Builtin functions of GCC compiler

These are three important builtin 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 
  2. __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 occurence 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

  3. __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 occurence of one(set bit).

    a = 16
    Binay 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
  4. // 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;
    
        pop = __builtin_popcount(num);
        printf("Number of one's in %d is %d\n", num, pop);
    
        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
    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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