Re: pure and const functions

["Kris Warkentin" <kewarken at qnx dot com>]

----- Original Message -----
From: "Chris Lattner" <sabre@nondot.org>
To: <gcc@gcc.gnu.org>
Sent: Friday, April 26, 2002 12:50 PM
Subject: RE: pure and const functions


>
> > int fib(int n) {
> >   if (n==0 || n==1) return n;
> >   return fib(n-1) + fib(n-2);
> > }
>
> > According to the definitions given above, this function is const,
> > but not total, because it does not return for negative n.
>
> While this is true in a practical sense, given infinite resources, this is
> certainly not the case.  Assuming you had a whole lot of stack space and
> bunch of time, the negative values for 'n' would wrap around to positive
> values.  Eventually it would get to 1 and terminate.
>
> As this is the case, for this example...
>
> int double_or_nothing (int n) {
>   fib(n);
>   return 2*n;
> }
>
> ... it should be optimizable to:
>
> int double_or_nothing (int n) {
>   return 2*n;
> }
>
> ...because on an ideal, theoretical, machine, they results are exactly the
> same.
>
> IOW: we don't try to predict stack overflow in other cases, so why should
>      we have to worry about that possibility here?
>
> There may be other cases where functions are "partial", can you give
> another example?

One that calls a longjmp() or does an exec()?

>
> -Chris
>
> http://www.nondot.org/~sabre/os/
> http://www.nondot.org/~sabre/Projects/
>
>