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 * version 2 for more details (a copy is included in the LICENSE file that

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 *

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 * 2 along with this work; if not, write to the Free Software Foundation,

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#ifndef SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP

#define SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP

#include "gc/shared/oopStorage.hpp"

#include "utilities/macros.hpp"

//////////////////////////////////////////////////////////////////////////////

// Support for parallel and optionally concurrent state iteration.

//

// Parallel iteration is for the exclusive use of the GC.  Other iteration

// clients must use serial iteration.

//

// Concurrent Iteration

//

// Iteration involves the _active_list, which contains all of the blocks owned

// by a storage object.  This is a doubly-linked list, linked through

// dedicated fields in the blocks.

//

// At most one concurrent ParState can exist at a time for a given storage

// object.

//

// A concurrent ParState sets the associated storage's

// _concurrent_iteration_active flag true when the state is constructed, and

// sets it false when the state is destroyed.  These assignments are made with

// _active_mutex locked.  Meanwhile, empty block deletion is not done while

// _concurrent_iteration_active is true.  The flag check and the dependent

// removal of a block from the _active_list is performed with _active_mutex

// locked.  This prevents concurrent iteration and empty block deletion from

// interfering with with each other.

//

// Both allocate() and delete_empty_blocks_concurrent() lock the

// _allocate_mutex while performing their respective list manipulations,

// preventing them from interfering with each other.

//

// When allocate() creates a new block, it is added to the front of the

// _active_list.  Then _active_head is set to the new block.  When concurrent

// iteration is started (by a parallel worker thread calling the state's

// iterate() function), the current _active_head is used as the initial block

// for the iteration, with iteration proceeding down the list headed by that

// block.

//

// As a result, the list over which concurrent iteration operates is stable.

// However, once the iteration is started, later allocations may add blocks to

// the front of the list that won't be examined by the iteration.  And while

// the list is stable, concurrent allocate() and release() operations may

// change the set of allocated entries in a block at any time during the

// iteration.

//

// As a result, a concurrent iteration handler must accept that some

// allocations and releases that occur after the iteration started will not be

// seen by the iteration.  Further, some may overlap examination by the

// iteration.  To help with this, allocate() and release() have an invariant

// that an entry's value must be NULL when it is not in use.

//

// An in-progress delete_empty_blocks_concurrent() operation can contend with

// the start of a concurrent iteration over the _active_mutex.  Since both are

// under GC control, that potential contention can be eliminated by never

// scheduling both operations to run at the same time.

//

// ParState<concurrent, is_const>

//   concurrent must be true if iteration is concurrent with the

//   mutator, false if iteration is at a safepoint.

//

//   is_const must be true if the iteration is over a constant storage

//   object, false if the iteration may modify the storage object.

//

// ParState([const] OopStorage* storage)

//   Construct an object for managing an iteration over storage.  For a

//   concurrent ParState, empty block deletion for the associated storage

//   is inhibited for the life of the ParState.  There can be no more

//   than one live concurrent ParState at a time for a given storage object.

//

// template<typename F> void iterate(F f)

//   Repeatedly claims a block from the associated storage that has

//   not been processed by this iteration (possibly by other threads),

//   and applies f to each entry in the claimed block. Assume p is of

//   type const oop* or oop*, according to is_const. Then f(p) must be

//   a valid expression whose value is ignored.  Concurrent uses must

//   be prepared for an entry's value to change at any time, due to

//   mutator activity.

//

// template<typename Closure> void oops_do(Closure* cl)

//   Wrapper around iterate, providing an adaptation layer allowing

//   the use of OopClosures and similar objects for iteration.  Assume

//   p is of type const oop* or oop*, according to is_const.  Then

//   cl->do_oop(p) must be a valid expression whose value is ignored.

//   Concurrent uses must be prepared for the entry's value to change

//   at any time, due to mutator activity.

//

// Optional operations, provided only if !concurrent && !is_const.

// These are not provided when is_const, because the storage object

// may be modified by the iteration infrastructure, even if the

// provided closure doesn't modify the storage object.  These are not

// provided when concurrent because any pre-filtering behavior by the

// iteration infrastructure is inappropriate for concurrent iteration;

// modifications of the storage by the mutator could result in the

// pre-filtering being applied (successfully or not) to objects that

// are unrelated to what the closure finds in the entry.

//

// template<typename Closure> void weak_oops_do(Closure* cl)

// template<typename IsAliveClosure, typename Closure>

// void weak_oops_do(IsAliveClosure* is_alive, Closure* cl)

//   Wrappers around iterate, providing an adaptation layer allowing

//   the use of is-alive closures and OopClosures for iteration.

//   Assume p is of type oop*.  Then

//

//   - cl->do_oop(p) must be a valid expression whose value is ignored.

//

//   - is_alive->do_object_b(*p) must be a valid expression whose value

//   is convertible to bool.

//

//   If *p == NULL then neither is_alive nor cl will be invoked for p.

//   If is_alive->do_object_b(*p) is false, then cl will not be

//   invoked on p.

class OopStorage::BasicParState {

  OopStorage* _storage;

  void* volatile _next_block;

  bool _concurrent;

  // Noncopyable.

  BasicParState(const BasicParState&);

  BasicParState& operator=(const BasicParState&);

  void update_iteration_state(bool value);

  void ensure_iteration_started();

  Block* claim_next_block();

  // Wrapper for iteration handler; ignore handler result and return true.

  template<typename F> class AlwaysTrueFn;

public:

  BasicParState(OopStorage* storage, bool concurrent);

  ~BasicParState();

  template<bool is_const, typename F> void iterate(F f);

};

template<bool concurrent, bool is_const>

class OopStorage::ParState {

  BasicParState _basic_state;

public:

  ParState(const OopStorage* storage) :

    // For simplicity, always recorded as non-const.

    _basic_state(const_cast<OopStorage*>(storage), concurrent)

  {}

  template<typename F> void iterate(F f);

  template<typename Closure> void oops_do(Closure* cl);

};

template<>

class OopStorage::ParState<false, false> {

  BasicParState _basic_state;

public:

  ParState(OopStorage* storage) :

    _basic_state(storage, false)

  {}

  template<typename F> void iterate(F f);

  template<typename Closure> void oops_do(Closure* cl);

  template<typename Closure> void weak_oops_do(Closure* cl);

  template<typename IsAliveClosure, typename Closure>

  void weak_oops_do(IsAliveClosure* is_alive, Closure* cl);

};

#endif // SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP