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

third_party/WebKit/Source/wtf/ThreadingPthreads.cpp

Index: third_party/WebKit/Source/wtf/ThreadingPthreads.cpp
diff --git a/third_party/WebKit/Source/wtf/ThreadingPthreads.cpp b/third_party/WebKit/Source/wtf/ThreadingPthreads.cpp
index 85a3976090948bb06a944969aa7b771ef9d023f1..b86de9b1763553a01d2a9a91701e5a5f5e12602e 100644
--- a/third_party/WebKit/Source/wtf/ThreadingPthreads.cpp
+++ b/third_party/WebKit/Source/wtf/ThreadingPthreads.cpp
@@ -85,8 +85,29 @@ void initializeThreading() {
   double_conversion::DoubleToStringConverter::EcmaScriptConverter();
 }
 
+namespace {
+ThreadSpecificKey s_currentThreadKey;
+bool s_currentThreadKeyInitialized = false;
+}  // namespace
+
+void initializeCurrentThread() {
+  DCHECK(!s_currentThreadKeyInitialized);
+  threadSpecificKeyCreate(&s_currentThreadKey, [](void*) {});
+  s_currentThreadKeyInitialized = true;
+}
+
 ThreadIdentifier currentThread() {
-  return wtfThreadData().threadId();

[haraken, 2017/03/06 19:21:36]

A stupid question: Why can't we use wtfThreadData() whereas other places are
using wtfThreadData()?

[jbroman, 2017/03/06 20:28:09]

Cyclic dependency. If wtfThreadData() is used here, we can loop like this (until
stack overflow):

currentThread -> wtfThreadData -> ThreadSpecific::operator* -> isMainThread ->
currentThread

There are a few ways of resolving this cycle, of which this seemed simplest to
me (especially storing the thread ID doesn't require supporting destructors
etc.). It also happens to remove one pointer indirection from currentThread, and
avoids reinitializing the atomic string table et al. if the thread ID is used
during thread shutdown, which are nice side effects.

[haraken, 2017/03/06 20:46:42]

Ah, got it. Maybe it's worth having a comment :)

[jbroman, 2017/03/07 16:03:30]

Done.

+  static_assert(sizeof(ThreadIdentifier) <= sizeof(void*),
+                "ThreadIdentifier must fit in a void*.");
+  DCHECK(s_currentThreadKeyInitialized);
+  void* value = threadSpecificGet(s_currentThreadKey);
+  if (UNLIKELY(!value)) {
+    value = reinterpret_cast<void*>(
+        static_cast<intptr_t>(internal::currentThreadSyscall()));
+    DCHECK(value);
+    threadSpecificSet(s_currentThreadKey, value);
+  }
+  return reinterpret_cast<intptr_t>(threadSpecificGet(s_currentThreadKey));
 }
 
 MutexBase::MutexBase(bool recursive) {