说明：
GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
GCC includes built-in versions of many of the functions in the standard C library. The versions prefixed with __builtin_ will always be treated as having the same meaning as the C library function even if you specify the-fno-builtinoption. (see C Dialect Options) Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function will be emitted.
— Built-in Function: int __builtin_ffs (unsigned int x)
Returns one plus the index of the least significant 1-bit of x, or
if x is zero, returns zero.
— Built-in Function: int __builtin_clz (unsigned int x)
Returns the number of leading 0-bits in x, starting at the most
significant bit position. If x is 0, the result is undefined.
— Built-in Function: int __builtin_ctz (unsigned int x)
Returns the number of trailing 0-bits in x, starting at the least
significant bit position. If x is 0, the result is undefined.
— Built-in Function: int __builtin_popcount (unsigned int x)
Returns the number of 1-bits in x.
— Built-in Function: int __builtin_parity (unsigned int x)
Returns the parity of x, i.e. the number of 1-bits in x
modulo 2.
（以上内容来源见参考哦）
1.__builtin_parity(unsigned int x)
统计一个数二进制表示中1的个数是偶数还是奇数
2.__builtin_popcount(unsigned int x)
统计一个数的二进制表示中1的个数
3.__builtin_ffs(unsigned int x)
找出一个数的二进制表示中从末尾开始到遇见第一个1的的位置
4.__builtin_clz(unsigned int x)
返回一个数二进制表示中前导零的数目
5.__builtin_ctz(unsigned int x)
返回一个数二进制表示中尾零的数目
试验代码如下：
#include <iostream>
#include <map> #define max_n 200005
using namespace std;
map<int,int> mp;
long long a[max_n];
int n;
int main()
{
cout << __builtin_popcount() << endl; //3:11 output:2
cout << __builtin_popcount() << endl; //7:111 output:3 cout << __builtin_parity() << endl; //output:0
cout << __builtin_parity() << endl; //output:1 cout << __builtin_ffs() << endl; //output:1
cout << __builtin_ffs() << endl;//10:1010 output:2 cout << __builtin_ctz() << endl; //output:0
cout << __builtin_ctz() << endl;//output:1 cout << __builtin_clz() << endl;//output:30
cout << __builtin_clz() << endl;//output:28
return ;
}
更多内置函数参见：http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html
