minix/lib/libc/arch/i386/int64/mul64.c

20 lines
571 B
C

#include <minix/u64.h>
u64_t mul64(u64_t i, u64_t j)
{
u64_t result;
/* Compute as follows:
* i * j =
* (i.hi << 32 + i.lo) * (j.hi << 32 + j.lo) =
* (i.hi << 32) * (j.hi << 32 + j.lo) + i.lo * (j.hi << 32 + j.lo) =
* (i.hi * j.hi) << 64 + (i.hi * j.lo) << 32 + (i.lo * j.hi << 32) + i.lo * j.lo
*
* 64-bit-result multiply only needed for (i.lo * j.lo)
* upper 32 bits overflow for (i.lo * j.hi) and (i.hi * j.lo)
* all overflows for (i.hi * j.hi)
*/
result = mul64u(i.lo, j.lo);
result.hi += i.hi * j.lo + i.lo * j.hi;
return result;
}