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Re: Ada subtypes and base types


On 3/16/06, Geert Bosch <bosch@adacore.com> wrote:
>
> On Mar 16, 2006, at 05:09, Robert Dewar wrote:
> > Not quite right. If you have an uninitialized variable, the value is
> > invalid and may be out of bounds, but this is a bounded error
> > situation,
> > not an erroneous program. So the possible effects are definitely NOT
> > unbounded, and the use of such values cannot turn a program erroneous.
> > (that's an Ada 95 change, this used to be erroneous in Ada 83).
>
> Actually, that's a good point and raises some potential issues:
> if we're never establish the invariant that a value of a type is in
> range, we can only use the base range for variables that might be
> used uninitialized. Any read of such a variable would then involve
> a range check.
>
>    package Uninitialized is
>       N : Positive;
>    end Uninitialized;
>
>    with Uninitialized;
>    procedure Test is
>       for J in 1 .. Uninitialized.N loop
>          ...
>       end loop;
>    end Test;
>
> In this case, GCC might replace the loop with
>     declare
>        J : Integer := 1;
>     begin
>        while J /= Uninitialized.N loop
>           ...
>           J := J + 1;
>        end loop;
>     end;
>
> which would be incorrect for N = 0.

Uh - what do you expect here??  Does the Ada standard require a out-of-range
exception upon the first use of N?  In this case, the frontend needs
to insert a proper
check.  You cannot expect the middle-end to avoid the above transformation, so
this is a frontend bug.

Richard.


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