[FFmpeg-devel] [PATCH] avcodec/aac_tablegen: speed up table initialization

[Rostislav Pehlivanov]

LGTM, but could you leave (just comment it out) the old code in there
so it's a little easier to follow?
>         //ff_aac_pow2sf_tab[i] = pow(2, (i - POW_SF2_ZERO) / 4.0);
>         //ff_aac_pow34sf_tab[i] = pow(ff_aac_pow2sf_tab[i], 3.0/4.0);

The accuracy increase is always nice.

On Thu, 2015-11-26 at 16:31 -0500, Ganesh Ajjanagadde wrote:
> This speeds up aac_tablegen to a ludicruous degree (~97%), i.e to the
> point
> where it can be argued that runtime initialization can always be done
> instead of
> hard-coded tables. The only cost is essentially a trivial increase in
> the stack size.
> 
> Even if one does not care about this, the patch also improves
> accuracy
> as detailed below.
> 
> Performance:
> Benchmark obtained by looping 10^4 times over ff_aac_tableinit.
> 
> Sample benchmark (x86-64, Haswell, GNU/Linux):
> old:
> 1295292 decicycles in ff_aac_tableinit,     512 runs,      0 skips
> 1275981 decicycles in ff_aac_tableinit,    1024 runs,      0 skips
> 1272932 decicycles in ff_aac_tableinit,    2048 runs,      0 skips
> 1262164 decicycles in ff_aac_tableinit,    4096 runs,      0 skips
> 1256720 decicycles in ff_aac_tableinit,    8192 runs,      0 skips
> 
> new:
> 25691 decicycles in ff_aac_tableinit,     505 runs,      7 skips
> 25130 decicycles in ff_aac_tableinit,    1016 runs,      8 skips
> 25973 decicycles in ff_aac_tableinit,    2036 runs,     12 skips
> 25911 decicycles in ff_aac_tableinit,    4078 runs,     18 skips
> 25816 decicycles in ff_aac_tableinit,    8154 runs,     38 skips
> 
> Accuracy:
> The previous code was resulting in needless loss of
> accuracy due to the pow being called in succession. As an
> illustration
> of this:
> ff_aac_pow34sf_tab[3]
> old : 0.000000000007598092294225
> new : 0.000000000007598091426864
> real: 0.000000000007598091778545
> 
> truncated to float
> old : 0.000000000007598092294225
> new : 0.000000000007598091426864
> real: 0.000000000007598091426864
> 
> showing that the old value was not correctly rounded. This affects a
> large number of elements of the array.
> 
> Patch tested with FATE.
> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavcodec/aac_tablegen.h | 38 ++++++++++++++++++++++++++++++++++++-
> -
>  1 file changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/aac_tablegen.h b/libavcodec/aac_tablegen.h
> index 8b223f9..255723b 100644
> --- a/libavcodec/aac_tablegen.h
> +++ b/libavcodec/aac_tablegen.h
> @@ -35,9 +35,43 @@ float ff_aac_pow34sf_tab[428];
>  av_cold void ff_aac_tableinit(void)
>  {
>      int i;
> +
> +    /* 2^(i/16) for 0 <= i <= 15 */
> +    const double exp2_lut[] = {
> +        1.00000000000000000000,
> +        1.04427378242741384032,
> +        1.09050773266525765921,
> +        1.13878863475669165370,
> +        1.18920711500272106672,
> +        1.24185781207348404859,
> +        1.29683955465100966593,
> +        1.35425554693689272830,
> +        1.41421356237309504880,
> +        1.47682614593949931139,
> +        1.54221082540794082361,
> +        1.61049033194925430818,
> +        1.68179283050742908606,
> +        1.75625216037329948311,
> +        1.83400808640934246349,
> +        1.91520656139714729387,
> +    };
> +    double t1 = 8.8817841970012523233890533447265625e-16; // 2^(-50)
> +    double t2 = 3.63797880709171295166015625e-12; // 2^(-38)
> +    int t1_inc_cur, t2_inc_cur;
> +    int t1_inc_prev = 0;
> +    int t2_inc_prev = 8;
> +
>      for (i = 0; i < 428; i++) {
> -        ff_aac_pow2sf_tab[i] = pow(2, (i - POW_SF2_ZERO) / 4.0);
> -        ff_aac_pow34sf_tab[i] = pow(ff_aac_pow2sf_tab[i], 3.0/4.0);
> +        t1_inc_cur = 4 * (i % 4);
> +        t2_inc_cur = (8 + 3*i) % 16;
> +        if (t1_inc_cur < t1_inc_prev)
> +            t1 *= 2;
> +        if (t2_inc_cur < t2_inc_prev)
> +            t2 *= 2;
> +        ff_aac_pow2sf_tab[i] = t1 * exp2_lut[t1_inc_cur];
> +        ff_aac_pow34sf_tab[i] = t2 * exp2_lut[t2_inc_cur];
> +        t1_inc_prev = t1_inc_cur;
> +        t2_inc_prev = t2_inc_cur;
>      }
>  }
>  #endif /* CONFIG_HARDCODED_TABLES */