cpp-compiler-flags

Development

C++ compiler/linker flag guidance for Serial Studio's build (GCC, Clang, AppleClang, MSVC cl.exe, clang-cl, MinGW, IntelLLVM). Use when reading, changing, or reasoning about the cmake flag modules (Optimization/Hardening/Sanitizers/MiMalloc), tuning -O/-march/LTO/PGO, adding a per-toolchain branch, debugging a flag that one compiler rejects, or explaining what a flag does. Encodes this repo's actual flag layout and its non-negotiable invariants (IEEE-stable math, Lua unwind tables, x86-64-v2 baseline, the two-stage PGO flow). It does NOT build, configure, or run cmake -- the developer does that.

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Serial Studio -- C++ compiler flags

Serial Studio's release builds are aggressively optimized and hardened across seven toolchains. The flag surface lives in four cmake modules, not scattered through CMakeLists.txt or per-target calls, and those modules are the single source of truth -- read them before answering anything flag-related:

Module	Owns
`cmake/Optimization.cmake`	`-O3`/`/O2`, `-march`, SIMD, LTO, and the full PGO (GENERATE/USE) flow, per toolchain.
`cmake/Hardening.cmake`	Stack protectors, FORTIFY, CFI/CET, RELRO/DEP, ASLR, `serial_studio_harden()` per-target deltas.
`cmake/Sanitizers.cmake`	ASan+UBSan (`DEBUG_SANITIZER`) and the separate TSan build (`ENABLE_TSAN`).
`cmake/MiMalloc.cmake`	The process-wide mimalloc allocator override (Windows/MSVC + Linux; macOS opts out).

Wiring: the four user-facing options (DEBUG_SANITIZER, PRODUCTION_OPTIMIZATION, ENABLE_HARDENING, ENABLE_PGO) are declared in CMakeLists.txt (around lines 79-87); PGO_STAGE and the internal DISABLE_LTO live in Optimization.cmake, ENABLE_TSAN in Sanitizers.cmake. The modules are include()d in CMakeLists.txt (MiMalloc -> Optimization -> Sanitizers, then Hardening, then Signing). The PGO two-stage flow is driven by .github/workflows/ci.yml (the only workflow file).

This skill does NOT build

Per CLAUDE.md, the developer runs cmake, the compiler, and the benchmark themselves. Never invoke cmake/jom/clang/cl/the compiler to "test" a flag. Reason from the modules and the references; propose the edit; hand off. Flag changes that touch the hotpath are gated by --benchmark-hotpath (256 kHz) -- flag it for the developer to re-run; see [[ss-hotpath]].

Build-option matrix (all default OFF except where noted)

Option	Effect	Set by
`PRODUCTION_OPTIMIZATION`	The aggressive per-toolchain release branch (`-O3`/`/O2`, `-march=x86-64-v2`, LTO).	Release CI; manual.
`ENABLE_PGO` + `PGO_STAGE`	Profile-guided opt; `GENERATE` then `USE`. Needs `PRODUCTION_OPTIMIZATION`.	CI two-pass; manual.
`ENABLE_HARDENING`	Defense-in-depth (canaries, FORTIFY, CFI, RELRO, CFG).	CI; auto-on for Release/RelWithDebInfo and sandboxed builds.
`DEBUG_SANITIZER`	ASan + UBSan (Debug only, GCC/Clang).	Manual.
`ENABLE_TSAN`	ThreadSanitizer (proves the lock-free hotpath); mutually exclusive with ASan and MSVC.	Manual / CI thread-safety job.

DISABLE_LTO is internal: forced ON for Flatpak/sandboxed builds (which also auto-enable hardening). LTO is otherwise ON whenever PRODUCTION_OPTIMIZATION is.

