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perf(semantic): avoid heap alloc for var hoist scope ids#22603

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perf(semantic): avoid heap alloc for var hoist scope ids#22603
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claude/perf-semantic-var-hoist-smallvec

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@Dunqing Dunqing commented May 19, 2026

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Summary

VariableDeclarator::bind allocated a fresh Vec<ScopeId> for every var declarator (binder.rs:48), used to collect ancestor scope ids on the path to the enclosing var / function scope. Nesting depth from a declarator to its hoist target is small in practice (1–5 scopes), so the Vec almost never grew past its first 16-byte chunk — but the first push always allocates.

Replace with SmallVec<[ScopeId; 8]> — inline storage covers typical depths without touching the heap.

-let mut var_scope_ids = vec![];
+let mut var_scope_ids: SmallVec<[ScopeId; 8]> = SmallVec::new();

Allocation snapshot impact

Measured against the kitchen-sink baseline (parent of this commit). Sys allocs:

File Pipeline Before After Δ
antd.js (6.69 MB ES5, var-heavy) semantic 2,455 1,072 −1,383 (−56%)
pdf.mjs (567 kB) semantic 1,401 1,366 −35
kitchen-sink.tsx (733 kB) semantic 1,326 1,320 −6
antd.js minifier (runs semantic internally) 7,624 4,652 −2,972 (−39%)
Other files semantic unchanged unchanged

For ES5-bundled code like antd.js, this single line accounted for a clear majority of the heap allocations in SemanticBuilder::build.

How I found it

Backtrace-attributed allocation profiling of a fresh SemanticBuilder::build against antd.js: 1,383 of 2,459 captured allocations converged on one Vec::push site inside VariableDeclarator::bind's var-hoist branch. Every visit path (block_statement, if_statement, for_statement_init, function_body, ...) eventually called through the same VariableDeclarator::bindVec::push line, so a single change here zeros them all out.

Why this matters for downstream consumers

Rolldown rebuilds Scoping 3–4× per file across hundreds of files per bundle. Files containing legacy / bundled / var-heavy JS (very common in real-world npm packages) hit this path on every declarator. Cutting the binder's per-declarator heap allocation directly shrinks the per-rebuild alloc cost.

Why SmallVec<[ScopeId; 8]>

  • ScopeId is u32 (4 bytes), so 8 inline slots = 32 bytes plus 8 bytes header / discriminant; cheap on a stack frame.
  • Real-world depth distribution: top-level var = 0 ancestors collected; var inside for { if { … } } ≈ 2–4; arbitrarily deep nesting is rare and falls back to heap correctly.
  • SmallVec is already in the workspace dependencies; pulled in here for the first time in oxc_semantic.

Test plan

  • cargo test -p oxc_semantic --lib --tests — pass
  • cargo clippy -p oxc_semantic --release — clean
  • cargo run -p oxc_track_memory_allocations — snapshots updated to reflect the new numbers

AI disclosure: drafted with Claude Code, reviewed manually.

@github-actions github-actions Bot added the A-semantic Area - Semantic label May 19, 2026
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codspeed-hq Bot commented May 19, 2026

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Merging this PR will not alter performance

✅ 57 untouched benchmarks
⏩ 3 skipped benchmarks1


Comparing claude/perf-semantic-var-hoist-smallvec (7abee9b) with main (a15be79)

Open in CodSpeed

Footnotes

  1. 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.

@Dunqing Dunqing changed the base branch from main to graphite-base/22603 May 20, 2026 01:29
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from feae2b1 to 81d2fee Compare May 20, 2026 01:29
@Dunqing Dunqing changed the base branch from graphite-base/22603 to claude/bench-kitchen-sink-fixture May 20, 2026 01:29

