[gentoo-amd64] Re: GCC 4.5.1 with FLTO and Graphite

[Duncan]

Frank Peters posted on Mon, 20 Sep 2010 21:49:33 -0400 as excerpted:

> After doing my periodical emerge update, I notice that gcc-4.5.1 is now
> unmasked and presumably stable.  I also notice that a couple of new
> options are possible with this new release, namely Graphite and FLTO
> optimizations.  (Actually, Graphite has been available for a while but I
> haven't been following gcc development too closely and only now do I
> become aware of it.)
>
> Does anyone have any comments on the use of Graphite and/or FLTO? Are
> they capable of significant improvements on a amd64 system?
>
> Supposedly FLTO will greatly increase file size and thus could be
> selectively enabled for only a few packages.  Would building gcc itself
> with FLTO be advisable?
>
> There are reports that many packages fail to build with FLTO:
> http://blog.flameeyes.eu/2010/04/25/tinderbox-getting-friendly-with-
gcc-4-5
>
> Since I have encountered little detailed discussion on this topic, I am
> posting my question to this list.

Two excerpts from $equery u gcc

 - - graphite    : Add support for the framework for loop optimizations 
based on a polyhedral intermediate representation

 - - lto         : Add support for link-time optimizations (unsupported, 
use at your own risk).

So at minimum, you'll need to enable the appropriate USE flags for those 
features.  (As you can see, I haven't.)  And, link-time-optimization isn't 
Gentoo supported at all -- probably partly as a result of flameeyes' 
testing.

Personally, I've been waiting for some general level documentation to come 
across my radar before I get involved with either of these.  I've been 
running 4.5 for some time now, without issue (except that the new version 
makes use of more instructions for certain later Intel chips, and I had a 
problem as a result in my 32-bit chroot, trying to build for my atom based 
netbook, until I dialed back the allowed hardware instructions to what was 
available on my older amd opteron based build machine, gcc 4.4 apparently 
didn't use the instructions at all, even if they were marked as 
available), but that's partly because I've been a bit conservative in 
enabling the newly available features such as graphite and link-time-
optimization.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman