Review fix(compiler): preserve -0/0 distinction through phi constant propagation and codegen (#37583)

This work implements React Compiler loses negative zero through phi optimization and codegen (open)

Description

Two compiler paths collapse JavaScript’s observable distinction between 0 and -0: constant propagation compares phi constants with !==, and reactive-function code generation only emits a unary negative expression when a number is < 0.

Reproduction

Compile a memoized component whose phi alternates between 0 and -0, then observe the value through 1 / value. Current output reports positive infinity for both paths instead of positive then negative infinity.

Expected behavior

The compiler should preserve signed zero through optimization and emitted code.

sleitor authored

of work during September 11

facebook/react

fix-37447

Diff Delta:

About 0 Diff Delta/hour

Classified as: Feature Add, General

sleitor's Description of Work #37447 React Compiler loses negative zero through phi optimization and codegen

Summary

Fixes #37447. Two paths in the React Compiler collapsed JavaScript's observable distinction between 0 and -0:

Constant propagation (Optimization/ConstantPropagation.ts): evaluatePhi compared phi operand constant values with !==. Since 0 !== -0 evaluates to false in JS, a phi merging 0 on one branch and -0 on another was incorrectly folded into a single "constant" value, silently discarding whichever branch didn't "win", regardless of which branch actually executed at runtime.

Codegen (ReactiveScopes/CodegenReactiveFunction.ts): codegenValue only emitted a unary negation (-0) when value < 0. Since -0 < 0 is false, a literal -0 was re-emitted as plain 0, losing its sign.

Fix

Use Object.is() instead of !== when comparing phi constant candidates in evaluatePhi, so operands with different sign but "equal" primitive values (i.e. 0/-0) are correctly treated as non-constant (and thus preserved as a genuine runtime phi instead of being folded to one arbitrary branch's value).

Emit a unary negation in codegen when Object.is(value, -0) is true, in addition to the existing value < 0 check, so -0 literals round-trip correctly.

Tests

Added negative-zero-phi-constant-propagation.js/.expect.md: a component whose phi alternates between 0 and -0 across renders. 1 / x is used to observe the sign of zero (Infinity vs -Infinity), and the fixture harness compares compiled vs. uncompiled eval output, verifying Object.is parity is preserved pre/post compile.

Updated the pre-existing constant-propagation-unary-number snapshot: it already contained a -0 literal that was silently being emitted as 0 in codegen prior to this fix — that regression is now fixed and visible in the updated snapshot (-2, -0, true, ... instead of -2, 0, true, ...).

Ran the full babel-plugin-react-compiler snap test suite (1817 fixtures) — all pass with only the one expected snapshot update above.

yarn workspace babel-plugin-react-compiler lint passes.

Test plan

cd compiler/packages/babel-plugin-react-compiler

yarn build

yarn workspace snap run snap

yarn lint

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fix(compiler): preserve -0/0 distinction through phi constant propagation and codegen #37583