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

Tue Jul 7 07:11:12 PDT 2020

Ayal added a comment.

In D81416#2096034 <https://reviews.llvm.org/D81416#2096034>, @AaronLiu wrote:

> In D81416#2095961 <https://reviews.llvm.org/D81416#2095961>, @spatel wrote:
>
> > IIUC, we should add a test under test/Transforms/PhaseOrdering with -O2 to show the cooperative effect of the 2 vectorizers rather than a stand-alone SLP test.
> >  If you can push that test with full baseline CHECK lines and then apply this patch and show test diffs, that would make it much easier to tell what is intended with this patch.
>
>
> Thanks for the comment. This patch does not intend to change or test phase ordering. In this patch, we interleave for small loops with scalar reductions which cannot be vectorized by LV, and later on SLP captures the opportunities. Interleaving is done by LV, and vectorization is done by SLP.

LV *cannot* vectorize the loop, or will not do so? If LV can interleave the loop, it should be able to also/instead vectorize it, unless there are some other obstacles? Is this an issue of LV's cost-model being more conservative than SLP's? If so, would updating LV's cost-model be a (more) appropriate remedy, than convincing LV to unroll for SLP?
The term "small loops" may be confusing; it presumably relates to loops having small number of instructions or low ILP(?), rather than small trip-count.

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https://reviews.llvm.org/D81416