Built-in shapes via source generator: one-liner opt-in (Math, JSON, Reflect, Atomics)

[lahma]

Summary

Generalize the Math built-in-shape pilot (#2554) into a reusable runtime and a source-generator opt-in, so a namespace built-in adopts shape storage with a one-liner — [JsObject(UseShape = true)] — instead of a hand-written shape file. Math, JSON, Reflect, and Atomics opt in here.

A shape-backed built-in stores its string-keyed own properties as a shared immutable layout + a per-realm lazily-filled descriptor array, instead of building a per-realm dictionary of descriptors in Initialize.

What changed

• BuiltinShape — immutable, process-shared layout (names, attributes, name→slot index) + a builder.
• BuiltinShapeObject — abstract base holding the per-realm lazily-filled descriptor array and the GetOwnProperty / SetOwnProperty / Delete / GetOwnPropertyKeys / GetOwnProperties / deopt logic.
• Source generator — [JsObject(UseShape = true)] makes the generated CreateProperties_Generated install the shared layout + per-realm descriptor array (InitializeBuiltinShape) instead of building a property dictionary, and emits the BuiltinShape + MakeBuiltinFunction hooks from the same key/descriptor/dispatcher metadata the dictionary path uses. The host just derives from BuiltinShapeObject. Supports [JsFunction]s, static-immutable [JsProperty] constants, and [JsSymbol]s.
• Math (replacing the pilot's hand-written MathInstance.Shape.cs), JSON, Reflect, Atomics opt in — no hand-written per-built-in shape code remains.

Redefining an existing property's attributes mutates the per-realm descriptor in place (so verifyProperty needs no deopt); adding or deleting an own property deopts to the ordinary dictionary, after which the unchanged base behavior runs. Prototypes / constructors (other bases, accessors, instance refs) are a later step.

Benchmarks (BuiltinShapeBenchmark, default job, vs main)

EngineOnly control is identical (13.69 KB) on both; the generated code allocates byte-identically to the earlier hand-written version.

Benchmark	Allocation	Time
EngineInitMath (Math)	15.51 KB == 15.51 KB (neutral refactor)	neutral
EngineInitBuiltins (Math + JSON + Reflect + Atomics)	22.87 → 20.20 KB (−11.7%)	3.81 → 3.30 µs (−13%)

The 4-built-in storage overhead (delta over EngineOnly) drops from 9.18 KB to 6.51 KB — −29%.

Conformance & tests

• Targeted Test262: Math 662/662, JSON 502/502, Reflect 306/306, Atomics 780/780.
• Full Test262: 0 new failures (the Atomics.waitAsync no-spurious-wakeup tests are pre-existing timing flakes under concurrent load — a different variant flakes each run and passes in isolation; already serialized via NonParallelFeatures).
• Jint.Tests 3138 (net10.0) / 3076 (net472); Jint.Tests.PublicInterface 79/79 — no public API change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RDS23QeSSNRkaUB2KL74U9