GCC编译器的内置函数

这些是GCC编译器中的四个重要的内置函数：

1. _builtin_popcount(x)：此函数用于对一个整数(设置位)的个数进行计数。
   例子：

   if x = 4
   binary value of 4 is 100
   Output: No of ones is 1.

   // C program to illustrate _builtin_popcount(x)
     
   #include 
   int main()
   {
       int n = 5;
         
       printf("Count of 1s in binary of %d is %d ",
              n, __builtin_popcount(n));
       return 0;
   }
   

   输出：

   Count of 1s in binary of 5 is 2
   

   注意：同样，对于长数据类型和长长数据类型，您可以使用__builtin_popcountl(x)和__builtin_popcountll(x)。

2. _builtin_parity(x)：此函数用于检查数字的奇偶校验。如果数字具有奇偶校验，则此函数返回true(1)，否则返回奇偶校验的false(0)。
   例子：

   if x = 7
   7 has odd no. of 1's in its binary(111).
   Output: Parity of 7 is 1 

   // C program to illustrate _builtin_parity(x)
     
   #include 
   int main()
   {
       int n = 7;
         
       printf("Parity of %d is %d ",
              n, __builtin_parity(n));
       return 0;
   }
   

   输出：

   Parity of 7 is 1
   

   注意：同样，对于长数据类型和长长数据类型，您可以使用__builtin_parityl(x)和__builtin_parityll(x)。

3. __builtin_clz(x)：此函数用于计算整数的前导零。注意：clz =计数前导零
   示例：它在第一次出现一个(置位)之前对零的数目进行计数。

   a = 16
   Binary form of 16 is 00000000 00000000 00000000 00010000
   Output: 27

   // C program to illustrate __builtin_clz(x)
   #include 
   int main()
   {
       int n = 16;
         
       printf("Count of leading zeros before 1 in %d is %d",
              n, __builtin_clz(n));
       return 0;
   }
   

   输出：

   Count of leading zeros before 1 in 16 is 27
   

   注意： __builtin_clz(x)此函数仅接受无符号值
   注意：同样，对于长数据类型和长长数据类型，您可以使用__builtin_clzl(x)和__builtin_clzll(x)。

4. __builtin_ctz(x)：此函数用于计算给定整数的尾随零。注意：ctz =计数尾随零。
   示例：从上次出现到第一次出现(置位)时不计零。

   a = 16
   Binary form of 16 is 00000000 00000000 00000000 00010000
   Output: ctz = 4
   

   // C program to illustrate __builtin_ctz(x)
   #include 
   int main()
   {
       int n = 16;
         
       printf("Count of zeros from last to first "
              "occurrence of one is %d",
              __builtin_ctz(n));
       return 0;
   }
   

   输出：

   Count of zeros from last to first occurrence of one is 4
   

   注意：同样，对于长数据类型和长长数据类型，您可以使用__builtin_ctzl(x)和__builtin_ctzll(x)。

   // C program to illustrate builtin functions of
   // GCC compiler
   #include 
   #include 
     
   int main()
   {
       int num = 4;
       int clz = 0;
       int ctz = 0;
       int pop = 0;
       int parity = 0;
     
       pop = __builtin_popcount(num);
       printf("Number of one's in %d is %d\n", num, pop);
     
       parity = __builtin_parity(num);
       printf("Parity of %d is %d\n", num, parity);
     
       clz = __builtin_clz(num);
       printf("Number of leading zero's in %d is %d\n", num, clz);
     
       // It only works for unsigned values
       clz = __builtin_clz(-num);
       printf("Number of leading zero's in %d is %d\n", -num, clz);
     
       ctz = __builtin_ctz(num);
       printf("Number of trailing zero's in %d is %d\n", num, ctz);
     
       return 0;
   }
   

   输出：

   Number of one's in 4 is 1
   Parity of 4 is 1
   Number of leading zero's in 4 is 29
   Number of leading zero's in -4 is 0
   Number of trailing zero's in 4 is 2
   

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