Reverse bits of a given 32 bits unsigned integer

For example, given input 43261596(represented in binary as 0000010100101000001111010011100).return 964176192(represented in binary as 00111001011110000010100101000000) 
Follow up: 
If this function is called many times, hou would you optimize it? 

Related problem:Reverse Integer

#include <iostream>
using namespace std;

unsigned int reverse32bits(const unsigned int& m){

        unsigned int n = m;
//首先，将32位的bit分为两部分，前16bit和后16bit。并交换这两部分
n = (n >> 16) | (n << 16);
//接下来，再对前16bit和后16bit，分别交换将它们各自的前8bit和后8bit交换
n = ((n & 0xff00ff00) >> 8) | ((n & 0x00ff00ff) << 8);
n = ((n & 0xf0f0f0f0) >> 4) | ((n & 0x0f0f0f0f) << 4);
n = ((n & 0xcccccccc) >> 2) | ((n & 0x33333333) << 2);
n = ((n & 0xaaaaaaaa) >> 1) | ((n & 0x55555555) << 1);
return n;
}


int main(){
unsigned int n;
cin >> n;
cout << n << endl;
cout << reverse32bits(n) << endl;
return 0;
}