[gcc 3.4] AMD's Recommended CFLAGS

gian · Apprentice Joined: 26 Jul 2004 Posts: 212 Location: Europe

I did get strange results using the --fast-math option with our home made circuit simulator .... (we use some lapack, blas and nag-like functions)

toofastforyahuh · Apprentice Joined: 18 May 2004 Posts: 165

Just to be fair I just ran another datapoint. Since I also built my box for video, it's interesting to see what kind of performance I get.
I compiled mjpegtools 3 times and tested each with kino.

This is realistic since I actually use kino to do this stuff and building a DVD with mpeg2enc takes basically all day, so any performance difference is noticeable and welcome.

I took a 30.33 second DV file and ran it through kino's export MPEG tool.
I set it to type 8 (DVD) and used the following flags:

toofastforyahuh · Apprentice Joined: 18 May 2004 Posts: 165

Aha! And a good case where it seems to backfire is gzip.
For me gzip is consistently faster with -O2 than with -O3 or my nitro flags.
So there you have it. No universal answer except: stay sane. Build your system sanely and safely, then only focus on the few programs you really care about and do some careful benchmark profiling. Don't add random CFLAGS just because they sound good. You'll only give the "Gentoo is Rice" crowd a laugh if it blows up on you.

barry · Apprentice Joined: 01 May 2002 Posts: 170 Location: UK

You should avoid -funroll-all-loops generally. The GCC documentation advises against using it. -funroll-loops has more chance of being beneficial, but shouldn't be used in CFLAGS because it can break packages and may slow down just as much software as it speeds up.

-O2 together with -fweb and -frename-registers is probably the safest bet.

toofastforyahuh · Apprentice Joined: 18 May 2004 Posts: 165

Check the benchmarks above. So far, for the programs I've cared about, I've seen only a positive trend with unrolling loops, and with -O3, so it's unlikely -O2 -frename-registers is better still, but you're welcome to try xmame and mjpegtools to see if you can rise above my nitro flags. Again, I don't use crazy flags for the whole system. I refuse to compromise stability. Benchmarks on specific programs, though, definitely point toward some gravy to be had. So far that's all I can call it, though---gravy. The real meal is in going amd64 in the first place.

barry · Apprentice Joined: 01 May 2002 Posts: 170 Location: UK

The only difference between -O2 and -O3 is the inclusion of -fweb, frename-registers and -finline-functions. -funroll-loops can certainly help speed up certain applications, so it's a good idea to use it on those, but definitely not system-wide. The same goes for -ffast-math.

zinion · Posted: Fri Dec 10, 2004 10:37 am Post subject:

What exactly can happen if I use them system-wide? Because I do it and my system runs really fine the last time...
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toofastforyahuh · Apprentice Joined: 18 May 2004 Posts: 165

Are you sure? How do you know? I could play tempest on xmame if I compiled it with -O3 on my old SGI but the vector coordinates and colors were corrupted! Just because you haven't had a failure yet doesn't mean you don't have one just waiting, lurking in the shadows.

Using wonky CFLAGS can lead to disaster if you aren't careful.

Besides, it isn't always a win. -O2 is sometimes better (as I just proved with gzip), and if you believe acovea sometimes -O1 with a few flags is better than anything else. If you just use some tricked out quirky flags blindly you're just gambling that all programs will 1. compile at all, 2. compile correctly, 3. work better. All programs do not behave the same.

It is much wiser to build a stable system with safer CFLAGS and then profile your usage.
It's not worth your time to build a 3% faster alsamixer and risk its stability, but it definitely is worth your time to understand how a bottleneck program behaves. For example, if you spend a day just waiting for mpeg2enc to finish encoding a DVD for you. A 7% speedup makes sense there because you really need it.

NismoC32 · Apprentice Joined: 07 Apr 2003 Posts: 214

What CFLAG should I use during Installastion og Gentoo -x86_64 ?
Having problem getting it installed.
It stops on "gettext" if i use stage 1 and "cracklib" on stage 2 and "ucl" on stage 3.

Dont't know if it has someting to do with CFLAG setting or other things though.

I have used this settings:
"-02 -march=athlon64 -ftrace -fprefetch-loop-arrays -pipe".

toofastforyahuh · Apprentice Joined: 18 May 2004 Posts: 165

Start with -02 -march=athlon64 -pipe and see if that helps.

superwutze · Posted: Fri Dec 10, 2004 5:40 pm Post subject:

i did a stage1 with 2004.3 and used this settings from start on:
CFLAGS="-O3 -march=opteron -funroll-loops -pipe -ftracer"
there was no problem and still there is none.
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borkdox · Tux's lil' helper Joined: 16 Jan 2004 Posts: 123

teilo · Posted: Sat Dec 11, 2004 6:47 pm Post subject:

NismoC32 · Apprentice Joined: 07 Apr 2003 Posts: 214

I had to remove all CFLAGS eccept -02 and -pipe and skip emerge sync during installation to get it to work.
After reboot I could put inn more flags, I now have:
-O2 -march=athlon64 -fweb -funroll-loops -pipe and it works OK.
I have synced the portage tree and emerge all new packages.

And I now have a smooth working fast speeding fine tuned monster file server...

Thanks guys...

cybrjackle · Apprentice Joined: 09 Jan 2003 Posts: 248 Location: USA

silverpig · Posted: Wed Feb 02, 2005 6:08 am Post subject:

-march=athlon64 -02 -pipe -fomit-frame-pointer

I did the install initially with -03 and almost everything worked. I couldn't compile firefox though. I changed to -02 and it emerged just fine.
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WuppieCat · n00b Joined: 17 Oct 2002 Posts: 38 Location: Cheshire, UK

hvengel · Guru Joined: 19 Sep 2004 Posts: 515

The AMD document also recommends the use of thier AMD Core Math Library (ACML). Just for giggles I tried emerge -pv acml and it is masked by ~amd64. I have some stuff that I run that I think might benifit from faster math libraries. Has anyone unmasked this? Does it help? Do you need to set a use flag?

ozbird · Apprentice Joined: 21 Oct 2003 Posts: 185

hvengel · Guru Joined: 19 Sep 2004 Posts: 515

I have one application, libpano12, that will seg fault with anything other than -O0. But everything else on my systems has been built with -O2 without problems. Other amd64 users have also reported the same problem with libpano12 on Gentoo and other distros. Just an FYI that the correct optimizations are more dependant on the specific piece of software than anything else.

Joffer · Posted: Sun Aug 07, 2005 10:24 pm Post subject:

Reading this tread have made me see I'm probably a lucky guy. What I mean is I've compiled my entire system with an insane amount of CFLAGS and it is _stable_

:

nxsty · Veteran Joined: 23 Jun 2004 Posts: 1556 Location: .se

lightvhawk0 · Guru Joined: 07 Nov 2003 Posts: 388

I think the reason why most people with crazy flags get away with them is because many packages filter out "Unstable" CFLAGS
also is there an overlay for the patched version of glibc
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revertex · l33t Joined: 23 Apr 2003 Posts: 806

@Joffer:

"--march=athlon64" doesn't include "mfpmath=sse"?

and why does ppl still add "-fomit-frame-pointer" to their cflags when use -O, -O2, -O3 or-Os?

I've found it in gcc onlinedocs page:

crazycat · Posted: Fri Oct 21, 2005 9:25 am Post subject:

-march=athlon64 uses -mfpmath=sse by default (mfpmath=i387 generally has better performance on my system) , -fomit-frame-pointer does nothing on amd64 and people who are using it just don't know much about cflags. I don't know exactly about -pipe but i think it prevents gcc to use temporary files and pipes instead.