[x86] simplify cmov with same true/false operands

llvm-svn: 365998

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -36783,6 +36783,10 @@ static SDValue combineCMov(SDNode *N, SelectionDAG &DAG,
   X86::CondCode CC = (X86::CondCode)N->getConstantOperandVal(2);
   SDValue Cond = N->getOperand(3);
 
+  // cmov X, X, ?, ? --> X
+  if (TrueOp == FalseOp)
+    return TrueOp;
+
   // Try to simplify the EFLAGS and condition code operands.
   // We can't always do this as FCMOV only supports a subset of X86 cond.
   if (SDValue Flags = combineSetCCEFLAGS(Cond, CC, DAG, Subtarget)) {

llvm/test/CodeGen/X86/combine-sbb.ll

@@ -291,9 +291,8 @@ define i32 @PR40483_sub5(i32*, i32) {
 ;
 ; X64-LABEL: PR40483_sub5:
 ; X64:       # %bb.0:
-; X64-NEXT:    xorl %eax, %eax
 ; X64-NEXT:    subl %esi, (%rdi)
-; X64-NEXT:    cmovael %eax, %eax
+; X64-NEXT:    xorl %eax, %eax
 ; X64-NEXT:    retq
   %3 = load i32, i32* %0, align 8
   %4 = tail call { i8, i32 } @llvm.x86.subborrow.32(i8 0, i32 %3, i32 %1)