[PATCH] D33720: [PowerPC] Eliminate compares - add i64 sext/zext handling for SETNE

[Nemanja Ivanovic via Phabricator via llvm-commits]

nemanjai added a comment.

In https://reviews.llvm.org/D33720#788613, @Carrot wrote:

>   SDValue Xor = IsRHSZero ? LHS :
>        SDValue(CurDAG->getMachineNode(PPC::XOR8, dl, MVT::i64, LHS, RHS), 0);
>
> SDValue AC =
>
>     SDValue(CurDAG->getMachineNode(PPC::ADDIC8, dl, MVT::i64, MVT::Glue,
>                                    Xor, getI32Imm(~0U, dl)), 0);
>   return SDValue(CurDAG->getMachineNode(PPC::SUBFE8, dl, MVT::i64, AC,
>                                         Xor, AC.getValue(1)), 0);
>   
>
> Suppose we have RHS literal 0, LHS is a var, its runtime value is also 0, the expected value of the expression should be 0. But the runtime values are:
>
>   Xor  = LHS = 0
>   AC = Xor + ~0 = ~0, no carry
>   result = Xor - AC - carry = 0 - (~0) = 1
>   
>
> So the code sequence computes out wrong result.


That's actually not what the SUBFE instruction does. As per the ISA:

  subfe RT,RA,RB
  computes
  RT = ~(RA) + (RB) + CA

So in your example, it will be:

  ~AC + XOR + CARRY
  ~(~0) + 0 + 0

Which of course will just be a zero.


Repository:
  rL LLVM

https://reviews.llvm.org/D33720