[fpc-devel] Compile time functions

[Martok]

Am 22.07.2018 um 06:03 schrieb J. Gareth Moreton:
> Pure functions cannot be
> evaluated at compile time if one of the arguments is a variable (or at the very
> least, not deterministic)
Do you have an idea how to prove that yet? The way I see it now, this is the
only hard issue, every other component is already present somewhere in some form.

> although there
> might still be issues in exceptional conditions (e.g. division by zero or trying
> to calculate tan(pi/2), for example).
Good point - what should happen in that case?
    const
      x  = Pi / 0;
    begin
      writeln(x);
    end.
That writes "+Inf".

> The other point with a "pure" modifier is that, assuming the compiler determines
> that the function is indeed pure, it would permit their assignment to a constant.
So: on processing a call to a pure function,
  if arguments are constant, emit (interpreted) result of function
  otherwise, emit regular call
?
What would error/warning cases be? I'd want to avoid a situation like that with
the insightful, but unfixable and thus spammy "06058_N_Call to subroutine "$1"
marked as inline is not inlined" message.

> The final issue is that while a function might be pure, you might not want to
> inline it because of its complexity, especially if it's called multiple times
> with variable arguments.
That is very true. Should the "interpretation complexity" be limited, or should
the compiler keep going and maybe run into a stack overflow?


-- 
Regards,
Martok