对于x64,我可以使用这个：
{
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;
}
但我想以可移植的方式执行相同的计算.例如,在x86上工作.
解决方法
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;
}
