Re: Generics and parameter modes

[Martin Dowie <martin@dowie-cs.demon.co.uk>]

well, if this helps, i've now discovered that the object being passed was a
component of a record, but if i do a rename of that component use that in the
call, it changes to a by-reference!

what do you suspect now?!

(thanks for your help & matthew!)

Tucker Taft wrote:

> Martin Dowie wrote:
> >
> > i have an instance of a package similar to the generic listed below.
> > when calling the 'size' routine the actual parameter is passed by copy
> > and not by reference as i had expected. this is really annoying as the
> > data object is a few hundred kbytes in size! is there anything in Ada95
> > that allows/requires this?
>
> Non-limited, non-tagged record types may be passed either by reference
> or by copy.  On the other hand, I am quite surprised that a compiler
> would go out of its way to pass such a large object by copy, since it
> requires more work.  The only reasons I can imagine for doing this are:
>
>    1) the compiler is trying to enable more generic code sharing
>      (but that seems a bit far-fetched, since even in a shared
>       generic it "knows" a_list is a record type);
>    2) there is some kind of alignment problem, where the actual parameter
>      is not properly aligned for the code inside the generic (again
>      a bit far-fetched);
>    3) the actual parameter is marked volatile or atomic (see RM95 C.6(19));
>    4) convention C_Pass_By_Copy applies to the type (I don't see such
>        a pragma below);
>    5) the compiler has a bug ;-).
>
> Are you really sure that it is passing the parameter by copy?  I would
> contact Rational and file a lousy-code-quality-but-not-quite-a-bug report.
>
> > generic
> >     type an_item is private;
> >     maximum_number_of_items : positive;
> > package generic_list is
> >     subtype a_number_of_items is natural range 0 ..
> > maximum_number_of_items;
> >     subtype an_item_index is natural range 1 .. maximum_number_of_items;
> >
> >     type a_list is private;
> >     -- Lots of other stuff...
> >     function size (list : in a_list) return a_number_of_items;
> > private
> >     type an_array_of_items is array (an_item_index) of an_item;
> >     type a_list is record
> >         current_size, current_item : a_number_of_items;
> >         items : an_array_of_items;
> >     end record;
> > end generic_list;
> >
> > the type used in the declaration is a variant record which it self
> > contains variant fields within a number of the variant branches (approx.
> > 300 bytes storage required).
> >
> > i'm using Rational Apex v3 and have tried a mini version at home (using
> > the above) and ObjectAda seems to pass it by reference (not 100% on this
> > as my Intel assembler knowledge is zilch!).
>
> ObjectAda would pass such an object by reference unless the actual
> were misaligned (maladjusted? ;-), marked atomic or volatile, or had
> a C_Pass_By_Copy convention.
>
> --
> -Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
> Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
> AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA