feat(minifier): remove unreachable code after terminating statements by Dunqing · Pull Request #24441 · oxc-project/oxc

Dunqing · 2026-07-13T03:47:28Z

Folding a define-replaced branch can leave if (true) { ... return fn; } as a bare block: the lexical declaration inside pins the block (flattening it could collide with sibling bindings), and the statements after it survived even though the block always returns. rolldown's per-module tree-shake hits this shape and ships the dead branch (rolldown/rolldown#10184).

minimize_statements only flagged dead control flow on direct jump statements, each arm setting the flag by hand. The driver now flips the flag whenever the statement it just emitted never completes normally: a direct jump, a block ending in a jump, an if/else where both branches jump, or a try where every path jumps. That single check subsumes the per-arm writes, so the &mut bool threaded through the return/throw handlers is gone. The removal reuses the existing dead-zone machinery, so var hoisting and kept function declarations behave as before, and both DCE and full-minify modes are covered.

A minimized dead zone keeps hoisting survivors after the jump — function declarations and the initializer-less var stub KeepVar re-emits — so the block predicate skips those from the back instead of testing only the literal last statement; nothing after an aborting statement ever runs, so they cannot change how the block completes.

The helper is conservative on purpose: labeled statements never count (a: { break a; } completes normally), and loops and switches are left to their specific folds. Terser removes the same tails; esbuild keeps them. The leftover { ... } block wrapper is also kept — flattening a block whose lexical declarations may collide with siblings is a separate rewrite, and terser and esbuild keep the braces too.

Example

__SSR__ defined to true (rolldown treeshake, minify: false):

export function test1() {
  if (true) {
    const fn = () => console.log("ssr");
    fn.stop = fn;
    return fn;
  }
  const fn = () => console.log("client");
  fn.stop = fn;
  return fn;
}

Before, the unreachable client tail stayed in the output. After:

export function test1() {
	{
		const fn = () => console.log("ssr");
		fn.stop = fn;
		return fn;
	}
}

Verified with the minifier unit suite, test262/babel minifier conformance (no changes), DCE --twice idempotency, and just minsize (antd gzip 455.51 kB -> 455.50 kB, iteration counts unchanged).

Fixes rolldown/rolldown#10184.

Implemented with AI assistance (Claude Code); design, review, and verification driven by the author per the repo AI-usage policy.

Dunqing · 2026-07-13T03:47:41Z

feat(minifier): remove unreachable code after terminating statements #24441 👈 (View in Graphite)
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oxc-guard · 2026-07-13T03:51:10Z

Monitor Oxc

Commit: 6cd22be527fb

suite	result
Build	✅
Test262	✅
Isolated Declarations	✅
Runtime Bundles	✅
Uglify Corpus	✅
(compressor)	✅
(transformer)	✅
(whitespace)	✅
(dce)	✅
(mangler)	❌
(codegen)	✅
(formatter_dcr)	✅
(formatter)	❌
Runtime (vue)	✅
Runtime (zod)	✅
Runtime (es-toolkit)	✅
Runtime (solid)	✅
Runtime (svelte)	✅

codspeed-hq · 2026-07-13T03:51:36Z

Merging this PR will not alter performance

✅ 52 untouched benchmarks
⏩ 19 skipped benchmarks¹

_{Comparing minifier-unreachable-after-terminating-statements (6cd22be) with main (cfed680)}

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 · 2026-07-13T08:42:54Z

I've reviewed the code myself and understand it.

Dunqing · 2026-07-13T08:43:04Z

Merge activity

Jul 13, 8:43 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 13, 8:43 AM UTC: Dunqing added this pull request to the Graphite merge queue.
Jul 13, 8:48 AM UTC: Merged by the Graphite merge queue.

…24441) Folding a define-replaced branch can leave `if (true) { ... return fn; }` as a bare block: the lexical declaration inside pins the block (flattening it could collide with sibling bindings), and the statements after it survived even though the block always returns. rolldown's per-module tree-shake hits this shape and ships the dead branch (rolldown/rolldown#10184). `minimize_statements` only flagged dead control flow on direct jump statements, each arm setting the flag by hand. The driver now flips the flag whenever the statement it just emitted never completes normally: a direct jump, a block ending in a jump, an if/else where both branches jump, or a try where every path jumps. That single check subsumes the per-arm writes, so the `&mut bool` threaded through the return/throw handlers is gone. The removal reuses the existing dead-zone machinery, so `var` hoisting and kept function declarations behave as before, and both DCE and full-minify modes are covered. A minimized dead zone keeps hoisting survivors after the jump — `function` declarations and the initializer-less `var` stub KeepVar re-emits — so the block predicate skips those from the back instead of testing only the literal last statement; nothing after an aborting statement ever runs, so they cannot change how the block completes. The helper is conservative on purpose: labeled statements never count (`a: { break a; }` completes normally), and loops and switches are left to their specific folds. Terser removes the same tails; esbuild keeps them. The leftover `{ ... }` block wrapper is also kept — flattening a block whose lexical declarations may collide with siblings is a separate rewrite, and terser and esbuild keep the braces too. ## Example `__SSR__` defined to `true` (rolldown treeshake, `minify: false`): ```js export function test1() { if (true) { const fn = () => console.log("ssr"); fn.stop = fn; return fn; } const fn = () => console.log("client"); fn.stop = fn; return fn; } ``` Before, the unreachable client tail stayed in the output. After: ```js export function test1() { { const fn = () => console.log("ssr"); fn.stop = fn; return fn; } } ``` Verified with the minifier unit suite, test262/babel minifier conformance (no changes), DCE `--twice` idempotency, and `just minsize` (antd gzip 455.51 kB -> 455.50 kB, iteration counts unchanged). Fixes rolldown/rolldown#10184. Implemented with AI assistance (Claude Code); design, review, and verification driven by the author per the repo AI-usage policy.

