optimize merge add if input rows of all selected rows is not duplicated

paddle/fluid/operators/math/selected_rows_functor.cc

@@ -296,6 +296,7 @@ struct MergeAdd<platform::CPUDeviceContext, T> {
     auto input_height = has_value_input->height();
     framework::SelectedRows& out = *output;
     std::set<int64_t> merged_row_set;
+    size_t row_num = 0;
     for (auto* input : inputs) {
       if (input->rows().size() == 0) {
         continue;
@@ -305,42 +306,71 @@ struct MergeAdd<platform::CPUDeviceContext, T> {
                         "dimension except for the first one");
       PADDLE_ENFORCE_EQ(input_height, input->height(),
                         "all input should have same height");
+      row_num += input->rows().size();
       merged_row_set.insert(input->rows().begin(), input->rows().end());
     }
-    std::vector<int64_t> merge_rows(merged_row_set.begin(),
-                                    merged_row_set.end());
-    if (sorted_result) {
-      std::sort(merge_rows.begin(), merge_rows.end());
-    }
-    std::unordered_map<int64_t, size_t> rows_to_id;
-    for (size_t i = 0; i < merge_rows.size(); ++i) {
-      rows_to_id[merge_rows[i]] = i;
-    }
-    out.set_rows(merge_rows);
+
     out.set_height(input_height);
     out.mutable_value()->mutable_data<T>(
         framework::make_ddim(
-            {static_cast<int64_t>(merge_rows.size()), input_width}),
+            {static_cast<int64_t>(merged_row_set.size()), input_width}),
         context.GetPlace());
+    auto* out_data = out.mutable_value()->data<T>();
 
-    math::SetConstant<platform::CPUDeviceContext, T> constant_functor;
-    constant_functor(context, out.mutable_value(), 0.0);
+    if (merged_row_set.size() == row_num && !sorted_result) {
+      // no duplicated ids, just concat the result together
+      std::vector<int64_t> merge_rows;
+      merge_rows.reserve(row_num);
+      // concat rows
+      for (auto* in : inputs) {
+        merge_rows.insert(merge_rows.end(), in->rows().begin(),
+                          in->rows().end());
+      }
+      out.set_rows(merge_rows);
+      auto in_place = inputs[0]->place();
+      auto out_place = out.place();
+      int64_t copied_numel = 0;
+      for (auto* in : inputs) {
+        auto* in_data = in->value().data<T>();
+        auto in_numel = in->value().numel();
+        memory::Copy(boost::get<platform::CPUPlace>(out_place),
+                     out_data + copied_numel,
+                     boost::get<platform::CPUPlace>(in_place), in_data,
+                     in_numel * sizeof(T));
+        copied_numel += in_numel;
+      }
+    } else {
+      std::vector<int64_t> merge_rows(merged_row_set.begin(),
+                                      merged_row_set.end());
 
-    auto* out_data = out.mutable_value()->data<T>();
+      if (sorted_result) {
+        std::sort(merge_rows.begin(), merge_rows.end());
+      }
 
-    auto blas = math::GetBlas<platform::CPUDeviceContext, T>(context);
-    for (auto* input : inputs) {
-      if (input->rows().size() == 0) {
-        continue;
+      out.set_rows(merge_rows);
+
+      math::SetConstant<platform::CPUDeviceContext, T> constant_functor;
+      constant_functor(context, out.mutable_value(), 0.0);
+
+      std::unordered_map<int64_t, size_t> rows_to_id;
+      for (size_t i = 0; i < merge_rows.size(); ++i) {
+        rows_to_id[merge_rows[i]] = i;
       }
-      auto* input_data = input->value().data<T>();
-      auto& input_rows = input->rows();
-
-      for (size_t i = 0; i < input_rows.size(); i++) {
-        size_t out_i = rows_to_id[input_rows[i]];
-        elementwise_add_to<platform::CPUDeviceContext, T>(
-            context, &blas, static_cast<size_t>(input_width),
-            &input_data[i * input_width], &out_data[out_i * input_width]);
+
+      auto blas = math::GetBlas<platform::CPUDeviceContext, T>(context);
+      for (auto* input : inputs) {
+        if (input->rows().size() == 0) {
+          continue;
+        }
+        auto* input_data = input->value().data<T>();
+        auto& input_rows = input->rows();
+
+        for (size_t i = 0; i < input_rows.size(); i++) {
+          size_t out_i = rows_to_id[input_rows[i]];
+          elementwise_add_to<platform::CPUDeviceContext, T>(
+              context, &blas, static_cast<size_t>(input_width),
+              &input_data[i * input_width], &out_data[out_i * input_width]);
+        }
       }
     }
   }