fix(react_compiler): align memoization cache-slot allocation with Babel by Boshen · Pull Request #24157 · oxc-project/oxc

Boshen · 2026-07-04T15:13:25Z

Root cause

On ~57 ecosystem files, oxc allocated one fewer _c() cache slot than babel-plugin-react-compiler in a function (e.g. oxc _c(25) where Babel emits _c(26); oxc [26,40] where Babel [27,40]), shifting every subsequent $[i] index by −1.

The construct responsible is the regex literal. In react_compiler_lowering/build_hir.rs, lower_expression had no arm for oxc::Expression::RegExpLiteral, so every regex literal fell through the catch-all that "bails to undefined":

_ => {
    // not-yet-ported arms bail to undefined
    Ok(InstructionValue::Primitive { value: PrimitiveValue::Undefined, loc })
}

Two consequences:

Correctness: the regex was dropped from the output — e.g. useId().replace(/:/g, "") compiled to t1.replace(undefined, ""), and pattern.test(value) compiled to undefined.test(value).
Slot count: an undefined primitive does not allocate, so may_allocate never gave it a reactive scope. Babel treats a regex literal as an allocation and memoizes it (a nested Symbol.for("react.memo_cache_sentinel") scope when it has no deps), reserving one extra slot. That missing slot was the −1.

All the downstream machinery for regex was already implemented — the HIR variant InstructionValue::RegExpLiteral, its codegen (codegen_reactive_function.rs), may_allocate (returns true), aliasing/type inference, and every visitor. Only the lowering arm was missing.

The fix

Add the single missing arm in lower_expression, mirroring TS BuildHIR's case 'RegExpLiteral':

oxc::Expression::RegExpLiteral(regexp) => Ok(InstructionValue::RegExpLiteral {
    pattern: regexp.regex.pattern.text.as_str().to_string(),
    flags: regexp.regex.flags.to_string(),
    loc: builder.source_location(regexp.span),
}),

The regex now lowers to a real value and receives its own reactive scope, matching Babel's sentinel-memoized nested scope. Minimal repro output now equals Babel byte-for-byte:

// before: t1 = t0.replace(undefined, "");   _c(4)
// after (matches Babel):                     _c(5)
if ($[0] !== t0) {
  let t2;
  if ($[2] === Symbol.for("react.memo_cache_sentinel")) {
    t2 = /:/g;
    $[2] = t2;
  } else { t2 = $[2]; }
  t1 = t0.replace(t2, "");
  ...
}

Before/after `_c(N)` on ecosystem files

file	before	after / Babel
`create-better-t-stack/.../charts/dash-tail-stroke.tsx`	`_c(25)`	`_c(26)`
`create-better-t-stack/.../charts/area-chart.tsx`	`_c(26)`,`_c(40)`	`_c(27)`,`_c(40)`
`.../day-picker.tsx`	`_c(77)`	`_c(78)`
`.../DynamicLink.tsx`	`_c(15)`	`_c(16)`
`.../Image.jsx`	`_c(23)`	`_c(24)`

Across the 420 regex-containing ecosystem source files: 48 went from mismatch → matching Babel's slot multiset, with 0 regressions (files that matched Babel before still match). The handful still differing do so for unrelated pre-existing reasons (whole extra/missing scopes), not the regex slot.

Snapshot note

Exactly one fixture snapshot changes: regexp-literal.snap — the dedicated regex fixture, whose old golden captured the undefined.test(value) bug. The new snapshot is byte-identical to the upstream Babel regexp-literal.expect.md golden (const pattern = /foo/g; … pattern.test(value)). CI=1 cargo test -p oxc_react_compiler is green; cargo fmt / cargo clippy --all-targets -D warnings clean.

`lower_expression` had no arm for `Expression::RegExpLiteral`, so every regex literal fell through the catch-all and was lowered to an `undefined` primitive. This dropped the regex from the output (e.g. `x.replace(undefined, "")`) and, because a primitive does not allocate, denied it the reactive scope that `babel-plugin-react-compiler` gives it. The result was one fewer `_c()` cache slot per affected function, shifting all subsequent `$[i]` indices by -1. The HIR variant `InstructionValue::RegExpLiteral`, its codegen, `may_allocate`, aliasing/type inference and all visitors were already implemented; only the lowering arm was missing. Add it (mirrors TS `BuildHIR` `case 'RegExpLiteral'`). A RegExp literal now lowers to a real value and receives its own reactive scope, matching Babel's sentinel-memoized nested scope.

