optimize consecutive cast

theano/tensor/opt.py

@@ -1449,6 +1449,29 @@ def local_alloc_unary(node):
             return [T.alloc(T.cast(v, node.outputs[0].dtype), *shp)]
 
 
+@register_canonicalize
+@register_specialize
+@gof.local_optimizer([T.Elemwise])
+def local_cast_cast(node):
+    """cast(cast(x, dtype1), dtype2)
+
+    when those contrain:
+    dtype1 == dtype2
+    TODO: the base dtype is the same (int, uint, float, complex)
+          and the first cast cause an upcast.
+    """
+    if (not isinstance(node.op, T.Elemwise) or
+        not isinstance(node.op.scalar_op, scalar.Cast)):
+        return
+    x = node.inputs[0]
+    if (not x.owner or
+        not isinstance(x.owner.op, T.Elemwise) or
+        not isinstance(x.owner.op.scalar_op, scalar.Cast)):
+        return
+    if node.op.scalar_op.o_type == x.owner.op.scalar_op.o_type:
+        return [x]
+
+
 class Assert(T.Op):
     """
     Implements assertion in a computational graph.

theano/tensor/tests/test_opt.py

@@ -3599,6 +3599,31 @@ class T_useless_elemwise(unittest.TestCase):
         assert topo[0].op == deep_copy_op
 
 
+class T_cast_cast(unittest.TestCase):
+    def setUp(self):
+        mode = theano.compile.get_default_mode()
+        self.mode = mode.including('local_cast_cast')
+
+    def test(self):
+        x = T.fmatrix()
+        o = T.Elemwise(scal.Cast(scal.Scalar("float64")))(x.astype("float64"))
+        f = theano.function([x], o, mode=self.mode)
+        dx = numpy.random.rand(5, 4).astype("float32")
+        f(dx)
+        topo = f.maker.fgraph.toposort()
+        assert len(topo) == 1
+        assert isinstance(topo[0].op, T.Elemwise)
+
+        x = T.dmatrix()
+        o = T.Elemwise(scal.Cast(scal.Scalar("float32")))(x.astype("float32"))
+        f = theano.function([x], o, mode=self.mode)
+        dx = numpy.random.rand(5, 4)
+        f(dx)
+        topo = f.maker.fgraph.toposort()
+        assert len(topo) == 1
+        assert isinstance(topo[0].op, T.Elemwise)
+
+
 def test_constant_folding():
     """ Test that constant folding get registered at fast_compile