RNN backward create (#3490)

* insert rnn's backward into Backward()

* add device_context into backward_test

paddle/framework/CMakeLists.txt

@@ -38,7 +38,7 @@ add_custom_command(TARGET framework_py_proto POST_BUILD
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 
 cc_library(backward SRCS backward.cc DEPS net_op)
-cc_test(backward_test SRCS backward_test.cc DEPS backward)
+cc_test(backward_test SRCS backward_test.cc DEPS backward recurrent_op device_context)
 
 if(WITH_PYTHON)
 cc_library(paddle_pybind SHARED

paddle/framework/backward.cc

@@ -17,6 +17,7 @@
 #include <list>
 #include "paddle/framework/op_registry.h"
 #include "paddle/operators/net_op.h"
+#include "paddle/operators/recurrent_op.h"
 
 namespace paddle {
 namespace framework {
@@ -178,6 +179,22 @@ std::shared_ptr<OperatorBase> BackwardRecursive(
                      return false;
                    });
 
+    // process recurrent gradient op as a special operator.
+    if (forwardOp.Type() == "recurrent_op") {
+      // NOTE clean up cycle call somewhere (RNN's stepnet constains itself), or
+      // this will result in infinite loop.
+      const auto& rnnop =
+          *static_cast<const operators::RecurrentOp*>(&forwardOp);
+      auto rnn_grad_op =
+          static_cast<operators::RecurrentGradientOp*>(grad_op.get());
+      const auto& stepnet_op =
+          *static_cast<const OperatorBase*>(&rnnop.stepnet());
+      // create stepnet's gradient op
+      auto grad_stepnet = BackwardRecursive(stepnet_op, no_grad_names, uniq_id);
+      rnn_grad_op->set_stepnet(
+          std::static_pointer_cast<operators::NetOp>(grad_stepnet));
+    }
+
     if (net->ops_.empty()) {  // Current no aux op is added to network
       return grad_op;
     }

paddle/operators/recurrent_op.h

@@ -127,7 +127,7 @@ class RecurrentOp final : public framework::OperatorBase {
   }
 
   void set_stepnet(std::shared_ptr<NetOp> net) { stepnet_ = net; }
-  const NetOp* stepnet() const { return stepnet_.get(); }
+  const NetOp& stepnet() const { return *stepnet_; }
 
   static const rnn::ArgumentName kArgName;
 
@@ -158,7 +158,7 @@ class RecurrentGradientOp final : public framework::OperatorBase {
   static const rnn::ArgumentName kArgName;
 
   void set_stepnet(const std::shared_ptr<NetOp>& net) { stepnet_ = net; }
-  const NetOp* stepnet() const { return stepnet_.get(); }
+  const NetOp& stepnet() const { return *stepnet_; }
 
  private:
   RecurrentGradientAlgorithm alg_;