如何将64位操作数相乘并获得128位结果？

{
   uint64_t hi,lo;
  // hi,lo = 64bit x 64bit multiply of c[0] and b[0]

   __asm__("mulq %3\n\t"
    : "=d" (hi),"=a" (lo)
    : "%a" (c[0]),"rm" (b[0])
    : "cc" );

   a[0] += hi;
   a[1] += lo;
 }

据我所知,你想要一个64位乘法的便携式纯C实现,输出为128位值,存储在两个64位值中.在这种情况下,这个 article声称拥有你需要的东西.该代码是为C编写的.将它变成C代码并不需要太多：

void mult64to128(uint64_t op1,uint64_t op2,uint64_t *hi,uint64_t *lo)
{
    uint64_t u1 = (op1 & 0xffffffff);
    uint64_t v1 = (op2 & 0xffffffff);
    uint64_t t = (u1 * v1);
    uint64_t w3 = (t & 0xffffffff);
    uint64_t k = (t >> 32);

    op1 >>= 32;
    t = (op1 * v1) + k;
    k = (t & 0xffffffff);
    uint64_t w1 = (t >> 32);

    op2 >>= 32;
    t = (u1 * op2) + k;
    k = (t >> 32);

    *hi = (op1 * op2) + w1 + k;
    *lo = (t << 32) + w3;
}