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 // ReferenceProcessor class encapsulates the per-"collector" processing

 // of java.lang.Reference objects for GC. The interface is useful for supporting

 // a generational abstraction, in particular when there are multiple

 // generations that are being independently collected -- possibly

 // concurrently and/or incrementally.  Note, however, that the

 // ReferenceProcessor class abstracts away from a generational setting

 // by using only a heap interval (called "span" below), thus allowing

 // its use in a straightforward manner in a general, non-generational

 // setting.

 //

 // The basic idea is that each ReferenceProcessor object concerns

 // itself with ("weak") reference processing in a specific "span"

 // of the heap of interest to a specific collector. Currently,

 // the span is a convex interval of the heap, but, efficiency

 // apart, there seems to be no reason it couldn't be extended

 // (with appropriate modifications) to any "non-convex interval".

 // forward references

 class ReferencePolicy;

 class AbstractRefProcTaskExecutor;

 class DiscoveredList;

 class ReferenceProcessor : public CHeapObj {

  protected:

   // End of list marker

   static oop  _sentinelRef;

   MemRegion   _span; // (right-open) interval of heap

                      // subject to wkref discovery

   bool        _discovering_refs;      // true when discovery enabled

   bool        _discovery_is_atomic;   // if discovery is atomic wrt

                                       // other collectors in configuration

   bool        _discovery_is_mt;       // true if reference discovery is MT.

   // If true, setting "next" field of a discovered refs list requires

   // write barrier(s).  (Must be true if used in a collector in which

   // elements of a discovered list may be moved during discovery: for

   // example, a collector like Garbage-First that moves objects during a

   // long-term concurrent marking phase that does weak reference

   // discovery.)

   bool        _discovered_list_needs_barrier;

   BarrierSet* _bs;                    // Cached copy of BarrierSet.

   bool        _enqueuing_is_done;     // true if all weak references enqueued

   bool        _processing_is_mt;      // true during phases when

                                       // reference processing is MT.

   int         _next_id;               // round-robin counter in

                                       // support of work distribution

   // For collectors that do not keep GC marking information

   // in the object header, this field holds a closure that

   // helps the reference processor determine the reachability

   // of an oop (the field is currently initialized to NULL for

   // all collectors but the CMS collector).

   BoolObjectClosure* _is_alive_non_header;

   // Soft ref clearing policies

   // . the default policy

   static ReferencePolicy*   _default_soft_ref_policy;

   // . the "clear all" policy

   static ReferencePolicy*   _always_clear_soft_ref_policy;

   // . the current policy below is either one of the above

   ReferencePolicy*          _current_soft_ref_policy;

   // The discovered ref lists themselves

   // The MT'ness degree of the queues below

   int             _num_q;

   // Arrays of lists of oops, one per thread

   DiscoveredList* _discoveredSoftRefs;

   DiscoveredList* _discoveredWeakRefs;

   DiscoveredList* _discoveredFinalRefs;

   DiscoveredList* _discoveredPhantomRefs;

  public:

   int num_q()                            { return _num_q; }

   DiscoveredList* discovered_soft_refs() { return _discoveredSoftRefs; }

   static oop  sentinel_ref()             { return _sentinelRef; }

   static oop* adr_sentinel_ref()         { return &_sentinelRef; }

   ReferencePolicy* setup_policy(bool always_clear) {

     _current_soft_ref_policy = always_clear ?

       _always_clear_soft_ref_policy : _default_soft_ref_policy;

     _current_soft_ref_policy->setup();   // snapshot the policy threshold

     return _current_soft_ref_policy;

   }

  public:

   // Process references with a certain reachability level.

   void process_discovered_reflist(DiscoveredList               refs_lists[],

                                   ReferencePolicy*             policy,

                                   bool                         clear_referent,

                                   BoolObjectClosure*           is_alive,

                                   OopClosure*                  keep_alive,

                                   VoidClosure*                 complete_gc,

                                   AbstractRefProcTaskExecutor* task_executor);

   void process_phaseJNI(BoolObjectClosure* is_alive,

                         OopClosure*        keep_alive,

                         VoidClosure*       complete_gc);

   // Work methods used by the method process_discovered_reflist

   // Phase1: keep alive all those referents that are otherwise

   // dead but which must be kept alive by policy (and their closure).

   void process_phase1(DiscoveredList&     refs_list,

                       ReferencePolicy*    policy,

                       BoolObjectClosure*  is_alive,

                       OopClosure*         keep_alive,

                       VoidClosure*        complete_gc);

   // Phase2: remove all those references whose referents are

   // reachable.

