Compile-time function execution

Compile time function execution is the ability of a compiler, that would normally compile a function to machine code and execute it at run time, to execute the function at compile time. This is possible if the arguments to the function are known at compile time, and the function does not make any reference to or attempt to modify any global state (is a pure function). Even if the value of only some of the arguments are known, the compiler may still be able to perform some level of compile time function execution (partial evaluation), possibly producing more optimized code than if no arguments were known.

Example 1

This example code is in the D programming language^[1]:

int square(int x)
{
   return x * x;
}

const int y = square(3);  // y is set to 9 at compile time

Example 2

In C++, template metaprogramming is often used to compute values at compile time, such as:

template <int N>
struct Factorial 
{
    enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> 
{
    enum { value = 1 };
};

// Factorial<4>::value == 24
// Factorial<0>::value == 1
void foo()
{
    int x = Factorial<4>::value; // == 24
    int y = Factorial<0>::value; // == 1
}

With compile time function evaluation, the code used to generate the factorial would be exactly the same as what one would write for run time evaluation (example is in D):

int factorial(int n) 
{
    if (n == 0)
       return 1;
    return n * factorial(n - 1);
}

void foo()
{
    static const int x = factorial(4); // == (4 * 3 * 2 * 1) == 24
    static const int y = factorial(0); // == 0! == 1
}

The use of static const tells the compiler that the initializer for the variables must be computed at compile time^[2].

References

[1] D 1.0 language specification: Functions

[2] D 1.0 language specification: Attributes

[1]

[2]