src/hotspot/share/gc/shared/oopStorage.hpp

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#ifndef SHARE_GC_SHARED_OOPSTORAGE_HPP
#define SHARE_GC_SHARED_OOPSTORAGE_HPP

#include "memory/allocation.hpp"
#include "oops/oop.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/macros.hpp"
#include "utilities/singleWriterSynchronizer.hpp"

class Mutex;
class outputStream;

// OopStorage supports management of off-heap references to objects allocated
// in the Java heap.  An OopStorage object provides a set of Java object
// references (oop values), which clients refer to via oop* handles to the
// associated OopStorage entries.  Clients allocate entries to create a
// (possibly weak) reference to a Java object, use that reference, and release
// the reference when no longer needed.
//
// The garbage collector must know about all OopStorage objects and their
// reference strength.  OopStorage provides the garbage collector with support
// for iteration over all the allocated entries.
//
// There are several categories of interaction with an OopStorage object.
//
// (1) allocation and release of entries, by the mutator or the VM.
// (2) iteration by the garbage collector, possibly concurrent with mutator.
// (3) iteration by other, non-GC, tools (only at safepoints).
// (4) cleanup of unused internal storage, possibly concurrent with mutator.
//
// A goal of OopStorage is to make these interactions thread-safe, while
// minimizing potential lock contention issues within and between these
// categories.  In particular, support for concurrent iteration by the garbage
// collector, under certain restrictions, is required.  Further, it must not
// block nor be blocked by other operations for long periods.
//
// Internally, OopStorage is a set of Block objects, from which entries are
// allocated and released.  A block contains an oop[] and a bitmask indicating
// which entries are in use (have been allocated and not yet released).  New
// blocks are constructed and added to the storage object when an entry
// allocation request is made and there are no blocks with unused entries.
// Blocks may be removed and deleted when empty.
//
// There are two important (and somewhat intertwined) protocols governing
// concurrent access to a storage object.  These are the Concurrent Iteration
// Protocol and the Allocation Protocol.  See the ParState class for a
// discussion of concurrent iteration and the management of thread
// interactions for this protocol.  Similarly, see the allocate() function for
// a discussion of allocation.

class OopStorage : public CHeapObjBase {
public:
  static OopStorage* create(const char* name, MemTag mem_tag);
  ~OopStorage();

  // These count and usage accessors are racy unless at a safepoint.

  // The number of allocated and not yet released entries.
  size_t allocation_count() const;

  // The number of blocks of entries.  Useful for sizing parallel iteration.
  size_t block_count() const;

  // Total number of blocks * memory allocation per block, plus
  // bookkeeping overhead, including this storage object.
  size_t total_memory_usage() const;

  // The memory tag for allocations.
  MemTag mem_tag() const;

  enum EntryStatus {
    INVALID_ENTRY,
    UNALLOCATED_ENTRY,
    ALLOCATED_ENTRY
  };

  // Locks _allocation_mutex.
  // precondition: ptr != nullptr.
  EntryStatus allocation_status(const oop* ptr) const;

  // Allocates and returns a new entry.  Returns null if memory allocation
  // failed.  Locks _allocation_mutex.
  // postcondition: result == nullptr or *result == nullptr.
  oop* allocate();

  // Maximum number of entries that can be obtained by one call to
  // allocate(oop**, size_t).
  static const size_t bulk_allocate_limit = BitsPerWord;

  // Allocates multiple entries, returning them in the ptrs buffer. Possibly
  // faster than making repeated calls to allocate(). Always make maximal
  // requests for best efficiency. Returns the number of entries allocated,
  // which may be less than requested. A result of zero indicates failure to
  // allocate any entries.
  // Locks _allocation_mutex.
  // precondition: size > 0.
  // postcondition: result <= min(size, bulk_allocate_limit).
  // postcondition: ptrs[i] is an allocated entry for i in [0, result).
  // postcondition: *ptrs[i] == nullptr for i in [0, result).
  size_t allocate(oop** ptrs, size_t size);

  // Deallocates ptr.  No locking.
  // precondition: ptr is a valid allocated entry.
  // precondition: *ptr == nullptr.
  void release(const oop* ptr);

  // Releases all the ptrs.  Possibly faster than individual calls to
  // release(oop*).  Best if ptrs is sorted by address.  No locking.
  // precondition: All elements of ptrs are valid allocated entries.
  // precondition: *ptrs[i] == nullptr, for i in [0,size).
  void release(const oop* const* ptrs, size_t size);

  // Applies f to each allocated entry's location.  f must be a function or
  // function object.  Assume p is either a const oop* or an oop*, depending
  // on whether the associated storage is const or non-const, respectively.
  // Then f(p) must be a valid expression.  The result of invoking f(p) must
  // be implicitly convertible to bool.  Iteration terminates and returns
  // false if any invocation of f returns false.  Otherwise, the result of
  // iteration is true.
  // precondition: at safepoint.
  template<typename F> inline bool iterate_safepoint(F f);
  template<typename F> inline bool iterate_safepoint(F f) const;

  // oops_do and weak_oops_do are wrappers around iterate_safepoint, providing
  // an adaptation layer allowing the use of existing is-alive closures and
  // OopClosures.  Assume p is either const oop* or oop*, depending on whether
  // the associated storage is const or non-const, respectively.  Then
  //
  // - closure->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.
  //
  // For weak_oops_do, if *p == nullptr then neither is_alive nor closure will be
  // invoked for p.  If is_alive->do_object_b(*p) is false, then closure will
  // not be invoked on p, and *p will be set to null.

