Add MergeOptimizer as a part of canonicalize

theano/tensor/opt.py

@@ -31,6 +31,7 @@ from theano import compile  # to register the optimizer built by this file
 from theano.gof.python25 import any, all
 from theano.gof.opt import (Optimizer, pre_constant_merge,
                             pre_greedy_local_optimizer)
+from theano.gof.opt import merge_optimizer
 from theano.gof import toolbox, DestroyHandler
 from basic import get_constant_value, ShapeError
 
@@ -301,6 +302,11 @@ def register_specialize_device(lopt, *tags, **kwargs):
     return lopt
 
 
+## Register merge_optimizer as a global opt during canonicalize
+compile.optdb['canonicalize'].register(
+        'canon_merge', merge_optimizer, 'fast_run')
+
+
 #####################
 # Dot optimizations #
 #####################