1 /*
   2  * Copyright (c) 2001, 2013, 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 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  40 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  41 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  42 
  43 void referenceProcessor_init() {
  44   ReferenceProcessor::init_statics();
  45 }
  46 
  47 void ReferenceProcessor::init_statics() {
  48   // We need a monotonically non-deccreasing time in ms but
  49   // os::javaTimeMillis() does not guarantee monotonicity.
  50   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  51 
  52   // Initialize the soft ref timestamp clock.
  53   _soft_ref_timestamp_clock = now;
  54   // Also update the soft ref clock in j.l.r.SoftReference
  55   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  56 
  57   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  58   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  59                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  60   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  61     vm_exit_during_initialization("Could not allocate reference policy object");
  62   }
  63   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  64             RefDiscoveryPolicy == ReferentBasedDiscovery,
  65             "Unrecongnized RefDiscoveryPolicy");
  66   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  67 }
  68 
  69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  70 #ifdef ASSERT
  71   // Verify that we're not currently discovering refs
  72   assert(!verify_disabled || !_discovering_refs, "nested call?");
  73 
  74   if (check_no_refs) {
  75     // Verify that the discovered lists are empty
  76     verify_no_references_recorded();
  77   }
  78 #endif // ASSERT
  79 
  80   // Someone could have modified the value of the static
  81   // field in the j.l.r.SoftReference class that holds the
  82   // soft reference timestamp clock using reflection or
  83   // Unsafe between GCs. Unconditionally update the static
  84   // field in ReferenceProcessor here so that we use the new
  85   // value during reference discovery.
  86 
  87   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  88   _discovering_refs = true;
  89 }
  90 
  91 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  92                                        bool      mt_processing,
  93                                        uint      mt_processing_degree,
  94                                        bool      mt_discovery,
  95                                        uint      mt_discovery_degree,
  96                                        bool      atomic_discovery,
  97                                        BoolObjectClosure* is_alive_non_header,
  98                                        bool      discovered_list_needs_barrier)  :
  99   _discovering_refs(false),
 100   _enqueuing_is_done(false),
 101   _is_alive_non_header(is_alive_non_header),
 102   _discovered_list_needs_barrier(discovered_list_needs_barrier),
 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 
 122   // Initialize all entries to NULL
 123   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 124     _discovered_refs[i].set_head(NULL);
 125     _discovered_refs[i].set_length(0);
 126   }
 127 
 128   setup_policy(false /* default soft ref policy */);
 129 }
 130 
 131 #ifndef PRODUCT
 132 void ReferenceProcessor::verify_no_references_recorded() {
 133   guarantee(!_discovering_refs, "Discovering refs?");
 134   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 135     guarantee(_discovered_refs[i].is_empty(),
 136               "Found non-empty discovered list");
 137   }
 138 }
 139 #endif
 140 
 141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 142   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 143     if (UseCompressedOops) {
 144       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 145     } else {
 146       f->do_oop((oop*)_discovered_refs[i].adr_head());
 147     }
 148   }
 149 }
 150 
 151 void ReferenceProcessor::update_soft_ref_master_clock() {
 152   // Update (advance) the soft ref master clock field. This must be done
 153   // after processing the soft ref list.
 154 
 155   // We need a monotonically non-deccreasing time in ms but
 156   // os::javaTimeMillis() does not guarantee monotonicity.
 157   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 158   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 159   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 160 
 161   NOT_PRODUCT(
 162   if (now < _soft_ref_timestamp_clock) {
 163     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 164             _soft_ref_timestamp_clock, now);
 165   }
 166   )
 167   // The values of now and _soft_ref_timestamp_clock are set using
 168   // javaTimeNanos(), which is guaranteed to be monotonically
 169   // non-decreasing provided the underlying platform provides such
 170   // a time source (and it is bug free).
 171   // In product mode, however, protect ourselves from non-monotonicty.
 172   if (now > _soft_ref_timestamp_clock) {
 173     _soft_ref_timestamp_clock = now;
 174     java_lang_ref_SoftReference::set_clock(now);
 175   }
 176   // Else leave clock stalled at its old value until time progresses
 177   // past clock value.
 178 }
 179 
 180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 181   size_t total = 0;
 182   for (uint i = 0; i < _max_num_q; ++i) {
 183     total += lists[i].length();
 184   }
 185   return total;
 186 }
 187 
 188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 189   BoolObjectClosure*           is_alive,
 190   OopClosure*                  keep_alive,
 191   VoidClosure*                 complete_gc,
 192   AbstractRefProcTaskExecutor* task_executor,
 193   GCTimer*                     gc_timer) {
 194   NOT_PRODUCT(verify_ok_to_handle_reflists());
 195 
 196   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 197   // Stop treating discovered references specially.
 198   disable_discovery();
 199 
 200   // If discovery was concurrent, someone could have modified
 201   // the value of the static field in the j.l.r.SoftReference
 202   // class that holds the soft reference timestamp clock using
 203   // reflection or Unsafe between when discovery was enabled and
 204   // now. Unconditionally update the static field in ReferenceProcessor
 205   // here so that we use the new value during processing of the
 206   // discovered soft refs.
 207 
 208   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 209 
 210   bool trace_time = PrintGCDetails && PrintReferenceGC;
 211 
 212   // Soft references
 213   size_t soft_count = 0;
 214   {
 215     GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
 216     soft_count =
 217       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 218                                  is_alive, keep_alive, complete_gc, task_executor);
 219   }
 220 
 221   update_soft_ref_master_clock();
 222 
 223   // Weak references
 224   size_t weak_count = 0;
 225   {
 226     GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
 227     weak_count =
 228       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 229                                  is_alive, keep_alive, complete_gc, task_executor);
 230   }
 231 
 232   // Final references
 233   size_t final_count = 0;
 234   {
 235     GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
 236     final_count =
 237       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 238                                  is_alive, keep_alive, complete_gc, task_executor);
 239   }
 240 
 241   // Phantom references
 242   size_t phantom_count = 0;
 243   {
 244     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
 245     phantom_count =
 246       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 247                                  is_alive, keep_alive, complete_gc, task_executor);
 248   }
 249 
 250   // Weak global JNI references. It would make more sense (semantically) to
 251   // traverse these simultaneously with the regular weak references above, but
 252   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 253   // thus use JNI weak references to circumvent the phantom references and
 254   // resurrect a "post-mortem" object.
 255   {
 256     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 257     if (task_executor != NULL) {
 258       task_executor->set_single_threaded_mode();
 259     }
 260     process_phaseJNI(is_alive, keep_alive, complete_gc);
 261   }
 262 
 263   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 264 }
 265 
 266 #ifndef PRODUCT
 267 // Calculate the number of jni handles.
 268 uint ReferenceProcessor::count_jni_refs() {
 269   class AlwaysAliveClosure: public BoolObjectClosure {
 270   public:
 271     virtual bool do_object_b(oop obj) { return true; }
 272   };
 273 
 274   class CountHandleClosure: public OopClosure {
 275   private:
 276     int _count;
 277   public:
 278     CountHandleClosure(): _count(0) {}
 279     void do_oop(oop* unused)       { _count++; }
 280     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 281     int count() { return _count; }
 282   };
 283   CountHandleClosure global_handle_count;
 284   AlwaysAliveClosure always_alive;
 285   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 286   return global_handle_count.count();
 287 }
 288 #endif
 289 
 290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 291                                           OopClosure*        keep_alive,
 292                                           VoidClosure*       complete_gc) {
 293 #ifndef PRODUCT
 294   if (PrintGCDetails && PrintReferenceGC) {
 295     unsigned int count = count_jni_refs();
 296     gclog_or_tty->print(", %u refs", count);
 297   }
 298 #endif
 299   JNIHandles::weak_oops_do(is_alive, keep_alive);
 300   complete_gc->do_void();
 301 }
 302 
 303 
 304 template <class T>
 305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 306                                    AbstractRefProcTaskExecutor* task_executor) {
 307 
 308   // Remember old value of pending references list
 309   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 310   T old_pending_list_value = *pending_list_addr;
 311 
 312   // Enqueue references that are not made active again, and
 313   // clear the decks for the next collection (cycle).
 314   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 315   // Do the post-barrier on pending_list_addr missed in
 316   // enqueue_discovered_reflist.
 317   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 318 
 319   // Stop treating discovered references specially.
 320   ref->disable_discovery();
 321 
 322   // Return true if new pending references were added
 323   return old_pending_list_value != *pending_list_addr;
 324 }
 325 
 326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 327   NOT_PRODUCT(verify_ok_to_handle_reflists());
 328   if (UseCompressedOops) {
 329     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 330   } else {
 331     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 332   }
 333 }
 334 
 335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 336                                                     HeapWord* pending_list_addr) {
 337   // Given a list of refs linked through the "discovered" field
 338   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 339   // thus distinguishing them from active References, then
 340   // prepend them to the pending list.
 341   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 342   // the "next" field is used to chain the pending list, not the discovered
 343   // field.
 344 
 345   if (TraceReferenceGC && PrintGCDetails) {
 346     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 347                            INTPTR_FORMAT, (address)refs_list.head());
 348   }
 349 
 350   oop obj = NULL;
 351   oop next_d = refs_list.head();
 352   if (pending_list_uses_discovered_field()) { // New behaviour
 353     // Walk down the list, self-looping the next field
 354     // so that the References are not considered active.
 355     while (obj != next_d) {
 356       obj = next_d;
 357       assert(obj->is_instanceRef(), "should be reference object");
 358       next_d = java_lang_ref_Reference::discovered(obj);
 359       if (TraceReferenceGC && PrintGCDetails) {
 360         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 361                                (void *)obj, (void *)next_d);
 362       }
 363       assert(java_lang_ref_Reference::next(obj) == NULL,
 364              "Reference not active; should not be discovered");
 365       // Self-loop next, so as to make Ref not active.
 366       java_lang_ref_Reference::set_next_raw(obj, obj);
 367       if (next_d == obj) {  // obj is last
 368         // Swap refs_list into pendling_list_addr and
 369         // set obj's discovered to what we read from pending_list_addr.
 370         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 371         // Need post-barrier on pending_list_addr above;
 372         // see special post-barrier code at the end of
 373         // enqueue_discovered_reflists() further below.
 374         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 375         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 376       }
 377     }
 378   } else { // Old behaviour
 379     // Walk down the list, copying the discovered field into
 380     // the next field and clearing the discovered field.
 381     while (obj != next_d) {
 382       obj = next_d;
 383       assert(obj->is_instanceRef(), "should be reference object");
 384       next_d = java_lang_ref_Reference::discovered(obj);
 385       if (TraceReferenceGC && PrintGCDetails) {
 386         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 387                                (void *)obj, (void *)next_d);
 388       }
 389       assert(java_lang_ref_Reference::next(obj) == NULL,
 390              "The reference should not be enqueued");
 391       if (next_d == obj) {  // obj is last
 392         // Swap refs_list into pendling_list_addr and
 393         // set obj's next to what we read from pending_list_addr.
 394         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 395         // Need oop_check on pending_list_addr above;
 396         // see special oop-check code at the end of
 397         // enqueue_discovered_reflists() further below.
 398         if (old == NULL) {
 399           // obj should be made to point to itself, since
 400           // pending list was empty.
