### _fyl2x -- Calculate y * log2(x)

Format

>     #include <builtin.h>
>     double _fyl2x(double x, double y);

Language Level: Extension
_fyl2x computes the base-2 logarithm of x and multiplies it by y (y * log2(x)). The variable x cannot be negative. This instruction is designed with built-in multiplication to optimize the calculation of logarithms with arbitrary positive base: log b (x) = (log2(b)**-1) * log2(x)

Because it is a built-in function and has no backing code in the library:

- You cannot take the address of _fyl2x.
- You cannot parenthesize a _fyl2x function call, because parentheses specify a call to the backing code for the function in the library.

Return Value
_fyl2x returns the result of the formula y * log2(x).

Example
This example calculates (y * log2(x)), after prompting the user for values of x and y.

    #include <builtin.h>
    #include <stdio.h>

    int main(void)
    {
       double x, y;

       printf("Enter a value for x:\n");
       scanf("%lf", &x);
       printf("Enter a value for y:\n");
       scanf("%lf", &y);
       printf("%lf * log2(%lf) is %lf.\n", y, x, _fyl2x(x, y));
       return 0;

       /*******************************************************
          Assuming you enter 4.0 for x and 3.0 for y.

          The output should be:

          3.000000 * log2(4.000000) is 6.000000.
       *******************************************************/
    }

_fyl2xp1 -- Calculate y * log2(x + 1)
_f2xm1 -- Calculate (2**x) - 1
<builtin.h>