--- a/hotspot/src/share/vm/code/nmethod.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/code/nmethod.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -1079,6 +1079,10 @@
                   this, (address)_method, (address)cause);
     cause->klass()->print();
   }
+  // Unlink the osr method, so we do not look this up again
+  if (is_osr_method()) {
+    invalidate_osr_method();
+  }
   // If _method is already NULL the methodOop is about to be unloaded,
   // so we don't have to break the cycle. Note that it is possible to
   // have the methodOop live here, in case we unload the nmethod because
@@ -1148,7 +1152,7 @@
   // will never be used anymore. That the nmethods only gets removed when class unloading
   // happens, make life much simpler, since the nmethods are not just going to disappear
   // out of the blue.
-  if (is_osr_only_method()) {
+  if (is_osr_method()) {
     if (osr_entry_bci() != InvalidOSREntryBci) {
       // only log this once
       log_state_change(state);
@@ -1520,6 +1524,17 @@
 #endif // !PRODUCT
 }
 
+// This method is called twice during GC -- once while
+// tracing the "active" nmethods on thread stacks during
+// the (strong) marking phase, and then again when walking
+// the code cache contents during the weak roots processing
+// phase. The two uses are distinguished by means of the
+// do_nmethods() method in the closure "f" below -- which
+// answers "yes" in the first case, and "no" in the second
+// case. We want to walk the weak roots in the nmethod
+// only in the second case. The weak roots in the nmethod
+// are the oops in the ExceptionCache and the InlineCache
+// oops.
 void nmethod::oops_do(OopClosure* f) {
   // make sure the oops ready to receive visitors
   assert(!is_zombie() && !is_unloaded(),
@@ -1538,19 +1553,25 @@
 
   // Compiled code
   f->do_oop((oop*) &_method);
-  ExceptionCache* ec = exception_cache();
-  while(ec != NULL) {
-    f->do_oop((oop*)ec->exception_type_addr());
-    ec = ec->next();
-  }
+  if (!f->do_nmethods()) {
+    // weak roots processing phase -- update ExceptionCache oops
+    ExceptionCache* ec = exception_cache();
+    while(ec != NULL) {
+      f->do_oop((oop*)ec->exception_type_addr());
+      ec = ec->next();
+    }
+  } // Else strong roots phase -- skip oops in ExceptionCache
 
   RelocIterator iter(this, low_boundary);
+
   while (iter.next()) {
     if (iter.type() == relocInfo::oop_type ) {
       oop_Relocation* r = iter.oop_reloc();
       // In this loop, we must only follow those oops directly embedded in
       // the code.  Other oops (oop_index>0) are seen as part of scopes_oops.
-      assert(1 == (r->oop_is_immediate()) + (r->oop_addr() >= oops_begin() && r->oop_addr() < oops_end()), "oop must be found in exactly one place");
+      assert(1 == (r->oop_is_immediate()) +
+                   (r->oop_addr() >= oops_begin() && r->oop_addr() < oops_end()),
+             "oop must be found in exactly one place");
       if (r->oop_is_immediate() && r->oop_value() != NULL) {
         f->do_oop(r->oop_addr());
       }

--- a/hotspot/src/share/vm/code/nmethod.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/code/nmethod.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -314,7 +314,6 @@
   bool is_java_method() const                     { return !method()->is_native(); }
   bool is_native_method() const                   { return method()->is_native(); }
   bool is_osr_method() const                      { return _entry_bci != InvocationEntryBci; }
-  bool is_osr_only_method() const                 { return is_osr_method(); }
 
   bool is_compiled_by_c1() const;
   bool is_compiled_by_c2() const;

--- a/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsOopClosures.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsOopClosures.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -155,6 +155,12 @@
   Prefetch::style prefetch_style() {
     return Prefetch::do_read;
   }
+  // In support of class unloading
+  virtual const bool should_remember_mdo() const {
+    return false;
+    // return _should_remember_klasses;
+  }
+  virtual void remember_mdo(DataLayout* v);
 };
 
 // In the parallel case, the revisit stack, the bit map and the
@@ -185,6 +191,12 @@
   Prefetch::style prefetch_style() {
     return Prefetch::do_read;
   }
+  // In support of class unloading
+  virtual const bool should_remember_mdo() const {
+    return false;
+    // return _should_remember_klasses;
+  }
+  virtual void remember_mdo(DataLayout* v);
 };
 
 // The non-parallel version (the parallel version appears further below).
@@ -303,6 +315,13 @@
   virtual void do_oop(narrowOop* p);
   inline void do_oop_nv(oop* p)       { PushOrMarkClosure::do_oop_work(p); }
   inline void do_oop_nv(narrowOop* p) { PushOrMarkClosure::do_oop_work(p); }
+  // In support of class unloading
+  virtual const bool should_remember_mdo() const {
+    return false;
+    // return _should_remember_klasses;
+  }
+  virtual void remember_mdo(DataLayout* v);
+
   // Deal with a stack overflow condition
   void handle_stack_overflow(HeapWord* lost);
  private:
@@ -340,6 +359,13 @@
   virtual void do_oop(narrowOop* p);
   inline void do_oop_nv(oop* p)       { Par_PushOrMarkClosure::do_oop_work(p); }
   inline void do_oop_nv(narrowOop* p) { Par_PushOrMarkClosure::do_oop_work(p); }
+  // In support of class unloading
+  virtual const bool should_remember_mdo() const {
+    return false;
+    // return _should_remember_klasses;
+  }
+  virtual void remember_mdo(DataLayout* v);
+
   // Deal with a stack overflow condition
   void handle_stack_overflow(HeapWord* lost);
  private:

--- a/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsOopClosures.inline.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsOopClosures.inline.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -51,13 +51,22 @@
   check_remember_klasses();
 }
 
+inline void PushOrMarkClosure::remember_mdo(DataLayout* v) {
+  // TBD
+}
+
+
 void Par_KlassRememberingOopClosure::remember_klass(Klass* k) {
   if (!_revisit_stack->par_push(oop(k))) {
-    fatal("Revisit stack overflow in PushOrMarkClosure");
+    fatal("Revisit stack overflow in Par_KlassRememberingOopClosure");
   }
   check_remember_klasses();
 }
 
+inline void Par_PushOrMarkClosure::remember_mdo(DataLayout* v) {
+  // TBD
+}
+
 inline void PushOrMarkClosure::do_yield_check() {
   _parent->do_yield_check();
 }

--- a/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -7632,6 +7632,14 @@
 void Par_PushAndMarkClosure::do_oop(oop* p)       { Par_PushAndMarkClosure::do_oop_work(p); }
 void Par_PushAndMarkClosure::do_oop(narrowOop* p) { Par_PushAndMarkClosure::do_oop_work(p); }
 
+void PushAndMarkClosure::remember_mdo(DataLayout* v) {
+  // TBD
+}
+
+void Par_PushAndMarkClosure::remember_mdo(DataLayout* v) {
+  // TBD
+}
+
 void CMSPrecleanRefsYieldClosure::do_yield_work() {
   DEBUG_ONLY(RememberKlassesChecker mux(false);)
   Mutex* bml = _collector->bitMapLock();

--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -5064,7 +5064,7 @@
     return hr->is_in(p);
   }
 }
-#endif // PRODUCT
+#endif // !PRODUCT
 
 void G1CollectedHeap::g1_unimplemented() {
   // Unimplemented();

--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -859,7 +859,7 @@
     return _g1_committed;
   }
 
-  NOT_PRODUCT( bool is_in_closed_subset(const void* p) const; )
+  NOT_PRODUCT(bool is_in_closed_subset(const void* p) const;)
 
   // Dirty card table entries covering a list of young regions.
   void dirtyCardsForYoungRegions(CardTableModRefBS* ct_bs, HeapRegion* list);

--- a/hotspot/src/share/vm/gc_implementation/g1/g1MarkSweep.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1MarkSweep.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -102,9 +102,14 @@
   GenMarkSweep::_marking_stack =
     new (ResourceObj::C_HEAP) GrowableArray<oop>(4000, true);
 
-  size_t size = SystemDictionary::number_of_classes() * 2;
+  int size = SystemDictionary::number_of_classes() * 2;
   GenMarkSweep::_revisit_klass_stack =
-    new (ResourceObj::C_HEAP) GrowableArray<Klass*>((int)size, true);
+    new (ResourceObj::C_HEAP) GrowableArray<Klass*>(size, true);
+  // (#klass/k)^2 for k ~ 10 appears a better fit, but this will have to do
+  // for now until we have a chance to work out a more optimal setting.
+  GenMarkSweep::_revisit_mdo_stack =
+    new (ResourceObj::C_HEAP) GrowableArray<DataLayout*>(size*2, true);
+
 }
 
 void G1MarkSweep::mark_sweep_phase1(bool& marked_for_unloading,
@@ -139,13 +144,18 @@
   CodeCache::do_unloading(&GenMarkSweep::is_alive,
                                    &GenMarkSweep::keep_alive,
                                    purged_class);
-           GenMarkSweep::follow_stack();
+  GenMarkSweep::follow_stack();
 
   // Update subklass/sibling/implementor links of live klasses
   GenMarkSweep::follow_weak_klass_links();
   assert(GenMarkSweep::_marking_stack->is_empty(),
          "stack should be empty by now");
 
+  // Visit memoized MDO's and clear any unmarked weak refs
+  GenMarkSweep::follow_mdo_weak_refs();
+  assert(GenMarkSweep::_marking_stack->is_empty(), "just drained");
+
+
   // Visit symbol and interned string tables and delete unmarked oops
   SymbolTable::unlink(&GenMarkSweep::is_alive);
   StringTable::unlink(&GenMarkSweep::is_alive);

--- a/hotspot/src/share/vm/gc_implementation/includeDB_gc_parallelScavenge	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/includeDB_gc_parallelScavenge	Wed Sep 02 00:04:29 2009 -0700
@@ -253,10 +253,11 @@
 psParallelCompact.cpp			gcLocker.inline.hpp
 psParallelCompact.cpp                   gcTaskManager.hpp
 psParallelCompact.cpp			isGCActiveMark.hpp
+psParallelCompact.cpp			management.hpp
+psParallelCompact.cpp			memoryService.hpp
+psParallelCompact.cpp			methodDataOop.hpp
 psParallelCompact.cpp			oop.inline.hpp
 psParallelCompact.cpp			oop.pcgc.inline.hpp
-psParallelCompact.cpp			memoryService.hpp
-psParallelCompact.cpp			management.hpp
 psParallelCompact.cpp			parallelScavengeHeap.inline.hpp
 psParallelCompact.cpp			pcTasks.hpp
 psParallelCompact.cpp			psMarkSweep.hpp

