Re: [FFmpeg-devel] [PATCH 1/7] lavc/me_cmp: R-V V pix_abs

["Rémi Denis-Courmont" <remi@remlab.net>]

Le keskiviikkona 7. helmikuuta 2024, 2.01.23 EET flow gg a écrit :
> I think in most cases it is like this, but specifically for this function,
> using Reduction only once would be slower.
> 
> The currently submitted version roughly takes:
> pix_abs_0_0_rvv_i32: 136.2
> 
> The version that uses Reduction only once takes:
> pix_abs_0_0_rvv_i32: 169.2

You're only using one vector and half a vector respectively, so the 
logarithmic time of the sum is relatively small.

But are you sure that it wouldn't be faster to process multiple rows and 
larger group multiplers?

> Here is the implementation of the version that uses it only once:
> 
> func ff_pix_abs16_temp_rvv, zve32x
>         vsetivli        zero, 16, e32, m4, ta, ma
>         vmv.v.i         v24, 0
>         vmv.s.x         v0, zero
> 1:
>         vsetvli         zero, zero, e8, m1, tu, ma
>         vle8.v          v4, (a1)
>         vle8.v          v12, (a2)
>         addi            a4, a4, -1
>         vwsubu.vv       v16, v4, v12
>         add             a1, a1, a3
>         vwsubu.vv       v20, v12, v4
>         vsetvli         zero, zero, e16, m2, tu, ma
>         vmax.vv         v16, v16, v20
>         add             a2, a2, a3
>         vwadd.wv        v24, v24, v16
>         bnez            a4, 1b
> 
>         vsetvli         zero, zero, e32, m4, ta, ma
>         vwredsumu.vs    v0, v24, v0
>         vmv.x.s         a0, v0
>         ret
> endfunc
> 
> Rémi Denis-Courmont <remi@remlab.net> 于2024年2月7日周三 00:58写道：
> 
> > Hi,
> > 
> > To sum a vector, you should only reduce once at the end of the function,
> > c.f.
> > how it's done in existing scalar products. Reduction instructions are
> > (intrinsically) slow.
> > 
> > --
> > Rémi Denis-Courmont
> > http://www.remlab.net/
> 
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-- 
雷米‧德尼-库尔蒙
http://www.remlab.net/



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