1 /*
   2  * Copyright (c) 2001, 2010, 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 oop              ReferenceProcessor::_sentinelRef = NULL;
  39 const int        subclasses_of_ref                = REF_PHANTOM - REF_OTHER;
  40 
  41 // List of discovered references.
  42 class DiscoveredList {
  43 public:
  44   DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }
  45   oop head() const     {
  46      return UseCompressedOops ?  oopDesc::decode_heap_oop_not_null(_compressed_head) :
  47                                 _oop_head;
  48   }
  49   HeapWord* adr_head() {
  50     return UseCompressedOops ? (HeapWord*)&_compressed_head :
  51                                (HeapWord*)&_oop_head;
  52   }
  53   void   set_head(oop o) {
  54     if (UseCompressedOops) {
  55       // Must compress the head ptr.
  56       _compressed_head = oopDesc::encode_heap_oop_not_null(o);
  57     } else {
  58       _oop_head = o;
  59     }
  60   }
  61   bool   empty() const          { return head() == ReferenceProcessor::sentinel_ref(); }
  62   size_t length()               { return _len; }
  63   void   set_length(size_t len) { _len = len;  }
  64   void   inc_length(size_t inc) { _len += inc; assert(_len > 0, "Error"); }
  65   void   dec_length(size_t dec) { _len -= dec; }
  66 private:
  67   // Set value depending on UseCompressedOops. This could be a template class
  68   // but then we have to fix all the instantiations and declarations that use this class.
  69   oop       _oop_head;
  70   narrowOop _compressed_head;
  71   size_t _len;
  72 };
  73 
  74 void referenceProcessor_init() {
  75   ReferenceProcessor::init_statics();
  76 }
  77 
  78 void ReferenceProcessor::init_statics() {
  79   assert(_sentinelRef == NULL, "should be initialized precisely once");
  80   EXCEPTION_MARK;
  81   _sentinelRef = instanceKlass::cast(
  82                     SystemDictionary::Reference_klass())->
  83                       allocate_permanent_instance(THREAD);
  84 
  85   // Initialize the master soft ref clock.
  86   java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
  87 
  88   if (HAS_PENDING_EXCEPTION) {
  89       Handle ex(THREAD, PENDING_EXCEPTION);
  90       vm_exit_during_initialization(ex);
  91   }
  92   assert(_sentinelRef != NULL && _sentinelRef->is_oop(),
  93          "Just constructed it!");
  94   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  95   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  96                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  97   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  98     vm_exit_during_initialization("Could not allocate reference policy object");
  99   }
 100   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
 101             RefDiscoveryPolicy == ReferentBasedDiscovery,
 102             "Unrecongnized RefDiscoveryPolicy");
 103 }
 104 
 105 ReferenceProcessor::ReferenceProcessor(MemRegion span,
 106                                        bool      mt_processing,
 107                                        int       mt_processing_degree,
 108                                        bool      mt_discovery,
 109                                        int       mt_discovery_degree,
 110                                        bool      atomic_discovery,
 111                                        BoolObjectClosure* is_alive_non_header,
 112                                        bool      discovered_list_needs_barrier)  :
 113   _discovering_refs(false),
 114   _enqueuing_is_done(false),
 115   _is_alive_non_header(is_alive_non_header),
 116   _discovered_list_needs_barrier(discovered_list_needs_barrier),
 117   _bs(NULL),
 118   _processing_is_mt(mt_processing),
 119   _next_id(0)
 120 {
 121   _span = span;
 122   _discovery_is_atomic = atomic_discovery;
 123   _discovery_is_mt     = mt_discovery;
 124   _num_q               = mt_processing_degree;
 125   _max_num_q           = MAX2(mt_processing_degree, mt_discovery_degree);
 126   _discoveredSoftRefs  = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref);
 127   if (_discoveredSoftRefs == NULL) {
 128     vm_exit_during_initialization("Could not allocated RefProc Array");
 129   }
 130   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 131   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 132   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 133   assert(sentinel_ref() != NULL, "_sentinelRef is NULL");
 134   // Initialized all entries to _sentinelRef
 135   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 136         _discoveredSoftRefs[i].set_head(sentinel_ref());
 137     _discoveredSoftRefs[i].set_length(0);
 138   }
 139   // If we do barreirs, cache a copy of the barrier set.
