[release/8.0] JIT: Fix exponential blowup of memory dependency arrays in VNForMapSelectWork #93388
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue DetailsBackport of #93344 to release/8.0 /cc @jakobbotsch Customer ImpactTestingRiskIMPORTANT: If this backport is for a servicing release, please verify that:
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jeffschwMSFT
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Oct 12, 2023
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jeffschwMSFT
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approved. please get a code review. we will take for consideration in 8.0.x
AndyAyersMS
approved these changes
Oct 12, 2023
mangod9
approved these changes
Oct 12, 2023
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Backport of #93344 to release/8.0
/cc @jakobbotsch
Customer Impact
Code of a particular shape may hit exponential blow-up when the JIT processes it as part of one of its optimizations. This can cause extreme memory usage (many gigabytes) and many orders of magnitude of slowdown, potentially showing up as a hang during JIT.
The affected code shape requires a long sequence of
ifstatements (or other control flow constructs) where eachifstatement modifies memory in a way that the JIT is able to fully reason about it, for example by writing a particular class field and avoiding any calls to other functions.The exponential behavior occurs if the code after this long sequence of
ifstatements then accesses a field that was written before all theifstatements. The PR includes a test case showing an example. The code shape may also be exposed after inlining performed by the JIT.Reported by a customer with an issue opened on their behalf in #93342. Also hit by an internal customer.
Testing
Regression test is included. Also tested by internal customer and confirmed as fixing the issue. Stress testing also done as part of the original PR.
Risk
Low. The fix effectively replaces a dynamic array with a set.