[ENGINE] Change obj pool to normal struct instead of union

include/mxnet/base.h

@@ -42,7 +42,8 @@
  *  Used to support g++-4.6 and g++4.7
  */
 #if DMLC_USE_CXX11 && defined(__GNUC__) && !defined(__clang_version__)
-#if __GNUC__ == 4 && __GNUC_MINOR__ == 6
+#if __GNUC__ == 4 && __GNUC_MINOR__ < 8
+#error "Currently we need g++ 4.8 or higher to fully support c++11 features"
 #define override
 #define final
 #endif

scripts/travis_script.sh

@@ -81,6 +81,7 @@ if [ ${TASK} == "cpp_unittest" ]; then
     echo "GTEST_PATH="${CACHE_PREFIX} >> config.mk
     make test || exit -1
     export MXNET_ENGINE_TYPE=ThreadedEngine
+    export MXNET_ENGINE_INFO=true
     for test in tests/cpp/*_test; do
         ./$test || exit -1
     done

src/common/object_pool.h

@@ -46,10 +46,15 @@ class ObjectPool {
    * \brief Internal structure to hold pointers.
    */
   struct LinkedList {
+#if defined(_MSC_VER)
+    T t;
+    LinkedList* next{nullptr};
+#else
     union {
-      LinkedList* next{nullptr};
       T t;
+      LinkedList* next{nullptr};
     };
+#endif
   };
   /*!
    * \brief Page size of allocation.

src/engine/threaded_engine.cc

@@ -302,11 +302,17 @@ void ThreadedEngine::WaitForVar(VarHandle var) {
   if (threaded_var->ready_to_read()) return;
   std::atomic<bool> done{false};
   this->PushSync([this, &done](RunContext) {
+      if (engine_info_) {
+        LOG(INFO) << "Sync is executed";
+      }
       {
         std::unique_lock<std::mutex> lock{finished_m_};
         done.store(true);
       }
       finished_cv_.notify_all();
+      if (engine_info_) {
+        LOG(INFO) << "Sync is notified";
+      }
     }, Context::CPU(), {var}, {}, FnProperty::kNormal);
   {
     std::unique_lock<std::mutex> lock{finished_m_};

src/engine/threaded_engine.h

@@ -249,7 +249,9 @@ class ThreadedEngine : public Engine {
     shutdown_phase_.store(true);
   }
 
-  ThreadedEngine() {}
+  ThreadedEngine() {
+    engine_info_ = dmlc::GetEnv("MXNET_ENGINE_INFO", false);
+  }
   ~ThreadedEngine() {
     {
       std::unique_lock<std::mutex> lock{finished_m_};
@@ -319,6 +321,8 @@ class ThreadedEngine : public Engine {
   std::atomic<bool> kill_{false};
   /*! \brief whether it is during shutdown phase*/
   std::atomic<bool> shutdown_phase_{false};
+  /*!\brief show more information from engine actions */
+  bool engine_info_{false};
   /*!
    * \brief Mutex and condition_variable,
    *  used to Notify waits for single or all variables.