[gentoo-dev] Re: LTO use in the tree

[Ryan Hill]

On Tue, 22 Apr 2014 08:45:31 + (UTC)
Martin Vaeth  wrote:

> Ryan Hill  wrote:
> >
> > One thing I forgot to mention - LTO can also have detrimental effect on
> > certain architectures.  On some (eg. ppc), performance can actually
> > be degraded due to increased register pressure.
> 
> If this really is the case it is not the problem of LTO but
> of the optimizer: If the optimizer really produces *worse*
> code when he *can* see the full program instead of only parts of it,
> something is severely broken in the optimizer. Only decreasing the
> possibilities of the optimizer by removing LTO would be the wrong way
> to "solve" this problem.

Yes, this is a problem caused by aggressive inlining, and is being worked on
upstream[1].  I meant that currently released versions exhibit this behaviour.


[1] see for example http://gcc.gnu.org/ml/gcc-patches/2014-04/msg01098.html

-- 
Ryan Hillpsn: dirtyepic_sk
   gcc-porting/toolchain/wxwidgets @ gentoo.org

47C3 6D62 4864 0E49 8E9E  7F92 ED38 BD49 957A 8463


signature.asc
Description: PGP signature

[gentoo-dev] Re: LTO use in the tree

[Ryan Hill]

On Tue, 22 Apr 2014 11:10:19 -0700
Matt Turner  wrote:

> > One thing I forgot to mention - LTO can also have detrimental effect on
> > certain architectures.  On some (eg. ppc), performance can actually be
> > degraded due to increased register pressure.  On others like alpha it's
> > questionable if it'll even work at all...
> 
> Worked for me on alpha, at least for what I tried. It cut eix's binary
> from 2 to 1.3 MB as well.

Cool, thanks for the info.  I was going by the request we had back in 4.6 to
turn off LTO for alpha because an upstream developer mentioned it wasn't
expected to work.


-- 
Ryan Hillpsn: dirtyepic_sk
   gcc-porting/toolchain/wxwidgets @ gentoo.org

47C3 6D62 4864 0E49 8E9E  7F92 ED38 BD49 957A 8463


signature.asc
Description: PGP signature