GH-50026: [C++][Parquet] SIMD-accelerate SBBF probe via branchless autovec

[dmatth1]

Rationale for this change

BlockSplitBloomFilter::FindHash ships the scalar reference probe — an 8-iteration short-circuit loop. The short-circuit blocks autovectorization, and on miss-heavy workloads (Parquet row-group skipping) the per-lane branch-mispredict dominates probe latency.

Closes #50026. Dev list discussion: https://lists.apache.org/thread/omof0fq47tndfd80g5hwp2bvjmzvpb40. Sibling change in Rust: apache/arrow-rs#10011.

What changes are included in this PR?

• Rewrite FindHash as a branchless OR-accumulator reduction. The new shape autovectorizes to SSE on x86 and NEON on aarch64 at the baseline.
• Add bloom_filter_avx2.cc (xsimd kernel built with -mavx2) behind CpuInfo-based DynamicDispatch, mirroring the existing level_comparison_avx2 pattern. xsimd was a requirement from the dev thread; the AVX2 target spells the reduction explicitly because gcc/MSVC don't lower the autovec body to a single vptest.
• No on-disk format change, no public API change, and bit-identical to the scalar reference.
• Insert path uses the same loop shape and will land in a follow-up PR.

Performance

End-to-end FindHash via parquet/benches/bloom_filter_benchmark.cc.

M1 (Apple clang -O3, NEON via autovec, 10 reps, CV ≤ 0.4%):

Bench	upstream/main	this PR	Speedup
BM_FindExistingHash (hit-heavy)	3.85 ns/probe	2.41 ns/probe	1.60×
BM_FindNonExistingHash (miss-heavy)	9.04 ns/probe	2.41 ns/probe	3.75×

x86-64 (gcc 13.3, -O2 -mavx2 via AVX2 dispatch TU, 5 reps, CV ≤ 0.6%):

Bench	upstream/main	this PR	Speedup
BM_FindExistingHash (hit-heavy)	8.62 ns/probe	4.32 ns/probe	2.00×
BM_FindNonExistingHash (miss-heavy)	15.29 ns/probe	4.33 ns/probe	3.53×

The scalar miss path stalls on the data-dependent early-exit (slower than its own hit path on both archs); the branchless reduction is constant-time across hit/miss. InsertHash, BatchInsertHash, ComputeHash, BatchComputeHash all
unchanged (16 benches within ±0.6%, inside CV).

Are these changes tested?

Yes. New BloomFilterProbeKernel test calls both dispatch targets directly across 20K random blocks + 200 production-populated blocks per CI run, asserting bit-identical output. DynamicDispatch resolves once at static init, so without this
test the un-picked target would never be exercised in CI.

Existing BasicTest, FPPTest, and CompatibilityTest continue to pass on both the scalar baseline and the AVX2 dispatch path.

Are there any user-facing changes?

No. Read-path implementation change only.

• GitHub Issue: [C++][Parquet] SIMD-accelerate the SBBF probe in BlockSplitBloomFilter::FindHash #50026

[github-actions]

⚠️ GitHub issue #50026 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

This PR accelerates Parquet’s BlockSplitBloomFilter::FindHash probe by reshaping the scalar short-circuit loop into a branchless reduction that autovectorizes, and by adding an AVX2 runtime-dispatched probe kernel for x86 targets.

Changes:

• Rework BlockSplitBloomFilter::FindHash to call a dispatchable per-block probe (FindHashBlockImpl) implemented as a branchless OR-accumulator reduction.
• Add an AVX2-specific probe implementation in a separate translation unit (bloom_filter_avx2.cc) using xsimd, wired through DynamicDispatch.
• Add a kernel agreement test that compares baseline vs AVX2 implementations on AVX2-capable hosts.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cpp/src/parquet/CMakeLists.txt	Adds bloom_filter_avx2.cc to Parquet sources under runtime-AVX2 builds and applies AVX2 compile flags.
cpp/src/parquet/bloom_filter.cc	Introduces DynamicDispatch plumbing and routes FindHash through the new per-block probe kernels.
cpp/src/parquet/bloom_filter_test.cc	Adds an AVX2-only cross-kernel agreement test and includes the baseline/AVX2 probe entrypoints.
cpp/src/parquet/bloom_filter_block_inc.h	New header containing the baseline branchless per-block probe implementation.
cpp/src/parquet/bloom_filter_avx2.cc	New AVX2 probe kernel implementation using xsimd.
cpp/src/parquet/bloom_filter_avx2_internal.h	New internal header declaring the AVX2 probe entrypoint (exported for Windows/MinGW test usage).

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[mapleFU]

1. What would just branchless code bench result be with/without avx2?
2. What would the probe speed change when bloom filter size changing?

I wonder how would the avx2 path faster than scalar path 🤔

[dmatth1]

Branchless body alone (no xsimd kernel) on AVX2:

• on clang -mavx2 it's within noise of the hand-written xsimd kernel in every regime
• on gcc it matches except ~0.79× of xsimd in the out-of-L3 regime.
• That gap is why this PR ships a separate xsimd kernel for the AVX2 TU rather than relying on autovec alone — on clang-only builds the xsimd kernel is essentially a no-op but on gcc/MSVC it pins the vptest lowering.

Cache regime sweep: scalar vs xsimd, post-hash probe latency:

Regime	scalar	xsimd	Speedup
Small in-cache (0.5 MiB)	12.35 ns	2.48 ns	5.0×
Medium out-of-L3 (128 MiB)	18.40 ns	7.41 ns	2.5×
Large deep DRAM (1 GiB)	31.05 ns	22.10 ns	1.4×

These numbers are with the as_batch_bool xsimd form (~1 cycle faster in-cache than the shipped miss != 0 spelling — out-of-cache regimes unchanged) and the post-hash only (XXH64 excluded) so absolute values don't compare directly to the end-to-end commit-body table. The regime shape (biggest gain in-cache, smallest in DRAM) holds for the shipped form.

Can re-bench in-tree with the commit if you want directly-comparable numbers.