensions (C++ and Objective-C++ only) — you guessed it, the C++23 version 🌈. Compiling with an older standard and don't wanna hear about C++23 stuff? Pass this. But lowkey, some C++23 constructs will still only be diagnosed if -Wpedantic is on. 😌
• -Wcast-qual — warns whenever a pointer gets cast in a way that strips a type qualifier off the target type. Like, warn if a const char * gets cast to a plain old char *. 🙈

It also warns when you make a cast that introduces a type qualifier in an unsafe way. Example: casting char ** to const char ** is unsafe, as in this little cautionary tale:

          /* 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';

that last line is giving chaos, ngl 💀

• -Wcast-align — warns when a pointer is cast such that the required alignment of the target goes up. Classic case: warn if a char * gets cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. slay, but carefully 🎀
• -Wcast-align=strict — same warning, same idea (pointer cast that increases the required alignment of the target, like a char * → int *), but here it fires regardless of the target machine. no vibe check on the architecture, it just warns. 💅
• -Wcast-function-type — warns when a function pointer is 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 int vs. long on ILP32 targets. Type qualifiers are ignored too. The function type void (*) (void) is special and matches everything, which you can use to suppress this warning. In a cast involving pointer to member types, this warning fires whenever the type cast changes the pointer to member type. This warning is enabled by -Wextra. 👑
• -Wwrite-strings — when compiling C, this gives string constants the type const char[length], so copying the address of one into a non-const char * pointer produces a warning. These warnings help you catch at compile time any code that might try to write into a string constant — but only if you've been super careful about using const in declarations and prototypes. Otherwise it's just a nuisance. That's literally why -Wall doesn't request these warnings, fr fr.

When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs. ✨

• -Wclobbered — warns for variables that might get changed by longjmp or vfork. This warning is also enabled by -Wextra. 😭 sneaky little gremlins
• -Wconversion — warns for implicit conversions that may alter a value. This includes:
  • conversions between real and integer, like abs (x) when x is double;
  • conversions between signed and unsigned, like unsigned ui = -1;
  • conversions to smaller types, like sqrtf (M_PI).

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 turned off with -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 same type are only given with -Warith-conversion. 🫶

• -Wdangling-else — warns about constructions where there might be confusion over which if statement an else branch belongs to. Here's the example:

        {
          if (a)
            if (b)
              foo ();
          else
            bar ();
        }

In C/C++, every else branch belongs to the innermost possible if statement — which in this example is if (b). That's often not what the programmer expected, as the indentation above makes painfully clear 😭. When there's potential for this confusion, GCC issues a warning when this flag is specified. To kill the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. The glow-up looks like:

        {
          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. On top of that, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels, but it may yield different results with optimization than without. 👀

-Wdangling-pointer=1 At level 1, the warning diagnoses only unconditional uses of dangling pointers. For example:

                int f (int c1, int c2, x)
                {
                  char *p = strchr ((char[]){ c1, c2 }, c3);
                  return p ? *p : 'x';   // warning: dangling pointer to a compound literal
                }

In the following function, the store of the address of the local variable x into the escaped pointer *p also triggers the warning:

                void g (int **p)
                {
                  int x = 7;
                  *p = &x;   // warning: storing the address of a local variable in *p
                }

-Wdangling-pointer=2 At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers. 😳

For example, because the array a in the following function is out of scope by the time the pointer s that was set to point at it gets used, the warning triggers at this level:

                void f (char *s)
                {
                  if (!s)
                    {
                      char a[12] = "tmpname";
                      s = a;
                    }
                  strcat (s, ".tmp");   // warning: dangling pointer to may be used
                  ...
                }

-Wdangling-pointer=2 is included in -Wall. 🎀

• -Wdate-time — warns when the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, since they might stop you from getting bit-wise-identical reproducible compilations. ⏰ no cap
• -Wempty-body — warns if an empty body happens in an if, else or do while statement. This warning is also enabled by -Wextra. 🙈
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif. live your life, bestie 💜
• -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 -Wextra in C. ✨
• -Wjump-misses-init (C, Objective-C only) — warns if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. It only warns about variables that are initialized when they're declared. This warning is only supported for C and Objective-C — in C++ this sort of branch is an error in any case, so. 💀

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.

• -Wsign-compare — warns when a comparison between signed and unsigned values could produce an incorrect result once the signed value gets converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it's also enabled by -Wextra. 😭 the classic
• -Wsign-conversion — warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is also enabled by -Wconversion. 💅
• -Wfloat-conversion — warns for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion. 🦋
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order. 🔥
• -Wsizeof-array-div — warns about divisions of two sizeof operators when the first one is applied to an array and the divisor doesn't equal the size of the array element. In that case, the computation won't give you the number of elements in the array — which is probably what you were going for. It warns about stuff like:

        int fn ()
        {
          int arr[10];
          return sizeof (arr) / sizeof (short);
        }

This warning is enabled by -Wall. ⭐

• -Wsizeof-pointer-div — bestie, this one is out here catching suspicious divisions of two sizeof expressions where you divide the pointer size by the element size. That's the usual way to compute array size, but it will NOT work out correctly with pointers. It warns, for example, about sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by -Wall. 💅
• -Wsizeof-pointer-memaccess — okay so this warns about suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof. It triggers, for example, for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and it suggests a possible fix. Or about memcpy (&foo, ptr, sizeof (&foo));.

It also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake, and so the call is diagnosed.

void make_file (const char *name)
{
  char path[PATH_MAX];
  strncpy (path, name, sizeof path - 1);
  strncat (path, ".text", sizeof ".text");
  ...
}

The -Wsizeof-pointer-memaccess option is enabled by -Wall. ✨

• -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
• -Wmemset-elt-size — warns for suspicious calls to the memset built-in function if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall. 🫶
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall. 😭
• -Waddress — warns about suspicious uses of address expressions. These include comparing the address of a function or a declared object to the null pointer constant, such as in:

void f (void);
void g (void)
{
  if (!func)   // warning: expression evaluates to false
    abort ();
}

comparisons of a pointer to a string literal, such as in:

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, such as in:

void f (const int *p, int i)
{
  return p + i == NULL;
}

Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so their use in a conditional might indicate missing parentheses in a function call or a missing 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 inptr_t or uinptr_t. Comparisons against string literals result in unspecified behavior and are not portable, and suggest the intent was 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. This includes using logical operators in contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator are the same:

extern int a;
if (a < 0 && a < 0) { ... }

• -Wlogical-not-parentheses — 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 to be a boolean expression. Its purpose is to detect suspicious code like the following:

int a;
...
if (!a > 1) { ... }

It is possible to suppress the warning by wrapping the LHS into parentheses:

if ((!a) > 1) { ... }

This warning is enabled by -Wall. 🌸

• -Waggregate-return — warns if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
• -Wno-aggressive-loop-optimizations — warns if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.

Additionally, using -Wno-attributes=, it is possible to suppress warnings about unknown scoped attributes (in C++11 and C2X). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:

[[vendor::attr]] void f();

It is also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor:: which prevents warning 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 does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When -Wextra is specified, also warn when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into scope.

For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed as well.

extern void* memset ();
void f (void *d)
{
  memset (d, '\0', 32);
}

• -Wno-builtin-macro-redefined — do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra.
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there is a previous prototype. A definition using () is not considered an old-style definition in C2X mode, because it is equivalent to (void) in that case, but is considered an old-style definition for older standards.
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
• -Wmissing-declarations — warns if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 🦋
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

This option does not warn about designated initializers, so the following modification does not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C this option does not warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.

• -Wno-missing-requires — bestie, by default the compiler WILL clock a concept-id showing up as a C++20 simple-requirement, like this:

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 the "template" keyword if the parent object is dependent and the member being named is a template. no cap, that's the rule:

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. 🫠 so like… just don't.
• -Wnormalized=[none|id|nfc|nfkc] — okay so this one is so interesting. In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. But sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look exactly the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning. 👑

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default. 🙏

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 😭 like literally, you cannot see them, that's the whole problem.

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🌈

• -Wno-attribute-warning — do not warn about usage of functions declared with "warning" attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning or -Wno-error=attribute-warning can be used to disable the error when compiled with -Werror flag.
• -Wno-deprecated — do not warn about usage of deprecated features. they're deprecated for a reason bestie but okay 💀
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.
• -Wno-overflow — do not warn about compile-time overflow in constant expressions.
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default.
• -Wopenacc-parallelism — warn about potentially suboptimal choices related to OpenACC parallelism.
• -Wopenmp-simd — warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model.
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.

• -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. (yes, note the difference from the one above, they are literally opposite vibes and you cannot mix them up 😌)
• -Wpacked — warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

• -Wnopacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. 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 field a and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct S, like, warning: alignment 1 of 'struct S' is less than 8, in this code:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall.

• -Wpadded — warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. ✨ efficiency era ✨
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. like bestie why are you saying it twice 😭
• -Wrestrict — warn when an object referenced by a restrict-qualified parameter (or, in C++, a __restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed.

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall.

• -Wnested-externs (C and Objective-C only) — warn if an extern declaration is encountered within a function. nesting it in there?? bold move, and we're gonna say something about it 🫶
• -Winline — warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. 🔥 so if it flickers, that's not you, that's the heuristics.

• -Winterference-size — warn about use of C++17 std::hardware_destructive_interference_size without specifying its value with --param destructive-interference-size. Also warn about questionable values for that option.

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 intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line. 🧚

so by default, --param destructive-interference-size and --param constructive-interference-size get their values from whatever -mtune you're rocking right now — usually the L1 cache line size for that specific target CPU, and sometimes a whole range if you're tuning for a generic target. the consequences tho: every translation unit that cares about ABI compatibility for these variables has to be compiled with the same -mtune (or -mcpu). no mixies. 🙅

If ABI stability is important to you — like, say, the use is in a header for a library — you should probably just... not use the hardware interference size variables at all, bestie. 💅 Alternatively, you can force a particular value with --param.

And if you're confident your use of the variable doesn't affect ABI outside a single build of your project? Go ahead and turn the warning off with -Wno-interference-size. 🎀

---

• -Wint-in-bool-context — warns when you use integer values in a place where booleans are expected and it looks suspicious. Like conditional expressions (?:) with non-boolean integer constants in boolean context, e.g. if (a <= b ? 2 : 3). Or left shifting signed integers in boolean context, like for (a = 0; 1 << a; a++);. Same deal for all kinds of multiplications regardless of 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-cast is 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. 😭 the disappointment.
• -Wlong-long — warns if the long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
• -Wvariadic-macros — 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 -Wpedantic or -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 like va_start. These warnings are enabled by default.
• -Wvector-operation-performance — warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. A vector operation can be implemented:
  • "piecewise" — the scalar operation is performed on every vector element
  • "in parallel" — the vector operation is implemented using scalars of wider type, which normally is more performance efficient
  • "as a single scalar" — the vector fits into a scalar type
• -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.
• *-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. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays. 📏

Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays.

-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).

See also -Walloca-larger-than=byte-size.

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or 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]);
void f (int, int[]);     // warning: argument 2 previously declared as a VLA

void g (int n)
{
    if (n > 4)
      return;
    int a[n];
    f (sizeof a, a);     // warning: access to a by f may be out of bounds
  ...
}

-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.

• -Wvolatile-register-var — warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there's 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) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
• -Wstack-protector — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.
• -Wno-lto-type-mismatch — during the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires -flto to 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.

---

🧪 Options That Control Static Analysis 🧪

• -fanalyzer — this option enables an static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them. 🔍

Heads up bestie: this analysis is much more expensive than other GCC warnings. like, a lot.

Enabling this option effectively enables the following warnings:

-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-allocation-size -Wanalyzer-tainted-array-index
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free -Wanalyzer-use-of-uninitialized-value
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

This option is only available if GCC was configured with analyzer support enabled. 🙈

• -Wanalyzer-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code.

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 if this occurs.

• -Wno-analyzer-double-fclose — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. 😭

• -Wno-analyzer-double-free — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to 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.

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to 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 literally how passwords end up in log files, protect yourself 💅

• -Wno-analyzer-file-leak — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it.

This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked. your streams are out here living their best open life with nobody closing them 😭

• -Wno-analyzer-free-of-non-heap — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). freeing a stack buffer?? the way that is a whole crime scene fr fr

• -Wno-analyzer-malloc-leak — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.

This diagnostic warns for paths through the code in which a pointer allocated via an allocator is leaked: either malloc, or a function marked with attribute malloc. the memory said "and I never came back" 💀

• -Wno-analyzer-mismatching-deallocation — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-mismatching-deallocation to disable it.

This diagnostic warns for paths through the code in which the wrong deallocation function is called on a pointer value, based on which function was used to allocate the pointer value. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. you can't free what was new'd, that's not matching energy at all, it's giving chaos

• -Wno-analyzer-possible-null-argument — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. the function said "non-NULL only" and you showed up with a maybe-NULL… read the room 🙈

• -Wno-analyzer-possible-null-dereference — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced. dereferencing a maybe? in this economy? absolutely not

• -Wno-analyzer-null-argument — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-argument to disable it.

This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. not "possibly" this time bestie, the analyzer knows it's NULL, no cap

• -Wno-analyzer-null-dereference — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to disable it.

This diagnostic warns for paths through the code in which a value known to be NULL is dereferenced. straight-up confirmed NULL deref, it's giving segfault ✨

• -Wno-analyzer-shift-count-negative — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-negative to disable it.

This diagnostic warns for paths through the code in which a shift is attempted with a negative count. It is analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. shifting by a negative?? that's not a shift that's a crisis 💀 also lowkey the analyzer isn't even prioritizing catching it so false negatives are more likely, iykyk

• -Wno-analyzer-shift-count-overflow — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it.

This diagnostic warns for paths through the code in which a shift is attempted with a count greater than or equal to the precision of the operand's type. It is analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. shifting past the whole width of the type, iconic behaviour honestly, but it's a no from me. also: analyzer does NOT prioritize these, so false negatives are likelier, keep that in mind 🎀

• -Wno-analyzer-stale-setjmp-buffer — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it.

This diagnostic warns for paths through the code in which longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned.

When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. The stack frame becomes invalid when the function containing the setjmp call returns. Attempting to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse). okay this one is genuinely scary bestie. you're longjmp-ing back to a stack frame that literally doesn't exist anymore?? crash at best, "(or worse)" at worst, and the manpage said it like that for a REASON 💀

• -Wno-analyzer-tainted-allocation-size — This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use -Wno-analyzer-tainted-allocation-size to disable it.

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an allocation without being sanitized, so that an attacker could inject an excessively large allocation and potentially cause a denial of service attack.

See @url{https://cwe.mitre.org/data/definitions/789.html, CWE-789: Memory Allocation with Excessive Size Value}. an attacker choosing your allocation size?? that's a denial of service waiting to happen, sanitize it 🫶

• -Wno-analyzer-tainted-array-index — This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use -Wno-analyzer-tainted-array-index to disable it.

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the index of an array access without being sanitized, so that an attacker could inject an out-of-bounds access.

See @url{https://cwe.mitre.org/data/definitions/129.html, CWE-129: Improper Validation of Array Index}. attacker-controlled array index = out-of-bounds access, and you do NOT want that energy in your codebase 😭

• -Wno-analyzer-tainted-divisor — This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use -Wno-analyzer-tainted-divisor to disable it.

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the divisor in a division or modulus operation without being sanitized, so that an attacker could inject a division-by-zero. attacker picks the divisor and suddenly you're dividing by zero, slay I guess?? no, sanitize it fr

• -Wno-analyzer-tainted-offset — This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use -Wno-analyzer-tainted-offset to disable it.

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as a pointer offset without being sanitized, so that an attacker could inject an out-of-bounds access.

See @url{https://cwe.mitre.org/data/definitions/823.html, CWE-823: Use of Out-of-range Pointer Offset}. tainted pointer offset = out-of-bounds access, sanitize or perish bestie 💅

• -Wno-analyzer-tainted-size — This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use -Wno-analyzer-tainted-size to disable it.

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so that an attacker could inject an out-of-bounds access. attacker-supplied size going into memset?? that's an out-of-bounds injection, be so for real and sanitize it 🎀

• -Wno-analyzer-unsafe-call-within-signal-handler — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it.

This diagnostic warns for paths through the code in which a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. calling fprintf from a signal handler?? bestie that's async-signal-unsafe and the manpage literally called it out by name 😭

• -Wno-analyzer-use-after-free — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it.

This diagnostic warns for paths through the code in which a pointer is used after a deallocator is called on it: either free, or a deallocator referenced by attribute malloc. use-after-free, the classic. you freed it, and then you USED it?? the audacity, the audacity 💀

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

This diagnostic warns for paths through the code in which a pointer is dereferenced that points to a variable in a stale stack frame. pointer pointing at a stack frame that's already gone, dereferencing it anyway, main character energy but make it undefined behaviour 🌸

• -Wno-analyzer-write-to-const — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to disable it.

This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a const object. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. writing to a const?? bold. also the analyzer doesn't prioritize catching these so false negatives are more likely, just so you know 🦋

• -Wno-analyzer-write-to-string-literal — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-string-literal to disable it.

This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a string literal. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. writing through a pointer to a string literal, absolutely unhinged, love the confidence, hate the behaviour. and again: not prioritized, so false negatives likelier ✨

• -Wno-analyzer-use-of-uninitialized-value — This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.

This diagnostic warns for paths through the code in which an uninitialized value is used. using a value you never even initialized, it's giving mystery box but the box is undefined behaviour 🙈

Pertinent parameters for controlling the exploration are: --param analyzer-bb-explosion-factor=value, --param analyzer-max-enodes-per-program-point=value, --param analyzer-max-recursion-depth=value, and --param analyzer-min-snodes-for-call-summary=value. yes bestie there are knobs. there are always knobs.

The following options control the analyzer. ✨ analyzer girlies, this section is FOR you ✨

• -fanalyzer-call-summaries — Simplify interprocedural analysis by computing the effect of certain calls, rather than exploring all paths through the function from callsite to each possible return.

If 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). it's giving "let's not explore every single path, let's just summarize the vibe" — and it only does this for functions with more than one call site that are sufficiently complicated 💅

• -fanalyzer-checker=name — Restrict the analyzer to run just the named checker, and enable it.

Some checkers are disabled by default (even with -fanalyzer), such as the taint checker that implements -Wanalyzer-tainted-array-index, and this option is required to enable them. some checkers are lowkey off by default even with -fanalyzer on, like taint, so you literally have to come in here and enable them by name. that's the whole point of this flag 🫶

• -fno-analyzer-feasibility — This option is intended for analyzer developers. not for you unless you're literally building the analyzer, bestie. devs only, iykyk 🧚

okay so by default the analyzer is a tiny bit of a skeptic 😭 — before it hands you a diagnostic, it checks that there's actually a feasible control flow path for it, like, that the conditions it's holding onto aren't mutually exclusive. Diagnostics where no feasible path can be found? rejected, blocked, ghosted. 🚫

And if you wanna turn that filtering off, there's -fno-analyzer-feasibility — but bestie, that's for debugging issues in this code, not for your everyday vibe. 🛠️

---

• -fanalyzer-fine-grained — this one is for analyzer developers. Internally the analyzer builds an "exploded graph" that smashes control flow graphs together with data flow information. By default, an edge in that graph can hold the effects of a whole run of multiple statements inside a basic block. With -fanalyzer-fine-grained, every single statement gets its own edge. one statement, one edge, no sharing, periodt. ✨
• -fanalyzer-show-duplicate-count — also for analyzer developers: if multiple diagnostics got flagged as duplicates of each other, this emits a note when it reports the best diagnostic, telling you how many extra diagnostics got suppressed by the deduplication logic. 💅
• -fno-analyzer-state-merge — for analyzer developers. By default the analyzer tries to keep things simple by merging sufficiently similar states at each program point while it's building its "exploded graph". With -fno-analyzer-state-merge that merging can be suppressed — for debugging state-handling issues. 🔍
• -fno-analyzer-state-purge — for analyzer developers. By default the analyzer simplifies stuff by purging aspects of state at a program point that look like they don't matter anymore — e.g. the values of locals that aren't accessed later in the function and aren't relevant to leak analysis. With -fno-analyzer-state-purge that purging gets suppressed, again for debugging state-handling issues. 🧹🚫
• -fanalyzer-transitivity — enables transitivity of constraints within the analyzer. that's it, that's the whole thing. 🔗
• -fanalyzer-verbose-edges — for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. ✍️
• -fanalyzer-verbose-state-changes — for analyzer developers. More verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths. 🎛️
• *-fanalyzer-verbosity=level* — this controls the complexity of the control flow paths that get 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. ⭐
  3. As per the previous level, but show all control flow events, not just the significant ones. 🌊
  4. This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer. 🐞
• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. This option is overridden by -fdump-analyzer-stderr. 📄
• -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer. 🗣️
• -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. 🕸️
• -fdump-analyzer-exploded-graph — dumps a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. 🌈
• -fdump-analyzer-exploded-nodes — emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source. 📍
• -fdump-analyzer-exploded-nodes-2 — dumps a textual representation of the "exploded graph" to file.eg.txt. 📝
• -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files. 😳 (like, your directory is gonna be eating good)
• -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt. 🛤️
• -fdump-analyzer-feasibility — dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form file.*.fg.dot and file.*.tg.dot. 🔎
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. ⚠️ (so don't go building a whole pipeline on it, bestie)
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot. 🧼
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. These show all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the "exploded graph" and diagnostics associated with them. 🌐

---

🐛✨ Options for Debugging Your Program ✨🐛

okay so if you want GCC to emit extra info for a debugger to snack on, in almost all cases you just add -g to your other options and you're done. 💖 Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options. besties sharing a couch, no drama.

GCC lets you use -g with -O. and ngl, the shortcuts optimized code takes can be a little surprising sometimes: 😭

• some variables you declared may not exist at all
• flow of control may briefly move where you did not expect it
• some statements may not be executed because they compute constant results or their values are already at hand
• some statements may execute in different places because they have been moved out of loops

Nevertheless, it is possible to debug optimized output. Which is why it's reasonable to use the optimizer for programs that might have bugs. 🔥

If you're not using some other optimization option, consider using -Og with -g. Because with no -O option at all, some compiler passes that collect information useful for debugging don't run at all — so -Og may give you a better debugging experience. 👑

• -g — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. 💅

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below). 🫶

• -ggdb — produce debugging information for use by GDB. This means using the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. 🎀
• -gdwarf
• -gdwarf-version — produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin/Mac OS X, which default to version 2, and AIX, which defaults to version 4). 🌸

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. 💀

• -gbtf — request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. 🦋
• -gctf
• -gctflevel — request CTF debug information and use level to specify how much CTF debug information should be produced. If -gctf is specified without a value for level, the default level of CTF debug information is 2. 🍓

CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.

• Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf. 🚫
• Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information. 🧵
• Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only. 🗂️

• -gstabs — produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler. 😮‍💨
• -gstabs+ — produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. 🙈
• -gxcoff — produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems. 👑
• -gxcoff+ — okay so this one produces debug info in XCOFF format (if your setup even supports it 💅), but with GNU extensions that only GDB understands. And bestie, listen: using those extensions will likely make other debuggers crash or flat-out refuse to read your program 😭, and it may also make assemblers that aren't the GNU assembler (GAS) fail with an error. So it's giving "main character only plays nice with GDB" energy.
• -gvms — produces debug info in Alpha/VMS debug format (again, if supported). This is literally the format that DEBUG uses on Alpha/VMS systems. niche but valid ✨
• -glevel
• -ggdblevel
• -gstabslevel
• -gxcofflevel
• -gvmslevel — these ask for debug info and let you pass level to say how much info you want. The default level is 2. Here's the tea:
  • Level 0 produces no debug information at all. So -g0 negates -g, periodt.
  • Level 1 produces minimal info — just enough for making backtraces in the parts of the program you don't plan to debug. That includes descriptions of functions and external variables, and line number tables, but no info about local variables.
  • Level 3 includes extra info, like all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3 👑.

If you use multiple -g options, with or without level numbers, the last such option is the one that's effective. And heads up: -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.

• -fno-eliminate-unused-debug-symbols — by default, no debug info is produced for symbols that aren't actually used. Use this option if you want debug info for all symbols, no cap 🫶.
• -femit-class-debug-always — instead of emitting debugging information for a C++ class in only one object file, this emits it in all object files using the class. Only use this with debuggers that can't handle the way GCC normally emits debugging info for classes, because using it increases the size of debugging information by as much as a factor of two 💀. that's a lot.
• -fno-merge-debug-strings — tells the linker to not merge together strings in the debugging info that are identical across different object files. Merging isn't supported by all assemblers or linkers, fyi. Merging decreases the size of the debug info in the output file at the cost of increasing link processing time, and it's enabled by default.
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, this records debugging information describing them as if the files resided in directory new instead. You can use it to swap a build-time path for an install-time path in the debug info ✨. You can also use it to change an absolute path into a relative path by using . for new. This gives 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-map.
• -fvar-tracking — runs the variable tracking pass. It computes where variables are stored at each position in code, and then better debugging information gets generated (if the debugging info 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 tries to carry those annotations all the way to the end, all in an attempt to improve debug info while optimizing. Using -gdwarf-4 along with it is recommended. 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 info is enabled, this separates as much debugging info as possible into a separate output file with the extension .dwo. This lets the build system avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files 🔥.
• -gdwarf32
• -gdwarf64 — if DWARF debugging info is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. The default is target specific, though on most targets it's -gdwarf32. The 32-bit DWARF format is smaller, but it 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. pick your fighter ✨
• -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. giving them the identity they deserve 💅
• -gpubnames — generates DWARF .debug_pubnames and .debug_pubtypes sections.
• -ggnu-pubnames — generates .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.
• -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of being part of the .debug_info section. It's more efficient to put them in a separate comdat section, since the linker can then remove duplicates 👑. But not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging information. so... vibe check before you commit 😭
• -grecord-gcc-switches
• -gno-record-gcc-switches — this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file ✨.
• -gstrict-dwarf — disallows using extensions of a later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed.
• -gno-strict-dwarf — allows using extensions of a later DWARF standard version than the one selected with -gdwarf-version.
• -gas-loc-support — informs the compiler that the assembler supports .loc directives. It may then use them for the assembler to generate DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can generate itself 🌸. This option is enabled by default if, at GCC configure time, the assembler was found to support such directives.
• -gno-as-loc-support — forces 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 .loc directives. This option is enabled by default if, at GCC configure time, the assembler was found to support them.
• -gno-as-locview-support — forces GCC to assign view numbers internally, if -gvariable-location-views is explicitly requested.
• -gcolumn-info
• -gno-column-info — emits location column information into DWARF debugging information, rather than just file and line. This option is enabled by default ✨.
• -gstatement-frontiers
• -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. This is enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2 debug information at the normal level.
• -gvariable-location-views
• -gvariable-location-views=incompat5
• -gno-variable-location-views — augments variable location lists with progressive view numbers implied from the line number table. This enables debug info 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 causes 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 info consumers that aren't aware of these augmentations — but those consumers 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's assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support isn't 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 💀. dramatic but true.

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and it can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, and is only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists using it. so like, yikes, careful bestie 😭

• -ginternal-reset-location-views, -gno-internal-reset-location-views — okay so this one tries to figure out which location views can just… be left out of the location view lists entirely ✨. The catch? It needs the compiler to have really accurate insn length estimates, and bestie, that's not always a thing that happens 😭. And if you're using an assembler that doesn't support location view lists, it might silently generate wrong view lists — silently, which is the worst kind of wrong. The GNU assembler will call that out as a "view number mismatch". This is only enabled on ports that define a reliable estimation function, so it's giving ✨exclusive✨.
• -ginline-points, -gno-inline-points — generates extended debug info for inlined functions 💅. Location view tracking markers get inserted at inlined entry points, so address and view numbers can be computed and output in debug info. You can turn this on independently of location views — in which case the view numbers just won't be output — but it can only be enabled alongside statement frontiers, and it's only on by default if location views are enabled. it's giving "you can't sit with us unless statement frontiers is sitting with us" 🫶
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. If type isn't given, the default depends on what the assembler and linker you're using can actually do. type can be one of:
  • none — don't compress debug sections
  • zlib — use zlib compression in ELF gABI format
  • zlib-gnu — use zlib compression in traditional GNU format

If the linker doesn't support writing compressed debug sections, the option is rejected, periodt. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files. silence. iconic of it.

---

🌸 Options for Debugging Your Program — continued 🌸

• -femit-struct-debug-baseonly — only emits debug info for struct-like types when the base name of the compilation source file matches the base name of the file where the struct is defined.

This substantially reduces the size of your debugging info 🎀 but at significant potential loss in type information to the debugger. Peep -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed if you want the full detailed control situation.

This option works only with DWARF debug output. no DWARF, no dice 💀

• -femit-struct-debug-reduced — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined — unless the struct is a template or defined in a system header.

This significantly reduces the size of debugging info, with some potential loss in type info to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for the more detailed control vibe.

Also DWARF-only. same rules bestie ✨

• -femit-struct-debug-detailed[=spec-list] — lets you specify exactly which struct-like types the compiler generates debug info for. The whole intent here is to reduce duplicate struct debug info between different object files within the same program. it's giving "stop repeating yourself" energy 💅

This option is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly it 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's the type of a variable, member. Indirect uses arise through pointers to structs. Like, 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 spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain, ngl. 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. not yet, bestie 😭

The third word specifies the source files for those structs for which the compiler should emit debug info. The values none and any have the normal meaning. The value base means that the base of the 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 info 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. fr fr, go touch grass and run some builds 🌱

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF debug output. (seeing a pattern? 🦋)

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler-generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes though, it's useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether or not they're actually used in that compilation unit — for example, if in the debugger you want to cast a value to a type that isn't actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 🙈 choose wisely.

---

👑 Options That Control Optimization 👑

These options control various sorts of optimizations. the main character options, honestly ✨

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. predictable queen behavior 💅

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. trade-offs, bestie, they're everywhere 🎀

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. it's giving "we know things now" 🌈

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. so if you're setting random -f flags and no -O, lowkey nothing's happening 🫠

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. very "look it up yourself" of the docs but okay 👑

• -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. it ate and left no crumbs 🔥

-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 -fgcse about invoking -O2 on programs that use computed gotos. 👀 consider this your heads-up bestie

• -O3 — Optimize yet more. -O3 turns on all optimizations specified by -O2 and 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 is the default. the chill baseline ✨
• -Os — Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. small and mighty 🎀

-Ofast — okay so bestie, this one literally says "strict standards compliance? never heard of her." 💅 It turns on every single -O3 optimization, plus a whole bunch that are not valid for all standard-compliant programs (yes, that's the trade-off, it's giving "chaotic but fast"). Specifically it flips on -ffast-math, -fallow-store-data-races, and the Fortran-specific -fstack-arrays — unless you also passed -fmax-stack-var-size — plus -fno-protect-parens. And it turns off -fsemantic-interposition.

• -Og — this is the debugging-glow-up optimization level ✨ Optimize the debugging experience, periodt. -Og should be your go-to for the standard edit-compile-debug cycle: it gives you a reasonable amount of optimization while keeping compilation fast and debugging actually pleasant. It's lowkey a better choice than -O0 for producing debuggable code, because some compiler passes that collect debug info are disabled at -O0. 😭

Like -O0, -Og completely disables a bunch of optimization passes, so individual options controlling those have no effect. Otherwise, -Og enables all the -O1 optimization flags except the ones that might mess 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 instead of speed 🎀 Which is kinda wild because it can actually increase the number of instructions executed, if those instructions need fewer bytes to encode. -Oz behaves similarly to -Os, including enabling most -O2 optimizations.

---

If you use multiple -O options, with or without level numbers, the last such option is the one that's effective. Periodt. Last one wins, no arguing. 👑

Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form — the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed, the one you typically use. You can figure out the other form by either removing no- or adding it. 🧚

The following options control specific optimizations. They're either activated by -O options or related to ones that are. Use these flags in the rare cases when you want to "fine-tune" which optimizations get performed. 💅

• -fno-defer-pop — for machines that have to pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; that lets the compiler let arguments accumulate on the stack across 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 — okay so the style picks how floating-point expression contraction behaves:
  • -ffp-contract=off disables floating-point expression contraction.
  • -ffp-contract=fast enables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them.
  • -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off. 🙈

The default is -ffp-contract=fast.

• -fomit-frame-pointer — omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. ✨

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -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, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. 🔥

Enabled at levels -O2, -O3.

• -fno-inline — do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute.

• -finline-small-functions — integrate functions into their callers when their body is smaller than the expected function call code (so the overall size of the program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline. 👑

Enabled at levels -O2, -O3, -Os.

• -findirect-inlining — inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

• -finline-functions — consider all functions for inlining, even if they're not declared inline. The compiler heuristically decides which functions are worth integrating this way. 🧚

If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.

• -finline-functions-called-once — consider all static functions called once for inlining into their caller, even if they're not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.

• -fearly-inlining — inline functions marked by always_inline, and functions whose body seems smaller than the function call overhead, early — before doing -fprofile-generate instrumentation and the real inlining pass. Doing so makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. 💖

Enabled by default.

• -fipa-sra — perform interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value. 🦋

Enabled at levels -O2, -O3 and -Os.

• -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of that limit. n is the size of functions that can be inlined, in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters like this:

• max-inline-insns-single — is set to n/2.
• max-inline-insns-auto — is set to n/2.

See below for documentation of the individual parameters controlling inlining, and for the defaults of these parameters.

Note: there may be no value of -finline-limit that results in default behavior. 😭

Note: "pseudo instruction" represents, in this particular context, an abstract measurement of a function's size. In no way does it represent a count of assembly instructions, and as such its exact meaning might change from one release to another.

• -fno-keep-inline-dllexport — this is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec.
• -fkeep-inline-functions — in C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file.
• -fkeep-static-functions — emit static functions into the object file, even if the function is never used. 💀
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check whether a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

• -fmerge-constants — attempt to merge identical constants (string constants and floating-point constants) across compilation units. 🌈

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

Enabled at levels -O1, -O2, -O3, -Os.

• -fmerge-all-constants — attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants, this considers e.g. even constant-initialized arrays, or initialized constant variables with integral or floating-point types.

Languages like C or C++ require each variable — including multiple instances of the same variable in recursive calls — to have distinct locations, so using this option results in non-conforming behavior. 😭 Not me enabling undefined-vibes mode, 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. By setting this flag, certain anti-dependence edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled. 🫶
• -fno-branch-count-reg — bestie, this turns OFF the optimization pass that goes hunting for chances to use "decrement and branch" instructions on a count register instead of the whole rigmarole of decrementing a register, comparing it against zero, and then branching based on the result. Heads up though: this option only even means anything on architectures that actually support those instructions, and that list includes x86, PowerPC, IA-64 and S/390. And okay, important nuance — -fno-branch-count-reg does not remove the decrement and branch instructions from the generated instruction stream that got introduced by other optimization passes. So like, it's not a full purge, iykyk. 💅

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

• -fno-function-cse — do not put function addresses in registers, okay?? Make each instruction that calls a constant function contain the function's address explicitly. ✨

This option results in less efficient code (yeah, it's giving slower), but some strange hacks that alter the assembler output may get confused by the optimizations performed when this option is not used. So if your weird little hack is acting up... maybe this one's for you.

The default is -ffunction-cse

• -fno-zero-initialized-in-bss — so if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. That can save space in the resulting code, slay. 🎀

This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

The default is -fzero-initialized-in-bss.

• -fthread-jumps — perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. 🔥

Enabled at levels -O1, -O2, -O3, -Os.

• -fsplit-wide-types — when using a type that occupies multiple registers, like long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but ngl 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-types is 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 is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false. 🦋

Enabled at levels -O2, -O3, -Os.

• -fcse-skip-blocks — this is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

Enabled at levels -O2, -O3, -Os.

• -frerun-cse-after-loop — re-run common subexpression elimination after loop optimizations are performed. it's giving "run it back". 🔁

Enabled at levels -O2, -O3, -Os.

• -fgcse — perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. ✨

Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os.

• -fgcse-lm — when -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. 👑

Enabled by default when -fgcse is enabled.

• -fgcse-sm — when -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. 🌸

Not enabled at any optimization level.

• -fgcse-las — when -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

Not enabled at any optimization level.

• -fgcse-after-reload — when -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling. 🧚

Enabled by -O3, -fprofile-use and -fauto-profile.

• -faggressive-loop-optimizations — this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by, for example, causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default. ⚠️ (yes bestie, that "assumes" is doing a LOT of heavy lifting here — if your code has UB, this is where things get spicy.)
• -funconstrained-commons — this option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds.
• -fcrossjumping — perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping. 🤷

Enabled at levels -O2, -O3, -Os.

• -fauto-inc-dec — combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O1 and higher on architectures that support this. 💜
• -fdce — perform dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher.
• -fdse — perform dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher.
• -fif-conversion — attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by -fif-conversion2. 🔥

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -fif-conversion2 — use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

Enabled at levels -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 calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. 😭 With this option, the base and complete variants are changed to be thunks that call a common implementation. iconic.

Enabled by -Os.

• -fdelete-null-pointer-checks — assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null. 💀

Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled.

Passes that use the dataflow information are enabled independently at different optimization levels.

• -fdevirtualize — attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os. ✨
• -fdevirtualize-speculatively — attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form. it's giving "commit, but keep the receipt". 🧾
• -fdevirtualize-at-ltrans — stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. 🙈
• -fexpensive-optimizations — perform a number of minor optimizations that are relatively expensive. 💸

Enabled at levels -O2, -O3, -Os.

• -free — attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. 🌈

Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.

• -fno-lifetime-dse — okay so in C++ the value of an object is only affected by changes within its lifetime: the sec the constructor begins, the object has an indeterminate value, and any changes during the object's lifetime are dead once it's destroyed. Normally dead store elimination eats that up for breakfast 🍓 — but if your code relies on the value of the object storage persisting beyond the lifetime of the object, flip this flag to turn that optimization off. Want to preserve stores before the constructor starts (like, say, because your operator new clears the object storage) while still treating the object as dead after the destructor? Use -flifetime-dse=1. The default behavior can be explicitly picked with -flifetime-dse=2. And -flifetime-dse=0 is literally equivalent to -fno-lifetime-dse. ✨
• -flive-range-shrinkage — attempt to decrease register pressure through register live range shrinkage. It's giving "helpful for fast processors with small or moderate size register sets" 💅
• *-fira-algorithm=algorithm* — use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Ngl, Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it's the default because it generates better code. 👑
• *-fira-region=region* — use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...). 🦋
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. 🐌

This option is enabled at level -Os for all targets.

• -fira-loop-pressure — use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.

This option is enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger. 😭
• -fno-ira-share-spill-slots — disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. same energy, different registers bestie 💀
• -flra-remat — enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable. 🔥

Enabled at levels -O2, -O3, -Os.

• -fdelayed-branch — if supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. 🎀

Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.

• -fschedule-insns — if supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required. ✨

Enabled at levels -O2, -O3.

• -fschedule-insns2 — similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. 🌀

Enabled at levels -O2, -O3, -Os.

• -fno-sched-interblock — disable instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fno-sched-spec — disable speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-pressure — enable register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Using this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation. 💖
• -fsched-spec-load — allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allow speculative motion of more load instructions. 👀 This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns
• -fsched-stalled-insns=n — define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that **no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1. 🌸
• -fsched-stalled-insns-dep
• -fsched-stalled-insns-dep=n — define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This has an effect **only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1. 🧚
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. ⚡ This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

• -fsched-group-heuristic — enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 👑
• -fsched-critical-path-heuristic — enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🔥
• -fsched-spec-insn-heuristic — enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. ✨
• -fsched-rank-heuristic — enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🎀
• -fsched-last-insn-heuristic — enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🦋
• -fsched-dep-count-heuristic — enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 💅
• -freschedule-modulo-scheduled-loops — modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior. 🌈
• -fselective-scheduling — schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. 👑
• -fselective-scheduling2 — schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass. ⭐
• -fsel-sched-pipelining — enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on. 🚫
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on. 💜
• -fsemantic-interposition — okay so bestie, some object formats (looking at you, ELF 💅) let the dynamic linker interpose symbols, which basically means for symbols exported from the DSO the compiler cannot do interprocedural propagation, inlining and other optimizations, because the function or variable in question might change. Is that feature useful? Sure — like when you rewrite memory allocation functions with a debugging implementation — but it is expensive in terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Same energy for variables: if interposition happens for a variable, 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. This flag is enabled by default at -O and higher. ✨
• -fshrink-wrap-separate — shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed. This option is on by default, but lowkey has no effect unless -fshrink-wrap is also turned on and the target supports this. 🙈
• -fcaller-saves — enable 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 -O1 and higher.

• -fipa-ra — use caller save registers for allocation if those registers are not used by any called function. In that case it is not necessary to save and restore them around calls. This is only possible if called functions are part of same compilation unit as current function and they are compiled before it.

Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this happens on targets that do not expose prologues and epilogues in RTL).

• -fconserve-stack — attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
• -ftree-reassoc — perform reassociation on trees. This flag is enabled by default at -O1 and higher.
• -fcode-hoisting — perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. This flag is enabled by default at -O2 and higher. 🔥
• -ftree-pre — perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
• -ftree-partial-pre — make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
• -ftree-forwprop — perform forward propagation on trees. This flag is enabled by default at -O1 and higher.
• -ftree-fre — perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O1 and higher.
• -ftree-phiprop — perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O1 and higher.
• -fhoist-adjacent-loads — speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher.
• -ftree-copy-prop — perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O1 and higher.
• -fipa-pure-const — discover which functions are pure or constant. Enabled by default at -O1 and higher.
• -fipa-reference — discover which static variables do not escape the compilation unit. Enabled by default at -O1 and higher.
• -fipa-reference-addressable — discover read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher.
• -fipa-stack-alignment — reduce stack alignment on call sites if possible. Enabled by default.
• -fipa-pta — perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. 💀
• -fipa-profile — perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as cold, noreturn, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O1 and higher.
• -fipa-modref — perform interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. This flag is enabled by default at -O1 and higher.
• -fipa-cp — perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile. 🌈
• -fipa-cp-clone — perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
• -fipa-vrp — when enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf — perform Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold.

This flag is enabled by default at -O2 and -Os.

• -flive-patching=level — control GCC's optimizations to produce output suitable for live-patching.

If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too. 🫶

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels.

The level argument should be one of the following:

• inline-clone — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well.

-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref

• inline-only-static — only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto). 💅

• -fisolate-erroneous-paths-dereference — bestie, this one catches the paths in your code that do the unthinkable: dereferencing a null pointer. 💀 It detects paths that trigger erroneous or undefined behavior because of that, then isolates those paths away from the main control flow and turns that statement with erroneous or undefined behavior into a trap. It's enabled by default at -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled — no -fdelete-null-pointer-checks, no deal. 🫶
• -fisolate-erroneous-paths-attribute — same energy as above but for a null value being used in a way that's forbidden by a "returns_nonnull" or "nonnull" attribute. It detects those paths that trigger erroneous or undefined behavior, isolates them from the main control flow, and turns the statement with erroneous or undefined behavior into a trap. Heads up though: this is not currently enabled, but it may be enabled by -O2 in the future. ✨
• -ftree-sink — performs forward store motion on trees. Enabled by default at -O1 and higher. 🦋
• -ftree-bit-ccp — performs sparse conditional bit constant propagation on trees, and also propagates pointer alignment information. This pass only operates on local scalar variables, and it's enabled by default at -O1 and higher — except for -Og. It requires that -ftree-ccp is enabled. 🌸
• -ftree-ccp — performs sparse conditional constant propagation (CCP) on trees. Only operates on local scalar variables, and it's enabled by default at -O1 and higher. 💅
• -fssa-backprop — propagates info about uses of a value up the definition chain so the definitions can get simplified. Like, for example, this pass strips sign operations if the sign of a value never matters. Enabled by default at -O1 and higher. 🔥
• -fssa-phiopt — performs pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at -O1 and higher, except for -Og. ✨
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array. Enabled by default at -O2 and higher. 🎀
• -ftree-tail-merge — hunts for identical code sequences, and when it finds them, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at -O2 and 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 — performs dead code elimination (DCE) on trees. Enabled by default at -O1 and higher. 💜
• -ftree-builtin-call-dce — performs conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are otherwise free of side effects. Enabled by default at -O2 and higher if -Os is not also specified. 🌈
• -ffinite-loops — assumes that a loop with an exit will eventually take that exit and not loop forever. That lets the compiler remove loops that otherwise have no side-effects, since it's not considering eventual endless looping as such. 🌀 This option is enabled by default at -O2 for C++ with -std=c++11 or higher.
• -ftree-dominator-opts — performs a whole lil' variety of simple scalar cleanups based on a dominator tree traversal: constant/copy propagation, redundancy elimination, range propagation and expression simplification. It also performs jump threading (to reduce jumps to jumps). Enabled by default at -O1 and higher. 💖
• -ftree-dse — performs dead store elimination (DSE) on trees. A dead store is a store into a memory location that later gets overwritten by another store, with no intervening loads in between. In that case, the earlier store can be deleted. Enabled by default at -O1 and higher. 🍓
• -ftree-ch — performs loop header copying on trees. This is a good thing because it increases the effectiveness of code motion optimizations, and it also saves one jump. Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size. 💅
• -ftree-loop-optimize — performs loop optimizations on trees. Enabled by default at -O1 and higher. ⭐
• -ftree-loop-linear, -floop-strip-mine, -floop-block — perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. 🧚
• -fgraphite-identity — enables the identity transformation for graphite. For every SCoP, the polyhedral representation is generated and then transformed back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops. ✨
• -floop-nest-optimize — 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 bestie: this option is experimental. 🙈
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized, then parallelizes all the loops that can be analyzed to not contain loop carried dependences — without checking whether it's actually profitable to parallelize the loops. 😭
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. Lowkey important warning: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise. 💜
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops into branch-less equivalents. The intent is to remove control-flow from the innermost loops to improve the ability of the vectorization pass to handle these loops. Enabled by default if vectorization is enabled. 🌸
• -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop

       DO I = 1, N
         A(I) = B(I) + C
         D(I) = E(I) * F
       ENDDO

is transformed to

       DO I = 1, N
          A(I) = B(I) + C
       ENDDO
       DO I = 1, N
          D(I) = E(I) * F
       ENDDO

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🔥

• -ftree-loop-distribute-patterns — performs loop distribution of patterns that can be code generated with calls to a library. Enabled by default at -O2 and higher, and by -fprofile-use and -fauto-profile. 🎀

This pass distributes the initialization loops and generates a call to memset zero. For example, the loop

       DO I = 1, N
         A(I) = 0
         B(I) = A(I) + I
       ENDDO

is transformed to

       DO I = 1, N
          A(I) = 0
       ENDDO
       DO I = 1, N
          B(I) = A(I) + I
       ENDDO

and the initialization loop is transformed into a call to memset zero. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. ✨

• -floop-interchange — performs loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop

       for (int i = 0; i < N; i++)
         for (int j = 0; j < N; j++)
           for (int k = 0; k < N; k++)
             c[i][j] = c[i][j] + a[i][k]*b[k][j];

is transformed to

       for (int i = 0; i < N; i++)
         for (int k = 0; k < N; k++)
           for (int j = 0; j < N; j++)
             c[i][j] = c[i][j] + a[i][k]*b[k][j];

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 💅

• -floop-unroll-and-jam — applies unroll and jam transformations on feasible loops. In a loop nest, this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🌈
• -ftree-loop-im — performs loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion. 💖
• -ftree-loop-ivcanon — creates a canonical counter for the number of iterations in loops where figuring out the number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling. 🦋
• -ftree-scev-cprop — performs final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, then uses of the final value get replaced by such a computation — provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher. ⭐
• -fivopts — performs induction variable optimizations on trees: strength reduction, induction variable merging and induction variable elimination. 🔥
• -ftree-parallelize-loops=n — parallelizes loops, i.e., splits 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 — does function-local points-to analysis on trees 🎯 like it's tracking where every pointer is pointing, locally, no drama. On by default at -O1 and up, except for -Og.
• -ftree-sra — scalar replacement of aggregates, bestie ✨ this pass swaps structure references out for scalars so we're not committing whole structures to memory before we absolutely have to. On by default at -O1 and higher, except for -Og.
• -fstore-merging — merges narrow stores that land on consecutive memory addresses 🫶 it takes a bunch of contiguous stores of immediate values that are each narrower than a word and smushes them into fewer, wider stores, so the instruction count goes down. On by default at -O2 and higher, plus -Os.
• -ftree-ter — temporary expression replacement, happening during the SSA->normal phase 💅 single-use/single-def temporaries get replaced at their use location with their defining expression. Yeah this produces non-GIMPLE code, but it hands the expanders way more complex trees to work with, which means better RTL generation. On by default at -O1 and higher.
• -ftree-slsr — straight-line strength reduction on trees 🔥 it spots related expressions that involve multiplications and swaps them for cheaper calculations whenever it can. On by default at -O1 and higher.
• -ftree-vectorize — vectorization on trees, periodt ✨ this flag turns on -ftree-loop-vectorize and -ftree-slp-vectorize if you didn't explicitly specify them yourself.
• -ftree-loop-vectorize — loop vectorization on trees 🌈 on by default at -O2, and also enabled by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — basic block vectorization on trees 🌸 on by default at -O2, and also enabled by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftrivial-auto-var-init=choice — initializes automatic variables with either a pattern or with zeroes 😌 the whole point is upping the security and predictability of a program by preventing uninitialized memory disclosure and use. Real talk though: GCC still considers an automatic variable that has no explicit initializer to be uninitialized, so -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still warn you about those automatic variables. With this option, GCC also initializes any padding of automatic variables that have structure or union types to zeroes. But heads up: the current implementation cannot initialize automatic variables declared between the controlling expression and the first case of a "switch" statement. Use -Wtrivial-auto-var-init to 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 that will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and are not values 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.

You can control this behavior for one specific variable by using the variable attribute "uninitialized" 🎀

• -fvect-cost-model=model — changes up the cost model used for vectorization 💫 model should be one of unlimited, dynamic, cheap or very-cheap. With unlimited the vectorized code-path is just assumed to be profitable. With dynamic, a runtime check guards the vectorized code-path so it only kicks in for iteration counts that will likely execute faster than running the original scalar loop. cheap disables vectorization of loops where doing so would be cost prohibitive — like when you'd need runtime checks for data dependence or alignment — but otherwise it's the same as dynamic. very-cheap only allows vectorization if the vector code would entirely replace the scalar code being vectorized. Example: if each iteration of a vectorized loop could only handle exactly four iterations of the scalar loop, very-cheap would only allow vectorization if the scalar iteration count is known to be a multiple of four.

The default cost model depends on other optimization flags and is either dynamic or cheap.

• -fsimd-cost-model=model — changes the cost model used for vectorization of loops marked with the OpenMP simd directive 🦋 model should be one of unlimited, 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-model gets used.
• -ftree-vrp — Value Range Propagation on trees 👑 similar to the constant propagation pass, except instead of values, ranges of values get propagated. This lets the optimizers delete unnecessary range checks like array bound checks and null pointer checks. On by default at -O2 and higher. Null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled.
• -fsplit-paths — splits paths leading to loop backedges 🔀 can improve dead code elimination and common subexpression elimination. On by default at -O3 and 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 makes the scheduling passes more efficient.

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. It also straight up doesn't work 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 makes multiple copies of some local variables when it unrolls a loop, which can give you superior code 💖

This optimization is enabled by default for PowerPC targets, but disabled by default everywhere else.

• -fpartial-inlining — inline parts of functions 🙈 this option only does anything when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

• -fpredictive-commoning — predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) that were performed in previous iterations of loops 🔁

This option is enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile.

• -fprefetch-loop-arrays — if the target machine supports it, generate instructions to prefetch memory so loops that access large arrays perform better 🏃♀️

This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.

Disabled at level -Os.

• -fno-printf-return-value — do not substitute constants for the known return value of formatted output functions like "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" or "fprintf") 🚫 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-value is in effect, both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer, because the return value is guaranteed to be at most 8.

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and gives the best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.

• -fno-peephole
• -fno-peephole2 — disable any machine-specific peephole optimizations 💀 the difference between -fno-peephole and -fno-peephole2 is 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 profiling feedback (-fprofile-arcs) doesn't provide them. 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 sometimes it's useful to disable the heuristics so the effects of "__builtin_expect" are easier to understand.

You can also specify the expected probability of 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 — reorders 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 ✨ algorithm can be simple, which doesn't increase code size (except sometimes from secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all the often-executed code together, minimizing the number of branches executed by making extra copies of code.

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.

• -freorder-blocks-and-partition — bestie, this one is not just shuffling your basic blocks around (which already cuts down on taken branches, cute). It goes further and partitions hot and cold basic blocks into separate sections of the assembly and .o files, so your paging and cache locality get a glow-up. 💅

But the tea ☕ — this optimization gets automatically turned off in a bunch of situations:

• when exception handling or unwind tables are present (on targets using setjump/longjump or some target-specific scheme)
• for linkonce sections
• for functions with a user-defined section attribute
• on any architecture that does not support named sections

And when -fsplit-stack is used, this is not enabled by default (it would cause linker errors, tragic 😭), but you can enable it explicitly if you're using a working linker.

Enabled for x86 at levels -O2, -O3, -Os.

• -freorder-functions — reorders functions inside the object file so code locality gets better, iykyk. It does this with special subsections: .text.hot for the functions that run the most, and .text.unlikely for the ones that basically never run. The linker does the reordering, so your object file format must support named sections and the linker has to place them in a reasonable way. No named sections, no reorder, no cap. 🧚

Lowkey important 🙈: this option isn't effective unless you either provide profile feedback (peep -fprofile-arcs for the 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 that apply to whatever language you're compiling. For C (and C++), this switches on optimizations based on the type of expressions. The vibe: an object of one type is assumed to never live at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type can 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;
}

Reading from a different union member than the one you most recently wrote to (this is called type-punning, iconic behavior) is super common. Even with -fstrict-aliasing, type-punning is allowed as long as the memory is accessed through the union type. So the code above works as expected. 🫶 However, this next one might not:

int f() {
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Same energy: accessing by taking the address, casting the resulting pointer, and dereferencing the result has undefined behavior, even if the cast uses a union type, like:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

• -fipa-strict-aliasing — controls whether the rules of -fstrict-aliasing get 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. 🦋

The -fipa-strict-aliasing option 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 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This guarantees that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. Slay. 🎀

If m isn't specified, it defaults to n.

Examples, because we love receipts:

• -falign-functions=32 aligns functions to the next 32-byte boundary
• -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less
• -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less

The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two; in that case it gets rounded up.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.

If n isn't specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -flimit-function-alignment — with this on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. Politeness, honestly. 🍓
• -falign-labels, -falign-labels=n, -falign-labels=n:m, -falign-labels=n:m:n2, -falign-labels=n:m:n2:m2 — aligns all branch targets to a power-of-two boundary. ✨

Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

If n isn't specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -falign-loops, -falign-loops=n, -falign-loops=n:m, -falign-loops=n:m:n2, -falign-loops=n:m:n2:m2 — aligns loops to a power-of-two boundary. If the loops get executed many times, this makes up for any execution of the dummy padding instructions. Math 💅

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.

If n isn't specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

• -falign-jumps, -falign-jumps=n, -falign-jumps=n:m, -falign-jumps=n:m:n2, -falign-jumps=n:m:n2:m2 — aligns branch targets to a power-of-two boundary, but only for branch targets where the target can only be reached by jumping. In this case, no dummy operations need to be executed. Chef's kiss. 👑 🎀

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n isn't specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -fno-allocation-dce — do not remove unused C++ allocations in dead code elimination. That's it, that's the whole thing. 🙈
• -fallow-store-data-races — lets the compiler perform optimizations that may 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 know the global data won't 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. ⚠️ And note: 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 around for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

Enabled by default.

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. They get output in the exact same order they appear in the input file. When this option is used, unreferenced static variables are not removed. This option exists to support existing code that relies on a particular ordering — for new code, it's better to use attributes when possible. 💜

-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally, -fno-toplevel-reorder implies -fno-section-anchors.

• -fweb — constructs webs the way they're 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, but it also strengthens several other optimization passes, like CSE, the loop optimizer, and the trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register". 😭 💀

Enabled by default with -funroll-loops.

• -fwhole-program — assume that the current compilation unit represents the whole program being compiled. All public functions and variables — with the exception of main and those merged by the attribute externally_visible — become static functions and in effect get optimized more aggressively by interprocedural optimizers. Main character energy for your one file. 👑

This option should not be used in combination with -flto. Instead, relying on a linker plugin should provide safer and more precise information.

• -flto[=n] — this option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. Fr fr, it's giving ✨whole-program✨ but make it legal. 🌈

okay so to actually USE the link-time optimizer, you gotta pass -flto and your optimization options both at compile time and during the final link. The recommended vibe: compile every single file that's gonna be in the same link with the same options, and also drop those options at link time. Like so:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

So the first two calls to GCC stash a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o 🫶. Then that final invocation reads the GIMPLE bytecode back out of foo.o and bar.o, merges the two files into one single internal image, and compiles the result like normal. And since both foo.o and bar.o got merged into one image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were literally one file. Which means, for example, the inliner can inline functions in bar.o into functions in foo.o and vice-versa 💅.

There's another (simpler) way to turn on link-time optimization:

gcc -o myprog -flto -O2 foo.c bar.c

That one generates bytecode for foo.c and bar.c, merges them into a single GIMPLE representation, and optimizes them like 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 do the link step. GCC automatically does link-time optimization if any of the objects involved were compiled with the -flto command-line option. And you can always override that automatic decision by passing -fno-lto to the link command, bestie.

To make whole program optimization actually effective, you need to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When the linker supports it, the linker plugin (see -fuse-linker-plugin) passes info to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used to let the compiler make these assumptions — which leads to more aggressive optimization decisions 🔥.

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is bigger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). So that means object files with LTO info can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations get applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them. 🫠

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can mix and match object files and libraries with GIMPLE bytecodes and final object code, no cap. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing ✨.

Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.

If you don't specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. But note that it's generally ineffective to specify an optimization level option only at link time and not at compile time — for two reasons:

1. Compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time.
2. Some early optimization passes can be performed only at compile time and not at link time.

There are some code generation flags that GCC preserves when generating bytecodes, because they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on this scheme:

-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpie

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time 💅.

Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not consistent with the setting at compile time. 🙈

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.

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 🎀.

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.

Another feature of LTO is that it'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 -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation 🦋.

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it'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; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized ✨.

If you're not using a linker with plugin support and/or don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.

Link-time optimizations don't 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 allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities 🔥. 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 is 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. 💀

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. 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. ⭐

• -flto-partition=alg — bestie this one picks the partitioning algorithm for the link-time optimizer, okay? So the value is either 1to1 (that's a partitioning that mirrors your original source files, one-for-one, cute), or balanced (chops things into equally sized chunks, whenever that's even possible), or max (makes a brand new partition for every single symbol where possible — highkey a lot). And if you pass none as the algorithm, partitioning and streaming get disabled completely, like gone. Default vibe is balanced. So while 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only 🙈 — do not treat it as a lifestyle. And there's also the value one, which specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase. So yes, none and one are different things, don't @ me.
• -flto-compression-level=n — sets the compression level used for the intermediate language that gets written into LTO object files. Important: it's only meaningful in conjunction with LTO mode (-flto), so without that it's giving nothing. GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range are clamped to either the minimum or the maximum of the supported values. If you don't give the option at all, a default balanced compression setting is used. Balanced, like her. 💅
• -fuse-linker-plugin — enables the use of a linker plugin during link-time optimization. This one relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. So old linker? Not invited. 💀

This option enables the extraction of object files with GIMPLE bytecode out of library archives. That improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). Default queen behavior. 👑

• -ffat-lto-objects — Fat LTO objects are object files that contain both the intermediate language and the object code. That makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them. Yes, you actually have to edit the makefile. Sorry bestie. 😭

Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).

The default is -fno-fat-lto-objects on targets with linker plugin support.

• -fcompare-elim — after register allocation and post-register allocation instruction splitting, this pass identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, it eliminates the explicit comparison operation. Efficiency, slay.

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. So it's a niche icon, not a universal one.

Enabled at levels -O1, -O2, -O3, -Os.

• -fcprop-registers — after register allocation and post-register allocation instruction splitting, this performs a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Occasionally! It's an opportunist. ✨

Enabled at levels -O1, -O2, -O3, -Os.

• -fprofile-correction — so profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates (the counters simply ghosted us 🙈). When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected — like, full on complaining.

This option is enabled by -fauto-profile.

• -fprofile-partial-training — with -fprofile-use, all portions of programs not executed during the train run are optimized agressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, program may contain functions specific for a given hardware and trianing may not cover all hardware configurations program is run on.) With -fprofile-partial-training profile feedback will be ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when the train run is not representative, but also leads to significantly bigger code. So: faster in the weird cases, chunkier binaries. Pick your fighter. 💀
• -fprofile-use
• -fprofile-use=path — enables profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Before you can use this option, you must first generate profiling information. No profile data, no profile feedback — that's the whole deal.

By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

• -fauto-profile
• -fauto-profile=path — enables sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.

E.g.

        perf record -e br_inst_retired:near_taken -b -o perf.data \
            -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.

E.g.

        create_gcov --binary=your_program.unstripped --profile=perf.data \
            --gcov=profile.afdo

---

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. And hear me out bestie: all of them must be specifically enabled. They don't just show up on their own. 🌸

• -ffloat-store — do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory.

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables. Yes, after modifying them. The flag is not a magic wand, it's a whole plan. 🪄

• -fexcess-precision=style — this option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used.

-fexcess-precision=standard is not implemented for languages other than C. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. Unpredictable!! Noted, flagged, and feared. 💀

• -ffast-math — sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast. A whole bundle, one flag. Iconic efficiency. ✨

This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. So like: if you need the math to be exactly right, do not do this. If you just need it fast, go off. 🔥

• -fno-math-errno — bestie, this one says don't set errno after calling math functions that run in a single instruction, like sqrt. If your program handles math errors via IEEE exceptions instead, you might want this flag for the speed boost while still keeping IEEE arithmetic compatibility. ✨

Heads up tho: no -O option turns this on, because it can genuinely produce incorrect output for programs that need an exact implementation of IEEE or ISO rules/specs for math functions. But it can give you faster code for programs that don't need those guarantees. 💅

The default is -fmath-errno.

Oh and on Darwin systems? The math library never sets errno. So there's literally no reason for the compiler to consider that it might — -fno-math-errno is the default there. 🍓

• -funsafe-math-optimizations — allow floating-point optimizations that (a) assume arguments and results are valid and (b) may violate IEEE or ANSI standards. When you use it at link time, it may pull in libraries or startup files that change the default FPU control word, or other similar optimizations. 😭

Not turned on by any -O option, since it can result in incorrect output for programs depending on an exact implementation of IEEE or ISO rules/specs for math functions. It may, however, yield faster code for programs that don't need those guarantees. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

• -fassociative-math — allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation results. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — so it cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. It may also reorder floating-point comparisons, so it may not be used when ordered comparisons are required. 🫠 This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Also, it doesn't make much sense with -frounding-math. For Fortran, the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

• -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if that enables optimizations. So like, x / y can become x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 💀

The default is -fno-reciprocal-math.

• -ffinite-math-only — allow floating-point optimizations that assume arguments and results are not NaNs or +-Infs.

Not turned on by any -O option, because it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specs for math functions. It may, however, yield faster code for programs that don't require those guarantees.

The default is -fno-finite-math-only.

• -fno-signed-zeros — allow floating-point optimizations that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant. 🌸

The default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if you're relying 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/specs for math functions.

The default is -ftrapping-math.

• -frounding-math — disable transformations and optimizations that assume default floating-point rounding behavior. That default is round-to-zero for all floating-point-to-integer conversions, and round-to-nearest for all other arithmetic truncations. You should specify this option for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. 👑

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used to specify the default state for FENV_ACCESS.

• -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. 🎀

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

• -fno-fp-int-builtin-inexact — do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C2X, does not allow these functions to do so. 🦋

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C2X 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 — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. 🧚
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math. 💜

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. 🌈

The default is -fno-cx-fortran-rules.

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code. 💀

• -fbranch-probabilities — after running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. 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 — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register". 😭

Enabled by default with -funroll-loops.

• -fschedule-fusion — bestie this one runs a target dependent pass over the instruction stream and lines up instructions of the same type right next to each other, because your target machine can execute them way more efficiently when they're adjacent in the instruction flow. It's giving ✨organized chaos✨.

Enabled at levels -O2, -O3, -Os.

• -ftracer — does tail duplication to make the superblock bigger. This transformation simplifies the control flow of the function so the other optimizations can do a better job. Teamwork fr fr 🫶.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — unrolls loops whose number of iterations can be figured out at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). Heads up though: this option makes code larger, and it may or may not make it run faster. So like… vibe check before you commit 💅.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unrolls all the loops, even when the number of iterations is uncertain when the loop is entered. Not gonna lie, this usually makes programs run more slowly 😭. -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops where there's enough info that they don't roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). Peeling off the layers like a lil clementine 🍓.

Enabled by -O3, -fprofile-use, and -fauto-profile.

• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.
• -fmove-loop-stores — enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop, in exchange for carrying the stored value in a register across the iteration. Note: for this option to have an effect, -ftree-loop-im has to be enabled as well. Enabled at level -O1 and 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. It's giving main character splitting into two mains 😌.

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 result of the condition).

Enabled by -fprofile-use and -fauto-profile.

• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one. Like, for example:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

This is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -ffunction-sections
• -fdata-sections — place each function or data item into its own section in the output file, if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Every function gets its own lil room, no roommates ✨🏠.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

• -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 out 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. Security girlie behavior, we stan 🔥🔒.

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 the amount of optimization that 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 --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. So like… don't get attached 💀.

In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q options.

In each case, the value is an integer. The 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 internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro.
• max-crossjump-edges — the maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.
• min-crossjump-insns — the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.
• max-grow-copy-bb-insns — the maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.
• max-goto-duplication-insns — the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.
• max-delay-slot-insn-search — the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.
• max-delay-slot-live-search — when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
• max-gcse-memory — the approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.

• max-gcse-insertion-ratio — okay so if the ratio of expression insertions to deletions goes past this value for any expression, RTL PRE just goes ahead and inserts or removes the expression, which means it leaves partially redundant computations chillin' in the instruction stream. that's the whole vibe of this param fr
• max-pending-list-length — the max number of pending dependencies scheduling is allowed to have before it flushes the current state and starts all over. heads up bestie: big functions with barely any branches or calls can build up excessively large lists, and those needlessly eat memory and resources. 💀
• max-modulo-backtrack-attempts — how many backtrack attempts the scheduler gets to make when it's doing modulo scheduling on a loop. bigger values can exponentially increase compilation time, so like… be careful. 😭
• max-inline-functions-called-once-loop-depth — the maximal loop depth of a call that the inline heuristics care about when they're trying to inline all functions called once.
• max-inline-functions-called-once-insns — the maximal estimated size of the functions that get produced while inlining functions called once.
• max-inline-insns-single — so several parameters control the tree inliner used in GCC, and this number sets the max number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will even consider for inlining. tiny detail but it matters: this only affects functions declared inline and methods implemented in a class declaration (C++).
• max-inline-insns-auto — when you use -finline-functions (which is included in -O3), a lot of functions that would otherwise never get looked at for inlining by the compiler do get investigated. For those functions a different, more restrictive limit — compared to functions declared inline — can be applied, and that's --param max-inline-insns-auto.
• max-inline-insns-small — this is the bound that gets applied to calls which are considered relevant with -finline-small-functions.
• max-inline-insns-size — this is the bound applied to calls which are optimized for size. A little growth may be desirable so you can anticipate the optimization opportunities that get exposed by inlining. ✨
• uninlined-function-insns — the number of instructions the inliner accounts for function overhead like the function prologue and epilogue.
• uninlined-function-time — the extra time the inliner accounts for function overhead, like the time needed to actually execute that function prologue and epilogue.
• inline-heuristics-hint-percent — the scale (in percents) that gets applied to inline-insns-single, inline-insns-single-O2 and inline-insns-auto when the inline heuristics hint that inlining is very profitable, i.e. it'll enable later optimizations. 🤑
• uninlined-thunk-insns, uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time, but applied to function thunks instead.
• inline-min-speedup — when the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can get inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. it's giving main character exemption. 👑
• large-function-insns — the limit that specifies what counts as a <em>really</em> large function. For functions larger than this limit after inlining, inlining gets constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by the non-linear algorithms the back end uses. ⏳
• large-function-growth — specifies the maximal growth of a large function caused by inlining, in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size.
• large-unit-insns — the limit specifying a large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth.
• lazy-modules — the maximum number of concurrently open C++ module files when lazy loading. 🗂️
• inline-unit-growth — specifies the maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. fun lil detail: cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.
• ipa-cp-unit-growth — specifies the maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size.
• ipa-cp-large-unit-insns — the size of translation unit that the IPA-CP pass considers large.
• large-stack-frame — the limit that specifies large stack frames. While inlining, the algorithm is trying not to grow past this limit too much.
• large-stack-frame-growth — specifies the maximal growth of large stack frames caused by inlining, in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size.
• max-inline-insns-recursive, max-inline-insns-recursive-auto — specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled, and --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, and --param max-inline-recursive-depth-auto applies instead.

• min-inline-recursive-probability — recursive inlining is profitable only for functions that have deep recursion on average, and it can hurt functions that have little recursion depth, by increasing the prologue size or the complexity of the function body for the other optimizers.

When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that the 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 the growth that the early inliner can make. In effect it increases the amount of inlining for code that has a large abstraction penalty.
• max-early-inliner-iterations — the limit on 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 — the probability (in percent) that C++ inline functions 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 the alias oracle can perform to disambiguate memory locations using the mod/ref information. note: this parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs.
• modref-max-depth — specifies the maximum depth of the DFS walk used by modref escape analysis. Setting it to 0 disables 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 gets enlarged during modref dataflow analysis.
• profile-func-internal-id — a parameter controlling whether to use the function internal id in profile database lookup. If the value is 0, the compiler uses an id that's based on the function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. 🫶
• min-vect-loop-bound — the minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.
• gcse-cost-distance-ratio — the scaling factor in the calculation of the 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 a cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions.
• gcse-unrestricted-cost — the cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 lets all expressions travel unrestricted distances. no cap, free range expressions 🦋
• max-hoist-depth — okay so this one sets how deep we dig into the dominator tree looking for expressions to hoist. it's there to stop the hoisting algo from going quadratic on us. and bestie, if you set it to 0, that means no limit at all — but fair warning, it can make compiling huge functions slow as heck 😭
• max-tail-merge-comparisons — this is the max number of similar basic blocks (bbs) we'll compare a bb against. keeps tree tail merging from going quadratic, periodt. 💅
• max-tail-merge-iterations — the max number of times the pass runs over the function. it's just here to keep tree tail merging from eating all your compile time. no cap.
• store-merging-allow-unaligned — if you turn this on, the store merging pass is allowed to introduce unaligned stores — but only when it's legal, obviously. slay.
• max-stores-to-merge — the max number of stores we'll try to smoosh together into wider stores in the store merging pass. the more the merrier? maybe. 🍓
• max-store-chains-to-track — how many store chains we keep an eye on at once while trying to merge them into wider stores in the store merging pass. multitasking queen 👑
• max-stores-to-track — the max number of stores we track simultaneously in the attempt to merge them into wider stores in the store merging pass. (original had a typo, but we're not judging 💀)
• max-unrolled-insns — the max number of instructions a loop can have before it's considered for unrolling. and if it does get unrolled, this also decides how many times the loop code gets duplicated. double duty, slay. ✨
• max-average-unrolled-insns — same vibe as above, but the instruction count is weighted by how likely each one is to execute. and yeah, if the loop gets unrolled, this also sets how many times the code is unrolled. math is mathing. 🧮
• max-unroll-times — the absolute max number of times a single loop can be unrolled. that's it. iconic. 💖
• max-peeled-insns — the max number of instructions a loop can have to be eligible for peeling. and if it gets peeled, this also says how many times the loop code gets peeled. peeling is like unrolling but different, iykyk. 🍊
• max-peel-times — the max number of peelings for one single loop. don't over-peel, bestie. 🙈
• max-peel-branches — the max number of branches allowed on the hot path through the peeled sequence. keep it hot but not too complicated 🔥
• max-completely-peeled-insns — the max number of instructions (insns) in a completely peeled loop. completely peeled, like a clementine 🍊
• max-completely-peel-times — the max number of iterations a loop can have and still be considered suitable for complete peeling. if it loops more than this, no full peel for you. 🚫
• max-completely-peel-loop-nest-depth — the deepest a loop nest can be and still be eligible for complete peeling. deep nests need not apply. 🌊
• max-unswitch-insns — the max number of instructions in a loop that's been unswitched. unswitch is a whole vibe. 🌈
• max-unswitch-level — the max number of branches we can unswitch in one loop. don't go overboard. 💅
• lim-expensive — the minimum cost threshold for an expression to be considered expensive in loop invariant motion. only the bougie expressions get moved. 💎
• min-loop-cond-split-prob — when you've got FDO profile info, this sets the minimum probability threshold for a semi-invariant condition statement to trigger a loop split. FDO = feedback, and this is the gatekeeper. 🚪
• iv-consider-all-candidates-bound — this is the cutoff for induction variable candidates. if you have fewer than this many, we consider all of them for each use in induction variable optimizations. if you have more, we only look at the most relevant ones to avoid quadratic time complexity. keeping it efficient, bestie. ⚡
• iv-max-considered-uses — if a loop has more induction variable uses than this, the induction variable optimizations just give up on it. sometimes you gotta know when to fold 'em. 🃏
• iv-always-prune-cand-set-bound — if the candidate set is smaller than this number, we always try to prune unnecessary induction variables (ivs) when adding a new one. tidy queen. 🧹
• avg-loop-niter — just the average number of iterations of a loop. for when you need a ballpark. ⚾
• dse-max-object-size — the max size (in bytes) of objects that dead store elimination will track byte by byte. bigger values = longer compile times, so don't go crazy. 🐌
• dse-max-alias-queries-per-store — the max number of queries to the alias oracle per store. crank it up and you might remove more dead stores, but your compile time will suffer. tradeoffs, amirite? ⚖️
• scev-max-expr-size — this bounds how big expressions can get in the scalar evolutions analyzer. big expressions = slow analyzer. keep it cute and small. 🎀
• scev-max-expr-complexity — same energy, but for complexity of expressions in the scalar evolutions analyzer. complex expressions slow it down. simple is better. ✨
• max-tree-if-conversion-phi-args — the max number of arguments in a PHI node that TREE if conversion will support — unless the loop has the simd pragma, then all bets are off. PHI nodes, not the sorority kind 💅
• vect-max-version-for-alignment-checks — the max number of run-time checks we'll do when loop versioning for alignment in the vectorizer. too many checks and it's not worth it. 🛑
• vect-max-version-for-alias-checks — same but for alias checks in loop versioning in the vectorizer. alias checks are like, is this pointer the same as that one? and we only do so many. 🔍
• vect-max-peeling-for-alignment — the max number of loop peels to help with access alignment for the vectorizer. set it to -1 and there's no limit. go wild. 🦋
• max-iterations-to-track — the max number of loop iterations that the brute-force algorithm will try to evaluate when analyzing how many iterations the loop has. brute force, but make it bounded. 💪
• hot-bb-count-fraction — okay so this is the denominator n in the fraction 1/n. a basic block needs to have at least 1/n of the maximal execution count in the whole program to be considered hot. default is 10000, so a bb is hot if its count is > 1/10000 of the max. set it to 0 and nothing is ever hot. used in non-LTO mode. 🔥
• hot-bb-count-ws-permille — this one's for LTO mode. it's the number of most executed permilles (from 0 to 1000) of the whole program's profiled execution that a bb's execution count must fall into to be hot. default is 990, meaning a bb is hot if its count is in the top 990 permilles, aka 99.0%, of the profiled execution. 0 means never hot. permilles, not percents, keep up bestie. 📊
• hot-bb-frequency-fraction — denominator n again, but this time for a function's entry block frequency. a bb is hot if its execution frequency is at least 1/n of the entry block's frequency. default 1000 means hot if executed more than 1/1000 of the entry block's frequency. 0 means never hot. got it? 💁‍♀️
• unlikely-bb-count-fraction — denominator n for the fraction 1/n of the number of profiled runs of the whole program. if a bb's execution count is below that fraction, it's considered unlikely executed. default 20 means a bb is unlikely executed if it runs in fewer than 1/20 (or 5%) of the program's runs. 0 means it's always considered unlikely executed. harsh but fair. 😔
• max-predicted-iterations — the max number of loop iterations we'll predict without any runtime info. it's helpful when a function has one loop with a known bound and another with an unknown bound. the known one gets predicted right, but the unknown one averages out to roughly 10. so the unbounded loop looks artificially cold next to the other one. sad for it. ❄️
• builtin-expect-probability — this controls the probability that an expression has the specified value. it takes a percentage from 0 to 100. so like, builtin_expect but with a dial. 🎛️
• builtin-string-cmp-inline-length — the max length of a constant string in a builtin string comparison call that can still be inlined. longer strings? no inlining for you. 📏
• align-threshold — pick a fraction of the max execution frequency of a bb in a function to decide whether to align that bb. alignment queen 👑
• align-loop-iterations — if a loop is expected to iterate at least this many times, it gets aligned. give it the spotlight. 🌟
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — these two are a duo. tracer-dynamic-coverage limits superblock formation once a given percentage of executed instructions is covered — it keeps code size from ballooning unnecessarily. and tracer-dynamic-coverage-feedback only kicks in when you have profile feedback. real profiles (not the static estimates) are way less balanced, so the threshold can be a larger value. dynamic duo, literally. 🦸‍♀️🦸‍♀️
• tracer-max-code-growth — stop tail duplication once code growth hits this percentage. it's a bit of an artificial cap because most duplicates get eliminated later in cross jumping, so you can set it way higher than the code growth you actually want. it's giving 'temporary measure'. ⏳
• tracer-min-branch-ratio — stop reverse growth when the reverse probability of the best edge drops below this threshold (in percent). if it's not popping off, we stop. ✋
• tracer-min-branch-probability, tracer-min-branch-probability-feedback — another duo! tracer-min-branch-probability stops forward growth if the best edge's probability is lower than this threshold. just like tracer-dynamic-coverage, there are two versions: tracer-min-branch-probability-feedback for when you have profile feedback, and tracer-min-branch-probability for when you don't. with profile feedback, you need a more conservative (higher) value to make tracer actually do its thing. balance is key. ⚖️
• stack-clash-protection-guard-size — set the size of the OS-provided stack guard as 2 raised to num bytes. higher values might mean fewer explicit probes, but if you go larger than the OS-provided guard, your code is vulnerable to stack clash style attacks. so like, don't do that. safety first bestie 🫶
• stack-clash-protection-probe-interval — so stack clash protection is all about probing the stack while it's being allocated, and this param sets the max distance between those probes, expressed as 2 raised to num bytes. ✨

Higher values can cut down the number of explicit probes (less work, we love that), but a value larger than the guard your operating system provides will leave the code vulnerable to stack clash style attacks. So don't get greedy, bestie. 💀

• max-cse-path-length — the maximum number of basic blocks on a path that CSE will actually consider.
• max-cse-insns — the maximum number of instructions CSE processes before it flushes. 🚽
• ggc-min-expand — okay so GCC runs a garbage collector to manage its own memory allocation, main character energy tbh. This parameter is the minimum percentage by which the collector's heap is allowed to expand between collections. Tuning it may improve compilation speed — and it has no effect on code generation, periodt. 🎀

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. And setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity — which is extremely slow, but can be useful for debugging. 🐛

• ggc-min-heapsize — minimum size of the garbage collector's heap before it even begins bothering to collect garbage. The first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed and has no effect on code generation. 🦋

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure 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 can't calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. And setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

• max-reload-search-insns — the maximum number of instructions reload should look backward for an equivalent register. Higher values = more aggressive optimization, which means compilation time goes up with probably slightly better performance. 📈
• max-cselib-memory-locations — the maximum number of memory locations cselib should take into account. Higher values = more aggressive optimization, compilation time increases, probably slightly better performance. Same vibe as above. 💅
• max-sched-ready-insns — the maximum number of instructions that are ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Higher values mean more thorough searches, which makes compilation time go up with probably little benefit. 😭
• max-sched-region-blocks — the maximum number of blocks in a region to be considered for interblock scheduling.
• max-pipeline-region-blocks — the maximum number of blocks in a region to be considered for pipelining in the selective scheduler.
• max-sched-region-insns — the maximum number of insns in a region to be considered for interblock scheduling.
• max-pipeline-region-insns — the maximum number of insns in a region to be considered for pipelining in the selective scheduler.
• min-spec-prob — the minimum probability (in percents) of reaching a source block for interblock speculative scheduling.
• max-sched-extend-regions-iters — the maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions. 🙈
• max-sched-insn-conflict-delay — the maximum conflict delay for an insn to be considered for speculative motion.
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns get 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 a store and a load targeting the same memory locations. ⏱️
• selsched-max-lookahead — the maximum size of the lookahead window of selective scheduling. Basically it's a depth of search for available instructions.
• selsched-max-sched-times — the maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.
• selsched-insns-to-rename — the maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler.
• sms-min-sc — the minimum value of stage count that the swing modulo scheduler generates.
• max-last-value-rtl — the maximum size, measured as number of RTLs, that can be recorded in an expression in combiner for a pseudo register as the last known value of that register.
• max-combine-insns — the maximum number of instructions the RTL combiner tries to combine.
• integer-share-limit — small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed. This sets the maximum value of a shared integer constant. 🔥
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used.
• min-size-for-stack-sharing — the minimum size of variables taking part in stack slot sharing when not optimizing.
• max-jump-thread-duplication-stmts — maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
• max-fields-for-field-sensitive — maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.
• prefetch-latency — an estimate of the average number of instructions executed before a prefetch finishes. The distance prefetched ahead is proportional to this constant. Bumping this number up may also lead to fewer streams being prefetched (peep simultaneous-prefetches). 👀
• simultaneous-prefetches — maximum number of prefetches that can run at the same time.
• l1-cache-line-size — the size of a cache line in the L1 data cache, in bytes.
• l1-cache-size — the size of the L1 data cache, in kilobytes.
• l2-cache-size — the size of the L2 data cache, in kilobytes.
• prefetch-dynamic-strides — whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact that the stride is non-constant may make it hard to predict when there's a clear benefit to issuing these hints. 🤔

Set to 1 if 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, because in that 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-size, constructive-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 — like 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's 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 maximum number of stmts in a loop to be interchanged.
• loop-interchange-stride-ratio — the minimum ratio between the 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 — do we want the compiler to use the "canonical" type system? Short answer: yes, bestie. It should always be 1, which uses a way more efficient internal mechanism for comparing types in C++ and Objective-C++. But — and hear me out — if bugs in the canonical type system are making your compiles fail, set this to 0 to disable canonical types. That's the escape hatch. 💅
• switch-conversion-max-branch-ratio — switch initialization conversion simply refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. She has a limit and she's sticking to it.
• max-partial-antic-length — the max length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when you're optimizing at -O3 and above. Why do we need this? Because for some kinds of source code, the enhanced partial redundancy elimination optimization can literally run away and eat ALL the memory available on the host machine. 😭 This parameter caps the length of the sets that get computed, which stops the runaway behavior. And setting it to 0 means unlimited set length — chaotic, but your call.
• rpo-vn-max-loop-depth — the maximum loop depth that gets value-numbered optimistically. Once the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't. Not everyone makes the cut, sorry. ✨
• sccvn-max-alias-queries-per-access — the max number of alias-oracle queries we perform when hunting for redundancies in loads and stores. If this limit gets hit, the search is aborted and that load or store is not considered redundant. Also, the number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. So like, it's bounded either way.
• ira-max-loops-num — IRA uses regional register allocation by default. If a function contains more loops than the number you give here, only at most that many of the most frequently-executed loops form regions for regional register allocation. Popular loops only, basically. 👑
• ira-max-conflict-table-size — okay so IRA uses a sophisticated algorithm to compress the conflict table, but the table can still demand 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 switches to a faster, simpler, and lower-quality algorithm that doesn't require building a pseudo-register conflict table. Speed over quality, sometimes that's the move. 💀
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops so it can decide whether to move loop invariants (peep -O3). This parameter gives the number of available registers reserved for some other purposes. The default is the best value found from numerous experiments. Science. 🧪
• ira-consider-dup-in-all-alts — makes IRA consider the matching constraint (duplicated operand number) heavily in all available alternatives for the preferred register class. Set it to zero and 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.
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values that were reloaded in registers in subsequent insns. This optimization is called inheritance, and EBB is used as the region where it happens. This parameter defines the minimal fall-through edge probability, in percentage, used to add a BB to the inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. She's been stress-tested, bestie. 🫶
• loop-invariant-max-bbs-in-loop — loop invariant motion can be very expensive, both in compilation time and in the amount of compile-time memory it needs, when you've got very large loops. Loops with more basic blocks than this parameter just don't get loop invariant motion optimization performed on them. Big loop? No motion for you.
• 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 get considered for data dependence analysis. These large loops aren't handled by the optimizations that use loop data dependencies.
• max-vartrack-size — sets a max number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded while variable tracking at assignments is enabled, the 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 that function. Setting the parameter to zero makes it unlimited. 🌀
• max-vartrack-expr-depth — sets a max number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This is a trade: compilation time in exchange for more complete debug information. Set it too low and value expressions that are available and could've been represented in debug info may end up not being used; set it higher and the compiler may find more complex debug expressions, but compile time and memory use may grow. Balance, babe. ⚖️
• max-debug-marker-count — sets a threshold on the number of debug markers (like 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, those stmts get dropped from the inlined copy of the function, and from its RTL expansion. 🗑️
• min-nondebug-insn-uid — use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. No glow-ups bigger than that. 💅
• ipa-sra-max-replacements — the max 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 the aggregate considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize) respectively.
• sra-max-propagations — the 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. Also FYI: a variable whose value is unknown at compilation time and defined outside a SCoP counts as a parameter of the SCoP.
• loop-block-tile-size — loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter.
• ipa-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 so it can propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. 📝
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics, and performs those cloning opportunities whose scores exceed ipa-cp-eval-threshold.
• ipa-cp-max-recursive-depth — the maximum depth of recursive cloning for a self-recursive function. 🌀
• ipa-cp-min-recursive-probability — recursive cloning only happens when the probability of the call being executed exceeds this parameter.
• ipa-cp-profile-count-base — when using the -fprofile-use option, 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.
• ipa-cp-recursive-freq-factor — the number of times interprocedural copy propagation expects recursive functions to call themselves.
• ipa-cp-recursion-penalty — the percentage penalty recursive functions receive when they're evaluated for cloning.
• ipa-cp-single-call-penalty — the percentage penalty functions containing a single call to another function receive when they're evaluated for cloning.
• ipa-max-agg-items — IPA-CP is also capable of propagating a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter. ✨
• ipa-cp-loop-hint-bonus — so when IPA-CP clocks that a cloning candidate would make a loop's iteration count known, it sprinkles a lil bonus of ipa-cp-loop-hint-bonus onto that candidate's profitability score. basically a glow-up for candidates that spill the tea about loops 💅
• ipa-max-loop-predicates — the max number of different predicates IPA will bother using to describe when loops inside a function have known properties.
• ipa-max-aa-steps — while IPA-CP is out here analyzing function bodies, it uses alias analysis to track the values that function parameters point at. but bestie, it's not about to spend all day on a giant function 😭 so after it's examined ipa-max-aa-steps statements that modify memory, it just gives up and considers all memory clobbered. no cap.
• ipa-max-switch-predicate-bounds — the max number of boundary endpoints of case ranges in a switch statement. if a switch goes over this limit, IPA-CP will not construct a cloning cost predicate for the default case of that switch — and that predicate is the whole thing it uses to estimate cloning benefit. so yeah, no predicate for you 🫶
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter, to estimate the benefit of cloning on a certain constant value. but if the number of operations in a parameter expression goes over ipa-max-param-expr-ops, the expression gets labeled complicated and IPA analysis just… doesn't handle it. it's giving "not my problem" 💀
• lto-partitions — pick your desired number of partitions to be produced during WHOPR compilation. heads up: the number of partitions should exceed the number of CPUs used for compilation.
• lto-min-partition — the size of the minimal partition for WHOPR, measured in estimated instructions. this exists so we don't waste effort splitting tiny programs into a million lil partitions.
• lto-max-partition — the size of the max partition for WHOPR (in estimated instructions), so there's an upper bound on how big any individual partition gets. meant to be used only with balanced partitioning, iykyk.
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming.
• cxx-max-namespaces-for-diagnostic-help — the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. (yes bestie, it's literally hunting for a namespace that might explain your typo 🌸)
• sink-frequency-threshold — the maximum relative execution frequency, in percents, of the target block relative to a statement's original block, for statement sinking of a statement to be allowed. bigger numbers = more aggressive statement sinking. and a small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink. periodt.
• max-stores-to-sink — the max number of conditional store pairs that can be sunk. set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.
• case-values-threshold — the smallest number of different values for which it's best to use a jump-table instead of a tree of conditional branches. if the value is 0, use the default for the machine.
• jump-table-max-growth-ratio-for-size — the maximum code size growth ratio when expanding into a jump table, in percent. this parameter is used when optimizing for size.
• jump-table-max-growth-ratio-for-speed — the maximum code size growth ratio when expanding into a jump table, in percent. this one's used when optimizing for speed. ✨
• tree-reassoc-width — set the maximum number of instructions executed in parallel in a reassociated tree. this parameter overrides the target-dependent heuristics used by default, if it has a non-zero value.
• sched-pressure-algorithm — pick between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation, and it's the one more likely to prevent instructions from being reordered. Algorithm 2 was designed as a compromise between algorithm 1's relatively conservative approach and the rather aggressive approach the default scheduler takes. it leans way harder on having a regular register file and accurate register pressure classes. for the full tea, see haifa-sched.cc in the GCC sources.

The default choice depends on the target.

• max-slsr-cand-scan — set the maximum number of existing candidates that get considered when you're seeking a basis for a new straight-line strength reduction candidate.
• asan-globals — enable buffer overflow detection for global objects. this kind of protection is on by default if you're using -fsanitize=address. to turn off global objects protection, use --param asan-globals=0.
• asan-stack — enable buffer overflow detection for stack objects. on by default when using -fsanitize=address. to disable stack protection, use --param asan-stack=0.
• asan-instrument-reads — enable buffer overflow detection for memory reads. on by default when using -fsanitize=address. to disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — enable buffer overflow detection for memory writes. on by default when using -fsanitize=address. to disable memory writes protection, use --param asan-instrument-writes=0.
• asan-memintrin — enable detection for built-in functions. on by default when using -fsanitize=address. to disable built-in functions protection, use --param asan-memintrin=0.
• asan-use-after-return — enable detection of use-after-return. on by default when using the -fsanitize=address option. to disable it, use --param asan-use-after-return=0.

Note: by default the check is disabled at run time. to enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. don't forget bestie, or the check just sits there doing nothing 😭

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than 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.
• hwasan-instrument-stack — enable hwasan instrumentation of statically sized stack-allocated variables. this instrumentation is enabled by default when using -fsanitize=hwaddress and 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.
• hwasan-random-frame-tag — when using stack instrumentation, decide tags for stack variables using a deterministic sequence that begins at a random tag for each frame. with this parameter unset, tags get chosen using the same sequence but beginning from 1. it's enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress. to disable it, use --param hwasan-random-frame-tag=0.
• hwasan-instrument-allocas — enable hwasan instrumentation of dynamically sized stack-allocated variables. enabled by default when using -fsanitize=hwaddress and 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.
• hwasan-instrument-reads — enable hwasan checks on memory reads. instrumentation of reads is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to disable checking memory reads, use --param hwasan-instrument-reads=0.
• hwasan-instrument-writes — enable hwasan checks on memory writes. instrumentation of writes is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to disable checking memory writes, use --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=hwaddress and -fsanitize=kernel-hwaddress. to disable instrumentation of builtin functions, use --param hwasan-instrument-mem-intrinsics=0.
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. slay, skip the middleman 💅
• tsan-distinguish-volatile — emit special instrumentation for accesses to volatiles.
• tsan-instrument-func-entry-exit — emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit().
• max-fsm-thread-path-insns — the maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
• max-fsm-thread-length — the maximum number of basic blocks on a jump thread path.
• threader-debug — threader-debug=[none|all] enables verbose dumping of the threader solver.
• parloops-chunk-size — the chunk size of the omp schedule for loops parallelized by parloops.
• parloops-schedule — the schedule type of the omp schedule for loops parallelized by parloops: static, dynamic, guided, auto, runtime.
• parloops-min-per-thread — the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. ✨
• max-ssa-name-query-depth — maximum depth of recursion when querying properties of SSA names in things like fold routines. one level of recursion corresponds to following a use-def chain.
• max-speculative-devirt-maydefs — The max number of may-defs we analyze when we're looking for a must-def that specifies the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. 🔍
• max-vrp-switch-assertions — The max number of assertions to add along the default edge of a switch statement during VRP. 💅
• evrp-sparse-threshold — Max number of basic blocks before EVRP uses a sparse cache. ✨
• evrp-mode — Specifies the mode Early VRP should operate in. 🎀
• vrp1-mode — Specifies the mode VRP pass 1 should operate in. 🦋
• vrp2-mode — Specifies the mode VRP pass 2 should operate in. 🌸
• ranger-debug — Specifies the type of debug output to be issued for ranges. 💖
• evrp-switch-limit — Specifies the maximum number of switch cases before EVRP ignores a switch. 💀
• unroll-jam-min-percent — The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable. 🔥
• unroll-jam-max-unroll — The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation. 👑
• max-rtl-if-conversion-unpredictable-cost — Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable. 🌈
• max-variable-expansions-in-unroller — If -fvariable-expansion-in-unroller is used, the maximum number of times that an individual variable will be expanded during loop unrolling. ⭐
• partial-inlining-entry-probability — Maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen. 🫶
• max-tracked-strlens — Maximum number of strings for which strlen optimization pass will track string lengths. 💜
• gcse-after-reload-partial-fraction — The threshold ratio for performing partial redundancy elimination after reload. 🧚
• gcse-after-reload-critical-fraction — The threshold ratio of critical edges execution count that permit performing redundancy elimination after reload. 🍓
• max-loop-header-insns — The maximum number of insns in loop header duplicated by the copy loop headers pass. 🙈
• vect-epilogues-nomask — Enable loop epilogue vectorization using smaller vector size. 💅
• 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.
  • 0 stops the vectorizer from ever using partial vector loads and stores.
  • 1 allows partial vector loads and stores if vectorization removes the need for the code to iterate.
  • 2 allows partial vector loads and stores in all loops. The parameter only has an effect on targets that support partial vector loads and stores. ✨
• 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 is 50. 🔥
• vect-induction-float — Enable loop vectorization of floating point inductions. 💖
• avoid-fma-max-bits — Maximum number of bits for which we avoid creating FMAs. 🎀
• sms-loop-average-count-threshold — A threshold on the average loop count considered by the swing modulo scheduler. 🦋
• sms-dfa-history — The number of cycles the swing modulo scheduler considers when checking conflicts using DFA. 🌸
• graphite-allow-codegen-errors — Whether codegen errors should be ICEs when -fchecking. 💀
• sms-max-ii-factor — A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop. 👑
• lra-max-considered-reload-pseudos — The max number of reload pseudos which are considered during spilling a non-reload pseudo. ⭐
• max-pow-sqrt-depth — Maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. 🌈
• max-dse-active-local-stores — Maximum number of active local stores in RTL dead store elimination. 🫶
• asan-instrument-allocas — Enable asan allocas/VLAs protection. 💜
• max-iterations-computation-cost — Bound on the cost of an expression to compute the number of iterations. 🧚
• max-isl-operations — Maximum number of isl operations, 0 means unlimited. 🍓
• graphite-max-arrays-per-scop — Maximum number of arrays per scop. 🙈
• max-vartrack-reverse-op-size — Max. size of loc list for which reverse ops should be added. 💅
• fsm-scale-path-stmts — Scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks. ✨
• uninit-control-dep-attempts — Maximum number of nested calls to search for control dependencies during uninitialized variable analysis. 🔥
• fsm-scale-path-blocks — Scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements. 💖
• sched-autopref-queue-depth — Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic. 🎀
• loop-versioning-max-inner-insns — The maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy. 🦋
• loop-versioning-max-outer-insns — The maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning. 🌸
• ssa-name-def-chain-limit — The maximum number of SSA_NAME assignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics. 💀
• store-merging-max-size — Maximum size of a single store merging region in bytes. 👑
• hash-table-verification-limit — The number of elements for which hash table verification is done for each searched element. ⭐
• max-find-base-term-values — Maximum number of VALUEs handled during a single find_base_term call. 🌈
• analyzer-max-enodes-per-program-point — The maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point. 🫶
• analyzer-max-constraints — The maximum number of constraints per state. 💜
• 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. 🧚
• 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. 🍓
• 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. 🙈
• analyzer-max-svalue-depth — The maximum depth of a symbolic value, before approximating the value as unknown. 💅
• analyzer-max-infeasible-edges — The maximum number of infeasible edges to reject before declaring a diagnostic as 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. 🔥
• analyzer-bb-explosion-factor — The maximum number of 'after supernode' exploded nodes within the analyzer per supernode, before terminating analysis. 💖
• ranger-logical-depth — Maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges. 🎀
• relation-block-limit — Maximum number of relations the oracle will register in a basic block. 🦋
• 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 — Specify mode of OpenACC privatization diagnostics for -fopt-info-omp-note and applicable -fdump-tree-*-details.
  • 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-sve-compare-costs — When vectorizing for SVE, consider using "unpacked" vectors for smaller elements and use the cost model to pick the cheapest approach. Also use the cost model to choose between SVE and Advanced SIMD vectorization. Using unpacked vectors 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 proportional to this param when division approximation is enabled. The default value is 2. 👑
• aarch64-autovec-preference — Force an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4, inclusive. ⭐
  • 0 — Use the default heuristics.

1. Advanced SIMD only, bestie — auto-vectorization uses only Advanced SIMD. ✨
2. SVE only — auto-vectorization uses only SVE. ✨
3. Both, but SIMD-leaning — use both Advanced SIMD and SVE, and prefer Advanced SIMD when the costs are deemed equal. 💅
4. Both, but SVE-leaning — use both Advanced SIMD and SVE, and prefer SVE when the costs are deemed equal. 💅

The default value is 0. 🫶

aarch64-loop-vect-issue-rate-niters

Okay so the tuning for some AArch64 CPUs is out here trying to take both latencies and issue rates into account when it decides whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or just… 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. no cap.

---

🎀 Program Instrumentation Options 🎀

GCC supports a whole bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, why would you want that? Bestie, plenty of reasons: one purpose of instrumentation is to collect profiling statistics for use in finding program hot spots, doing code coverage analysis, or doing 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 — and also to catch deliberately hostile attacks such as stack smashing or C++ vtable hijacking. 😤

And there's also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. it's giving "we thought of everything." 👑

---

-p

-pg

Generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). 🫶

You must use this option when compiling the source files you want data about, and you must also use it when linking. (Yes, both. Don't skip the link step, I'm begging. 😭)

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. handy!

-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. very thorough of it honestly.

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.

--coverage

This option is used to compile and link code instrumented for coverage analysis. It's basically a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details. 👀

• Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program. 🙌
• Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
• Link your object files with -lgcov or -fprofile-arcs (the latter implies the former). 🔗
• Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting.

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 gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. 🧚

When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftest-coverage

Produce a notes file that the gcov code-coverage utility can use to show program coverage. 📓 Each source file's note file is called auxname.gcno.

Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. (I know, I know — no optimizing feels wrong, but trust. 😭)

-fprofile-abs-path

Automatically convert relative source file names to absolute path names in the .gcno files. ✨ This allows gcov to find the correct sources in projects where compilations occur with different working directories.

-fprofile-dir=path

Set the directory to search for the profile data files in to path. 📂

This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file.

In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. 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 variable VAR.

-fprofile-generate

-fprofile-generate=path

Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. 💖

You must use -fprofile-generate both when compiling and when linking your program. (Again, both. the way that linking keeps mattering… 😭)

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

Register 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-arcs is 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. 🚫

This option is not intended to be used in hosted environments such as GNU/Linux. It targets free-standing 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 *__gcov_info_start[];
extern const struct gcov_info *__gcov_info_end[];

static void
filename (const char *f, void *arg)
{
  puts (f);
}

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 *
allocate (unsigned length, void *arg)
{
  return malloc (length);
}

static void
dump_gcov_info (void)
{
  const struct gcov_info **info = __gcov_info_start;
  const struct gcov_info **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()
                   {
                     dump_gcov_info();
                     return 0;
                   }

• -fprofile-note=path — bestie, if you pass a path here, GCC saves the .gcno file right into that path location. 💾 But heads up: if you combine this option with multiple source files, the .gcno file will be overwritten — so like, only one of them gets to survive. 🙈
• -fprofile-prefix-path=path — okay so this one is the bestie of profile-generate=profile_dir and profile-use=profile_dir, and its whole job is telling GCC where the base directory of your built source tree is. By default, profile_dir is gonna be full of files with mangled absolute paths of every object file in the built project 😭 which is not it when the directory used to build the instrumented binary is different from the directory used to build the binary optimized with profile feedback — because then the profile data literally will not be found during the optimized build 💀. In setups like that, -fprofile-prefix-path=path with path pointing to the base directory of the build strips the irrelevant part of the path and keeps all file names relative to the main build directory. ✨ iconic.
• -fprofile-prefix-map=old=new — when compiling files that live in directory old, record profiling information (with --coverage) describing them as if the files lived in directory new instead. Also peep -ffile-prefix-map while you're here. 🫶
• -fprofile-update=method — changes up the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic, or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. 🧵

⚠️ Warning: when an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted. no cap, this is real.

Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line. 💅

• -fprofile-filter-files=regex — instruments only functions from files whose name matches any of the regular expressions (separated by semi-colons). ✂️

For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'.

• -fprofile-exclude-files=regex — instruments 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.

• -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, like, 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 program run but also before the fork function is invoked. 👀

Also note it's quite common that execution counts of some part of programs depend on stuff like, for example, length of temporary file names or memory space randomization (which may affect hash-table collision rate). Such non-reproducible parts of programs may be annotated by the no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible. 🔍

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible regardless of the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (such as with make -j). It reduces the quality of gathered data, in particular of indirect call profiling. 📉

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector. 🕵️‍♀️ Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable — when set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. The option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. And note that the only target -fsanitize=hwaddress is currently supported on is AArch64. 💀
• -fsanitize=kernel-address — enables AddressSanitizer for the Linux kernel. See https://github.com/google/kasan for more details. 🐧
• -fsanitize=hwaddress — enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead. ✨ Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html for more details. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable — when set to help=1, the available options are shown at startup of the instrumented program. The option cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64.
• -fsanitize=kernel-hwaddress — enables Hardware-assisted AddressSanitizer for compilation of the Linux kernel. 🐧 Similar to -fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the Linux kernel. These differences are to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.

Note: this option has different defaults to the -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options --param hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. Using a random frame tag is not implemented for kernel instrumentation.

• -fsanitize=pointer-compare — instruments comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. 🔎
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time. To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS. Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null. 🔎
• -fsanitize=shadow-call-stack — enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (e.g. stack buffer overflows). 🛡️ It works by saving a function's return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack.

Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For the user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. 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 is 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 — enables ThreadSanitizer, a fast data race detector. 🏁 Memory access instructions are instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak.

Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).

• -fsanitize=leak — enables LeakSanitizer, a memory leak detector. 💧 This option only matters for linking of executables and the executable is linked against a library that overrides malloc and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread.

---

• -fsanitize=undefined — bestie, this turns on UndefinedBehaviorSanitizer, aka the fast lil undefined-behavior detector 💅. It instruments a bunch of computations so it can clock undefined behavior at runtime. Peep https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html for the deep lore. The runtime vibe can be tweaked with the UBSAN_OPTIONS environment variable. Current suboptions, bestie, are:
  • -fsanitize=shift — checks that a shift operation's result isn't undefined ✨. Ngl, what counts as "undefined" wiggles a bit between C and C++, and between ISO C90 and C99, etc. This one has two sub-suboptions: -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — checks that the second argument of a shift op is not negative and is smaller than the precision of the promoted first argument 🌸.
  • -fsanitize=shift-base — if that second argument is within range, checks that the shift result isn't undefined. Same disclaimer as before: what exactly counts as undefined differs slightly between C and C++, and between ISO C90 and C99, etc. 🙈
  • -fsanitize=integer-divide-by-zero — detects integer division by zero 💀.
  • -fsanitize=unreachable — with this, the compiler flips the __builtin_unreachable call into a diagnostics message call instead. When you actually reach that __builtin_unreachable call, the behavior is undefined 🫠.
  • -fsanitize=vla-bound — tells the compiler to check that the size of a variable length array is positive 👑.
  • -fsanitize=null — pointer checking, periodt. An app built with this on will throw an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) gets bound to a NULL pointer, or if a method is invoked on an object pointed at by a NULL pointer 😭.
  • -fsanitize=return — return statement checking ✨. Programs built with this issue an error message when the end of a non-void function is reached without actually returning a value. Heads up: this one works in C++ only.
  • -fsanitize=signed-integer-overflow — signed integer overflow checking 🔥. It checks that the result of +, *, and both unary and binary - doesn't overflow in signed arithmetics. It also detects INT_MIN / -1 signed division. Ngl though, integer promotion rules have to be taken into account — meaning this next thing is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — instruments array bounds 🎀. Various out-of-bounds accesses get detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.
• -fsanitize=bounds-strict — the strict girlie version of array bounds instrumentation 💅. Most out-of-bounds accesses get detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are still not instrumented.
• -fsanitize=alignment — checks alignment of pointers when they get 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 ✨.
• -fsanitize=object-size — instruments memory references using the __builtin_object_size function 🦋. Various out-of-bounds pointer accesses get detected.
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. Unlike the other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because fp division by zero can be a totally legit way of obtaining infinities and NaNs 💜.
• -fsanitize=float-cast-overflow — floating-point type to integer conversion checking 🌈. It checks the conversion result doesn't overflow. Unlike the other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also it does not work well with FE_INVALID exceptions enabled 🙈.
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values get passed to arguments marked as requiring a non-null value by the nonnull function attribute ⭐.
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to catch returning null values from those functions 💖.
• -fsanitize=bool — instruments loads from bool 🍓. If a value other than 0/1 gets loaded, a run-time error is issued.
• -fsanitize=enum — instruments loads from an enum type 🌸. If a value outside the range of values for that enum type gets loaded, a run-time error is issued.
• -fsanitize=vptr — instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type 👑.
• -fsanitize=pointer-overflow — instruments pointer arithmetics ✨. If the pointer arithmetics overflows, a run-time error is issued.
• -fsanitize=builtin — instruments arguments to selected builtin functions 🔥. If an invalid value gets passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option.

So here's the tea: while -ftrapv makes traps for signed overflows get emitted, -fsanitize=undefined gives you a diagnostic message instead 💅. And this currently works only for the C family of languages.

• -fno-sanitize=all — this disables all previously enabled sanitizers 🚫. Note that -fsanitize=all is not allowed, because some sanitizers can't be used together, iykyk.
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks 🌟. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections 🎀. Each si may contain wildcards.
• -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts ✨. Enabling this for a sanitizer component makes it try to keep running the program as if nothing happened. Meaning multiple runtime errors can get reported in a single program run, and the exit code may indicate success even when errors were reported 😳. The -fno-sanitize-recover= option can alter this behavior: only the first detected error gets reported and then the program 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 recovery mode is on compiler-side, it also needs to be enabled on the runtime library side, otherwise the failures are still fatal 💀. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default for AddressSanitizer is halt_on_error=1. You can override this by setting the halt_on_error flag in the corresponding environment variable.

Heads up bestie: syntax without an explicit opts parameter is deprecated 🙈. It's equivalent to specifying an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs 🌸. This option sets -fstack-reuse to none.
• -fsanitize-undefined-trap-on-error — tells the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine 🎀. The glow-up here is that the libubsan library isn't needed and isn't linked in, so this is usable even in freestanding environments ✨.
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation 🦋. It inserts a call to __sanitizer_cov_trace_pc into every basic block.
• -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation 🔥. It inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, or __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements 💅.
• -fcf-protection=[full|branch|return|none|check] — okay bestie, this one turns on code instrumentation for control-flow transfers so the program can vibe-check that target addresses of control-flow transfer instructions are actually valid. We’re talking indirect function calls, function returns, indirect jumps — all of them. It stops control flow from getting diverted to some unexpected target, which is the whole point. This is meant to protect against threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). No cap, it’s giving security.

The value "branch" tells the compiler to check validity at the point of indirect branch instructions, aka call/jmp instructions. The value "return" checks validity at the point of returning from a function. The value "full" is just an alias for specifying both "branch" and "return". The value "none" turns off instrumentation completely.

The value "check" is used for the final link with link-time optimization (LTO). An error gets issued if LTO object files are compiled with different -fcf-protection values. The value "check" is ignored at compile time.

The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value "branch" and the second bit of __CET__ is set to 1 for the "return".

You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. Periodt.

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. 💅

• -fharden-compares — For every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call __builtin_trap if the results do not match. Use with -fharden-conditional-branches to cover all conditionals. ✨
• -fharden-conditional-branches — For every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call __builtin_trap if the result is unexpected. Use with -fharden-compares to cover all conditionals. 🦋
• -fstack-protector — Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 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-protector except that all functions are protected. 👑
• -fstack-protector-strong — Like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don’t count. 🫶
• -fstack-protector-explicit — Like -fstack-protector but only protects those functions which have the stack_protect attribute. 🌸
• -fstack-check — Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada’s needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. ⚠️

• -fstack-clash-protection — Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system.

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific. 🔥

• -fstack-limit-register=reg, -fstack-limit-symbol=sym, -fno-stack-limit — Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the no_stack_limit function attribute. 🍓

• -fsplit-stack — Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux.

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. 🌈

• -fvtable-verify=[std|preinit|none] — This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. 💀

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.

If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. 👑

• -fvtv-debug — When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise.

Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. 🙈

• -fvtv-counts — This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. 🧚

Little note before we go any further: this feature appends data to the log files. So if you want fresh log files, bestie, you gotta delete the old ones first. No leftovers. 💅

---

• -finstrument-functions — okay so this one generates instrumentation calls for entry and exit to functions. Like, right after a function is entered and right before it exits, these profiling functions get called with the address of the current function and its call site:

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

(quick aside: on some platforms __builtin_return_address doesn't work past the current function, so the call site info might just… not be available to the profiling functions. sad but true.)

The first argument is the address of the start of the current function, and you can look that up exactly in the symbol table. 👑

This instrumentation also happens for functions that get expanded inline inside other functions — the profiling calls show where, conceptually, the inline function is entered and exited. Which means addressable versions of those functions must be available. If literally every use of a function gets expanded inline, you might be looking at extra code size. And if you use extern inline in your C code, an addressable version of such functions must be provided. (Normally that's the case anyway, but if you get lucky and the optimizer always inlines them, you might have gotten away without providing static copies. 😌)

A function can be given the attribute no_instrument_function, and then this instrumentation just doesn't happen to it. Use it for stuff like the profiling functions above, high-priority interrupt routines, and any function the profiling functions can't safely be called from — think signal handlers, if your profiling routines generate output or allocate memory. 🫶

• -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions excluded from instrumentation (peep the -finstrument-functions description up there ⬆️). If the file containing a function definition matches one of file, that function is not instrumented. And the match is done on substrings: if the file parameter is a substring of the file name, that counts as a match.

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

And if, for some reason, you want to include the 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,... — same vibe as -finstrument-functions-exclude-file-list, except this one sets the list of function names excluded from instrumentation. The name matched is the user-visible one, like vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). Again, substring matching: if the sym parameter is a substring of the function name, it's a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not with universal character names. ✨
• -fpatchable-function-entry=N[,M] — generates N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted it defaults to 0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space you can use to patch in whatever instrumentation you want at run time — provided the code segment is writable. 🔥 The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary.

Heads up: 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. If N=0, no pad location is recorded.

The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue.

The maximum value of N and M is 65535.

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which runs on each C source file before actual compilation. 🧚

If you use the -E option, nothing happens except preprocessing. And some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation. 🙈

On top of the options listed here, there are a bunch of options controlling search paths for include files, documented over in Directory Options. Options controlling preprocessor diagnostics live in Warning Options.

• -D name — predefine name as a macro, with definition 1.
• -D name=definition — the contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters.

If you're invoking the preprocessor from a shell or shell-like program, you may need the shell's quoting syntax to protect characters like spaces that mean something in shell syntax.

Wanna 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 quote the option. With sh and csh, -D'name(args...)=definition' works. 💅

-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all -D and -U options.

• -U name — cancel any previous definition of name, whether it was built in or provided with a -D option.
• -include file — process file as if #include "file" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory, not 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 any output produced by scanning file is thrown away. 🗑️ Macros it defines stay defined. This lets you grab all the macros from a header without also processing its declarations.

All files specified by -imacros are processed before all files specified by -include.

• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. 🌸
• -pthread — define additional macros required for using the POSIX threads library. Use this option consistently for both compilation and linking, bestie, I'm serious. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
• -M — instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files — including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with 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, like -dM. To avoid mixing such debug output with the dependency rules, you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. 😭

• -MM — like -M, but do not mention header files found in system header directories, nor header files included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header shows up in -MM dependency output. 🤔

• -MF file — when used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout. ✨

• -MG — in conjunction with an option like -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, since a missing header file renders that 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 errors make gives you 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 — okay so this one changes the target of the rule that dependency generation spits out. ✨ By default CPP grabs the name of the main input file, yeets any directory components and any file suffix like .c, then appends the platform's usual object suffix. That result is the target, no cap.

An -MT option sets the target to be exactly the string you specify. Want multiple targets? You can hand them over as a single argument to -MT, or just stack up multiple -MT options. 💅

Like, -MT '$(objpfx)foo.o' might give you

$(objpfx)foo.o: foo.c

• -MQ target — same as -MT bestie, except it quotes any characters that are special to Make. So -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. Iconic behaviour tbh.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver figures out file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix. 🎀

If -MD is used together 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. Lowkey important, don't mix that up.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Multi-tasking queen. 👑

• -MMD — like -MD except it only mentions user header files, not system header files.
• -fpreprocessed — tells the preprocessor that the input file has already been preprocessed. This suppresses stuff like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments though, so you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. 💀

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives, but do not expand macros. The option's behavior depends on the -E and -fpreprocessed options. 🌸

With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as "__LINE__", which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with "-E -fdirectives-only".

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with "-E -fdirectives-only".

• -fdollars-in-identifiers — accept $ in identifiers. money moves 💸
• -fextended-identifiers — accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization.
• -fmax-include-depth=depth — set the maximum depth of the nested #include. The default is 200.
• -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
• -ftrack-macro-expansion[=level] — tracks locations of tokens across macro expansions. This lets the compiler emit a diagnostic about the current macro expansion stack when a compilation error happens inside a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. 😭

The level parameter picks how precise the token location tracking is, so you can dial down the memory consumption if you need to:

• Value 0 of level de-activates this option.
• Value 1 tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location.
• Value 2 tracks token locations completely. This value is the most memory hungry.

When this option is given with no argument, the default parameter value is 2.

Note that "-ftrack-macro-expansion=2" is activated by default.

• -fmacro-prefix-map=old=new — when preprocessing files residing in directory old, expand the "__FILE__" and "__BASE_FILE__" macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new, which can result in more reproducible builds that are location independent. This option also affects "__builtin_FILE()" during compilation. See also -ffile-prefix-map. 🦋
• -fexec-charset=charset — set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.
• -fwide-exec-charset=charset — set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t". 🙈
• -finput-charset=charset — set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.
• -fpch-deps — when using precompiled headers, this flag makes the dependency-output flags also list the files from the precompiled header's dependencies. If not specified, only the precompiled header is listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 🧚
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC pch_preprocess "filename"", in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.

• -fworking-directory — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no "#line" directives are emitted whatsoever. 💅
• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. ✨
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. 😳

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
• -traditional
• -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. 🌈

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.

• -trigraphs — support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. 🍓

okay so the nine trigraphs and their replacements are literally:

Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
Replacement:      [    ]    {    }    #    \    ^    |    ~

By default GCC ignores trigraphs, like fully does not see them. But in standard-conforming modes it converts them. See the -std and -ansi options.

• -remap — turns on special code to work around file systems that only let you have super short file names, like MS-DOS. ancient behavior, but okay bestie 💅
• -H — prints the name of every single header file used, on top of all the normal stuff it does. Each name gets indented so you can see how deep in the #include stack it is. Precompiled header files get printed too, even if they turn out to be invalid — an invalid precompiled header prints with ...x and a valid one prints with ...! . 🎀
• -dletters — tells GCC to make debugging dumps during compilation, as specified by letters. The flags documented right here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. And if you specify letters whose behavior conflicts? The result is undefined. 😭 not a vibe.
  • -dM — instead of the normal output, generates a list of #define directives for all the macros defined while the preprocessor ran, predefined macros included. This is your way to find out what's predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.

• -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — like -dD, but emits only the macro names, not their expansions.
• -dI — outputs #include directives in addition to the result of preprocessing.
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, get output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.

• -fdebug-cpp — this one is only useful for debugging GCC itself, fr. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

When used from GCC without -E, this option has no effect. 🫠

• -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. However — and this is the part people ignore — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. no cap, listen to the driver. 👑
• -Xpreprocessor option — passes option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument. twice. like, do it twice. 🍓

• -no-integrated-cpp — performs preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. ✨
• -flarge-source-files — adjusts GCC to expect large source files, at the expense of slower compilation and higher memory usage. highkey a tradeoff.

Specifically, GCC normally tracks both column numbers and line numbers within source files, and it normally prints both of these numbers in diagnostics. However, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. This means that diagnostics for later lines do not include column numbers. It also means that options like -Wmisleading-indentation cease to work at that point, although the compiler prints a note if this happens. Passing -flarge-source-files significantly increases the number of source lines that GCC can process before it stops tracking columns. 🌸

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🌈 Passing Options to the Assembler 🌈

You can pass options to the assembler. yes, the assembler specifically.

• -Wa,option — passes option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
• -Xassembler option — passes option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument. (same double-tap energy as -Xpreprocessor, iykyk 💅)

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💖 Options for Linking 💖

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. so like, don't come for them if nothing's linking.

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
• -c
• -S
• -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments.
• -flinker-output=type — this option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. For debugging the compiler and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually. 🔥

If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled.

If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This allows to build shared libraries without position-independent code on architectures where this is possible, i.e. on x86.

If type is pie, code generation produces an -fpie executable. This results in similar optimizations as exec except that -fpie is not disabled if specified at compilation time.

If type is rel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if -ffat-lto-objects is specified, 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-rel configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of benefits of whole program optimizations are lost.

During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of object files in 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 static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects. 🫶

• -fuse-ld=bfd — use the bfd linker instead of the default linker.
• -fuse-ld=gold — use the gold linker instead of the default linker.
• -fuse-ld=lld — use the LLVM lld linker instead of the default linker.
• -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker. 🧚
• -llibrary
• -l library — search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. 😭 order matters bestie, always has.

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used. 💜
• -nodefaultlibs — bestie, this one says do NOT touch the standard system libraries when linking. Only the libraries you name get handed to the linker, and any option that tries to wire up system libs — think -static-libgcc or -shared-libgcc — just gets ignored. The standard startup files still show up like normal though, unless you also pass -nostartfiles. 💅

Heads up: the compiler might still generate calls to memcmp, memset, memcpy and memmove. Those usually get resolved by entries in libc, so if you're skipping libc you gotta supply those entry points through some other mechanism. No cap. 😭

• -nolibc — this is the "no C library and none of the system libraries that are glued to it" vibe, specifically for linking. But it still links the startup files, libgcc, and toolchain-provided language support libs like libgnat, libgfortran or libstdc++ — unless you also pass options that stop those from coming in. 🦋

Usually this means -lc gets yanked off the link command line, plus the system libraries that normally tag along and become meaningless once you assume there's no C library — like -lpthread or -lm in some configurations. This one is for bare-board targets where there genuinely is no C library available. periodt.

• -nostdlib — the full send 🙈: no standard system startup files and no libraries when linking. Zero startup files, and only the libraries you specify get passed to the linker. Same deal as before — options that specify linkage of the system libraries, like -static-libgcc or -shared-libgcc, are ignored.

And again: the compiler may generate calls to memcmp, memset, memcpy and memmove, which normally get resolved by libc entries. You gotta supply those entry points another way when you use this.

Okay so here's the tea ☕: one of the standard libraries that -nostdlib and -nodefaultlibs bypass is libgcc.a — a library of internal subroutines GCC uses to work around shortcomings of particular machines, or special needs for some languages.

In most cases you actually need libgcc.a even when you're avoiding other standard libraries. So when you say -nostdlib or -nodefaultlibs, you should usually also say -lgcc. That way you have no unresolved references to internal GCC library subroutines. (One example of such an internal subroutine is __main, which is used to make sure C++ constructors get called.) 🫶

• -e entry
• --entry=entry — tells everyone the program entry point is entry. The argument gets interpreted by the linker; the GNU linker accepts either a symbol name or an address. iconic.
• -pie — produce a dynamically linked position independent executable on targets that support it. For predictable results you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you pass this linker option. fr fr.
• -no-pie — don't produce a dynamically linked position independent executable. that's it, that's the whole thing. 💀
• -static-pie — produce a static position independent executable on targets that support it. A static PIE is similar to a static executable, but it can be loaded at any address without a dynamic linker. Same rule as -pie: for predictable results you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. ✨
• -pthread — link with the POSIX threads library. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so use it consistently for both compilation and linking. don't be inconsistent bestie, it's giving chaos. 🌀
• -r — produce a relocatable object as output. Also known as partial linking. 🎀
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This tells the linker to add all symbols, not just used ones, to the dynamic symbol table. Needed for some uses of dlopen, or to allow obtaining backtraces from within a program. 🔥
• -s — remove all symbol table and relocation information from the executable. bye bye debug info 👋
• -static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect. simple as.
• -shared — produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1] 💅
• -shared-libgcc
• -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect. (mutually exclusive energy, pick a lane bestie ✋)

There are several situations where an app should use the shared libgcc instead of the static version. The most common is when the application wants to throw and catch exceptions across different shared libraries. In that case, each of the libraries and the application itself should use the shared libgcc.

So the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. 👑

If instead you use the GCC driver to create shared libraries, you may find 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 incurring relocation costs at library load time. 🌈

However — and this is important — if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, so that it's linked with the shared libgcc.

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and -static is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. 🧚
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and -static is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. 🌸
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and -static is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. 💜
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and -static is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically. 🍓
• -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and -static is not used, then this links against the shared version of libstdc++. That's normally fine. ✨

But sometimes it's useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.

• -symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. ⚠️
• -T script — use script as the linker script. Supported by most systems using the GNU linker. On some targets, like bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. 💖
• -Xlinker option — pass option as an option to the linker. 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. 🙅

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 — pass option as an option to the linker. If option contains commas, it gets split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. slay. 🎀

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• -u symbol — bestie, pretend symbol is undefined, so the linker is forced to link library modules that define it. You can use -u multiple times with different symbols to force loading of additional library modules. 💅
• -z keyword — -z is passed directly on to the linker along with the keyword. Go check the section in the documentation of your linker for permitted values and their meanings. no cap. 🫶

✨ Options for Directory Search ✨

These options specify directories to search for header files, for libraries and for parts of the compiler. So like, the ultimate treasure map for your includes. 🗺️💖

• -I dir
• -iquote dir
• -isystem dir
• -idirafter dir — Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive, "#include "file"". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the "#include "file"" and "#include <file>" directives.

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

1. For the quote form of the include directive, the directory of the current file is searched first. 👑
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. 🚫

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories. ✨

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the "#include_next" directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 💅

• -I- — Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option. 🚨

Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". There is no way to override this effect of -I-. 😭

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 🎀
• -iwithprefix dir
• -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. 🌈
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🍓
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. 💜
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. 🙈
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 👑
• -iplugindir=dir — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🤫
• -Ldir — Add directory dir to the list of directories to be searched for -l. 🔍
• -Bprefix — This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. 🛠️

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable. 🌟

The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix. 💅

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. 😭

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. 🌈

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 🎀

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. 🚫
• --sysroot=dir — Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. 🏠

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files. 💖

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. ⚠️

• --no-sysroot-suffix — For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. ✨

👑 Options for Code Generation Conventions 👑

These machine-independent options control the interface conventions used in code generation. So like, the vibe of how your code talks to the machine. 💅

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. 🧚

• -fstack-reuse=reuse-level — This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 💀

For example,

                      int *p;
                      {
                        int local1;

                        p = &local1;
                        local1 = 10;
                        ....
                      }
                      {
                         int local2;
                         local2 = 20;
                         ...
                      }

                      if (*p == 10)  // out of scope use of local1
                        {

                        }

Another example:

                      struct A
                      {
                          A(int k) : i(k), j(k) { }
                          int i;
                          int j;
                      };

                      A *ap;

                      void foo(const A& ar)
                      {
                         ap = &ar;
                      }

                      void bar()
                      {
                         foo(A(10)); // temp object's lifetime ends when foo returns

{ A a(20); .... } ap->i+= 10; // ap references out of scope temp whose space // is reused with a. What is the value of ap->i? }

so like. bestie. the lifetime of a compiler-generated temporary is <pink>fully, legally, defined</pink> by the C++ standard, okay, that part is not vibes-based. When a temporary's lifetime ends, and if that temporary lives in memory, the optimizing compiler is <yellow>allowed</yellow> to reuse its stack space with other temporaries or scoped local variables whose live range doesn't overlap with it. That's the whole game.

but <red>plot twist</red> 💀 some legacy code out there is out here relying on how older compilers behaved, where temporaries' stack space was <rainbow>never</rainbow> reused. so the aggressive stack reuse can absolutely lead to runtime errors. this option is the vibe check on the temporary stack reuse optimization. 🫶

---

- **`-ftrapv`** — this one makes the compiler <hotpink>generate traps</hotpink> for signed overflow on addition, subtraction, and multiplication. 🚨 Now listen: `-ftrapv` and `-fwrapv` override each other, so passing `-ftrapv -fwrapv` on the command line means `-fwrapv` is the one that's actually in effect. and ngl the sneaky part is that <purple>only active options override</purple>, so passing `-ftrapv -fwrapv -fno-wrapv` on the command line means `-ftrapv` is the one that's effective. ikykyk. 💅

- **`-fwrapv`** — this tells the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication <mint>wraps around using twos-complement representation</mint>. it enables some optimizations and disables others. same override drama as above: `-ftrapv` and `-fwrapv` override each other, so `-ftrapv -fwrapv` on the command line leaves `-fwrapv` effective, and since only active options override, `-ftrapv -fwrapv -fno-wrapv` on the command line leaves `-ftrapv` effective. ✨

- **`-fwrapv-pointer`** — the pointer-flavored sibling 🦋 this instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. it disables some optimizations which assume pointer overflow is invalid.

- **`-fstrict-overflow`** — this option implies `-fno-wrapv -fno-wrapv-pointer`, and when negated it implies `-fwrapv -fwrapv-pointer`. that's it, that's the whole personality. 👑

- **`-fexceptions`** — enable exception handling. it generates the extra code needed to propagate exceptions. for some targets this implies GCC generates frame unwind information for <hotpink>all</hotpink> functions, which can produce significant data size overhead, although it does not affect execution. if you don't specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that don't normally require it. <lavender>but</lavender> you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. and you may also wish to disable this option if you're compiling older C++ programs that don't use exception handling. flexible queen energy fr fr. 🌸

- **`-fnon-call-exceptions`** — generate code that allows trapping instructions to throw exceptions. ⚠️ note that this requires <red>platform-specific runtime support that does not exist everywhere</red>. moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. it does <purple>not</purple> allow exceptions to be thrown from arbitrary signal handlers such as `"SIGALRM"`. this enables `-fexceptions`.

- **`-fdelete-dead-exceptions`** — consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. this 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. optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. 🧚

- **`-funwind-tables`** — similar to `-fexceptions`, except that it just generates any needed static data, but does not affect the generated code in any other way. you normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. 🫶

- **`-fasynchronous-unwind-tables`** — generate unwind table in DWARF format, if supported by target machine. the table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). ⭐

- **`-fno-gnu-unique`** — on systems with recent GNU assembler and C library, the C++ compiler uses the `"STB_GNU_UNIQUE"` binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of `"RTLD_LOCAL"`; this is necessary to avoid problems with a library used by two different `"RTLD_LOCAL"` plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. <red>but</red> this causes `"dlclose"` to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via `"dlclose"` and `"dlopen"`, you can use `-fno-gnu-unique`. 💀

- **`-fpcc-struct-return`** — return `"short"` `"struct"` and `"union"` values in memory like longer ones, rather than in registers. this convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). 🎀

  the precise convention for returning structures in memory depends on the target configuration macros.

  short structures and unions are those whose size and alignment match that of some integer type.

  > warning: code compiled with the `-fpcc-struct-return` switch is <red>not binary compatible</red> with code compiled with the `-freg-struct-return` switch. use it to conform to a non-default application binary interface.

- **`-freg-struct-return`** — return `"struct"` and `"union"` values in registers when possible. this is more efficient for small structures than `-fpcc-struct-return`. 💅

  if you specify neither `-fpcc-struct-return` nor `-freg-struct-return`, GCC defaults to whichever convention is standard for the target. if there is no standard convention, GCC defaults to `-fpcc-struct-return`, except on targets where GCC is the principal compiler. in those cases, we can choose the standard, and we chose the more efficient register return alternative. <rainbow>main character decision-making, honestly</rainbow> ✨

  > warning: code compiled with the `-freg-struct-return` switch is <red>not binary compatible</red> with code compiled with the `-fpcc-struct-return` switch. use it to conform to a non-default application binary interface.

- **`-fshort-enums`** — allocate to an `"enum"` type only as many bytes as it needs for the declared range of possible values. specifically, the `"enum"` type is equivalent to the smallest integer type that has enough room. 🍓

  > warning: the `-fshort-enums` switch causes GCC to generate code that is <red>not binary compatible</red> with code generated without that switch. use it to conform to a non-default application binary interface.

- **`-fshort-wchar`** — override the underlying type for `"wchar_t"` to be `"short unsigned int"` instead of the default for the target. this option is useful for building programs to run under WINE. 🍷

  > warning: the `-fshort-wchar` switch causes GCC to generate code that is <red>not binary compatible</red> 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 <cyan>tentative definitions</cyan> in the C standard. tentative definitions are distinct from declarations of a variable with the `"extern"` keyword, which do not allocate storage. 🫶

  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 <red>multiple-definition error</red> if the same variable is accidentally defined in more than one compilation unit. 💀

  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. 👑

- **`-fno-ident`** — ignore the `"#ident"` directive. that's it. short and iconic. 💅

- **`-finhibit-size-directive`** — don't output a `".size"` assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. this option is used when compiling `crtstuff.c`; you should not need to use it for anything else. 🙈

- **`-fverbose-asm`** — put extra commentary information in the generated assembly code to make it more readable. this option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). ✨

  `-fno-verbose-asm`, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

  the added comments include:

  - information on the compiler version and command-line options,

  - the source code lines associated with the assembly instructions, in the form `FILENAME:LINENUMBER:CONTENT OF LINE`,

  - hints on which high-level expressions correspond to the various assembly instruction operands.

  for example, given this C source file:

int test (int n) { int i; int total = 0;

for (i = 0; i < n; i++) total += i * i;

return total; }

compiling to (x86_64) assembly via `-S` and emitting the result direct to stdout via `-o -`

gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

.file "test.c"

GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)

[...snip...]

options passed:

[...snip...]

okay bestie, real talk — this is what `-fverbose-asm` actually cooks up for you, comments and all 💅✨:

.text .globl test .type test, @function test: .LFB0: .cfi_startproc # test.c:4: int total = 0; xorl %eax, %eax # <retval> # test.c:6: for (i = 0; i < n; i++) xorl %edx, %edx # i .L2: # test.c:6: for (i = 0; i < n; i++) cmpl %edi, %edx # n, i jge .L5 #, # test.c:7: total += i * i; movl %edx, %ecx # i, tmp92 imull %edx, %ecx # i, tmp92 # test.c:6: for (i = 0; i < n; i++) incl %edx # i # test.c:7: total += i * i; addl %ecx, %eax # tmp92, <retval> jmp .L2 # .L5: # test.c:10: } ret .cfi_endproc .LFE0: .size test, .-test .ident "GCC: (GNU) 7.0.0 20160809 (experimental)" .section .note.GNU-stack,"",@progbits

<purple>The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change.</purple>

Like, adorable for *you* to read, but don't you dare build a parser on them — they can shift whenever, no cap 🙈.

---

- **`-frecord-gcc-switches`** — this one makes the command line you used to call the compiler get **recorded right into the object file** it's building, iconic behavior honestly. Only implemented on <cyan>some</cyan> targets, and the exact format of the recording depends on the target and the binary file format — usually it ends up as a section holding ASCII text. It's cousins with `-fverbose-asm`, but that one only drops the info into the assembler output file as comments, so it <hotpink>never</hotpink> reaches the object file. Also peep `-grecord-gcc-switches` for another way of storing compiler options into the object file 👑.

- **`-fpic`** — generate position-independent code (PIC), the kind that's suitable for use in a shared library, if it's supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts — and heads up, the dynamic loader is <red>not</red> part of GCC, it's part of the operating system, so don't come for GCC about it 😭. If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying that `-fpic` does not work; in that case, recompile with `-fPIC` instead. (Those maximums are **8k on the SPARC**, **28k on AArch64** and **32k on the m68k and RS/6000**. The x86 has no such limit, slay 💅.)

  Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but <red>not</red> for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent, periodt.

  When this flag is set, the macros `"__pic__"` and `"__PIC__"` are defined to **1**.

- **`-fPIC`** — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on **AArch64**, **m68k**, **PowerPC** and **SPARC**. Big main-character energy for those targets ✨.

  Position-independent code requires special support, and therefore works only on certain machines.

  When this flag is set, the macros `"__pic__"` and `"__PIC__"` are defined to **2**.

- **`-fpie`**, **`-fPIE`** — these are the glow-up siblings of `-fpic` and `-fPIC`, except the generated position-independent code can **only be linked into executables**. Usually you use these to compile code that'll be linked using the `-pie` GCC option. Both `-fpie` and `-fPIE` define the macros `"__pie__"` and `"__PIE__"`. The macros have the value **1** for `-fpie` and **2** for `-fPIE`.

- **`-fno-plt`** — do <red>not</red> use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as **32-bit x86** where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with `-fno-plt` all external symbols are resolved at load time 🫶.

  Alternatively, the function attribute `"noplt"` can be used to avoid calls through the PLT for specific external functions.

  In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

- **`-fno-jump-tables`** — do not use jump tables for `switch` statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with `-fpic` or `-fPIC` for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed — so like, vibe check your target first 🦋.

- **`-fno-bit-tests`** — do not use bit tests for `switch` statements even where it would be more efficient than other code generation strategies. That's it, that's the whole thing 🎀.

- **`-ffixed-reg`** — treat the register named *reg* as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

  *reg* must be the name of a register. The register names accepted are machine-specific and are defined in the `"REGISTER_NAMES"` macro in the machine description macro file.

  > This flag does **not** have a negative form, because it specifies a **three-way choice**. No `-fno-` version for you, bestie 💀.

- **`-fcall-used-reg`** — treat the register named *reg* as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do <red>not</red> save and restore the register *reg*.

  It is an **error** to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces **disastrous results** 😭💀.

  > This flag does not have a negative form, because it specifies a three-way choice.

- **`-fcall-saved-reg`** — treat the register named *reg* as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register *reg* if they use it.

  It is an **error** to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces **disastrous results**.

  A <hotpink>different sort of disaster</hotpink> results from the use of this flag for a register in which function values may be returned. Yeah. Don't 🙈.

  > This flag does not have a negative form, because it specifies a three-way choice.

- **`-fpack-struct[=n]`** — without a value specified, pack all structure members together **without holes**. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

  > Warning: the `-fpack-struct` switch causes GCC to generate code that is **not binary compatible** with code generated without that switch. Additionally, it makes the code **suboptimal**. Use it to conform to a non-default application binary interface.

- **`-fleading-underscore`** — this option and its counterpart, `-fno-leading-underscore`, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code, which is lowkey a rescue mission 💜.

  > Warning: the `-fleading-underscore` switch causes GCC to generate code that is **not binary compatible** with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

- **`-ftls-model=`***model* — alter the thread-local storage model to be used. The *model* argument should be one of `global-dynamic`, `local-dynamic`, `initial-exec` or `local-exec`. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if `-fpic` is not given on the command line. Sneaky-smart, honestly 🌈.

  The default without `-fpic` is `initial-exec`; with `-fpic` the default is `global-dynamic`.

- **`-ftrampolines`** — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them, like for example **HP-PA** or **IA-64**, do nothing.

  A **trampoline** is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made **executable** in order for the program to work properly 🔥.

  `-fno-trampolines` is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, `-fno-trampolines` is enabled by default **only for Ada**.

  Moreover, code compiled with `-ftrampolines` and code compiled with `-fno-trampolines` are **not binary compatible** if nested functions are present. This option must therefore be used on a **program-wide basis** and be manipulated with **extreme care** 😭.

  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.

- **`-fvisibility=[default|internal|hidden|protected]`** — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is **strongly recommended** that you use this in any shared objects you distribute 👑💖.

---

<pink>okay so</pink> despite the nomenclature, `default` literally always means <rainbow>public</rainbow> — i.e., available to be linked against from outside the shared object. Periodt.

`protected` and `internal` are <purple>pretty useless</purple> in real-world usage, ngl, so the only other commonly used option is `hidden`. And the default if `-fvisibility` isn't specified is `default`, i.e., make every symbol public. ✨

A <green>good</green> 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. <hotpink>This is a great boon to those working with cross-platform projects.</hotpink> 🫶

For those adding visibility support to existing code, you may find `#pragma GCC visibility` of use. This works by you enclosing the declarations you wish to set visibility for with (for example) `#pragma GCC visibility push(hidden)` and `#pragma GCC visibility pop`. <yellow>Bear in mind</yellow> that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. <cyan>Note</cyan> that due to ISO C++ specification requirements, `operator new` and `operator delete` must always be of default visibility.

Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may <red>not</red> be expecting to be compiled with visibility other than the default. You may need to explicitly say `#pragma GCC visibility push(default)` before including any such headers. 😭

`extern` declarations are <lavender>not</lavender> affected by `-fvisibility`, so a lot of code can be recompiled with `-fvisibility=hidden` with no modifications. However, this means that calls to `extern` functions with no explicit visibility use the PLT, so it is more effective to use `__attribute ((visibility))` and/or `#pragma GCC visibility` to tell the compiler which `extern` declarations should be treated as hidden. 💅

<purple>Note</purple> that `-fvisibility` does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the `type_info` nodes are unified between the DSOs.

An overview of these techniques, their benefits and how to use them is at `https://gcc.gnu.org/wiki/Visibility`. 🌈

---

- **`-fstrict-volatile-bitfields`** — <pink>bestie, use this one</pink> if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a **single** access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as `unsigned short` (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. 🎀

  If this option is disabled, the compiler uses the <hotpink>most efficient</hotpink> instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 💀

  In some cases, such as when the `packed` attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.

  > Note: Due to restrictions of the C/C++11 memory model, write accesses are **not allowed** to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members. 🙈

  The default value of this option is determined by the application binary interface for the target processor.

- **`-fsync-libcalls`** — this option controls whether any out-of-line instance of the `__sync` family of functions may be used to implement the C++11 `__atomic` family of functions.

  The default value of this option is **enabled**, thus the only useful form of the option is `-fno-sync-libcalls`. This option is used in the implementation of the libatomic runtime library. ⭐

---

## 🌸 GCC Developer Options 🌸

This section describes command-line options that are primarily of interest to <purple>GCC developers</purple>, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should <yellow>rarely</yellow> need to use any of these options for ordinary compilation and linking tasks, bestie. 🧚

Many developer options that cause GCC to dump output to a file take an optional `=filename` suffix. You can specify `stdout` or `-` to dump to standard output, and `stderr` for standard error. 💖

If `=filename` is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named `final`, even if they are executed earlier. The phase letter is one of `i` (inter-procedural analysis), `l` (language-specific), `r` (RTL), or `t` (tree). The files are created in the directory of the output file. 🦋

- **`-fcallgraph-info`**
- **`-fcallgraph-info=MARKERS`**
  — Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated *markers* is additionally specified. When the `su` marker is specified, the callgraph is decorated with stack usage information; it is equivalent to `-fstack-usage`. When the `da` marker is specified, the callgraph is decorated with information about dynamically allocated objects. ✨

  When compiling with `-flto`, <red>no</red> 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 <hotpink>different meaning</hotpink> when `-E` is used for preprocessing.

  Debug dumps can be enabled with a `-fdump-rtl` switch or some `-d` option *letters*. Here are the possible *letters* for use in *pass* and *letters*, and their meanings:

  - **`-fdump-rtl-alignments`** — Dump after branch alignments have been computed.
  - **`-fdump-rtl-asmcons`** — Dump after fixing rtl statements that have unsatisfied in/out constraints.
  - **`-fdump-rtl-auto_inc_dec`** — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
  - **`-fdump-rtl-barriers`** — Dump after cleaning up the barrier instructions.
  - **`-fdump-rtl-bbpart`** — Dump after partitioning hot and cold basic blocks.
  - **`-fdump-rtl-bbro`** — Dump after block reordering.
  - **`-fdump-rtl-btl1`**, **`-fdump-rtl-btl2`** — `-fdump-rtl-btl1` and `-fdump-rtl-btl2` enable dumping after the two branch target load optimization passes.
  - **`-fdump-rtl-bypass`** — Dump after jump bypassing and control flow optimizations.
  - **`-fdump-rtl-combine`** — Dump after the RTL instruction combination pass.
  - **`-fdump-rtl-compgotos`** — Dump after duplicating the computed gotos.
  - **`-fdump-rtl-ce1`**, **`-fdump-rtl-ce2`**, **`-fdump-rtl-ce3`** — `-fdump-rtl-ce1`, `-fdump-rtl-ce2`, and `-fdump-rtl-ce3` enable dumping after the three if conversion passes.
  - **`-fdump-rtl-cprop_hardreg`** — Dump after hard register copy propagation.
  - **`-fdump-rtl-csa`** — Dump after combining stack adjustments.
  - **`-fdump-rtl-cse1`**, **`-fdump-rtl-cse2`** — `-fdump-rtl-cse1` and `-fdump-rtl-cse2` enable dumping after the two common subexpression elimination passes.
  - **`-fdump-rtl-dce`** — Dump after the standalone dead code elimination passes.
  - **`-fdump-rtl-dbr`** — Dump after delayed branch scheduling.
  - **`-fdump-rtl-dce1`**, **`-fdump-rtl-dce2`** — `-fdump-rtl-dce1` and `-fdump-rtl-dce2` enable dumping after the two dead store elimination passes.
  - **`-fdump-rtl-eh`** — Dump after finalization of EH handling code.
  - **`-fdump-rtl-eh_ranges`** — Dump after conversion of EH handling range regions.
  - **`-fdump-rtl-expand`** — Dump after RTL generation.
  - **`-fdump-rtl-fwprop1`**, **`-fdump-rtl-fwprop2`** — `-fdump-rtl-fwprop1` and `-fdump-rtl-fwprop2` enable dumping after the two forward propagation passes.
  - **`-fdump-rtl-gcse1`**, **`-fdump-rtl-gcse2`** — `-fdump-rtl-gcse1` and `-fdump-rtl-gcse2` enable dumping after global common subexpression elimination.
  - **`-fdump-rtl-init-regs`** — Dump after the initialization of the registers.
  - **`-fdump-rtl-initvals`** — Dump after the computation of the initial value sets.
  - **`-fdump-rtl-into_cfglayout`** — Dump after converting to cfglayout mode.

- **`-fdump-rtl-ira`** — dumps stuff after <purple>iterated register allocation</purple>. yes bestie, that's a whole pass. 💅

- **`-fdump-rtl-jump`** — dumps after the <cyan>second jump optimization</cyan>. round two, fight! 🥊

- **`-fdump-rtl-loop2`** — `-fdump-rtl-loop2` enables dumping after the rtl loop optimization passes. it's giving second-pass loop energy 🌈

- **`-fdump-rtl-mach`** — dumps after the <peach>machine dependent reorganization pass</peach>, if that pass even exists. (conditional queen — only shows up if she's invited 🎀)

- **`-fdump-rtl-mode_sw`** — dumps after removing redundant mode switches. bye bye redundancy ✨

- **`-fdump-rtl-rnreg`** — dumps after register renumbering. fresh numbers, fresh me 💅

- **`-fdump-rtl-outof_cfglayout`** — dumps after converting from cfglayout mode. we're leaving that mode behind, no cap 🦋

- **`-fdump-rtl-peephole2`** — dumps after the peephole pass. peep peep 👀

- **`-fdump-rtl-postreload`** — dumps after post-reload optimizations. post-reload glow-up 🌸

- **`-fdump-rtl-pro_and_epilogue`** — dumps after generating the function prologues and epilogues. intros and outros, very podcast of them 🎙️

- **`-fdump-rtl-sched1`**, **`-fdump-rtl-sched2`** — these two enable dumping after the basic block scheduling passes. scheduled and slaying 📅

- **`-fdump-rtl-ree`** — dumps after sign/zero extension elimination. extensions? eliminated. ✂️

- **`-fdump-rtl-seqabstr`** — dumps after common sequence discovery. finding the matching pieces like a true detective 🕵️‍♀️

- **`-fdump-rtl-shorten`** — dumps after shortening branches. short branches, tall vibes 🌴

- **`-fdump-rtl-sibling`** — dumps after sibling call optimizations. family optimization fr fr 👯

- **`-fdump-rtl-split1`**, **`-fdump-rtl-split2`**, **`-fdump-rtl-split3`**, **`-fdump-rtl-split4`**, **`-fdump-rtl-split5`** — these options enable dumping after <hotpink>five rounds</hotpink> of instruction splitting. five. rounds. iconic endurance 💀

- **`-fdump-rtl-sms`** — dumps after modulo scheduling. <yellow>heads up:</yellow> this pass is only run on some architectures. not everyone gets invited, and that's okay 🫶

- **`-fdump-rtl-stack`** — dumps after conversion from GCC's `"flat register file"` registers to the x87's stack-like registers. <yellow>only run on x86 variants</yellow> — very exclusive, very x86-only 💅

- **`-fdump-rtl-subreg1`**, **`-fdump-rtl-subreg2`** — these two enable dumping after the <purple>two subreg expansion passes</purple>. two for two 🎀

- **`-fdump-rtl-unshare`** — dumps after all rtl has been unshared. sharing is over, we're independent now 💅

- **`-fdump-rtl-vartrack`** — dumps after variable tracking. following the variables around like a loyal bestie 👑

- **`-fdump-rtl-vregs`** — dumps after converting virtual registers to hard registers. virtual → hard, the ultimate glow-up transformation ✨

- **`-fdump-rtl-web`** — dumps after live range splitting. web of ranges, spinning 🕸️

- **`-fdump-rtl-regclass`**, **`-fdump-rtl-subregs_of_mode_init`**, **`-fdump-rtl-subregs_of_mode_finish`**, **`-fdump-rtl-dfinit`**, **`-fdump-rtl-dfinish`** — <red>plot twist bestie:</red> these dumps are defined but **always produce empty files**. so much effort for literally nothing 😭 not me crying over empty files

- **`-da`**, **`-fdump-rtl-all`** — produce <rainbow>all the dumps listed above</rainbow>. the everything bagel of dump flags 🥯

- **`-dA`** — annotate the assembler output with miscellaneous debugging information. little notes everywhere, like margin doodles 🖊️

- **`-dD`** — dump all macro definitions, at the end of preprocessing, in addition to normal output. macros get their moment 🎤

- **`-dH`** — produce a core dump whenever an error occurs. <red>yes, an actual core dump.</red> dramatic? maybe. but that's literally what it does 💀

- **`-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. receipts for every instruction 🧾

- **`-dP`** — dump the RTL in the assembler output as a comment before each instruction. also turns on `-dp` annotation. bonus feature activated ✨

- **`-dx`** — just generate RTL for a function instead of compiling it. usually used with `-fdump-rtl-expand`. RTL only, no compiling, we're just vibing 🦋

---

- **`-fdump-debug`** — dump debugging information generated during the debug generation phase. debug info about debug info, very meta 💅

- **`-fdump-earlydebug`** — dump debugging information generated during the <peach>early debug generation phase</peach>. the early bird version 🐦

- **`-fdump-noaddr`** — when doing debugging dumps, suppress address output. this makes it way more feasible to use `diff` on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. diff-able dumps, we love to see it 🫶

- **`-freport-bug`** — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. <cyan>when things go wrong, we get receipts</cyan> 🧊

- **`-fdump-unnumbered`** — when doing debugging dumps, suppress instruction numbers and address output. this makes it more feasible to use `diff` on debugging dumps for compiler invocations with different options, in particular with and without `-g`. comparing apples to apples, periodt 🍎

- **`-fdump-unnumbered-links`** — when doing debugging dumps (see `-d` option above), suppress instruction numbers for the links to the previous and next instructions in a sequence. no numbers on the chain links 🔗

- **`-fdump-ipa-switch`**, **`-fdump-ipa-switch-options`** — control the dumping at various stages of <purple>inter-procedural analysis language tree</purple> 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. here's what you can dump:

  - `all` — enables <hotpink>all</hotpink> inter-procedural analysis dumps.
  - `cgraph` — dumps information about call-graph optimization, unused function removal, and inlining decisions. the whole social network of your functions 🕸️
  - `inline` — dumps after function inlining. inlined and thriving ✨

  also, 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. the wins and the losses, documented 🎀

  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 `.lang` to the source file name. cute suffix, love that for us 💅

- **`-fdump-lang-all`**, **`-fdump-lang-switch`**, **`-fdump-lang-switch-options`**, **`-fdump-lang-switch-options=filename`** — control the dumping of language-specific information. the options and filename portions behave as described in the `-fdump-tree` option. the following switch values are accepted:

  - `all` — enable all language-specific dumps.
  - `class` — dump class hierarchy information. virtual table information is emitted unless `'slim'` is specified. <yellow>this option is applicable to C++ only.</yellow>
  - `module` — dump module information. options `lineno` (locations), `graph` (reachability), `blocks` (clusters), `uid` (serialization), `alias` (mergeable), `asmname` (Elrond), `eh` (mapper) & `vops` (macros) may provide additional information. <yellow>this option is applicable to C++ only.</yellow>
  - `raw` — dump the raw internal tree data. <yellow>this option is applicable to C++ only.</yellow>

- **`-fdump-passes`** — print on `stderr` the list of optimization passes that are turned on and off by the current command-line options. a full rundown of who's on and who's off 📋

- **`-fdump-statistics-option`** — enable and control dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in `.statistics` to the source file name, and the file is created in the same directory as the output file. if the `-option` form is used, `-stats` causes counters to be summed over the whole compilation unit while `-details` dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled. 📊

- **`-fdump-tree-all`**, **`-fdump-tree-switch`**, **`-fdump-tree-switch-options`**, **`-fdump-tree-switch-options=filename`** — control the dumping at various stages of processing the intermediate language tree to a file. if the `-options` form is used, `options` is a list of `-` separated options which control the details of the dump. <cyan>not all options are applicable to all dumps</cyan>; those that are not meaningful are just ignored, no drama. here's the full menu:

  - `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. addresses be moving, lowkey unreliable 🫠
  - `asmname` — if `"DECL_ASSEMBLER_NAME"` has been set for a given decl, use that in the dump instead of `"DECL_NAME"`. its primary use is ease of use working backward from mangled names in the assembly file.
  - `slim` — when dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.
    when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
    when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation. condensed queen 💅
  - `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.
    <yellow>this option currently only works for RTL dumps, and the RTL is always dumped in slim form.</yellow>
  - `vops` — enable showing virtual operands for every statement.
  - `lineno` — enable showing line numbers for statements.
  - `uid` — enable showing the unique ID (`"DECL_UID"`) for each variable.
  - `verbose` — enable showing the tree dump for each statement.
  - `eh` — enable showing the EH region number holding each statement.
  - `scev` — enable showing scalar evolution analysis details.
  - `optimized` — enable showing optimization information (only available in certain passes).
  - `missed` — enable showing missed optimization information (only available in certain passes).
  - `note` — enable other detailed optimization information (only available in certain passes).
  - `all` — turn on all options, except `raw`, `slim`, `verbose` and `lineno`. the all-inclusive package, minus a few 🎁
  - `optall` — turn on all optimization options, i.e., `optimized`, `missed`, and `note`. optimization overload ✨

  to determine what tree dumps are available or find the dump for a pass of interest follow the steps below. 🪜

1.  Run GCC with `-fdump-passes` and go stare at the <cyan>stderr</cyan> output, bestie — hunt for the code that matches the pass you're obsessed with. Like, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" are the three Value Range Propagation passes, okay? And that lil number at the end? That's just distinguishing different invocations of the *same* pass. She contains multitudes. 💅

2.  To make the dump file actually happen, append the pass code to the `-fdump-` option prefix and invoke GCC with it. So say you want the dump from the Early Value Range Propagation pass — you'd invoke GCC with the `-fdump-tree-evrp` option. You *can* also specify the name of the dump file if you want. If you don't, GCC creates it as described below. No notes. ✨

3.  Find the pass dump in a file whose name is three components separated by a period, in this exact order: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter `t` for tree passes (and the letter `r` for RTL passes), and finally the pass code. So the Early VRP pass dump might be sitting in a file named `myfile.c.038t.evrp` in the current working directory. 🫶

> Heads up though — those numeric codes are **not stable** and may change from one version of GCC to another. Do not be building your whole workflow on them, iykyk. 💀

---

- **`-fopt-info`**, **`-fopt-info-options`**, **`-fopt-info-options=filename`** — this one controls optimization dumps from all the various optimization passes. If you use the `-options` form, then *options* is a list of `-`-separated option keywords that select the dump details and the optimizations. 🎀

The options split into three little groups:

1.  options describing what kinds of messages should be emitted,
2.  options describing the verbosity of the dump, and
3.  options describing which optimizations should be included.

And here's the cute part — the options from each group can be freely mixed, because they're non-overlapping. But if there's *any* conflict, the later options override the earlier options on the command line. Later = louder. Periodt. 👑

The following options control which kinds of messages should be emitted:

- **`optimized`** — print information when an optimization is successfully applied. It's up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized. Slay. 🔥
- **`missed`** — print information about missed optimizations. Individual passes control which information to include in the output. 😭
- **`note`** — print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
- **`all`** — print detailed optimization information. This includes `optimized`, `missed`, and `note`. The whole buffet, bestie. ✨

The following option controls the dump verbosity:

- **`internals`** — by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. (Not us. Unless it *is* us. 🙈)

One or more of the following option keywords can be used to describe a group of optimizations:

- **`ipa`** — enable dumps from all interprocedural optimizations.
- **`loop`** — enable dumps from all loop optimizations.
- **`inline`** — enable dumps from all inlining optimizations.
- **`omp`** — enable dumps from all OMP (Offloading and Multi Processing) optimizations.
- **`vec`** — enable dumps from all vectorization optimizations.
- **`optall`** — enable dumps from all optimizations. This is a superset of the optimization groups listed above. 👑

If *options* is omitted, it defaults to `optimized-optall`, which means: dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If the *filename* is provided, then the dumps from all the applicable optimizations are concatenated into the *filename*. Otherwise the dump is output onto `stderr`. And like — 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. Sorry babes, that's the rule. 🫶

> Note that the output filename is **overwritten** in case of multiple translation units. If you want a combined output from multiple translation units, `stderr` should be used instead. Big deal, okay, read that again. 💀

In the following example, the optimization info is output to `stderr`:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into `missed.all`, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on `stderr`. Note that `-fopt-info-vec-missed` is equivalent to `-fopt-info-missed-vec`. The order of the optimization group names and message types listed after `-fopt-info` does not matter. Chaos is welcome here. 🌈

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into `inline.txt`.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames `vec.miss` and `loop.opt` are in **conflict**, since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only `vec.miss` is produced, which contains dumps from the vectorizer about missed opportunities. First come, first served, no cap. 💅

---

- **`-fsave-optimization-record`** — write a `SRCFILE.opt-record.json.gz` file detailing what optimizations were performed, for those optimizations that support `-fopt-info`. 🦋

> This option is **experimental** and the format of the data within the compressed JSON file is **subject to change**. Consider yourself warned, bestie.

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, with the following additional data for each message:

- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. Nested like matryoshka dolls, iconic. 🎀

---

- **`-fsched-verbose=n`** — on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files. 📊

For *n* greater than zero, `-fsched-verbose` outputs the same information as `-fdump-rtl-sched1` and `-fdump-rtl-sched2`. For *n* greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For *n* greater than two, it includes RTL at abort point, control-flow and regions info. And for *n* over four, `-fsched-verbose` also includes dependence info. The more n, the more she tells you. 💅

---

- **`-fenable-kind-pass`**, **`-fdisable-kind-pass=range-list`** — this is a whole *set* of options used to explicitly disable/enable optimization passes. 🧚

> These options are intended for use for **debugging GCC**. Compiler users should use regular options for enabling/disabling passes instead. Like, this is backstage access, not the main entrance, okay?

- **`-fdisable-ipa-pass`** — disable IPA pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from `1`.

- **`-fdisable-rtl-pass`**, **`-fdisable-rtl-pass=range-list`** — disable RTL pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from `1`. *range-list* is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option `-fdump-passes`. 🫶

- **`-fdisable-tree-pass`**, **`-fdisable-tree-pass=range-list`** — disable tree pass *pass*. See `-fdisable-rtl` for the description of option arguments.

- **`-fenable-ipa-pass`** — enable IPA pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from `1`.

- **`-fenable-rtl-pass`**, **`-fenable-rtl-pass=range-list`** — enable RTL pass *pass*. See `-fdisable-rtl` for option argument description and examples.

- **`-fenable-tree-pass`**, **`-fenable-tree-pass=range-list`** — enable tree pass *pass*. See `-fdisable-rtl` for the description of option arguments.

Here are some examples showing uses of these options. Feast your eyes, 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`**

<pink>okay so</pink> this one turns on the compiler's own internal consistency checking — like, GCC checking *itself* for vibes. The default depends on the compiler configuration. And `-fchecking=2` enables <purple>further</purple> internal consistency checking that might affect code generation. ✨

---

- **`-frandom-seed=`*`string`*

<pink>bestie</pink> this option gives GCC a seed that it uses instead of random numbers when it's generating certain symbol names — the ones that have to be different in every compiled file, yk? It's also used to place unique stamps in coverage data files and the object files that produce them. So basically you can use `-frandom-seed` to produce <green>reproducibly identical</green> object files. slay. 💅

The *`string`* can either be a number (decimal, octal or hex) or an arbitrary string — and if it's an arbitrary string, it gets converted to a number by computing CRC32. 🧮

> <red>heads up:</red> the *`string`* should be **different for every file you compile.** no cap.

---

- **`-save-temps`**

Stores the usual "temporary" intermediate files **permanently** instead of yeeting them. Names them as auxiliary output files, as specified described under `-dumpbase` and `-dumpdir`. 📁

When used in combination with the `-x` command-line option, `-save-temps` is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using `-save-temps`. 🫶

- **`-save-temps=cwd`**

Equivalent to `-save-temps -dumpdir ./.`

- **`-save-temps=obj`**

Equivalent to `-save-temps -dumpdir outdir/`, where *`outdir/`* is the directory of the output file specified after the `-o` option, including any directory separators. If the `-o` option is not used, the `-save-temps=obj` switch behaves like `-save-temps=cwd`.

---

- **`-time[=`*`file`*`]`**

Reports the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). ⏱️

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01 # as 0.00 0.01

The first number on each line is the "user time" — that is, time spent executing the program itself. The second number is "system time" — time spent executing operating system routines on behalf of the program. Both numbers are in **seconds**.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options> 0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. iykyk 🔥

---

- **`-fdump-final-insns[=`*`file`*`]`**

Dumps the final internal representation (RTL) to *`file`*. If the optional argument is omitted (or if *`file`* is `.`), the name of the dump file is determined by appending `.gkd` to the dump base name — see `-dumpbase`.

---

- **`-fcompare-debug[=`*`opts`*`]`**

If no error occurs during compilation, run the compiler a **second time**, adding *`opts`* and `-fcompare-debug-second` to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ. 👀

If the equal sign is omitted, the default `-gtoggle` is used.

The environment variable `GCC_COMPARE_DEBUG`, if defined, non-empty and nonzero, implicitly enables `-fcompare-debug`. If `GCC_COMPARE_DEBUG` is defined to a string starting with a dash, then it is used for *`opts`*, otherwise the default `-gtoggle` is used.

`-fcompare-debug=`, with the equal sign but without *`opts`*, is equivalent to `-fno-compare-debug`, which disables the dumping of the final representation and the second compilation, preventing even `GCC_COMPARE_DEBUG` from taking effect. 💀

To verify full coverage during `-fcompare-debug` testing, set `GCC_COMPARE_DEBUG` to say `-fcompare-debug-not-overridden`, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting `GCC_COMPARE_DEBUG` to `-w%n-fcompare-debug not overridden` will do.

---

- **`-fcompare-debug-second`**

This option is implicitly passed to the compiler for the second compilation requested by `-fcompare-debug`, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the `.gk` additional extension during the second compilation, to avoid overwriting those generated by the first. 🎀

When this option is passed to the compiler driver, it causes the **first** compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

---

- **`-gtoggle`**

Turns **off** generation of debug info, if leaving out this option generates it, or turns it **on** at level 2 otherwise. The position of this argument in the command line does not matter — it takes effect after all other options are processed, and it does so only **once**, no matter how many times it is given. This is mainly intended to be used with `-fcompare-debug`. 🌈

---

- **`-fvar-tracking-assignments-toggle`**

Toggles `-fvar-tracking-assignments`, in the same way that `-gtoggle` toggles `-g`.

---

- **`-Q`**

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. 📊

---

- **`-ftime-report`**

Makes the compiler print some statistics about the time consumed by each pass when it finishes. ⏳

---

- **`-ftime-report-details`**

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. The default value is `5`. If the value *`n`* is greater or equal to `10`, the dump output is sent to `stderr` using the same format as *`n`* minus 10.

---

- **`-flto-report`**

Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via `-flto`). 🔗

> Disabled by default.

---

- **`-flto-report-wpa`**

Like `-flto-report`, but only print for the WPA phase of link-time optimization.

---

- **`-fmem-report`**

Makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠

---

- **`-fmem-report-wpa`**

Makes the compiler print some statistics about permanent memory allocation for the WPA phase **only**.

---

- **`-fpre-ipa-mem-report`**
- **`-fpost-ipa-mem-report`**

Makes the compiler print some statistics about permanent memory allocation **before** or **after** interprocedural optimization.

---

- **`-fprofile-report`**

Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. 📈

---

- **`-fstack-usage`**

Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending `.su` to the *`auxname`*. *`auxname`* is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields: 📚

- The name of the function.

- A number of bytes.

- One or more qualifiers: `static`, `dynamic`, `bounded`.

The qualifier `static` means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. 🧱

The qualifier `dynamic` means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier `bounded` is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 🌊

---

- **`-fstats`**

Emit statistics about front-end processing at the end of the compilation. This option is supported **only by the C++ front end**, and the information is generally only useful to the G++ development team. 💖

---

- **`-fdbg-cnt-list`**

Print the name and the counter upper bound for all debug counters. 🔢

---

- **`-fdbg-cnt=`*`counter-value-list`*

Set the internal debug counter lower and upper bound. *`counter-value-list`* is a comma-separated list of `name:lower_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. For `dbg_cnt(tail_call)` true is returned for first 10 invocations. 🎯

---

- **`-print-file-name=`*`library`*

Print the full absolute name of the library file *`library`* that would be used when linking — and **don't do anything else**. With this option, GCC does not compile or link anything; it just prints the file name. 🖨️

---

- **`-print-multi-directory`**

Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in `GCC_EXEC_PREFIX`.

---

- **`-print-multi-lib`**

Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by `;`, and each switch starts with an `@` instead of the `-`, without spaces between multiple switches. This is supposed to ease shell processing. 🐚

---

- **`-print-multi-os-directory`**

Print the path to OS libraries for the selected multilib, relative to some `lib` subdirectory. If OS libraries are present in the `lib` subdirectory and no multilibs are used, this is usually just `.`, if OS libraries are present in `lib`*`suffix`* sibling directories this prints e.g. `../lib64`, `../lib` or `../lib32`, or if OS libraries are present in `lib/`*`subdir`* subdirectories it prints e.g. `amd64`, `sparcv9` or `ev6`. 🌸

- **`-print-multiarch`** — prints the path to the OS libraries for the multiarch you picked, relative to some `lib` subdirectory. ✨

- **`-print-prog-name=program`** — it's giving `-print-file-name` energy but for a *program*, like `cpp`. 🫶

- **`-print-libgcc-file-name`** — literally the same as `-print-file-name=libgcc.a`. no cap.

  This one is lowkey a lifesaver when you're out here with `-nostdlib` or `-nodefaultlibs` but you still want to link with `libgcc.a`. So you can do:

gcc -nostdlib <files>... gcc -print-libgcc-file-name

periodt. 💅

- **`-print-search-dirs`** — prints the name of the configured installation directory plus a list of program and library directories that `gcc` searches — and then does absolutely nothing else. main character behaviour.

Super useful when `gcc` hits you with the error message `installation problem, cannot exec cpp0: No such file or directory`. To fix that vibe you either gotta put `cpp0` and the other compiler components exactly where `gcc` expects them, or you can set the environment variable `GCC_EXEC_PREFIX` to the directory where you installed them. <hotpink>Don't forget the trailing `/`.</hotpink> 😭

- **`-print-sysroot`** — prints the target sysroot directory used during compilation. This is the target sysroot you specified either at configure time or with the `--sysroot` option, possibly with an extra suffix that depends on your compilation options. If no target sysroot is specified, this option prints *nothing*. it's giving silence. 💀

- **`-print-sysroot-headers-suffix`** — prints the suffix that gets added to the target sysroot when searching for headers, or throws an error if the compiler isn't configured with such a suffix — and does nothing else. 🦋

- **`-dumpmachine`** — prints the compiler's target machine (for example, `i686-pc-linux-gnu`) — and don't do anything else. ✨

- **`-dumpversion`** — prints the compiler version (for example, `3.0`, `6.3.0` or `7`) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version). iykyk. 🌸

- **`-dumpfullversion`** — prints the *full* compiler version — and don't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version. 👑

- **`-dumpspecs`** — prints the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built.) 🎀

---

## 🌈 Machine-Dependent Options 🌈

Every target machine that GCC supports can have its own lil options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations that are specific to that machine. By convention, the names of machine-specific options start with `-m`. 💅

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. 🫶

---

### 🦋 AArch64 Options 🦋

These options are defined for AArch64 implementations:

- **`-mabi=name`** — generates code for the data model you name. The permissible values are `ilp32` for the SysV-like data model where `int`, `long int` and pointers are 32 bits, and `lp64` for the SysV-like data model where `int` is 32 bits but `long int` and pointers are 64 bits. ✨

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are <red>not link-compatible</red>; you must compile your entire program with the same ABI, and link with a compatible set of libraries. no mixing bestie. 😭

- **`-mbig-endian`** — generates big-endian code. This is the default when GCC is configured for an `aarch64_be-*-*` target. 🌸

- **`-mgeneral-regs-only`** — generates code that uses *only* the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it will not impose any restrictions on the assembler. 💅

- **`-mlittle-endian`** — generates little-endian code. This is the default when GCC is configured for an `aarch64-*-*` but not an `aarch64_be-*-*` target. 🦋

- **`-mcmodel=tiny`** — generates code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked. 🎀

- **`-mcmodel=small`** — generates code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. 👑

- **`-mcmodel=large`** — generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The `-mcmodel=large` option is <red>incompatible with</red> `-mabi=ilp32`, `-fpic` and `-fPIC`. 🙈

- **`-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 a canary at *guard*. Supported locations are `global` for a global canary or `sysreg` for a canary in an appropriate system register. 💜

With the latter choice, the options `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is <hotpink>no default register or offset</hotpink> 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 the bit size of immediate TLS offsets. Valid values are `12`, `24`, `32`, `48`. This option requires binutils 2.26 or newer. 💅

- **`-mfix-cortex-a53-835769`**, **`-mno-fix-cortex-a53-835769`** — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. 🦋

- **`-mfix-cortex-a53-843419`**, **`-mno-fix-cortex-a53-843419`** — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker. 🎀

- **`-mlow-precision-recip-sqrt`**, **`-mno-low-precision-recip-sqrt`** — enable or disable the reciprocal square root approximation. This option only has an effect if `-ffast-math` or `-funsafe-math-optimizations` is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. ⭐

- **`-mlow-precision-sqrt`**, **`-mno-low-precision-sqrt`** — enable or disable the square root approximation. This option only has an effect if `-ffast-math` or `-funsafe-math-optimizations` is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies `-mlow-precision-recip-sqrt`. 🍓

- **`-mlow-precision-div`**, **`-mno-low-precision-div`** — enable or disable the division approximation. This option only has an effect if `-ffast-math` or `-funsafe-math-optimizations` is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision. 💜

- **`-mtrack-speculation`**, **`-mno-track-speculation`** — enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to `"__builtin_speculation_safe_copy"` to permit a more efficient code sequence to be generated. 🔥

- **`-moutline-atomics`**, **`-mno-outline-atomics`** — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. 🌈

This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. `-march=armv8.1-a` or `-march=armv8-a+lse`, the ARMv8.1-Atomics instructions will be used directly. The same applies when using `-mcpu=` when the selected cpu supports the `lse` feature. 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:

arch value : Architecture : Includes by default armv8-a : Armv8-A : +fp, +simd armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma armv8.2-a : Armv8.2-A : armv8.1-a armv8.3-a : Armv8.3-A : armv8.2-a, +pauth armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm armv8.7-a : Armv8.7-A : armv8.6-a, +ls64 armv8.8-a : Armv8.8-a : armv8.7-a, +mops armv9-a : Armv9-A : armv8.5-a, +sve, +sve2 armv8-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`** — bestie, this is where you tell GCC which processor to *tune the performance of the code* for. Like, you're not changing what instructions it can use, you're just telling it what kind of CPU is gonna be running this so it can be a little smarter about it. ✨ The values you're allowed to pass 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, 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-x2, cortex-a510, cortex-a710, ampere1, native

Okay so those values <hotpink>`cortex-a57.cortex-a53`</hotpink>, <hotpink>`cortex-a72.cortex-a53`</hotpink>, <hotpink>`cortex-a73.cortex-a35`</hotpink>, <hotpink>`cortex-a73.cortex-a53`</hotpink>, <hotpink>`cortex-a75.cortex-a55`</hotpink>, <hotpink>`cortex-a76.cortex-a55`</hotpink> — the ones with the dot in the middle — they're saying "tune this for a <purple>big.LITTLE</purple> system". Periodt, that's the whole point of the dot.

And <mint>`neoverse-512tvb`</mint>? Okay that one is *iconic* and specific. It means tune for Neoverse cores that (a) implement <lavender>SVE</lavender> and (b) have a total vector bandwidth of <yellow>512 bits per cycle</yellow>. In other words, it tells GCC to tune for Neoverse cores that can execute <yellow>4</yellow> 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (<yellow>2</yellow> for 256-bit SVE, <yellow>4</yellow> for 128-bit SVE). So it's more general than tuning for a specific core like Neoverse V1 but it's more specific than the default tuning described below. Slay. 💅

Also, on native <cyan>AArch64 GNU/Linux</cyan> systems the value <mint>`native`</mint> tunes performance to the host system. And heads up — this option has <red>no effect</red> if the compiler is unable to recognize the processor of the host system. Like if it doesn't know your CPU, it just shrugs. 😭

Where none of `-mtune=`, `-mcpu=` or `-march=` are specified, the code is tuned to perform well across a range of target processors.

> 🚨 Also important: this option **cannot** be suffixed by feature modifiers. No add-ons, no extras, don't even try it bestie.

- **`-mcpu=name`** — this one's the bigger sibling. You specify the name of the target processor, optionally suffixed by one or more feature modifiers. The form is `-mcpu=cpu{+[no]feature}*`, where the values allowed for *cpu* are the same as the ones available for `-mtune`. The values allowed for *feature* live in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. And if you pass conflicting feature modifiers? The <hotpink>right-most</hotpink> feature wins. Just like group chat, the last message counts. 🫶

GCC uses *name* to figure out what kind of instructions it can emit when generating assembly code (as if by `-march`) and to figure out the target processor to tune for performance (as if by `-mtune`). But the tea is: where this option is used together with `-march` or `-mtune`, those options take precedence over the appropriate part of this one. Okay so they override you. Noted. 💀

Now `-mcpu=neoverse-512tvb` is *special* — it does not refer to a specific core, it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless it's overridden by `-march`, `-mcpu=neoverse-512tvb` generates code that can run on a <purple>Neoverse V1</purple> core, since Neoverse V1 is the first Neoverse core with these properties. And unless overridden by `-mtune`, `-mcpu=neoverse-512tvb` tunes code in the same way as for `-mtune=neoverse-512tvb`. Consistent queen. 👑

- **`-moverride=string`** — this one overrides tuning decisions made by the back-end in response to a `-mtune=` switch. But the syntax, semantics, and accepted values for *string* in this option are <red>not guaranteed to be consistent across releases</red>. So like, don't build your whole personality on it.

This option is only intended to be useful when developing GCC. It's a compiler-dev thing, iykyk. 🙈

- **`-mverbose-cost-dump`** — turns on verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. Very much a "for the developers" moment. ✨

- **`-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. Which caps the maximum size of functions at <yellow>1MB</yellow>. This is enabled by default for `-mcmodel=tiny`. 🎀

- **`-msign-return-address=scope`** — picks the function scope on which return address signing gets applied. The values you can pass are:
- `none` — disables return address signing
- `non-leaf` — enables pointer signing for functions which are not leaf functions
- `all` — enables pointer signing for all functions

The default value is `none`. And bestie, listen: <red>this option has been deprecated by `-mbranch-protection`</red>. 🚩

- **`-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti`** — selects 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 <cyan>a-key</cyan>. 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 <cyan>B-key</cyan> instead of the <cyan>A-key</cyan>.
- `bti` turns on branch target identification mechanism.

- **`-mharden-sls=opts`** — enables compiler hardening against straight line speculation (<purple>SLS</purple>). *opts* is a comma-separated list of the following options:

retbr blr

And on top of those, `-mharden-sls=all` enables all SLS hardening while `-mharden-sls=none` disables all SLS hardening. Simple as. 🌸

- **`-msve-vector-bits=bits`** — specifies the number of bits in an SVE vector register. Heads up though: this option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: <mint>"vector-length agnostic"</mint> output that works with any size of vector register, and <mint>"vector-length specific"</mint> output that lets GCC make assumptions about the vector length when it's 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. And the behavior of these values may change in future releases, and <red>no value except `scalable` should be relied on for producing code that is portable across different hardware SVE vector lengths</red>. So like, `scalable` is the safe bestie here. 💖

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 <purple>CRC</purple> extension. This is on by default for `-march=armv8.1-a`.
- `crypto` — enable <purple>Crypto</purple> extension. This also enables Advanced SIMD and floating-point instructions.
- `fp` — enable floating-point instructions. This is on by default for all possible values for options `-march` and `-mcpu`.
- `simd` — enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options `-march` and `-mcpu`.
- `sve` — enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
- `lse` — enable Large System Extension instructions. This is on by default for `-march=armv8.1-a`.
- `rdma` — enable Round Double Multiply Accumulate instructions. This is on by default for `-march=armv8.1-a`.
- `fp16` — enable <purple>FP16</purple> extension. This also enables floating-point instructions.
- `fp16fml` — enable <purple>FP16 fmla</purple> extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for `-march=armv8.4-a`. 🚨 Use of this option with architectures prior to Armv8.2-A is <red>not supported</red>.
- `rcpc` — enable the <purple>RcPc</purple> extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
- `dotprod` — enable the <purple>Dot Product</purple> extension. This also enables Advanced SIMD instructions.
- `aes` — enable the Armv8-a <purple>aes</purple> and <purple>pmull</purple> crypto extension. This also enables Advanced SIMD instructions.
- `sha2` — enable the Armv8-a <purple>sha2</purple> crypto extension. This also enables Advanced SIMD instructions.
- `sha3` — enable the <purple>sha512</purple> and <purple>sha3</purple> crypto extension. This also enables Advanced SIMD instructions. 🚨 Use of this option with architectures prior to Armv8.2-A is <red>not supported</red>.
- `sm4` — enable the <purple>sm3</purple> and <purple>sm4</purple> crypto extension. This also enables Advanced SIMD instructions. 🚨 Use of this option with architectures prior to Armv8.2-A is <red>not supported</red>.
- `profile` — enable the <purple>Statistical Profiling</purple> extension. This option is only to enable the extension at the assembler level and does not affect code generation.
- `rng` — enable the Armv8.5-a <purple>Random Number</purple> instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
- `memtag` — enable the Armv8.5-a <purple>Memory Tagging Extensions</purple>. 🚨 Use of this option with architectures prior to Armv8.5-A is <red>not supported</red>.
- `sb` — enable the Armv8-a <purple>Speculation Barrier</purple> instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for `-march=armv8.5-a`.
- `ssbs` — enable the Armv8-a <purple>Speculative Store Bypass Safe</purple> instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for `-march=armv8.5-a`.
- `predres` — enable the Armv8-a <purple>Execution and Data Prediction Restriction</purple> instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for `-march=armv8.5-a`.
- `sve2` — enable the Armv8-a <purple>Scalable Vector Extension 2</purple>. This also enables SVE instructions.
- `sve2-bitperm` — enable <purple>SVE2 bitperm</purple> instructions. This also enables SVE2 instructions.
- `sve2-sm4` — enable <purple>SVE2 sm4</purple> instructions. This also enables SVE2 instructions.

