软件乘法

#include <stdint.h>
#include <stdbool.h>

uint64_t _soft_umul_bad(uint32_t a, uint32_t b)
{
uint64_t ret = 0;
int i, j;

for (i = 0; i < sizeof(a)*8; i++)
for (j = 0; j < sizeof(b)*8; j++)
{
ret += (uint64_t)((a & (1 << i)) && (b & (1 << j))) << (i + j);
}
return ret;
}

uint64_t _soft_umul(uint32_t a, uint32_t b)
{
uint64_t ret = 0, shift;
int i;

shift = b;

for (i = 0; i < sizeof(b)*8; i++)
{
if(a & (1 << i))
{
ret += shift;
}
shift <<= 1;
}
return ret;
}

int64_t _soft_mul(int32_t a, int32_t b)
{
bool neg_a, neg_b, neg_re;
int64_t ret;

neg_a = (a < 0);
neg_b = (b < 0);
neg_re = neg_a ^ neg_b;

a = neg_a ? (-a) : (a);
b = neg_b ? (-b) : (b);

ret = (int64_t)_soft_umul((uint32_t)a, (uint32_t)b);
return neg_re ? (-ret) : (ret);
}

#include <stdio.h>

int main(int argc, char *argv[])
{
int32_t a, b;
int64_t re_soft, re;
if (argc != 0)
{
fprintf(stderr, "softmul [num1] [num2]\n");
return -1;
}

sscanf(argv[1], "%d", &a);
sscanf(argv[2], "%d", &b);

re = (int64_t)a * b;
re_soft = _soft_mul(a, b);

printf("a = %d, b = %d\n", a, b);
printf("re = %lld, re_soft = %lld\n", re, re_soft);

return 0;
}