perf(lexer): fix perf of `Token::set_*` methods on Rust 1.95.0 by overlookmotel · Pull Request #21659 · oxc-project/oxc

overlookmotel · 2026-04-22T23:15:40Z

Token's set_* methods used safe bitwise ops to write "fields" of Token.

Unfortunately this regressed heavily in Rust 1.95.0, due to a bug in LLVM. Methods which were a single scalar op turned into a string of heavy SIMD ops, impacting performance.

Re-implement these methods using unsafe pointer manipulation, which recovers the original assembly, and reverses the perf regression - gains 3% on parser benchmarks.

The original implementations are left in the code as comments, in case we want to revert to them once the LLVM bug is fixed.

overlookmotel · 2026-04-22T23:15:53Z

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codspeed-hq · 2026-04-22T23:22:06Z

Merging this PR will improve performance by 3.27%

⚡ 4 improved benchmarks
✅ 44 untouched benchmarks
⏩ 3 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	Simulation	`parser[cal.com.tsx]`	26.5 ms	25.6 ms	+3.27%
⚡	Simulation	`parser[RadixUIAdoptionSection.jsx]`	83.8 µs	81.3 µs	+3.05%
⚡	Simulation	`parser[react.development.js]`	1.3 ms	1.3 ms	+3.24%
⚡	Simulation	`parser[binder.ts]`	3.4 ms	3.3 ms	+3.02%

_{Comparing om/04-23-perf_lexer_fix_perf_of_token_set__methods_on_rust_1.95.0 (f7a428f) with main (30e0ad3)}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

Copilot

Pull request overview

This PR addresses a Rust 1.95.0 performance regression in the lexer by replacing Token’s safe bitfield-update logic with unsafe, direct memory writes to restore the previously tight codegen.

Changes:

Reworks Token::set_* methods to write “fields” via pointer-based stores instead of u128 bitwise masking/shifting.
Adds compile-time layout/alignment assertions to justify the pointer-based field access.
Preserves the previous safe implementations as comments for potential future rollback.

Copilot

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

overlookmotel · 2026-04-23T00:07:54Z

@copilot See response to your critique in #21659 (comment). Do you agree/disagree?

Copilot · 2026-04-23T00:09:24Z

@copilot See response to your critique in #21659 (comment). Do you agree/disagree?

Agreed — the strict-aliasing/TBAA concern I raised was incorrect. I was incorrectly applying C's type-based alias analysis rules to Rust. Rust's aliasing model is based on borrows (Stacked Borrows / Tree Borrows), not on the declared types of pointers, so reading the same bytes through both u128 and u32/bool pointers is well-defined. The fact that Miri passes confirms the code is sound.

camc314

Thanks! I meant to do this following #21523 , but didn't manage to

camc314 · 2026-04-23T07:27:49Z

Merge activity

Apr 23, 7:27 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Apr 23, 7:27 AM UTC: camc314 added this pull request to the Graphite merge queue.
Apr 23, 7:35 AM UTC: Merged by the Graphite merge queue.

`Token`'s `set_*` methods used safe bitwise ops to write "fields" of `Token`. Unfortunately this regressed heavily in Rust 1.95.0, due to a bug in LLVM. Methods which were a single scalar op turned into a string of heavy SIMD ops, impacting performance. * #21509 * rust-lang/rust#155422 Re-implement these methods using unsafe pointer manipulation, which recovers the original assembly, and reverses the perf regression - gains 3% on parser benchmarks. The original implementations are left in the code as comments, in case we want to revert to them once the LLVM bug is fixed.

### 💥 BREAKING CHANGES - 502e804 ast: [**BREAKING**] Reduce size of `TSTypePredicateName` (#21711) (overlookmotel) - 5651539 ast: [**BREAKING**] Reduce size of `JSXExpression` (#21710) (overlookmotel) - c44e280 ast: [**BREAKING**] Reduce size of `ArrayExpressionElement` (#21709) (overlookmotel) - c5b3deb syntax: [**BREAKING**] Remove `CommentNodeId` (#21679) (overlookmotel) ### 🚀 Features - b738a39 allocator: Add `Allocator::cursor_ptr` method (#21773) (overlookmotel) - 678767e ast: Generate node_id accessors for AST enum wrappers (#21653) (camc314) - f091d77 minifier: Inline constant spread elements into arrays (#21095) (Armano) ### 🐛 Bug Fixes - 0d608c2 minifier: Preserve raw CR in template literals (#21645) (Dunqing) - a889ea9 minifier: Track pure functions in DCE mode (#21722) (Dunqing) - 674dfac allocator: `Arena` retry allocation when chunk size approaches maximum (#21777) (overlookmotel) - f130cc0 allocator: Fix arithmetic overflow in `Arena::new_chunk_memory_details` (#21745) (overlookmotel) - b9bf239 allocator: Fix UB in `Arena::grow_zeroed` (#21739) (overlookmotel) - d2b9389 allocator: Clippy warning when building without `testing` feature (#21681) (camc314) - 503dc86 codegen: Map sourcemaps from visible output starts (#21662) (Dunqing) - c92bd3b transformer: Use SPAN for synthesized helper calls to prevent comment misattribution (#21578) (Dunqing) - 0d80441 codegen: Add mapping before printing `#` for private ident (#21619) (camc314) ### ⚡ Performance - 9fa362e napi/parser: Do not generate tokens except in tests (#21811) (overlookmotel) - 0044392 allocator: Reduce branches when allocating new chunk (#21776) (overlookmotel) - 7896bd0 allocator: `Allocator::used_bytes` do not use chunk iterator (#21771) (overlookmotel) - a5c562f allocator: Remove check in `Arena::new_chunk_memory_details` (#21750) (overlookmotel) - 35bbe1f allocator: `Arena` use unchecked size round up where guaranteed no overflow (#21743) (overlookmotel) - ffe229b allocator: Remove unnecessary check from `Arena::try_alloc_layout_slow_impl` (#21732) (overlookmotel) - 72fece5 allocator: Use `NonNull::offset_from_unsigned` in `Arena::chunk_capacity` (#21731) (overlookmotel) - cab32ae ast: Add `#[inline(always)]` to `node_id` methods on enums with all variants unboxed (#21707) (overlookmotel) - b179688 parser: Allocate `TriviaBuilder` comments in the arena (#21512) (Boshen) - 2290f31 lexer: Fix perf of `Token::set_*` methods on Rust 1.95.0 (#21659) (overlookmotel) - 1b58029 allocator: Move code into cold path in `Arena::alloc_layout` (#21622) (overlookmotel) - 3cf7cef allocator: Reduce instructions on allocation hot path (#21510) (overlookmotel) ### 📚 Documentation - ce65070 data_structures: Document why `as_ref` and `as_mut` on `NonNullConst` and `NonNullMut` take `self` (#21800) (overlookmotel) - 93b7dbd allocator: Improve doc comments for `ChunkFooter` (#21733) (overlookmotel) - 295db8d transformer: Fix comment (#21717) (overlookmotel) - 5c93af8 ast: Add comments explaining `#[inline(always)]` to `node_id` methods on enums (#21706) (overlookmotel) - e4cea25 transform: Use the `node:` namespace in the example (#19998) (루밀LuMir) ### 🛡️ Security - d8076c9 deps: Update rolldown (#21639) (renovate) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>

overlookmotel marked this pull request as ready for review April 22, 2026 23:20

Copilot AI review requested due to automatic review settings April 22, 2026 23:20

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Comment thread crates/oxc_parser/src/lexer/token.rs

Comment thread crates/oxc_parser/src/lexer/token.rs Outdated

Comment thread crates/oxc_parser/src/lexer/token.rs Outdated

Comment thread crates/oxc_parser/src/lexer/token.rs Outdated

overlookmotel marked this pull request as draft April 22, 2026 23:29

overlookmotel force-pushed the om/04-23-perf_lexer_fix_perf_of_token_set__methods_on_rust_1.95.0 branch from 9a4e526 to 1068f82 Compare April 22, 2026 23:59

overlookmotel mentioned this pull request Apr 22, 2026

refactor(lexer): Token::read_bool use NonNull pointer #21660

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overlookmotel marked this pull request as ready for review April 23, 2026 00:00

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overlookmotel requested a review from camc314 April 23, 2026 00:09

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overlookmotel force-pushed the om/04-23-perf_lexer_fix_perf_of_token_set__methods_on_rust_1.95.0 branch from f1248de to f7a428f Compare April 23, 2026 00:11

overlookmotel added A-parser Area - Parser C-performance Category - Solution not expected to change functional behavior, only performance labels Apr 23, 2026

camc314 approved these changes Apr 23, 2026

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graphite-app Bot force-pushed the om/04-23-perf_lexer_fix_perf_of_token_set__methods_on_rust_1.95.0 branch from f7a428f to 2290f31 Compare April 23, 2026 07:28

graphite-app Bot merged commit 2290f31 into main Apr 23, 2026
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graphite-app Bot deleted the om/04-23-perf_lexer_fix_perf_of_token_set__methods_on_rust_1.95.0 branch April 23, 2026 07:35

oxc-guard Bot mentioned this pull request Apr 27, 2026

release(crates): oxc v0.128.0 #21837

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perf(lexer): fix perf of `Token::set_*` methods on Rust 1.95.0#21659

perf(lexer): fix perf of `Token::set_*` methods on Rust 1.95.0#21659
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Merging this PR will improve performance by 3.27%

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