fix comments

Author: NHZlX

paddle/fluid/inference/analysis/data_flow_graph_to_fluid_pass.cc

@@ -87,7 +87,7 @@ void DataFlowGraphToFluidPass::AddFluidOp(Node *node) {
 }
 
 void CreateTrtEngineOp(Node *node, const DataFlowGraph &graph,
-                       framework::proto::BlockDesc &block) {
+                       framework::proto::BlockDesc *block) {
   static int counter{0};
   PADDLE_ENFORCE(node->IsFunctionBlock());
   framework::OpDesc desc;
@@ -112,22 +112,33 @@ void CreateTrtEngineOp(Node *node, const DataFlowGraph &graph,
   desc.SetType("tensorrt_engine");
 
   std::unordered_map<std::string, std::string> output_name_map;
-  auto subgraph_nodes = func->subgraph;
 
-  for (int index = 0; index < block.ops_size(); index++) {
-    framework::proto::OpDesc *op = block.mutable_ops(index);
-    // auto &op = block.mutable_ops(index);
+  // The following procedure is used to rename all the intermediate
+  // variables and the output variables of the subgraph.
+  // Why we do this?
+  // During the transition from fluid OP to tensorrt OP, we map
+  // the input and output Tensor(fluid data structure) of fluid OP
+  // to the correspondin ITensor (trt data structure) through the
+  // Tensor name. When we set up ITensor for an variable, we must
+  // ensure that it has not been set before.
+  // If there is variable in the fluid graph, which is not only the
+  // input of a OP, but also the output of a Op, there will be problems.
+  // So we have to rename the variable in the subgraph to make sure
+  // it is either an OP's input or an OP's output.
+
+  auto subgraph_nodes = func->subgraph;
+  for (int index = 0; index < block->ops_size(); index++) {
+    framework::proto::OpDesc *op = block->mutable_ops(index);
     auto correspond_node = subgraph_nodes[index];
     PADDLE_ENFORCE_EQ(correspond_node->name(), op->type());
 
     std::unordered_map<std::string, size_t> var2id;
     for (auto *in_var : correspond_node->inlinks) {
       var2id[in_var->name()] = in_var->id();
     }
-    // TODO(zhaolong): add comments
+    // rename for the input variables of op inside subgraph
     for (int i = 0; i < op->inputs_size(); i++) {
       framework::proto::OpDesc_Var *in_var = op->mutable_inputs(i);
-      // auto &in_var = op->mutable_inputs(i);
       std::vector<std::string> replaced_names;
       for (int k = 0; k < in_var->arguments_size(); k++) {
         std::string arg_value = in_var->arguments(k);
@@ -148,6 +159,7 @@ void CreateTrtEngineOp(Node *node, const DataFlowGraph &graph,
       var2id[out_var->name()] = out_var->id();
     }
 
+    // rename for the output variables of op inside subgraph
     for (int i = 0; i < op->outputs_size(); i++) {
       framework::proto::OpDesc_Var *out_var = op->mutable_outputs(i);
       std::vector<std::string> replaced_names;
@@ -165,15 +177,18 @@ void CreateTrtEngineOp(Node *node, const DataFlowGraph &graph,
       }
     }
   }
+  // When tensorrt engine runs at the end of the operation,
+  // output_mapping help us copy the data from the renamed ITensor
+  // to Tensor.
   std::vector<std::string> output_mapping;
   for (auto name : output_names) {
     PADDLE_ENFORCE(output_name_map.count(name) != 0);
     output_mapping.push_back(output_name_map[name]);
   }
 
-  PADDLE_ENFORCE(!block.vars().empty(), "the block has no var-desc");
+  PADDLE_ENFORCE(!block->vars().empty(), "the block has no var-desc");
   // Set attrs
-  SetAttr(desc.Proto(), "subgraph", block.SerializeAsString());
+  SetAttr(desc.Proto(), "subgraph", block->SerializeAsString());
   SetAttr(desc.Proto(), "engine_uniq_key", "trt-" + std::to_string(counter++));
   SetAttr(desc.Proto(), "max_batch", FLAGS_tensorrt_max_batchsize);
   SetAttr(desc.Proto(), "max_workspace", FLAGS_tensorrt_workspace_size);
@@ -220,7 +235,7 @@ void DataFlowGraphToFluidPass::AddEngineOp(Node *node) {
   *block_desc.Proto()->mutable_vars() =
       argument_->origin_program_desc->blocks(0).vars();
   PADDLE_ENFORCE(!block_desc.Proto()->vars().empty());
-  CreateTrtEngineOp(node, *argument_->main_dfg, *block_desc.Proto());
+  CreateTrtEngineOp(node, *argument_->main_dfg, block_desc.Proto());
   auto *main_block = desc_->mutable_blocks(framework::kRootBlockIndex);
   auto *op = main_block->add_ops();
   PADDLE_ENFORCE(!node->pb_msg().empty(), "failed to set desc for block");