Store RouteTree slots in a Map to keep slot access monomorphic#96168
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acdlite
marked this pull request as ready for review
July 24, 2026 20:35
samselikoff
approved these changes
Jul 24, 2026
acdlite
enabled auto-merge (squash)
July 24, 2026 20:51
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Root cause
Parallel route slot names are app-defined, so a
RouteTree.slotsstored as a plain object has an unbounded set of hidden classes: every distinct combination of slot names ({children},{children, side}, ...) produces a different shape. Keyed access over these objects makes the inline cache megamorphic. This was the singleic-megamorphicfinding reported bypnpm bench:deopt --scenario segment-cache, at the keyed store inconvertTreePrefetchToRouteTree(cache.ts:1531, keyschildren,side).This PR converts
RouteTree.slotsfrom{ [parallelRouteKey: string]: RouteTree }toMap<string, RouteTree>. All reads, writes, and iterations over RouteTree slots now go through monomorphicMapmethod calls, and iteration order is preserved (Map iterates in insertion order, matching plain-object string-key order).Scope
This was an approved design decision:
RouteTreeis an in-memory-only structure, so only it changes.CacheNode.slots(spread into JSX props) and the wire formats (TreePrefetch.slots,FlightRouterState[1]) remain plain objects; the existing conversion functions bridge them into and out of the Map.Before/after findings diff
The targeted finding at
cache.ts:1531is cleared. The remaining megamorphic site (stable across two runs) isscheduler.ts:1068:23inpingSharedPartOfCacheComponentsTree— the keyed loadoldTreeChildren[parallelRouteKey], whereoldTreeChildrenisFlightRouterState[1], i.e. the wire-format plain object that is intentionally out of scope for this change. That access existed before this PR; previously thefor...inloop key qualified the sibling load for V8's fast enum-cache path, and with Map iteration keys it now surfaces as a generic keyed load. Eliminating it would require changing theFlightRouterStatechildren format, which is a separate decision. The new info-level lines are one-time warm-up/prototype-registration deopts from the introducedMapusage, not steady-state IC misses.Tests
test/e2e/app-dir/segment-cache/basic(start-turbo): 10 passed, 1 skippedtest/e2e/app-dir/segment-cache/optimistic-routing-parallel-slot-catchall-regression(start-turbo): 1 passedtest/e2e/app-dir/segment-cache/cached-navigations(start-turbo): 16 passedpnpm --filter=next typespasses (also re-run after rebasing onto canary)