 140   if (discovered_list_needs_barrier) {
 141     _bs = Universe::heap()->barrier_set();
 142   }
 143   setup_policy(false /* default soft ref policy */);
 144 }
 145 
 146 #ifndef PRODUCT
 147 void ReferenceProcessor::verify_no_references_recorded() {
 148   guarantee(!_discovering_refs, "Discovering refs?");
 149   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 150     guarantee(_discoveredSoftRefs[i].empty(),
 151               "Found non-empty discovered list");
 152   }
 153 }
 154 #endif
 155 
 156 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 157   // Should this instead be
 158   // for (int i = 0; i < subclasses_of_ref; i++_ {
 159   //   for (int j = 0; j < _num_q; j++) {
 160   //     int index = i * _max_num_q + j;
 161   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 162     if (UseCompressedOops) {
 163       f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
 164     } else {
 165       f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
 166     }
 167   }
 168 }
 169 
 170 void ReferenceProcessor::oops_do(OopClosure* f) {
 171   f->do_oop(adr_sentinel_ref());
 172 }
 173 
 174 void ReferenceProcessor::update_soft_ref_master_clock() {
 175   // Update (advance) the soft ref master clock field. This must be done
 176   // after processing the soft ref list.
 177   jlong now = os::javaTimeMillis();
 178   jlong clock = java_lang_ref_SoftReference::clock();
 179   NOT_PRODUCT(
 180   if (now < clock) {
 181     warning("time warp: %d to %d", clock, now);
 182   }
 183   )
 184   // In product mode, protect ourselves from system time being adjusted
 185   // externally and going backward; see note in the implementation of
 186   // GenCollectedHeap::time_since_last_gc() for the right way to fix
 187   // this uniformly throughout the VM; see bug-id 4741166. XXX
 188   if (now > clock) {
 189     java_lang_ref_SoftReference::set_clock(now);
 190   }
 191   // Else leave clock stalled at its old value until time progresses
 192   // past clock value.
 193 }
 194 
 195 void ReferenceProcessor::process_discovered_references(
 196   BoolObjectClosure*           is_alive,
 197   OopClosure*                  keep_alive,
 198   VoidClosure*                 complete_gc,
 199   AbstractRefProcTaskExecutor* task_executor) {
 200   NOT_PRODUCT(verify_ok_to_handle_reflists());
 201 
 202   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 203   // Stop treating discovered references specially.
 204   disable_discovery();
 205 
 206   bool trace_time = PrintGCDetails && PrintReferenceGC;
 207   // Soft references
 208   {
 209     TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
 210     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 211                                is_alive, keep_alive, complete_gc, task_executor);
 212   }
 213 
 214   update_soft_ref_master_clock();
 215 
 216   // Weak references
 217   {
 218     TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
 219     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 220                                is_alive, keep_alive, complete_gc, task_executor);
 221   }
 222 
 223   // Final references
 224   {
 225     TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
 226     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 227                                is_alive, keep_alive, complete_gc, task_executor);
 228   }
 229 
 230   // Phantom references
 231   {
 232     TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
 233     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 234                                is_alive, keep_alive, complete_gc, task_executor);
 235   }
 236 
 237   // Weak global JNI references. It would make more sense (semantically) to
 238   // traverse these simultaneously with the regular weak references above, but
 239   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 240   // thus use JNI weak references to circumvent the phantom references and
 241   // resurrect a "post-mortem" object.
 242   {
 243     TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
 244     if (task_executor != NULL) {
 245       task_executor->set_single_threaded_mode();
 246     }
 247     process_phaseJNI(is_alive, keep_alive, complete_gc);
 248   }
 249 }
 250 
 251 #ifndef PRODUCT
 252 // Calculate the number of jni handles.
 253 uint ReferenceProcessor::count_jni_refs() {
 254   class AlwaysAliveClosure: public BoolObjectClosure {
 255   public:
 256     virtual bool do_object_b(oop obj) { return true; }
 257     virtual void do_object(oop obj) { assert(false, "Don't call"); }
 258   };
 259 
 260   class CountHandleClosure: public OopClosure {
 261   private:
 262     int _count;
 263   public:
 264     CountHandleClosure(): _count(0) {}
 265     void do_oop(oop* unused)       { _count++; }
 266     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 267     int count() { return _count; }
 268   };
 269   CountHandleClosure global_handle_count;
 270   AlwaysAliveClosure always_alive;
 271   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 272   return global_handle_count.count();
 273 }
 274 #endif
 275 
 276 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 277                                           OopClosure*        keep_alive,
 278                                           VoidClosure*       complete_gc) {
 279 #ifndef PRODUCT
 280   if (PrintGCDetails && PrintReferenceGC) {
 281     unsigned int count = count_jni_refs();
 282     gclog_or_tty->print(", %u refs", count);
 283   }
 284 #endif
 285   JNIHandles::weak_oops_do(is_alive, keep_alive);
 286   // Finally remember to keep sentinel around
 287   keep_alive->do_oop(adr_sentinel_ref());
 288   complete_gc->do_void();
 289 }
 290 
 291 
 292 template <class T>
 293 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 294                                    AbstractRefProcTaskExecutor* task_executor) {
 295 
 296   // Remember old value of pending references list
 297   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 298   T old_pending_list_value = *pending_list_addr;
 299 
 300   // Enqueue references that are not made active again, and
 301   // clear the decks for the next collection (cycle).
