8264742: member variable _monitor of MonitorLocker is redundant

Reviewed-by: coleenp, dholmes, phh

src/hotspot/share/runtime/mutexLocker.hpp

@@ -198,7 +198,6 @@ void assert_locked_or_safepoint_or_handshake(const Mutex* lock, const JavaThread
 class MutexLocker: public StackObj {
  protected:
   Mutex* _mutex;
- private:
  public:
   MutexLocker(Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
     _mutex(mutex) {
@@ -242,36 +241,41 @@ class MutexLocker: public StackObj {
 
 class MonitorLocker: public MutexLocker {
   Mutex::SafepointCheckFlag _flag;
-  Monitor* _monitor;
+
+ protected:
+  Monitor* as_monitor() const {
+    return static_cast<Monitor*>(_mutex);
+  }
+
  public:
   MonitorLocker(Monitor* monitor, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
-    MutexLocker(monitor, flag), _flag(flag), _monitor(monitor) {
+    MutexLocker(monitor, flag), _flag(flag) {
     // Superclass constructor did locking
-    assert(_monitor != NULL, "NULL monitor not allowed");
+    assert(monitor != NULL, "NULL monitor not allowed");
   }
 
   MonitorLocker(Thread* thread, Monitor* monitor, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
-    MutexLocker(thread, monitor, flag), _flag(flag), _monitor(monitor)  {
+    MutexLocker(thread, monitor, flag), _flag(flag) {
     // Superclass constructor did locking
-    assert(_monitor != NULL, "NULL monitor not allowed");
+    assert(monitor != NULL, "NULL monitor not allowed");
   }
 
   bool wait(int64_t timeout = 0,
             bool as_suspend_equivalent = !Mutex::_as_suspend_equivalent_flag) {
     if (_flag == Mutex::_safepoint_check_flag) {
-      return _monitor->wait(timeout, as_suspend_equivalent);
+      return as_monitor()->wait(timeout, as_suspend_equivalent);
     } else {
-      return _monitor->wait_without_safepoint_check(timeout);
+      return as_monitor()->wait_without_safepoint_check(timeout);
     }
     return false;
   }
 
   void notify_all() {
-    _monitor->notify_all();
+    as_monitor()->notify_all();
   }
 
   void notify() {
-    _monitor->notify();
+    as_monitor()->notify();
   }
 };