perf(ast): place `NodeId` field after `Span` in structs by overlookmotel · Pull Request #20584 · oxc-project/oxc

overlookmotel · 2026-03-20T21:00:28Z

Alter the code in ast_tools which re-orders struct fields, to make NodeId always occupy a consistent "slot" in every AST node - at byte position 8, just after Span.

This will be a sizeable gain once we start utilizing the NodeId fields more. Because, for example, every variant of Expression has its NodeId stored in same location as all the other variants, Expression::node_id() is just 1 operation - a pointer read - rather than a nest of branches, or a lookup table.

Also alter the algorithm for ordering struct fields to fill in the 4-byte gap after NodeId field with other field(s), to avoid excess padding.

The new algorithm also prioritizes keeping fields in definition order as much as possible, rather than sorting them strictly in order of size and alignment. This is mildly advantageous because field definition order is the order the AST is walked in, so it avoids bouncing between cache lines while iterating through the fields of a struct when visiting the AST.

No types change size in the process. Fields remain packed to keep type sizes the minimum they can be - they just change order.

overlookmotel · 2026-03-20T21:00:44Z

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codspeed-hq · 2026-03-20T21:07:09Z

Merging this PR will not alter performance

✅ 53 untouched benchmarks
⏩ 3 skipped benchmarks¹

_{Comparing om/02-15-perf_ast_place_nodeid_field_after_span_in_structs (2064ade) with om/02-15-refactor_ast_re-order_layout_assertions_in_memory_order (2dfbe32)²}

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. ↩
No successful run was found on om/02-15-refactor_ast_re-order_layout_assertions_in_memory_order (cc4867c) during the generation of this report, so 9405508 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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graphite-app · 2026-03-20T22:15:28Z

Merge activity

Mar 20, 10:15 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
Mar 20, 10:29 PM UTC: Merged by the Graphite merge queue.

Alter the code in `ast_tools` which re-orders struct fields, to make `NodeId` always occupy a consistent "slot" in every AST node - at byte position 8, just after `Span`. This will be a sizeable gain once we start utilizing the `NodeId` fields more. Because, for example, every variant of `Expression` has its `NodeId` stored in same location as all the other variants, `Expression::node_id()` is just 1 operation - a pointer read - rather than a nest of branches, or a lookup table. Also alter the algorithm for ordering struct fields to fill in the 4-byte gap after `NodeId` field with other field(s), to avoid excess padding. The new algorithm also prioritizes keeping fields in definition order as much as possible, rather than sorting them strictly in order of size and alignment. This is mildly advantageous because field definition order is the order the AST is walked in, so it avoids bouncing between cache lines while iterating through the fields of a struct when visiting the AST. No types change size in the process. Fields remain packed to keep type sizes the minimum they can be - they just change order.

…ods (#20582) Pure refactor. Memory layout calculation was implemented as a series of free functions. Convert them to a `struct` with methods, so they can pass data between them in `self` rather than many function params. This prepares the way for the changes in #20584.

Now that `AstKind`'s variants are all structs (no enums any more), and #20584 has ensured that `Span` is in a consistent position in all structs, `AstKind::span` should boil down to a single operation - a pointer read. Therefore, mark it `#[inline]`. Note: `Span` was already in a consistent position prior to #20584, but that was more by accident than anything else. Now it's guaranteed. But this change won't make a difference to perf, it just guards against a perf regression in future.

Follow-on after #20584. Simplify implementation of struct field ordering in `ast_tools`. This change does not alter any layouts, just shortens the code by taking a simpler approach to handling ZST fields.

…ods (oxc-project#20582) Pure refactor. Memory layout calculation was implemented as a series of free functions. Convert them to a `struct` with methods, so they can pass data between them in `self` rather than many function params. This prepares the way for the changes in oxc-project#20584.