 401           java_lang_ref_Reference::set_next(obj, obj);
 402         } else {
 403           java_lang_ref_Reference::set_next(obj, old);
 404         }
 405       } else {
 406         java_lang_ref_Reference::set_next(obj, next_d);
 407       }
 408       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 409     }
 410   }
 411 }
 412 
 413 // Parallel enqueue task
 414 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 415 public:
 416   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 417                      DiscoveredList      discovered_refs[],
 418                      HeapWord*           pending_list_addr,
 419                      int                 n_queues)
 420     : EnqueueTask(ref_processor, discovered_refs,
 421                   pending_list_addr, n_queues)
 422   { }
 423 
 424   virtual void work(unsigned int work_id) {
 425     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 426     // Simplest first cut: static partitioning.
 427     int index = work_id;
 428     // The increment on "index" must correspond to the maximum number of queues
 429     // (n_queues) with which that ReferenceProcessor was created.  That
 430     // is because of the "clever" way the discovered references lists were
 431     // allocated and are indexed into.
 432     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 433     for (int j = 0;
 434          j < ReferenceProcessor::number_of_subclasses_of_ref();
 435          j++, index += _n_queues) {
 436       _ref_processor.enqueue_discovered_reflist(
 437         _refs_lists[index], _pending_list_addr);
 438       _refs_lists[index].set_head(NULL);
 439       _refs_lists[index].set_length(0);
 440     }
 441   }
 442 };
 443 
 444 // Enqueue references that are not made active again
 445 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 446   AbstractRefProcTaskExecutor* task_executor) {
 447   if (_processing_is_mt && task_executor != NULL) {
 448     // Parallel code
 449     RefProcEnqueueTask tsk(*this, _discovered_refs,
 450                            pending_list_addr, _max_num_q);
 451     task_executor->execute(tsk);
 452   } else {
 453     // Serial code: call the parent class's implementation
 454     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 455       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 456       _discovered_refs[i].set_head(NULL);
 457       _discovered_refs[i].set_length(0);
 458     }
 459   }
 460 }
 461 
 462 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 463   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 464   oop discovered = java_lang_ref_Reference::discovered(_ref);
 465   assert(_discovered_addr && discovered->is_oop_or_null(),
 466          "discovered field is bad");
 467   _next = discovered;
 468   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 469   _referent = java_lang_ref_Reference::referent(_ref);
 470   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 471          "Wrong oop found in java.lang.Reference object");
 472   assert(allow_null_referent ?
 473              _referent->is_oop_or_null()
 474            : _referent->is_oop(),
 475          "bad referent");
 476 }
 477 
 478 void DiscoveredListIterator::remove() {
 479   assert(_ref->is_oop(), "Dropping a bad reference");
 480   oop_store_raw(_discovered_addr, NULL);
 481 
 482   // First _prev_next ref actually points into DiscoveredList (gross).
 483   oop new_next;
 484   if (_next == _ref) {
 485     // At the end of the list, we should make _prev point to itself.
 486     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 487     // and _prev will be NULL.
 488     new_next = _prev;
 489   } else {
 490     new_next = _next;
 491   }
 492 
 493   if (UseCompressedOops) {
 494     // Remove Reference object from list.
 495     oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
 496   } else {
 497     // Remove Reference object from list.
 498     oopDesc::store_heap_oop((oop*)_prev_next, new_next);
 499   }
 500   NOT_PRODUCT(_removed++);
 501   _refs_list.dec_length(1);
 502 }
 503 
 504 // Make the Reference object active again.
 505 void DiscoveredListIterator::make_active() {
 506   // For G1 we don't want to use set_next - it
 507   // will dirty the card for the next field of
 508   // the reference object and will fail
 509   // CT verification.
 510   if (UseG1GC) {
 511     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 512     if (UseCompressedOops) {
 513       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 514     } else {
 515       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 516     }
 517     java_lang_ref_Reference::set_next_raw(_ref, NULL);
 518   } else {
 519     java_lang_ref_Reference::set_next(_ref, NULL);
 520   }
 521 }
 522 
 523 void DiscoveredListIterator::clear_referent() {
 524   oop_store_raw(_referent_addr, NULL);
 525 }
 526 
 527 // NOTE: process_phase*() are largely similar, and at a high level
 528 // merely iterate over the extant list applying a predicate to
 529 // each of its elements and possibly removing that element from the
 530 // list and applying some further closures to that element.
 531 // We should consider the possibility of replacing these
 532 // process_phase*() methods by abstracting them into
 533 // a single general iterator invocation that receives appropriate
 534 // closures that accomplish this work.
 535 
 536 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 537 // referents are not alive, but that should be kept alive for policy reasons.
 538 // Keep alive the transitive closure of all such referents.
 539 void
 540 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 541                                    ReferencePolicy*   policy,
 542                                    BoolObjectClosure* is_alive,
 543                                    OopClosure*        keep_alive,
 544                                    VoidClosure*       complete_gc) {
 545   assert(policy != NULL, "Must have a non-NULL policy");
 546   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 547   // Decide which softly reachable refs should be kept alive.
 548   while (iter.has_next()) {
 549     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 550     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 551     if (referent_is_dead &&
 552         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 553       if (TraceReferenceGC) {
 554         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 555                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 556       }
 557       // Remove Reference object from list
 558       iter.remove();
 559       // Make the Reference object active again
 560       iter.make_active();
 561       // keep the referent around
 562       iter.make_referent_alive();
 563       iter.move_to_next();
 564     } else {
 565       iter.next();
 566     }
 567   }
 568   // Close the reachable set
 569   complete_gc->do_void();
 570   NOT_PRODUCT(
 571     if (PrintGCDetails && TraceReferenceGC) {
 572       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 573         "discovered Refs by policy, from list " INTPTR_FORMAT,
 574         iter.removed(), iter.processed(), (address)refs_list.head());
 575     }
 576   )
 577 }
 578 
 579 // Traverse the list and remove any Refs that are not active, or
 580 // whose referents are either alive or NULL.
 581 void
 582 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 583                              BoolObjectClosure* is_alive,
 584                              OopClosure*        keep_alive) {
 585   assert(discovery_is_atomic(), "Error");
 586   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 587   while (iter.has_next()) {
 588     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 589     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 590     assert(next == NULL, "Should not discover inactive Reference");
 591     if (iter.is_referent_alive()) {
 592       if (TraceReferenceGC) {
 593         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 594                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 595       }
 596       // The referent is reachable after all.
 597       // Remove Reference object from list.
 598       iter.remove();
 599       // Update the referent pointer as necessary: Note that this
 600       // should not entail any recursive marking because the
 601       // referent must already have been traversed.
 602       iter.make_referent_alive();
 603       iter.move_to_next();
 604     } else {
 605       iter.next();
 606     }
 607   }
 608   NOT_PRODUCT(
 609     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 610       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 611         "Refs in discovered list " INTPTR_FORMAT,
 612         iter.removed(), iter.processed(), (address)refs_list.head());
 613     }
 614   )
 615 }
 616 
 617 void
 618 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 619                                                   BoolObjectClosure* is_alive,
 620                                                   OopClosure*        keep_alive,
 621                                                   VoidClosure*       complete_gc) {
 622   assert(!discovery_is_atomic(), "Error");
 623   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 624   while (iter.has_next()) {
 625     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 626     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 627     oop next = java_lang_ref_Reference::next(iter.obj());
 628     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 629          next != NULL)) {
 630       assert(next->is_oop_or_null(), "bad next field");
 631       // Remove Reference object from list
 632       iter.remove();
 633       // Trace the cohorts
 634       iter.make_referent_alive();
 635       if (UseCompressedOops) {
 636         keep_alive->do_oop((narrowOop*)next_addr);
 637       } else {
 638         keep_alive->do_oop((oop*)next_addr);
 639       }
 640       iter.move_to_next();
 641     } else {
 642       iter.next();
 643     }
 644   }
 645   // Now close the newly reachable set
 646   complete_gc->do_void();
 647   NOT_PRODUCT(
 648     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 649       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 650         "Refs in discovered list " INTPTR_FORMAT,
 651         iter.removed(), iter.processed(), (address)refs_list.head());
 652     }
 653   )
 654 }
 655 
 656 // Traverse the list and process the referents, by either
 657 // clearing them or keeping them (and their reachable
 658 // closure) alive.
 659 void
 660 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 661                                    bool               clear_referent,
 662                                    BoolObjectClosure* is_alive,
 663                                    OopClosure*        keep_alive,
 664                                    VoidClosure*       complete_gc) {
 665   ResourceMark rm;
 666   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 667   while (iter.has_next()) {
 668     iter.update_discovered();
 669     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 670     if (clear_referent) {
 671       // NULL out referent pointer
 672       iter.clear_referent();
 673     } else {
 674       // keep the referent around
 675       iter.make_referent_alive();
 676     }
 677     if (TraceReferenceGC) {
 678       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 679                              clear_referent ? "cleared " : "",
 680                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 681     }
 682     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 683     iter.next();
 684   }
 685   // Remember to update the next pointer of the last ref.
 686   iter.update_discovered();
 687   // Close the reachable set
 688   complete_gc->do_void();
 689 }
 690 
 691 void
 692 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 693   oop obj = NULL;
 694   oop next = refs_list.head();
 695   while (next != obj) {
 696     obj = next;
 697     next = java_lang_ref_Reference::discovered(obj);
 698     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 699   }
 700   refs_list.set_head(NULL);
 701   refs_list.set_length(0);
 702 }
 703 
 704 void
 705 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 706   clear_discovered_references(refs_list);
 707 }
 708 
 709 void ReferenceProcessor::abandon_partial_discovery() {
 710   // loop over the lists
 711   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 712     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 713       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 714     }
 715     abandon_partial_discovered_list(_discovered_refs[i]);
 716   }
 717 }
 718 
 719 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 720 public:
 721   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 722                     DiscoveredList      refs_lists[],
 723                     ReferencePolicy*    policy,
 724                     bool                marks_oops_alive)
 725     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 726       _policy(policy)
 727   { }
 728   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 729                     OopClosure& keep_alive,
 730                     VoidClosure& complete_gc)
 731   {
 732     Thread* thr = Thread::current();
 733     int refs_list_index = ((WorkerThread*)thr)->id();
 734     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 735                                   &is_alive, &keep_alive, &complete_gc);
 736   }
 737 private:
 738   ReferencePolicy* _policy;
 739 };
 740 
 741 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 742 public:
 743   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 744                     DiscoveredList      refs_lists[],
 745                     bool                marks_oops_alive)
 746     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 747   { }
 748   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 749                     OopClosure& keep_alive,
 750                     VoidClosure& complete_gc)
 751   {
 752     _ref_processor.process_phase2(_refs_lists[i],
 753                                   &is_alive, &keep_alive, &complete_gc);
 754   }
 755 };
 756 
 757 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 758 public:
 759   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 760                     DiscoveredList      refs_lists[],
 761                     bool                clear_referent,
 762                     bool                marks_oops_alive)
 763     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 764       _clear_referent(clear_referent)
 765   { }
 766   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 767                     OopClosure& keep_alive,
 768                     VoidClosure& complete_gc)
 769   {
 770     // Don't use "refs_list_index" calculated in this way because
 771     // balance_queues() has moved the Ref's into the first n queues.
