[release/9.0-preview4] JIT: Avoid relying on bbNum to find lexical loop boundaries
#101724
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Switch `FlowGraphNaturalLoop::GetLexicallyTopMostBlock` and `FlowGraphNaturalLoop::GetLexicallyBottomMostBlock` to more robust implementations that scan the basic block list forwards (and backwards) to find the boundary blocks. Fix #101695
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
AndyAyersMS
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Apr 30, 2024
wtgodbe
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Apr 30, 2024
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@jeffschwMSFT, please review and approve this backporting that fixes preview 4 build break. |
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Merging optimistically to unblock p4, @jeffschwMSFT please still take a look. |
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Backport of #101714 to release/9.0-preview4
/cc @jakobbotsch
Customer Impact
The JIT may crash when compiling functions involving a particular code pattern with multiple intertwined loops. In these cases the JIT internally fails to find the correct first basic block corresponding to a loop, and unexpectedly runs out of basic blocks as a result. Reported in #101695 and blocks flow into ASP.NET.
Regression
Introduced in #96995 and exposed subsequently by #99827.
Testing
Manually verified that the ASP.NET test case no longer crashes.
Risk
Low. The fix makes the query to find the first basic block corresponding to a loop more robust.