[VectorCombine] Use std::iota for shuffle mask construction

Author: laurenmchin

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -848,24 +848,17 @@ bool VectorCombine::foldVectorInsertToShuffle(Instruction &I) {
  // shufflevector requires the two input vectors to be the same width.
  // Elements beyond the bounds of SubVec within the widened vector are
  // undefined.
- SmallVector<int, 8> WidenMask;
- unsigned int i = 0;
- for (i = 0; i != SubVecNumElts; ++i)
- WidenMask.push_back(i);
- for (; i != VecNumElts; ++i)
- WidenMask.push_back(PoisonMaskElem);
+ SmallVector<int, 8> WidenMask(VecNumElts, PoisonMaskElem);
+ std::iota(WidenMask.begin(), WidenMask.begin() + SubVecNumElts, 0);
+ std::fill(WidenMask.begin() + SubVecNumElts, WidenMask.end(), PoisonMaskElem);
 
  auto *WidenShuffle = Builder.CreateShuffleVector(SubVec, WidenMask);
  Worklist.pushValue(WidenShuffle);
 
- SmallVector<int, 8> Mask;
- unsigned int j;
- for (i = 0; i != SubVecIdx; ++i)
- Mask.push_back(i);
- for (j = 0; j != SubVecNumElts; ++j)
- Mask.push_back(DstNumElts + j);
- for (i = SubVecIdx + SubVecNumElts; i != DstNumElts; ++i)
- Mask.push_back(i);
+ SmallVector<int, 8> Mask(DstNumElts);
+ std::iota(Mask.begin(), Mask.begin() + SubVecIdx, 0);
+ std::iota(Mask.begin() + SubVecIdx, Mask.begin() + SubVecIdx + SubVecNumElts, DstNumElts);
+ std::iota(Mask.begin() + SubVecIdx + SubVecNumElts, Mask.end(), SubVecIdx + SubVecNumElts);
 
  auto *Shuffle = Builder.CreateShuffleVector(Vec, WidenShuffle, Mask);
  replaceValue(I, *Shuffle);