On Sun, 11 Oct 2015 09:59:39 -0400 Ganesh Ajjanagadde <gajja...@mit.edu> wrote:
> On Sun, Oct 11, 2015 at 9:34 AM, wm4 <nfx...@googlemail.com> wrote: > > On Sat, 10 Oct 2015 21:58:47 -0400 > > Ganesh Ajjanagadde <gajjanaga...@gmail.com> wrote: > > > >> This uses Stein's binary GCD algorithm: > >> https://en.wikipedia.org/wiki/Binary_GCD_algorithm > >> to get a roughly 4x speedup over Euclidean GCD on standard architectures > >> with a compiler intrinsic for ctzll, and a roughly 2x speedup otherwise. > >> At the moment, the compiler intrinsic is used on GCC and Clang due to > >> its easy availability. > >> > >> Quick note regarding overflow: yes, subtractions on int64_t can, but the > >> llabs takes care of that. The llabs is also guaranteed to be safe, with > >> no annoying INT64_MIN business since INT64_MIN being a power of 2, is > >> shifted down before being sent to llabs. > >> > >> The binary GCD needs ff_ctzll, an extension of ff_ctz for long long > >> (int64_t). On > >> GCC, this is provided by a built-in. On Microsoft, there is a > >> BitScanForward64 analog of BitScanForward that should work; but I can't > >> confirm. > >> Apparently it is not available on 32 bit builds; so this may or may not > >> work correctly. On Intel, per the documentation there is only an > >> intrinsic for _bit_scan_forward and people have posted on forums > >> regarding _bit_scan_forward64, but often their documentation is > >> woeful. Again, I don't have it, so I can't test. > >> > >> As such, to be safe, for now only the GCC/Clang intrinsic is added, the > >> rest > >> use a compiled version based on the De-Bruijn method of Leiserson et al: > >> http://supertech.csail.mit.edu/papers/debruijn.pdf. > >> > >> Tested with FATE, sample benchmark (x86-64, GCC 5.2.0, Haswell) > >> with a START_TIMER and STOP_TIMER in libavutil/rationsl.c, followed by a > >> make fate. > >> > >> aac-am00_88.err: > >> builtin: > >> 714 decicycles in av_gcd, 4095 runs, 1 skips > >> > >> de-bruijn: > >> 1440 decicycles in av_gcd, 4096 runs, 0 skips > >> > >> previous: > >> 2889 decicycles in av_gcd, 4096 runs, 0 skips > >> > >> Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> > >> --- > >> libavutil/intmath.h | 19 +++++++++++++++++++ > >> libavutil/mathematics.c | 26 +++++++++++++++++++++----- > >> 2 files changed, 40 insertions(+), 5 deletions(-) > >> > >> diff --git a/libavutil/intmath.h b/libavutil/intmath.h > >> index 08d54a6..b412385 100644 > >> --- a/libavutil/intmath.h > >> +++ b/libavutil/intmath.h > >> @@ -114,6 +114,9 @@ static av_always_inline av_const int > >> ff_log2_16bit_c(unsigned int v) > >> #ifndef ff_ctz > >> #define ff_ctz(v) __builtin_ctz(v) > >> #endif > >> +#ifndef ff_ctzll > >> +#define ff_ctzll(v) __builtin_ctzll(v) > >> +#endif > >> #endif > >> #endif > >> > >> @@ -158,6 +161,22 @@ static av_always_inline av_const int ff_ctz_c( int v ) > >> #endif > >> #endif > >> > >> +#ifndef ff_ctzll > >> +#define ff_ctzll ff_ctzll_c > >> +/* We use the De-Bruijn method outlined in: > >> + * http://supertech.csail.mit.edu/papers/debruijn.pdf. */ > >> +static av_always_inline av_const int ff_ctzll_c(long long v) > >> +{ > >> + static const int debruijn_ctz64[64] = { > >> + 0, 1, 2, 53, 3, 7, 54, 27, 4, 38, 41, 8, 34, 55, 48, 28, > >> + 62, 5, 39, 46, 44, 42, 22, 9, 24, 35, 59, 56, 49, 18, 29, 11, > >> + 63, 52, 6, 26, 37, 40, 33, 47, 61, 45, 43, 21, 23, 58, 17, 10, > >> + 51, 25, 36, 32, 60, 20, 57, 16, 50, 31, 19, 15, 30, 14, 13, 12 > >> + }; > >> + return debruijn_ctz64[(uint64_t)((v & -v) * 0x022FDD63CC95386D) >> > >> 58]; > >> +} > >> +#endif > >> + > > > > Is this duplicated from somewhere? > > It may be obtained from a number of sources (or generated onself from > the link I gave, which actually is an original source): > "The De Bruijn bitscan was devised in 1997, according to Donald Knuth > [3] by Martin Läuter, and independently by Charles Leiserson, Harald > Prokop and Keith H. Randall a few month later." (I use the Leiserson > et al reference in the code). > > It is not a unique sequence, there are many available on the web: > https://chessprogramming.wikispaces.com/BitScan, > https://gist.github.com/deffi420/e700f0adefc82f28c0d7 (the sequence I > randomly picked). I just thought I've seen this somewhere else in libav*, but I didn't find anything. Ignore me. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
