1 /*
   2  * Copyright (c) 2001, 2011, 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_interface/collectedHeap.hpp"
  29 #include "gc_interface/collectedHeap.inline.hpp"
  30 #include "memory/referencePolicy.hpp"
  31 #include "memory/referenceProcessor.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/java.hpp"
  34 #include "runtime/jniHandles.hpp"
  35 
  36 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  37 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  38 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  39 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  40 
  41 void referenceProcessor_init() {
  42   ReferenceProcessor::init_statics();
  43 }
  44 
  45 void ReferenceProcessor::init_statics() {
  46   jlong now = os::javaTimeMillis();
  47 
  48   // Initialize the soft ref timestamp clock.
  49   _soft_ref_timestamp_clock = now;
  50   // Also update the soft ref clock in j.l.r.SoftReference
  51   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  52 
  53   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  54   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  55                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  56   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  57     vm_exit_during_initialization("Could not allocate reference policy object");
  58   }
  59   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  60             RefDiscoveryPolicy == ReferentBasedDiscovery,
  61             "Unrecongnized RefDiscoveryPolicy");
  62   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  63 }
  64 
  65 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  66 #ifdef ASSERT
  67   // Verify that we're not currently discovering refs
  68   assert(!verify_disabled || !_discovering_refs, "nested call?");
  69 
  70   if (check_no_refs) {
  71     // Verify that the discovered lists are empty
  72     verify_no_references_recorded();
  73   }
  74 #endif // ASSERT
  75 
  76   // Someone could have modified the value of the static
  77   // field in the j.l.r.SoftReference class that holds the
  78   // soft reference timestamp clock using reflection or
  79   // Unsafe between GCs. Unconditionally update the static
  80   // field in ReferenceProcessor here so that we use the new
  81   // value during reference discovery.
  82 
  83   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  84   _discovering_refs = true;
  85 }
  86 
  87 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  88                                        bool      mt_processing,
  89                                        int       mt_processing_degree,
  90                                        bool      mt_discovery,
  91                                        int       mt_discovery_degree,
  92                                        bool      atomic_discovery,
  93                                        BoolObjectClosure* is_alive_non_header,
  94                                        bool      discovered_list_needs_barrier)  :
  95   _discovering_refs(false),
  96   _enqueuing_is_done(false),
  97   _is_alive_non_header(is_alive_non_header),
  98   _discovered_list_needs_barrier(discovered_list_needs_barrier),
  99   _bs(NULL),
 100   _processing_is_mt(mt_processing),
 101   _next_id(0)
 102 {
 103   _span = span;
 104   _discovery_is_atomic = atomic_discovery;
 105   _discovery_is_mt     = mt_discovery;
 106   _num_q               = MAX2(1, mt_processing_degree);
 107   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 108   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 109                                           _max_num_q * number_of_subclasses_of_ref());
 110   if (_discovered_refs == NULL) {
 111     vm_exit_during_initialization("Could not allocated RefProc Array");
 112   }
 113   _discoveredSoftRefs    = &_discovered_refs[0];
 114   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 115   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 116   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 117 
 118   // Initialize all entries to NULL
 119   for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 120     _discovered_refs[i].set_head(NULL);
 121     _discovered_refs[i].set_length(0);
 122   }
 123 
 124   // If we do barriers, cache a copy of the barrier set.
 125   if (discovered_list_needs_barrier) {
 126     _bs = Universe::heap()->barrier_set();
 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 (int 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 (int 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   jlong now = os::javaTimeMillis();
 155   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 156   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 157 
 158   NOT_PRODUCT(
 159   if (now < _soft_ref_timestamp_clock) {
 160     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 161             _soft_ref_timestamp_clock, now);
 162   }
 163   )
 164   // In product mode, protect ourselves from system time being adjusted
 165   // externally and going backward; see note in the implementation of
 166   // GenCollectedHeap::time_since_last_gc() for the right way to fix
 167   // this uniformly throughout the VM; see bug-id 4741166. XXX
 168   if (now > _soft_ref_timestamp_clock) {
 169     _soft_ref_timestamp_clock = now;
 170     java_lang_ref_SoftReference::set_clock(now);
 171   }
 172   // Else leave clock stalled at its old value until time progresses
 173   // past clock value.
 174 }
 175 
 176 void ReferenceProcessor::process_discovered_references(
 177   BoolObjectClosure*           is_alive,
 178   OopClosure*                  keep_alive,
 179   VoidClosure*                 complete_gc,
 180   AbstractRefProcTaskExecutor* task_executor) {
 181   NOT_PRODUCT(verify_ok_to_handle_reflists());
 182 
 183   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 184   // Stop treating discovered references specially.
 185   disable_discovery();
 186 
 187   // If discovery was concurrent, someone could have modified
 188   // the value of the static field in the j.l.r.SoftReference
 189   // class that holds the soft reference timestamp clock using
 190   // reflection or Unsafe between when discovery was enabled and
 191   // now. Unconditionally update the static field in ReferenceProcessor
 192   // here so that we use the new value during processing of the
 193   // discovered soft refs.
 194 
 195   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 196 
 197   bool trace_time = PrintGCDetails && PrintReferenceGC;
 198   // Soft references
 199   {
 200     TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
 201     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 202                                is_alive, keep_alive, complete_gc, task_executor);
 203   }
 204 
 205   update_soft_ref_master_clock();
 206 
 207   // Weak references
 208   {
 209     TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
 210     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 211                                is_alive, keep_alive, complete_gc, task_executor);
 212   }
 213 
 214   // Final references
 215   {
 216     TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
 217     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 218                                is_alive, keep_alive, complete_gc, task_executor);
 219   }
 220 
 221   // Phantom references
 222   {
 223     TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
 224     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 225                                is_alive, keep_alive, complete_gc, task_executor);
 226   }
 227 
 228   // Weak global JNI references. It would make more sense (semantically) to
 229   // traverse these simultaneously with the regular weak references above, but
 230   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 231   // thus use JNI weak references to circumvent the phantom references and
 232   // resurrect a "post-mortem" object.
 233   {
 234     TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
 235     if (task_executor != NULL) {
 236       task_executor->set_single_threaded_mode();
 237     }
 238     process_phaseJNI(is_alive, keep_alive, complete_gc);
 239   }
 240 }
 241 
 242 #ifndef PRODUCT
 243 // Calculate the number of jni handles.
 244 uint ReferenceProcessor::count_jni_refs() {
 245   class AlwaysAliveClosure: public BoolObjectClosure {
 246   public:
 247     virtual bool do_object_b(oop obj) { return true; }
 248     virtual void do_object(oop obj) { assert(false, "Don't call"); }
 249   };
 250 
 251   class CountHandleClosure: public OopClosure {
 252   private:
 253     int _count;
 254   public:
 255     CountHandleClosure(): _count(0) {}
 256     void do_oop(oop* unused)       { _count++; }
 257     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 258     int count() { return _count; }
 259   };
 260   CountHandleClosure global_handle_count;
 261   AlwaysAliveClosure always_alive;
 262   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 263   return global_handle_count.count();
 264 }
 265 #endif
 266 
 267 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 268                                           OopClosure*        keep_alive,
 269                                           VoidClosure*       complete_gc) {
 270 #ifndef PRODUCT
 271   if (PrintGCDetails && PrintReferenceGC) {
 272     unsigned int count = count_jni_refs();
 273     gclog_or_tty->print(", %u refs", count);
 274   }
 275 #endif
 276   JNIHandles::weak_oops_do(is_alive, keep_alive);
 277   complete_gc->do_void();
 278 }
 279 
 280 
 281 template <class T>
 282 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 283                                    AbstractRefProcTaskExecutor* task_executor) {
 284 
 285   // Remember old value of pending references list
 286   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 287   T old_pending_list_value = *pending_list_addr;
 288 
 289   // Enqueue references that are not made active again, and
 290   // clear the decks for the next collection (cycle).
 291   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 292   // Do the oop-check on pending_list_addr missed in
 293   // enqueue_discovered_reflist. We should probably
 294   // do a raw oop_check so that future such idempotent
 295   // oop_stores relying on the oop-check side-effect
 296   // may be elided automatically and safely without
 297   // affecting correctness.
 298   oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 299 
 300   // Stop treating discovered references specially.
 301   ref->disable_discovery();
 302 
 303   // Return true if new pending references were added
 304   return old_pending_list_value != *pending_list_addr;
 305 }
 306 
 307 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 308   NOT_PRODUCT(verify_ok_to_handle_reflists());
 309   if (UseCompressedOops) {
 310     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 311   } else {
 312     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 313   }
 314 }
 315 
 316 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 317                                                     HeapWord* pending_list_addr) {
 318   // Given a list of refs linked through the "discovered" field
 319   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 320   // thus distinguishing them from active References, then
 321   // prepend them to the pending list.
 322   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 323   // the "next" field is used to chain the pending list, not the discovered
 324   // field.
 325 
 326   if (TraceReferenceGC && PrintGCDetails) {
 327     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 328                            INTPTR_FORMAT, (address)refs_list.head());
 329   }
 330 
 331   oop obj = NULL;
 332   oop next_d = refs_list.head();
 333   if (pending_list_uses_discovered_field()) { // New behaviour
 334     // Walk down the list, self-looping the next field
 335     // so that the References are not considered active.
 336     while (obj != next_d) {
 337       obj = next_d;
 338       assert(obj->is_instanceRef(), "should be reference object");
 339       next_d = java_lang_ref_Reference::discovered(obj);
 340       if (TraceReferenceGC && PrintGCDetails) {
 341         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 342                                obj, next_d);
 343       }
 344       assert(java_lang_ref_Reference::next(obj) == NULL,
 345              "Reference not active; should not be discovered");
 346       // Self-loop next, so as to make Ref not active.
 347       java_lang_ref_Reference::set_next(obj, obj);
 348       if (next_d == obj) {  // obj is last
 349         // Swap refs_list into pendling_list_addr and
 350         // set obj's discovered to what we read from pending_list_addr.
 351         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 352         // Need oop_check on pending_list_addr above;
 353         // see special oop-check code at the end of
 354         // enqueue_discovered_reflists() further below.
 355         java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
 356       }
 357     }
 358   } else { // Old behaviour
 359     // Walk down the list, copying the discovered field into
 360     // the next field and clearing the discovered field.
 361     while (obj != next_d) {
 362       obj = next_d;
 363       assert(obj->is_instanceRef(), "should be reference object");
 364       next_d = java_lang_ref_Reference::discovered(obj);
 365       if (TraceReferenceGC && PrintGCDetails) {
 366         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 367                                obj, next_d);
 368       }
 369       assert(java_lang_ref_Reference::next(obj) == NULL,
 370              "The reference should not be enqueued");
 371       if (next_d == obj) {  // obj is last
 372         // Swap refs_list into pendling_list_addr and
 373         // set obj's next to what we read from pending_list_addr.
 374         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 375         // Need oop_check on pending_list_addr above;
 376         // see special oop-check code at the end of
 377         // enqueue_discovered_reflists() further below.
 378         if (old == NULL) {
 379           // obj should be made to point to itself, since
 380           // pending list was empty.
 381           java_lang_ref_Reference::set_next(obj, obj);
 382         } else {
 383           java_lang_ref_Reference::set_next(obj, old);
 384         }
 385       } else {
 386         java_lang_ref_Reference::set_next(obj, next_d);
 387       }
 388       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 389     }
 390   }
 391 }
 392 
 393 // Parallel enqueue task
 394 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 395 public:
 396   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 397                      DiscoveredList      discovered_refs[],
 398                      HeapWord*           pending_list_addr,
 399                      int                 n_queues)
 400     : EnqueueTask(ref_processor, discovered_refs,
 401                   pending_list_addr, n_queues)
 402   { }
 403 
 404   virtual void work(unsigned int work_id) {
 405     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 406     // Simplest first cut: static partitioning.
 407     int index = work_id;
 408     // The increment on "index" must correspond to the maximum number of queues
 409     // (n_queues) with which that ReferenceProcessor was created.  That
 410     // is because of the "clever" way the discovered references lists were
 411     // allocated and are indexed into.
 412     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 413     for (int j = 0;
 414          j < ReferenceProcessor::number_of_subclasses_of_ref();
 415          j++, index += _n_queues) {
 416       _ref_processor.enqueue_discovered_reflist(
 417         _refs_lists[index], _pending_list_addr);
 418       _refs_lists[index].set_head(NULL);
 419       _refs_lists[index].set_length(0);
 420     }
 421   }
 422 };
 423 
 424 // Enqueue references that are not made active again
 425 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 426   AbstractRefProcTaskExecutor* task_executor) {
 427   if (_processing_is_mt && task_executor != NULL) {
 428     // Parallel code
 429     RefProcEnqueueTask tsk(*this, _discovered_refs,
 430                            pending_list_addr, _max_num_q);
 431     task_executor->execute(tsk);
 432   } else {
 433     // Serial code: call the parent class's implementation
 434     for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 435       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 436       _discovered_refs[i].set_head(NULL);
 437       _discovered_refs[i].set_length(0);
 438     }
 439   }
 440 }
 441 
 442 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 443   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 444   oop discovered = java_lang_ref_Reference::discovered(_ref);
 445   assert(_discovered_addr && discovered->is_oop_or_null(),
 446          "discovered field is bad");
 447   _next = discovered;
 448   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 449   _referent = java_lang_ref_Reference::referent(_ref);
 450   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 451          "Wrong oop found in java.lang.Reference object");
 452   assert(allow_null_referent ?
 453              _referent->is_oop_or_null()
 454            : _referent->is_oop(),
 455          "bad referent");
 456 }
 457 
 458 void DiscoveredListIterator::remove() {
 459   assert(_ref->is_oop(), "Dropping a bad reference");
 460   oop_store_raw(_discovered_addr, NULL);
 461 
 462   // First _prev_next ref actually points into DiscoveredList (gross).
 463   oop new_next;
 464   if (_next == _ref) {
 465     // At the end of the list, we should make _prev point to itself.
 466     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 467     // and _prev will be NULL.
 468     new_next = _prev;
 469   } else {
 470     new_next = _next;
 471   }
 472 
 473   if (UseCompressedOops) {
 474     // Remove Reference object from list.
 475     oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
 476   } else {
 477     // Remove Reference object from list.
 478     oopDesc::store_heap_oop((oop*)_prev_next, new_next);
 479   }
 480   NOT_PRODUCT(_removed++);
 481   _refs_list.dec_length(1);
 482 }
 483 
 484 // Make the Reference object active again.
 485 void DiscoveredListIterator::make_active() {
 486   // For G1 we don't want to use set_next - it
 487   // will dirty the card for the next field of
 488   // the reference object and will fail
 489   // CT verification.
 490   if (UseG1GC) {
 491     BarrierSet* bs = oopDesc::bs();
 492     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 493 
 494     if (UseCompressedOops) {
 495       bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
 496     } else {
 497       bs->write_ref_field_pre((oop*)next_addr, NULL);
 498     }
 499     java_lang_ref_Reference::set_next_raw(_ref, NULL);
 500   } else {
 501     java_lang_ref_Reference::set_next(_ref, NULL);
 502   }
 503 }
 504 
 505 void DiscoveredListIterator::clear_referent() {
 506   oop_store_raw(_referent_addr, NULL);
 507 }
 508 
 509 // NOTE: process_phase*() are largely similar, and at a high level
 510 // merely iterate over the extant list applying a predicate to
 511 // each of its elements and possibly removing that element from the
 512 // list and applying some further closures to that element.
 513 // We should consider the possibility of replacing these
 514 // process_phase*() methods by abstracting them into
 515 // a single general iterator invocation that receives appropriate
 516 // closures that accomplish this work.
 517 
 518 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 519 // referents are not alive, but that should be kept alive for policy reasons.
 520 // Keep alive the transitive closure of all such referents.
 521 void
 522 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 523                                    ReferencePolicy*   policy,
 524                                    BoolObjectClosure* is_alive,
 525                                    OopClosure*        keep_alive,
 526                                    VoidClosure*       complete_gc) {
 527   assert(policy != NULL, "Must have a non-NULL policy");
 528   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 529   // Decide which softly reachable refs should be kept alive.
 530   while (iter.has_next()) {
 531     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 532     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 533     if (referent_is_dead &&
 534         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 535       if (TraceReferenceGC) {
 536         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 537                                iter.obj(), iter.obj()->blueprint()->internal_name());
 538       }
 539       // Remove Reference object from list
 540       iter.remove();
 541       // Make the Reference object active again
 542       iter.make_active();
 543       // keep the referent around
 544       iter.make_referent_alive();
 545       iter.move_to_next();
 546     } else {
 547       iter.next();
 548     }
 549   }
 550   // Close the reachable set
 551   complete_gc->do_void();
 552   NOT_PRODUCT(
 553     if (PrintGCDetails && TraceReferenceGC) {
 554       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 555         "discovered Refs by policy, from list " INTPTR_FORMAT,
 556         iter.removed(), iter.processed(), (address)refs_list.head());
 557     }
 558   )
 559 }
 560 
 561 // Traverse the list and remove any Refs that are not active, or
 562 // whose referents are either alive or NULL.
