[FFmpeg-cvslog] r12593 - trunk/libavcodec/ac3dec.c

[Michael Niedermayer]

On Tue, Mar 25, 2008 at 08:49:41PM -0400, Justin Ruggles wrote:
> M?ns Rullg?rd wrote:
> > Justin Ruggles <justinruggles at bellsouth.net> writes:
> > 
> >> Uoti Urpala wrote:
> >>> On Tue, 2008-03-25 at 19:38 -0400, Justin Ruggles wrote:
> >>>> jbr wrote:
> >>>>> @@ -191,6 +194,7 @@ typedef struct {
> >>>>>      GetBitContext gbc;                      ///< bitstream reader
> >>>>>      AVRandomState dith_state;               ///< for dither generation
> >>>>>      AVCodecContext *avctx;                  ///< parent context
> >>>>> +    uint8_t input_buffer[AC3_MAX_FRAME_SIZE];   ///< temp buffer to prevent overread
> >>>>>  } AC3DecodeContext;
> >>>> Right after I applied this, it occurred to me that it might be better to
> >>>> allocate this with av_malloc() at decoder init depending on
> >>>> error_resiliance.  Does that sound like a good idea?
> >>> I haven't looked at the specific code in this case, but generally moving
> >>> a commonly used buffer out of a context struct could hurt performance.
> >>> The compiler must either generate extra indirection through the struct
> >>> or reserve another register to keep the location of the buffer. OTOH if
> >>> time is mostly spent elsewhere, or if accesses to be buffer are not
> >>> interleaved with code which would also need to access the struct, then
> >>> it might not matter.
> >> Thanks Uoti. This is definitely a commonly used buffer, so maybe it is
> >> not a big enough trade-off just to reduce memory footprint when running
> >> with zero error_resilience.  I will test the speed, but if what you say
> >> is true, it will probably be slower.
> > 
> > On most systems, the extra allocation won't matter (much), since
> > physical pages are allocated only when written to.
> 
> Indeed, this change had pretty much zero affect on performance on my system.
> 
> So does the attached look ok?
> 
> -Justin
> 

> Index: libavcodec/ac3dec.c
> ===================================================================
> --- libavcodec/ac3dec.c	(revision 12594)
> +++ libavcodec/ac3dec.c	(working copy)
> @@ -194,7 +194,7 @@
>      GetBitContext gbc;                      ///< bitstream reader
>      AVRandomState dith_state;               ///< for dither generation
>      AVCodecContext *avctx;                  ///< parent context
> -    uint8_t input_buffer[AC3_MAX_FRAME_SIZE];   ///< temp buffer to prevent overread
> +    uint8_t *input_buffer;                  ///< temp buffer to prevent overread
>  } AC3DecodeContext;
>  
>  /**
> @@ -294,6 +294,13 @@
>      }
>      s->downmixed = 1;
>  
> +    /* allocate context input buffer */
> +    if (avctx->error_resilience >= FF_ER_CAREFUL) {
> +        s->input_buffer = av_mallocz(AC3_MAX_FRAME_SIZE);
> +        if (!s->input_buffer)
> +            return AVERROR_NOMEM;
> +    }

You dont need the if() as you check it later anyway, also you do need a
av_free() somewhere.
and then a + FF_INPUT_BUFFER_PADDING_SIZE might be needed as well


> +
>      return 0;
>  }
>  
> @@ -1137,7 +1144,7 @@
>      int i, blk, ch, err;
>  
>      /* initialize the GetBitContext with the start of valid AC-3 Frame */
> -    if(avctx->error_resilience >= FF_ER_CAREFUL) {
> +    if(avctx->error_resilience >= FF_ER_CAREFUL && s->input_buffer) {

I do not think the error_resilience check here is needed anymore.

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

I count him braver who overcomes his desires than him who conquers his
enemies for the hardest victory is over self. -- Aristotle
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