Fast path for ThreadSpecific for main thread on TLS-slow platforms

third_party/WebKit/Source/wtf/WTFThreadData.h

Index: third_party/WebKit/Source/wtf/WTFThreadData.h
diff --git a/third_party/WebKit/Source/wtf/WTFThreadData.h b/third_party/WebKit/Source/wtf/WTFThreadData.h
index a3351de1a4782adffc7051de70bf835f2becd445..bf49b3713a805ba87862a8058c3d510a763ef5f8 100644
--- a/third_party/WebKit/Source/wtf/WTFThreadData.h
+++ b/third_party/WebKit/Source/wtf/WTFThreadData.h
@@ -46,7 +46,8 @@ class WTF_EXPORT WTFThreadData {
   WTF_MAKE_NONCOPYABLE(WTFThreadData);
 
  public:
-  WTFThreadData();
+  enum ThreadType { MainThread, Other };
+  explicit WTFThreadData(ThreadType = Other);
   ~WTFThreadData();
 
   AtomicStringTable& getAtomicStringTable() { return *m_atomicStringTable; }
@@ -55,27 +56,44 @@ class WTF_EXPORT WTFThreadData {
 
   ThreadIdentifier threadId() const { return m_threadId; }
 
+#if OS(WIN) && COMPILER(MSVC)
+  size_t threadStackSize();
+#endif
+
+  static void initialize();
+
+  static WTFThreadData& current();
+
+  static bool isMainThread();
+
+  // Returns true if this is the main thread, via a stack address hueristic.
+  // This is a subtle method and most callers should prefer isMainThread.
+  static bool stackBasedIsMainThread();

[haraken, 2017/01/16 04:42:30]

Let's mention that this may return false even if it's the main thread.

To clarify the semantics, we might want to rename it to mayNotBeMainThread and
flip the returned true/false.

[Charlie Harrison, 2017/01/18 01:49:48]

Done.

+
  private:
+  static WTFThreadData& mainThreadData();
+  // m_threadId must be initialized first, as following members need it for
+  // initialization.
+  const ThreadIdentifier m_threadId;
+
+#if OS(WIN) && COMPILER(MSVC)
+  size_t m_threadStackSize = 0u;
+#endif
+
   std::unique_ptr<AtomicStringTable> m_atomicStringTable;
   std::unique_ptr<ICUConverterWrapper> m_cachedConverterICU;
 
-  ThreadIdentifier m_threadId;
+  static uintptr_t s_mainThreadStackStart;
+  static uintptr_t s_mainThreadUnderestimatedStackSize;
+  static uint8_t s_mainThreadDataStorage[];
 
   static ThreadSpecific<WTFThreadData>* staticData;
-  friend WTFThreadData& wtfThreadData();
 };
 
-inline WTFThreadData& wtfThreadData() {
-  // WTFThreadData is used on main thread before it could possibly be used
-  // on secondary ones, so there is no need for synchronization here.
-  if (!WTFThreadData::staticData)
-    WTFThreadData::staticData = new ThreadSpecific<WTFThreadData>;
-  return **WTFThreadData::staticData;
-}
+bool operator==(const WTFThreadData& data1, const WTFThreadData& data2);
 
 }  // namespace WTF
 
 using WTF::WTFThreadData;
-using WTF::wtfThreadData;
 
 #endif  // WTFThreadData_h