glibc的几个有用的处理二进制位的内置函数

 

— 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】

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#include <stdio.h>

#include <iostream>

#include <bitset>

#include <string>

#include <limits.h>

  

usingnamespace 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

//};

  

intmain()

{

/*    for (int k = 0; k < 5; k ++) {

        printf("%s:\n", 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\n", t, b.to_string().c_str(), fp_func(t));

        }

  

        puts("");

    }

*/

  

        // ffs

        printf("%s:\n", g_str_func[0].c_str());

        for(int i = 0; i < 12; i ++) {

            intt = g_arr[i];

            bitset<32> b(t);

            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ffs(t));

        }

        puts("");

  

        // clz

        printf("%s:\n", g_str_func[1].c_str());

        for(int i = 0; i < 12; i ++) {

            intt = g_arr[i];

            bitset<32> b(t);

            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_clz(t));

        }

        puts("");

  

        // ctz

        printf("%s:\n", g_str_func[2].c_str());

        for(int i = 0; i < 12; i ++) {

            intt = g_arr[i];

            bitset<32> b(t);

            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ctz(t));

        }

        puts("");

  

        // popcount

        printf("%s:\n", g_str_func[3].c_str());

        for(int i = 0; i < 12; i ++) {

            intt = g_arr[i];

            bitset<32> b(t);

            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_popcount(t));

        }

        puts("");

  

        // parity

        printf("%s:\n", g_str_func[4].c_str());

        for(int i = 0; i < 12; i ++) {

            intt = g_arr[i];

            bitset<32> b(t);

            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_parity(t));

        }

        puts("");

    return0;

}

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。

 

【测试结果】

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__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)   executiontime : 2.405 s

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