Builtin functions of GCC compiler

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
   

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