[Ffmpeg-devel] [PATCH]: Too much alignment assumed by H264 decoder

[David S. Miller]

From: Michael Niedermayer <michaelni at gmx.at>
Date: Mon, 26 Dec 2005 05:57:40 +0100

> >         for(y=0; y<4; y++){
> >             *(uint64_t*)s->current_picture.motion_val[list][b_xy + 0 + y*h->b_stride]= *(uint64_t*)h->mv_cache[list][scan8[0]+0 + 8*y];
> >             *(uint64_t*)s->current_picture.motion_val[list][b_xy + 2 + y*h->b_stride]= *(uint64_t*)h->mv_cache[list][scan8[0]+2 + 8*y];
> >         }
> > 
> > The problematic access is the h->mv_cache[] one, h->mv_cache itself
> > is 8-byte aligned, but the indexed value here is only 4-byte aligned.
> > 
> > Is the problem at least a little bit clearer to folks now? :-)
> 
> no it was always clear, fix b_stride so its a multiple of 2

b_stride is not involved in the indexing of h->mv_cache[],
and h->mv_cache[] is where the unaligned access is happening.

The accesses to s->current_picture.motion_val[] are just fine.

Any index into h->mv_cache[] which is a multiple of 2, but not
a multiple of 4, will not be 8-byte aligned.  The first trap
I get while decoding has us with scan8[0] == 12, so we end up
indexing 14 + 8*y, which is the problematic index type (a multiple
of 2 but not a multiple of 4).  The resulting address is 4-byte
but not 8-byte aligned, and we trap on that.