Fuse strict-equality guards: x === undefined/null and typeof x === "literal"

[lahma]

Third PR of the quickjs-ng learnings campaign (#2654 lanes, #2656 for-in array fast path). QuickJS dedicates opcodes (is_undefined, is_null, typeof_is_undefined, typeof_is_function) to these guard idioms, produced by its peephole pass; this is the AST-interpreter equivalent — a build-time specialized node.

What

When one side of ===/!== is statically known, the generic strict-equality node (evaluate both operands, generic dispatch) is replaced at build time:

• x === undefined / x === null (either orientation): evaluates only the interesting operand; the answer is a single internal-type test. The undefined form keys off the same compile-time folding the identifier itself already evaluates through (BindingName.CalculatedValue), so semantics match the unfused path exactly. Strict equality against these singletons is precisely a type-identity check (document.all-style IsHTMLDDA values are only loosely equal to them, so the strict path needs no carve-out).
• typeof x === "literal": the typeof side still evaluates through JintTypeOfExpression — unresolvable identifiers and host-object classification untouched — and since typeof only ever produces the interned JsString singletons, the comparison is a reference test against the singleton the literal maps to at build time. An unmapped literal ("bogus") can never match, but the operand still evaluates for its side effects.

Literal-vs-literal expressions keep the existing constant fold; loose equality (== null) is untouched.

Numbers

New GuardComparisonBenchmark (LCG-mixed inputs the branch predictor cannot memorize), default job:

Lane	Before	After	Δ
IsUndefinedGuard	13.54 ms	10.36 ms	−23.5%
IsNullGuard	12.42 ms	10.29 ms	−17.1%
TypeofStringGuard	17.06 ms	14.24 ms	−16.5%
TypeofUndefinedGuard	11.87 ms	10.73 ms	−9.6%
TypeofSwitchMixed (existing lane)	22.13 ms	18.65 ms	−15.7%
LooseNullGuard (untouched path)	10.13 ms	10.26 ms	noise

linq-js is within run-to-run noise (its Execute lane varies ±5% between identical binaries).

Tests

New GuardFusionTests pin: both orientations, every typeof result string, undeclared-identifier typeof (no throw), impossible literals evaluating side effects exactly once, CLR interop callables classifying as "function", fused guards across await suspension, and the if-statement boolean fast path.

Full gate green: Jint.Tests, PublicInterface, CommonScripts, and a clean 99,429-test Test262 run.

🤖 Generated with Claude Code