[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.5                       |4.4.6

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING
                 CC|                            |steven at gcc dot gnu.org

--- Comment #18 from Steven Bosscher <steven at gcc dot gnu.org> 2011-02-20 15:22:26 UTC ---
Hello Joost, could you please check if this is still a problem in GCC 4.6?

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #19 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-02-20 16:17:33 UTC ---
(In reply to comment #18)
> Hello Joost, could you please check if this is still a problem in GCC 4.6?

I think it still is a minor problem, but (without   -fschedule-insns) somewhat
less pronounced (the old hardware is gone, this might make a difference):

4.3 branch

> gfortran -O3 -march=native -funroll-loops  -ffast-math   -fschedule-insns test.f90 ; ./a.out 
Time for evaluation [s]:                        3.478
> gfortran -O3 -march=native -funroll-loops  -ffast-math   test.f90 ; ./a.out 
Time for evaluation [s]:                        4.367

4.5 branch

> gfortran -O3 -march=native -funroll-loops  -ffast-math   -fschedule-insns test.f90 ; ./a.out 
Time for evaluation [s]:                        4.839
> gfortran -O3 -march=native -funroll-loops  -ffast-math  test.f90 ; ./a.out 
Time for evaluation [s]:                        4.524

4.6 branch
> gfortran -O3 -march=native -funroll-loops  -ffast-math   -fschedule-insns test.f90 ; ./a.out 
Time for evaluation [s]:                        4.997
> gfortran -O3 -march=native -funroll-loops  -ffast-math   test.f90 ; ./a.out 
Time for evaluation [s]:                        4.547

FYI: -march=amdfam10 -mcx16 -msahf -mpopcnt -mabm
model name      : AMD Opteron(tm) Processor 6176 SE

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #20 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-02-20 16:28:00 UTC ---
additionally for trunk, lto/profile-use seem not to help:

> gfortran -O3 -march=native -funroll-loops  -ffast-math  -flto -fprofile-use test.f90 ; ./a.out 
Time for evaluation [s]:                        4.664

> gfortran -O3 -march=native -funroll-loops  -ffast-math   -fprofile-use test.f90 ; ./a.out 
Time for evaluation [s]:                        4.665

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #21 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-02-20 16:32:38 UTC ---
... however, the following works great:

> gfortran -O2 -march=native -funroll-loops  -ffast-math  -ftree-vectorize test.f90 ; ./a.out 
Time for evaluation [s]:                        2.700

(notice -O2 instead of -O3, -O2 is thus twice as fast as -O3)

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[bonzini at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #22 from Paolo Bonzini <bonzini at gnu dot org> 2011-02-21 07:55:35 UTC ---
What is the performance with 4.3 -O2?  A regression that is limited to -O3 is
(a bit) less important since -O3 is still a "mixing bag" of optimizations that
might or might not be proficient.

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #23 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-02-21 12:53:30 UTC ---
(In reply to comment #22)
> What is the performance with 4.3 -O2?  

4.3:
> gfortran -O2 -march=native -funroll-loops -ffast-math test.f90 ; ./a.out
Time for evaluation [s]:                        4.373

4.6:
>  gfortran -O2 -march=native -funroll-loops -ffast-math test.f90 ; ./a.out
Time for evaluation [s]:                        4.347

so, same performance. 

Given that vectorization only happens at -O3, it is an important optimization
level for numerical codes. Nevertheless, I would propose to remove the
regression tag, and instead refocus the bug on the what current trunk does at
-O3 vs -O2 -ftree-vectorize as noted in comment #21

> gfortran -O2 -march=native -funroll-loops  -ffast-math  -ftree-vectorize test.f90 ; ./a.out
Time for evaluation [s]:                        2.694

> gfortran -O3 -march=native -funroll-loops  -ffast-math  -ftree-vectorize test.f90 ; ./a.out
Time for evaluation [s]:                        4.536

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.6                       |4.4.7

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2011-02-20 19:01:16         |2011-09-09 19:01:16

--- Comment #24 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-09-09 19:06:50 UTC ---
checked again current trunk, the situation remains that -O2 is much faster than
-O3:

> gfortran -O2 -march=native -funroll-loops  -ffast-math  -ftree-vectorize pr38306.f90  ; ./a.out
Time for evaluation [s]:                        2.830

> gfortran -O3 -march=native -funroll-loops  -ffast-math  -ftree-vectorize pr38306.f90  ; ./a.out
Time for evaluation [s]:                        4.593

The issue is that at -O3 the subroutine PD2VAL is not vectorized, while it is
at -O2.

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #25 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-09-10 09:43:58 UTC ---
(In reply to comment #24)
> 
> The issue is that at -O3 the subroutine PD2VAL is not vectorized, while it is
> at -O2.

If you are interested in investigating why this is so by yourself, I would
suggest that you use the various -fdump- options to check what GCC is doing
differently between the two variants. 

1) Dump everything you can dump.

2) Then find the earliest optimization pass where they differ (you may even use
diff to make this faster).

3) Check subsequent dumps to see if that difference is actually what makes -O3
to not vectorize. (At this point you can play with -f* -fno-* to reduce the
differences further and isolate the trigger).

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #26 from Uros Bizjak <ubizjak at gmail dot com> 2011-09-10 12:31:23 UTC ---
At -O3, vectorizer says:

pr38306.f90:246: note: not vectorized: the size of group of strided accesses is
not a power of 2D.2258_518 = *c0_193(D)[D.2257_517];

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[Joost.VandeVondele at pci dot uzh.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

--- Comment #27 from Joost VandeVondele <Joost.VandeVondele at pci dot uzh.ch> 2011-09-13 07:59:06 UTC ---
(In reply to comment #25)
> 2) Then find the earliest optimization pass where they differ (you may even use
> diff to make this faster).

The first point where things differ for PD2VAL is 

pr38306_xxx.f90.057t.cunrolli

afterwards, everything seems fully different.

[Bug target/38306] [4.4/4.5/4.6/4.7 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WORKSFORME

--- Comment #28 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-16 12:38:17 UTC ---
Using original flags (-O3 -march=native -funroll-loops -ffast-math) on
a AMD Athlon(tm) 64 X2 (that's close enough to "Opteron") (same family 15):

4.2.4: 5.78s
4.3.6: 5.77s
4.4.6: 5.84s
4.5.3: 5.77s
4.6.2: 5.85s
trunk: 5.75s

seems to be a wash for me and I cannot reproduce even the originally
reported slowdown in 4.4.  There are very many different flag measurements
in this report which makes it unlikely that this bug will be ever
properly triaged (or even "fixed").

Note that in general we'd like to concentrate monitoring performance
for standard flags (thus, not including -fschedule-insns if not enabled
by default).  -Ofast -funroll-loops is a reasonable flag set, as we (still)
do not enable loop unrolling by default at -O3.

I'm closing this as worksforme.  Please try to not make too much of a mess
out of regression bugreports ;)

[Bug target/38306] [4.4/4.5/4.6 Regression] 15% slowdown w.r.t. 4.3 of computational kernel on some architectures

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.4                       |4.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38306