…20584) Alter the code in `ast_tools` which re-orders struct fields, to make `NodeId` always occupy a consistent "slot" in every AST node - at byte position 8, just after `Span`. This will be a sizeable gain once we start utilizing the `NodeId` fields more. Because, for example, every variant of `Expression` has its `NodeId` stored in same location as all the other variants, `Expression::node_id()` is just 1 operation - a pointer read - rather than a nest of branches, or a lookup table. Also alter the algorithm for ordering struct fields to fill in the 4-byte gap after `NodeId` field with other field(s), to avoid excess padding. The new algorithm also prioritizes keeping fields in definition order as much as possible, rather than sorting them strictly in order of size and alignment. This is mildly advantageous because field definition order is the order the AST is walked in, so it avoids bouncing between cache lines while iterating through the fields of a struct when visiting the AST. No types change size in the process. Fields remain packed to keep type sizes the minimum they can be - they just change order.

Now that `AstKind`'s variants are all structs (no enums any more), and oxc-project#20584 has ensured that `Span` is in a consistent position in all structs, `AstKind::span` should boil down to a single operation - a pointer read. Therefore, mark it `#[inline]`. Note: `Span` was already in a consistent position prior to oxc-project#20584, but that was more by accident than anything else. Now it's guaranteed. But this change won't make a difference to perf, it just guards against a perf regression in future.

) Follow-on after oxc-project#20584. Simplify implementation of struct field ordering in `ast_tools`. This change does not alter any layouts, just shortens the code by taking a simpler approach to handling ZST fields.

This was referenced Mar 20, 2026

refactor(ast_tools): search all crates which depend on oxc_ast_macros #20577

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perf(ast_tools): parse files in parallel #20578

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perf(ast_tools): reduce data passing between functions #20579

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github-actions bot added A-linter Area - Linter A-parser Area - Parser labels Mar 20, 2026

overlookmotel mentioned this pull request Mar 20, 2026

fix(ast_tools): correctly parse define_index_type! macros #20580

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github-actions bot added A-cli Area - CLI A-ast Area - AST A-ast-tools Area - AST tools A-linter-plugins Area - Linter JS plugins labels Mar 20, 2026

overlookmotel mentioned this pull request Mar 20, 2026

fix(ast_tools): remove NonMax32 primitive #20581

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github-actions bot added the C-performance Category - Solution not expected to change functional behavior, only performance label Mar 20, 2026

overlookmotel marked this pull request as ready for review March 20, 2026 21:28

overlookmotel requested a review from camc314 as a code owner March 20, 2026 21:28

Copilot AI review requested due to automatic review settings March 20, 2026 21:28

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overlookmotel self-assigned this Mar 20, 2026

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overlookmotel force-pushed the om/02-15-refactor_ast_re-order_layout_assertions_in_memory_order branch from 2dfbe32 to cc4867c Compare March 20, 2026 22:02

overlookmotel force-pushed the om/02-15-perf_ast_place_nodeid_field_after_span_in_structs branch from b258884 to 2064ade Compare March 20, 2026 22:02

graphite-app bot added the 0-merge Merge with Graphite Merge Queue label Mar 20, 2026

graphite-app bot force-pushed the om/02-15-refactor_ast_re-order_layout_assertions_in_memory_order branch from cc4867c to e984666 Compare March 20, 2026 22:19

graphite-app bot requested a review from leaysgur as a code owner March 20, 2026 22:19

graphite-app bot force-pushed the om/02-15-perf_ast_place_nodeid_field_after_span_in_structs branch from 2064ade to c6ea0a0 Compare March 20, 2026 22:20

graphite-app bot removed the 0-merge Merge with Graphite Merge Queue label Mar 20, 2026

Base automatically changed from om/02-15-refactor_ast_re-order_layout_assertions_in_memory_order to main March 20, 2026 22:28

graphite-app bot merged commit c6ea0a0 into main Mar 20, 2026
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graphite-app bot deleted the om/02-15-perf_ast_place_nodeid_field_after_span_in_structs branch March 20, 2026 22:29

This was referenced Mar 23, 2026

release(crates): oxc v0.122.0 #20648

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release(apps): oxlint v1.57.0 && oxfmt v0.42.0 #20649

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release(apps): oxlint v1.57.0 && oxfmt v0.42.0 #20680

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perf(ast): place `NodeId` field after `Span` in structs#20584

perf(ast): place `NodeId` field after `Span` in structs#20584
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