From 9f5e1686397ffcd35b14ef25e48f1d7e3f2e1ea8 Mon Sep 17 00:00:00 2001
From: mogemimi <mogemimi@enginetrouble.net>
Date: Wed, 18 Sep 2024 01:00:29 +0900
Subject: [PATCH] Improve spin lock for C++20

---
 pomdog/utility/spin_lock.cpp | 34 +++++++++++-----------------------
 pomdog/utility/spin_lock.h   |  9 ++-------
 2 files changed, 13 insertions(+), 30 deletions(-)

diff --git a/pomdog/utility/spin_lock.cpp b/pomdog/utility/spin_lock.cpp
index 02facae11..d18f00664 100644
--- a/pomdog/utility/spin_lock.cpp
+++ b/pomdog/utility/spin_lock.cpp
@@ -1,16 +1,6 @@
 // Copyright mogemimi. Distributed under the MIT license.
 
 #include "pomdog/utility/spin_lock.h"
-#include "pomdog/basic/conditional_compilation.h"
-#include "pomdog/basic/platform.h"
-
-POMDOG_SUPPRESS_WARNINGS_GENERATED_BY_STD_HEADERS_BEGIN
-#if defined(POMDOG_PLATFORM_LINUX) && (defined(__x86_64__) || defined(_M_X64) || defined(_M_AMD64))
-#include <emmintrin.h>
-#else
-#include <thread>
-#endif
-POMDOG_SUPPRESS_WARNINGS_GENERATED_BY_STD_HEADERS_END
 
 namespace pomdog::detail {
 
@@ -18,24 +8,22 @@ SpinLock::SpinLock() noexcept = default;
 
 void SpinLock::lock() noexcept
 {
-    while (flag.test_and_set(std::memory_order_acquire)) {
-#if defined(POMDOG_PLATFORM_LINUX) && (defined(__x86_64__) || defined(_M_X64) || defined(_M_AMD64))
-        // NOTE: For both performance and power consumption, we use the
-        // pause instruction in the loop instead of fully spinning.
-        // https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops/
-        // http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0514r0.pdf
-        _mm_pause();
-#else
-        // NOTE: It is much faster to use `std::this_thread::yield()` than to
-        // call `_mm_pause()` directly on Windows and macOS.
-        std::this_thread::yield();
-#endif
+    while (flag_.test_and_set(std::memory_order_acquire)) {
+        // NOTE: Instead of a busy loop, a wait instruction is inserted
+        //       to put the CPU into low power consumption mode.
+        //       When implementing spin, it is possible to call the __mm_pause(),
+        //       _umwait(), TPAUSE, or std::this_thread::yield(), but here wait() is called for portability.
+        //
+        //       https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops/
+        //       http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0514r0.pdf
+        flag_.wait(true, std::memory_order_relaxed);
     }
 }
 
 void SpinLock::unlock() noexcept
 {
-    flag.clear(std::memory_order_release);
+    flag_.clear(std::memory_order_release);
+    flag_.notify_one();
 }
 
 } // namespace pomdog::detail
diff --git a/pomdog/utility/spin_lock.h b/pomdog/utility/spin_lock.h
index dd5f68f76..b422dc9d4 100644
--- a/pomdog/utility/spin_lock.h
+++ b/pomdog/utility/spin_lock.h
@@ -12,13 +12,8 @@ POMDOG_SUPPRESS_WARNINGS_GENERATED_BY_STD_HEADERS_END
 namespace pomdog::detail {
 
 class POMDOG_EXPORT SpinLock final {
-#if defined(__EMSCRIPTEN__)
-    // NOTE: for C++20
-    std::atomic_flag flag;
-#else
-    // NOTE: for C++17
-    std::atomic_flag flag = ATOMIC_FLAG_INIT;
-#endif
+    std::atomic_flag flag_;
+    static_assert(__cplusplus >= 202002L, "ATOMIC_FLAG_INIT was deprecated in C++20, https://cplusplus.github.io/LWG/issue3659");
 
 public:
     SpinLock() noexcept;