Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11425,36 +11425,51 @@
     }
   }
 
+  // Fold any combination of BUILD_VECTOR or UNDEF nodes into one BUILD_VECTOR.
+  // We have already tested above for an UNDEF only concatenation.
   // fold (concat_vectors (BUILD_VECTOR A, B, ...), (BUILD_VECTOR C, D, ...))
   // -> (BUILD_VECTOR A, B, ..., C, D, ...)
-  if (N->getNumOperands() == 2 &&
-      N->getOperand(0).getOpcode() == ISD::BUILD_VECTOR &&
-      N->getOperand(1).getOpcode() == ISD::BUILD_VECTOR) {
-    EVT VT = N->getValueType(0);
-    SDValue N0 = N->getOperand(0);
-    SDValue N1 = N->getOperand(1);
+  auto IsBuildVectorOrUndef = [](const SDValue &Op) {
+    return ISD::UNDEF == Op.getOpcode() || ISD::BUILD_VECTOR == Op.getOpcode();
+  };
+  bool AllBuildVectorsOrUndefs =
+      std::all_of(N->op_begin(), N->op_end(), IsBuildVectorOrUndef);
+  if (AllBuildVectorsOrUndefs) {
     SmallVector<SDValue, 8> Opnds;
-    unsigned BuildVecNumElts =  N0.getNumOperands();
+    EVT SVT = VT.getScalarType();
 
-    EVT SclTy0 = N0.getOperand(0)->getValueType(0);
-    EVT SclTy1 = N1.getOperand(0)->getValueType(0);
-    if (SclTy0.isFloatingPoint()) {
-      for (unsigned i = 0; i != BuildVecNumElts; ++i)
-        Opnds.push_back(N0.getOperand(i));
-      for (unsigned i = 0; i != BuildVecNumElts; ++i)
-        Opnds.push_back(N1.getOperand(i));
-    } else {
+    EVT MinVT = SVT;
+    if (!SVT.isFloatingPoint())
       // If BUILD_VECTOR are from built from integer, they may have different
       // operand types. Get the smaller type and truncate all operands to it.
-      EVT MinTy = SclTy0.bitsLE(SclTy1) ? SclTy0 : SclTy1;
-      for (unsigned i = 0; i != BuildVecNumElts; ++i)
-        Opnds.push_back(DAG.getNode(ISD::TRUNCATE, SDLoc(N), MinTy,
-                        N0.getOperand(i)));
-      for (unsigned i = 0; i != BuildVecNumElts; ++i)
-        Opnds.push_back(DAG.getNode(ISD::TRUNCATE, SDLoc(N), MinTy,
-                        N1.getOperand(i)));
+      for (const SDValue &Op : N->ops()) {
+        EVT OpSVT = Op.getValueType().getScalarType();
+        MinVT = MinVT.bitsLE(OpSVT) ? MinVT : OpSVT;
+      }
+
+    for (const SDValue &Op : N->ops()) {
+      EVT OpVT = Op.getValueType();
+      unsigned NumElts = OpVT.getVectorNumElements();
+
+      if (ISD::UNDEF == Op.getOpcode())
+        for (unsigned i = 0; i != NumElts; ++i)
+          Opnds.push_back(DAG.getUNDEF(MinVT));
+
+      if (ISD::BUILD_VECTOR == Op.getOpcode()) {
+        if (SVT.isFloatingPoint()) {
+          assert(SVT == OpVT.getScalarType() && "Concat vector type mismatch");
+          for (unsigned i = 0; i != NumElts; ++i)
+            Opnds.push_back(Op.getOperand(i));
+        } else {
+          for (unsigned i = 0; i != NumElts; ++i)
+            Opnds.push_back(
+                DAG.getNode(ISD::TRUNCATE, SDLoc(N), MinVT, Op.getOperand(i)));
+        }
+      }
     }
 
+    assert(VT.getVectorNumElements() == Opnds.size() &&
+           "Concat vector type mismatch");
     return DAG.getNode(ISD::BUILD_VECTOR, SDLoc(N), VT, Opnds);
   }
 