Dunqing commented May 20, 2026

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@Dunqing Dunqing force-pushed the claude/bench-kitchen-sink-fixture branch from f6a07b0 to 3622d8a Compare May 20, 2026 01:38
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from 81d2fee to dc53600 Compare May 20, 2026 01:40
@Dunqing Dunqing changed the base branch from claude/bench-kitchen-sink-fixture to graphite-base/22603 May 20, 2026 02:49
@Dunqing Dunqing force-pushed the graphite-base/22603 branch from 3622d8a to 3cce4d9 Compare May 20, 2026 02:50
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from dc53600 to 223eaaf Compare May 20, 2026 02:50
@Dunqing Dunqing changed the base branch from graphite-base/22603 to claude/bench-kitchen-sink-fixture May 20, 2026 02:50
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from 223eaaf to a25c5d7 Compare May 20, 2026 09:37
@Dunqing Dunqing changed the base branch from claude/bench-kitchen-sink-fixture to graphite-base/22603 May 20, 2026 10:13
@Dunqing Dunqing force-pushed the graphite-base/22603 branch from 3cce4d9 to f75b4e4 Compare May 20, 2026 10:15
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from a25c5d7 to fc5257e Compare May 20, 2026 10:15
@Dunqing Dunqing changed the base branch from graphite-base/22603 to claude/bench-kitchen-sink-fixture May 20, 2026 10:15
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from fc5257e to f0f2f65 Compare May 20, 2026 10:21
@Dunqing Dunqing force-pushed the claude/bench-kitchen-sink-fixture branch from f75b4e4 to 87dcb4f Compare May 20, 2026 15:20
@Dunqing Dunqing force-pushed the claude/perf-semantic-var-hoist-smallvec branch from f0f2f65 to e8731d7 Compare May 20, 2026 15:20
@graphite-app graphite-app Bot changed the base branch from claude/bench-kitchen-sink-fixture to graphite-base/22603 May 20, 2026 15:56
graphite-app Bot pushed a commit that referenced this pull request May 20, 2026
## Summary

Adds `kitchen-sink.tsx` — a comprehensive synthetic TypeScript+JSX fixture maintained at [oxc-project/benchmark-files](https://github.com/oxc-project/benchmark-files) — to both `TestFiles::minimal()` (bench input set) and `TestFiles::complicated()` (alloc-tracking input set). The existing files in each set are untouched; this is a strict append.

## Why

The existing bench input set didn't reliably surface general-purpose perf wins above the ~1-2% measurement noise floor:
- #22580 (semantic pre-reserve) — visible because `binder.ts` exercises it
- #22594 (formatter buffer) — visible
- #22596 (minifier `try_fold_concat`) — **not visible** on the old set
- #22599 (semantic resolve-refs no-temp-Vec) — **not visible**
- #22603 (semantic var-hoist SmallVec) — **not visible**

The kitchen-sink fixes that by exercising every AST node, every transformer plugin, every minifier optimization opportunity, and every semantic step in one large file. Verified by re-benching #22596 against this fixture: **minifier mean −1.5%, min −3.7%** — above noise, signal confirmed.

## Fixture stats (cross-checked locally)

| Metric | Value |
|---|---|
| Source size | 21,117 lines / 732.90 kB |
| AST nodes | ~133,000 |
| Scopes | ~4,750 |
| Symbols | ~7,000 |
| Resolved references | ~16,000 |
| Semantic diagnostics | 0 errors / 0 warnings |

## Snap baselines

`tasks/track_memory_allocations/allocs_*.snap` updated with the kitchen-sink row across all 5 pipelines (parser / semantic / transformer / minifier / formatter). Future PRs that change allocation behavior on this fixture will produce a snap diff in CI.

## Bench-cleaner fix

`tasks/benchmark/benches/lexer.rs`'s `SourceCleaner` was missing `visit_ts_template_literal_type` — TypeScript type-level template literals (e.g. `` `${T}-${U}` `` in conditional / mapped types) are syntactically identical to value-level template literals, so the bench-mode lexer (without parser context) cannot distinguish them. Without the cleaner converting them to plain strings, kitchen-sink's type-level templates caused the lexer bench to swallow ~1 KB spans as a single `TemplateHead` and produce spurious "Unterminated string" / "Invalid Unicode escape" errors. One-line fix to mirror the existing `visit_template_literal` handling.

