[SDAG] refactor folds for scalar-to-vector; NFCI

Fix typos, add comments, improve variable names,
rearrange code, add early exits.

Author: rotateright

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -23504,51 +23504,54 @@ SDValue DAGCombiner::visitVECTOR_SHUFFLE(SDNode *N) {
 }
 
 SDValue DAGCombiner::visitSCALAR_TO_VECTOR(SDNode *N) {
-  SDValue InVal = N->getOperand(0);
+  SDValue Scalar = N->getOperand(0);
   EVT VT = N->getValueType(0);
+  if (!VT.isFixedLengthVector())
+    return SDValue();
 
   // Replace a SCALAR_TO_VECTOR(EXTRACT_VECTOR_ELT(V,C0)) pattern
   // with a VECTOR_SHUFFLE and possible truncate.
-  if (InVal.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
-      VT.isFixedLengthVector() &&
-      InVal->getOperand(0).getValueType().isFixedLengthVector()) {
-    SDValue InVec = InVal->getOperand(0);
-    SDValue EltNo = InVal->getOperand(1);
-    auto InVecT = InVec.getValueType();
-    if (ConstantSDNode *C0 = dyn_cast<ConstantSDNode>(EltNo)) {
-      SmallVector<int, 8> NewMask(InVecT.getVectorNumElements(), -1);
-      int Elt = C0->getZExtValue();
-      NewMask[0] = Elt;
-      // If we have an implict truncate do truncate here as long as it's legal.
-      // if it's not legal, this should
-      if (VT.getScalarType() != InVal.getValueType() &&
-          InVal.getValueType().isScalarInteger() &&
-          isTypeLegal(VT.getScalarType())) {
-        SDValue Val =
-            DAG.getNode(ISD::TRUNCATE, SDLoc(InVal), VT.getScalarType(), InVal);
-        return DAG.getNode(ISD::SCALAR_TO_VECTOR, SDLoc(N), VT, Val);
-      }
-      if (VT.getScalarType() == InVecT.getScalarType() &&
-          VT.getVectorNumElements() <= InVecT.getVectorNumElements()) {
-        SDValue LegalShuffle =
-          TLI.buildLegalVectorShuffle(InVecT, SDLoc(N), InVec,
-                                      DAG.getUNDEF(InVecT), NewMask, DAG);
-        if (LegalShuffle) {
-          // If the initial vector is the correct size this shuffle is a
-          // valid result.
-          if (VT == InVecT)
-            return LegalShuffle;
-          // If not we must truncate the vector.
-          if (VT.getVectorNumElements() != InVecT.getVectorNumElements()) {
-            SDValue ZeroIdx = DAG.getVectorIdxConstant(0, SDLoc(N));
-            EVT SubVT = EVT::getVectorVT(*DAG.getContext(),
-                                         InVecT.getVectorElementType(),
-                                         VT.getVectorNumElements());
-            return DAG.getNode(ISD::EXTRACT_SUBVECTOR, SDLoc(N), SubVT,
-                               LegalShuffle, ZeroIdx);
-          }
-        }
-      }
+  if (Scalar.getOpcode() != ISD::EXTRACT_VECTOR_ELT ||
+      !Scalar.getOperand(0).getValueType().isFixedLengthVector())
+    return SDValue();
+
+  // If we have an implicit truncate, truncate here if it is legal.
+  if (VT.getScalarType() != Scalar.getValueType() &&
+      Scalar.getValueType().isScalarInteger() &&
+      isTypeLegal(VT.getScalarType())) {
+    SDValue Val =
+        DAG.getNode(ISD::TRUNCATE, SDLoc(Scalar), VT.getScalarType(), Scalar);
+    return DAG.getNode(ISD::SCALAR_TO_VECTOR, SDLoc(N), VT, Val);
+  }
+
+  auto *ExtIndexC = dyn_cast<ConstantSDNode>(Scalar.getOperand(1));
+  if (!ExtIndexC)
+    return SDValue();
+
+  SDValue SrcVec = Scalar.getOperand(0);
+  EVT SrcVT = SrcVec.getValueType();
+  unsigned SrcNumElts = SrcVT.getVectorNumElements();
+  unsigned VTNumElts = VT.getVectorNumElements();
+  if (VT.getScalarType() == SrcVT.getScalarType() && VTNumElts <= SrcNumElts) {
+    // Create a shuffle equivalent for scalar-to-vector: {ExtIndex, -1, -1, ...}
+    SmallVector<int, 8> Mask(SrcNumElts, -1);
+    Mask[0] = ExtIndexC->getZExtValue();
+    SDValue LegalShuffle = TLI.buildLegalVectorShuffle(
+        SrcVT, SDLoc(N), SrcVec, DAG.getUNDEF(SrcVT), Mask, DAG);
+    if (!LegalShuffle)
+      return SDValue();
+
+    // If the initial vector is the same size, the shuffle is the result.
+    if (VT == SrcVT)
+      return LegalShuffle;
+
+    // If not, shorten the shuffled vector.
+    if (VTNumElts != SrcNumElts) {
+      SDValue ZeroIdx = DAG.getVectorIdxConstant(0, SDLoc(N));
+      EVT SubVT = EVT::getVectorVT(*DAG.getContext(),
+                                   SrcVT.getVectorElementType(), VTNumElts);
+      return DAG.getNode(ISD::EXTRACT_SUBVECTOR, SDLoc(N), SubVT, LegalShuffle,
+                         ZeroIdx);
     }
   }