[FFmpeg-devel] [PATCH] Common ACELP routines (2/3) - filters

[Michael Niedermayer]

On Fri, Apr 25, 2008 at 08:22:15AM +0700, Vladimir Voroshilov wrote:
> On Fri, Apr 25, 2008 at 1:17 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Fri, Apr 25, 2008 at 12:07:15AM +0700, Vladimir Voroshilov wrote:
> >  > On Thu, Apr 24, 2008 at 10:13 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >  [...]
> >
> >
> > > >  > > > +        filter_data[10+n] = out[n] = sum;
> >  > >  > >
> >  > >  > > This duplicated storeage is unacceptable.
> >  > >  >
> >  > >  > First for all assigned to filter data values will be used in loop later.
> >  > >  > Thus filter_data can not be eliminated.
> >  > >  > I can't use "out" instead of it due to necessary 10 items
> >  > >  > with data from previous subframe at top).
> >  > >  > Extending out with 10 items at top will require another temporary buffer
> >  > >  > one memcpy somewhere later (because i will not be able to use output buffer
> >  > >  > directly).
> >  > >
> >  > >  The double write is definitly useless after the first 10 iterations as
> >  > >  after that you can just work in the out buffer.
> >  > >
> >  > >  foobar_filter(filter_data+10, 10);
> >  > >  memcpy(out, filter_data+10, 10);
> >  > >  foobar_filter(out+10, N-10);
> >  > >
> >  > >  should work fine and will for large N (dunno how large it is, so maybe
> >  > >  this isnt worth it ...) be faster. Also it allows filter_data to be smaller.
> >  >
> >  > ... and code will look like :(
> >  >
> >  > if(foobar_filter(filter_data+10, 10)!=OVERFLOW)
> >  > {
> >  >   memcpy(out, filter_data+10, 10);
> >  >   if(foobar_filter(out+10, N-10)==OVERFLOW)
> >  >   {
> >  >      for(i=0;i<len;i++) out>>=2;
> >  >      foobar_filter(filter_data+10, 10);
> >  >      memcpy(out, filter_data+10, 10);
> >  >      foobar_filter(out+10, N-10);
> >  >   }
> >  > }
> >  > else
> >  > {
> >  >      for(i=0;i<len;i++) out>>=2;
> >  >      foobar_filter(filter_data+10, 10);
> >  >      memcpy(out, filter_data+10, 10);
> >  >      foobar_filter(out+10, N-10);
> >  > }
> >
> >  for(;;){
> >     overflow= foobar_filter(filter_data+10, 10);
> >
> >     memcpy(out, filter_data+10, 10);
> >     overflow|= foobar_filter(out+10, N-10);
> >     if(!overflow)
> >         break;
> >
> >     for(i=0;i<len;i++) out>>=2;
> >  }
> 
> This will change filter_data even if overflow occuried.
> Which cause wrong synthesis result on second iteration.
> Current code on overflow case just downscales
> excitation signal (without touching filter data).

well its a matter of adding if(!overflow)

Also note that a memcpy is likely faster than the one by one element writing
in the loop. So if you dislike spliting it in 2 like above, then a seperate
memcpy is definitly prefered over the double writing.


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

It is not what we do, but why we do it that matters.
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