JIT: Fix empty-block cycle hang - #132795
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@jakobbotsch PTAL Fixes a fuzzing-created example; haven't seen this in real code. |
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Pull request overview
This PR updates CoreCLR JIT flow-graph optimization to avoid non-terminating rewrites when a branch targets an empty BBJ_ALWAYS block that participates in an empty-block cycle, and adds a focused regression test to cover the hang scenario.
Changes:
- Extend
fgOptimizeBranchToEmptyUnconditionalto detect cycles in empty unconditional chains (using a slow/fast pointer walk) and suppress the redirect when a cycle is found. - Add a new JitBlue regression test project
Runtime_132785that runs withDOTNET_TieredCompilation=0to ensure the optimizing JIT path is exercised deterministically.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| src/coreclr/jit/fgopt.cpp | Prevents repeated edge redirection around empty-block cycles so flow optimization terminates. |
| src/tests/JIT/Regression/JitBlue/Runtime_132785/Runtime_132785.csproj | Adds isolated JIT regression test project configuration, including tiered compilation disabled. |
| src/tests/JIT/Regression/JitBlue/Runtime_132785/Runtime_132785.cs | Adds an xUnit-based repro-derived test that asserts the expected result and guards against the compilation hang. |
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jakobbotsch
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Aug 27, 2026
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
# Conflicts: # src/tests/JIT/Regression/Regression_ro_2.csproj
jakobbotsch
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Aug 27, 2026
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/ba-g OSX timeouts |
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Fixes #132785.
fgOptimizeBranchToEmptyUnconditionalonly recognized two-block empty cycles. Detect any empty unconditional cycle before redirecting the edge so flow optimization terminates.Adds the regression test to the shared JIT regression runner and uses
AggressiveOptimizationto exercise FullOpts.Validation:
Regression_ro_2buildjit-formatNote
This pull request description was generated by GitHub Copilot.