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

third_party/WebKit/Source/wtf/StackUtil.h

Index: third_party/WebKit/Source/wtf/StackUtil.h
diff --git a/third_party/WebKit/Source/wtf/StackUtil.h b/third_party/WebKit/Source/wtf/StackUtil.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef StackUtil_h
+#define StackUtil_h
+
+#include "wtf/Compiler.h"
+#include "wtf/WTFExport.h"
+#include "wtf/build_config.h"
+#include <stddef.h>
+
+namespace WTF {
+
+WTF_EXPORT size_t getUnderestimatedStackSize();
+WTF_EXPORT void* getStackStart();
+
+namespace internal {
+
+// Returns true if the function is not called on the main thread. Note carefully
+// that this function may have false positives, i.e. it can return true even if
+// we are on the main thread. If the function returns false, we are certainly
+// not on the main thread. Must be WTF_EXPORT due to template inlining in
+// ThreadSpecific.
+WTF_EXPORT bool mayNotBeMainThread();
+
+#if OS(WIN) && COMPILER(MSVC)
+size_t threadStackSize();
+#endif
+
+}  // namespace internal
+
+}  // namespace WTF
+
+#endif  // StackUtil_h