AI disclosure: drafted with Claude Code, reviewed manually.
@graphite-app graphite-app Bot force-pushed the claude/perf-semantic-var-hoist-smallvec branch from e8731d7 to 7aba78a Compare May 20, 2026 16:02
@graphite-app graphite-app Bot force-pushed the graphite-base/22603 branch from 87dcb4f to a15be79 Compare May 20, 2026 16:02
@graphite-app graphite-app Bot changed the base branch from graphite-base/22603 to main May 20, 2026 16:03
@graphite-app graphite-app Bot force-pushed the claude/perf-semantic-var-hoist-smallvec branch from 7aba78a to 7abee9b Compare May 20, 2026 16:03
Comment thread crates/oxc_semantic/src/binder.rs
@graphite-app graphite-app Bot added the 0-merge Merge with Graphite Merge Queue label May 21, 2026
Comment thread tasks/track_memory_allocations/allocs_formatter.snap
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## Summary

`VariableDeclarator::bind` allocated a fresh `Vec<ScopeId>` for every `var` declarator (`binder.rs:48`), used to collect ancestor scope ids on the path to the enclosing var / function scope. Nesting depth from a declarator to its hoist target is small in practice (1–5 scopes), so the Vec almost never grew past its first 16-byte chunk — but the first push always allocates.

Replace with `SmallVec<[ScopeId; 8]>` — inline storage covers typical depths without touching the heap.

```diff
-let mut var_scope_ids = vec![];
+let mut var_scope_ids: SmallVec<[ScopeId; 8]> = SmallVec::new();
```

## Allocation snapshot impact

Measured against the kitchen-sink baseline (parent of this commit). Sys allocs:

| File | Pipeline | Before | After | Δ |
|---|---|---|---|---|
| `antd.js` (6.69 MB ES5, var-heavy) | semantic | 2,455 | **1,072** | **−1,383 (−56%)** |
| `pdf.mjs` (567 kB) | semantic | 1,401 | 1,366 | −35 |
| `kitchen-sink.tsx` (733 kB) | semantic | 1,326 | 1,320 | −6 |
| `antd.js` | minifier (runs semantic internally) | 7,624 | **4,652** | **−2,972 (−39%)** |
| Other files | semantic | unchanged | unchanged | — |

For ES5-bundled code like antd.js, this single line accounted for a clear majority of the heap allocations in `SemanticBuilder::build`.

## How I found it

Backtrace-attributed allocation profiling of a fresh `SemanticBuilder::build` against `antd.js`: 1,383 of 2,459 captured allocations converged on one `Vec::push` site inside `VariableDeclarator::bind`'s var-hoist branch. Every visit path (`block_statement`, `if_statement`, `for_statement_init`, `function_body`, ...) eventually called through the same `VariableDeclarator::bind` → `Vec::push` line, so a single change here zeros them all out.

## Why this matters for downstream consumers

Rolldown rebuilds `Scoping` 3–4× per file across hundreds of files per bundle. Files containing legacy / bundled / `var`-heavy JS (very common in real-world npm packages) hit this path on every declarator. Cutting the binder's per-declarator heap allocation directly shrinks the per-rebuild alloc cost.

## Why `SmallVec<[ScopeId; 8]>`

- `ScopeId` is `u32` (4 bytes), so 8 inline slots = 32 bytes plus 8 bytes header / discriminant; cheap on a stack frame.
- Real-world depth distribution: top-level var = 0 ancestors collected; var inside `for { if { … } }` ≈ 2–4; arbitrarily deep nesting is rare and falls back to heap correctly.
- `SmallVec` is already in the workspace dependencies; pulled in here for the first time in `oxc_semantic`.

## Test plan

- [x] `cargo test -p oxc_semantic --lib --tests` — pass
- [x] `cargo clippy -p oxc_semantic --release` — clean
- [x] `cargo run -p oxc_track_memory_allocations` — snapshots updated to reflect the new numbers

AI disclosure: drafted with Claude Code, reviewed manually.
@graphite-app graphite-app Bot force-pushed the claude/perf-semantic-var-hoist-smallvec branch from 223ae1a to e862c15 Compare May 21, 2026 07:56
@graphite-app graphite-app Bot merged commit e862c15 into main May 21, 2026
30 checks passed
@graphite-app graphite-app Bot removed the 0-merge Merge with Graphite Merge Queue label May 21, 2026
@graphite-app graphite-app Bot deleted the claude/perf-semantic-var-hoist-smallvec branch May 21, 2026 08:01
Dunqing added a commit that referenced this pull request May 26, 2026
### 🚀 Features

