linux/include/asm-generic/div64.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_GENERIC_DIV64_H
#define _ASM_GENERIC_DIV64_H
/*
 * Copyright (C) 2003 Bernardo Innocenti <[email protected]>
 * Based on former asm-ppc/div64.h and asm-m68knommu/div64.h
 *
 * Optimization for constant divisors on 32-bit machines:
 * Copyright (C) 2006-2015 Nicolas Pitre
 *
 * The semantics of do_div() is, in C++ notation, observing that the name
 * is a function-like macro and the n parameter has the semantics of a C++
 * reference:
 *
 * uint32_t do_div(uint64_t &n, uint32_t base)
 * {
 * 	uint32_t remainder = n % base;
 * 	n = n / base;
 * 	return remainder;
 * }
 *
 * NOTE: macro parameter n is evaluated multiple times,
 *       beware of side effects!
 */

#include <linux/types.h>
#include <linux/compiler.h>

#if BITS_PER_LONG == 64

/**
 * do_div - returns 2 values: calculate remainder and update new dividend
 * @n: uint64_t dividend (will be updated)
 * @base: uint32_t divisor
 *
 * Summary:
 * ``uint32_t remainder = n % base;``
 * ``n = n / base;``
 *
 * Return: (uint32_t)remainder
 *
 * NOTE: macro parameter @n is evaluated multiple times,
 * beware of side effects!
 */
#define do_div(n,base) …

#elif BITS_PER_LONG == 32

#include <linux/log2.h>

/*
 * If the divisor happens to be constant, we determine the appropriate
 * inverse at compile time to turn the division into a few inline
 * multiplications which ought to be much faster.
 *
 * (It is unfortunate that gcc doesn't perform all this internally.)
 */

#define __div64_const32 …

#ifndef __arch_xprod_64
/*
 * Default C implementation for __arch_xprod_64()
 *
 * Prototype: uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
 * Semantic:  retval = ((bias ? m : 0) + m * n) >> 64
 *
 * The product is a 128-bit value, scaled down to 64 bits.
 * Assuming constant propagation to optimize away unused conditional code.
 * Architectures may provide their own optimized assembly implementation.
 */
static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
{
	uint32_t m_lo = m;
	uint32_t m_hi = m >> 32;
	uint32_t n_lo = n;
	uint32_t n_hi = n >> 32;
	uint64_t res;
	uint32_t res_lo, res_hi, tmp;

	if (!bias) {
		res = ((uint64_t)m_lo * n_lo) >> 32;
	} else if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
		/* there can't be any overflow here */
		res = (m + (uint64_t)m_lo * n_lo) >> 32;
	} else {
		res = m + (uint64_t)m_lo * n_lo;
		res_lo = res >> 32;
		res_hi = (res_lo < m_hi);
		res = res_lo | ((uint64_t)res_hi << 32);
	}

	if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
		/* there can't be any overflow here */
		res += (uint64_t)m_lo * n_hi;
		res += (uint64_t)m_hi * n_lo;
		res >>= 32;
	} else {
		res += (uint64_t)m_lo * n_hi;
		tmp = res >> 32;
		res += (uint64_t)m_hi * n_lo;
		res_lo = res >> 32;
		res_hi = (res_lo < tmp);
		res = res_lo | ((uint64_t)res_hi << 32);
	}

	res += (uint64_t)m_hi * n_hi;

	return res;
}
#endif

#ifndef __div64_32
extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
#endif

/* The unnecessary pointer compare is there
 * to check for type safety (n must be 64bit)
 */
#define do_div …

#else /* BITS_PER_LONG == ?? */

# error do_div() does not yet support the C64

#endif /* BITS_PER_LONG */

#endif /* _ASM_GENERIC_DIV64_H */