Features/ArchitectureSupport - changing what we build for

[Dominik 'Rathann' Mierzejewski]

On Tuesday, 03 February 2009 at 07:01, Gregory Maxwell wrote:
> 2009/2/2 King InuYasha <ngompa13 at gmail.com>:
> [snip]
> > Honestly, I think Fedora needs to stop
> > bottlenecking its performance by not optimizing its code for i586 or i686. I
> > would prefer the code to be optimized to i686 when built as RPMs.
> 
> -march != -mtune.  The code is already tuned for more modern CPUs.
> Allow me to repeat this: (loudly, not to be mean but because people
> seem to miss it) FEDORA x86 IS ALREADY OPTIMIZED FOR MODERN CPUS.
> 
> mtune should result in the overwhelming majority of the performance
> difference with none of the compatibility issues.  -march=i686 adds a
> single instruction of disputable value, cmov.

It is not of disputable value. While FFmpeg is not part of Fedora (it's
in RPMFusion), its H.264 decoder uses cmov to gain measurable speed
increase in decoding.

> In exchange for getting
> this one additional instruction it breaks compatibility with pentium,
> VIA C3 (Nehemiah has cmov, but eden does not; I have an eden600 board
> here which is only ~3 years old!) , AMD K6, etc and potential
> performance problems on Pentium 4.

Pentium 4 is one big performance problem. Nothing optimized (-mtune)
for P4 will run efficiently on everything else. IMHO we shouldn't care
too much for P4, because doing so will be at the cost of everyone else.

> Do you have benchmarks that show given a constant -mtune that
> -march=i686 makes a material difference for any significant userspace
> apps vs -march=i586,  if not why are you being so insistent and
> damning of the current compatible behavior?

Which applications do you suggest for testing this hypothesis?

Regards,
R.

-- 
Fedora http://fedoraproject.org/wiki/User:Rathann
RPMFusion http://rpmfusion.org | MPlayer http://mplayerhq.hu
"Faith manages."
        -- Delenn to Lennier in Babylon 5:"Confessions and Lamentations"