[FFmpeg-devel] [PATCH] VP8 V simple loopfilter in MMX/MMX2/SSE2

[Ronald S. Bultje]

Hi,

On Thu, Jul 1, 2010 at 12:41 PM, Pascal Massimino
<pascal.massimino at gmail.com> wrote:
> On Thu, Jul 1, 2010 at 8:10 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> + ? ?mova ? ? ?m0, [pb_80]
> + ? ?pxor ? ? ?m2, m0
> + ? ?pxor ? ? ?m4, m0
> + ? ?psubsb ? ?m2, m4 ? ? ? ?; m2=p1-q1 (signed) backup for below
> + ? ?pand ? ? ?m3, [pb_FE]
> + ? ?psrlq ? ? m3, 1 ? ? ? ? ; m3=FFABS(p1-q1)/2, this can be used signed
>
> i think you can avoid loading pb_FE by re-using pb_80 as:
>
> + ? ?mova ? ? ?m0, [pb_80]
> + ? ?pxor ? ? ?m2, m0
> + ? ?pxor ? ? ?m4, m0
> + ? ?psubsb ? ?m2, m4 ? ? ? ?; m2=p1-q1 (signed) backup for below
> + ? ?psrlq ? ? m3, 1 ? ? ? ? ; m3=FFABS(p1-q1)/2, this can be used signed
> + ? pandn m0

That wouldn't work, The key thing here is that we're doing a
byte-based right-shift, but there's no such psrlb instruction. So
we're using psrlq (or any psrlX) after clearing the least significant
bit, to prevent overflows in the next (lower) byte if it was set.

> also: have you considered using something along the lines of :
>
> pxor m0,m0
> pavbg(m3, m0)
>
> for computing fabs(p1-q1)/2 ?

Then we lose the sign... How do we put it back in?

Ronald