### 🚀 Features - 616bfa2 minifier: Remove unreachable code after terminating statements (#24441) (Dunqing) - ddab89a data_structures: Add `likely` and `unlikely` functions (#24368) (overlookmotel) - a3a39f9 react_compiler: Implement enableEmitHookGuards codegen (#24329) (Boshen) - b79eef7 minifier: Apply De Morgan's law to negated comparison chains in jump guards and loop tests (#24279) (Dunqing) - 34ff7b4 minifier: Drop write-only property assignments to unused local bindings by default (#24112) (Dunqing) - 1b829d8 semantic: Record const enums in EnumData (#24268) (Dunqing) - ba0944c semantic: Add `Scoping::set_symbol_span` (#24221) (camc314) ### 🐛 Bug Fixes - 7d33363 minifier: Preserve guaranteed throws from class heritage evaluation (#24349) (Dunqing) - 058a62f semantic: Track ambient contexts in `SemanticBuilder` (#24327) (camc314) - 721eb0b transformer/decorator: Scope accessor class binding (#24330) (camc314) - 1ebdce3 semantic: Allow reserved keywords in ambient declaration types (#24325) (camc314) - 460176a track-memory-allocations: Exclude arena chunks from Sys allocs (#24292) (Dunqing) - af4922b transformer: Clear lowered namespace redeclarations (#24300) (camc314) - ffd2765 semantic: Mark declared computed `MethodDefinition`s as type references (#24296) (camc314) - f17514b isolated-declarations: Emit const readonly fields as types (#24288) (camc314) - 40f769d minifier: Make `__proto__` write tracking execution-order independent (#24280) (Dunqing) - 6371fed transformer: Remove stale enum member bindings (#24272) (camc314) - f05dfab transformer: Correct symbol flags for lowered namespaces (#24271) (Dunqing) - 84eeb55 transformer: Correct symbol flags for lowered enums (#24269) (Dunqing) - c3057da transformer: Preserve generated class binding spans (#24220) (camc314) - 8260096 transformer: Correct span for lowered namespace symbol (#24222) (camc314) - 42d00d3 semantic: Mark declared class heritage as type references (#24237) (camc314) - 588d997 semantic: Mark TS `PropertyDefinition`s computed fields as type references (#24233) (camc314) - 9b95632 semantic: Mark computed method keys in `TSMethodSignature`s as type references (#24232) (camc314) ### ⚡ Performance - 5b26643 transformer_plugins: Dispatch global defines by trailing name (#23666) (Boshen) - dce0f29 react_compiler: Replace all compiled functions in a single AST walk (#24403) (Boshen) - f85f0d8 ast: Delegate inherited enum variants in clone_in and estree derives (#23555) (Boshen) - 3ff0234 allocator: Remove `unwrap` from `ReplaceWith` (#24365) (overlookmotel) - ab22e80 transformer: Fix Rust 1.97 performance regression (#24354) (camc314) - b47585c parser: Use `ReplaceWith` instead of `TakeIn` (#24018) (overlookmotel) - b227a06 minifier: Use `ReplaceWith` instead of `TakeIn` (#24017) (overlookmotel) Co-authored-by: Boshen <[email protected]>

github-actions Bot added the A-minifier Area - Minifier label Jul 13, 2026

Dunqing added the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 13, 2026

oxc-guard Bot removed the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 13, 2026

Dunqing marked this pull request as ready for review July 13, 2026 08:00

Dunqing requested a review from overlookmotel as a code owner July 13, 2026 08:00

Dunqing force-pushed the minifier-unreachable-after-terminating-statements branch from 17f80bf to 6cd22be Compare July 13, 2026 08:10

Dunqing marked this pull request as draft July 13, 2026 08:11

Dunqing added the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 13, 2026

oxc-guard Bot removed the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 13, 2026

Dunqing marked this pull request as ready for review July 13, 2026 08:42

Dunqing added the 0-merge Merge with Graphite Merge Queue label Jul 13, 2026

graphite-app Bot force-pushed the minifier-unreachable-after-terminating-statements branch from 6cd22be to 616bfa2 Compare July 13, 2026 08:44

graphite-app Bot merged commit 616bfa2 into main Jul 13, 2026
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graphite-app Bot removed the 0-merge Merge with Graphite Merge Queue label Jul 13, 2026

graphite-app Bot deleted the minifier-unreachable-after-terminating-statements branch July 13, 2026 08:48

oxc-guard Bot mentioned this pull request Jul 13, 2026

release(crates): oxc v0.140.0 #24453

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feat(minifier): remove unreachable code after terminating statements#24441

feat(minifier): remove unreachable code after terminating statements#24441
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