> * Steven Rostedt (
rostedt@goodmis.org) wrote:
> >
> > On Fri, 8 Aug 2008, Linus Torvalds wrote:
> > >
> > >
> > > On Fri, 8 Aug 2008, Jeremy Fitzhardinge wrote:
> > > >
> > > > Steven Rostedt wrote:
> > > > > I wish we had a true 5 byte nop.
> > > >
> > > > 0x66 0x66 0x66 0x66 0x90
> > >
> > > I don't think so. Multiple redundant prefixes can be really expensive on
> > > some uarchs.
> > >
> > > A no-op that isn't cheap isn't a no-op at all, it's a slow-op.
> >
> >
> > A quick meaningless benchmark showed a slight perfomance hit.
> >
>
> Hi Steven,
>
> I tried to run my own tests to see if I could get to know if these
> numbers are actually meaningful at all. My results seems to show that
> there is not any significant difference between the various
> configurations, and actually that the only one tendency I see is that
> the 2-bytes jump offset 0x03 would be slightly faster than the 3/2 nop
> on Intel Xeon. But we would have to run these a bit more often to
> confirm that I guess.
>
> I am just trying to get a sense of whether we are really trying hard to
> optimize something worthless in practice, and to me it looks like it.
> But it could be the architecture I am using that brings these results.
>
> Mathieu
>
> Intel Xeon dual quad-core
> Intel(R) Xeon(R) CPU E5405 @ 2.00GHz
>
> 3/2 nop used :
> K8_NOP3 K8_NOP2
> #define K8_NOP2 ".byte 0x66,0x90\n"
> #define K8_NOP3 ".byte 0x66,0x66,0x90\n"
>
