[release/9.0-staging] JIT: fix local assertion prop error for partial local comparisons#112539
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jeffschwMSFT merged 2 commits intorelease/9.0-stagingfrom Feb 28, 2025
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If a JTRUE comparison only involves part of a local value we cannot make assertions about the local as a whole. Fixes #111352.
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lgtm. please get a code review. we will take this for consideration in 9.0.x
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Backport of #112506 to release/9.0-staging
/cc @AndyAyersMS
Customer Impact
We got a report of incorrect behavior in 9.0 on github: #111352
This is silent bad code and could impact customers unexpectedly.
Regression
Introduced by #94741. We enabled more aggressive assertion propagation in the JIT "morph" phase, and one of the new classes of assertions could incorrectly draw inferences from operations on just the low 32 bits of
longvariables.For example; if the program had code like
Then the JIT would mistakenly optimize the
WriteLinetoWriteLine($"{x},{x}");Testing
Verified on a private repo case from the user that opened the issue, and extracted the problematic pattern and added a new test case.
Risk
Low. This optimization was enabled in our mainline in Dec 2023 and has just surfaced now. Also, no SPMI hits for this pattern.