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Builtin functions of GCC compiler
  • Difficulty Level : Easy
  • Last Updated : 20 Nov, 2019

These are four important built-in functions in GCC compiler:

  1. _builtin_popcount(x): This function is used to count the number of one’s(set bits) in an integer.
    Example:
    if x = 4
    binary value of 4 is 100
    Output: No of ones is 1.




    // C program to illustrate _builtin_popcount(x)
      
    #include <stdio.h>
    int main()
    {
        int n = 5;
          
        printf("Count of 1s in binary of %d is %d ",
               n, __builtin_popcount(n));
        return 0;
    }
    Output:
    Count of 1s in binary of 5 is 2
    

    Note: Similarly you can use __builtin_popcountl(x) & __builtin_popcountll(x) for long and long long data types.

  2. _builtin_parity(x): This function is used to check the parity of a number. This function returns true(1) if the number has odd parity else it returns false(0) for even parity.
    Example:
    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 <stdio.h>
    int main()
    {
        int n = 7;
          
        printf("Parity of %d is %d ",
               n, __builtin_parity(n));
        return 0;
    }
    Output:



    Parity of 7 is 1
    

    Note: Similarly you can use __builtin_parityl(x) & __builtin_parityll(x) for long and long long data types.

  3. __builtin_clz(x): This function is used to count the leading zeros of the integer. Note : clz = count leading zero’s
    Example: It counts number of zeros before the first occurrence of one(set bit).
    a = 16
    Binary form of 16 is 00000000 00000000 00000000 00010000
    Output: 27




    // C program to illustrate __builtin_clz(x)
    #include <stdio.h>
    int main()
    {
        int n = 16;
          
        printf("Count of leading zeros before 1 in %d is %d",
               n, __builtin_clz(n));
        return 0;
    }
    Output:
    Count of leading zeros before 1 in 16 is 27
    

    Note: __builtin_clz(x) This function only accept unsigned values
    Note: Similarly you can use __builtin_clzl(x) & __builtin_clzll(x) for long and long long data types.

  4. __builtin_ctz(x): This function is used to count the trailing zeros of the given integer. Note : ctz = count trailing zeros.
    Example: Count no of zeros from last to first occurrence of one(set bit).
    a = 16
    Binary form of 16 is 00000000 00000000 00000000 00010000
    Output: ctz = 4
    




    // C program to illustrate __builtin_ctz(x)
    #include <stdio.h>
    int main()
    {
        int n = 16;
          
        printf("Count of zeros from last to first "
               "occurrence of one is %d",
               __builtin_ctz(n));
        return 0;
    }
    Output:
    Count of zeros from last to first occurrence of one is 4
    

    Note: Similarly you can use __builtin_ctzl(x) & __builtin_ctzll(x) for long and long long data types.




    // C program to illustrate builtin functions of
    // GCC compiler
    #include <stdio.h>
    #include <stdlib.h>
      
    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;
    }
    Output:
    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
    

    This article is contributed by Maxx selva k. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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