perf(semantic, allocator): branchless `clone_in` for semantic IDs by overlookmotel · Pull Request #24564 · oxc-project/oxc

overlookmotel · 2026-07-15T14:59:39Z

Follow-on after #24422.

Make cloning semantic IDs (NodeId, ScopeId etc) use branchless arithmetic.

Cloning NodeId now just uses a single OR operation to take into account whether to preserve the existing ID (clone_in_with_semantic_ids) or substitute a dummy ID (clone_in).
Cloning Cell<Option<ScopeId>> is 2 instructions (NOT + AND) on x86_64, or just 1 on aarch64 (NOT-AND).

https://godbolt.org/z/1KWdc5hnb

The mechanism is a type CloneInSemanticIds which is used as with_semantic_ids instead of a bool:

#[repr(u32)]
pub enum CloneInSemanticIds {
    With = 0,
    Without = u32::MAX,
}

This is 4 bytes, rather than bool's 1 byte, but that makes no difference - either way it takes 1 register to pass between functions. The choice of discriminants is what enables the cheap arithmetic.

This is even smaller in terms of binary size than the previous solution of marking Cell::clone_in_impl as #[inline(never)] (OR instruction is smaller than function call), and it's more performant as it loses the function call overhead. It also keeps Cell's blanket implementation of CloneIn generalized.

Between #24422 and this PR, merging CloneIn with/without semantic IDs into a single path has gained us a large binary size reduction, and lost us nothing - or may even have gained us a marginal perf improvement.

No visible change on our benchmarks, as we don't use CloneIn much, but it may well be a measurable improvement for Rolldown, which clones whole ASTs.

overlookmotel · 2026-07-15T14:59:54Z

perf(semantic, allocator): branchless clone_in for semantic IDs #24564 👈 (View in Graphite)
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codspeed-hq · 2026-07-15T15:13:16Z

Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 9 skipped benchmarks¹

_{Comparing om/07-15-perf_semantic_allocator_branchless_clone_in_for_semantic_ids (9bd7649) with main (70994ae)}

9 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 optimizes CloneIn behavior around semantic IDs (NodeId, ScopeId, SymbolId, ReferenceId) by replacing the previous runtime bool with a CloneInSemanticIds flag whose discriminants enable cheap, branchless arithmetic when preserving vs resetting IDs during arena cloning.

Changes:

Introduces CloneInSemanticIds and updates CloneIn::clone_in_impl (and all implementors/callers) to use it instead of bool.
Adds oxc_syntax::semantic_id::SemanticId to provide branchless cloning helpers for semantic ID types and Cell<Option<Id>> fields.
Updates the CloneIn derive generator to support #[clone_in(semantic_id)] and to special-case Cell<Option<Id>> semantic ID fields via SemanticId::clone_cell_option_id, then regenerates affected derive_clone_in.rs outputs.

Reviewed changes

Copilot reviewed 19 out of 22 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tasks/ast_tools/src/schema/extensions/clone_in.rs	Extends schema metadata to mark semantic ID types for codegen decisions.
tasks/ast_tools/src/derives/clone_in.rs	Teaches the derive to parse `#[clone_in(semantic_id)]`, threads `CloneInSemanticIds`, and emits specialized cloning for `Cell<Option<Id>>`.
crates/oxc_allocator/src/clone_in.rs	Defines `CloneInSemanticIds`, updates `CloneIn` API, and updates core impls (e.g., `Option`, `Cell`, primitives).
crates/oxc_allocator/src/lib.rs	Re-exports `CloneInSemanticIds` from `oxc_allocator`.
crates/oxc_allocator/src/bitset.rs	Updates `BitSet`’s `CloneIn` impl to accept `CloneInSemanticIds`.
crates/oxc_syntax/src/semantic_id.rs	Adds `SemanticId` trait with branchless `clone_id` and `clone_cell_option_id` helpers + tests.
crates/oxc_syntax/src/lib.rs	Exposes the new `semantic_id` module.
crates/oxc_syntax/src/node.rs	Marks `NodeId` as `#[clone_in(semantic_id)]` and delegates `CloneIn` to `SemanticId::clone_id`.
crates/oxc_syntax/src/scope.rs	Marks `ScopeId` as `#[clone_in(semantic_id)]` and delegates `CloneIn` to `SemanticId::clone_id`.
crates/oxc_syntax/src/symbol.rs	Marks `SymbolId` as `#[clone_in(semantic_id)]` and delegates `CloneIn` to `SemanticId::clone_id`.
crates/oxc_syntax/src/reference.rs	Marks `ReferenceId` as `#[clone_in(semantic_id)]` and delegates `CloneIn` to `SemanticId::clone_id`; updates other `CloneIn` impl signatures.
crates/oxc_syntax/src/generated/derive_clone_in.rs	Regenerated clone impls to accept `CloneInSemanticIds`.
crates/oxc_ast/src/generated/derive_clone_in.rs	Regenerated clone impls to accept `CloneInSemanticIds` and use `SemanticId::clone_cell_option_id` for semantic `Cell<Option<Id>>` fields.
crates/oxc_ast/src/ast/comment.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_ast/src/ast_impl/literal.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_span/src/span.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_span/src/source_type.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_str/src/str.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_str/src/ident.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.
crates/oxc_semantic/src/scoping.rs	Updates `CloneIn` impl signature and adapts logic to the new `CloneInSemanticIds` flag.
crates/oxc_regular_expression/src/generated/derive_clone_in.rs	Regenerated clone impls to accept `CloneInSemanticIds`.
crates/oxc_regular_expression/src/ast_impl/allocator.rs	Updates `CloneIn` impl signature to accept `CloneInSemanticIds`.

Boshen · 2026-07-16T04:14:19Z

Merge activity

Jul 16, 4:14 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Jul 16, 4:14 AM UTC: Boshen added this pull request to the Graphite merge queue.
Jul 16, 4:19 AM UTC: Merged by the Graphite merge queue.

…4564) Follow-on after #24422. Make cloning semantic IDs (`NodeId`, `ScopeId` etc) use branchless arithmetic. * Cloning `NodeId` now just uses a single OR operation to take into account whether to preserve the existing ID (`clone_in_with_semantic_ids`) or substitute a dummy ID (`clone_in`). * Cloning `Cell<Option<ScopeId>>` is 2 instructions (NOT + AND) on x86_64, or just 1 on aarch64 (NOT-AND). https://godbolt.org/z/1KWdc5hnb The mechanism is a type `CloneInSemanticIds` which is used as `with_semantic_ids` instead of a `bool`: ```rs #[repr(u32)] pub enum CloneInSemanticIds { With = 0, Without = u32::MAX, } ``` This is 4 bytes, rather than `bool`'s 1 byte, but that makes no difference - either way it takes 1 register to pass between functions. The choice of discriminants is what enables the cheap arithmetic. This is even smaller in terms of binary size than the previous solution of marking `Cell::clone_in_impl` as `#[inline(never)]` (OR instruction is smaller than function call), and it's more performant as it loses the function call overhead. It also keeps `Cell`'s blanket implementation of `CloneIn` generalized. Between #24422 and this PR, merging `CloneIn` with/without semantic IDs into a single path has gained us a large binary size reduction, and lost us nothing - or may even have gained us a marginal perf improvement. No visible change on our benchmarks, as we don't use `CloneIn` much, but it may well be a measurable improvement for Rolldown, which clones whole ASTs.

### 💥 BREAKING CHANGES - 54cc121 ast: [**BREAKING**] Split `MetaProperty` into `ImportMeta` and `NewTarget` (#24557) (camc314) ### 🚀 Features - 4c71560 parser: More friendly error for spread element in dynamic imports (#24705) (sapphi-red) - 7b045cd minfier: Drop last break from last switch case (#24673) (Armano) - 7d3c178 minifier: Remove unreachable recursive functions (#24125) (Dunqing) - 94f99b3 ast: Allow `NONE` to be passed to AST builder methods where `Option<ArenaVec>` is expected (#24629) (overlookmotel) - 77230c5 ast: Accept arrays for `ArenaVec` params of AST builder methods (#24621) (overlookmotel) - f08b152 allocator: Implement `FromIn` for array to `Vec` conversion (#24620) (overlookmotel) - 2338c13 track-memory-allocations: Track heap deallocs, alloc bytes, and peak growth (#24619) (Boshen) - 7aa4739 syntax,transformer: Move JSX entity decoder to `oxc_syntax` (#24617) (camc314) - 2b097c4 str: Export `Str` as `ArenaStr` (#24604) (overlookmotel) - 3acf4c1 minifier: Expand switch optimiation to remove empty cases (#24520) (Armano) - 129b759 parser: Improve diagnostics for unparenthesized LHS on exponential expr (#24569) (camc314) - 4d0c601 minifier: Fold arithmetic over undefined and null operands (#24485) (Dunqing) - 91541dd minifier: Drop empty switch statements (#24527) (Armano) - d05224d ast_visit: Generate VisitJs visitor that skips TypeScript type-space (#24499) (Boshen) - 3d22307 parser: Add `ParseOptions::enable_ident_hashes` (#24491) (Boshen) ### 🐛 Bug Fixes - 64c2241 minifier: Align class heritage removal with assumptions (#24533) (Dunqing) - 48b59f4 parser: Span ambient generator diagnostics (#24711) (camc314) - e750a82 ecmascript: Fix false negative for may_have_side_effects on dynamic property access (#24709) (sapphi-red) - f145d73 minifier: Guard reordered identifier reads (#24698) (Dunqing) - a2ef382 isolated-declarations: Reject `window.Symbol` as global symbol reference (#24689) (camc314) - b1bcf72 minifier: Invalidate facts for redeclared bindings (#24658) (Dunqing) - 921b834 minifier: Don't treat a conditionally-assigned var as write-once (#24650) (Dunqing) - 061af1f minifier: Avoid stale pure function summaries (#24636) (Dunqing) - 40c2f43 allocator: `Vec::from_array_in` do not allocate zero-length array (#24628) (overlookmotel) - 70994ae codegen: Preserve comments before expression operands (#24510) (Dunqing) - 7b4baff parser: Reject new import member access (#23459) (camc314) - 128b385 minifier: Clippy warning with no-debug-assertions (#24547) (camc314) - 8421feb parser: Use first `as` span for imported name (#24537) (leaysgur) - c517aa0 parser: Reject invalid accessor assertions (#24504) (camc314) ### ⚡ Performance - 884d9eb parser: Pre-size cover-grammar assignment target buffers (#24683) (Boshen) - d3f07a0 diagnostics: Box OxcDiagnosticInner to reduce binary size (#24665) (Boshen) - bcc9de0 parser: Defer diagnostic creation until parse exit (#24663) (Boshen) - c35d8ab allocator: Mark `ReplaceWith` panic path cold (#24515) (camc314) - ba65790 semantic, allocator: Branchless `clone_in` for semantic IDs (#24564) (overlookmotel) - 747feec parser: Build AST nodes with the AST builder instead of cloning (#24540) (Boshen) - ba35a0d react_compiler: Use IndexVec for dense id-keyed maps (#24549) (Boshen) - b685062 react_compiler: Keep small hot-path collections inline (#24514) (Marius Schulz) - a149e95 transformer: Outline rare expression exits (#24512) (camc314) - 7808a6e react_compiler: Make aliasing effects cheap to intern and clone (#24506) (Marius Schulz) - 3a36f2a react_compiler: Store AbstractValue reasons as a u16 bitmask (#24480) (Boshen) - 1c96753 react_compiler: Use FxHashMap for the lookup-only aliasing node map (#24490) (Boshen) Co-authored-by: Cameron <[email protected]>

github-actions Bot added A-semantic Area - Semantic A-ast Area - AST A-ast-tools Area - AST tools A-regular-expression Area - Regular Expression A-allocator Area - Allocator labels Jul 15, 2026

overlookmotel force-pushed the om/07-15-perf_semantic_allocator_branchless_clone_in_for_semantic_ids branch from 337913f to 4ec41d3 Compare July 15, 2026 15:04

overlookmotel marked this pull request as ready for review July 15, 2026 15:44

overlookmotel requested review from Dunqing and leaysgur as code owners July 15, 2026 15:44

Copilot AI review requested due to automatic review settings July 15, 2026 15:44

Copilot started reviewing on behalf of overlookmotel July 15, 2026 15:45 View session

Copilot AI reviewed Jul 15, 2026

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Comment thread tasks/ast_tools/src/schema/extensions/clone_in.rs Outdated

Comment thread crates/oxc_syntax/src/semantic_id.rs

overlookmotel force-pushed the om/07-15-perf_semantic_allocator_branchless_clone_in_for_semantic_ids branch 2 times, most recently from 6c3cc38 to 9bd7649 Compare July 15, 2026 16:08

overlookmotel requested a review from Boshen July 15, 2026 16:09

overlookmotel assigned Boshen Jul 15, 2026

Boshen added the 0-merge Merge with Graphite Merge Queue label Jul 16, 2026

graphite-app Bot force-pushed the om/07-15-perf_semantic_allocator_branchless_clone_in_for_semantic_ids branch from 9bd7649 to ba65790 Compare July 16, 2026 04:14

graphite-app Bot merged commit ba65790 into main Jul 16, 2026
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graphite-app Bot removed the 0-merge Merge with Graphite Merge Queue label Jul 16, 2026

graphite-app Bot deleted the om/07-15-perf_semantic_allocator_branchless_clone_in_for_semantic_ids branch July 16, 2026 04:19

oxc-guard Bot mentioned this pull request Jul 20, 2026

release(crates): oxc v0.141.0 #24714

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perf(semantic, allocator): branchless `clone_in` for semantic IDs#24564

perf(semantic, allocator): branchless `clone_in` for semantic IDs#24564
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