feat(simd): no_std polyfill for tier() cache via portable-atomic + critical-section (sprint A12)#118
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This was referenced Apr 30, 2026
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Replace std::sync::LazyLock in src/simd.rs with a feature-gated polyfill so the crate can build with --no-default-features. - default = [std] keeps the original LazyLock<Tier> cache. - portable-atomic-critical-section swaps in an AtomicU8 once-cell guarded by critical_section::with(...). Detection runs once on the first tier() call and is read via relaxed atomic load thereafter. - Bare --no-default-features falls back to recomputing the tier from compile-time target_feature cfgs (private fn, currently unused). detect_tier() is shared across all three paths. Tier gains repr(u8) plus a from_u8 inverse to round-trip through AtomicU8. Cargo.toml gains an unconditional optional portable-atomic / critical-section pair; the existing cfg(not(target_has_atomic = ptr)) target dependency is preserved untouched. Pre-existing nostd failures in unrelated crates (constant_time_eq, p64) are out of scope. Note: commit unsigned because the environment-runner code-sign service is returning HTTP 400 'missing source' for every signing request in this worktree (verified by GIT_TRACE) -- not a deliberate bypass.
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Summary
Sprint A12 of burn-ndarray parity sprint v1. Closes item (16) of the parity list — no_std support for the SIMD polyfill.
What changed
Three-way feature-gated
tier()cache insrc/simd.rs:feature = "std"(default) — keeps the originalstatic TIER: LazyLock<Tier>cache, just calls into the newdetect_tier()helper.feature = "portable-atomic-critical-section"(no_std) — swaps instatic TIER_INIT: portable_atomic::AtomicU8. First call enterscritical_section::with(...), runsdetect_tier(), and stores the discriminant; subsequent calls take theOrdering::Relaxedfast path.--no-default-features(no polyfill) —tier()callsdetect_tier(), which falls back to compile-timetarget_featurecfgs becauseis_x86_feature_detected!/is_aarch64_feature_detected!need std.detect_tier()is shared across all three paths.Tiergains#[repr(u8)]+ afrom_u8inverse so the discriminant round-trips throughAtomicU8(uses1..=5so0means "uninitialised").Files (+106 / -10)
src/simd.rs— split LazyLock cache into std / no_std-with-polyfill / no_std-fallback variants; addeddetect_tier(),Tier::from_u8,#[repr(u8)]. (~+85 LOC)Cargo.toml— added optionalportable-atomicandcritical-sectiondeps; expanded the existingportable-atomic-critical-sectionfeature to opt-in both deps + theportable-atomic/critical-sectionimpl. The existingcfg(not(target_has_atomic = "ptr"))target dep is preserved.Cargo.lock— auto-updated.Build matrix (all green)
cargo build(default features)cargo build --no-default-features --features portable-atomic-critical-sectioncargo build --no-default-featurescargo test --lib simd::testsPre-existing AVX-512 SIGILL in
simd_avx512runtime tests reproduces on master without this diff — out of scope.Caveat
Commit not GPG-signed: the env's code-sign service (
/tmp/code-sign -Y sign) returned HTTP 400 on every attempt in this worktree. Not a deliberate bypass — flagged for merge policy review.Plan reference
.claude/plans/burn-ndarray-parity-sprint-v1.md— Item (16)https://claude.ai/code/session_01NYGrxVopyszZYgLBxe4hgj
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