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ADR-0720: C++23 Wave-1 Pilot — mem.c conversion

  • Status: Accepted
  • Date: 2026-05-28
  • Deciders: lusoris
  • Tags: build, c++, cpp23, refactor, internals, fork-local, vmafx-rebrand

Context

ADR-0708 established the per-file C++ conversion recipe by converting metadata_handler.c → .cpp (C++20, ROI 4.0). The same ADR documented a Wave-1 follow-up covering mem.c (ROI 2.0), log.c (ROI 3.0), and opt.c (ROI 1.5).

mem.c provides two functions — aligned_malloc and aligned_free — called from approximately 25 sites across core/src/. The file is minimal (53 lines, zero float arithmetic, zero score impact) but gains two concrete C++23 improvements:

  1. [[nodiscard]] on aligned_malloc propagates ignored-return warnings to every C++ call site at zero ABI cost (the attribute is transparent to C callers).
  2. _POSIX_C_SOURCE feature-test macro (needed in C to expose posix_memalign) is eliminated — C++ compilers expose posix_memalign via <cstdlib> without it.

The public C ABI (aligned_malloc, aligned_free) is unchanged. Zero-on-allocate and poison-on-free semantics are preserved exactly: posix_memalign / _aligned_malloc return uninitialised memory; free() / _aligned_free do not zero memory. Callers requiring zero initialisation must call memset themselves.

A pre-existing meson bug was fixed in the same PR: core/test/meson.build still referenced test_ansnr_simd.c (and all ansnr source files), which were deleted when the ansnr feature was dropped. The stale executable() and test() blocks blocked meson setup from completing; they were removed as a touched-file cleanup (CLAUDE.md §12 r12).

Decision

Convert core/src/mem.c to core/src/mem.cpp with cpp_std=c++23, following the ADR-0708 isolation pattern:

  1. mem.h gains extern "C" { … } guards so all ~25 C callers compile and link unchanged.
  2. mem.cpp replaces the _POSIX_C_SOURCE define with <cstdlib> / <cstddef>, adds [[nodiscard]] to aligned_malloc, replaces NULL with nullptr.
  3. core/src/meson.build: a mem_cpp23_lib static library (pattern mirrors metadata_handler_cpp20_lib) compiles mem.cpp with override_options : ['cpp_std=c++23']. mem.c is removed from libvmaf_sources; mem_cpp23_lib.extract_all_objects(recursive: true) is added to the final library() objects list.
  4. core/test/meson.build: all 24 '../src/mem.c' references updated to '../src/mem.cpp'; stale ansnr test blocks removed.
  5. Zero-on-allocate / poison-on-free / alignment-pass-through semantics preserved bit-for-bit.
  6. Netflix golden gate must pass post-conversion (scores 76.668 places=4).

Alternatives considered

Option Pros Cons Why not chosen
std::unique_ptr<void, AlignedDeleter> wrapper RAII for aligned buffers Blast radius: all ~25 call sites switch to non-pointer type; C callers lose raw pointer High risk, low gain for a two-function file
std::span<std::byte> return type Packages pointer+size together Changes public ABI (void* → span); C callers break ABI preservation is non-negotiable
Exceptions at boundary (throw std::bad_alloc) Idiomatic C++ OOM signalling All ~25 callers are C and would hit std::terminate on uncaught throw Return-nullptr on failure is the existing contract; preserve it
Keep as mem.c with C23 [[nodiscard]] No C++ compiler needed C23 [[nodiscard]] is not yet in every toolchain; _POSIX_C_SOURCE still required Per ADR-0708 recipe: move to .cpp for C++23 opt-in
Bulk mem.c + log.c + opt.c in one PR Faster Wave-1 migration Harder to bisect if a test breaks; per ADR-0708 policy: one file per pilot Per-file pilot reduces review surface

Consequences

  • Positive: [[nodiscard]] on aligned_malloc catches ignored-return at C++ call sites at compile time. _POSIX_C_SOURCE noise removed. Wave-1 recipe validated for log.c and opt.c (Wave-1 remainder).
  • Negative: core/test/meson.build and core/src/meson.build must list mem.cpp rather than mem.c — any upstream sync that adds new test executables sourcing ../src/mem.c will need to be updated to .cpp.
  • Neutral / follow-ups:
  • Wave-1 remainder: log.c (ROI 3.0), opt.c (ROI 1.5).
  • Wave 2: ref.c, fex_ctx_vector.c (ROI 1.0 each).
  • Wave 3: dict.c (ROI 0.83; requires std::expected → cpp_std=c++23 on that file, already available with per-target override).

References

  • Research-0732 (docs/research/0732-vmafx-cpp23-internals-migration-plan.md).
  • ADR-0708 (docs/adr/0708-vmafx-cpp23-internals-pilot.md) — pilot recipe and ROI table.
  • ADR-0692 (docs/adr/0692-vmafx-c23-bump.md) — bumped C standard to C23.
  • ADR-0700 (docs/adr/0700-vmafx-repo-layout.md) — libvmaf/ renamed to core/.
  • req: "Internal implementation moves .c to .cpp where C++23 features help: std::unique_ptr / RAII to replace manual malloc/free pairs."