Index: test/CodeGen/X86/vector-zext.ll
===================================================================
--- test/CodeGen/X86/vector-zext.ll
+++ test/CodeGen/X86/vector-zext.ll
@@ -359,21 +359,15 @@
 ; AVX1-LABEL: shuf_zext_8i16_to_8i32:
 ; AVX1:       # BB#0: # %entry
 ; AVX1-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX1-NEXT:    vpshuflw {{.*#+}} xmm1 = xmm1[0,0,0,0,4,5,6,7]
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm2 = xmm0[2,3,0,1]
-; AVX1-NEXT:    vpunpcklwd {{.*#+}} xmm2 = xmm2[0],xmm1[0],xmm2[1],xmm1[1],xmm2[2],xmm1[2],xmm2[3],xmm1[3]
-; AVX1-NEXT:    vpunpcklwd {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1],xmm0[2],xmm1[2],xmm0[3],xmm1[3]
-; AVX1-NEXT:    vinsertf128 $1, %xmm2, %ymm0, %ymm0
+; AVX1-NEXT:    vpunpckhwd {{.*#+}} xmm1 = xmm0[4],xmm1[4],xmm0[5],xmm1[5],xmm0[6],xmm1[6],xmm0[7],xmm1[7]
+; AVX1-NEXT:    vpmovzxwd {{.*#+}} xmm0 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero
+; AVX1-NEXT:    vinsertf128 $1, %xmm1, %ymm0, %ymm0
 ; AVX1-NEXT:    retq
 ;
 ; AVX2-LABEL: shuf_zext_8i16_to_8i32:
 ; AVX2:       # BB#0: # %entry
-; AVX2-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX2-NEXT:    vpbroadcastw %xmm1, %xmm1
-; AVX2-NEXT:    vpshufd {{.*#+}} xmm2 = xmm0[2,3,0,1]
-; AVX2-NEXT:    vpunpcklwd {{.*#+}} xmm2 = xmm2[0],xmm1[0],xmm2[1],xmm1[1],xmm2[2],xmm1[2],xmm2[3],xmm1[3]
-; AVX2-NEXT:    vpunpcklwd{{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1],xmm0[2],xmm1[2],xmm0[3],xmm1[3]
-; AVX2-NEXT:    vinserti128 $1, %xmm2, %ymm0, %ymm0
+; AVX2-NEXT:    # kill
+; AVX2-NEXT:    vpmovzxwd {{.*#+}} ymm0 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero,xmm0[4],zero,xmm0[5],zero,xmm0[6],zero,xmm0[7],zero
 ; AVX2-NEXT:    retq
 entry:
   %B = shufflevector <8 x i16> %A, <8 x i16> zeroinitializer, <16 x i32> <i32 0, i32 8, i32 1, i32 8, i32 2, i32 8, i32 3, i32 8, i32 4, i32 8, i32 5, i32 8, i32 6, i32 8, i32 7, i32 8>
@@ -410,22 +404,17 @@
 ;
 ; AVX1-LABEL: shuf_zext_4i32_to_4i64:
 ; AVX1:       # BB#0: # %entry
-; AVX1-NEXT:    vxorps %xmm1, %xmm1, %xmm1
-; AVX1-NEXT:    vshufps {{.*#+}} xmm2 = xmm0[0,1],xmm1[0,0]
-; AVX1-NEXT:    vshufps {{.*#+}} xmm2 = xmm2[0,2,1,3]
-; AVX1-NEXT:    vblendpd {{.*#+}} xmm0 = xmm1[0],xmm0[1]
+; AVX1-NEXT:    vinsertps {{.*#+}} xmm1 = xmm0[0],zero,xmm0[1],zero
+; AVX1-NEXT:    vxorpd %xmm2, %xmm2, %xmm2
+; AVX1-NEXT:    vblendpd {{.*#+}} xmm0 = xmm2[0],xmm0[1]
 ; AVX1-NEXT:    vpermilps {{.*#+}} xmm0 = xmm0[2,0,3,0]
-; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm2, %ymm0
+; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm1, %ymm0
 ; AVX1-NEXT:    retq
 ;
 ; AVX2-LABEL: shuf_zext_4i32_to_4i64:
 ; AVX2:       # BB#0: # %entry
 ; AVX2-NEXT:    # kill
 ; AVX2-NEXT:    vpmovzxdq {{.*#+}} ymm0 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero
-; AVX2-NEXT:    xorl %eax, %eax
-; AVX2-NEXT:    vmovd %eax, %xmm1
-; AVX2-NEXT:    vpbroadcastd %xmm1, %ymm1
-; AVX2-NEXT:    vpblendd {{.*#+}} ymm0 = ymm0[0],ymm1[1],ymm0[2],ymm1[3],ymm0[4],ymm1[5],ymm0[6],ymm1[7]
 ; AVX2-NEXT:    retq
 entry:
   %B = shufflevector <4 x i32> %A, <4 x i32> zeroinitializer, <8 x i32> <i32 0, i32 4, i32 1, i32 4, i32 2, i32 4, i32 3, i32 4>