[PATCH] D67805: [LV] Allow vectorization of hot short trip count loops with epilog

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added a comment.

> Ideally we should be using loop size and expected perf gain from cost model in addition to loop hotness to make the decision but that would be much bigger change and can be a follow up step.

The original motivation to vectorize loops of tiny trip counts as if under opt-for-size, is not to save code size, nor is it related to how hot the loop is. It stems from the fact that LV's cost model currently focuses on the body of the vector loop only, disregarding overheads associated with runtime guards and code outside this body, under the assumption that the execution of the vector loop will dominate. If the trip count is tiny, this assumption no longer holds, and the overheads may outweigh the benefits, w/o LV's cost-model noticing, leading to slowdowns (which become more severe as the loop gets hotter). Under opt-for-size LV generates only the vector loop, with no runtime guards nor scalar epilog, so this assumption holds.

In order to vectorize loops of tiny trip count with a scalar epilogue, the cost model should be enhanced to account for both plus associated overheads.

Regarding hotness, that could help the cost model in general to compare the expected performance with that of the original loop, but not sure how in this case where the trip count is a known constant.


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https://reviews.llvm.org/D67805