Add C2C.atom_needs_GOT_access

This is used on ELF platforms to determine which symbols are relocatable.

Author: xavierleroy

cfrontend/C2C.ml

@@ -52,7 +52,10 @@ let atom_is_static a =
   with Not_found ->
     false
 
-(* Is it possible for symbol [a] to be defined in a DLL? *)
+(* Is it possible for symbol [a] to be defined in a DLL?
+   Yes, unless [a] is defined in the current compilation unit, or is static.
+   (This criterion is appropriate for macOS and for Cygwin; for ELF,
+    see [atom_needs_GOT_access] below.)  *)
 let atom_is_external a =
   match Hashtbl.find decl_atom a with
   | { a_defined = true } -> false
@@ -61,6 +64,18 @@ let atom_is_external a =
   | _ -> true
   | exception Not_found -> true
 
+(* In ELF PIC code, the rules are different: all non-static symbols
+   must be accessed through the GOT, even if they are defined in the
+   current compilation unit.  (This is to allow symbol interposition by
+   the dynamic loader.)  *)
+let atom_needs_GOT_access a =
+  !Clflags.option_fpic &&
+  begin match Hashtbl.find decl_atom a with
+  | { a_storage = C.Storage_static } -> false
+  | _ -> true
+  | exception Not_found -> true
+  end
+
 let atom_alignof a =
   try
     (Hashtbl.find decl_atom a).a_alignment