[Ada] Fix issue with Import_Valued_Procedure pragma

[Arnaud Charlet <charlet at adacore dot com>]

The problem pertains to passing components of records as by-reference
parameters to procedures, the  types of the components being by-copy
types; the trick here is pragma Import_Valued_Procedure and its siblings,
which let the user specify the passing mechanism on a per-procedure basis.
That is basically the same problem for Gigi as the Reference trick, because
it may wind up trying to take the address of something that was not meant
to  be addressed in the first place.
So Gigi has an ad-hoc mechanism to deal with non-addressable Out or In Out
parameters passed by-ref: it first creates a temporary, passes its address
to the procedure and then generates an explicit backwards copy operation in
order to update the actual parameter.
Clearly this mechanism is on the fringe of legality: 6.2 (2) reads "When a
parameter is passed by reference, the formal parameter denotes (a view of)
the object denoted by the actual parameter; reads and updates of the formal
parameter directly reference the actual parameter object."  Moreover we have
discovered that Starlet very likely makes questionable assumptions about
pragma Import_Valued_Procedure, which in particular are not fulfilled if the
above mechanism is triggered.  So it must be used very sparingly and the
compiler now issues a warning when it is triggered in presence of pragma
Import_Valued_Procedure or its siblings.
The predicate for the mechanism essentially is addressable_p from trans.c.
As of today, addressable_p is equivalent to !DECL_NONADDRESSABLE_P for
fields that are not bitfields; as a consequence, we now create temporaries
for any non-aliased components of records whose type is by-copy and for
which the ABI does not mandate by-reference, in particular non-aliased
scalar components.
The fix is to differentiate "semantics addressability" as conveyed by
DECL_NONADDRESSABLE_P from "technical addressability" as predicated by
addressable_p.  The former would of course imply the latter, but the latter
could be true even in the absence of the former.

Test case, only relevant to VMS should output:
  Status from ASCTOID:  1

pragma Extend_System (Aux_DEC);
with SYSTEM;
use  SYSTEM;
procedure p is
  subtype RIGHTS_ID_TYPE is SYSTEM.UNSIGNED_LONGWORD;
  subtype COND_VALUE_TYPE is SYSTEM.UNSIGNED_LONGWORD;
  procedure ASCTOID (
    STATUS : out COND_VALUE_TYPE;
    ID     : out RIGHTS_ID_TYPE
  );
  pragma INTERFACE (EXTERNAL, ASCTOID);
  pragma IMPORT_VALUED_PROCEDURE (ASCTOID, "SYS$ASCTOID",
    (COND_VALUE_TYPE, RIGHTS_ID_TYPE),
    (VALUE, REFERENCE));
  type ace_type is record
    size : unsigned_byte;
    key  : rights_id_type;
  end record;
  status         : cond_value_type;
  client_ace     : ace_type := (size  => 12, key => 0);
begin
    ASCTOID (status => status, id => client_ace.key);
end p;

2005-08-29  Arnaud Charlet  <charlet@adacore.com>
	    Eric Botcazou  <ebotcazou@adacore.com>

	* trans.c: Protect < in error msg with quote
	Replace GCC_ZCX by Back_End_Exceptions.
	(addressable_p) <COMPONENT_REF>: Also return 1 if the field
	has been sufficiently aligned in the record.

Attachment: difs.3
Description: Text document