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

third_party/WebKit/Source/wtf/ThreadSpecific.h

Index: third_party/WebKit/Source/wtf/ThreadSpecific.h
diff --git a/third_party/WebKit/Source/wtf/ThreadSpecific.h b/third_party/WebKit/Source/wtf/ThreadSpecific.h
index 377a82f4e24e405aafb5c707a672de58ddc39ce6..5b197de00b5e533550547a6575b3cc4d7f3dfa1a 100644
--- a/third_party/WebKit/Source/wtf/ThreadSpecific.h
+++ b/third_party/WebKit/Source/wtf/ThreadSpecific.h
@@ -44,6 +44,7 @@
 
 #include "wtf/Allocator.h"
 #include "wtf/Noncopyable.h"
+#include "wtf/StackUtil.h"
 #include "wtf/StdLibExtras.h"
 #include "wtf/WTF.h"
 #include "wtf/WTFExport.h"
@@ -113,6 +114,8 @@ class ThreadSpecific {
 #elif OS(WIN)
   int m_index;
 #endif
+  // This member must only be accessed or modified on the main thread.
+  T* m_mainThreadStorage = nullptr;
 };
 
 #if OS(POSIX)
@@ -261,17 +264,29 @@ inline bool ThreadSpecific<T>::isSet() {
 
 template <typename T>
 inline ThreadSpecific<T>::operator T*() {
-  T* ptr = static_cast<T*>(get());
-  if (!ptr) {
-    // Set up thread-specific value's memory pointer before invoking
-    // constructor, in case any function it calls
-    // needs to access the value, to avoid recursion.
-    ptr = static_cast<T*>(Partitions::fastZeroedMalloc(
+  T* offThreadPtr;
+#if defined(__GLIBC__) || OS(ANDROID) || OS(FREEBSD)
+  // TLS is fast on these platforms.
+  // TODO(csharrison): Qualify this statement for Android.
+  T** ptr = &offThreadPtr;
+  offThreadPtr = static_cast<T*>(get());
+#else
+  T** ptr = &m_mainThreadStorage;
+  if (UNLIKELY(internal::mayNotBeMainThread())) {
+    offThreadPtr = static_cast<T*>(get());
+    ptr = &offThreadPtr;
+  }
+#endif
+  // Set up thread-specific value's memory pointer before invoking constructor,
+  // in case any function it calls needs to access the value, to avoid
+  // recursion.
+  if (UNLIKELY(!*ptr)) {
+    *ptr = static_cast<T*>(Partitions::fastZeroedMalloc(
         sizeof(T), WTF_HEAP_PROFILER_TYPE_NAME(T)));
-    set(ptr);
-    new (NotNull, ptr) T;
+    set(*ptr);
+    new (NotNull, *ptr) T;
   }
-  return ptr;
+  return *ptr;
 }
 
 template <typename T>