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

We often generate X86ISD::SBB(X, 0) for carry flag arithmetic.

I had tried to create test cases for the ADC equivalent (which often uses the same pattern) but haven't managed to find anything yet.

Differential Revision: https://reviews.llvm.org/D57169

llvm-svn: 352288

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index a069b579f9d..ec652e5ca5e 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -40566,6 +40566,15 @@ static SDValue combineSBB(SDNode *N, SelectionDAG &DAG) {
                        Flags);
   }
 
+  // Fold SBB(SUB(X,Y),0,Carry) -> SBB(X,Y,Carry)
+  // iff the flag result is dead.
+  SDValue Op0 = N->getOperand(0);
+  SDValue Op1 = N->getOperand(1);
+  if (Op0.getOpcode() == ISD::SUB && isNullConstant(Op1) &&
+      !N->hasAnyUseOfValue(1))
+    return DAG.getNode(X86ISD::SBB, SDLoc(N), N->getVTList(), Op0.getOperand(0),
+                       Op0.getOperand(1), N->getOperand(2));
+
   return SDValue();
 }