codspeed-hq · 2026-07-04T15:21:27Z

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 61 skipped benchmarks¹

_{Comparing fix/rc-cache-slot-alignment (0db23a3) with main (3205cac)}

61 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. ↩

### 🚀 Features - 260425f semantic/examples: Include unresolved references (#24214) (camc314) - 2d9b0b3 minifier: Fold boolean-literal ternary branches in value contexts (#24110) (Dunqing) - 61fbf10 ast: Implement `ReplaceWith` on all AST types (#24013) (overlookmotel) - 7db7a29 allocator: Add `ReplaceWith` trait (#24012) (overlookmotel) - 4eb074e mangler: Add `reserved` option for names that must not be mangled (#24041) (Dunqing) - 2e62012 data_structures: Add `StringExt` trait (#24006) (overlookmotel) - 60e7160 minifier: Drop side-effect-free IIFEs whose result is unused (#23967) (Dunqing) - 26dd9e2 ast: Add method to widen inherited enum ref to parent ref (#23961) (overlookmotel) ### 🐛 Bug Fixes - e8b50ee transformer: Clean up semantics for stripped TypeScript syntax (#24180) (camc314) - d966d0b react_compiler: Remove clippy allows (#24168) (Boshen) - 854ef8d react_compiler: Compile generic functions instead of over-bailing on type-param hoisting (#24158) (Boshen) - 093586c react_compiler: Align memoization cache-slot allocation with Babel (#24157) (Boshen) - 09c8f59 react_compiler: Normalize snapshot fixture paths (#24142) (camc314) - f13df97 react_compiler: Drop stray empty statement from catch bindings (#24133) (Boshen) - cb2a505 react_compiler: Codegen destructuring reassignment targets (#24131) (Boshen) - b82c394 react_compiler: Propagate codegen invariants instead of emitting empty bodies (#24128) (Boshen) - 5771982 react_compiler: Render unchanged programs as source in fixture snapshots (#24129) (Boshen) - 4b16e1a transformer/async-to-generator: Preserve direct eval scope flags (#24136) (camc314) - 4e9194f react_compiler: Lower `delete obj.prop` to Property/ComputedDelete (#24123) (Boshen) - 0b25582 ast: Type binding node `typeAnnotation` as `TSTypeAnnotation | null` (#23113) (Boshen) - 018c0e5 transformer: Hoist lowered async declarations (#22770) (camc314) - 652fbaf mangler: Keep names of destructured exported bindings (#24036) (Dunqing) - e274415 minifier: Don't drop global calls that throw despite pure arguments (#23917) (Dunqing) - 59abb30 minifier: Only merge string literals in `try_fold_add` when the inner operator is `+` (#23622) (Jerry Zhao) ### ⚡ Performance - c5ca77b transformer: Avoid cloning refresh options (#24191) (camc314) - bf1a151 react_compiler: Compile out debug printers (#24184) (Boshen) - abb44a0 transformer: Build fixed object-rest arguments (#24190) (camc314) - a4db731 isolated_declarations: Use `ReplaceWith` instead of `TakeIn` (#24016) (overlookmotel) - ff10855 transformer: Use `ReplaceWith` instead of `TakeIn` (#24015) (overlookmotel) - bd49aff ecmascript: Avoid heap-allocating Math.min/max/imul operands (#23941) (Lawrence Lin) - e4b708b react_compiler: Skip compiled files before prefilters (#24171) (Boshen) - c59f2fe rust: Return impl ExactSizeIterator from slice-backed accessors (#24144) (Boshen) - 5d6d04a codegen: SWAR-skip boring byte runs in sourcemap line/column scan (#24023) (Boshen) - a55e0be traverse: Reduce string operations in `get_var_name_from_node` (#24007) (overlookmotel) - e6d48e1 transformer/nullish_coalescing: Move cold path into separate function (#23989) (overlookmotel) - c4e35b5 transformer/object_rest_spread: Pre-allocate capacity in `Vec` (#23988) (overlookmotel) - 527b8e5 transformer/decorators: Narrow type earlier (#23987) (overlookmotel) ### 📚 Documentation - 30d17f5 allocator: Clarify docs for `TakeIn::take_in_box` (#24093) (overlookmotel) - 675e6a8 ast: Correct doc comment for `PrivateFieldExpression` (#24008) (overlookmotel) - e4c30e6 minifier: Explain what `dce` mode means (#23994) (Dunqing) - 37cbf88 ast_macros: Document fields of `StructDetails` (#23959) (overlookmotel) - 4de3e54 ast: Correct doc comment (#23948) (overlookmotel) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>

Boshen marked this pull request as ready for review July 5, 2026 01:22

Boshen merged commit 093586c into main Jul 5, 2026
30 checks passed

Boshen deleted the fix/rc-cache-slot-alignment branch July 5, 2026 01:25

oxc-guard Bot mentioned this pull request Jul 6, 2026

release(crates): oxc v0.139.0 #24218

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fix(react_compiler): align memoization cache-slot allocation with Babel#24157

fix(react_compiler): align memoization cache-slot allocation with Babel#24157
Boshen merged 1 commit into
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fix/rc-cache-slot-alignment

Boshen commented Jul 4, 2026

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Boshen commented Jul 4, 2026

Root cause

The fix

Before/after _c(N) on ecosystem files

Snapshot note

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codspeed-hq Bot commented Jul 4, 2026

Merging this PR will not alter performance

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Before/after `_c(N)` on ecosystem files