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[] = {, , , , , , , , , , UINT_MAX-, 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:\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[].c_str());
         for (int i = ; i < ; i ++) {
             int t = g_arr[i];
             bitset<> b(t);
             printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ffs(t));
         }
         puts("");

         // clz
         printf("%s:\n", g_str_func[].c_str());
         for (int i = ; i < ; i ++) {
             int t = g_arr[i];
             bitset<> b(t);
             printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_clz(t));
         }
         puts("");

         // ctz
         printf("%s:\n", g_str_func[].c_str());
         for (int i = ; i < ; i ++) {
             int t = g_arr[i];
             bitset<> b(t);
             printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ctz(t));
         }
         puts("");

         // popcount
         printf("%s:\n", g_str_func[].c_str());
         for (int i = ; i < ; i ++) {
             int t = g_arr[i];
             bitset<> b(t);
             printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_popcount(t));
         }
         puts("");

         // parity
         printf("%s:\n", g_str_func[].c_str());
         for (int i = ; i < ; i ++) {
             int t = g_arr[i];
             bitset<> b(t);
             printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_parity(t));
         }
         puts("");
     return ;
 }

【测试结果】

 __builtin_ffs:
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 __builtin_clz:
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 __builtin_ctz:
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 __builtin_popcount:
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 __builtin_parity:
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 Process returned  (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