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.
Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.
Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.
Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.
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：
1 #include <iostream> 2 #include <map> 3 4 #define max_n 200005 5 using namespace std; 6 map<int,int> mp; 7 long long a[max_n]; 8 int n; 9 int main() 10 { 11 cout << __builtin_popcount(3) << endl; //3:11 output:2 12 cout << __builtin_popcount(7) << endl; //7:111 output:3 13 14 cout << __builtin_parity(3) << endl; //output:0 15 cout << __builtin_parity(7) << endl; //output:1 16 17 cout << __builtin_ffs(3) << endl; //output:1 18 cout << __builtin_ffs(10) << endl;//10:1010 output:2 19 20 cout << __builtin_ctz(3) << endl; //output:0 21 cout << __builtin_ctz(10) << endl;//output:1 22 23 cout << __builtin_clz(3) << endl;//output:30 24 cout << __builtin_clz(10) << endl;//output:28 25 return 0; 26 }
更多内置函数参见：http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html
