Greg Kroah-Hartman
598cf27219
Link: https://lore.kernel.org/r/20260217200002.683975158@linuxfoundation.org Tested-by: Florian Fainelli <florian.fainelli@broadcom.com> Tested-by: Takeshi Ogasawara <takeshi.ogasawara@futuring-girl.com> Tested-by: Peter Schneider <pschneider1968@googlemail.com> Tested-by: Jon Hunter <jonathanh@nvidia.com> Tested-by: Salvatore Bonaccorso <carnil@debian.org> Tested-by: Brett A C Sheffield <bacs@librecast.net> Tested-by: Mark Brown <broonie@kernel.org> Tested-by: Luna Jernberg <droidbittin@gmail.com> Tested-by: Ronald Warsow <rwarsow@gmx.de> Tested-by: Justin M. Forbes <jforbes@fedoraproject.org> Tested-by: Ron Economos <re@w6rz.net> Tested-by: Miguel Ojeda <ojeda@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
89 lines
1.5 KiB
C
89 lines
1.5 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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#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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DEFINE_STATIC_KEY_TRUE(efficient_ffs_key);
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#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
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/* If __ffs is available, the even/odd algorithm benchmarks slower. */
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static unsigned long binary_gcd(unsigned long a, unsigned long b)
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{
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unsigned long r = a | b;
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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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#endif
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/* If normalization is done by loops, the even/odd algorithm is a win. */
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/**
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* gcd - calculate and return the greatest common divisor of 2 unsigned longs
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* @a: first value
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* @b: second value
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*/
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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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#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
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if (static_branch_likely(&efficient_ffs_key))
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return binary_gcd(a, b);
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#endif
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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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EXPORT_SYMBOL_GPL(gcd);
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