[Bug middle-end/64099] [5 Regression] ~15% runtime increase for fatigue.f90.

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64099

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Dominique d'Humieres from comment #2)
> > I don't see this on any of our testers.  What CPU do you have and what default
> > -march gets used for you?  (thus please show -v output)
> 
> My CPU is a 2.8 GHz Intel Core i7. All the versions reported in comment 0
> have been configured with
> 
> ../p_work/configure --prefix=/opt/gcc/gcc4.10p-#rev.p#patch
> --enable-languages=c,c++,lto,fortran,ada,objc,obj-c++ --with-gmp=/opt/mp
> --with-system-zlib --enable-checking=release --with-isl=/opt/mp --enable-lto
> --enable-plugin --with-arch=core2 --with-cpu=core2

Ok, so it is -march=core2

> but r216631 for which --enable-checking=release has been omitted. #rev. is
> the revision and #patch is the number of patches required to bootstrap.
> 
> > Btw, -flto should be redundant for a single-file benchmark - -fwhole-program
> > is enough.  
> 
> I know, however I have seen in the past some regressions when -flto is
> added. Since I can afford to double the compile time, I keep it in my
> reference options.
> 
> > Does -ftree-loop-linear make a difference for you?
> 
> AFAICT it does not on fatigue.f90, but I see some (minor) improvements for
> other tests in the suite.
> 
> > Our testers use -ffast-math -funroll-loops -O3.
> 
> Using '-O3 -ffast-math' instead of '-Ofast' almost double the runtime:

Ugh - -Ofast is an alias for -O3 -ffast-math.  Ah, no - it also enables
--param allow-store-data-races=1 ...

> [Book15] lin/test% gfortran -O3 -ffast-math -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 2.648u 0.002s 0:02.65 99.6%	0+0k 0+3io 38pf+0w
> [Book15] lin/test% gfortran -Ofast -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 1.385u 0.002s 0:01.38 100.0%	0+0k 0+1io 0pf+0w
> [Book15] lin/test% gfc -O3 -ffast-math -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 2.952u 0.002s 0:02.96 99.6%	0+0k 0+0io 40pf+0w
> [Book15] lin/test% gfc -Ofast -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 1.643u 0.001s 0:01.64 100.0%	0+0k 0+1io 0pf+0w
> 
> (gfortran is 4.9.2 and gfc is 5.0 r218134).
> 
> The runtime increase with '-O3 -ffast-math' is ~0.4s between r217816 and
> r217833
> 
> [Book15] lin/test% /opt/gcc/gcc4.10p-217816p2/bin/gfortran -O3 -ffast-math
> -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 2.654u 0.002s 0:02.66 99.6%	0+0k 0+1io 41pf+0w
> [Book15] lin/test% /opt/gcc/gcc4.10p-217833p1/bin/gfortran -O3 -ffast-math
> -fwhole-program fatigue.f90
> [Book15] lin/test% time a.out > /dev/null
> 2.962u 0.001s 0:02.97 99.6%	0+0k 0+1io 39pf+0w
> 
> > Can you bisect the regressions to a single commit?
> 
> I can do it for the range r217816-r217833 (the candidates are r217824 and
> r217827, may be r217828 also). As indicated by the p? in my coding scheme, I
> cannot bootstrap in the range r216631-r216747 without at least two patches,
> so bisecting this range will take much longer.
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From: "dominiq at lps dot ens.fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/64099] [5 Regression] ~15% runtime increase for fatigue.f90.
Date: Fri, 28 Nov 2014 13:22:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd099

--- Comment #4 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> Ugh - -Ofast is an alias for -O3 -ffast-math.  Ah, no - it also enables
> --param allow-store-data-races=1 ...

-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It
also enables optimizations that are not valid for all standard-compliant
programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens
and -fstack-arrays.

My guess is that -fstack-arrays makes the difference.