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 /*

  * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.

  * 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

  * or visit www.oracle.com if you need additional information or have any

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 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP

 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP

 #include "gc_implementation/g1/g1StringDedupStat.hpp"

 #include "runtime/mutexLocker.hpp"

 class G1StringDedupEntryCache;

 //

 // 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 G1StringDedupEntry : public CHeapObj<mtGC> {

 private:

   G1StringDedupEntry* _next;

   unsigned int      _hash;

   typeArrayOop      _obj;

 public:

   G1StringDedupEntry() :

     _next(NULL),

     _hash(0),

     _obj(NULL) {

   }

   G1StringDedupEntry* next() {

     return _next;

   }

   G1StringDedupEntry** next_addr() {

     return &_next;

   }

   void set_next(G1StringDedupEntry* next) {

     _next = next;

   }

   unsigned int hash() {

     return _hash;

   }

   void set_hash(unsigned int hash) {

     _hash = hash;

   }

   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 G1StringDedupTable : public CHeapObj<mtGC> {

 private:

   // The currently active hashtable instance. Only modified when

   // the table is resizes or rehashed.

   static G1StringDedupTable*      _table;

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

   static G1StringDedupEntryCache* _entry_cache;

   G1StringDedupEntry**            _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;

   G1StringDedupTable(size_t size, jint hash_seed = 0);

   ~G1StringDedupTable();

   // Returns the hash bucket at the given index.

   G1StringDedupEntry** 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, unsigned int hash, G1StringDedupEntry** list);

   // Removes the given table entry from the table.

   void remove(G1StringDedupEntry** pentry, uint worker_id);

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

   void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest);

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

   // if no matching character array exists.

   typeArrayOop lookup(typeArrayOop value, unsigned int hash,

                       G1StringDedupEntry** 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, unsigned int hash);

   // Thread safe lookup or add of table entry

   static typeArrayOop lookup_or_add(typeArrayOop value, 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, 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);

   static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,

                                  size_t partition_begin,

                                  size_t partition_end,

                                  uint worker_id);

 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, G1StringDedupStat& stat);

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

   // hashtable of the proper size.

   static G1StringDedupTable* prepare_resize();

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

   // and deletes the previously active table.

   static void finish_resize(G1StringDedupTable* resized_table);

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

   // hashtable and updates the hash seed.

   static G1StringDedupTable* 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(G1StringDedupTable* rehashed_table);

   // If the table entry cache has grown too large, trim it down according to policy

   static void trim_entry_cache();

   static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);

   static void print_statistics(outputStream* st);

   static void verify();

 };

 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP