Add DestroyHandler if there are inplace ops

theano/compile/debugmode.py

@@ -677,8 +677,13 @@ def _optcheck_fgraph(input_specs, output_specs, accept_inplace=False):
     inputs, outputs = gof.graph.clone(orig_inputs, orig_outputs)
     equivalence_tracker = _VariableEquivalenceTracker()
     fgraph = gof.fg.FunctionGraph(inputs, outputs,
-            #DestroyHandler is not needed because it is actually installed by an optimization
-            # after canonicalization.  This variables in a big speed gain.
+            # DestroyHandler may not be needed yet, as there is usually no
+            # inplace operation in the graph at this stage. DestroyHandler
+            # will be installed by an optimization after canonicalization,
+            # before the inplace operations are applied.
+            # This results in a big speed gain.
+            # If inplace operations are accepted and present, however,
+            # DestroyHandler will be inserted in the loop below.
             #features=[equivalence_tracker, gof.DestroyHandler(do_imports_on_attach=False)])
             features=[equivalence_tracker])
 
@@ -687,6 +692,13 @@ def _optcheck_fgraph(input_specs, output_specs, accept_inplace=False):
             if getattr(node.op, 'destroy_map', None):
                 raise TypeError("Graph must not contain inplace operations",
                                 node)
+    else:
+        # However, if some inplace ops are already in the graph,
+        # DestroyHandler is needed for the Supervisor below to work correctly.
+        for node in fgraph.apply_nodes:
+            if getattr(node.op, 'destroy_map', None):
+                fgraph.attach_feature(gof.DestroyHandler())
+                break
 
     # We need to protect all immutable inputs from inplace operations.
     fgraph.attach_feature(Supervisor(input for spec, input in zip(input_specs, inputs)