These are three important builtin functions in GCC compiler:

**_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

**__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**__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

// 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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