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

Tue Jul 7 11:29:09 PDT 2020

Ayal added a comment.

In D81416#2136421 <https://reviews.llvm.org/D81416#2136421>, @AaronLiu wrote:

> In the application we try, LV refuse to vectorize due to not profitable, but if we force LV to vectorize and it will crash. Apparently there are some obstacles. There are cases that even if LV fails, SLP could succeed.

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.

> Yes, the term small loop is a little bit of confusing. For example a loop which has a small number of instructions but has a huge loop trip count, is the loop small or big? In our example, the loop trip count is small, and also the instruction number is small.

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

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D81416/new/

https://reviews.llvm.org/D81416