[DAG] Extend visitSCALAR_TO_VECTOR optimization to truncated vector.

Summary:
Allow SCALAR_TO_VECTOR of EXTRACT_VECTOR_ELT to reduce to
EXTRACT_SUBVECTOR of vector shuffle when output is smaller. Marginally
improves vector shuffle computations.

Reviewers: efriedma, RKSimon, spatel

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D35566

llvm-svn: 309680

1 files changed, 26 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 85055862917..5c9db976a42 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15710,23 +15710,38 @@ SDValue DAGCombiner::visitSCALAR_TO_VECTOR(SDNode *N) {
   EVT VT = N->getValueType(0);
 
   // Replace a SCALAR_TO_VECTOR(EXTRACT_VECTOR_ELT(V,C0)) pattern
-  // with a VECTOR_SHUFFLE.
+  // with a VECTOR_SHUFFLE and possible truncate.
   if (InVal.getOpcode() == ISD::EXTRACT_VECTOR_ELT) {
     SDValue InVec = InVal->getOperand(0);
     SDValue EltNo = InVal->getOperand(1);
-
-    // FIXME: We could support implicit truncation if the shuffle can be
-    // scaled to a smaller vector scalar type.
+    auto InVecT = InVec.getValueType();
     ConstantSDNode *C0 = dyn_cast<ConstantSDNode>(EltNo);
-    if (C0 && VT == InVec.getValueType() &&
-        VT.getScalarType() == InVal.getValueType()) {
-      SmallVector<int, 8> NewMask(VT.getVectorNumElements(), -1);
+
+    if (C0) {
+      SmallVector<int, 8> NewMask(InVecT.getVectorNumElements(), -1);
       int Elt = C0->getZExtValue();
       NewMask[0] = Elt;
-
-      if (TLI.isShuffleMaskLegal(NewMask, VT))
-        return DAG.getVectorShuffle(VT, SDLoc(N), InVec, DAG.getUNDEF(VT),
-                                    NewMask);
+      SDValue Val;
+      if (VT.getVectorNumElements() <= InVecT.getVectorNumElements() &&
+          TLI.isShuffleMaskLegal(NewMask, VT)) {
+        Val = DAG.getVectorShuffle(InVecT, SDLoc(N), InVec,
+                                   DAG.getUNDEF(InVecT), NewMask);
+        // If the initial vector is the correct size this shuffle is a
+        // valid result.
+        if (VT == InVecT)
+          return Val;
+        // If not we must truncate the vector.
+        if (VT.getVectorNumElements() != InVecT.getVectorNumElements()) {
+          MVT IdxTy = TLI.getVectorIdxTy(DAG.getDataLayout());
+          SDValue ZeroIdx = DAG.getConstant(0, SDLoc(N), IdxTy);
+          EVT SubVT =
+              EVT::getVectorVT(*DAG.getContext(), InVecT.getVectorElementType(),
+                               VT.getVectorNumElements());
+          Val = DAG.getNode(ISD::EXTRACT_SUBVECTOR, SDLoc(N), SubVT, Val,
+                            ZeroIdx);
+          return Val;
+        }
+      }
     }
   }