[PATCH] D36498: [InstCombine] Teach foldSelectICmpAnd to recognize a (icmp slt trunc X, 0) and (icmp sgt trunc X, -1) as equivalent to an and with the sign bit of the truncated type

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

This patch is really just making InstCombine self consistent. We currently optimize this case differently depending on whether i8 is legal in datalayout.

define i32 @test71(i32 %x) {
; CHECK-LABEL: @test71(
; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[X:%.*]], 6
; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP1]], 2
; CHECK-NEXT:    [[TMP3:%.*]] = or i32 [[TMP2]], 40
; CHECK-NEXT:    ret i32 [[TMP3]]
;

  %1 = and i32 %x, 128
  %2 = icmp eq i32 %1, 0
  %3 = select i1 %2, i32 40, i32 42
  ret i32 %3

}

If we want to remove foldSelectICmpAnd that's a different question.


https://reviews.llvm.org/D36498