[all-commits] [llvm/llvm-project] c6b88c: [InstCombine] Push freeze through recurrence phi

Nikita Popov via All-commits all-commits at lists.llvm.org
Fri Jun 17 06:03:24 PDT 2022

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c6b88cb9184fc6c17bb3d9c2c9568646dc46638d
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2022-06-17 (Fri, 17 Jun 2022)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineInternal.h
    M llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    M llvm/test/Transforms/InstCombine/freeze.ll

  Log Message:
  [InstCombine] Push freeze through recurrence phi

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.

Differential Revision: https://reviews.llvm.org/D127960

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