Non-negotiable invariants (do not regress these)

These are deliberate choices baked into the modules. A flag change that breaks one is a bug, not an optimization:

IEEE-stable math, always. Every non-MSVC branch pins -fno-fast-math -fno-unsafe-math-optimizations; MSVC/clang-cl use /fp:precise. Never add -ffast-math, /fp:fast, -funsafe-math-optimizations, -Ofast, or -ffp-contract=fast -- telemetry output must be bit-stable across builds and platforms. This is the one rule most likely to be "helpfully" violated.
Unwind tables on every TU (non-MSVC). -fexceptions -funwind-tables -fasynchronous-unwind-tables; on macOS the lua54 target additionally opts out of LTO with -fno-lto -femit-dwarf-unwind=no-compact-unwind (Xcode 26's ld drops fallback DWARF unwind under -flto, llvm#135888, and made -Wl,-keep_dwarf_unwind a no-op). Lua is compiled as C++ and throws across the VM stack; without unwind metadata, --gc-sections/-dead_strip
- LTO can drop a personality routine and turn a routine Lua error into std::terminate. See [[ss-cpp-modern]] and the Lua exception-safety setup. Do not remove these to "shrink" the binary.
x86-64-v2 is the conservative baseline (SSE4.2, 2012+ CPUs). Every non-MSVC x86-64 branch uses -march=x86-64-v2; clang-cl passes /clang:-march=x86-64-v2; cl.exe gets no -march (the x64 ABI's SSE2 baseline stands). Do not bump to v3/v4 or -march=native for shipped binaries -- it would SIGILL on older CPUs. ARM baselines: aarch64 -> armv8-a (+-latomic); armv7l -> armv7-a -mfpu=neon -mfloat-abi=hard (hardfloat pinned on purpose).
-msse4.1 is added for DSP/FFT on non-MSVC x86-64, separate from the -march baseline. MSVC/clang-cl are skipped (the x64 ABI guarantees SSE2; no explicit SSE4.1 there).
Hardening auto-enables for optimized configs. A manual Release that forgets -DENABLE_HARDENING=ON still gets hardened (belt-and-suspenders in Hardening.cmake). Don't "simplify" that away.
Per-toolchain branches are exclusive and order-sensitive. clang-cl reports MSVC=ON, so its branch must precede cl.exe's; MinGW-Clang precedes MinGW-GCC. Adding a branch in the wrong order silently routes a compiler to the wrong flags.

The clang-cl / lld-link gotcha (Windows, MSVC ABI)

This trips up almost everyone, so it is stated once here and detailed in the reference: CMake links this target with lld-link directly, not through the clang-cl driver. So:

LTO/PGO compile flags use the /clang: passthrough (/clang:-flto=thin, /clang:-fprofile-generate=, /clang:-fprofile-use=) and go on the compile step only.
The link step gets native lld-link flags only (/lldltocache:, /OPT:REF, /OPT:ICF) -- never driver-style -flto=thin / -fprofile-*, which lld-link rejects as bogus input.
ThinLTO needs no link flag: bitcode .objs are auto-detected; PGO instrumentation embeds an /INCLUDE: that pulls in clang_rt.profile automatically.
clang-cl rejects /Qspectre /sdl /ZH:SHA_256 (those are cl.exe-only); it takes /GS /guard:cf and the link-side /GUARD:CF /DYNAMICBASE /CETCOMPAT.

When to use which reference

Changing this repo's build, or "why is this flag here / who sets it" -> references/serial-studio-build-flags.md. The authoritative per-module, per-toolchain map of what Serial Studio actually passes, the PGO CI flow, mimalloc, and the rationale for each invariant.
"What does flag X do" / "what's the Clang equivalent of /GL" / picking a new flag -> references/flag-catalog.md. A cross-compiler catalog (GCC / Clang / MSVC families + the PGO concept) distilled from the canonical upstream docs, with per-source accuracy caveats.

Always cross-check the catalog against the repo module before recommending a change: a flag being valid is not a reason to add it here (see the IEEE-math invariant). Before proposing any flag edit, name in chat which invariant above it sits nearest to and why it is safe against it — an invariant named at the point of the proposal is one the proposal respects (doc/claude/j-space.md); "it doesn't touch any" is an acceptable answer only after checking each.

Output expectations

Follow CLAUDE.md's handoff rules: one-line statement of intent before a non-trivial change, a one/two-sentence summary when you stop, no new doc files unless asked. Propose targeted Edits to the cmake modules -- do not rewrite a whole module. If a change could shift hotpath throughput, say so and point the developer at --benchmark-hotpath ([[ss-hotpath]]); you do not run it. For style/structure on any C++ you touch alongside, defer to [[ss-verify]].

References

references/serial-studio-build-flags.md -- this repo's real flag layout and invariants.
references/flag-catalog.md -- cross-compiler flag catalog from upstream GCC/Clang/MSVC docs
- the PGO concept, with citations and caveats.