feat(minifier): inline const value for read-only vars by Dunqing · Pull Request #22593 · oxc-project/oxc

Dunqing · 2026-05-19T14:15:18Z

Summary

The minifier skipped constant-inlining for every var, blaming TDZ. The real constraint is narrower — var hoisting — and most vars are still safe to inline. This lifts the blanket skip and inlines a var's constant whenever no read can observe its hoisted undefined. Closes #13051.

Why

A var x = <literal>; binds x to undefined at the top of its scope and assigns the literal only in source order. A read in that window sees undefined, so inlining the literal everywhere would change behavior there — which is why the old code skipped all vars. But that also blocked a common, perfectly safe pattern from optimizing in DCE mode:

var DEBUG = false;
function check() { if (DEBUG) console.log('debug'); }
console.log(check());

vite 8's topLevelVar: true rewrites top-level let/const to var, so users' flag patterns silently stopped tree-shaking — vitejs/vite#22363 → rolldown/rolldown#9279.

The safety principle

Inline only when the value at the read is provably the assigned literal, never the hoisted undefined. Concretely, no user code may run between scope entry and the assignment that could read x. A var x = <literal>; qualifies when all of:

It is in a still-declarative body. It sits directly in the current function/program body, and every preceding statement is declarative — runs no user code (is_declarative_body_statement: function/empty/type-only declarations, module re-exports, and literal-initialised simple vars). The first non-declarative statement ends the body's "declarative prelude"; this is also enforced per-declarator for multi-declarator statements.
Its initializer is a ConstantValue.
It is not a top-level script var (those alias the global object).
At program scope, the module loads no other module (import / export … from / export * from) — a cyclic importer could call into our exports and observe a captured var before assignment, regardless of export status.
It has exactly one read, crossing a function boundary — the gap that substitute_single_use_symbol (same call frame) and the small-value inliner don't fill. This also keeps the change byte-positive: dropping var x = L; and replacing one read with L always saves bytes.

The prelude flag is a per-body stack pushed/popped at function boundaries, so the analysis runs independently for nested functions. Reassignment is handled by the existing write_references_count guard, and direct-eval scopes are excluded.

Verification. A battery of hoisting / closure / reassignment programs was run through the minifier and then executed, checking every output reproduces the original's observable behavior. All pass — output is semantically equal to source on each, matching Terser.

Comparison with other minifiers

Behaviour checked against Terser 5.48, esbuild 0.24, and SWC 1.15:

esbuild only propagates const primitives — it never inlines a var cross-scope. So const DEBUG = false; … folds to console.log(void 0) but var DEBUG = false; … stays. That is exactly the regression path: topLevelVar: true rewrites const→var, defeating const-only propagation. oxc had the same blanket-var skip; this PR lifts it for the provably-safe subset.
Terser / SWC reach the same result (and more — flow-sensitive reassignment, call inlining) via multi-pass reduce_vars/collapse_vars. This PR recovers the common flag pattern with one syntactic prelude check instead of that machinery, staying single-pass.

Results

minsize shows small gzip improvements on lodash / victory / antd / typescript; jquery is byte-identical.

Closes #13051

Dunqing · 2026-05-19T14:15:34Z

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codspeed-hq · 2026-05-19T14:19:30Z

Merging this PR will not alter performance

✅ 52 untouched benchmarks
⏩ 19 skipped benchmarks¹

_{Comparing feat/minifier-inline-readonly-vars (729741a) with minifier-stack/incremental-scoping (db7b22e)}

19 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 · 2026-05-19T14:52:40Z

Verification: oxc (this PR) vs esbuild 0.27.4

Ran the same 7 edge cases through both minifiers — covers the motivating case, the four hoisting hazards the predicate is supposed to block, and the two holes Codex flagged during review.

#	Case	oxc	esbuild	Verdict
1	`var used = false; function test() { if (used) return 123; return 321; } console.log(test())`	Inlines → `function test(){return 321}`	`var used=!1; … used?123:321` (no inline)	oxc folds the dead branch
2	`output(); var foo = true; function output() { if (!foo) console.log('foo') }` (preceding-call hazard)	Preserves `foo`	Preserves `foo`	both correct
3	`var y = foo; var foo = 1; console.log(y)` (same-scope pre-init read)	Preserves	Preserves	both correct
4	`var foo = 1; foo = 2; console.log(foo)` (reassigned)	Preserves	Preserves	both correct
5	`var [x = inner()] = ''; var flag = true; function inner() { return flag }; console.log(x)` (destructuring default — Codex finding 1)	Preserves `flag`	Preserves `flag`	both correct ✓
6	`import './b.js'; var flag = true; export function check() { return flag }` (exported closure under imports — Codex finding 2)	Preserves `flag`	Preserves `flag`	both correct ✓
7	`function outer() { var flag = false; function inner() { return flag ? 1 : 2 } return inner() }` (function-body single-use cross-function)	Inlines → `function inner(){return 2}`	`var n=!1; … n?1:2` (no inline)	oxc folds inside the function body

