[mirror_ubuntu-artful-kernel.git] / lib / gcd.c

#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) && !defined(CPU_NO_EFFICIENT_FFS)

/* If __ffs is available, the even/odd algorithm benchmarks slower. */
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);
Commit	Line	Data
d2829224 FF	1	#include <linux/kernel.h>
d2829224 FF	2	#include <linux/gcd.h>
8bc3bcc9	3	#include <linux/export.h>
d2829224	4
fff7fb0b ZZ	5	/*
	6	* This implements the binary GCD algorithm. (Often attributed to Stein,
	7	* but as Knuth has noted, appears in a first-century Chinese math text.)
	8	*
	9	* This is faster than the division-based algorithm even on x86, which
	10	* has decent hardware division.
	11	*/
	12
	13	#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) && !defined(CPU_NO_EFFICIENT_FFS)
	14
	15	/* If __ffs is available, the even/odd algorithm benchmarks slower. */
d2829224 FF	16	unsigned long gcd(unsigned long a, unsigned long b)
d2829224 FF	17	{
fff7fb0b ZZ	18	unsigned long r = a \| b;
	19
	20	if (!a \|\| !b)
	21	return r;
d2829224	22
fff7fb0b ZZ	23	b >>= __ffs(b);
	24	if (b == 1)
	25	return r & -r;
e9687567	26
fff7fb0b ZZ	27	for (;;) {
	28	a >>= __ffs(a);
	29	if (a == 1)
	30	return r & -r;
	31	if (a == b)
	32	return a << __ffs(r);
	33
	34	if (a < b)
	35	swap(a, b);
	36	a -= b;
d2829224	37	}
d2829224	38	}
fff7fb0b ZZ	39
	40	#else
	41
	42	/* If normalization is done by loops, the even/odd algorithm is a win. */
	43	unsigned long gcd(unsigned long a, unsigned long b)
	44	{
	45	unsigned long r = a \| b;
	46
	47	if (!a \|\| !b)
	48	return r;
	49
	50	/* Isolate lsbit of r */
	51	r &= -r;
	52
	53	while (!(b & r))
	54	b >>= 1;
	55	if (b == r)
	56	return r;
	57
	58	for (;;) {
	59	while (!(a & r))
	60	a >>= 1;
	61	if (a == r)
	62	return r;
	63	if (a == b)
	64	return a;
	65
	66	if (a < b)
	67	swap(a, b);
	68	a -= b;
	69	a >>= 1;
	70	if (a & r)
	71	a += b;
	72	a >>= 1;
	73	}
	74	}
	75
	76	#endif
	77
d2829224	78	EXPORT_SYMBOL_GPL(gcd);