1 /*
   2  * Copyright (c) 2014, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/altHashing.hpp"
  27 #include "classfile/javaClasses.inline.hpp"
  28 #include "gc/shared/stringdedup/stringDedup.hpp"
  29 #include "gc/shared/stringdedup/stringDedupTable.hpp"
  30 #include "gc/shared/suspendibleThreadSet.hpp"
  31 #include "logging/log.hpp"
  32 #include "memory/padded.inline.hpp"
  33 #include "oops/access.inline.hpp"
  34 #include "oops/arrayOop.inline.hpp"
  35 #include "oops/oop.inline.hpp"
  36 #include "oops/typeArrayOop.hpp"
  37 #include "runtime/mutexLocker.hpp"
  38 #include "runtime/safepointVerifiers.hpp"
  39 
  40 //
  41 // List of deduplication table entries. Links table
  42 // entries together using their _next fields.
  43 //
  44 class StringDedupEntryList : public CHeapObj<mtGC> {
  45 private:
  46   StringDedupEntry*   _list;
  47   size_t              _length;
  48 
  49 public:
  50   StringDedupEntryList() :
  51     _list(NULL),
  52     _length(0) {
  53   }
  54 
  55   void add(StringDedupEntry* entry) {
  56     entry->set_next(_list);
  57     _list = entry;
  58     _length++;
  59   }
  60 
  61   StringDedupEntry* remove() {
  62     StringDedupEntry* entry = _list;
  63     if (entry != NULL) {
  64       _list = entry->next();
  65       _length--;
  66     }
  67     return entry;
  68   }
  69 
  70   StringDedupEntry* remove_all() {
  71     StringDedupEntry* list = _list;
  72     _list = NULL;
  73     return list;
  74   }
  75 
  76   size_t length() {
  77     return _length;
  78   }
  79 };
  80 
  81 //
  82 // Cache of deduplication table entries. This cache provides fast allocation and
  83 // reuse of table entries to lower the pressure on the underlying allocator.
  84 // But more importantly, it provides fast/deferred freeing of table entries. This
  85 // is important because freeing of table entries is done during stop-the-world
  86 // phases and it is not uncommon for large number of entries to be freed at once.
  87 // Tables entries that are freed during these phases are placed onto a freelist in
  88 // the cache. The deduplication thread, which executes in a concurrent phase, will
  89 // later reuse or free the underlying memory for these entries.
  90 //
  91 // The cache allows for single-threaded allocations and multi-threaded frees.
  92 // Allocations are synchronized by StringDedupTable_lock as part of a table
  93 // modification.
  94 //
  95 class StringDedupEntryCache : public CHeapObj<mtGC> {
  96 private:
  97   // One cache/overflow list per GC worker to allow lock less freeing of
  98   // entries while doing a parallel scan of the table. Using PaddedEnd to
  99   // avoid false sharing.
 100   size_t                             _nlists;
 101   size_t                             _max_list_length;
 102   PaddedEnd<StringDedupEntryList>*   _cached;
 103   PaddedEnd<StringDedupEntryList>*   _overflowed;
 104 
 105 public:
 106   StringDedupEntryCache(size_t max_size);
 107   ~StringDedupEntryCache();
 108 
 109   // Set max number of table entries to cache.
 110   void set_max_size(size_t max_size);
 111 
 112   // Get a table entry from the cache, or allocate a new entry if the cache is empty.
 113   StringDedupEntry* alloc();
 114 
 115   // Insert a table entry into the cache.
 116   void free(StringDedupEntry* entry, uint worker_id);
 117 
 118   // Returns current number of entries in the cache.
 119   size_t size();
 120 
 121   // Deletes overflowed entries.
 122   void delete_overflowed();
 123 };
 124 
 125 StringDedupEntryCache::StringDedupEntryCache(size_t max_size) :
 126   _nlists(ParallelGCThreads),
 127   _max_list_length(0),
 128   _cached(PaddedArray<StringDedupEntryList, mtGC>::create_unfreeable((uint)_nlists)),
 129   _overflowed(PaddedArray<StringDedupEntryList, mtGC>::create_unfreeable((uint)_nlists)) {
 130   set_max_size(max_size);
 131 }
 132 
 133 StringDedupEntryCache::~StringDedupEntryCache() {
 134   ShouldNotReachHere();
 135 }
 136 
 137 void StringDedupEntryCache::set_max_size(size_t size) {
 138   _max_list_length = size / _nlists;
 139 }
 140 
 141 StringDedupEntry* StringDedupEntryCache::alloc() {
 142   for (size_t i = 0; i < _nlists; i++) {
 143     StringDedupEntry* entry = _cached[i].remove();
 144     if (entry != NULL) {
 145       return entry;
 146     }
 147   }
 148   return new StringDedupEntry();
 149 }
 150 
 151 void StringDedupEntryCache::free(StringDedupEntry* entry, uint worker_id) {
 152   assert(entry->obj() != NULL, "Double free");
 153   assert(worker_id < _nlists, "Invalid worker id");
 154 
 155   entry->set_obj(NULL);
 156   entry->set_hash(0);
 157 
 158   if (_cached[worker_id].length() < _max_list_length) {
 159     // Cache is not full
 160     _cached[worker_id].add(entry);
 161   } else {
 162     // Cache is full, add to overflow list for later deletion
 163     _overflowed[worker_id].add(entry);
 164   }
 165 }
 166 
 167 size_t StringDedupEntryCache::size() {
 168   size_t size = 0;
 169   for (size_t i = 0; i < _nlists; i++) {
 170     size += _cached[i].length();
 171   }
 172   return size;
 173 }
 174 
 175 void StringDedupEntryCache::delete_overflowed() {
 176   double start = os::elapsedTime();
 177   uintx count = 0;
 178 
 179   for (size_t i = 0; i < _nlists; i++) {
 180     StringDedupEntry* entry;
 181 
 182     {
 183       // The overflow list can be modified during safepoints, therefore
 184       // we temporarily join the suspendible thread set while removing
 185       // all entries from the list.
 186       SuspendibleThreadSetJoiner sts_join;
 187       entry = _overflowed[i].remove_all();
 188     }
 189 
 190     // Delete all entries
 191     while (entry != NULL) {
 192       StringDedupEntry* next = entry->next();
 193       delete entry;
 194       entry = next;
 195       count++;
 196     }
 197   }
 198 
 199   double end = os::elapsedTime();
 200   log_trace(gc, stringdedup)("Deleted " UINTX_FORMAT " entries, " STRDEDUP_TIME_FORMAT_MS,
 201                              count, STRDEDUP_TIME_PARAM_MS(end - start));
 202 }
 203 
 204 StringDedupTable*        StringDedupTable::_table = NULL;
 205 StringDedupEntryCache*   StringDedupTable::_entry_cache = NULL;
 206 
 207 const size_t             StringDedupTable::_min_size = (1 << 10);   // 1024
 208 const size_t             StringDedupTable::_max_size = (1 << 24);   // 16777216
 209 const double             StringDedupTable::_grow_load_factor = 2.0; // Grow table at 200% load
 210 const double             StringDedupTable::_shrink_load_factor = _grow_load_factor / 3.0; // Shrink table at 67% load
 211 const double             StringDedupTable::_max_cache_factor = 0.1; // Cache a maximum of 10% of the table size
 212 const uintx              StringDedupTable::_rehash_multiple = 60;   // Hash bucket has 60 times more collisions than expected
 213 const uintx              StringDedupTable::_rehash_threshold = (uintx)(_rehash_multiple * _grow_load_factor);
 214 
 215 uintx                    StringDedupTable::_entries_added = 0;
 216 uintx                    StringDedupTable::_entries_removed = 0;
 217 uintx                    StringDedupTable::_resize_count = 0;
 218 uintx                    StringDedupTable::_rehash_count = 0;
 219 
 220 StringDedupTable*        StringDedupTable::_resized_table = NULL;
 221 StringDedupTable*        StringDedupTable::_rehashed_table = NULL;
 222 volatile size_t          StringDedupTable::_claimed_index = 0;
 223 
 224 StringDedupTable::StringDedupTable(size_t size, jint hash_seed) :
 225   _size(size),
 226   _entries(0),
 227   _grow_threshold((uintx)(size * _grow_load_factor)),
 228   _shrink_threshold((uintx)(size * _shrink_load_factor)),
 229   _rehash_needed(false),
 230   _hash_seed(hash_seed) {
 231   assert(is_power_of_2(size), "Table size must be a power of 2");
 232   _buckets = NEW_C_HEAP_ARRAY(StringDedupEntry*, _size, mtGC);
 233   memset(_buckets, 0, _size * sizeof(StringDedupEntry*));
 234 }
 235 
 236 StringDedupTable::~StringDedupTable() {
 237   FREE_C_HEAP_ARRAY(G1StringDedupEntry*, _buckets);
 238 }
 239 
 240 void StringDedupTable::create() {
 241   assert(_table == NULL, "One string deduplication table allowed");
 242   _entry_cache = new StringDedupEntryCache(_min_size * _max_cache_factor);
 243   _table = new StringDedupTable(_min_size);
 244 }
 245 
 246 void StringDedupTable::add(typeArrayOop value, bool latin1, unsigned int hash, StringDedupEntry** list) {
 247   StringDedupEntry* entry = _entry_cache->alloc();
 248   entry->set_obj(value);
 249   entry->set_hash(hash);
 250   entry->set_latin1(latin1);
 251   entry->set_next(*list);
 252   *list = entry;
 253   _entries++;
 254 }
 255 
 256 void StringDedupTable::remove(StringDedupEntry** pentry, uint worker_id) {
 257   StringDedupEntry* entry = *pentry;
 258   *pentry = entry->next();
 259   _entry_cache->free(entry, worker_id);
 260 }
 261 
 262 void StringDedupTable::transfer(StringDedupEntry** pentry, StringDedupTable* dest) {
 263   StringDedupEntry* entry = *pentry;
 264   *pentry = entry->next();
 265   unsigned int hash = entry->hash();
 266   size_t index = dest->hash_to_index(hash);
 267   StringDedupEntry** list = dest->bucket(index);
 268   entry->set_next(*list);
 269   *list = entry;
 270 }
 271 
 272 bool StringDedupTable::equals(typeArrayOop value1, typeArrayOop value2) {
 273   return (oopDesc::equals(value1, value2) ||
 274           (value1->length() == value2->length() &&
 275           (!memcmp(value1->base(T_BYTE),
 276                     value2->base(T_BYTE),
 277                     value1->length() * sizeof(jbyte)))));
 278 }
 279 
 280 typeArrayOop StringDedupTable::lookup(typeArrayOop value, bool latin1, unsigned int hash,
 281                                       StringDedupEntry** list, uintx &count) {
 282   for (StringDedupEntry* entry = *list; entry != NULL; entry = entry->next()) {
 283     if (entry->hash() == hash && entry->latin1() == latin1) {
 284       oop* obj_addr = (oop*)entry->obj_addr();
 285       oop obj = NativeAccess<ON_PHANTOM_OOP_REF | AS_NO_KEEPALIVE>::oop_load(obj_addr);
 286       if (equals(value, static_cast<typeArrayOop>(obj))) {
 287         obj = NativeAccess<ON_PHANTOM_OOP_REF>::oop_load(obj_addr);
 288         return static_cast<typeArrayOop>(obj);
 289       }
 290     }
 291     count++;
 292   }
 293 
 294   // Not found
 295   return NULL;
 296 }
 297 
 298 typeArrayOop StringDedupTable::lookup_or_add_inner(typeArrayOop value, bool latin1, unsigned int hash) {
 299   size_t index = hash_to_index(hash);
 300   StringDedupEntry** list = bucket(index);
 301   uintx count = 0;
 302 
 303   // Lookup in list
 304   typeArrayOop existing_value = lookup(value, latin1, hash, list, count);
 305 
 306   // Check if rehash is needed
 307   if (count > _rehash_threshold) {
 308     _rehash_needed = true;
 309   }
 310 
 311   if (existing_value == NULL) {
 312     // Not found, add new entry
 313     add(value, latin1, hash, list);
 314 
 315     // Update statistics
 316     _entries_added++;
 317   }
 318 
 319   return existing_value;
 320 }
 321 
 322 unsigned int StringDedupTable::hash_code(typeArrayOop value, bool latin1) {
 323   unsigned int hash;
 324   int length = value->length();
 325   if (latin1) {
 326     const jbyte* data = (jbyte*)value->base(T_BYTE);
 327     if (use_java_hash()) {
 328       hash = java_lang_String::hash_code(data, length);
 329     } else {
 330       hash = AltHashing::murmur3_32(_table->_hash_seed, data, length);
 331     }
 332   } else {
 333     length /= sizeof(jchar) / sizeof(jbyte); // Convert number of bytes to number of chars
 334     const jchar* data = (jchar*)value->base(T_CHAR);
 335     if (use_java_hash()) {
 336       hash = java_lang_String::hash_code(data, length);
 337     } else {
 338       hash = AltHashing::murmur3_32(_table->_hash_seed, data, length);
 339     }
 340   }
 341 
 342   return hash;
 343 }
 344 
 345 void StringDedupTable::deduplicate(oop java_string, StringDedupStat* stat) {
 346   assert(java_lang_String::is_instance(java_string), "Must be a string");
 347   NoSafepointVerifier nsv;
 348 
 349   stat->inc_inspected();
 350 
 351   typeArrayOop value = java_lang_String::value(java_string);
 352   if (value == NULL) {
 353     // String has no value
 354     stat->inc_skipped();
 355     return;
 356   }
 357 
 358   bool latin1 = java_lang_String::is_latin1(java_string);
 359   unsigned int hash = 0;
 360 
 361   if (use_java_hash()) {
 362     // Get hash code from cache
 363     hash = java_lang_String::hash(java_string);
 364   }
 365 
 366   if (hash == 0) {
 367     // Compute hash
 368     hash = hash_code(value, latin1);
 369     stat->inc_hashed();
 370 
 371     if (use_java_hash() && hash != 0) {
 372       // Store hash code in cache
 373       java_lang_String::set_hash(java_string, hash);
 374     }
 375   }
 376 
 377   typeArrayOop existing_value = lookup_or_add(value, latin1, hash);
 378   if (existing_value == value) {
 379     // Same value, already known
 380     stat->inc_known();
 381     return;
 382   }
 383 
 384   // Get size of value array
 385   uintx size_in_bytes = value->size() * HeapWordSize;
 386   stat->inc_new(size_in_bytes);
 387 
 388   if (existing_value != NULL) {
 389     // Existing value found, deduplicate string
 390     java_lang_String::set_value(java_string, existing_value);
 391     stat->deduped(value, size_in_bytes);
 392   }
 393 }
 394 
 395 bool StringDedupTable::is_resizing() {
 396   return _resized_table != NULL;
 397 }
 398 
 399 bool StringDedupTable::is_rehashing() {
 400   return _rehashed_table != NULL;
 401 }
 402 
 403 StringDedupTable* StringDedupTable::prepare_resize() {
 404   size_t size = _table->_size;
 405 
 406   // Check if the hashtable needs to be resized
 407   if (_table->_entries > _table->_grow_threshold) {
 408     // Grow table, double the size
 409     size *= 2;
 410     if (size > _max_size) {
 411       // Too big, don't resize
 412       return NULL;
 413     }
 414   } else if (_table->_entries < _table->_shrink_threshold) {
 415     // Shrink table, half the size
 416     size /= 2;
 417     if (size < _min_size) {
 418       // Too small, don't resize
 419       return NULL;
 420     }
 421   } else if (StringDeduplicationResizeALot) {
 422     // Force grow
 423     size *= 2;
 424     if (size > _max_size) {
 425       // Too big, force shrink instead
 426       size /= 4;
 427     }
 428   } else {
 429     // Resize not needed
 430     return NULL;
 431   }
 432 
 433   // Update statistics
 434   _resize_count++;
 435 
 436   // Update max cache size
 437   _entry_cache->set_max_size(size * _max_cache_factor);
 438 
 439   // Allocate the new table. The new table will be populated by workers
 440   // calling unlink_or_oops_do() and finally installed by finish_resize().
 441   return new StringDedupTable(size, _table->_hash_seed);
 442 }
 443 
 444 void StringDedupTable::finish_resize(StringDedupTable* resized_table) {
 445   assert(resized_table != NULL, "Invalid table");
 446 
 447   resized_table->_entries = _table->_entries;
 448 
 449   // Free old table
 450   delete _table;
 451 
 452   // Install new table
 453   _table = resized_table;
 454 }
 455 
 456 void StringDedupTable::unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id) {
 457   // The table is divided into partitions to allow lock-less parallel processing by
 458   // multiple worker threads. A worker thread first claims a partition, which ensures
 459   // exclusive access to that part of the table, then continues to process it. To allow
 460   // shrinking of the table in parallel we also need to make sure that the same worker
 461   // thread processes all partitions where entries will hash to the same destination
 462   // partition. Since the table size is always a power of two and we always shrink by
 463   // dividing the table in half, we know that for a given partition there is only one
 464   // other partition whoes entries will hash to the same destination partition. That
 465   // other partition is always the sibling partition in the second half of the table.
 466   // For example, if the table is divided into 8 partitions, the sibling of partition 0
 467   // is partition 4, the sibling of partition 1 is partition 5, etc.
 468   size_t table_half = _table->_size / 2;
 469 
 470   // Let each partition be one page worth of buckets
 471   size_t partition_size = MIN2(table_half, os::vm_page_size() / sizeof(StringDedupEntry*));
 472   assert(table_half % partition_size == 0, "Invalid partition size");
 473 
 474   // Number of entries removed during the scan
 475   uintx removed = 0;
 476 
 477   for (;;) {
 478     // Grab next partition to scan
 479     size_t partition_begin = claim_table_partition(partition_size);
 480     size_t partition_end = partition_begin + partition_size;
 481     if (partition_begin >= table_half) {
 482       // End of table
 483       break;
 484     }
 485 
 486     // Scan the partition followed by the sibling partition in the second half of the table
 487     removed += unlink_or_oops_do(cl, partition_begin, partition_end, worker_id);
 488     removed += unlink_or_oops_do(cl, table_half + partition_begin, table_half + partition_end, worker_id);
 489   }
 490 
 491   // Delayed update to avoid contention on the table lock
 492   if (removed > 0) {
 493     MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
 494     _table->_entries -= removed;
 495     _entries_removed += removed;
 496   }
 497 }
 498 
 499 uintx StringDedupTable::unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl,
 500                                           size_t partition_begin,
 501                                           size_t partition_end,
 502                                           uint worker_id) {
 503   uintx removed = 0;
 504   for (size_t bucket = partition_begin; bucket < partition_end; bucket++) {
 505     StringDedupEntry** entry = _table->bucket(bucket);
 506     while (*entry != NULL) {
 507       oop* p = (oop*)(*entry)->obj_addr();
 508       if (cl->is_alive(*p)) {
 509         cl->keep_alive(p);
 510         if (is_resizing()) {
 511           // We are resizing the table, transfer entry to the new table
 512           _table->transfer(entry, _resized_table);
 513         } else {
 514           if (is_rehashing()) {
 515             // We are rehashing the table, rehash the entry but keep it
 516             // in the table. We can't transfer entries into the new table
 517             // at this point since we don't have exclusive access to all
 518             // destination partitions. finish_rehash() will do a single
 519             // threaded transfer of all entries.
 520             typeArrayOop value = (typeArrayOop)*p;
 521             bool latin1 = (*entry)->latin1();
 522             unsigned int hash = hash_code(value, latin1);
 523             (*entry)->set_hash(hash);
 524           }
 525 
 526           // Move to next entry
 527           entry = (*entry)->next_addr();
 528         }
 529       } else {
 530         // Not alive, remove entry from table
 531         _table->remove(entry, worker_id);
 532         removed++;
 533       }
 534     }
 535   }
 536 
 537   return removed;
 538 }
 539 
 540 void StringDedupTable::gc_prologue(bool resize_and_rehash_table) {
 541   assert(!is_resizing() && !is_rehashing(), "Already in progress?");
 542 
 543   _claimed_index = 0;
 544   if (resize_and_rehash_table) {
 545     // If both resize and rehash is needed, only do resize. Rehash of
 546     // the table will eventually happen if the situation persists.
 547     _resized_table = StringDedupTable::prepare_resize();
 548     if (!is_resizing()) {
 549       _rehashed_table = StringDedupTable::prepare_rehash();
 550     }
 551   }
 552 }
 553 
 554 void StringDedupTable::gc_epilogue() {
 555   assert(!is_resizing() || !is_rehashing(), "Can not both resize and rehash");
 556   assert(_claimed_index >= _table->_size / 2 || _claimed_index == 0, "All or nothing");
 557 
 558   if (is_resizing()) {
 559     StringDedupTable::finish_resize(_resized_table);
 560     _resized_table = NULL;
 561   } else if (is_rehashing()) {
 562     StringDedupTable::finish_rehash(_rehashed_table);
 563     _rehashed_table = NULL;
 564   }
 565 }
 566 
 567 StringDedupTable* StringDedupTable::prepare_rehash() {
 568   if (!_table->_rehash_needed && !StringDeduplicationRehashALot) {
 569     // Rehash not needed
 570     return NULL;
 571   }
 572 
 573   // Update statistics
 574   _rehash_count++;
 575 
 576   // Compute new hash seed
