Add Theano flag scan.debug=False.

doc/library/config.txt

@@ -239,6 +239,14 @@ import theano and print the config variable, as in:
     ``False``, then we will gc the inner of scan after all
     iterations. This is the default.
 
+.. attribute:: config.scan.debug
+
+    Bool value, either ``True`` or ``False``
+
+    Default: ``False``
+
+    If True, we will print extra scan debug information.
+
 .. attribute:: openmp
 
     Bool value: either True or False

theano/configdefaults.py

@@ -1427,6 +1427,11 @@ AddConfigVar('scan.allow_output_prealloc',
              BoolParam(True),
              in_c_key=False)
 
+AddConfigVar('scan.debug',
+             "If True, enable extra verbose output related to scan",
+             BoolParam(False),
+             in_c_key=False)
+
 AddConfigVar('pycuda.init',
              """If True, always initialize PyCUDA when Theano want to
                 initilize the GPU.  Currently, we must always initialize

theano/gof/toolbox.py

@@ -304,6 +304,9 @@ class ReplaceValidate(History, Validator):
         chk = fgraph.checkpoint()
         if verbose is None:
             verbose = config.optimizer_verbose
+        if config.scan.debug:
+            nb = len([n for n in fgraph.apply_nodes if isinstance(n.op, theano.scan_module.scan_op.Scan)])
+
         for r, new_r in replacements:
             try:
                 fgraph.replace(r, new_r, reason=reason, verbose=False)
@@ -337,6 +340,12 @@ class ReplaceValidate(History, Validator):
             if verbose:
                 print("validate failed on node %s.\n Reason: %s, %s" % (r, reason, e))
             raise
+        if config.scan.debug:
+            nb2 = len([n for n in fgraph.apply_nodes if isinstance(n.op, theano.scan_module.scan_op.Scan)])
+            if nb2 > nb:
+                print("Extra scan introduced", nb, nb2, getattr(reason, 'name', reason), r, new_r)
+            elif nb2 < nb:
+                print("Scan removed", nb, nb2, getattr(reason, 'name', reason), r, new_r)
         if verbose:
             print(reason, r, new_r)
         # The return is needed by replace_all_validate_remove