  template<typename Closure> inline void oops_do(Closure* closure);
  template<typename Closure> inline void oops_do(Closure* closure) const;
  template<typename Closure> inline void weak_oops_do(Closure* closure);

  template<typename IsAliveClosure, typename Closure>
  inline void weak_oops_do(IsAliveClosure* is_alive, Closure* closure);

  // Parallel iteration is for the exclusive use of the GC.
  // Other clients must use serial iteration.
  template<bool concurrent, bool is_const> class ParState;

  // Support GC callbacks reporting dead entries.  This lets clients respond
  // to entries being cleared.

  typedef void (*NumDeadCallback)(size_t num_dead);

  // Used by a client to register a callback function with the GC.
  // precondition: No more than one registration per storage object.
  void register_num_dead_callback(NumDeadCallback f);

  // Called by the GC after an iteration that may clear dead referents.
  // This calls the registered callback function, if any.  num_dead is the
  // number of entries which were either already null or were cleared by the
  // iteration.
  void report_num_dead(size_t num_dead) const;

  // Used by the GC to test whether a callback function has been registered.
  bool should_report_num_dead() const;

  // Service thread cleanup support.

  // Called by the service thread to process any pending cleanups for this
  // storage object.  Drains the _deferred_updates list, and deletes empty
  // blocks.  Stops deleting if there is an in-progress concurrent
  // iteration.  Locks both the _allocation_mutex and the _active_mutex, and
  // may safepoint.  Deletion may be throttled, with only some available
  // work performed, in order to allow other Service thread subtasks to run.
  // Returns true if there may be more work to do, false if nothing to do.
  bool delete_empty_blocks();

  // Called by safepoint cleanup to notify the service thread (via
  // Service_lock) that there may be some OopStorage objects with pending
  // cleanups to process.
  static void trigger_cleanup_if_needed();

  // Called by the service thread (while holding Service_lock) to test
  // for pending cleanup requests, and resets the request state to allow
  // recognition of new requests.  Returns true if there was a pending
  // request.
  static bool has_cleanup_work_and_reset();

  // Debugging and logging support.
  const char* name() const;
  void print_on(outputStream* st) const PRODUCT_RETURN;
  bool print_containing(const oop* addr, outputStream* st);

  // Provides access to storage internals, for unit testing.
  // Declare, but not define, the public class OopStorage::TestAccess.
  // That class is defined as part of the unit-test. It "exports" the needed
  // private types by providing public typedefs for them.
  class TestAccess;

private:
  class Block;                  // Fixed-size array of oops, plus bookkeeping.
  class ActiveArray;            // Array of Blocks, plus bookkeeping.
  class AllocationListEntry;    // Provides AllocationList links in a Block.

  // Doubly-linked list of Blocks.  For all operations with a block
  // argument, the block must be from the list's OopStorage.
  class AllocationList {
    const Block* _head;
    const Block* _tail;

    NONCOPYABLE(AllocationList);

  public:
    AllocationList();
    ~AllocationList();

    Block* head();
    Block* tail();
    const Block* chead() const;
    const Block* ctail() const;

    Block* prev(Block& block);
    Block* next(Block& block);

    const Block* prev(const Block& block) const;
    const Block* next(const Block& block) const;

    void push_front(const Block& block);
    void push_back(const Block& block);
    void unlink(const Block& block);

    bool contains(const Block& block) const;
  };

private:
  const char* _name;
  ActiveArray* _active_array;
  AllocationList _allocation_list;
  Block* volatile _deferred_updates;
  Mutex* _allocation_mutex;
  Mutex* _active_mutex;
  NumDeadCallback _num_dead_callback;

  // Volatile for racy unlocked accesses.
  volatile size_t _allocation_count;

  // Protection for _active_array.
  mutable SingleWriterSynchronizer _protect_active;

  // mutable because this gets set even for const iteration.
  mutable int _concurrent_iteration_count;

  // The memory tag for allocations.
  MemTag _mem_tag;

  // Flag indicating this storage object is a candidate for empty block deletion.
  volatile bool _needs_cleanup;

  // Clients construct via "create" factory function.
  OopStorage(const char* name, MemTag mem_tag);
  NONCOPYABLE(OopStorage);

  bool try_add_block();
  Block* block_for_allocation();
  void  log_block_transition(Block* block, const char* new_state) const;

  Block* block_for_ptr(const oop* ptr) const;
  void delete_empty_block(const Block& block);
  bool reduce_deferred_updates();
  void record_needs_cleanup();

  // Managing _active_array.
  bool expand_active_array();
  void replace_active_array(ActiveArray* new_array);
  ActiveArray* obtain_active_array() const;
  void relinquish_block_array(ActiveArray* array) const;
  class WithActiveArray;        // RAII helper for active array access.

  template<typename F, typename Storage>
  static bool iterate_impl(F f, Storage* storage);

  // Implementation support for parallel iteration
  class BasicParState;

  // Wrapper for OopClosure-style function, so it can be used with
  // iterate.  Assume p is of type oop*.  Then cl->do_oop(p) must be a
  // valid expression whose value may be ignored.
  template<typename Closure> class OopFn;
  template<typename Closure> static OopFn<Closure> oop_fn(Closure* cl);

  // Wrapper for BoolObjectClosure + iteration handler pair, so they
  // can be used with iterate.
  template<typename IsAlive, typename F> class IfAliveFn;
  template<typename IsAlive, typename F>
  static IfAliveFn<IsAlive, F> if_alive_fn(IsAlive* is_alive, F f);

  // Wrapper for iteration handler, automatically skipping null entries.
  template<typename F> class SkipNullFn;
  template<typename F> static SkipNullFn<F> skip_null_fn(F f);
};

#endif // SHARE_GC_SHARED_OOPSTORAGE_HPP