[C++ PATCH] extern "C" checking

[C++ PATCH] extern "C" checking

[Nathan Sidwell]

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This patch breaks a bit more out of pushdecl.  When pushing an extern 
"C" function, we have to go check for other instances of it in different 
namespaces.  We had lookup_extern_c_fun_in_all_ns to go find it in a 
different namespace.  Looking for a name across all namespaces is the 
one thing our current data representation could do well.  Pity we only 
need it for this case, (and the other use cases suck).

This patch adds a new hash table to record such extern C lists -- and 
commonly of course there are no other instances.  We have to have a new 
hash trait specialization for identifier_node, so that we can use the 
identifier's hash value and not the raw pointer -- because of PCH 
remapping possibilities.  I'll be using this hash table type more later 
on, so this isn't a one-off just for this use.

If it wasn't for redefine_extname pragma we'd only ever need to record a 
single decl per hash slot.

Later patches will use check_extern_c_conflict on another code path, 
where we fail to check properly at the moment.

nathan
-- 
Nathan Sidwell

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2017-05-17  Nathan Sidwell  <nathan@acm.org>

	* cp-tree.h (default_hash_traits <lang_identifier *>): New
	specialization.
	* name-lookup.c (lookup_extern_c_fun_in_all_ns): Delete.
	(extern_c_fns): New hash table.
	(check_extern_c_conflict): New, broken out of ...
	(pushdecl_maybe_friend_1): ... here.  Call it.
	(c_linkage_bindings): Just look in hash table.

Index: cp-tree.h
===================================================================
--- cp-tree.h	(revision 248151)
+++ cp-tree.h	(working copy)
@@ -535,6 +535,26 @@ identifier_p (tree t)
   return NULL;
 }
 
+/* Hash trait specialization for lang_identifiers.  This allows
+   PCH-safe maps keyed by DECL_NAME.  If it wasn't for PCH, we could
+   just use a regular tree key.  */
+
+template <>
+struct default_hash_traits <lang_identifier *>
+  : pointer_hash <tree_node>, ggc_remove <tree>
+{
+  /* Use a regular tree as the type, to make using the hash table
+     simpler.  We'll get dynamic type checking with the hash function
+     itself.  */
+  GTY((skip)) typedef tree value_type;
+  GTY((skip)) typedef tree compare_type;
+
+  static hashval_t hash (const value_type &id)
+  {
+    return IDENTIFIER_HASH_VALUE (id);
+  }
+};
+
 /* In an IDENTIFIER_NODE, nonzero if this identifier is actually a
    keyword.  C_RID_CODE (node) is then the RID_* value of the keyword.  */
 
Index: name-lookup.c
===================================================================
--- name-lookup.c	(revision 248151)
+++ name-lookup.c	(working copy)
@@ -60,7 +60,6 @@ static void consider_binding_level (tree
 				    enum lookup_name_fuzzy_kind kind);
 static tree lookup_type_current_level (tree);
 static tree push_using_directive (tree);
-static tree lookup_extern_c_fun_in_all_ns (tree);
 static void diagnose_name_conflict (tree, tree);
 
 /* Add DECL to the list of things declared in B.  */
@@ -1184,6 +1183,75 @@ supplement_binding (cxx_binding *binding
   return ret;
 }
 
+/* Map of identifiers to extern C functions (or LISTS thereof).  */
+
+static GTY(()) hash_map<lang_identifier *, tree> *extern_c_fns;
+
+/* DECL has C linkage. If we have an existing instance, make sure it
+   has the same exception specification [7.5, 7.6].  If there's no
+   instance, add DECL to the map.  */
+
+static void
+check_extern_c_conflict (tree decl)
+{
+  /* Ignore artificial or system header decls.  */
+  if (DECL_ARTIFICIAL (decl) || DECL_IN_SYSTEM_HEADER (decl))
+    return;
+
+  if (!extern_c_fns)
+    extern_c_fns = hash_map<lang_identifier *,tree>::create_ggc (127);
+
+  bool existed;
+  tree *slot = &extern_c_fns->get_or_insert (DECL_NAME (decl), &existed);
+  if (!existed)
+    *slot = decl;
+  else
+    {
+      tree old = *slot;
+      if (TREE_CODE (old) == TREE_LIST)
+	old = TREE_VALUE (old);
+
+      int mismatch = 0;
+      if (DECL_CONTEXT (old) == DECL_CONTEXT (decl))
+	; /* If they're in the same context, we'll have already complained
+	     about a (possible) mismatch, when inserting the decl.  */
+      else if (!decls_match (decl, old))
+	mismatch = 1;
+      else if (!comp_except_specs (TYPE_RAISES_EXCEPTIONS (TREE_TYPE (old)),
+				   TYPE_RAISES_EXCEPTIONS (TREE_TYPE (decl)),
+				   ce_normal))
+	mismatch = -1;
+      else if (DECL_ASSEMBLER_NAME_SET_P (old))
+	SET_DECL_ASSEMBLER_NAME (decl, DECL_ASSEMBLER_NAME (old));
+
+      if (mismatch)
+	{
+	  pedwarn (input_location, 0,
+		   "declaration of %q#D with C language linkage", decl);
+	  pedwarn (DECL_SOURCE_LOCATION (old), 0,
+		   "conflicts with previous declaration %q#D", old);
+	  if (mismatch < 0)
+	    pedwarn (input_location, 0,
+		     "due to different exception specifications");
+	}
+      else
+	/* Chain it on for c_linkage_binding's use.  */
+	*slot = tree_cons (NULL_TREE, decl, *slot);
+    }
+}
+
+/* Returns a list of C-linkage decls with the name NAME.  Used in
+   c-family/c-pragma.c to implement redefine_extname pragma.  */
+
+tree
+c_linkage_bindings (tree name)
+{
+  if (extern_c_fns)
+    if (tree *slot = extern_c_fns->get (name))
+      return *slot;
+  return NULL_TREE;
+}
+
 /* DECL is being declared at a local scope.  Emit suitable shadow
    warnings.  */
 
@@ -1591,63 +1659,6 @@ pushdecl_maybe_friend_1 (tree x, bool is
 	    }
 	}
 
-      /* If x has C linkage-specification, (extern "C"),
-	 lookup its binding, in case it's already bound to an object.
-	 The lookup is done in all namespaces.
-	 If we find an existing binding, make sure it has the same
-	 exception specification as x, otherwise, bail in error [7.5, 7.6].  */
-      if ((TREE_CODE (x) == FUNCTION_DECL)
-	  && DECL_EXTERN_C_P (x)
-          /* We should ignore declarations happening in system headers.  */
-	  && !DECL_ARTIFICIAL (x)
-	  && !DECL_IN_SYSTEM_HEADER (x))
-	{
-	  tree previous = lookup_extern_c_fun_in_all_ns (x);
-	  if (previous
-	      && !DECL_ARTIFICIAL (previous)
-              && !DECL_IN_SYSTEM_HEADER (previous)
-	      && DECL_CONTEXT (previous) != DECL_CONTEXT (x))
-	    {
-	      /* In case either x or previous is declared to throw an exception,
-	         make sure both exception specifications are equal.  */
-	      if (decls_match (x, previous))
-		{
-		  tree x_exception_spec = NULL_TREE;
-		  tree previous_exception_spec = NULL_TREE;
-
-		  x_exception_spec =
-				TYPE_RAISES_EXCEPTIONS (TREE_TYPE (x));
-		  previous_exception_spec =
-				TYPE_RAISES_EXCEPTIONS (TREE_TYPE (previous));
-		  if (!comp_except_specs (previous_exception_spec,
-					  x_exception_spec,
-					  ce_normal))
-		    {
-		      pedwarn (input_location, 0,
-                               "declaration of %q#D with C language linkage",
-			       x);
-		      pedwarn (DECL_SOURCE_LOCATION (previous), 0,
-                               "conflicts with previous declaration %q#D",
-			       previous);
-		      pedwarn (input_location, 0,
-                               "due to different exception specifications");
-		      return error_mark_node;
-		    }
-		  if (DECL_ASSEMBLER_NAME_SET_P (previous))
-		    SET_DECL_ASSEMBLER_NAME (x,
-					     DECL_ASSEMBLER_NAME (previous));
-		}
-	      else
-		{
-		  pedwarn (input_location, 0,
-			   "declaration of %q#D with C language linkage", x);
-		  pedwarn (DECL_SOURCE_LOCATION (previous), 0,
-			   "conflicts with previous declaration %q#D",
-			   previous);
-		}
-	    }
-	}
-
       check_template_shadow (x);
 
