Re: intermediate results causing constraint errors

[kst@alsys.com (Keith Thompson)]

In <D0Cp98.1AK@inmet.camb.inmet.com> bobduff@dsd.camb.inmet.com (Bob Duff) writes:
> If I write:
> 
>     type T is range 1..10_000;
>     X: T := 10_000;
>     Five: T := 5;
>     Ten: T := 10;
>     ...
>     X := (X * Five) / Ten;
> 
> one implementation might overflow on the multiply and another might not.
> 
> It's true that there are ways to avoid this (define a wider-ranged type
> first, and define T as a subtype of that), but I would say that that
> requires some "Ada experience".  Furthermore, the Ada compiler won't
> necessarily tell you about the problem, either at compiler or run-time
> -- you'll find out when you try to port the code from a 32-bit machine
> to a 16-bit machine (a rarity these days, I suppose).

Actually, you're likely to find out when you try to port the code from
an implementation that derived T from a predefined 32-bit type to one
that derives it from a predefined 16-bit type.

Ada 83 specifies that a user-defined integer type is derived from a
predefined integer type "implicitly selected by the implementation"
so as to include the declared bounds; it doesn't specify exactly how
the parent type is selected.  I know of at least one implementation
that derives T from Integer (32 bits), and another that derives it from
Short_Integer (16 bits); both implementations have 32-bit Integer and
16-bit Short_Integer.

The behavior can also depend on whether the intermediate result is stored
in a register (typically 32 bits) or in a memory object (typically the
size of type T).

-- 
Keith Thompson (The_Other_Keith)  kst@alsys.com
TeleSoft^H^H^H^H^H^H^H^H Alsys, Inc.
10251 Vista Sorrento Parkway, Suite 300, San Diego, CA, USA, 92121-2718
/user/kst/.signature: I/O error (core dumped)