linux-stable/lib/gcd.c
Paul Burton d0894409d1
lib/gcd: Remove use of CPU_NO_EFFICIENT_FFS macro
The CPU_NO_EFFICIENT_FFS pre-processor macro is no longer used, with all
architectures toggling the equivalent Kconfig symbol
CONFIG_CPU_NO_EFFICIENT_FFS instead. Remove our check for the unused
macro.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21046/
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
2018-11-12 14:26:21 -08:00

84 lines
1.4 KiB
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)
/* 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);