[PATCH] D92401: [BasicAA] Handle two unknown sizes for GEPs

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: asbirlea, jdoerfert.
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If we have two unknown sizes and one GEP operand and one non-GEP operand, then we currently simply return MayAlias. The comment says we can't do anything useful ... but we can! We can still check that the underlying objects are different (and do so for the GEP-GEP case).

To reduce the compile-time impact, this a) checks this early, before doing the relatively expensive GEP decomposition that will not be used and b) doesn't do the check if the other operand is a phi or select. In that case, the phi/select will already recurse, so this would just do two slightly different recursive walks that arrive at the same roots.

Compile-time is still a bit of a mixed bag: https://llvm-compile-time-tracker.com/compare.php?from=624af932a808b363a888139beca49f57313d9a3b&to=845356e14adbe651a553ed11318ddb5e79a24bcd&stat=instructions On average this is a small improvement, but sqlite with ThinLTO has a 0.5% regression (lencod has a 1% improvement).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92401

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/phi-aa.ll
  llvm/test/Transforms/LoopVectorize/ARM/pointer_iv.ll
  llvm/test/Transforms/LoopVersioning/exit-block-dominates-rt-check-block.ll

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