1 /*
   2  * Copyright (c) 2001, 2015, 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/shared/collectedHeap.hpp"
  29 #include "gc/shared/collectedHeap.inline.hpp"
  30 #include "gc/shared/gcTimer.hpp"
  31 #include "gc/shared/gcTraceTime.hpp"
  32 #include "gc/shared/referencePolicy.hpp"
  33 #include "gc/shared/referenceProcessor.hpp"
  34 #include "memory/allocation.hpp"
  35 #include "oops/oop.inline.hpp"
  36 #include "runtime/java.hpp"
  37 #include "runtime/jniHandles.hpp"
  38 
  39 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  40 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  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-decreasing 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 #if defined(COMPILER2) || INCLUDE_JVMCI
  59   _default_soft_ref_policy      = new LRUMaxHeapPolicy();
  60 #else
  61   _default_soft_ref_policy      = new LRUCurrentHeapPolicy();
  62 #endif
  63   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  64     vm_exit_during_initialization("Could not allocate reference policy object");
  65   }
  66   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  67             RefDiscoveryPolicy == ReferentBasedDiscovery,
  68             "Unrecognized RefDiscoveryPolicy");
  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: " JLONG_FORMAT " to " JLONG_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 static void log_ref_count(size_t count, bool doit) {
 190   if (doit) {
 191     gclog_or_tty->print(", " SIZE_FORMAT " refs", count);
 192   }
 193 }
 194 
 195 class GCRefTraceTime : public StackObj {
 196   GCTraceTimeImpl _gc_trace_time;
 197  public:
 198   GCRefTraceTime(const char* title, bool doit, GCTimer* timer, size_t count) :
 199     _gc_trace_time(title, doit, false, timer) {
 200     log_ref_count(count, doit);
 201   }
 202 };
 203 
 204 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 205   BoolObjectClosure*           is_alive,
 206   OopClosure*                  keep_alive,
 207   VoidClosure*                 complete_gc,
 208   AbstractRefProcTaskExecutor* task_executor,
 209   GCTimer*                     gc_timer) {
 210 
 211   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 212   // Stop treating discovered references specially.
 213   disable_discovery();
 214 
 215   // If discovery was concurrent, someone could have modified
 216   // the value of the static field in the j.l.r.SoftReference
 217   // class that holds the soft reference timestamp clock using
 218   // reflection or Unsafe between when discovery was enabled and
 219   // now. Unconditionally update the static field in ReferenceProcessor
 220   // here so that we use the new value during processing of the
 221   // discovered soft refs.
 222 
 223   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 224 
 225   bool trace_time = PrintGCDetails && PrintReferenceGC;
 226 
 227   // Include cleaners in phantom statistics.  We expect Cleaner
 228   // references to be temporary, and don't want to deal with
 229   // possible incompatibilities arising from making it more visible.
 230   ReferenceProcessorStats stats(
 231       total_count(_discoveredSoftRefs),
 232       total_count(_discoveredWeakRefs),
 233       total_count(_discoveredFinalRefs),
 234       total_count(_discoveredPhantomRefs) + total_count(_discoveredCleanerRefs));
 235 
 236   // Soft references
 237   {
 238     GCRefTraceTime tt("SoftReference", trace_time, gc_timer, stats.soft_count());
 239     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 240                                is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   update_soft_ref_master_clock();
 244 
 245   // Weak references
 246   {
 247     GCRefTraceTime tt("WeakReference", trace_time, gc_timer, stats.weak_count());
 248     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 249                                is_alive, keep_alive, complete_gc, task_executor);
 250   }
 251 
 252   // Final references
 253   {
 254     GCRefTraceTime tt("FinalReference", trace_time, gc_timer, stats.final_count());
 255     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 256                                is_alive, keep_alive, complete_gc, task_executor);
 257   }
 258 
 259   // Phantom references
 260   {
 261     GCRefTraceTime tt("PhantomReference", trace_time, gc_timer, stats.phantom_count());
 262     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 263                                is_alive, keep_alive, complete_gc, task_executor);
 264 
 265     // Process cleaners, but include them in phantom timing.  We expect
 266     // Cleaner references to be temporary, and don't want to deal with
 267     // possible incompatibilities arising from making it more visible.
 268     process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
 269                                  is_alive, keep_alive, complete_gc, task_executor);
 270   }
 271 
 272   // Weak global JNI references. It would make more sense (semantically) to
 273   // traverse these simultaneously with the regular weak references above, but
 274   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 275   // thus use JNI weak references to circumvent the phantom references and
 276   // resurrect a "post-mortem" object.
 277   {
 278     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 279     NOT_PRODUCT(log_ref_count(count_jni_refs(), trace_time);)
 280     if (task_executor != NULL) {
 281       task_executor->set_single_threaded_mode();
 282     }
 283     process_phaseJNI(is_alive, keep_alive, complete_gc);
 284   }
 285 
 286   return stats;
 287 }
 288 
 289 #ifndef PRODUCT
 290 // Calculate the number of jni handles.
 291 uint ReferenceProcessor::count_jni_refs() {
 292   class AlwaysAliveClosure: public BoolObjectClosure {
 293   public:
 294     virtual bool do_object_b(oop obj) { return true; }
 295   };
 296 
 297   class CountHandleClosure: public OopClosure {
 298   private:
 299     int _count;
 300   public:
 301     CountHandleClosure(): _count(0) {}
 302     void do_oop(oop* unused)       { _count++; }
 303     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 304     int count() { return _count; }
 305   };
 306   CountHandleClosure global_handle_count;
 307   AlwaysAliveClosure always_alive;
 308   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 309   return global_handle_count.count();
 310 }
 311 #endif
 312 
 313 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 314                                           OopClosure*        keep_alive,
 315                                           VoidClosure*       complete_gc) {
 316   JNIHandles::weak_oops_do(is_alive, keep_alive);
 317   complete_gc->do_void();
 318 }
 319 
 320 
 321 template <class T>
 322 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 323                                    AbstractRefProcTaskExecutor* task_executor) {
 324 
 325   // Remember old value of pending references list
 326   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 327   T old_pending_list_value = *pending_list_addr;
 328 
 329   // Enqueue references that are not made active again, and
 330   // clear the decks for the next collection (cycle).
 331   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 332   // Do the post-barrier on pending_list_addr missed in
 333   // enqueue_discovered_reflist.
 334   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 335 
 336   // Stop treating discovered references specially.
 337   ref->disable_discovery();
 338 
 339   // Return true if new pending references were added
 340   return old_pending_list_value != *pending_list_addr;
 341 }
 342 
 343 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 344   if (UseCompressedOops) {
 345     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 346   } else {
 347     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 348   }
 349 }
 350 
 351 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 352                                                     HeapWord* pending_list_addr) {
 353   // Given a list of refs linked through the "discovered" field
 354   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 355   // thus distinguishing them from active References, then
 356   // prepend them to the pending list.
 357   //
 358   // The Java threads will see the Reference objects linked together through
 359   // the discovered field. Instead of trying to do the write barrier updates
 360   // in all places in the reference processor where we manipulate the discovered
 361   // field we make sure to do the barrier here where we anyway iterate through
 362   // all linked Reference objects. Note that it is important to not dirty any
 363   // cards during reference processing since this will cause card table
 364   // verification to fail for G1.
 365   if (TraceReferenceGC && PrintGCDetails) {
 366     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 367                            INTPTR_FORMAT, p2i(refs_list.head()));
 368   }
 369 
 370   oop obj = NULL;
 371   oop next_d = refs_list.head();
 372   // Walk down the list, self-looping the next field
 373   // so that the References are not considered active.
 374   while (obj != next_d) {
 375     obj = next_d;
 376     assert(obj->is_instance(), "should be an instance object");
 377     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 378     next_d = java_lang_ref_Reference::discovered(obj);
 379     if (TraceReferenceGC && PrintGCDetails) {
 380       gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 381                              p2i(obj), p2i(next_d));
 382     }
 383     assert(java_lang_ref_Reference::next(obj) == NULL,
 384            "Reference not active; should not be discovered");
 385     // Self-loop next, so as to make Ref not active.
 386     java_lang_ref_Reference::set_next_raw(obj, obj);
 387     if (next_d != obj) {
 388       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 389     } else {
 390       // This is the last object.
 391       // Swap refs_list into pending_list_addr and
 392       // set obj's discovered to what we read from pending_list_addr.
 393       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 394       // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 395       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 396       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 397     }
 398   }
 399 }
 400 
 401 // Parallel enqueue task
 402 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 403 public:
 404   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 405                      DiscoveredList      discovered_refs[],
 406                      HeapWord*           pending_list_addr,
 407                      int                 n_queues)
 408     : EnqueueTask(ref_processor, discovered_refs,
 409                   pending_list_addr, n_queues)
 410   { }
 411 
 412   virtual void work(unsigned int work_id) {
 413     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 414     // Simplest first cut: static partitioning.
 415     int index = work_id;
 416     // The increment on "index" must correspond to the maximum number of queues
 417     // (n_queues) with which that ReferenceProcessor was created.  That
 418     // is because of the "clever" way the discovered references lists were
 419     // allocated and are indexed into.
 420     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 421     for (int j = 0;
 422          j < ReferenceProcessor::number_of_subclasses_of_ref();
 423          j++, index += _n_queues) {
 424       _ref_processor.enqueue_discovered_reflist(
 425         _refs_lists[index], _pending_list_addr);
 426       _refs_lists[index].set_head(NULL);
 427       _refs_lists[index].set_length(0);
 428     }
 429   }
 430 };
 431 
 432 // Enqueue references that are not made active again
 433 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 434   AbstractRefProcTaskExecutor* task_executor) {
 435   if (_processing_is_mt && task_executor != NULL) {
 436     // Parallel code
 437     RefProcEnqueueTask tsk(*this, _discovered_refs,
 438                            pending_list_addr, _max_num_q);
 439     task_executor->execute(tsk);
 440   } else {
 441     // Serial code: call the parent class's implementation
 442     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 443       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 444       _discovered_refs[i].set_head(NULL);
 445       _discovered_refs[i].set_length(0);
 446     }
 447   }
 448 }
 449 
 450 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 451   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 452   oop discovered = java_lang_ref_Reference::discovered(_ref);
 453   assert(_discovered_addr && discovered->is_oop_or_null(),
 454          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 455   _next = discovered;
 456   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 457   _referent = java_lang_ref_Reference::referent(_ref);
 458   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 459          "Wrong oop found in java.lang.Reference object");
 460   assert(allow_null_referent ?
 461              _referent->is_oop_or_null()
 462            : _referent->is_oop(),
 463          "Expected an oop%s for referent field at " PTR_FORMAT,
 464          (allow_null_referent ? " or NULL" : ""),
 465          p2i(_referent));
 466 }
 467 
 468 void DiscoveredListIterator::remove() {
 469   assert(_ref->is_oop(), "Dropping a bad reference");
 470   oop_store_raw(_discovered_addr, NULL);
 471 
 472   // First _prev_next ref actually points into DiscoveredList (gross).
 473   oop new_next;
 474   if (_next == _ref) {
 475     // At the end of the list, we should make _prev point to itself.
 476     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 477     // and _prev will be NULL.
 478     new_next = _prev;
 479   } else {
 480     new_next = _next;
 481   }
 482   // Remove Reference object from discovered list. Note that G1 does not need a
 483   // pre-barrier here because we know the Reference has already been found/marked,
 484   // that's how it ended up in the discovered list in the first place.
 485   oop_store_raw(_prev_next, new_next);
 486   NOT_PRODUCT(_removed++);
 487   _refs_list.dec_length(1);
 488 }
 489 
 490 void DiscoveredListIterator::clear_referent() {
 491   oop_store_raw(_referent_addr, NULL);
 492 }
 493 
 494 // NOTE: process_phase*() are largely similar, and at a high level
 495 // merely iterate over the extant list applying a predicate to
 496 // each of its elements and possibly removing that element from the
 497 // list and applying some further closures to that element.
 498 // We should consider the possibility of replacing these
 499 // process_phase*() methods by abstracting them into
 500 // a single general iterator invocation that receives appropriate
 501 // closures that accomplish this work.
 502 
 503 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 504 // referents are not alive, but that should be kept alive for policy reasons.
 505 // Keep alive the transitive closure of all such referents.
 506 void
 507 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 508                                    ReferencePolicy*   policy,
 509                                    BoolObjectClosure* is_alive,
 510                                    OopClosure*        keep_alive,
 511                                    VoidClosure*       complete_gc) {
 512   assert(policy != NULL, "Must have a non-NULL policy");
 513   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 514   // Decide which softly reachable refs should be kept alive.
 515   while (iter.has_next()) {
 516     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 517     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 518     if (referent_is_dead &&
 519         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 520       if (TraceReferenceGC) {
 521         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 522                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
 523       }
 524       // Remove Reference object from list
 525       iter.remove();
 526       // keep the referent around
 527       iter.make_referent_alive();
 528       iter.move_to_next();
 529     } else {
 530       iter.next();
 531     }
 532   }
 533   // Close the reachable set
 534   complete_gc->do_void();
 535   NOT_PRODUCT(
 536     if (PrintGCDetails && TraceReferenceGC) {
 537       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
 538         " discovered Refs by policy, from list " INTPTR_FORMAT,
 539         iter.removed(), iter.processed(), p2i(refs_list.head()));
 540     }
 541   )
 542 }
 543 
 544 // Traverse the list and remove any Refs that are not active, or
 545 // whose referents are either alive or NULL.
 546 void
 547 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 548                              BoolObjectClosure* is_alive,
 549                              OopClosure*        keep_alive) {
 550   assert(discovery_is_atomic(), "Error");
 551   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 552   while (iter.has_next()) {
 553     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 554     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 555     assert(next == NULL, "Should not discover inactive Reference");
 556     if (iter.is_referent_alive()) {
 557       if (TraceReferenceGC) {
 558         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 559                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
 560       }
 561       // The referent is reachable after all.
 562       // Remove Reference object from list.
 563       iter.remove();
 564       // Update the referent pointer as necessary: Note that this
 565       // should not entail any recursive marking because the
 566       // referent must already have been traversed.
