buffer: restore swap16/32/64 and multi-byte indexOf throughput - #39616
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…hroughput Buffer.swap16/32/64 were 2-5.5x slower than 1.3.14 on x64 because the byte loops relied on -march auto-vectorization; they are now Highway kernels (ReverseLaneBytes) with runtime dispatch, so AVX2/AVX-512 is used regardless of the baseline build. Buffer.indexOf/lastIndexOf/includes with a multi-byte needle were ~2x slower than 1.3.14: the two-anchor memmem filter loaded both anchors for every block. It now tests one anchor first and only loads the second on a hit, replaces the scalar tail (up to 63 starts on AVX-512) with an overlapping final block plus a 128-bit pass for short haystacks, drops a per-call division, and verifies short candidates with in-bounds scalar loads instead of glibc memcmp (whose masked load takes a microcode assist when a match sits at the end of a buffer).
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You can disable this status message by setting the WalkthroughThe PR adds cached Highway dispatch, reworks forward and reverse SIMD substring search, adds UTF-16 search testing, and implements Highway byte-swapping for Buffer methods. Static allowlists and dispatch documentation are updated. ChangesHighway runtime and SIMD operations
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I reviewed this PR and didn't find any bugs. Because it rewrites hot-path SIMD substring search (overlapping tail blocks, hand-rolled sub-32-byte compare, byte-denominated budget) and adds new Highway bswap kernels, a human look from someone with Highway/perf context would still be worthwhile.
What was reviewed
- Traced load bounds for the overlapping final block in
MemMemForwardVec/MemMemReverseVec: withend = haystack_len - needle_len + 1andanchor < needle_len, the last block atend - Nreads up tohaystack + haystack_lenexactly — in bounds. - Checked that overlap re-verification of already-rejected candidates only wastes budget, never returns a wrong position or skips one; on budget exhaustion in the overlap the fallback
resumestill yields the correct result. - Verified
MemMemVerify's overlapping scalar loads stay within[a, a+n)for every size bucket, and callers guaranteen >= 2. - Confirmed
BSwapImplscalar-tail alignment holds (N is a multiple of 8 on every compiled target) andlen == 0/ null vector on detached buffers is a no-op.
Extended reasoning...
Overview
Two independent perf restorations in native Buffer code: (1) Buffer.swap16/32/64 become Highway ReverseLaneBytes kernels behind HWY_DYNAMIC_DISPATCH, replacing scalar loops in JSBuffer.cpp; (2) the two-anchor SIMD memmem/memrmem filter in highway_strings.cpp is restructured to load one anchor per block, use an overlapping final block instead of a scalar tail, add a 128-bit path for short haystacks, track the false-positive budget in bytes, and verify sub-32-byte candidates with in-bounds overlapping scalar loads instead of memcmp. A new buffer.test.js case exercises swap across vector-boundary lengths and odd byteOffsets.
Security risks
None identified. Inputs are already length-checked by the existing callers (haystack_len >= needle_len, length % elemSize == 0). The new MemMemVerify was written specifically to keep reads inside [a, a+n), and I traced each size bucket to confirm the highest load ends at exactly a + n. The overlapping-block loads were traced against end + anchor <= haystack_len. No new user-controlled arithmetic that could overflow on 64-bit size_t.
Level of scrutiny
High. This is hot-path native SIMD with manual bounds management, a hand-rolled replacement for memcmp, and a rewritten budget/fallback trigger — the exact class of change where an off-by-one reads past a buffer or silently changes worst-case complexity. The forward/reverse and 8-/16-bit variants multiply the surface. It also carries perf claims that only someone able to reproduce on the cited hardware can validate.
Other factors
- The overlap-block design means already-rejected candidates in the overlap region get re-verified; I confirmed this is correctness-neutral (a real match there would have been returned earlier, and a fallback triggered mid-overlap still hands a safe
resumeto the two-way search). - The byte-denominated budget (
haystack_len*2 + needle_len*32, decremented byneedle_lenper miss) is equivalent to the old count-denominated one, so the two-way fallback trigger point is unchanged. Repartition<T, D8>inBSwapImplis valid on every target Bun compiles (baseline ≥ 16-byte vectors on x86-64/aarch64;HWY_SCALARis not in the target set).- Test coverage is good for swap (boundary lengths, odd offsets, neighbour-byte assertions) and the PR reports fuzzing memmem against a naive reference across SSSE3/AVX2/AVX-512, plus the existing
highway-strings.test.tsand Node'stest-buffer-*suite.
Given the complexity and hot-path nature, deferring to a human reviewer rather than auto-approving.
HWY_DYNAMIC_DISPATCH calls hwy::GetChosenTarget() out of line and recomputes the table index on every call. BUN_HWY_DISPATCH caches the resolved pointer in a function-local static, so each highway_* wrapper is a guard load plus an indirect jump (~20 fewer instructions, ~7 cycles per call measured on Buffer.swap16 / indexOf(byte)). Applied to every dispatch site (strings, json, sourcemap, xml, image, xxhash3). memmem: fold the forward/reverse drivers into one MemMemSearch<kForward>, credit the false-positive budget for the starts the overlapping tail block re-tests, and add memmem16/memrmem16 to the test shim with a UTF-16 sweep (planted needles across lane boundaries, low-byte decoys). bswap: single full-width loop plus a 128-bit pass before the scalar tail, std::byteswap for the tail.
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Updated 1:00 AM PT - Aug 19th, 2026
❌ @Jarred-Sumner, your commit a62ea71 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 39616That installs a local version of the PR into your bun-39616 --bun |
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Inline comments:
In `@scripts/verify-baseline-static/allowlist-aarch64.txt`:
- Line 8: Update the symbol count metadata on the allowlist header from 166 to
168, matching the 168 Highway symbols listed below it.
In `@scripts/verify-baseline-static/CLAUDE.md`:
- Line 159: Update the Highway (Bun) row in the gate table so its source
reference points to the BUN_HWY_DISPATCH helper definition in highway_dispatch.h
rather than highway_strings.cpp, while preserving the existing
hwy::SupportedTargets() description.
In `@test/js/bun/util/highway-strings.test.ts`:
- Around line 257-258: Extend the highwayStringsForTesting operation union in
internal-for-testing.ts to include the memmem16 and memrmem16 symbols,
preserving the existing native handler behavior and making these test calls
valid under the declared API.
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- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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scripts/verify-baseline-static/CLAUDE.mdscripts/verify-baseline-static/allowlist-aarch64.txtscripts/verify-baseline-static/allowlist-x64-windows.txtscripts/verify-baseline-static/allowlist-x64.txtsrc/jsc/bindings/JSBuffer.cppsrc/jsc/bindings/highway_dispatch.hsrc/jsc/bindings/highway_json.cppsrc/jsc/bindings/highway_sourcemap.cppsrc/jsc/bindings/highway_strings.cppsrc/jsc/bindings/highway_strings_testing.cppsrc/jsc/bindings/highway_xml.cppsrc/jsc/bindings/image_resize.cppsrc/jsc/bindings/xxhash3.cpptest/js/bun/util/highway-strings.test.tstest/js/node/buffer.test.js
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…ew nits - Tail block masks off the starts the previous block covered instead of crediting the budget (same cost measured on SSSE3/AVX2/AVX-512). - BSwapLanes: restore the two-vector main loop; clang does not unroll it. - Test shim: memmem16/memrmem16 accept any alignment/odd length like JSBuffer.cpp; declare the ops in internal-for-testing.ts. - highway-strings.test.ts: reuse the 8-bit filler/reference helpers for UTF-16, add an odd-base-pointer variant, and make the decoy test actually pass the anchor filter (short: first == last unit; long: low-byte histogram anchors) so verification and the two-way fallback run. - verify-baseline allowlists: BMI2 (bzhi from FirstN) on the forward memmem targets; fix group symbol counts; point the gate table at highway_dispatch.h.
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Thanks for addressing the allowlist entries — I re-reviewed at a62ea71 and didn't find any bugs. Given this is a substantial rewrite of the hot-path SIMD memmem/memrmem search (overlapping tail blocks, new skip-lane masking, custom in-bounds MemMemVerify) plus a new dispatch macro applied across every Highway TU, a human pass on the boundary reasoning would still be worthwhile.
What was reviewed:
- Traced load bounds for the forward and reverse overlapping tail blocks in
MemMemSearchVecagainstend >= Lanes(d)— anchor loads stay within[haystack, haystack+haystack_len). - Checked
MemMemVerify's overlapping-scalar-load coverage for n in each size bucket (16..31, 8..15, 4..7, 2..3) — the union covers[0, n). - Confirmed
BSwapImpl's scalar tail stays element-aligned given callers pre-checklength % elemSize;BUN_HWY_DISPATCH's function-local static is thread-safe under C++11 magic statics.
