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

third_party/WebKit/Source/wtf/StackUtil.cpp

 // 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.
 
 #include "wtf/StackUtil.h"
 
 #include "wtf/Assertions.h"
 #include "wtf/Threading.h"
 #include "wtf/WTFThreadData.h"
 
@@ skipping 125 matching lines @@
 #else
   return reinterpret_cast<void*>(__readfsdword(offsetof(NT_TIB, StackBase)));
 #endif
 #else
 #error Unsupported getStackStart on this platform.
 #endif
 }
 
 namespace internal {
 
-uintptr_t mainThreadUnderestimatedStackSize() {
+uintptr_t s_mainThreadStackStart = 0;
+uintptr_t s_mainThreadUnderestimatedStackSize = 0;
+
+void initializeMainThreadStackEstimate() {
+  // 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 below.
+  s_mainThreadStackStart =
+      reinterpret_cast<uintptr_t>(getStackStart()) - sizeof(void*);
+
   size_t underestimatedStackSize = getUnderestimatedStackSize();
-  // See comment in mayNotBeMainThread as to why we subtract here.
   if (underestimatedStackSize > sizeof(void*)) {
     underestimatedStackSize = underestimatedStackSize - sizeof(void*);
   }
-  return underestimatedStackSize;
+  s_mainThreadUnderestimatedStackSize = underestimatedStackSize;
 }
 
 #if OS(WIN) && COMPILER(MSVC)
 size_t threadStackSize() {
   // Notice that we cannot use the TIB's StackLimit for the stack end, as i
   // tracks the end of the committed range. We're after the end of the reserved
   // stack area (most of which will be uncommitted, most times.)
   MEMORY_BASIC_INFORMATION stackInfo;
   memset(&stackInfo, 0, sizeof(MEMORY_BASIC_INFORMATION));
   size_t resultSize =
@@ skipping 17 matching lines @@
   // explains the details.
   RELEASE_ASSERT(s_threadStackSize > 4 * 0x1000);
   s_threadStackSize -= 4 * 0x1000;
   return s_threadStackSize;
 }
 #endif
 
 }  // namespace internal
 
 }  // namespace WTF