[FFmpeg-devel] [PATCH] Fix 4XM decoding on big-endian and unaligned reads

[Reimar Döffinger]

On Thu, Nov 11, 2010 at 09:31:51PM +0100, Vitor Sessak wrote:
> Index: libavcodec/4xm.c
> ===================================================================
> --- libavcodec/4xm.c	(revision 25719)
> +++ libavcodec/4xm.c	(working copy)
> @@ -260,6 +260,21 @@
>      }
>  }
>  
> +#if HAVE_BIGENDIAN
> +#define LE_CENTRIC_MUL(dst, src, scale, dc) \
> +    { \
> +        unsigned tmpval = ((src)[1] << 16) + (src)[0];  \
> +        tmpval = tmpval * (scale) + (dc);               \
> +        (dst)[0] = tmpval & 0xFFFF;                     \
> +        (dst)[1] = tmpval >> 16;                        \
> +    }
> +#else
> +#define LE_CENTRIC_MUL(dst, src, scale, dc) \
> +    { \
> +        *((uint32_t *) (dst)) = AV_RL32(src) * (scale) + (dc); \
> +    }
> +#endif


Hmm.. Isn't this the same as
uint32_t tmp = AV_RN32(src);
#if HAVE_BIGENDIAN
tmp = (tmp << 16) | (tmp >> 16);
#endif
tmp = tmp * scale + dc;
#if HAVE_BIGENDIAN
tmp = (tmp << 16) | (tmp >> 16);
#endif
AV_RN32A(dst, tmp);

Note that the two things under #if should compile
into a single rotate instruction.
Which one is faster overall depends on whether unaligned
accesses are fast or not...