 563 void
 564 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 565                              BoolObjectClosure* is_alive,
 566                              OopClosure*        keep_alive) {
 567   assert(discovery_is_atomic(), "Error");
 568   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 569   while (iter.has_next()) {
 570     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 571     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 572     assert(next == NULL, "Should not discover inactive Reference");
 573     if (iter.is_referent_alive()) {
 574       if (TraceReferenceGC) {
 575         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 576                                iter.obj(), iter.obj()->blueprint()->internal_name());
 577       }
 578       // The referent is reachable after all.
 579       // Remove Reference object from list.
 580       iter.remove();
 581       // Update the referent pointer as necessary: Note that this
 582       // should not entail any recursive marking because the
 583       // referent must already have been traversed.
 584       iter.make_referent_alive();
 585       iter.move_to_next();
 586     } else {
 587       iter.next();
 588     }
 589   }
 590   NOT_PRODUCT(
 591     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 592       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 593         "Refs in discovered list " INTPTR_FORMAT,
 594         iter.removed(), iter.processed(), (address)refs_list.head());
 595     }
 596   )
 597 }
 598 
 599 void
 600 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 601                                                   BoolObjectClosure* is_alive,
 602                                                   OopClosure*        keep_alive,
 603                                                   VoidClosure*       complete_gc) {
 604   assert(!discovery_is_atomic(), "Error");
 605   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 606   while (iter.has_next()) {
 607     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 608     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 609     oop next = java_lang_ref_Reference::next(iter.obj());
 610     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 611          next != NULL)) {
 612       assert(next->is_oop_or_null(), "bad next field");
 613       // Remove Reference object from list
 614       iter.remove();
 615       // Trace the cohorts
 616       iter.make_referent_alive();
 617       if (UseCompressedOops) {
 618         keep_alive->do_oop((narrowOop*)next_addr);
 619       } else {
 620         keep_alive->do_oop((oop*)next_addr);
 621       }
 622       iter.move_to_next();
 623     } else {
 624       iter.next();
 625     }
 626   }
 627   // Now close the newly reachable set
 628   complete_gc->do_void();
 629   NOT_PRODUCT(
 630     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 631       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 632         "Refs in discovered list " INTPTR_FORMAT,
 633         iter.removed(), iter.processed(), (address)refs_list.head());
 634     }
 635   )
 636 }
 637 
 638 // Traverse the list and process the referents, by either
 639 // clearing them or keeping them (and their reachable
 640 // closure) alive.
 641 void
 642 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 643                                    bool               clear_referent,
 644                                    BoolObjectClosure* is_alive,
 645                                    OopClosure*        keep_alive,
 646                                    VoidClosure*       complete_gc) {
 647   ResourceMark rm;
 648   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 649   while (iter.has_next()) {
 650     iter.update_discovered();
 651     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 652     if (clear_referent) {
 653       // NULL out referent pointer
 654       iter.clear_referent();
 655     } else {
 656       // keep the referent around
 657       iter.make_referent_alive();
 658     }
 659     if (TraceReferenceGC) {
 660       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 661                              clear_referent ? "cleared " : "",
 662                              iter.obj(), iter.obj()->blueprint()->internal_name());
 663     }
 664     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 665     iter.next();
 666   }
 667   // Remember to update the next pointer of the last ref.
 668   iter.update_discovered();
 669   // Close the reachable set
 670   complete_gc->do_void();
 671 }
 672 
 673 void
 674 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 675   oop obj = NULL;
 676   oop next = refs_list.head();
 677   while (next != obj) {
 678     obj = next;
 679     next = java_lang_ref_Reference::discovered(obj);
 680     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 681   }
 682   refs_list.set_head(NULL);
 683   refs_list.set_length(0);
 684 }
 685 
 686 void
 687 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 688   clear_discovered_references(refs_list);
 689 }
 690 
 691 void ReferenceProcessor::abandon_partial_discovery() {
 692   // loop over the lists
 693   for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 694     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 695       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 696     }
 697     abandon_partial_discovered_list(_discovered_refs[i]);
 698   }
 699 }
 700 
 701 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 702 public:
 703   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 704                     DiscoveredList      refs_lists[],
 705                     ReferencePolicy*    policy,
 706                     bool                marks_oops_alive)
 707     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 708       _policy(policy)
 709   { }
 710   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 711                     OopClosure& keep_alive,
 712                     VoidClosure& complete_gc)
 713   {
 714     Thread* thr = Thread::current();
 715     int refs_list_index = ((WorkerThread*)thr)->id();
 716     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 717                                   &is_alive, &keep_alive, &complete_gc);
 718   }
 719 private:
 720   ReferencePolicy* _policy;
 721 };
 722 
 723 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 724 public:
 725   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 726                     DiscoveredList      refs_lists[],
 727                     bool                marks_oops_alive)
 728     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 729   { }
 730   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 731                     OopClosure& keep_alive,
 732                     VoidClosure& complete_gc)
 733   {
 734     _ref_processor.process_phase2(_refs_lists[i],
 735                                   &is_alive, &keep_alive, &complete_gc);
 736   }
 737 };
 738 
 739 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 740 public:
 741   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 742                     DiscoveredList      refs_lists[],
 743                     bool                clear_referent,
 744                     bool                marks_oops_alive)
 745     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 746       _clear_referent(clear_referent)
 747   { }
 748   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 749                     OopClosure& keep_alive,
 750                     VoidClosure& complete_gc)
 751   {
 752     // Don't use "refs_list_index" calculated in this way because
 753     // balance_queues() has moved the Ref's into the first n queues.
