[PATCH] D136174: [BasicAA] Replace VisitedPhiBBs with a single flag

[Nikita Popov via Phabricator via llvm-commits]

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When looking through phis, BasicAA has to guard against the possibility that values from two separate cycle iterations are being compared -- in this case, even though the SSA values may be the same, they cannot be considered as equal.

This is currently done by keeping a set of VisitedPhiBBs for any phis we looked through, and then checking whether the relevant instruction is reachable from one of the phis.

This patch replaces this set with a single flag. If the flag is set, then we will not assume equality for any instruction part of a cycle. While this is nominally less accurate, it makes essentially no difference in practice. Here are the AA stats for test-suite:

  aa.NumMayAlias  |   3072005 |  3072016
  aa.NumMustAlias |    337858 |   337854
  aa.NumNoAlias   |  13255345 | 13255349

The motivation for the change is to expose the MayBeCrossIteration flag to AA users, which will allow fixing miscompiles related to incorrect handling of cross-iteration AA queries.


https://reviews.llvm.org/D136174

Files:
  llvm/include/llvm/Analysis/BasicAliasAnalysis.h
  llvm/lib/Analysis/BasicAliasAnalysis.cpp

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