[EXECUTOR] PruneGraph pass (#274)

apps/graph_executor/src/graph_pass.cc

@@ -499,5 +499,61 @@ NNVM_REGISTER_OP(layout_transform)
 .set_num_outputs(1)
 .add_argument("data", "NDArray-or-Symbol", "Input data")
 .add_arguments(LayoutTransformParam::__FIELDS__());
+
+
+nnvm::Graph PruneGraph(nnvm::Graph src) {
+  const auto& params = src.GetAttr<std::unordered_set<std::string>>("params");
+
+  std::unordered_set<nnvm::Node*> pruned;
+  nnvm::NodeEntryMap<nnvm::NodePtr> entry_var;
+  DFSVisit(src.outputs, [&](const nnvm::NodePtr& n) {
+    bool can_be_pruned = true;
+    if (n->is_variable()) {
+      if (params.count(n->attrs.name)) {
+        pruned.emplace(n.get());
+      }
+      can_be_pruned = false;
+    }
+
+    for (const auto& e : n->inputs) {
+      if (!pruned.count(e.node.get())) {
+        can_be_pruned = false;
+      }
+    }
+    if (can_be_pruned) {
+      pruned.emplace(n.get());
+    } else {
+      // scan again to find edge nodes
+      for (auto& e : n->inputs) {
+        if (pruned.count(e.node.get())) {
+          if (!entry_var.count(e)) {
+            nnvm::NodePtr var = nnvm::Node::Create();
+            var->attrs.name = e.node->attrs.name + "_" + std::to_string(e.index);
+            entry_var.emplace(e, var);
+          }
+          e = nnvm::NodeEntry{entry_var.at(e), 0, 0};
+        }
+      }
+    }
+  });
+
+  nnvm::Graph pre_graph;
+  pre_graph.outputs.reserve(entry_var.size());
+  std::vector<std::string> output_names;
+  output_names.reserve(entry_var.size());
+  for (auto kv : entry_var) {
+    pre_graph.outputs.emplace_back(kv.first);
+    output_names.emplace_back(kv.second->attrs.name);
+  }
+
+  pre_graph.attrs["pruned_params"] =
+    std::make_shared<dmlc::any>(std::move(output_names));
+  src.attrs["pre_graph"] =
+    std::make_shared<dmlc::any>(std::move(pre_graph));
+  return src;
+}
+
+NNVM_REGISTER_PASS(PruneGraph)
+.set_body(PruneGraph);
 }  // namespace contrib
 }  // namespace tvm