79 lines
1.3 KiB
C
79 lines
1.3 KiB
C
#include <linux/kernel.h>
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#include <linux/gcd.h>
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#include <linux/export.h>
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/*
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* This implements the binary GCD algorithm. (Often attributed to Stein,
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* but as Knuth has noted, appears in a first-century Chinese math text.)
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*
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* This is faster than the division-based algorithm even on x86, which
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* has decent hardware division.
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*/
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#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) && !defined(CPU_NO_EFFICIENT_FFS)
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/* If __ffs is available, the even/odd algorithm benchmarks slower. */
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unsigned long gcd(unsigned long a, unsigned long b)
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{
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unsigned long r = a | b;
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if (!a || !b)
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return r;
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b >>= __ffs(b);
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if (b == 1)
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return r & -r;
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for (;;) {
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a >>= __ffs(a);
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if (a == 1)
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return r & -r;
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if (a == b)
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return a << __ffs(r);
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if (a < b)
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swap(a, b);
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a -= b;
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}
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}
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#else
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/* If normalization is done by loops, the even/odd algorithm is a win. */
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unsigned long gcd(unsigned long a, unsigned long b)
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{
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unsigned long r = a | b;
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if (!a || !b)
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return r;
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/* Isolate lsbit of r */
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r &= -r;
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while (!(b & r))
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b >>= 1;
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if (b == r)
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return r;
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for (;;) {
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while (!(a & r))
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a >>= 1;
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if (a == r)
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return r;
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if (a == b)
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return a;
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if (a < b)
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swap(a, b);
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a -= b;
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a >>= 1;
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if (a & r)
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a += b;
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a >>= 1;
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}
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}
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#endif
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EXPORT_SYMBOL_GPL(gcd);
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