Make NS_IsMainThread faster on our major platforms. (Bug 521750) r=dbaron

configure.in

@@ -4160,7 +4160,15 @@ if test "$ac_cv_trouble_comparing_to_zero" = yes ; then
   AC_DEFINE(HAVE_CPP_TROUBLE_COMPARING_TO_ZERO)
 fi
 
-
+AC_CACHE_CHECK(for __thread keyword for TLS variables,
+               ac_cv_thread_keyword,
+               [AC_TRY_COMPILE([__thread bool tlsIsMainThread = false;],
+                               [return tlsIsMainThread;],
+                               ac_cv_thread_keyword=yes,
+                               ac_cv_thread_keyword=no)])
+if test "$ac_cv_thread_keyword" = yes; then
+  AC_DEFINE(HAVE_THREAD_TLS_KEYWORD)
+fi
 
 dnl End of C++ language/feature checks
 AC_LANG_C

xpcom/base/nscore.h

@@ -466,6 +466,12 @@ typedef PRUint32 nsrefcnt;
 #define XPCOM_GLUE_AVOID_NSPR
 #endif
 
+#if defined(_MSC_VER) && !defined(WINCE)
+#define NS_TLS __declspec(thread)
+#elif defined(HAVE_THREAD_TLS_KEYWORD)
+#define NS_TLS __thread
+#endif
+
 /**
  * Static type annotations, enforced when static-checking is enabled:
  *

xpcom/glue/nsThreadUtils.cpp

@@ -116,20 +116,29 @@ NS_GetMainThread(nsIThread **result)
 #endif
 }
 
-NS_METHOD_(PRBool)
+#ifndef MOZILLA_INTERNAL_API
-NS_IsMainThread()
+bool NS_IsMainThread()
 {
   PRBool result = PR_FALSE;
-#ifdef MOZILLA_INTERNAL_API
-  nsThreadManager::get()->nsThreadManager::GetIsMainThread(&result);
-#else
   nsCOMPtr<nsIThreadManager> mgr =
-      do_GetService(NS_THREADMANAGER_CONTRACTID);
+    do_GetService(NS_THREADMANAGER_CONTRACTID);
   if (mgr)
     mgr->GetIsMainThread(&result);
-#endif
+  return bool(result);
-  return result;
 }
+#elif !defined(NS_TLS)
+bool NS_IsMainThread()
+{
+  PRBool result = PR_FALSE;
+  nsThreadManager::get()->nsThreadManager::GetIsMainThread(&result);
+  return bool(result);
+}
+#elif !defined(MOZ_ENABLE_LIBXUL)
+bool NS_IsMainThread()
+{
+  return gTLSIsMainThread;
+}
+#endif
 
 NS_METHOD
 NS_DispatchToCurrentThread(nsIRunnable *event)

xpcom/glue/nsThreadUtils.h

@@ -98,14 +98,28 @@ NS_GetCurrentThread(nsIThread **result);
 extern NS_COM_GLUE NS_METHOD
 NS_GetMainThread(nsIThread **result);
 
+#if defined(MOZILLA_INTERNAL_API) && defined(NS_TLS)
+// This is defined in nsThreadManager.cpp and initialized to `true` for the
+// main thread by nsThreadManager::Init.
+extern NS_TLS bool gTLSIsMainThread;
+
+#ifdef MOZ_ENABLE_LIBXUL
+inline bool NS_IsMainThread()
+{
+  return gTLSIsMainThread;
+}
+#else
+NS_COM bool NS_IsMainThread();
+#endif
+#else
 /**
  * Test to see if the current thread is the main thread.
  *
  * @returns PR_TRUE if the current thread is the main thread, and PR_FALSE
  * otherwise.
  */
-extern NS_COM_GLUE NS_METHOD_(PRBool)
+extern NS_COM_GLUE bool NS_IsMainThread();
-NS_IsMainThread();
+#endif
 
 /**
  * Dispatch the given event to the current thread.

xpcom/threads/nsThreadManager.cpp

@@ -38,11 +38,16 @@
 
 #include "nsThreadManager.h"
 #include "nsThread.h"
+#include "nsThreadUtils.h"
 #include "nsIClassInfoImpl.h"
 #include "nsTArray.h"
 #include "nsAutoPtr.h"
 #include "nsAutoLock.h"
 
+#ifdef NS_TLS
+NS_TLS bool gTLSIsMainThread = false;
+#endif
+
 typedef nsTArray< nsRefPtr<nsThread> > nsThreadArray;
 
 //-----------------------------------------------------------------------------
@@ -101,6 +106,10 @@ nsThreadManager::Init()
   // GetIsMainThread calls that occur post-Shutdown.
   mMainThread->GetPRThread(&mMainPRThread);
 
+#ifdef NS_TLS
+  gTLSIsMainThread = true;
+#endif
+
   mInitialized = PR_TRUE;
   return NS_OK;
 }