 567       iter.make_referent_alive();
 568       iter.move_to_next();
 569     } else {
 570       iter.next();
 571     }
 572   }
 573   NOT_PRODUCT(
 574     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 575       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 576         " Refs in discovered list " INTPTR_FORMAT,
 577         iter.removed(), iter.processed(), p2i(refs_list.head()));
 578     }
 579   )
 580 }
 581 
 582 void
 583 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 584                                                   BoolObjectClosure* is_alive,
 585                                                   OopClosure*        keep_alive,
 586                                                   VoidClosure*       complete_gc) {
 587   assert(!discovery_is_atomic(), "Error");
 588   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 589   while (iter.has_next()) {
 590     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 591     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 592     oop next = java_lang_ref_Reference::next(iter.obj());
 593     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 594          next != NULL)) {
 595       assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 596       // Remove Reference object from list
 597       iter.remove();
 598       // Trace the cohorts
 599       iter.make_referent_alive();
 600       if (UseCompressedOops) {
 601         keep_alive->do_oop((narrowOop*)next_addr);
 602       } else {
 603         keep_alive->do_oop((oop*)next_addr);
 604       }
 605       iter.move_to_next();
 606     } else {
 607       iter.next();
 608     }
 609   }
 610   // Now close the newly reachable set
 611   complete_gc->do_void();
 612   NOT_PRODUCT(
 613     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 614       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 615         " Refs in discovered list " INTPTR_FORMAT,
 616         iter.removed(), iter.processed(), p2i(refs_list.head()));
 617     }
 618   )
 619 }
 620 
 621 // Traverse the list and process the referents, by either
 622 // clearing them or keeping them (and their reachable
 623 // closure) alive.
 624 void
 625 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 626                                    bool               clear_referent,
 627                                    BoolObjectClosure* is_alive,
 628                                    OopClosure*        keep_alive,
 629                                    VoidClosure*       complete_gc) {
 630   ResourceMark rm;
 631   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 632   while (iter.has_next()) {
 633     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 634     if (clear_referent) {
 635       // NULL out referent pointer
 636       iter.clear_referent();
 637     } else {
 638       // keep the referent around
 639       iter.make_referent_alive();
 640     }
 641     if (TraceReferenceGC) {
 642       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 643                              clear_referent ? "cleared " : "",
 644                              p2i(iter.obj()), iter.obj()->klass()->internal_name());
 645     }
 646     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 647     iter.next();
 648   }
 649   // Close the reachable set
 650   complete_gc->do_void();
 651 }
 652 
 653 void
 654 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 655   oop obj = NULL;
 656   oop next = refs_list.head();
 657   while (next != obj) {
 658     obj = next;
 659     next = java_lang_ref_Reference::discovered(obj);
 660     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 661   }
 662   refs_list.set_head(NULL);
 663   refs_list.set_length(0);
 664 }
 665 
 666 void ReferenceProcessor::abandon_partial_discovery() {
 667   // loop over the lists
 668   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 669     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 670       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 671     }
 672     clear_discovered_references(_discovered_refs[i]);
 673   }
 674 }
 675 
 676 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 677 public:
 678   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 679                     DiscoveredList      refs_lists[],
 680                     ReferencePolicy*    policy,
 681                     bool                marks_oops_alive)
 682     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 683       _policy(policy)
 684   { }
 685   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 686                     OopClosure& keep_alive,
 687                     VoidClosure& complete_gc)
 688   {
 689     Thread* thr = Thread::current();
 690     int refs_list_index = ((WorkerThread*)thr)->id();
 691     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 692                                   &is_alive, &keep_alive, &complete_gc);
 693   }
 694 private:
 695   ReferencePolicy* _policy;
 696 };
 697 
 698 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 699 public:
 700   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 701                     DiscoveredList      refs_lists[],
 702                     bool                marks_oops_alive)
 703     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 704   { }
 705   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 706                     OopClosure& keep_alive,
 707                     VoidClosure& complete_gc)
 708   {
 709     _ref_processor.process_phase2(_refs_lists[i],
 710                                   &is_alive, &keep_alive, &complete_gc);
 711   }
 712 };
 713 
 714 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 715 public:
 716   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 717                     DiscoveredList      refs_lists[],
 718                     bool                clear_referent,
 719                     bool                marks_oops_alive)
 720     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 721       _clear_referent(clear_referent)
 722   { }
 723   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 724                     OopClosure& keep_alive,
 725                     VoidClosure& complete_gc)
 726   {
 727     // Don't use "refs_list_index" calculated in this way because
 728     // balance_queues() has moved the Ref's into the first n queues.
 729     // Thread* thr = Thread::current();
 730     // int refs_list_index = ((WorkerThread*)thr)->id();
 731     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 732     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 733                                   &is_alive, &keep_alive, &complete_gc);
 734   }
 735 private:
 736   bool _clear_referent;
 737 };
 738 
 739 // Balances reference queues.
 740 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 741 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 742 // corresponding to the active workers will be processed.
 743 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 744 {
 745   // calculate total length
 746   size_t total_refs = 0;
 747   if (TraceReferenceGC && PrintGCDetails) {
 748     gclog_or_tty->print_cr("\nBalance ref_lists ");
 749   }
 750 
 751   for (uint i = 0; i < _max_num_q; ++i) {
 752     total_refs += ref_lists[i].length();
 753     if (TraceReferenceGC && PrintGCDetails) {
 754       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
 755     }
 756   }
 757   if (TraceReferenceGC && PrintGCDetails) {
 758     gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
 759   }
 760   size_t avg_refs = total_refs / _num_q + 1;
 761   uint to_idx = 0;
 762   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 763     bool move_all = false;
 764     if (from_idx >= _num_q) {
 765       move_all = ref_lists[from_idx].length() > 0;
 766     }
 767     while ((ref_lists[from_idx].length() > avg_refs) ||
 768            move_all) {
 769       assert(to_idx < _num_q, "Sanity Check!");
 770       if (ref_lists[to_idx].length() < avg_refs) {
 771         // move superfluous refs
 772         size_t refs_to_move;
 773         // Move all the Ref's if the from queue will not be processed.
 774         if (move_all) {
 775           refs_to_move = MIN2(ref_lists[from_idx].length(),
 776                               avg_refs - ref_lists[to_idx].length());
 777         } else {
 778           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 779                               avg_refs - ref_lists[to_idx].length());
 780         }
 781 
 782         assert(refs_to_move > 0, "otherwise the code below will fail");
 783 
 784         oop move_head = ref_lists[from_idx].head();
 785         oop move_tail = move_head;
 786         oop new_head  = move_head;
 787         // find an element to split the list on
 788         for (size_t j = 0; j < refs_to_move; ++j) {
 789           move_tail = new_head;
 790           new_head = java_lang_ref_Reference::discovered(new_head);
 791         }
 792 
 793         // Add the chain to the to list.
 794         if (ref_lists[to_idx].head() == NULL) {
 795           // to list is empty. Make a loop at the end.
 796           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 797         } else {
 798           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 799         }
 800         ref_lists[to_idx].set_head(move_head);
 801         ref_lists[to_idx].inc_length(refs_to_move);
 802 
 803         // Remove the chain from the from list.
 804         if (move_tail == new_head) {
 805           // We found the end of the from list.
 806           ref_lists[from_idx].set_head(NULL);
 807         } else {
 808           ref_lists[from_idx].set_head(new_head);
 809         }
 810         ref_lists[from_idx].dec_length(refs_to_move);
 811         if (ref_lists[from_idx].length() == 0) {
 812           break;
 813         }
 814       } else {
 815         to_idx = (to_idx + 1) % _num_q;
 816       }
 817     }
 818   }
 819 #ifdef ASSERT
 820   size_t balanced_total_refs = 0;
 821   for (uint i = 0; i < _max_num_q; ++i) {
 822     balanced_total_refs += ref_lists[i].length();
 823     if (TraceReferenceGC && PrintGCDetails) {
 824       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
 825     }
 826   }
 827   if (TraceReferenceGC && PrintGCDetails) {
 828     gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
 829     gclog_or_tty->flush();
 830   }
 831   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 832 #endif
 833 }
 834 
 835 void ReferenceProcessor::balance_all_queues() {
 836   balance_queues(_discoveredSoftRefs);
 837   balance_queues(_discoveredWeakRefs);
 838   balance_queues(_discoveredFinalRefs);
 839   balance_queues(_discoveredPhantomRefs);
 840   balance_queues(_discoveredCleanerRefs);
 841 }
 842 
 843 void ReferenceProcessor::process_discovered_reflist(
 844   DiscoveredList               refs_lists[],
 845   ReferencePolicy*             policy,
 846   bool                         clear_referent,
 847   BoolObjectClosure*           is_alive,
 848   OopClosure*                  keep_alive,
 849   VoidClosure*                 complete_gc,
 850   AbstractRefProcTaskExecutor* task_executor)
 851 {
 852   bool mt_processing = task_executor != NULL && _processing_is_mt;
 853   // If discovery used MT and a dynamic number of GC threads, then
 854   // the queues must be balanced for correctness if fewer than the
 855   // maximum number of queues were used.  The number of queue used
 856   // during discovery may be different than the number to be used
 857   // for processing so don't depend of _num_q < _max_num_q as part
 858   // of the test.
 859   bool must_balance = _discovery_is_mt;
 860 
 861   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 862       must_balance) {
 863     balance_queues(refs_lists);
 864   }
 865 
 866   // Phase 1 (soft refs only):
 867   // . Traverse the list and remove any SoftReferences whose
 868   //   referents are not alive, but that should be kept alive for
 869   //   policy reasons. Keep alive the transitive closure of all
 870   //   such referents.
 871   if (policy != NULL) {
 872     if (mt_processing) {
 873       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 874       task_executor->execute(phase1);
 875     } else {
 876       for (uint i = 0; i < _max_num_q; i++) {
 877         process_phase1(refs_lists[i], policy,
 878                        is_alive, keep_alive, complete_gc);
 879       }
 880     }
 881   } else { // policy == NULL
 882     assert(refs_lists != _discoveredSoftRefs,
 883            "Policy must be specified for soft references.");
 884   }
 885 
 886   // Phase 2:
 887   // . Traverse the list and remove any refs whose referents are alive.
 888   if (mt_processing) {
 889     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 890     task_executor->execute(phase2);
 891   } else {
 892     for (uint i = 0; i < _max_num_q; i++) {
 893       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 894     }
 895   }
 896 
 897   // Phase 3:
 898   // . Traverse the list and process referents as appropriate.
 899   if (mt_processing) {
 900     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 901     task_executor->execute(phase3);
 902   } else {
 903     for (uint i = 0; i < _max_num_q; i++) {
 904       process_phase3(refs_lists[i], clear_referent,
 905                      is_alive, keep_alive, complete_gc);
 906     }
 907   }
 908 }
 909 
 910 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 911   uint id = 0;
 912   // Determine the queue index to use for this object.
 913   if (_discovery_is_mt) {
 914     // During a multi-threaded discovery phase,
 915     // each thread saves to its "own" list.
 916     Thread* thr = Thread::current();
 917     id = thr->as_Worker_thread()->id();
 918   } else {
 919     // single-threaded discovery, we save in round-robin
 920     // fashion to each of the lists.
 921     if (_processing_is_mt) {
 922       id = next_id();
 923     }
 924   }
 925   assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
 926 
 927   // Get the discovered queue to which we will add
 928   DiscoveredList* list = NULL;
 929   switch (rt) {
 930     case REF_OTHER:
 931       // Unknown reference type, no special treatment
 932       break;
 933     case REF_SOFT:
 934       list = &_discoveredSoftRefs[id];
 935       break;
 936     case REF_WEAK:
 937       list = &_discoveredWeakRefs[id];
 938       break;
 939     case REF_FINAL:
 940       list = &_discoveredFinalRefs[id];
 941       break;
 942     case REF_PHANTOM:
 943       list = &_discoveredPhantomRefs[id];
 944       break;
 945     case REF_CLEANER:
 946       list = &_discoveredCleanerRefs[id];
 947       break;
 948     case REF_NONE:
 949       // we should not reach here if we are an InstanceRefKlass
 950     default:
 951       ShouldNotReachHere();
 952   }
 953   if (TraceReferenceGC && PrintGCDetails) {
 954     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 955   }
 956   return list;
 957 }
 958 
 959 inline void
 960 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 961                                               oop             obj,
 962                                               HeapWord*       discovered_addr) {
 963   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 964   // First we must make sure this object is only enqueued once. CAS in a non null
 965   // discovered_addr.
 966   oop current_head = refs_list.head();
 967   // The last ref must have its discovered field pointing to itself.
 968   oop next_discovered = (current_head != NULL) ? current_head : obj;
 969 
 970   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
 971                                                     NULL);
 972   if (retest == NULL) {
 973     // This thread just won the right to enqueue the object.
 974     // We have separate lists for enqueueing, so no synchronization
 975     // is necessary.
 976     refs_list.set_head(obj);
 977     refs_list.inc_length(1);
 978 
 979     if (TraceReferenceGC) {
 980       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 981                              p2i(obj), obj->klass()->internal_name());
 982     }
 983   } else {
 984     // If retest was non NULL, another thread beat us to it:
 985     // The reference has already been discovered...
 986     if (TraceReferenceGC) {
 987       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
 988                              p2i(obj), obj->klass()->internal_name());
 989     }
 990   }
 991 }
 992 
 993 #ifndef PRODUCT
 994 // Non-atomic (i.e. concurrent) discovery might allow us
 995 // to observe j.l.References with NULL referents, being those
 996 // cleared concurrently by mutators during (or after) discovery.
 997 void ReferenceProcessor::verify_referent(oop obj) {
 998   bool da = discovery_is_atomic();
 999   oop referent = java_lang_ref_Reference::referent(obj);
1000   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1001          "Bad referent " INTPTR_FORMAT " found in Reference "
1002          INTPTR_FORMAT " during %satomic discovery ",
1003          p2i(referent), p2i(obj), da ? "" : "non-");
1004 }
1005 #endif
1006 
1007 // We mention two of several possible choices here:
1008 // #0: if the reference object is not in the "originating generation"
1009 //     (or part of the heap being collected, indicated by our "span"
1010 //     we don't treat it specially (i.e. we scan it as we would
1011 //     a normal oop, treating its references as strong references).