 577   _table->_hash_seed = AltHashing::compute_seed();
 578 
 579   // Allocate the new table, same size and hash seed
 580   return new StringDedupTable(_table->_size, _table->_hash_seed);
 581 }
 582 
 583 void StringDedupTable::finish_rehash(StringDedupTable* rehashed_table) {
 584   assert(rehashed_table != NULL, "Invalid table");
 585 
 586   // Move all newly rehashed entries into the correct buckets in the new table
 587   for (size_t bucket = 0; bucket < _table->_size; bucket++) {
 588     StringDedupEntry** entry = _table->bucket(bucket);
 589     while (*entry != NULL) {
 590       _table->transfer(entry, rehashed_table);
 591     }
 592   }
 593 
 594   rehashed_table->_entries = _table->_entries;
 595 
 596   // Free old table
 597   delete _table;
 598 
 599   // Install new table
 600   _table = rehashed_table;
 601 }
 602 
 603 size_t StringDedupTable::claim_table_partition(size_t partition_size) {
 604   return Atomic::add(partition_size, &_claimed_index) - partition_size;
 605 }
 606 
 607 void StringDedupTable::verify() {
 608   for (size_t bucket = 0; bucket < _table->_size; bucket++) {
 609     // Verify entries
 610     StringDedupEntry** entry = _table->bucket(bucket);
 611     while (*entry != NULL) {
 612       typeArrayOop value = (*entry)->obj();
 613       guarantee(value != NULL, "Object must not be NULL");
 614       guarantee(Universe::heap()->is_in_reserved(value), "Object must be on the heap");
 615       guarantee(!value->is_forwarded(), "Object must not be forwarded");
 616       guarantee(value->is_typeArray(), "Object must be a typeArrayOop");
 617       bool latin1 = (*entry)->latin1();
 618       unsigned int hash = hash_code(value, latin1);
 619       guarantee((*entry)->hash() == hash, "Table entry has inorrect hash");
 620       guarantee(_table->hash_to_index(hash) == bucket, "Table entry has incorrect index");
 621       entry = (*entry)->next_addr();
 622     }
 623 
 624     // Verify that we do not have entries with identical oops or identical arrays.
 625     // We only need to compare entries in the same bucket. If the same oop or an
 626     // identical array has been inserted more than once into different/incorrect
 627     // buckets the verification step above will catch that.
 628     StringDedupEntry** entry1 = _table->bucket(bucket);
 629     while (*entry1 != NULL) {
 630       typeArrayOop value1 = (*entry1)->obj();
 631       bool latin1_1 = (*entry1)->latin1();
 632       StringDedupEntry** entry2 = (*entry1)->next_addr();
 633       while (*entry2 != NULL) {
 634         typeArrayOop value2 = (*entry2)->obj();
 635         bool latin1_2 = (*entry2)->latin1();
 636         guarantee(latin1_1 != latin1_2 || !equals(value1, value2), "Table entries must not have identical arrays");
 637         entry2 = (*entry2)->next_addr();
 638       }
 639       entry1 = (*entry1)->next_addr();
 640     }
 641   }
 642 }
 643 
 644 void StringDedupTable::clean_entry_cache() {
 645   _entry_cache->delete_overflowed();
 646 }
 647 
 648 void StringDedupTable::print_statistics() {
 649   Log(gc, stringdedup) log;
 650   log.debug("  Table");
 651   log.debug("    Memory Usage: " STRDEDUP_BYTES_FORMAT_NS,
 652             STRDEDUP_BYTES_PARAM(_table->_size * sizeof(StringDedupEntry*) + (_table->_entries + _entry_cache->size()) * sizeof(StringDedupEntry)));
 653   log.debug("    Size: " SIZE_FORMAT ", Min: " SIZE_FORMAT ", Max: " SIZE_FORMAT, _table->_size, _min_size, _max_size);
 654   log.debug("    Entries: " UINTX_FORMAT ", Load: " STRDEDUP_PERCENT_FORMAT_NS ", Cached: " UINTX_FORMAT ", Added: " UINTX_FORMAT ", Removed: " UINTX_FORMAT,
 655             _table->_entries, percent_of((size_t)_table->_entries, _table->_size), _entry_cache->size(), _entries_added, _entries_removed);
 656   log.debug("    Resize Count: " UINTX_FORMAT ", Shrink Threshold: " UINTX_FORMAT "(" STRDEDUP_PERCENT_FORMAT_NS "), Grow Threshold: " UINTX_FORMAT "(" STRDEDUP_PERCENT_FORMAT_NS ")",
 657             _resize_count, _table->_shrink_threshold, _shrink_load_factor * 100.0, _table->_grow_threshold, _grow_load_factor * 100.0);
 658   log.debug("    Rehash Count: " UINTX_FORMAT ", Rehash Threshold: " UINTX_FORMAT ", Hash Seed: 0x%x", _rehash_count, _rehash_threshold, _table->_hash_seed);
 659   log.debug("    Age Threshold: " UINTX_FORMAT, StringDeduplicationAgeThreshold);
 660 }