[FFmpeg-cvslog] r18599 - trunk/libavcodec/xan.c

[Michael Niedermayer]

On Fri, Apr 17, 2009 at 10:07:31PM +0200, Reimar D?ffinger wrote:
> On Fri, Apr 17, 2009 at 10:05:27PM +0200, reimar wrote:
> > Author: reimar
> > Date: Fri Apr 17 22:05:27 2009
> > New Revision: 18599
> > 
> > Log:
> > Use sign_extend function instead of reimplementing it.
> > 
> > Modified:
> >    trunk/libavcodec/xan.c
> > 
> > Modified: trunk/libavcodec/xan.c
> > ==============================================================================
> > --- trunk/libavcodec/xan.c	Fri Apr 17 22:01:45 2009	(r18598)
> > +++ trunk/libavcodec/xan.c	Fri Apr 17 22:05:27 2009	(r18599)
> > @@ -333,16 +333,10 @@ static void xan_wc3_decode_frame(XanCont
> >              }
> >          } else {
> >              /* run-based motion compensation from last frame */
> > -            motion_x = *vector_segment >> 4;
> > -            motion_y = *vector_segment & 0xF;
> > +            motion_x = sign_extend(*vector_segment >> 4,  4);
> > +            motion_y = sign_extend(*vector_segment & 0xF, 4);
> 
> Someone forgot to document sign_extend, should I be doing the "& 0xF"
> or can I assume that any future optimized implementation will flush
> the upper bits, too?

the upper bits must be cleared

its possible to implement sign extend with 1 + and 1 xor if the shift
is constant, and this may be faster than the 2 shifts ...

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

While the State exists there can be no freedom; when there is freedom there
will be no State. -- Vladimir Lenin
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