Fix misoptimization at -O2 in LTO mode

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.

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

diff --git a/gcc/ada/gcc-interface/decl.cc b/gcc/ada/gcc-interface/decl.cc
index ca174bff009730df0a2444ec1d910679f170f83d..53b5e5962181d0187efe338c0a314469e36b987c 100644 (file)
--- a/gcc/ada/gcc-interface/decl.cc
+++ b/gcc/ada/gcc-interface/decl.cc
@@ -3564,6 +3564,14 @@ 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