Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

[H. Peter Anvin]

On 09/16/2013 01:07 AM, Peter Zijlstra wrote:
> On Fri, Sep 13, 2013 at 11:18:21PM -0700, Andi Kleen wrote:
> > tip-bot for Peter Zijlstra <tipbot@xxxxxxxxx> writes:
> > > +
> > > +	at  = (struct pebs_record_nhm *)(unsigned long)ds->pebs_buffer_base;
> > > +	top = (struct pebs_record_nhm *)(unsigned long)ds->pebs_index;
> > >
> > >   	ds->pebs_index = ds->pebs_buffer_base;
> > >
> > > +	n = (top - at) / x86_pmu.pebs_record_size;
> >
> >
> > This adds a full slow division to the PEBS hot path.
> >
> > Does not seem like a improvement to me.
>
> There already was an implicit division there, and
> sizeof(pebs_record_hsw) = 176, can it really optimize that constant
> division?

gcc can optimize ANY constant division, if nothing else then by 
reciprocal multiplication.

	-hpa


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