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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 *

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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

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

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

#define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

//

// String Deduplication

//

// String deduplication aims to reduce the heap live-set by deduplicating identical

// instances of String so that they share the same backing character array.

//

// The deduplication process is divided in two main parts, 1) finding the objects to

// deduplicate, and 2) deduplicating those objects. The first part is done as part of

// a normal GC cycle when objects are marked or evacuated. At this time a check is

// applied on each object to check if it is a candidate for deduplication. If so, the

// object is placed on the deduplication queue for later processing. The second part,

// processing the objects on the deduplication queue, is a concurrent phase which

// starts right after the stop-the-wold marking/evacuation phase. This phase is

// executed by the deduplication thread, which pulls deduplication candidates of the

// deduplication queue and tries to deduplicate them.

//

// A deduplication hashtable is used to keep track of all unique character arrays

// used by String objects. When deduplicating, a lookup is made in this table to see

// if there is already an identical character array somewhere on the heap. If so, the

// String object is adjusted to point to that character array, releasing the reference

// to the original array allowing it to eventually be garbage collected. If the lookup

// fails the character array is instead inserted into the hashtable so that this array

// can be shared at some point in the future.

//

// Candidate selection

//

// An object is considered a deduplication candidate if all of the following

// statements are true:

//

// - The object is an instance of java.lang.String

//

// - The object is being evacuated from a young heap region

//

// - The object is being evacuated to a young/survivor heap region and the

//   object's age is equal to the deduplication age threshold

//

//   or

//

//   The object is being evacuated to an old heap region and the object's age is

//   less than the deduplication age threshold

//

// Once an string object has been promoted to an old region, or its age is higher

// than the deduplication age threshold, is will never become a candidate again.

// This approach avoids making the same object a candidate more than once.

//

// Interned strings are a bit special. They are explicitly deduplicated just before

// being inserted into the StringTable (to avoid counteracting C2 optimizations done

// on string literals), then they also become deduplication candidates if they reach

// the deduplication age threshold or are evacuated to an old heap region. The second

// attempt to deduplicate such strings will be in vain, but we have no fast way of

// filtering them out. This has not shown to be a problem, as the number of interned

// strings is usually dwarfed by the number of normal (non-interned) strings.

//

// For additional information on string deduplication, please see JEP 192,

// http://openjdk.java.net/jeps/192

//

#include "memory/allocation.hpp"

#include "oops/oop.hpp"

class OopClosure;

class BoolObjectClosure;

class ThreadClosure;

class outputStream;

class G1StringDedupTable;

//

// Main interface for interacting with string deduplication.

//

class G1StringDedup : public AllStatic {

private:

  // Single state for checking if both G1 and string deduplication is enabled.

  static bool _enabled;

  // Candidate selection policies, returns true if the given object is

  // candidate for string deduplication.

  static bool is_candidate_from_mark(oop obj);

  static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);

public:

  // Returns true if both G1 and string deduplication is enabled.

  static bool is_enabled() {

    return _enabled;

  }

  static void initialize();

  // Immediately deduplicates the given String object, bypassing the

  // the deduplication queue.

  static void deduplicate(oop java_string);

  // Enqueues a deduplication candidate for later processing by the deduplication

  // thread. Before enqueuing, these functions apply the appropriate candidate

  // selection policy to filters out non-candidates.

  static void enqueue_from_mark(oop java_string);

  static void enqueue_from_evacuation(bool from_young, bool to_young,

                                      unsigned int queue, oop java_string);

  static void oops_do(OopClosure* keep_alive);

  static void unlink(BoolObjectClosure* is_alive);

  static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,

                                bool allow_resize_and_rehash = true);

  static void threads_do(ThreadClosure* tc);

  static void print_worker_threads_on(outputStream* st);

  static void verify();

};

//

// This closure encapsulates the state and the closures needed when scanning

// the deduplication queue and table during the unlink_or_oops_do() operation.

// A single instance of this closure is created and then shared by all worker

// threads participating in the scan. The _next_queue and _next_bucket fields

// provide a simple mechanism for GC workers to claim exclusive access to a

// queue or a table partition.

//

class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {

private:

  BoolObjectClosure*  _is_alive;

  OopClosure*         _keep_alive;

  G1StringDedupTable* _resized_table;

  G1StringDedupTable* _rehashed_table;

  size_t              _next_queue;

  size_t              _next_bucket;

public:

  G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,

                                     OopClosure* keep_alive,

                                     bool allow_resize_and_rehash);

  ~G1StringDedupUnlinkOrOopsDoClosure();

  bool is_resizing() {

    return _resized_table != NULL;

  }

  G1StringDedupTable* resized_table() {

    return _resized_table;

  }

  bool is_rehashing() {

    return _rehashed_table != NULL;

  }

  // Atomically claims the next available queue for exclusive access by

  // the current thread. Returns the queue number of the claimed queue.

  size_t claim_queue() {

    return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;

  }

  // Atomically claims the next available table partition for exclusive

  // access by the current thread. Returns the table bucket number where

  // the claimed partition starts.

  size_t claim_table_partition(size_t partition_size) {

    return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;

  }

  // Applies and returns the result from the is_alive closure, or

  // returns true if no such closure was provided.

  bool is_alive(oop o) {

    if (_is_alive != NULL) {

      return _is_alive->do_object_b(o);

    }

    return true;

  }

  // Applies the keep_alive closure, or does nothing if no such

  // closure was provided.

  void keep_alive(oop* p) {

    if (_keep_alive != NULL) {

      _keep_alive->do_oop(p);

    }

  }

};

#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP