經常使用技巧

快速判斷一個數是不是2的冪

#define is_power_of_2(x)    (!((x) & (x - 1)))

已知一個數n,求m,使m >= n,且m是2的冪

unsigned int fixsize(unsigned long size){
    if(is_power_of_2(size)) return size;
    size |= size >> 1;
    size |= size >> 2;
    size |= size >> 4;
    size |= size >> 8;
    size |= size >> 16;
    return size + 1;    
}