// SPDX-License-Identifier: GPL-2.0-only
#include <linux/kernel.h>
#include <linux/gcd.h>
#include <linux/export.h>

/*
 * 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.
 */

DEFINE_STATIC_KEY_TRUE(efficient_ffs_key);

#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)

/* If __ffs is available, the even/odd algorithm benchmarks slower. */

static unsigned long binary_gcd(unsigned long a, unsigned long b)
{
        unsigned long r = a | b;

        b >>= __ffs(b);
        if (b == 1)
                return r & -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;
        }
}

#endif

/* If normalization is done by loops, the even/odd algorithm is a win. */

/**
 * gcd - calculate and return the greatest common divisor of 2 unsigned longs
 * @a: first value
 * @b: second value
 */
unsigned long gcd(unsigned long a, unsigned long b)
{
        unsigned long r = a | b;

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

#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
        if (static_branch_likely(&efficient_ffs_key))
                return binary_gcd(a, b);
#endif

        /* 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;
        }
}

EXPORT_SYMBOL_GPL(gcd);