[gcc r15-1322] ada: Do not create null GCC thunks

[Marc Poulhi?s]

https://gcc.gnu.org/g:55ceb87a72fee45a9a3e547a4e688f31d376a95a

commit r15-1322-g55ceb87a72fee45a9a3e547a4e688f31d376a95a
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Mon Apr 29 09:15:13 2024 +0200

    ada: Do not create null GCC thunks
    
    This prevents Gigi from creating null GCC thunks, i.e. thunks that have all
    their internal parameters set to zero, replacing them with aliases.  They
    can arise in degenerate cases and null thunks would trip on an assertion in
    former_thunk_p when they are later optimized.
    
    gcc/ada/
    
            PR ada/109817
            * gcc-interface/trans.cc (maybe_make_gnu_thunk): Create an alias
            instead of a null thunk.

Diff:
---
 gcc/ada/gcc-interface/trans.cc | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/gcc/ada/gcc-interface/trans.cc b/gcc/ada/gcc-interface/trans.cc
index 93978c0f0ba8..5256095dfeb6 100644
--- a/gcc/ada/gcc-interface/trans.cc
+++ b/gcc/ada/gcc-interface/trans.cc
@@ -11093,6 +11093,16 @@ maybe_make_gnu_thunk (Entity_Id gnat_thunk, tree gnu_thunk)
   tree gnu_interface_offset
     = gnu_interface_tag ? byte_position (gnu_interface_tag) : NULL_TREE;
 
+  /* But we generate a call to the Thunk_Entity in the thunk.  */
+  tree gnu_target
+    = gnat_to_gnu_entity (Thunk_Entity (gnat_thunk), NULL_TREE, false);
+
+  /* If the target is local, then thunk and target must have the same context
+     because cgraph_node::expand_thunk can only forward the static chain.  */
+  if (DECL_STATIC_CHAIN (gnu_target)
+      && DECL_CONTEXT (gnu_thunk) != DECL_CONTEXT (gnu_target))
+    return false;
+
   /* There are three ways to retrieve the offset between the interface view
      and the base object.  Either the controlling type covers the interface
      type and the offset of the corresponding tag is fixed, in which case it
@@ -11111,6 +11121,15 @@ maybe_make_gnu_thunk (Entity_Id gnat_thunk, tree gnu_thunk)
       virtual_value = 0;
       virtual_offset = NULL_TREE;
       indirect_offset = 0;
+
+      /* Do not create a null thunk, instead make it an alias.  */
+      if (fixed_offset == 0)
+	{
+	  SET_DECL_ASSEMBLER_NAME (gnu_thunk, DECL_ASSEMBLER_NAME (gnu_target));
+	  (void) cgraph_node::get_create (gnu_target);
+	  (void) cgraph_node::create_alias (gnu_thunk, gnu_target);
+	  return true;
+	}
     }
   else if (!gnu_interface_offset
 	   && !Is_Variable_Size_Record (gnat_controlling_type))
@@ -11132,16 +11151,6 @@ maybe_make_gnu_thunk (Entity_Id gnat_thunk, tree gnu_thunk)
       indirect_offset = (HOST_WIDE_INT) (POINTER_SIZE / BITS_PER_UNIT);
     }
 
-  /* But we generate a call to the Thunk_Entity in the thunk.  */
-  tree gnu_target
-    = gnat_to_gnu_entity (Thunk_Entity (gnat_thunk), NULL_TREE, false);
-
-  /* If the target is local, then thunk and target must have the same context
-     because cgraph_node::expand_thunk can only forward the static chain.  */
-  if (DECL_STATIC_CHAIN (gnu_target)
-      && DECL_CONTEXT (gnu_thunk) != DECL_CONTEXT (gnu_target))
-    return false;
-
   /* If the target returns by invisible reference and is external, apply the
      same transformation as Subprogram_Body_to_gnu here.  */
   if (TREE_ADDRESSABLE (TREE_TYPE (gnu_target))