ref: f0f2d452c873255dec4b3359b3f93e349be0f2c5
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)
{
ulong bh, bl, ah, al;
bl = b;
bh = b >> 32;
al = a;
ah = a >> 32;
*r = (((uvlong)al*(uvlong)bl)>>32)
+ ((uvlong)al*(uvlong)bh)
+ ((uvlong)ah*(uvlong)bl)
+ (((uvlong)ah*(uvlong)bh)<<32);
}