[c++] convert keyed_classes to a VEC

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 73bcb75..6f9d308 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -112,6 +112,10 @@  static GTY (()) tree sizeof_biggest_empty_class;
    declaration order.  */
 VEC(tree,gc) *local_classes;
 
+/* A list of the dynamic classes whose vtables may have to be
+   emitted in this translation unit.  */
+VEC(tree,gc) *keyed_classes;
+
 static tree get_vfield_name (tree);
 static void finish_struct_anon (tree);
 static tree get_vtable_name (tree);
@@ -5450,7 +5454,7 @@  finish_struct_1 (tree t)
       /* If a polymorphic class has no key method, we may emit the vtable
 	 in every translation unit where the class definition appears.  */
       if (CLASSTYPE_KEY_METHOD (t) == NULL_TREE)
-	keyed_classes = tree_cons (NULL_TREE, t, keyed_classes);
+	VEC_safe_push (tree, gc, keyed_classes, t);
     }
 
   /* Layout the class itself.  */
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index baa6656..b669856 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -793,8 +793,6 @@  enum cp_tree_index
     CPTI_DSO_HANDLE,
     CPTI_DCAST,
 
-    CPTI_KEYED_CLASSES,
-
     CPTI_NULLPTR,
     CPTI_NULLPTR_TYPE,
 
@@ -906,11 +904,6 @@  extern GTY(()) tree cp_global_trees[CPTI_MAX];
    destructors.  */
 #define vtt_parm_type			cp_global_trees[CPTI_VTT_PARM_TYPE]
 
-/* A TREE_LIST of the dynamic classes whose vtables may have to be
-   emitted in this translation unit.  */
-
-#define keyed_classes			cp_global_trees[CPTI_KEYED_CLASSES]
-
 /* Node to indicate default access. This must be distinct from the
    access nodes in tree.h.  */
 
@@ -4029,6 +4022,10 @@  extern int current_class_depth;
 /* An array of all local classes present in this translation unit, in
    declaration order.  */
 extern GTY(()) VEC(tree,gc) *local_classes;
+
+/* A list of the dynamic classes whose vtables may have to be
+   emitted in this translation unit.  */
+extern GTY(()) VEC(tree,gc) *keyed_classes;
 
 /* Here's where we control how name mangling takes place.  */
 
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 4e4a277..5db4215 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -12577,7 +12577,7 @@  record_key_method_defined (tree fndecl)
     {
       tree fnclass = DECL_CONTEXT (fndecl);
       if (fndecl == CLASSTYPE_KEY_METHOD (fnclass))
-	keyed_classes = tree_cons (NULL_TREE, fnclass, keyed_classes);
+	VEC_safe_push (tree, gc, keyed_classes, fnclass);
     }
 }
 
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index a768877..c3a93ad 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -3643,30 +3643,38 @@  cp_write_global_declarations (void)
 	 vtables then we remove the class from our list so we don't
 	 have to look at it again.  */
 
-      while (keyed_classes != NULL_TREE
-	     && maybe_emit_vtables (TREE_VALUE (keyed_classes)))
+      while (!VEC_empty (tree, keyed_classes)
+	     && maybe_emit_vtables (VEC_last (tree, keyed_classes)))
 	{
 	  reconsider = true;
-	  keyed_classes = TREE_CHAIN (keyed_classes);
+	  VEC_pop (tree, keyed_classes);
 	}
 
-      t = keyed_classes;
-      if (t != NULL_TREE)
+      /* The last element of the vector (if it exists) has not been
+	 written out.  We still need to iterate through any remaining
+	 members and write them out if necessary.  Do this by
+	 maintaining two indices into the vector: one for the next
+	 candidate to consider and another for the point at which known
+	 unwritten entries start.  */
+
+      if (!VEC_empty (tree, keyed_classes))
 	{
-	  tree next = TREE_CHAIN (t);
+	  int unwritten = VEC_length (tree, keyed_classes) - 1;
+	  int next_idx = unwritten - 1;
 
-	  while (next)
+	  while (next_idx >= 0)
 	    {
-	      if (maybe_emit_vtables (TREE_VALUE (next)))
-		{
-		  reconsider = true;
-		  TREE_CHAIN (t) = TREE_CHAIN (next);
-		}
+	      tree candidate = VEC_index (tree, keyed_classes, next_idx--);
+	      if (maybe_emit_vtables (candidate))
+		reconsider = true;
 	      else
-		t = next;
-
-	      next = TREE_CHAIN (t);
+		/* We didn't write out CANDIDATE.  We need to slide it
+		   to the front of the unwritten entries.  */
+		VEC_replace (tree, keyed_classes, --unwritten, candidate);
 	    }
+
+	  /* All done.  Slide all the unwritten elements down.  */
+	  VEC_block_remove (tree, keyed_classes, 0, unwritten);
 	}
 
       /* Write out needed type info variables.  We have to be careful
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index bb6b1a0..ef9e759 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8251,7 +8251,7 @@  instantiate_class_template (tree type)
      method, however, finish_struct_1 will already have added TYPE to
      the keyed_classes list.  */
   if (TYPE_CONTAINS_VPTR_P (type) && CLASSTYPE_KEY_METHOD (type))
-    keyed_classes = tree_cons (NULL_TREE, type, keyed_classes);
+    VEC_safe_push (tree, gc, keyed_classes, type);
 
   return type;
 }