[all-commits] [llvm/llvm-project] e63643: [BasicAA][LAA] Don't use same-block phis in cross ...

[Nikita Popov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e636434bdf74cf40071b776ef05aafbf161ce4cd
      https://github.com/llvm/llvm-project/commit/e636434bdf74cf40071b776ef05aafbf161ce4cd
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2024-11-27 (Wed, 27 Nov 2024)

  Changed paths:
    M llvm/lib/Analysis/BasicAliasAnalysis.cpp
    M llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll

  Log Message:
  -----------
  [BasicAA][LAA] Don't use same-block phis in cross iteration mode (#116802)

In 4de3184f07fd8c548125d315dd306d4afa7c9698 we exposed BasicAA's
cross-iteration mode for use in LAA, so we can handle selects with equal
conditions correctly (where the select condition is not actually equal
across iterations).

However, if we replace the selects with equivalent phis, the issue still
exists. In the phi case, we effectively still have an assumption that
the condition(s) that control which phi arg is used will be the same
across iterations. Fix this by disabling this phi handling in
cross-iteration mode.

(I'm not entirely sure whether this is also needed when BasicAA enables
cross-iteration mode during internal phi recursion, but I wouldn't be
surprised if that's the case.)



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