How to do an if check in a function only 1 time on the first call?

Question

Algorithms

Suppose there is such a function in which it is known that only at its very first execution, the parameter can be null, then the if comparison will be meaningless.
The question is whether there is any approach, or optimization pattern.

void fun(string str)
    {
        if (str == null) /// после 1 выполнения функции, str != null всегда
            return;
        //// fast work
    }

Answer 1

This is called "premature optimization". Checking for null costs nothing.

Answer 2

And what's the difference whether to check the argument for null or for the first run?

Answer 3

Only write 2 functions: one with a check, the other without. And call the first one only the first time, and then the second. If the call is in a loop, then you need to expand the loop - pull out the first iteration before the loop and start the loop from the second iteration.
But this optimization will be practically useless in all cases. Stupidly increasing code can change cache hits and slow down your program much more.