Changing the default 32-bit x86 arch for Fedora 12 (#2)

[Gregory Maxwell]

On Tue, Jun 23, 2009 at 4:19 PM, Clemens Eisserer<linuxhippy at gmail.com> wrote:
>> 1) Optimizing for P4 is ... messy
>> 2) If you're using C2D, etc., you can already use the 64-bit distro.
> So why not stay with generic, where most users would benefit.
>
> Sure I could use 64-bit, as could all the others using 32-bit on
> 64-bit capable CPUs (I guess 50% of all fedora-x86 users).

Fedora x86_64 is the solution for good performance for those systems.
The difference between 32bit mode and 64bit mode dwarfs all the little
compiler tweaks we could discuss.

Optimizing for atom makes sense because it's the most modern hardware
which doesn't have a higher performing alternative than the 32bit
build.

Moreover, as an in-order core it atom should gain more from
optimization than most cpus and generally optimizations for atom are
harmless or even beneficial for other CPUs, while optimization for
highly out of order CPUs can be devastating for in-order cores. As you
can see in Bill's post upthread optimizing for atom is mildly
beneficial even to P4.

Amusingly, on my own code at least -mtune=atom produces significantly
faster code than -mtune=geode on my geode LX.

P4 is pretty much a lost cause. The move to i686 from i586 itself will
make P4 slower, while helping most everything else by about the same
margin that it hurt p4. Optimizing for P4 will probably hurt
everything, certainly atom.

Atom systems are frequently battery powered, so improvements there can
also to increased battery life.  P4, OTOH, already requires a locally
installed atomic power plant so energy isn't an issue there.

...