6dc6ca3302
The legality check in LoopInterchange allows two loops to be exchanged if all direction vectors are lexicographically positive (or zero) for both before and after the swap. The current implementation performs this routine naively. However, if a direction vector is lexicographically positive due to an element corresponding to a loop that is outside the given two loops (i.e., if there is an element `<` before the loops we are trying to interchange), then obviously it is also positive after exchanging them. For example, for a direction vector `[< < >]`, swapping the last two elements doesn't make it lexicographically negative because the first element is `<`. This patch adds a code to skip legality check if surrounding loops already guarantee that the direction vector is lexicographically positive. Note that this is only a small improvement on its own, but it's necessary to relax the legality check I'm working on. Split off from #118267 --------- Co-authored-by: Michael Kruse <llvm-project@meinersbur.de>
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