linux-stable/lib/math/gcd.c
Thomas Gleixner 457c899653 treewide: Add SPDX license identifier for missed files
Add SPDX license identifiers to all files which:

 - Have no license information of any form

 - Have EXPORT_.*_SYMBOL_GPL inside which was used in the
   initial scan/conversion to ignore the file

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

  GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2019-05-21 10:50:45 +02:00

85 lines
1.4 KiB
C

// SPDX-License-Identifier: GPL-2.0-only
#include <linux/kernel.h>
#include <linux/gcd.h>
#include <linux/export.h>
/*
* This implements the binary GCD algorithm. (Often attributed to Stein,
* but as Knuth has noted, appears in a first-century Chinese math text.)
*
* This is faster than the division-based algorithm even on x86, which
* has decent hardware division.
*/
#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
/* If __ffs is available, the even/odd algorithm benchmarks slower. */
/**
* gcd - calculate and return the greatest common divisor of 2 unsigned longs
* @a: first value
* @b: second value
*/
unsigned long gcd(unsigned long a, unsigned long b)
{
unsigned long r = a | b;
if (!a || !b)
return r;
b >>= __ffs(b);
if (b == 1)
return r & -r;
for (;;) {
a >>= __ffs(a);
if (a == 1)
return r & -r;
if (a == b)
return a << __ffs(r);
if (a < b)
swap(a, b);
a -= b;
}
}
#else
/* If normalization is done by loops, the even/odd algorithm is a win. */
unsigned long gcd(unsigned long a, unsigned long b)
{
unsigned long r = a | b;
if (!a || !b)
return r;
/* Isolate lsbit of r */
r &= -r;
while (!(b & r))
b >>= 1;
if (b == r)
return r;
for (;;) {
while (!(a & r))
a >>= 1;
if (a == r)
return r;
if (a == b)
return a;
if (a < b)
swap(a, b);
a -= b;
a >>= 1;
if (a & r)
a += b;
a >>= 1;
}
}
#endif
EXPORT_SYMBOL_GPL(gcd);