[llvm] [SLP] Avoid crash in computeExtractCost (PR #93188)

[Björn Pettersson via llvm-commits]

bjope wrote:

Hi @alexey-bataev , do you think this patch looks OK despite not having any in-tree reproducer?

I could not really find any matching piece of code doing the slicing based on EltsPerVector like that, but I wonder a bit if the cost estimates here need to match some other piece of code used to emit the actual shuffles?

---------------

When I looked at this I also noticed some other things that looked suspect to me.

ShuffleCostEstimator::adjustExtracts is calculating `SliceSize = VL.size() / NumParts`. But afaict this isn't always an exact division. For example test/Transforms/SLPVectorizer/AArch64/vec3-reorder-reshuffle.ll will end up in a situation when divide 3 by 2. And then
```
   for (unsigned Part = 0; Part < NumParts; ++Part) {
      ArrayRef<int> SubMask = Mask.slice(Part * SliceSize, SliceSize);
      for (auto [I, V] : enumerate(VL.slice(Part * SliceSize, SliceSize))) {
```
won't cover for the full VL/mask. There are a couple of more places with similar code (e.g. BoUpSLP::tryToGatherExtractElements). It is not clear to me if this is a real problem. But the code seems to have some assumptions regarding how NumParts relates to VL.size() and Mask.size() etc. But it is not quite clear what those assumptions are.

https://github.com/llvm/llvm-project/pull/93188