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avutil/mathematics: speed up av_gcd by using Stein's binary GCD algorithm

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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 <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Ganesh Ajjanagadde 2015-10-10 21:58:47 -04:00 • committed by Michael Niedermayer
parent 1e7e4f13f9
commit 971d12b7f9
2 changed files with 40 additions and 5 deletions
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26
libavutil/mathematics.c
View file

@ -27,16 +27,32 @@
#include <limits.h>
#include "mathematics.h"
#include "libavutil/intmath.h"
#include "libavutil/common.h"
#include "avassert.h"
#include "version.h"
int64_t av_gcd(int64_t a, int64_t b)
{
if (b)
return av_gcd(b, a % b);
else
/* Stein's binary GCD algorithm:
* https://en.wikipedia.org/wiki/Binary_GCD_algorithm */
int64_t av_gcd(int64_t a, int64_t b) {
int za, zb, k;
int64_t u, v;
if (a == 0)
return b;
if (b == 0)
return a;
za = ff_ctzll(a);
zb = ff_ctzll(b);
k = FFMIN(za, zb);
u = llabs(a >> za);
v = llabs(b >> zb);
while (u != v) {
if (u > v)
FFSWAP(int64_t, v, u);
v -= u;
v >>= ff_ctzll(v);
}
return u << k;
}
int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd)