Added optimization to remove useless subtensor.

theano/tensor/opt.py

@@ -1088,6 +1088,21 @@ def local_upcast_elemwise_constant_inputs(node):
 # Subtensor opts #
 ##################
 
+@register_canonicalize
+@register_specialize
+@gof.local_optimizer([T.Subtensor])
+def local_useless_subtensor(node):
+    """
+    Remove Subtensor if it take the full input
+    """
+    if (isinstance(node.op, T.Subtensor) and
+        all([isinstance(idx, slice) and
+             idx.start in [0, None] and
+             idx.stop in [sys.maxint, None]
+             and idx.step in [1, None] for idx in node.op.idx_list])):
+        x = node.inputs[0]
+        return [x]
+
 @register_canonicalize
 @gof.local_optimizer([])
 def local_subtensor_lift(node):

theano/tensor/tests/test_opt.py

@@ -1145,6 +1145,15 @@ def test_log_add():
 
     #TODO: (write and) test that the optimization works with Sum in addition to working with Add.
 
+def test_local_useless_subtensor():
+    x = TT.matrix('x')
+    f = function([x], TT.exp(x)[0:], mode=mode_opt)
+
+    prog=f.maker.env.toposort()
+    assert prog[0].op == TT.exp
+    assert len(prog)==1
+    f([[0,1],[2,3]]) # let debugmode test something
+
 class test_local_subtensor_lift(unittest.TestCase):
 
     def test0(self):