Re: [PATCH][AArch64] Cleanup memset

[Andrew Pinski <pinskia@gmail.com>]

On Tue, Mar 3, 2020 at 6:42 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
> Hi Siddhesh,
>
> > This looks OK in general, although can you please elaborate on the
> > following:
> >
> > - What cores did you test on to conclude that 160 is a better threshold
> > than 256?
>
> I've mostly done testing on Neoverse N1, Cortex-A72 and Cortex-A53.
> (the latter seems to be always faster with DC ZVA disabled, so the threshold
> doesn't really matter). I wrote a random memset benchmark similar to the
> memcpy one, and performance is unchanged there given there is no change
> in the way small cases are handled.
>
> > - Is the intention to support non-64-byte zva sizes once there is actual
> > hardware that implements it and not bother with it for now?  I agree
> > with the idea if that's the case, just that it would be nice to have
> > that documented in the git commit message.
>
> Yes, otherwise it's hard to test or prove it helps performance after all. We've
> had issues with the non-64 ZVA sizes before, so it's best to keep it simple.
>
> I'm also trying to reduce the amount of code and avoid unnecessary proliferation
> of almost identical ifuncs. I think we can remove most of the memset ifuncs,
> it seems we need one version without ZVA and a ZVA version for size 64.

I am trying to understand what was the decision here.  The main reason
is OcteonTX2 does ZVA 128 which was faster than doing one without
(OcteonTX1 is similar but has an errata which causes ZVA to be turned
off).

Thanks,
Andrew Pinski

>
> Cheers,
> Wilco