Merge pull request Theano#1549 from lamblin/fix_shape_cycle

[WIP] Fix shape cycle

theano/tensor/opt.py

@@ -898,22 +898,29 @@ def update_shape(self, r, other_r):
             r_shape = self.shape_of[r]
         else:
             # If no info is known on r's shape, use other_shape
-            self.shape_of[r] = other_shape
-            for sv in other_shape:
-                self.shape_of_reverse_index.setdefault(sv, set()).add(r)
+            self.set_shape(r, other_shape)
             return
 
         # Merge other_shape with r_shape, giving the priority to other_shape
         merged_shape = []
         for i, ps in enumerate(other_shape):
-            # If other_shape[i] is uninformative, use r_shape[i].
-            # For now, we consider 2 cases of uninformative other_shape[i]:
-            #  - Shape_i(i)(other_r);
-            #  - Shape_i(i)(r).
             if (ps.owner
                     and isinstance(getattr(ps.owner, 'op', None), Shape_i)
                     and ps.owner.op.i == i
                     and ps.owner.inputs[0] in (r, other_r)):
+                # If other_shape[i] is uninformative, use r_shape[i].
+                # For now, we consider 2 cases of uninformative other_shape[i]:
+                #  - Shape_i(i)(other_r);
+                #  - Shape_i(i)(r).
+                merged_shape.append(r_shape[i])
+            elif r_shape[i] in theano.gof.graph.ancestors([other_shape[i]]):
+                # Another case where we want to use r_shape[i] is when
+                # other_shape[i] actually depends on r_shape[i]. In that case,
+                # we do not want to substitute an expression with another that
+                # is strictly more complex. Such a substitution could also lead
+                # to cycles: if (in the future) r_shape[i] gets replaced by an
+                # expression of other_shape[i], other_shape[i] may end up
+                # depending on itself.
                 merged_shape.append(r_shape[i])
             else:
                 merged_shape.append(other_shape[i])
@@ -1107,6 +1114,12 @@ def on_change_input(self, fgraph, node, i, r, new_r):
                     # replacement.
                     continue
 
+                if shpnode.outputs[0] in theano.gof.graph.ancestors([repl]):
+                    raise AssertionError(
+                        "This substitution would insert a cycle in the graph:"
+                        "node: %s, i: %i, r: %s, new_r: %s"
+                        % (node, i, r, new_r))
+
                 self.scheduled[shpnode] = new_r
         # In case 2, if r is a variable that we've scheduled for shape update,
         # then we should cancel it.