[llvm] [SLP] Make getSameOpcode support interchangeable instructions. (PR #133888)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> At EuroLLVM I spoke to multiple other individuals for whom the SLP vectorizer is the #1 source of toolchain instability.

@rnk: Thank you for raising this!

This is the case for us. We have an out of tree backend and we merge from upstream trunk llvm several times per day, and fairly often we see new problems with the SLP vectorizer.

When I report a problem it does get fixed quickly though so thumbs up for that, but that the issues appear in the first place is turning into a problem. Digging out a reproducer, and then converting it to something that reproduces on an in-tree target often takes an effort since the pass depends quite a bit on the target.

We have even raised the question internally if we should disable SLP vectorizer since the maintenance cost of dealing with the crashes is sticking out.

Of course bugs and problems will always occur but SLP vectorizer unfortunately stands out in a negative way for us due to the asserts we sometimes see after merging new patches.

https://github.com/llvm/llvm-project/pull/133888