[MicroPerf] Avoid per-call closure allocation in type-hierarchy traversal - #20384
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[MicroPerf] Avoid per-call closure allocation in type-hierarchy traversal#20384T-Gro wants to merge 2 commits into
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FoldHierarchyOfTypeAux runs on a hot path (InfoReader member/property/field lookup and type inference). Its inner 'let rec loop' was handed to List.foldBack / Option.foldBack / List.exists as a partial application, so a fresh closure was allocated on every traversal. Add a ListInline module (illib.fs) with exists / foldBack whose function argument is inlined at the call site via InlineIfLambda, so the closure never materializes. foldBack folds lists up to length five directly (no allocation) and only copies to an array for longer lists, so it stays stack-safe. Length five is measured: over a real compile GetImmediateInterfacesOfType returns <=5 elements ~96% of the time (81% are empty), so short interface lists allocate nothing at all. At the call sites the top-level 'typeEquiv g ty' stays partially applied (the optimizer fuses it after inlining), but the local 'loop' is passed as a lambda so InlineIfLambda inlines it rather than allocating it as a closure. Fold order is unchanged, so visit order, dedup and the ndeep>100 error are preserved; compiler output is byte-identical under --deterministic+. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: c6610fae-a96e-4421-a920-e73de4fc94dc
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This was referenced Aug 27, 2026
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FoldHierarchyOfTypeAuxis on a hot path (member/property/field resolution and type inference). Its innerlet rec loopwas handed toList.foldBack/Option.foldBack/List.existsas a partial application, so a fresh closure was allocated per traversal. Routing the folds through a smallListInlinemodule (exists/foldBackwith[<InlineIfLambda>]) inlines the function argument at the call site, so the closures never materialize; fold order is unchanged.Closures allocated in
FoldHierarchyOfTypeAux(dotnet-tracegc-verbose, 6 self-compiles of a 120-file corpus):loopenvironment (7 captures)loop (ndeep+1)→foldBacktypeEquiv g ty→existsListInline.foldBackfolds lengths ≤ 5 directly (no allocation) and only arrays longer lists (stack-safe, likeList.foldBack). The ≤ 5 cutoff is measured —GetImmediateInterfacesOfTypelength over a real compile (1.0M calls):Output is byte-identical under
--deterministic+(SHA-256 vs base on the same corpus); visit order, thevisitedTycon/visiteddedup,AllowMultiIntfInstantiationshandling and thendeep > 100error are unchanged; interface / inheritance / member-resolution / equality tests pass.