diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d4af722e7e4..edbd9f3e1ef 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2844,18 +2844,20 @@ static SDValue FoldCONCAT_VECTORS(SDLoc DL, EVT VT, ArrayRef<SDValue> Ops,
                   [](SDValue Op) { return Op.isUndef(); }))
     return DAG.getUNDEF(VT);
 
-  // A CONCAT_VECTOR with all operands BUILD_VECTOR can be simplified
-  // to one big BUILD_VECTOR.
-  // FIXME: Add support for UNDEF and SCALAR_TO_VECTOR as well.
-  if (!std::all_of(Ops.begin(), Ops.end(), [](SDValue Op) {
-        return Op.getOpcode() == ISD::BUILD_VECTOR;
-      }))
-    return SDValue();
-
+  // A CONCAT_VECTOR with all UNDEF/BUILD_VECTOR operands can be
+  // simplified to one big BUILD_VECTOR.
+  // FIXME: Add support for SCALAR_TO_VECTOR as well.
   EVT SVT = VT.getScalarType();
   SmallVector<SDValue, 16> Elts;
-  for (SDValue Op : Ops)
-    Elts.append(Op->op_begin(), Op->op_end());
+  for (SDValue Op : Ops) {
+    EVT OpVT = Op.getValueType();
+    if (Op.isUndef())
+      Elts.append(OpVT.getVectorNumElements(), DAG.getUNDEF(SVT));
+    else if (Op.getOpcode() == ISD::BUILD_VECTOR)
+      Elts.append(Op->op_begin(), Op->op_end());
+    else
+      return SDValue();
+  }
 
   // BUILD_VECTOR requires all inputs to be of the same type, find the
   // maximum type and extend them all.
diff --git a/test/CodeGen/X86/avx-cast.ll b/test/CodeGen/X86/avx-cast.ll
index 34c5dfaa016..8dd3529eb48 100644
--- a/test/CodeGen/X86/avx-cast.ll
+++ b/test/CodeGen/X86/avx-cast.ll
@@ -31,8 +31,8 @@ define <4 x double> @castB(<2 x double> %m) nounwind uwtable readnone ssp {
 define <4 x i64> @castC(<2 x i64> %m) nounwind uwtable readnone ssp {
 ; AVX1-LABEL: castC:
 ; AVX1:       ## BB#0:
-; AVX1-NEXT:    vxorps %ymm1, %ymm1, %ymm1
-; AVX1-NEXT:    vblendps {{.*#+}} ymm0 = ymm0[0,1,2,3],ymm1[4,5,6,7]
+; AVX1-NEXT:    vxorpd %ymm1, %ymm1, %ymm1
+; AVX1-NEXT:    vblendpd {{.*#+}} ymm0 = ymm0[0,1],ymm1[2,3]
 ; AVX1-NEXT:    retq
 ;
 ; AVX2-LABEL: castC: