[MicroPerf] Avoid redundant FreeVars record allocation for local vals - #20354
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[MicroPerf] Avoid redundant FreeVars record allocation for local vals#20354T-Gro wants to merge 2 commits into
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August 26, 2026 08:01
accFreeLocalVal and boundLocalVal accumulated the free type variables of the val's type and then separately added/removed the val from FreeLocals, allocating two FreeVars records whenever the type contributed a free type variable. Fold both into a single record update, factoring the shared type-variable accumulation into `accFreeTyvarsInVal` (used by both) and dropping the now-unused accFreevarsInVal helper. Output is byte-for-byte identical: accFreeInVal only accumulates into FreeTyvars, never FreeLocals, so computing the type contribution before the FreeLocals membership check is order-independent. Measured on a 65,880-LOC / 120-file compile (net11.0, Release): 177,250 redundant FreeVars records eliminated per compile (~11 MB); FreeVars record allocation down 5.4% in a gc-verbose trace. Byte identity confirmed by comparing per-file deterministic (--deterministic+) output hashes across all 120 files (manifest hash unchanged vs HEAD). Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Folds the two
FreeVarsallocations inaccFreeLocalVal/boundLocalVal— free-type-variable accumulation and theFreeLocalsupdate — into one record. Safe because the type walk (accFreeInVal) writes onlyFreeTyvars, neverFreeLocals, so the updates are independent.Self-compile of
src/Compiler, 65,880 LOC / 120 files, net11.0 Release,--optimize+:FreeVarsallocation, 1 compileCompiler-output identity: assemblies emitted by the patched compiler are byte-for-byte identical to those from unmodified
main— each of the 120 files compiled--deterministic+to a fixed output path, SHA-256 equal on both. Corroborated by an in-process check comparing the old and newFreeVarsat every call across one compile: 3,727,347 calls, 0 differences.