gcc的__builtin_函数（注意前面是两个下划线）

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