Re: Unconstrained Arrays

["Randy Brukardt" <randy@rrsoftware.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message 
news:wccy6ut1bi8.fsf@shell01.TheWorld.com...
> sjw <simon.j.wright@mac.com> writes:
...
> Well, GNAT is right in the sense that it obeys the Ada RM on this point.
> But some people (including some of the folks who wrote GNAT) think the
> language designers were wrong on this point.  It's an efficiency issue.
> You can certainly come up with cases that make a big difference:
>
>    subtype S is String (1..4);
>    type A is array (Positive range <>) of aliased S;
>    X : A (1..1_000_000);
>
>    type S_Ref is access all S;
>
> An object of subtype S is likely 4 bytes without dope, 12 bytes with
> dope.  Removing this restriction from the language would triple
> the size of X from 4 million bytes to 12 million.  If you only
> wanted to point at components of X from objects of type S_Ref,
> (as opposed to some "access all String" type) then that would be
> annoying (why store 1 million copies of the compile-time-known
> values 1 and 4?).

There is another option if you use non-contiguous dope (as Janus/Ada does): 
create the dope on the fly when the 'Access is encountered. That's what we 
did in our Ada 9x prototype compiler (before the "static matching" rule was 
added to Ada 9x). The problem is that recovering the dope's memory is hard 
in that case (you can't do it until either the object or the access type go 
out of scope), and we didn't have a way to do it then (we hadn't implemented 
finalization at that time).

As I recall, when I complained about the problem and asked for advice, you 
guys added the "static matching" rule. Which eliminated this problem but 
also made many safe and easy cases illegal. (OT3H, some of those "safe" 
cases have other problems - see my response to Adam on Ada-Comment.) It is 
interesting, though, that virtually every complaint I've seen on this has 
always been asking about the case which is prohibitively expensive to 
implement (either in complexity or in space used) -- no one seems concerned 
about the "easy" cases that are illegal.

                                       Randy.