Fix corner case in cvm updates, add test.

theano/compile/tests/test_pfunc.py

@@ -618,6 +618,42 @@ class Test_pfunc(unittest.TestCase):
         self.assertRaises(theano.compile.UnusedInputError,
                 theano.function, [x, x, x], x)
 
+    def test_update_same(self):
+        # There was a bug in CVM, triggered when a shared variable
+        # was its own update expression.
+        a = shared(1., 'a')
+        b = shared(numpy.ones((2, 3)), 'b')
+
+        # The order of the variables is not determined, so we try
+        # both shared variables.
+        f = theano.function([], [], updates={a: a, b: (2 * b)})
+        g = theano.function([], [], updates={a: (a * 2), b: b})
+
+        f()
+        assert a.get_value(borrow=True).shape == (), a.get_value()
+        assert b.get_value(borrow=True).shape == (2, 3), b.get_value()
+        g()
+        assert a.get_value(borrow=True).shape == (), a.get_value()
+        assert b.get_value(borrow=True).shape == (2, 3), b.get_value()
+
+    def test_update_equiv(self):
+        # Like test_update_same, but the update expression is simplified until
+        # it is found to be equal to the original variable
+        a = shared(1., 'a')
+        b = shared(numpy.ones((2, 3)), 'b')
+
+        # The order of the variables is not determined, so we try
+        # both shared variables.
+        f = theano.function([], [], updates={a: a, b: (2 * b - b)})
+        g = theano.function([], [], updates={a: (a * 2 - a), b: b})
+
+        f()
+        assert a.get_value(borrow=True).shape == (), a.get_value()
+        assert b.get_value(borrow=True).shape == (2, 3), b.get_value()
+        g()
+        assert a.get_value(borrow=True).shape == (), a.get_value()
+        assert b.get_value(borrow=True).shape == (2, 3), b.get_value()
+
 
 class Test_aliasing_rules(unittest.TestCase):
     """

theano/gof/vm.py

@@ -716,7 +716,6 @@ class VM_Linker(link.LocalLinker):
             update_storage = []
             update_in_from_out = {}
             for (ivar, ovar) in updated_vars.items():
-                if ivar != ovar:
                 update_in_from_out[vars_idx[ovar]] = vars_idx[ivar]
             for oidx in output_vars:
                 if oidx in update_in_from_out: