glibc的几个有用的处理二进制位的内置函数（转）

— 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.

返回右起第一个‘1’的位置。

— 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.

返回左起第一个‘1’之前0的个数。

— 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.

返回右起第一个‘1’之后的0的个数。

— Built-in Function: int __builtin_popcount (unsigned int x)

Returns the number of 1-bits in x.

返回‘1’的个数。

— 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.

返回‘1’的个数的奇偶性。

— Built-in Function: int __builtin_ffsl (unsigned long)

Similar to __builtin_ffs, except the argument type is unsigned long. 

— Built-in Function: int __builtin_clzl (unsigned long)

Similar to __builtin_clz, except the argument type is unsigned long. 

— Built-in Function: int __builtin_ctzl (unsigned long)

Similar to __builtin_ctz, except the argument type is unsigned long. 

— Built-in Function: int __builtin_popcountl (unsigned long)

Similar to __builtin_popcount, except the argument type is unsigned long. 

— Built-in Function: int __builtin_parityl (unsigned long)

Similar to __builtin_parity, except the argument type is unsigned long. 

— Built-in Function: int __builtin_ffsll (unsigned long long)

Similar to __builtin_ffs, except the argument type is unsigned long long. 

— Built-in Function: int __builtin_clzll (unsigned long long)

Similar to __builtin_clz, except the argument type is unsigned long long. 

— Built-in Function: int __builtin_ctzll (unsigned long long)

Similar to __builtin_ctz, except the argument type is unsigned long long. 

— Built-in Function: int __builtin_popcountll (unsigned long long)

Similar to __builtin_popcount, except the argument type is unsigned long long. 

— Built-in Function: int __builtin_parityll (unsigned long long)

Similar to __builtin_parity, except the argument type is unsigned long long.

 

【实验程序】

#include <stdio.h>
#include <iostream>
#include <bitset>
#include <string>
#include <limits.h>
 
using namespace std;
 
uint32_t g_arr[12] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, UINT_MAX-1, UINT_MAX};
 
string g_str_func[] = {
    "__builtin_ffs",
    "__builtin_clz",
    "__builtin_ctz",
    "__builtin_popcount",
    "__builtin_parity"
};
 
//typedef int (*fp_builtin_t)(unsigned int);
 
//fp_builtin_t g_func[] = {
    //__builtin_ffs,
    //__builtin_clz,
    //__builtin_ctz,
    //__builtin_popcount,
    //__builtin_parity
//};
 
int main()
{
/*    for (int k = 0; k < 5; k ++) {
        printf("%s:
", g_str_func[k].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            fp_builtin_t fp_func = g_func[k];
            printf("%u(%s): %d
", t, b.to_string().c_str(), fp_func(t));
        }
 
        puts("");
    }
*/
 
        // ffs
        printf("%s:
", g_str_func[0].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            printf("%u(%s): %d
", t, b.to_string().c_str(), __builtin_ffs(t));
        }
        puts("");
 
        // clz
        printf("%s:
", g_str_func[1].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            printf("%u(%s): %d
", t, b.to_string().c_str(), __builtin_clz(t));
        }
        puts("");
 
        // ctz
        printf("%s:
", g_str_func[2].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            printf("%u(%s): %d
", t, b.to_string().c_str(), __builtin_ctz(t));
        }
        puts("");
 
        // popcount
        printf("%s:
", g_str_func[3].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            printf("%u(%s): %d
", t, b.to_string().c_str(), __builtin_popcount(t));
        }
        puts("");
 
        // parity
        printf("%s:
", g_str_func[4].c_str());
        for (int i = 0; i < 12; i ++) {
            int t = g_arr[i];
            bitset<32> b(t);
            printf("%u(%s): %d
", t, b.to_string().c_str(), __builtin_parity(t));
        }
        puts("");
    return 0;
}

【测试结果】

__builtin_ffs:
0(00000000000000000000000000000000): 0
1(00000000000000000000000000000001): 1
2(00000000000000000000000000000010): 2
3(00000000000000000000000000000011): 1
4(00000000000000000000000000000100): 3
5(00000000000000000000000000000101): 1
6(00000000000000000000000000000110): 2
7(00000000000000000000000000000111): 1
8(00000000000000000000000000001000): 4
9(00000000000000000000000000001001): 1
4294967294(11111111111111111111111111111110): 2
4294967295(11111111111111111111111111111111): 1
 
__builtin_clz:
0(00000000000000000000000000000000): 31
1(00000000000000000000000000000001): 31
2(00000000000000000000000000000010): 30
3(00000000000000000000000000000011): 30
4(00000000000000000000000000000100): 29
5(00000000000000000000000000000101): 29
6(00000000000000000000000000000110): 29
7(00000000000000000000000000000111): 29
8(00000000000000000000000000001000): 28
9(00000000000000000000000000001001): 28
4294967294(11111111111111111111111111111110): 0
4294967295(11111111111111111111111111111111): 0
 
__builtin_ctz:
0(00000000000000000000000000000000): 0
1(00000000000000000000000000000001): 0
2(00000000000000000000000000000010): 1
3(00000000000000000000000000000011): 0
4(00000000000000000000000000000100): 2
5(00000000000000000000000000000101): 0
6(00000000000000000000000000000110): 1
7(00000000000000000000000000000111): 0
8(00000000000000000000000000001000): 3
9(00000000000000000000000000001001): 0
4294967294(11111111111111111111111111111110): 1
4294967295(11111111111111111111111111111111): 0
 
__builtin_popcount:
0(00000000000000000000000000000000): 0
1(00000000000000000000000000000001): 1
2(00000000000000000000000000000010): 1
3(00000000000000000000000000000011): 2
4(00000000000000000000000000000100): 1
5(00000000000000000000000000000101): 2
6(00000000000000000000000000000110): 2
7(00000000000000000000000000000111): 3
8(00000000000000000000000000001000): 1
9(00000000000000000000000000001001): 2
4294967294(11111111111111111111111111111110): 31
4294967295(11111111111111111111111111111111): 32
 
__builtin_parity:
0(00000000000000000000000000000000): 0
1(00000000000000000000000000000001): 1
2(00000000000000000000000000000010): 1
3(00000000000000000000000000000011): 0
4(00000000000000000000000000000100): 1
5(00000000000000000000000000000101): 0
6(00000000000000000000000000000110): 0
7(00000000000000000000000000000111): 1
8(00000000000000000000000000001000): 1
9(00000000000000000000000000001001): 0
4294967294(11111111111111111111111111111110): 1
4294967295(11111111111111111111111111111111): 0
 
 
Process returned 0 (0x0)   execution time : 2.405 s
Press any key to continue.

这里存在一个为题，希望知道的朋友能够解释，就是为什么不能用函数指针指向这些内置函数。

答：为了性能，这些函数都是内联的。
比如很多平台都有特定的指令，所以这些“函数”都只要一条指令就完成了，做成函数得不偿失。

转自：https://www.cnblogs.com/nysanier/archive/2011/04/19/2020778.html