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

[suyog sarda via llvm-commits]

As far as i understand this patch tries to vectorize the horizontal sum in
an unrolled loop in following manner :

vec0 = shuffle<p[0], p[1], p[2], p[3], p[4], p[5], p[6], p[7]>
vec1 = shuffle vec0 <p[4], p[5], p[6], p[7],undef, undef, undef, undef>
vec2 = add vec0, vec1                ---------> this will result in
<p[0]+p[4], p[1]+p[5], p[2]+p[6], p[3]+p[7], undef, undef, undef, undef>
vec3 = shuffle vec2 <p[2]+p[6], p[3]+p[7], undef, undef, undef, undef,
undef, undef>
vec4 = add vec2, vec3                ---------> this will result in
<p[0]+p[4]+p[2]+p[6], p[1]+p[5]+p[3]+p[7], undef, undef, undef, undef,
undef, undef>
vec5 = shuffle vec4<p[1]+p[5]+p[3]+p[7], undef, undef, undef, undef, undef,
undef, undef>
vec6 = add vec4, vec5                ---------> this will result in
<p[0]+p[4]+p[2]+p[6] +p[1]+p[5]+p[3]+p[7], undef, undef, undef, undef,
undef, undef, undef>
sum = extractelement vec6, 0

This was discussed earlier too (
https://marc.info/?l=llvm-dev&m=141106671810521&w=4)
in the similar manner and it was suggested to generate intrinsic for
horizontal sum so that it can be lowered to target specific code.

Also, does this patch takes care of floating point ops as described in
https://marc.info/?l=llvm-commits&m=141892087031143&w=3
I haven't checked the patch. Just pitching in with some relevant data in
the past.

Regards,
Suyog

On Tue, Oct 4, 2016 at 5:31 PM, Alexey Bataev <a.bataev at hotmail.com> wrote:

> ABataev updated this revision to Diff 73458.
> ABataev added a comment.
>
> Added a comment + updated
>
>
> https://reviews.llvm.org/D24796
>
> Files:
>   lib/Transforms/Vectorize/SLPVectorizer.cpp
>   test/Transforms/SLPVectorizer/X86/reduction_unrolled.ll
>   test/Transforms/SLPVectorizer/X86/scheduling.ll
>
>
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