allreduce_robust.cc: Allow num_global_replica to be 0 (#38)

In some cases, users may not want to have any global replica of
the data being broadcasted/all-reduced. In such cases, set the
result_buffer_round to -1 as a flag that this is not necessary
and check for it.

src/allreduce_robust.cc

@@ -33,7 +33,11 @@ AllreduceRobust::AllreduceRobust(void) {
 }
 void AllreduceRobust::Init(int argc, char* argv[]) {
   AllreduceBase::Init(argc, argv);
-  result_buffer_round = std::max(world_size / num_global_replica, 1);
+  if (num_global_replica == 0) {
+    result_buffer_round = -1;
+  } else {
+    result_buffer_round = std::max(world_size / num_global_replica, 1);
+  }
 }
 /*! \brief shutdown the engine */
 void AllreduceRobust::Shutdown(void) {
@@ -86,7 +90,8 @@ void AllreduceRobust::Allreduce(void *sendrecvbuf_,
   bool recovered = RecoverExec(sendrecvbuf_, type_nbytes * count, 0, seq_counter);
   // now we are free to remove the last result, if any
   if (resbuf.LastSeqNo() != -1 &&
-      (resbuf.LastSeqNo() % result_buffer_round != rank % result_buffer_round)) {
+      (result_buffer_round == -1 ||
+       resbuf.LastSeqNo() % result_buffer_round != rank % result_buffer_round)) {
     resbuf.DropLast();
   }
   if (!recovered && prepare_fun != NULL) prepare_fun(prepare_arg);
@@ -118,7 +123,8 @@ void AllreduceRobust::Broadcast(void *sendrecvbuf_, size_t total_size, int root)
   bool recovered = RecoverExec(sendrecvbuf_, total_size, 0, seq_counter);
   // now we are free to remove the last result, if any
   if (resbuf.LastSeqNo() != -1 &&
-      (resbuf.LastSeqNo() % result_buffer_round != rank % result_buffer_round)) {
+      (result_buffer_round == -1 ||
+       resbuf.LastSeqNo() % result_buffer_round != rank % result_buffer_round)) {
     resbuf.DropLast();
   }
   void *temp = resbuf.AllocTemp(1, total_size);