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -58,9 +58,8 @@
     PrintGCDetails && TraceParallelOldGCTasks, true, gclog_or_tty));
   ParCompactionManager* cm =
     ParCompactionManager::gc_thread_compaction_manager(which);
-  // cm->allocate_stacks();
   assert(cm->stacks_have_been_allocated(),
-    "Stack space has not been allocated");
+         "Stack space has not been allocated");
   PSParallelCompact::MarkAndPushClosure mark_and_push_closure(cm);
 
   switch (_root_type) {
@@ -129,9 +128,8 @@
     PrintGCDetails && TraceParallelOldGCTasks, true, gclog_or_tty));
   ParCompactionManager* cm =
     ParCompactionManager::gc_thread_compaction_manager(which);
-  // cm->allocate_stacks();
   assert(cm->stacks_have_been_allocated(),
-    "Stack space has not been allocated");
+         "Stack space has not been allocated");
   PSParallelCompact::MarkAndPushClosure mark_and_push_closure(cm);
   PSParallelCompact::FollowStackClosure follow_stack_closure(cm);
   _rp_task.work(_work_id, *PSParallelCompact::is_alive_closure(),

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -61,12 +61,16 @@
   int size =
     (SystemDictionary::number_of_classes() * 2) * 2 / ParallelGCThreads;
   _revisit_klass_stack = new (ResourceObj::C_HEAP) GrowableArray<Klass*>(size, true);
+  // From some experiments (#klass/k)^2 for k = 10 seems a better fit, but this will
+  // have to do for now until we are able to investigate a more optimal setting.
+  _revisit_mdo_stack = new (ResourceObj::C_HEAP) GrowableArray<DataLayout*>(size*2, true);
 
 }
 
 ParCompactionManager::~ParCompactionManager() {
   delete _overflow_stack;
   delete _revisit_klass_stack;
+  delete _revisit_mdo_stack;
   // _manager_array and _stack_array are statics
   // shared with all instances of ParCompactionManager
   // should not be deallocated.
@@ -195,6 +199,7 @@
 void ParCompactionManager::reset() {
   for(uint i=0; i<ParallelGCThreads+1; i++) {
     manager_array(i)->revisit_klass_stack()->clear();
+    manager_array(i)->revisit_mdo_stack()->clear();
   }
 }
 
@@ -296,6 +301,7 @@
 
 #ifdef ASSERT
 bool ParCompactionManager::stacks_have_been_allocated() {
-  return (revisit_klass_stack()->data_addr() != NULL);
+  return (revisit_klass_stack()->data_addr() != NULL &&
+          revisit_mdo_stack()->data_addr() != NULL);
 }
 #endif

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -93,6 +93,7 @@
 
 #if 1  // does this happen enough to need a per thread stack?
   GrowableArray<Klass*>*        _revisit_klass_stack;
+  GrowableArray<DataLayout*>*   _revisit_mdo_stack;
 #endif
   static ParMarkBitMap* _mark_bitmap;
 
@@ -154,6 +155,7 @@
 #if 1
   // Probably stays as a growable array
   GrowableArray<Klass*>* revisit_klass_stack() { return _revisit_klass_stack; }
+  GrowableArray<DataLayout*>* revisit_mdo_stack() { return _revisit_mdo_stack; }
 #endif
 
   // Save oop for later processing.  Must not fail.

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/psMarkSweep.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/psMarkSweep.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -482,6 +482,9 @@
 
   int size = SystemDictionary::number_of_classes() * 2;
   _revisit_klass_stack = new (ResourceObj::C_HEAP) GrowableArray<Klass*>(size, true);
+  // (#klass/k)^2, for k ~ 10 appears a better setting, but this will have to do for
+  // now until we investigate a more optimal setting.
+  _revisit_mdo_stack   = new (ResourceObj::C_HEAP) GrowableArray<DataLayout*>(size*2, true);
 }
 
 
@@ -495,6 +498,7 @@
 
   delete _marking_stack;
   delete _revisit_klass_stack;
+  delete _revisit_mdo_stack;
 }
 
 void PSMarkSweep::mark_sweep_phase1(bool clear_all_softrefs) {
@@ -540,6 +544,10 @@
   follow_weak_klass_links();
   assert(_marking_stack->is_empty(), "just drained");
 
+  // Visit memoized mdo's and clear unmarked weak refs
+  follow_mdo_weak_refs();
+  assert(_marking_stack->is_empty(), "just drained");
+
   // Visit symbol and interned string tables and delete unmarked oops
   SymbolTable::unlink(is_alive_closure());
   StringTable::unlink(is_alive_closure());

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -2378,7 +2378,10 @@
 
   // Update subklass/sibling/implementor links of live klasses
   // revisit_klass_stack is used in follow_weak_klass_links().
-  follow_weak_klass_links(cm);
+  follow_weak_klass_links();
+
+  // Revisit memoized MDO's and clear any unmarked weak refs
+  follow_mdo_weak_refs();
 
   // Visit symbol and interned string tables and delete unmarked oops
   SymbolTable::unlink(is_alive_closure());
@@ -2721,17 +2724,25 @@
 }
 
 void
-PSParallelCompact::follow_weak_klass_links(ParCompactionManager* serial_cm) {
+PSParallelCompact::follow_weak_klass_links() {
   // All klasses on the revisit stack are marked at this point.
   // Update and follow all subklass, sibling and implementor links.
-  for (uint i = 0; i < ParallelGCThreads+1; i++) {
+  if (PrintRevisitStats) {
+    gclog_or_tty->print_cr("#classes in system dictionary = %d", SystemDictionary::number_of_classes());
+  }
+  for (uint i = 0; i < ParallelGCThreads + 1; i++) {
     ParCompactionManager* cm = ParCompactionManager::manager_array(i);
     KeepAliveClosure keep_alive_closure(cm);
-    for (int i = 0; i < cm->revisit_klass_stack()->length(); i++) {
-      cm->revisit_klass_stack()->at(i)->follow_weak_klass_links(
+    int length = cm->revisit_klass_stack()->length();
+    if (PrintRevisitStats) {
+      gclog_or_tty->print_cr("Revisit klass stack[%d] length = %d", i, length);
+    }
+    for (int j = 0; j < length; j++) {
+      cm->revisit_klass_stack()->at(j)->follow_weak_klass_links(
         is_alive_closure(),
         &keep_alive_closure);
     }
+    // revisit_klass_stack is cleared in reset()
     follow_stack(cm);
   }
 }
@@ -2741,6 +2752,35 @@
   cm->revisit_klass_stack()->push(k);
 }
 
+#if ( defined(COMPILER1) || defined(COMPILER2) )
+void PSParallelCompact::revisit_mdo(ParCompactionManager* cm, DataLayout* p) {
+  cm->revisit_mdo_stack()->push(p);
+}
+
+void PSParallelCompact::follow_mdo_weak_refs() {
+  // All strongly reachable oops have been marked at this point;
+  // we can visit and clear any weak references from MDO's which
+  // we memoized during the strong marking phase.
+  if (PrintRevisitStats) {
+    gclog_or_tty->print_cr("#classes in system dictionary = %d", SystemDictionary::number_of_classes());
+  }
+  for (uint i = 0; i < ParallelGCThreads + 1; i++) {
+    ParCompactionManager* cm = ParCompactionManager::manager_array(i);
+    GrowableArray<DataLayout*>* rms = cm->revisit_mdo_stack();
+    int length = rms->length();
+    if (PrintRevisitStats) {
+      gclog_or_tty->print_cr("Revisit MDO stack[%d] length = %d", i, length);
+    }
+    for (int j = 0; j < length; j++) {
+      rms->at(j)->follow_weak_refs(is_alive_closure());
+    }
+    // revisit_mdo_stack is cleared in reset()
+    follow_stack(cm);
+  }
+}
+#endif //  ( COMPILER1 || COMPILER2 )
+
+
 #ifdef VALIDATE_MARK_SWEEP
 
 void PSParallelCompact::track_adjusted_pointer(void* p, bool isroot) {

--- a/hotspot/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -901,7 +901,8 @@
   static void marking_phase(ParCompactionManager* cm,
                             bool maximum_heap_compaction);
   static void follow_stack(ParCompactionManager* cm);
-  static void follow_weak_klass_links(ParCompactionManager* cm);
+  static void follow_weak_klass_links();
+  static void follow_mdo_weak_refs();
 
   template <class T> static inline void adjust_pointer(T* p, bool is_root);
   static void adjust_root_pointer(oop* p) { adjust_pointer(p, true); }
@@ -1221,6 +1222,9 @@
   // Update subklass/sibling/implementor links at end of marking.
   static void revisit_weak_klass_link(ParCompactionManager* cm, Klass* k);
 
+  // Clear unmarked oops in MDOs at the end of marking.
+  static void revisit_mdo(ParCompactionManager* cm, DataLayout* p);
+
 #ifndef PRODUCT
   // Debugging support.
   static const char* space_names[last_space_id];

--- a/hotspot/src/share/vm/gc_implementation/shared/markSweep.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/shared/markSweep.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -27,6 +27,7 @@
 
 GrowableArray<oop>*     MarkSweep::_marking_stack       = NULL;
 GrowableArray<Klass*>*  MarkSweep::_revisit_klass_stack = NULL;
+GrowableArray<DataLayout*>*  MarkSweep::_revisit_mdo_stack = NULL;
 
 GrowableArray<oop>*     MarkSweep::_preserved_oop_stack = NULL;
 GrowableArray<markOop>* MarkSweep::_preserved_mark_stack= NULL;
@@ -62,12 +63,37 @@
 void MarkSweep::follow_weak_klass_links() {
   // All klasses on the revisit stack are marked at this point.
   // Update and follow all subklass, sibling and implementor links.
+  if (PrintRevisitStats) {
+    gclog_or_tty->print_cr("#classes in system dictionary = %d", SystemDictionary::number_of_classes());
+    gclog_or_tty->print_cr("Revisit klass stack length = %d", _revisit_klass_stack->length());
+  }
   for (int i = 0; i < _revisit_klass_stack->length(); i++) {
     _revisit_klass_stack->at(i)->follow_weak_klass_links(&is_alive,&keep_alive);
   }
   follow_stack();
 }
 
