Index: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8064,10 +8064,15 @@
   unsigned NumElts = VT.getVectorNumElements();
   SDLoc DL(N);
 
-  for (unsigned i=0; i != NumElts; ++i) {
-    SDValue Op = N0->getOperand(i);
-    if (Op->isUndef()) {
-      Elts.push_back(DAG.getUNDEF(SVT));
+  // For zero-extensions, UNDEF elements still guarantee to have the upper
+  // bits set to zero.
+  bool IsZext =
+      Opcode == ISD::ZERO_EXTEND || Opcode == ISD::ZERO_EXTEND_VECTOR_INREG;
+
+  for (unsigned i = 0; i != NumElts; ++i) {
+    SDValue Op = N0.getOperand(i);
+    if (Op.isUndef()) {
+      Elts.push_back(IsZext ? DAG.getConstant(0, DL, SVT) : DAG.getUNDEF(SVT));
       continue;
     }
 
Index: llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1848,6 +1848,13 @@
       return true;
     KnownZero = SrcZero.zextOrTrunc(NumElts);
     KnownUndef = SrcUndef.zextOrTrunc(NumElts);
+
+    if (Op.getOpcode() == ISD::ZERO_EXTEND_VECTOR_INREG) {
+      // zext(undef) upper bits are guaranteed to be zero.
+      if (DemandedElts.isSubsetOf(KnownUndef))
+        return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+      KnownUndef.clearAllBits();
+    }
     break;
   }
   case ISD::OR:
@@ -1892,6 +1899,13 @@
     if (SimplifyDemandedVectorElts(Op.getOperand(0), DemandedElts, KnownUndef,
                                    KnownZero, TLO, Depth + 1))
       return true;
+
+    if (Op.getOpcode() == ISD::ZERO_EXTEND) {
+      // zext(undef) upper bits are guaranteed to be zero.
+      if (DemandedElts.isSubsetOf(KnownUndef))
+        return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+      KnownUndef.clearAllBits();
+    }
     break;
   default: {
     if (Op.getOpcode() >= ISD::BUILTIN_OP_END) {
Index: llvm/trunk/test/CodeGen/AArch64/pr40091.ll
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/pr40091.ll
+++ llvm/trunk/test/CodeGen/AArch64/pr40091.ll
@@ -4,7 +4,7 @@
 define i64 @test(i64 %aa) {
 ; CHECK-LABEL: test:
 ; CHECK:       // %bb.0: // %entry
-; CHECK-NEXT:    movi v0.2d, #0xffffffffffffffff
+; CHECK-NEXT:    movi v0.8b, #137
 ; CHECK-NEXT:    fmov x0, d0
 ; CHECK-NEXT:    ret
 entry:
Index: llvm/trunk/test/CodeGen/X86/fold-vector-sext-zext.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/fold-vector-sext-zext.ll
+++ llvm/trunk/test/CodeGen/X86/fold-vector-sext-zext.ll
@@ -261,12 +261,12 @@
 define <4 x i16> @test_zext_4i8_4i16_undef() {
 ; X32-LABEL: test_zext_4i8_4i16_undef:
 ; X32:       # %bb.0:
-; X32-NEXT:    vmovaps {{.*#+}} xmm0 = <u,255,u,253>
+; X32-NEXT:    vmovaps {{.*#+}} xmm0 = [0,255,0,253]
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test_zext_4i8_4i16_undef:
 ; X64:       # %bb.0:
-; X64-NEXT:    vmovaps {{.*#+}} xmm0 = <u,255,u,253>
+; X64-NEXT:    vmovaps {{.*#+}} xmm0 = [0,255,0,253]
 ; X64-NEXT:    retq
   %1 = insertelement <4 x i8> undef, i8 undef, i32 0
   %2 = insertelement <4 x i8> %1, i8 -1, i32 1
@@ -279,12 +279,12 @@
 define <4 x i32> @test_zext_4i8_4i32_undef() {
 ; X32-LABEL: test_zext_4i8_4i32_undef:
 ; X32:       # %bb.0:
-; X32-NEXT:    vmovaps {{.*#+}} xmm0 = <0,u,2,u>
+; X32-NEXT:    vmovaps {{.*#+}} xmm0 = [0,0,2,0]
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test_zext_4i8_4i32_undef:
 ; X64:       # %bb.0:
-; X64-NEXT:    vmovaps {{.*#+}} xmm0 = <0,u,2,u>
+; X64-NEXT:    vmovaps {{.*#+}} xmm0 = [0,0,2,0]
 ; X64-NEXT:    retq
   %1 = insertelement <4 x i8> undef, i8 0, i32 0
   %2 = insertelement <4 x i8> %1, i8 undef, i32 1
@@ -297,12 +297,12 @@
 define <4 x i64> @test_zext_4i8_4i64_undef() {
 ; X32-LABEL: test_zext_4i8_4i64_undef:
 ; X32:       # %bb.0:
-; X32-NEXT:    vmovaps {{.*#+}} ymm0 = <u,u,255,0,2,0,u,u>
+; X32-NEXT:    vmovaps {{.*#+}} ymm0 = [0,0,255,0,2,0,0,0]
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test_zext_4i8_4i64_undef:
 ; X64:       # %bb.0:
-; X64-NEXT:    vmovaps {{.*#+}} ymm0 = <u,255,2,u>
+; X64-NEXT:    vmovaps {{.*#+}} ymm0 = [0,255,2,0]
 ; X64-NEXT:    retq
   %1 = insertelement <4 x i8> undef, i8 undef, i32 0
   %2 = insertelement <4 x i8> %1, i8 -1, i32 1
@@ -359,12 +359,12 @@
 define <8 x i16> @test_zext_8i8_8i16_undef() {
 ; X32-LABEL: test_zext_8i8_8i16_undef:
 ; X32:       # %bb.0:
-; X32-NEXT:    vmovaps {{.*#+}} xmm0 = <u,255,u,253,u,251,u,249>
+; X32-NEXT:    vmovaps {{.*#+}} xmm0 = [0,255,0,253,0,251,0,249]
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test_zext_8i8_8i16_undef:
 ; X64:       # %bb.0:
-; X64-NEXT:    vmovaps {{.*#+}} xmm0 = <u,255,u,253,u,251,u,249>
+; X64-NEXT:    vmovaps {{.*#+}} xmm0 = [0,255,0,253,0,251,0,249]
 ; X64-NEXT:    retq
   %1 = insertelement <8 x i8> undef, i8 undef, i32 0
   %2 = insertelement <8 x i8> %1, i8 -1, i32 1
@@ -381,12 +381,12 @@
 define <8 x i32> @test_zext_8i8_8i32_undef() {
 ; X32-LABEL: test_zext_8i8_8i32_undef:
 ; X32:       # %bb.0:
-; X32-NEXT:    vmovaps {{.*#+}} ymm0 = <0,u,2,253,4,u,6,u>
+; X32-NEXT:    vmovaps {{.*#+}} ymm0 = [0,0,2,253,4,0,6,0]
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: test_zext_8i8_8i32_undef:
 ; X64:       # %bb.0:
-; X64-NEXT:    vmovaps {{.*#+}} ymm0 = <0,u,2,253,4,u,6,u>
+; X64-NEXT:    vmovaps {{.*#+}} ymm0 = [0,0,2,253,4,0,6,0]
 ; X64-NEXT:    retq
   %1 = insertelement <8 x i8> undef, i8 0, i32 0
   %2 = insertelement <8 x i8> %1, i8 undef, i32 1