[PATCH] D114462: [InstSimplify] fold xor logic of 2 variables

[Stanislav Mekhanoshin via Phabricator via llvm-commits]

rampitec accepted this revision.
rampitec added a comment.
This revision is now accepted and ready to land.

LGTM.

Thanks Sanjay! This really helps D113216 <https://reviews.llvm.org/D113216> with -O3, but then not a more complex version of it which needs demorganing and reassociation:

  ; (~a & ~b & c) | ~(a | (b | c)) -> ~(a | b)
  ; (~a & ~b & c) | ~(b | (a | c)) -> ~(a | b)
  
  define i32 @not_and_not_and_or_not_or_or(i32 %a, i32 %b, i32 %c) {
  ; CHECK-LABEL: @not_and_not_and_or_not_or_or(
  ; CHECK-NEXT:    [[OR3:%.*]] = or i32 [[B:%.*]], [[A:%.*]]
  ; CHECK-NEXT:    [[NOT2:%.*]] = xor i32 [[OR3]], -1
  ; CHECK-NEXT:    ret i32 [[NOT2]]
  ;
    %or1 = or i32 %b, %c
    %or2 = or i32 %or1, %a
    %not1 = xor i32 %or2, -1
    %nota = xor i32 %a, -1
    %notb = xor i32 %b, -1
    %and1 = and i32 %nota, %notb
    %and2 = and i32 %and1, %c
    %or4 = or i32 %and2, %not1
    call void @use(i32 %nota)
    call void @use(i32 %notb)
    ret i32 %or4
  }
  
  declare void @use(i32)

In this case reassociation is blocked by uses, so I was looking into extending LHS match in D113216 <https://reviews.llvm.org/D113216> to catch `(~a & ~b & c)` in addition to `(~(a | b) & c)`. I still need to look into that, but this is clearly LGTM.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D114462/new/

https://reviews.llvm.org/D114462