[GISel][SDAG] Avoid push_back in loops for some shuffle mask handling. (llvm#119434)

Each call to push_back contains a check to see if the vector needs to
grow. Using resize or giving the size to the constructor can reduce
the number of checks for growing.

Author: topperc

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -6172,9 +6172,8 @@ LegalizerHelper::equalizeVectorShuffleLengths(MachineInstr &MI) {
   if (MaskNumElts < SrcNumElts) {
     // Extend mask to match new destination vector size with
     // undef values.
-    SmallVector<int, 16> NewMask(Mask);
-    for (unsigned I = MaskNumElts; I < SrcNumElts; ++I)
-      NewMask.push_back(-1);
+    SmallVector<int, 16> NewMask(SrcNumElts, -1);
+    llvm::copy(Mask, NewMask.begin());
 
     moreElementsVectorDst(MI, SrcTy, 0);
     MIRBuilder.setInstrAndDebugLoc(MI);
@@ -6254,16 +6253,14 @@ LegalizerHelper::moreElementsVectorShuffle(MachineInstr &MI,
   moreElementsVectorSrc(MI, MoreTy, 2);
 
   // Adjust mask based on new input vector length.
-  SmallVector<int, 16> NewMask;
+  SmallVector<int, 16> NewMask(WidenNumElts, -1);
   for (unsigned I = 0; I != NumElts; ++I) {
     int Idx = Mask[I];
     if (Idx < static_cast<int>(NumElts))
-      NewMask.push_back(Idx);
+      NewMask[I] = Idx;
     else
-      NewMask.push_back(Idx - NumElts + WidenNumElts);
+      NewMask[I] = Idx - NumElts + WidenNumElts;
   }
-  for (unsigned I = NumElts; I != WidenNumElts; ++I)
-    NewMask.push_back(-1);
   moreElementsVectorDst(MI, MoreTy, 0);
   MIRBuilder.setInstrAndDebugLoc(MI);
   MIRBuilder.buildShuffleVector(MI.getOperand(0).getReg(),

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -6421,16 +6421,14 @@ SDValue DAGTypeLegalizer::WidenVecRes_VECTOR_SHUFFLE(ShuffleVectorSDNode *N) {
   SDValue InOp2 = GetWidenedVector(N->getOperand(1));
 
   // Adjust mask based on new input vector length.
-  SmallVector<int, 16> NewMask;
+  SmallVector<int, 16> NewMask(WidenNumElts, -1);
   for (unsigned i = 0; i != NumElts; ++i) {
     int Idx = N->getMaskElt(i);
     if (Idx < (int)NumElts)
-      NewMask.push_back(Idx);
+      NewMask[i] = Idx;
     else
-      NewMask.push_back(Idx - NumElts + WidenNumElts);
+      NewMask[i] = Idx - NumElts + WidenNumElts;
   }
-  for (unsigned i = NumElts; i != WidenNumElts; ++i)
-    NewMask.push_back(-1);
   return DAG.getVectorShuffle(WidenVT, dl, InOp1, InOp2, NewMask);
 }
 
@@ -6478,12 +6476,8 @@ SDValue DAGTypeLegalizer::WidenVecRes_VECTOR_REVERSE(SDNode *N) {
 
   // Use VECTOR_SHUFFLE to combine new vector from 'ReverseVal' for
   // fixed-vectors.
-  SmallVector<int, 16> Mask;
-  for (unsigned i = 0; i != VTNumElts; ++i) {
-    Mask.push_back(IdxVal + i);
-  }
-  for (unsigned i = VTNumElts; i != WidenNumElts; ++i)
-    Mask.push_back(-1);
+  SmallVector<int, 16> Mask(WidenNumElts, -1);
+  std::iota(Mask.begin(), Mask.begin() + VTNumElts, IdxVal);
 
   return DAG.getVectorShuffle(WidenVT, dl, ReverseVal, DAG.getUNDEF(WidenVT),
                               Mask);