Findings

Every hazard case (2–6) behaves identically — same observably-correct output by different mechanisms (esbuild doesn't inline var at all; oxc's predicate proves safety case-by-case).
Both Codex-flagged hazards are correctly avoided — neither tool falls into the destructuring-default trap or the cyclic-closure trap.
oxc is more aggressive on cases 1 and 7 — these are bytes esbuild leaves on the table, and they're exactly what esbuild's open feature request evanw/esbuild#1981 is asking for.
oxc's behavior is a strict superset of esbuild's: every elision esbuild produces, oxc also produces, plus the two safe-to-fold cases above.

Reproduction

# install / locate esbuild 0.27.4 and run with --minify --bundle=false
# run oxc with `cargo run -p oxc_minifier --example dce -- <file>`

Each row above was generated from a fresh .mjs file containing only that snippet.

oxc-guard · 2026-06-15T14:43:25Z

Monitor Oxc

Commit: 729741a0b2b5

suite	result
Build	✅
Isolated Declarations	✅
(whitespace)	✅
(codegen)	✅
(transformer)	❌
(compressor)	✅
(dce)	✅
(formatter)	❌
(formatter_dcr)	✅
(mangler)	❌

Dunqing · 2026-06-15T15:06:05Z

Merge activity

Jun 15, 3:06 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Jun 16, 2:08 AM UTC: Dunqing added this pull request to the Graphite merge queue.
Jun 16, 2:13 AM UTC: Merged by the Graphite merge queue.

## Summary The minifier skipped constant-inlining for **every** `var`, blaming TDZ. The real constraint is narrower — `var` *hoisting* — and most `var`s are still safe to inline. This lifts the blanket skip and inlines a `var`'s constant whenever no read can observe its hoisted `undefined`. Closes #13051. ## Why A `var x = <literal>;` binds `x` to `undefined` at the top of its scope and assigns the literal only in source order. A read in that window sees `undefined`, so inlining the literal everywhere would change behavior there — which is why the old code skipped all `var`s. But that also blocked a common, perfectly safe pattern from optimizing in DCE mode: ```js var DEBUG = false; function check() { if (DEBUG) console.log('debug'); } console.log(check()); ``` vite 8's `topLevelVar: true` rewrites top-level `let`/`const` to `var`, so users' flag patterns silently stopped tree-shaking — vitejs/vite#22363 → rolldown/rolldown#9279. ## The safety principle Inline only when the value at the read is provably the assigned literal, never the hoisted `undefined`. Concretely, no user code may run between scope entry and the assignment that could read `x`. A `var x = <literal>;` qualifies when all of: - **It is in a still-declarative body.** It sits directly in the current function/program body, and every preceding statement is *declarative* — runs no user code (`is_declarative_body_statement`: function/empty/type-only declarations, module re-exports, and literal-initialised simple `var`s). The first non-declarative statement ends the body's "declarative prelude"; this is also enforced per-declarator for multi-declarator statements. - **Its initializer is a `ConstantValue`.** - **It is not a top-level script var** (those alias the global object). - **At program scope, the module loads no other module** (`import` / `export … from` / `export * from`) — a cyclic importer could call into our exports and observe a captured var before assignment, regardless of export status. - **It has exactly one read, crossing a function boundary** — the gap that `substitute_single_use_symbol` (same call frame) and the small-value inliner don't fill. This also keeps the change byte-positive: dropping `var x = L;` and replacing one read with `L` always saves bytes. The prelude flag is a per-body stack pushed/popped at function boundaries, so the analysis runs independently for nested functions. Reassignment is handled by the existing `write_references_count` guard, and direct-eval scopes are excluded. **Verification.** A battery of hoisting / closure / reassignment programs was run through the minifier and then *executed*, checking every output reproduces the original's observable behavior. All pass — output is semantically equal to source on each, matching Terser. ## Comparison with other minifiers Behaviour checked against Terser 5.48, esbuild 0.24, and SWC 1.15: - **esbuild** only propagates `const` primitives — it never inlines a `var` cross-scope. So `const DEBUG = false; …` folds to `console.log(void 0)` but `var DEBUG = false; …` stays. That is exactly the regression path: `topLevelVar: true` rewrites `const`→`var`, defeating const-only propagation. oxc had the same blanket-`var` skip; this PR lifts it for the provably-safe subset. - **Terser / SWC** reach the same result (and more — flow-sensitive reassignment, call inlining) via multi-pass `reduce_vars`/`collapse_vars`. This PR recovers the common flag pattern with one syntactic prelude check instead of that machinery, staying single-pass. ## Results `minsize` shows small gzip improvements on lodash / victory / antd / typescript; jquery is byte-identical. Closes #13051

