Fix cut graph

python/mxnet/symbol/contrib.py

@@ -133,25 +133,11 @@ def _construct_subgraph(sym_out, sym_states):
     all_outputs = []
     all_outputs.extend(sym_out)
     all_outputs.extend(sym_states)
-    g = symbol.Group(all_outputs)
-
     flat_out = []
-    all_input_names = g.list_inputs()
-    output_names = [o.name for o in sym_out]
     for o in sym_out:
-        if o.name in all_input_names:
-            flat_out.append(symbol.op.identity(o))
-        else:
-            flat_out.append(o)
-
+        flat_out.append(symbol.op.identity(o))
     for s in sym_states:
-        if s.name in all_input_names or s.name in output_names:
-            # There is a problem if the outputs are the same as the inputs
-            # or the first output. By calling identity, we can make sure that
-            # all symbols will refer to different NDArrays.
-            flat_out.append(symbol.op.identity(s))
-        else:
-            flat_out.append(s)
+        flat_out.append(symbol.op.identity(s))
     return symbol.Group(flat_out)
 
 def foreach(body, data, init_states, name="foreach"):
@@ -469,10 +455,8 @@ def _create_subgraph(graph_vars, graph_func, subgraph_name):
             num_outputs = len(outputs) + len(final_state)
             # nnvm cut-graph does not allow inputs and outputs overlap
             # so we calculate the name of inputs, and copy outputs once it overlaps with inputs
-            all_input_names = symbol.Group(outputs + final_state).list_inputs()
-            make_identity = lambda x: symbol.op.identity(x) if x.name in all_input_names else x
             # group all outputs of graph_func
-            graph = symbol.Group(list(map(make_identity, outputs + final_state)))
+            graph = symbol.Group(list(map(symbol.op.identity, outputs + final_state)))
         return graph, num_out_data, num_outputs
 
     def _union_inputs(*graphs):
@@ -627,10 +611,8 @@ def _create_subgraph(graph_vars, graph_func, subgraph_name):
             num_outputs = len(outputs)
             # nnvm cut-graph does not allow inputs and outputs overlap
             # so we calculate the name of inputs, and copy outputs once it overlaps with inputs
-            all_input_names = symbol.Group(outputs).list_inputs()
-            make_identity = lambda x: symbol.op.identity(x) if x.name in all_input_names else x
             # group all outputs of graph_func
-            graph = symbol.Group(list(map(make_identity, outputs)))
+            graph = symbol.Group(list(map(symbol.op.identity, outputs)))
         return graph, num_outputs
 
     def _union_inputs(*graphs):

src/c_api/c_api_symbolic.cc

@@ -372,13 +372,13 @@ int MXSymbolCutSubgraph(SymbolHandle sym, SymbolHandle **input_symbols,
   // a subgraph.
   API_BEGIN();
   nnvm::Symbol *s = static_cast<nnvm::Symbol*>(sym);
-  std::string subg_attr = "__subgraph_name__";
+  const std::string subg_attr = "__subgraph_name__";
   auto out_node = s->outputs[0].node;
   auto it = out_node->attrs.dict.find(subg_attr);
   if (it != out_node->attrs.dict.end()) {
-    std::string subg_name = it->second;
+    const std::string &subg_name = it->second;
     std::vector<nnvm::NodeEntry *> input_entries;
-    DFSVisit(s->outputs, [subg_attr, subg_name, &input_entries]
+    DFSVisit(s->outputs, [&subg_attr, &subg_name, &input_entries]
              (nnvm::NodePtr n) {
       // If the node itself isn't in the subgraph, we ignore it.
       auto it = n->attrs.dict.find(subg_attr);