 302   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 303   // Do the oop-check on pending_list_addr missed in
 304   // enqueue_discovered_reflist. We should probably
 305   // do a raw oop_check so that future such idempotent
 306   // oop_stores relying on the oop-check side-effect
 307   // may be elided automatically and safely without
 308   // affecting correctness.
 309   oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 310 
 311   // Stop treating discovered references specially.
 312   ref->disable_discovery();
 313 
 314   // Return true if new pending references were added
 315   return old_pending_list_value != *pending_list_addr;
 316 }
 317 
 318 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 319   NOT_PRODUCT(verify_ok_to_handle_reflists());
 320   if (UseCompressedOops) {
 321     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 322   } else {
 323     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 324   }
 325 }
 326 
 327 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 328                                                     HeapWord* pending_list_addr) {
 329   // Given a list of refs linked through the "discovered" field
 330   // (java.lang.ref.Reference.discovered) chain them through the
 331   // "next" field (java.lang.ref.Reference.next) and prepend
 332   // to the pending list.
 333   if (TraceReferenceGC && PrintGCDetails) {
 334     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 335                            INTPTR_FORMAT, (address)refs_list.head());
 336   }
 337   oop obj = refs_list.head();
 338   // Walk down the list, copying the discovered field into
 339   // the next field and clearing it (except for the last
 340   // non-sentinel object which is treated specially to avoid
 341   // confusion with an active reference).
 342   while (obj != sentinel_ref()) {
 343     assert(obj->is_instanceRef(), "should be reference object");
 344     oop next = java_lang_ref_Reference::discovered(obj);
 345     if (TraceReferenceGC && PrintGCDetails) {
 346       gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,
 347                              obj, next);
 348     }
 349     assert(java_lang_ref_Reference::next(obj) == NULL,
 350            "The reference should not be enqueued");
 351     if (next == sentinel_ref()) {  // obj is last
 352       // Swap refs_list into pendling_list_addr and
 353       // set obj's next to what we read from pending_list_addr.
 354       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 355       // Need oop_check on pending_list_addr above;
 356       // see special oop-check code at the end of
 357       // enqueue_discovered_reflists() further below.
 358       if (old == NULL) {
 359         // obj should be made to point to itself, since
 360         // pending list was empty.
 361         java_lang_ref_Reference::set_next(obj, obj);
 362       } else {
 363         java_lang_ref_Reference::set_next(obj, old);
 364       }
 365     } else {
 366       java_lang_ref_Reference::set_next(obj, next);
 367     }
 368     java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 369     obj = next;
 370   }
 371 }
 372 
 373 // Parallel enqueue task
 374 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 375 public:
 376   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 377                      DiscoveredList      discovered_refs[],
 378                      HeapWord*           pending_list_addr,
 379                      oop                 sentinel_ref,
 380                      int                 n_queues)
 381     : EnqueueTask(ref_processor, discovered_refs,
 382                   pending_list_addr, sentinel_ref, n_queues)
 383   { }
 384 
 385   virtual void work(unsigned int work_id) {
 386     assert(work_id < (unsigned int)_ref_processor.num_q(), "Index out-of-bounds");
 387     // Simplest first cut: static partitioning.
 388     int index = work_id;
 389     // The increment on "index" must correspond to the maximum number of queues
 390     // (n_queues) with which that ReferenceProcessor was created.  That
 391     // is because of the "clever" way the discovered references lists were
 392     // allocated and are indexed into.  That number is ParallelGCThreads
 393     // currently.  Assert that.
 394     assert(_n_queues == (int) ParallelGCThreads, "Different number not expected");
 395     for (int j = 0;
 396          j < subclasses_of_ref;
 397          j++, index += _n_queues) {
 398       _ref_processor.enqueue_discovered_reflist(
 399         _refs_lists[index], _pending_list_addr);
 400       _refs_lists[index].set_head(_sentinel_ref);
 401       _refs_lists[index].set_length(0);
 402     }
 403   }
 404 };
 405 
 406 // Enqueue references that are not made active again
 407 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 408   AbstractRefProcTaskExecutor* task_executor) {
 409   if (_processing_is_mt && task_executor != NULL) {
 410     // Parallel code
 411     RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
 412                            pending_list_addr, sentinel_ref(), _max_num_q);
 413     task_executor->execute(tsk);
 414   } else {
 415     // Serial code: call the parent class's implementation
 416     for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 417       enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
 418       _discoveredSoftRefs[i].set_head(sentinel_ref());
 419       _discoveredSoftRefs[i].set_length(0);
 420     }
 421   }
 422 }
 423 
 424 // Iterator for the list of discovered references.
 425 class DiscoveredListIterator {
 426 public:
 427   inline DiscoveredListIterator(DiscoveredList&    refs_list,
 428                                 OopClosure*        keep_alive,
 429                                 BoolObjectClosure* is_alive);
 430 
 431   // End Of List.
 432   inline bool has_next() const { return _next != ReferenceProcessor::sentinel_ref(); }
 433 
 434   // Get oop to the Reference object.
 435   inline oop obj() const { return _ref; }
 436 
 437   // Get oop to the referent object.
 438   inline oop referent() const { return _referent; }
 439 
 440   // Returns true if referent is alive.
 441   inline bool is_referent_alive() const;
 442 
 443   // Loads data for the current reference.
 444   // The "allow_null_referent" argument tells us to allow for the possibility
 445   // of a NULL referent in the discovered Reference object. This typically
 446   // happens in the case of concurrent collectors that may have done the
 447   // discovery concurrently, or interleaved, with mutator execution.
 448   inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
 449 
 450   // Move to the next discovered reference.
 451   inline void next();
 452 
 453   // Remove the current reference from the list
 454   inline void remove();
 455 
 456   // Make the Reference object active again.
 457   inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }
 458 
 459   // Make the referent alive.
 460   inline void make_referent_alive() {
 461     if (UseCompressedOops) {
 462       _keep_alive->do_oop((narrowOop*)_referent_addr);
 463     } else {
 464       _keep_alive->do_oop((oop*)_referent_addr);
 465     }
 466   }
 467 
 468   // Update the discovered field.
 469   inline void update_discovered() {
 470     // First _prev_next ref actually points into DiscoveredList (gross).
 471     if (UseCompressedOops) {
 472       _keep_alive->do_oop((narrowOop*)_prev_next);
 473     } else {
 474       _keep_alive->do_oop((oop*)_prev_next);
 475     }
 476   }
 477 
 478   // NULL out referent pointer.
 479   inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
 480 
 481   // Statistics
 482   NOT_PRODUCT(
 483   inline size_t processed() const { return _processed; }
 484   inline size_t removed() const   { return _removed; }
 485   )
 486 
 487   inline void move_to_next();
 488 
 489 private:
 490   DiscoveredList&    _refs_list;
 491   HeapWord*          _prev_next;
 492   oop                _ref;
 493   HeapWord*          _discovered_addr;
 494   oop                _next;
 495   HeapWord*          _referent_addr;
 496   oop                _referent;
 497   OopClosure*        _keep_alive;
 498   BoolObjectClosure* _is_alive;
 499   DEBUG_ONLY(
 500   oop                _first_seen; // cyclic linked list check
 501   )
 502   NOT_PRODUCT(
 503   size_t             _processed;
 504   size_t             _removed;
 505   )
 506 };
 507 
 508 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList&    refs_list,
 509                                                       OopClosure*        keep_alive,
 510                                                       BoolObjectClosure* is_alive)
 511   : _refs_list(refs_list),
 512     _prev_next(refs_list.adr_head()),
 513     _ref(refs_list.head()),
 514 #ifdef ASSERT
 515     _first_seen(refs_list.head()),
 516 #endif
 517 #ifndef PRODUCT
 518     _processed(0),
 519     _removed(0),
 520 #endif
 521     _next(refs_list.head()),
 522     _keep_alive(keep_alive),
 523     _is_alive(is_alive)
 524 { }
 525 
 526 inline bool DiscoveredListIterator::is_referent_alive() const {
 527   return _is_alive->do_object_b(_referent);
 528 }
 529 
 530 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 531   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 532   oop discovered = java_lang_ref_Reference::discovered(_ref);
 533   assert(_discovered_addr && discovered->is_oop_or_null(),
 534          "discovered field is bad");
 535   _next = discovered;
 536   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 537   _referent = java_lang_ref_Reference::referent(_ref);
 538   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 539          "Wrong oop found in java.lang.Reference object");
 540   assert(allow_null_referent ?
 541              _referent->is_oop_or_null()
 542            : _referent->is_oop(),
 543          "bad referent");
 544 }
 545 
 546 inline void DiscoveredListIterator::next() {
 547   _prev_next = _discovered_addr;
 548   move_to_next();
 549 }
 550 
 551 inline void DiscoveredListIterator::remove() {
 552   assert(_ref->is_oop(), "Dropping a bad reference");
 553   oop_store_raw(_discovered_addr, NULL);
 554   // First _prev_next ref actually points into DiscoveredList (gross).
 555   if (UseCompressedOops) {
 556     // Remove Reference object from list.
 557     oopDesc::encode_store_heap_oop_not_null((narrowOop*)_prev_next, _next);
 558   } else {
 559     // Remove Reference object from list.
 560     oopDesc::store_heap_oop((oop*)_prev_next, _next);
 561   }
 562   NOT_PRODUCT(_removed++);
 563   _refs_list.dec_length(1);
 564 }
 565 
 566 inline void DiscoveredListIterator::move_to_next() {
 567   _ref = _next;
 568   assert(_ref != _first_seen, "cyclic ref_list found");
 569   NOT_PRODUCT(_processed++);
 570 }
 571 
 572 // NOTE: process_phase*() are largely similar, and at a high level
 573 // merely iterate over the extant list applying a predicate to
 574 // each of its elements and possibly removing that element from the
 575 // list and applying some further closures to that element.
