[vm] Use C++11 thread_local instead of platform TLS for Thread::Current().

Allows the C compiler more freedom to optimize.

Change-Id: I274f5b6c4f7bfb152ba9d3432c9ae7ecc550c6d9
Reviewed-on: https://dart-review.googlesource.com/63270
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/os_thread.cc

@@ -19,6 +19,10 @@ OSThread* OSThread::thread_list_head_ = NULL;
 Mutex* OSThread::thread_list_lock_ = NULL;
 bool OSThread::creation_enabled_ = false;
 
+#if defined(HAS_C11_THREAD_LOCAL)
+thread_local Thread* OSThread::current_vm_thread_ = NULL;
+#endif
+
 OSThread::OSThread()
     : BaseThread(true),
       id_(OSThread::GetCurrentThreadId()),
@@ -269,8 +273,18 @@ void OSThread::RemoveThreadFromList(OSThread* thread) {
   }
 }
 
-void OSThread::SetCurrent(OSThread* current) {
-  OSThread::SetThreadLocal(thread_key_, reinterpret_cast<uword>(current));
+void OSThread::SetCurrentTLS(BaseThread* value) {
+  // Provides thread-local destructors.
+  SetThreadLocal(thread_key_, reinterpret_cast<uword>(value));
+
+#if defined(HAS_C11_THREAD_LOCAL)
+  // Allows the C compiler more freedom to optimize.
+  if ((value != NULL) && !value->is_os_thread()) {
+    current_vm_thread_ = static_cast<Thread*>(value);
+  } else {
+    current_vm_thread_ = NULL;
+  }
+#endif
 }
 
 OSThreadIterator::OSThreadIterator() {

runtime/vm/os_thread.h

@@ -11,6 +11,10 @@
 #include "vm/allocation.h"
 #include "vm/globals.h"
 
+#if !HOST_OS_IOS
+#define HAS_C11_THREAD_LOCAL 1
+#endif
+
 // Declare the OS-specific types ahead of defining the generic classes.
 #if defined(HOST_OS_ANDROID)
 #include "vm/os_thread_android.h"
@@ -154,7 +158,11 @@ class OSThread : public BaseThread {
     }
     return os_thread;
   }
-  static void SetCurrent(OSThread* current);
+  static void SetCurrent(OSThread* current) { SetCurrentTLS(current); }
+
+#if defined(HAS_C11_THREAD_LOCAL)
+  static Thread* CurrentVMThread() { return current_vm_thread_; }
+#endif
 
   // TODO(5411455): Use flag to override default value and Validate the
   // stack size by querying OS.
@@ -166,7 +174,7 @@ class OSThread : public BaseThread {
   static BaseThread* GetCurrentTLS() {
     return reinterpret_cast<BaseThread*>(OSThread::GetThreadLocal(thread_key_));
   }
-  static void SetCurrentTLS(uword value) { SetThreadLocal(thread_key_, value); }
+  static void SetCurrentTLS(BaseThread* value);
 
   typedef void (*ThreadStartFunction)(uword parameter);
   typedef void (*ThreadDestructor)(void* parameter);
@@ -265,6 +273,10 @@ class OSThread : public BaseThread {
   static OSThread* thread_list_head_;
   static bool creation_enabled_;
 
+#if defined(HAS_C11_THREAD_LOCAL)
+  static thread_local Thread* current_vm_thread_;
+#endif
+
   friend class Isolate;  // to access set_thread(Thread*).
   friend class OSThreadIterator;
   friend class ThreadInterrupterWin;

runtime/vm/thread.h

@@ -212,11 +212,15 @@ class Thread : public BaseThread {
 
   // The currently executing thread, or NULL if not yet initialized.
   static Thread* Current() {
+#if defined(HAS_C11_THREAD_LOCAL)
+    return OSThread::CurrentVMThread();
+#else
     BaseThread* thread = OSThread::GetCurrentTLS();
     if (thread == NULL || thread->is_os_thread()) {
       return NULL;
     }
     return reinterpret_cast<Thread*>(thread);
+#endif
   }
 
   // Makes the current thread enter 'isolate'.
@@ -902,9 +906,7 @@ class Thread : public BaseThread {
   void ExitSafepointUsingLock();
   void BlockForSafepoint();
 
-  static void SetCurrent(Thread* current) {
-    OSThread::SetCurrentTLS(reinterpret_cast<uword>(current));
-  }
+  static void SetCurrent(Thread* current) { OSThread::SetCurrentTLS(current); }
 
   void DeferOOBMessageInterrupts();
   void RestoreOOBMessageInterrupts();