ref: 7d7650dffc20c3853eb1acd551c9a02f73751ae3
dir: /sys/src/9/port/mul64fract.c/
#include <u.h>
/* mul64fract(uvlong*r, uvlong a, uvlong 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(uvlong *r, uvlong a, uvlong b)
{
uvlong bh, bl;
uvlong ah, al;
uvlong 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;
}