Re: [PATCH] Set AVX128_OPTIMAL for all avx targets.

[Hongtao Liu <crazylht at gmail dot com>]

On Tue, Nov 12, 2019 at 4:29 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Tue, Nov 12, 2019 at 9:19 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Tue, Nov 12, 2019 at 8:36 AM Hongtao Liu <crazylht@gmail.com> wrote:
> > >
> > > Hi:
> > >   This patch is about to set X86_TUNE_AVX128_OPTIMAL as default for
> > > all AVX target because we found there's still performance gap between
> > > 128-bit auto-vectorization and 256-bit auto-vectorization even with
> > > epilog vectorized.
> > >   The performance influence of setting avx128_optimal as default on
> > > SPEC2017 with option `-march=native -funroll-loops -Ofast -flto" on
> > > CLX is as bellow:
> > >
> > >     INT rate
> > >     500.perlbench_r         -0.32%
> > >     502.gcc_r                       -1.32%
> > >     505.mcf_r                       -0.12%
> > >     520.omnetpp_r                   -0.34%
> > >     523.xalancbmk_r         -0.65%
> > >     525.x264_r                      2.23%
> > >     531.deepsjeng_r         0.81%
> > >     541.leela_r                     -0.02%
> > >     548.exchange2_r         10.89%  ----------> big improvement
> > >     557.xz_r                        0.38%
> > >     geomean for intrate             1.10%
> > >
> > >     FP rate
> > >     503.bwaves_r                    1.41%
> > >     507.cactuBSSN_r         -0.14%
> > >     508.namd_r                      1.54%
> > >     510.parest_r                    -0.87%
> > >     511.povray_r                    0.28%
> > >     519.lbm_r                       0.32%
> > >     521.wrf_r                       -0.54%
> > >     526.blender_r                   0.59%
> > >     527.cam4_r                      -2.70%
> > >     538.imagick_r                   3.92%
> > >     544.nab_r                       0.59%
> > >     549.fotonik3d_r         -5.44%  -------------> regression
> > >     554.roms_r                      -2.34%
> > >     geomean for fprate              -0.28%
> > >
> > > The 10% improvement of 548.exchange_r is because there is 9-layer
> > > nested loop, and the loop count for innermost layer is small(enough
> > > for 128-bit vectorization, but not for 256-bit vectorization).
> > > Since loop count is not statically analyzed out, vectorizer will
> > > choose 256-bit vectorization which would never never be triggered. The
> > > vectorization of epilog will introduced some extra instructions,
> > > normally it will bring back some performance, but since it's 9-layer
> > > nested loop, costs of extra instructions will cover the gain.
> > >
> > > The 5.44% regression of 549.fotonik3d_r is because 256-bit
> > > vectorization is better than 128-bit vectorization. Generally when
> > > enabling 256-bit or 512-bit vectorization, there will be instruction
> > > clocksticks reduction also with frequency reduction. when frequency
> > > reduction is less than instructions clocksticks reduction, long vector
> > > width vectorization would be better than shorter one, otherwise the
> > > opposite. The regression of 549.fotonik3d_r is due to this, similar
> > > for 554.roms_r, 528.cam4_r, for those 3 benchmarks, 512-bit
> > > vectorization is best.
> > >
> > > Bootstrap and regression test on i386 is ok.
> > > Ok for trunk?
> >
> > I don't think 128_optimal does what you think it does.  If you want to
> > prefer 128bit AVX adjust the preference, but 128_optimal describes
> > a microarchitectural detail (AVX256 ops are split into two AVX128 ops)
> > and is _not_ intended for "tuning".
>
> So yes, it's poorly named.  A preparatory patch to clean this up
> (and maybe split it into TARGET_AVX256_SPLIT_REGS and TARGET_AVX128_OPTIMAL)
> would be nice.
>
> And I'm not convinced that a single SPEC benchmark is good enough to
> penaltize this for all users.  GCC isn't a benchmark compiler and GCC
> does exactly what you expect it to do - try FDO if you want to tell it more.
Yes, you're right, it's just benchmark result.
>
> Richard.
>
> > Richard.
> >
> > > Changelog
> > >     gcc/
> > >             * config/i386/i386-option.c (m_CORE_AVX): New macro.
> > >             * config/i386/x86-tune.def: Enable 128_optimal for avx and
> > >             replace m_SANDYBRIDGE | m_CORE_AVX2 with m_CORE_AVX.
> > >             * testsuite/gcc.target/i386/pr84413-1.c: Adjust testcase.
> > >             * testsuite/gcc.target/i386/pr84413-2.c: Ditto.
> > >             * testsuite/gcc.target/i386/pr84413-3.c: Ditto.
> > >             * testsuite/gcc.target/i386/pr70021.c: Ditto.
> > >             * testsuite/gcc.target/i386/pr90579.c: New test.
> > >
> > >
> > > --
> > > BR,
> > > Hongtao



-- 
BR,
Hongtao