opt:inc_subtensor:alloc new space when broadcastable

theano/tensor/opt.py

@@ -1911,7 +1911,13 @@ def local_subtensor_inc_subtensor(node):
             return
 
         if x.owner.inputs[2:] == node.inputs[1:] and tuple(x.owner.op.idx_list) == tuple(node.op.idx_list):
-            return [x.owner.inputs[1]]
+            # if x[idx] and y have the same ndim (and shape), directly return y
+            if x.owner.inputs[0].ndim-(len(node.op.idx_list)-sum([isinstance(idx, slice) for idx in node.op.idx_list])) == x.owner.inputs[1].ndim:
+                return [x.owner.inputs[1]]
+            # else y is broadcastable, return alloc of broadcastable y
+            else:
+                x_subtensor = Subtensor(node.op.idx_list)(x.owner.inputs[0], *x.owner.inputs[2:])
+                return [T.alloc(x.owner.inputs[1], *x_subtensor.shape)]
         else:
             return