 576 // We should consider the possibility of replacing these
 577 // process_phase*() methods by abstracting them into
 578 // a single general iterator invocation that receives appropriate
 579 // closures that accomplish this work.
 580 
 581 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 582 // referents are not alive, but that should be kept alive for policy reasons.
 583 // Keep alive the transitive closure of all such referents.
 584 void
 585 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 586                                    ReferencePolicy*   policy,
 587                                    BoolObjectClosure* is_alive,
 588                                    OopClosure*        keep_alive,
 589                                    VoidClosure*       complete_gc) {
 590   assert(policy != NULL, "Must have a non-NULL policy");
 591   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 592   // Decide which softly reachable refs should be kept alive.
 593   while (iter.has_next()) {
 594     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 595     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 596     if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
 597       if (TraceReferenceGC) {
 598         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 599                                iter.obj(), iter.obj()->blueprint()->internal_name());
 600       }
 601       // Remove Reference object from list
 602       iter.remove();
 603       // Make the Reference object active again
 604       iter.make_active();
 605       // keep the referent around
 606       iter.make_referent_alive();
 607       iter.move_to_next();
 608     } else {
 609       iter.next();
 610     }
 611   }
 612   // Close the reachable set
 613   complete_gc->do_void();
 614   NOT_PRODUCT(
 615     if (PrintGCDetails && TraceReferenceGC) {
 616       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 617         "discovered Refs by policy  list " INTPTR_FORMAT,
 618         iter.removed(), iter.processed(), (address)refs_list.head());
 619     }
 620   )
 621 }
 622 
 623 // Traverse the list and remove any Refs that are not active, or
 624 // whose referents are either alive or NULL.
 625 void
 626 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 627                              BoolObjectClosure* is_alive,
 628                              OopClosure*        keep_alive) {
 629   assert(discovery_is_atomic(), "Error");
 630   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 631   while (iter.has_next()) {
 632     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 633     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 634     assert(next == NULL, "Should not discover inactive Reference");
 635     if (iter.is_referent_alive()) {
 636       if (TraceReferenceGC) {
 637         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 638                                iter.obj(), iter.obj()->blueprint()->internal_name());
 639       }
 640       // The referent is reachable after all.
 641       // Remove Reference object from list.
 642       iter.remove();
 643       // Update the referent pointer as necessary: Note that this
 644       // should not entail any recursive marking because the
 645       // referent must already have been traversed.
 646       iter.make_referent_alive();
 647       iter.move_to_next();
 648     } else {
 649       iter.next();
 650     }
 651   }
 652   NOT_PRODUCT(
 653     if (PrintGCDetails && TraceReferenceGC) {
 654       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 655         "Refs in discovered list " INTPTR_FORMAT,
 656         iter.removed(), iter.processed(), (address)refs_list.head());
 657     }
 658   )
 659 }
 660 
 661 void
 662 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 663                                                   BoolObjectClosure* is_alive,
 664                                                   OopClosure*        keep_alive,
 665                                                   VoidClosure*       complete_gc) {
 666   assert(!discovery_is_atomic(), "Error");
 667   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 668   while (iter.has_next()) {
 669     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 670     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 671     oop next = java_lang_ref_Reference::next(iter.obj());
 672     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 673          next != NULL)) {
 674       assert(next->is_oop_or_null(), "bad next field");
 675       // Remove Reference object from list
 676       iter.remove();
 677       // Trace the cohorts
 678       iter.make_referent_alive();
 679       if (UseCompressedOops) {
 680         keep_alive->do_oop((narrowOop*)next_addr);
 681       } else {
 682         keep_alive->do_oop((oop*)next_addr);
 683       }
 684       iter.move_to_next();
 685     } else {
 686       iter.next();
 687     }
 688   }
 689   // Now close the newly reachable set
 690   complete_gc->do_void();
 691   NOT_PRODUCT(
 692     if (PrintGCDetails && TraceReferenceGC) {
 693       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 694         "Refs in discovered list " INTPTR_FORMAT,
 695         iter.removed(), iter.processed(), (address)refs_list.head());
 696     }
 697   )
 698 }
 699 
 700 // Traverse the list and process the referents, by either
 701 // clearing them or keeping them (and their reachable
 702 // closure) alive.
