[SYCL] Load PI plugins only once. Make pi::initialize return const reference to the global vector of plugins, not a temporary copy of it.

Change-Id: Ic1373059c9697b0b4d318548b79be7bcb9d4f2a9
Signed-off-by: Sergey V Maslov <[email protected]>

Author: smaslov-intel

sycl/include/CL/sycl/detail/pi.hpp

@@ -135,7 +135,7 @@ template <class To, class From> To cast(From value);
 extern std::shared_ptr<plugin> GlobalPlugin;
 
 // Performs PI one-time initialization.
-vector_class<plugin> initialize();
+const vector_class<plugin> &initialize();
 
 // Utility Functions to get Function Name for a PI Api.
 template <PiApiKind PiApiOffset> struct PiFuncInfo {};

sycl/source/detail/pi.cpp

@@ -255,7 +255,13 @@ bool trace(TraceLevel Level) {
 }
 
 // Initializes all available Plugins.
-vector_class<plugin> initialize() {
+// The returned reference lifetime is through the end of the process.
+//
+// TODO: make sure that the global Plugins is destroyed last, at least
+// after other tear-down relying on plugins (like destructors of global
+// buffers) is completed.
+//
+const vector_class<plugin> &initialize() {
   static bool PluginsInitDone = false;
   static vector_class<plugin> Plugins;
   if (PluginsInitDone) {

sycl/source/detail/platform_impl.cpp

@@ -49,7 +49,7 @@ static bool IsBannedPlatform(platform Platform) {
 
 vector_class<platform> platform_impl::get_platforms() {
   vector_class<platform> Platforms;
-  vector_class<plugin> Plugins = RT::initialize();
+  const vector_class<plugin> &Plugins = RT::initialize();
 
   info::device_type ForcedType = detail::get_forced_type();
   for (unsigned int i = 0; i < Plugins.size(); i++) {