On Monday 18 September 2006 18:12, Benjamin LaHaise wrote:
> On Mon, Sep 18, 2006 at 05:29:23PM +0200, Andi Kleen wrote:
> > > - asm volatile("pushl %%ebp\n\t" \
> > > + asm volatile("pushfl\n\t" /* Save flags */ \
> > > + "pushl %%ebp\n\t" \
> >
> > We used to do that pushfl/popfl some time ago, but Ben removed it because
> > it was slow on P4. Ok, nobody thought of that case back then.
>
> It's the pushfl that will be slow on any OoO CPU, as it has dependancies on
> any previous instructions that modified the flags, which ends up bringing
> all of the memory ordering dependancies into play. Doing a popfl to set the
> flags to some known value is much less expensive.
Yes it's never fast, but on basically all non P4 CPUs it is still fast enough
to not be a problem. I suppose it causes a trace cache flush or something like
that there.
-Andi
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