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? _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
