perf(simd): vectorize I8x16::saturating_abs (VPABSB) + binding W1a tests

[AdaWorldAPI]

Summary

Follow-up hardening of the W1a SIMD primitives merged in #203 — turning the AVX-512-baseline I8x16::saturating_abs from a scalar loop into real SIMD, and adding the binding parity tests #203 shipped without.

• I8x16::saturating_abs → _mm_abs_epi8 + _mm_min_epu8 (simd_avx512.rs). The contract's VPABSB correction: bare VPABSB returns 0x80 for i8::MIN; VPMINUB clamps it to 0x7f (= i8::MAX). 16 lanes, branchless, vs the prior per-lane branching scalar loop. (I8x32::saturating_abs was already real.)
• Binding W1a unit tests added (only rust,ignore doctests existed before): saturating_abs(i8::MIN)==i8::MAX for I8x16+I8x32, a scalar-reference corpus, i4 sign-extension, U64x8::popcnt/xor_popcount, and gather_u16. All 6 pass on the v3 build.

Measured, not assumed (deliberately not changed)

• U64x8::popcnt on AVX2 already lowers to the hardware POPCNT instruction via u64::count_ones() — a VPSHUFB-Mula rewrite adds complexity for ~zero gain at 8 lanes.
• gather_u16 stays scalar: a 32-bit _mm256_i32gather_epi32 over a &[u16] over-reads 2 bytes past the last valid index (UB even for in-bounds indices), and no 16-bit hardware gather exists (AVX2/AVX-512 gather granularity is 32/64-bit). The safe SIMD path for small palettes (≤32 u16) would be _mm512_permutexvar_epi16 (VPERMW, register permute) — a possible follow-up.

Posture

Compile-time dispatch only (runtime dispatch deferred). Consumer site: lance-graph:crates/lance-graph-contract/src/mul.rs (i4 saturating-abs classifier). The AVX-512 path is CI-verified — it can't be compiled on a non-AVX-512 runner (the v4 build SIGILLs in build scripts).

Test plan

• cargo test --lib w1a_ — 6/6 pass on default v3
• cargo fmt --all --check clean
• cargo clippy --lib clean
• CI: tier4-avx512-check compiles the AVX-512 path; NEON job covers aarch64

https://claude.ai/code/session_017GFLBnDy23AWBqvkbHHC41

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Generated by Claude Code

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Fixed saturation behavior for minimum integer values in SIMD absolute value operations.

• Performance

  • Optimized integer absolute value saturation computation using hardware-accelerated SIMD instructions.

• Tests

  • Added comprehensive test coverage for saturating arithmetic operations and SIMD primitives.

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

The PR optimizes I8x16::saturating_abs in the AVX-512 backend by replacing a scalar per-lane loop with x86 SIMD intrinsics, and adds comprehensive test coverage for the optimized function alongside related W1a primitives to validate correctness and behavior.

Changes

I8x16 Saturating Abs Optimization

Layer / File(s)	Summary
I8x16::saturating_abs SIMD implementation src/simd_avx512.rs	Replaced scalar loop with x86 intrinsics: _mm_abs_epi8 computes raw absolute values, then _mm_min_epu8 clamps unsigned results to 0x7f to enforce i8::MIN → i8::MAX saturation across all 16 lanes.
W1a primitive test coverage src/simd_avx512.rs	Added test cases for I8x16::saturating_abs (i8::MIN saturation and scalar parity), I8x32::saturating_abs saturation, I8x16::from_i4_packed_u64 sign-extension, U64x8::popcnt and xor_popcount Hamming-distance behavior, and U16x8::gather_u16 in-bounds correctness.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

A rabbit hops through bytes with care, 🐰
Where i8::MIN floats through the air,
SIMD intrinsics clamp it tight,
To i8::MAX with all their might,
Tests ensure saturation's fair!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main changes: vectorizing I8x16::saturating_abs using VPABSB and adding W1a binding tests.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/splat3d-cpu-simd-renderer-MAOO0

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