Perf: per-function-scope candidacy in ArrayLocalPromotionAnalyzer#871
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The typed-array-local promotion (#857/#860) keyed candidacy/disqualification whole-program per lexeme: a clean push/index/length array was poisoned by an unrelated, escaping array of the SAME NAME elsewhere (a different function, or a different module in a bundle). Common array names (`arr`, `data`, `items`, `result`) therefore frequently failed to promote in real multi-module programs — the optimization only landed when the name happened to be globally unique. Key candidacy by (function scope, lexeme) instead: each Stmt.Function / Expr.ArrowFunction body is its own scope (module top level is scope 0), mirroring StringAccumulatorPromotionAnalyzer. Cross-scope references are captures, already handled by the IsVariableCaptured guard. Use-site emission is slot-type-keyed and thus inherently scope-correct, so promoting more arrays is safe. Effect (verified by IL): a clean `const xs = []` (push/index/length) now promotes to List<double> even when another function has an escaping `const xs` — previously both stayed boxed/$Array. No behavior change where names were already unique. This is the prerequisite step (1) for the array-methods typed-HOF-pipeline work (docs/plans/issue-861-array-methods-typed-pipeline.md): common array names must promote before HOF receivers can stay typed. Green on dotnet test (24 ArrayLocalPromotionTests incl. a new per-scope name-collision case, both modes) except the pre-existing stale/flaky Test262 baselines (drift is Array.isArray/proxy, present in interpreter mode too → unrelated to this compile-only change).
This was referenced Jun 21, 2026
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Part of #856 (perf epic). Prerequisite for the array-methods typed-HOF-pipeline work.
Problem
The typed-array-local promotion (#857/#860) keyed candidacy whole-program per lexeme. A clean
push/[i]/.lengtharray was poisoned by an unrelated, escaping array of the same name elsewhere — another function, or another module in a bundle. So common array names (arr,data,items,result) routinely failed to promote in real multi-module programs; the optimization only landed when the name happened to be globally unique. (This is the same latent bug fixed for strings in #870 — surfaced there becausestringWork'sscollided withperf_hooks'sconst s.)Fix
Key candidacy/disqualification by (function scope, lexeme): each
Stmt.Function/Expr.ArrowFunctionbody is its own scope (module top level is scope 0), mirroringStringAccumulatorPromotionAnalyzer. Cross-scope references are captures, already handled by theIsVariableCapturedguard. Use-site emission is slot-type-keyed (so it's inherently scope-correct), which makes promoting more arrays safe.Effect (IL-verified)
A clean
const xs = [](push/index/length) now promotes toList<double>even when another function has an escapingconst xs— previously both stayed boxed/$Array. No change where names were already unique (e.g. count-primes'isPrime).Tests / validation
ArrayLocalPromotionTests: +1 per-scope name-collision case; 24/24 green (both modes), no regression in the existing cases.dotnet test: 13968 passed, 0 real regressions (the only failures are 2 tests that pass in isolation under parallel load, plus the documented stale Test262 baselines — drift isArray.isArray/proxy, present in interpreter mode too, i.e. unrelated to this compile-only change).--verifypasses on the promoted output.Context
Step 1 of
docs/plans/issue-861-array-methods-typed-pipeline.md(the last remaining benchmark gap, array-methods @ 2.79× Node). Common array names must promote before HOF receivers can be kept typed.