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-rw-r--r--lib/gcd.c77
1 files changed, 67 insertions, 10 deletions
diff --git a/lib/gcd.c b/lib/gcd.c
index 3657f129d7b8..135ee6407a5e 100644
--- a/lib/gcd.c
+++ b/lib/gcd.c
@@ -2,20 +2,77 @@
#include <linux/gcd.h>
#include <linux/export.h>
-/* Greatest common divisor */
+/*
+ * 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) && !defined(CPU_NO_EFFICIENT_FFS)
+
+/* If __ffs is available, the even/odd algorithm benchmarks slower. */
unsigned long gcd(unsigned long a, unsigned long b)
{
- unsigned long r;
+ unsigned long r = a | b;
+
+ if (!a || !b)
+ return r;
- if (a < b)
- swap(a, b);
+ b >>= __ffs(b);
+ if (b == 1)
+ return r & -r;
- if (!b)
- return a;
- while ((r = a % b) != 0) {
- a = b;
- b = 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;
}
- return 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);