diff --git a/include/llvm/CodeGen/SelectionDAGNodes.h b/include/llvm/CodeGen/SelectionDAGNodes.h
index 4bf37b6..d9f930d 100644
--- a/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -1499,14 +1499,16 @@
 
   bool isSplat() const { return isSplatMask(Mask, getValueType(0)); }
 
-  int  getSplatIndex() const {
+  int getSplatIndex() const {
     assert(isSplat() && "Cannot get splat index for non-splat!");
     EVT VT = getValueType(0);
-    for (unsigned i = 0, e = VT.getVectorNumElements(); i != e; ++i) {
+    for (unsigned i = 0, e = VT.getVectorNumElements(); i != e; ++i)
       if (Mask[i] >= 0)
         return Mask[i];
-    }
-    llvm_unreachable("Splat with all undef indices?");
+
+    // We can choose any index value here and be correct because all elements
+    // are undefined. Return 0 for better potential for callers to simplify.
+    return 0;
   }
 
   static bool isSplatMask(const int *Mask, EVT VT);
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 5689a15..b2de2a4 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -9350,7 +9350,10 @@
   for (i = 0, e = VT.getVectorNumElements(); i != e && Mask[i] < 0; ++i)
     /* search */;
 
-  assert(i != e && "VECTOR_SHUFFLE node with all undef indices!");
+  // If all elements are undefined, this shuffle can be considered a splat
+  // (although it should eventually get simplified away completely).
+  if (i == e)
+    return true;
 
   // Make sure all remaining elements are either undef or the same as the first
   // non-undef value.
diff --git a/test/CodeGen/AArch64/shuffle-mask-legal.ll b/test/CodeGen/AArch64/shuffle-mask-legal.ll
new file mode 100644
index 0000000..7aa7013
--- /dev/null
+++ b/test/CodeGen/AArch64/shuffle-mask-legal.ll
@@ -0,0 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=aarch64-- | FileCheck %s
+
+; A shuffle mask with all undef elements is always legal.
+
+define <4 x i32> @PR41535(<2 x i32> %p1, <2 x i32> %p2) {
+; CHECK-LABEL: PR41535:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ext v0.8b, v0.8b, v1.8b, #4
+; CHECK-NEXT:    mov v0.d[1], v0.d[0]
+; CHECK-NEXT:    ret
+  %cat1 = shufflevector <2 x i32> %p1, <2 x i32> undef, <4 x i32> <i32 undef, i32 1, i32 undef, i32 undef>
+  %cat2 = shufflevector <2 x i32> %p2, <2 x i32> undef, <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
+  %r = shufflevector <4 x i32> %cat1, <4 x i32> %cat2, <4 x i32> <i32 undef, i32 undef, i32 1, i32 4>
+  ret <4 x i32> %r
+}