* Generics and parameter modes
@ 1999-08-16 0:00 Martin Dowie
1999-08-17 0:00 ` Matthew Heaney
1999-08-17 0:00 ` Tucker Taft
0 siblings, 2 replies; 6+ messages in thread
From: Martin Dowie @ 1999-08-16 0:00 UTC (permalink / raw)
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?
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!).
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Generics and parameter modes
1999-08-16 0:00 Generics and parameter modes Martin Dowie
1999-08-17 0:00 ` Matthew Heaney
@ 1999-08-17 0:00 ` Tucker Taft
1999-08-21 0:00 ` Martin Dowie
1 sibling, 1 reply; 6+ messages in thread
From: Tucker Taft @ 1999-08-17 0:00 UTC (permalink / raw)
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Generics and parameter modes
1999-08-16 0:00 Generics and parameter modes Martin Dowie
@ 1999-08-17 0:00 ` Matthew Heaney
1999-08-17 0:00 ` Tucker Taft
1 sibling, 0 replies; 6+ messages in thread
From: Matthew Heaney @ 1999-08-17 0:00 UTC (permalink / raw)
In article <37B85F88.C87638C4@dowie-cs.demon.co.uk> , Martin Dowie
<martin@dowie-cs.demon.co.uk> 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?
>
> 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 ...
Do you mean "in the instantiation"?
> ...is a variant record which it self
> contains variant fields within a number of the variant branches (approx.
> 300 bytes storage required).
o You could make the list type limited, and implement the full view of the
type as limited:
type A_List is limited private;
...
private
...
type A_List is limited record ... end record;
You lose the assignment operator, but that's no big deal. Just declare a
Copy operation:
procedure Copy
(From : in A_List;
To : in out A_List);
o You can privately implement the type as tagged. Tagged types are always
passed by reference:
type A_List is private;
...
private
...
type A_List is tagged record ... end record;
--
Matt
It is impossible to feel great confidence in a negative theory which has
always rested its main support on the weak points of its opponent.
Joseph Needham, "A Mechanistic Criticism of Vitalism"
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Generics and parameter modes
1999-08-17 0:00 ` Tucker Taft
@ 1999-08-21 0:00 ` Martin Dowie
1999-08-21 0:00 ` Robert Dewar
0 siblings, 1 reply; 6+ messages in thread
From: Martin Dowie @ 1999-08-21 0:00 UTC (permalink / raw)
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Generics and parameter modes
1999-08-21 0:00 ` Martin Dowie
@ 1999-08-21 0:00 ` Robert Dewar
1999-08-21 0:00 ` Martin Dowie
0 siblings, 1 reply; 6+ messages in thread
From: Robert Dewar @ 1999-08-21 0:00 UTC (permalink / raw)
Tucker said
> > 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.
As would GNAT, indeed it is hard to imagine any other sensible
implementation choice!
Robert Dewar
Ada Core Technologies
Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Generics and parameter modes
1999-08-21 0:00 ` Robert Dewar
@ 1999-08-21 0:00 ` Martin Dowie
0 siblings, 0 replies; 6+ messages in thread
From: Martin Dowie @ 1999-08-21 0:00 UTC (permalink / raw)
sensible - yes
is it happening - no
:-(
Robert Dewar wrote:
> Tucker said
>
> > > 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.
>
> As would GNAT, indeed it is hard to imagine any other sensible
> implementation choice!
>
> Robert Dewar
> Ada Core Technologies
>
> Sent via Deja.com http://www.deja.com/
> Share what you know. Learn what you don't.
^ permalink raw reply [flat|nested] 6+ messages in thread
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1999-08-16 0:00 Generics and parameter modes Martin Dowie
1999-08-17 0:00 ` Matthew Heaney
1999-08-17 0:00 ` Tucker Taft
1999-08-21 0:00 ` Martin Dowie
1999-08-21 0:00 ` Robert Dewar
1999-08-21 0:00 ` Martin Dowie
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