Add opt local_useless_composite that remove Composite outputs that aren't needed.

theano/tensor/opt.py

@@ -7335,6 +7335,26 @@ else:
                            'FusionOptimizer')
 
 
+@register_canonicalize
+@gof.local_optimizer([Elemwise])
+def local_useless_composite(node):
+    """For elemwise Composite that have multiple outputs, remove the
+    outputs that are not used.
+
+    """
+    if (not isinstance(node.op, Elemwise) or
+            not isinstance(node.op.scalar_op, scalar.Composite)):
+        return
+    comp = node.op.scalar_op
+    idx = [i for i, o_extern in enumerate(node.outputs)
+           if o_extern.clients]
+    if len(idx) < len(node.outputs):
+        new_outputs = [comp.outputs[i] for i in idx]
+        c = scalar.Composite(inputs=comp.inputs,
+                             outputs=new_outputs)
+        e = Elemwise(scalar_op=c)(*node.inputs, return_list=True)
+        return dict(zip([node.outputs[i] for i in idx], e))
+
 # ############################
 # # Remove consider_constant #
 # ############################

theano/tensor/tests/test_opt.py

@@ -1526,6 +1526,26 @@ class TestCompositeCodegen(unittest.TestCase):
         fval = numpy.asarray(f([1, 2, 3]))
         assert numpy.all(fval == [6, 12, 18]), fval
 
+    def test_local_useless_composite(self):
+        x = theano.scalar.float32()
+        c = theano.scalar.Composite([x], [x+1, x-1])
+        X = theano.tensor.matrix()
+        o = theano.tensor.Elemwise(scalar_op=c)(X)
+        mode = theano.compile.mode.get_default_mode().including(
+            'local_useless_composite')
+
+        f = theano.function([X], o[0], mode=mode)
+        topo = f.maker.fgraph.toposort()
+        assert len(topo) == 1
+        assert len(topo[0].outputs) == 1
+        utt.assert_allclose(f([[1.]]), [[2.]])
+
+        f = theano.function([X], o[1], mode=mode)
+        topo = f.maker.fgraph.toposort()
+        assert len(topo) == 1
+        assert len(topo[0].outputs) == 1
+        utt.assert_allclose(f([[1.]]), [[0.]])
+
 
 def test_log1p():
     m = theano.config.mode