Memoize slot-cache chain reachability for closure reads#2726
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Closure-variable reads at hops 1-3 re-validated their slot location on every access: CanReachAtOuterHop re-probed every intermediate declarative environment (linear slot-name scan + dictionary probe per hop) on each read, ~6% of dromaeo-string-base64-modern (3.5% CanReachAtOuterHop + 2.4% TryResolveNonLocal). A successful walk is now published as an immutable SlotChainMemo pinning the probed chain links by identity plus Engine._envBindingInjectionEpoch, mirroring the global read cache's NestedChainMemo. Steady-state closure reads then validate with a handful of reference compares instead of re-walking. Soundness: a declarative environment's name set is a deterministic function of its defining AST node/definition (every reuse channel re-initializes an instance with the identical name set), the only mutations that grow a pre-existing environment's name set (sloppy direct eval hoisting, AnnexB block-function var copies) bump the injection epoch, deletions only shrink it, and an environment's class is immutable - so pinned "intermediate does not own the name" probes stay false while identity and epoch hold. Validation follows the CURRENT _outerEnv pointers against the pinned links, so pooled environments re-attached under different outers and with-ObjectEnvironments (which break link identity by construction) fall through to the authoritative walk, which also stays in charge of every miss and re-publishes on success. ResolveAndPopulate publishes the memo directly when it lands at hops 1-3 since its traversal proves the same condition. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
lahma
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Jul 21, 2026
Single-session re-measure of all lanes on main with #2725/#2726 merged. Extends the campaign notes with the round-2 wins (JSON scan fast paths, closure-read chain memoization) and records the measured-and-dropped direct-dispatch experiment: the builtin-call path is already at its cost floor, so the residual string-row gap is interpreter dispatch itself, not call ceremony. Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
This was referenced Jul 22, 2026
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Campaign-2 item (follow-up to #2716–#2723). Profiles show closure-variable reads re-validating their slot location on every access —
SlotLocationCache.CanReachAtOuterHop3.5% +TryResolveNonLocal2.4% ondromaeo-string-base64-modern, driven fromJintIdentifierExpression.GetValue(closure-captured lookup tables read in tight loops).Change
Plain per-node
(lastEnv, hop, slot)caching guarded by environment identity alone is unsound — environment shapes are not immutable (sloppy directevalinjects vars into pre-existing function environments; AnnexB block-function semantics create var-scope bindings mid-execution). Instead this extends the guard pattern the codebase already ships for exactly this hazard in the global-read path (NestedChainMemo+Engine._envBindingInjectionEpoch):SlotChainMemopins the probed intermediate chain links by identity plus the injection epoch after a successful hop-1..3 walk. Steady-state closure reads validate with ~5 reference compares + 1 int compare instead of per-hopHasBindingprobes.withinsertion breaks link identity by construction; pooled/reused environments re-initialize with per-definition-deterministic name sets (every reuse channel audited); all dynamic injections bump the epoch (everyCreateMutableBindingcaller audited); deletions only shrink name sets, so a cached "not here" verdict stays valid.Numbers (launchCount-5, both sides, default job otherwise)
Gates
Jint.Tests 3706/3643 (net10/net472) including 6 new soundness tests (alternating closure instances at one callsite, sloppy direct-eval shadowing mid-loop,
witharound closure reads with mid-loop injection, deep nesting past the memo depth, pooled loop environments re-attached across entries); PublicInterface 114/114; Test262 99,431 / 0 failed; CommonScripts 28/28.🤖 Generated with Claude Code