Extended reasoning...
Overview
This PR restores Buffer perf on baseline x64 by (1) replacing the scalar swap16/32/64 loops in JSBuffer.cpp with new Highway ReverseLaneBytes kernels, (2) rewriting the two-anchor SIMD memmem/memrmem filter in highway_strings.cpp — unifying MemMemForward/MemMemReverse into a templated MemMemSearch<kForward, Char>, replacing the scalar tail with an overlapping final vector block (with skip-lane masking), adding a 128-bit CappedTag fallback for short haystacks, switching the false-positive budget from a per-call division to byte counting, and replacing memcmp verification with in-bounds overlapping scalar loads for n < 32 — and (3) introducing BUN_HWY_DISPATCH (function-local static caching the resolved target pointer) across all six Highway TUs. Allowlists and the internal-for-testing shim are updated to match; new tests cover swap across vector boundaries/odd offsets and memmem16/memrmem16.
Security risks
None identified. No untrusted-input parsing, no auth/crypto, no allocation. Bounds are the only concern: I traced the anchor-load addresses for both the forward tail block (i = end - N) and the reverse tail block (i = 0, valid < N) against the end >= Lanes(d) precondition and both stay within the haystack; MemMemVerify's overlapping loads never read past a + n.
Level of scrutiny
High. highway_memmem/highway_memrmem back Buffer.indexOf/lastIndexOf/includes and the bundler/lexer's substring search — an off-by-one in the tail-block skip mask or the reverse valid clamp would be a silent wrong-result bug across the runtime, not a crash. The new MemMemVerify bypasses memcmp for a hand-rolled overlapping-load compare with five size buckets. The dispatch-macro change touches every Highway call site. This is exactly the class of intricate boundary reasoning the review guide flags for human verification.
Other factors
My earlier finding (missing BSwap*Impl allowlist entries) and CodeRabbit's three nits are all resolved in commits 4317990/2d22245/a62ea71. Test coverage is good — the new swap test sweeps 0..288 plus 4K/64K at odd byteOffsets against a byte-reversal reference and checks neighbouring bytes; the new memmem16/memrmem16 tests exercise decoys, odd base pointers, and the two-way fallback. The PR description reports a 600k-case fuzz against a naive reference on SSSE3/AVX2/AVX-512 and a diff against Node. Deferring solely because the algorithmic changes are non-trivial and hot-path, not because anything looks wrong.
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What does this PR do?
Fixes two Buffer perf regressions in canary relative to 1.3.14 (x64):
Buffer.swap16/32/64were 2–5.5x slower because the byte loops depended on-marchauto-vectorization, which the baseline-only x64 build no longer gets. They are now Highway kernels (ReverseLaneBytes) behindHWY_DYNAMIC_DISPATCH, so AVX2/AVX-512 is picked at runtime.Buffer.indexOf/lastIndexOf/includeswith a multi-byte needle were ~2x slower: the two-anchor SIMD memmem filter loaded both anchor vectors for every block. Now it:memcmp— glibc's EVEX memcmp does a masked 32-byte load that takes a microcode assist when the masked-off tail crosses into a non-resident page, i.e. whenever the match sits at the end of a buffer (~100 ns per call)The two-way fallback and the pathological-input wins from #37052 are unchanged.
Highway dispatch overhead.
HWY_DYNAMIC_DISPATCHcallshwy::GetChosenTarget()out of line and recomputes the table index on every call (call/ret + ~10 instructions before the indirect jump). A newBUN_HWY_DISPATCH(highway_dispatch.h) resolves the per-CPU entry once per call site into a function-local static, so eachhighway_*wrapper is a guard load +jmp *ptr. Applied to all dispatch sites (strings, json, sourcemap, xml, image, xxhash3); measured −20 instructions / −7 cycles per call onBuffer.swap16andindexOf(byte)tight loops.Sapphire Rapids, median of 5, vs 1.3.14 / current canary:
'a'*.indexOf('ab')How did you verify your code works?
bun bd test test/js/bun/util/highway-strings.test.ts(memmem/memrmem boundary + decoy coverage, plus a new memmem16/memrmem16 UTF-16 sweep through the test shim) andbun bd test test/js/node/buffer.test.jswith a new swap test covering lengths across vector boundaries and oddbyteOffsets; node'stest-buffer-{indexof,swap,includes}.jspass.