Keep builtin-shape deopt lazy: unmaterialized function slots stay deferred#2588
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…erred DeoptBuiltinShape previously materialized EVERY slot, forcing all of a host''s dispatcher functions into existence the moment user code added or removed one own property (e.g. `Math.x = 1` instantiated all ~35 Math functions). For upcoming shaped hosts where deopt is routine — the global object deopts on any top-level var — an eager deopt would be strictly worse than the dictionary path it replaces. Now already-materialized slots keep their descriptor instance (inline caches and spec identity depend on it); unmaterialized function slots become LazyBuiltinSlotDescriptor wrappers that create the dispatcher on first value read; unmaterialized accessors (rare) still materialize eagerly since a data-descriptor wrapper cannot defer them; aliases share their target''s instance — materialized or lazy — so name pairs keep one function identity across the deopt. Like LazyPropertyDescriptor, the wrapper clears CustomJsValue once materialized so it becomes a plain data descriptor. Deopt-triggering script (Math/JSON/Reflect mutation, fresh engine): 43.7 -> 32.3 KB/iter (-26%). BuiltinShape/EngineConstruction/MinimalScript suites: allocations byte-identical, times within run-to-run noise. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This was referenced Jul 6, 2026
This was referenced Jul 13, 2026
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DeoptBuiltinShapepreviously materialized every slot when a shaped built-in fell back to dictionary mode, forcing all of the host's dispatcher functions into existence the moment user code added or removed one own property —Math.x = 1instantiated all ~35 Math functions nobody asked for. For upcoming shaped hosts where deopt is routine (the global object deopts on any top-levelvar), an eager deopt would be strictly worse than the dictionary path it replaces, so this is a prerequisite for flippingGlobalObject/FunctionPrototypeto shapes.LazyBuiltinSlotDescriptorwrappers that create the dispatcher on first value read (mirroringLazyPropertyDescriptor, including clearingCustomJsValueonce materialized).Set.prototype.keys === Set.prototype.valueskeep one function identity across the deopt.Measurements
Deopt-triggering script (
Math.x = 1; JSON.x = 1; Reflect.x = 1+ calls, fresh engine per iteration): 43.7 -> 32.3 KB/iter (-26%), matching the dispatcher functions no longer force-created.No-regression gates (BenchmarkDotNet default jobs, A/B vs main + A/B/A noise check):
BuiltinShapeBenchmark,EngineConstructionBenchmark,MinimalScriptBenchmark— allocations byte-identical on every row, times within the suites'' observed run-to-run bands (the deopt path is not on any of their execution paths).Verification
Jint.Testsgreen (net10.0 + net472),Jint.Tests.PublicInterfacegreengetOwnPropertyNamesverified against a live engine🤖 Generated with Claude Code