This uses Stein's binary GCD algorithm:
https://en.wikipedia.org/wiki/Binary_GCD_algorithm
to get a reported 1.7-2.1x speedup over Euclidean GCD on standard architectures.
Have not benchmarked, so I can't comment.
Note that there may be some room for optimization of the bit operations.
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 intrinsic is added, the rest
use a compiled version.
Tested with FATE.
Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com>
---
libavutil/intmath.h | 39 +++++++++++++++++++++++++++++++++++++++
libavutil/mathematics.c | 26 +++++++++++++++++++++-----
2 files changed, 60 insertions(+), 5 deletions(-)
diff --git a/libavutil/intmath.h b/libavutil/intmath.h
index 08d54a6..63657d9 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 +161,42 @@ static av_always_inline av_const int ff_ctz_c( int v )
#endif
#endif
+#ifndef ff_ctzll
+#define ff_ctzll ff_ctzll_c
+static av_always_inline av_const int ff_ctzll_c(long long v)
+{
+ long long c;
+
+ if (v & 0x1)
+ return 0;
+
+ c = 1;
+ if (!(v & 0xffffffff)) {
+ v >>= 32;
+ c += 32;
+ }
+ if (!(v & 0xffff)) {
+ v >>= 16;
+ c += 16;
+ }
+ if (!(v & 0xff)) {
+ v >>= 8;
+ c += 8;
+ }
+ if (!(v & 0xf)) {
+ v >>= 4;
+ c += 4;
+ }
+ if (!(v & 0x3)) {
+ v >>= 2;
+ c += 2;
+ }
+ c -= v & 0x1;
+
+ return c;
+}
+#endif
+
/**
* Trailing zero bit count.
*
diff --git a/libavutil/mathematics.c b/libavutil/mathematics.c
index 252794e..16e4eba 100644
--- a/libavutil/mathematics.c
+++ b/libavutil/mathematics.c
@@ -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)
--
2.6.1
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