### 💥 BREAKING CHANGES - 7a76cd3 estree: [**BREAKING**] Make whether to include TS fields a runtime option (#23574) (overlookmotel) - e7b6b68 estree: [**BREAKING**] `ESTree` config use methods not consts (#23573) (overlookmotel) ### 🚀 Features - 556cc6d data_structures: Add `CodeBuffer::as_str` method (#23571) (overlookmotel) - 38c4b06 parser: Add friendly error for adjacent JSX elements (#23378) (sapphi-red) - 53509a8 minifier: Treeshake pure typed arrays and Set/Map array literals (#23469) (Dunqing) - 09762d9 minifier: Inline const value for read-only vars (#22593) (Dunqing) ### 🐛 Bug Fixes - 20375f9 react_compiler: Keep imports referenced only by a computed key (#23586) (Boshen) - 31bfd9b minifier: Keep Object introspection calls on a possible Proxy (#23483) (Dunqing) - 837a395 parser: Treat a line comment after ':' as leading, not trailing (#23515) (Dunqing) - e409fe0 minifier: Keep `new Map`/`WeakSet`/`WeakMap` with a string argument (#23470) (Dunqing) - ae02b4e ci/parser: Use `minimal` for vitest reporter (#23457) (camc314) ### ⚡ Performance - cf24329 mangler: Compile slot sort once instead of per CAPACITY (#23577) (Boshen) - 4058a6a parser: Reduce code bloat from verify_modifiers monomorphization (#23576) (Boshen) - 053b0c1 estree: Remove pointless `mem::take` (#23572) (overlookmotel) - dfb52b6 transformer: Pre-size statement vecs in TS enum & namespace lowering (#23516) (Yunfei He) - 970e09a minifier: Compute template-literal inline checks in a single pass (#23467) (Yunfei He) - 3170c0e semantic,mangler,minifier: Fix `Semantic::stats` node count and reuse stats in mangler builds (#23352) (Boshen) - d1fa6e0 minifier: Evaluate ternary branches once in minimize_conditional_expression (#23479) (Yunfei He) - 3fa8051 transformer: Pre-size JSX props vec to attribute count (#23466) (Yunfei He) - 488b382 react_compiler: Borrow binding names in prefilter instead of allocating (#23471) (Yunfei He) - bcb3894 minifier: Incremental scoping refresh, delete LiveUsageCollector (#23197) (Dunqing) ### 📚 Documentation - f68641e data_structures: Improve docs on safety contract (#23575) (overlookmotel) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>

## Summary The minifier skipped constant-inlining for **every** `var`, blaming TDZ. The real constraint is narrower — `var` *hoisting* — and most `var`s are still safe to inline. This lifts the blanket skip and inlines a `var`'s constant whenever no read can observe its hoisted `undefined`. Closes #13051. ## Why A `var x = <literal>;` binds `x` to `undefined` at the top of its scope and assigns the literal only in source order. A read in that window sees `undefined`, so inlining the literal everywhere would change behavior there — which is why the old code skipped all `var`s. But that also blocked a common, perfectly safe pattern from optimizing in DCE mode: ```js var DEBUG = false; function check() { if (DEBUG) console.log('debug'); } console.log(check()); ``` vite 8's `topLevelVar: true` rewrites top-level `let`/`const` to `var`, so users' flag patterns silently stopped tree-shaking — vitejs/vite#22363 → rolldown/rolldown#9279. ## The safety principle Inline only when the value at the read is provably the assigned literal, never the hoisted `undefined`. Concretely, no user code may run between scope entry and the assignment that could read `x`. A `var x = <literal>;` qualifies when all of: - **It is in a still-declarative body.** It sits directly in the current function/program body, and every preceding statement is *declarative* — runs no user code (`is_declarative_body_statement`: function/empty/type-only declarations, module re-exports, and literal-initialised simple `var`s). The first non-declarative statement ends the body's "declarative prelude"; this is also enforced per-declarator for multi-declarator statements. - **Its initializer is a `ConstantValue`.** - **It is not a top-level script var** (those alias the global object). - **At program scope, the module loads no other module** (`import` / `export … from` / `export * from`) — a cyclic importer could call into our exports and observe a captured var before assignment, regardless of export status. - **It has exactly one read, crossing a function boundary** — the gap that `substitute_single_use_symbol` (same call frame) and the small-value inliner don't fill. This also keeps the change byte-positive: dropping `var x = L;` and replacing one read with `L` always saves bytes. The prelude flag is a per-body stack pushed/popped at function boundaries, so the analysis runs independently for nested functions. Reassignment is handled by the existing `write_references_count` guard, and direct-eval scopes are excluded. **Verification.** A battery of hoisting / closure / reassignment programs was run through the minifier and then *executed*, checking every output reproduces the original's observable behavior. All pass — output is semantically equal to source on each, matching Terser. ## Comparison with other minifiers Behaviour checked against Terser 5.48, esbuild 0.24, and SWC 1.15: - **esbuild** only propagates `const` primitives — it never inlines a `var` cross-scope. So `const DEBUG = false; …` folds to `console.log(void 0)` but `var DEBUG = false; …` stays. That is exactly the regression path: `topLevelVar: true` rewrites `const`→`var`, defeating const-only propagation. oxc had the same blanket-`var` skip; this PR lifts it for the provably-safe subset. - **Terser / SWC** reach the same result (and more — flow-sensitive reassignment, call inlining) via multi-pass `reduce_vars`/`collapse_vars`. This PR recovers the common flag pattern with one syntactic prelude check instead of that machinery, staying single-pass. ## Results `minsize` shows small gzip improvements on lodash / victory / antd / typescript; jquery is byte-identical. Closes #13051

### 💥 BREAKING CHANGES - 7a76cd3 estree: [**BREAKING**] Make whether to include TS fields a runtime option (#23574) (overlookmotel) - e7b6b68 estree: [**BREAKING**] `ESTree` config use methods not consts (#23573) (overlookmotel) ### 🚀 Features - 556cc6d data_structures: Add `CodeBuffer::as_str` method (#23571) (overlookmotel) - 38c4b06 parser: Add friendly error for adjacent JSX elements (#23378) (sapphi-red) - 53509a8 minifier: Treeshake pure typed arrays and Set/Map array literals (#23469) (Dunqing) - 09762d9 minifier: Inline const value for read-only vars (#22593) (Dunqing) ### 🐛 Bug Fixes - 20375f9 react_compiler: Keep imports referenced only by a computed key (#23586) (Boshen) - 31bfd9b minifier: Keep Object introspection calls on a possible Proxy (#23483) (Dunqing) - 837a395 parser: Treat a line comment after ':' as leading, not trailing (#23515) (Dunqing) - e409fe0 minifier: Keep `new Map`/`WeakSet`/`WeakMap` with a string argument (#23470) (Dunqing) - ae02b4e ci/parser: Use `minimal` for vitest reporter (#23457) (camc314) ### ⚡ Performance - cf24329 mangler: Compile slot sort once instead of per CAPACITY (#23577) (Boshen) - 4058a6a parser: Reduce code bloat from verify_modifiers monomorphization (#23576) (Boshen) - 053b0c1 estree: Remove pointless `mem::take` (#23572) (overlookmotel) - dfb52b6 transformer: Pre-size statement vecs in TS enum & namespace lowering (#23516) (Yunfei He) - 970e09a minifier: Compute template-literal inline checks in a single pass (#23467) (Yunfei He) - 3170c0e semantic,mangler,minifier: Fix `Semantic::stats` node count and reuse stats in mangler builds (#23352) (Boshen) - d1fa6e0 minifier: Evaluate ternary branches once in minimize_conditional_expression (#23479) (Yunfei He) - 3fa8051 transformer: Pre-size JSX props vec to attribute count (#23466) (Yunfei He) - 488b382 react_compiler: Borrow binding names in prefilter instead of allocating (#23471) (Yunfei He) - bcb3894 minifier: Incremental scoping refresh, delete LiveUsageCollector (#23197) (Dunqing) ### 📚 Documentation - f68641e data_structures: Improve docs on safety contract (#23575) (overlookmotel) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>

github-actions Bot added the A-minifier Area - Minifier label May 19, 2026

Dunqing force-pushed the feat/minifier-inline-readonly-vars branch from 462eaa9 to 06d1393 Compare June 13, 2026 14:48

Dunqing changed the base branch from main to minifier-stack/incremental-scoping June 13, 2026 14:48

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This was referenced Jun 15, 2026

perf(minifier): incremental scoping refresh, delete LiveUsageCollector #23197

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refactor(minifier): route all peephole mutations through typed helpers #23196

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Dunqing marked this pull request as ready for review June 15, 2026 14:57

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Dunqing mentioned this pull request Jun 16, 2026

refactor(minifier): remove dead SymbolValue::scope_id field #23464

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release(crates): oxc v0.137.0 #23591

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Verification: oxc (this PR) vs esbuild 0.27.4

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