TTG task migration

[josephjohnjj]

I am trying to migrate a TTG task to another process. To do this, migrate() calls migrate_data() for each input data index of the task.

    void migrate(parsec_task_t* parsec_task, int dst )
    {
      int nb_input = parsec_task->task_class->dependencies_goal;

      if constexpr (!ttg::meta::is_void_v<keyT> )
      {
        keyT key;
        unpack(key, parsec_task->ttg_key, 0);
      
        for(int i = 0; i < nb_input; i++)
          migrate_data(i, key, parsec_task, dst);
        
        auto &world_impl = world.impl();
        world_impl.taskpool()->tdm.module->taskpool_addto_nb_tasks(world_impl.taskpool(), -1);
      }
      
    }


    template <typename Key>
    void migrate_data(int data_index, const Key &key, parsec_task_t* parsec_task, int dst )
    {
        using valueT = typename std::tuple_element<data_index, input_terminals_type>::type::value_type;

        parsec_data_copy_t *copy;
        copy = parsec_task->data[data_index].data_in;
        auto data = copy->device_private;

        using decvalueT = std::decay_t<valueT>;
        decvalueT val;
        pack(val, data, 0);

        set_arg_impl<data_index>(key, std::move(val), dst);
    }

To get the type of data to use pack() I used

using valueT = typename std::tuple_element<data_index, input_terminals_type>::type::value_type;

But when I do this I get an error:

ttg/ttg/ttg/parsec/ttg.h:568:84: error: ‘data_index’ is not a constant expression
  568 |         using valueT = typename std::tuple_element<data_index, input_terminals_type>::type::value_type;

Is there a way to overcome this?

[devreal]

The problem is that the argument to tuple_element has to be a compile-time constant. Try something like this:

    template <typename Key, int data_index>
    void migrate_data(const Key &key, parsec_task_t* parsec_task, int dst )
    {
        using valueT = typename std::tuple_element<data_index, input_terminals_type>::type::value_type;
        ...
    }

You can then recursively iterate over the std::tuple_size_v<input_terminals_type> using make_integer_sequence. For an example, check out register_input_callbacks in the PaRSEC ttg backend :)