[llvm] [LV] optimize VF for low TC, when tail-folding (PR #91253)

[David Green via llvm-commits]

davemgreen wrote:

Hi. One quick question - If there is a target that can vectorize naturally at, say v16i8, but has more trouble vectorizing at v8i8 or v4i8, then why should the vectorizer not be considering larger vector factors? So long as it's a single iteration, could the higher factor not be lower cost?

I doubt it should matter a lot, but MVE has 128bit vector lengths and smaller factors can be less natural for it. Does the cost models not already handle picking the best factor?

https://github.com/llvm/llvm-project/pull/91253