 772     // Thread* thr = Thread::current();
 773     // int refs_list_index = ((WorkerThread*)thr)->id();
 774     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 775     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 776                                   &is_alive, &keep_alive, &complete_gc);
 777   }
 778 private:
 779   bool _clear_referent;
 780 };
 781 
 782 void ReferenceProcessor::set_discovered(oop ref, oop value) {
 783   java_lang_ref_Reference::set_discovered_raw(ref, value);
 784   if (_discovered_list_needs_barrier) {
 785     oopDesc::bs()->write_ref_field(ref, value);
 786   }
 787 }
 788 
 789 // Balances reference queues.
 790 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 791 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 792 // corresponding to the active workers will be processed.
 793 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 794 {
 795   // calculate total length
 796   size_t total_refs = 0;
 797   if (TraceReferenceGC && PrintGCDetails) {
 798     gclog_or_tty->print_cr("\nBalance ref_lists ");
 799   }
 800 
 801   for (uint i = 0; i < _max_num_q; ++i) {
 802     total_refs += ref_lists[i].length();
 803     if (TraceReferenceGC && PrintGCDetails) {
 804       gclog_or_tty->print("%d ", ref_lists[i].length());
 805     }
 806   }
 807   if (TraceReferenceGC && PrintGCDetails) {
 808     gclog_or_tty->print_cr(" = %d", total_refs);
 809   }
 810   size_t avg_refs = total_refs / _num_q + 1;
 811   uint to_idx = 0;
 812   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 813     bool move_all = false;
 814     if (from_idx >= _num_q) {
 815       move_all = ref_lists[from_idx].length() > 0;
 816     }
 817     while ((ref_lists[from_idx].length() > avg_refs) ||
 818            move_all) {
 819       assert(to_idx < _num_q, "Sanity Check!");
 820       if (ref_lists[to_idx].length() < avg_refs) {
 821         // move superfluous refs
 822         size_t refs_to_move;
 823         // Move all the Ref's if the from queue will not be processed.
 824         if (move_all) {
 825           refs_to_move = MIN2(ref_lists[from_idx].length(),
 826                               avg_refs - ref_lists[to_idx].length());
 827         } else {
 828           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 829                               avg_refs - ref_lists[to_idx].length());
 830         }
 831 
 832         assert(refs_to_move > 0, "otherwise the code below will fail");
 833 
 834         oop move_head = ref_lists[from_idx].head();
 835         oop move_tail = move_head;
 836         oop new_head  = move_head;
 837         // find an element to split the list on
 838         for (size_t j = 0; j < refs_to_move; ++j) {
 839           move_tail = new_head;
 840           new_head = java_lang_ref_Reference::discovered(new_head);
 841         }
 842 
 843         // Add the chain to the to list.
 844         if (ref_lists[to_idx].head() == NULL) {
 845           // to list is empty. Make a loop at the end.
 846           set_discovered(move_tail, move_tail);
 847         } else {
 848           set_discovered(move_tail, ref_lists[to_idx].head());
 849         }
 850         ref_lists[to_idx].set_head(move_head);
 851         ref_lists[to_idx].inc_length(refs_to_move);
 852 
 853         // Remove the chain from the from list.
 854         if (move_tail == new_head) {
 855           // We found the end of the from list.
 856           ref_lists[from_idx].set_head(NULL);
 857         } else {
 858           ref_lists[from_idx].set_head(new_head);
 859         }
 860         ref_lists[from_idx].dec_length(refs_to_move);
 861         if (ref_lists[from_idx].length() == 0) {
 862           break;
 863         }
 864       } else {
 865         to_idx = (to_idx + 1) % _num_q;
 866       }
 867     }
 868   }
 869 #ifdef ASSERT
 870   size_t balanced_total_refs = 0;
 871   for (uint i = 0; i < _max_num_q; ++i) {
 872     balanced_total_refs += ref_lists[i].length();
 873     if (TraceReferenceGC && PrintGCDetails) {
 874       gclog_or_tty->print("%d ", ref_lists[i].length());
 875     }
 876   }
 877   if (TraceReferenceGC && PrintGCDetails) {
 878     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 879     gclog_or_tty->flush();
 880   }
 881   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 882 #endif
 883 }
 884 
 885 void ReferenceProcessor::balance_all_queues() {
 886   balance_queues(_discoveredSoftRefs);
 887   balance_queues(_discoveredWeakRefs);
 888   balance_queues(_discoveredFinalRefs);
 889   balance_queues(_discoveredPhantomRefs);
 890 }
 891 
 892 size_t
 893 ReferenceProcessor::process_discovered_reflist(
 894   DiscoveredList               refs_lists[],
 895   ReferencePolicy*             policy,
 896   bool                         clear_referent,
 897   BoolObjectClosure*           is_alive,
 898   OopClosure*                  keep_alive,
 899   VoidClosure*                 complete_gc,
 900   AbstractRefProcTaskExecutor* task_executor)
 901 {
 902   bool mt_processing = task_executor != NULL && _processing_is_mt;
 903   // If discovery used MT and a dynamic number of GC threads, then
 904   // the queues must be balanced for correctness if fewer than the
 905   // maximum number of queues were used.  The number of queue used
 906   // during discovery may be different than the number to be used
 907   // for processing so don't depend of _num_q < _max_num_q as part
 908   // of the test.
 909   bool must_balance = _discovery_is_mt;
 910 
 911   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 912       must_balance) {
 913     balance_queues(refs_lists);
 914   }
 915 
 916   size_t total_list_count = total_count(refs_lists);
 917 
 918   if (PrintReferenceGC && PrintGCDetails) {
 919     gclog_or_tty->print(", %u refs", total_list_count);
 920   }
 921 
 922   // Phase 1 (soft refs only):
 923   // . Traverse the list and remove any SoftReferences whose
 924   //   referents are not alive, but that should be kept alive for
 925   //   policy reasons. Keep alive the transitive closure of all
 926   //   such referents.
 927   if (policy != NULL) {
 928     if (mt_processing) {
 929       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 930       task_executor->execute(phase1);
 931     } else {
 932       for (uint i = 0; i < _max_num_q; i++) {
 933         process_phase1(refs_lists[i], policy,
 934                        is_alive, keep_alive, complete_gc);
 935       }
 936     }
 937   } else { // policy == NULL
 938     assert(refs_lists != _discoveredSoftRefs,
 939            "Policy must be specified for soft references.");
 940   }
 941 
 942   // Phase 2:
 943   // . Traverse the list and remove any refs whose referents are alive.
 944   if (mt_processing) {
 945     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 946     task_executor->execute(phase2);
 947   } else {
 948     for (uint i = 0; i < _max_num_q; i++) {
 949       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 950     }
 951   }
 952 
 953   // Phase 3:
 954   // . Traverse the list and process referents as appropriate.
 955   if (mt_processing) {
 956     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 957     task_executor->execute(phase3);
 958   } else {
 959     for (uint i = 0; i < _max_num_q; i++) {
 960       process_phase3(refs_lists[i], clear_referent,
 961                      is_alive, keep_alive, complete_gc);
 962     }
 963   }
 964 
 965   return total_list_count;
 966 }
 967 
 968 void ReferenceProcessor::clean_up_discovered_references() {
 969   // loop over the lists
 970   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 971     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 972       gclog_or_tty->print_cr(
 973         "\nScrubbing %s discovered list of Null referents",
 974         list_name(i));
 975     }
 976     clean_up_discovered_reflist(_discovered_refs[i]);
 977   }
 978 }
 979 
 980 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 981   assert(!discovery_is_atomic(), "Else why call this method?");
 982   DiscoveredListIterator iter(refs_list, NULL, NULL);
 983   while (iter.has_next()) {
 984     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 985     oop next = java_lang_ref_Reference::next(iter.obj());
 986     assert(next->is_oop_or_null(), "bad next field");
 987     // If referent has been cleared or Reference is not active,
 988     // drop it.
 989     if (iter.referent() == NULL || next != NULL) {
 990       debug_only(
 991         if (PrintGCDetails && TraceReferenceGC) {
 992           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 993             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 994             " and referent: " INTPTR_FORMAT,
 995             (void *)iter.obj(), (void *)next, (void *)iter.referent());
 996         }
 997       )
 998       // Remove Reference object from list
 999       iter.remove();
1000       iter.move_to_next();
1001     } else {
1002       iter.next();
1003     }
1004   }
1005   NOT_PRODUCT(
1006     if (PrintGCDetails && TraceReferenceGC) {
1007       gclog_or_tty->print(
1008         " Removed %d Refs with NULL referents out of %d discovered Refs",
1009         iter.removed(), iter.processed());
1010     }
1011   )
1012 }
1013 
1014 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1015   uint id = 0;
1016   // Determine the queue index to use for this object.
1017   if (_discovery_is_mt) {
1018     // During a multi-threaded discovery phase,
1019     // each thread saves to its "own" list.
1020     Thread* thr = Thread::current();
1021     id = thr->as_Worker_thread()->id();
1022   } else {
1023     // single-threaded discovery, we save in round-robin
1024     // fashion to each of the lists.
1025     if (_processing_is_mt) {
1026       id = next_id();
1027     }
1028   }
1029   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1030 
1031   // Get the discovered queue to which we will add
1032   DiscoveredList* list = NULL;
1033   switch (rt) {
1034     case REF_OTHER:
1035       // Unknown reference type, no special treatment
1036       break;
1037     case REF_SOFT:
1038       list = &_discoveredSoftRefs[id];
1039       break;
1040     case REF_WEAK:
1041       list = &_discoveredWeakRefs[id];
1042       break;
1043     case REF_FINAL:
1044       list = &_discoveredFinalRefs[id];
1045       break;
1046     case REF_PHANTOM:
1047       list = &_discoveredPhantomRefs[id];
1048       break;
1049     case REF_NONE:
1050       // we should not reach here if we are an InstanceRefKlass
1051     default:
1052       ShouldNotReachHere();
1053   }
1054   if (TraceReferenceGC && PrintGCDetails) {
1055     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1056   }
1057   return list;
1058 }
1059 
1060 inline void
1061 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1062                                               oop             obj,
1063                                               HeapWord*       discovered_addr) {
1064   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1065   // First we must make sure this object is only enqueued once. CAS in a non null
1066   // discovered_addr.
1067   oop current_head = refs_list.head();
1068   // The last ref must have its discovered field pointing to itself.
1069   oop next_discovered = (current_head != NULL) ? current_head : obj;
1070 
1071   // Note: In the case of G1, this specific pre-barrier is strictly
1072   // not necessary because the only case we are interested in
1073   // here is when *discovered_addr is NULL (see the CAS further below),
1074   // so this will expand to nothing. As a result, we have manually
1075   // elided this out for G1, but left in the test for some future
1076   // collector that might have need for a pre-barrier here, e.g.:-
1077   // oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1078   assert(!_discovered_list_needs_barrier || UseG1GC,
1079          "Need to check non-G1 collector: "
1080          "may need a pre-write-barrier for CAS from NULL below");
1081   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1082                                                     NULL);
1083   if (retest == NULL) {
1084     // This thread just won the right to enqueue the object.
1085     // We have separate lists for enqueueing, so no synchronization
1086     // is necessary.
1087     refs_list.set_head(obj);
1088     refs_list.inc_length(1);
1089     if (_discovered_list_needs_barrier) {
1090       oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1091     }
1092 
1093     if (TraceReferenceGC) {
1094       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1095                              (void *)obj, obj->klass()->internal_name());
1096     }
1097   } else {
1098     // If retest was non NULL, another thread beat us to it:
1099     // The reference has already been discovered...
1100     if (TraceReferenceGC) {
1101       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1102                              (void *)obj, obj->klass()->internal_name());
1103     }
1104   }
1105 }
1106 
1107 #ifndef PRODUCT
1108 // Non-atomic (i.e. concurrent) discovery might allow us
1109 // to observe j.l.References with NULL referents, being those
1110 // cleared concurrently by mutators during (or after) discovery.
1111 void ReferenceProcessor::verify_referent(oop obj) {
1112   bool da = discovery_is_atomic();
1113   oop referent = java_lang_ref_Reference::referent(obj);
1114   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1115          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1116                  INTPTR_FORMAT " during %satomic discovery ",
1117                  (void *)referent, (void *)obj, da ? "" : "non-"));
1118 }
1119 #endif
1120 
1121 // We mention two of several possible choices here:
1122 // #0: if the reference object is not in the "originating generation"
1123 //     (or part of the heap being collected, indicated by our "span"
1124 //     we don't treat it specially (i.e. we scan it as we would
1125 //     a normal oop, treating its references as strong references).
1126 //     This means that references can't be discovered unless their
1127 //     referent is also in the same span. This is the simplest,
1128 //     most "local" and most conservative approach, albeit one
1129 //     that may cause weak references to be enqueued least promptly.
1130 //     We call this choice the "ReferenceBasedDiscovery" policy.
1131 // #1: the reference object may be in any generation (span), but if
1132 //     the referent is in the generation (span) being currently collected
1133 //     then we can discover the reference object, provided
1134 //     the object has not already been discovered by
1135 //     a different concurrently running collector (as may be the
1136 //     case, for instance, if the reference object is in CMS and
1137 //     the referent in DefNewGeneration), and provided the processing
1138 //     of this reference object by the current collector will
1139 //     appear atomic to every other collector in the system.
1140 //     (Thus, for instance, a concurrent collector may not
1141 //     discover references in other generations even if the
1142 //     referent is in its own generation). This policy may,
1143 //     in certain cases, enqueue references somewhat sooner than
1144 //     might Policy #0 above, but at marginally increased cost
1145 //     and complexity in processing these references.
1146 //     We call this choice the "RefeferentBasedDiscovery" policy.
1147 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1148   // Make sure we are discovering refs (rather than processing discovered refs).
1149   if (!_discovering_refs || !RegisterReferences) {
1150     return false;
1151   }
1152   // We only discover active references.
1153   oop next = java_lang_ref_Reference::next(obj);
1154   if (next != NULL) {   // Ref is no longer active
1155     return false;
1156   }
1157 
1158   HeapWord* obj_addr = (HeapWord*)obj;
1159   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1160       !_span.contains(obj_addr)) {
1161     // Reference is not in the originating generation;
1162     // don't treat it specially (i.e. we want to scan it as a normal
1163     // object with strong references).
1164     return false;
1165   }
1166 
1167   // We only discover references whose referents are not (yet)
1168   // known to be strongly reachable.
1169   if (is_alive_non_header() != NULL) {
1170     verify_referent(obj);
1171     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1172       return false;  // referent is reachable
1173     }
1174   }
1175   if (rt == REF_SOFT) {
1176     // For soft refs we can decide now if these are not
1177     // current candidates for clearing, in which case we
1178     // can mark through them now, rather than delaying that
1179     // to the reference-processing phase. Since all current
1180     // time-stamp policies advance the soft-ref clock only
1181     // at a major collection cycle, this is always currently
1182     // accurate.
1183     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1184       return false;
1185     }
1186   }
1187 
1188   ResourceMark rm;      // Needed for tracing.
1189 
1190   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1191   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1192   assert(discovered->is_oop_or_null(), "bad discovered field");
1193   if (discovered != NULL) {
1194     // The reference has already been discovered...
1195     if (TraceReferenceGC) {
1196       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1197                              (void *)obj, obj->klass()->internal_name());
1198     }
1199     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1200       // assumes that an object is not processed twice;
1201       // if it's been already discovered it must be on another
1202       // generation's discovered list; so we won't discover it.
1203       return false;
1204     } else {
1205       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1206              "Unrecognized policy");
1207       // Check assumption that an object is not potentially
1208       // discovered twice except by concurrent collectors that potentially
1209       // trace the same Reference object twice.
1210       assert(UseConcMarkSweepGC || UseG1GC,
1211              "Only possible with a concurrent marking collector");
1212       return true;
1213     }
1214   }
1215 
1216   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1217     verify_referent(obj);
1218     // Discover if and only if EITHER:
1219     // .. reference is in our span, OR
1220     // .. we are an atomic collector and referent is in our span
1221     if (_span.contains(obj_addr) ||
1222         (discovery_is_atomic() &&
1223          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1224       // should_enqueue = true;
1225     } else {
1226       return false;
1227     }
1228   } else {
1229     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1230            _span.contains(obj_addr), "code inconsistency");
1231   }
1232 
1233   // Get the right type of discovered queue head.
1234   DiscoveredList* list = get_discovered_list(rt);
1235   if (list == NULL) {
1236     return false;   // nothing special needs to be done
1237   }
1238 
1239   if (_discovery_is_mt) {
1240     add_to_discovered_list_mt(*list, obj, discovered_addr);
1241   } else {
1242     // If "_discovered_list_needs_barrier", we do write barriers when
1243     // updating the discovered reference list.  Otherwise, we do a raw store
1244     // here: the field will be visited later when processing the discovered
1245     // references.
