[Ada] Fix misoptimization at -O2 in LTO mode

[Eric Botcazou]

This is a regression in recent releases. The problem is that the IPA mod/ref 
pass looks through the (nominal) type of a pointer-to-discriminated-type 
parameter in a call to a subprogram in order to see the (actual) type used for 
the dereferences of the parameter in the callee, which is a pointer-to-
constrained-subtype.

Historically the discriminated type is marked with the may_alias attribute 
because of the symmetric effect for the argument in the caller, so we mark
the constrained subtype with the attribute now for the sake of the callee.

Tested on x86-64/Linux, applied on the mainline, 14 and 13 branches.


2025-03-19  Eric Botcazou  <ebotca...@adacore.com>

        * gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Record_Subtype>:
        Set the may_alias attribute if a specific GCC type is built.

-- 
Eric Botcazou
diff --git a/gcc/ada/gcc-interface/decl.cc b/gcc/ada/gcc-interface/decl.cc
index 1854c587543..1694b4eb106 100644
--- a/gcc/ada/gcc-interface/decl.cc
+++ b/gcc/ada/gcc-interface/decl.cc
@@ -3651,6 +3651,14 @@ gnat_to_gnu_entity (Entity_Id gnat_entity, tree gnu_expr, bool definition)
 	      TYPE_PACKED (gnu_type) = TYPE_PACKED (gnu_base_type);
 	      TYPE_REVERSE_STORAGE_ORDER (gnu_type)
 		= Reverse_Storage_Order (gnat_entity);
+
+	      /* Do the same for subtypes as for the base type, since pointers
+		 to them may symmetrically also point to the latter.  */
+	      prepend_one_attribute
+		(&attr_list, ATTR_MACHINE_ATTRIBUTE,
+		 get_identifier ("may_alias"), NULL_TREE,
+		 gnat_entity);
+
 	      process_attributes (&gnu_type, &attr_list, true, gnat_entity);
 
 	      /* Set the size, alignment and alias set of the type to match