   inline void process_phase2(DiscoveredList&    refs_list,

                              BoolObjectClosure* is_alive,

                              OopClosure*        keep_alive,

                              VoidClosure*       complete_gc) {

     if (discovery_is_atomic()) {

       // complete_gc is ignored in this case for this phase

       pp2_work(refs_list, is_alive, keep_alive);

     } else {

       assert(complete_gc != NULL, "Error");

       pp2_work_concurrent_discovery(refs_list, is_alive,

                                     keep_alive, complete_gc);

     }

   }

   // Work methods in support of process_phase2

   void pp2_work(DiscoveredList&    refs_list,

                 BoolObjectClosure* is_alive,

                 OopClosure*        keep_alive);

   void pp2_work_concurrent_discovery(

                 DiscoveredList&    refs_list,

                 BoolObjectClosure* is_alive,

                 OopClosure*        keep_alive,

                 VoidClosure*       complete_gc);

   // Phase3: process the referents by either clearing them

   // or keeping them alive (and their closure)

   void process_phase3(DiscoveredList&    refs_list,

                       bool               clear_referent,

                       BoolObjectClosure* is_alive,

                       OopClosure*        keep_alive,

                       VoidClosure*       complete_gc);

   // Enqueue references with a certain reachability level

   void enqueue_discovered_reflist(DiscoveredList& refs_list, HeapWord* pending_list_addr);

   // "Preclean" all the discovered reference lists

   // by removing references with strongly reachable referents.

   // The first argument is a predicate on an oop that indicates

   // its (strong) reachability and the second is a closure that

   // may be used to incrementalize or abort the precleaning process.

   // The caller is responsible for taking care of potential

   // interference with concurrent operations on these lists

   // (or predicates involved) by other threads. Currently

   // only used by the CMS collector.  should_unload_classes is

   // used to aid assertion checking when classes are collected.

   void preclean_discovered_references(BoolObjectClosure* is_alive,

                                       OopClosure*        keep_alive,

                                       VoidClosure*       complete_gc,

                                       YieldClosure*      yield,

                                       bool               should_unload_classes);

   // Delete entries in the discovered lists that have

   // either a null referent or are not active. Such

   // Reference objects can result from the clearing

   // or enqueueing of Reference objects concurrent

   // with their discovery by a (concurrent) collector.

   // For a definition of "active" see java.lang.ref.Reference;

   // Refs are born active, become inactive when enqueued,

   // and never become active again. The state of being

   // active is encoded as follows: A Ref is active

   // if and only if its "next" field is NULL.

   void clean_up_discovered_references();

   void clean_up_discovered_reflist(DiscoveredList& refs_list);

   // Returns the name of the discovered reference list

   // occupying the i / _num_q slot.

   const char* list_name(int i);

   void enqueue_discovered_reflists(HeapWord* pending_list_addr, AbstractRefProcTaskExecutor* task_executor);

  protected:

   // "Preclean" the given discovered reference list

   // by removing references with strongly reachable referents.

   // Currently used in support of CMS only.

   void preclean_discovered_reflist(DiscoveredList&    refs_list,

                                    BoolObjectClosure* is_alive,

                                    OopClosure*        keep_alive,

                                    VoidClosure*       complete_gc,

                                    YieldClosure*      yield);

   int next_id() {

     int id = _next_id;

     if (++_next_id == _num_q) {

       _next_id = 0;

     }

     return id;

   }

   DiscoveredList* get_discovered_list(ReferenceType rt);

   inline void add_to_discovered_list_mt(DiscoveredList& refs_list, oop obj,

                                         HeapWord* discovered_addr);

   void verify_ok_to_handle_reflists() PRODUCT_RETURN;

   void abandon_partial_discovered_list(DiscoveredList& refs_list);

   // Calculate the number of jni handles.

   unsigned int count_jni_refs();

   // Balances reference queues.

   void balance_queues(DiscoveredList ref_lists[]);

   // Update (advance) the soft ref master clock field.

   void update_soft_ref_master_clock();

  public:

   // constructor

   ReferenceProcessor():

     _span((HeapWord*)NULL, (HeapWord*)NULL),

     _discoveredSoftRefs(NULL),  _discoveredWeakRefs(NULL),

     _discoveredFinalRefs(NULL), _discoveredPhantomRefs(NULL),

     _discovering_refs(false),

     _discovery_is_atomic(true),

     _enqueuing_is_done(false),

     _discovery_is_mt(false),

     _discovered_list_needs_barrier(false),

     _bs(NULL),

     _is_alive_non_header(NULL),

     _num_q(0),

     _processing_is_mt(false),

     _next_id(0)

   {}

   ReferenceProcessor(MemRegion span, bool atomic_discovery,

                      bool mt_discovery,

                      int mt_degree = 1,

                      bool mt_processing = false,

                      bool discovered_list_needs_barrier = false);

   // Allocates and initializes a reference processor.

   static ReferenceProcessor* create_ref_processor(

     MemRegion          span,

     bool               atomic_discovery,

     bool               mt_discovery,

     BoolObjectClosure* is_alive_non_header = NULL,

     int                parallel_gc_threads = 1,

     bool               mt_processing = false,

     bool               discovered_list_needs_barrier = false);

   // RefDiscoveryPolicy values

   enum DiscoveryPolicy {

     ReferenceBasedDiscovery = 0,

     ReferentBasedDiscovery  = 1,

     DiscoveryPolicyMin      = ReferenceBasedDiscovery,

     DiscoveryPolicyMax      = ReferentBasedDiscovery

   };

   static void init_statics();

  public:

   // get and set "is_alive_non_header" field

   BoolObjectClosure* is_alive_non_header() {

     return _is_alive_non_header;

   }

   void set_is_alive_non_header(BoolObjectClosure* is_alive_non_header) {

     _is_alive_non_header = is_alive_non_header;

   }

   // get and set span

   MemRegion span()                   { return _span; }

   void      set_span(MemRegion span) { _span = span; }

   // start and stop weak ref discovery

   void enable_discovery()   { _discovering_refs = true;  }

   void disable_discovery()  { _discovering_refs = false; }

   bool discovery_enabled()  { return _discovering_refs;  }

   // whether discovery is atomic wrt other collectors

   bool discovery_is_atomic() const { return _discovery_is_atomic; }

   void set_atomic_discovery(bool atomic) { _discovery_is_atomic = atomic; }

   // whether discovery is done by multiple threads same-old-timeously

   bool discovery_is_mt() const { return _discovery_is_mt; }

   void set_mt_discovery(bool mt) { _discovery_is_mt = mt; }

   // Whether we are in a phase when _processing_ is MT.

   bool processing_is_mt() const { return _processing_is_mt; }

   void set_mt_processing(bool mt) { _processing_is_mt = mt; }

   // whether all enqueuing of weak references is complete

   bool enqueuing_is_done()  { return _enqueuing_is_done; }

   void set_enqueuing_is_done(bool v) { _enqueuing_is_done = v; }

   // iterate over oops

   void weak_oops_do(OopClosure* f);       // weak roots

   static void oops_do(OopClosure* f);     // strong root(s)

   // Discover a Reference object, using appropriate discovery criteria

   bool discover_reference(oop obj, ReferenceType rt);

   // Process references found during GC (called by the garbage collector)

   void process_discovered_references(BoolObjectClosure*           is_alive,

                                      OopClosure*                  keep_alive,

                                      VoidClosure*                 complete_gc,

                                      AbstractRefProcTaskExecutor* task_executor);

  public:

   // Enqueue references at end of GC (called by the garbage collector)

   bool enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor = NULL);

   // If a discovery is in process that is being superceded, abandon it: all

   // the discovered lists will be empty, and all the objects on them will

   // have NULL discovered fields.  Must be called only at a safepoint.

   void abandon_partial_discovery();

   // debugging

   void verify_no_references_recorded() PRODUCT_RETURN;

   static void verify();

   // clear the discovered lists (unlinking each entry).

   void clear_discovered_references() PRODUCT_RETURN;

 };

 // A utility class to disable reference discovery in

 // the scope which contains it, for given ReferenceProcessor.

 class NoRefDiscovery: StackObj {

  private:

   ReferenceProcessor* _rp;

   bool _was_discovering_refs;

  public:

   NoRefDiscovery(ReferenceProcessor* rp) : _rp(rp) {

     if (_was_discovering_refs = _rp->discovery_enabled()) {

       _rp->disable_discovery();

     }

   }

   ~NoRefDiscovery() {

     if (_was_discovering_refs) {

       _rp->enable_discovery();

     }

   }

 };

 // A utility class to temporarily mutate the span of the

 // given ReferenceProcessor in the scope that contains it.

 class ReferenceProcessorSpanMutator: StackObj {

  private:

   ReferenceProcessor* _rp;

   MemRegion           _saved_span;

  public:

   ReferenceProcessorSpanMutator(ReferenceProcessor* rp,

                                 MemRegion span):

     _rp(rp) {

     _saved_span = _rp->span();

     _rp->set_span(span);

   }

   ~ReferenceProcessorSpanMutator() {

     _rp->set_span(_saved_span);

   }

 };

 // A utility class to temporarily change the MT'ness of

 // reference discovery for the given ReferenceProcessor

 // in the scope that contains it.

 class ReferenceProcessorMTMutator: StackObj {

  private:

   ReferenceProcessor* _rp;

   bool                _saved_mt;

  public:

   ReferenceProcessorMTMutator(ReferenceProcessor* rp,

                               bool mt):

     _rp(rp) {

     _saved_mt = _rp->discovery_is_mt();

     _rp->set_mt_discovery(mt);

   }

   ~ReferenceProcessorMTMutator() {

     _rp->set_mt_discovery(_saved_mt);

   }

 };

 // A utility class to temporarily change the disposition

 // of the "is_alive_non_header" closure field of the

 // given ReferenceProcessor in the scope that contains it.

 class ReferenceProcessorIsAliveMutator: StackObj {

  private:

   ReferenceProcessor* _rp;

   BoolObjectClosure*  _saved_cl;

  public:

   ReferenceProcessorIsAliveMutator(ReferenceProcessor* rp,

                                    BoolObjectClosure*  cl):

     _rp(rp) {

     _saved_cl = _rp->is_alive_non_header();

     _rp->set_is_alive_non_header(cl);

   }

   ~ReferenceProcessorIsAliveMutator() {

     _rp->set_is_alive_non_header(_saved_cl);

   }

 };

 // A utility class to temporarily change the disposition

 // of the "discovery_is_atomic" field of the

 // given ReferenceProcessor in the scope that contains it.

 class ReferenceProcessorAtomicMutator: StackObj {

  private:

   ReferenceProcessor* _rp;

   bool                _saved_atomic_discovery;

  public:

   ReferenceProcessorAtomicMutator(ReferenceProcessor* rp,

                                   bool atomic):

     _rp(rp) {

     _saved_atomic_discovery = _rp->discovery_is_atomic();

     _rp->set_atomic_discovery(atomic);

   }

   ~ReferenceProcessorAtomicMutator() {

     _rp->set_atomic_discovery(_saved_atomic_discovery);

   }

 };

 // A utility class to temporarily change the MT processing

 // disposition of the given ReferenceProcessor instance

 // in the scope that contains it.

 class ReferenceProcessorMTProcMutator: StackObj {

  private:

   ReferenceProcessor* _rp;

   bool  _saved_mt;

  public:

   ReferenceProcessorMTProcMutator(ReferenceProcessor* rp,

                                   bool mt):

     _rp(rp) {

     _saved_mt = _rp->processing_is_mt();

     _rp->set_mt_processing(mt);

   }

   ~ReferenceProcessorMTProcMutator() {

     _rp->set_mt_processing(_saved_mt);

   }

 };

 // This class is an interface used to implement task execution for the

 // reference processing.

 class AbstractRefProcTaskExecutor {

 public:

   // Abstract tasks to execute.

   class ProcessTask;

   class EnqueueTask;

   // Executes a task using worker threads.

   virtual void execute(ProcessTask& task) = 0;

   virtual void execute(EnqueueTask& task) = 0;

   // Switch to single threaded mode.

   virtual void set_single_threaded_mode() { };

 };

 // Abstract reference processing task to execute.

 class AbstractRefProcTaskExecutor::ProcessTask {

 protected:

   ProcessTask(ReferenceProcessor& ref_processor,

               DiscoveredList      refs_lists[],

               bool                marks_oops_alive)

     : _ref_processor(ref_processor),

       _refs_lists(refs_lists),

       _marks_oops_alive(marks_oops_alive)

   { }

 public:

   virtual void work(unsigned int work_id, BoolObjectClosure& is_alive,

                     OopClosure& keep_alive,

                     VoidClosure& complete_gc) = 0;

   // Returns true if a task marks some oops as alive.

   bool marks_oops_alive() const

   { return _marks_oops_alive; }

 protected:

   ReferenceProcessor& _ref_processor;

   DiscoveredList*     _refs_lists;

   const bool          _marks_oops_alive;

 };

 // Abstract reference processing task to execute.

 class AbstractRefProcTaskExecutor::EnqueueTask {

 protected:

   EnqueueTask(ReferenceProcessor& ref_processor,

               DiscoveredList      refs_lists[],

               HeapWord*           pending_list_addr,

               oop                 sentinel_ref,

               int                 n_queues)

     : _ref_processor(ref_processor),

       _refs_lists(refs_lists),

       _pending_list_addr(pending_list_addr),

       _sentinel_ref(sentinel_ref),

       _n_queues(n_queues)

   { }

 public:

   virtual void work(unsigned int work_id) = 0;

 protected:

   ReferenceProcessor& _ref_processor;

   DiscoveredList*     _refs_lists;

   HeapWord*           _pending_list_addr;

   oop                 _sentinel_ref;

   int                 _n_queues;

 };