Re: [PATCH 1/1] GCD: add binary GCD algorithm

[George Spelvin]

Here's a variant using the even-odd speedup:
(Feel free to add my S-o-b if you use it.)

/*
 * This implements Brent & Kung's "even-odd" variant of the binary GCD
 * algorithm.  (Often attributed to Stein, but as Knith has noted, appears
 * a first-century Chinese math text.)
 * 
 * First, find the common powers of 2 from a and b.  Here, they are not
 * divided by that common power, but rather we form a bitmap r which
 * covers the least-significant bit set in either.
 * Let s = (r+1)/2 be the common factor of 2.
 *
 * Then, shift down both a and b until both are odd multiples of s.
 * Assuming a > b, then gcd(a, b) = gcd(a-b, b).  But a-b is an even
 * multiple of s and we can divide it by two at least once.
 *
 * Further, either a-b or a+b is a multiple of 4s, so by choosing between
 * the two we can divde by 4.  This is done by (in the case that a > b):
 * 1. Compute (a - b) / 2
 * 2. If this is an odd multiple of s (AND with r is non-zero), then
 *    add b to make an even multiiple of s.  (This cannot overflow,
 *    as it equals (a + b) / 2, which must be less than max(a, b).)
 * 3. Divide by 2 one or more more times until we are left with an odd
 *    multiple of r.
 * 4. The result replaces a, and we go back to finding the larger.
 *
 * While the algorothm is equally correct without step2 and the first
 * divide by 2, the benefit of adding it it can be evaluated with a CMOV
 * rather than a full conditional branch.
 */
unsigned long bgcd2(unsigned long a, unsigned long b)
{
	unsigned long r = a | b;	/* To find common powers of 2 */

	if (!a || !b)
		return r;

	r ^= r-1;	/* Only the msbit of r (r/2 + 1) really matters */

	if (!(a & r))
		goto a_even;
	if (!(b & r))
		goto b_even;

	while (a != b) {
		/* (a & r) == (b & r) == (r >> 1) != 0 */
		if (a > b) {
			a = (a - b) >> 1;
			if (a & r)
				a += b;
a_even:                 do
				a >>= 1;
			while (!(a & r));
		} else {
			b = (b - a) >> 1;
			if (b & r)
				b += a;
b_even:                 do
				b >>= 1;
			while (!(b & r));
		}
	}
	return b;
}
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