+#if ( defined(COMPILER1) || defined(COMPILER2) )
+void MarkSweep::revisit_mdo(DataLayout* p) {
+  _revisit_mdo_stack->push(p);
+}
+
+void MarkSweep::follow_mdo_weak_refs() {
+  // All strongly reachable oops have been marked at this point;
+  // we can visit and clear any weak references from MDO's which
+  // we memoized during the strong marking phase.
+  assert(_marking_stack->is_empty(), "Marking stack should be empty");
+  if (PrintRevisitStats) {
+    gclog_or_tty->print_cr("#classes in system dictionary = %d", SystemDictionary::number_of_classes());
+    gclog_or_tty->print_cr("Revisit MDO stack length = %d", _revisit_mdo_stack->length());
+  }
+  for (int i = 0; i < _revisit_mdo_stack->length(); i++) {
+    _revisit_mdo_stack->at(i)->follow_weak_refs(&is_alive);
+  }
+  follow_stack();
+}
+#endif //  ( COMPILER1 || COMPILER2 )
+
 MarkSweep::FollowRootClosure  MarkSweep::follow_root_closure;
 
 void MarkSweep::FollowRootClosure::do_oop(oop* p)       { follow_root(p); }

--- a/hotspot/src/share/vm/gc_implementation/shared/markSweep.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_implementation/shared/markSweep.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -23,6 +23,7 @@
  */
 
 class ReferenceProcessor;
+class DataLayout;
 
 // MarkSweep takes care of global mark-compact garbage collection for a
 // GenCollectedHeap using a four-phase pointer forwarding algorithm.  All
@@ -65,6 +66,8 @@
     virtual void do_oop(oop* p);
     virtual void do_oop(narrowOop* p);
     virtual const bool do_nmethods() const { return true; }
+    virtual const bool should_remember_mdo() const { return true; }
+    virtual void remember_mdo(DataLayout* p) { MarkSweep::revisit_mdo(p); }
   };
 
   class FollowStackClosure: public VoidClosure {
@@ -103,6 +106,7 @@
   friend class KeepAliveClosure;
   friend class VM_MarkSweep;
   friend void marksweep_init();
+  friend class DataLayout;
 
   //
   // Vars
@@ -112,6 +116,8 @@
   static GrowableArray<oop>*             _marking_stack;
   // Stack for live klasses to revisit at end of marking phase
   static GrowableArray<Klass*>*          _revisit_klass_stack;
+  // Set (stack) of MDO's to revisit at end of marking phase
+  static GrowableArray<DataLayout*>*    _revisit_mdo_stack;
 
   // Space for storing/restoring mark word
   static GrowableArray<markOop>*         _preserved_mark_stack;
@@ -157,6 +163,10 @@
   // Class unloading. Update subklass/sibling/implementor links at end of marking phase.
   static void follow_weak_klass_links();
 
+  // Class unloading. Clear weak refs in MDO's (ProfileData)
+  // at the end of the marking phase.
+  static void follow_mdo_weak_refs();
+
   // Debugging
   static void trace(const char* msg) PRODUCT_RETURN;
 
@@ -213,7 +223,10 @@
 #endif
 
   // Call backs for class unloading
-  static void revisit_weak_klass_link(Klass* k);  // Update subklass/sibling/implementor links at end of marking.
+  // Update subklass/sibling/implementor links at end of marking.
+  static void revisit_weak_klass_link(Klass* k);
+  // For weak refs clearing in MDO's
+  static void revisit_mdo(DataLayout* p);
 };
 
 class PreservedMark VALUE_OBJ_CLASS_SPEC {

--- a/hotspot/src/share/vm/gc_interface/collectedHeap.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/gc_interface/collectedHeap.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -239,6 +239,9 @@
     return p == NULL || is_in_closed_subset(p);
   }
 
+  // XXX is_permanent() and is_in_permanent() should be better named
+  // to distinguish one from the other.
+
   // Returns "TRUE" if "p" is allocated as "permanent" data.
   // If the heap does not use "permanent" data, returns the same
   // value is_in_reserved() would return.
@@ -247,13 +250,17 @@
   // space). If you need the more conservative answer use is_permanent().
   virtual bool is_in_permanent(const void *p) const = 0;
 
+  bool is_in_permanent_or_null(const void *p) const {
+    return p == NULL || is_in_permanent(p);
+  }
+
   // Returns "TRUE" if "p" is in the committed area of  "permanent" data.
   // If the heap does not use "permanent" data, returns the same
   // value is_in() would return.
   virtual bool is_permanent(const void *p) const = 0;
 
-  bool is_in_permanent_or_null(const void *p) const {
-    return p == NULL || is_in_permanent(p);
+  bool is_permanent_or_null(const void *p) const {
+    return p == NULL || is_permanent(p);
   }
 
   // Returns "TRUE" if "p" is a method oop in the

--- a/hotspot/src/share/vm/includeDB_core	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/includeDB_core	Wed Sep 02 00:04:29 2009 -0700
@@ -2684,6 +2684,7 @@
 markOop.inline.hpp                      markOop.hpp
 
 markSweep.cpp                           compileBroker.hpp
+markSweep.cpp                           methodDataOop.hpp
 
 markSweep.hpp                           collectedHeap.hpp
 

--- a/hotspot/src/share/vm/interpreter/interpreterRuntime.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/interpreter/interpreterRuntime.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -849,8 +849,25 @@
 }
 #endif // !PRODUCT
 
+nmethod* InterpreterRuntime::frequency_counter_overflow(JavaThread* thread, address branch_bcp) {
+  nmethod* nm = frequency_counter_overflow_inner(thread, branch_bcp);
+  assert(branch_bcp != NULL || nm == NULL, "always returns null for non OSR requests");
+  if (branch_bcp != NULL && nm != NULL) {
+    // This was a successful request for an OSR nmethod.  Because
+    // frequency_counter_overflow_inner ends with a safepoint check,
+    // nm could have been unloaded so look it up again.  It's unsafe
+    // to examine nm directly since it might have been freed and used
+    // for something else.
+    frame fr = thread->last_frame();
+    methodOop method =  fr.interpreter_frame_method();
+    int bci = method->bci_from(fr.interpreter_frame_bcp());
+    nm = method->lookup_osr_nmethod_for(bci);
+  }
+  return nm;
+}
+
 IRT_ENTRY(nmethod*,
-          InterpreterRuntime::frequency_counter_overflow(JavaThread* thread, address branch_bcp))
+          InterpreterRuntime::frequency_counter_overflow_inner(JavaThread* thread, address branch_bcp))
   // use UnlockFlagSaver to clear and restore the _do_not_unlock_if_synchronized
   // flag, in case this method triggers classloading which will call into Java.
   UnlockFlagSaver fs(thread);
@@ -923,7 +940,6 @@
         }
         BiasedLocking::revoke(objects_to_revoke);
       }
-
       return osr_nm;
     }
   }

--- a/hotspot/src/share/vm/interpreter/interpreterRuntime.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/interpreter/interpreterRuntime.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -49,6 +49,9 @@
   static ConstantPoolCacheEntry* cache_entry(JavaThread *thread)            { return cache_entry_at(thread, Bytes::get_native_u2(bcp(thread) + 1)); }
   static void      note_trap(JavaThread *thread, int reason, TRAPS);
 
+  // Inner work method for Interpreter's frequency counter overflow
+  static nmethod* frequency_counter_overflow_inner(JavaThread* thread, address branch_bcp);
+
  public:
   // Constants
   static void    ldc           (JavaThread* thread, bool wide);

--- a/hotspot/src/share/vm/memory/genMarkSweep.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/memory/genMarkSweep.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -162,6 +162,9 @@
 
   int size = SystemDictionary::number_of_classes() * 2;
   _revisit_klass_stack = new (ResourceObj::C_HEAP) GrowableArray<Klass*>(size, true);
+  // (#klass/k)^2 for k ~ 10 appears to be a better fit, but this will have to do for
+  // now until we have had a chance to investigate a more optimal setting.
+  _revisit_mdo_stack   = new (ResourceObj::C_HEAP) GrowableArray<DataLayout*>(2*size, true);
 
 #ifdef VALIDATE_MARK_SWEEP
   if (ValidateMarkSweep) {
@@ -206,6 +209,7 @@
 
   delete _marking_stack;
   delete _revisit_klass_stack;
+  delete _revisit_mdo_stack;
 
 #ifdef VALIDATE_MARK_SWEEP
   if (ValidateMarkSweep) {
@@ -262,6 +266,10 @@
   follow_weak_klass_links();
   assert(_marking_stack->is_empty(), "just drained");
 
+  // Visit memoized MDO's and clear any unmarked weak refs
+  follow_mdo_weak_refs();
+  assert(_marking_stack->is_empty(), "just drained");
+
   // Visit symbol and interned string tables and delete unmarked oops
   SymbolTable::unlink(&is_alive);
   StringTable::unlink(&is_alive);

--- a/hotspot/src/share/vm/memory/iterator.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/memory/iterator.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -25,6 +25,7 @@
 // The following classes are C++ `closures` for iterating over objects, roots and spaces
 
 class ReferenceProcessor;
+class DataLayout;
 
 // Closure provides abortability.
 
@@ -62,6 +63,12 @@
 
   virtual void remember_klass(Klass* k) { /* do nothing */ }
 
+  // In support of post-processing of weak references in
+  // ProfileData (MethodDataOop) objects; see, for example,
+  // VirtualCallData::oop_iterate().
+  virtual const bool should_remember_mdo() const { return false; }
+  virtual void remember_mdo(DataLayout* v) { /* do nothing */ }
+
   // If "true", invoke on nmethods (when scanning compiled frames).
   virtual const bool do_nmethods() const { return false; }
 

--- a/hotspot/src/share/vm/oops/methodDataOop.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/oops/methodDataOop.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -49,6 +49,12 @@
   }
 }
 
+void DataLayout::follow_weak_refs(BoolObjectClosure* cl) {
+  ResourceMark m;
+  data_in()->follow_weak_refs(cl);
+}
+
+
 // ==================================================================
 // ProfileData
 //
@@ -145,42 +151,92 @@
 // which are used to store a type profile for the receiver of the check.
 
 void ReceiverTypeData::follow_contents() {
-  for (uint row = 0; row < row_limit(); row++) {
-    if (receiver(row) != NULL) {
-      MarkSweep::mark_and_push(adr_receiver(row));
-    }
-  }
+  // This is a set of weak references that need
+  // to be followed at the end of the strong marking
+  // phase. Memoize this object so it can be visited
+  // in the weak roots processing phase.
+  MarkSweep::revisit_mdo(data());
 }
 
 #ifndef SERIALGC
 void ReceiverTypeData::follow_contents(ParCompactionManager* cm) {
-  for (uint row = 0; row < row_limit(); row++) {
-    if (receiver(row) != NULL) {
-      PSParallelCompact::mark_and_push(cm, adr_receiver(row));
-    }
-  }
+  // This is a set of weak references that need
+  // to be followed at the end of the strong marking
+  // phase. Memoize this object so it can be visited
+  // in the weak roots processing phase.
+  PSParallelCompact::revisit_mdo(cm, data());
 }
 #endif // SERIALGC
 
 void ReceiverTypeData::oop_iterate(OopClosure* blk) {
-  for (uint row = 0; row < row_limit(); row++) {
-    if (receiver(row) != NULL) {
-      blk->do_oop(adr_receiver(row));
-    }
-  }
-}
-
-void ReceiverTypeData::oop_iterate_m(OopClosure* blk, MemRegion mr) {
-  for (uint row = 0; row < row_limit(); row++) {
-    if (receiver(row) != NULL) {
-      oop* adr = adr_receiver(row);
-      if (mr.contains(adr)) {
+  if (blk->should_remember_mdo()) {
+    // This is a set of weak references that need
+    // to be followed at the end of the strong marking
+    // phase. Memoize this object so it can be visited
+    // in the weak roots processing phase.
+    blk->remember_mdo(data());
+  } else { // normal scan
+    for (uint row = 0; row < row_limit(); row++) {
+      if (receiver(row) != NULL) {
+        oop* adr = adr_receiver(row);
         blk->do_oop(adr);
       }
     }
   }
 }
 
+void ReceiverTypeData::oop_iterate_m(OopClosure* blk, MemRegion mr) {
+  // Currently, this interface is called only during card-scanning for
+  // a young gen gc, in which case this object cannot contribute anything,
+  // since it does not contain any references that cross out of
+  // the perm gen. However, for future more general use we allow
+  // the possibility of calling for instance from more general
+  // iterators (for example, a future regionalized perm gen for G1,
+  // or the possibility of moving some references out of perm in
+  // the case of other collectors). In that case, you will need
+  // to relax or remove some of the assertions below.
+#ifdef ASSERT
+  // Verify that none of the embedded oop references cross out of
+  // this generation.
+  for (uint row = 0; row < row_limit(); row++) {
+    if (receiver(row) != NULL) {
+      oop* adr = adr_receiver(row);
+      CollectedHeap* h = Universe::heap();
+      assert(h->is_permanent(adr) && h->is_permanent_or_null(*adr), "Not intra-perm");
+    }
+  }
+#endif // ASSERT
+  assert(!blk->should_remember_mdo(), "Not expected to remember MDO");
+  return;   // Nothing to do, see comment above
+#if 0
+  if (blk->should_remember_mdo()) {
+    // This is a set of weak references that need
+    // to be followed at the end of the strong marking
+    // phase. Memoize this object so it can be visited
+    // in the weak roots processing phase.
+    blk->remember_mdo(data());
+  } else { // normal scan
+    for (uint row = 0; row < row_limit(); row++) {
+      if (receiver(row) != NULL) {
+        oop* adr = adr_receiver(row);
+        if (mr.contains(adr)) {
+          blk->do_oop(adr);
+        } else if ((HeapWord*)adr >= mr.end()) {
+          // Test that the current cursor and the two ends of the range
+          // that we may have skipped iterating over are monotonically ordered;
+          // this is just a paranoid assertion, just in case represetations
+          // should change in the future rendering the short-circuit return
+          // here invalid.
+          assert((row+1 >= row_limit() || adr_receiver(row+1) > adr) &&
+                 (row+2 >= row_limit() || adr_receiver(row_limit()-1) > adr_receiver(row+1)), "Reducing?");
+          break; // remaining should be outside this mr too
+        }
+      }
+    }
+  }
+#endif
+}
+
 void ReceiverTypeData::adjust_pointers() {
   for (uint row = 0; row < row_limit(); row++) {
     if (receiver(row) != NULL) {
@@ -189,6 +245,15 @@
   }
 }
 
+void ReceiverTypeData::follow_weak_refs(BoolObjectClosure* is_alive_cl) {
+  for (uint row = 0; row < row_limit(); row++) {
+    klassOop p = receiver(row);
+    if (p != NULL && !is_alive_cl->do_object_b(p)) {
+      clear_row(row);
+    }
+  }
+}
+
 #ifndef SERIALGC
 void ReceiverTypeData::update_pointers() {
   for (uint row = 0; row < row_limit(); row++) {
@@ -625,30 +690,33 @@
     return NULL;
   }
   DataLayout* data_layout = data_layout_at(data_index);
+  return data_layout->data_in();
+}
 
-  switch (data_layout->tag()) {
+ProfileData* DataLayout::data_in() {
+  switch (tag()) {
   case DataLayout::no_tag:
   default:
     ShouldNotReachHere();
     return NULL;
   case DataLayout::bit_data_tag:
-    return new BitData(data_layout);
+    return new BitData(this);
   case DataLayout::counter_data_tag:
-    return new CounterData(data_layout);
+    return new CounterData(this);
   case DataLayout::jump_data_tag:
-    return new JumpData(data_layout);
+    return new JumpData(this);
   case DataLayout::receiver_type_data_tag:
-    return new ReceiverTypeData(data_layout);
+    return new ReceiverTypeData(this);
   case DataLayout::virtual_call_data_tag:
-    return new VirtualCallData(data_layout);
+    return new VirtualCallData(this);
   case DataLayout::ret_data_tag:
-    return new RetData(data_layout);
+    return new RetData(this);
   case DataLayout::branch_data_tag:
-    return new BranchData(data_layout);
+    return new BranchData(this);
   case DataLayout::multi_branch_data_tag:
-    return new MultiBranchData(data_layout);
+    return new MultiBranchData(this);
   case DataLayout::arg_info_data_tag:
-    return new ArgInfoData(data_layout);
+    return new ArgInfoData(this);
   };
 }
 

--- a/hotspot/src/share/vm/oops/methodDataOop.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/oops/methodDataOop.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -55,6 +55,9 @@
 // with invocation counter incrementation.  None of these races harm correct
 // execution of the compiled code.
 
+// forward decl
+class ProfileData;
+
 // DataLayout
 //
 // Overlay for generic profiling data.
@@ -231,6 +234,10 @@
     temp._header._struct._flags = byte_constant;
     return temp._header._bits;
   }
+
+  // GC support
+  ProfileData* data_in();
+  void follow_weak_refs(BoolObjectClosure* cl);
 };
 
 
@@ -430,6 +437,7 @@
   virtual void oop_iterate(OopClosure* blk) {}
   virtual void oop_iterate_m(OopClosure* blk, MemRegion mr) {}
   virtual void adjust_pointers() {}
+  virtual void follow_weak_refs(BoolObjectClosure* is_alive_closure) {}
 
 #ifndef SERIALGC
   // Parallel old support
@@ -667,11 +675,27 @@
     return recv;
   }
 
+  void set_receiver(uint row, oop p) {
+    assert((uint)row < row_limit(), "oob");
+    set_oop_at(receiver_cell_index(row), p);
+  }
+
   uint receiver_count(uint row) {
     assert(row < row_limit(), "oob");
     return uint_at(receiver_count_cell_index(row));
   }
 
+  void set_receiver_count(uint row, uint count) {
+    assert(row < row_limit(), "oob");
+    set_uint_at(receiver_count_cell_index(row), count);
+  }
+
+  void clear_row(uint row) {
+    assert(row < row_limit(), "oob");
+    set_receiver(row, NULL);
+    set_receiver_count(row, 0);
+  }
+
   // Code generation support
   static ByteSize receiver_offset(uint row) {
     return cell_offset(receiver_cell_index(row));
@@ -688,6 +712,7 @@
   virtual void oop_iterate(OopClosure* blk);
   virtual void oop_iterate_m(OopClosure* blk, MemRegion mr);
   virtual void adjust_pointers();
+  virtual void follow_weak_refs(BoolObjectClosure* is_alive_closure);
 
 #ifndef SERIALGC
   // Parallel old support

--- a/hotspot/src/share/vm/runtime/globals.hpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/runtime/globals.hpp	Wed Sep 02 00:04:29 2009 -0700
@@ -1707,6 +1707,9 @@
   product(bool, TLABStats, true,                                            \
           "Print various TLAB related information")                         \
                                                                             \
+  product(bool, PrintRevisitStats, false,                                   \
+          "Print revisit (klass and MDO) stack related information")        \
+                                                                            \
   product_pd(bool, NeverActAsServerClassMachine,                            \
           "Never act like a server-class machine")                          \
                                                                             \

--- a/hotspot/src/share/vm/runtime/sweeper.cpp	Mon Aug 31 05:27:29 2009 -0700
+++ b/hotspot/src/share/vm/runtime/sweeper.cpp	Wed Sep 02 00:04:29 2009 -0700
@@ -125,8 +125,14 @@
     // there are no inline caches that referes to it.
     if (nm->is_marked_for_reclamation()) {
       assert(!nm->is_locked_by_vm(), "must not flush locked nmethods");
+      if (PrintMethodFlushing && Verbose) {
+        tty->print_cr("### Nmethod 0x%x (marked for reclamation) being flushed", nm);
+      }
       nm->flush();
     } else {
+      if (PrintMethodFlushing && Verbose) {
+        tty->print_cr("### Nmethod 0x%x (zombie) being marked for reclamation", nm);
+      }
       nm->mark_for_reclamation();
       _rescan = true;
     }
@@ -134,6 +140,9 @@
     // If there is no current activations of this method on the
     // stack we can safely convert it to a zombie method
     if (nm->can_not_entrant_be_converted()) {
+      if (PrintMethodFlushing && Verbose) {
+        tty->print_cr("### Nmethod 0x%x (not entrant) being made zombie", nm);
+      }
       nm->make_zombie();
       _rescan = true;
     } else {
@@ -146,7 +155,9 @@
     }
   } else if (nm->is_unloaded()) {
     // Unloaded code, just make it a zombie
-    if (nm->is_osr_only_method()) {
+    if (PrintMethodFlushing && Verbose)
+      tty->print_cr("### Nmethod 0x%x (unloaded) being made zombie", nm);
+    if (nm->is_osr_method()) {
       // No inline caches will ever point to osr methods, so we can just remove it
       nm->flush();
     } else {