diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -1258,22 +1258,25 @@
   GetSplitVector(Vec, Lo, Hi);
 
   EVT VecVT = Vec.getValueType();
+  EVT LoVT = Lo.getValueType();
   unsigned VecElems = VecVT.getVectorNumElements();
   unsigned SubElems = SubVec.getValueType().getVectorNumElements();
+  unsigned LoElems = LoVT.getVectorNumElements();
 
-  // If we know the index is 0, and we know the subvector doesn't cross the
-  // boundary between the halves, we can avoid spilling the vector, and insert
-  // into the lower half of the split vector directly.
-  // TODO: The IdxVal == 0 constraint is artificial, we could do this whenever
-  // there is no boundary crossing. But those cases don't seem to get hit in
-  // practice.
+  // If we know the index is in the first half, and we know the subvector
+  // doesn't cross the boundary between the halves, we can avoid spilling the
+  // vector, and insert into the lower half of the split vector directly.
+  // Similarly if the subvector is fully in the high half.
   unsigned IdxVal = cast<ConstantSDNode>(Idx)->getZExtValue();
-  if ((IdxVal == 0) && (IdxVal + SubElems <= VecElems / 2)) {
-    EVT LoVT, HiVT;
-    std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
+  if (IdxVal >= 0 && (IdxVal + SubElems <= LoElems)) {
     Lo = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, LoVT, Lo, SubVec, Idx);
     return;
   }
+  if ((IdxVal >= LoElems) && (IdxVal + SubElems <= VecElems)) {
+    Hi = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, Hi.getValueType(), Hi, SubVec,
+                     DAG.getVectorIdxConstant(IdxVal - LoElems, dl));
+    return;
+  }
 
   // Spill the vector to the stack.
   // In cases where the vector is illegal it will be broken down into parts