1 /*
   2  * Copyright (c) 2001, 2014, 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/javaClasses.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc_implementation/shared/gcTimer.hpp"
  29 #include "gc_implementation/shared/gcTraceTime.hpp"
  30 #include "gc_interface/collectedHeap.hpp"
  31 #include "gc_interface/collectedHeap.inline.hpp"
  32 #include "memory/referencePolicy.hpp"
  33 #include "memory/referenceProcessor.hpp"
  34 #include "oops/oop.inline.hpp"
  35 #include "runtime/java.hpp"
  36 #include "runtime/jniHandles.hpp"
  37 
  38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  39 
  40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  42 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  43 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  44 
  45 void referenceProcessor_init() {
  46   ReferenceProcessor::init_statics();
  47 }
  48 
  49 void ReferenceProcessor::init_statics() {
  50   // We need a monotonically non-decreasing time in ms but
  51   // os::javaTimeMillis() does not guarantee monotonicity.
  52   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  53 
  54   // Initialize the soft ref timestamp clock.
  55   _soft_ref_timestamp_clock = now;
  56   // Also update the soft ref clock in j.l.r.SoftReference
  57   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  58 
  59   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  60   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  61                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  62   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  63     vm_exit_during_initialization("Could not allocate reference policy object");
  64   }
  65   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  66             RefDiscoveryPolicy == ReferentBasedDiscovery,
  67             "Unrecognized RefDiscoveryPolicy");
  68   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  69 }
  70 
  71 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
  72 #ifdef ASSERT
  73   // Verify that we're not currently discovering refs
  74   assert(!_discovering_refs, "nested call?");
  75 
  76   if (check_no_refs) {
  77     // Verify that the discovered lists are empty
  78     verify_no_references_recorded();
  79   }
  80 #endif // ASSERT
  81 
  82   // Someone could have modified the value of the static
  83   // field in the j.l.r.SoftReference class that holds the
  84   // soft reference timestamp clock using reflection or
  85   // Unsafe between GCs. Unconditionally update the static
  86   // field in ReferenceProcessor here so that we use the new
  87   // value during reference discovery.
  88 
  89   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  90   _discovering_refs = true;
  91 }
  92 
  93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  94                                        bool      mt_processing,
  95                                        uint      mt_processing_degree,
  96                                        bool      mt_discovery,
  97                                        uint      mt_discovery_degree,
  98                                        bool      atomic_discovery,
  99                                        BoolObjectClosure* is_alive_non_header)  :
 100   _discovering_refs(false),
 101   _enqueuing_is_done(false),
 102   _is_alive_non_header(is_alive_non_header),
 103   _processing_is_mt(mt_processing),
 104   _next_id(0)
 105 {
 106   _span = span;
 107   _discovery_is_atomic = atomic_discovery;
 108   _discovery_is_mt     = mt_discovery;
 109   _num_q               = MAX2(1U, mt_processing_degree);
 110   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 111   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 112             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 113 
 114   if (_discovered_refs == NULL) {
 115     vm_exit_during_initialization("Could not allocated RefProc Array");
 116   }
 117   _discoveredSoftRefs    = &_discovered_refs[0];
 118   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 119   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 120   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 121   _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
 122 
 123   // Initialize all entries to NULL
 124   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 125     _discovered_refs[i].set_head(NULL);
 126     _discovered_refs[i].set_length(0);
 127   }
 128 
 129   setup_policy(false /* default soft ref policy */);
 130 }
 131 
 132 #ifndef PRODUCT
 133 void ReferenceProcessor::verify_no_references_recorded() {
 134   guarantee(!_discovering_refs, "Discovering refs?");
 135   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 136     guarantee(_discovered_refs[i].is_empty(),
 137               "Found non-empty discovered list");
 138   }
 139 }
 140 #endif
 141 
 142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 143   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 144     if (UseCompressedOops) {
 145       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 146     } else {
 147       f->do_oop((oop*)_discovered_refs[i].adr_head());
 148     }
 149   }
 150 }
 151 
 152 void ReferenceProcessor::update_soft_ref_master_clock() {
 153   // Update (advance) the soft ref master clock field. This must be done
 154   // after processing the soft ref list.
 155 
 156   // We need a monotonically non-decreasing time in ms but
 157   // os::javaTimeMillis() does not guarantee monotonicity.
 158   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 159   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 160   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 161 
 162   NOT_PRODUCT(
 163   if (now < _soft_ref_timestamp_clock) {
 164     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 165             _soft_ref_timestamp_clock, now);
 166   }
 167   )
 168   // The values of now and _soft_ref_timestamp_clock are set using
 169   // javaTimeNanos(), which is guaranteed to be monotonically
 170   // non-decreasing provided the underlying platform provides such
 171   // a time source (and it is bug free).
 172   // In product mode, however, protect ourselves from non-monotonicity.
 173   if (now > _soft_ref_timestamp_clock) {
 174     _soft_ref_timestamp_clock = now;
 175     java_lang_ref_SoftReference::set_clock(now);
 176   }
 177   // Else leave clock stalled at its old value until time progresses
 178   // past clock value.
 179 }
 180 
 181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 182   size_t total = 0;
 183   for (uint i = 0; i < _max_num_q; ++i) {
 184     total += lists[i].length();
 185   }
 186   return total;
 187 }
 188 
 189 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 190   BoolObjectClosure*           is_alive,
 191   OopClosure*                  keep_alive,
 192   VoidClosure*                 complete_gc,
 193   AbstractRefProcTaskExecutor* task_executor,
 194   GCTimer*                     gc_timer,
 195   GCId                         gc_id) {
 196   NOT_PRODUCT(verify_ok_to_handle_reflists());
 197 
 198   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 199   // Stop treating discovered references specially.
 200   disable_discovery();
 201 
 202   // If discovery was concurrent, someone could have modified
 203   // the value of the static field in the j.l.r.SoftReference
 204   // class that holds the soft reference timestamp clock using
 205   // reflection or Unsafe between when discovery was enabled and
 206   // now. Unconditionally update the static field in ReferenceProcessor
 207   // here so that we use the new value during processing of the
 208   // discovered soft refs.
 209 
 210   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 211 
 212   bool trace_time = PrintGCDetails && PrintReferenceGC;
 213 
 214   // Soft references
 215   size_t soft_count = 0;
 216   {
 217     GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
 218     soft_count =
 219       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 220                                  is_alive, keep_alive, complete_gc, task_executor);
 221   }
 222 
 223   update_soft_ref_master_clock();
 224 
 225   // Weak references
 226   size_t weak_count = 0;
 227   {
 228     GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
 229     weak_count =
 230       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 231                                  is_alive, keep_alive, complete_gc, task_executor);
 232   }
 233 
 234   // Final references
 235   size_t final_count = 0;
 236   {
 237     GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
 238     final_count =
 239       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 240                                  is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   // Phantom references
 244   size_t phantom_count = 0;
 245   {
 246     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
 247     phantom_count =
 248       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 249                                  is_alive, keep_alive, complete_gc, task_executor);
 250 
 251     // Process cleaners, but include them in phantom statistics.  We expect
 252     // Cleaner references to be temporary, and don't want to deal with
 253     // possible incompatibilities arising from making it more visible.
 254     phantom_count +=
 255       process_discovered_reflist(_discoveredCleanerRefs, NULL, false,
 256                                  is_alive, keep_alive, complete_gc, task_executor);
 257   }
 258 
 259   // Weak global JNI references. It would make more sense (semantically) to
 260   // traverse these simultaneously with the regular weak references above, but
 261   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 262   // thus use JNI weak references to circumvent the phantom references and
 263   // resurrect a "post-mortem" object.
 264   {
 265     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
 266     if (task_executor != NULL) {
 267       task_executor->set_single_threaded_mode();
 268     }
 269     process_phaseJNI(is_alive, keep_alive, complete_gc);
 270   }
 271 
 272   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 273 }
 274 
 275 #ifndef PRODUCT
 276 // Calculate the number of jni handles.
 277 uint ReferenceProcessor::count_jni_refs() {
 278   class AlwaysAliveClosure: public BoolObjectClosure {
 279   public:
 280     virtual bool do_object_b(oop obj) { return true; }
 281   };
 282 
 283   class CountHandleClosure: public OopClosure {
 284   private:
 285     int _count;
 286   public:
 287     CountHandleClosure(): _count(0) {}
 288     void do_oop(oop* unused)       { _count++; }
 289     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 290     int count() { return _count; }
 291   };
 292   CountHandleClosure global_handle_count;
 293   AlwaysAliveClosure always_alive;
 294   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 295   return global_handle_count.count();
 296 }
 297 #endif
 298 
 299 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 300                                           OopClosure*        keep_alive,
 301                                           VoidClosure*       complete_gc) {
 302 #ifndef PRODUCT
 303   if (PrintGCDetails && PrintReferenceGC) {
 304     unsigned int count = count_jni_refs();
 305     gclog_or_tty->print(", %u refs", count);
 306   }
 307 #endif
 308   JNIHandles::weak_oops_do(is_alive, keep_alive);
 309   complete_gc->do_void();
 310 }
 311 
 312 
 313 template <class T>
 314 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 315                                    AbstractRefProcTaskExecutor* task_executor) {
 316 
 317   // Remember old value of pending references list
 318   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 319   T old_pending_list_value = *pending_list_addr;
 320 
 321   // Enqueue references that are not made active again, and
 322   // clear the decks for the next collection (cycle).
 323   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 324   // Do the post-barrier on pending_list_addr missed in
 325   // enqueue_discovered_reflist.
 326   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 327 
 328   // Stop treating discovered references specially.
 329   ref->disable_discovery();
 330 
 331   // Return true if new pending references were added
 332   return old_pending_list_value != *pending_list_addr;
 333 }
 334 
 335 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 336   NOT_PRODUCT(verify_ok_to_handle_reflists());
 337   if (UseCompressedOops) {
 338     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 339   } else {
 340     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 341   }
 342 }
 343 
 344 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 345                                                     HeapWord* pending_list_addr) {
 346   // Given a list of refs linked through the "discovered" field
 347   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 348   // thus distinguishing them from active References, then
 349   // prepend them to the pending list.
 350   //
 351   // The Java threads will see the Reference objects linked together through
 352   // the discovered field. Instead of trying to do the write barrier updates
 353   // in all places in the reference processor where we manipulate the discovered
 354   // field we make sure to do the barrier here where we anyway iterate through
 355   // all linked Reference objects. Note that it is important to not dirty any
 356   // cards during reference processing since this will cause card table
 357   // verification to fail for G1.
 358   //
 359   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 360   // the "next" field is used to chain the pending list, not the discovered
 361   // field.
 362   if (TraceReferenceGC && PrintGCDetails) {
 363     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 364                            INTPTR_FORMAT, (address)refs_list.head());
 365   }
 366 
 367   oop obj = NULL;
 368   oop next_d = refs_list.head();
 369   if (pending_list_uses_discovered_field()) { // New behavior
 370     // Walk down the list, self-looping the next field
 371     // so that the References are not considered active.
