Why vectorization didn't turn on by -O2

Hongtao Liu crazylht@gmail.com
Wed Aug 4 08:21:05 GMT 2021


On Tue, May 18, 2021 at 4:27 AM Richard Sandiford via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> Jan Hubicka <hubicka@ucw.cz> writes:
> > Hi,
> > here are updated scores.
> > https://lnt.opensuse.org/db_default/v4/SPEC/latest_runs_report?younger_in_days=14&older_in_days=0&all_elf_detail_stats=on&min_percentage_change=0.001&revisions=9388fc7bf0da61a8104e8501e5965120e9159e12%2Cea21f32198432a490dd490696322838d94b3d3b2%2C4f5431c5768bbba81a422f6fed6a6e2454c700ee%2C&include_user_branches=on
> > compares
> >   base:  mainline
> >   1st column: mainline with very cheap vectorization at -O2 and -O3
> >   2nd column: mainline with cheap vectorization at -O2 and -O3.
> >
> > The short story is:
> >
> > 1) -O2 generic performance
> >     kabylake (Intel):
> >                               very    cheap
> >         SPEC/SPEC2006/FP/total        ~       8.32%
> >       SPEC/SPEC2006/total     -0.38%  4.74%
> >       SPEC/SPEC2006/INT/total -0.91%  -0.14%
> >
> >       SPEC/SPEC2017/INT/total 4.71%   7.11%
> >       SPEC/SPEC2017/total     2.22%   6.52%
> >       SPEC/SPEC2017/FP/total  0.34%   6.06%
> >     zen
> >         SPEC/SPEC2006/FP/total        0.61%   10.23%
> >       SPEC/SPEC2006/total     0.26%   6.27%
> >       SPEC/SPEC2006/INT/total 34.006  -0.24%  0.90%
> >
> >         SPEC/SPEC2017/INT/total       3.937   5.34%   7.80%
> >       SPEC/SPEC2017/total     3.02%   6.55%
> >       SPEC/SPEC2017/FP/total  1.26%   5.60%
> >
> >  2) -O2 size:
> >      -0.78% (very cheap) 6.51% (cheap) for spec2k2006
> >      -0.32% (very cheap) 6.75% (cheap) for spec2k2017
> >  3) build times:
> >      0%, 0.16%, 0.71%, 0.93% (very cheap) 6.05% 4.80% 6.75% 7.15% (cheap) for spec2k2006
> >      0.39% 0.57% 0.71%       (very cheap) 5.40% 6.23% 8.44%       (cheap) for spec2k2017
> >     here I simply copied data from different configuratoins
> >
> > So for SPEC i would say that most of compile time costs are derrived
> > from code size growth which is a problem with cheap model but not with
> > very cheap.  Very cheap indeed results in code size improvements and
> > compile time impact is probably somewhere around 0.5%
> >
> > So from these scores alone this would seem that vectorization makes
> > sense at -O2 with very cheap model to me (I am sure we have other
> > optimizations with worse benefits to compile time tradeoffs).
>
> Thanks for running these.
>
> The biggest issue I know of for enabling very-cheap at -O2 is:
>
>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100089
>
> Perhaps we could get around that by (hopefully temporarily) disabling
> BB SLP within loop vectorisation for the very-cheap model.  This would
> purely be a workaround and we should remove it once the PR is fixed.
> (It would even be a compile-time win in the meantime :-))
>
> Thanks,
> Richard
>
> > However there are usual arguments against:
> >
> >   1) Vectorizer being tuned for SPEC.  I think the only way to overcome
> >      that argument is to enable it by default :)
> >   2) Workloads improved are more of -Ofast type workloads
> >
> > Here are non-spec benchmarks we track:
> > https://lnt.opensuse.org/db_default/v4/CPP/latest_runs_report?younger_in_days=14&older_in_days=0&min_percentage_change=0.02&revisions=9388fc7bf0da61a8104e8501e5965120e9159e12%2Cea21f32198432a490dd490696322838d94b3d3b2%2C4f5431c5768bbba81a422f6fed6a6e2454c700ee%2C&include_user_branches=on
> >
> > I also tried to run Firefox some time ago. Results are not surprising -
> > vectorizaiton helps rendering benchmarks which are those compiler with
> > aggressive flags anyway.
> >
> > Honza

Hi:
  I would like to ask if we can turn on O2 vectorization now?


-- 
BR,
Hongtao


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