[PATCH] D81416: [LV][SLP] Interleave to expose ILP for small loops with scalar reductions.

[Aaron H Liu via Phabricator via llvm-commits]

AaronLiu added a comment.

> In that case, best understand why LV's cost model claims vectorizing the loop is not profitable, which you and SLP know it is; and ideally fix LV's cost model.
>  A crash due to forced vectorization sounds like a bug, which best be reported and/or fixed.
>  If cases with concrete "obstacles" are identified preventing LV from vectorizing a loop but allowing SLP to vectorize (part of) it, after LV interleaves the loop, such obstacles could potentially be used to (further) drive LV to interleave the loop.

Agree, ideally LV's cost model and its vectorization functionality should be improved in the future to be able to vectorize a lot more instructions. 
We see some applications keep being crashed, due to some changes in LV and probably being fixed later on, or because of its own weakness in some aspects. 
But all the above are beyond of this patch. 
Currently, LV and SLP complement each other, and there are cases that LV fails to vectorize (functionally not being able to do it) but SLP succeed.

> Hence the term "small loop" should be more specific; as in "vectorizer-min-trip-count" / "TinyTripCountVectorThreshold".

The "small or tiny" values are relative, and will keep on changing. In the situations we see, it is even more dynamic, the exact trip count is not known, but we know that it is relatively small.


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  https://reviews.llvm.org/D81416/new/

https://reviews.llvm.org/D81416