- **`sve2-aes`** — turns on SVE2 aes instructions 💅 and honestly? it also turns on SVE2 instructions while it's at it.
- **`sve2-sha3`** — turns on SVE2 sha3 instructions ✨ and yeah, it also brings SVE2 instructions along for the ride.
- **`tme`** — enables the Transactional Memory Extension 🎀
- **`i8mm`** — enables 8-bit Integer Matrix Multiply instructions 🌸 this one also enables Advanced SIMD and floating-point instructions. It's **on by default** for `-march=armv8.6-a` 💖. Using it with architectures older than Armv8.2-A? Not supported, bestie 🚫
- **`f32mm`** — enables 32-bit Floating point Matrix Multiply instructions 💫 this also enables SVE instructions. Using it with architectures prior to Armv8.2-A is not supported.
- **`f64mm`** — enables 64-bit Floating point Matrix Multiply instructions ✨ also enables SVE instructions. Pre-Armv8.2-A architectures: not supported 🙅♀️
- **`bf16`** — enables 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`** — enables the 64-byte atomic load and store instructions for accelerators 🔥 on by default for `-march=armv8.7-a`.
- **`mops`** — enables the instructions that accelerate memory operations like `memcpy`, `memmove`, `memset` 🏃‍♀️💨 on by default for `-march=armv8.8-a`
- **`flagm`** — enables the Flag Manipulation instructions Extension 🚩
- **`pauth`** — enables the Pointer Authentication Extension 🔐

> The feature `crypto` implies aes, sha2, and simd, which implies fp. Flip it around and `nofp` implies nosimd, which implies nocrypto, noaes and nosha2. It's giving domino effect 💀

---

## 🌈 Adapteva Epiphany Options 🌈

These `-m` options are defined for Adapteva Epiphany:

- **`-mhalf-reg-file`** — don't allocate any register in the range `r32`...`r63` 🚫 that's what lets your code run on hardware variants that just don't have those registers.
- **`-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 overall code size. Could go either way, lowkey.
- **`-mbranch-cost=num`** — set the cost of branches to roughly *num* "simple" instructions. This cost is only a heuristic, and it's not guaranteed to produce consistent results across releases 🎲
- **`-mcmove`** — enable the generation of conditional moves ✨
- **`-mnops=num`** — emit *num* NOPs before every other generated instruction 🫠
- **`-mno-soft-cmpsf`** — for single-precision floating-point comparisons, emit an `fsub` instruction and test the flags. This is faster than a software comparison, but it can get incorrect results when NaNs are around, or when two different small numbers get compared such that their difference calculates out to 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. Like, a value of 8 means the eight bytes in the range `sp+0...sp+7` can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work 💀. Also heads up, this option **changes the ABI**; compiling a program with a different stack offset than the libraries were compiled with generally does not work. This option can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it's recommended you configure the toolchain with the appropriate `--with-stack-offset=num` option.
- **`-mno-round-nearest`** — make the scheduler assume the rounding mode has been set to truncating. The default is `-mround-nearest`.
- **`-mlong-calls`** — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the `b` / `bl` instructions, and therefore load the function address into a register before performing an (otherwise direct) call. **This is the default** 👑
- **`-mshort-calls`** — if not otherwise specified by an attribute, assume all direct calls are in the range of the `b` / `bl` instructions, so use these instructions for direct calls. The default is `-mlong-calls`.
- **`-msmall16`** — assume addresses can be loaded as 16-bit unsigned values. This doesn't apply to function addresses, for which `-mlong-calls` semantics are in effect.
- **`-mfp-mode=mode`** — set the prevailing mode of the floating-point unit 🌊 this determines the floating-point mode that's 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 are msplit-lohi, `-mpost-inc`, and `-mpost-modify`.
- **`-mnovect-double`** — change the preferred SIMD mode to SImode. The default is `-mvect-double`, which uses DImode as preferred SIMD mode.
- **`-max-vect-align=num`** — the maximum alignment for SIMD vector mode types. *num* may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types ⚠️
- **`-msplit-vecmove-early`** — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case 🙃
- **`-m1reg-reg`** — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for *reg* are `r43` and `r63`, which specify use of that register as a fixed register, and `none`, which means that no register is used for this purpose. The default is `-m1reg-none` 👑

---

## 🎮 AMD GCN Options 🎮

These options are defined specifically for the AMD GCN port:

- **`-march=gpu`**
- **`-mtune=gpu`** — set architecture type or tuning for *gpu*. Supported values for *gpu* are:

- **`fiji`** — compile for GCN3 Fiji devices (`gfx803`).
- **`gfx900`** — compile for GCN5 Vega 10 devices (`gfx900`).
- **`gfx906`** — compile for GCN5 Vega 20 devices (`gfx906`).

- **`-msram-ecc=on`**
- **`-msram-ecc=off`**
- **`-msram-ecc=any`** — compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode 😤. The default is `any`, for devices that support it.
- **`-mstack-size=bytes`** — specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may be many threads and limited memory available 🫣. The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
- **`-mxnack`** — compile binaries suitable for devices with the XNACK feature enabled. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is `-mno-xnack`. At present this option is a placeholder for support that is not yet implemented 🚧

---

## 🏹 ARC Options 🏹

The following options control the architecture variant for which code is being compiled:

- **`-mbarrel-shifter`** — generate instructions supported by barrel shifter ✨ this is the default unless `-mcpu=ARC601` or `-mcpu=ARCEM` is in effect.
- **`-mjli-always`** — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture 👑
- **`-mcpu=cpu`** — set architecture type, register usage, and instruction scheduling parameters for *cpu*. There are also shortcut alias options available for backward compatibility and convenience. Supported values for *cpu* are:

- **`arc600`** — compile for ARC600. Aliases: `-mA6`, `-mARC600`.
- **`arc601`** — compile for ARC601. Alias: `-mARC601`.
- **`arc700`** — compile for ARC700. Aliases: `-mA7`, `-mARC700`. This is the default when configured with `--with-cpu=arc700`.
- **`arcem`** — compile for ARC EM.
- **`archs`** — compile for ARC HS.
- **`em`** — compile for ARC EM CPU with no hardware extensions.
- **`em4`** — compile for ARC EM4 CPU.
- **`em4_dmips`** — compile for ARC EM4 DMIPS CPU.
- **`em4_fpus`** — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
- **`em4_fpuda`** — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
- **`hs`** — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
- **`hs34`** — compile for ARC HS34 CPU.
- **`hs38`** — compile for ARC HS38 CPU.
- **`hs38_linux`** — compile for ARC HS38 CPU with all hardware extensions on.
- **`arc600_norm`** — compile for ARC 600 CPU with `norm` instructions enabled.

- **`arc600_mul32x16`** — compile for ARC 600 CPU with `"norm"` and 32x16-bit multiply instructions turned on. ✨

- **`arc600_mul64`** — compile for ARC 600 CPU with `"norm"` and `"mul64"`-family instructions enabled. 💅

- **`arc601_norm`** — compile for ARC 601 CPU with `"norm"` instructions enabled. 🦋

- **`arc601_mul32x16`** — compile for ARC 601 CPU with `"norm"` and 32x16-bit multiply instructions enabled. 🦋

- **`arc601_mul64`** — compile for ARC 601 CPU with `"norm"` and `"mul64"`-family instructions enabled. 🦋

- **`nps400`** — compile for ARC 700 on the NPS400 chip. 👑

- **`em_mini`** — compile for the ARC EM minimalist config, the one with the reduced register set. 🎀

- **`-mdpfp`**, **`-mdpfp-compact`** — generate double-precision FPX instructions, tuned for the *compact* implementation. 🍓

- **`-mdpfp-fast`** — generate double-precision FPX instructions, but tuned for the *fast* implementation this time. 🔥

- **`-mno-dpfp-lrsr`** — stops `"lr"` and `"sr"` instructions from using FPX extension aux registers. no cap. 🙈

- **`-mea`** — generate extended arithmetic instructions. rn it only supports `"divaw"`, `"adds"`, `"subs"`, and `"sat16"`. <red>only valid for `-mcpu=ARC700`</red>. ⚠️

- **`-mno-mpy`** — do **not** generate `"mpy"`-family instructions for ARC700. > heads up bestie: this option is deprecated. 💀

- **`-mmul32x16`** — generate 32x16-bit multiply and multiply-accumulate instructions. ✨

- **`-mmul64`** — generate `"mul64"` and `"mulu64"` instructions. <red>only valid for `-mcpu=ARC600`</red>. ⚠️

- **`-mnorm`** — generate `"norm"` instructions. this is the default if `-mcpu=ARC700` is in effect. the default!! 💅

- **`-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. <red>only valid for `-mcpu=ARC700`</red>. ⚠️

- **`-msoft-float`** — okay so this one is <yellow>ignored</yellow>, it's provided for compatibility purposes **only**. software floating-point code is emitted by default, and that default can be overridden by the FPX options: `-mspfp`, `-mspfp-compact`, or `-mspfp-fast` for single precision, and `-mdpfp`, `-mdpfp-compact`, or `-mdpfp-fast` for double precision. 🫶

- **`-mswap`** — generate `"swap"` instructions. 🔄

- **`-matomic`** — this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. <red>not available for ARC 6xx or ARC EM cores.</red> ⚠️

- **`-mdiv-rem`** — enable `"div"` and `"rem"` instructions for ARCv2 cores. ➗

- **`-mcode-density`** — enable code density instructions for ARC EM. this one is **on by default for ARC HS**. 💖

- **`-mll64`** — enable 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 the option using either a string or a numeric value for *multo*. `wlh1` is the default value. the recognized values are, and bestie there are a lot of them: 👑

- **`0`**, **`none`** — no multiplier available. 💀

- **`1`**, **`w`** — 16x16 multiplier, fully pipelined. the following instructions are enabled: `"mpyw"` and `"mpyuw"`. ✨

- **`2`**, **`wlh1`** — 32x32 multiplier, fully pipelined (1 stage). the following instructions are *additionally* enabled: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`. 💅

- **`3`**, **`wlh2`** — 32x32 multiplier, fully pipelined (2 stages). the following instructions are additionally enabled: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`. 💅

- **`4`**, **`wlh3`** — two 16x16 multipliers, blocking, sequential. the following instructions are additionally enabled: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`. 💅

- **`5`**, **`wlh4`** — one 16x16 multiplier, blocking, sequential. the following instructions are additionally enabled: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`. 💅

- **`6`**, **`wlh5`** — one 32x4 multiplier, blocking, sequential. the following instructions are additionally enabled: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`. 💅

- **`7`**, **`plus_dmpy`** — ARC HS SIMD support. 🌈

- **`8`**, **`plus_macd`** — ARC HS SIMD support. 🌈

- **`9`**, **`plus_qmacw`** — ARC HS SIMD support. 🌈

this option is <red>only available for ARCv2 cores</red>. ⚠️

- **`-mfpu=`*fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. the 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. <red>not available for ARC EM.</red> ⚠️

- **`fpuda`** — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point extension is also enabled. <red>this option is only available for ARC EM.</red> ⚠️

- **`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. <red>this option is only available for ARC EM.</red> ⚠️

- **`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. <red>this option is only available for ARC EM.</red> ⚠️

- **`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. <red>this option is only available for ARC EM.</red> ⚠️

- **`fpus_div`** — enables support for single-precision floating-point, square-root and divide hardware extensions. ➗

- **`fpud_div`** — enables support for double-precision floating-point, square-root and divide hardware extensions. this option includes option `fpus_div`. <red>not available for ARC EM.</red> ⚠️

- **`fpus_fma`** — enables support for single-precision floating-point and fused multiply and add hardware extensions. ✨

- **`fpud_fma`** — enables support for double-precision floating-point and fused multiply and add hardware extensions. this option includes option `fpus_fma`. <red>not available for ARC EM.</red> ⚠️

- **`fpus_all`** — enables support for **all** single-precision floating-point hardware extensions. 💖

- **`fpud_all`** — enables support for all single- **and** double-precision floating-point hardware extensions. <red>not available for ARC EM.</red> ⚠️

- **`-mirq-ctrl-saved=`*register-range*, *blink*, *lp_count* — specifies general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. *register-range* is specified as two registers separated by a dash. the register range always starts with `"r0"`, and the upper limit is the `"fp"` register. *blink* and *lp_count* are optional. <red>this option is only valid for ARC EM and ARC HS cores.</red> ⚠️

- **`-mrgf-banked-regs=`*number* — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. fast interrupts are interrupts with the highest priority level P0. these interrupts save **only** PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. use this option when you're using fast interrupts in an ARC V2 family processor. the permitted values are `4`, `8`, `16`, and `32`. 🔥

- **`-mlpc-width=`*width* — specify the width of the `"lp_count"` register. valid values for *width* are `8`, `16`, `20`, `24`, `28` and `32` bits. the default width is fixed to 32 bits. if the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it's known that the `"lp_count"` register can hold the required loop-counter value. depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. this option defines macro `"__ARC_LPC_WIDTH__"` with the value of *width*. 🎀

- **`-mrf16`** — this option instructs the compiler to generate code for a 16-entry register file. this option defines the `"__ARC_RF16__"` preprocessor macro. 🦋

- **`-mbranch-index`** — enable use of `"bi"` or `"bih"` instructions to implement jump tables. 👑

---

<purple>the following options are passed through to the assembler, and they also define preprocessor macro symbols.</purple> 📎

- **`-mdsp-packa`** — passed down to the assembler to enable the DSP Pack A extensions. also sets the preprocessor symbol `"__Xdsp_packa"`. > this option is deprecated, bestie. 💀

- **`-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"`. 🌈

---

<purple>and these ones control how the assembly code gets annotated:</purple> 📝

- **`-misize`** — annotate assembler instructions with estimated addresses. 🎀

- **`-mannotate-align`** — explain what alignment considerations lead to the decision to make an instruction short or long. the way that it just *tells you why*?? iconic. 💅

okay so these next options are passed straight through to the linker:

- **`-marclinux`** — passed through to the linker, to specify use of the `"arclinux"` emulation. this one 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 is enabled by default in tool chains built for `"arc-linux-uclibc"` and `"arceb-linux-uclibc"` targets when profiling is requested. ✨

---

<pink>these next ones control the semantics of the generated code:</pink>

- **`-mlong-calls`** — generate calls as register indirect calls, so you get access to the full 32-bit address range. no cap. 💖

- **`-mmedium-calls`** — don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. this is the default for tool chains built for `"arc-linux-uclibc"` and `"arceb-linux-uclibc"` targets. 🎀

- **`-G num`** — put definitions of externally-visible data in a small data section if that data is no bigger than *num* bytes. the default value of *num* is `4` for any ARC configuration, or `8` when we have double load/store operations. 🍓

- **`-mno-sdata`** — do not generate sdata references. this is the default for tool chains built for `"arc-linux-uclibc"` and `"arceb-linux-uclibc"` targets. 🦋

- **`-mvolatile-cache`** — use ordinarily cached memory accesses for volatile references. this is the default. 💅

- **`-mno-volatile-cache`** — enable cache bypass for volatile references. 👑

---

<cyan>these next ones fine-tune code generation:</cyan>

- **`-malign-call`** — does nothing. preserved for backward compatibility. 💀

- **`-mauto-modify-reg`** — enable the use of pre/post modify with register displacement. ✨

- **`-mbbit-peephole`** — enable bbit peephole2. 🧚

- **`-mno-brcc`** — this option disables a target-specific pass in `arc_reorg` to generate compare-and-branch (`"brcc"`) instructions. it has no effect on generation of these instructions driven by the combiner pass. 🌸

- **`-mcase-vector-pcrel`** — use PC-relative switch case tables to enable case table shortening. this is the default for `-Os`. 🎀

- **`-mcompact-casesi`** — enable compact `"casesi"` pattern. this is the default for `-Os`, and only available for ARCv1 cores. this option is deprecated. ⚠️

- **`-mno-cond-exec`** — disable the ARCompact-specific pass to generate conditional execution instructions. due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. this pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. if you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using `-mmedium-calls` instead. 😭

- **`-mearly-cbranchsi`** — enable pre-reload use of the `"cbranchsi"` pattern. 💜

- **`-mexpand-adddi`** — expand `"adddi3"` and `"subdi3"` at RTL generation time into `"add.f"`, `"adc"` etc. this option is deprecated. 💀

- **`-mindexed-loads`** — enable the use of indexed loads. this can be problematic because some optimizers then assume that indexed stores exist, which is not the case. 🙈

- **`-mlra`** — enable Local Register Allocation. this is still experimental for ARC, so by default the compiler uses standard reload (i.e. `-mno-lra`). 🔥

- **`-mlra-priority-none`** — don't indicate any priority for target registers. 🌈

- **`-mlra-priority-compact`** — indicate target register priority for `r0..r3` / `r12..r15`. ⭐

- **`-mlra-priority-noncompact`** — reduce target register priority for `r0..r3` / `r12..r15`. 💫

- **`-mmillicode`** — when optimizing for size (using `-Os`), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function in `libgcc`; this is referred to as a millicode call. as these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation. 🛠️

- **`-mcode-density-frame`** — this option enables the compiler to emit `"enter"` and `"leave"` instructions. these instructions are only valid for CPUs with code-density feature. 👑

- **`-mmixed-code`** — does nothing. preserved for backward compatibility. 💀

- **`-mq-class`** — this option is deprecated. enable q instruction alternatives. this is the default for `-Os`. 💅

- **`-mRcq`** — enable Rcq constraint handling. most short code generation depends on this. this is the default. 💅

- **`-mRcw`** — enable Rcw constraint handling. most ccfsm condexec mostly depends on this. this is the default. ✨

- **`-msize-level=level`** — fine-tune size optimization with regards to instruction lengths and alignment. the recognized values for *level* are:
- `0` — no size optimization. this level is deprecated and treated like `1`. 💀
- `1` — short instructions are used opportunistically. 🎀
- `2` — in addition, alignment of loops and of code after barriers are dropped. 🦋
- `3` — in addition, optional data alignment is dropped, and the option `Os` is enabled. 👑
This defaults to `3` when `-Os` is in effect. otherwise, the behavior when this is not set is equivalent to level `1`. 💖

- **`-mtune=cpu`** — set instruction scheduling parameters for *cpu*, overriding any implied by `-mcpu=`. supported values for *cpu* are:
- `ARC600` — tune for ARC600 CPU. ⭐
- `ARC601` — tune for ARC601 CPU. ⭐
- `ARC700` — tune for ARC700 CPU with standard multiplier block. ⭐
- `ARC700-xmac` — tune for ARC700 CPU with XMAC block. ⭐
- `ARC725D` — tune for ARC725D CPU. ⭐
- `ARC750D` — tune for ARC750D CPU. ⭐

- **`-mmultcost=num`** — cost to assume for a multiply instruction, with `4` being equal to a normal instruction. 🍓

- **`-munalign-prob-threshold=probability`** — does nothing. preserved for backward compatibility. 💀

---

<yellow>these next ones are still hanging around for backward compatibility, but they're deprecated and will be removed in a future release:</yellow>

- **`-margonaut`** — obsolete FPX. 💀

- **`-mbig-endian`, `-EB`** — compile code for big-endian targets. use of these options is now deprecated. big-endian code is supported by configuring GCC to build `"arceb-elf32"` and `"arceb-linux-uclibc"` targets, for which big endian is the default. 🌍

- **`-mlittle-endian`, `-EL`** — compile code for little-endian targets. use of these options is now deprecated. little-endian code is supported by configuring GCC to build `"arc-elf32"` and `"arc-linux-uclibc"` targets, for which little endian is the default. 🌎

- **`-mbarrel_shifter`** — replaced by `-mbarrel-shifter`. 🔁

- **`-mdpfp_compact`** — replaced by `-mdpfp-compact`. 🔁

- **`-mdpfp_fast`** — replaced by `-mdpfp-fast`. 🔁

- **`-mdsp_packa`** — replaced by `-mdsp-packa`. 🔁

- **`-mEA`** — replaced by `-mea`. 🔁

- **`-mmac_24`** — replaced by `-mmac-24`. 🔁

- **`-mmac_d16`** — replaced by `-mmac-d16`. 🔁

- **`-mspfp_compact`** — replaced by `-mspfp-compact`. 🔁

- **`-mspfp_fast`** — replaced by `-mspfp-fast`. 🔁

- **`-mtune=cpu`** — values `arc600`, `arc601`, `arc700` and `arc700-xmac` for *cpu* are replaced by `ARC600`, `ARC601`, `ARC700` and `ARC700-xmac` respectively. 🔁

- **`-multcost=num`** — replaced by `-mmultcost`. 🔁

---

## ✨ ARM Options ✨

these `-m` options are defined for the ARM port:

- **`-mabi=name`** — generate code for the specified ABI. permissible values are: `apcs-gnu`, `atpcs`, `aapcs`, `aapcs-linux` and `iwmmxt`. 💅

- **`-mapcs-frame`** — generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. specifying `-fomit-frame-pointer` with this option causes the stack frames not to be generated for leaf functions. the default is `-mno-apcs-frame`. this option is deprecated. ⚠️

- **`-mapcs`** — this is a synonym for `-mapcs-frame` and is deprecated. 💀

- **`-mthumb-interwork`** — generate code that supports calling between the ARM and Thumb instruction sets. without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. the default is `-mno-thumb-interwork`, since slightly larger code is generated when `-mthumb-interwork` is specified. in AAPCS configurations this option is meaningless. 🔄

- **`-mno-sched-prolog`** — prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body. this means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. the default is `-msched-prolog`. 🧐

- **`-mfloat-abi=name`** — specifies which floating-point ABI to use. permissible values are: `soft`, `softfp` and `hard`. specifying `soft` causes GCC to generate output containing library calls for floating-point operations. `softfp` allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. `hard` allows generation of floating-point instructions and uses FPU-specific calling conventions. the default depends on the specific target configuration. note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. ⚠️

- **`-mgeneral-regs-only`** — generate code which uses only the general-purpose registers. this will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. 💪

- **`-mlittle-endian`** — generate code for a processor running in little-endian mode. this is the default for all standard configurations. 🌎

- **`-mbig-endian`** — bestie, this one makes code for a processor chugging along in big-endian mode. And the default? Compiling for a little-endian processor, periodt. 💅

- **`-mbe8`, `-mbe32`** — okay so when you're linking a big-endian image, this picks between BE8 and BE32 formats. For little-endian images it does literally nothing and gets ignored (it's giving "not my problem"). The default depends on whichever target architecture you picked — for ARMv6 and later it's BE8, for older ones it's BE32. And heads up: <red>BE32 format has been deprecated by ARM</red>, so like… rip to it. 💀

- **`-march=`*name*`[+`*extension*`...]`** — this is where you name the target ARM architecture. GCC peeks at this name to figure out what kind of instructions it's allowed to emit when it's generating assembly code. You can use it alongside `-mcpu=` or instead of it, dealer's choice. ✨

The 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`.

Also — and this is important — the following architectures, which do <red>not</red> support the Thumb execution state, are recognized but their support is **deprecated**: `armv4`. 🫠

Lots of these architectures support extensions, and you add them by tacking `+`*extension* onto the architecture name. Extension options get processed in order and the capabilities just keep accumulating, no cap. An extension will also switch on any base extensions it depends on — like, the `+crypto` extension will *always* enable `+simd`. The one plot twist: extensions prefixed with `+no...` disable the thing you named and anything else that depends on it being there.

So for example, `-march=armv7-a+simd+nofp+vfpv4` is equivalent to writing `-march=armv7-a+vfpv4`, because the `+simd` option gets completely disabled by the `+nofp` that comes after it. Iconic cascade. 💅

Most extension names are generic-sounding but what they actually do depends on the architecture you slap them onto. Case in point: `+simd` can go on both `armv7-a` and `armv8-a`, but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for `armv7-a`, and the ARMv8-A variant for `armv8-a`. Same name, different vibe. 🦋

The list below shows the supported extensions for each architecture. Architectures that aren't mentioned? They don't support any extensions, sorry bestie.

### armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk

- **`+fp`** — the VFPv2 floating-point instructions. You can use `+vfpv2` as an alias for this extension.
- **`+nofp`** — disables 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. `+vfpv3-d16` works as an alias for this. Quick note tho: floating-point isn't supported by the base ARMv7-M architecture, but it *is* compatible with both ARMv7-A and ARMv7-R.
- **`+nofp`** — disables 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. `+vfpv3-d16` is an alias for this.
- **`+simd`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. `+neon` and `+neon-vfpv3` are aliases for this.
- **`+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`** — disables the Advanced SIMD instructions (it does *not* disable floating point, fyi).
- **`+nofp`** — disables 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. `+vfpv4-d16` is an alias for this.
- **`+simd`** — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. `+neon-vfpv4` is an alias for this.
- **`+vfpv3-d16`** — the VFPv3 floating-point instructions, with 16 double-precision registers.
- **`+vfpv3`** — the VFPv3 floating-point instructions, with 32 double-precision registers.
- **`+vfpv3-d16-fp16`** — the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
- **`+vfpv3-fp16`** — the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
- **`+vfpv4-d16`** — the VFPv4 floating-point instructions, with 16 double-precision registers.
- **`+vfpv4`** — the VFPv4 floating-point instructions, with 32 double-precision registers.
- **`+neon`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. `+neon-vfpv3` is an alias for this.
- **`+neon-fp16`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
- **`+nosimd`** — disables the Advanced SIMD instructions (nope, it doesn't touch floating point).
- **`+nofp`** — disables the floating-point and Advanced SIMD instructions.

### armv8-a

- **`+crc`** — the Cyclic Redundancy Check (CRC) instructions.
- **`+simd`** — the ARMv8-A Advanced SIMD and floating-point instructions.
- **`+crypto`** — the cryptographic instructions.
- **`+nocrypto`** — disables the cryptographic instructions.
- **`+nofp`** — disables the floating-point, Advanced SIMD *and* cryptographic instructions.
- **`+sb`** — Speculation Barrier Instruction.
- **`+predres`** — Execution and Data Prediction Restriction Instructions.

### armv8.1-a

- **`+simd`** — the ARMv8.1-A Advanced SIMD and floating-point instructions.
- **`+crypto`** — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
- **`+nocrypto`** — disables the cryptographic instructions.
- **`+nofp`** — disables the floating-point, Advanced SIMD and cryptographic instructions.
- **`+sb`** — Speculation Barrier Instruction.
- **`+predres`** — Execution and Data Prediction Restriction Instructions.

### armv8.2-a, armv8.3-a

- **`+fp16`** — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
- **`+fp16fml`** — the half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.
- **`+simd`** — the ARMv8.1-A Advanced SIMD and floating-point instructions.
- **`+crypto`** — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
- **`+dotprod`** — 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.
- **`+bf16`** — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

### armv8.4-a

- **`+fp16`** — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
- **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- **`+crypto`** — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- **`+nocrypto`** — disables the cryptographic extension.

- **`+nofp`** — turns off the floating-point, Advanced SIMD *and* cryptographic instructions. all of it, gone 💀
- **`+sb`** — Speculation Barrier Instruction.
- **`+predres`** — Execution and Data Prediction Restriction Instructions.
- **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. this one also turns on Advanced SIMD and floating-point instructions 🫶
- **`+bf16`** — Brain half-precision floating-point instructions. and yeah, 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, plus the Dot Product extension, plus the half-precision floating-point `fmla` extension. it's giving ✨everything✨
- **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension.
- **`+crypto`** — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- **`+nocrypto`** — disable the cryptographic extension. bye bestie 👋
- **`+nofp`** — disable the floating-point, Advanced SIMD and cryptographic instructions.
- **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
- **`+bf16`** — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.

- **`armv8.6-a`** 💫

- **`+fp16`** — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, plus the Dot Product extension, plus the half-precision floating-point `fmla` extension.
- **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- **`+crypto`** — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- **`+nocrypto`** — disable the cryptographic extension.
- **`+nofp`** — disable the floating-point, Advanced SIMD and cryptographic instructions.
- **`+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. lowkey cute alias alert: the extension `+vfpv3xd` can be used as an alias for this extension.
- **`+fp`** — the VFPv3 floating-point instructions with 16 double-precision registers. and surprise, the extension `+vfpv3-d16` can be used as an alias for this extension.
- **`+vfpv3xd-d16-fp16`** — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
- **`+vfpv3-d16-fp16`** — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
- **`+nofp`** — disable the floating-point extension.
- **`+idiv`** — the ARM-state integer division instructions.
- **`+noidiv`** — disable the ARM-state integer division extension.

- **`armv7e-m`** 🌸

- **`+fp`** — the single-precision VFPv4 floating-point instructions.
- **`+fpv5`** — the single-precision FPv5 floating-point instructions.
- **`+fp.dp`** — the single- and double-precision FPv5 floating-point instructions.
- **`+nofp`** — disable the floating-point extensions.

- **`armv8.1-m.main`** 🦋

- **`+dsp`** — the DSP instructions.
- **`+mve`** — the M-Profile Vector Extension (MVE) integer instructions.
- **`+mve.fp`** — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
- **`+fp`** — the single-precision floating-point instructions.
- **`+fp.dp`** — the single- and double-precision floating-point instructions.
- **`+nofp`** — disable the floating-point extension.
- **`+cdecp0`, `+cdecp1`, ... , `+cdecp7`** — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.

- **`armv8-m.main`** 💜

- **`+dsp`** — the DSP instructions.
- **`+nodsp`** — disable the DSP extension.
- **`+fp`** — the single-precision floating-point instructions.
- **`+fp.dp`** — the single- and double-precision floating-point instructions.
- **`+nofp`** — disable the floating-point extension.
- **`+cdecp0`, `+cdecp1`, ... , `+cdecp7`** — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.

- **`armv8-r`** 🔥

- **`+crc`** — the Cyclic Redundancy Check (CRC) instructions.
- **`+fp.sp`** — the single-precision FPv5 floating-point instructions.
- **`+simd`** — the ARMv8-A Advanced SIMD and floating-point instructions.
- **`+crypto`** — the cryptographic instructions.
- **`+nocrypto`** — disable the cryptographic instructions.
- **`+nofp`** — disable the floating-point, Advanced SIMD and cryptographic instructions.

---

<pink>`-march=native`</pink> makes the compiler ✨auto-detect✨ the architecture of the build computer. but bestie, real talk: at present this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful the option just... has no effect. it's giving 🫥

---

## <hotpink>`-mtune=name`</hotpink> 🎀

this option specifies the name of the target ARM processor for which GCC should tune the performance of the code. for some ARM implementations you can get better performance by using this option. the 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-m55, cortex-x1, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.

additionally, this option can specify that GCC should tune the performance of the code for a **big.LITTLE** system. the permissible names there 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.

<purple>`-mtune=generic-arch`</purple> 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. no cap, the effects of this option may change in future GCC versions as CPU models come and go. it's giving 🌊 the tide comes in, the tide goes out 🌊

`-mtune` permits the same extension options as `-mcpu`, but heads up: 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.

---

## <hotpink>`-mcpu=name[+extension...]`</hotpink> 👑

this specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by `-march`) and the ARM processor type for which to tune for performance (as if specified by `-mtune`). where this option is used in conjunction with `-march` or `-mtune`, those options take precedence over the appropriate part of this option. so they win, periodt 💅

many of the supported CPUs implement optional architectural extensions. where this is so, the architectural extensions are normally enabled by default. if implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. for floating-point and Advanced SIMD (Neon) instructions, the settings of the options `-mfloat-abi` and `-mfpu` must also be considered: floating-point and Advanced SIMD instructions will only be used if `-mfloat-abi` is not set to `soft`; and any setting of `-mfpu` other than `auto` will override the available floating-point and SIMD extension instructions.

for example, `cortex-a9` can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. the default is to enable all the instructions, but the extensions `+nosimd` and `+nofp` can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively.

the permissible names for this option are the same as those for `-mtune`. 🔁

the following extension options are common to the listed CPUs:

- **`+nodsp`** — disable the DSP instructions on `cortex-m33`, `cortex-m35p`.

- **`+nofp`** — turns the floating-point instructions **off** on `arm9e`, `arm946e-s`, `arm966e-s`, `arm968e-s`, `arm10e`, `arm1020e`, `arm1022e`, `arm926ej-s`, `arm1026ej-s`, `cortex-r5`, `cortex-r7`, `cortex-r8`, `cortex-m4`, `cortex-m7`, `cortex-m33` and `cortex-m35p`. And on `generic-armv7-a`, `cortex-a5`, `cortex-a7`, `cortex-a8`, `cortex-a9`, `cortex-a12`, `cortex-a15`, `cortex-a17`, `cortex-a15.cortex-a7`, `cortex-a17.cortex-a7`, `cortex-a32`, `cortex-a35`, `cortex-a53` and `cortex-a55` it kills the floating-point **and** the SIMD instructions. like, both. gone. 💀

- **`+nofp.dp`** — disables the double-precision component of the floating-point instructions on `cortex-r5`, `cortex-r7`, `cortex-r8`, `cortex-r52`, `cortex-r52plus` and `cortex-m7`. just the double-precision part, bestie, single-precision stays 💅

- **`+nosimd`** — disables the SIMD instructions (but <rainbow>not</rainbow> the floating-point ones, ok?? just SIMD) on `generic-armv7-a`, `cortex-a5`, `cortex-a7` and `cortex-a9`.

- **`+crypto`** — enables the cryptographic instructions on `cortex-a32`, `cortex-a35`, `cortex-a53`, `cortex-a55`, `cortex-a57`, `cortex-a72`, `cortex-a73`, `cortex-a75`, `exynos-m1`, `xgene1`, `cortex-a57.cortex-a53`, `cortex-a72.cortex-a53`, `cortex-a73.cortex-a35`, `cortex-a73.cortex-a53` and `cortex-a75.cortex-a55`. 🔐✨

---

<purple>Also</purple> the `generic-armv7-a` pseudo target defaults to <cyan>VFPv3</cyan> with 16 double-precision registers, bestie, that's just the vibe it ships with. And it supports these 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`. same energy, different spelling 🦋

`-mcpu=generic-arch` is also permissible, and it's equivalent to `-march=arch -mtune=generic-arch`. Go peep `-mtune` if you want the lore on that one.

`-mcpu=native` makes the compiler auto-detect the CPU of the build computer. <yellow>heads up tho:</yellow> 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. it just. does nothing. iconic behaviour honestly 💀

## ✨ OPTIONS ✨ (still going, we're so back)

- **`-mfpu=`*name*** — this one 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`. Oh and pspsps: `neon` is an alias for `neon-vfpv3`, and `vfp` is an alias for `vfpv2`. cute little nicknames 🎀

  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 floating-point hardware you picked includes the NEON extension (like, say, `-mfpu=neon`), note that floating-point operations are **not** generated by GCC's auto-vectorization pass unless `-funsafe-math-optimizations` is *also* specified. Why? Because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so using NEON instructions may lead to a **loss of precision**. no cap, that's the tradeoff 😭

  You can also set the fpu name at function level by using the `target("fpu=")` function attributes or pragmas. per-function fpu? she contains multitudes 💅

- **`-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 at all. it's giving nonexistent 🫥

- **`-mstructure-size-boundary=`*n*** — the sizes of all structures and unions get 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. respect the ABI's boundaries bestie 🌸

  Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. so like, mismatch = chaos 🙈

  > This option is **deprecated**. rip bozo 🪦

- **`-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. you said you'd never return and then you tried to return?? embarrassing, abort 💀

- **`-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. 64 megabytes?? she's too far away, need a whole extra register for the journey 🚗

  Even if this switch is enabled, <hotpink>not all</hotpink> function calls are turned into long calls. The heuristic is that static functions, functions that have the `short_call` attribute, functions that are inside the scope of a `#pragma no_long_calls` directive, and functions whose definitions have already been compiled within the current compilation unit are **not** turned into long calls. The exceptions to this rule are that weak function definitions, functions with the `long_call` attribute or the `section` attribute, and functions that are within the scope of a `#pragma long_calls` directive are **always** turned into long calls. it's giving rules with exceptions to the rules ✨

  This feature is not enabled by default. Specifying `-mno-long-calls` restores the default behavior, as does placing the function calls within the scope of a `#pragma long_calls_off` directive. Note these switches have **no** effect on how the compiler generates code to handle function calls via function pointers. pointers said "not my problem" 🙅‍♀️

- **`-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 the runtime gotta do its own homework, the compiler is off the clock 💅

- **`-mpic-register=`*reg*** — specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler (she picks, she has taste). For single PIC base case, the default is `R9` if target is EABI based or stack-checking is enabled, otherwise the default is `R10`.

- **`-mpic-data-is-text-relative`** — assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable `-msingle-pic-base` by default. one option turning on another option, it's giving codependency 🦋

- **`-mpoke-function-name`** — write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

t0 .ascii "arm_poke_function_name", 0 .align t1 .word 0xff000000 + (t1 - t0) arm_poke_function_name mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4

  When performing a stack backtrace, code can inspect the value of `pc` stored at `fp + 0`. If the trace function then looks at location `pc - 12` and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length `((pc[-3]) & 0xff000000)`. forensic detective energy 🔍✨

- **`-mthumb`**, **`-marm`** — select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the `--with-mode=state` configure option.

  You can also override the ARM and Thumb mode for each function by using the `target("thumb")` and `target("arm")` function attributes or pragmas. per-function mode switch?? main character energy fr 👑

- **`-mflip-thumb`** — switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is **not** intended for ordinary use in compiling code. it's a test tool bestie, do not put this in your build system 😭

- **`-mtpcs-frame`** — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-tpcs-frame`. 🍃

- **`-mtpcs-leaf-frame`** — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-apcs-leaf-frame`. yes the default name doesn't match, that's just how it is, we don't question the elders 💀

- **`-mcallee-super-interworking`** — gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is **not valid** in AAPCS configurations because interworking is enabled by default. already enabled, so this would be doing the most for no reason 🙈

- **`-mcaller-super-interworking`** — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. small price, big correctness ✨ This option is **not valid** in AAPCS configurations because interworking is enabled by default.

- **`-mtp=`*name*** — specify the access model for the thread local storage pointer. The valid models are `soft`, which generates calls to `__aeabi_read_tp`, `cp15`, which fetches the thread pointer from `cp15` directly (supported in the arm6k architecture), and `auto`, which uses the best available method for the selected processor. The default setting is `auto`. she'll figure it out herself, she always does 💅

- **`-mtls-dialect=`*dialect*** — specify the dialect to use for accessing thread local storage. Two dialects are supported---`gnu` and `gnu2`. The `gnu` dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The `gnu2` dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. og scheme stays winning for those two, no notes 🌈

- **`-mword-relocations`** — only makes absolute relocations on word-sized values (that's `R_ARM_ABS32` btw). It's <green>on by default</green> on targets where the runtime loader is like "no ma'am" (uClinux, SymbianOS), and also whenever you pass `-fpic` or `-fPIC`. Heads up: this one beefs with `-mslow-flash-data`.

- **`-mfix-cortex-m3-ldrd`** — some Cortex-M3 cores will straight up corrupt your data when `ldrd` instructions have overlapping destination and base registers 💀 so this option makes GCC not generate those. <green>On by default</green> when you specify `-mcpu=cortex-m3`.

- **`-mfix-cortex-a57-aes-1742098`**, **`-mno-fix-cortex-a57-aes-1742098`**, **`-mfix-cortex-a72-aes-1655431`**, **`-mno-fix-cortex-a72-aes-1655431`** — turns the mitigation for a Cortex-A57 and Cortex-A72 erratum <pink>on</pink> (or <pink>off</pink>), and that erratum messes with the AES cryptographic instructions. <green>Enabled by default</green> when either `-mcpu=cortex-a57` or `-mcpu=cortex-a72` is specified.

- **`-munaligned-access`**, **`-mno-unaligned-access`** — enables (or disables) reading and writing 16- and 32-bit values from addresses that are **not** 16- or 32-bit aligned. By default unaligned access is <red>off</red> for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and <green>on</green> for literally every other architecture. If unaligned access isn't enabled, then words inside packed data structures get accessed a byte at a time. 😭

  The ARM attribute `"Tag_CPU_unaligned_access"` gets set in the generated object file to either true or false, depending on how you set this option. And if unaligned access is enabled, the preprocessor symbol `"__ARM_FEATURE_UNALIGNED"` is also defined.

- **`-mneon-for-64bits`** — this option is <red>deprecated</red> and does absolutely nothing. Not me giving you a flag with no job 🙈

- **`-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 <green>off by default</green>. 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 <pink>no impact on Thumb2</pink>. But fr, this may change in future releases of GCC. And divided syntax? Yeah, that should be considered <red>deprecated</red>.

- **`-mrestrict-it`** — restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This one's <green>on by default</green> 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 is not intended for ordinary use when compiling code. <red>Disabled by default.</red>

- **`-mverbose-cost-dump`** — enables verbose cost model dumping in the debug dump files. This one's here for debugging the compiler, bestie.

- **`-mpure-code`** — does not allow constant data to be placed in code sections. On top of that, 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 lives over at <https://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). <green>Enabled by default</green> when the `-mcpu=` option is used with `"cortex-m33"`, `"cortex-m35p"` or `"cortex-m55"`. And you can flip it off with `-mno-fix-cmse-cve-2021-35465`. 💅

- **`-mstack-protector-guard=guard`**, **`-mstack-protector-guard-offset=offset`** — generates stack protection code using a canary at *guard*. Supported locations are `global` for a global canary, or `tls` for a canary you reach via the TLS register. The `-mstack-protector-guard-offset=` option is for use with `-fstack-protector-guard=tls` and <red>not</red> for use in user-land code.

- **`-mfdpic`**, **`-mno-fdpic`** — selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for `"arm-*-uclinuxfdpiceabi"` targets, this option is <green>on by default</green> and it implies `-fPIE` if none of the PIC/PIE-related options is provided. On other targets, it only turns on the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed.

  > Note that static linking is <red>not supported</red>, because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use `-static` and `-Wl,-dynamic-linker` options.

  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.

---

## ✨ AVR Options ✨

These options are defined for AVR implementations, bestie:

- **`-mmcu=mcu`** — specifies the Atmel AVR instruction set architecture (ISA) or MCU type.

  The default for this option is `avr2`.

  GCC supports the following AVR devices and ISAs:

  - **`"avr2"`** — <lavender>"Classic"</lavender> devices with up to 8 KiB of program memory. *mcu* = `"attiny22"`, `"attiny26"`, `"at90s2313"`, `"at90s2323"`, `"at90s2333"`, `"at90s2343"`, `"at90s4414"`, `"at90s4433"`, `"at90s4434"`, `"at90c8534"`, `"at90s8515"`, `"at90s8535"`.

  - **`"avr25"`** — <lavender>"Classic"</lavender> 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"`** — <lavender>"Classic"</lavender> devices with 16 KiB up to 64 KiB of program memory. *mcu* = `"at76c711"`, `"at43usb355"`.

  - **`"avr31"`** — <lavender>"Classic"</lavender> devices with 128 KiB of program memory. *mcu* = `"atmega103"`, `"at43usb320"`.

  - **`"avr35"`** — <lavender>"Classic"</lavender> 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"`** — <mint>"Enhanced"</mint> devices with up to 8 KiB of program memory. *mcu* = `"atmega48"`, `"atmega48a"`, `"atmega48p"`, `"atmega48pa"`, `"atmega48pb"`, `"atmega8"`, `"atmega8a"`, `"atmega8hva"`, `"atmega88"`, `"atmega88a"`, `"atmega88p"`, `"atmega88pa"`, `"atmega88pb"`, `"atmega8515"`, `"atmega8535"`, `"ata6285"`, `"ata6286"`, `"ata6289"`, `"ata6612c"`, `"at90pwm1"`, `"at90pwm2"`, `"at90pwm2b"`, `"at90pwm3"`, `"at90pwm3b"`, `"at90pwm81"`.

  - **`"avr5"`** — <mint>"Enhanced"</mint> 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"`, `"ata5795"`, `"ata5790"`, `"ata5790n"`, `"ata5791"`, `"ata6613c"`, `"ata6614q"`, `"ata5782"`, `"ata5831"`, `"ata8210"`, `"ata8510"`, `"ata5702m322"`, `"at90pwm161"`, `"at90pwm216"`, `"at90pwm316"`, `"at90can32"`, `"at90can64"`, `"at90scr100"`, `"at90usb646"`, `"at90usb647"`, `"at94k"`, `"m3000"`.

  - **`"avr51"`** — <mint>"Enhanced"</mint> devices with 128 KiB of program memory. *mcu* = `"atmega128"`, `"atmega128a"`, `"atmega128rfa1"`, `"atmega128rfr2"`, `"atmega1280"`, `"atmega1281"`, `"atmega1284"`, `"atmega1284p"`, `"atmega1284rfr2"`, `"at90can128"`, `"at90usb1286"`, `"at90usb1287"`.

  - **`"avr6"`** — <mint>"Enhanced"</mint> devices with a 3-byte PC, i.e. with more than 128 KiB of program memory. *mcu* = `"atmega256rfr2"`, `"atmega2560"`, `"atmega2561"`, `"atmega2564rfr2"`.

  - **`"avrxmega2"`** — <cyan>"XMEGA"</cyan> devices with more than 8 KiB and up to 64 KiB of program memory. *mcu* = `"atxmega8e5"`, `"atxmega16a4"`, `"atxmega16a4u"`, `"atxmega16c4"`, `"atxmega16d4"`, `"atxmega16e5"`, `"atxmega32a4"`, `"atxmega32a4u"`, `"atxmega32c3"`, `"atxmega32c4"`, `"atxmega32d3"`, `"atxmega32d4"`, `"atxmega32e5"`.

  - **`"avrxmega3"`** — <cyan>"XMEGA"</cyan> devices with up to 64 KiB of combined program memory **and** RAM, and with program memory visible in the RAM address space. *mcu* = `"attiny202"`, `"attiny204"`, `"attiny212"`, `"attiny214"`, `"attiny402"`, `"attiny404"`, `"attiny406"`, `"attiny412"`, `"attiny414"`, `"attiny416"`, `"attiny417"`, `"attiny804"`, `"attiny806"`, `"attiny807"`, `"attiny814"`, `"attiny816"`, `"attiny817"`, `"attiny1604"`, `"attiny1606"`, `"attiny1607"`, `"attiny1614"`, `"attiny1616"`, `"attiny1617"`, `"attiny3214"`, `"attiny3216"`, `"attiny3217"`, `"atmega808"`, `"atmega809"`, `"atmega1608"`, `"atmega1609"`, `"atmega3208"`, `"atmega3209"`, `"atmega4808"`, `"atmega4809"`.

- **`"avrxmega4"`** — <purple>"XMEGA"</purple> girlies with <yellow>more than 64 KiB and up to 128 KiB</yellow> of program memory. mcu = `"atxmega64a3"`, `"atxmega64a3u"`, `"atxmega64a4u"`, `"atxmega64b1"`, `"atxmega64b3"`, `"atxmega64c3"`, `"atxmega64d3"`, `"atxmega64d4"`. 💖

- **`"avrxmega5"`** — <purple>"XMEGA"</purple> besties with more than 64 KiB and up to 128 KiB of program memory <hotpink>and</hotpink> more than 64 KiB of RAM. mcu = `"atxmega64a1"`, `"atxmega64a1u"`. ✨

- **`"avrxmega6"`** — <purple>"XMEGA"</purple> cuties 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"`. That's a whole roster, no cap. 🎀

- **`"avrxmega7"`** — <purple>"XMEGA"</purple> icons with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = `"atxmega128a1"`, `"atxmega128a1u"`, `"atxmega128a4u"`. 💅

- **`"avrtiny"`** — <purple>"TINY"</purple> Tiny core devices, tiny but mighty, with 512 B up to 4 KiB of program memory. mcu = `"attiny4"`, `"attiny5"`, `"attiny9"`, `"attiny10"`, `"attiny20"`, `"attiny40"`. 🦋

- **`"avr1"`** — this ISA is implemented by the minimal AVR core and is supported <red>for assembler only</red>. mcu = `"attiny11"`, `"attiny12"`, `"attiny15"`, `"attiny28"`, `"at90s1200"`. 👑

---

- **`-mabsdata`** — assume that all data in static storage can be reached by `LDS` / `STS` instructions. This one only does anything on reduced Tiny devices like `ATtiny40`. Also peep the `absdata` AVR Variable Attributes variable attribute. iykyk 🌸

- **`-maccumulate-args`** — stack up your outgoing function arguments and grab/release the stack space they need <pink>once</pink>, in the function prologue/epilogue. Without this, outgoing args get pushed before calling a function and popped after. 🙈

  Popping args after the call can be <red>expensive</red> on AVR, so accumulating the stack space might give you smaller executables, because those arguments don't need to be yanked off the stack after a call like that.

  This option can lowkey shrink your code for functions that make several calls to functions which get their arguments on the stack — like calls to printf-ish functions. 💖

- **`-mbranch-cost=cost`** — set the branch costs for conditional branch instructions to *cost*. Reasonable values for *cost* are small, non-negative integers. Default branch cost is `0`. ✨

- **`-mcall-prologues`** — function prologues/epilogues get expanded as calls to the appropriate subroutines. Code size goes <green>smaller</green>. 🎀

- **`-mdouble=bits`**
- **`-mlong-double=bits`** — set the size (in bits) of the `double` or `long double` type respectively. Possible values for *bits* are `32` and `64`. Whether a specific *bits* value is even allowed depends on the `--with-double=` and `--with-long-double=` configure options (`https://gcc.gnu.org/install/configure.html#avr`), and same energy applies to the default values of these options. 🦋

- **`-mgas-isr-prologues`** — interrupt service routines (ISRs) may use the `__gcc_isr` pseudo instruction supported by GNU Binutils. If this option is on, you can still turn the feature off for individual ISRs with the `no_gccisr` AVR Function Attributes function attribute. This feature activates by default if optimization is on (but <red>not</red> with `-Og`, `@pxref{Optimize Options}`), and if GNU Binutils supports PR21683 (`https://sourceware.org/PR21683`). 🔥

- **`-mint8`** — assume `int` is an 8-bit integer. This messes with the sizes of <pink>all</pink> types: a `char` is 1 byte, an `int` is 1 byte, a `long` is 2 bytes, and `long long` is 4 bytes. Heads up: this option does <red>not</red> conform to the C standards, but it does give you smaller code size. 💀

- **`-mmain-is-OS_task`** — don't save registers in `main`. Same effect as attaching the `OS_task` AVR Function Attributes attribute to `main`. It activates by default if optimization is on. ✨

- **`-mn-flash=num`** — assume the flash memory has a size of *num* times 64 KiB. 💅

- **`-mno-interrupts`** — generated code is <red>not</red> compatible with hardware interrupts. Code size is smaller though. 🫶

- **`-mrelax`** — try to swap a `CALL` resp. `JMP` instruction for the shorter `RCALL` resp. `RJMP` instruction when it applies. Setting `-mrelax` just adds the `--mlink-relax` option to the assembler's command line and the `--relax` option to the linker's command line. 🌈

  Jump relaxing happens in the <purple>linker</purple>, because jump offsets aren't known before the code is located. So the assembler code the compiler spits out is the same, but the instructions in the executable may differ from the instructions in the assembler code. 🤯

  Relaxing <hotpink>must</hotpink> be turned on if linker stubs are needed — see the section on `EIND` and linker stubs below. 👀

- **`-mrmw`** — assume the device supports the Read-Modify-Write instructions `XCH`, `LAC`, `LAS` and `LAT`. 💜

- **`-mshort-calls`** — assume `RJMP` and `RCALL` can target the whole program memory.

  This option is used internally for multilib selection. It is <red>not</red> an optimization option, and you don't need to set it by hand. 🙅‍♀️

- **`-msp8`** — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.

  This option is used internally by the compiler to select and build multilibs for architectures `"avr2"` and `"avr25"`. Those architectures mix devices with and without `SPH`. For any setting other than `-mmcu=avr2` or `-mmcu=avr25` the compiler driver adds or removes this option from the compiler proper's command line, because then the compiler knows whether the device or architecture has an 8-bit stack pointer and thus no `SPH` register or not. 🧚

- **`-mstrict-X`** — use the address register `X` the way the hardware proposes. That means `X` only gets 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 library `lib<mcu>.a`. 🚫

- **`-nodevicespecs`** — don't add `-specs=device-specs/specs-mcu` to the compiler driver's command line. The user takes responsibility for supplying the sub-processes — like compiler proper, assembler and linker — with appropriate command line options. This means the user has to supply her private device specs file via `-specs=path-to-specs-file`. There's no more need for the option `-mmcu=mcu`. 👑

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 in the case where the resulting address space is not contained in the incoming address space. ⚠️

- **`-Wmisspelled-isr`** — warn if the ISR is misspelled, i.e. without `__vector` prefix. Enabled by default. 💖

---

## 🌈 `"EIND"` and Devices with More Than 128 Ki Bytes of Flash 🌈

Pointers in the implementation are <pink>16 bits wide</pink>. The address of a function or label is represented as a <purple>word address</purple>, so that indirect jumps and calls can target any code address in the range of 64 Ki words. ✨

To make indirect jumps work on devices with more than 128 Ki bytes of program memory space, there's a special function register called `EIND` 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 like this by the compiler, and they come with some limitations, bestie:

- The compiler <red>never</red> sets `EIND`. 🚫
- The compiler uses `EIND` implicitly in `EICALL`/`EIJMP` instructions, or might read `EIND` directly to emulate an indirect call/jump by means of a `RET` instruction. 👀
- The compiler assumes that `EIND` never changes during the startup code or during the application. In particular, `EIND` is <red>not</red> saved/restored in function or interrupt service routine prologue/epilogue. 😭
- For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🦋
- Linker relaxation <hotpink>must</hotpink> be turned on so that the linker generates the stubs correctly in all situations. See the compiler option `-mrelax` and the linker option `--relax`. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀
- The default linker script is arranged for code with `EIND = 0`. If code is supposed to work for a setup with `EIND != 0`, a custom linker script has to be used in order to place the sections whose name starts with `.trampolines` into the segment where `EIND` points to. 🌸
- The startup code from libgcc never sets `EIND`. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on `EIND`, see the AVR-LibC user manual (`http://nongnu.org/avr-libc/user-manual/`). 📖
- It is legitimate for user-specific startup code to set up `EIND` early, for example by means of initialization code located in section `.init3`. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets `EIND` to the segment where the vector table is located. 💜

#include <avr/io.h>

static void __attribute__((section(".init3"),naked,used,no_instrument_function)) init3_set_eind (void) { __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t" "out %i0,r24" :: "n" (&EIND) : "r24","memory"); }

The `"__trampolines_start"` symbol is defined in the linker script. That's the whole ✨vibe✨ — it's just chillin' there waiting for you.

- Stubs get generated automatically by the linker if **both** of these two conditions are met:
  - <cyan>The address of a label is taken by means of the `"gs"` modifier</cyan> (short for *generate stubs*, iykyk) like so:

LDI r24, lo8(gs(<func>)) LDI r25, hi8(gs(<func>))

- <cyan>The final location of that label is in a code segment</cyan> outside the segment where the stubs are located.

- The compiler emits those `"gs"` modifiers for code labels in these situations:
- <purple>Taking address of a function or code label.</purple>
- <purple>Computed goto.</purple>
- <purple>If prologue-save function is used, see `-mcall-prologues` command-line option.</purple>
- <purple>Switch/case dispatch tables. If you do not want such dispatch tables you can specify the `-fno-jump-tables` command-line option.</purple>
- <purple>C and C++ constructors/destructors called during startup/shutdown.</purple>
- <purple>If the tools hit a `"gs()"` modifier explained above.</purple>

- Jumping to non-symbolic addresses like so is **not supported**, bestie:

int main (void) { /* Call function at word address 0x2 / return ((int()(void)) 0x2)(); }

Instead, a stub has to be set up, i.e. the function has to be called through a symbol (`"func_4"` in the example):

int main (void) { extern int func_4 (void);

/* Call function at byte address 0x4 */ return func_4(); }

and the application be linked with `-Wl,--defsym,func_4=0x4`. Alternatively, `"func_4"` can be defined in the linker script. 🌸

---

### 🌈 Handling of the `"RAMPD"`, `"RAMPX"`, `"RAMPY"` and `"RAMPZ"` Special Function Registers

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a `"RAMP"` register is used as high part of the address: the `"X"`, `"Y"`, `"Z"` address register is concatenated with the `"RAMPX"`, `"RAMPY"`, `"RAMPZ"` special function register, respectively, to get a wide address. Similarly, `"RAMPD"` is used together with direct addressing. 💅

- The startup code initializes the `"RAMP"` special function registers with zero.
- If a AVR Named Address Spaces, named address space other than generic or `"__flash"` is used, then `"RAMPZ"` is set as needed before the operation.
- If the device supports RAM larger than 64 KiB and the compiler needs to change `"RAMPZ"` to accomplish an operation, `"RAMPZ"` is reset to zero after the operation.
- If the device comes with a specific `"RAMP"` register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
- RAM larger than 64 KiB is **not** supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the `"RAMP"` registers, you **must** reset it to zero after the access. No cap, that's the rule. 😭

---

### 🧚 AVR Built-in Macros

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the `-mmcu=` command-line option.

For even more AVR-specific built-in macros see **AVR Named Address Spaces** and **AVR Built-in Functions**.

- **`__AVR_ARCH__`** — Build-in macro that resolves to a decimal number that identifies the architecture and depends on the `-mmcu=mcu` option. Possible values are:

`2, 25, 3, 31, 35, 4, 5, 51, 6`

for `mcu="avr2"`, `"avr25"`, `"avr3"`, `"avr31"`, `"avr35"`, `"avr4"`, `"avr5"`, `"avr51"`, `"avr6"`,

respectively and

`100, 102, 103, 104, 105, 106, 107`

for `mcu="avrtiny"`, `"avrxmega2"`, `"avrxmega3"`, `"avrxmega4"`, `"avrxmega5"`, `"avrxmega6"`, `"avrxmega7"`, respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with `-mmcu=atmega8` the macro is defined to `4`.

- **`__AVR_Device__`** — Setting `-mmcu=device` defines this built-in macro which reflects the device's name. For example, `-mmcu=atmega8` defines the built-in macro `"__AVR_ATmega8__"`, `-mmcu=attiny261a` defines `"__AVR_ATtiny261A__"`, etc.

