Add write after write dependence (#12632)

* Add write after write

* follow comment

Author: chengduo

paddle/fluid/framework/ir/graph.cc

@@ -182,9 +182,11 @@ Graph::Graph(const ProgramDesc &program) : program_(program) {
   }
 
   /**
-   * We only handle write after read(WAR), since it should not have a write
-   * after write in program. If there are write after write operators, we need
-   * prune them.
+   * We should handle write after read(WAR) and write after write(WAW) here.
+   * Because some of the operators of the program can be executed parallelly.
+   * So, to make the program running in the right order, we should add the
+   * dependence of WAR and WAW.
+   *
    *
    * https://en.wikipedia.org/wiki/Hazard_(computer_architecture)#Write_after_read_(WAR)
    */
@@ -201,6 +203,19 @@ Graph::Graph(const ProgramDesc &program) : program_(program) {
           (*it_new)->inputs.empty() ? nullptr : (*it_new)->inputs[0];
       const auto &read_ops = (*it_old)->outputs;
 
+      PADDLE_ENFORCE(write_op, "The write_op should not be empty.");
+
+      // Add write after write dependence
+      ir::Node *upstream_op =
+          (*it_old)->inputs.empty() ? nullptr : (*it_old)->inputs[0];
+      if (upstream_op) {
+        ir::Node *dep_var = CreateControlDepVar();
+        write_op->inputs.push_back(dep_var);
+        upstream_op->outputs.push_back(dep_var);
+        dep_var->outputs.push_back(write_op);
+        dep_var->inputs.push_back(upstream_op);
+      }
+
       for (auto *read_op : read_ops) {
         // Manually add a dependency var from read_op to write_op;
         if (read_op == write_op) {