Keep optimistic-route param handling monomorphic#96169
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Normalize ResolvedParams values to strings at the resolution boundary
(join catch-all parts at set time) and replace split('/')+filter with a
manual splitter that always produces packed-elements arrays. This
removes the wrong-map deopts in reifyRouteTree and matchKnownRoutePart
caused by the string|string[] union and V8's packed/holey elements-kind
instability propagating through slice/join.
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acdlite
marked this pull request as ready for review
July 24, 2026 20:38
samselikoff
approved these changes
Jul 24, 2026
acdlite
enabled auto-merge (squash)
July 24, 2026 20:52
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Found by
pnpm bench:deopt --scenario segment-cache(V8 deopt/IC analysis of the client segment cache), with map-level IC evidence. Two related sources of megamorphism in the optimistic route matching path: (1)String.prototype.splitreturns arrays whose elements kind alternates between PACKED_ELEMENTS and HOLEY_ELEMENTS depending on internal fast paths, andfilter/slicepropagate the kind, so the arrays flowing throughmatchKnownRoutePartandreifyRouteTreehad unstable maps, causing repeated "wrong map" eager deopts at theslicesite inmatchKnownRoutePartand thejoinsite inreifyRouteTree(which runs once per tree node on every optimistic match); and (2) the privateResolvedParamsmap typed asMap<string, string | string[]>forced a polymorphicArray.isArray/joinbranch on every dynamic segment during reification.Semantics are preserved exactly. Catch-all param values are now joined into a single string at the moment they are resolved in
matchKnownRoutePartinstead of at read time inreifyRouteTree— the value was only ever read once and immediately collapsed to a joined string cache key, and the empty optional catch-all case writes'', which is identical to[].join('/'). The newsplitPathnameIntoPartshelper is byte-for-byte equivalent topathname.split('/').filter((p) => p !== ''): empty tokens from leading, trailing, or consecutive slashes are skipped, but the parts array is built by pushing into an array literal so it always has a packed elements kind.Before/after
findings.txtdiff from the bench scenario:Both high-severity wrong-map deopts in
optimistic-routes.tsare gone (confirmed across two independent after-runs); the remaining new lines are info-level string-receiver ICs inside the splitter helper. Total ic-polymorphic findings dropped from 90 to 80.Tests (all passing,
test-start-turbo):test/e2e/app-dir/segment-cache/optimistic-route-cache-keying-regression— 1 passedtest/e2e/app-dir/segment-cache/optimistic-routing-parallel-slot-catchall-regression— 1 passedtest/e2e/app-dir/segment-cache/encoded-slash-params— 2 passed