[x86] fold sext (xor Bool, -1) --> sub (zext Bool), 1

This is the same transform that is current used for:
select Bool, 0, -1

llvm-svn: 295568

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 57d937dd545..5522c7138b9 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -33436,6 +33436,16 @@ static SDValue combineSext(SDNode *N, SelectionDAG &DAG,
     return SDValue();
   }
 
+  if (InVT == MVT::i1 && N0.getOpcode() == ISD::XOR &&
+      isAllOnesConstant(N0.getOperand(1)) && N0.hasOneUse()) {
+    // Invert and sign-extend a boolean is the same as zero-extend and subtract
+    // 1 because 0 becomes -1 and 1 becomes 0. The subtract is efficiently
+    // lowered with an LEA or a DEC. This is the same as: select Bool, 0, -1.
+    // sext (xor Bool, -1) --> sub (zext Bool), 1
+    SDValue Zext = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, N0.getOperand(0));
+    return DAG.getNode(ISD::SUB, DL, VT, Zext, DAG.getConstant(1, DL, VT));
+  }
+
   if (SDValue V = combineToExtendVectorInReg(N, DAG, DCI, Subtarget))
     return V;