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Treat Wasm Vector<T> layout as fixed - #135804
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davidwrighton merged 2 commits intoOct 19, 2026
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Skip runtime-dependent vector layout checks on Wasm while retaining checks on native targets and for types outside the version bubble. Add cross-target compiler regressions for vectors, wrappers, and tuples. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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davidwrighton
marked this pull request as ready for review
October 14, 2026 21:53
davidwrighton
requested review from
jtschuster and
tannergooding
and
a balanced review from Copilot
October 14, 2026 21:53
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🟡 Changes recommended
The unconditional regression theory expects compiled vector methods on targets where their layout is indeterminate.
1 open finding
What changed in this PR
Treats Wasm32 Vector<T> layout as fixed in ReadyToRun compilation, avoiding redundant checks while preserving other targets’ behavior and version-bubble restrictions.
Changes:
- Recognizes Wasm32 vectors as fixed-layout types.
- Adds regular/composite regression cases for vectors, wrappers, and tuples.
- Documents the Wasm layout rule.
Builds and tests were not run during this review.
| File | Description |
|---|---|
| src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/ReadyToRunCodegenCompilation.cs | Adds the Wasm32 fixed-layout decision. |
| src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/Webcil/WasmSimdModule.cs | Adds vector tuple fixtures. |
| src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/R2RTestSuites.cs | Checks layout fixups across compilation modes. |
| docs/design/coreclr/botr/readytorun-format.md | Documents fixed Wasm vector layout. |
🧠 Review effort: Balanced
jkotas
reviewed
Oct 14, 2026
This was referenced Oct 15, 2026
Closed
Preserve the Vector<T> optimism policy in InstructionSetSupport and use it for fixed-layout decisions instead of special-casing Wasm. Cover fixed and optimistic configurations in regular and composite tests, and gate the tests on target vector support. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8f7879c0-2733-4f0e-a679-07cc5a1b6ad2
jkotas
approved these changes
Oct 19, 2026
jtschuster
approved these changes
Oct 19, 2026
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Description
Extract the independent Wasm vector-layout change from #135553 onto
main.Treat
System.Numerics.Vector<T>as having fixed layout on Wasm32, where its width is fixed at 128 bits. Avoid redundant runtime-dependent vector layout checks on that target. Containing types can also have fixed layout when their remaining layout is fixed within the version bubble.Other targets retain runtime-dependent vector layout checks, and ordinary version-bubble restrictions remain in place for containing types. Add regular/composite regression cases covering vectors, wrappers, and vector-containing tuples, and document the Wasm layout rule.
Source commit:
6a269adea70a2ea611bc2a3da78c43cdc08545c7. This PR does not include the separate canonical SIMD-root exclusion, two-phase compilation, direct calls, or the prolog/frame-identity changes.Historical size measurements
These matched before/after Wasm measurements were collected on the original development branch, not on this newly extracted/rebased PR. The recorded JIT hashes match between baseline and updated builds. Brotli used quality 11 and window 22.
These are historical size measurements, not throughput benchmarks or a guarantee of equivalent savings on current main.
Validation
git diff --check upstream/main...split/wasm-vector-layout— passed.This remains a draft; build and runtime validation on the independent branch are outstanding.
Note
This PR was prepared with GitHub Copilot.