[DAGCombiner] Don't ignore N2's undef elements in foldVSelectOfConstants

Author: dtcxzyw

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -12535,9 +12535,10 @@ SDValue DAGCombiner::foldVSelectOfConstants(SDNode *N) {
   for (unsigned i = 0; i != Elts; ++i) {
     SDValue N1Elt = N1.getOperand(i);
     SDValue N2Elt = N2.getOperand(i);
-    if (N1Elt.isUndef() || N2Elt.isUndef())
+    if (N1Elt.isUndef())
       continue;
-    if (N1Elt.getValueType() != N2Elt.getValueType()) {
+    // N2 should not contain undef values since it will be reused in the fold.
+    if (N2Elt.isUndef() || N1Elt.getValueType() != N2Elt.getValueType()) {
       AllAddOne = false;
       AllSubOne = false;
       break;

llvm/test/CodeGen/X86/vselect-constants.ll

@@ -306,10 +306,12 @@ define i32 @wrong_min_signbits(<2 x i16> %x) {
 define i32 @pr129181() {
 ; SSE-LABEL: pr129181:
 ; SSE:       # %bb.0: # %entry
+; SSE-NEXT:    xorl %eax, %eax
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: pr129181:
 ; AVX:       # %bb.0: # %entry
+; AVX-NEXT:    xorl %eax, %eax
 ; AVX-NEXT:    retq
 entry:
   %x = insertelement <4 x i32> zeroinitializer, i32 0, i32 0