llvm-project

281028e3 - [LoopInterchange] Prevent from undoing its own transformation (#127473)

[LoopInterchange] Prevent from undoing its own transformation (#127473) LoopInterchange uses the bubble-sort fashion algorithm to sort the loops in a loop nest. For two loops `A` and `B`, it calls `isProfitable(A, B)` to determine whether these loops should be exchanged. The result of `isProfitable(A, B)` should be conservative, that is, it should return true only when we are sure exchanging `A` and `B` will improve performance. If it's not conservative enough, it's possible that a subsequent `isProfitable(B, A)` will also return true, in which case LoopInterchange will undo its previous transformation. To avoid such cases, `isProfitable(B, A)` must not return true if `isProfitable(A, B)` returned true in the past. However, the current implementation can be in such a situation. This patch resolves it by correcting the handling of two loops that have the same cache cost. This resolve the problem mentioned in https://github.com/llvm/llvm-project/pull/118267#issuecomment-2510759354.