[llvm] [SLP][REVEC] Fix CommonMask is transformed into vector form but used outside finalize. (PR #120952)

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

> > Take the following code as an example.
> > ```
> >     if (Action) {
> >       Value *Vec = InVectors.front();
> >       if (InVectors.size() == 2) {
> >         Vec = createShuffle(Vec, InVectors.back(), CommonMask);
> >         InVectors.pop_back();
> >       } else {
> >         Vec = createShuffle(Vec, nullptr, CommonMask);
> >       }
> >       for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
> >         if (CommonMask[Idx] != PoisonMaskElem)
> >           CommonMask[Idx] = Idx;
> >       assert(VF > 0 &&
> >              "Expected vector length for the final value before action.");
> >       unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();
> >       if (VecVF < VF) {
> >         SmallVector<int> ResizeMask(VF, PoisonMaskElem);
> >         std::iota(ResizeMask.begin(), std::next(ResizeMask.begin(), VecVF), 0);
> >         Vec = createShuffle(Vec, nullptr, ResizeMask);
> >       }
> >       Action(Vec, CommonMask);
> >       InVectors.front() = Vec;
> >     }
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > How do you know `<4 x Ty>` from `unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();` is two `<2 x Ty>` (so VF is 2) instead of four `Ty` (so VF is 4)?
> 
> It should be considered as four `Ty`, we should operate on actual types here

If we have VL with two `<2 x Ty>`.
```
      Res = ShuffleBuilder.finalize(
          E->ReuseShuffleIndices, SubVectors, SubVectorsMask, E->Scalars.size(),
          [&](Value *&Vec, SmallVectorImpl<int> &Mask) {
            TryPackScalars(NonConstants, Mask, /*IsRootPoison=*/false);
            Vec = ShuffleBuilder.gather(NonConstants, Mask.size(), Vec);
          });
```
The VF it expects is 2. But `unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();` here is 4? And we compare `VecVF` (4) with the incoming `VF` (2)?

https://github.com/llvm/llvm-project/pull/120952