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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 ``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 ``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 ``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 ``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 ``#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;``}`
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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