8304820: Statically allocate ObjectSynchronizer mutexes

Reviewed-by: dcubed, dholmes

Author: jcking

src/hotspot/share/runtime/synchronizer.cpp

@@ -245,12 +245,20 @@ int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, JavaThread* thr) {
   return 0;
 }
 
-static const int NINFLATIONLOCKS = 256;
-static PlatformMutex* gInflationLocks[NINFLATIONLOCKS];
+static constexpr size_t inflation_lock_count() {
+  return 256;
+}
+
+// Static storage for an array of PlatformMutex.
+alignas(PlatformMutex) static uint8_t _inflation_locks[inflation_lock_count()][sizeof(PlatformMutex)];
+
+static inline PlatformMutex* inflation_lock(size_t index) {
+  return reinterpret_cast<PlatformMutex*>(_inflation_locks[index]);
+}
 
 void ObjectSynchronizer::initialize() {
-  for (int i = 0; i < NINFLATIONLOCKS; i++) {
-    gInflationLocks[i] = new PlatformMutex();
+  for (size_t i = 0; i < inflation_lock_count(); i++) {
+    ::new(static_cast<void*>(inflation_lock(i))) PlatformMutex();
   }
   // Start the ceiling with the estimate for one thread.
   set_in_use_list_ceiling(AvgMonitorsPerThreadEstimate);
@@ -740,11 +748,11 @@ static markWord read_stable_mark(oop obj) {
         // then for each thread on the list, set the flag and unpark() the thread.
 
         // Index into the lock array based on the current object address.
-        static_assert(is_power_of_2(NINFLATIONLOCKS), "must be");
-        int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
+        static_assert(is_power_of_2(inflation_lock_count()), "must be");
+        size_t ix = (cast_from_oop<intptr_t>(obj) >> 5) & (inflation_lock_count() - 1);
         int YieldThenBlock = 0;
-        assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
-        gInflationLocks[ix]->lock();
+        assert(ix < inflation_lock_count(), "invariant");
+        inflation_lock(ix)->lock();
         while (obj->mark_acquire() == markWord::INFLATING()) {
           // Beware: naked_yield() is advisory and has almost no effect on some platforms
           // so we periodically call current->_ParkEvent->park(1).
@@ -755,7 +763,7 @@ static markWord read_stable_mark(oop obj) {
             os::naked_yield();
           }
         }
-        gInflationLocks[ix]->unlock();
+        inflation_lock(ix)->unlock();
       }
     } else {
       SpinPause();       // SMP-polite spinning