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

[Michael Niedermayer]

On Wed, Mar 26, 2008 at 12:31:44AM +0000, 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 ive not considered this, but as the context is allocated via common
code it is sadly allocated by av_mallocz() which zeros it and thus forces
asisgnment to physical pages.

[...]

-- 
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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