Linux kernel mirror (for testing)
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linux
1#include <linux/kernel.h>
2#include <linux/gcd.h>
3#include <linux/export.h>
4
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. */
16unsigned long gcd(unsigned long a, unsigned long b)
17{
18 unsigned long r = a | b;
19
20 if (!a || !b)
21 return r;
22
23 b >>= __ffs(b);
24 if (b == 1)
25 return r & -r;
26
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;
37 }
38}
39
40#else
41
42/* If normalization is done by loops, the even/odd algorithm is a win. */
43unsigned 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
78EXPORT_SYMBOL_GPL(gcd);