Oilpan: Avoid looking up TLS in ThreadState::current()

Source/platform/heap/ThreadState.cpp

 /*
  * 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 83 matching lines @@
 #else
 #error Unsupported getStackStart on this platform.
 #endif
 }
 
 // The maximum number of WrapperPersistentRegions to keep around in the
 // m_pooledWrapperPersistentRegions pool.
 static const size_t MaxPooledWrapperPersistentRegionCount = 2;
 
 WTF::ThreadSpecific<ThreadState*>* ThreadState::s_threadSpecific = 0;
+intptr_t ThreadState::s_mainThreadStackStart = 0;
 uint8_t ThreadState::s_mainThreadStateStorage[sizeof(ThreadState)];
 SafePointBarrier* ThreadState::s_safePointBarrier = 0;
 bool ThreadState::s_inGC = false;
 
 static Mutex& threadAttachMutex()
 {
     AtomicallyInitializedStatic(Mutex&, mutex = *new Mutex);
     return mutex;
 }
 
@@ skipping 190 matching lines @@
     , m_shouldFlushHeapDoesNotContainCache(false)
     , m_lowCollectionRate(false)
     , m_numberOfSweeperTasks(0)
 #if defined(ADDRESS_SANITIZER)
     , m_asanFakeStack(__asan_get_current_fake_stack())
 #endif
 {
     ASSERT(!**s_threadSpecific);
     **s_threadSpecific = this;
 
+    if (isMainThread())
+        s_mainThreadStackStart = reinterpret_cast<intptr_t>(m_startOfStack);
+
     InitializeHeaps<NumberOfHeaps>::init(m_heaps, this);
 
     m_weakCallbackStack = new CallbackStack();
 
     if (blink::Platform::current())
         m_sweeperThread = adoptPtr(blink::Platform::current()->createThread("Blink GC Sweeper"));
 }
 
 ThreadState::~ThreadState()
 {
@@ skipping 937 matching lines @@
             return gcInfo;
         }
     }
     if (needLockForIteration)
         threadAttachMutex().unlock();
     return 0;
 }
 #endif
 
 }