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JIT: expand all stack array allocations in a statement - #133531
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fgExpandStackArrayAllocations stopped after the first expansion in a statement, but splitting only moves the preceding part of the tree out, so any remaining NEWARR helper calls stayed behind with a StackArrayLocal arg. Fixes dotnet#133523 Co-authored-by: Copilot <[email protected]> Copilot-Session: 3b9596b9-f3b3-47d1-b1ca-1a8781e56de7
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🟢 Approval recommended
The fix is localized, maintains statement side-effect flag correctness via gtUpdateStmtSideEffects, and is backed by a targeted regression test covering the reported scenario.
Pull request overview
This PR fixes a CoreCLR JIT correctness bug where stack-array allocation expansion would only expand the first eligible NEWARR helper call within a statement, potentially leaving later stack-array allocations in the same statement unexpanded and malformed.
Changes:
- Update
Compiler::fgExpandStackArrayAllocationsto rescan a statement after each successful expansion until no more stack-array allocations remain. - Add a JIT regression test covering multiple stack-allocated arrays in one statement (including a 3-array variant).
- Register the new regression test source file in
Regression_ro_2.csproj.
File summaries
| File | Description |
|---|---|
| src/coreclr/jit/helperexpansion.cpp | Rescans a statement until all stack-array allocation helper calls in that statement are expanded. |
| src/tests/JIT/Regression/Regression_ro_2.csproj | Includes the new Runtime_133523 regression test in the build. |
| src/tests/JIT/Regression/JitBlue/Runtime_133523/Runtime_133523.cs | Adds an xUnit regression test reproducing multiple stack-array allocations within one statement. |
Review details
- Files reviewed: 3/3 changed files
- Comments generated: 0
- Review effort level: Lite
AndyAyersMS
approved these changes
Sep 9, 2026
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Fixes #133523
fgExpandStackArrayAllocationsstopped after expanding the first allocation in a statement.gtSplitTreeonly moves the part of the tree evaluated before the split point into new statements, so any furtherNEWARRhelper calls stayed in the statement and were never expanded - leaving a helper call with a bogusStackArrayLocalarg (asserts in lowering, AVs in release).No SPMI asm diffs.