[FFmpeg-devel] [PATCH 2/2] ac3enc: add SIMD-optimized shifting functions for use with the fixed-point AC3 encoder.

[Michael Niedermayer]

On Sat, Mar 12, 2011 at 11:40:04AM +0000, M?ns Rullg?rd wrote:
> Ivan Kalvachev <ikalvachev at gmail.com> writes:
> 
> > On 3/12/11, M?ns Rullg?rd <mans at mansr.com> wrote:
> >> Justin Ruggles <justin.ruggles at gmail.com> writes:
> >>
> >>> ---
> >>>  libavcodec/ac3dsp.c         |   24 +++++++++++++++++++++
> >>>  libavcodec/ac3dsp.h         |   22 +++++++++++++++++++
> >>>  libavcodec/ac3enc_fixed.c   |   40 ++---------------------------------
> >>>  libavcodec/x86/ac3dsp.asm   |   48
> >>> +++++++++++++++++++++++++++++++++++++++++++
> >>>  libavcodec/x86/ac3dsp_mmx.c |   12 ++++++++++
> >>>  5 files changed, 109 insertions(+), 37 deletions(-)
> >>>
> >>>
> >>> diff --git a/libavcodec/ac3dsp.c b/libavcodec/ac3dsp.c
> >>> index da3a123..83f48a6 100644
> >>> --- a/libavcodec/ac3dsp.c
> >>> +++ b/libavcodec/ac3dsp.c
> >>> @@ -50,10 +50,34 @@ static int ac3_max_msb_abs_int16_c(const int16_t *src,
> >>> int len)
> >>>      return v;
> >>>  }
> >>>
> >>> +static void ac3_lshift_int16_c(int16_t *src, unsigned int len,
> >>> +                               unsigned int shift)
> >>> +{
> >>> +    int i;
> >>> +
> >>> +    if (shift > 0) {
> >>> +        for (i = 0; i < len; i++)
> >>> +            src[i] <<= shift;
> >>> +    }
> >>> +}
> >
> > You can shift 2 values at once if you read them as int32_t, shift and
> > then mask (masking could even be skipped if you are sure they will
> > never overflow).

These are signed values, you cannot skip the mask sadly


> 
> There's no point optimising the C version beyond comprehension.  We have
> the asm hooks for that.

We can keep the simple & understandable variant though i see no need for it.
Still if we can make the
C code 2x as fast we should, we dont have asm for every C function and every
cpu architecture.

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

Many that live deserve death. And some that die deserve life. Can you give
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