 703 void
 704 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 705                                    bool               clear_referent,
 706                                    BoolObjectClosure* is_alive,
 707                                    OopClosure*        keep_alive,
 708                                    VoidClosure*       complete_gc) {
 709   ResourceMark rm;
 710   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 711   while (iter.has_next()) {
 712     iter.update_discovered();
 713     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 714     if (clear_referent) {
 715       // NULL out referent pointer
 716       iter.clear_referent();
 717     } else {
 718       // keep the referent around
 719       iter.make_referent_alive();
 720     }
 721     if (TraceReferenceGC) {
 722       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 723                              clear_referent ? "cleared " : "",
 724                              iter.obj(), iter.obj()->blueprint()->internal_name());
 725     }
 726     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 727     iter.next();
 728   }
 729   // Remember to keep sentinel pointer around
 730   iter.update_discovered();
 731   // Close the reachable set
 732   complete_gc->do_void();
 733 }
 734 
 735 void
 736 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 737   oop obj = refs_list.head();
 738   while (obj != sentinel_ref()) {
 739     oop discovered = java_lang_ref_Reference::discovered(obj);
 740     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 741     obj = discovered;
 742   }
 743   refs_list.set_head(sentinel_ref());
 744   refs_list.set_length(0);
 745 }
 746 
 747 void ReferenceProcessor::abandon_partial_discovery() {
 748   // loop over the lists
 749   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 750     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 751       gclog_or_tty->print_cr("\nAbandoning %s discovered list",
 752                              list_name(i));
 753     }
 754     abandon_partial_discovered_list(_discoveredSoftRefs[i]);
 755   }
 756 }
 757 
 758 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 759 public:
 760   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 761                     DiscoveredList      refs_lists[],
 762                     ReferencePolicy*    policy,
 763                     bool                marks_oops_alive)
 764     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 765       _policy(policy)
 766   { }
 767   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 768                     OopClosure& keep_alive,
 769                     VoidClosure& complete_gc)
 770   {
 771     Thread* thr = Thread::current();
 772     int refs_list_index = ((WorkerThread*)thr)->id();
 773     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 774                                   &is_alive, &keep_alive, &complete_gc);
 775   }
 776 private:
 777   ReferencePolicy* _policy;
 778 };
 779 
 780 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 781 public:
 782   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 783                     DiscoveredList      refs_lists[],
 784                     bool                marks_oops_alive)
 785     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 786   { }
 787   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 788                     OopClosure& keep_alive,
 789                     VoidClosure& complete_gc)
 790   {
 791     _ref_processor.process_phase2(_refs_lists[i],
 792                                   &is_alive, &keep_alive, &complete_gc);
 793   }
 794 };
 795 
 796 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 797 public:
 798   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 799                     DiscoveredList      refs_lists[],
 800                     bool                clear_referent,
 801                     bool                marks_oops_alive)
 802     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 803       _clear_referent(clear_referent)
 804   { }
 805   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 806                     OopClosure& keep_alive,
 807                     VoidClosure& complete_gc)
 808   {
 809     // Don't use "refs_list_index" calculated in this way because
 810     // balance_queues() has moved the Ref's into the first n queues.
 811     // Thread* thr = Thread::current();
 812     // int refs_list_index = ((WorkerThread*)thr)->id();
 813     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 814     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 815                                   &is_alive, &keep_alive, &complete_gc);
 816   }
 817 private:
 818   bool _clear_referent;
 819 };
 820 
 821 // Balances reference queues.
 822 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 823 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 824 // corresponding to the active workers will be processed.
 825 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 826 {
 827   // calculate total length
 828   size_t total_refs = 0;
 829   if (TraceReferenceGC && PrintGCDetails) {
 830     gclog_or_tty->print_cr("\nBalance ref_lists ");
 831   }
 832 
 833   for (int i = 0; i < _max_num_q; ++i) {
 834     total_refs += ref_lists[i].length();
 835     if (TraceReferenceGC && PrintGCDetails) {
 836       gclog_or_tty->print("%d ", ref_lists[i].length());
 837     }
 838   }
 839   if (TraceReferenceGC && PrintGCDetails) {
 840     gclog_or_tty->print_cr(" = %d", total_refs);
 841   }
 842   size_t avg_refs = total_refs / _num_q + 1;
 843   int to_idx = 0;
 844   for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
 845     bool move_all = false;
 846     if (from_idx >= _num_q) {
 847       move_all = ref_lists[from_idx].length() > 0;
 848     }
 849     while ((ref_lists[from_idx].length() > avg_refs) ||
 850            move_all) {
 851       assert(to_idx < _num_q, "Sanity Check!");
 852       if (ref_lists[to_idx].length() < avg_refs) {
 853         // move superfluous refs
 854         size_t refs_to_move;
 855         // Move all the Ref's if the from queue will not be processed.
