From a326eeaada649b3515ea992959459d0960c1b1a1 Mon Sep 17 00:00:00 2001
From: Andrea Bocci <andrea.bocci@cern.ch>
Date: Wed, 26 Jul 2023 13:37:02 +0200
Subject: [PATCH] DO NOT MERGE: minimal revert of #2020 and #1988 for the SYCL
 platform

---
 include/alpaka/pltf/PltfGenericSycl.hpp | 121 +++++++++++++++++-------
 1 file changed, 86 insertions(+), 35 deletions(-)

diff --git a/include/alpaka/pltf/PltfGenericSycl.hpp b/include/alpaka/pltf/PltfGenericSycl.hpp
index 8c67f5f50c31..c2aeec69e3ce 100644
--- a/include/alpaka/pltf/PltfGenericSycl.hpp
+++ b/include/alpaka/pltf/PltfGenericSycl.hpp
@@ -14,6 +14,8 @@
 #if ALPAKA_DEBUG >= ALPAKA_DEBUG_MINIMAL
 #    include <iostream>
 #endif
+#include <mutex>
+#include <optional>
 #include <sstream>
 #include <stdexcept>
 #include <vector>
@@ -29,64 +31,113 @@ namespace alpaka
     struct PltfGenericSycl : concepts::Implements<ConceptPltf, PltfGenericSycl<TSelector>>
     {
         PltfGenericSycl()
-            : platform{TSelector{}}
-            , devices(platform.get_devices())
-            , context{sycl::context{
-                  devices,
-                  [](sycl::exception_list exceptions)
-                  {
-                      auto ss_err = std::stringstream{};
-                      ss_err << "Caught asynchronous SYCL exception(s):\n";
-                      for(std::exception_ptr e : exceptions)
-                      {
-                          try
-                          {
-                              std::rethrow_exception(e);
-                          }
-                          catch(sycl::exception const& err)
-                          {
-                              ss_err << err.what() << " (" << err.code() << ")\n";
-                          }
-                      }
-                      throw std::runtime_error(ss_err.str());
-                  }}}
         {
         }
 
         [[nodiscard]] auto syclPlatform() -> sycl::platform&
         {
-            return platform;
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
+            return *platform_opt;
         }
 
         [[nodiscard]] auto syclPlatform() const -> sycl::platform const&
         {
-            return platform;
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
+            return *platform_opt;
         }
 
         [[nodiscard]] auto syclDevices() -> std::vector<sycl::device>&
         {
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
             return devices;
         }
 
         [[nodiscard]] auto syclDevices() const -> std::vector<sycl::device> const&
         {
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
             return devices;
         }
 
         [[nodiscard]] auto syclContext() -> sycl::context&
         {
-            return context;
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
+            return *context_opt;
         }
 
         [[nodiscard]] auto syclContext() const -> sycl::context const&
         {
-            return context;
+            auto lock = std::scoped_lock<std::mutex>{mutex};
+
+            if(!initialized)
+                do_initialization();
+
+            return *context_opt;
         }
 
     private:
-        sycl::platform platform;
-        std::vector<sycl::device> devices;
-        sycl::context context;
+        static auto do_initialization()
+        {
+            if(initialized)
+                return;
+
+            platform_opt = sycl::platform{TSelector{}};
+            devices = platform_opt->get_devices();
+            context_opt = sycl::context{
+                devices,
+                [](sycl::exception_list exceptions)
+                {
+                    auto ss_err = std::stringstream{};
+                    ss_err << "Caught asynchronous SYCL exception(s):\n";
+                    for(std::exception_ptr e : exceptions)
+                    {
+                        try
+                        {
+                            std::rethrow_exception(e);
+                        }
+                        catch(sycl::exception const& err)
+                        {
+                            ss_err << err.what() << " (" << err.code() << ")\n";
+                        }
+                    }
+                    throw std::runtime_error(ss_err.str());
+                }};
+
+            initialized = true;
+        }
+
+#    if BOOST_COMP_CLANG
+#        pragma clang diagnostic push
+#        pragma clang diagnostic ignored "-Wexit-time-destructors"
+#    endif
+        inline static std::mutex mutex;
+        inline static bool initialized{false};
+
+        inline static std::optional<sycl::platform> platform_opt{std::nullopt};
+        inline static std::vector<sycl::device> devices;
+        inline static std::optional<sycl::context> context_opt{std::nullopt};
+#    if BOOST_COMP_CLANG
+#        pragma clang diagnostic pop
+#    endif
     };
 } // namespace alpaka
 
@@ -106,22 +157,23 @@ namespace alpaka::trait
 
     //! The SYCL platform device get trait specialization.
     template<typename TSelector>
-    struct GetDevByIdx<alpaka::PltfGenericSycl<TSelector>>
+    struct GetDevByIdx<PltfGenericSycl<TSelector>>
     {
         static auto getDevByIdx(PltfGenericSycl<TSelector> const& platform, std::size_t const& devIdx)
         {
             ALPAKA_DEBUG_FULL_LOG_SCOPE;
 
-            auto const& devices = platform.syclDevices();
-            if(devIdx >= devices.size())
+            using SyclPltf = PltfGenericSycl<TSelector>;
+            auto const dev_num = platform.syclDevices().size();
+            if(devIdx >= dev_num)
             {
                 auto ss_err = std::stringstream{};
-                ss_err << "Unable to return device handle for device " << devIdx << ". There are only "
-                       << devices.size() << " SYCL devices!";
+                ss_err << "Unable to return device handle for device " << devIdx << ". There are only " << dev_num
+                       << " SYCL devices!";
                 throw std::runtime_error(ss_err.str());
             }
 
-            auto sycl_dev = devices.at(devIdx);
+            auto sycl_dev = platform.syclDevices().at(devIdx);
 
             // Log this device.
 #    if ALPAKA_DEBUG >= ALPAKA_DEBUG_FULL
@@ -129,7 +181,6 @@ namespace alpaka::trait
 #    elif ALPAKA_DEBUG >= ALPAKA_DEBUG_MINIMAL
             std::cout << __func__ << sycl_dev.template get_info<sycl::info::device::name>() << '\n';
 #    endif
-            using SyclPltf = alpaka::PltfGenericSycl<TSelector>;
             return typename DevType<SyclPltf>::type{sycl_dev, platform.syclContext()};
         }