[DAG] Improve legalization of INSERT_SUBVECTOR

When the index is known to be constant 0, insert directly into the the low half,
instead of spilling, performing the insert in-memory, and reloading.

Differential Revision: http://reviews.llvm.org/D20763

llvm-svn: 271428

1 files changed, 22 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
index fc487e25516..9ba68bf2be3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -851,15 +851,34 @@ void DAGTypeLegalizer::SplitVecRes_INSERT_SUBVECTOR(SDNode *N, SDValue &Lo,
   SDLoc dl(N);
   GetSplitVector(Vec, Lo, Hi);
 
-  // Spill the vector to the stack.
   EVT VecVT = Vec.getValueType();
-  EVT SubVecVT = VecVT.getVectorElementType();
+  EVT VecElemVT = VecVT.getVectorElementType();
+  unsigned VecElems = VecVT.getVectorNumElements();
+  unsigned SubElems = SubVec.getValueType().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
+  // the index is constant and there is no boundary crossing. But those cases
+  // don't seem to get hit in practice.
+  if (ConstantSDNode *ConstIdx = dyn_cast<ConstantSDNode>(Idx)) {
+    unsigned IdxVal = ConstIdx->getZExtValue();
+    if ((IdxVal == 0) && (IdxVal + SubElems <= VecElems / 2)) {
+      EVT LoVT, HiVT;
+      std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
+      Lo = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, LoVT, Lo, SubVec, Idx);
+      return;
+    }
+  }
+
+  // Spill the vector to the stack.
   SDValue StackPtr = DAG.CreateStackTemporary(VecVT);
   SDValue Store = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr,
                                MachinePointerInfo(), false, false, 0);
 
   // Store the new subvector into the specified index.
-  SDValue SubVecPtr = GetVectorElementPointer(StackPtr, SubVecVT, Idx);
+  SDValue SubVecPtr = GetVectorElementPointer(StackPtr, VecElemVT, Idx);
   Type *VecType = VecVT.getTypeForEVT(*DAG.getContext());
   unsigned Alignment = DAG.getDataLayout().getPrefTypeAlignment(VecType);
   Store = DAG.getStore(Store, dl, SubVec, SubVecPtr, MachinePointerInfo(),