harvey/sys/src/9/port/mul64fract.c

/*
 * This file is part of the UCB release of Plan 9. It is subject to the license
 * terms in the LICENSE file found in the top-level directory of this
 * distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
 * part of the UCB release of Plan 9, including this file, may be copied,
 * modified, propagated, or distributed except according to the terms contained
 * in the LICENSE file.
 */

#include "u.h"

/* mul64fract(uint64_t*r, uint64_t a, uint64_t b)
 *
 * Multiply two 64 numbers and return the middle 64 bits of the 128 bit result.
 *
 * The assumption is that one of the numbers is a
 * fixed point number with the integer portion in the
 * high word and the fraction in the low word.
 *
 * There should be an assembler version of this routine
 * for each architecture.  This one is intended to
 * make ports easier.
 *
 *	ignored		r0 = lo(a0*b0)
 *	lsw of result	r1 = hi(a0*b0) +lo(a0*b1) +lo(a1*b0)
 *	msw of result	r2 = 		hi(a0*b1) +hi(a1*b0) +lo(a1*b1)
 *	ignored		r3 = hi(a1*b1)
 */

void
mul64fract(uint64_t *r, uint64_t a, uint64_t b)
{
	uint64_t bh, bl;
	uint64_t ah, al;
	uint64_t res;

	bl = b & 0xffffffffULL;
	bh = b >> 32;
	al = a & 0xffffffffULL;
	ah = a >> 32;

	res = (al*bl)>>32;
	res += (al*bh);
	res += (ah*bl);
	res += (ah*bh)<<32;

	*r = res;
}