[all-commits] [llvm/llvm-project] 472558: [ValueTracking] A and (B & ~A) have no common bits...

Nikita Popov via All-commits all-commits at lists.llvm.org
Tue May 3 02:33:43 PDT 2022

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 47255834e720b735ff1358a74fdf32345bb4daca
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2022-05-03 (Tue, 03 May 2022)

  Changed paths:
    M llvm/lib/Analysis/ValueTracking.cpp
    M llvm/test/Transforms/InstCombine/add.ll

  Log Message:
  [ValueTracking] A and (B & ~A) have no common bits set

This extends haveNoCommonBitsSet() to two additional cases, allowing
the following folds:

 * `A + (B & ~A)` --> `A | (B & ~A)`
 * `A + ((A & B) ^ B)` --> `A | ((A & B) ^ B)`

These should further fold to just `A | B`, though this currently
only works in the first case.

The reason why the second fold is necessary is that we consider
this to be the canonical form if B is a constant. (I did check
whether we can change that, but it looks like a number of folds
depend on the current canonicalization, so I ended up adding both
patterns here.)

Differential Revision: https://reviews.llvm.org/D124763

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