qom: clean up/optimize object_dynamic_cast

The interface loop can be performed only on the parent object.  It
does not need to be done on each interface.  Similarly, we can
simplify the code by switching early from the implementation
object to the parent object.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qom/object.c

@@ -368,11 +368,9 @@ void object_delete(Object *obj)
     g_free(obj);
 }
 
-static bool object_is_type(Object *obj, const char *typename)
+static bool type_is_ancestor(TypeImpl *type, TypeImpl *target_type)
 {
-    TypeImpl *target_type = type_get_by_name(typename);
-    TypeImpl *type = obj->class->type;
-    GSList *i;
+    assert(target_type);
 
     /* Check if typename is a direct ancestor of type */
     while (type) {
@@ -383,24 +381,50 @@ static bool object_is_type(Object *obj, const char *typename)
         type = type_get_parent(type);
     }
 
-    /* Check if obj has an interface of typename */
-    for (i = obj->interfaces; i; i = i->next) {
-        Interface *iface = i->data;
+    return false;
+}
 
-        if (object_is_type(OBJECT(iface), typename)) {
-            return true;
-        }
-    }
+static bool object_is_type(Object *obj, const char *typename)
+{
+    TypeImpl *target_type;
 
-    return false;
+    if (typename == TYPE_OBJECT) {
+        return true;
+    }
+    target_type = type_get_by_name(typename);
+    return type_is_ancestor(obj->class->type, target_type);
 }
 
 Object *object_dynamic_cast(Object *obj, const char *typename)
 {
     GSList *i;
 
-    /* Check if typename is a direct ancestor */
-    if (object_is_type(obj, typename)) {
+    /* Check if typename is a direct ancestor.  Special-case TYPE_OBJECT,
+     * we want to go back from interfaces to the parent.
+    */
+    if (typename && object_is_type(obj, typename)) {
+        return obj;
+    }
+
+    /* Check if obj is an interface and its containing object is a direct
+     * ancestor of typename.  In principle we could do this test at the very
+     * beginning of object_dynamic_cast, avoiding a second call to
+     * object_is_type.  However, casting between interfaces is relatively
+     * rare, and object_is_type(obj, TYPE_INTERFACE) would fail almost always.
+     *
+     * Perhaps we could add a magic value to the object header for increased
+     * (run-time) type safety and to speed up tests like this one.  If we ever
+     * do that we can revisit the order here.
+     */
+    if (object_is_type(obj, TYPE_INTERFACE)) {
+        assert(!obj->interfaces);
+        obj = INTERFACE(obj)->obj;
+        if (object_is_type(obj, typename)) {
+            return obj;
+        }
+    }
+
+    if (typename == TYPE_OBJECT) {
         return obj;
     }
 
@@ -413,16 +437,6 @@ Object *object_dynamic_cast(Object *obj, const char *typename)
         }
     }
 
-    /* Check if obj is an interface and its containing object is a direct
-     * ancestor of typename */
-    if (object_is_type(obj, TYPE_INTERFACE)) {
-        Interface *iface = INTERFACE(obj);
-
-        if (object_is_type(iface->obj, typename)) {
-            return iface->obj;
-        }
-    }
-
     return NULL;
 }