-fomit-frame-pointer

dwilson · Tux's lil' helper Joined: 10 Feb 2003 Posts: 115

I have read that this optimization should be used. The problem is I haven't build anything on my system with it, and I am wondering if it is ok to just turn it on now and let it slowly permeate through as everything gets recompiled during updates.

Also, I am kind of curious as to what it actually does and if it really improves speed that much.

Jimbow · Posted: Fri Mar 28, 2003 3:53 am Post subject:

It is fine to turn it on and off whenever you want. The compiler uses up a register as a frame pointer in order to enable stack trace debugging. If you omit the frame pointer, it frees up this register (which makes programs run faster because they don't need to access RAM as much) but it means that you don't get stack traces when programs crash.
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bssteph · l33t Joined: 26 Feb 2003 Posts: 652 Location: Wisconsin

It should be perfectly safe to let work its way into your system, but if you wanted to be a bit zealous, you could 'emerge -e world' and rebuild -everything- (assuming you have the time of course)... I did it and never had a problem aside from forgetting about a couple ebuilds that were quirky with my USE flags in make.conf. It's a perfectly safe flag, in my experience.

The rule for fomit-frame-pointer that I've seen is if you're going to be developing and/or testing lots of code, you'll probably want the frame pointer so you can actually trace back any problems... but if you're mainly just general use, it's worth the performance gain to use the flag.

timbo · Posted: Fri Mar 28, 2003 9:38 pm Post subject:

bssteph

I've built my whole system with this flag cause it was recomended. Now reading the last couple of posts was this a waste of time as I don't care if a program crashes, not interested in finding out why.....

Should I remove this flag from any future emerges?

Regards
Tim

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michaelb · Posted: Fri Mar 28, 2003 9:45 pm Post subject:

Timbo,

If you do not want to do any traceback debugging, then you probably want to have the -fomit-frame-pointer flag in your C flags. This potentially breaks debugging, but opens up an extra register for the app at runtime.

Hope that clears it up for you

Michael

gilesjuk · Posted: Fri Mar 28, 2003 9:57 pm Post subject:

The rule is if you like to (and do) submit detailed bug reports then you should not use this flag. Otherwise use it.
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masseya · Posted: Fri Mar 28, 2003 10:47 pm Post subject:

gilesjuk · Posted: Fri Mar 28, 2003 11:01 pm Post subject:

masseya · Posted: Fri Mar 28, 2003 11:08 pm Post subject:

Yeah, kinda. However, the devs don't use -fomit-frame-pointer, so if you submit a bug report and you expect them to be able to reproduce the bug then you're guaranteed (to an extent) that your bug report will be unreliable in the case that the problem is with -fomit-frame-pointer. In general, you should try testing your setup with a set of minimal compilation flags before submitting a bug report. This helps devs a lot, but I can certainly see how it's not always possible.
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GojiraST · n00b Joined: 14 Mar 2003 Posts: 41 Location: Florida

Well... as a person who does software QA for a living, we usually test "retail" builds, that is, the binaries the customers will get. Granted most of what I test is Windows based (unfortunately), but most of the time only the developers use "debug" builds. One reason for this (and I can't speak to this being true in the case of this particular pointer), is that we have had bugs in the past that don't show up with the debug code in place, but do for the binaries that ship. If it becomes necessary, it's a simple matter to put the debug binaries in place.

I would think that if someone wanted to submit a bug, it should be relatively simple for them to re-emerge with the fomit flag turned off, if that's useful in tracking the bug.

And yes, I'm aware that the debug code that's placed in developmental builds is a mite different from the register reserved for traces in this case, but it's a similar concept. If most end users are likely to build with that flag in place for performance reasons, most non-developer testing should likely be done that way. I know I'm willing to set whatever flags and compile however is requested by the developer in the course of researching the bug, but I'm not usually going to do that before submitting it...

As a side note, I have learned more about how Linux systems work in the time I've been using Gentoo than in my previous 5+ years of Linux experience.

I taught a small class last year to co-workers on Linux installation and use based solely on the Gentoo Install and Desktop config docs.

As I get more proficient and confident, I'll do everything I can to contribute both here on the forums, and through bug-submission/research, and one day development

masseya · Posted: Sat Mar 29, 2003 2:23 pm Post subject:

It would be nice if the Gentoo team was large enough to have a QA department that could be soley dedicated to testing the ebuilds with as many different flag combinations as possible. However, that's probably a little bit out of their capabilities for the foreseeable future. As you said, it shouldn't be too bad to just try the ebuild with a set of reduced compiler optimizations before submitting a bug report.

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Narada · Guru Joined: 12 Dec 2002 Posts: 300 Location: London, UK

CFLAGS can be changed at anytime and will then become applicable to all future emerge compilations. The performance of -fomit-frame-pointer has received a good consensus from all who use it and is generally a good idea. If CFLAG changes are considerable and you wish to recompile all packages with the new CFLAGS use command below. Beware that this is time consuming and will generate many configuration files to sort through.

GojiraST · n00b Joined: 14 Mar 2003 Posts: 41 Location: Florida

yup. I won't submit one without baselining it against a debug compile (I've already inserted a commented out set of CFLAG, CXFLAG, etc lines in my make.conf). I'll probably post here to get more feedback while I'm researching/before I debug. I won't file on Bugzilla without full debug. Thanks for reminding me that this isn't the same as the projects I work on for money

pblinux · n00b Joined: 07 Jan 2006 Posts: 17

At the risk of waking a long-dead thread, I have a question:

nixnut · Posted: Mon Feb 06, 2006 5:50 pm Post subject:

Moved from Installing Gentoo to Portage & Programming.
Not an installation support question, so moved here.
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predatorfreak · Posted: Mon Mar 06, 2006 7:57 pm Post subject:

Even though I'm not a regular C developer, there is near no way I can recommend -fomit-frame-pointer because it can increase binary size by a good margine and really I can't notice the performance difference with or without it. I don't think the I/O bashing you go through with large applications is worth the one extra register gain from using the flag.
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nmbrthry · Posted: Tue Mar 07, 2006 3:30 am Post subject:

predatorfreak · Posted: Tue Mar 07, 2006 10:35 pm Post subject:

nmbrthry · Posted: Wed Mar 08, 2006 3:34 am Post subject:

predatorfreak · Posted: Wed Mar 08, 2006 5:37 am Post subject:

chtephan · Posted: Wed Mar 08, 2006 8:48 am Post subject:

Perhaps it has something to do with the fact the the frame pointer operations are one-byte commands while most basic register commands are larger on i386 (which doesn't mean they're slower). Comparing the assembler core side by side might give an explanation.

PrakashP · Posted: Wed Mar 08, 2006 1:31 pm Post subject:

IIRC to free the frame pointer, you need to add a bit of code - for every function doing this trick.

predatorfreak · Posted: Thu Mar 09, 2006 12:13 am Post subject:

Seppel · Tux's lil' helper Joined: 10 Nov 2002 Posts: 113

PrakashP · Posted: Thu Mar 09, 2006 8:15 am Post subject: