Re: pure and const functions

[dewar@gnat.com (Robert Dewar)]

> 
> This seems nice and simple, but #2 confuses things.  As I understand it,
> GCC automatically trys to prove functions pure.  Obviously it will not be
> able to figure out the structure of a memoizing function in general, so it
> would punt on these functions, thus not automatically marking it pure.
> 
> The programmer, however, can manually add the pure attribute, so the
> actual _notion_ of being pure is still intact, even if the actual
> implementation isn't "smart enough" (and there isn't really any reasonable
> way to make it so).

Given that a programmer can mark things as pure, you need to decide

1. If the compiler can check that this designation is correct. The answer
is pretty clearly no.

2. What exactly is the criterion for "correct" designation of Pure (in
Ada by the way this is the Pure_Function pragma).

Yes, the compiler can try to prove some functions pure, but it will always
end up being too conservative, which is fine.