       /* If this is a function conjured up by the back end, massage it
@@ -1848,6 +1859,9 @@ pushdecl_maybe_friend_1 (tree x, bool is
 
       if (VAR_P (x))
 	maybe_register_incomplete_var (x);
+      if (TREE_CODE (x) == FUNCTION_DECL && DECL_EXTERN_C_P (x))
+	/* We need to check and register the fn now.  */
+	check_extern_c_conflict (x);
     }
 
   if (need_new_binding)
@@ -2722,74 +2736,6 @@ binding_for_name (cp_binding_level *scop
   return result;
 }
 
-/* Walk through the bindings associated to the name of FUNCTION,
-   and return the first declaration of a function with a
-   "C" linkage specification, a.k.a 'extern "C"'.
-   This function looks for the binding, regardless of which scope it
-   has been defined in. It basically looks in all the known scopes.
-   Note that this function does not lookup for bindings of builtin functions
-   or for functions declared in system headers.  */
-static tree
-lookup_extern_c_fun_in_all_ns (tree function)
-{
-  tree name;
-  cxx_binding *iter;
-
-  gcc_assert (function && TREE_CODE (function) == FUNCTION_DECL);
-
-  name = DECL_NAME (function);
-  gcc_assert (name && identifier_p (name));
-
-  for (iter = IDENTIFIER_NAMESPACE_BINDINGS (name);
-       iter;
-       iter = iter->previous)
-    {
-      tree ovl;
-      for (ovl = iter->value; ovl; ovl = OVL_NEXT (ovl))
-	{
-	  tree decl = OVL_CURRENT (ovl);
-	  if (decl
-	      && TREE_CODE (decl) == FUNCTION_DECL
-	      && DECL_EXTERN_C_P (decl)
-	      && !DECL_ARTIFICIAL (decl))
-	    {
-	      return decl;
-	    }
-	}
-    }
-  return NULL;
-}
-
-/* Returns a list of C-linkage decls with the name NAME.  */
-
-tree
-c_linkage_bindings (tree name)
-{
-  tree decls = NULL_TREE;
-  cxx_binding *iter;
-
-  for (iter = IDENTIFIER_NAMESPACE_BINDINGS (name);
-       iter;
-       iter = iter->previous)
-    {
-      tree ovl;
-      for (ovl = iter->value; ovl; ovl = OVL_NEXT (ovl))
-	{
-	  tree decl = OVL_CURRENT (ovl);
-	  if (decl
-	      && DECL_EXTERN_C_P (decl)
-	      && !DECL_ARTIFICIAL (decl))
-	    {
-	      if (decls == NULL_TREE)
-		decls = decl;
-	      else
-		decls = tree_cons (NULL_TREE, decl, decls);
-	    }
-	}
-    }
-  return decls;
-}
-
 /* Insert another USING_DECL into the current binding level, returning
    this declaration. If this is a redeclaration, do nothing, and
    return NULL_TREE if this not in namespace scope (in namespace