[PATCH] D38514: [InstCombine] improve folds for icmp gt/lt (shr X, C1), C2

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

I think this changes behavior of the current code with "exact" shifts. Did we not have tests for that?

For example:

  define i1 @lshrugt_01_01_exact(i4 %x) {
  ; CHECK-LABEL: @lshrugt_01_01(
  ; CHECK-NEXT:    [[C:%.*]] = icmp ugt i4 %x, 2
  ; CHECK-NEXT:    ret i1 [[C]]
  ;
    %s = lshr exact i4 %x, 1
    %c = icmp ugt i4 %s, 1
    ret i1 %c
  }
  
  define i1 @lshrugt_01_01(i4 %x) {
  ; CHECK-LABEL: @lshrugt_01_01(
  ; CHECK-NEXT:    [[C:%.*]] = icmp ugt i4 %x, 3
  ; CHECK-NEXT:    ret i1 [[C]]
  ;
    %s = lshr i4 %x, 1
    %c = icmp ugt i4 %s, 1
    ret i1 %c
  }


https://reviews.llvm.org/D38514