[PATCH] D127960: [InstCombine] Push freeze through recurrence phi

[Nikita Popov via Phabricator via llvm-commits]

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We really want to push freezes through recurrence phis, so that we freeze only the start value, rather than the IV value on every iteration. foldOpIntoPhi() already handles this for the case where the transfer function doesn't produce poison, e.g. `%iv.next = add %iv, 1`. However, this does not work if nowrap flags are present, e.g. the very common `%iv.next = add nuw %iv, 1` case.

This patch adds a fold that pushes freeze instructions to the start value by checking whether all backedge values will be non-poison after poison generating flags have been dropped. This allows pushing freezes out of loops in most cases. I suspect that this also obsoletes the CanonicalizeFreezeInLoops pass, and we can probably drop it.

Fixes https://github.com/llvm/llvm-project/issues/56048.


https://reviews.llvm.org/D127960

Files:
  llvm/lib/Transforms/InstCombine/InstCombineInternal.h
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/freeze.ll

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