[clang] [llvm] [llvm][ARM] Emit MVE .arch_extension after .fpu directive if it does not include MVE features (PR #71545)

[David Green via cfe-commits]

https://github.com/davemgreen commented:

Hi. From what I can tell the logic looks OK. We add the archextension in places we expect to now?

It is generally considered best practice to not have clang test that run the entire pass pipeline. In this case it looks like it's trying to SLP vectorize the code to make sure MVE operations are produced?  The problem can be that decision like that can change, and it is better if the tests are more narrowly focussed than replying on the whole pipeline. You do loose end-to-end testing but it may be better to have a clang test that checks the IR generated is what is expected, and an assembly test to make sure the `.arch_extension mve.fp` is recognized and turns on the MVE instructions it should do.

https://github.com/llvm/llvm-project/pull/71545