 754     // Thread* thr = Thread::current();
 755     // int refs_list_index = ((WorkerThread*)thr)->id();
 756     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 757     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 758                                   &is_alive, &keep_alive, &complete_gc);
 759   }
 760 private:
 761   bool _clear_referent;
 762 };
 763 
 764 void ReferenceProcessor::set_discovered(oop ref, oop value) {
 765   if (_discovered_list_needs_barrier) {
 766     java_lang_ref_Reference::set_discovered(ref, value);
 767   } else {
 768     java_lang_ref_Reference::set_discovered_raw(ref, value);
 769   }
 770 }
 771 
 772 // Balances reference queues.
 773 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 774 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 775 // corresponding to the active workers will be processed.
 776 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 777 {
 778   // calculate total length
 779   size_t total_refs = 0;
 780   if (TraceReferenceGC && PrintGCDetails) {
 781     gclog_or_tty->print_cr("\nBalance ref_lists ");
 782   }
 783 
 784   for (int i = 0; i < _max_num_q; ++i) {
 785     total_refs += ref_lists[i].length();
 786     if (TraceReferenceGC && PrintGCDetails) {
 787       gclog_or_tty->print("%d ", ref_lists[i].length());
 788     }
 789   }
 790   if (TraceReferenceGC && PrintGCDetails) {
 791     gclog_or_tty->print_cr(" = %d", total_refs);
 792   }
 793   size_t avg_refs = total_refs / _num_q + 1;
 794   int to_idx = 0;
 795   for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
 796     bool move_all = false;
 797     if (from_idx >= _num_q) {
 798       move_all = ref_lists[from_idx].length() > 0;
 799     }
 800     while ((ref_lists[from_idx].length() > avg_refs) ||
 801            move_all) {
 802       assert(to_idx < _num_q, "Sanity Check!");
 803       if (ref_lists[to_idx].length() < avg_refs) {
 804         // move superfluous refs
 805         size_t refs_to_move;
 806         // Move all the Ref's if the from queue will not be processed.
 807         if (move_all) {
 808           refs_to_move = MIN2(ref_lists[from_idx].length(),
 809                               avg_refs - ref_lists[to_idx].length());
 810         } else {
 811           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 812                               avg_refs - ref_lists[to_idx].length());
 813         }
 814 
 815         assert(refs_to_move > 0, "otherwise the code below will fail");
 816 
 817         oop move_head = ref_lists[from_idx].head();
 818         oop move_tail = move_head;
 819         oop new_head  = move_head;
 820         // find an element to split the list on
 821         for (size_t j = 0; j < refs_to_move; ++j) {
 822           move_tail = new_head;
 823           new_head = java_lang_ref_Reference::discovered(new_head);
 824         }
 825 
 826         // Add the chain to the to list.
 827         if (ref_lists[to_idx].head() == NULL) {
 828           // to list is empty. Make a loop at the end.
 829           set_discovered(move_tail, move_tail);
 830         } else {
 831           set_discovered(move_tail, ref_lists[to_idx].head());
 832         }
 833         ref_lists[to_idx].set_head(move_head);
 834         ref_lists[to_idx].inc_length(refs_to_move);
 835 
 836         // Remove the chain from the from list.
 837         if (move_tail == new_head) {
 838           // We found the end of the from list.
 839           ref_lists[from_idx].set_head(NULL);
 840         } else {
 841           ref_lists[from_idx].set_head(new_head);
 842         }
 843         ref_lists[from_idx].dec_length(refs_to_move);
 844         if (ref_lists[from_idx].length() == 0) {
 845           break;
 846         }
 847       } else {
 848         to_idx = (to_idx + 1) % _num_q;
 849       }
 850     }
 851   }
 852 #ifdef ASSERT
 853   size_t balanced_total_refs = 0;
 854   for (int i = 0; i < _max_num_q; ++i) {
 855     balanced_total_refs += ref_lists[i].length();
 856     if (TraceReferenceGC && PrintGCDetails) {
 857       gclog_or_tty->print("%d ", ref_lists[i].length());
 858     }
 859   }
 860   if (TraceReferenceGC && PrintGCDetails) {
 861     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 862     gclog_or_tty->flush();
 863   }
 864   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 865 #endif
 866 }
 867 
 868 void ReferenceProcessor::balance_all_queues() {
 869   balance_queues(_discoveredSoftRefs);
 870   balance_queues(_discoveredWeakRefs);
 871   balance_queues(_discoveredFinalRefs);
 872   balance_queues(_discoveredPhantomRefs);
 873 }
 874 
 875 void
 876 ReferenceProcessor::process_discovered_reflist(
 877   DiscoveredList               refs_lists[],
 878   ReferencePolicy*             policy,
 879   bool                         clear_referent,
 880   BoolObjectClosure*           is_alive,
 881   OopClosure*                  keep_alive,
 882   VoidClosure*                 complete_gc,
 883   AbstractRefProcTaskExecutor* task_executor)
 884 {
 885   bool mt_processing = task_executor != NULL && _processing_is_mt;
 886   // If discovery used MT and a dynamic number of GC threads, then
 887   // the queues must be balanced for correctness if fewer than the
 888   // maximum number of queues were used.  The number of queue used
 889   // during discovery may be different than the number to be used
 890   // for processing so don't depend of _num_q < _max_num_q as part
 891   // of the test.
 892   bool must_balance = _discovery_is_mt;
 893 
 894   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 895       must_balance) {
 896     balance_queues(refs_lists);
 897   }
 898   if (PrintReferenceGC && PrintGCDetails) {
 899     size_t total = 0;
 900     for (int i = 0; i < _max_num_q; ++i) {
 901       total += refs_lists[i].length();
 902     }
 903     gclog_or_tty->print(", %u refs", total);
 904   }
 905 
 906   // Phase 1 (soft refs only):
 907   // . Traverse the list and remove any SoftReferences whose
 908   //   referents are not alive, but that should be kept alive for
 909   //   policy reasons. Keep alive the transitive closure of all
 910   //   such referents.
 911   if (policy != NULL) {
 912     if (mt_processing) {
 913       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 914       task_executor->execute(phase1);
 915     } else {
 916       for (int i = 0; i < _max_num_q; i++) {
 917         process_phase1(refs_lists[i], policy,
 918                        is_alive, keep_alive, complete_gc);
 919       }
 920     }
 921   } else { // policy == NULL
 922     assert(refs_lists != _discoveredSoftRefs,
 923            "Policy must be specified for soft references.");
 924   }
 925 
 926   // Phase 2:
 927   // . Traverse the list and remove any refs whose referents are alive.
 928   if (mt_processing) {
 929     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 930     task_executor->execute(phase2);
 931   } else {
 932     for (int i = 0; i < _max_num_q; i++) {
 933       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 934     }
 935   }
 936 
 937   // Phase 3:
 938   // . Traverse the list and process referents as appropriate.
 939   if (mt_processing) {
 940     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 941     task_executor->execute(phase3);
 942   } else {
 943     for (int i = 0; i < _max_num_q; i++) {
 944       process_phase3(refs_lists[i], clear_referent,
 945                      is_alive, keep_alive, complete_gc);
 946     }
 947   }
 948 }
 949 
 950 void ReferenceProcessor::clean_up_discovered_references() {
 951   // loop over the lists
 952   for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 953     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 954       gclog_or_tty->print_cr(
 955         "\nScrubbing %s discovered list of Null referents",
 956         list_name(i));
 957     }
 958     clean_up_discovered_reflist(_discovered_refs[i]);
 959   }
 960 }
 961 
 962 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 963   assert(!discovery_is_atomic(), "Else why call this method?");
 964   DiscoveredListIterator iter(refs_list, NULL, NULL);
 965   while (iter.has_next()) {
 966     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 967     oop next = java_lang_ref_Reference::next(iter.obj());
 968     assert(next->is_oop_or_null(), "bad next field");
 969     // If referent has been cleared or Reference is not active,
 970     // drop it.
 971     if (iter.referent() == NULL || next != NULL) {
 972       debug_only(
 973         if (PrintGCDetails && TraceReferenceGC) {
 974           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 975             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 976             " and referent: " INTPTR_FORMAT,
 977             iter.obj(), next, iter.referent());
 978         }
 979       )
 980       // Remove Reference object from list
 981       iter.remove();
 982       iter.move_to_next();
 983     } else {
 984       iter.next();
 985     }
 986   }
 987   NOT_PRODUCT(
 988     if (PrintGCDetails && TraceReferenceGC) {
 989       gclog_or_tty->print(
 990         " Removed %d Refs with NULL referents out of %d discovered Refs",
 991         iter.removed(), iter.processed());
 992     }
 993   )
 994 }
 995 
 996 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 997   int id = 0;
 998   // Determine the queue index to use for this object.
 999   if (_discovery_is_mt) {
1000     // During a multi-threaded discovery phase,
1001     // each thread saves to its "own" list.
1002     Thread* thr = Thread::current();
1003     id = thr->as_Worker_thread()->id();
1004   } else {
1005     // single-threaded discovery, we save in round-robin
1006     // fashion to each of the lists.
1007     if (_processing_is_mt) {
1008       id = next_id();
1009     }
1010   }
1011   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1012 
1013   // Get the discovered queue to which we will add
1014   DiscoveredList* list = NULL;
1015   switch (rt) {
1016     case REF_OTHER:
1017       // Unknown reference type, no special treatment
1018       break;
1019     case REF_SOFT:
1020       list = &_discoveredSoftRefs[id];
1021       break;
1022     case REF_WEAK:
1023       list = &_discoveredWeakRefs[id];
1024       break;
1025     case REF_FINAL:
1026       list = &_discoveredFinalRefs[id];
1027       break;
1028     case REF_PHANTOM:
1029       list = &_discoveredPhantomRefs[id];
1030       break;
1031     case REF_NONE:
1032       // we should not reach here if we are an instanceRefKlass
1033     default:
1034       ShouldNotReachHere();
1035   }
1036   if (TraceReferenceGC && PrintGCDetails) {
1037     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1038   }
1039   return list;
1040 }
1041 
1042 inline void
1043 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1044                                               oop             obj,
1045                                               HeapWord*       discovered_addr) {
1046   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1047   // First we must make sure this object is only enqueued once. CAS in a non null
1048   // discovered_addr.
1049   oop current_head = refs_list.head();
1050   // The last ref must have its discovered field pointing to itself.
1051   oop next_discovered = (current_head != NULL) ? current_head : obj;
1052 
1053   // Note: In the case of G1, this specific pre-barrier is strictly
1054   // not necessary because the only case we are interested in
1055   // here is when *discovered_addr is NULL (see the CAS further below),
1056   // so this will expand to nothing. As a result, we have manually
1057   // elided this out for G1, but left in the test for some future
1058   // collector that might have need for a pre-barrier here, e.g.:-
1059   // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1060   assert(!_discovered_list_needs_barrier || UseG1GC,
1061          "Need to check non-G1 collector: "
1062          "may need a pre-write-barrier for CAS from NULL below");
1063   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1064                                                     NULL);
1065   if (retest == NULL) {
1066     // This thread just won the right to enqueue the object.
1067     // We have separate lists for enqueueing, so no synchronization
1068     // is necessary.
