GCC provides quite a lot of builtin functions. These functions are part of standard C offered by the compiler and may come in various variants as per the gcc. These are also termed as hardware specific functions which are internally implemented in assembly or we can say machine instructions, with wide usage in low level programming and are generally target optimized.

These functions are prefixed with “__builtin”. There are a huge lot of builtin functions by GCC, however here in this article, we shall be learning about two such builtin functions.

Lets learn more about these methods.

This builtin method is provided by GCC to count the number of leading zero’s in variable.

The Syntax:

It takes the input parameter as a number for which the the count of leading zero’s is to be determined. It returns the count of leading zero’s as expected.

Taking for example, lets take a number 16. An int takes 4 bytes in gcc. Its binary representation is

Counting the number of leading zero’s is 27 which should be our result for this case..

This builtin method by GCC determines the count of trailing zero in the binary representation of a number.

The Syntax:

The input parameter is the number for which the the count of trailing zero’s is to be determined. It returns the count of trailing zero’s as expected.

Taking for example, lets take the same number 16. Again, an int takes 4 bytes in gcc. Its binary representation is

Counting the number of trailing zero’s is 4 which should be our result for this case..

This builtin method by GCC determines the number of one’s in the binary representation of a number.

The Syntax:

The input parameter is the number for which the the number of 1’s is to be determined. It returns the count of set bits as expected..

Taking for example, lets take the same number 16. Again, an int takes 4 bytes in gcc. Its binary representation is

Counting the number of one’s is just 1, which should be our result for this case..

Here is a complete example program which demonstrates the use of these builtin methods.

On running the above example program, here is our output:

Great! it works fine as per our understanding. Note, it even worked fine for negative numbers which are stored as two’s complement.

These two builtin functions are really helpful for low level bitwise operations and manipulation computations. Many advanced and optimized algorithms need to count the leading or trailing zero’s of a number. However, there are many variants available for these builtin functions to cater input parameters as unsigned int, unsigned long, etc.

These functions are prefixed with “__builtin”. There are a huge lot of builtin functions by GCC, however here in this article, we shall be learning about two such builtin functions.

Code:

__builtin_clz(); __builtin_ctz(); __builtin_popcount();

### Function __builtin_clz

This builtin method is provided by GCC to count the number of leading zero’s in variable.

The Syntax:

Code:

int __builtin_clz (unsigned int x)

Taking for example, lets take a number 16. An int takes 4 bytes in gcc. Its binary representation is

Code:

00000000 00000000 00000000 00010000

### Function __builtin_ctz

This builtin method by GCC determines the count of trailing zero in the binary representation of a number.

The Syntax:

Code:

int __builtin_ctz (unsigned int x)

Taking for example, lets take the same number 16. Again, an int takes 4 bytes in gcc. Its binary representation is

Code:

00000000 00000000 00000000 00010000

### Function __builtin_popcount

This builtin method by GCC determines the number of one’s in the binary representation of a number.

The Syntax:

Code:

int __builtin_popcount (unsigned int x)

Taking for example, lets take the same number 16. Again, an int takes 4 bytes in gcc. Its binary representation is

Code:

00000000 00000000 00000000 00010000

### Complete Example

Here is a complete example program which demonstrates the use of these builtin methods.

Code:

#include <stdio.h> #include <stdlib.h> int main() { int num = 16; int clz = 0; int ctz = 0; int ones = 0; clz = __builtin_clz(num); printf("Number of leading zero's in %d is %d\n", num, clz); 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); ones = __builtin_popcount(num); printf("Number of one's in %d is %d\n", num, ones); return 0; }

Code:

Number of leading zero's in 16 is 27 Number of leading zero's in -16 is 0 Number of trailing zero's in 16 is 4 Number of one's in 16 is 1

### Conclusion

These two builtin functions are really helpful for low level bitwise operations and manipulation computations. Many advanced and optimized algorithms need to count the leading or trailing zero’s of a number. However, there are many variants available for these builtin functions to cater input parameters as unsigned int, unsigned long, etc.

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