Index: lib/Target/ARM/ARMISelLowering.cpp
===================================================================
--- lib/Target/ARM/ARMISelLowering.cpp
+++ lib/Target/ARM/ARMISelLowering.cpp
@@ -5050,10 +5050,16 @@
   if (M.size() != NumElts && M.size() != NumElts*2)
     return false;
 
-  // If the mask is twice as long as the result then we need to check the upper
-  // and lower parts of the mask
+  // If the mask is twice as long as the input vector then we need to check the
+  // upper and lower parts of the mask with a matching value for WhichResult
+  // FIXME: A mask with only even values will be rejected in case the first
+  // element is undefined, e.g. [-1, 4, 2, 6] will be rejected, because only
+  // M[0] is used to determine WhichResult
   for (unsigned i = 0; i < M.size(); i += NumElts) {
-    WhichResult = M[i] == 0 ? 0 : 1;
+    if (M.size() == NumElts * 2)
+      WhichResult = i / NumElts;
+    else
+      WhichResult = M[i] == 0 ? 0 : 1;
     for (unsigned j = 0; j < NumElts; j += 2) {
       if ((M[i+j] >= 0 && (unsigned) M[i+j] != j + WhichResult) ||
           (M[i+j+1] >= 0 && (unsigned) M[i+j+1] != j + NumElts + WhichResult))
@@ -5080,7 +5086,10 @@
     return false;
 
   for (unsigned i = 0; i < M.size(); i += NumElts) {
-    WhichResult = M[i] == 0 ? 0 : 1;
+    if (M.size() == NumElts * 2)
+      WhichResult = i / NumElts;
+    else
+      WhichResult = M[i] == 0 ? 0 : 1;
     for (unsigned j = 0; j < NumElts; j += 2) {
       if ((M[i+j] >= 0 && (unsigned) M[i+j] != j + WhichResult) ||
           (M[i+j+1] >= 0 && (unsigned) M[i+j+1] != j + WhichResult))
Index: test/CodeGen/ARM/vtrn.ll
===================================================================
--- test/CodeGen/ARM/vtrn.ll
+++ test/CodeGen/ARM/vtrn.ll
@@ -371,3 +371,31 @@
   %rv = select <8 x i1> %c, <8 x i8> %tr0, <8 x i8> %tr1
   ret <8 x i8> %rv
 }
+
+; Negative test that should not generate a vtrn
+define void @lower_twice_no_vtrn(<4 x i16>* %A, <4 x i16>* %B, <8 x i16>* %C) {
+entry:
+  ; CHECK-LABEL: lower_twice_no_vtrn
+  ; CHECK: @ BB#0:
+  ; CHECK-NOT: vtrn
+  ; CHECK: mov pc, lr
+  %tmp1 = load <4 x i16>, <4 x i16>* %A
+  %tmp2 = load <4 x i16>, <4 x i16>* %B
+  %0 = shufflevector <4 x i16> %tmp1, <4 x i16> %tmp2, <8 x i32> <i32 undef, i32 5, i32 3, i32 7, i32 1, i32 5, i32 3, i32 7>
+  store <8 x i16> %0, <8 x i16>* %C
+  ret void
+}
+
+; Negative test that should not generate a vtrn
+define void @upper_twice_no_vtrn(<4 x i16>* %A, <4 x i16>* %B, <8 x i16>* %C) {
+entry:
+  ; CHECK-LABEL: upper_twice_no_vtrn
+  ; CHECK: @ BB#0:
+  ; CHECK-NOT: vtrn
+  ; CHECK: mov pc, lr
+  %tmp1 = load <4 x i16>, <4 x i16>* %A
+  %tmp2 = load <4 x i16>, <4 x i16>* %B
+  %0 = shufflevector <4 x i16> %tmp1, <4 x i16> %tmp2, <8 x i32> <i32 0, i32 undef, i32 2, i32 6, i32 0, i32 4, i32 2, i32 6>
+  store <8 x i16> %0, <8 x i16>* %C
+  ret void
+}
Index: test/CodeGen/ARM/vuzp.ll
===================================================================
--- test/CodeGen/ARM/vuzp.ll
+++ test/CodeGen/ARM/vuzp.ll
@@ -286,6 +286,18 @@
   ret <4 x i32> %0
 }
 
+define void @vuzp_rev_shufflemask_vtrn(<2 x i32>* %A, <2 x i32>* %B, <4 x i32>* %C) {
+entry:
+  ; CHECK-LABEL: vuzp_rev_shufflemask_vtrn
+  ; CHECK-NOT: vtrn
+  ; CHECK: vuzp
+  %tmp1 = load <2 x i32>, <2 x i32>* %A
+  %tmp2 = load <2 x i32>, <2 x i32>* %B
+  %0 = shufflevector <2 x i32> %tmp1, <2 x i32> %tmp2, <4 x i32> <i32 1, i32 3, i32 0, i32 2>
+  store <4 x i32> %0, <4 x i32>* %C
+  ret void
+}
+
 define <8 x i8> @vuzp_trunc(<8 x i8> %in0, <8 x i8> %in1, <8 x i32> %cmp0, <8 x i32> %cmp1) {
 ; In order to create the select we need to truncate the vcgt result from a vector of i32 to a vector of i8.
 ; This results in a build_vector with mismatched types. We will generate two vmovn.i32 instructions to
Index: test/CodeGen/ARM/vzip.ll
===================================================================
--- test/CodeGen/ARM/vzip.ll
+++ test/CodeGen/ARM/vzip.ll
@@ -295,3 +295,13 @@
   ret <4 x i32> %0
 }
 
+define void @vzip_undef_rev_shufflemask_vtrn(<2 x i32>* %A, <4 x i32>* %B) {
+entry:
+  ; CHECK-LABEL: vzip_undef_rev_shufflemask_vtrn
+  ; CHECK-NOT: vtrn
+  ; CHECK: vzip
+  %tmp1 = load <2 x i32>, <2 x i32>* %A
+  %0 = shufflevector <2 x i32> %tmp1, <2 x i32> undef, <4 x i32> <i32 1, i32 1, i32 0, i32 0>
+  store <4 x i32> %0, <4 x i32>* %B
+  ret void
+}