[PATCH] D24796: [SLPVectorizer] Fix for PR25748: reduction vectorization after loop unrolling.

[Michael Kuperstein via llvm-commits]

mkuper added a comment.

Hi Alexey,
Sorry for the slow response time, I'm intermittently on vacation.



> ABataev wrote in SLPVectorizer.cpp:1797
> It is required to handle the next kind of hor reduction:
> 
>   sum = a + b + c + ...
> 
> It comes from:
> 
>   int test(unsigned int *p) {
>     int sum = 0;
>     for (int i = 0; i < 8; i++)
>       sum += p[i];
>     return sum;
>   }

What I'm asking is, how do you even get trees of height 1?

> ABataev wrote in SLPVectorizer.cpp:4171
> Ok, will do it

The code below is really not entirely obvious (at least to me - admittedly, I'm not an expert on the SLP vectorizer...). 
Could you actually update the comment to explain what it does - that is, why you have that specific set of conditions - instead of just removing it?

> ABataev wrote in SLPVectorizer.cpp:4586
> Michael,
> I'm working on a general patch, that is able to vectorize your example (I have it already but need some time to improve it). But this patch is still required as a first step for the second patch. Later we can rework this code for better compatibility

Ok, that sounds reasonable.

https://reviews.llvm.org/D24796