 372     while (obj != next_d) {
 373       obj = next_d;
 374       assert(obj->is_instanceRef(), "should be reference object");
 375       next_d = java_lang_ref_Reference::discovered(obj);
 376       if (TraceReferenceGC && PrintGCDetails) {
 377         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 378                                (void *)obj, (void *)next_d);
 379       }
 380       assert(java_lang_ref_Reference::next(obj) == NULL,
 381              "Reference not active; should not be discovered");
 382       // Self-loop next, so as to make Ref not active.
 383       java_lang_ref_Reference::set_next_raw(obj, obj);
 384       if (next_d != obj) {
 385         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 386       } else {
 387         // This is the last object.
 388         // Swap refs_list into pending_list_addr and
 389         // set obj's discovered to what we read from pending_list_addr.
 390         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 391         // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 392         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 393         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 394       }
 395     }
 396   } else { // Old behavior
 397     // Walk down the list, copying the discovered field into
 398     // the next field and clearing the discovered field.
 399     while (obj != next_d) {
 400       obj = next_d;
 401       assert(obj->is_instanceRef(), "should be reference object");
 402       next_d = java_lang_ref_Reference::discovered(obj);
 403       if (TraceReferenceGC && PrintGCDetails) {
 404         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 405                                (void *)obj, (void *)next_d);
 406       }
 407       assert(java_lang_ref_Reference::next(obj) == NULL,
 408              "The reference should not be enqueued");
 409       if (next_d == obj) {  // obj is last
 410         // Swap refs_list into pending_list_addr and
 411         // set obj's next to what we read from pending_list_addr.
 412         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 413         // Need oop_check on pending_list_addr above;
 414         // see special oop-check code at the end of
 415         // enqueue_discovered_reflists() further below.
 416         if (old == NULL) {
 417           // obj should be made to point to itself, since
 418           // pending list was empty.
 419           java_lang_ref_Reference::set_next(obj, obj);
 420         } else {
 421           java_lang_ref_Reference::set_next(obj, old);
 422         }
 423       } else {
 424         java_lang_ref_Reference::set_next(obj, next_d);
 425       }
 426       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 427     }
 428   }
 429 }
 430 
 431 // Parallel enqueue task
 432 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 433 public:
 434   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 435                      DiscoveredList      discovered_refs[],
 436                      HeapWord*           pending_list_addr,
 437                      int                 n_queues)
 438     : EnqueueTask(ref_processor, discovered_refs,
 439                   pending_list_addr, n_queues)
 440   { }
 441 
 442   virtual void work(unsigned int work_id) {
 443     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 444     // Simplest first cut: static partitioning.
 445     int index = work_id;
 446     // The increment on "index" must correspond to the maximum number of queues
 447     // (n_queues) with which that ReferenceProcessor was created.  That
 448     // is because of the "clever" way the discovered references lists were
 449     // allocated and are indexed into.
 450     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 451     for (int j = 0;
 452          j < ReferenceProcessor::number_of_subclasses_of_ref();
 453          j++, index += _n_queues) {
 454       _ref_processor.enqueue_discovered_reflist(
 455         _refs_lists[index], _pending_list_addr);
 456       _refs_lists[index].set_head(NULL);
 457       _refs_lists[index].set_length(0);
 458     }
 459   }
 460 };
 461 
 462 // Enqueue references that are not made active again
 463 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 464   AbstractRefProcTaskExecutor* task_executor) {
 465   if (_processing_is_mt && task_executor != NULL) {
 466     // Parallel code
 467     RefProcEnqueueTask tsk(*this, _discovered_refs,
 468                            pending_list_addr, _max_num_q);
 469     task_executor->execute(tsk);
 470   } else {
 471     // Serial code: call the parent class's implementation
 472     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 473       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 474       _discovered_refs[i].set_head(NULL);
 475       _discovered_refs[i].set_length(0);
 476     }
 477   }
 478 }
 479 
 480 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 481   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 482   oop discovered = java_lang_ref_Reference::discovered(_ref);
 483   assert(_discovered_addr && discovered->is_oop_or_null(),
 484          err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
 485   _next = discovered;
 486   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 487   _referent = java_lang_ref_Reference::referent(_ref);
 488   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 489          "Wrong oop found in java.lang.Reference object");
 490   assert(allow_null_referent ?
 491              _referent->is_oop_or_null()
 492            : _referent->is_oop(),
 493          err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
 494                  (allow_null_referent ? " or NULL" : ""),
 495                  p2i(_referent)));
 496 }
 497 
 498 void DiscoveredListIterator::remove() {
 499   assert(_ref->is_oop(), "Dropping a bad reference");
 500   oop_store_raw(_discovered_addr, NULL);
 501 
 502   // First _prev_next ref actually points into DiscoveredList (gross).
 503   oop new_next;
 504   if (_next == _ref) {
 505     // At the end of the list, we should make _prev point to itself.
 506     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 507     // and _prev will be NULL.
 508     new_next = _prev;
 509   } else {
 510     new_next = _next;
 511   }
 512   // Remove Reference object from discovered list. Note that G1 does not need a
 513   // pre-barrier here because we know the Reference has already been found/marked,
 514   // that's how it ended up in the discovered list in the first place.
 515   oop_store_raw(_prev_next, new_next);
 516   NOT_PRODUCT(_removed++);
 517   _refs_list.dec_length(1);
 518 }
 519 
 520 // Make the Reference object active again.
 521 void DiscoveredListIterator::make_active() {
 522   // The pre barrier for G1 is probably just needed for the old
 523   // reference processing behavior. Should we guard this with
 524   // ReferenceProcessor::pending_list_uses_discovered_field() ?
 525   if (UseG1GC) {
 526     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 527     if (UseCompressedOops) {
 528       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 529     } else {
 530       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 531     }
 532   }
 533   java_lang_ref_Reference::set_next_raw(_ref, NULL);
 534 }
 535 
 536 void DiscoveredListIterator::clear_referent() {
 537   oop_store_raw(_referent_addr, NULL);
 538 }
 539 
 540 // NOTE: process_phase*() are largely similar, and at a high level
 541 // merely iterate over the extant list applying a predicate to
 542 // each of its elements and possibly removing that element from the
 543 // list and applying some further closures to that element.
 544 // We should consider the possibility of replacing these
 545 // process_phase*() methods by abstracting them into
 546 // a single general iterator invocation that receives appropriate
 547 // closures that accomplish this work.
 548 
 549 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 550 // referents are not alive, but that should be kept alive for policy reasons.
 551 // Keep alive the transitive closure of all such referents.
 552 void
 553 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 554                                    ReferencePolicy*   policy,
 555                                    BoolObjectClosure* is_alive,
 556                                    OopClosure*        keep_alive,
 557                                    VoidClosure*       complete_gc) {
 558   assert(policy != NULL, "Must have a non-NULL policy");
 559   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 560   // Decide which softly reachable refs should be kept alive.
 561   while (iter.has_next()) {
 562     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 563     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 564     if (referent_is_dead &&
 565         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 566       if (TraceReferenceGC) {
 567         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 568                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 569       }
 570       // Remove Reference object from list
 571       iter.remove();
 572       // Make the Reference object active again
 573       iter.make_active();
 574       // keep the referent around
 575       iter.make_referent_alive();
 576       iter.move_to_next();
 577     } else {
 578       iter.next();
 579     }
 580   }
 581   // Close the reachable set
 582   complete_gc->do_void();
 583   NOT_PRODUCT(
 584     if (PrintGCDetails && TraceReferenceGC) {
 585       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 586         "discovered Refs by policy, from list " INTPTR_FORMAT,
 587         iter.removed(), iter.processed(), (address)refs_list.head());
 588     }
 589   )
 590 }
 591 
 592 // Traverse the list and remove any Refs that are not active, or
 593 // whose referents are either alive or NULL.
 594 void
 595 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 596                              BoolObjectClosure* is_alive,
 597                              OopClosure*        keep_alive) {
 598   assert(discovery_is_atomic(), "Error");
 599   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 600   while (iter.has_next()) {
 601     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 602     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 603     assert(next == NULL, "Should not discover inactive Reference");
 604     if (iter.is_referent_alive()) {
 605       if (TraceReferenceGC) {
 606         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 607                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 608       }
 609       // The referent is reachable after all.
 610       // Remove Reference object from list.
 611       iter.remove();
 612       // Update the referent pointer as necessary: Note that this
 613       // should not entail any recursive marking because the
 614       // referent must already have been traversed.
 615       iter.make_referent_alive();
 616       iter.move_to_next();
 617     } else {
 618       iter.next();
 619     }
 620   }
 621   NOT_PRODUCT(
 622     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 623       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 624         "Refs in discovered list " INTPTR_FORMAT,
 625         iter.removed(), iter.processed(), (address)refs_list.head());
 626     }
 627   )
 628 }
 629 
 630 void
 631 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 632                                                   BoolObjectClosure* is_alive,
 633                                                   OopClosure*        keep_alive,
 634                                                   VoidClosure*       complete_gc) {
 635   assert(!discovery_is_atomic(), "Error");
 636   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 637   while (iter.has_next()) {
 638     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 639     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 640     oop next = java_lang_ref_Reference::next(iter.obj());
 641     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 642          next != NULL)) {
 643       assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
 644       // Remove Reference object from list
 645       iter.remove();
 646       // Trace the cohorts
 647       iter.make_referent_alive();
 648       if (UseCompressedOops) {
 649         keep_alive->do_oop((narrowOop*)next_addr);
 650       } else {
 651         keep_alive->do_oop((oop*)next_addr);
 652       }
 653       iter.move_to_next();
 654     } else {
 655       iter.next();
 656     }
 657   }
 658   // Now close the newly reachable set
 659   complete_gc->do_void();
 660   NOT_PRODUCT(
 661     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 662       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 663         "Refs in discovered list " INTPTR_FORMAT,
 664         iter.removed(), iter.processed(), (address)refs_list.head());
 665     }
 666   )
 667 }
 668 
 669 // Traverse the list and process the referents, by either
 670 // clearing them or keeping them (and their reachable
 671 // closure) alive.
 672 void
 673 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 674                                    bool               clear_referent,
 675                                    BoolObjectClosure* is_alive,
 676                                    OopClosure*        keep_alive,
 677                                    VoidClosure*       complete_gc) {
 678   ResourceMark rm;
 679   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 680   while (iter.has_next()) {
 681     iter.update_discovered();
 682     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 683     if (clear_referent) {
 684       // NULL out referent pointer
 685       iter.clear_referent();
 686     } else {
 687       // keep the referent around
 688       iter.make_referent_alive();
 689     }
 690     if (TraceReferenceGC) {
 691       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 692                              clear_referent ? "cleared " : "",
 693                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 694     }
 695     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 696     iter.next();
 697   }
 698   // Remember to update the next pointer of the last ref.
 699   iter.update_discovered();
 700   // Close the reachable set
 701   complete_gc->do_void();
 702 }
 703 
 704 void
 705 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 706   oop obj = NULL;
 707   oop next = refs_list.head();
 708   while (next != obj) {
 709     obj = next;
 710     next = java_lang_ref_Reference::discovered(obj);
 711     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 712   }
 713   refs_list.set_head(NULL);
 714   refs_list.set_length(0);
 715 }
 716 
 717 void
 718 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 719   clear_discovered_references(refs_list);
 720 }
 721 
 722 void ReferenceProcessor::abandon_partial_discovery() {
 723   // loop over the lists
 724   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 725     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 726       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 727     }
 728     abandon_partial_discovered_list(_discovered_refs[i]);
 729   }
 730 }
 731 
 732 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 733 public:
 734   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 735                     DiscoveredList      refs_lists[],
 736                     ReferencePolicy*    policy,
 737                     bool                marks_oops_alive)
 738     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 739       _policy(policy)
 740   { }
 741   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 742                     OopClosure& keep_alive,
 743                     VoidClosure& complete_gc)
 744   {
 745     Thread* thr = Thread::current();
 746     int refs_list_index = ((WorkerThread*)thr)->id();
 747     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 748                                   &is_alive, &keep_alive, &complete_gc);
 749   }
 750 private:
 751   ReferencePolicy* _policy;
 752 };
 753 
 754 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 755 public:
 756   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 757                     DiscoveredList      refs_lists[],
 758                     bool                marks_oops_alive)
 759     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 760   { }
 761   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 762                     OopClosure& keep_alive,
 763                     VoidClosure& complete_gc)
 764   {
 765     _ref_processor.process_phase2(_refs_lists[i],
 766                                   &is_alive, &keep_alive, &complete_gc);
 767   }
 768 };
 769 
 770 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 771 public:
 772   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 773                     DiscoveredList      refs_lists[],
 774                     bool                clear_referent,
 775                     bool                marks_oops_alive)
 776     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 777       _clear_referent(clear_referent)
 778   { }
 779   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 780                     OopClosure& keep_alive,
 781                     VoidClosure& complete_gc)
 782   {
 783     // Don't use "refs_list_index" calculated in this way because
 784     // balance_queues() has moved the Ref's into the first n queues.
 785     // Thread* thr = Thread::current();
 786     // int refs_list_index = ((WorkerThread*)thr)->id();
 787     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 788     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 789                                   &is_alive, &keep_alive, &complete_gc);
 790   }
 791 private:
 792   bool _clear_referent;
 793 };
 794 
 795 // Balances reference queues.
 796 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 797 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 798 // corresponding to the active workers will be processed.
 799 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 800 {
 801   // calculate total length
 802   size_t total_refs = 0;
 803   if (TraceReferenceGC && PrintGCDetails) {
 804     gclog_or_tty->print_cr("\nBalance ref_lists ");
 805   }
 806 
 807   for (uint i = 0; i < _max_num_q; ++i) {
 808     total_refs += ref_lists[i].length();
 809     if (TraceReferenceGC && PrintGCDetails) {
 810       gclog_or_tty->print("%d ", ref_lists[i].length());
 811     }
 812   }
 813   if (TraceReferenceGC && PrintGCDetails) {
 814     gclog_or_tty->print_cr(" = %d", total_refs);
 815   }
 816   size_t avg_refs = total_refs / _num_q + 1;
 817   uint to_idx = 0;
 818   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 819     bool move_all = false;
 820     if (from_idx >= _num_q) {
 821       move_all = ref_lists[from_idx].length() > 0;
 822     }
 823     while ((ref_lists[from_idx].length() > avg_refs) ||
 824            move_all) {
 825       assert(to_idx < _num_q, "Sanity Check!");
 826       if (ref_lists[to_idx].length() < avg_refs) {
 827         // move superfluous refs
 828         size_t refs_to_move;
 829         // Move all the Ref's if the from queue will not be processed.
 830         if (move_all) {
 831           refs_to_move = MIN2(ref_lists[from_idx].length(),
 832                               avg_refs - ref_lists[to_idx].length());
 833         } else {
 834           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 835                               avg_refs - ref_lists[to_idx].length());
 836         }
 837 
 838         assert(refs_to_move > 0, "otherwise the code below will fail");
 839 
 840         oop move_head = ref_lists[from_idx].head();
 841         oop move_tail = move_head;
 842         oop new_head  = move_head;
 843         // find an element to split the list on
 844         for (size_t j = 0; j < refs_to_move; ++j) {
 845           move_tail = new_head;
 846           new_head = java_lang_ref_Reference::discovered(new_head);
 847         }
 848 
 849         // Add the chain to the to list.
 850         if (ref_lists[to_idx].head() == NULL) {
 851           // to list is empty. Make a loop at the end.
 852           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 853         } else {
 854           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 855         }
 856         ref_lists[to_idx].set_head(move_head);
 857         ref_lists[to_idx].inc_length(refs_to_move);
 858 
 859         // Remove the chain from the from list.
 860         if (move_tail == new_head) {
 861           // We found the end of the from list.
 862           ref_lists[from_idx].set_head(NULL);
 863         } else {
 864           ref_lists[from_idx].set_head(new_head);
 865         }
 866         ref_lists[from_idx].dec_length(refs_to_move);
 867         if (ref_lists[from_idx].length() == 0) {
 868           break;
 869         }
 870       } else {
 871         to_idx = (to_idx + 1) % _num_q;
 872       }
 873     }
 874   }
 875 #ifdef ASSERT
 876   size_t balanced_total_refs = 0;
 877   for (uint i = 0; i < _max_num_q; ++i) {
 878     balanced_total_refs += ref_lists[i].length();
 879     if (TraceReferenceGC && PrintGCDetails) {
 880       gclog_or_tty->print("%d ", ref_lists[i].length());
 881     }
 882   }
 883   if (TraceReferenceGC && PrintGCDetails) {
 884     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 885     gclog_or_tty->flush();
 886   }
 887   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 888 #endif
 889 }
 890 
 891 void ReferenceProcessor::balance_all_queues() {
 892   balance_queues(_discoveredSoftRefs);
 893   balance_queues(_discoveredWeakRefs);
 894   balance_queues(_discoveredFinalRefs);
 895   balance_queues(_discoveredPhantomRefs);
 896   balance_queues(_discoveredCleanerRefs);
 897 }
 898 
 899 size_t
 900 ReferenceProcessor::process_discovered_reflist(
 901   DiscoveredList               refs_lists[],
 902   ReferencePolicy*             policy,
 903   bool                         clear_referent,
 904   BoolObjectClosure*           is_alive,
 905   OopClosure*                  keep_alive,
 906   VoidClosure*                 complete_gc,
 907   AbstractRefProcTaskExecutor* task_executor)
 908 {
 909   bool mt_processing = task_executor != NULL && _processing_is_mt;
 910   // If discovery used MT and a dynamic number of GC threads, then
 911   // the queues must be balanced for correctness if fewer than the
 912   // maximum number of queues were used.  The number of queue used
 913   // during discovery may be different than the number to be used
 914   // for processing so don't depend of _num_q < _max_num_q as part
 915   // of the test.
 916   bool must_balance = _discovery_is_mt;
 917 
 918   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 919       must_balance) {
 920     balance_queues(refs_lists);
 921   }
 922 
 923   size_t total_list_count = total_count(refs_lists);
 924 
 925   if (PrintReferenceGC && PrintGCDetails) {
 926     gclog_or_tty->print(", %u refs", total_list_count);
 927   }
 928 
 929   // Phase 1 (soft refs only):
 930   // . Traverse the list and remove any SoftReferences whose
 931   //   referents are not alive, but that should be kept alive for
 932   //   policy reasons. Keep alive the transitive closure of all
 933   //   such referents.
 934   if (policy != NULL) {
 935     if (mt_processing) {
 936       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 937       task_executor->execute(phase1);
 938     } else {
 939       for (uint i = 0; i < _max_num_q; i++) {
 940         process_phase1(refs_lists[i], policy,
 941                        is_alive, keep_alive, complete_gc);
 942       }
 943     }
 944   } else { // policy == NULL
 945     assert(refs_lists != _discoveredSoftRefs,
 946            "Policy must be specified for soft references.");
 947   }
 948 
 949   // Phase 2:
 950   // . Traverse the list and remove any refs whose referents are alive.
 951   if (mt_processing) {
 952     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 953     task_executor->execute(phase2);
 954   } else {
 955     for (uint i = 0; i < _max_num_q; i++) {
 956       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 957     }
 958   }
 959 
 960   // Phase 3:
 961   // . Traverse the list and process referents as appropriate.
 962   if (mt_processing) {
 963     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 964     task_executor->execute(phase3);
 965   } else {
 966     for (uint i = 0; i < _max_num_q; i++) {
 967       process_phase3(refs_lists[i], clear_referent,
 968                      is_alive, keep_alive, complete_gc);
 969     }
 970   }
 971 
 972   return total_list_count;
 973 }
 974 
 975 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 976   uint id = 0;
 977   // Determine the queue index to use for this object.
 978   if (_discovery_is_mt) {
 979     // During a multi-threaded discovery phase,
 980     // each thread saves to its "own" list.
 981     Thread* thr = Thread::current();
 982     id = thr->as_Worker_thread()->id();
 983   } else {
 984     // single-threaded discovery, we save in round-robin
 985     // fashion to each of the lists.
 986     if (_processing_is_mt) {
 987       id = next_id();
 988     }
 989   }
 990   assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
 991 
 992   // Get the discovered queue to which we will add
 993   DiscoveredList* list = NULL;
 994   switch (rt) {
 995     case REF_OTHER:
 996       // Unknown reference type, no special treatment
 997       break;
 998     case REF_SOFT:
 999       list = &_discoveredSoftRefs[id];
1000       break;
1001     case REF_WEAK:
1002       list = &_discoveredWeakRefs[id];
1003       break;
1004     case REF_FINAL:
1005       list = &_discoveredFinalRefs[id];
1006       break;
1007     case REF_PHANTOM:
1008       list = &_discoveredPhantomRefs[id];
1009       break;
1010     case REF_CLEANER:
1011       list = &_discoveredCleanerRefs[id];
1012       break;
1013     case REF_NONE:
1014       // we should not reach here if we are an InstanceRefKlass
1015     default:
1016       ShouldNotReachHere();
1017   }
1018   if (TraceReferenceGC && PrintGCDetails) {
1019     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1020   }
1021   return list;
1022 }
1023 
1024 inline void
1025 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1026                                               oop             obj,
1027                                               HeapWord*       discovered_addr) {
1028   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1029   // First we must make sure this object is only enqueued once. CAS in a non null
1030   // discovered_addr.
1031   oop current_head = refs_list.head();
1032   // The last ref must have its discovered field pointing to itself.
1033   oop next_discovered = (current_head != NULL) ? current_head : obj;
1034 
1035   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1036                                                     NULL);
1037   if (retest == NULL) {
1038     // This thread just won the right to enqueue the object.
1039     // We have separate lists for enqueueing, so no synchronization
1040     // is necessary.
1041     refs_list.set_head(obj);
1042     refs_list.inc_length(1);
1043 
1044     if (TraceReferenceGC) {
1045       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1046                              (void *)obj, obj->klass()->internal_name());
1047     }
1048   } else {
1049     // If retest was non NULL, another thread beat us to it:
1050     // The reference has already been discovered...
1051     if (TraceReferenceGC) {
1052       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1053                              (void *)obj, obj->klass()->internal_name());
1054     }
1055   }
1056 }
1057 
1058 #ifndef PRODUCT
1059 // Non-atomic (i.e. concurrent) discovery might allow us
1060 // to observe j.l.References with NULL referents, being those
1061 // cleared concurrently by mutators during (or after) discovery.
1062 void ReferenceProcessor::verify_referent(oop obj) {
1063   bool da = discovery_is_atomic();
1064   oop referent = java_lang_ref_Reference::referent(obj);
1065   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1066          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1067                  INTPTR_FORMAT " during %satomic discovery ",
1068                  (void *)referent, (void *)obj, da ? "" : "non-"));
1069 }
1070 #endif
1071 
1072 // We mention two of several possible choices here:
1073 // #0: if the reference object is not in the "originating generation"
1074 //     (or part of the heap being collected, indicated by our "span"
1075 //     we don't treat it specially (i.e. we scan it as we would
1076 //     a normal oop, treating its references as strong references).
1077 //     This means that references can't be discovered unless their
1078 //     referent is also in the same span. This is the simplest,
1079 //     most "local" and most conservative approach, albeit one
1080 //     that may cause weak references to be enqueued least promptly.
1081 //     We call this choice the "ReferenceBasedDiscovery" policy.
1082 // #1: the reference object may be in any generation (span), but if
1083 //     the referent is in the generation (span) being currently collected
1084 //     then we can discover the reference object, provided
1085 //     the object has not already been discovered by
1086 //     a different concurrently running collector (as may be the
1087 //     case, for instance, if the reference object is in CMS and
1088 //     the referent in DefNewGeneration), and provided the processing
1089 //     of this reference object by the current collector will
1090 //     appear atomic to every other collector in the system.
1091 //     (Thus, for instance, a concurrent collector may not
1092 //     discover references in other generations even if the
1093 //     referent is in its own generation). This policy may,
1094 //     in certain cases, enqueue references somewhat sooner than
1095 //     might Policy #0 above, but at marginally increased cost
1096 //     and complexity in processing these references.
1097 //     We call this choice the "RefeferentBasedDiscovery" policy.
1098 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1099   // Make sure we are discovering refs (rather than processing discovered refs).
1100   if (!_discovering_refs || !RegisterReferences) {
1101     return false;
1102   }
1103   // We only discover active references.
1104   oop next = java_lang_ref_Reference::next(obj);
1105   if (next != NULL) {   // Ref is no longer active
1106     return false;
1107   }
1108 
1109   HeapWord* obj_addr = (HeapWord*)obj;
1110   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1111       !_span.contains(obj_addr)) {
1112     // Reference is not in the originating generation;
1113     // don't treat it specially (i.e. we want to scan it as a normal
1114     // object with strong references).
1115     return false;
1116   }
1117 
1118   // We only discover references whose referents are not (yet)
1119   // known to be strongly reachable.
1120   if (is_alive_non_header() != NULL) {
1121     verify_referent(obj);
1122     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1123       return false;  // referent is reachable
1124     }
1125   }
1126   if (rt == REF_SOFT) {
1127     // For soft refs we can decide now if these are not
1128     // current candidates for clearing, in which case we
1129     // can mark through them now, rather than delaying that
1130     // to the reference-processing phase. Since all current
1131     // time-stamp policies advance the soft-ref clock only
1132     // at a major collection cycle, this is always currently
1133     // accurate.
1134     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1135       return false;
1136     }
1137   }
1138 
1139   ResourceMark rm;      // Needed for tracing.
1140 
1141   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1142   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1143   assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1144   if (discovered != NULL) {
1145     // The reference has already been discovered...
1146     if (TraceReferenceGC) {
1147       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1148                              (void *)obj, obj->klass()->internal_name());
1149     }
1150     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1151       // assumes that an object is not processed twice;
1152       // if it's been already discovered it must be on another
1153       // generation's discovered list; so we won't discover it.
1154       return false;
1155     } else {
1156       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1157              "Unrecognized policy");
1158       // Check assumption that an object is not potentially
1159       // discovered twice except by concurrent collectors that potentially
1160       // trace the same Reference object twice.
1161       assert(UseConcMarkSweepGC || UseG1GC,
1162              "Only possible with a concurrent marking collector");
1163       return true;
1164     }
1165   }
1166 
1167   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1168     verify_referent(obj);
1169     // Discover if and only if EITHER:
1170     // .. reference is in our span, OR
1171     // .. we are an atomic collector and referent is in our span
1172     if (_span.contains(obj_addr) ||
1173         (discovery_is_atomic() &&
1174          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1175       // should_enqueue = true;
1176     } else {
1177       return false;
1178     }
1179   } else {
1180     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1181            _span.contains(obj_addr), "code inconsistency");
1182   }
1183 
1184   // Get the right type of discovered queue head.
1185   DiscoveredList* list = get_discovered_list(rt);
1186   if (list == NULL) {
1187     return false;   // nothing special needs to be done
1188   }
1189 
1190   if (_discovery_is_mt) {
1191     add_to_discovered_list_mt(*list, obj, discovered_addr);
1192   } else {
1193     // We do a raw store here: the field will be visited later when processing
1194     // the discovered references.
1195     oop current_head = list->head();
1196     // The last ref must have its discovered field pointing to itself.
1197     oop next_discovered = (current_head != NULL) ? current_head : obj;
1198 
1199     assert(discovered == NULL, "control point invariant");
1200     oop_store_raw(discovered_addr, next_discovered);
1201     list->set_head(obj);
1202     list->inc_length(1);
1203 
1204     if (TraceReferenceGC) {
1205       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1206                                 (void *)obj, obj->klass()->internal_name());
1207     }
1208   }
1209   assert(obj->is_oop(), "Discovered a bad reference");
1210   verify_referent(obj);
1211   return true;
1212 }
1213 
1214 // Preclean the discovered references by removing those
1215 // whose referents are alive, and by marking from those that
1216 // are not active. These lists can be handled here
1217 // in any order and, indeed, concurrently.
1218 void ReferenceProcessor::preclean_discovered_references(
1219   BoolObjectClosure* is_alive,
1220   OopClosure* keep_alive,
1221   VoidClosure* complete_gc,
1222   YieldClosure* yield,
1223   GCTimer* gc_timer,
1224   GCId     gc_id) {
1225 
1226   NOT_PRODUCT(verify_ok_to_handle_reflists());
1227 
1228   // Soft references
1229   {
1230     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1231               false, gc_timer, gc_id);
1232     for (uint i = 0; i < _max_num_q; i++) {
1233       if (yield->should_return()) {
1234         return;
1235       }
1236       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1237                                   keep_alive, complete_gc, yield);
1238     }
1239   }
1240 
1241   // Weak references
1242   {
1243     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1244               false, gc_timer, gc_id);
1245     for (uint i = 0; i < _max_num_q; i++) {
1246       if (yield->should_return()) {
1247         return;
1248       }
1249       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1250                                   keep_alive, complete_gc, yield);
1251     }
1252   }
1253 
1254   // Final references
1255   {
1256     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1257               false, gc_timer, gc_id);
1258     for (uint i = 0; i < _max_num_q; i++) {
1259       if (yield->should_return()) {
1260         return;
1261       }
1262       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1263                                   keep_alive, complete_gc, yield);
1264     }
1265   }
1266 
1267   // Phantom references
1268   {
1269     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1270               false, gc_timer, gc_id);
1271     for (uint i = 0; i < _max_num_q; i++) {
1272       if (yield->should_return()) {
1273         return;
1274       }
1275       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1276                                   keep_alive, complete_gc, yield);
1277     }
1278 
1279     // Cleaner references.  Included in timing for phantom references.  We
1280     // expect Cleaner references to be temporary, and don't want to deal with
1281     // possible incompatibilities arising from making it more visible.
1282     for (uint i = 0; i < _max_num_q; i++) {
1283       if (yield->should_return()) {
1284         return;
1285       }
1286       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1287                                   keep_alive, complete_gc, yield);
1288     }
1289   }
1290 }
1291 
1292 // Walk the given discovered ref list, and remove all reference objects
1293 // whose referents are still alive, whose referents are NULL or which
1294 // are not active (have a non-NULL next field). NOTE: When we are
1295 // thus precleaning the ref lists (which happens single-threaded today),
1296 // we do not disable refs discovery to honor the correct semantics of
1297 // java.lang.Reference. As a result, we need to be careful below
1298 // that ref removal steps interleave safely with ref discovery steps
1299 // (in this thread).
1300 void
1301 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1302                                                 BoolObjectClosure* is_alive,
1303                                                 OopClosure*        keep_alive,
1304                                                 VoidClosure*       complete_gc,
1305                                                 YieldClosure*      yield) {
1306   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1307   while (iter.has_next()) {
1308     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1309     oop obj = iter.obj();
1310     oop next = java_lang_ref_Reference::next(obj);
1311     if (iter.referent() == NULL || iter.is_referent_alive() ||
1312         next != NULL) {
1313       // The referent has been cleared, or is alive, or the Reference is not
1314       // active; we need to trace and mark its cohort.
1315       if (TraceReferenceGC) {
1316         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1317                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1318       }
1319       // Remove Reference object from list
1320       iter.remove();
1321       // Keep alive its cohort.
1322       iter.make_referent_alive();
1323       if (UseCompressedOops) {
1324         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1325         keep_alive->do_oop(next_addr);
1326       } else {
1327         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1328         keep_alive->do_oop(next_addr);
1329       }
1330       iter.move_to_next();
1331     } else {
1332       iter.next();
1333     }
1334   }
1335   // Close the reachable set
1336   complete_gc->do_void();
1337 
1338   NOT_PRODUCT(
1339     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1340       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1341         "Refs in discovered list " INTPTR_FORMAT,
1342         iter.removed(), iter.processed(), (address)refs_list.head());
1343     }
1344   )
1345 }
1346 
1347 const char* ReferenceProcessor::list_name(uint i) {
1348    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1349           "Out of bounds index");
1350 
1351    int j = i / _max_num_q;
1352    switch (j) {
1353      case 0: return "SoftRef";
1354      case 1: return "WeakRef";
1355      case 2: return "FinalRef";
1356      case 3: return "PhantomRef";
1357      case 4: return "CleanerRef";
1358    }
1359    ShouldNotReachHere();
1360    return NULL;
1361 }
1362 
1363 #ifndef PRODUCT
1364 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1365   // empty for now
1366 }
1367 #endif
1368 
1369 #ifndef PRODUCT
1370 void ReferenceProcessor::clear_discovered_references() {
1371   guarantee(!_discovering_refs, "Discovering refs?");
1372   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1373     clear_discovered_references(_discovered_refs[i]);
1374   }
1375 }
1376 
1377 #endif // PRODUCT