1246     oop current_head = list->head();
1247     // The last ref must have its discovered field pointing to itself.
1248     oop next_discovered = (current_head != NULL) ? current_head : obj;
1249 
1250     // As in the case further above, since we are over-writing a NULL
1251     // pre-value, we can safely elide the pre-barrier here for the case of G1.
1252     // e.g.:- oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1253     assert(discovered == NULL, "control point invariant");
1254     assert(!_discovered_list_needs_barrier || UseG1GC,
1255            "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1256     oop_store_raw(discovered_addr, next_discovered);
1257     if (_discovered_list_needs_barrier) {
1258       oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1259     }
1260     list->set_head(obj);
1261     list->inc_length(1);
1262 
1263     if (TraceReferenceGC) {
1264       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1265                                 (void *)obj, obj->klass()->internal_name());
1266     }
1267   }
1268   assert(obj->is_oop(), "Discovered a bad reference");
1269   verify_referent(obj);
1270   return true;
1271 }
1272 
1273 // Preclean the discovered references by removing those
1274 // whose referents are alive, and by marking from those that
1275 // are not active. These lists can be handled here
1276 // in any order and, indeed, concurrently.
1277 void ReferenceProcessor::preclean_discovered_references(
1278   BoolObjectClosure* is_alive,
1279   OopClosure* keep_alive,
1280   VoidClosure* complete_gc,
1281   YieldClosure* yield,
1282   GCTimer* gc_timer) {
1283 
1284   NOT_PRODUCT(verify_ok_to_handle_reflists());
1285 
1286   // Soft references
1287   {
1288     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1289               false, gc_timer);
1290     for (uint i = 0; i < _max_num_q; i++) {
1291       if (yield->should_return()) {
1292         return;
1293       }
1294       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1295                                   keep_alive, complete_gc, yield);
1296     }
1297   }
1298 
1299   // Weak references
1300   {
1301     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1302               false, gc_timer);
1303     for (uint i = 0; i < _max_num_q; i++) {
1304       if (yield->should_return()) {
1305         return;
1306       }
1307       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1308                                   keep_alive, complete_gc, yield);
1309     }
1310   }
1311 
1312   // Final references
1313   {
1314     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1315               false, gc_timer);
1316     for (uint i = 0; i < _max_num_q; i++) {
1317       if (yield->should_return()) {
1318         return;
1319       }
1320       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1321                                   keep_alive, complete_gc, yield);
1322     }
1323   }
1324 
1325   // Phantom references
1326   {
1327     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1328               false, gc_timer);
1329     for (uint i = 0; i < _max_num_q; i++) {
1330       if (yield->should_return()) {
1331         return;
1332       }
1333       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1334                                   keep_alive, complete_gc, yield);
1335     }
1336   }
1337 }
1338 
1339 // Walk the given discovered ref list, and remove all reference objects
1340 // whose referents are still alive, whose referents are NULL or which
1341 // are not active (have a non-NULL next field). NOTE: When we are
1342 // thus precleaning the ref lists (which happens single-threaded today),
1343 // we do not disable refs discovery to honour the correct semantics of
1344 // java.lang.Reference. As a result, we need to be careful below
1345 // that ref removal steps interleave safely with ref discovery steps
1346 // (in this thread).
1347 void
1348 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1349                                                 BoolObjectClosure* is_alive,
1350                                                 OopClosure*        keep_alive,
1351                                                 VoidClosure*       complete_gc,
1352                                                 YieldClosure*      yield) {
1353   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1354   while (iter.has_next()) {
1355     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1356     oop obj = iter.obj();
1357     oop next = java_lang_ref_Reference::next(obj);
1358     if (iter.referent() == NULL || iter.is_referent_alive() ||
1359         next != NULL) {
1360       // The referent has been cleared, or is alive, or the Reference is not
1361       // active; we need to trace and mark its cohort.
1362       if (TraceReferenceGC) {
1363         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1364                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1365       }
1366       // Remove Reference object from list
1367       iter.remove();
1368       // Keep alive its cohort.
1369       iter.make_referent_alive();
1370       if (UseCompressedOops) {
1371         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1372         keep_alive->do_oop(next_addr);
1373       } else {
1374         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1375         keep_alive->do_oop(next_addr);
1376       }
1377       iter.move_to_next();
1378     } else {
1379       iter.next();
1380     }
1381   }
1382   // Close the reachable set
1383   complete_gc->do_void();
1384 
1385   NOT_PRODUCT(
1386     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1387       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1388         "Refs in discovered list " INTPTR_FORMAT,
1389         iter.removed(), iter.processed(), (address)refs_list.head());
1390     }
1391   )
1392 }
1393 
1394 const char* ReferenceProcessor::list_name(uint i) {
1395    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1396           "Out of bounds index");
1397 
1398    int j = i / _max_num_q;
1399    switch (j) {
1400      case 0: return "SoftRef";
1401      case 1: return "WeakRef";
1402      case 2: return "FinalRef";
1403      case 3: return "PhantomRef";
1404    }
1405    ShouldNotReachHere();
1406    return NULL;
1407 }
1408 
1409 #ifndef PRODUCT
1410 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1411   // empty for now
1412 }
1413 #endif
1414 
1415 #ifndef PRODUCT
1416 void ReferenceProcessor::clear_discovered_references() {
1417   guarantee(!_discovering_refs, "Discovering refs?");
1418   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1419     clear_discovered_references(_discovered_refs[i]);
1420   }
1421 }
1422 
1423 #endif // PRODUCT