Re: Why no constraint error?

["Samuel A. Mize" <smize@magellan.bgm.link.com>]

Tucker Taft wrote:
> Well Bob, I believe GNAT is technically right in this one, though
> I think it may be pessimizing the code overall if it follows
> the approach implied by the above (see below for more discussion).

Sorry -- "pessimizing?"  Could you supply a definition for the
less experienced?  Do you mean "making less efficient through
excessive pessimism?"  Thanks.


> One thing to keep in mind is that "out of the box" GNAT suppresses
> certain run-time checks (not my favorite feature of GNAT, I might say ;-).
> I trust this was compiled with all checks *on*...

It was.


> In this particular case, the propagation of an uninitialized
> value from one variable to another is relatively benign, since
> no memory is being trashed by this propagation.  The real question
> is what happens when "Target" is used as an index into an array.
> Does GNAT remember that Target might become deinitialized by the
> assignment, and hence do the check when Target is used as an index?

No.


> Or does GNAT see that Target is initialized to a valid value, and
> presume that it never becomes deinitialized.

No.  I tested the array case with a straight uninitialized
variable, and it indexed to it.  (I used Normalize_Scalars
to get a predictable value, but I don't *think* this would
impact the presence/absence of this range check.)


> In our AdaMagic front end, objects are identified by the compiler
> as either "reliable" or "unreliable."  ...

This sounds slick.  I'll look at your web site.


> I'm curious what approach GNAT adopts, and whether the above
> would in fact reveal a bug if Target were used as an array index...

No, it doesn't.


> 
> -Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
> Intermetrics, Inc.  Burlington, MA  USA


Sam Mize
(same Sam Mize, different account -- opinions are my own)
-- 
Samuel Mize           (817) 619-8622               "Team Ada"
Hughes Training Inc.  PO Box 6171 m/s 400, Arlington TX 76005