[llvm] [LoopInterchange] Stop performing unprofitable interchange (PR #127473)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur requested changes to this pull request.

>From the Summary:

>  This patch fixes the problem and prevents the interchange from performing unprofitable transformations.

Please eloborate on how this PR does this. Due to refactorings in the diff I haven't yet found what the actual change is.

LoopInterchange should indeed not undo its own optimization, which indicates a problem with the cost model (which should be conservative, never apply an transformation we are not pretty sure it will improve performance; if we do we were not conservative enough). However, this is a greedy algorithm on neighboring loops, properties that need to take the entire loop nest into consideration is beyond this scope. Feel free to replace the greedy algorithm, but you cannot ensure global properties with just local information.

https://github.com/llvm/llvm-project/pull/127473