/*

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

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * 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.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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

#include "runtime/mutexLocker.hpp"

class StringDedupEntryCache;

class StringDedupUnlinkOrOopsDoClosure;

//

// Table entry in the deduplication hashtable. Points weakly to the

// character array. Can be chained in a linked list in case of hash

// collisions or when placed in a freelist in the entry cache.

//

class StringDedupEntry : public CHeapObj<mtGC> {

private:

  StringDedupEntry* _next;

  unsigned int      _hash;

  bool              _latin1;

  typeArrayOop      _obj;

public:

  StringDedupEntry() :

    _next(NULL),

    _hash(0),

    _latin1(false),

    _obj(NULL) {

  }

  StringDedupEntry* next() {

    return _next;

  }

  StringDedupEntry** next_addr() {

    return &_next;

  }

  void set_next(StringDedupEntry* next) {

    _next = next;

  }

  unsigned int hash() {

    return _hash;

  }

  void set_hash(unsigned int hash) {

    _hash = hash;

  }

  bool latin1() {

    return _latin1;

  }

  void set_latin1(bool latin1) {

    _latin1 = latin1;

  }

  typeArrayOop obj() {

    return _obj;

  }

  typeArrayOop* obj_addr() {

    return &_obj;

  }

  void set_obj(typeArrayOop obj) {

    _obj = obj;

  }

};

//

// The deduplication hashtable keeps track of all unique character arrays used

// by String objects. Each table entry weakly points to an character array, allowing

// otherwise unreachable character arrays to be declared dead and pruned from the

// table.

//

// The table is dynamically resized to accommodate the current number of table entries.

// The table has hash buckets with chains for hash collision. If the average chain

// length goes above or below given thresholds the table grows or shrinks accordingly.

//

// The table is also dynamically rehashed (using a new hash seed) if it becomes severely

// unbalanced, i.e., a hash chain is significantly longer than average.

//

// All access to the table is protected by the StringDedupTable_lock, except under

// safepoints in which case GC workers are allowed to access a table partitions they

// have claimed without first acquiring the lock. Note however, that this applies only

// the table partition (i.e. a range of elements in _buckets), not other parts of the

// table such as the _entries field, statistics counters, etc.

//

class StringDedupTable : public CHeapObj<mtGC> {

private:

  // The currently active hashtable instance. Only modified when

  // the table is resizes or rehashed.

  static StringDedupTable*        _table;

  // Cache for reuse and fast alloc/free of table entries.

  static StringDedupEntryCache*   _entry_cache;

  StringDedupEntry**              _buckets;

  size_t                          _size;

  uintx                           _entries;

  uintx                           _shrink_threshold;

  uintx                           _grow_threshold;

  bool                            _rehash_needed;

  // The hash seed also dictates which hash function to use. A

  // zero hash seed means we will use the Java compatible hash

  // function (which doesn't use a seed), and a non-zero hash

  // seed means we use the murmur3 hash function.

  jint                            _hash_seed;

  // Constants governing table resize/rehash/cache.

  static const size_t             _min_size;

  static const size_t             _max_size;

  static const double             _grow_load_factor;

  static const double             _shrink_load_factor;

  static const uintx              _rehash_multiple;

  static const uintx              _rehash_threshold;

  static const double             _max_cache_factor;

  // Table statistics, only used for logging.

  static uintx                    _entries_added;

  static uintx                    _entries_removed;

  static uintx                    _resize_count;

  static uintx                    _rehash_count;

  static volatile size_t          _claimed_index;

  static StringDedupTable*        _resized_table;

  static StringDedupTable*        _rehashed_table;

  StringDedupTable(size_t size, jint hash_seed = 0);

  ~StringDedupTable();

  // Returns the hash bucket at the given index.

  StringDedupEntry** bucket(size_t index) {

    return _buckets + index;

  }

  // Returns the hash bucket index for the given hash code.

  size_t hash_to_index(unsigned int hash) {

    return (size_t)hash & (_size - 1);

  }

  // Adds a new table entry to the given hash bucket.

  void add(typeArrayOop value, bool latin1, unsigned int hash, StringDedupEntry** list);

  // Removes the given table entry from the table.

  void remove(StringDedupEntry** pentry, uint worker_id);

  // Transfers a table entry from the current table to the destination table.

  void transfer(StringDedupEntry** pentry, StringDedupTable* dest);

  // Returns an existing character array in the given hash bucket, or NULL

  // if no matching character array exists.

  typeArrayOop lookup(typeArrayOop value, bool latin1, unsigned int hash,

                      StringDedupEntry** list, uintx &count);

  // Returns an existing character array in the table, or inserts a new

  // table entry if no matching character array exists.

  typeArrayOop lookup_or_add_inner(typeArrayOop value, bool latin1, unsigned int hash);

  // Thread safe lookup or add of table entry

  static typeArrayOop lookup_or_add(typeArrayOop value, bool latin1, unsigned int hash) {

    // Protect the table from concurrent access. Also note that this lock

    // acts as a fence for _table, which could have been replaced by a new

    // instance if the table was resized or rehashed.

    MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);

    return _table->lookup_or_add_inner(value, latin1, hash);

  }

  // Returns true if the hashtable is currently using a Java compatible

  // hash function.

  static bool use_java_hash() {

    return _table->_hash_seed == 0;

  }

  static bool equals(typeArrayOop value1, typeArrayOop value2);

  // Computes the hash code for the given character array, using the

  // currently active hash function and hash seed.

  static unsigned int hash_code(typeArrayOop value, bool latin1);

  static uintx unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl,

                                 size_t partition_begin,

                                 size_t partition_end,

                                 uint worker_id);

  static size_t claim_table_partition(size_t partition_size);

  static bool is_resizing();

  static bool is_rehashing();

  // If a table resize is needed, returns a newly allocated empty

  // hashtable of the proper size.

  static StringDedupTable* prepare_resize();

  // Installs a newly resized table as the currently active table

  // and deletes the previously active table.

  static void finish_resize(StringDedupTable* resized_table);

  // If a table rehash is needed, returns a newly allocated empty

  // hashtable and updates the hash seed.

  static StringDedupTable* prepare_rehash();

  // Transfers rehashed entries from the currently active table into

  // the new table. Installs the new table as the currently active table

  // and deletes the previously active table.

  static void finish_rehash(StringDedupTable* rehashed_table);

public:

  static void create();

  // Deduplicates the given String object, or adds its backing

  // character array to the deduplication hashtable.

  static void deduplicate(oop java_string, StringDedupStat* stat);

  static void unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);

  static void print_statistics();

  static void verify();

  // If the table entry cache has grown too large, delete overflowed entries.

  static void clean_entry_cache();

  // GC support

  static void gc_prologue(bool resize_and_rehash_table);

  static void gc_epilogue();

};