Re: 2.6.13/14 x86 Makefile - Pentiums penalized ?

[Denis Vlasenko <vda@ilport.com.ua>]

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