 856         if (move_all) {
 857           refs_to_move = MIN2(ref_lists[from_idx].length(),
 858                               avg_refs - ref_lists[to_idx].length());
 859         } else {
 860           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 861                               avg_refs - ref_lists[to_idx].length());
 862         }
 863         oop move_head = ref_lists[from_idx].head();
 864         oop move_tail = move_head;
 865         oop new_head  = move_head;
 866         // find an element to split the list on
 867         for (size_t j = 0; j < refs_to_move; ++j) {
 868           move_tail = new_head;
 869           new_head = java_lang_ref_Reference::discovered(new_head);
 870         }
 871         java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
 872         ref_lists[to_idx].set_head(move_head);
 873         ref_lists[to_idx].inc_length(refs_to_move);
 874         ref_lists[from_idx].set_head(new_head);
 875         ref_lists[from_idx].dec_length(refs_to_move);
 876         if (ref_lists[from_idx].length() == 0) {
 877           break;
 878         }
 879       } else {
 880         to_idx = (to_idx + 1) % _num_q;
 881       }
 882     }
 883   }
 884 #ifdef ASSERT
 885   size_t balanced_total_refs = 0;
 886   for (int i = 0; i < _max_num_q; ++i) {
 887     balanced_total_refs += ref_lists[i].length();
 888     if (TraceReferenceGC && PrintGCDetails) {
 889       gclog_or_tty->print("%d ", ref_lists[i].length());
 890     }
 891   }
 892   if (TraceReferenceGC && PrintGCDetails) {
 893     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 894     gclog_or_tty->flush();
 895   }
 896   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 897 #endif
 898 }
 899 
 900 void ReferenceProcessor::balance_all_queues() {
 901   balance_queues(_discoveredSoftRefs);
 902   balance_queues(_discoveredWeakRefs);
 903   balance_queues(_discoveredFinalRefs);
 904   balance_queues(_discoveredPhantomRefs);
 905 }
 906 
 907 void
 908 ReferenceProcessor::process_discovered_reflist(
 909   DiscoveredList               refs_lists[],
 910   ReferencePolicy*             policy,
 911   bool                         clear_referent,
 912   BoolObjectClosure*           is_alive,
 913   OopClosure*                  keep_alive,
 914   VoidClosure*                 complete_gc,
 915   AbstractRefProcTaskExecutor* task_executor)
 916 {
 917   bool mt_processing = task_executor != NULL && _processing_is_mt;
 918   // If discovery used MT and a dynamic number of GC threads, then
 919   // the queues must be balanced for correctness if fewer than the
 920   // maximum number of queues were used.  The number of queue used
 921   // during discovery may be different than the number to be used
 922   // for processing so don't depend of _num_q < _max_num_q as part
 923   // of the test.
 924   bool must_balance = _discovery_is_mt;
 925 
 926   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 927       must_balance) {
 928     balance_queues(refs_lists);
 929   }
 930   if (PrintReferenceGC && PrintGCDetails) {
 931     size_t total = 0;
 932     for (int i = 0; i < _max_num_q; ++i) {
 933       total += refs_lists[i].length();
 934     }
 935     gclog_or_tty->print(", %u refs", total);
 936   }
 937 
 938   // Phase 1 (soft refs only):
 939   // . Traverse the list and remove any SoftReferences whose
 940   //   referents are not alive, but that should be kept alive for
 941   //   policy reasons. Keep alive the transitive closure of all
 942   //   such referents.
 943   if (policy != NULL) {
 944     if (mt_processing) {
 945       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 946       task_executor->execute(phase1);
 947     } else {
 948       for (int i = 0; i < _max_num_q; i++) {
 949         process_phase1(refs_lists[i], policy,
 950                        is_alive, keep_alive, complete_gc);
 951       }
 952     }
 953   } else { // policy == NULL
 954     assert(refs_lists != _discoveredSoftRefs,
 955            "Policy must be specified for soft references.");
 956   }
 957 
 958   // Phase 2:
 959   // . Traverse the list and remove any refs whose referents are alive.
 960   if (mt_processing) {
 961     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 962     task_executor->execute(phase2);
 963   } else {
 964     for (int i = 0; i < _max_num_q; i++) {
 965       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 966     }
 967   }
 968 
 969   // Phase 3:
 970   // . Traverse the list and process referents as appropriate.
 971   if (mt_processing) {
 972     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 973     task_executor->execute(phase3);
 974   } else {
 975     for (int i = 0; i < _max_num_q; i++) {
 976       process_phase3(refs_lists[i], clear_referent,
 977                      is_alive, keep_alive, complete_gc);
 978     }
 979   }
 980 }
 981 
 982 void ReferenceProcessor::clean_up_discovered_references() {
 983   // loop over the lists
 984   // Should this instead be
 985   // for (int i = 0; i < subclasses_of_ref; i++_ {
 986   //   for (int j = 0; j < _num_q; j++) {
 987   //     int index = i * _max_num_q + j;
 988   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
 989     if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {
 990       gclog_or_tty->print_cr(
 991         "\nScrubbing %s discovered list of Null referents",
 992         list_name(i));
 993     }
 994     clean_up_discovered_reflist(_discoveredSoftRefs[i]);
 995   }
 996 }
 997 
 998 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 999   assert(!discovery_is_atomic(), "Else why call this method?");
1000   DiscoveredListIterator iter(refs_list, NULL, NULL);
1001   while (iter.has_next()) {
1002     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1003     oop next = java_lang_ref_Reference::next(iter.obj());
1004     assert(next->is_oop_or_null(), "bad next field");
1005     // If referent has been cleared or Reference is not active,
1006     // drop it.
