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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+++ 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>
+#include <stdint.h>
+
+namespace WTF {
+
+WTF_EXPORT size_t getUnderestimatedStackSize();
+WTF_EXPORT void* getStackStart();
+
+namespace internal {
+
+WTF_EXPORT uintptr_t mainThreadUnderestimatedStackSize();
+
+#if OS(WIN) && COMPILER(MSVC)
+size_t threadStackSize();
+#endif
+
+// 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.
+inline WTF_EXPORT bool mayNotBeMainThread() {
+  // getStackStart is exclusive, not inclusive (i.e. it points past the last
+  // page of the stack in linear order). So, to ensure an inclusive comparison,
+  // subtract here and in mainThreadUnderestimatedStackSize().
+  static uintptr_t s_mainThreadStackStart =
+      reinterpret_cast<uintptr_t>(getStackStart()) - sizeof(void*);
+  static uintptr_t s_mainThreadUnderestimatedStackSize =
+      mainThreadUnderestimatedStackSize();
+  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.
+  return addressDiff >= s_mainThreadUnderestimatedStackSize;
+}
+
+}  // namespace internal
+
+}  // namespace WTF
+
+#endif  // StackUtil_h