Index: lib/Target/ARM/ARMISelLowering.cpp
===================================================================
--- lib/Target/ARM/ARMISelLowering.cpp
+++ lib/Target/ARM/ARMISelLowering.cpp
@@ -5982,8 +5982,22 @@
     }
   }
 
-  if (M.size() == NumElts*2)
+  if (M.size() == NumElts*2) {
+    // If the output is a two register pair, we assume WhichResult to be 0
+    // for the first output reg, and 1 for the other one.
     WhichResult = 0;
+    // Check this assumption - if M[NumElts] == 0, WhichRegister was 0
+    // for the second half.
+    if (M[NumElts] == 0)
+      return false;
+    // If M[0] was nonzero, WhichRegister was 1 for the first half. That's ok
+    // only if all of them were undef.
+    if (M[0] != 0) {
+      for (unsigned i = 0; i < NumElts; i++)
+        if (M[i] >= 0)
+          return false;
+    }
+  }
 
   // VZIP.32 for 64-bit vectors is a pseudo-instruction alias for VTRN.32.
   if (VT.is64BitVector() && EltSz == 32)
Index: test/CodeGen/ARM/vzip.ll
===================================================================
--- test/CodeGen/ARM/vzip.ll
+++ test/CodeGen/ARM/vzip.ll
@@ -282,6 +282,23 @@
   ret <8 x i16> %0
 }
 
+define <8 x i16> @vzip_lower_shufflemask_undef2(<4 x i16>* %A, <4 x i16>* %B) {
+; CHECK-LABEL: vzip_lower_shufflemask_undef2:
+; CHECK:       @ BB#0: @ %entry
+; CHECK-NEXT:    vldr d16, [r1]
+; CHECK-NEXT:    vldr d19, [r0]
+; CHECK-NEXT:    vtrn.16 d19, d16
+; CHECK-NEXT:    vmov r0, r1, d18
+; CHECK-NEXT:    vmov r2, r3, d19
+; CHECK-NEXT:    mov pc, lr
+entry:
+  %tmp1 = load <4 x i16>, <4 x i16>* %A
+  %tmp2 = load <4 x i16>, <4 x i16>* %B
+  %cat = shufflevector <4 x i16> %tmp1, <4 x i16> %tmp2, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
+  %res = shufflevector <8 x i16> %cat, <8 x i16> undef, <8 x i32> <i32 undef, i32 undef, i32 undef, i32 undef, i32 0, i32 4, i32 undef, i32 undef>
+  ret <8 x i16> %res
+}
+
 define <4 x i32> @vzip_lower_shufflemask_zeroed(<2 x i32>* %A) {
 ; CHECK-LABEL: vzip_lower_shufflemask_zeroed:
 ; CHECK:       @ BB#0: @ %entry