Benefits of pure function

Question

Today i was reading about pure function, got confused with its use:

A function is said to be pure if it returns same set of values for same set of inputs and does not have any observable side effects.

e.g. strlen() is a pure function while rand() is an impure one.

__attribute__ ((pure)) int fun(int i)
{
    return i*i;
}

int main()
{
    int i=10;
    printf("%d",fun(i));//outputs 100
    return 0;
}

http://ideone.com/33XJU

The above program behaves in the same way as in the absence of pure declaration.

What are the benefits of declaring a function as pure[if there is no change in output]?

Answer 1

pure lets the compiler know that it can make certain optimisations about the function: imagine a bit of code like

for (int i = 0; i < 1000; i++)
{
    printf("%d", fun(10));
}

With a pure function, the compiler can know that it needs to evaluate fun(10) once and once only, rather than 1000 times. For a complex function, that's a big win.