Re: [Qemu-arm] [RFC PATCH v2 2/2] utils: Add prefetch for Thunderx platform

[Richard Henderson]

On 08/16/2016 04:45 PM, Vijay Kilari wrote:
> On Tue, Aug 16, 2016 at 11:32 PM, Richard Henderson <address@hidden> wrote:
> > On 08/16/2016 05:02 AM, address@hidden wrote:
> > > +static inline void prefetch_vector_loop(const VECTYPE *p, int index)
> > > +{
> > > +#if defined(__aarch64__)
> > > +    if (is_thunderx_pass2_cpu()) {
> > > +        /* Prefetch 4 cache lines ahead from index */
> > > +        VEC_PREFETCH(p, index + (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
> > > * 4));
> > > +    }
> > > +#endif
> > > +}
> >
> >
> > Oh come now.  This is even worse than before.  A function call protecting a
> > mere prefetch within the main body of an inner loop?
> >
> > Did you not understand what I was asking for?
>
> No, Could you please detail the problem?.

The thunderx check, *if it even needs to exist at all*, must happen outside the 
loop.  Preferably not more than once, at startup time.

I strongly suspect that you do not need any check at all.  That even for cpus 
which automatically detect the streaming loop, adding a prefetch will not hurt.

You should repeat your same benchmark, with and without the prefetch, on (1) an 
A57 or suchlike, and (2) an x86 of some variety.


r~