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Compact type mappers and coallocate inference mapper storage - #64796
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Anders Hejlsberg (ahejlsberg) merged 2 commits intoOct 14, 2026
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Jake Bailey (jakebailey)
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The compact representation preserves existing behavior and has targeted coverage for its unsafe layout and lifetime invariants.
0 open findings
What changed in this PR
Compacts type mapper dispatch and coallocates inference mappers to reduce compiler allocations.
Changes:
- Replaces interface-backed mappers with kind-based concrete dispatch.
- Embeds both inference mappers in their context.
- Adds behavior, layout, lifetime, and allocation tests.
| File | Description |
|---|---|
tsc/internal/checker/mapper.go |
Implements compact mapper dispatch. |
tsc/internal/checker/inference.go |
Initializes embedded inference mappers. |
tsc/internal/checker/checker.go |
Adds coallocated mapper storage. |
tsc/internal/checker/utilities.go |
Updates concrete mapper casts. |
tsc/internal/checker/types_test.go |
Tests dispatch, layout, and allocations. |
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Anders Hejlsberg (ahejlsberg)
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We be nice to only have one kind type.
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Every mapper currently carries a self-referential interface, and each
inference context separately allocates two mappers with the same
lifetime. A concrete kind header eliminates the interface and lets
inference contexts own their fixing and non-fixing mappers in one
allocation.
Fresh parse, bind, and check versus reusable normalization workspaces:
│ before │ after │
│ B/op │ B/op vs base │
CheckFreshProgram-20 277.2Mi 276.0Mi -0.44% (p=0.002 n=6)
│ before │ after │
│ allocs/op │ allocs/op vs base │
CheckFreshProgram-20 1.139M 1.094M -3.90% (p=0.002 n=6)
VS Code allocations decrease from 28.52M to 27.17M with identical
compiler counts and diagnostics. Retained memory is effectively
unchanged. Three interleaved fixture runs show fewer fixed-work
instructions, but a single VS Code run is noisy; no wall-time claim is
made.
Keeping a public kind enum next to the private header tag meant two parallel kind types and a translation switch between them. Exposing the tag itself gives callers every mapper kind directly. Mapper comparison still only orders simple, array, and merged mappers, so type ordering is unchanged.
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Every mapper carries a self-referential interface, and each inference context separately allocates two mappers with the same lifetime.
Part of the memory optimization stack summarized in #64734.
Replaces #64779, which GitHub closed as merged when the stack was reordered.