- e857b0c napi/minify: Expose legalComments option and result (#20370)
(Boshen)
- 661132d parser: More friendly error messages for rest assignment
target and rest binding element (#22719) (sapphi-red)
- ee659b6 transformer/legacy-decorator: Add `strictNullChecks` option
for nullable-union design:type (#22266) (Kyle Cannon)

### 🐛 Bug Fixes

- e1d064e transformer/class-properties: Reparent lifted private method
helpers (#22716) (Cameron)
- 4ac0fca minifier: Preserve `0 && (module.exports = { ... })`
cjs-module-lexer hint (#22729) (Dunqing)
- 40ff611 minifier: Mark peephole loop changed when dropping
dead-after-throw statement (#22722) (Dunqing)
- 2f7b210 codegen: Emit pife-arrow/function leading comments inside the
wrap (#22720) (Dunqing)
- e184f74 parser: Improve invalid `import` property access diagnostic
(#22693) (camc314)
- 7baed9c transformer/private-method: Clear inherited strict flags
(#22508) (camc314)
- a9ad27e parser: Keep annotation comments leading without preceding
newline (#22711) (Dunqing)
- 9ea4d64 minifier: Re-evaluate pure/no-side-effects flags after
peephole inlining (#22595) (Dunqing)
- 07afbb6 minifier: Drop empty-body IIFE wrapper when called with
arguments (#22589) (Dunqing)
- fa7c463 semantic: Correct TS enum member symbol spans (#22689)
(camc314)
- 26b9396 semantic: Resolve parameter decorators outside parameter scope
(#22623) (camc314)
- b284045 parser: Switch to module goal eagerly on `export` (#22684)
(Boshen)
- dfa931d semantic: Propagate unresolved auto-increment enum value
instead of defaulting to 0 (#22646) (Dunqing)
- 69a6ba6 transformer/legacy-decorator: Emit Array for ReadonlyArray<T>
in decorator metadata (#22265) (Kyle Cannon)
- e421ef0 transformer/legacy-decorator: Return runtime binding for
design:type (#22640) (Dunqing)
- d61e1d7 codegen: Preserve verbatim text of pure/no-side-effects
comments (#22525) (Dunqing)
- 702b14e minifier: Preserve IIFE structure in DCE-only mode (#22547)
(Dunqing)
- 917da24 parser: Apply PURE comment through member-access chains
(#22566) (Dunqing)
- a069b1c codegen: Preserve quotes for cjs-module-lexer equality strings
(#22551) (Dunqing)

### ⚡ Performance

- 2f623b0 semantic: Skip unresolved checks for re-exports (#22660)
(camc314)
- 0d9553d semantic: Early-exit `check_object_expression` for objects
with <2 properties (#22668) (Dunqing)
- d721ad9 semantic: Use direct grandparent lookup for TS type parameters
(#22658) (camc314)
- 0aff288 semantic: Reorder numeric literal strict mode checks (#22657)
(camc314)
- 4d5ddb1 semantic: Reorder binding identifier checks (#22656) (camc314)
- e32acd8 semantic: Reorder identifier ambient binding check (#22653)
(camc314)
- 09fe178 semantic: Reorder ident reference strict mode check (#22652)
(camc314)
- 4b6add2 semantic: Avoid duplicate ident clone for bindings (#22663)
(camc314)
- 82f9662 parser: Check identifier kind before context flag (#22662)
(camc314)
- d7cd951 parser: Fast path identifier parsing and inline operator
helpers (#22650) (Boshen)
- 7b84314 semantic: Use direct byte access for numeric leading-zero
check (#22642) (camc314)
- 0345a31 semantic: Pre-size class elements hash map (#22618) (camc314)
- 04d3065 minifier: Drop per-call buffers in try_fold_concat (#22596)
(Dunqing)
- 4f289f1 semantic: Resolve_references_for_current_scope without a temp
Vec (#22599) (Dunqing)
- e862c15 semantic: Avoid heap alloc for var hoist scope ids (#22603)
(Dunqing)
- 8ff8674 semantic: Early return if `excess` is `0` in
`Stats::increase_by` (#22616) (camc314)
- 7a4120e semantic: Pre-reserve unresolved_references using
Stats::references (#22580) (Dunqing)

Co-authored-by: Dunqing <29533304+Dunqing@users.noreply.github.com>
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