[FFmpeg-devel] [PATCH] pixdesc: use 8-bit accesses when possible in av_read/write_image_line()

[Stefano Sabatini]

On date Sunday 2010-09-12 16:59:40 +0100, Mans Rullgard encoded:
> This fixes out of bounds accesses for big endian formats and should be
> a little faster.
> ---
>  libavutil/pixdesc.c |   20 ++++++++++++++++++++
>  1 files changed, 20 insertions(+), 0 deletions(-)
> 
> diff --git a/libavutil/pixdesc.c b/libavutil/pixdesc.c
> index e2bc649..41bf717 100644
> --- a/libavutil/pixdesc.c
> +++ b/libavutil/pixdesc.c
> @@ -54,6 +54,17 @@ void av_read_image_line(uint16_t *dst, const uint8_t *data[4], const int linesiz
>      } else {
>          const uint8_t *p = data[plane]+ y*linesize[plane] + x*step + comp.offset_plus1-1;
>  
> +        if (shift + depth <= 8) {
> +            p += !!(flags & PIX_FMT_BE);
> +            while(w--) {
> +                int val = *p;
> +                val = (val>>shift) & mask;
> +                if(read_pal_component)
> +                    val= data[1][4*val + c];
> +                p+= step;
> +                *dst++= val;
> +            }
> +        } else {
>          while(w--){
>              int val;
>              if(flags & PIX_FMT_BE) val= AV_RB16(p);
> @@ -64,6 +75,7 @@ void av_read_image_line(uint16_t *dst, const uint8_t *data[4], const int linesiz
>              p+= step;
>              *dst++= val;
>          }
> +        }
>      }
>  }

I'd like to avoid the code duplication.
What about something like this:

diff --git a/libavutil/pixdesc.c b/libavutil/pixdesc.c
index e2bc649..3496787 100644
--- a/libavutil/pixdesc.c
+++ b/libavutil/pixdesc.c
@@ -52,12 +52,16 @@ void av_read_image_line(uint16_t *dst, const uint8_t *data[4], const int linesiz
             *dst++= val;
         }
     } else {
+        int is_8bit = 0;
         const uint8_t *p = data[plane]+ y*linesize[plane] + x*step + comp.offset_plus1-1;
 
+        if (shift + depth <= 8) {
+            p += !!(flags & PIX_FMT_BE);
+            is_8bit = 1;
+        }
         while(w--){
             int val;
-            if(flags & PIX_FMT_BE) val= AV_RB16(p);
-            else                   val= AV_RL16(p);
+            val = is_8bit ? *p : flags & PIX_FMT_BE ? AV_RB16(p) : AV_RL16(p);
             val = (val>>shift) & mask;
             if(read_pal_component)
                 val= data[1][4*val + c];
?

Regards.
-- 
FFmpeg = Faithful & Fostering Monstrous Powerful Elegant Generator