[llvm] [Offload] Move RPC server handling to a dedicated thread (PR #112988)

[Ivan Butygin via llvm-commits]

================
@@ -21,8 +21,64 @@ using namespace llvm;
 using namespace omp;
 using namespace target;
 
+void RPCServerTy::ServerThread::shutDown() {
+#ifdef LIBOMPTARGET_RPC_SUPPORT
+  {
+    std::lock_guard<decltype(Mutex)> Lock(Mutex);
+    Running.store(false, std::memory_order_release);
+    CV.notify_all();
+  }
+  if (Worker.joinable())
+    Worker.join();
+#endif
+}
+
+void RPCServerTy::ServerThread::run() {
+#ifdef LIBOMPTARGET_RPC_SUPPORT
+  for (;;) {
+    std::unique_lock<decltype(Mutex)> Lock(Mutex);
+    CV.wait(Lock, [&]() {
+      return NumUsers.load(std::memory_order_acquire) > 0 ||
+             !Running.load(std::memory_order_acquire);
+    });
+
+    if (!Running.load(std::memory_order_acq_rel))
+      return;
+
+    while (NumUsers.load(std::memory_order_relaxed) > 0 &&
+           Running.load(std::memory_order_relaxed)) {
+      Lock.unlock();
+      for (const auto &Handle : Handles) {
+        rpc_device_t RPCDevice{Handle};
+        [[maybe_unused]] rpc_status_t Err = rpc_handle_server(RPCDevice);
+        assert(Err == RPC_STATUS_SUCCESS &&
+               "Checking the RPC server should not fail");
+      }
+      Lock.lock();
+    }
+  }
+#endif
+}
+
+RPCServerTy::ServerThread::ServerThread(std::atomic<uintptr_t> Handles[],
+                                        size_t Length)
+    : Running(true), NumUsers(0), CV(), Mutex(), Handles(Handles, Length) {
+#ifdef LIBOMPTARGET_RPC_SUPPORT
+  Worker = std::thread([this]() { run(); });
+#endif
+}
+
 RPCServerTy::RPCServerTy(plugin::GenericPluginTy &Plugin)
-    : Handles(Plugin.getNumDevices()) {}
+    : Handles(
+          std::make_unique<std::atomic<uintptr_t>[]>(Plugin.getNumDevices())),
+      Thread(new ServerThread(Handles.get(), Plugin.getNumDevices())) {}
+
+llvm::Error RPCServerTy::shutDown() {
+#ifdef LIBOMPTARGET_RPC_SUPPORT
+  Thread->shutDown();
+  return Error::success();
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Hardcode84 wrote:

Return should be after `#endif`

https://github.com/llvm/llvm-project/pull/112988