[ENH] Do not always compile c code when doing constant_folding

theano/tensor/elemwise.py

@@ -1193,6 +1193,13 @@ class Elemwise(OpenMPOp):
         else:
             return ()
 
+    def python_constant_folding(self, node):
+        """
+        Return True if we do not want to compile c code
+        when doing constant folding of this node.
+        """
+        return node.outputs[0].ndim == 0
+
 # def elemwise_to_scal(fgraph):
 # TODO: why is this commented out? should it be removed?
 #       it has needed maintenance despite being commented

theano/tensor/opt.py

@@ -4508,7 +4508,19 @@ def constant_folding(node):
     for o in node.outputs:
         storage_map[o] = [None]
         compute_map[o] = [False]
+    if (hasattr(node.op, 'python_constant_folding') and
+        node.op.python_constant_folding(node)):
 
+        old_value = getattr(node.op, '_op_use_c_code', False)
+        try:
+            node.op._op_use_c_code = False
+            thunk = node.op.make_thunk(node,
+                                       storage_map,
+                                       compute_map,
+                                       [])
+        finally:
+            node.op._op_use_c_code = old_value
+    else:
         thunk = node.op.make_thunk(node, storage_map, compute_map,
                                    no_recycling=[])
 

theano/tensor/tests/test_opt.py

@@ -3671,6 +3671,17 @@ def test_constant_folding():
     topo = f.maker.fgraph.toposort()
     assert len(topo) == 2
 
+    # Test that we do not crash when constant folding elemwise scalar
+    # as they should not generate c code.
+
+    x = tensor.constant(3)
+    assert x.ndim == 0
+    mode = theano.compile.get_mode("FAST_COMPILE").excluding("fusion")
+    f = theano.function([], [x * 2, x + x], mode=mode)
+    topo = f.maker.fgraph.toposort()
+    assert len(topo) == 2
+    assert all([isinstance(n.op, DeepCopyOp) for n in topo])
+
 
 def test_constant_get_stabilized():
     """