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

third_party/WebKit/Source/wtf/ThreadSpecific.h

Index: third_party/WebKit/Source/wtf/ThreadSpecific.h
diff --git a/third_party/WebKit/Source/wtf/ThreadSpecific.h b/third_party/WebKit/Source/wtf/ThreadSpecific.h
index 5b197de00b5e533550547a6575b3cc4d7f3dfa1a..57c6392d3b3708febb032e0ee0a3fc6b4c3f1a36 100644
--- a/third_party/WebKit/Source/wtf/ThreadSpecific.h
+++ b/third_party/WebKit/Source/wtf/ThreadSpecific.h
@@ -268,11 +268,13 @@ inline ThreadSpecific<T>::operator T*() {
 #if defined(__GLIBC__) || OS(ANDROID) || OS(FREEBSD)
   // TLS is fast on these platforms.
   // TODO(csharrison): Qualify this statement for Android.
+  const bool mainThreadAlwaysChecksTLS = true;
   T** ptr = &offThreadPtr;
   offThreadPtr = static_cast<T*>(get());
 #else
+  const bool mainThreadAlwaysChecksTLS = false;
   T** ptr = &m_mainThreadStorage;
-  if (UNLIKELY(internal::mayNotBeMainThread())) {
+  if (UNLIKELY(mayNotBeMainThread())) {
     offThreadPtr = static_cast<T*>(get());
     ptr = &offThreadPtr;
   }
@@ -283,6 +285,14 @@ inline ThreadSpecific<T>::operator T*() {
   if (UNLIKELY(!*ptr)) {
     *ptr = static_cast<T*>(Partitions::fastZeroedMalloc(
         sizeof(T), WTF_HEAP_PROFILER_TYPE_NAME(T)));
+
+    // Even if we didn't realize we're on the main thread, we might still be.
+    // We need to double-check so that |m_mainThreadStorage| is populated.
+    if (!mainThreadAlwaysChecksTLS && UNLIKELY(ptr != &m_mainThreadStorage) &&
+        isMainThread()) {
+      m_mainThreadStorage = *ptr;

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

Or is there any option to populate m_mainThreadStorage when initializing the
main thread? Will it mess up the code?

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

We have no registry of all ThreadSpecific objects, and some of them require
other parts of Blink to be initialized (e.g. many are Persistent handles into
the Oilpan heap). Some of these objects may even allocate resources when an
instance per thread is created, so I don't think it would be a good blanket
policy to initialize all of them on thread start.

As you say, this code is only hit when the object does not yet exist on the
calling thread and so isn't hot. We could possible out-of-line this at some
point if it becomes a code size issue, but most of these checks should be pretty
small, and isMainThread is already out-of-line.

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

Ah, sorry, you're totally right. Given that ThreadSpecific exists per TLS, it's
never a good idea to initialize it eagerly.

+    }
+
     set(*ptr);
     new (NotNull, *ptr) T;
   }