Work around missing notify / wait on std::atomic

Author: davidchisnall

spinlock.h

@@ -1,6 +1,17 @@
 #include "visibility.h"
 #include <atomic>
+#include <chrono>
 #include <mutex>
+#include <thread>
+
+
+// Not all supported targets implement the wait / notify instructions on
+// atomics.  Provide simple spinning fallback for ones that don't.
+template<typename T>
+concept Waitable = requires(T t)
+{
+	t.notify_all();
+};
 
 /**
  * Lightweight spinlock that falls back to using the operating system's futex
@@ -20,6 +31,35 @@ class ThinLock
 	};
 	// The lock word
 	std::atomic<LockState> lockWord;
+
+	// Call notify if it exists
+	template<typename T>
+	static void notify(T &atomic) requires (Waitable<T>)
+	{
+		atomic.notify_all();
+	}
+
+	// Simply ignore notify if we don't have one.
+	template<typename T>
+	static void notify(T &atomic) requires (!Waitable<T>)
+	{
+	}
+
+	// Wait if we have a wait method.
+	template<typename T>
+	static void wait(T &atomic, LockState expected) requires (Waitable<T>)
+	{
+		atomic.wait(expected);
+	}
+
+	// Short sleep if we don't.
+	template<typename T>
+	static void wait(T &atomic, LockState expected) requires (!Waitable<T>)
+	{
+		using namespace std::chrono_literals;
+		std::this_thread::sleep_for(1ms);
+	}
+
 	public:
 	// Acquire the lock
 	void lock()
@@ -41,7 +81,7 @@ class ThinLock
 					continue;
 				}
 			}
-			lockWord.wait(LockState::LockedWithWaiters);
+			wait(lockWord, LockState::LockedWithWaiters);
 		}
 	}
 
@@ -51,7 +91,7 @@ class ThinLock
 		auto old = lockWord.exchange(LockState::Unlocked);
 		if (old == LockState::LockedWithWaiters)
 		{
-			lockWord.notify_all();
+			notify(lockWord);
 		}
 	}