Revert "remove dead code"

This reverts commit d1bab4d.

Author: Esan5

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -191,6 +191,7 @@ class SelectionDAGLegalize {
   SDValue ExpandExtractFromVectorThroughStack(SDValue Op);
   SDValue ExpandInsertToVectorThroughStack(SDValue Op);
   SDValue ExpandVectorBuildThroughStack(SDNode* Node);
+  SDValue ExpandConcatVectors(SDNode *Node);
 
   SDValue ExpandConstantFP(ConstantFPSDNode *CFP, bool UseCP);
   SDValue ExpandConstant(ConstantSDNode *CP);
@@ -1517,6 +1518,27 @@ SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {
                      BaseVecAlignment);
 }
 
+SDValue SelectionDAGLegalize::ExpandConcatVectors(SDNode *Node) {
+  assert(Node->getOpcode() == ISD::CONCAT_VECTORS && "Unexpected opcode!");
+  SDLoc DL(Node);
+  SmallVector<SDValue, 16> Ops;
+  unsigned NumOperands = Node->getNumOperands();
+  MVT VectorIdxType = TLI.getVectorIdxTy(DAG.getDataLayout());
+  EVT VectorValueType = Node->getOperand(0).getValueType();
+  unsigned NumSubElem = VectorValueType.getVectorNumElements();
+  EVT ElementValueType = TLI.getTypeToTransformTo(
+      *DAG.getContext(), VectorValueType.getVectorElementType());
+  for (unsigned I = 0; I < NumOperands; ++I) {
+    SDValue SubOp = Node->getOperand(I);
+    for (unsigned Idx = 0; Idx < NumSubElem; ++Idx) {
+      Ops.push_back(DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, ElementValueType,
+                                SubOp,
+                                DAG.getConstant(Idx, DL, VectorIdxType)));
+    }
+  }
+  return DAG.getBuildVector(Node->getValueType(0), DL, Ops);
+}
+
 SDValue SelectionDAGLegalize::ExpandVectorBuildThroughStack(SDNode* Node) {
   assert((Node->getOpcode() == ISD::BUILD_VECTOR ||
           Node->getOpcode() == ISD::CONCAT_VECTORS) &&
@@ -3375,7 +3397,12 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     Results.push_back(ExpandInsertToVectorThroughStack(SDValue(Node, 0)));
     break;
   case ISD::CONCAT_VECTORS:
-    Results.push_back(ExpandVectorBuildThroughStack(Node));
+    if (EVT VectorValueType = Node->getOperand(0).getValueType();
+        VectorValueType.isScalableVector() ||
+        TLI.isOperationExpand(ISD::EXTRACT_VECTOR_ELT, VectorValueType))
+      Results.push_back(ExpandVectorBuildThroughStack(Node));
+    else
+      Results.push_back(ExpandConcatVectors(Node));
     break;
   case ISD::SCALAR_TO_VECTOR:
     Results.push_back(ExpandSCALAR_TO_VECTOR(Node));