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