WTF: Initialize main-thread stack estimates when WTF starts up.

third_party/WebKit/Source/wtf/StackUtil.h

 // 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();
+WTF_EXPORT extern uintptr_t s_mainThreadStackStart;
+WTF_EXPORT extern uintptr_t s_mainThreadUnderestimatedStackSize;
+
+WTF_EXPORT void initializeMainThreadStackEstimate();
 
 #if OS(WIN) && COMPILER(MSVC)
 size_t threadStackSize();
 #endif
 
+}  // 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
-// 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();
+// on the main thread.
+inline bool mayNotBeMainThread() {
   uintptr_t dummy;
   uintptr_t addressDiff =
-      s_mainThreadStackStart - reinterpret_cast<uintptr_t>(&dummy);
+      internal::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;
+  return addressDiff >= internal::s_mainThreadUnderestimatedStackSize;
 }
 
-}  // namespace internal
-
 }  // namespace WTF
 
 #endif  // StackUtil_h