1012 //     This means that references can't be discovered unless their
1013 //     referent is also in the same span. This is the simplest,
1014 //     most "local" and most conservative approach, albeit one
1015 //     that may cause weak references to be enqueued least promptly.
1016 //     We call this choice the "ReferenceBasedDiscovery" policy.
1017 // #1: the reference object may be in any generation (span), but if
1018 //     the referent is in the generation (span) being currently collected
1019 //     then we can discover the reference object, provided
1020 //     the object has not already been discovered by
1021 //     a different concurrently running collector (as may be the
1022 //     case, for instance, if the reference object is in CMS and
1023 //     the referent in DefNewGeneration), and provided the processing
1024 //     of this reference object by the current collector will
1025 //     appear atomic to every other collector in the system.
1026 //     (Thus, for instance, a concurrent collector may not
1027 //     discover references in other generations even if the
1028 //     referent is in its own generation). This policy may,
1029 //     in certain cases, enqueue references somewhat sooner than
1030 //     might Policy #0 above, but at marginally increased cost
1031 //     and complexity in processing these references.
1032 //     We call this choice the "RefeferentBasedDiscovery" policy.
1033 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1034   // Make sure we are discovering refs (rather than processing discovered refs).
1035   if (!_discovering_refs || !RegisterReferences) {
1036     return false;
1037   }
1038   // We only discover active references.
1039   oop next = java_lang_ref_Reference::next(obj);
1040   if (next != NULL) {   // Ref is no longer active
1041     return false;
1042   }
1043 
1044   HeapWord* obj_addr = (HeapWord*)obj;
1045   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1046       !_span.contains(obj_addr)) {
1047     // Reference is not in the originating generation;
1048     // don't treat it specially (i.e. we want to scan it as a normal
1049     // object with strong references).
1050     return false;
1051   }
1052 
1053   // We only discover references whose referents are not (yet)
1054   // known to be strongly reachable.
1055   if (is_alive_non_header() != NULL) {
1056     verify_referent(obj);
1057     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1058       return false;  // referent is reachable
1059     }
1060   }
1061   if (rt == REF_SOFT) {
1062     // For soft refs we can decide now if these are not
1063     // current candidates for clearing, in which case we
1064     // can mark through them now, rather than delaying that
1065     // to the reference-processing phase. Since all current
1066     // time-stamp policies advance the soft-ref clock only
1067     // at a full collection cycle, this is always currently
1068     // accurate.
1069     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1070       return false;
1071     }
1072   }
1073 
1074   ResourceMark rm;      // Needed for tracing.
1075 
1076   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1077   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1078   assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1079   if (discovered != NULL) {
1080     // The reference has already been discovered...
1081     if (TraceReferenceGC) {
1082       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1083                              p2i(obj), obj->klass()->internal_name());
1084     }
1085     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1086       // assumes that an object is not processed twice;
1087       // if it's been already discovered it must be on another
1088       // generation's discovered list; so we won't discover it.
1089       return false;
1090     } else {
1091       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1092              "Unrecognized policy");
1093       // Check assumption that an object is not potentially
1094       // discovered twice except by concurrent collectors that potentially
1095       // trace the same Reference object twice.
1096       assert(UseConcMarkSweepGC || UseG1GC,
1097              "Only possible with a concurrent marking collector");
1098       return true;
1099     }
1100   }
1101 
1102   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1103     verify_referent(obj);
1104     // Discover if and only if EITHER:
1105     // .. reference is in our span, OR
1106     // .. we are an atomic collector and referent is in our span
1107     if (_span.contains(obj_addr) ||
1108         (discovery_is_atomic() &&
1109          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1110       // should_enqueue = true;
1111     } else {
1112       return false;
1113     }
1114   } else {
1115     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1116            _span.contains(obj_addr), "code inconsistency");
1117   }
1118 
1119   // Get the right type of discovered queue head.
1120   DiscoveredList* list = get_discovered_list(rt);
1121   if (list == NULL) {
1122     return false;   // nothing special needs to be done
1123   }
1124 
1125   if (_discovery_is_mt) {
1126     add_to_discovered_list_mt(*list, obj, discovered_addr);
1127   } else {
1128     // We do a raw store here: the field will be visited later when processing
1129     // the discovered references.
1130     oop current_head = list->head();
1131     // The last ref must have its discovered field pointing to itself.
1132     oop next_discovered = (current_head != NULL) ? current_head : obj;
1133 
1134     assert(discovered == NULL, "control point invariant");
1135     oop_store_raw(discovered_addr, next_discovered);
1136     list->set_head(obj);
1137     list->inc_length(1);
1138 
1139     if (TraceReferenceGC) {
1140       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1141                                 p2i(obj), obj->klass()->internal_name());
1142     }
1143   }
1144   assert(obj->is_oop(), "Discovered a bad reference");
1145   verify_referent(obj);
1146   return true;
1147 }
1148 
1149 // Preclean the discovered references by removing those
1150 // whose referents are alive, and by marking from those that
1151 // are not active. These lists can be handled here
1152 // in any order and, indeed, concurrently.
1153 void ReferenceProcessor::preclean_discovered_references(
1154   BoolObjectClosure* is_alive,
1155   OopClosure* keep_alive,
1156   VoidClosure* complete_gc,
1157   YieldClosure* yield,
1158   GCTimer* gc_timer) {
1159 
1160   // Soft references
1161   {
1162     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1163               false, gc_timer);
1164     for (uint i = 0; i < _max_num_q; i++) {
1165       if (yield->should_return()) {
1166         return;
1167       }
1168       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1169                                   keep_alive, complete_gc, yield);
1170     }
1171   }
1172 
1173   // Weak references
1174   {
1175     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1176               false, gc_timer);
1177     for (uint i = 0; i < _max_num_q; i++) {
1178       if (yield->should_return()) {
1179         return;
1180       }
1181       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1182                                   keep_alive, complete_gc, yield);
1183     }
1184   }
1185 
1186   // Final references
1187   {
1188     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1189               false, gc_timer);
1190     for (uint i = 0; i < _max_num_q; i++) {
1191       if (yield->should_return()) {
1192         return;
1193       }
1194       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1195                                   keep_alive, complete_gc, yield);
1196     }
1197   }
1198 
1199   // Phantom references
1200   {
1201     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1202               false, gc_timer);
1203     for (uint i = 0; i < _max_num_q; i++) {
1204       if (yield->should_return()) {
1205         return;
1206       }
1207       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1208                                   keep_alive, complete_gc, yield);
1209     }
1210 
1211     // Cleaner references.  Included in timing for phantom references.  We
1212     // expect Cleaner references to be temporary, and don't want to deal with
1213     // possible incompatibilities arising from making it more visible.
1214     for (uint i = 0; i < _max_num_q; i++) {
1215       if (yield->should_return()) {
1216         return;
1217       }
1218       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1219                                   keep_alive, complete_gc, yield);
1220     }
1221   }
1222 }
1223 
1224 // Walk the given discovered ref list, and remove all reference objects
1225 // whose referents are still alive, whose referents are NULL or which
1226 // are not active (have a non-NULL next field). NOTE: When we are
1227 // thus precleaning the ref lists (which happens single-threaded today),
1228 // we do not disable refs discovery to honor the correct semantics of
1229 // java.lang.Reference. As a result, we need to be careful below
1230 // that ref removal steps interleave safely with ref discovery steps
1231 // (in this thread).
1232 void
1233 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1234                                                 BoolObjectClosure* is_alive,
1235                                                 OopClosure*        keep_alive,
1236                                                 VoidClosure*       complete_gc,
1237                                                 YieldClosure*      yield) {
1238   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1239   while (iter.has_next()) {
1240     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1241     oop obj = iter.obj();
1242     oop next = java_lang_ref_Reference::next(obj);
1243     if (iter.referent() == NULL || iter.is_referent_alive() ||
1244         next != NULL) {
1245       // The referent has been cleared, or is alive, or the Reference is not
1246       // active; we need to trace and mark its cohort.
1247       if (TraceReferenceGC) {
1248         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1249                                p2i(iter.obj()), iter.obj()->klass()->internal_name());
1250       }
1251       // Remove Reference object from list
1252       iter.remove();
1253       // Keep alive its cohort.
1254       iter.make_referent_alive();
1255       if (UseCompressedOops) {
1256         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1257         keep_alive->do_oop(next_addr);
1258       } else {
1259         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1260         keep_alive->do_oop(next_addr);
1261       }
1262       iter.move_to_next();
1263     } else {
1264       iter.next();
1265     }
1266   }
1267   // Close the reachable set
1268   complete_gc->do_void();
1269 
1270   NOT_PRODUCT(
1271     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1272       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1273         " Refs in discovered list " INTPTR_FORMAT,
1274         iter.removed(), iter.processed(), p2i(refs_list.head()));
1275     }
1276   )
1277 }
1278 
1279 const char* ReferenceProcessor::list_name(uint i) {
1280    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1281           "Out of bounds index");
1282 
1283    int j = i / _max_num_q;
1284    switch (j) {
1285      case 0: return "SoftRef";
1286      case 1: return "WeakRef";
1287      case 2: return "FinalRef";
1288      case 3: return "PhantomRef";
1289      case 4: return "CleanerRef";
1290    }
1291    ShouldNotReachHere();
1292    return NULL;
1293 }
1294