[PATCH] D57169: [X86] Fold X86ISD::SBB(ISD::SUB(X, Y), 0) -> X86ISD::SBB(X, Y) (PR25858)

Thu Jan 24 12:53:38 PST 2019

RKSimon planned changes to this revision.
RKSimon marked an inline comment as done.
RKSimon added inline comments.

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Comment at: lib/Target/X86/X86ISelLowering.cpp:40510
+  SDValue Op1 = N->getOperand(1);
+  if (Op0.getOpcode() == ISD::SUB && isNullConstant(Op1))
+    return DAG.getNode(X86ISD::SBB, SDLoc(N), N->getVTList(), Op0.getOperand(0),
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craig.topper wrote:
> This is only valid if the flag result of the SBB is dead. If the sub(X,Y) had a borrow, that information was silently dropped. The SBB output flag should only be calculating the borrow from the result of the subtract minus the C flag. By folding them it would calculate the C flag based on the combined subtraction.
> 
> This is the same reason DAGCombiner::visitADDCARRYLike has a check that the ADDCARRY flag result isn't used.
LOL - I realised that walking home. I'll take another look.

Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D57169/new/

https://reviews.llvm.org/D57169