Develop backward building precess of single op

paddle/framework/backward.cc

@@ -12,8 +12,9 @@
    See the License for the specific language governing permissions and
    limitations under the License. */
 
-#include <paddle/framework/backward.h>
-#include <paddle/framework/net.h>
+#include "paddle/framework/backward.h"
+#include "paddle/framework/net.h"
+#include "paddle/framework/op_registry.h"
 
 namespace paddle {
 namespace framework {
@@ -71,6 +72,24 @@ static std::shared_ptr<OperatorBase> BackwardImpl(
     //! TODO(dzh)
   } else {
     //! TODO(fjy)
+    std::shared_ptr<OperatorBase> grad_op = OpRegistry::CreateGradOp(forwardOp);
+    for (std::string& grad_input : grad_op->inputs_) {
+      if (no_grad_names.count(grad_input)) {
+        std::string prefix = grad_input.substr(
+            0, grad_input.size() - OperatorBase::GRAD_VAR_SUFFIX().size());
+        grad_input = prefix + OperatorBase::ZERO_VAR_SUFFIX();
+        std::vector<std::string> fill_zeros_in = {prefix};
+        std::vector<std::string> fill_zeros_out = {grad_input};
+        net.AddOp(OpRegistry::CreateOp("fill_zeros_like", fill_zeros_in,
+                                       fill_zeros_out, AttributeMap()));
+      }
+    }
+    for (std::string& grad_output : grad_op->output_) {
+      if (no_grad_names.count(grad_output)) {
+        grad_output = OperatorBase::EMPTY_VAR_NAME();
+      }
+    }
+    net.AddOp(grad_op);
   }
 
   net->CompleteAddOp();

paddle/framework/operator.h

@@ -67,6 +67,9 @@ class OperatorBase {
   /// e.g. Variable "x@GRAD" is the gradient of varibale "x".
   static std::string GRAD_VAR_SUFFIX() { return "@GRAD"; }
 
+  /// Variables with this suffix are supposed to be filled up with zeros.
+  static std::string ZERO_VAR_SUFFIX() { return "@ZERO"; }
+
   virtual ~OperatorBase() {}
 
   template <typename T>