From: Eric Botcazou Date: Wed, 18 Dec 2019 07:14:18 +0000 (+0000) Subject: [Ada] Document the introduction of the Object_Size attribute in Ada 2020 X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=c846eedd56a5ecf28233d5e8fe938d44b3add5f3;p=gcc.git [Ada] Document the introduction of the Object_Size attribute in Ada 2020 2019-12-18 Eric Botcazou gcc/ada/ * einfo.ads (Handling of Type'Size Value): Add references to the introduction of Object_Size in Ada 2020. * sem_eval.adb (Subtypes_Statically_Match): Likewise. From-SVN: r279506 --- diff --git a/gcc/ada/ChangeLog b/gcc/ada/ChangeLog index 6f6caf45642..2b845d4cb57 100644 --- a/gcc/ada/ChangeLog +++ b/gcc/ada/ChangeLog @@ -1,3 +1,9 @@ +2019-12-18 Eric Botcazou + + * einfo.ads (Handling of Type'Size Value): Add references to the + introduction of Object_Size in Ada 2020. + * sem_eval.adb (Subtypes_Statically_Match): Likewise. + 2019-12-18 Bob Duff * sem_ch3.adb (Derive_Subprogram): Do not set the diff --git a/gcc/ada/einfo.ads b/gcc/ada/einfo.ads index d5a26d1d638..cb0736aa085 100644 --- a/gcc/ada/einfo.ads +++ b/gcc/ada/einfo.ads @@ -132,19 +132,23 @@ package Einfo is -- 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 @@ -158,9 +162,11 @@ package Einfo is -- 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 @@ -182,7 +188,7 @@ package Einfo is -- 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 @@ -194,14 +200,15 @@ package Einfo is -- 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 diff --git a/gcc/ada/sem_eval.adb b/gcc/ada/sem_eval.adb index 5f26ecd834d..a08284759f8 100644 --- a/gcc/ada/sem_eval.adb +++ b/gcc/ada/sem_eval.adb @@ -5905,7 +5905,8 @@ package body Sem_Eval is -- 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; @@ -5921,8 +5922,6 @@ package body Sem_Eval is -- 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)