On Wednesday 14 September 2005 09:48, Zwane Mwaikambo wrote:
> On Wed, 14 Sep 2005, Chris White wrote:
>
> > That's correct, gcc 3.4 started the -mtune flag. Chances are if you really
> > want the -mtune optimizations you're going to have to upgrade to gcc 3.4 or
> > greater.
> >
> > > This, of course, heavily penalizes P4's (the notorious inc/dec).
> >
> > Are you referring to cpu cycle counts? Is there certain code that causes the
> > kernel to perform that unfavorably by a large scale?
>
> It's documented as being suboptimal to use inc/dec due to it modifying all
> of eflags resulting in dependency related stalls. add/sub only modifies
> one bit of eflags so is more optimal. However there is a problem of
?! add/sub doesn't modify "only one bit in eflags", it modifies all.
In fact, it's dec/inc which does not modify all bits.
It doesn't touch 'carry' bit (IIRC).
If inc/dec is slower on P4, it must be just another P4 quirk.
> increased code size with add/sub.
>
> But i've never benchmarked all of this ;)
I don't even have one to test this.
--
vda
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