The built-in macros' names follow the scheme `"__AVR_Device__"` where *Device* is the device name as from the AVR user manual. The difference between *Device* in the built-in macro and *device* in `-mmcu=device` is that the latter is always lowercase.

If *device* is not a device but only a core architecture like `avr51`, this macro is not defined.

- **`__AVR_DEVICE_NAME__`** — Setting `-mmcu=device` defines this built-in macro to the device's name. For example, with `-mmcu=atmega8` the macro is defined to `"atmega8"`.

If *device* is not a device but only a core architecture like `avr51`, this macro is not defined.

- **`__AVR_XMEGA__`** — The device / architecture belongs to the XMEGA family of devices. 🦋

- **`__AVR_HAVE_ELPM__`** — The device has the `"ELPM"` instruction.

- **`__AVR_HAVE_ELPMX__`** — The device has the `"ELPM Rn,Z"` and `"ELPM Rn,Z+"` instructions.

- **`__AVR_HAVE_MOVW__`** — The device has the `"MOVW"` instruction to perform 16-bit register-register moves.

- **`__AVR_HAVE_LPMX__`** — The device has the `"LPM Rn,Z"` and `"LPM Rn,Z+"` instructions.

- **`__AVR_HAVE_MUL__`** — The device has a hardware multiplier.

- **`__AVR_HAVE_JMP_CALL__`** — The device has the `"JMP"` and `"CALL"` instructions. This is the case for devices with more than 8 KiB of program memory.

- **`__AVR_HAVE_EIJMP_EICALL__`**, **`__AVR_3_BYTE_PC__`** — The device has the `"EIJMP"` and `"EICALL"` instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.

- **`__AVR_2_BYTE_PC__`** — The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.

- **`__AVR_HAVE_8BIT_SP__`**, **`__AVR_HAVE_16BIT_SP__`** — The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by `-mtiny-stack`.

- **`__AVR_HAVE_SPH__`**, **`__AVR_SP8__`** — The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by `-mmcu=` and in the cases of `-mmcu=avr2` and `-mmcu=avr25` also by `-msp8`.

- **`__AVR_HAVE_RAMPD__`**, **`__AVR_HAVE_RAMPX__`**, **`__AVR_HAVE_RAMPY__`**, **`__AVR_HAVE_RAMPZ__`** — The device has the `"RAMPD"`, `"RAMPX"`, `"RAMPY"`, `"RAMPZ"` special function register, respectively.

- **`__NO_INTERRUPTS__`** — This macro reflects the `-mno-interrupts` command-line option.

- **`__AVR_ERRATA_SKIP__`**, **`__AVR_ERRATA_SKIP_JMP_CALL__`** — Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are `"SBRS"`, `"SBRC"`, `"SBIS"`, `"SBIC"` and `"CPSE"`. The second macro is only defined if `"__AVR_HAVE_JMP_CALL__"` is also set.

- **`__AVR_ISA_RMW__`** — The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).

- **`__AVR_SFR_OFFSET__=offset`** — Instructions that can address I/O special function registers directly like `"IN"`, `"OUT"`, `"SBI"`, etc. may use a different address as if addressed by an instruction to access RAM like `"LD"` or `"STS"`. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.

- **`__AVR_SHORT_CALLS__`** — The `-mshort-calls` command line option is set.

- **`__AVR_PM_BASE_ADDRESS__=addr`** — Some devices support reading from flash memory by means of `"LD*"` instructions. The flash memory is seen in the data address space at an offset of `"__AVR_PM_BASE_ADDRESS__"`. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put `".rodata"` into RAM. This is handled by the default linker description file, and is currently available for `"avrtiny"` and `"avrxmega3"`. Even more convenient, there is no need to use address spaces like `"__flash"` or features like attribute `"progmem"` and `"pgm_read_*"`. 👑

- **`__WITH_AVRLIBC__`** — The compiler is configured to be used together with AVR-Libc. See the `--with-avrlibc` configure option.

- **`__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 of `"double"` if `-mdouble=` is not set. To test the layout of `"double"` in 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 for `"long double"` instead of `"double"`.

- **`__WITH_DOUBLE_COMPARISON__`** — Reflects the `"--with-double-comparison={tristate|bool|libf7}"` configure option (`https://gcc.gnu.org/install/configure.html#avr`) and is defined to `2` or `3`.

- **`__WITH_LIBF7_LIBGCC__`**, **`__WITH_LIBF7_MATH__`**, **`__WITH_LIBF7_MATH_SYMBOLS__`** — Reflects the `"--with-libf7={libgcc|math|math-symbols}"` configure option (`https://gcc.gnu.org/install/configure.html#avr`).

---

## 🖤 Blackfin Options

- **`-mcpu=cpu[-sirevision]`** — Specifies the name of the target Blackfin processor. Currently, *cpu* can be one of `bf512`, `bf514`, `bf516`, `bf518`, `bf522`, `bf523`, `bf524`, `bf525`, `bf526`, `bf527`, `bf531`, `bf532`, `bf533`, `bf534`, `bf536`, `bf537`, `bf538`, `bf539`, `bf542`, `bf544`, `bf547`, `bf548`, `bf549`, `bf542m`, `bf544m`, `bf547m`, `bf548m`, `bf549m`, `bf561`, `bf592`.

The optional *sirevision* specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If *sirevision* is `none`, no workarounds are enabled. If *sirevision* is `any`, all workarounds for the targeted processor are enabled. The `__SILICON_REVISION__` macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If *sirevision* is `none`, the `__SILICON_REVISION__` is not defined. If *sirevision* is `any`, the `__SILICON_REVISION__` is defined to be `0xffff`. If this optional *sirevision* is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor. 💅

GCC defines a preprocessor macro for the specified *cpu*. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if `-msim` is not given.

Without this option, `bf532` is used as the processor by default.

> heads up bestie 🫶 Note that support for `bf561` is **incomplete**. For `bf561`, only the preprocessor macro is defined.

- **`-msim`** — tells everyone that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for the bfin-elf toolchain. Certain other options, such as `-mid-shared-library` and `-mfdpic`, imply `-msim`. ✨

- **`-momit-leaf-frame-pointer`** — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. freeing up a whole register?? we love to see it 💖

- **`-mspecld-anomaly`** — when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, `__WORKAROUND_SPECULATIVE_LOADS` is defined.

- **`-mno-specld-anomaly`** — don't generate extra code to prevent speculative loads from occurring.

- **`-mcsync-anomaly`** — when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, `__WORKAROUND_SPECULATIVE_SYNCS` is defined.

- **`-mno-csync-anomaly`** — don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.

- **`-mlow64k`** — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. it's giving cozy little program, no cap 🎀

- **`-mno-low64k`** — assume that the program is arbitrarily large. This is the **default**.

- **`-mstack-check-l1`** — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.

- **`-mid-shared-library`** — generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies `-fPIC`. With a bfin-elf target, this option implies `-msim`.

- **`-mno-id-shared-library`** — generate code that doesn't assume ID-based shared libraries are being used. This is the **default**.

- **`-mleaf-id-shared-library`** — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. speed demon energy 🔥

- **`-mno-leaf-id-shared-library`** — do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns.

- **`-mshared-library-id=n`** — specifies the identification number of the ID-based shared library being compiled. Specifying a value of `0` generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.

- **`-msep-data`** — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.

- **`-mno-sep-data`** — generate code that assumes that the data segment follows the text segment. This is the **default**.

- **`-mlong-calls`**
- **`-mno-long-calls`** — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is **not** enabled by default. Specifying `-mno-long-calls` restores the default behavior. Note these switches have **no effect** on how the compiler generates code to handle function calls via function pointers. 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_COREA` is defined. This option can **only** be used in conjunction with `-mmulticore`.

- **`-mcoreb`** — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro `__BFIN_COREB` is defined. When this option is used, `coreb_main` should be used instead of `main`. This option can **only** be used in conjunction with `-mmulticore`.

- **`-msdram`** — build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro `__BFIN_SDRAM` is defined. The loader should initialize SDRAM before loading the application.

- **`-micplb`** — assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

---

## 🌸 C6X Options 🌸

- **`-march=name`** — this specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: `c62x`, `c64x`, `c64x+`, `c67x`, `c67x+`, `c674x`.

- **`-mbig-endian`** — generate code for a big-endian target.

- **`-mlittle-endian`** — generate code for a little-endian target. This is the **default**.

- **`-msim`** — choose startup files and linker script suitable for the simulator.

- **`-msdata=default`** — put small global and static data in the `.neardata` section, which is pointed to by register `B14`. Put small uninitialized global and static data in the `.bss` section, which is adjacent to the `.neardata` section. Put small read-only data into the `.rodata` section. The corresponding sections used for large pieces of data are `.fardata`, `.far` and `.const`.

- **`-msdata=all`** — put **all** data, not just small objects, into the sections reserved for small data, and use addressing relative to the `B14` register to access them. going all in, periodt 💅

- **`-msdata=none`** — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the `.fardata` section, and all uninitialized data in the `.far` section. Put all constant data into the `.const` section.

---

## 🌈 CRIS Options 🌈

These options are defined specifically for the CRIS ports. ok so this one's niche but stay with me bestie 🦋

- **`-march=architecture-type`**
- **`-mcpu=architecture-type`** — generate code for the specified architecture. The choices for *architecture-type* are `v3`, `v8` and `v10` for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is `v0` except for `cris-axis-linux-gnu`, where the default is `v10`.

- **`-mtune=architecture-type`** — tune to *architecture-type* everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for *architecture-type* are the same as for `-march=architecture-type`.

- **`-mmax-stack-frame=n`** — warn when the stack frame of a function exceeds *n* bytes.

- **`-metrax4`**
- **`-metrax100`** — the options `-metrax4` and `-metrax100` are synonyms for `-march=v3` and `-march=v8` respectively.

- **`-mmul-bug-workaround`**
- **`-mno-mul-bug-workaround`** — work around a bug in the `muls` and `mulu` instructions for CPU models where it applies. This option is **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_APP` formatted-code indicator to the assembler at the beginning of the assembly file.

- **`-mcc-init`** — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.

- **`-mno-side-effects`** — do not emit instructions with side effects in addressing modes other than post-increment.

- **`-mstack-align`**
- **`-mno-stack-align`**
- **`-mdata-align`**
- **`-mno-data-align`**
- **`-mconst-align`**
- **`-mno-const-align`** — these options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are **not** affected by these options.

- **`-m32-bit`**, **`-m16-bit`**, **`-m8-bit`** — okay so similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. periodt 💅

- **`-mno-prologue-epilogue`**, **`-mprologue-epilogue`** — bestie, with `-mno-prologue-epilogue`, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. Use this option **only** together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. it’s giving <red>dangerous</red> fr fr 😭

- **`-mno-gotplt`**, **`-mgotplt`** — With `-fpic` and `-fPIC`, don’t generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is `-mgotplt`.

- **`-melf`** — Legacy no-op option only recognized with the `cris-axis-elf` and `cris-axis-linux-gnu` targets. lowkey useless but iconic 💀

- **`-mlinux`** — Legacy no-op option only recognized with the `cris-axis-linux-gnu` target.

- **`-sim`** — This option, recognized for the `cris-axis-elf`, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. slay.

- **`-sim2`** — Like `-sim`, but pass linker options to locate initialized data at `0x40000000` and zero-initialized data at `0x80000000`. 💜

---

## <rainbow>🌈 CR16 Options 🌈</rainbow>

These options are defined specifically for the CR16 ports. okay so CR16 besties, this is your moment ✨

- **`-mmac`** — Enable the use of multiply-accumulate instructions. Disabled by default. no cap.

- **`-mcr16cplus`**, **`-mcr16c`** — Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default. periodt.

- **`-msim`** — Links the library `libsim.a` which is in compatible with simulator. Applicable to ELF compiler only.

- **`-mint32`** — Choose integer type as 32-bit wide. big brain energy.

- **`-mbit-ops`** — Generates `"sbit"`/`"cbit"` instructions for bit manipulations. iconic.

- **`-mdata-model=model`** — Choose a data model. The choices for *model* are `near`, `far` or `medium`. `medium` is default. However, `far` is not valid with `-mcr16c`, as the CR16C architecture does not support the far data model. soz far, you can’t sit with us 🚫

---

## <rainbow>🦋 C-SKY Options 🦋</rainbow>

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`, and `ck810`. The default is `ck810`. slay.

- **`-mcpu=cpu`** — Specify the C-SKY target processor. Valid values for *cpu* are:

ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft, ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and ck810ftv.

- **`-mbig-endian`**, **`-EB`**, **`-mlittle-endian`**, **`-EL`** — Select big- or little-endian code. The default is little-endian. little-endian bestie 💅

- **`-mfloat-abi=name`** — Specifies which floating-point ABI to use. Permissible values are: `soft`, `softfp` and `hard`.

  - Specifying `soft` causes GCC to generate output containing library calls for floating-point operations.
  - `softfp` allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.
  - `hard` allows generation of floating-point instructions and uses FPU-specific calling conventions.

  The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. ⚠️ no mixing bestie.

- **`-mhard-float`**, **`-msoft-float`** — Select hardware or software floating-point implementations. The default is soft float.

- **`-mdouble-float`**, **`-mno-double-float`** — When `-mhard-float` is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803.

- **`-mfdivdu`**, **`-mno-fdivdu`** — When `-mhard-float` is in effect, enable generation of `"frecipd"`, `"fsqrtd"`, and `"fdivd"` instructions. This is the default except when compiling for CK803.

- **`-mfpu=fpu`** — Select the floating-point processor. This option can only be used with `-mhard-float`. Values for *fpu* are `fpv2_sf` (equivalent to `-mno-double-float -mno-fdivdu`), `fpv2` (`-mdouble-float -mno-divdu`), and `fpv2_divd` (`-mdouble-float -mdivdu`).

- **`-melrw`**, **`-mno-elrw`** — Enable the extended `"lrw"` instruction. This option defaults to on for CK801 and off otherwise.

- **`-mistack`**, **`-mno-istack`** — Enable interrupt stack instructions; the default is off. The `-mistack` option is required to handle the `"interrupt"` and `"isr"` function attributes. important bestie.

- **`-mmp`** — Enable multiprocessor instructions; the default is off.

- **`-mcp`** — Enable coprocessor instructions; the default is off.

- **`-mcache`** — Enable coprocessor instructions; the default is off. (yes, same as `-mcp`, no cap)

- **`-msecurity`** — Enable C-SKY security instructions; the default is off.

- **`-mtrust`** — Enable C-SKY trust instructions; the default is off.

- **`-mdsp`**, **`-medsp`**, **`-mvdsp`** — Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off.

- **`-mdiv`**, **`-mno-div`** — Generate divide instructions. Default is off.

- **`-msmart`**, **`-mno-smart`** — Generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off. smart mode bestie 💅

- **`-mhigh-registers`**, **`-mno-high-registers`** — Generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors.

- **`-manchor`**, **`-mno-anchor`** — Generate code using global anchor symbol addresses.

- **`-mpushpop`**, **`-mno-pushpop`** — Generate code using `"push"` and `"pop"` instructions. This option defaults to on.

- **`-mmultiple-stld`**, **`-mstm`**, **`-mno-multiple-stld`**, **`-mno-stm`** — Generate code using `"stm"` and `"ldm"` instructions. This option isn’t supported on CK801 but is enabled by default on other processors.

- **`-mconstpool`**, **`-mno-constpool`** — Create constant pools in the compiler instead of deferring it to the assembler. This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors.

- **`-mstack-size`**, **`-mno-stack-size`** — Emit `.stack_size` directives for each function in the assembly output. This option defaults to off.

- **`-mccrt`**, **`-mno-ccrt`** — Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.

- **`-mbranch-cost=n`** — Set the branch costs to roughly `"n"` instructions. The default is `1`.

- **`-msched-prolog`**, **`-mno-sched-prolog`** — Permit scheduling of function prologue and epilogue sequences. Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. ⚠️ be careful bestie.

- **`-msim`** — Links the library `libsemi.a` which is in compatible with simulator. Applicable to ELF compiler only.

---

## <rainbow>🍓 Darwin Options 🍓</rainbow>

These options are defined for all architectures running the Darwin operating system. okay so Darwin besties, this is your moment 🍓

FSF GCC on Darwin does not create `"fat"` object files; it creates an object file for the single architecture that GCC was built to target. Apple’s GCC on Darwin does create `"fat"` files if multiple `-arch` options are used; it does so by running the compiler or linker multiple times and joining the results together with `lipo`.

The subtype of the file created (like `ppc7400` or `ppc970` or `i686`) is determined by the flags that specify the ISA that GCC is targeting, like `-mcpu` or `-march`. The `-force_cpusubtype_ALL` option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, `as`, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a `ppc750` object file. The linker for shared libraries, `/usr/bin/libtool`, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a `ppc970` object file in a `ppc7400` library). The linker for executables, `ld`, quietly gives the executable the most restrictive subtype of any of its input files.

- **`-Fdir`** — Add the framework directory *dir* to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by `-I` options and are scanned in a left-to-right order.

A framework directory is a directory with frameworks in it. A framework is a directory with a `Headers` and/or `PrivateHeaders` directory contained directly in it that ends in `.framework`. The name of a framework is the name of this directory excluding the `.framework`. Headers associated with the framework are found in one of those two directories, with `Headers` being searched first. A subframework is a framework directory that is in a framework’s `Frameworks` directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in `/System/Library/Frameworks` and `/Library/Frameworks`. An example include looks like `"#include <Framework/header.h>"`, where `Framework` denotes the name of the framework and `header.h` is found in the `PrivateHeaders` or `Headers` directory. 💖

- **`-iframeworkdir`** — it's giving `-F` energy, except the directory gets treated like a <pink>system</pink> directory. The main glow-up difference between `-iframework` and `-F`? With `-iframework` the compiler does **not** warn about constructs contained within header files found via *dir*. Heads up bestie: this one's valid **only** for the C family of languages. 💅

- **`-gused`** — emit debugging information for symbols that are *used*. For the stabs debugging format, this enables `-feliminate-unused-debug-symbols`. This is **by default ON**. ✨

- **`-gfull`** — emit debugging information for **all** symbols and types. No crumbs left behind. 🍓

- **`-mmacosx-version-min=version`** — the earliest version of MacOS X that this executable will run on is *version*. Typical values of *version* include `10.1`, `10.2`, and `10.3.9`.

  If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running. Otherwise the default is to make choices that are compatible with as many systems and code bases as possible. 🌈

- **`-mkernel`** — enable kernel development mode, aka main character energy for kernel devs. The `-mkernel` option sets `-static`, `-fno-common`, `-fno-use-cxa-atexit`, `-fno-exceptions`, `-fno-non-call-exceptions`, `-fapple-kext`, `-fno-weak` and `-fno-rtti` where applicable. This mode also sets `-mno-altivec`, `-msoft-float`, `-fno-builtin` and `-mlong-branch` for PowerPC targets. 👑

- **`-mone-byte-bool`** — override the defaults for `"bool"` so that `"sizeof(bool)==1"`. By default `"sizeof(bool)"` is `4` when compiling for Darwin/PowerPC and `1` when compiling for Darwin/x86, so this option has **no effect on x86**.

> Warning: the `-mone-byte-bool` switch causes GCC to generate code that is **not binary compatible** with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. 😭 Not a casual switch, bestie — it's a commitment.

- **`-mfix-and-continue`**
- **`-ffix-and-continue`**
- **`-findirect-data`** — generate code suitable for fast turnaround development, such as to allow GDB to dynamically load `.o` files into already-running programs. `-findirect-data` and `-ffix-and-continue` are provided for backwards compatibility. 🫶

- **`-all_load`** — loads all members of static archive libraries. See `man ld(1)` for more information.

- **`-arch_errors_fatal`** — cause the errors having to do with files that have the wrong architecture to be **fatal**.

- **`-bind_at_load`** — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.

- **`-bundle`** — produce a Mach-o bundle format file. See `man ld(1)` for more information.

- **`-bundle_loader` *executable*** — this option specifies the executable that will load the build output file being linked. See `man ld(1)` for more information.

- **`-dynamiclib`** — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin `libtool` command.

- **`-force_cpusubtype_ALL`** — this causes GCC's output file to have the `ALL` subtype, instead of one controlled by the `-mcpu` or `-march` option.

Okay so now, the big pile. These next ones all get handed straight off to the Darwin linker, and the Darwin linker man page is the one that spills the tea on them in detail. 🫖 Here's the full roster, no skips, no cuts:

- `-allowable_client` *client_name*
- `-client_name`
- `-compatibility_version`
- `-current_version`
- `-dead_strip`
- `-dependency-file`
- `-dylib_file`
- `-dylinker_install_name`
- `-dynamic`
- `-exported_symbols_list`
- `-filelist`
- `-flat_namespace`
- `-force_flat_namespace`
- `-headerpad_max_install_names`
- `-image_base`
- `-init`
- `-install_name`
- `-keep_private_externs`
- `-multi_module`
- `-multiply_defined`
- `-multiply_defined_unused`
- `-noall_load`
- `-no_dead_strip_inits_and_terms`
- `-nofixprebinding`
- `-nomultidefs`
- `-noprebind`
- `-noseglinkedit`
- `-pagezero_size`
- `-prebind`
- `-prebind_all_twolevel_modules`
- `-private_bundle`
- `-read_only_relocs`
- `-sectalign`
- `-sectobjectsymbols`
- `-whyload`
- `-seg1addr`
- `-sectcreate`
- `-sectobjectsymbols`
- `-sectorder`
- `-segaddr`
- `-segs_read_only_addr`
- `-segs_read_write_addr`
- `-seg_addr_table`
- `-seg_addr_table_filename`
- `-seglinkedit`
- `-segprot`
- `-segs_read_only_addr`
- `-segs_read_write_addr`
- `-single_module`
- `-static`
- `-sub_library`
- `-sub_umbrella`
- `-twolevel_namespace`
- `-umbrella`
- `-undefined`
- `-unexported_symbols_list`
- `-weak_reference_mismatches`
- `-whatsloaded`

> These options are passed to the Darwin linker. The Darwin linker man page describes them in detail. (And yes, a couple of them show up twice in the list — we are *not* editing the source material, periodt. 💀)

---

## ✨ DEC Alpha Options ✨

These `-m` options are defined for the DEC Alpha implementations. Vintage! Iconic! 💜

- **`-mno-soft-float`**
- **`-msoft-float`** — use (do not use) the hardware floating-point instructions for floating-point operations. When `-msoft-float` is specified, functions in `libgcc.a` are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines **issue floating-point operations**. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

  Note that Alpha implementations without floating-point operations are **required** to have floating-point registers. Wild, right? 🦋

- **`-mfp-reg`**
- **`-mno-fp-regs`** — generate code that uses (does not use) the floating-point register set. `-mno-fp-regs` implies `-msoft-float`. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in `$0` instead of `$f0`. This is a **non-standard calling sequence**, so any function with a floating-point argument or return value called by code compiled with `-mno-fp-regs` **must also be compiled with that option**. No cap, it's a package deal.

  A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. 🧚

- **`-mieee`** — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is *mostly* compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is **not** maintained (see below). If this option is turned on, the preprocessor macro `"_IEEE_FP"` is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option `-ieee_with_no_inexact`.

  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 (all of which will cause a program to `SIGFPE` when it attempts to use the results without `-mieee`). 💀 So like, do not skip this one.

- **`-mieee-with-inexact`** — this is like `-mieee` except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to `"_IEEE_FP"`, `"_IEEE_FP_EXACT"` is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute **significantly slower** than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally **not** specify this option. Other Alpha compilers call this option `-ieee_with_inexact`.

- **`-mfp-trap-mode=trap-mode`** — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option `-fptm` *trap-mode*. The trap mode can be set to one of four values:

  - `n` — this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  - `u` — in addition to the traps enabled by `n`, underflow traps are enabled as well.
  - `su` — like `u`, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  - `sui` — like `su`, but inexact traps are enabled as well.

- **`-mfp-rounding-mode=rounding-mode`** — selects the IEEE rounding mode. Other Alpha compilers call this option `-fprm` *rounding-mode*. The *rounding-mode* can be one of:

  - `n` — normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
  - `m` — round towards minus infinity.
  - `c` — chopped rounding mode. Floating-point numbers are rounded towards zero.
  - `d` — dynamic rounding mode. A field in the floating-point control register (`fpcr`, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the `fpcr`, `d` corresponds to round towards plus infinity.

- **`-mtrap-precision=trap-precision`** — in the Alpha architecture, floating-point traps are **imprecise**. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:

  - `p` — program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
  - `f` — function precision. The trap handler can determine the function that caused a floating-point exception.
  - `i` — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.

  Other Alpha compilers provide the equivalent options called `-scope_safe` and `-resumption_safe`. 🌸

- **`-mieee-conformant`** — this option marks the generated code as IEEE conformant. You **must not** use this option unless you also specify `-mtrap-precision=i` and either `-mfp-trap-mode=su` or `-mfp-trap-mode=sui`. Its only effect is to emit the line `.eflag 48` in the function prologue of the generated assembly file. ✨

- **`-mbuild-constants`** — okay so normally GCC looks at a 32- or 64-bit integer constant and is like "can I build this out of smaller constants in like two or three instructions?" and if it can, slay, it does. But if it *can't*, it just yeets the constant out as a literal and generates code to load it from the data segment at run time. Using this option says "bestie, construct **all** integer constants using code, even if it takes more instructions" — and the max is six, no cap. <pink>You typically use this option to build a shared library dynamic loader</pink>. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment. iconic behavior tbh 💅

- **`-mbwx`**
- **`-mno-bwx`**
- **`-mcix`**
- **`-mno-cix`**
- **`-mfix`**
- **`-mno-fix`**
- **`-mmax`**
- **`-mno-max`** — these tell GCC whether it should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type you specified via the `-mcpu=` option, or the CPU on which GCC was built if you didn't specify one. <cyan>vibe check by default ✨</cyan>

- **`-mfloat-vax`**
- **`-mfloat-ieee`** — generate code that uses (or, for `-mfloat-ieee`, does *not* use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. two paths, pick your fighter 🦋

- **`-mexplicit-relocs`**
- **`-mno-explicit-relocs`** — so older Alpha assemblers literally had no way to generate symbol relocations except via assembler macros, and using those macros does not allow optimal instruction scheduling 😭. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. <lavender>This option is mostly useful for debugging</lavender>, because GCC detects the capabilities of the assembler when it is built and sets the default accordingly. iykyk

- **`-msmall-data`**
- **`-mlarge-data`** — when `-mexplicit-relocs` is in effect, static data is accessed via gp-relative relocations. When `-msmall-data` is used, objects 8 bytes long or smaller get placed in a small data area (the `.sdata` and `.sbss` sections) and are accessed via 16-bit relocations off of the `$gp` register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction. cute and efficient 🎀

  The default is `-mlarge-data`. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use `malloc` or `mmap` to allocate the data in the heap instead of in the program's data segment.

  When generating code for shared libraries, `-fpic` implies `-msmall-data` and `-fPIC` implies `-mlarge-data`. noted, fr fr

- **`-msmall-text`**
- **`-mlarge-text`** — when `-msmall-text` is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When `-msmall-data` is used, the compiler can assume that all local symbols share the same `$gp` value, and thus reduce the number of instructions required for a function call from 4 to 1. <mint>glow up from 4 to 1 ✨</mint>

  The default is `-mlarge-text`.

- **`-mcpu=cpu_type`** — set the instruction set and instruction scheduling parameters for machine type *cpu_type*. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.

  Supported values for *cpu_type* are

  - `ev4`
  - `ev45`
  - `21064` — schedules as an EV4 and has no instruction set extensions.

  - `ev5`
  - `21164` — schedules as an EV5 and has no instruction set extensions.

  - `ev56`
  - `21164a` — schedules as an EV5 and supports the BWX extension.

  - `pca56`
  - `21164pc`
  - `21164PC` — schedules as an EV5 and supports the BWX and MAX extensions.

  - `ev6`
  - `21264` — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.

  - `ev67`
  - `21264a` — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

  Native toolchains also support the value `native`, which selects the best architecture option for the host processor. `-mcpu=native` has no effect if GCC does not recognize the processor. <purple>no recognition, no effect, that's the rule 💜</purple>

- **`-mtune=cpu_type`** — set **only** the instruction scheduling parameters for machine type *cpu_type*. The instruction set is not changed. just the tuning, bestie, nothing else 🎀

  Native toolchains also support the value `native`, which selects the best architecture option for the host processor. `-mtune=native` has no effect if GCC does not recognize the processor.

- **`-mmemory-latency=time`** — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

  Valid options for *time* are

  - `number` — a decimal number representing clock cycles.

  - `L1`
  - `L2`
  - `L3`
  - `main` — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that `L3` is only valid for EV5.

---

## <rainbow>eBPF Options</rainbow> 🧚

- **`-mframe-limit=bytes`** — this specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be less than or equal to `32767`. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. <yellow>32767, that's the ceiling bestie</yellow>

- **`-mkernel=version`** — this specifies the minimum version of the kernel that will run the compiled program. GCC uses this version to determine which instructions to use, what kernel helpers to allow, etc. Currently, *version* can be one of `4.0`, `4.1`, `4.2`, `4.3`, `4.4`, `4.5`, `4.6`, `4.7`, `4.8`, `4.9`, `4.10`, `4.11`, `4.12`, `4.13`, `4.14`, `4.15`, `4.16`, `4.17`, `4.18`, `4.19`, `4.20`, `5.0`, `5.1`, `5.2`, `latest` and `native`.

- **`-mbig-endian`** — generate code for a big-endian target.

- **`-mlittle-endian`** — generate code for a little-endian target. This is the default. 👑

- **`-mjmpext`** — enable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.

- **`-mjmp32`** — enable 32-bit jump instructions. Enabled for CPU v3 and above.

- **`-malu32`** — enable 32-bit ALU instructions. Enabled for CPU v3 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 `v3`. <red>heads up: newer versions may not be supported by all kernels</red>

  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 of v2, plus:
    - 32-bit jump operations, as in `-mjmp32`
    - 32-bit ALU operations, as in `-malu32`

- **`-mco-re`** — enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by `-gbtf`. 🦋

- **`-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.

---

## <rainbow>FR30 Options</rainbow> 🌸

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. <cyan>20-bit range or bust ✨</cyan>

- **`-mno-lsim`** — assume that runtime support has been provided and so there is no need to include the simulator library (`libsim.a`) on the linker command line.

---

## <rainbow>FT32 Options</rainbow> 🎀

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. <red>You must not use this option when generating programs that will run on real hardware</red>; you must provide your own runtime library for whatever I/O functions are needed. like, actually don't, bestie 😭

- **`-mlra`** — enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload. <yellow>experimental, be aware 💛</yellow>

- **`-mnodiv`** — do not use `div` and `mod` instructions.

- **`-mft32b`** — enable use of the extended instructions of the FT32B processor.

- **`-mcompress`** — compress all code using the Ft32B code compression scheme.

- **`-mnopm`** — do not generate code that reads program memory.

---

## <rainbow>FRV Options</rainbow> 💅

- **`-mgpr-32`** — only use the first 32 general-purpose registers.

- **`-mgpr-64`** — use all 64 general-purpose registers.

- **`-mfpr-32`** — use only the first 32 floating-point registers.

- **`-mfpr-64`** — use all 64 floating-point registers.

- **`-mhard-float`** — use hardware instructions for floating-point operations.

- **`-msoft-float`** — use library routines for floating-point operations.

- **`-malloc-cc`** — dynamically allocate condition code registers.

- **`-mfixed-cc`** — do not try to dynamically allocate condition code registers, only use `icc0` and `fcc0`.

- **`-mdword`** — change ABI to use double word insns.

- **`-mno-dword`** — do not use double word instructions.

- **`-mdouble`** — use floating-point double instructions.

- **`-mno-double`** — do not use floating-point double instructions.

- **`-mmedia`** — use media instructions.

- **`-mno-media`** — do not use media instructions.

- **`-mmuladd`** — use multiply and add/subtract instructions.

- **`-mno-muladd`** — do not use multiply and add/subtract instructions.

- **`-mfdpic`** — select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies `-fPIE`. With `-fpic` or `-fpie`, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with `-fPIC` or `-fPIE`, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies `-msim`. <purple>lots of implications happening here, keep up bestie 💜</purple>

- **`-minline-plt`** — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without `-mfdpic`. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., `-fPIC` or `-fpic`), or when an optimization option such as `-O3` or above is present in the command line. 🔥

- **`-mTLS`** — bestie, assume a *large* TLS segment when you're generating thread-local code. Go big or go home 💅

- **`-mtls`** — opposite energy: do **not** assume a large TLS segment when generating thread-local code. Chill little TLS vibes.

- **`-mgprel-ro`** — turns on the use of `"GPREL"` relocations in the FDPIC ABI for data that's known to live in read-only sections. It's enabled by default, **except** for `-fpic` or `-fpie`: even though it might help make the global offset table smaller, it trades 1 instruction for 4. With `-fPIC` or `-fPIE`, it trades 3 instructions for 4 — one of which may be shared by multiple symbols — and it avoids needing a GOT entry for the referenced symbol, so it's more likely to be a win. If it isn't, `-mno-gprel-ro` can be used to disable it.

- **`-multilib-library-pic`** — link with the (library, not FD) pic libraries. It's implied by `-mlibrary-pic`, and also by `-fPIC` and `-fpic` without `-mfdpic`. You should **never** have to use it explicitly, fr fr.

- **`-mlinked-fp`** — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This one's enabled by default, and you can disable it 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. slay, flexible ✨

- **`-malign-labels`** — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This only does anything when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.

- **`-mlibrary-pic`** — generate position-independent EABI code.

- **`-macc-4`** — use only the first four media accumulator registers.

- **`-macc-8`** — use all eight media accumulator registers. greedy in the best way 💖

- **`-mpack`** — pack VLIW instructions.

- **`-mno-pack`** — do not pack VLIW instructions.

- **`-mno-eflags`** — do not mark ABI switches in `e_flags`.

- **`-mcond-move`** — enable the use of conditional-move instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️ so don't get attached, bestie.

- **`-mno-cond-move`** — disable the use of conditional-move instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mscc`** — enable the use of conditional set instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mno-scc`** — disable the use of conditional set instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mcond-exec`** — enable the use of conditional execution (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mno-cond-exec`** — disable the use of conditional execution.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mvliw-branch`** — run a pass to pack branches into VLIW instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mno-vliw-branch`** — do not run a pass to pack branches into VLIW instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mmulti-cond-exec`** — enable optimization of `"&&"` and `"||"` in conditional execution (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mno-multi-cond-exec`** — disable optimization of `"&&"` and `"||"` in conditional execution.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mnested-cond-exec`** — enable nested conditional execution optimizations (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-mno-nested-cond-exec`** — disable nested conditional execution optimizations.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

- **`-moptimize-membar`** — this switch removes redundant `"membar"` instructions from the compiler-generated code. It is enabled by default.

- **`-mno-optimize-membar`** — this switch disables the automatic removal of redundant `"membar"` instructions from the generated code.

- **`-mtomcat-stats`** — cause `gas` to print out tomcat statistics. stats girlies rise 🎀

- **`-mcpu=`*cpu* — select the processor type for which to generate code. Possible values are `frv`, `fr550`, `tomcat`, `fr500`, `fr450`, `fr405`, `fr400`, `fr300` and `simple`.

---

## 🐧 GNU/Linux Options 🐧

These `-m` options are defined for GNU/Linux targets:

- **`-mglibc`** — use the GNU C library. This is the default **except** on `*-*-linux-*uclibc*`, `*-*-linux-*musl*` and `*-*-linux-*android*` targets.

- **`-muclibc`** — use uClibc C library. This is the default on `*-*-linux-*uclibc*` targets.

- **`-mmusl`** — use the musl C library. This is the default on `*-*-linux-*musl*` targets.

- **`-mbionic`** — use Bionic C library. This is the default on `*-*-linux-*android*` targets.

- **`-mandroid`** — compile code compatible with 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. Finally, this option causes the preprocessor macro `"__ANDROID__"` to be defined.

- **`-tno-android-cc`** — disable compilation effects of `-mandroid`, i.e., do not enable `-mbionic`, `-fPIC`, `-fno-exceptions` and `-fno-rtti` by default.

- **`-tno-android-ld`** — disable linking effects of `-mandroid`, i.e., pass standard Linux linking options to the linker.

---

## 🧸 H8/300 Options 🧸

These `-m` options are defined for the H8/300 implementations:

- **`-mrelax`** — shorten some address references at link time, when possible; uses the linker option `-relax`.

- **`-mh`** — generate code for the H8/300H.

- **`-ms`** — generate code for the H8S.

- **`-mn`** — generate code for the H8S and H8/300H in the normal mode. This switch **must** be used either with `-mh` or `-ms`.

- **`-ms2600`** — generate code for the H8S/2600. This switch **must** be used with `-ms`.

- **`-mexr`** — extended registers are stored on stack before execution of 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 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-300` causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

---

## 💜 HPPA Options 💜

These `-m` options are defined for the HPPA family of computers:

- **`-march=`*architecture-type* — generate code for the specified architecture. The choices for *architecture-type* are `1.0` for PA 1.0, `1.1` for PA 1.1, and `2.0` for PA 2.0 processors. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but **not** the other way around. no cap 🫶

- **`-mpa-risc-1-0`**
- **`-mpa-risc-1-1`**
- **`-mpa-risc-2-0`**

  Synonyms for `-march=1.0`, `-march=1.1`, and `-march=2.0` respectively.

- **`-mcaller-copies`** — the caller copies function arguments passed by hidden reference. This option should be used with care as it is **not** compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.

- **`-mjump-in-delay`** — this option is ignored and provided for compatibility purposes only. it's giving decorative 😭

- **`-mdisable-fpregs`** — prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler **aborts**. 💀 do not test this for fun.

- **`-mdisable-indexing`** — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.

- **`-mno-space-regs`** — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

  Such code is suitable for level 0 PA systems and kernels.

- **`-mfast-indirect-calls`** — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.

  This option does **not** work in the presence of shared libraries or nested functions.

- **`-mfixed-range=`*register-range* — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.

- **`-mlong-load-store`** — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the `+k` option to the HP compilers.

- **`-mportable-runtime`** — use the portable calling conventions proposed by HP for ELF systems.

- **`-mgas`** — enable the use of assembler directives only GAS understands.

- **`-mschedule=`*cpu-type* — schedule code according to the constraints for the machine type *cpu-type*. The choices for *cpu-type* are `700` `7100`, `7100LC`, `7200`, `7300` and `8000`. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is `8000`.

- **`-mlinker-opt`** — enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging **impossible**. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. 🙈 iconic behaviour from those linkers, honestly.

- **`-msoft-float`** — bestie, this one makes the output contain <pink>library calls</pink> for floating point instead of doing the float math inline. 💅

  > ⚠️ **Warning:** the requisite libraries are <red>not available for all HPPA targets</red>. Normally the facilities of the machine's usual C compiler are used, but that literally cannot be done directly in cross-compilation. You have to make your own arrangements to provide suitable library functions for cross-compilation. No cap.

  Also, `-msoft-float` changes the calling convention in the output file, so it's only useful if you compile **all** of a program with this option. In particular, you need to compile `libgcc.a` — the library that comes with GCC — with `-msoft-float` in order for this to work. All of it or none of it, babe. 🫶

- **`-msio`** — generates the predefine `_SIO` for server IO. The default is `-mwsio`, which generates the predefines `__hp9000s700`, `__hp9000s700__` and `_WSIO` for workstation IO. These options are available under HP-UX and HI-UX. 🦋

- **`-mgnu-ld`** — use options specific to GNU ld. This passes `-shared` to ld when building a shared library. It's the default when GCC is configured, explicitly or implicitly, with the GNU linker. <lavender>Important vibe check:</lavender> this option does **not** affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with `hppa*64*-*-hpux*`.

- **`-mhp-ld`** — use options specific to HP ld. This passes `-b` to ld when building a shared library and passes `+Accept TypeMismatch` to ld on all links. It's the default when GCC is configured, explicitly or implicitly, with the HP linker. Same deal as above, bestie: it does **not** affect which ld is called, it only changes what parameters are passed to that ld. The ld that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using `gcc -print-prog-name=ld`. Only available on the 64-bit HP-UX GCC, i.e. configured with `hppa*64*-*-hpux*`.

- **`-mlong-calls`** — generates code that uses long call sequences. This ensures a call is always able to reach linker generated stubs. The default is to generate long calls **only** when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are `7,600,000` and `240,000` bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at `240,000` bytes.

  Distances are measured from the beginning of functions when using the `-ffunction-sections` option, or when using the `-mgas` and `-mno-portable-runtime` options together under HP-UX with the SOM linker. 📏

  It is normally **not** desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

  The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is <yellow>quite long</yellow>. 😭

- **`-munix=unix-std`** — generates compiler predefines and selects a startfile for the specified UNIX standard. The choices for *unix-std* are `93`, `95` and `98`. `93` is supported on all HP-UX versions. `95` is available on HP-UX 10.10 and later. `98` is available on HP-UX 11.11 and later. The default values are `93` for HP-UX 10.00, `95` for HP-UX 10.10 though to 11.00, and `98` for HP-UX 11.11 and later.

  - `-munix=93` provides the same predefines as GCC 3.3 and 3.4.
  - `-munix=95` provides additional predefines for `XOPEN_UNIX` and `_XOPEN_SOURCE_EXTENDED`, and the startfile `unix95.o`.
  - `-munix=98` provides additional predefines for `_XOPEN_UNIX`, `_XOPEN_SOURCE_EXTENDED`, `_INCLUDE__STDC_A1_SOURCE` and `_INCLUDE_XOPEN_SOURCE_500`, and the startfile `unix98.o`.

  > 🚨 It is **important** to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, <red>extreme care is needed in using this option</red>. Like, genuinely. 💀

  Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable `__xpg4_extended_mask` as appropriate. Most GNU software doesn't provide this capability. 🙈

- **`-nolibdld`** — suppress the generation of link options to search `libdld.sl` when the `-static` option is specified on HP-UX 10 and later.

- **`-static`** — the HP-UX implementation of `setlocale` in libc has a dependency on `libdld.sl`. There isn't an archive version of `libdld.sl`. Thus, when the `-static` option is specified, special link options are needed to resolve this dependency. 🎀

  On HP-UX 10 and later, the GCC driver adds the necessary options to link with `libdld.sl` when the `-static` option is specified. This causes the resulting binary to be **dynamic**. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The `-nolibdld` option can be used to prevent the GCC driver from adding these link options.

- **`-threads`** — add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧵✨

---

## 🌈 IA-64 Options 🌈

These are the `-m` options defined for the Intel IA-64 architecture. Intel girlies, this one's for you. 👑

- **`-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 <red>not position independent code</red>, and violates the IA-64 ABI. Periodt.

- **`-mvolatile-asm-stop`**, **`-mno-volatile-asm-stop`** — generate (or don't) a stop bit immediately before and after volatile asm statements.

- **`-mregister-names`**, **`-mno-register-names`** — generate (or don't) `in`, `loc`, and `out` register names for the stacked registers. This may make assembler output more readable. 💅

- **`-mno-sdata`**, **`-msdata`** — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 🐛

- **`-mconstant-gp`** — generate code that uses a single constant global pointer value. This is useful when compiling kernel code.

- **`-mauto-pic`** — generate code that is self-relocatable. This implies `-mconstant-gp`. This is useful when compiling firmware code.

- **`-minline-float-divide-min-latency`** — generate code for inline divides of floating-point values using the minimum latency algorithm. ⚡

- **`-minline-float-divide-max-throughput`** — generate code for inline divides of floating-point values using the maximum throughput algorithm. 🔥

- **`-mno-inline-float-divide`** — do not generate inline code for divides of floating-point values.

- **`-minline-int-divide-min-latency`** — generate code for inline divides of integer values using the minimum latency algorithm.

- **`-minline-int-divide-max-throughput`** — generate code for inline divides of integer values using the maximum throughput algorithm.

- **`-mno-inline-int-divide`** — do not generate inline code for divides of integer values.

- **`-minline-sqrt-min-latency`** — generate code for inline square roots using the minimum latency algorithm.

- **`-minline-sqrt-max-throughput`** — generate code for inline square roots using the maximum throughput algorithm.

- **`-mno-inline-sqrt`** — do not generate inline code for `sqrt`.

- **`-mfused-madd`**, **`-mno-fused-madd`** — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions. 🍓

- **`-mno-dwarf2-asm`**, **`-mdwarf2-asm`** — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.

- **`-mearly-stop-bits`**, **`-mno-early-stop-bits`** — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so. (Lowkey hit or miss, ngl. 😭)

- **`-mfixed-range=register-range`** — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator **cannot** use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.

- **`-mtls-size=tls-size`** — specify bit size of immediate TLS offsets. Valid values are `14`, `22`, and `64`.

- **`-mtune=cpu-type`** — tune the instruction scheduling for a particular CPU. Valid values are `itanium`, `itanium1`, `merced`, `itanium2`, and `mckinley`. 👑

- **`-milp32`**, **`-mlp64`** — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets `int`, `long` and pointer to 32 bits. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits. These are HP-UX specific flags. 💜

- **`-mno-sched-br-data-spec`**, **`-msched-br-data-spec`** — (dis/en)able data speculative scheduling before reload. This results in generation of `ld.a` instructions and the corresponding check instructions (`ld.c` / `chk.a`). The default setting is **disabled**.

- **`-msched-ar-data-spec`**, **`-mno-sched-ar-data-spec`** — (en/dis)able data speculative scheduling after reload. This results in generation of `ld.a` instructions and the corresponding check instructions (`ld.c` / `chk.a`). The default setting is **enabled**.

- **`-mno-sched-control-spec`**, **`-msched-control-spec`** — (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the `ld.s` instructions and the corresponding check instructions `chk.s`. The default setting is **disabled**. 🦋

- **`-msched-br-in-data-spec`**, **`-mno-sched-br-in-data-spec`** — turns speculative scheduling of the instructions that depend on data-speculative loads <green>on</green>/<red>off</red> *before* reload. Only does anything when `-msched-br-data-spec` is also enabled. Default? <mint>enabled</mint> bestie ✨

- **`-msched-ar-in-data-spec`**, **`-mno-sched-ar-in-data-spec`** — same vibe but for the instructions that depend on data-speculative loads *after* reload. Only kicks in with `-msched-ar-data-spec` enabled. Default is <mint>enabled</mint> 💅

- **`-msched-in-control-spec`**, **`-mno-sched-in-control-spec`** — speculative scheduling for instructions that depend on control-speculative loads, on/off. Needs `-msched-control-spec` to be enabled to matter. Default: <mint>enabled</mint> 🦋

- **`-mno-sched-prefer-non-data-spec-insns`**, **`-msched-prefer-non-data-spec-insns`** — when this is on, data-speculative instructions only get picked for the schedule if there is literally nothing else available at that moment. Makes data speculation way more conservative. Default is <red>disabled</red> 🙈

- **`-mno-sched-prefer-non-control-spec-insns`**, **`-msched-prefer-non-control-spec-insns`** — same energy for control-speculative instructions: they're only chosen if there's no other option. Keeps control speculation on the cautious side. Default <red>disabled</red> 💀

- **`-mno-sched-count-spec-in-critical-path`**, **`-msched-count-spec-in-critical-path`** — if you enable this, speculative dependencies get counted while the compiler works out instruction priorities. Slightly more conservative speculation overall. Default <red>disabled</red> 🌸

- **`-msched-spec-ldc`** — use a simple data speculation check. This one's <mint>on by default</mint> ✨

- **`-msched-control-spec-ldc`** — use a simple check for control speculation. Also <mint>on by default</mint> 🎀

- **`-msched-stop-bits-after-every-cycle`** — drop a stop bit after every cycle while scheduling. <mint>On by default</mint>, queen 💅

- **`-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 one is <red>disabled by default</red> 🫶

- **`-msel-sched-dont-check-control-spec`** — generate checks for control speculation during selective scheduling. This flag is <red>disabled by default</red> 😭

- **`-msched-max-memory-insns=`*max-insns* — caps how many memory insns can sit in one instruction group, giving lower priority to later memory insns trying to schedule into the same group. Super handy for dodging cache bank conflicts. Default value is `1` ⭐

- **`-msched-max-memory-insns-hard-limit`** — makes the limit from `msched-max-memory-insns` a **hard** limit, so no more than that number can be in an instruction group, periodt. Without it the limit is "soft" — non-memory operations get preferred once you hit the limit, but memory operations can still be scheduled 🌈

---

## 👑 LM32 Options 👑

These `-m` options are defined for the LatticeMico32 architecture:

- **`-mbarrel-shift-enabled`** — enable barrel-shift instructions ✨

- **`-mdivide-enabled`** — enable divide and modulus instructions 💅

- **`-mmultiply-enabled`** — enable multiply instructions 🎀

- **`-msign-extend-enabled`** — enable sign extend instructions 🦋

- **`-muser-enabled`** — enable user-defined instructions 🌸

---

## 🍓 M32C Options 🍓

- **`-mcpu=`*name* — pick the CPU to generate code for. *name* can be `r8c` for the R8C/Tiny series, `m16c` for the M16C (up to /60) series, `m32cm` for the M16C/80 series, or `m32c` for the M32C/80 series 👑

- **`-msim`** — says the program will be run on the simulator. This links in an alternate runtime library that supports stuff like file I/O. You **must not** use this option when generating programs that will run on real hardware — for 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 behave like real registers, so there's a tradeoff between GCC's ability to fit code into available registers and the performance hit of using memory instead of registers. Note: every module 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 <mint>default</mint> 💅

- **`-mmodel=small`** — assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the `ld24` instruction), and assume all subroutines are reachable with the `bl` instruction. This is the <mint>default</mint>.

  The addressability of a particular object can be set with the `model` attribute 🦋

- **`-mmodel=medium`** — assume objects may be anywhere in the 32-bit address space (the compiler generates `seth/add3` instructions to load their addresses), and assume all subroutines are reachable with the `bl` instruction 🌸

- **`-mmodel=large`** — assume objects may be anywhere in the 32-bit address space (compiler generates `seth/add3` instructions to load their addresses), and assume subroutines may **not** be reachable with the `bl` instruction — so the compiler generates the much slower `seth/add3/jl` instruction sequence 😭

- **`-msdata=none`** — disable use of the small data area. Variables get put into one of `.data`, `.bss`, or `.rodata` (unless the `section` attribute has been specified). This is the <mint>default</mint>.

  The small data area consists of sections `.sdata` and `.sbss`. Objects may be explicitly put in the small data area with the `section` attribute using one of these sections 💅

- **`-msdata=sdata`** — put small global and static data in the small data area, but 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 less than or equal to *num* bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of *num* is `8`. The `-msdata` option must be set to one of `sdata` or `use` for this option to have any effect.

  All modules should be compiled with the same `-G` *num* value. Compiling with different values of *num* may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated 👑

- **`-mdebug`** — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs 🔍

- **`-malign-loops`** — align all loops to a 32-byte boundary ⭐

- **`-mno-align-loops`** — don't enforce a 32-byte alignment for loops. This is the <mint>default</mint> 🙈

- **`-missue-rate=`*number* — issue *number* instructions per cycle. *number* can only be `1` or `2` 💅

- **`-mbranch-cost=`*number* — *number* can only be `1` or `2`. If it's `1`, branches are preferred over conditional code; if it's `2`, the opposite applies 🦋

- **`-mflush-trap=`*number* — specifies the trap number to use to flush the cache. Default is `12`. Valid numbers are between `0` and `15` inclusive 🌸

- **`-mno-flush-trap`** — specifies that the cache cannot be flushed by using a trap 💀

- **`-mflush-func=`*name* — specifies the name of the operating system function to call to flush the cache. Default is `_flush_cache`, but a function call is only used if a trap is not available 🎀

- **`-mno-flush-func`** — indicates that there is no OS function for flushing the cache 😭

---

## 🌈 M680x0 Options 🌈

These are the `-m` options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below ✨

- **`-march=`*arch* — generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of *arch* for M680x0 architectures are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060` and `cpu32`. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: `isaa`, `isaaplus`, `isab` and `isac`.

  GCC defines a macro `__mcfarch__` whenever it is generating code for a ColdFire target. The *arch* in this macro is one of the `-march` arguments given above.

  When used together, `-march` and `-mtune` select code that runs on a family of similar processors but that is optimized for a particular microarchitecture 💅

- **`-mcpu=`*cpu* — generate code for a specific M680x0 or ColdFire processor. The M680x0 *cpu*s are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060`, `68302`, `68332` and `cpu32`. The ColdFire *cpu*s are given by the table below, which also classifies the CPUs into families:

  - Family `51`: `51` `51ac` `51ag` `51cn` `51em` `51je` `51jf` `51jg` `51jm` `51mm` `51qe` `51qm`
  - Family `5206`: `5202` `5204` `5206`
  - Family `5206e`: `5206e`
  - Family `5208`: `5207` `5208`
  - Family `5211a`: `5210a` `5211a`
  - Family `5213`: `5211` `5212` `5213`
  - Family `5216`: `5214` `5216`
  - Family `52235`: `52230` `52231` `52232` `52233` `52234` `52235`
  - Family `5225`: `5224` `5225`
  - Family `52259`: `52252` `52254` `52255` `52256` `52258` `52259`
  - Family `5235`: `5232` `5233` `5234` `5235` `523x`
  - Family `5249`: `5249`
  - Family `5250`: `5250`
  - Family `5271`: `5270` `5271`
  - Family `5272`: `5272`
  - Family `5275`: `5274` `5275`
  - Family `5282`: `5280` `5281` `5282` `528x`
  - Family `53017`: `53011` `53012` `53013` `53014` `53015` `53016` `53017`
  - Family `5307`: `5307`
  - Family `5329`: `5327` `5328` `5329` `532x`
  - Family `5373`: `5372` `5373` `537x`
  - Family `5407`: `5407`
  - Family `5475`: `5470` `5471` `5472` `5473` `5474` `5475` `547x` `5480` `5481` `5482` `5483` `5484` `5485`

  `-mcpu=`*cpu* overrides `-march=`*arch* if *arch* is compatible with *cpu*. Other combinations of `-mcpu` and `-march` are rejected 😤

  GCC defines the macro `__mcf_cpu_cpu` when ColdFire target *cpu* is selected. It also defines `__mcf_family_family`, where the value of *family* is given by the table above 🎀

- **`-mtune=`*tune* — tune the code for a particular microarchitecture within the constraints set by `-march` and `-mcpu`. The M680x0 microarchitectures are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060` and `cpu32`. The ColdFire microarchitectures are: `cfv1`, `cfv2`, `cfv3`, `cfv4` and `cfv4e` ⭐

<purple>okay so</purple> you can also use `-mtune=68020-40` for code that needs to run <pink>relatively well</pink> on 68020, 68030 and 68040 targets. And `-mtune=68020-60` is the same idea but it throws 68060 targets in there too. These two select the <hotpink>exact same tuning decisions</hotpink> as `-m68020-40` and `-m68020-60` respectively — fr fr, identical.

GCC defines the macros `__mcarch` and `__mcarch__` when it's tuning for 680x0 architecture *arch*. It also defines `mcarch` unless you're using `-ansi` or a non-GNU `-std` option. And if GCC is tuning for a *range* of architectures — which is what you get with `-mtune=68020-40` or `-mtune=68020-60` — it defines the macros for <rainbow>every architecture in the range</rainbow>. 💖

GCC also defines the macro `__muarch__` when tuning for ColdFire microarchitecture *uarch*, where *uarch* is one of the arguments given above.

---

- **`-m68000`, `-mc68000`** — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. Equivalent to `-march=68000`. 🎀

  Use this one for microcontrollers with a 68000 or EC000 core — that's the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. ✨

- **`-m68010`** — generate output for a 68010. Default when the compiler is configured for 68010-based systems. Equivalent to `-march=68010`.

- **`-m68020`, `-mc68020`** — generate output for a 68020. Default when the compiler is configured for 68020-based systems. Equivalent to `-march=68020`.

- **`-m68030`** — generate output for a 68030. Default when the compiler is configured for 68030-based systems. Equivalent to `-march=68030`.

- **`-m68040`** — generate output for a 68040. Default when the compiler is configured for 68040-based systems. Equivalent to `-march=68040`. 🌸

  Heads up: this option <red>inhibits</red> the use of 68881/68882 instructions that have to be emulated by software on the 68040. So use it if your 68040 doesn't have code to emulate those instructions. 👑

- **`-m68060`** — generate output for a 68060. Default when the compiler is configured for 68060-based systems. Equivalent to `-march=68060`.

  This one <red>inhibits</red> the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 doesn't have code to emulate those instructions.

- **`-mcpu32`** — generate output for a CPU32. Default when the compiler is configured for CPU32-based systems. Equivalent to `-march=cpu32`. 🦋

  Use this for microcontrollers with a CPU32 or CPU32+ core — that's the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

- **`-m5200`** — generate output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. Equivalent to `-mcpu=5206`, and it's <yellow>now deprecated</yellow> in favor of that option. 😭

  Use it for microcontrollers with a 5200 core — the MCF5202, MCF5203, MCF5204 and MCF5206.

- **`-m5206e`** — generate output for a 5206e ColdFire CPU. <yellow>Now deprecated</yellow> in favor of the equivalent `-mcpu=5206e`.

- **`-m528x`** — generate output for a member of the ColdFire 528X family. <yellow>Now deprecated</yellow> in favor of the equivalent `-mcpu=528x`.

- **`-m5307`** — generate output for a ColdFire 5307 CPU. <yellow>Now deprecated</yellow> in favor of the equivalent `-mcpu=5307`.

- **`-m5407`** — generate output for a ColdFire 5407 CPU. <yellow>Now deprecated</yellow> in favor of the equivalent `-mcpu=5407`.

- **`-mcfv4e`** — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. Equivalent to `-mcpu=547x`, and it's <yellow>now deprecated</yellow> in favor of that option.

- **`-m68020-40`** — generate output for a 68040, <pink>without</pink> using any of the new instructions. This gives you code that can run relatively efficiently on either a 68020/68881, a 68030, or a 68040. The generated code *does* use the 68881 instructions that are emulated on the 68040. 💅

  Equivalent to `-march=68020 -mtune=68020-40`.

- **`-m68020-60`** — generate output for a 68060, <pink>without</pink> using any of the new instructions. Same deal: code that runs relatively efficiently on a 68020/68881, a 68030, or a 68040. The generated code *does* use the 68881 instructions that are emulated on the 68060.

  Equivalent to `-march=68020 -mtune=68020-60`.

- **`-mhard-float`, `-m68881`** — generate floating-point instructions. Default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro `__HAVE_68881__` on M680x0 targets and `__mcffpu__` on ColdFire targets. ⭐

- **`-msoft-float`** — do <red>not</red> generate floating-point instructions; use library calls instead. Default for 68000, 68010, and 68832 targets. Also the default for ColdFire devices that have no FPU.

- **`-mdiv`, `-mno-div`** — generate (or don't generate) ColdFire hardware divide and remainder instructions. If `-march` is used without `-mcpu`, the default is `"on"` for ColdFire architectures and `"off"` for M680x0 architectures. Otherwise the default is taken from the target CPU — either the default CPU or the one you specified with `-mcpu`. For example, the default is `"off"` for `-mcpu=5206` and `"on"` for `-mcpu=5206e`. 🧚

  GCC defines the macro `__mcfhwdiv__` when this option is enabled.

- **`-mshort`** — consider type `int` to be 16 bits wide, like `short int`. Bonus: 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 <red>not</red> consider type `int` to be 16 bits wide. This is the default.

- **`-mnobitfield`, `-mno-bitfield`** — do not use the bit-field instructions. The `-m68000`, `-mcpu32` and `-m5200` options imply `-mnobitfield`.

- **`-mbitfield`** — do use the bit-field instructions. The `-m68020` option implies `-mbitfield`. This is the default if you use a configuration designed for a 68020.

- **`-mrtd`** — use a different function-calling convention, where functions that take a fixed number of arguments return with the `rtd` instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. 🔥

  > This calling convention is <red>incompatible</red> with the one normally used on Unix, so you <red>cannot</red> use it if you need to call libraries compiled with the Unix compiler.

  Also, you <hotpink>must</hotpink> 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, <red>seriously incorrect code</red> results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

  The `rtd` instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but <red>not</red> by the 68000 or 5200.

  The default is `-mno-rtd`.

- **`-malign-int`, `-mno-align-int`** — control whether GCC aligns `int`, `long`, `long long`, `float`, `double`, and `long double` variables on a 32-bit boundary (`-malign-int`) or a 16-bit boundary (`-mno-align-int`). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory. 🫶

  > Warning: if you use the `-malign-int` switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k.

And now the pc-relative tea continues: use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies `-fpic`, allowing at most a 16-bit offset for pc-relative addressing. `-fPIC` is not presently supported with `-mpcrel`, though this could be supported for 68020 and higher processors. 💜

- **`-mno-strict-align`, `-mstrict-align`** — do not (do) assume that unaligned memory references are handled by the system.

- **`-msep-data`** — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies `-fPIC`. ✨

- **`-mno-sep-data`** — generate code that assumes the data segment follows the text segment. This is the default.

- **`-mid-shared-library`** — generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies `-fPIC`. 🌈

- **`-mno-id-shared-library`** — generate code that doesn't assume ID-based shared libraries are being used. This is the default.

- **`-mshared-library-id=`*n* — specifies the identification number of the ID-based shared library being compiled. Specifying a value of `0` generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. 🤔

- **`-mxgot`, `-mno-xgot`** — when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is <pink>not needed</pink> — `-fPIC` suffices. 💅

  GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with `-mxgot`. It should then work with very large GOTs. <yellow>However</yellow>, 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 <red>no effect</red> unless GCC is generating position-independent code.

- **`-mlong-jump-table-offsets`** — use 32-bit offsets in `switch` tables. The default is to use 16-bit offsets. 🎀

---

## 🍓 MCore Options 🍓

---

<pink>okay so</pink> these are the `-m` options that Motorola defined for the <rainbow>M*Core</rainbow> processors 💖

- **`-mhardlit`, `-mno-hardlit`** — squish constants directly into the code stream *if* it can be done in two instructions or fewer. No more, no less. 🎀
- **`-mdiv`, `-mno-div`** — use the divide instruction. (It's enabled by default, bestie, it's already on.) ✨
- **`-mrelax-immediate`, `-mno-relax-immediate`** — let immediates be whatever size they want in bit operations. free-range immediates fr.
- **`-mwide-bitfields`, `-mno-wide-bitfields`** — always treat bit-fields as `"int"`-sized, no matter what.
- **`-m4byte-functions`, `-mno-4byte-functions`** — force every single function 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 you're 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 the runtime support is already handled, so just <pink>omit</pink> the simulator library (`libsim.a`) from the linker command line.
- **`-mstack-increment=size`** — set the max amount for a single stack increment operation. Big values can speed up programs that have functions needing a *lot* of stack space… but they can also trigger a segmentation fault if the stack gets extended too much. 😭 The default value is `0x1000`.

---

## 🧚 MeP Options 🧚

- **`-mabsdiff`** — turns on the `"abs"` instruction, which is the absolute difference between two registers. iconic.
- **`-mall-opts`** — turns on <hotpink>ALL</hotpink> the optional instructions: average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. the full menu, no skips. 👑
- **`-maverage`** — turns on the `"ave"` instruction, which computes the average of two registers.
- **`-mbased=n`** — variables of size *n* bytes or smaller get placed in the `.based` section by default. Based variables use the `$tp` register as a base register, and there's a 128-byte limit to the `.based` section. (yes it's called based. we love that for it.)
- **`-mbitops`** — turns on the bit operation instructions: bit test (`"btstm"`), set (`"bsetm"`), clear (`"bclrm"`), invert (`"bnotm"`), and test-and-set (`"tas"`).
- **`-mc=name`** — picks which section constant data goes in. *name* can be `tiny`, `near`, or `far`.
- **`-mclip`** — turns on the `"clip"` instruction. ⚠️ heads up: `-mclip` isn't useful unless you *also* pass `-mminmax`. they're a package deal, iykyk.
- **`-mconfig=name`** — picks one of the built-in core configurations. Every MeP chip has one or more modules in it; each module has a core CPU plus a bunch of coprocessors, optional instructions, and peripherals. The `"MeP-Integrator"` tool (not part of GCC) provides these configurations through this option — using this option is the same as using all the corresponding command-line options. The default configuration is `default`.
- **`-mcop`** — turns on the coprocessor instructions. By default this is a 32-bit coprocessor. ngl the coprocessor is normally enabled via the `-mconfig=` option.
- **`-mcop32`** — turns on the 32-bit coprocessor's instructions.
- **`-mcop64`** — turns on the 64-bit coprocessor's instructions.
- **`-mivc2`** — turns on IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor. ✨
- **`-mdc`** — makes constant variables get placed in the `.near` section.
- **`-mdiv`** — turns on the `"div"` and `"divu"` instructions.
- **`-meb`** — generate big-endian code.
- **`-mel`** — generate little-endian code.
- **`-mio-volatile`** — tells the compiler that any variable marked with the `"io"` attribute is to be considered volatile.
- **`-ml`** — makes variables get assigned to the `.far` section by default.
- **`-mleadz`** — turns on the `"leadz"` (leading zero) instruction.
- **`-mm`** — makes variables get assigned to the `.near` section by default.
- **`-mminmax`** — turns on the `"min"` and `"max"` instructions.
- **`-mmult`** — turns on the multiplication and multiply-accumulate instructions.
- **`-mno-opts`** — turns <red>OFF</red> all the optional instructions that `-mall-opts` turns on. the anti-`-mall-opts`, if you will.
- **`-mrepeat`** — turns on the `"repeat"` and `"erepeat"` instructions, which are used for low-overhead looping. 🔁
- **`-ms`** — makes all variables default to the `.tiny` section. note that there's a 65536-byte limit to this section. accesses to these variables use the `%gp` base register.
- **`-msatur`** — turns on the saturation instructions. note that the compiler doesn't currently generate these itself, but this option is here for compatibility with other tools, like `"as"`.
- **`-msdram`** — link the SDRAM-based runtime instead of the default ROM-based runtime.
- **`-msim`** — link the simulator run-time libraries.
- **`-msimnovec`** — link the simulator runtime libraries, but *excluding* built-in support for reset and exception vectors and tables.
- **`-mtf`** — makes all functions default to the `.far` section. Without this option, functions default to the `.near` section.
- **`-mtiny=n`** — variables that are *n* bytes or smaller get allocated to the `.tiny` section. These variables use the `$gp` base register. The default for this option is `4`, but note that there's a 65536-byte limit to the `.tiny` section.

---

## 🦋 MicroBlaze Options 🦋

- **`-msoft-float`** — use software emulation for floating point (this is the default).
- **`-mhard-float`** — use hardware floating-point instructions.
- **`-mmemcpy`** — don't optimize block moves, just use `"memcpy"`.
- **`-mno-clearbss`** — ⚠️ this option is <red>deprecated</red>. Use `-fno-zero-initialized-in-bss` instead.
- **`-mcpu=cpu-type`** — use features of, and schedule code for, the given CPU. Supported values are in the format `vX.YY.Z`, where *X* is a major version, *YY* is the minor version, and *Z* is compatibility code. Example values are `v3.00.a`, `v4.00.b`, `v5.00.a`, `v5.00.b`, `v6.00.a`.
- **`-mxl-soft-mul`** — use software multiply emulation (default).
- **`-mxl-soft-div`** — use software emulation for divides (default).
- **`-mxl-barrel-shift`** — use the hardware barrel shifter.
- **`-mxl-pattern-compare`** — use pattern compare instructions.
- **`-msmall-divides`** — use table lookup optimization for small signed integer divisions.
- **`-mxl-stack-check`** — ⚠️ this option is <red>deprecated</red>. Use `-fstack-check` instead.
- **`-mxl-gp-opt`** — use GP-relative `.sdata`/`.sbss` sections.
- **`-mxl-multiply-high`** — use multiply high instructions for the high part of a 32x32 multiply.
- **`-mxl-float-convert`** — use hardware floating-point conversion instructions.
- **`-mxl-float-sqrt`** — use the hardware floating-point square root instruction.
- **`-mbig-endian`** — generate code for a big-endian target.
- **`-mlittle-endian`** — generate code for a little-endian target.
- **`-mxl-reorder`** — use reorder instructions (swap and byte reversed load/store).
- **`-mxl-mode-app-model`** — select application model *app-model*. Valid models are:

- `executable` — normal executable (default), uses startup code `crt0.o`.
- `xmdstub` — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called `xmdstub`. This uses startup file `crt1.o` and sets the start address of the program to `0x800`.
- `bootstrap` — for applications that are loaded using a bootloader. This model uses startup file `crt2.o` which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
- `novectors` — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses `crt3.o` as a startup file.

> Option `-xl-mode-app-model` is a deprecated alias for `-mxl-mode-app-model`.

- **`-mpic-data-is-text-relative`** — assume that the displacement between the text and data segments is fixed at static link time. This lets data be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing is not supported. 🫶

---

## 🌈 MIPS Options 🌈

- **`-EB`** — generate big-endian code.
- **`-EL`** — generate little-endian code. This is the default for `mips*el-*-*` configurations.
- **`-march=arch`** — generate code that runs on *arch*, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: `mips1`, `mips2`, `mips3`, `mips4`, `mips32`, `mips32r2`, `mips32r3`, `mips32r5`, `mips32r6`, `mips64`, `mips64r2`, `mips64r3`, `mips64r5` and `mips64r6`. The processor names are: `4kc`, `4km`, `4kp`, `4ksc`, `4kec`, `4kem`, `4kep`, `4ksd`, `5kc`, `5kf`, `20kc`, `24kc`, `24kf2_1`, `24kf1_1`, `24kec`, `24kef2_1`, `24kef1_1`, `34kc`, `34kf2_1`, `34kf1_1`, `34kn`, `74kc`, `74kf2_1`, `74kf1_1`, `74kf3_2`, `1004kc`, `1004kf2_1`, `1004kf1_1`, `i6400`, `i6500`, `interaptiv`, `loongson2e`, `loongson2f`, `loongson3a`, `gs464`, `gs464e`, `gs264e`, `m4k`, `m14k`, `m14kc`, `m14ke`, `m14kec`, `m5100`, `m5101`, `octeon`, `octeon+`, `octeon2`, `octeon3`, `orion`, `p5600`, `p6600`, `r2000`, `r3000`, `r3900`, `r4000`, `r4400`, `r4600`, `r4650`, `r4700`, `r5900`, `r6000`, `r8000`, `rm7000`, `rm9000`, `r10000`, `r12000`, `r14000`, `r16000`, `sb1`, `sr71000`, `vr4100`, `vr4111`, `vr4120`, `vr4130`, `vr4300`, `vr5000`, `vr5400`, `vr5500`, `xlr` and `xlp`. The special value `from-abi` selects the most compatible architecture for the selected ABI (that is, `mips1` for 32-bit ABIs and `mips3` for 64-bit ABIs).

The native Linux/GNU toolchain also supports the value `native`, which selects the best architecture option for the host processor. `-march=native` has no effect if GCC does not recognize the processor.

In processor names, a final `000` can be abbreviated as `k` (for example, `-march=r2k`). Prefixes are optional, and `vr` may be written `r`.

Names of the form *nf2_1* refer to processors with FPUs clocked at half the rate of the core, names of the form *nf1_1* refer to processors with FPUs clocked at the same rate as the core, and names of the form *nf3_2* refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, `nf` is accepted as a synonym for `nf2_1` while `nx` and `bfx` are accepted as synonyms for `nf1_1`.

GCC defines two macros based on the value of this option. The first is `_MIPS_ARCH`, which gives the name of the target architecture, as a string. The second has the form `_MIPS_ARCH_foo`, where *foo* is the capitalized value of `_MIPS_ARCH`. For example, `-march=r2000` sets `_MIPS_ARCH` to `"r2000"` and defines the macro `_MIPS_ARCH_R2000`. 💅

- Note that the <pink>`_MIPS_ARCH`</pink> macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate `000` as `k`. In the case of `from-abi`, the macro names the resolved architecture (either `"mips1"` or `"mips3"`). It names the default architecture when no `-march` option is given.

- **`-mtune=arch`** — optimize for *arch*. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of *arch* values is the same as for `-march`.

When this option is not used, GCC optimizes for the processor specified by `-march`. By using `-march` and `-mtune` together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family.

`-mtune` defines the macros `_MIPS_TUNE` and `_MIPS_TUNE_foo`, which work in the same way as the `-march` ones described above.

- **`-mips1`** — equivalent to `-march=mips1`.

- **`-mips2`** — equivalent to `-march=mips2`.

- **`-mips3`** — equivalent to `-march=mips3`.

- **`-mips4`** — equivalent to `-march=mips4`.

- **`-mips32`** — equivalent to `-march=mips32`.

- **`-mips32r3`** — equivalent to `-march=mips32r3`.

- **`-mips32r5`** — equivalent to `-march=mips32r5`.

- **`-mips32r6`** — equivalent to `-march=mips32r6`.

- **`-mips64`** — equivalent to `-march=mips64`.

- **`-mips64r2`** — equivalent to `-march=mips64r2`.

- **`-mips64r3`** — equivalent to `-march=mips64r3`.

- **`-mips64r5`** — equivalent to `-march=mips64r5`.

- **`-mips64r6`** — equivalent to `-march=mips64r6`.

- **`-mips16`**, **`-mno-mips16`** — generate (or don't generate, respectively) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. ✨

MIPS16 code generation can also be controlled on a per-function basis by means of `"mips16"` and `"nomips16"` attributes.

- **`-mflip-mips16`** — generate MIPS16 code on alternating functions. <yellow>This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.</yellow> So like, it exists, but it's not for you bestie.

- **`-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. `-minterlink-compressed` therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

- **`-minterlink-mips16`**, **`-mno-interlink-mips16`** — aliases of `-minterlink-compressed` and `-mno-interlink-compressed`. These options predate the microMIPS ASE and are retained for backwards compatibility.

- **`-mabi=32`**, **`-mabi=o64`**, **`-mabi=n32`**, **`-mabi=64`**, **`-mabi=eabi`** — generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use `-mgp32` to get 32-bit code instead.

For info 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. `-mabicalls` is 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 foobar

If this happens, you should recompile your code with `-mxgot`. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. 💀

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use `-mxgot` when a single object file accesses more than 64k's worth of GOT entries. Very few do.

> These options have no effect unless GCC is generating position independent code.

- **`-mgp32`** — assume that general-purpose registers are 32 bits wide.

- **`-mgp64`** — assume that general-purpose registers are 64 bits wide.

- **`-mfp32`** — assume that floating-point registers are 32 bits wide.

- **`-mfp64`** — assume that floating-point registers are 64 bits wide.

- **`-mfpxx`** — do not assume the width of floating-point registers.

- **`-mhard-float`** — use floating-point coprocessor instructions.

- **`-msoft-float`** — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.

- **`-mno-float`** — equivalent to `-msoft-float`, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point `printf` formats). <red>If code compiled with `-mno-float` accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.</red> 😭

- **`-msingle-float`** — assume that the floating-point coprocessor only supports single-precision operations.

- **`-mdouble-float`** — assume that the floating-point coprocessor supports double-precision operations. This is the default.

- **`-modd-spreg`**, **`-mno-odd-spreg`** — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, `-mno-odd-spreg` is set by default.

- **`-mabs=2008`**, **`-mabs=legacy`** — these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the `abs.fmt` and `neg.fmt` machine instructions.

By default or when `-mabs=legacy` is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the `-ffinite-math-only` option has also been specified.

The `-mabs=2008` option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. ✨

- **`-mnan=2008`**, **`-mnan=legacy`** — these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

The `-mnan=legacy` option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being `0`, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being `1`.

The `-mnan=2008` option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being `1`, whereas sNaNs are denoted by the first bit of their trailing significand field being `0`.

The default is `-mnan=legacy` unless GCC has been configured with `--with-nan=2008`. 🌸

- **`-mllsc`**, **`-mno-llsc`** — do (or don't) use <pink>`ll`</pink>, <pink>`sc`</pink>, and <pink>`sync`</pink> instructions to implement atomic memory built-in functions. If you specify neither one, GCC just uses those instructions whenever the target architecture actually supports them, periodt. <pink>`-mllsc`</pink> is the move when your runtime environment can emulate the instructions, and <pink>`-mno-llsc`</pink> lowkey saves you when you're compiling for nonstandard ISAs. Wanna make either one the default? Configure GCC with `--with-llsc` or `--without-llsc` respectively. `--with-llsc` is the default for some configurations — see the installation documentation for the deets. 💅

- **`-mdsp`**, **`-mno-dsp`** — use (don't use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro `__mips_dsp`. It also defines `__mips_dsp_rev` to 1. ✨

- **`-mdspr2`**, **`-mno-dspr2`** — use (don't use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros `__mips_dsp` **and** `__mips_dspr2`. It also defines `__mips_dsp_rev` to 2. two for two bestie 🎀

- **`-msmartmips`**, **`-mno-smartmips`** — use (don't use) the MIPS SmartMIPS ASE.

- **`-mpaired-single`**, **`-mno-paired-single`** — use (don't use) paired-single floating-point instructions. Heads up tho: this option requires hardware floating-point support to be enabled. ⚠️

- **`-mdmx`**, **`-mno-mdmx`** — use (don't use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code, and it requires hardware floating-point support to be enabled. both conditions, no skipping 🫶

- **`-mips3d`**, **`-mno-mips3d`** — use (don't use) the MIPS-3D ASE. The option `-mips3d` implies `-mpaired-single`. package deal fr

- **`-mmicromips`**, **`-mno-micromips`** — generate (don't generate) microMIPS code. You can also control microMIPS code generation on a per-function basis by means of `micromips` and `nomicromips` attributes. per-function vibes ✨

- **`-mmt`**, **`-mno-mt`** — use (don't use) MT Multithreading instructions.

- **`-mmcu`**, **`-mno-mcu`** — use (don't use) the MIPS MCU ASE instructions.

- **`-meva`**, **`-mno-eva`** — use (don't use) the MIPS Enhanced Virtual Addressing instructions.

- **`-mvirt`**, **`-mno-virt`** — use (don't use) the MIPS Virtualization (VZ) instructions.

- **`-mxpa`**, **`-mno-xpa`** — use (don't use) the MIPS eXtended Physical Address (XPA) instructions.

- **`-mcrc`**, **`-mno-crc`** — use (don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.

- **`-mginv`**, **`-mno-ginv`** — use (don't use) the MIPS Global INValidate (GINV) instructions.

- **`-mloongson-mmi`**, **`-mno-loongson-mmi`** — use (don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).

- **`-mloongson-ext`**, **`-mno-loongson-ext`** — use (don't use) the MIPS Loongson EXTensions (EXT) instructions.

- **`-mloongson-ext2`**, **`-mno-loongson-ext2`** — use (don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.

- **`-mlong64`** — force `long` types to be 64 bits wide. See `-mlong32` for an explanation of the default and the way that the pointer size is determined. 👑

- **`-mlong32`** — force `long`, `int`, and pointer types to be 32 bits wide.

The default size of `int`s, `long`s and pointers depends on the ABI. All the supported ABIs use 32-bit `int`s. The n64 ABI uses 64-bit `long`s, as does the 64-bit EABI; the others use 32-bit `long`s. Pointers are the same size as `long`s, or the same size as integer registers, whichever is smaller. math is mathing 💅

- **`-msym32`**, **`-mno-sym32`** — assume (don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with `-mabi=64` and `-mno-abicalls` because it lets GCC generate shorter and faster references to symbolic addresses. speed AND brevity, we love 🏎️

- **`-G`** *num* — put definitions of externally-visible data in a small data section if that data is no bigger than *num* bytes. Then GCC can generate more efficient accesses to the data; see `-mgpopt` for the details. The default `-G` option depends on the configuration.

- **`-mlocal-sdata`**, **`-mno-local-sdata`** — extend (don't extend) the `-G` behavior to local data too, like to static variables in C. `-mlocal-sdata` is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with `-mno-local-sdata`. You might also want to build large libraries with `-mno-local-sdata`, so that the libraries leave more room for the main program. sharing is caring but so is leaving room 🫶

- **`-mextern-sdata`**, **`-mno-extern-sdata`** — assume (don't assume) that externally-defined data is in a small data section if the size of that data is within the `-G` limit. `-mextern-sdata` is the default for all configurations.

If you compile a module *Mod* with `-mextern-sdata` `-G` *num* `-mgpopt`, and *Mod* references a variable *Var* that is no bigger than *num* bytes, you **must** make sure that *Var* is placed in a small data section. If *Var* is defined by another module, you must either compile that module with a high-enough `-G` setting or attach a `section` attribute to *Var*'s definition. If *Var* is common, you must link the application with a high-enough `-G` setting. no cap, this one is strict 😤

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 (don't use) GP-relative accesses for symbols that are known to be in a small data section; see `-G`, `-mlocal-sdata` and `-mextern-sdata`. `-mgpopt` is the default for all configurations.

`-mno-gpopt` is useful for cases where the `$gp` register might not hold the value of `_gp`. For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in `$gp`. (In such situations, the boot monitor itself is usually compiled with `-G0`.)

`-mno-gpopt` implies `-mno-local-sdata` and `-mno-extern-sdata`. chain reaction fr 💀

- **`-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 it reduces the amount of RAM required when executing, and so it may be preferred for some embedded systems. slower but lighter, the tradeoff is real ✨

- **`-muninit-const-in-rodata`**, **`-mno-uninit-const-in-rodata`** — put uninitialized `const` variables in the read-only data section. This option is only meaningful in conjunction with `-membedded-data`. only works as a duo 🎀

- **`-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. go crazy 💅
- **`-mcode-readable=pcrel`** — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
- **`-mcode-readable=no`** — instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.

- **`-msplit-addresses`**, **`-mno-split-addresses`** — enable (disable) use of the `%hi()` and `%lo()` assembler relocation operators. This option has been superseded by `-mexplicit-relocs` but is retained for backwards compatibility. legacy icon, still here for you 🙈

- **`-mexplicit-relocs`**, **`-mno-explicit-relocs`** — use (don't use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by `-mno-explicit-relocs`, is to use assembler macros instead.

`-mexplicit-relocs` is the default if GCC was configured to use an assembler that supports relocation operators.

- **`-mcheck-zero-division`**, **`-mno-check-zero-division`** — trap (don't trap) on integer division by zero. The default is `-mcheck-zero-division`. 🚨

- **`-mdivide-traps`**, **`-mdivide-breaks`** — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but it's only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (`SIGFPE`). Use `-mdivide-traps` to allow conditional traps on architectures that support them and `-mdivide-breaks` to force the use of breaks.

The default is usually `-mdivide-traps`, but this can be overridden at configure time using `--with-divide=breaks`. Divide-by-zero checks can be completely disabled using `-mno-check-zero-division`. 💀

- **`-mload-store-pairs`**, **`-mno-load-store-pairs`** — enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. bestie pairing energy 🔥

- **`-munaligned-access`**, **`-mno-unaligned-access`** — enable (disable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So `-mno-unaligned-access` may be needed by kernel. 🌸

- **`-mmemcpy`**, **`-mno-memcpy`** — force (don't force) the use of `memcpy` for non-trivial block moves. The default is `-mno-memcpy`, which allows GCC to inline most constant-sized copies.

- **`-mlong-calls`**, **`-mno-long-calls`** — disable (don't disable) use of the `jal` instruction. Calling functions using `jal` is more efficient but requires the caller and callee to be in the same 256 megabyte segment. close quarters only, 256MB of it 😭

This option has no effect on abicalls code. The default is `-mno-long-calls`.

- **`-mmad`**, **`-mno-mad`** — enable (disable) use of the `mad`, `madu` and `mul` instructions, as provided by the R4650 ISA. ✨

- **`-mimadd`**, **`-mno-imadd`** — turns the `"madd"` and `"msub"` integer instructions on (or off). Default is `-mimadd` on any architecture that supports `"madd"` and `"msub"`… except the 74k, bestie, because on that one it was found to generate <red>slower code</red> 😭 so we don't do that there.

- **`-mfused-madd`**, **`-mno-fused-madd`** — turns on (or off) the floating-point multiply-accumulate instructions, whenever they're actually available. Default is `-mfused-madd` 💖.

<yellow>Heads up though:</yellow> on the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product gets calculated to infinite precision and is *not* subject to the FCSR Flush to Zero bit. That can be a whole vibe killer in some circumstances 🙈. On other processors, the result is numerically identical to the equivalent computation done with separate multiply, add, subtract and negate instructions — so it's giving same math, different fit.

- **`-nocpp`** — tells the MIPS assembler to <pink>not</pink> run its preprocessor over your user assembler files (the ones with a `.s` suffix) when it assembles them. No preprocessing, we said what we said 💅.

- **`-mfix-24k`**, **`-mno-fix-24k`** — works around the 24K E48 errata (that's *lost data on stores during refill*, iconic in the worst way 💀). These workarounds are implemented by the assembler rather than by GCC, fyi.

- **`-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. (yeah, same energy as the R4000 one, not a typo 🫶)

- **`-mfix-r10000`**, **`-mno-fix-r10000`** — works around certain R10000 errata:

- `"ll"`/`"sc"` sequences may not behave atomically on revisions prior to 3.0. They may <red>deadlock</red> on revisions 2.6 and earlier. not the deadlock bestie 😭

This option can only be used if the target architecture supports branch-likely instructions. `-mfix-r10000` is the default when `-march=r10000` is used; `-mno-fix-r10000` is the default otherwise.

- **`-mfix-r5900`**, **`-mno-fix-r5900`** — do <pink>not</pink> attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a `"nop"` instruction there instead. The short loop bug, under certain conditions, causes loops to execute only once or twice, all thanks to a hardware bug in the R5900 chip 💀. The workaround is implemented by the assembler rather than by GCC.

- **`-mfix-rm7000`**, **`-mno-fix-rm7000`** — works around the RM7000 `"dmult"`/`"dmultu"` errata. Again, the workarounds are implemented by the assembler rather than by GCC.

- **`-mfix-vr4120`**, **`-mno-fix-vr4120`** — works around certain VR4120 errata:

- `"dmultu"` does not always produce the correct result.
- `"div"` and `"ddiv"` do not always produce the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in `libgcc.a`. At present, these functions are only provided by the `"mips64vr*-elf"` configurations. (only those, fr fr 🥲)

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

- **`-mfix-vr4130`** — works around the VR4130 `"mflo"`/`"mfhi"` errata. The workarounds are implemented by the assembler rather than by GCC — although GCC avoids using `"mflo"` and `"mfhi"` if the VR4130 `"macc"`, `"macchi"`, `"dmacc"` and `"dmacchi"` instructions are available instead. slay, alternative instructions ✨

- **`-mfix-sb1`**, **`-mno-fix-sb1`** — works around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 `"F1"` and `"F2"` floating-point errata.)

- **`-mr10k-cache-barrier=setting`** — specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. okay this one needs a whole story, sit down 🍓

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the `"taken"` branch. It later aborts these instructions if the predicted outcome is wrong. <hotpink>However</hotpink>, on the R10K, even aborted instructions can have side effects. the audacity 💀

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the `"dirty"` line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. `-mr10k-cache-barrier=setting` controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. (kernel, that's on you bestie 🫵)

If the input program contains a function declaration such as:

void foo (void);

then the implementation of `"foo"` must allow `j foo` and `jal foo` to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same. no cap.

The option has three forms:

- **`-mr10k-cache-barrier=load-store`** — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
- **`-mr10k-cache-barrier=store`** — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
- **`-mr10k-cache-barrier=none`** — disable the insertion of cache barriers. This is the default setting. 🎀

- **`-mflush-func=func`**, **`-mno-flush-func`** — specifies the function to call to flush the I and D caches, or to not call any such function at all. If called, the function must take the same arguments as the common `"_flush_func"`, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number `3` (to flush both caches). The default depends on the target GCC was configured for, but commonly is either `"_flush_func"` or `"__cpu_flush"`.

- **`mbranch-cost=num`** — set the cost of branches to roughly *num* `"simple"` instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the `-mtune` setting. (heuristic queen 👑 — vibes, not promises)

- **`-mbranch-likely`**, **`-mno-branch-likely`** — enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they're supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use. deprecated, so we're not doing it by default 🙅

- **`-mcompact-branches=never`**, **`-mcompact-branches=optimal`**, **`-mcompact-branches=always`** — these options control which form of branches will be generated. The default is `-mcompact-branches=optimal`.

The `-mcompact-branches=never` option ensures that compact branch instructions will never be generated. <pink>never ever</pink>.

The `-mcompact-branches=always` option ensures that a compact branch instruction will be generated if available. If a compact branch instruction is not available, a delay slot form of the branch will be used instead.

This option is supported from MIPS Release 6 onwards.

The `-mcompact-branches=optimal` option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available.

- **`-mfp-exceptions`**, **`-mno-fp-exceptions`** — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.

For instance, on the SB-1, if FP exceptions are disabled, and we're emitting 64-bit code, then we can use <rainbow>both</rainbow> FP pipes ✨. Otherwise, we can only use one FP pipe. one pipe era vs two pipe era, the way that matters 😭

- **`-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. team work makes the dream work 💖

This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level `-O3`.

- **`-msynci`**, **`-mno-synci`** — enable (disable) generation of `"synci"` instructions on architectures that support it. The `"synci"` instructions (if enabled) are generated when `"__builtin___clear_cache"` is compiled.

This option defaults to `-mno-synci`, but the default can be overridden by configuring GCC with `--with-synci`.

When compiling code for single processor systems, it's generally safe to use `"synci"`. <red>However</red>, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. undefined behavior bestie, we don't play like that 💀

- **`-mrelax-pic-calls`**, **`-mno-relax-pic-calls`** — try to turn PIC calls that are normally dispatched via register `$25` into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. link-time destiny, iykyk 🔥

- **`-mrelax-pic-calls`** — <green>this one's the default</green> if GCC was built to use an assembler and a linker that support the `.reloc` assembly directive *and* `-mexplicit-relocs` is in effect. But if you're running `-mno-explicit-relocs`, then lowkey the assembler and the linker can do this optimization all by themselves, no compiler help needed. team effort bestie 🫶

- **`-mmcount-ra-address`**, **`-mno-mcount-ra-address`** — emit (or don't emit) code that lets `_mcount` mess with the calling function's return address. When it's on, the usual `_mcount` interface gets a lil upgrade: a new *ra-address* parameter, type `intptr_t *`, passed in register `$12`. then `_mcount` can change the return address by doing both of these:

* returning the new address in register `$31`.

* storing the new address in `*ra-address`, if *ra-address* isn't null.

default? that's `-mno-mcount-ra-address`. no cap.

- **`-mframe-header-opt`**, **`-mno-frame-header-opt`** — enable (disable) frame header optimization in the o32 ABI. so when you're on o32, calling functions allocate 16 bytes on the stack so the called function can write out register arguments. turn this on and the optimization will suppress that frame header allocation *if* it figures out nobody's using it. iconic.

> heads up: this optimization is <red>off by default at all optimization levels</red>. every single one. 💀

- **`-mlxc1-sxc1`**, **`-mno-lxc1-sxc1`** — when applicable, enable (disable) generating `lwxc1`, `swxc1`, `ldxc1`, `sdxc1` instructions. <green>enabled by default</green>.

- **`-mmadd4`**, **`-mno-madd4`** — when applicable, enable (disable) generating 4-operand `madd.s`, `madd.d` and related instructions. <green>enabled by default</green> ✨

---

## 🦋 MMIX Options 🦋

these ones are defined for the MMIX, bestie:

- **`-mlibfuncs`**, **`-mno-libfuncs`** — specify that intrinsic library functions are what's being compiled, so all values go in registers, no matter the size. periodt.

- **`-mepsilon`**, **`-mno-epsilon`** — generate floating-point comparison instructions that compare with respect to the `rE` epsilon register. 🎀

- **`-mabi=mmixware`**, **`-mabi=gnu`** — generate code that passes function parameters and return values that (inside the called function) show up as registers `$0` and up, as opposed to the GNU ABI which uses global registers `$231` and up. two completely different vibes.

- **`-mzero-extend`**, **`-mno-zero-extend`** — when reading data from memory in sizes shorter than 64 bits, use (don't use) zero-extending load instructions by default, instead of sign-extending ones.

- **`-mknuthdiv`**, **`-mno-knuthdiv`** — make the result of a division that yields a remainder have the same sign as the divisor. with the default, `-mno-knuthdiv`, the sign of the remainder follows the sign of the dividend. both methods are arithmetically valid, but the latter is used almost exclusively. iykyk 💅

- **`-mtoplevel-symbols`**, **`-mno-toplevel-symbols`** — prepend (don't prepend) a `:` to all global symbols, so the assembly code can be used with the `PREFIX` assembly directive.

- **`-melf`** — generate an executable in the ELF format instead of the default mmo format that the mmix simulator uses.

- **`-mbranch-predict`**, **`-mno-branch-predict`** — use (don't use) the probable-branch instructions when static branch prediction says it's a probable branch.

- **`-mbase-addresses`**, **`-mno-base-addresses`** — generate (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 the number of different data items you can address is limited. so a program using lots of static data may need `-mno-base-addresses`. tradeoffs, always tradeoffs 😭

- **`-msingle-exit`**, **`-mno-single-exit`** — force (don't force) generated code to have a single exit point in each function.

---

## 🌸 MN10300 Options 🌸

these `-m` options are defined for Matsushita MN10300 architectures:

- **`-mmult-bug`** — generate code to avoid bugs in the multiply instructions for the MN10300 processors. this is the default. <green>we stay safe out here.</green>

- **`-mno-mult-bug`** — don't generate code to avoid bugs in the multiply instructions for the MN10300 processors.

- **`-mam33`** — generate code using features specific to the AM33 processor.

- **`-mno-am33`** — don't generate code using features specific to the AM33 processor. this is the default.

- **`-mam33-2`** — generate code using features specific to the AM33/2.0 processor.

- **`-mam34`** — generate code using features specific to the AM34 processor.

- **`-mtune=cpu-type`** — use the timing characteristics of the indicated CPU type when scheduling instructions. this does *not* change the targeted processor type. the CPU type has to be one of `mn10300`, `am33`, `am33-2` or `am34`.

- **`-mreturn-pointer-on-d0`** — when generating a function that returns a pointer, return the pointer in both `a0` and `d0`. otherwise the pointer comes back only in `a0`, and attempts to call such functions without a prototype result in errors. note that this option is <green>on by default</green>; use `-mno-return-pointer-on-d0` to disable it.

- **`-mno-crt0`** — do not link in the C run-time initialization object file.

- **`-mrelax`** — tell the linker it should do a relaxation optimization pass to shorten branches, calls and absolute memory addresses. this option only does anything when it's used on the command line for the final link step.

> ⚠️ this option makes symbolic debugging <red>impossible</red>. like, gone. 💀

- **`-mliw`** — allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. this is the default. this option defines the preprocessor macro `__LIW__`.

- **`-mno-liw`** — do not allow the compiler to generate Long Instruction Word instructions. this option defines the preprocessor macro `__NO_LIW__`.

- **`-msetlb`** — allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. this is the default. this option defines the preprocessor macro `__SETLB__`.

- **`-mno-setlb`** — do not allow the compiler to generate SETLB or Lcc instructions. this option defines the preprocessor macro `__NO_SETLB__`.

---

## 🧚 Moxie Options 🧚

- **`-meb`** — generate big-endian code. this is the default for `moxie-*-*` configurations.

- **`-mel`** — generate little-endian code.

- **`-mmul.x`** — generate `mul.x` and `umul.x` instructions. this is the default for `moxiebox-*-*` configurations.

- **`-mno-crt0`** — do not link in the C run-time initialization object file.

---

## 💜 MSP430 Options 💜

these options are defined for the MSP430:

- **`-masm-hex`** — force assembly output to always use hex constants. normally such constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. aesthetic. purposes. we love a practical queen ✨

- **`-mmcu=`** — select the MCU to target. this is 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 then used by the `msp430.h` header file to select an MCU-specific supplementary header file.

the option also sets the ISA to use. if the MCU name is one that's known to only support the 430 ISA, then that one's selected; otherwise the 430X ISA gets 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 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 search for a script called `xxx.ld`.

the ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. *but* 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 precedence order, where the first method takes precedence over the second, which takes precedence over the third.

- *Include path specified with `-I` and `-L`* — `devices.csv` will 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`* — define the value of the global environment variable `MSP430_GCC_INCLUDE_DIR` to the full path to the directory containing `devices.csv`, and GCC will search this directory for `devices.csv`. if `devices.csv` is found, this directory will also be registered as an include path and linker library path. so header files and linker scripts in this directory can be used without manually specifying `-I` and `-L` on the command line. <rainbow>so convenient, obsessed</rainbow>

- *The `msp430-elf{,bare}/include/devices` directory* — finally, GCC will examine `msp430-elf{,bare}/include/devices` from the toolchain root directory. this directory does not exist in a default installation, but if the user has created it and copied `devices.csv` there, then the MCU data will be read. as above, this directory will also be 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 used.

- **`-mwarn-mcu`**, **`-mno-warn-mcu`** — this option enables or disables warnings about conflicts between the MCU name specified by the `-mmcu` option and the ISA set by the `-mcpu` option and/or the hardware multiply support set by the `-mhwmult` option. it also toggles warnings about unrecognized MCU names. this option is <green>on by default</green>.

- **`-mcpu=`** — specifies the ISA to use. accepted values are `msp430`, `msp430x` and `msp430xv2`. > this option is <red>deprecated</red>. the `-mmcu=` option should be used to select the ISA instead, bestie.

- **`-msim`** — link to the simulator runtime libraries and linker script. overrides any scripts that would be selected by the `-mmcu=` option.

- **`-mlarge`** — use large-model addressing (20-bit pointers, 20-bit `size_t`).

- **`-msmall`** — use small-model addressing (16-bit pointers, 16-bit `size_t`).

- **`-mrelax`** — this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. 🦋

- **`mhwmult=`** — <pink>okay so this one tells GCC what kind of hardware multiply your target actually supports</pink> 💅 Accepted values are `none` (no hardware multiply at all), `16bit` (the original 16-bit-only multiply that early MCUs had), `32bit` (the 16/32-bit multiply later MCUs support), and `f5series` (the 16/32-bit multiply that F5-series MCUs support). You can also pass `auto`, which tells GCC to deduce the hardware multiply support based on the MCU name you gave with `-mmcu`. If you don't specify `-mmcu` or the MCU name isn't recognized, then no hardware multiply support is assumed. `"auto"` is the default setting, bestie. ✨

Hardware multiplies are normally done by calling a library routine. That saves space in the generated code. But when you compile at `-O3` or higher, the hardware multiplier is invoked inline. That makes for bigger, but faster code. 🔥

The hardware multiply routines disable interrupts while they run and restore the previous interrupt state when they finish. So they're safe to use inside interrupt handlers as well as in normal code. No cap. 🫶

- **`-minrt`** — <mint>Enable the use of a minimum runtime environment</mint> — like, no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🎀

- **`-mtiny-printf`** — <lavender>Enable reduced code size `printf` and `puts` library functions</lavender>. The tiny implementations of these functions are **not reentrant**, so you must use them with caution in multi-threaded applications. 😭

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=`** — <cyan>This option takes an integer between 0 and 64 inclusive</cyan>, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an `mspabi` helper function is used instead. The default value is `4`. ✨

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=`** — <peach>These options tell the compiler where to place functions and data</peach> that do not have one of the `"lower"`, `"upper"`, `"either"` or `"section"` attributes. Possible values are `"lower"`, `"upper"`, `"either"` or `"any"`. The first three behave like the corresponding attribute. The fourth possible value — `"any"` — is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (`.text`, `.data`, etc) to the memory regions. 🌈

- **`-msilicon-errata=`** — <hotpink>This option passes on a request to assembler to enable the fixes for the named silicon errata</hotpink>. 💖

- **`-msilicon-errata-warn=`** — <purple>This option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied</purple>. 🦋

- **`-mwarn-devices-csv`**, **`-mno-warn-devices-csv`** — <yellow>Warn if `devices.csv` is not found or there are problem parsing it (default: on)</yellow>. 👑

---

## ✨ NDS32 Options ✨

<rainbow>These options are defined for NDS32 implementations:</rainbow> 💜

- **`-mbig-endian`** — Generate code in big-endian mode. 🌸
- **`-mlittle-endian`** — Generate code in little-endian mode. 🌸
- **`-mreduced-regs`** — Use reduced-set registers for register allocation. 🎀
- **`-mfull-regs`** — Use full-set registers for register allocation. 🎀
- **`-mcmov`** — Generate conditional move instructions. ✨
- **`-mno-cmov`** — Do not generate conditional move instructions. ✨
- **`-mext-perf`** — Generate performance extension instructions. 🔥
- **`-mno-ext-perf`** — Do not generate performance extension instructions. 🔥
- **`-mext-perf2`** — Generate performance extension 2 instructions. 🔥
- **`-mno-ext-perf2`** — Do not generate performance extension 2 instructions. 🔥
- **`-mext-string`** — Generate string extension instructions. 🧚
- **`-mno-ext-string`** — Do not generate string extension instructions. 🧚
- **`-mv3push`** — Generate v3 push25/pop25 instructions. 💅
- **`-mno-v3push`** — Do not generate v3 push25/pop25 instructions. 💅
- **`-m16-bit`** — Generate 16-bit instructions. 🦋
- **`-mno-16-bit`** — Do not generate 16-bit instructions. 🦋
- **`-misr-vector-size=num`** — Specify the size of each interrupt vector, which must be `4` or `16`. 💖
- **`-mcache-block-size=num`** — Specify the size of each cache block, which must be a power of `2` between `4` and `512`. 💖
- **`-march=arch`** — Specify the name of the target architecture. 🌈
- **`-mcmodel=code-model`** — Set the code model to one of:

- `small` — All the data and read-only data segments must be within `512KB` addressing space. The text segment must be within `16MB` addressing space. 🍓
- `medium` — The data segment must be within `512KB` while the read-only data segment can be within `4GB` addressing space. The text segment should be still within `16MB` addressing space. 🍓
- `large` — All the text and data segments can be within `4GB` addressing space. 🍓

- **`-mctor-dtor`** — Enable constructor/destructor feature. 🎀
- **`-mrelax`** — Guide linker to relax instructions. 💜

---

## 🌸 Nios II Options 🌸

<rainbow>These are the options defined for the Altera Nios II processor.</rainbow> 💖

- **`-G num`** — Put global and static objects less than or equal to `num` bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of `num` is `8`. ✨

- **`-mgpopt=option`**, **`-mgpopt`**, **`-mno-gpopt`** — Generate (do not generate) GP-relative accesses. The following option names are recognized:

- `none` — Do not generate GP-relative accesses. 🙈
- `local` — Generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a `"section"` attribute. 💅
- `global` — As for `local`, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same `-G` setting. 🫶
- `data` — Generate GP-relative accesses for all data objects in the program. If you use this option, the entire data and BSS segments of your program must fit in `64K` of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. 😭
- `all` — Generate GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program must fit in `64K` of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. 😭

`-mgpopt` is equivalent to `-mgpopt=local`, and `-mno-gpopt` is equivalent to `-mgpopt=none`.

The default is `-mgpopt` except when `-fpic` or `-fPIC` is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries. 🚫

You may need to specify `-mno-gpopt` explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. 💀

- **`-mgprel-sec=regexp`** — This option specifies additional section names that can be accessed via GP-relative addressing. It is most useful in conjunction with `"section"` attributes on variable declarations and a custom linker script. The `regexp` is a POSIX Extended Regular Expression. 🧚

This option does not affect the behavior of the `-G` option, and the specified sections are in addition to the standard `.sdata` and `.sbss` small-data sections that are recognized by `-mgpopt`. ✨

- **`-mr0rel-sec=regexp`** — This option specifies names of sections that can be accessed via a 16-bit offset from `r0`; that is, in the low `32K` or high `32K` of the 32-bit address space. It is most useful in conjunction with `"section"` attributes on variable declarations and a custom linker script. The `regexp` is a POSIX Extended Regular Expression. 🦋

In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory. 💅

- **`-mel`**, **`-meb`** — Generate little-endian (default) or big-endian (experimental) code, respectively. 🌈

- **`-march=arch`** — This specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: `r1`, `r2`. 🔥

The preprocessor macro `__nios2_arch__` is available to programs, with value `1` or `2`, indicating the targeted ISA level. ✨

- **`-mbypass-cache`**, **`-mno-bypass-cache`** — Force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache. 🎀

- **`-mno-cache-volatile`**, **`-mcache-volatile`** — Volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache. 🎀

- **`-mno-fast-sw-div`**, **`-mfast-sw-div`** — Do not use table-based fast divide for small numbers. The default is to use the fast divide at `-O3` and above. 💅

- **`-mno-hw-mul`**, **`-mhw-mul`**, **`-mno-hw-mulx`**, **`-mhw-mulx`**, **`-mno-hw-div`**, **`-mhw-div`** — Enable or disable emitting `mul`, `mulx` and `div` family of instructions by the compiler. The default is to emit `mul` and not emit `div` and `mulx`. 🔥

- **`-mbmx`**, **`-mno-bmx`**, **`-mcdx`**, **`-mno-cdx`** — Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Enabling these instructions also requires `-march=r2`. Since these instructions are optional extensions to the R2 architecture, the default is not to emit them. 💅

- **`-mcustom-insn=N`**
- **`-mno-custom-insn`**

okay so bestie, each `-mcustom-insn=N` option lets you turn on a <pink>custom instruction</pink> with encoding `N` whenever GCC is generating code that uses `insn` ✨. Like, for example, `-mcustom-fadds=253` makes it emit custom instruction `253` for single-precision floating-point add operations instead of doing the default thing, which is... calling a library function 💀. Yeah. A whole function call. For an add. Not slay.

Here are all the values of `insn` that are supported. Unless it says otherwise, floating-point operations are assumed to be implemented with normal IEEE 754 semantics and map directly onto the C operators or the equivalent GCC built-in functions 🫶.

**Single-precision floating point:**

- `fadds`, `fsubs`, `fdivs`, `fmuls` — binary arithmetic operations. the classics 💅
- `fnegs` — unary negation.
- `fabss` — unary absolute value.
- `fcmpeqs`, `fcmpges`, `fcmpgts`, `fcmples`, `fcmplts`, `fcmpnes` — comparison operations.
- `fmins`, `fmaxs` — floating-point minimum and maximum. These instructions are **only** generated if `-ffinite-math-only` is specified 👑.
- `fsqrts` — unary square root operation.
- `fcoss`, `fsins`, `ftans`, `fatans`, `fexps`, `flogs` — floating-point trigonometric and exponential functions. These instructions are **only** generated if `-funsafe-math-optimizations` is also specified 🌸.

**Double-precision floating point:**

- `faddd`, `fsubd`, `fdivd`, `fmuld` — binary arithmetic operations.
- `fnegd` — unary negation.
- `fabsd` — unary absolute value.
- `fcmpeqd`, `fcmpged`, `fcmpgtd`, `fcmpled`, `fcmpltd`, `fcmpned` — comparison operations.
- `fmind`, `fmaxd` — double-precision minimum and maximum. These instructions are **only** generated if `-ffinite-math-only` is specified.
- `fsqrtd` — unary square root operation.
- `fcosd`, `fsind`, `ftand`, `fatand`, `fexpd`, `flogd` — double-precision trigonometric and exponential functions. These instructions are **only** generated if `-funsafe-math-optimizations` is also specified.

**Conversions:**

- `fextsd` — conversion from single precision to double precision.
- `ftruncds` — conversion from double precision to single precision.
- `fixsi`, `fixsu`, `fixdi`, `fixdu` — conversion from floating point to signed or unsigned integer types, with truncation towards zero.
- `round` — conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the `"__builtin_lroundf"` function when `-fno-math-errno` is used.
- `floatis`, `floatus`, `floatid`, `floatud` — conversion from signed or unsigned integer types to floating-point types.

Plus, listen, <purple>all</purple> of the following transfer instructions for internal registers `X` and `Y` must be provided in order to use **any** of the double-precision floating-point instructions 😤. Custom instructions that take two double-precision source operands expect the first operand in the 64-bit register `X`. The other operand (or the only operand, for a unary operation) is handed to the custom arithmetic instruction with the least significant half in source register `src1` and the most significant half in `src2`. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register `Y`. GCC automatically generates the necessary code sequences to write register `X` and/or read register `Y` when double-precision floating-point instructions are used 🧚.

- `fwrx` — write `src1` into the least significant half of `X` and `src2` into the most significant half of `X`.
- `fwry` — write `src1` into `Y`.
- `frdxhi`, `frdxlo` — read the most or least (respectively) significant half of `X` and store it in `dest`.
- `frdy` — read the value of `Y` and store it into `dest`.

> lowkey important: you can gain more local control over generation of Nios II custom instructions by using the `"target("custom-insn=N")"` and `"target("no-custom-insn")"` function attributes or pragmas 💖

- **`-mcustom-fpu-cfg=name`**

This one turns on a predefined, named set of custom instruction encodings (peep `-mcustom-insn` above 👆). Right now, these are the sets that are defined:

`-mcustom-fpu-cfg=60-1` is equivalent to: `-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant`

`-mcustom-fpu-cfg=60-2` is equivalent to: `-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant`

`-mcustom-fpu-cfg=72-3` is equivalent to: `-mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant`

`-mcustom-fpu-cfg=fph2` is equivalent to: `-mcustom-fabss=224 -mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227 -mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230 -mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233 -mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250 -mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255`

Custom instruction assignments given by individual `-mcustom-insn=` options **override** those given by `-mcustom-fpu-cfg=`, no matter what order the options appear in on the command line. periodt 💅

> and again, 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. Just so you know: 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 unlike 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 together with `-msys-crt0=` to specify where the alternate startup code provided by the HAL BSP lives.
- **`-msmallc`** — link with a limited version of the C library, `-lsmallc`, instead of 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 the 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 the 64-bit ABI is supported 🫠.
- **`-misa=ISA-string`** — generate code for the specified PTX ISA (e.g. `sm_35`). ISA strings must be lower-case. Valid ISA strings include `sm_30` and `sm_35`. The default ISA is `sm_35`.
- **`-mptx=version-string`** — generate code for the specified PTX version (e.g. `7.0`). Valid version strings include `3.1`, `6.0`, `6.3`, and `7.0`. The default PTX version is `6.0`, unless a higher minimal version is required for the specified PTX ISA via option `-misa=`.
- **`-mmainkernel`** — link in code for a `__main` kernel. This is for stand-alone instead of offloading execution.
- **`-moptimize`** — apply partitioned execution optimizations. This is the default when any level of optimization is selected.
- **`-msoft-stack`** — generate code that does not use `".local"` memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or `"alloca"`), and when global memory is used for underlying storage, it 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 a code generation variant that allows executing all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (`malloc`, `free`, `vprintf`) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array `"int __nvptx_uni[]"` stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position `"tid.y"` with current lane index to compute the master lane index.
- **`-mgomp`** — generate code for use in OpenMP offloading: enables `-msoft-stack` and `-muniform-simt` options, and selects the 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`** — this option is ignored; it is for compatibility purposes only. It used to select linker and preprocessor options for use with newlib.
- **`-msoft-div`**
- **`-mhard-div`**

Select software or hardware divide (`"l.div"`, `"l.divu"`) instructions. This default is hardware divide.

- **`-msoft-mul`**
- **`-mhard-mul`**

Select software or hardware multiply (`"l.mul"`, `"l.muli"`) instructions. This default is hardware multiply.

- **`-msoft-float`**
- **`-mhard-float`**

Select software or hardware for floating point operations. The default is software.

- **`-mdouble-float`** — when `-mhard-float` is selected, enables generation of double-precision floating point instructions. By default functions from libgcc are used to perform double-precision floating point operations 💜.

- **`-munordered-float`** — when `-mhard-float` is selected, this turns on generation of unordered floating point compare and set flag (`lf.sfun*`) instructions. By default? functions from `libgcc` get used to do the unordered floating point compare and set flag ops. 💫

- **`-mcmov`** — enables generation of conditional move (`l.cmov`) instructions. Default behaviour is that the equivalent gets generated using set and branch instead.

- **`-mror`** — enables generation of rotate right (`l.ror`) instructions. By default, functions from `libgcc` do the rotate right work for you.

- **`-mrori`** — enables generation of rotate right with immediate (`l.rori`) instructions. Default is `libgcc` functions handling rotate right with immediate operations.

- **`-msext`** — enables generation of sign extension (`l.ext*`) instructions. By default, memory loads are what perform sign extension.

- **`-msfimm`** — enables generation of compare and set flag with immediate (`l.sf*i`) instructions. Default: extra instructions get generated to store the immediate into a register first.

- **`-mshftimm`** — enables generation of shift with immediate (`l.srai`, `l.srli`, `l.slli`) instructions. Default: extra instructions get generated to stash the immediate in a register first.

- **`-mcmodel=small`** — generate OpenRISC code for the small model: the GOT is limited to 64k. <pink>This is the default model.</pink>

- **`-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 <red>not supported</red>.) Implies `-m45`.

- **`-msoft-float`** — do not use hardware floating point.

- **`-mac0`** — return floating-point results in ac0 (fr0 in Unix assembler syntax).

- **`-mno-ac0`** — return floating-point results in memory. This is the default.

- **`-m40`** — generate code for a PDP-11/40. Implies `-msoft-float -mno-split`.

- **`-m45`** — generate code for a PDP-11/45. This is the default.

- **`-m10`** — generate code for a PDP-11/10. Implies `-msoft-float -mno-split`.

- **`-mint16`**, **`-mno-int32`** — use 16-bit `int`. This is the default.

- **`-mint32`**, **`-mno-int16`** — use 32-bit `int`.

- **`-msplit`** — target has split instruction and data space. Implies `-m45`.

- **`-munix-asm`** — use Unix assembler syntax.

- **`-mdec-asm`** — use DEC assembler syntax.

- **`-mgnu-asm`** — use GNU assembler syntax. This is the default.

- **`-mlra`** — use the new LRA register allocator. By default, the old `reload` allocator is used.

---

# 🧚 picoChip Options 🧚

These `-m` options are defined for picoChip implementations:

- **`-mae=ae_type`** — set the instruction set, register set, and instruction scheduling parameters for array element type *ae_type*. Supported values for *ae_type* are `ANY`, `MUL`, and `MAC`.

- `-mae=ANY` selects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.

- `-mae=MUL` selects a MUL AE type. This is the most useful AE type for compiled code, and is <pink>the default</pink>.

- `-mae=MAC` selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores. 😭

- **`-msymbol-as-address`** — enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Typically, using this option generates larger programs, which run faster than when the option isn't used. But ngl the results vary from program to program, so it's left as a user option rather than being permanently enabled.

- **`-mno-inefficient-warnings`** — disables warnings about the generation of inefficient code. These warnings can pop up, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. That's inefficient and a warning gets generated to tell you to rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables those warnings. 🔇

---

# 👑 PowerPC Options 👑

These are listed under

---

# 🦋 PRU Options 🦋

These command-line options are defined for PRU target:

- **`-minrt`** — link with a minimum runtime environment, with no support for static initializers and constructors. Using this option can significantly reduce the size of the final ELF binary. <red>Beware</red> 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.

- **`-mmcu=mcu`** — specify the PRU MCU variant to use. Check Newlib for the exact list of supported MCUs.

- **`-mno-relax`** — make GCC pass the `--no-relax` command-line option to the linker instead of the `--relax` option.

- **`-mloop`** — allow (or do not allow) GCC to use the LOOP instruction.

- **`-mabi=variant`** — specify the ABI variant to output code for. `-mabi=ti` selects the unmodified TI ABI while `-mabi=gnu` selects 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=lp64d` means that long and pointers are 64-bit (implicitly defining `int` to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with `-march=rv64ifd -mabi=lp64f`, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or `-march=rv64ifd -mabi=lp64`, in which no floating-point arguments will be passed in registers.

The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: `ilp32`, `ilp32f`, `ilp32d`, `lp64`, `lp64f`, and `lp64d`. Some calling conventions are impossible to implement on some ISAs: for example, `-march=rv32if -mabi=ilp32d` is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There is also the `ilp32e` ABI that can only be used with the `rv32e` architecture. This ABI is not well specified at present, and is subject to change. 🙈

- **`-mfdiv`**, **`-mno-fdiv`** — do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.

- **`-mdiv`**, **`-mno-div`** — do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.

- **`-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 include `rv64i`, `rv32g`, `rv32e`, and `rv32imaf`.

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, users who want a specific architecture extensions should specify one explicitly.

- **`-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`, and `sifive-u74`.

- **`-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`, `size`, 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 <rainbow>good performance</rainbow>.

- **`-mpreferred-stack-boundary=num`** — Attempt to keep the stack boundary aligned to a <pink>2 raised to `num` byte boundary</pink>. If `-mpreferred-stack-boundary` is not specified, the default is `4` (16 bytes or 128-bits).

> Warning: If you use this switch, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. <red>Do not skip this</red> 💀.

- **`-msmall-data-limit=n`** — Put global and static data smaller than `n` bytes into a special section (on some targets). <lavender>cute</lavender>.

- **`-msave-restore`**, **`-mno-save-restore`** — Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use <mint>fast inline prologues and epilogues</mint>.

- **`-mshorten-memrefs`**, **`-mno-shorten-memrefs`** — Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of `'base register + large offset'` with a new load/store of `'new base + small offset'`. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets <cyan>32-bit integer load/stores only</cyan>.

- **`-mstrict-align`**, **`-mno-strict-align`** — Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.

- **`-mcmodel=medlow`** — Generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the <yellow>default code model</yellow>. Periodt.

- **`-mcmodel=medany`** — Generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.

The code generated by the medium-any code model is position-independent, but is <red>not guaranteed to function correctly</red> when linked into position-independent executables or libraries.

- **`-mexplicit-relocs`**, **`-mno-exlicit-relocs`** — Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization.

- **`-mrelax`**, **`-mno-relax`** — Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to <green>take advantage of linker relaxations</green>.

- **`-memit-attribute`**, **`-mno-emit-attribute`** — Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least <purple>binutils 2.32</purple>.

- **`-malign-data=type`** — Control how GCC aligns variables and constants of array, structure, or union types. Supported values for `type` are `xlen` which uses x register width as the alignment value, and `natural` which uses natural alignment. `xlen` is the default.

- **`-mbig-endian`** — Generate big-endian code. This is the default when GCC is configured for a `riscv64be-*-*` or `riscv32be-*-*` target.

- **`-mlittle-endian`** — Generate little-endian code. This is the default when GCC is configured for a `riscv64-*-*` or `riscv32-*-*` but not a `riscv64be-*-*` or `riscv32be-*-*` target.

- **`-mstack-protector-guard=guard`**, **`-mstack-protector-guard-reg=reg`**, **`-mstack-protector-guard-offset=offset`** — Generate stack protection code using canary at `guard`. Supported locations are `global` for a global canary or `tls` for per-thread canary in the TLS block.

With the latter choice the options `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is <hotpink>no default register or offset</hotpink> as this is entirely for use within the Linux kernel. Kernel girlies, this one's for you 🐧.

---

## ✨ RL78 Options ✨

- **`-msim`** — Links in additional target libraries to support operation within a simulator.

- **`-mmul=none`**, **`-mmul=g10`**, **`-mmul=g13`**, **`-mmul=g14`**, **`-mmul=rl78`** — Specifies the type of hardware multiplication and division support to be used. The simplest is `"none"`, which uses software for both multiplication and division. This is the default. The `"g13"` value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The `"g14"` value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value `"rl78"` is an alias for `"g14"` and the value `"mg10"` is an alias for `"none"`.

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: `"__RL78_MUL_NONE__"`, `"__RL78_MUL_G13__"` or `"__RL78_MUL_G14__"`.

- **`-mcpu=g10`**, **`-mcpu=g13`**, **`-mcpu=g14`**, **`-mcpu=rl78`** — Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.

If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit `-mmul=none` option on the command line. Thus specifying `-mcpu=g13` enables the use of the G13 hardware multiply peripheral and specifying `-mcpu=g10` disables the use of hardware multiplications altogether.

Note, although the RL78/G14 core is the default target, specifying `-mcpu=g14` or `-mcpu=rl78` on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support.

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: `"__RL78_G10__"`, `"__RL78_G13__"` or `"__RL78_G14__"`.

- **`-mg10`**, **`-mg13`**, **`-mg14`**, **`-mrl78`** — These are aliases for the corresponding `-mcpu=` option. They are provided for backwards compatibility.

- **`-mallregs`** — Allow the compiler to use all of the available registers. By default registers `"r24..r31"` are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well.

- **`-m64bit-doubles`**, **`-m32bit-doubles`** — Make the `"double"` data type be 64 bits (`-m64bit-doubles`) or 32 bits (`-m32bit-doubles`) in size. The default is `-m32bit-doubles`.

- **`-msave-mduc-in-interrupts`**, **`-mno-save-mduc-in-interrupts`** — Specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option `-mg13` needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function.

---

## 💖 IBM RS/6000 and PowerPC Options 💖

These `-m` options are defined for the IBM RS/6000 and PowerPC:

- **`-mpowerpc-gpopt`**, **`-mno-powerpc-gpopt`**, **`-mpowerpc-gfxopt`**, **`-mno-powerpc-gfxopt`**, **`-mpowerpc64`**, **`-mno-powerpc64`**, **`-mmfcrf`**, **`-mno-mfcrf`**, **`-mpopcntb`**, **`-mno-popcntb`**, **`-mpopcntd`**, **`-mno-popcntd`**, **`-mfprnd`**, **`-mno-fprnd`**, **`-mcmpb`**, **`-mno-cmpb`**, **`-mhard-dfp`**, **`-mno-hard-dfp`** — You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the `-mcpu=cpu_type` overrides the specification of these options. We recommend you use the `-mcpu=cpu_type` option rather than the options listed above.

Specifying `-mpowerpc-gpopt` allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt` allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

The `-mmfcrf` option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The `-mpopcntb` option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The `-mpopcntd` option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The `-mfprnd` option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The `-mcmpb` option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The `-mhard-dfp` option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.

The `-mpowerpc64` option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to `-mno-powerpc64`.

- **`-mcpu=cpu_type`** — okay so this one sets the architecture type, register usage, AND instruction scheduling parameters for the machine type you pass as *cpu_type*. Bestie, the supported values are *a lot*:

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

  <pink>`-mcpu=powerpc`</pink>, <pink>`-mcpu=powerpc64`</pink>, and <pink>`-mcpu=powerpc64le`</pink> are the vibe-check options: they specify pure 32-bit PowerPC (either endian 💫), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types respectively — with an appropriate, generic processor model assumed for scheduling purposes.

  Specifying `native` as the cpu type detects and selects the architecture option that matches the host processor of the system doing the compiling. <hotpink>But heads up</hotpink> — `-mcpu=native` has <red>no effect</red> 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 <red>not run at all</red> on others. No cap.

  The `-mcpu` options automatically enable or disable the following options:

-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion -mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protect

  The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — it doesn't necessarily reflect the actual hardware's capabilities. So don't come for me. 😭 If you wish to set an individual option to a particular value, you may specify it <purple>after</purple> the `-mcpu` option, like `-mcpu=970 -mno-altivec`.

  On AIX, the `-maltivec` and `-mpowerpc64` options are <yellow>not</yellow> 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`** — sets the instruction scheduling parameters for machine type *cpu_type*, but does <red>not</red> 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`. It's giving teamwork. 🤝

- **`-mcmodel=small`** — generates PowerPC64 code for the small model: the TOC is limited to 64k.

- **`-mcmodel=medium`** — generates 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 <green>default for 64-bit Linux</green>. ✨

- **`-mcmodel=large`** — generates 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. She's got range. 💅

- **`-maltivec`**, **`-mno-altivec`** — generate code that uses (does not use) AltiVec instructions, and <purple>also</purple> 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 <mint>leftmost</mint> element in a vector register when targeting a big-endian platform, and identifies the <peach>rightmost</peach> element in a vector register when targeting a little-endian platform. 🦋

- **`-mvrsave`**, **`-mno-vrsave`** — generate VRSAVE instructions when generating AltiVec code.

- **`-msecure-plt`** — generate code that allows `ld` and `ld.so` to build executables and shared libraries with non-executable `.plt` and `.got` sections. This is a PowerPC 32-bit SYSV ABI option. 🔒

- **`-mbss-plt`** — generate code that uses a BSS `.plt` section that `ld.so` fills in, and requires `.plt` and `.got` sections that are both writable <red>and</red> executable. This is a PowerPC 32-bit SYSV ABI option. Lowkey spicy but okay.

- **`-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, "does not keeps" — that's literally the manpage, don't @ me. 💀)

- **`-mpower8-vector`**, **`-mno-power8-vector`** — generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector instructions.

- **`-mquad-memory`**, **`-mno-quad-memory`** — generate code that uses (does not use) the non-atomic quad word memory instructions. The `-mquad-memory` option <yellow>requires use of 64-bit mode</yellow>.

- **`-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 <yellow>requires use of 64-bit mode</yellow>. Same energy, atomic edition. ⚛️

- **`-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`) <red>must</red> 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 (`-mpower9-vector` or `-mcpu=power9`) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation. 🧚

- **`-mfloat128-hardware`**, **`-mno-float128-hardware`** — enable/disable using ISA 3.0 hardware instructions to support the `__float128` data type.

  The default for `-mfloat128-hardware` is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.

- **`-m32`**, **`-m64`** — generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets `int`, `long` and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits, and generates code for PowerPC64, as for `-mpowerpc64`. 🌈

- **`-mfull-toc`**, **`-mno-fp-in-toc`**, **`-mno-sum-in-toc`**, **`-mminimal-toc`** — modify generation of the TOC (Table Of Contents), which is created for every executable file. The `-mfull-toc` option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. <hotpink>However, only 16,384 entries are available in the TOC.</hotpink> 😭

  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 <italic>still</italic> 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 <purple>one</purple> 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. It's giving "last resort but make it cute". 💅

- **`-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`. <cyan>GCC defaults to `-maix32`.</cyan>

- **`-mxl-compat`**, **`-mno-xl-compat`** — produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to `double`. Use XL symbol names for long double support routines.

  The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is <yellow>not enabled by default</yellow> and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. Bestie if you don't need it, leave it alone. 🙈

- **`-mpe`** — bestie this one's for the IBM RS/6000 SP Parallel Environment (PE). It links an app written to use message passing with special startup code so it can actually run. <red>Heads up tho:</red> your system needs PE installed in the standard spot (`/usr/lpp/ppe.poe/`), or the specs file has to be overridden with the `-specs=` option to point at the right directory location. And PE does <hotpink>not</hotpink> support threads, so `-mpe` and `-pthread` are literally incompatible, no cap 💅

- **`-malign-natural`**, **`-malign-power`** — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, `-malign-natural` overrides the ABI-defined alignment of larger types (like floating-point doubles) onto their natural size-based boundary. `-malign-power` tells GCC to follow the ABI-specified alignment rules instead. GCC defaults to the standard alignment defined in the ABI. <purple>Plot twist:</purple> on 64-bit Darwin, natural alignment *is* the default, and `-malign-power` is not supported there. 🌸

- **`-msoft-float`**, **`-mhard-float`** — generate code that does not use (or does use) the floating-point register set. Software floating-point emulation is provided if you use `-msoft-float`, and you gotta pass the option to GCC when linking too.

- **`-mmultiple`**, **`-mno-multiple`** — generate code that uses (or 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. <red>Do not</red> use `-mmultiple` on little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode.

- **`-mupdate`**, **`-mno-update`** — generate code that uses (or does not use) the load or store instructions that update the base register to the address of the calculated memory location. These are generated by default. 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. 😭

- **`-mavoid-indexed-addresses`**, **`-mno-avoid-indexed-addresses`** — generate code that tries to avoid (or not avoid) the use of indexed load or store instructions. These can incur a performance penalty on Power6 processors in certain situations, like when stepping through large arrays that cross a 16M boundary. Enabled by default when targeting Power6 and disabled otherwise.

- **`-mfused-madd`**, **`-mno-fused-madd`** — generate code that uses (or does not use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is used. <cyan>Lowkey an identity crisis moment:</cyan> the machine-dependent `-mfused-madd` option is now mapped to the machine-independent `-ffp-contract=fast` option, and `-mno-fused-madd` is mapped to `-ffp-contract=off`.

- **`-mmulhw`**, **`-mno-mulhw`** — generate code that uses (or does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These get generated by default when targeting those processors.

- **`-mdlmzb`**, **`-mno-dlmzb`** — generate code that uses (or does not use) the string-search `dlmzb` instruction on the IBM 405, 440, 464 and 476 processors. This instruction gets generated by default when targeting those processors.

- **`-mno-bit-align`**, **`-mbit-align`** — on System V.4 and embedded PowerPC systems, do not (or do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. For example, by default a structure containing nothing but 8 `unsigned` bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using `-mno-bit-align`, the structure is aligned to a 1-byte boundary and is 1 byte in size. 🎀

- **`-mno-strict-align`**, **`-mstrict-align`** — on System V.4 and embedded PowerPC systems, do not (or do) assume that unaligned memory references are handled by the system.

- **`-mrelocatable`**, **`-mno-relocatable`** — generate code that allows (or does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of `.got2` and 4-byte locations listed in the `.fixup` section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with `-mrelocatable` or `-mrelocatable-lib`. `-mrelocatable` code aligns the stack to an 8-byte boundary.

- **`-mrelocatable-lib`**, **`-mno-relocatable-lib`** — like `-mrelocatable`, `-mrelocatable-lib` generates a `.fixup` section to allow static executables to be relocated at run time, but `-mrelocatable-lib` does not use the smaller stack alignment of `-mrelocatable`. Objects compiled with `-mrelocatable-lib` may be linked with objects compiled with any combination of the `-mrelocatable` options. 🫶

- **`-mno-toc`**, **`-mtoc`** — on System V.4 and embedded PowerPC systems, do not (or do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

- **`-mlittle`**, **`-mlittle-endian`** — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The `-mlittle-endian` option is the same as `-mlittle`.

- **`-mbig`**, **`-mbig-endian`** — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The `-mbig-endian` option is the same as `-mbig`.

- **`-mdynamic-no-pic`** — on Darwin and Mac OS X systems, compile code so that it is not relocatable, but its external references are relocatable. The resulting code is suitable for applications, but not shared libraries.

- **`-msingle-pic-base`** — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.

- **`-mprioritize-restricted-insns=priority`** — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument *priority* takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. <yellow>Pick your fighter ✨</yellow>

- **`-msched-costly-dep=dependence_type`** — this option controls which dependences are considered costly by the target during instruction scheduling. The argument *dependence_type* takes one of the following values:

  - `no` — no dependence is costly.
  - `all` — all dependences are costly.
  - `true_store_to_load` — a true dependence from store to load is costly.
  - `store_to_load` — any dependence from store to load is costly.
  - *number* — any dependence for which the latency is greater than or equal to *number* is costly.

- **`-minsert-sched-nops=scheme`** — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument *scheme* takes one of the following values:

  - `no` — don't insert NOPs.
  - `pad` — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  - `regroup_exact` — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
  - *number* — insert NOPs to force costly dependent insns into separate groups. Insert *number* NOPs to force an insn to a new group.

- **`-mcall-sysv`** — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using `powerpc-*-eabiaix`.

- **`-mcall-sysv-eabi`**, **`-mcall-eabi`** — specify both `-mcall-sysv` and `-meabi` options. <mint>Two-for-one, we love a combo deal</mint> 💖

- **`-mcall-sysv-noeabi`** — specify both `-mcall-sysv` and `-mno-eabi` options.

- **`-mcall-aixdesc`** — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.

- **`-mcall-linux`** — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system.

- **`-mcall-freebsd`** — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system.

- **`-mcall-netbsd`** — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system.

- **`-mcall-openbsd`** — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system. 🌈

- **`-mtraceback=traceback_type`** — select the type of traceback table. Valid values for *traceback_type* are `full`, `part`, and `no`.

- **`-maix-struct-return`** — return all structures in memory (as specified by the AIX ABI).

- **`-msvr4-struct-return`** — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).

- **`-mabi=abi-type`** — extend the current ABI with a particular extension, or remove such extension. Valid values are: `altivec`, `no-altivec`, `ibmlongdouble`, `ieeelongdouble`, `elfv1`, `elfv2`, and for AIX: `vec-extabi`, `vec-default`. 👑

- **`-mabi=ibmlongdouble`** — change the current ABI to use IBM extended-precision `long double`. This is not likely to work if your system defaults to using IEEE extended-precision `long double`. If you change the `long double` type from IEEE extended-precision, the compiler will issue a warning unless you use the `-Wno-psabi` option. Requires `-mlong-double-128` to be enabled.

- **`-mabi=ieeelongdouble`** — change the current ABI to use IEEE extended-precision `long double`. This is not likely to work if your system defaults to using IBM extended-precision `long double`. If you change the `long double` type from IBM extended-precision, the compiler will issue a warning unless you use the `-Wno-psabi` option. Requires `-mlong-double-128` to be enabled. 🔥

- **`-mabi=elfv1`** — change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. <red>Warning bestie:</red> overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀 Not "oops a lil bug" fail — like, *spectacular*. You've been warned.

- **`-mabi=elfv2`** — bestie, this switches the current ABI over to the ELFv2 ABI 💫. It's the default ABI for little-endian PowerPC 64-bit Linux, so if that's your setup you're already living the dream. Overriding the default ABI needs special system support and is <red>likely to fail in spectacular ways</red> 😭 so like… don't.

- **`-mgnu-attribute`**, **`-mno-gnu-attribute`** — turns on (or off) emitting `.gnu_attribute` assembly directives, which set tag/value pairs in a `.gnu.attributes` section that spell out ABI variations in function parameters or return values ✨.

- **`-mprototype`**, **`-mno-prototype`** — on System V.4 and embedded PowerPC systems, assume every call to a variable argument function is 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`), telling everyone whether floating-point values are being passed in the floating-point registers in case the function takes variable arguments 🙈. With `-mprototype`, only calls to prototyped variable argument functions bother setting or clearing that bit.

- **`-msim`** — on embedded PowerPC systems, assume the startup module is called `sim-crt0.o` and that the standard C libraries are `libsim.a` and `libc.a` 🫶. This is the default for `powerpc-*-eabisim` configurations.

- **`-mmvme`** — on embedded PowerPC systems, assume the startup module is called `crt0.o` and the standard C libraries are `libmvme.a` and `libc.a`.

- **`-mads`** — on embedded PowerPC systems, assume the startup module is called `crt0.o` and the standard C libraries are `libads.a` and `libc.a`.

- **`-myellowknife`** — on embedded PowerPC systems, assume the startup module is called `crt0.o` and the standard C libraries are `libyk.a` and `libc.a` 💛 (yellowknife the name, iconic the vibe).

- **`-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 signal that eabi extended relocations are in play.

- **`-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 whole set of modifications to the System V.4 specs 📜. Picking `-meabi` means:
  - the stack is aligned to an 8-byte boundary,
  - a function `__eabi` gets called from `main` to set up the EABI environment,
  - and the `-msdata` option can use both `r2` and `r13` to point to two separate small data areas.

  Picking `-mno-eabi` means:
  - the stack is aligned to a 16-byte boundary,
  - no EABI initialization function is called from `main`,
  - and `-msdata` only uses `r13` to point to a single small data area.

  The `-meabi` option is on by default if you configured GCC using one of the `powerpc*-*-eabi*` options.

- **`-msdata=eabi`** — on System V.4 and embedded PowerPC systems, drop small initialized `const` global and static data into the `.sdata2` section, which register `r2` points to. Small initialized non-`const` global and static data goes in `.sdata`, pointed to by register `r13`. Small uninitialized global and static data lands in `.sbss`, which sits adjacent to `.sdata`. Heads up: `-msdata=eabi` is <red>incompatible with `-mrelocatable`</red> 💀. It also sets `-memb` for you.

- **`-msdata=sysv`** — on System V.4 and embedded PowerPC systems, small global and static data goes in `.sdata`, pointed to by register `r13`, and small uninitialized global and static data goes in `.sbss`, adjacent to `.sdata`. Also <red>incompatible with `-mrelocatable`</red>, bestie.

- **`-msdata=default`**, **`-msdata`** — on System V.4 and embedded PowerPC systems: if `-meabi` is in use, compile code exactly like `-msdata=eabi`; otherwise compile it exactly like `-msdata=sysv` 🌈.

- **`-msdata=data`** — on System V.4 and embedded PowerPC systems, small global data goes in `.sdata`, small uninitialized global data goes in `.sbss`. But do **not** use register `r13` to address small data here. This is the default behavior unless some other `-msdata` option gets used.

- **`-msdata=none`**, **`-mno-sdata`** — on embedded PowerPC systems, put all initialized global and static data in `.data`, and all uninitialized data in `.bss` 🎀.

- **`-mreadonly-in-sdata`** — also put read-only objects in the `.sdata` section. This is the default, so enjoy 💅.

- **`-mblock-move-inline-limit=num`** — inline all block moves (think 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 to handle the remainder of the block. The default value is target-specific.

- **`-mstring-compare-inline-limit=num`** — compare at most *num* string bytes with inline code. If the difference or the end of the string isn't found by the end of that inline compare, a call to `strcmp` or `strncmp` takes care of the rest of the comparison. The default is 64 bytes.

- **`-G num`** — on embedded PowerPC systems, put global and static items less than or equal to *num* bytes into the small data or BSS sections instead of the normal data or BSS section. By default, *num* is 8. The `-G num` switch also gets passed to the linker, and <yellow>all modules should be compiled with the same `-G num` value</yellow> ⭐ — no mixing, bestie.

- **`-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 this assumes all calls are far away, so a longer and more expensive calling sequence is required 😮‍💨. That's needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. This setting can be overridden by the `shortcall` function attribute, or by `#pragma longcall(0)`.

  Some linkers can detect out-of-range calls and generate glue code on the fly 🧚. On those systems, long calls are unnecessary and just make 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 is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it 🦋.

  On Mach-O (Darwin) systems, this option directs the compiler to emit 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 don't implement) `-fno-plt` and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries 🌸. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and 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`. Warning bestie: `-mpltseq` code and `-mbss-plt` 32-bit PowerPC relocatable objects may <red>not</red> be linked together 💀.

- **`-mtls-markers`**, **`-mno-tls-markers`** — mark (or don't mark) calls to `__tls_get_addr` with a relocation specifying the function argument 🎯. That relocation lets the linker reliably associate a function call with argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better.

- **`-mrecip`**, **`-mno-recip`** — this enables use of the reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to boost precision, instead of doing a divide — or a square root and divide — for floating-point arguments 🧮. You should use `-ffast-math` when you use `-mrecip` (or at least `-funsafe-math-optimizations`, `-ffinite-math-only`, `-freciprocal-math` and `-fno-trapping-math`). Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp 😭 — like, the inverse of 1.0 equals 0.99999994 — for reciprocal square roots.

- **`-mrecip=opt`** — this controls which reciprocal estimate instructions may be used. *opt* is a comma-separated list of options, and any of them may 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 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` — turns on the single-precision reciprocal square root approximation instructions. 🧮
- `rsqrtd` — same energy but for the double-precision reciprocal square root approximation instructions.

So like, for example, `-mrecip=all,!rsqrtd` enables all of the reciprocal estimate instructions, <pink>except</pink> for the `"FRSQRTE"`, `"XSRSQRTEDP"`, and `"XVRSQRTEDP"` instructions, which are the ones handling the double-precision reciprocal square root calculations. 💅

- **`-mrecip-precision`**, **`-mno-recip-precision`** — assume (or do <hotpink>not</hotpink> assume) that the reciprocal estimate instructions give higher-precision estimates than the PowerPC ABI says they have to. Picking `-mcpu=power6`, `-mcpu=power7` or `-mcpu=power8` automatically selects `-mrecip-precision`. Heads up bestie: the double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't give an estimate that converges after three steps. 😭

- **`-mveclibabi=type`** — says which ABI type to use for vectorizing intrinsics with an external library. The only `type` supported right now is `mass`, which means use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics via external libraries. GCC currently emits calls to `"acosd2"`, `"acosf4"`, `"acoshd2"`, `"acoshf4"`, `"asind2"`, `"asinf4"`, `"asinhd2"`, `"asinhf4"`, `"atan2d2"`, `"atan2f4"`, `"atand2"`, `"atanf4"`, `"atanhd2"`, `"atanhf4"`, `"cbrtd2"`, `"cbrtf4"`, `"cosd2"`, `"cosf4"`, `"coshd2"`, `"coshf4"`, `"erfcd2"`, `"erfcf4"`, `"erfd2"`, `"erff4"`, `"exp2d2"`, `"exp2f4"`, `"expd2"`, `"expf4"`, `"expm1d2"`, `"expm1f4"`, `"hypotd2"`, `"hypotf4"`, `"lgammad2"`, `"lgammaf4"`, `"log10d2"`, `"log10f4"`, `"log1pd2"`, `"log1pf4"`, `"log2d2"`, `"log2f4"`, `"logd2"`, `"logf4"`, `"powd2"`, `"powf4"`, `"sind2"`, `"sinf4"`, `"sinhd2"`, `"sinhf4"`, `"sqrtd2"`, `"sqrtf4"`, `"tand2"`, `"tanf4"`, `"tanhd2"`, and `"tanhf4"` when generating code for `power7`. Both `-ftree-vectorize` and `-funsafe-math-optimizations` must also be enabled. And the MASS libraries must be specified at link time. 🫶

- **`-mfriz`**, **`-mno-friz`** — generate (or don't generate) the `"friz"` instruction when `-funsafe-math-optimizations` is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Big yikes moment: the `"friz"` instruction does <red>not</red> return the same value if the floating-point number is too large to fit in an integer. 💀

- **`-mpointers-to-nested-functions`**, **`-mno-pointers-to-nested-functions`** — generate (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 register `"r2"`, and the static chain value to be loaded in register `"r11"`. `-mpointers-to-nested-functions` is on by default. And fr fr, you <hotpink>cannot</hotpink> call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use `-mno-pointers-to-nested-functions`. 🚫

- **`-msave-toc-indirect`**, **`-mno-save-toc-indirect`** — generate (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 gets saved just before the call through the pointer. `-mno-save-toc-indirect` 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.

  Older versions of GCC (before 4.9.0) incorrectly did <em>not</em> align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That got corrected in more recent versions of GCC. This option can be used to generate code that's compatible with functions compiled with older versions of GCC.

  `-mno-compat-align-parm` 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`. Supported locations are `global` for a global canary, or `tls` for a per-thread canary in the TLS block (that's the default with GNU libc version 2.4 or later). 🐤

  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. The default for those is as specified in the relevant ABI. And `-mstack-protector-guard-symbol=symbol` overrides the offset with a symbol reference to a canary in the TLS block. 💅

- **`-mpcrel`**, **`-mno-pcrel`** — generate (or don't generate) pc-relative addressing. The `-mpcrel` option requires that the medium code model (`-mcmodel=medium`) and prefixed addressing (`-mprefixed`) options are enabled. 👑

- **`-mprefixed`**, **`-mno-prefixed`** — generate (or don't generate) addressing modes using prefixed load and store instructions. The `-mprefixed` option requires that `-mcpu=power10` (or later) is enabled. ⭐

- **`-mmma`**, **`-mno-mma`** — generate (or don't generate) the MMA instructions. The `-mma` option 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`). 🛡️

- **`-mprivileged`**, **`-mno-privileged`** — generate (or don't generate) code that will run in privileged state.

- **`-mblock-ops-unaligned-vsx`**, **`-mno-block-ops-unaligned-vsx`** — generate (or don't generate) unaligned vsx loads and stores for inline expansion of `"memcpy"` and `"memmove"`. 📦

---

## 🌈 RX Options 🌈

These command-line options are defined for RX targets, bestie:

- **`-m64bit-doubles`**, **`-m32bit-doubles`** — make the `"double"` data type be 64 bits (`-m64bit-doubles`) or 32 bits (`-m32bit-doubles`) in size. The default is `-m32bit-doubles`. And note: RX floating-point hardware only works on 32-bit values, which is literally why the default is `-m32bit-doubles`. 💁‍♀️

- **`-fpu`**, **`-nofpu`** — enables (`-fpu`) or disables (`-nofpu`) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

  Floating-point instructions are only generated for 32-bit floating-point values though, so the FPU hardware is not used for doubles if the `-m64bit-doubles` option is used.

  > Note: if `-fpu` is enabled then `-funsafe-math-optimizations` is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe. 😬 No cap.

- **`-mcpu=name`** — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600.

  The only difference between RX600 and RX610 is that the RX610 does not support the `"MVTIPL"` instruction.

  The RX200 series has no hardware floating-point unit and so `-nofpu` is enabled by default when this type is selected.

- **`-mbig-endian-data`**, **`-mlittle-endian-data`** — store data (but <em>not</em> code) in big-endian format. The default is `-mlittle-endian-data`, i.e. storing data in little-endian format. 🔄

- **`-msmall-data-limit=N`** — specifies the maximum size in bytes of global and static variables that 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's up to the programmer to make sure the area doesn't overflow. Also, when the small data area is used, one of the RX's registers (usually `"r13"`) is reserved for use pointing to this area, so it's no longer available for the compiler to use. This could result in slower and/or larger code if variables get pushed onto the stack instead of being held in this register. 😭

  > Note: common variables (variables that haven't been initialized) and constants are <hotpink>not</hotpink> placed into the small data area, since they're assigned to other sections in the output executable.

  The default value is zero, which disables this feature. And note, this feature is not enabled by default with higher optimization levels (`-O2` etc) because of the potentially detrimental effects of reserving a register. It's up to the programmer to experiment and discover whether this feature benefits their program. See the description of the `-mpid` option for how the actual register to hold the small data area pointer is chosen. 🧪

- **`-msim`**, **`-mno-sim`** — use the simulator runtime. The default is to use the libgloss board-specific runtime. 🖥️

- **`-mas100-syntax`**, **`-mno-as100-syntax`** — when generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it is not generated by default. 🙈

- **`-mmax-constant-size=N`** — specifies the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction. Even though the RX instruction set does allow constants up to 4 bytes long to be used in instructions, a longer value means a longer instruction. So in some circumstances it can be beneficial to restrict the size of constants used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection. 📏

  The value `N` can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed.

- **`-mrelax`** — enable linker relaxation. Linker relaxation is a process where the linker tries to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. 🎀

- **`-mint-register=N`** — specify the number of registers to reserve for fast interrupt handler functions. The value `N` can be between 0 and 4. A value of 1 means register `"r13"` is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves `"r13"` and `"r12"`. A value of 3 reserves `"r13"`, `"r12"` and `"r11"`, and a value of 4 reserves `"r13"` through `"r10"`. A value of 0, the default, doesn't reserve any registers. 👑

- **`-msave-acc-in-interrupts`** — bestie, this one says your interrupt handler functions should keep the accumulator register safe. You only need this if your normal code might actually use that accumulator, like when it's out here doing 64-bit multiplications. Default? It just ignores the accumulator entirely, because that makes the interrupt handlers *faster*. speed 💅

- **`-mpid`**, **`-mno-pid`** — turns on generating position independent data. When it's on, any access to constant data happens via an offset from a base address that lives in a register. Why do we care? Because it lets the location of constant data be figured out at *run time*, no relocating the executable required — which is giving major main-character energy for embedded apps with tight memory constraints. Data that can be modified? Not affected by this option at all, she's unbothered. ✨

  Heads up though: this feature reserves a register, usually `r13`, to hold that constant data base address. That can make your code slower and/or bigger, especially in complicated functions. 😭

  Which register actually gets picked for the constant data base address depends on whether `-msmall-data-limit` and/or `-mint-register` are enabled. Starting at register `r13` and walking *downwards*, registers get allocated first to satisfy `-mint-register`, then `-mpid`, and finally `-msmall-data-limit`. So yes, it's literally possible for the small data area register to be `r8` if you specify both `-mint-register=4` and `-mpid` on the command line. iconic.

  By default this feature is **not** enabled, and you can restore the default with the `-mno-pid` command-line option.

- **`-mno-warn-multiple-fast-interrupts`**, **`-mwarn-multiple-fast-interrupts`** — this stops GCC from throwing a warning message when it finds more than one fast interrupt handler while compiling a file. The default is to warn for *each extra* fast interrupt handler found, because the RX only supports one such interrupt. one only, she said no sharing 🫶

- **`-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. Here's the thing: these instructions may prefetch data, which is **not safe** if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info, iykyk.)

  Default is to allow these instructions, but GCC can't reliably detect every single circumstance where a string instruction might hit an I/O register, so their use can't be disabled automatically. Instead it's on *you*, the programmer, to use `-mno-allow-string-insns` if your program accesses I/O space.

  When the instructions are enabled, GCC defines the C preprocessor symbol `__RX_ALLOW_STRING_INSNS__`, otherwise it defines `__RX_DISALLOW_STRING_INSNS__`.

- **`-mjsr`**, **`-mno-jsr`** — use only (or not only) `JSR` instructions to access functions. This is your girl when code size exceeds the range of `BSR` instructions. Note that `-mno-jsr` does **not** mean "don't use `JSR`" — it means any type of branch may be used. 🦋

> Note: the generic GCC command-line option `-ffixed-reg` has special significance to the RX port when 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-reg` or `-mint-register` command-line options. no cap.

---

## 💜 S/390 and zSeries Options 💜

These are the `-m` options defined for the S/390 and zSeries architecture. let's gooo 🎀

- **`-mhard-float`**, **`-msoft-float`** — use (or don't use) the hardware floating-point instructions and registers for floating-point operations. When `-msoft-float` is specified, functions in `libgcc.a` are used to perform floating-point operations. When `-mhard-float` is specified, the compiler generates IEEE floating-point instructions. This is the default. ⭐

- **`-mhard-dfp`**, **`-mno-hard-dfp`** — use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. With `-mno-hard-dfp`, functions in `libgcc.a` handle the decimal-floating-point operations. With `-mhard-dfp`, the compiler generates decimal-floating-point hardware instructions. This is the default for `-march=z9-ec` or higher. ✨

- **`-mlong-double-64`**, **`-mlong-double-128`** — these switches control the size of the `long double` type. A size of 64 bits makes `long double` equivalent to the `double` type. This is the default. 🫶

- **`-mbackchain`**, **`-mno-backchain`** — store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. You might need a backchain to allow debugging with tools that don't understand DWARF call frame information. When `-mno-packed-stack` is in effect, the backchain pointer is stored at the *bottom* of the stack frame; when `-mpacked-stack` is in effect, the backchain goes into the topmost word of the 96/160 byte register save area. 💅

  Generally, code compiled with `-mbackchain` is call-compatible with code compiled with `-mno-backchain`; however, using the backchain for debugging usually requires the *whole* binary to be built with `-mbackchain`. Note that combining `-mbackchain`, `-mpacked-stack` and `-mhard-float` is **not supported**. To build a linux kernel, use `-msoft-float`. 👑

  The default is to not maintain the backchain.

- **`-mpacked-stack`**, **`-mno-packed-stack`** — use (or don't use) the packed stack layout. With `-mno-packed-stack`, 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. With `-mpacked-stack`, register save slots are densely packed at the top of the register save area, and unused space gets reused for other purposes — more efficient use of the available stack space, slay. BUT when `-mbackchain` is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 🌸

  As long as the stack frame backchain isn't used, code generated with `-mpacked-stack` is call-compatible with code generated with `-mno-packed-stack`. Careful though: 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. That code is **not** call-compatible with code compiled with `-mpacked-stack`. Also note that combining `-mbackchain`, `-mpacked-stack` and `-mhard-float` is **not supported**. 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 don't generate) code using the `bras` instruction for subroutine calls. This only works reliably if the total executable size doesn't exceed 64k. The default is to use the `basr` instruction instead, which doesn't have that limitation. 🔥

- **`-m64`**, **`-m31`** — with `-m31`, generate code compliant to the GNU/Linux for S/390 ABI. With `-m64`, generate code compliant to the GNU/Linux for zSeries ABI. This lets GCC in particular generate 64-bit instructions. For s390 targets the default is `-m31`, while s390x targets default to `-m64`. ⭐

- **`-mzarch`**, **`-mesa`** — with `-mzarch`, generate code using the instructions available on z/Architecture. With `-mesa`, generate code using the instructions available on ESA/390. Note that `-mesa` is **not possible** with `-m64`. When generating code compliant to the GNU/Linux for S/390 ABI, the default is `-mesa`. When generating code compliant to the GNU/Linux for zSeries ABI, the default is `-mzarch`. 🦋

- **`-mhtm`**, **`-mno-htm`** — `-mhtm` enables a set of builtins that make use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. `-mhtm` is enabled by default when using `-march=zEC12`. 💖

- **`-mvx`**, **`-mno-vx`** — with `-mvx`, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values regarding alignment and calling conventions. If vector type values are used in an ABI-relevant context, a GAS `.gnu_attribute` command gets added to mark the resulting binary with the ABI used. `-mvx` is enabled by default when using `-march=z13`. ✨

- **`-mzvector`**, **`-mno-zvector`** — `-mzvector` enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option adds support for `vector` to be used as a keyword defining vector type variables and arguments. `vector` is only available when GNU extensions are enabled — it will *not* be expanded when you request strict standard compliance, e.g. with `-std=c99`. On top of the GCC low-level builtins, `-mzvector` enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To use these builtins, the header file `vecintrin.h` needs to be included. `-mzvector` is disabled by default. 🎀

- **`-mmvcle`**, **`-mno-mvcle`** — generate (or don't generate) code using the `mvcle` instruction to perform block moves. With `-mno-mvcle`, use a `mvc` loop instead. This is the default unless optimizing for size. 💅

- **`-mdebug`**, **`-mno-debug`** — print (or don't print) additional debug information when compiling. The default is to not print debug information. 🙈

- **`-march=`*`cpu-type`* — generate code that runs on *`cpu-type`*, which is the name of a system representing a certain processor type. Possible values for *`cpu-type`* are `z900/arch5`, `z990/arch6`, `z9-109`, `z9-ec/arch7`, `z10/arch8`, `z196/arch9`, `zEC12`, `z13/arch11`, `z14/arch12`, `z15/arch13`, and `native`. 👑

  The default is `-march=z900`.

  Specifying `native` as the cpu type can be used to select the best architecture option for the host processor. `-march=native` has no effect if GCC doesn't recognize the processor. 🌈

- **`-mtune=cpu-type`** — ok so this tunes *everything applicable* about the generated code for `cpu-type`, EXCEPT the ABI and the set of available instructions. The list of `cpu-type` values is literally the same as for `-march`. Default? Whatever value you used for `-march`, bestie. ✨

- **`-mtpf-trace`**, **`-mno-tpf-trace`** — makes the code add (or *not* add) TPF OS specific branches to trace routines in the OS. It's <yellow>off by default</yellow>, even when you're compiling for the TPF OS. Periodt.

- **`-mtpf-trace-skip`**, **`-mno-tpf-trace-skip`** — generates code that changes (or doesn't change) the default branch targets that `-mtpf-trace` enabled, so they point at specialized trace routines that can selectively *skip* function trace entries for the TPF OS. Also <yellow>off by default</yellow> — even when you compile for TPF OS AND pass `-mtpf-trace`. She's shy fr fr.

- **`-mfused-madd`**, **`-mno-fused-madd`** — use (or don't use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is in play. 🔥

- **`-mwarn-framesize=framesize`** — emits a warning if the current function exceeds the given frame size. Since it's a compile-time check it doesn't have to be a real problem at runtime — it's just there to sniff out functions that most probably cause a stack overflow. Super useful in environments with limited stack size, e.g. the linux kernel. 🫶

- **`-mwarn-dynamicstack`** — emits a warning if the function calls `alloca` or uses dynamically-sized arrays. Which is generally a bad idea when your stack size is limited, ngl.

- **`-mstack-guard=stack-guard`**, **`-mstack-size=stack-size`** — if you give these options, the S/390 back end emits extra instructions in the function prologue that trigger a trap if the stack size is `stack-guard` bytes above the `stack-size` (remember: the stack on S/390 grows *downward*, bestie 📉). If you skip the `stack-guard` option, it picks the smallest power of 2 larger than the frame size of the compiled function. These are meant to help debug stack overflow problems. The extra code adds only a little overhead, so you can totally use it in production-like systems without major performance degradation. The values you give <red>have to be exact powers of 2</red>, and `stack-size` has to be greater than `stack-guard` without exceeding 64k. To be efficient, the extra code assumes the stack starts at an address aligned to the value given by `stack-size`. And `stack-guard` can only be used *in conjunction with* `stack-size`. No solo missions. 💅

- **`-mhotpatch=pre-halfwords,post-halfwords`** — if hotpatch is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The function label gets prepended with the given number of two-byte NOP instructions (`pre-halfwords`, max 1000000). After the label, `2 * post-halfwords` bytes are appended, using the largest NOP-like instructions the architecture allows (max 1000000). If both arguments are zero, hotpatching is disabled. And you can override this for individual functions with the `"hotpatch"` attribute. 🌈

---

# 🎯 Score Options 🎯

These options are defined for Score implementations, bestie:

- **`-meb`** — compile code for big-endian mode. This is the default. 👑

- **`-mel`** — compile code for little-endian mode.

- **`-mnhwloop`** — disable generation of `"bcnz"` instructions.

- **`-muls`** — enable generation of unaligned load and store instructions.

- **`-mmac`** — enable the use of multiply-accumulate instructions. Disabled by default.

- **`-mscore5`** — specify the SCORE5 as the target architecture.

- **`-mscore5u`** — specify the SCORE5U of the target architecture.

- **`-mscore7`** — specify the SCORE7 as the target architecture. This is the default. ⭐

- **`-mscore7d`** — specify the SCORE7D as the target architecture.

---

# 🌸 SH Options 🌸

These `-m` options are defined for the SH implementations, so buckle up:

- **`-m1`** — generate code for the SH1.

- **`-m2`** — generate code for the SH2.

- **`-m2e`** — generate code for the SH2e.

- **`-m2a-nofpu`** — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.

- **`-m2a-single-only`** — generate code for the SH2a-FPU in such a way that no double-precision floating-point operations are used.

- **`-m2a-single`** — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.

- **`-m2a`** — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.

- **`-m3`** — generate code for the SH3.

- **`-m3e`** — generate code for the SH3e.

- **`-m4-nofpu`** — generate code for the SH4 without a floating-point unit.

- **`-m4-single-only`** — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.

- **`-m4-single`** — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.

- **`-m4`** — generate code for the SH4.

- **`-m4-100`** — generate code for SH4-100.

- **`-m4-100-nofpu`** — generate code for SH4-100 in such a way that the floating-point unit is not used.

- **`-m4-100-single`** — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.

- **`-m4-100-single-only`** — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.

- **`-m4-200`** — generate code for SH4-200.

- **`-m4-200-nofpu`** — generate code for SH4-200 without in such a way that the floating-point unit is not used.

- **`-m4-200-single`** — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.

- **`-m4-200-single-only`** — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.

- **`-m4-300`** — generate code for SH4-300.

- **`-m4-300-nofpu`** — generate code for SH4-300 without in such a way that the floating-point unit is not used.

- **`-m4-300-single`** — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.

- **`-m4-300-single-only`** — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.

- **`-m4-340`** — generate code for SH4-340 (no MMU, no FPU).

- **`-m4-500`** — generate code for SH4-500 (no FPU). Passes `-isa=sh4-nofpu` to the assembler.

- **`-m4a-nofpu`** — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.

- **`-m4a-single-only`** — generate code for the SH4a in such a way that no double-precision floating-point operations are used.

- **`-m4a-single`** — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.

- **`-m4a`** — generate code for the SH4a.

- **`-m4al`** — same as `-m4a-nofpu`, except that it implicitly passes `-dsp` to the assembler. GCC doesn't generate any DSP instructions at the moment, just so you know.

- **`-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. Heads up tho: this changes the calling conventions, so some functions from the standard C library won't work unless you recompile it first with `-mdalign`. 😭

- **`-mrelax`** — shorten some address references at link time when possible; uses the linker option `-relax`.

- **`-mbigtable`** — use 32-bit offsets in `"switch"` tables. The default is to use 16-bit offsets.

- **`-mbitops`** — enable the use of bit manipulation instructions on SH2A.

- **`-mfmovd`** — enable the use of the instruction `"fmovd"`. Check `-mdalign` for alignment constraints.

- **`-mrenesas`** — comply with the calling conventions defined by Renesas.

- **`-mno-renesas`** — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.

- **`-mnomacsave`** — mark the `"MAC"` register as call-clobbered, even if `-mrenesas` is given.

- **`-mieee`**, **`-mno-ieee`** — control the IEEE compliance of floating-point comparisons, which affects how cases where the result of a comparison is unordered are handled. By default `-mieee` is implicitly enabled. If `-ffinite-math-only` is enabled, `-mno-ieee` is implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons. You can override the implicit settings by specifying either `-mieee` or `-mno-ieee`. ✨

- **`-minline-ic_invalidate`** — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has <yellow>no effect</yellow> if `-musermode` is in effect and the selected code generation option (e.g. `-m4`) does not allow the use of the `"icbi"` instruction. If the selected code generation option does not allow the use of the `"icbi"` instruction, and `-musermode` is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped. 💀

- **`-misize`** — dump instruction size and location in the assembly code.

- **`-mpadstruct`** — > ⚠️ this option is **deprecated**. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.

- **`-matomic-model=model`** — sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see `__atomic Builtins`. The following models and parameters are supported:

  - `none` — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not `"sh*-*-linux*"`.

- **`soft-gusa`** — this one makes GNU/Linux-compatible gUSA software atomic sequences for the atomic built-in functions ✨. Lowkey important: those generated sequences need *extra support* from the interrupt/exception handling code of your system, and they're only suitable for SH3* and SH4* **single-core** systems. It's on by default when the target is `sh*-*-linux*` and SH3* or SH4*. And when the target is SH4A, it also partially uses the hardware atomic instructions `movli.l` and `movco.l` to make more efficient code — unless you say `strict`, bestie.

- **`soft-tcb`** — generates software atomic sequences that use a variable in the thread control block 🧵. It's a variation of the gUSA sequences that *also* works on SH1* and SH2* targets. Same deal though: the generated atomic sequences require additional support from the interrupt/exception handling code of the system, and they're only suitable for single-core systems. When you use this model, you **have to** specify the `gbr-offset=` parameter too, no skipping 💅.

- **`soft-imask`** — generates software atomic sequences that temporarily disable interrupts by setting `SR.IMASK = 1111` 🙈. This model works *only* when the program runs in privileged mode, and it's only suitable for single-core systems. Good news: it does **not** need additional support from the interrupt/exception handling code of the system. It's enabled by default when the target is `sh*-*-linux*` and SH1* or SH2*.

- **`hard-llcs`** — generates hardware atomic sequences using the `movli.l` and `movco.l` instructions **only** ⚡. This is 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. And for this model, additional support from the interrupt/exception handling code of the system is *not* required 👑

- **`gbr-offset=`** — this parameter specifies the offset **in bytes** of the variable in the thread control block structure that the generated atomic sequences should use when the `soft-tcb` model has been selected. For other models this parameter is ignored, periodt. The value you specify must be an integer multiple of four and in the range 0-1020 📏.

- **`strict`** — this parameter prevents mixed usage of multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model **only** 🚫. No mixing, no blending, we're being decisive.

- **`-mtas`** — generate the `tas.b` opcode for `__atomic_test_and_set`. Real talk though: depending on your particular hardware and software configuration, this can degrade overall performance because of the operand cache line flushes that the `tas.b` instruction implies 😭. And on multi-core SH4A processors, the `tas.b` instruction must be used with **caution**, since it can result in data corruption for certain cache configurations 💀.

- **`-mprefergot`** — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table 🎀.

- **`-musermode`**, **`-mno-usermode`** — don't allow (allow) the compiler to generate privileged mode code. Specifying `-musermode` also implies `-mno-inline-ic_invalidate` if the inlined code wouldn't work in user mode. `-musermode` is the default when the target is `sh*-*-linux*`. And heads up: if the target is SH1* or SH2*, `-musermode` has **no effect**, because there's no user mode to speak of 🤷.

- **`-multcost=number`** — set the cost to assume for a multiply insn. Yes, `number`, that's the vibe 🔢.

- **`-mdiv=strategy`** — set the division strategy to be used for integer division operations. `strategy` can be one of these lil guys:

  - **`call-div1`** — calls a library function that uses the single-step division instruction `div1` to perform the operation. Division by zero calculates an unspecified result and does **not** trap 😳. This is the default except for SH4, SH2A and SHcompact.
  - **`call-fp`** — calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception 💥. This is the default for SHcompact with FPU. Specifying this for targets that don't have a double precision FPU defaults to `call-div1`.
  - **`call-table`** — calls a library function that uses a lookup table for small divisors and the `div1` instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does **not** trap. This is the default for SH4. Specifying this for targets that don't have dynamic shift instructions defaults to `call-div1`.

  When a division strategy hasn't been specified, the default strategy is selected based on the current target. For SH2A the default strategy is to use the `divs` and `divu` instructions instead of library function calls ✨.

- **`-maccumulate-outgoing-args`** — reserve space **once** for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size, slay 💅. Also needed for unwinding, to avoid changing the stack frame around conditional code.

- **`-mdivsi3_libfunc=name`** — set the name of the library function used for 32-bit signed division to `name`. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option weren't present 👀.

- **`-mfixed-range=register-range`** — generate code treating the given register range as fixed registers. A fixed register is one the register allocator **cannot** use. This is useful when compiling kernel code 🐧. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.

- **`-mbranch-cost=num`** — assume `num` to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified, the value is selected depending on the processor type that's being compiled for.

- **`-mzdcbranch`**, **`-mno-zdcbranch`** — assume (do not assume) that zero displacement conditional branch instructions `bt` and `bf` are fast ⚡. If `-mzdcbranch` is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying `-mno-zdcbranch`.

- **`-mcbranch-force-delay-slot`** — force the usage of delay slots for conditional branches, which stuffs the delay slot with a `nop` if a suitable instruction 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-madd` option is now mapped to the machine-independent `-ffp-contract=fast` option, and `-mno-fused-madd` is mapped to `-ffp-contract=off` ✨.

- **`-mfsca`**, **`-mno-fsca`** — allow or disallow the compiler to emit the `fsca` instruction for sine and cosine approximations 🌊. The option `-mfsca` **must** be used in combination with `-funsafe-math-optimizations`. It's enabled by default when generating code for SH4A. Using `-mno-fsca` disables sine and cosine approximations even if `-funsafe-math-optimizations` is in effect.

- **`-mfsrra`**, **`-mno-fsrra`** — allow or disallow the compiler to emit the `fsrra` instruction for reciprocal square root approximations 🍓. The option `-mfsrra` **must** be used in combination with `-funsafe-math-optimizations` and `-ffinite-math-only`. It's enabled by default when generating code for SH4A. Using `-mno-fsrra` disables reciprocal square root approximations even if `-funsafe-math-optimizations` and `-ffinite-math-only` are in effect.

- **`-mpretend-cmove`** — prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor 🏎️.

- **`-mfdpic`** — generate code using the FDPIC ABI.

---

## 🌞 Solaris 2 Options 🌞

These `-m` options are supported on Solaris 2, bestie:

- **`-mclear-hwcap`** — `-mclear-hwcap` tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions **not supported** by the current machine, but check at runtime whether or not to use them 🧐.

- **`-mimpure-text`** — `-mimpure-text`, used in addition to `-shared`, tells the compiler to **not** pass `-z text` to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

  > `-mimpure-text` suppresses the `relocations remain against allocatable but non-writable sections` linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is **not actually shared** across processes 😭. Instead of using `-mimpure-text`, you should compile all source code with `-fpic` or `-fPIC`. That's the move, fr fr.

These switches are supported in addition to the above on Solaris 2:

- **`-pthreads`** — this is a synonym for `-pthread` 🧵.

---

## 🌈 SPARC Options 🌈

These `-m` options are supported on the SPARC:

- **`-mno-app-regs`**, **`-mapp-regs`** — specify `-mapp-regs` to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications 👑. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.

  To be fully SVR4 ABI-compliant at the cost of some performance loss, specify `-mno-app-regs`. You should compile libraries and system software with this option 💖.

- **`-mflat`**, **`-mno-flat`** — with `-mflat`, the compiler does **not** generate save/restore instructions and uses a "flat" or single register window model 🪟. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.

With `-mno-flat` (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode, bestie, no notes. 🫶

- **`-mfpu`**, **`-mhard-float`** — Generate output containing floating-point instructions. This is the default. 🌸

- **`-mno-fpu`**, **`-msoft-float`** — Generate output containing library calls for floating point.

  > ⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets `sparc-*-aout` and `sparclite-*-*` do provide software floating-point support.

  Heads up: `-msoft-float` changes the calling convention in the output file, so it's only useful if you compile **all** of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work. 👑

- **`-mhard-quad-float`** — Generate output containing quad-word (`long double`) floating-point instructions. ✨

- **`-msoft-quad-float`** — Generate output containing library calls for quad-word (`long double`) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

  And like, 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. So it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. 🙈

- **`-mstd-struct-return`**, **`-mno-std-struct-return`** — With `-mstd-struct-return`, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

  The default is `-mno-std-struct-return`. This option has no effect in 64-bit mode.

- **`-mlra`**, **`-mno-lra`** — Enable Local Register Allocation. This is the default for SPARC since GCC 7, so `-mno-lra` needs to be passed to get old Reload. 🔥

- **`-mcpu=`*`cpu_type`* — Set the instruction set, register set, and instruction scheduling parameters for machine type *`cpu_type`*. Supported values for *`cpu_type`* are `v7`, `cypress`, `v8`, `supersparc`, `hypersparc`, `leon`, `leon3`, `leon3v7`, `leon5`, `sparclite`, `f930`, `f934`, `sparclite86x`, `sparclet`, `tsc701`, `v9`, `ultrasparc`, `ultrasparc3`, `niagara`, `niagara2`, `niagara3`, `niagara4`, `niagara7` and `m8`.

  Native Solaris and GNU/Linux toolchains also support the value `native`, which selects the best architecture option for the host processor. `-mcpu=native` has no effect if GCC does not recognize the processor.

  Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are `v7`, `v8`, `sparclite`, `sparclet`, `v9`.

  Here's the full lineup of each supported architecture and their supported implementations, bestie:

  - `v7` — `cypress`, `leon3v7`
  - `v8` — `supersparc`, `hypersparc`, `leon`, `leon3`, `leon5`
  - `sparclite` — `f930`, `f934`, `sparclite86x`
  - `sparclet` — `tsc701`
  - `v9` — `ultrasparc`, `ultrasparc3`, `niagara`, `niagara2`, `niagara3`, `niagara4`, `niagara7`, `m8`

  By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With `-mcpu=cypress`, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

  With `-mcpu=v8`, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With `-mcpu=supersparc`, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

  With `-mcpu=sparclite`, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan (`ffs`) instructions which exist in SPARClite but not in SPARC-V7. With `-mcpu=f930`, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With `-mcpu=f934`, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

  With `-mcpu=sparclet`, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan (`ffs`) instructions which exist in SPARClet but not in SPARC-V7. With `-mcpu=tsc701`, the compiler additionally optimizes it for the TEMIC SPARClet chip.

  With `-mcpu=v9`, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With `-mcpu=ultrasparc`, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With `-mcpu=ultrasparc3`, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With `-mcpu=niagara`, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With `-mcpu=niagara2`, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With `-mcpu=niagara3`, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With `-mcpu=niagara4`, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With `-mcpu=niagara7`, the compiler additionally optimizes it for Oracle SPARC M7 chips. With `-mcpu=m8`, the compiler additionally optimizes it for Oracle M8 chips. 🦋

- **`-mtune=`*`cpu_type`*** — Set the instruction scheduling parameters for machine type *`cpu_type`*, but **do not** set the instruction set or register set that the option `-mcpu=`*`cpu_type`* does.

  The same values for `-mcpu=`*`cpu_type`* can be used for `-mtune=`*`cpu_type`*, but the only useful values are those that select a particular CPU implementation. Those are `cypress`, `supersparc`, `hypersparc`, `leon`, `leon3`, `leon3v7`, `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 `-mvis2` when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting `-mvis2` also sets `-mvis`. ✨

- **`-mvis3`**, **`-mno-vis3`** — With `-mvis3`, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is `-mvis3` when targeting a cpu that supports such instructions, such as `niagara-3` and later. Setting `-mvis3` also sets `-mvis2` and `-mvis`.

- **`-mvis4`**, **`-mno-vis4`** — With `-mvis4`, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is `-mvis4` when targeting a cpu that supports such instructions, such as `niagara-7` and later. Setting `-mvis4` also sets `-mvis3`, `-mvis2` and `-mvis`.

- **`-mvis4b`**, **`-mno-vis4b`** — With `-mvis4b`, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default is `-mvis4b` when targeting a cpu that supports such instructions, such as `m8` and later. Setting `-mvis4b` also sets `-mvis4`, `-mvis3`, `-mvis2` and `-mvis`.

- **`-mcbcond`**, **`-mno-cbcond`** — With `-mcbcond`, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is `-mcbcond` when targeting a CPU that supports such instructions, such as Niagara-4 and later. 🎀

- **`-mfmaf`**, **`-mno-fmaf`** — With `-mfmaf`, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is `-mfmaf` when targeting a CPU that supports such instructions, such as Niagara-3 and later.

- **`-mfsmuld`**, **`-mno-fsmuld`** — With `-mfsmuld`, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is `-mfsmuld` when targeting a CPU supporting the architecture versions V8 or V9 with FPU except `-mcpu=leon`. 🌈

- **`-mpopc`**, **`-mno-popc`** — okay so with `-mpopc` GCC pops off and generates code that takes advantage of the UltraSPARC Population Count instruction ✨ and the default is `-mpopc` when you're targeting a CPU that supports such an instruction, like Niagara-2 and later. No cap, it just knows 💅

- **`-msubxc`**, **`-mno-subxc`** — with `-msubxc`, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is `-msubxc` when targeting a CPU that supports such an instruction, such as Niagara-7 and later 👑

- **`-mfix-at697f`** — turn on the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor). It's giving "we fixed the oopsie" energy 🩹

- **`-mfix-ut699`** — turn on the documented workarounds for the floating-point errata <pink>and</pink> the data cache nullify errata of the UT699 processor. Two bugs, one flag, iconic behavior fr fr 💖

- **`-mfix-ut700`** — turn on the documented workaround for the back-to-back store errata of the UT699E/UT700 processor 🛠️

- **`-mfix-gr712rc`** — turn on the documented workaround for the back-to-back store errata of the GR712RC processor. Same vibe, different chip 🎀

<purple>These `-m` options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:</purple>

- **`-m32`**, **`-m64`** — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets `int`, `long` and pointer to 32 bits. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits. Classic glow-up moment 🌸

- **`-mcmodel=which`** — set the code model to one of these besties:

  - `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. Free-spirited ✨
  - `embmedany` — the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register `%g4` points to the base of the data segment. Programs are statically linked and PIC is <red>not</red> supported 🚫

- **`-mmemory-model=mem-model`** — set the memory model in force on the processor to one of:

  - `default` — the default memory model for the processor and operating system.
  - `rmo` — Relaxed Memory Order 🧘
  - `pso` — Partial Store Order
  - `tso` — Total Store Order
  - `sc` — Sequential Consistency

  These memory models are formally defined in <purple>Appendix D</purple> of the SPARC-V9 architecture manual, as set in the processor's `"PSTATE.MM"` field. Very official, very demure 📖

- **`-mstack-bias`**, **`-mno-stack-bias`** — with `-mstack-bias`, GCC assumes that the stack pointer, and frame pointer if present, are offset by `-2047` which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. Lowkey math but we move 🫶

---

## 🎀 Options for System V 🎀

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

- **`-G`** — create a shared object. It is recommended that `-symbolic` or `-shared` be used instead. <yellow>(bestie, use the newer ones, this one's a bit vintage)</yellow>

- **`-Qy`** — identify the versions of each tool used by the compiler, in a `".ident"` assembler directive in the output. Receipts, basically 🧾

- **`-Qn`** — refrain from adding `".ident"` directives to the output file (this is the default). No receipts, keep it mysterious 🙈

- **`-YP,dirs`** — search the directories *dirs*, and no others, for libraries specified with `-l`. Very exclusive guest list 💅

- **`-Ym,dir`** — look in the directory *dir* to find the M4 preprocessor. The assembler uses this option.

---

## 🌈 TILE-Gx Options 🌈

These `-m` options are supported on the TILE-Gx:

- **`-mcmodel=small`** — generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range. Smol but mighty 🍓

- **`-mcmodel=large`** — generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses. Go big or go home 👑

- **`-mcpu=name`** — selects the type of CPU to be targeted. Currently the only supported type is `tilegx`.

- **`-m32`**, **`-m64`** — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets `int`, `long`, and pointer to 32 bits. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits.

- **`-mbig-endian`**, **`-mlittle-endian`** — generate code in big/little endian mode, respectively. Pick your fighter ⚔️

---

## 💜 TILEPro Options 💜

These `-m` options are supported on the TILEPro:

- **`-mcpu=name`** — selects the type of CPU to be targeted. Currently the only supported type is `tilepro`.

