Make a separate merge_signature method.

theano/gof/graph.py

@@ -450,6 +450,9 @@ class Constant(Variable):
     def signature(self):
         return (self.type, self.data)
 
+    def merge_signature(self):
+        return self.signature()
+
     def __str__(self):
         if self.name is not None:
             return self.name

theano/gof/opt.py

@@ -499,7 +499,7 @@ class MergeFeature(object):
         """Check if a constant can be merged, and queue that replacement"""
         if id(c) in self.seen_constants:
             return
-        sig = c.signature()
+        sig = c.merge_signature()
         other_c = self.const_sig_inv.get(sig, None)
         if other_c is not None:
             # multiple names will clobber each other..

theano/gof/type.py

@@ -627,8 +627,13 @@ if (py_%(name)s == NULL) { %(freefunc)s(%(name)s); }
 
 
 class CDataTypeConstant(graph.Constant):
-    def signature(self):
-        # We can't check if certain constants are equal or not so just
-        # assume they are all unequal.
+    def merge_signature(self):
+        # We don't want to merge constants that don't point to the
+        # same object.
         return id(self.data)
+
+    def signature(self):
+        # There is no way to put the data in the signature, so we
+        # don't even try
+        return (self.type,)
 CDataType.Constant = CDataTypeConstant