diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index bd7d94e22433..c0d5337ecc33 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11086,8 +11086,9 @@ SDValue DAGCombiner::visitSIGN_EXTEND_VECTOR_INREG(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // sext_vector_inreg(undef) = 0 because the top bit will all be the same.
   if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+    return DAG.getConstant(0, SDLoc(N), VT);
 
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
@@ -11102,8 +11103,9 @@ SDValue DAGCombiner::visitZERO_EXTEND_VECTOR_INREG(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // zext_vector_inreg(undef) = 0 because the top bits will be zero.
   if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+    return DAG.getConstant(0, SDLoc(N), VT);
 
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
diff --git a/llvm/test/CodeGen/X86/pr46585.ll b/llvm/test/CodeGen/X86/pr46585.ll
index 65cd6b897ea3..7bea63176d1b 100644
--- a/llvm/test/CodeGen/X86/pr46585.ll
+++ b/llvm/test/CodeGen/X86/pr46585.ll
@@ -7,7 +7,7 @@
 define void @spam() local_unnamed_addr {
 ; CHECK-LABEL: spam:
 ; CHECK:       ## %bb.0: ## %bb
-; CHECK-NEXT:    pmovmskb %xmm0, %eax
+; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    testb %al, %al
 ; CHECK-NEXT:    je LBB0_2
 ; CHECK-NEXT:  ## %bb.1: ## %bb9