- **`-m32`** — generate code for a 32-bit environment, which sets `int`, `long`, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored. Not me showing up to a party that's already happening 😭

---

## 🦋 V850 Options 🦋

These `-m` options are defined for V850 implementations:

- **`-mlong-calls`**, **`-mno-long-calls`** — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. Long distance relationship energy 📞

- **`-mno-ep`**, **`-mep`** — do <red>not</red> optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the `"ep"` register, and use the shorter `"sld"` and `"sst"` instructions. The `-mep` option is on by default if you optimize.

- **`-mno-prolog-function`**, **`-mprolog-function`** — do <red>not</red> use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The `-mprolog-function` option is on by default if you optimize.

- **`-mspace`** — try to make the code as small as possible. At present, this just turns on the `-mep` and `-mprolog-function` options. Compact queen 💅

- **`-mtda=n`** — put static or global variables whose size is *n* bytes or less into the tiny data area that register `"ep"` points to. The tiny data area can hold up to `256` bytes in total (`128` bytes for byte references). Tiny data area, tiny but powerful 🎀

- **`-msda=n`** — put static or global variables whose size is *n* bytes or less into the small data area that register `"gp"` points to. The small data area can hold up to `64` kilobytes.

- **`-mzda=n`** — put static or global variables whose size is *n* bytes or less into the first `32` kilobytes of memory.

- **`-mv850`** — specify that the target processor is the V850.

- **`-mv850e3v5`** — specify that the target processor is the V850E3V5. The preprocessor constant `"__v850e3v5__"` is defined if this option is used. ✨

- **`-mv850e2v4`** — specify that the target processor is the V850E3V5. This is an alias for the `-mv850e3v5` option. (Yes bestie, it says V850E3V5 — that's what the page says, I don't make the rules 😭)

- **`-mv850e2v3`** — specify that the target processor is the V850E2V3. The preprocessor constant `"__v850e2v3__"` is defined if this option is used.

- **`-mv850e2`** — specify that the target processor is the V850E2. The preprocessor constant `"__v850e2__"` is defined if this option is used.

- **`-mv850e1`** — specify that the target processor is the V850E1. The preprocessor constants `"__v850e1__"` and `"__v850e__"` are defined if this option is used.

- **`-mv850es`** — specify that the target processor is the V850ES. This is an alias for the `-mv850e1` option.

- **`-mv850e`** — specify that the target processor is the V850E. The preprocessor constant `"__v850e__"` is defined if this option is used.

> If neither `-mv850` nor `-mv850e` nor `-mv850e1` nor `-mv850e2` nor `-mv850e2v3` nor `-mv850e3v5` are defined then a default target processor is chosen and the relevant `__v850*__` preprocessor constant is defined.

> The preprocessor constants `"__v850"` and `"__v851__"` are always defined, regardless of which processor variant is the target. Always. Periodt 💖

- **`-mdisable-callt`**, **`-mno-disable-callt`** — this option suppresses generation of the `"CALLT"` instruction for the `v850e`, `v850e1`, `v850e2`, `v850e2v3` and `v850e3v5` flavors of the v850 architecture.

  This option is enabled by default when the RH850 ABI is in use (see `-mrh850-abi`), and disabled by default when the GCC ABI is in use. If `"CALLT"` instructions are being generated then the C preprocessor symbol `"__V850_CALLT__"` is defined. 🛑

- **`-mrelax`**, **`-mno-relax`** — pass on (or do not pass on) the `-mrelax` command-line option to the assembler.

- **`-mlong-jumps`**, **-mno-long-jumps** — disable (or re-enable) the generation of PC-relative jump instructions.

- **`-msoft-float`**, **`-mhard-float`** — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol `"__FPU_OK__"` is defined, otherwise the symbol `"__NO_FPU__"` is defined. Float or no float, we stay cute 🌸

- **`-mloop`** — enables the use of the e3v5 LOOP instruction. The use of this instruction is <yellow>not enabled by default</yellow> when the e3v5 architecture is selected because its use is still <red>experimental</red>. 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-align` command-line option is supported.
  - The `-mdisable-callt` command-line option is enabled by default. The `-mno-disable-callt` command-line option is <red>not supported</red>.

  When this version of the ABI is enabled the C preprocessor symbol `"__V850_RH850_ABI__"` is defined. 👑

- **`-mgcc-abi`** — enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:

  - Integer sized structures and unions are returned in register `"r10"`.
  - Large structures and unions (more than 8 bytes in size) are passed by reference.

- Functions are aligned to 32-bit boundaries, unless optimizing for size.

- The `-m8byte-align` command-line option is not supported.

- The `-mdisable-callt` command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol `"__V850_GCC_ABI__"` is defined.

- **`-m8byte-align`**, **`-mno-8byte-align`** — lets `double` and `long long` types get aligned on 8-byte boundaries 💅. The default is to restrict the alignment of *all* objects to at most 4-bytes. When `-m8byte-align` is in effect the C preprocessor symbol `"__V850_8BYTE_ALIGN__"` is defined.

- **`-mbig-switch`** — generate code suitable for big switch tables. Use this option *only* if the assembler/linker complain about out of range branches within a switch table 🙈.

- **`-mapp-regs`** — this option causes `r2` and `r5` to be used in the code generated by the compiler. This setting is the default.

- **`-mno-app-regs`** — this option causes `r2` and `r5` to be treated as fixed registers.

---

## 🦋 VAX Options 🦋

These `-m` options are defined for the VAX:

- **`-munix`** — do *not* output certain jump instructions (`"aobleq"` and so on) that the Unix assembler for the VAX cannot handle across long ranges.

- **`-mgnu`** — *do* output those jump instructions, on the assumption that the GNU assembler is being used.

- **`-mg`** — output code for G-format floating-point numbers instead of D-format.

- **`-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 libraries `libc.a` and `libdebug.a` to be linked. The program should be run on the target under the control of the GDB remote debugging stub.

- **`-msim`** — a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries `libc.a` and `libsim.a` to be linked.

- **`-mfpu`**, **`-mhard-float`** — generate code containing floating-point instructions. This is the default.

- **`-mno-fpu`**, **`-msoft-float`** — generate code containing library calls for floating-point.

> `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile *all* of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work. fr fr, don't skip that part 😭

- **`-mcpu=`*cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type *cpu_type*. Supported values for *cpu_type* are `mcm`, `gr5` and `gr6`.

  `mcm` is a synonym of `gr5` present for backward compatibility.

  By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

  With `-mcpu=gr6`, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

- **`-mtune=`*cpu_type* — set the instruction scheduling parameters for machine type *cpu_type*, but do *not* set the instruction set or register set that the option `-mcpu=`*cpu_type* would.

- **`-msv-mode`** — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.

- **`-muser-mode`** — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers `r24` to `r31` cannot be accessed in this mode; on the GR6, only registers `r29` to `r31` are affected.

---

## 💜 VMS Options 💜

These `-m` options are defined for the VMS implementations:

- **`-mvms-return-codes`** — return VMS condition codes from `"main"`. The default is to return POSIX-style condition (e.g. error) codes.

- **`-mdebug-main=`*prefix* — flag the first routine whose name starts with *prefix* as the main routine for the debugger.

- **`-mmalloc64`** — default to 64-bit memory allocation routines.

- **`-mpointer-size=`*size* — set the default size of pointers. Possible options for *size* are `32` or `short` for 32 bit pointers, `64` or `long` for 64 bit pointers, and `no` for supporting only 32 bit pointers. The later option disables `"pragma pointer_size"`.

---

## 🎀 VxWorks Options 🎀

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.

- **`-mrtp`** — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro `"__RTP__"`.

- **`-non-static`** — link an RTP executable against shared libraries rather than static libraries. The options `-static` and `-shared` can also be used for RTPs; `-static` is the default.

- **`-Bstatic`**, **`-Bdynamic`** — these options are passed down to the linker. They are defined for compatibility with Diab.

- **`-Xbind-lazy`** — enable lazy binding of function calls. This option is equivalent to `-Wl,-z,now` and is defined for compatibility with Diab.

- **`-Xbind-now`** — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

---

## ⭐ x86 Options ⭐

These `-m` options are defined for the x86 family of computers.

- **`-march=`*cpu-type* — generate instructions for the machine type *cpu-type*. In contrast to `-mtune=`*cpu-type*, which merely tunes the generated code for the specified *cpu-type*, `-march=`*cpu-type* allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying `-march=`*cpu-type* implies `-mtune=`*cpu-type*, 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=native` enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using `-mtune=native` produces code optimized for the local machine under the constraints of the selected instruction set.

  - `x86-64` — a generic CPU with 64-bit extensions.

  - `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 as `generic`.

  - `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, ADCX and PREFETCHW instruction set support.

  - `skylake` — Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.

  - `bonnell` — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

- **`silvermont`** — Intel Silvermont CPU, and yes it's got 64-bit extensions, `MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND` instruction set support. <mint>tiny but mighty tbh</mint> 💚

  - **`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, CLFLUSHOPT and FSGSBASE` instruction set support. <cyan>glow up from silvermont fr fr</cyan> ✨

  - **`goldmont-plus`** — Intel Goldmont Plus CPU, 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 and SGX` instruction set support. <lavender>the "plus" is doing heavy lifting and honestly? it earned it</lavender> 💜

  - **`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, CLDEMOTE and WAITPKG` instruction set support. <pink>the list keeps growing bestie, she ate</pink> 🍓

  - **`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, AVX512CD and PREFETCHWT1` instruction set support. <purple>note the `MOVBE` showing up twice in there — that's just how the original rolls, not a typo from me okay 😭</purple>

  - **`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, AVX512CD and PREFETCHWT1, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ` instruction set support. <hotpink>yes `MOVBE` is in there twice again. iconic behavior honestly</hotpink> 👑

  - **`skylake-avx512`** — Intel Skylake Server CPU, 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 and AVX512CD` instruction set support. <blue>server core with main character energy</blue> 🔥

  - **`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, AVX512IFMA and SHA` instruction set support. <mint>new drops: PKU, AVX512VBMI, AVX512IFMA and SHA 💅</mint>

  - **`icelake-client`** — Intel Icelake Client CPU, 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 and AVX512VPOPCNTDQ` instruction set support. <lavender>and yes there's a lil space before that comma in `AVX512VBMI2 , VPCLMULQDQ` — I'm preserving the original's quirks, no cap 🫶</lavender>

  - **`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, WBNOINVD and CLWB` instruction set support. <cyan>same lil comma-space moment in `AVX512VBMI2 , VPCLMULQDQ`, keepin' it real</cyan> ✨

  - **`cascadelake`** — Intel Cascadelake CPU, 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 and AVX512VNNI` instruction set support. <peach>cascade slay</peach> 🌸

  - **`cooperlake`** — Intel cooperlake CPU (yes, lowercase in the source, we respect it) 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, AVX512VNNI and AVX512BF16` instruction set support. <green>AVX512BF16 joins the party 🎀</green>

  - **`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, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER` instruction set support. <hotpink>spot the `AVX512CD PKU` with no comma — original typo, preserved with love 💖</hotpink>

  - **`sapphirerapids`** — Intel sapphirerapids CPU (lowercase again, that's just the source's vibe) with 64-bit extensions, `MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16 and AVX512BF16` instruction set support. <purple>and there's `AVX512VBMI2 VPCLMULQDQ` with no comma here either — keeping that exact too, iykyk</purple> 🌈

  - **`alderlake`** — Intel Alderlake 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 and AVX-VNNI` instruction set support. <cyan>no `CX16` and no `SAHF` here, that's just how it is, don't come for me</cyan> 🦋

  - **`rocketlake`** — Intel Rocketlake CPU, 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, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ` instruction set support. <lavender>two lil quirks in here — the space in `SSSE3 , SSE4.1` and the missing comma in `AVX512CD PKU`. both preserved, both protected 💜</lavender>

  - **`k6`** — AMD K6 CPU, rockin' `MMX` instruction set support. <peach>throwback era, still cute</peach> 🎀

  - **`k6-2`**
  - **`k6-3`** — Improved versions of the AMD K6 CPU, with `MMX` and `3DNow!` instruction set support. <mint>the glow-up twins ✨</mint>

  - **`athlon`**
  - **`athlon-tbird`** — AMD Athlon CPU with `MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions` support. <blue>note the lowercase `3dNOW!` — that's the original, and I will not be touching it</blue> 💙

  - **`athlon-4`**
  - **`athlon-xp`**
  - **`athlon-mp`** — Improved AMD Athlon CPU, with `MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set` support. <purple>now it's `3DNow!` with a capital D. the source said so. we obey 🙈</purple>

  - **`k8`**
  - **`opteron`**
  - **`athlon64`**
  - **`athlon-fx`** — Processors based on the AMD K8 core, with x86-64 instruction set support — that's the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets `MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions`.) <hotpink>a whole squad in one bullet, obsessed</hotpink> 👑

  - **`k8-sse3`**
  - **`opteron-sse3`**
  - **`athlon64-sse3`** — Improved versions of the AMD K8 cores, with `SSE3` instruction set support. <cyan>same as before but with the SSE3 sprinkles on top 💅</cyan>

  - **`amdfam10`**
  - **`barcelona`** — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets `MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions`.) <yellow>barcelona said "i contain multitudes" 🌟</yellow>

  - **`bdver1`** — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets `FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions`.) <green>AVX has entered the chat 🔥</green>

  - **`bdver2`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets `BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions`.) <purple>more stuff than bdver1, as a treat 💜</purple>

  - **`bdver3`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets `BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions`.) <mint>`FSGSBASE` joins in ✨</mint>

  - **`bdver4`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets `BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions`.) <cyan>now with `AVX2` and `MOVBE`, she's thriving</cyan> 🦋

  - **`znver1`** — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets `BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions`.) <hotpink>yes there's a comma before the "and" in the original. keeping it. don't @ me 🫶</hotpink>

  - **`znver2`** — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets `BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, 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`.) <lavender>and yes, the comma-before-"and" energy is here too. consistent queen 💜</lavender>

- **`znver3`** — CPUs built on AMD Family 19h cores, x86-64 instruction set support and all 💅. (This supersets BMI, 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.) she's stacked bestie, no cap ✨

- **`btver1`** — CPUs on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)

- **`btver2`** — CPUs on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.

- **`winchip-c6`** — the IDT WinChip C6 CPU, and it gets handled in the same way as i486 except with extra MMX instruction set support.

- **`winchip2`** — the IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.

- **`c3`** — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip, sorry babe 💀)

- **`c3-2`** — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

- **`c7`** — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`samuel-2`** — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)

- **`nehemiah`** — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

- **`esther`** — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`eden-x2`** — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`eden-x4`** — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)

- **`nano`** — generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-1000`** — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-2000`** — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-3000`** — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-x2`** — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-x4`** — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`geode`** — AMD Geode embedded processor with MMX and 3DNow! instruction set support. vintage icon behavior honestly 🎀

---

## <purple>`-mtune=cpu-type`</purple> 🌸

okay so this one tunes *everything applicable about the generated code* to `cpu-type`, EXCEPT the ABI and the set of available instructions. So it's giving vibes-matching for one specific chip: while picking a specific `cpu-type` schedules things appropriately for that particular chip, the compiler does **not** generate any code that cannot run on the default machine type unless you use a `-march=cpu-type` option. Like, for example, if GCC is configured for `i686-pc-linux-gnu` then `-mtune=pentium4` generates code that is tuned for Pentium 4 but still runs on i686 machines. she's adaptable 💅

The choices for `cpu-type` are the same as for `-march`. In addition, `-mtune` supports 2 extra choices for `cpu-type`:

- **`generic`** — produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding `-mtune` or `-march` option instead of `-mtune=generic`. But, if you do not know exactly what CPU users of your application will have, then you should use this option. fr fr, it's the "idk what they're running so let's be safe" option 🫶

  As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

  There is no `-march=generic` option because `-march` indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, `-mtune` indicates the processor (or, in this case, collection of processors) for which the code is optimized. iykyk 🧚

- **`intel`** — produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding `-mtune` or `-march` option instead of `-mtune=intel`. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.

  As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

  There is no `-march=intel` option because `-march` indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, `-mtune` indicates the processor (or, in this case, collection of processors) for which the code is optimized.

---

## <purple>`-mcpu=cpu-type`</purple> 🦋

a **deprecated** synonym for `-mtune`. bestie she's retired, don't @ her, just use `-mtune` 😭

---

## <purple>`-mfpmath=unit`</purple> ✨

generate floating-point arithmetic for selected unit `unit`. The choices for `unit` are:

- **`387`** — use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See `-ffloat-store` for more detailed description.

  This is the default choice for non-Darwin x86-32 targets.

- **`sse`** — use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.

  For the x86-32 compiler, you must use `-march=cpu-type`, `-msse` or `-msse2` switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.

  The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. ⚠️

  This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when `-ffast-math` is enabled.

- **`sse,387`**
- **`sse+387`**
- **`both`**

  attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is **still experimental**, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. not her being unstable 😭 use with care bestie, she's in her experimental era 💀

---

## <purple>`-masm=dialect`</purple> 💜

output assembly instructions using selected `dialect`. Also affects which dialect is used for basic `asm` and extended `asm`. Supported choices (in dialect order) are `att` or `intel`. The default is `att`. Darwin does not support `intel`.

---

## <purple>`-mieee-fp`</purple> / <purple>`-mno-ieee-fp`</purple> 🎀

control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. iconic for not being messy about comparisons ngl ✨

---

## <purple>`-m80387`</purple> / <purple>`-mhard-float`</purple> 🔥

generate output containing 80387 instructions for floating point.

---

## <purple>`-mno-80387`</purple> / <purple>`-msoft-float`</purple> 🌈

generate output containing library calls for floating point.

> Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if `-msoft-float` is used. sneaky 😭

---

## <purple>`-mno-fp-ret-in-387`</purple> 🍓

do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types `float` and `double` in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option `-mno-fp-ret-in-387` causes such values to be returned in ordinary CPU registers instead.

- **`-mno-fancy-math-387`** — okay so some 387 emulators are lowkey *not* built for the `sin`, `cos` and `sqrt` instructions on the 387 🫠. Say the word and GCC won't generate those instructions at all. But heads up bestie: this option gets overridden whenever `-march` says the target CPU always has an FPU, because then the instruction doesn't need emulation anyway. Also, plot twist — these instructions aren't generated unless you *also* pass `-funsafe-math-optimizations`. 💅

- **`-malign-double`**, **`-mno-align-double`** — these two decide whether GCC aligns `double`, `long double` and `long long` variables on a **two-word boundary** or a **one-word boundary**. Aligning `double` variables on a two-word boundary gives you code that runs somewhat faster on a Pentium, at the cost of more memory. trade-off queen behavior, honestly. ⚖️

  On x86-64, `-malign-double` is enabled by default. ✨

  > Warning: if you use the `-malign-double` switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32, and are **not binary compatible** with structures in code compiled without that switch. 💀

- **`-m96bit-long-double`**, **`-m128bit-long-double`** — these switches control the size of the `long double` type. The x86-32 application binary interface specifies the size to be 96 bits, so `-m96bit-long-double` is the default in 32-bit mode. 📏

  Modern architectures (Pentium and newer) prefer `long double` to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying `-m128bit-long-double` aligns `long double` to a 16-byte boundary by padding the `long double` with an additional 32-bit zero. cute little padding moment 🎀

  In the x86-64 compiler, `-m128bit-long-double` is the default choice, since its ABI specifies that `long double` is aligned on a 16-byte boundary.

  Notice that **neither** of these options enable any extra precision over the x87 standard of 80 bits for a `long double`. nope, not a single extra bit of precision, bestie. 🙈

  > Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing `long double` variables, as well as modifying the function calling convention for functions taking `long double`. Hence they are not binary-compatible with code compiled without that switch. 💀

- **`-mlong-double-64`**, **`-mlong-double-80`**, **`-mlong-double-128`** — these switches control the size of the `long double` type. A size of 64 bits makes the `long double` type equivalent to the `double` type — and that's the default for the 32-bit Bionic C library. A size of 128 bits makes the `long double` type equivalent to the `__float128` type — and that's the default for the 64-bit Bionic C library. 🌈

  > Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing `long double` variables, as well as modifying the function calling convention for functions taking `long double`. Hence they are not binary-compatible with code compiled without that switch. 💀

- **`-malign-data=`** *type* — controls how GCC aligns variables. The supported values for *type* are: `compat`, which uses an increased alignment value compatible with GCC 4.8 and earlier; `abi`, which uses the alignment value as specified by the psABI; and `cacheline`, which uses an increased alignment value to match the cache line size. `compat` is the default. 👑

- **`-mlarge-data-threshold=`** *threshold* — when `-mcmodel=medium` is specified, data objects larger than *threshold* are placed in the large data section. This value must be the same across **all** objects linked into the binary, and it defaults to `65535`. 🔢

---

- **`-mrtd`** — uses a different function-calling convention, where functions that take a fixed number of arguments return with the `ret num` instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. efficiency icon ✨

  You can specify that an individual function is called with this calling sequence using 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.) no cap, this one bites. 💀

- **`-mregparm=`** *num* — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most **3** registers can be used. You can control this behavior for a specific function by using the function attribute `regparm`. 🎯

  > Warning: if you use this switch, and *num* is nonzero, then you must build **all** modules with the same value, including any libraries. This includes the system libraries and startup modules. 🔥

- **`-msseregparm`** — uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute `sseregparm`. 🦋

  > Warning: if you use this switch then you must build **all** modules with the same value, including any libraries. This includes the system libraries and startup modules. 🔥

- **`-mvect8-ret-in-mem`** — 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. legacy-code solidarity 🫶

---

- **`-mpc32`**, **`-mpc64`**, **`-mpc80`** — set 80387 floating-point precision to 32, 64 or 80 bits. When `-mpc32` is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); `-mpc64` rounds the significands of results of floating-point operations to 53 bits (double precision); and `-mpc80` rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are **not available** to the programmer without setting the FPU control word explicitly. 🙈

  Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by **2% or more**. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 😭

- **`-mstackrealign`** — realigns the stack at entry. On the x86, the `-mstackrealign` option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute `force_align_arg_pointer`, applicable to individual functions. 🧚

- **`-mpreferred-stack-boundary=`** *num* — attempts to keep the stack boundary aligned to a 2 raised to *num* byte boundary. If `-mpreferred-stack-boundary` is not specified, the default is **4** (16 bytes or 128 bits). 📐

  > Warning: when generating code for the x86-64 architecture with SSE extensions disabled, `-mpreferred-stack-boundary=3` can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and is intended to be used in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and `__int128`), leading to wrong results. You must build all modules with `-mpreferred-stack-boundary=3`, including any libraries. This includes the system libraries and startup modules. 💀

- **`-mincoming-stack-boundary=`** *num* — assumes the incoming stack is aligned to a 2 raised to *num* byte boundary. If `-mincoming-stack-boundary` is not specified, the one specified by `-mpreferred-stack-boundary` is used. 🎀

  On Pentium and Pentium Pro, `double` and `long double` values should be aligned to an 8-byte boundary (see `-malign-double`) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type `__m128` may not work properly if it is not 16-byte aligned. ⚠️

  To ensure proper alignment of these 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 bestie, buckle UP 🎀 because this is the x86 instruction-set switch buffet and it is *long*. These switches let GCC use instructions from the <pink>MMX</pink>, <pink>SSE</pink>, <pink>SSE2</pink>, <pink>SSE3</pink>, <pink>SSSE3</pink>, <pink>SSE4</pink>, <pink>SSE4A</pink>, <pink>SSE4.1</pink>, <pink>SSE4.2</pink>, <pink>AVX</pink>, <pink>AVX2</pink>, <pink>AVX512F</pink>, <pink>AVX512PF</pink>, <pink>AVX512ER</pink>, <pink>AVX512CD</pink>, <pink>AVX512VL</pink>, <pink>AVX512BW</pink>, <pink>AVX512DQ</pink>, <pink>AVX512IFMA</pink>, <pink>AVX512VBMI</pink>, <pink>SHA</pink>, <pink>AES</pink>, <pink>PCLMUL</pink>, <pink>CLFLUSHOPT</pink>, <pink>CLWB</pink>, <pink>FSGSBASE</pink>, <pink>PTWRITE</pink>, <pink>RDRND</pink>, <pink>F16C</pink>, <pink>FMA</pink>, <pink>PCONFIG</pink>, <pink>WBNOINVD</pink>, <pink>FMA4</pink>, <pink>PREFETCHW</pink>, <pink>RDPID</pink>, <pink>PREFETCHWT1</pink>, <pink>RDSEED</pink>, <pink>SGX</pink>, <pink>XOP</pink>, <pink>LWP</pink>, <pink>3DNow!</pink>, <pink>enhanced 3DNow!</pink>, <pink>POPCNT</pink>, <pink>ABM</pink>, <pink>ADX</pink>, <pink>BMI</pink>, <pink>BMI2</pink>, <pink>LZCNT</pink>, <pink>FXSR</pink>, <pink>XSAVE</pink>, <pink>XSAVEOPT</pink>, <pink>XSAVEC</pink>, <pink>XSAVES</pink>, <pink>RTM</pink>, <pink>HLE</pink>, <pink>TBM</pink>, <pink>MWAITX</pink>, <pink>CLZERO</pink>, <pink>PKU</pink>, <pink>AVX512VBMI2</pink>, <pink>GFNI</pink>, <pink>VAES</pink>, <pink>WAITPKG</pink>, <pink>VPCLMULQDQ</pink>, <pink>AVX512BITALG</pink>, <pink>MOVDIRI</pink>, <pink>MOVDIR64B</pink>, <pink>AVX512BF16</pink>, <pink>ENQCMD</pink>, <pink>AVX512VPOPCNTDQ</pink>, <pink>AVX5124FMAPS</pink>, <pink>AVX512VNNI</pink>, <pink>AVX5124VNNIW</pink>, <pink>SERIALIZE</pink>, <pink>UINTR</pink>, <pink>HRESET</pink>, <pink>AMXTILE</pink>, <pink>AMXINT8</pink>, <pink>AMXBF16</pink>, <pink>KL</pink>, <pink>WIDEKL</pink>, <pink>AVXVNNI</pink>, <pink>AVX512FP16</pink> or <pink>CLDEMOTE</pink> extended instruction sets. And literally every single one has a matching <cyan>`-mno-`</cyan> option so you can turn that instruction use back OFF. iykyk ✨

Here's the whole roster, in order, no skipping, no cap:

- **`-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`**

These extensions are also available as <purple>built-in functions</purple> — go peek at <cyan>x86 Built-in Functions</cyan> for the details on which functions each switch turns on and off. 🦋

Wanna make GCC spit out SSE/SSE2 instructions *automatically* from floating-point code (instead of using 387 instructions)? That's a <cyan>`-mfpmath=sse`</cyan> situation, bestie. 💅

And heads up: GCC <hotpink>depresses</hotpink> SSEx instructions when <cyan>`-mavx`</cyan> is in play. Instead it generates brand-new AVX instructions, or AVX equivalents, for all the SSEx instructions whenever it needs them. main character energy tbh 👑

These options let GCC use those extended instructions in generated code <yellow>even without</yellow> <cyan>`-mfpmath=sse`</cyan>. Apps that do run-time CPU detection have to compile separate files for each supported architecture using the right flags. And especially — the file holding the CPU detection code should be compiled <red>without</red> these options. 😭

---

- **`-mdump-tune-features`** — tells GCC to dump the names of the x86 performance tuning features and their default settings. Those names are exactly what you feed into <cyan>`-mtune-ctrl=feature-list`</cyan>. 🎀

- **`-mtune-ctrl=feature-list`** — fine-grain control over x86 code generation features. <purple>*feature-list*</purple> is a comma-separated list of feature names. Also see <cyan>`-mdump-tune-features`</cyan>. When you specify a feature, it gets turned <green>on</green> if it's not preceded by `^`, and turned <red>off</red> if it is. This one is <hotpink>intended to be used by GCC developers</hotpink> — using it may lead you into code paths that testing doesn't cover, and it can potentially result in compiler <red>ICEs</red> or runtime errors. 💀

- **`-mno-default`** — tells GCC to turn off all tunable features. See also <cyan>`-mtune-ctrl=feature-list`</cyan> and <cyan>`-mdump-tune-features`</cyan>. 🌸

- **`-mcld`** — makes GCC emit a `"cld"` instruction in the prologue of functions that use string instructions. String instructions depend on the <purple>DF flag</purple> to pick between autoincrement or autodecrement mode. The ABI says the DF flag is cleared on function entry, but some operating systems <red>violate</red> that spec by not clearing DF in their exception dispatchers. Then the exception handler can get invoked with DF set, which leads to wrong direction mode when string instructions get used. 😱 You can turn this on by default on 32-bit x86 targets by configuring GCC with the <cyan>`--enable-cld`</cyan> configure option — and in that case you suppress the `"cld"` instructions with the <cyan>`-mno-cld`</cyan> compiler option.

- **`-mvzeroupper`** — makes GCC emit a `"vzeroupper"` instruction before control flow transfers out of the function, so the AVX-to-SSE transition penalty is minimized and unnecessary `"zeroupper"` intrinsics get removed. 🧚

- **`-mprefer-avx128`** — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. ✨

- **`-mprefer-vector-width=`<purple>*opt*</purple> — tells GCC to use <purple>*opt*</purple>-bit vector width in instructions instead of whatever the default is on the selected platform.

- **`-mmove-max=`<purple>*bits*</purple> — sets the maximum number of bits that can be moved from memory to memory efficiently to <purple>*bits*</purple>. Valid values for <purple>*bits*</purple> are `128`, `256` and `512`.

- **`-mstore-max=`<purple>*bits*</purple> — sets the maximum number of bits that can be stored to memory efficiently to <purple>*bits*</purple>. Valid values for <purple>*bits*</purple> are `128`, `256` and `512`.

And here's what the <purple>*opt*</purple> values actually mean, bestie:

- `none` — no extra limitations applied to GCC beyond what the selected platform already defines. 🫶
- `128` — prefer 128-bit vector width for instructions.
- `256` — prefer 256-bit vector width for instructions.
- `512` — prefer 512-bit vector width for instructions.

---

- **`-mcx16`** — lets GCC generate `"CMPXCHG16B"` instructions 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 that are bigger than one machine word. The compiler uses this instruction to implement <cyan>`__sync` Builtins</cyan>. But for <cyan>`__atomic` Builtins</cyan> operating on 128-bit integers, a library call is <hotpink>always</hotpink> used. periodt 💅

- **`-msahf`** — enables generation of `"SAHF"` instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step landed in December 2005, <red>lacked</red> the `"LAHF"` and `"SAHF"` instructions that AMD64 supports. Those two are the load and store instructions, respectively, for certain status flags. In 64-bit mode, the `"SAHF"` instruction is used to optimize the `"fmod"`, `"drem"`, and `"remainder"` built-in functions — see <cyan>Other Builtins</cyan> for the tea. 🍓

- **`-mmovbe`** — enables use of the `"movbe"` instruction to implement `"__builtin_bswap32"` and `"__builtin_bswap64"`.

- **`-mshstk`** — enables shadow stack built-in functions from x86 <purple>Control-flow Enforcement Technology (CET)</purple>. 🔥

- **`-mcrc32`** — enables the built-in functions `"__builtin_ia32_crc32qi"`, `"__builtin_ia32_crc32hi"`, `"__builtin_ia32_crc32si"` and `"__builtin_ia32_crc32di"` so they generate the `"crc32"` machine instruction.

- **`-mmwait`** — enables the built-in functions `"__builtin_ia32_monitor"` and `"__builtin_ia32_mwait"` so they generate the `"monitor"` and `"mwait"` machine instructions.

- **`-mrecip`** — enables use of `"RCPSS"` and `"RSQRTSS"` instructions (plus their vectorized variants `"RCPPS"` and `"RSQRTPS"`) with an extra Newton-Raphson step to bump up precision, instead of `"DIVSS"` and `"SQRTSS"` (and their vectorized variants) for single-precision floating-point arguments. These instructions only get generated when <cyan>`-funsafe-math-optimizations`</cyan> is enabled together with <cyan>`-ffinite-math-only`</cyan> and <cyan>`-fno-trapping-math`</cyan>. Note that even though the throughput of the sequence beats the throughput of the non-reciprocal instruction, the precision of the sequence can drop by up to <hotpink>2 ulp</hotpink> — like, the inverse of `1.0` equals `0.99999994`. 💀

  Also note that GCC already implements `"1.0f/sqrtf(x)"` in terms of `"RSQRTSS"` (or `"RSQRTPS"`) with <cyan>`-ffast-math`</cyan> (or the option combo above), and it does <green>not</green> need `-mrecip`. 🎀

  And also note: GCC already emits that sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized `"sqrtf(x)"` with <cyan>`-ffast-math`</cyan> (or the combo above), and doesn't need `-mrecip` either. ✨

- **`-mrecip=`<purple>*opt*</purple> — controls which reciprocal estimate instructions may be used. <purple>*opt*</purple> is a comma-separated list of options, and any of them may 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.

  So, for example, `-mrecip=all,!sqrt` enables all of the reciprocal approximations except for square root. 🌈

- **`-mveclibabi=`<purple>*type*</purple> — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for <purple>*type*</purple> are `svml` for the Intel short vector math library and `acml` for the AMD math core library. To use this option, <yellow>both</yellow> <cyan>`-ftree-vectorize`</cyan> and <cyan>`-funsafe-math-optimizations`</cyan> have to be enabled, and an SVML- or ACML-ABI-compatible library must be specified at link time. 🫶

  Right now, when <cyan>`-mveclibabi=svml`</cyan> is used, GCC emits calls to `"vmldExp2"`, `"vmldLn2"`, `"vmldLog102"`, `"vmldPow2"`, `"vmldTanh2"`, `"vmldTan2"`, `"vmldAtan2"`, `"vmldAtanh2"`, `"vmldCbrt2"`, `"vmldSinh2"`, `"vmldSin2"`, `"vmldAsinh2"`, `"vmldAsin2"`, `"vmldCosh2"`, `"vmldCos2"`, `"vmldAcosh2"`, `"vmldAcos2"`, `"vmlsExp4"`, `"vmlsLn4"`, `"vmlsLog104"`, `"vmlsPow4"`, `"vmlsTanh4"`, `"vmlsTan4"`, `"vmlsAtan4"`, `"vmlsAtanh4"`, `"vmlsCbrt4"`, `"vmlsSinh4"`, `"vmlsSin4"`, `"vmlsAsinh4"`, `"vmlsAsin4"`, `"vmlsCosh4"`, `"vmlsCos4"`, `"vmlsAcosh4"` and `"vmlsAcos4"` for the corresponding function type.

  And when <cyan>`-mveclibabi=acml`</cyan> is used, it emits `"__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. 👑

- **`-mabi=name`** — bestie, this one picks the <pink>calling convention</pink> your code gets compiled for ✨. The only values you're allowed to pass are `sysv` (the ABI used on GNU/Linux and other systems) and `ms` (the Microsoft ABI). Default vibe? If you're targeting Microsoft Windows you get the Microsoft ABI, and literally everywhere else you get the SysV ABI. Wanna flip it for just *some* functions instead of the whole file? Use the function attributes `ms_abi` and `sysv_abi` and go off 💅.

- **`-mforce-indirect-call`** — forces <hotpink>every single call to functions to be indirect</hotpink>. Why would you want that?? Because when you're using Intel Processor Trace it generates more precise timing information for function calls. It's giving detective energy 🔍.

- **`-mmanual-endbr`** — drops the ENDBR instruction at function entry <purple>only</purple> via the `cf_check` function attribute. Super useful when you pair it with `-fcf-protection=branch`, so you get to control ENDBR insertion at the function entry yourself. You're the main character, you decide 👑.

- **`-mcall-ms2sysv-xlogues`** — okay so, because 64-bit ABIs are messy, any Microsoft ABI function that calls a System V ABI function **must** treat RSI, RDI and XMM6-15 as clobbered 😭. By default the code that saves and restores those registers is emitted inline, which gives you fairly lengthy prologues and epilogues (so long, honestly). If you turn this on with `-mcall-ms2sysv-xlogues`, the prologues and epilogues use stubs in the static portion of `libgcc` to do the saving and restoring instead — so your function size goes down at the cost of a few extra instructions. trade-off queen 🎀.

- **`-mtls-dialect=type`** — generates code to access thread-local storage using the `gnu` or `gnu2` conventions. `gnu` is the conservative default; `gnu2` is more efficient, but it may add compile- and run-time requirements that <red>cannot be satisfied on all systems</red> 🫣. So like, slay, but know your audience.

- **`-mpush-args`**, **`-mno-push-args`** — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. Sometimes turning it off may improve performance though, because of improved scheduling and reduced dependencies. lowkey a scheduling glow-up 💅.

- **`-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? A notable increase in code size 📈. Heads up: this switch implies `-mno-push-args`.

- **`-mthreads`** — supports thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling **must** compile and link all code with the `-mthreads` option (all of it, no skipping, fr fr). When compiling, `-mthreads` defines `-D_MT`; when linking, it links in a special thread helper library `-lmingwthrd` which cleans up per-thread exception-handling data 🧹.

- **`-mms-bitfields`**, **`-mno-ms-bitfields`** — enable/disable bit-field layout that's compatible with the native Microsoft Windows compiler. Here's the tea: 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 info, see <cyan>x86 Variable Attributes</cyan> and <cyan>x86 Type Attributes</cyan>.

  The Microsoft structure layout algorithm is fairly simple, with the exception of the bit-field packing. The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

  1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.

  2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the `aligned` attribute or the `pack` pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are **not** coalesced.

For example:

struct { unsigned long bf_1 : 12; unsigned long : 0; unsigned long bf_2 : 12; } t1;

The size of `t1` is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, `t1`'s size would be 4 bytes. wild, right? 😭

2. If a zero-length bit-field is inserted after a bit-field, `foo`, and the alignment of the zero-length bit-field is greater than the member that follows it, `bar`, `bar` is aligned as the type of the zero-length bit-field.

For example:

struct { char foo : 4; short : 0; char bar; } t2;

struct { char foo : 4; short : 0; double bar; } t3;

For `t2`, `bar` is placed at offset `2`, rather than offset `1`. Accordingly, the size of `t2` is `4`. For `t3`, the zero-length bit-field does not affect the alignment of `bar` or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example, `t2` has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type `short`.

2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:

struct { char foo : 6; long : 0; } t4;

Here, `t4` takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:

struct { char foo; long : 0; char bar; } t5;

Here, `t5` takes up 2 bytes.

- **`-mno-align-stringops`** — do **not** align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it 🙈. (It's giving "trust me bro, it's aligned.")

- **`-minline-all-stringops`** — by default GCC inlines string operations only when the destination is known to be aligned to at 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. Bonus: the option enables inline expansion of `strlen` for **all** pointer alignments ✨.

- **`-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. Best of both worlds, we love a hybrid 🌈.

- **`-mstringop-strategy=alg`** — overrides the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for *alg* are:

- `rep_byte`
- `rep_4byte`
- `rep_8byte`

Expand using i386 `rep` prefix of the specified size.

- `byte_loop`
- `loop`
- `unrolled_loop`

Expand into an inline loop.

- `libcall`

Always use a library call.

- **`-mmemcpy-strategy=strategy`** — overrides the internal decision heuristic to decide if `__builtin_memcpy` should be inlined and what inline algorithm to use when the expected size of the copy operation is known. *strategy* is a comma-separated list of *alg*:*max_size*:*dest_align* triplets. *alg* is specified in `-mstringop-strategy`, *max_size* specifies the max byte size with which inline algorithm *alg* is allowed. For the last triplet, the *max_size* **must** be `-1`. The *max_size* of the triplets in the list must be specified in increasing order 📊. The minimal byte size for *alg* is `0` for the first triplet and `max_size + 1` of the preceding range.

- **`-mmemset-strategy=strategy`** — the option is similar to `-mmemcpy-strategy=` except that it is to control `__builtin_memset` expansion. same energy, different builtin 💅.

- **`-momit-leaf-frame-pointer`** — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions 🎁. The option `-fomit-leaf-frame-pointer` removes the frame pointer for leaf functions, which might make debugging harder 🫣.

- **`-mtls-direct-seg-refs`**, **`-mno-tls-direct-seg-refs`** — controls whether TLS variables may be accessed with offsets from the TLS segment register (`%gs` for 32-bit, `%fs` for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. For systems that use the GNU C Library, the default is <green>on</green> ✅.

- **`-msse2avx`**, **`-mno-sse2avx`** — specifies that the assembler should encode SSE instructions with VEX prefix. The option `-mavx` turns this on by default. cute little automatic 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 `-mfentry` and `-pg` 😭.

- **`-mrecord-mcount`**, **`-mno-record-mcount`** — okay so if profiling is turned on (`-pg`), this makes a `__mcount_loc` section stuffed with pointers to every single profiling call. why do you care? bestie, it's literally for automatically patching and out calls. 💅

- **`-mnop-mcount`**, **`-mno-nop-mcount`** — if profiling is active (`-pg`), generate the calls to the profiling functions as NOPs instead. this is giving "patch me in later, dynamically" energy, and it's *likely only useful together with* `-mrecord-mcount`. no cap.

- **`-minstrument-return=type`** — instrument function exit inside `-pg -mfentry` instrumented functions with a call to a specified function. heads up: this only instruments true returns that end with `ret`, **not** sibling calls ending with `jump`. valid types are: `none` to not instrument at all, `call` to generate a call to `__return__`, or `nop5` to generate a 5 byte nop. slay, pick one. ✨

- **`-mrecord-return`**, **`-mno-record-return`** — generate a `__return_loc` section pointing to all return instrumentation code. little index of your return-instrumentation, iconic. 🎀

- **`-mfentry-name=name`** — sets the name of the `__fentry__` symbol that gets called at function entry for `-pg -mfentry` functions. name it whatever, it's your call. 💜

- **`-mfentry-section=name`** — sets the name of the section used to record `-mrecord-mcount` calls. the default is `__mcount_loc`.

- **`-mskip-rax-setup`**, **`-mno-skip-rax-setup`** — when you're generating code for the x86-64 architecture with SSE extensions **disabled**, `-mskip-rax-setup` can be used to skip setting up the RAX register when there are no variable arguments passed in vector registers. ✨

> **Warning:** Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't have those issues, regardless the RAX register value. 💀 so like, read that again if you're on something old.

- **`-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, and if both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. smart tbh. 🧠

- **`-mavx256-split-unaligned-load`**, **`-mavx256-split-unaligned-store`** — split 32-byte AVX unaligned load and store. that's the whole thing. short king. 🔥

- **`-mstack-protector-guard=guard`**, **`-mstack-protector-guard-reg=reg`**, **`-mstack-protector-guard-offset=offset`** — generate stack protection code using canary at *guard*. supported locations are `global` for global canary, or `tls` for per-thread canary in the TLS block (**the default**). this option has effect **only** when `-fstack-protector` or `-fstack-protector-all` is specified. 💅

with the latter choice, the options `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` furthermore specify which segment register (`%fs` or `%gs`) to use as base register for reading the canary, and from what offset from that base register. the default for those is as specified in the relevant ABI. 🦋

- **`-mgeneral-regs-only`** — generate code that uses *only* the general-purpose registers. this prevents the compiler from using floating-point, vector, mask and bound registers. sorry to the fancy registers, they're not invited. 🙈

- **`-mrelax-cmpxchg-loop`** — relax cmpxchg loop by emitting an early load and compare before `cmpxchg`, execute pause if load value is not expected. this reduces excessive cachline bouncing when and works for all atomic logic fetch builtins that generates compare and swap loop. 🔥

- **`-mindirect-branch=choice`** — convert indirect call and jump with *choice*. the default is `keep`, which keeps indirect call and jump unmodified. `thunk` converts indirect call and jump to call and return thunk. `thunk-inline` converts indirect call and jump to inlined call and return thunk. `thunk-extern` converts indirect call and jump to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attribute `"indirect_branch"`.

note that `-mcmodel=large` is **incompatible** with `-mindirect-branch=thunk` and `-mindirect-branch=thunk-extern` since the thunk function may not be reachable in the large code model. 🫠

note that `-mindirect-branch=thunk-extern` is compatible with `-fcf-protection=branch` since the external thunk can be made to enable control-flow check. ✨

- **`-mfunction-return=choice`** — convert function return with *choice*. the default is `keep`, which keeps function return unmodified. `thunk` converts function return to call and return thunk. `thunk-inline` converts function return to inlined call and return thunk. `thunk-extern` converts function return to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attribute `"function_return"`.

note that `-mindirect-return=thunk-extern` is compatible with `-fcf-protection=branch` since the external thunk can be made to enable control-flow check. 💖

note that `-mcmodel=large` is **incompatible** with `-mfunction-return=thunk` and `-mfunction-return=thunk-extern` since the thunk function may not be reachable in the large code model. 😭

- **`-mindirect-branch-register`** — force indirect call and jump via register. that's it, that's the vibe.

- **`-mharden-sls=choice`** — generate code to mitigate against straight line speculation (SLS) with *choice*. the default is `none` which disables all SLS hardening. `return` enables SLS hardening for function returns. `indirect-jmp` enables SLS hardening for indirect jumps. `all` enables all SLS hardening. pick your fighter. 👑

- **`-mindirect-branch-cs-prefix`** — add CS prefix to call and jmp to 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 with `lfence; call *%r8-r15` or `lfence; jmp *%r8-r15` at run-time. iconic byte math. ⭐

---

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. let's break it down bestie:

- the `-m32` option sets `"int"`, `"long"`, and pointer types to 32 bits, and generates code that runs on any i386 system.
- the `-m64` option sets `"int"` to 32 bits and `"long"` and pointer types to 64 bits, and generates code for the x86-64 architecture. for Darwin **only** the `-m64` option also turns off the `-fno-pic` and `-mdynamic-no-pic` options.
- the `-mx32` option sets `"int"`, `"long"`, and pointer types to 32 bits, and generates code for the x86-64 architecture.
- the `-m16` option is the same as `-m32`, except for that it outputs the `.code16gcc` assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.
- the `-miamcu` option generates code which conforms to Intel MCU psABI. it requires the `-m32` option to be turned on.

- **`-mno-red-zone`** — do **not** use a so-called "red zone" for x86-64 code. the red zone is mandated by the x86-64 ABI; it'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-zone` disables this red zone. 🚫🎀

- **`-mcmodel=small`** — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. pointers are 64 bits. programs can be statically or dynamically linked. **this is the default code model.** the main character, lowkey.

- **`-mcmodel=kernel`** — generate code for the kernel code model. the kernel runs in the negative 2 GB of the address space. this model has to be used for Linux kernel code. periodt. 🐧

- **`-mcmodel=medium`** — generate code for the medium model: the program is linked in the lower 2 GB of the address space. small symbols are also placed there. symbols with sizes larger than `-mlarge-data-threshold` are put into large data or BSS sections and can be located above 2GB. programs can be statically or dynamically linked.

- **`-mcmodel=large`** — generate code for the large model. this model makes **no assumptions** about addresses and sizes of sections. free spirit fr fr. 🦋

- **`-maddress-mode=long`** — generate code for long address mode. this is **only** supported for 64-bit and x32 environments. it is the default address mode for 64-bit environments.

- **`-maddress-mode=short`** — generate code for short address mode. this is **only** supported for 32-bit and x32 environments. it is the default address mode for 32-bit and x32 environments. 💜

- **`-mneeded`**, **`-mno-needed`** — emit `GNU_PROPERTY_X86_ISA_1_NEEDED` GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. it's giving "please check my requirements list". ✨

- **`-mno-direct-extern-access`** — without `-fpic` nor `-fPIC`, always use the GOT pointer to access external symbols. with `-fpic` or `-fPIC`, treat access to protected symbols as local symbols. the default is `-mdirect-extern-access`.

> **Warning:** shared libraries compiled with `-mno-direct-extern-access` and executable compiled with `-mdirect-extern-access` may **not** be binary compatible if protected symbols are used in shared libraries and executable. 😭 do not do this to yourself.

---

### 🪟 x86 Windows Options 🪟

these additional options are available for Microsoft Windows targets: 💻

- **`-mconsole`** — this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. this option is available for Cygwin and MinGW targets and is **enabled by default** on those targets. 🎀

- **`-mdll`** — this option is available for Cygwin and MinGW targets. it specifies that a DLL---a dynamic link library---is to be generated, enabling the selection of the required runtime startup object and entry point. ✨

- **`-mnop-fun-dllimport`** — this option is available for Cygwin and MinGW targets. it specifies that the `"dllimport"` attribute should be **ignored**. ngl, rude to the attribute but okay.

- **`-mthreads`** — this option is available for MinGW targets. it specifies that MinGW-specific thread support is to be used. 🧵

- **`-municode`** — this option is available for MinGW-w64 targets. it causes the `"UNICODE"` preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌈

- **`-mwin32`** — this option is available for Cygwin and MinGW targets. it specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does **not** influence the choice of runtime library/startup code. 🙈

- **`-mwindows`** — bestie, this one's for <cyan>Cygwin</cyan> and <cyan>MinGW</cyan> targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. Slay. 💅

- **`-fno-set-stack-executable`** — this option is available for <cyan>MinGW</cyan> targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the `User32` API, which is used to set executable privileges, isn't available. No cap. 🫶

- **`-fwritable-relocated-rdata`** — this option is available for <cyan>MinGW</cyan> and <cyan>Cygwin</cyan> targets. It specifies that relocated-data in read-only section is put into the `.data` section. This is a necessary for older runtimes not supporting modification of `.rdata` sections for pseudo-relocation. Periodt.

- **`-mpe-aligned-commons`** — this option is available for <cyan>Cygwin</cyan> and <cyan>MinGW</cyan> targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. Iconic.

See also under <cyan>x86 Options</cyan> for standard options, bestie. ✨

---

## 🦋 Xstormy16 Options 🦋

These options are defined for Xstormy16:

- **`-msim`** — choose startup files and linker script suitable for the simulator. Simple, clean, iconic. 💖

---

## 🌈 Xtensa Options 🌈

These options are supported for Xtensa targets:

- **`-mconst16`, `-mno-const16`** — enable or disable use of `CONST16` instructions for loading constant values. The `CONST16` instruction is currently not a standard option from Tensilica. When enabled, `CONST16` instructions are always used in place of the standard `L32R` instructions. The use of `CONST16` is enabled by default only if the `L32R` instruction is not available. 💀

- **`-mfused-madd`, `-mno-fused-madd`** — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. 😭

- **`-mserialize-volatile`, `-mno-serialize-volatile`** — when this option is enabled, GCC inserts `MEMW` instructions before `volatile` memory references to guarantee sequential consistency. The default is `-mserialize-volatile`. Use `-mno-serialize-volatile` to omit the `MEMW` instructions. 🌸

- **`-mforce-no-pic`** — for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. 🔥

- **`-mtext-section-literals`, `-mno-text-section-literals`** — these options control the treatment of literal pools. The default is `-mno-text-section-literals`, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With `-mtext-section-literals`, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. 💅

- **`-mauto-litpools`, `-mno-auto-litpools`** — these options control the treatment of literal pools. The default is `-mno-auto-litpools`, which places literals in a separate section in the output file unless `-mtext-section-literals` is used. With `-mauto-litpools` the literals are interspersed in the text section by the assembler. Compiler does not produce explicit `.literal` directives and loads literals into registers with `MOVI` instructions instead of `L32R` to let the assembler do relaxation and place literals as necessary. This option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with `-mtext-section-literals`. 🧚

- **`-mtarget-align`, `-mno-target-align`** — when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is `-mtarget-align`. These options do not affect the treatment of auto-aligned instructions like `LOOP`, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. 👑

- **`-mlongcalls`, `-mno-longcalls`** — when this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct `CALL` instruction into an `L32R` followed by a `CALLX` instruction. The default is `-mno-longcalls`. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. 🫶

- **`-mabi=name`** — generate code for the specified ABI. Permissible values are: `call0`, `windowed`. Default ABI is chosen by the Xtensa core configuration. ✨

- **`-mabi=call0`** — when this option is enabled function parameters are passed in registers `a2` through `a7`, registers `a12` through `a15` are caller-saved, and register `a15` may 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 registers `a10` through `a15`, and called function rotates register window by 8 registers on entry so that its arguments are found in registers `a2` through `a7`. Register `a7` may be used as a frame pointer. 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. 🌈

---

## 👑 zSeries Options 👑

These are listed under

# 🌸 ENVIRONMENT 🌸

okay bestie, this section is literally about all the env vars that change how `gcc` does its thing 💅 some of them are like "hey here's a directory to look in", some of them are like "hey here's a whole vibe for the compilation environment". all valid.

also quick heads up — you can *also* tell gcc where to look with options like `-B`, `-I` and `-L`. those **win** over env vars, and env vars **win** over whatever gcc was configured with at build time. priority order, periodt. 👑

---

## 🎀 Localization besties: `LANG`, `LC_CTYPE`, `LC_MESSAGES`, `LC_ALL`

- **`LANG`**, **`LC_CTYPE`**, **`LC_MESSAGES`**, **`LC_ALL`** — these control how GCC uses localization info so it can vibe with different national conventions. GCC peeks at the locale categories `LC_CTYPE` and `LC_MESSAGES` if it was configured to do so. these categories can be literally any value your installation supports. a typical one is `en_GB.UTF-8` for English in the United Kingdom encoded in UTF-8. 🦋

`LC_CTYPE` specifies character classification. GCC uses it to figure out character boundaries in a string — and that's needed for some multibyte encodings that contain quote and escape characters which would otherwise get read as a string end or an escape. no cap.

`LC_MESSAGES` specifies the language to use in diagnostic messages. (so your errors can yell at you in your language 💀)

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 the `LANG` environment variable. and if none of these are set at all? GCC defaults to traditional C English behavior. classic. 🙈

---

## 🧚 `TMPDIR`

- **`TMPDIR`** — if it's set, it tells gcc which directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's gonna be the input to the next stage. for example: the output of the preprocessor, which is the input to the compiler proper. pipeline energy. ✨

---

## 💜 `GCC_COMPARE_DEBUG`

- **`GCC_COMPARE_DEBUG`** — setting this is *nearly* equivalent to passing `-fcompare-debug` to the compiler driver. check out that option's docs for the deets.

---

## 👑 `GCC_EXEC_PREFIX`

- **`GCC_EXEC_PREFIX`** — if set, it specifies a prefix to use in the names of the subprograms executed by the compiler. no slash gets added when this prefix is combined with a subprogram name, but you *can* specify a prefix that ends with a slash if you want. your call bestie.

if `GCC_EXEC_PREFIX` is not set, GCC tries to figure out an appropriate prefix based on the pathname it was invoked with.

if GCC can't find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. it's giving determined. 🔥

the default value of `GCC_EXEC_PREFIX` is `prefix/lib/gcc/` where *prefix* is the prefix to the installed compiler. in many cases *prefix* is the value of `"prefix"` when you ran the `configure` script.

other prefixes specified with `-B` take precedence over this prefix.

this prefix is also used for finding files such as `crt0.o` that are used for linking.

and in addition, the prefix is used in an unusual way in finding the directories to search for header files. for each of the standard directories whose name normally begins with `/usr/local/lib/gcc` (more precisely, with the value of `GCC_INCLUDE_DIR`), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. 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. icon behavior, honestly. 💅

---

## 🌈 `COMPILER_PATH`

- **`COMPILER_PATH`** — its value is a colon-separated list of directories, much like `PATH`. GCC tries the directories you specify here when searching for subprograms, if it couldn't find the subprograms using `GCC_EXEC_PREFIX`.

---

## 🍓 `LIBRARY_PATH`

- **`LIBRARY_PATH`** — its value is a colon-separated list of directories, much like `PATH`. when configured as a native compiler, GCC tries these directories when searching for special linker files, if it can't find them using `GCC_EXEC_PREFIX`. linking using GCC also uses these directories when searching for ordinary libraries for the `-l` option — but directories specified with `-L` come first. 🫶

---

## 🌸 `LANG` (yes, again, she contains multitudes)

this variable is used to pass locale information to the compiler. one way that info gets used is determining the character set used when character literals, string literals and comments are parsed in C and C++. when the compiler is configured to allow multibyte characters, these values for `LANG` are recognized:

- **`C-JIS`** — recognize JIS characters.
- **`C-SJIS`** — recognize SJIS characters.
- **`C-EUCJP`** — recognize EUCJP characters.

if `LANG` is not defined, or if it has some other value, then the compiler uses `"mblen"` and `"mbtowc"` as defined by the default locale to recognize and translate multibyte characters.

---

## 💅 `GCC_EXTRA_DIAGNOSTIC_OUTPUT`

if `GCC_EXTRA_DIAGNOSTIC_OUTPUT` is set to one of these values, then extra text gets emitted to `stderr` when fix-it hints are emitted. heads up: `-fdiagnostics-parseable-fixits` and `-fno-diagnostics-parseable-fixits` take precedence over this environment variable. so say it with me — options win. periodt.

- **`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`** — same as `"fixits-v1"`, but columns are expressed as display columns, as per `-fdiagnostics-column-unit=display`.

---

a few more env vars affect the behavior of the preprocessor. 🧚

## 🎀 `CPATH`, `C_INCLUDE_PATH`, `CPLUS_INCLUDE_PATH`, `OBJC_INCLUDE_PATH`

- **`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's a semicolon, and for almost all other targets it's a colon.

`CPATH` specifies a list of directories to be searched as if specified with `-I`, but *after* any paths given with `-I` options on the command line. this env var is used regardless of which language is being preprocessed.

the remaining env vars apply only when preprocessing the particular language indicated. each specifies a list of directories to be searched as if specified with `-isystem`, but after any paths given with `-isystem` options on the command line.

in all these variables, an empty element tells 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`. slay. ✨

---

## 🔥 `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. (not them being excluded 💀)

the value of `DEPENDENCIES_OUTPUT` can be just a file name — in which case the Make rules are written to that file, guessing the target name from the source file name. or the value can have the form *file target*, in which case the rules are written to file *file* using *target* as the target name.

in other words, this environment variable is equivalent to combining the options `-MM` and `-MF`, with an optional `-MT` switch too.

---

## 🦋 `SUNPRO_DEPENDENCIES`

this variable is the same as `DEPENDENCIES_OUTPUT` (see above), except that system header files are **not** ignored, so it implies `-M` rather than `-MM`. *however* — the dependence on the main input file is omitted. plot twist.

---

## ⭐ `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. reproducible builds besties, we love her. 🫶

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/>. go off, tell them everything.

---

# 🎀 FOOTNOTES

1. on some systems, `gcc -shared` needs to build supplementary stub code for constructors to work. on multi-libbed systems, `gcc -shared` must select the correct support libraries to link against. failing to supply the correct flags may lead to subtle defects. supplying them in cases where they are not necessary is innocuous. so like, worst case you're fine. best case you're also fine. just supply the right ones. 🙈

---

# 🌈 SEE ALSO

`gpl(7)`, `gfdl(7)`, `fsf-funding(7)`, `cpp(1)`, `gcov(1)`, `as(1)`, `ld(1)`, `gdb(1)`, `dbx(1)` and the Info entries for `gcc`, `cpp`, `as`, `ld`, `binutils` and `gdb`.

---

# 👑 AUTHOR

see the Info entry for `gcc`, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. the whole squad, fr fr.

---

# 💜 COPYRIGHT

Copyright (c) 1988-2022 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 a wrap bestie, that's literally the whole page 🫶 this edition is `gcc-12`, dated `2022-03-19`, and it lives in section `GCC(1)` of the manual. go forth and compile something iconic. 💅✨