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//

// 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 criteria is GC specific.

//

// 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 "gc/shared/stringdedup/stringDedupQueue.hpp"

#include "gc/shared/stringdedup/stringDedupStat.hpp"

#include "gc/shared/stringdedup/stringDedupTable.hpp"

#include "memory/allocation.hpp"

#include "runtime/thread.hpp"

//

// Main interface for interacting with string deduplication.

//

class StringDedup : public AllStatic {

private:

  // Single state for checking if string deduplication is enabled.

  static bool _enabled;

public:

  // Returns true if string deduplication is enabled.

  static bool is_enabled() {

    return _enabled;

  }

  // Stop the deduplication thread.

  static void stop();

  // Immediately deduplicates the given String object, bypassing the

  // the deduplication queue.

  static void deduplicate(oop java_string);

  static void parallel_unlink(StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id);

  static void threads_do(ThreadClosure* tc);

  static void print_worker_threads_on(outputStream* st);

  static void verify();

  // GC support

  static void gc_prologue(bool resize_and_rehash_table);

  static void gc_epilogue();

protected:

  // Initialize string deduplication.

  // Q: String Dedup Queue implementation

  // S: String Dedup Stat implementation

  template <typename Q, typename S>

  static void initialize_impl();

};

//

// This closure encapsulates the closures needed when scanning

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

//

class StringDedupUnlinkOrOopsDoClosure : public StackObj {

private:

  BoolObjectClosure*  _is_alive;

  OopClosure*         _keep_alive;

public:

  StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,

                                     OopClosure* keep_alive);

  // 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);

    }

  }

};