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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

  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.