Don't merge ordering when there is only 1 ordering.

theano/gof/fg.py

@@ -655,7 +655,10 @@ class FunctionGraph(utils.object2):
 
         """
         ords = OrderedDict()
+        first_ordering = None  # first non empty ordering
         assert isinstance(self._features, list)
+        non_empty_ordering = 0
+
         for feature in self._features:
             if hasattr(feature, 'orderings'):
                 orderings = feature.orderings(self)
@@ -664,16 +667,34 @@ class FunctionGraph(utils.object2):
                                     str(feature.orderings) +
                                     ". Nondeterministic object is " +
                                     str(orderings))
-                for node, prereqs in iteritems(orderings):
-                    if not isinstance(prereqs, (list, OrderedSet)):
-                        raise TypeError(
-                            "prereqs must be a type with a "
-                            "deterministic iteration order, or toposort "
-                            " will be non-deterministic.")
-                    ords.setdefault(node, []).extend(prereqs)
-        # eliminate duplicate prereqs
-        for (node, prereqs) in iteritems(ords):
-            ords[node] = list(OrderedSet(prereqs))
+                if len(orderings) > 0:
+                    non_empty_ordering += 1
+                    # If we get only 1 ordering, we reuse it directly.
+                    if non_empty_ordering == 1:
+                        ords = orderings
+                        for node, prereqs in iteritems(orderings):
+                            if not isinstance(prereqs, (list, OrderedSet)):
+                                raise TypeError(
+                                    "prereqs must be a type with a "
+                                    "deterministic iteration order, or toposort "
+                                    " will be non-deterministic.")
+                    # If we get more then 1 orderings, we need to
+                    # combine them.
+                    elif non_empty_ordering == 2:
+                        ords = OrderedDict()
+                    if non_empty_ordering > 1:
+                        for node, prereqs in iteritems(orderings):
+                            if not isinstance(prereqs, (list, OrderedSet)):
+                                raise TypeError(
+                                    "prereqs must be a type with a "
+                                    "deterministic iteration order, or toposort "
+                                    " will be non-deterministic.")
+                            ords.setdefault(node, []).extend(prereqs)
+        if non_empty_ordering > 1:
+            # eliminate duplicate prereqs if there is more then one
+            # empty ordering
+            for (node, prereqs) in iteritems(ords):
+                ords[node] = list(OrderedSet(prereqs))
         return ords
 
     def check_integrity(self):