Enabling GCC Graphite and LTO ?

painteru · Tux's lil' helper Joined: 23 Aug 2007 Posts: 103

Hi,
Surfing the Internet, I found something about graphite and lto gcc optimizations.
I really don't know what good for are these but ... aren't they called OPTIMIZATIONS ? .... so I can't stay away from them and I decided to compile my system using them.
I used this how to :
http://yuguangzhang.com/blog/enabling-gcc-graphite-and-lto-on-gentoo/

Then, searching for the right "n" in -flto=n, I found this article, too:
http://realnc.blogspot.com/2012/06/building-gentoo-linux-with-gcc-47-and.html

The question is:
Are all those CFLAGS needed
or it's just enough USE="...graphite lto..." ?

And also , is there any visible benefit using these optimizations?

Thiose howtos are for gcc 4.7
and now, there is gcc4.8.3 ~ in portage (not to mention 4.9 hardmasked)
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NeddySeagoon · Posted: Wed Jul 23, 2014 5:26 pm Post subject:

painteru,

LTO is known to be broken with gcc-4.8.x
Its better with gcc-4.9. Also, it needs huge amounts of RAM.
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freke · Posted: Wed Jul 23, 2014 7:53 pm Post subject:

ulenrich · Veteran Joined: 10 Oct 2010 Posts: 1483

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ulenrich · Veteran Joined: 10 Oct 2010 Posts: 1483

Randy Andy · Posted: Tue Jul 29, 2014 7:01 am Post subject:

Coming back to the painterus theme now.

Painteru,

in this thread you could find some more pitfalls and needful references regarding LTO:
https://forums.gentoo.org/viewtopic-t-954376-postdays-0-postorder-asc-highlight-lto-start-0.html

So you're be warned now... :wink:

Andy.
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HerrSchafer · Tux's lil' helper Joined: 18 May 2011 Posts: 139

This thread is little old, but I'm having hard times dealing with GRAPHITE and LTO since a long time.

I have followed that links at first post; my idea was to enable the "Holy Combo" at the Gentoo's installation. The installation was going fine, until the point I emerged the DE (lxde). I had about 30 broken packages, but then, things like libcanberra and pulseaudio start to fail, even when I linked them to no-lto.conf.
The GRAPHITE flags, never worked; I've commented that line and the whole thing could start. Any linker (bfd and gold) fails.

I swear, I'm not a ricer, but, as I'm not a dev-guy, somethings sound complicated to me.

I've read that GRAPHITE should be applied in a "per package" base; ok! What packages should I compile with it?

My goal? Overall performance in a daily use notebook, but far beyond that, to understand how the stuff works and how to properly set it, so I can get a better performance from old machines.

About LTO, should I use it globaly or per package?

My installation is now broken and I will begin from scratch (again).

Any help will be welcome. Thanks guys!
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ct85711 · Veteran Joined: 27 Sep 2005 Posts: 1791

Well I can say, you shouldn't need to start over to get your computer working. The best way to get your computer working, is first start with the safe settings and get everything setup with that; then work on optimizing everything afterwards. Now if you need help getting your computer working, we'll help you get everything sorted out. I'd start a separate thread on getting your computer working with the safe settings; do provide output emerge --info and any/all build.log (full output please, if needed, provide a link to a pastebin with the output). As long as the system is kept up to date, it should be too difficult to get everything working. https://wiki.gentoo.org/wiki/CFLAGS

Now optimizing your system, I like starting from a stable foundation and tweak everything one at a time from there. Last time I read, there was several packages that does not like graphite/LTO. How many that does work with it, I have no clue. Best way to start with, is per package, and do one package/set at a time and test the software to see if it works (it would be a good idea to test one package/set at a time for a couple days to ensure no unforeseen issues pop up).

HerrSchafer · Tux's lil' helper Joined: 18 May 2011 Posts: 139

Thanks, for helping!

ct85711 · Veteran Joined: 27 Sep 2005 Posts: 1791

by stable foundation, I meant with the cflags, you can easily have a stable system running stable branch or unstable branch. For me, I've been running straight unstable branch, and my system has been stable since I've started using Gentoo. I've only encountered 2 packages that crashed during run time, both of them didn't matter that it is stable branch or not (one, was actually in the stable branch, the other is firefox (issue with gstreamer-1). The point I am getting at, is that it doesn't matter which branch you use and still get a stable system (that doesn't crash).

Now as for your crashes, I'd suspect it's because one of a dependency is failing. The best way to do it, is do a emerge -e system then do emerge -e world so that you rebuild the entire system rather then any packages after the change. In effect you are rebuilding the system (but you don't have to do all the steps all over again). The first should rebuild the system's toolchain (that that is setup correctly), then the other rebuilds everything afterwards. Last I read, the graphite and LTO USE flags does not actually affect the system, unless you change the packages to use graphite and LTO. However, if you applied any disable-LTO or stuff, you may want to disable them till after you recompiled the system.

HerrSchafer · Tux's lil' helper Joined: 18 May 2011 Posts: 139