1069     refs_list.set_head(obj);
1070     refs_list.inc_length(1);
1071     if (_discovered_list_needs_barrier) {
1072       _bs->write_ref_field((void*)discovered_addr, next_discovered);
1073     }
1074 
1075     if (TraceReferenceGC) {
1076       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1077                              obj, obj->blueprint()->internal_name());
1078     }
1079   } else {
1080     // If retest was non NULL, another thread beat us to it:
1081     // The reference has already been discovered...
1082     if (TraceReferenceGC) {
1083       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1084                              obj, obj->blueprint()->internal_name());
1085     }
1086   }
1087 }
1088 
1089 #ifndef PRODUCT
1090 // Non-atomic (i.e. concurrent) discovery might allow us
1091 // to observe j.l.References with NULL referents, being those
1092 // cleared concurrently by mutators during (or after) discovery.
1093 void ReferenceProcessor::verify_referent(oop obj) {
1094   bool da = discovery_is_atomic();
1095   oop referent = java_lang_ref_Reference::referent(obj);
1096   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1097          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1098                  INTPTR_FORMAT " during %satomic discovery ",
1099                  (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1100 }
1101 #endif
1102 
1103 // We mention two of several possible choices here:
1104 // #0: if the reference object is not in the "originating generation"
1105 //     (or part of the heap being collected, indicated by our "span"
1106 //     we don't treat it specially (i.e. we scan it as we would
1107 //     a normal oop, treating its references as strong references).
1108 //     This means that references can't be discovered unless their
1109 //     referent is also in the same span. This is the simplest,
1110 //     most "local" and most conservative approach, albeit one
1111 //     that may cause weak references to be enqueued least promptly.
1112 //     We call this choice the "ReferenceBasedDiscovery" policy.
1113 // #1: the reference object may be in any generation (span), but if
1114 //     the referent is in the generation (span) being currently collected
1115 //     then we can discover the reference object, provided
1116 //     the object has not already been discovered by
1117 //     a different concurrently running collector (as may be the
1118 //     case, for instance, if the reference object is in CMS and
1119 //     the referent in DefNewGeneration), and provided the processing
1120 //     of this reference object by the current collector will
1121 //     appear atomic to every other collector in the system.
1122 //     (Thus, for instance, a concurrent collector may not
1123 //     discover references in other generations even if the
1124 //     referent is in its own generation). This policy may,
1125 //     in certain cases, enqueue references somewhat sooner than
1126 //     might Policy #0 above, but at marginally increased cost
1127 //     and complexity in processing these references.
1128 //     We call this choice the "RefeferentBasedDiscovery" policy.
1129 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1130   // Make sure we are discovering refs (rather than processing discovered refs).
1131   if (!_discovering_refs || !RegisterReferences) {
1132     return false;
1133   }
1134   // We only discover active references.
1135   oop next = java_lang_ref_Reference::next(obj);
1136   if (next != NULL) {   // Ref is no longer active
1137     return false;
1138   }
1139 
1140   HeapWord* obj_addr = (HeapWord*)obj;
1141   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1142       !_span.contains(obj_addr)) {
1143     // Reference is not in the originating generation;
1144     // don't treat it specially (i.e. we want to scan it as a normal
1145     // object with strong references).
1146     return false;
1147   }
1148 
1149   // We only discover references whose referents are not (yet)
1150   // known to be strongly reachable.
1151   if (is_alive_non_header() != NULL) {
1152     verify_referent(obj);
1153     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1154       return false;  // referent is reachable
1155     }
1156   }
1157   if (rt == REF_SOFT) {
1158     // For soft refs we can decide now if these are not
1159     // current candidates for clearing, in which case we
1160     // can mark through them now, rather than delaying that
1161     // to the reference-processing phase. Since all current
1162     // time-stamp policies advance the soft-ref clock only
1163     // at a major collection cycle, this is always currently
1164     // accurate.
1165     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1166       return false;
1167     }
1168   }
1169 
1170   ResourceMark rm;      // Needed for tracing.
1171 
1172   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1173   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1174   assert(discovered->is_oop_or_null(), "bad discovered field");
1175   if (discovered != NULL) {
1176     // The reference has already been discovered...
1177     if (TraceReferenceGC) {
1178       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1179                              obj, obj->blueprint()->internal_name());
1180     }
1181     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1182       // assumes that an object is not processed twice;
1183       // if it's been already discovered it must be on another
1184       // generation's discovered list; so we won't discover it.
1185       return false;
1186     } else {
1187       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1188              "Unrecognized policy");
1189       // Check assumption that an object is not potentially
1190       // discovered twice except by concurrent collectors that potentially
1191       // trace the same Reference object twice.
1192       assert(UseConcMarkSweepGC || UseG1GC,
1193              "Only possible with a concurrent marking collector");
1194       return true;
1195     }
1196   }
1197 
1198   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1199     verify_referent(obj);
1200     // Discover if and only if EITHER:
1201     // .. reference is in our span, OR
1202     // .. we are an atomic collector and referent is in our span
1203     if (_span.contains(obj_addr) ||
1204         (discovery_is_atomic() &&
1205          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1206       // should_enqueue = true;
1207     } else {
1208       return false;
1209     }
1210   } else {
1211     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1212            _span.contains(obj_addr), "code inconsistency");
1213   }
1214 
1215   // Get the right type of discovered queue head.
1216   DiscoveredList* list = get_discovered_list(rt);
1217   if (list == NULL) {
1218     return false;   // nothing special needs to be done
1219   }
1220 
1221   if (_discovery_is_mt) {
1222     add_to_discovered_list_mt(*list, obj, discovered_addr);
1223   } else {
1224     // If "_discovered_list_needs_barrier", we do write barriers when
1225     // updating the discovered reference list.  Otherwise, we do a raw store
1226     // here: the field will be visited later when processing the discovered
1227     // references.
1228     oop current_head = list->head();
1229     // The last ref must have its discovered field pointing to itself.
1230     oop next_discovered = (current_head != NULL) ? current_head : obj;
1231 
1232     // As in the case further above, since we are over-writing a NULL
1233     // pre-value, we can safely elide the pre-barrier here for the case of G1.
1234     // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1235     assert(discovered == NULL, "control point invariant");
1236     assert(!_discovered_list_needs_barrier || UseG1GC,
1237            "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1238     oop_store_raw(discovered_addr, next_discovered);
1239     if (_discovered_list_needs_barrier) {
1240       _bs->write_ref_field((void*)discovered_addr, next_discovered);
1241     }
1242     list->set_head(obj);
1243     list->inc_length(1);
1244 
1245     if (TraceReferenceGC) {
1246       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1247                                 obj, obj->blueprint()->internal_name());
1248     }
1249   }
1250   assert(obj->is_oop(), "Discovered a bad reference");
1251   verify_referent(obj);
1252   return true;
1253 }
1254 
1255 // Preclean the discovered references by removing those
1256 // whose referents are alive, and by marking from those that
1257 // are not active. These lists can be handled here
1258 // in any order and, indeed, concurrently.
1259 void ReferenceProcessor::preclean_discovered_references(
1260   BoolObjectClosure* is_alive,
1261   OopClosure* keep_alive,
1262   VoidClosure* complete_gc,
1263   YieldClosure* yield,
1264   bool should_unload_classes) {
1265 
1266   NOT_PRODUCT(verify_ok_to_handle_reflists());
1267 
1268 #ifdef ASSERT
1269   bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1270                                CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1271                                ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1272                                  UseConcMarkSweepGC && should_unload_classes;
1273   RememberKlassesChecker mx(must_remember_klasses);
1274 #endif
1275   // Soft references
1276   {
1277     TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1278               false, gclog_or_tty);
1279     for (int i = 0; i < _max_num_q; i++) {
1280       if (yield->should_return()) {
1281         return;
1282       }
1283       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1284                                   keep_alive, complete_gc, yield);
1285     }
1286   }
1287 
1288   // Weak references
1289   {
1290     TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1291               false, gclog_or_tty);
1292     for (int i = 0; i < _max_num_q; i++) {
1293       if (yield->should_return()) {
1294         return;
1295       }
1296       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1297                                   keep_alive, complete_gc, yield);
1298     }
1299   }
1300 
1301   // Final references
1302   {
1303     TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1304               false, gclog_or_tty);
1305     for (int i = 0; i < _max_num_q; i++) {
1306       if (yield->should_return()) {
1307         return;
1308       }
1309       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1310                                   keep_alive, complete_gc, yield);
1311     }
1312   }
1313 
1314   // Phantom references
1315   {
1316     TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1317               false, gclog_or_tty);
1318     for (int i = 0; i < _max_num_q; i++) {
1319       if (yield->should_return()) {
1320         return;
1321       }
1322       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1323                                   keep_alive, complete_gc, yield);
1324     }
1325   }
1326 }
1327 
1328 // Walk the given discovered ref list, and remove all reference objects
1329 // whose referents are still alive, whose referents are NULL or which
1330 // are not active (have a non-NULL next field). NOTE: When we are
1331 // thus precleaning the ref lists (which happens single-threaded today),
1332 // we do not disable refs discovery to honour the correct semantics of
1333 // java.lang.Reference. As a result, we need to be careful below
1334 // that ref removal steps interleave safely with ref discovery steps
1335 // (in this thread).
1336 void
1337 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1338                                                 BoolObjectClosure* is_alive,
1339                                                 OopClosure*        keep_alive,
1340                                                 VoidClosure*       complete_gc,
1341                                                 YieldClosure*      yield) {
1342   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1343   while (iter.has_next()) {
1344     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1345     oop obj = iter.obj();
1346     oop next = java_lang_ref_Reference::next(obj);
1347     if (iter.referent() == NULL || iter.is_referent_alive() ||
1348         next != NULL) {
1349       // The referent has been cleared, or is alive, or the Reference is not
1350       // active; we need to trace and mark its cohort.
1351       if (TraceReferenceGC) {
1352         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1353                                iter.obj(), iter.obj()->blueprint()->internal_name());
1354       }
1355       // Remove Reference object from list
1356       iter.remove();
1357       // Keep alive its cohort.
1358       iter.make_referent_alive();
1359       if (UseCompressedOops) {
1360         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1361         keep_alive->do_oop(next_addr);
1362       } else {
1363         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1364         keep_alive->do_oop(next_addr);
1365       }
1366       iter.move_to_next();
1367     } else {
1368       iter.next();
1369     }
1370   }
1371   // Close the reachable set
1372   complete_gc->do_void();
1373 
1374   NOT_PRODUCT(
1375     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1376       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1377         "Refs in discovered list " INTPTR_FORMAT,
1378         iter.removed(), iter.processed(), (address)refs_list.head());
1379     }
1380   )
1381 }
1382 
1383 const char* ReferenceProcessor::list_name(int i) {
1384    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1385           "Out of bounds index");
1386 
1387    int j = i / _max_num_q;
1388    switch (j) {
1389      case 0: return "SoftRef";
1390      case 1: return "WeakRef";
1391      case 2: return "FinalRef";
1392      case 3: return "PhantomRef";
1393    }
1394    ShouldNotReachHere();
1395    return NULL;
1396 }
1397 
1398 #ifndef PRODUCT
1399 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1400   // empty for now
1401 }
1402 #endif
1403 
1404 #ifndef PRODUCT
1405 void ReferenceProcessor::clear_discovered_references() {
1406   guarantee(!_discovering_refs, "Discovering refs?");
1407   for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1408     clear_discovered_references(_discovered_refs[i]);
1409   }
1410 }
1411 
1412 #endif // PRODUCT