Docsting

theano/tensor/opt.py

@@ -1776,10 +1776,15 @@ def local_useless_alloc(node):
 @register_canonicalize
 @gof.local_optimizer([T.alloc])
 def local_canonicalize_alloc(node):
-    """
-    If the input type is the same as the output type (dtype and broadcast)
+    """If the input type is the same as the output type (dtype and broadcast)
     there is no change in the shape of the input. So this is just a simple copy
-    of the input. This is not needed.
+    of the input. This is not needed. (as local_useless_alloc)
+
+    Also, it will canonicalize alloc by creating Dimshuffle after the
+    alloc to introduce the dimensions of constant size 1.
+
+    See https://github.com/Theano/Theano/issues/4072 to know why this
+    is needed.
 
     """
     op = node.op
@@ -1799,6 +1804,7 @@ def local_canonicalize_alloc(node):
     for client, i in clients:
         if client != "output" and isinstance(client.op, Alloc):
             return
+
     # Check if alloc adds a broadcastable dimension with shape 1.
 
     output_shape = node.inputs[1:]
@@ -2996,6 +3002,7 @@ def local_subtensor_merge(node):
             return [out]
 
 
+@register_useless
 @register_canonicalize
 @register_specialize
 @gof.local_optimizer([Subtensor])
@@ -3639,6 +3646,7 @@ def local_join_1(node):
 
 
 # TODO: merge in local_useless_join
+@register_useless
 @register_specialize
 @register_canonicalize
 @gof.local_optimizer([T.Join])