编写一个函数比较两个整数大小，但不能使用任何比较操作符

有陷阱，要考虑溢出问题

int comp_int(int var1, int var2)
/* return value < 0: means var1 < var2; 
 *              = 0: means var1 = var2;
 *              > 0: means var1 > var2;
*/
{
    int bit_width, msb, rtnvar;
    bit_width = sizeof(int) << 3;    
    msb = var2 >> (bit_width -1) - 
          var1 >> (bit_width -1);
    if (msb)
        rtnvar = msb;
    else
        rtnvar = var1 - var2;
    return rtnvar;
}