如何将 64 位操作数相乘并轻松获得 128 位结果? [英] How can I multiply 64 bit operands and get 128 bit result portably?
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问题描述
对于 x64,我可以使用这个:
For x64 I can use this:
{
uint64_t hi, lo;
// hi,lo = 64bit x 64bit multiply of c[0] and b[0]
__asm__("mulq %3
"
: "=d" (hi),
"=a" (lo)
: "%a" (c[0]),
"rm" (b[0])
: "cc" );
a[0] += hi;
a[1] += lo;
}
但我想便携地执行相同的计算.例如在 x86 上工作.
But I'd like to perform the same calculation portably. For instance to work on x86.
推荐答案
据我所知,您需要一个可移植的纯 C 实现 64 位乘法,输出到 128 位值,存储在两个 64 位值中.在这种情况下,这篇文章 声称拥有您所需要的.该代码是为 C++ 编写的.把它转成C代码不需要太多:
As I understand the question, you want a portable pure C implementation of 64 bit multiplication, with output to a 128 bit value, stored in two 64 bit values. In which case this article purports to have what you need. That code is written for C++. It doesn't take much to turn it into C code:
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;
}
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