[gcc r16-1820] ada: Make class-wide Max_Size_In_Storage_Elements return a large value

[Marc Poulhies]

https://gcc.gnu.org/g:f2063c92d30d6b94f7d023ff0caf96c0b3e439ea

commit r16-1820-gf2063c92d30d6b94f7d023ff0caf96c0b3e439ea
Author: Bob Duff <duff@adacore.com>
Date:   Tue Apr 29 13:12:44 2025 -0400

    ada: Make class-wide Max_Size_In_Storage_Elements return a large value
    
    Max_Size_In_Storage_Elements is supposed to return a value greater or
    equal to what is passed for any heap allocation for an object of the
    type. For a tagged type T, we don't know the allocation size for
    descendants; therefore T'Class'Max_Size_In_Storage_Elements should
    return a huge number. In particular, it now returns Storage_Count'Last,
    which is greater than any possible heap allocation.
    
    Previously, T'Class'Max_Size_In_Storage_Elements was returning
    the same value as T'Max_Size_In_Storage_Elements, which was
    wrong.
    
    gcc/ada/ChangeLog:
    
            * exp_attr.adb (Attribute_Max_Size_In_Storage_Elements):
            Return Storage_Count'Last converted to universal_integer.

Diff:
---
 gcc/ada/exp_attr.adb | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
index 3d1bff93b408..0f09ba587acc 100644
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -5353,22 +5353,42 @@ package body Exp_Attr is
          Typ : constant Entity_Id := Etype (N);
 
       begin
-         --  If the prefix is X'Class, we transform it into a direct reference
-         --  to the class-wide type, because the back end must not see a 'Class
-         --  reference. See also 'Size.
+         --  Tranform T'Class'Max_Size_In_Storage_Elements (for any T) into
+         --  Storage_Count'Pos (Storage_Count'Last), because it must include
+         --  all descendants, which can be arbitrarily large. Note that the
+         --  back end must not see any 'Class attribute references.
+         --  The 'Pos is to make it be of type universal_integer.
+         --
+         --  ???If T'Class'Size is specified, it should probably affect
+         --  T'Class'Max_Size_In_Storage_Elements accordingly.
 
          if Is_Entity_Name (Pref)
            and then Is_Class_Wide_Type (Entity (Pref))
          then
-            Rewrite (Prefix (N), New_Occurrence_Of (Entity (Pref), Loc));
-            return;
-         end if;
+            declare
+               Storage_Count_Type : constant Entity_Id :=
+                 RTE (RE_Storage_Count);
+               Attr : constant Node_Id :=
+                 Make_Attribute_Reference (Loc,
+                   Prefix => New_Occurrence_Of (Storage_Count_Type, Loc),
+                   Attribute_Name => Name_Pos,
+                   Expressions => New_List (
+                     Make_Attribute_Reference (Loc,
+                       Prefix => New_Occurrence_Of (Storage_Count_Type, Loc),
+                       Attribute_Name => Name_Last)));
+            begin
+               Rewrite (N, Attr);
+               Analyze_And_Resolve (N, Typ);
+               return;
+            end;
 
          --  Heap-allocated controlled objects contain two extra pointers which
          --  are not part of the actual type. Transform the attribute reference
          --  into a runtime expression to add the size of the hidden header.
 
-         if Needs_Finalization (Ptyp) and then not Header_Size_Added (N) then
+         elsif Needs_Finalization (Ptyp)
+           and then not Header_Size_Added (N)
+         then
             Set_Header_Size_Added (N);
 
             --  Generate: