[FFmpeg-devel] [PATCH] mpeg2: fix block_last_index when mismatch control modifies last coeff

[Michael Niedermayer]

On Mon, Jun 21, 2010 at 02:47:16AM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Mon, Jun 21, 2010 at 12:40:14AM +0100, M?ns Rullg?rd wrote:
> >> Michael Niedermayer <michaelni at gmx.at> writes:
> >> 
> >> > On Sun, Jun 20, 2010 at 11:41:32PM +0100, Mans Rullgard wrote:
> >> >> ---
> >> >>  libavcodec/mpeg12.c |    4 ++--
> >> >>  1 files changed, 2 insertions(+), 2 deletions(-)
> >> >> 
> >> >> diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
> >> >> index bc9ddcc..bd579ac 100644
> >> >> --- a/libavcodec/mpeg12.c
> >> >> +++ b/libavcodec/mpeg12.c
> >> >> @@ -939,7 +939,7 @@ end:
> >> >>      }
> >> >>      block[63] ^= (mismatch & 1);
> >> >>  
> >> >> -    s->block_last_index[n] = i;
> >> >> +    s->block_last_index[n] = block[63]? 63: i;
> >> >>      return 0;
> >> >>  }
> >> >>  
> >> >> @@ -1090,7 +1090,7 @@ static inline int mpeg2_decode_block_intra(MpegEncContext *s,
> >> >>      }
> >> >>      block[63]^= mismatch&1;
> >> >>  
> >> >> -    s->block_last_index[n] = i;
> >> >> +    s->block_last_index[n] = block[63]? 63: i;
> >> >>      return 0;
> >> >
> >> > is this fixing a bug?
> >> 
> >> The value of block_last_index is wrong otherwise.  We're trying to add
> >> a dc-only idct (and perhaps other long zero tails).  That requires a
> >> correct block_last_index value.
> >> 
> >> > because, if it makes no difference, then it would cause a speedloss
> >> > and possibly not small if this is a conditional branch that fails to be
> >> > predicted
> >> 
> >> Do you have a better suggestion?
> >
> > ignore it?
> 
> Then the optimisation is impossible. 

you are unable to ignore block[63] in the dc idct ?

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Let us carefully observe those good qualities wherein our enemies excel us
and endeavor to excel them, by avoiding what is faulty, and imitating what
is excellent in them. -- Plutarch
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