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![Uwe Kleine-König](/assets/img/avatar_default.png)
As indicated in the added comment, the algorithm works better if b is big. As multiplication is commutative, a and b can be swapped. Do this if a is bigger than b. Link: https://lkml.kernel.org/r/20240303092408.662449-2-u.kleine-koenig@pengutronix.de Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Tested-by: Biju Das <biju.das.jz@bp.renesas.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
241 lines
5.4 KiB
C
241 lines
5.4 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Copyright (C) 2003 Bernardo Innocenti <bernie@develer.com>
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*
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* Based on former do_div() implementation from asm-parisc/div64.h:
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* Copyright (C) 1999 Hewlett-Packard Co
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* Copyright (C) 1999 David Mosberger-Tang <davidm@hpl.hp.com>
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*
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*
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* Generic C version of 64bit/32bit division and modulo, with
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* 64bit result and 32bit remainder.
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*
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* The fast case for (n>>32 == 0) is handled inline by do_div().
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*
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* Code generated for this function might be very inefficient
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* for some CPUs. __div64_32() can be overridden by linking arch-specific
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* assembly versions such as arch/ppc/lib/div64.S and arch/sh/lib/div64.S
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* or by defining a preprocessor macro in arch/include/asm/div64.h.
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*/
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#include <linux/bitops.h>
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#include <linux/export.h>
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#include <linux/math.h>
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#include <linux/math64.h>
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#include <linux/minmax.h>
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#include <linux/log2.h>
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/* Not needed on 64bit architectures */
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#if BITS_PER_LONG == 32
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#ifndef __div64_32
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uint32_t __attribute__((weak)) __div64_32(uint64_t *n, uint32_t base)
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{
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uint64_t rem = *n;
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uint64_t b = base;
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uint64_t res, d = 1;
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uint32_t high = rem >> 32;
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/* Reduce the thing a bit first */
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res = 0;
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if (high >= base) {
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high /= base;
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res = (uint64_t) high << 32;
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rem -= (uint64_t) (high*base) << 32;
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}
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while ((int64_t)b > 0 && b < rem) {
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b = b+b;
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d = d+d;
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}
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do {
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if (rem >= b) {
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rem -= b;
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res += d;
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}
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b >>= 1;
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d >>= 1;
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} while (d);
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*n = res;
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return rem;
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}
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EXPORT_SYMBOL(__div64_32);
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#endif
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#ifndef div_s64_rem
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s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder)
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{
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u64 quotient;
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if (dividend < 0) {
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quotient = div_u64_rem(-dividend, abs(divisor), (u32 *)remainder);
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*remainder = -*remainder;
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if (divisor > 0)
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quotient = -quotient;
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} else {
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quotient = div_u64_rem(dividend, abs(divisor), (u32 *)remainder);
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if (divisor < 0)
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quotient = -quotient;
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}
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return quotient;
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}
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EXPORT_SYMBOL(div_s64_rem);
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#endif
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/*
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* div64_u64_rem - unsigned 64bit divide with 64bit divisor and remainder
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* @dividend: 64bit dividend
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* @divisor: 64bit divisor
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* @remainder: 64bit remainder
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*
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* This implementation is a comparable to algorithm used by div64_u64.
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* But this operation, which includes math for calculating the remainder,
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* is kept distinct to avoid slowing down the div64_u64 operation on 32bit
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* systems.
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*/
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#ifndef div64_u64_rem
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u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
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{
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u32 high = divisor >> 32;
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u64 quot;
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if (high == 0) {
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u32 rem32;
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quot = div_u64_rem(dividend, divisor, &rem32);
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*remainder = rem32;
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} else {
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int n = fls(high);
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quot = div_u64(dividend >> n, divisor >> n);
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if (quot != 0)
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quot--;
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*remainder = dividend - quot * divisor;
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if (*remainder >= divisor) {
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quot++;
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*remainder -= divisor;
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}
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}
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return quot;
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}
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EXPORT_SYMBOL(div64_u64_rem);
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#endif
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/*
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* div64_u64 - unsigned 64bit divide with 64bit divisor
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* @dividend: 64bit dividend
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* @divisor: 64bit divisor
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*
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* This implementation is a modified version of the algorithm proposed
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* by the book 'Hacker's Delight'. The original source and full proof
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* can be found here and is available for use without restriction.
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*
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* 'http://www.hackersdelight.org/hdcodetxt/divDouble.c.txt'
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*/
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#ifndef div64_u64
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u64 div64_u64(u64 dividend, u64 divisor)
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{
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u32 high = divisor >> 32;
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u64 quot;
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if (high == 0) {
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quot = div_u64(dividend, divisor);
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} else {
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int n = fls(high);
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quot = div_u64(dividend >> n, divisor >> n);
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if (quot != 0)
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quot--;
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if ((dividend - quot * divisor) >= divisor)
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quot++;
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}
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return quot;
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}
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EXPORT_SYMBOL(div64_u64);
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#endif
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#ifndef div64_s64
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s64 div64_s64(s64 dividend, s64 divisor)
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{
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s64 quot, t;
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quot = div64_u64(abs(dividend), abs(divisor));
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t = (dividend ^ divisor) >> 63;
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return (quot ^ t) - t;
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}
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EXPORT_SYMBOL(div64_s64);
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#endif
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#endif /* BITS_PER_LONG == 32 */
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/*
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* Iterative div/mod for use when dividend is not expected to be much
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* bigger than divisor.
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*/
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u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
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{
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return __iter_div_u64_rem(dividend, divisor, remainder);
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}
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EXPORT_SYMBOL(iter_div_u64_rem);
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#ifndef mul_u64_u64_div_u64
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u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
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{
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u64 res = 0, div, rem;
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int shift;
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/* can a * b overflow ? */
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if (ilog2(a) + ilog2(b) > 62) {
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/*
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* Note that the algorithm after the if block below might lose
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* some precision and the result is more exact for b > a. So
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* exchange a and b if a is bigger than b.
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*
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* For example with a = 43980465100800, b = 100000000, c = 1000000000
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* the below calculation doesn't modify b at all because div == 0
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* and then shift becomes 45 + 26 - 62 = 9 and so the result
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* becomes 4398035251080. However with a and b swapped the exact
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* result is calculated (i.e. 4398046510080).
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*/
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if (a > b)
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swap(a, b);
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/*
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* (b * a) / c is equal to
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*
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* (b / c) * a +
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* (b % c) * a / c
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*
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* if nothing overflows. Can the 1st multiplication
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* overflow? Yes, but we do not care: this can only
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* happen if the end result can't fit in u64 anyway.
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*
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* So the code below does
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*
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* res = (b / c) * a;
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* b = b % c;
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*/
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div = div64_u64_rem(b, c, &rem);
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res = div * a;
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b = rem;
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shift = ilog2(a) + ilog2(b) - 62;
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if (shift > 0) {
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/* drop precision */
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b >>= shift;
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c >>= shift;
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if (!c)
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return res;
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}
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}
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return res + div64_u64(a * b, c);
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}
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EXPORT_SYMBOL(mul_u64_u64_div_u64);
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#endif
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