Reuse hoisted function definitions across re-evaluations on an engine

[lahma]

What

Declaration instantiation constructed a fresh JintFunctionDefinition for every hoisted function on
every evaluation — and the definition owns the lazily-built body handler tree, so re-running a
prepared script rebuilt each called function's interpreter subtree and reset every per-node inline
cache. Function-level and block-level DI paid the same cost per call for inner declarations.

Definitions are now cached per engine, keyed on the declaration AST node.

Why this exact shape (two rejected designs)

• Not the AST's shared UserData State: handler trees accumulate engine-affine cache entries,
  and sharing them across engines retains dead engines —
  GarbageCollectionTests.SharedPreparedScriptDoesNotRetainEngines failed immediately.
• Not a ConditionalWeakTable: a dependent handle keeps its value alive on key reachability
  alone, so with the key rooted by a live Prepared<Script>, the value → inline caches → engine
  chain pins the dropped engine — the same test failed again, in the opposite direction of the
  naive mental model.
• The shipped shape: a strong engine-owned Dictionary (dies with the engine; retention scoped
  to engine lifetime, mirroring Realm._templateMap), with the global-DI site engaging the cache
  only on re-evaluation of a script this engine has seen (one HashSet probe) — so
  fresh-engine-per-run hosts execute the exact uncached path. Block/function-level DI sites cache
  unconditionally since repeated calls within one evaluation already amortize them. A 2048-entry
  reset backstops hosts streaming endless distinct sources through one engine.

Results

Default job; the measurement window drifted uniformly +5–8% across all rows (including untouched
guards), so in-window ratios and the deterministic allocation columns carry the signal:

measurement	before	after
PreparedReusedEngine (new row: long-lived engine re-evaluating a cached Prepared<Script>)	14.61 KB/op	9.99 KB/op	−32% allocation; in-window time ratio 1.10× → 1.08× vs SourcePerOp
diagnostic probe (small function-declaring script, engine reuse)	3,288 B/op	1,740 B/op	−47% allocation
SourcePerOp / PreparedShared / PreparedPerOp / LinqJs / EvalExecution (fresh-engine guards)	—	—	allocation byte-identical (gated path is code-identical to base)

Gating

All-TFM build ✅ · Jint.Tests ×2 ✅ (including all GC retention pins) · PublicInterface ×2 ✅ ·
Test262 99,426/0 ✅

🤖 Generated with Claude Code