2004-08-13 Olivier Hainque <hainque@act-europe.fr>

	* decl.c (gnat_to_gnu_entity) <E_Variable>: When building an allocator
	for a global aliased object with a variable size and an unconstrained
	nominal subtype, pretend there is no initializer if the one we have is
	incomplete, and avoid referencing an inexistant component in there. The
	part we have will be rebuilt anyway and the reference may confuse
	further operations.

2004-08-13  Thomas Quinot  <quinot@act-europe.fr>

	* einfo.ads: Minor reformatting

	* lib-writ.adb (Output_Main_Program_Line): Do not set parameter
	restrictions in the ALI if we only want to warn about violations.

2004-08-13  Vincent Celier  <celier@gnat.com>

	* ali.adb (Scan_ALI): Initialize component Body_Needed_For_SAL to False
	when creating a new Unit_Record in table Units.

	* gnatls.adb (Output_Unit): In verbose mode, output the restrictions
	that are violated, if any.

	* prj-nmsc.adb (Ada_Check.Get_Path_Names_And_Record_Sources): Do not
	add directory separator if path already ends with a directory separator.

2004-08-13  Ed Schonberg  <schonberg@gnat.com>

	* rtsfind.adb (Entity_Not_Defined): If the error ocurrs in a predefined
	unit, this is an attempt to inline a construct that is not available in
	the current restricted mode, so abort rather than trying to continue.

	* sem_ch3.adb (Build_Underlying_Full_View): If the new type has
	discriminants that rename those of the parent, recover names of
	original discriminants for the constraint on the full view of the
	parent.
	(Complete_Private_Subtype): Do not create a subtype declaration if the
	subtype is an itype.

	* gnat_rm.texi: Added section on implementation of discriminated
	records with default values for discriminants.

2004-08-13  Ed Schonberg  <schonberg@gnat.com>

	PR ada/15601
	* sem_res.adb (Make_Call_Into_Operator): Handle properly the case where
	the second operand is overloaded.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@85934 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 94 insertions, 0 deletions

diff --git a/gcc/ada/gnat_rm.texi b/gcc/ada/gnat_rm.texi
index d3d28367e88..82c390ab34f 100644
--- a/gcc/ada/gnat_rm.texi
+++ b/gcc/ada/gnat_rm.texi
@@ -380,6 +380,7 @@ Implementation of Specific Ada Features
 * GNAT Implementation of Tasking::
 * GNAT Implementation of Shared Passive Packages::
 * Code Generation for Array Aggregates::
+* The Size of Discriminated Records with Default Discriminants::
 
 Project File Reference
 
@@ -12798,6 +12799,7 @@ facilities.
 * GNAT Implementation of Tasking::
 * GNAT Implementation of Shared Passive Packages::
 * Code Generation for Array Aggregates::
+* The Size of Discriminated Records with Default Discriminants::
 @end menu
 
 @node Machine Code Insertions
@@ -13342,6 +13344,98 @@ If any of these conditions are violated, the aggregate will be built in
 a temporary (created either by the front-end or the code generator) and then
 that temporary will be copied onto the target.
 
+
+@node The Size of Discriminated Records with Default Discriminants
+@section The Size of Discriminated Records with Default Discriminants
+
+@noindent
+If a discriminated type @code{T} has discriminants with default values, it is
+possible to declare an object of this type without providing an explicit
+constraint:
+
+@smallexample @c ada
+@group
+type Size is range 1..100;
+
+type Rec (D : Size := 15) is record
+   Name : String (1..D);
+end T;
+
+Word : Rec;
+@end group
+@end smallexample
+
+@noindent 
+Such an object is said to be @emph{unconstrained}.
+The discriminant of the object
+can be modified by a full assignment to the object, as long as it preserves the
+relation between the value of the discriminant, and the value of the components
+that depend on it:
+
+@smallexample @c ada
+@group
+Word := (3, "yes");
+
+Word := (5, "maybe");
+
+Word := (5, "no"); -- raises Constraint_Error
+@end group
+@end smallexample
+
+@noindent
+In order to support this behavior efficiently, an unconstrained object is
+given the maximum size that any value of the type requires. In the case
+above, @code{Word} has storage for the discriminant and for
+a @code{String} of length 100.
+It is important to note that unconstrained objects do not require dynamic
+allocation. It would be an improper implementation to place on the heap those
+components whose size depends on discriminants. (This improper implementation
+was used by some Ada83 compilers, where the @code{Name} component above
+would have
+been stored as a pointer to a dynamic string). Following the principle that
+dynamic storage management should never be introduced implicitly,
+an Ada95 compiler should reserve the full size for an unconstrained declared
+object, and place it on the stack.
+
+This maximum size approach
+has been a source of surprise to some users, who expect the default
+values of the discriminants to determine the size reserved for an
+unconstrained object: ``If the default is 15, why should the object occupy
+a larger size?''
+The answer, of course, is that the discriminant may be later modified,
+and its full range of values must be taken into account. This is why the
+declaration:
+
+@smallexample
+@group
+type Rec (D : Positive := 15) is record
+   Name : String (1..D);
+end record;
+
+Too_Large : Rec;
+@end group
+@end smallexample
+
+@noindent
+is flagged by the compiler with a warning:
+an attempt to create @code{Too_Large} will raise @code{Storage_Error},
+because the required size includes @code{Positive'Last}
+bytes. As the first example indicates, the proper approach is to declare an
+index type of ``reasonable'' range so that unconstrained objects are not too
+large.
+
+One final wrinkle: if the object is declared to be @code{aliased}, or if it is
+created in the heap by means of an allocator, then it is @emph{not}
+unconstrained:
+it is constrained by the default values of the discriminants, and those values
+cannot be modified by full assignment. This is because in the presence of
+aliasing all views of the object (which may be manipulated by different tasks,
+say) must be consistent, so it is imperative that the object, once created,
+remain invariant.
+
+
+
+
 @node Project File Reference
 @chapter Project File Reference