include/linux/reciprocal_div.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_RECIPROCAL_DIV_H
#define _LINUX_RECIPROCAL_DIV_H

#include <linux/types.h>

/*
 * This algorithm is based on the paper "Division by Invariant
 * Integers Using Multiplication" by Torbjörn Granlund and Peter
 * L. Montgomery.
 *
 * The assembler implementation from Agner Fog, which this code is
 * based on, can be found here:
 * http://www.agner.org/optimize/asmlib.zip
 *
 * This optimization for A/B is helpful if the divisor B is mostly
 * runtime invariant. The reciprocal of B is calculated in the
 * slow-path with reciprocal_value(). The fast-path can then just use
 * a much faster multiplication operation with a variable dividend A
 * to calculate the division A/B.
 */

struct reciprocal_value {
	u32 m;
	u8 sh1, sh2;
};

struct reciprocal_value reciprocal_value(u32 d);

static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
{
	u32 t = (u32)(((u64)a * R.m) >> 32);
	return (t + ((a - t) >> R.sh1)) >> R.sh2;
}

#endif /* _LINUX_RECIPROCAL_DIV_H */