Make an inverted atomic flag type and use it

The inverted atomic flag replaces test_and_set+clear with test_and_clear+set,
essentially inverting the flag status. This makes more logical sense for
flagging dirty state, which is less confusing than flagging clean state. The
one caveat is ATOMIC_FLAG_INIT (or default construction in C++20) initializes
the state to true rather than false.

1 files changed, 21 insertions, 0 deletions

diff --git a/common/atomic.h b/common/atomic.h
index 5e9b04c6..d70382ce 100644
--- a/common/atomic.h
+++ b/common/atomic.h
@@ -2,8 +2,29 @@
 #define AL_ATOMIC_H
 
 #include <atomic>
+#include <utility>
 
 
+namespace al {
+
+struct atomic_invflag : protected std::atomic_flag {
+    atomic_invflag() noexcept = default;
+    template<typename T>
+    atomic_invflag(T&& arg) noexcept : std::atomic_flag{std::forward<T>(arg)} { }
+
+    inline bool test_and_clear(std::memory_order m=std::memory_order_seq_cst) noexcept
+    { return !test_and_set(m); }
+    inline bool test_and_clear(std::memory_order m=std::memory_order_seq_cst) volatile noexcept
+    { return !test_and_set(m); }
+
+    inline void set(std::memory_order m=std::memory_order_seq_cst) noexcept
+    { clear(m); }
+    inline void set(std::memory_order m=std::memory_order_seq_cst) volatile noexcept
+    { clear(m); }
+};
+
+} // namespace al
+
 using RefCount = std::atomic<unsigned int>;
 
 inline void InitRef(RefCount &ref, unsigned int value)