[FFmpeg-devel] [PATCHv2 2/2] lavu/lfg: switch to Ziggurat algorithm for normal random number generation (WIP)

[Michael Niedermayer]

On Sat, Mar 12, 2016 at 08:58:41AM -0500, Ganesh Ajjanagadde wrote:
> Code taken from the Julia project, licensed under MIT:
> https://github.com/JuliaLang/julia/blob/master/base/random.jl, in turn
> derived from: "The Ziggurat Method for generating random variables" - Marsaglia and Tsang.
> 
> Paper and reference code: http://www.jstatsoft.org/v05/i08/. This is
> well known to be the fastest method empirically for generating normal random
> variables for a fixed PRNG source.
> 
> Note that there are a large number of implementations with
> various tunings, this was one of the simpler ones and also has a friendly
> license.
> 
> This results in ~ 3x speedup of random number generation:
> old:
>   15090 decicycles in av_bmg_get,       1 runs,      0 skips
>   13140 decicycles in av_bmg_get,       2 runs,      0 skips
>   10117 decicycles in av_bmg_get,       4 runs,      0 skips
>   [...]
>    2133 decicycles in av_bmg_get,  524268 runs,     20 skips=60.4x
>    2134 decicycles in av_bmg_get, 1048531 runs,     45 skips=61.3x
>    2135 decicycles in av_bmg_get, 2097061 runs,     91 skips=61.9x
> 
> new:
>    7650 decicycles in av_gaussian_get,       1 runs,      0 skips
>    5490 decicycles in av_gaussian_get,       2 runs,      0 skips
>    3982 decicycles in av_gaussian_get,       4 runs,      0 skips
>    [...]
>     812 decicycles in av_gaussian_get,  524281 runs,      7 skips
>     813 decicycles in av_gaussian_get, 1048563 runs,     13 skips
>     813 decicycles in av_gaussian_get, 2097131 runs,     21 skips
> 
> and accordingly a ~2% speedup in aac encoding (-march=native, Haswell, clang):
> 
> old:
> ffmpeg -f lavfi -i anoisesrc -t 300 -y sin_new.aac  5.30s user 0.02s system 99% cpu 5.322 total
> new:
> ffmpeg -f lavfi -i anoisesrc -t 300 -y sin_new.aac  5.16s user 0.03s system 99% cpu 5.198 total
> 
> Function added as av_gaussian_get with documentation, minor bumped.
> 
[...]

> +static inline double ziggurat(AVLFG *lfg)
> +{
> +    while (1) {

> +        uint64_t r = (((uint64_t)av_lfg_get(lfg) << 32) + av_lfg_get(lfg)) & 0x000fffffffffffff;

the order of Function evaluations is undefined here i think


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

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
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