[llvm] LCA: migrate from DA to LAA; fix cost computation (PR #107691)

[Sebastian Pop via llvm-commits]

sebpop wrote:

What's the motivation of this change?

> DependenceAnalysis, which hasn't been worked on for years
It's not because DA has not been worked on for several years that we need to rewrite the passes currently using DA.

> fixing a long-standing cost-computation issue, due to an underlying bug in DependenceAnalysis.

What's this "cost-computation issue" about?
Have you investigated how to properly fix this with DA distance vectors?
I'm pretty sure it's related to CacheAnalysis incorrectly reading the distance vectors.  

Instead of this patch we should do the opposite:
remove uses of LAA and move the loop vectorizer pass to use DA.
With DA the loop vectorizer would not depend on runtime checks to disambiguate the existence of data dependences.


https://github.com/llvm/llvm-project/pull/107691