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

third_party/WebKit/Source/platform/heap/ThreadState.h

Index: third_party/WebKit/Source/platform/heap/ThreadState.h
diff --git a/third_party/WebKit/Source/platform/heap/ThreadState.h b/third_party/WebKit/Source/platform/heap/ThreadState.h
index 1eb43ff1d3349894f20d17ac8f385744b2593c33..5de90fcbf9d64182a36154fa2ee9b18e8204cc55 100644
--- a/third_party/WebKit/Source/platform/heap/ThreadState.h
+++ b/third_party/WebKit/Source/platform/heap/ThreadState.h
@@ -45,6 +45,7 @@
 #include "wtf/ThreadSpecific.h"
 #include "wtf/Threading.h"
 #include "wtf/ThreadingPrimitives.h"
+#include "wtf/WTFThreadData.h"
 #include <memory>
 
 namespace v8 {
@@ -182,18 +183,10 @@ class PLATFORM_EXPORT ThreadState {
 
   static ThreadState* current() {
 #if defined(__GLIBC__) || OS(ANDROID) || OS(FREEBSD)
-    // TLS lookup is fast in these platforms.
     return **s_threadSpecific;
 #else
-    uintptr_t dummy;
-    uintptr_t addressDiff =
-        s_mainThreadStackStart - reinterpret_cast<uintptr_t>(&dummy);
-    // This is a fast way to judge if we are in the main thread.
-    // If |&dummy| is within |s_mainThreadUnderestimatedStackSize| byte from
-    // the stack start of the main thread, we judge that we are in
-    // the main thread.
-    if (LIKELY(addressDiff < s_mainThreadUnderestimatedStackSize)) {
-      ASSERT(**s_threadSpecific == mainThreadState());
+    if (LIKELY(WTF::WTFThreadData::stackBasedIsMainThread())) {
+      DCHECK_EQ(**s_threadSpecific, mainThreadState());

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

Ideally s_threadSpecific should be moved into WTFThreadData. Then we can just
use WTF::WTFThreadData::isMainThread() here. We don't need to expose
WTFThreadData::stackBasedIsMainThread.

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

I've just turned this into **s_threadSpecific because now all the logic is in
ThreadSpecific.h.

       return mainThreadState();
     }
     // TLS lookup is slow.
@@ -655,8 +648,6 @@ class PLATFORM_EXPORT ThreadState {
   friend class SafePointScope;
 
   static WTF::ThreadSpecific<ThreadState*>* s_threadSpecific;
-  static uintptr_t s_mainThreadStackStart;
-  static uintptr_t s_mainThreadUnderestimatedStackSize;
 
   // We can't create a static member of type ThreadState here
   // because it will introduce global constructor and destructor.
@@ -671,9 +662,6 @@ class PLATFORM_EXPORT ThreadState {
   ThreadIdentifier m_thread;
   std::unique_ptr<PersistentRegion> m_persistentRegion;
   BlinkGC::StackState m_stackState;
-#if OS(WIN) && COMPILER(MSVC)
-  size_t m_threadStackSize;
-#endif
   intptr_t* m_startOfStack;
   intptr_t* m_endOfStack;