Question - Who Is Using -ffast-math?

robnotts

Having been slapped down by one of the developers for reporting a bug where -ffast-math breaks a program... specifically games-sports/foobillard-3.0a and requesting that a new ebuild is generated with the -ffast-math flag filtered, I am curious to know how many people on AMD64 are using -ffast-math, and are they experiencing any problems, and do they notice any speed ups?

The reason I ask... I do a lot of video/sound work on this machine, and as most (now thankfully not all) software does not have AMD64 sse/sse2 optimisations I was using -ffast-math to gain some speed up. Apart from foobillard as listed above, I have not noticed any problems, and indeed seem to have a very stable machine.

Rob.
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Gentoo Athlon64 X2-4200+ on NForce 4 + Geforce 7600GT 2GB/750GB (Desktop)
+ MythTV (3xFreeview,1xFreesat HD) on 1080p
+ Soundblaster Audigy
Gentoo Turion64 X2 Geforce 6150 2GB/120GB (Laptop)

neuron · Veteran Joined: 28 May 2002 Posts: 2185

I seriously considered it, after reading an article (see http://forums.gentoo.org/viewtopic-t-332318.html ), but I dont have it on now.

I also made another thread a long time ago on the subject http://forums.gentoo.org/viewtopic-t-10535.html

robnotts · Posted: Sun Jul 03, 2005 12:44 pm Post subject:

An interesting article certainly... I think I'll stick with the -ffast-math, and just put into my /usr/local/portage directory anything that I need to change to filter out flags.

Rob.
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Gentoo Athlon64 X2-4200+ on NForce 4 + Geforce 7600GT 2GB/750GB (Desktop)
+ MythTV (3xFreeview,1xFreesat HD) on 1080p
+ Soundblaster Audigy
Gentoo Turion64 X2 Geforce 6150 2GB/120GB (Laptop)

crazycat · Posted: Sun Jul 03, 2005 1:16 pm Post subject:

A very interesting article on the topic: gentoo ont opteron with gcc 3.4.3
http://www.coyotegulch.com/products/acovea/aco5k8gcc34.html
You can see that -O2 is generally better than -O3 probably because -O3 uncludes -funroll-all-loops.
-funroll-all-loops looks like the worst optimization ever while -frename-registers and -fweb are quite usefull.
I also found that on -ffast-math while googling

neuron · Veteran Joined: 28 May 2002 Posts: 2185

the threads that benchmark linked to was very interesting, I especially liked this post:
http://gcc.gnu.org/ml/gcc/2004-03/msg01459.html

If it's good enough for people who "make a very nice
living from selling software to solve finite-difference Poison-Boltzmann
electrostatic calculations on regular grids, and molecular minimizations
using quasi-newtonian numerical optimizers" then hell, it's gotta be good enough for me

in fact I'm gonna recompile with

Earthwings · Posted: Sun Jul 03, 2005 2:17 pm Post subject:

Please don't create any bug reports if you're using it.
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crazycat · Posted: Sun Jul 03, 2005 3:06 pm Post subject:

I once posted a bug to gentoos bugzilla along with a link to the patch that fixes the problem http://bugs.gentoo.org/show_bug.cgi?id=87149 ,and it's still not fixed (3 months already). Since then I'd rather check/post on creators website than report a bug to gentoo's bugzilla.
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Birtz · Posted: Sun Jul 03, 2005 3:46 pm Post subject:

Interesting read neuron, thank you. I myself don't use the -ffast-math flag because I never did much FP intensive stuff myself. If I did, I would consider testing the overall speed versus efficiency, and make the conclusion for myself. That said, I follow directions set by Gentoo developers (and "vast majority" of GCC team developers). This way I don't step on toes and "in most cases" my bug-reports or objections are rendered serious

On the other hand, if the bug-report policy says that you shoud drop "those flags" and "test again" it is only few minutes more effort, no?

Regards
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thumper · Guru Joined: 05 Dec 2002 Posts: 303 Location: Venice FL

With the exception of a few packages that this breaks, I have been using this system wide without any obvious ill effects. (knock wood)

robnotts · Posted: Sun Jul 03, 2005 4:10 pm Post subject:

Well I must admit, I have no problem with taking out the -ffast-math flag for those packages that need it removing, the one so far, foobillard, but surely it only takes a few minutes effort to add the command into the ebuild, unless I miss understood something?

Anyway, never mind, I'll just carry on regardless... I seem to have very conservative CFLAGS with the exception of the -ffast-math flag anyway...

I'll get around to doing an emerge -Dvep world with something a bit more serious, I live the look of -fweb and -frename-registers, once I get the X2 processor and don't have to kill a day and a bit recompiling!

Rob.
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Gentoo Athlon64 X2-4200+ on NForce 4 + Geforce 7600GT 2GB/750GB (Desktop)
+ MythTV (3xFreeview,1xFreesat HD) on 1080p
+ Soundblaster Audigy
Gentoo Turion64 X2 Geforce 6150 2GB/120GB (Laptop)

Shapemaker · n00b Joined: 22 Aug 2004 Posts: 64 Location: Finland

Imago · Posted: Mon Jul 04, 2005 4:48 am Post subject: