| 1 | |
| 2 | #include <linux/kernel.h>
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| 3 | #include <linux/gcd.h>
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| 4 | #include <linux/export.h>
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| 5 |
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| 6 | |
| 7 | * This implements the binary GCD algorithm. (Often attributed to Stein,
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| 8 | * but as Knuth has noted, appears in a first-century Chinese math text.)
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| 9 | *
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| 10 | * This is faster than the division-based algorithm even on x86, which
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| 11 | * has decent hardware division.
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| 12 |
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| 13 |
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| 14 | DEFINE_STATIC_KEY_TRUE(efficient_ffs_key);
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| 15 |
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| 16 | #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
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| 17 |
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| 18 |
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| 19 |
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| 20 | static unsigned long binary_gcd(unsigned long a, unsigned long b)
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| 21 | {
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| 22 | unsigned long r = a | b;
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| 23 |
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| 24 | b >>= __ffs(b);
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| 25 | if (b == 1)
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| 26 | return r & -r;
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| 27 |
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| 28 | for (;;) {
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| 29 | a >>= __ffs(a);
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| 30 | if (a == 1)
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| 31 | return r & -r;
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| 32 | if (a == b)
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| 33 | return a << __ffs(r);
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| 34 |
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| 35 | if (a < b)
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| 36 | swap(a, b);
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| 37 | a -= b;
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| 38 | }
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| 39 | }
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| 40 |
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| 41 | #endif
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| 42 |
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| 43 |
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| 44 |
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| 45 | |
| 46 | * gcd - calculate and return the greatest common divisor of 2 unsigned longs
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| 47 | * @a: first value
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| 48 | * @b: second value
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| 49 |
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| 50 | unsigned long gcd(unsigned long a, unsigned long b)
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| 51 | {
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| 52 | unsigned long r = a | b;
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| 53 |
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| 54 | if (!a || !b)
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| 55 | return r;
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| 56 |
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| 57 | #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
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| 58 | if (static_branch_likely(&efficient_ffs_key))
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| 59 | return binary_gcd(a, b);
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| 60 | #endif
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| 61 |
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| 62 |
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| 63 | r &= -r;
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| 64 |
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| 65 | while (!(b & r))
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| 66 | b >>= 1;
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| 67 | if (b == r)
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| 68 | return r;
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| 69 |
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| 70 | for (;;) {
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| 71 | while (!(a & r))
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| 72 | a >>= 1;
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| 73 | if (a == r)
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| 74 | return r;
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| 75 | if (a == b)
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| 76 | return a;
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| 77 |
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| 78 | if (a < b)
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| 79 | swap(a, b);
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| 80 | a -= b;
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| 81 | a >>= 1;
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| 82 | if (a & r)
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| 83 | a += b;
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| 84 | a >>= 1;
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| 85 | }
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| 86 | }
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| 87 |
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| 88 | EXPORT_SYMBOL_GPL(gcd);
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