-- default size of objects, creates chaos, and major incompatibilities in
-- existing code.
+-- The Ada 2020 RM acknowledges it and adopts GNAT's Object_Size attribute
+-- for determining the default size of objects, but stops short of applying
+-- it universally like GNAT. Indeed the notable exceptions are nonaliased
+-- stand-alone objects, which are not covered by Object_Size in Ada 2020.
+
-- We proceed as follows, for discrete and fixed-point subtypes, we have
-- two separate sizes for each subtype:
-- The Object_Size, which is used for determining the default size of
-- objects and components. This size value can be referred to using the
-- Object_Size attribute. The phrase "is used" here means that it is
--- the basis of the determination of the size. The backend is free to
+-- the basis of the determination of the size. The back end is free to
-- pad this up if necessary for efficiency, e.g. an 8-bit stand-alone
-- character might be stored in 32 bits on a machine with no efficient
-- byte access instructions such as the Alpha.
--- The default rules for the value of Object_Size for fixed-point and
--- discrete types are as follows:
+-- The default rules for the value of Object_Size are as follows:
-- The Object_Size for base subtypes reflect the natural hardware
-- size in bits (see Ttypes and Cstand for integer types). For
-- base type, and the Object_Size of a derived first subtype is copied
-- from the parent first subtype.
--- The Value_Size which is the number of bits required to store a value
+-- The Ada 2020 RM defined attribute Object_Size uses this implementation.
+
+-- The Value_Size, which is the number of bits required to store a value
-- of the type. This size can be referred to using the Value_Size
--- attribute. This value is used to determine how tightly to pack
+-- attribute. This value is used for determining how tightly to pack
-- records or arrays with components of this type, and also affects
-- the semantics of unchecked conversion (unchecked conversions where
-- the Value_Size values differ generate a warning, and are potentially
-- dynamic bounds, it is assumed that the value can range down or up
-- to the corresponding bound of the ancestor.
--- The RM defined attribute Size corresponds to the Value_Size attribute.
+-- The Ada 95 RM defined attribute Size is identified with Value_Size.
-- The Size attribute may be defined for a first-named subtype. This sets
-- the Value_Size of the first-named subtype to the given value, and the
-- subtypes. The Value_Size of any other static subtypes is not affected.
-- Value_Size and Object_Size may be explicitly set for any subtype using
--- an attribute definition clause. Note that the use of these attributes
--- can cause the RM 13.1(14) rule to be violated. If two access types
--- reference aliased objects whose subtypes have differing Object_Size
--- values as a result of explicit attribute definition clauses, then it
--- is erroneous to convert from one access subtype to the other.
-
--- At the implementation level, Esize stores the Object_Size and the
--- RM_Size field stores the Value_Size (and hence the value of the
+-- an attribute definition clause. Note that the use of such a clause can
+-- cause the RM 13.1(14) rule to be violated, in Ada 95 and 2020 for the
+-- Value_Size attribute, but only in Ada 95 for the Object_Size attribute.
+-- If access types reference aliased objects whose subtypes have differing
+-- Object_Size values as a result of explicit attribute definition clauses,
+-- then it is erroneous to convert from one access subtype to the other.
+
+-- At the implementation level, the Esize field stores the Object_Size
+-- and the RM_Size field stores the Value_Size (hence the value of the
-- Size attribute, which, as noted above, is equivalent to Value_Size).
-- To get a feel for the difference, consider the following examples (note
-- In addition, in GNAT, the object size (Esize) values of the types must
-- match if they are set (unless checking an actual for a formal derived
-- type). The use of 'Object_Size can cause this to be false even if the
- -- types would otherwise match in the RM sense.
+ -- types would otherwise match in the Ada 95 RM sense, but this deviation
+ -- is adopted by AI12-059 which introduces Object_Size in Ada 2020.
function Subtypes_Statically_Match
(T1 : Entity_Id;
-- No match if sizes different (from use of 'Object_Size). This test
-- is excluded if Formal_Derived_Matching is True, as the base types
-- can be different in that case and typically have different sizes.
- -- ??? Frontend_Layout_On_Target used to set Esizes but this is no
- -- longer the case, consider removing the last test below.
elsif not Formal_Derived_Matching
and then Known_Static_Esize (T1)
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