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

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

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 27 matching lines @@
 #include "public/platform/WebThread.h"
 #include "wtf/AddressSanitizer.h"
 #include "wtf/Allocator.h"
 #include "wtf/Forward.h"
 #include "wtf/Functional.h"
 #include "wtf/HashMap.h"
 #include "wtf/HashSet.h"
 #include "wtf/ThreadSpecific.h"
 #include "wtf/Threading.h"
 #include "wtf/ThreadingPrimitives.h"
+#include "wtf/WTFThreadData.h"
 #include <memory>
 
 namespace v8 {
 class Isolate;
 };
 
 namespace blink {
 
 class BasePage;
 class CallbackStack;
@@ skipping 117 matching lines @@
   // call thread can start using the garbage collected heap infrastructure.
   // It also has to periodically check for safepoints.
   static void attachCurrentThread();
 
   // Disassociate attached ThreadState from the current thread. The thread
   // can no longer use the garbage collected heap after this call.
   static void detachCurrentThread();
 
   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.
     return **s_threadSpecific;
 #endif
   }
 
   static ThreadState* mainThreadState() {
     return reinterpret_cast<ThreadState*>(s_mainThreadStateStorage);
   }
@@ skipping 441 matching lines @@
   // Should only be called under protection of threadAttachMutex().
   const Vector<std::unique_ptr<BlinkGCInterruptor>>& interruptors() const {
     return m_interruptors;
   }
 
   friend class SafePointAwareMutexLocker;
   friend class SafePointBarrier;
   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.
   // We would like to manage lifetime of the ThreadState attached
   // to the main thread explicitly instead and still use normal
   // constructor and destructor for the ThreadState class.
   // For this we reserve static storage for the main ThreadState
   // and lazily construct ThreadState in it using placement new.
   static uint8_t s_mainThreadStateStorage[];
 
   ThreadHeap* m_heap;
   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;
 
   void* m_safePointScopeMarker;
   Vector<Address> m_safePointStackCopy;
   bool m_atSafePoint;
   Vector<std::unique_ptr<BlinkGCInterruptor>> m_interruptors;
   bool m_sweepForbidden;
   size_t m_noAllocationCount;
   size_t m_gcForbiddenCount;
@@ skipping 75 matching lines @@
 class ThreadStateFor<AnyThread> {
   STATIC_ONLY(ThreadStateFor);
 
  public:
   static ThreadState* state() { return ThreadState::current(); }
 };
 
 }  // namespace blink
 
 #endif  // ThreadState_h