1007     if (iter.referent() == NULL || next != NULL) {
1008       debug_only(
1009         if (PrintGCDetails && TraceReferenceGC) {
1010           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1011             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1012             " and referent: " INTPTR_FORMAT,
1013             iter.obj(), next, iter.referent());
1014         }
1015       )
1016       // Remove Reference object from list
1017       iter.remove();
1018       iter.move_to_next();
1019     } else {
1020       iter.next();
1021     }
1022   }
1023   NOT_PRODUCT(
1024     if (PrintGCDetails && TraceReferenceGC) {
1025       gclog_or_tty->print(
1026         " Removed %d Refs with NULL referents out of %d discovered Refs",
1027         iter.removed(), iter.processed());
1028     }
1029   )
1030 }
1031 
1032 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1033   int id = 0;
1034   // Determine the queue index to use for this object.
1035   if (_discovery_is_mt) {
1036     // During a multi-threaded discovery phase,
1037     // each thread saves to its "own" list.
1038     Thread* thr = Thread::current();
1039     id = thr->as_Worker_thread()->id();
1040   } else {
1041     // single-threaded discovery, we save in round-robin
1042     // fashion to each of the lists.
1043     if (_processing_is_mt) {
1044       id = next_id();
1045     }
1046   }
1047   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1048 
1049   // Get the discovered queue to which we will add
1050   DiscoveredList* list = NULL;
1051   switch (rt) {
1052     case REF_OTHER:
1053       // Unknown reference type, no special treatment
1054       break;
1055     case REF_SOFT:
1056       list = &_discoveredSoftRefs[id];
1057       break;
1058     case REF_WEAK:
1059       list = &_discoveredWeakRefs[id];
1060       break;
1061     case REF_FINAL:
1062       list = &_discoveredFinalRefs[id];
1063       break;
1064     case REF_PHANTOM:
1065       list = &_discoveredPhantomRefs[id];
1066       break;
1067     case REF_NONE:
1068       // we should not reach here if we are an instanceRefKlass
1069     default:
1070       ShouldNotReachHere();
1071   }
1072   if (TraceReferenceGC && PrintGCDetails) {
1073     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1074   }
1075   return list;
1076 }
1077 
1078 inline void
1079 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1080                                               oop             obj,
1081                                               HeapWord*       discovered_addr) {
1082   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1083   // First we must make sure this object is only enqueued once. CAS in a non null
1084   // discovered_addr.
1085   oop current_head = refs_list.head();
1086 
1087   // Note: In the case of G1, this specific pre-barrier is strictly
1088   // not necessary because the only case we are interested in
1089   // here is when *discovered_addr is NULL (see the CAS further below),
1090   // so this will expand to nothing. As a result, we have manually
1091   // elided this out for G1, but left in the test for some future
1092   // collector that might have need for a pre-barrier here.
1093   if (_discovered_list_needs_barrier && !UseG1GC) {
1094     if (UseCompressedOops) {
1095       _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1096     } else {
1097       _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1098     }
1099     guarantee(false, "Need to check non-G1 collector");
1100   }
1101   oop retest = oopDesc::atomic_compare_exchange_oop(current_head, discovered_addr,
1102                                                     NULL);
1103   if (retest == NULL) {
1104     // This thread just won the right to enqueue the object.
1105     // We have separate lists for enqueueing so no synchronization
1106     // is necessary.
1107     refs_list.set_head(obj);
1108     refs_list.inc_length(1);
1109     if (_discovered_list_needs_barrier) {
1110       _bs->write_ref_field((void*)discovered_addr, current_head);
1111     }
1112 
1113     if (TraceReferenceGC) {
1114       gclog_or_tty->print_cr("Enqueued reference (mt) (" INTPTR_FORMAT ": %s)",
1115                              obj, obj->blueprint()->internal_name());
1116     }
1117   } else {
1118     // If retest was non NULL, another thread beat us to it:
1119     // The reference has already been discovered...
1120     if (TraceReferenceGC) {
1121       gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1122                              obj, obj->blueprint()->internal_name());
1123     }
1124   }
1125 }
1126 
1127 #ifndef PRODUCT
1128 // Non-atomic (i.e. concurrent) discovery might allow us
1129 // to observe j.l.References with NULL referents, being those
1130 // cleared concurrently by mutators during (or after) discovery.
1131 void ReferenceProcessor::verify_referent(oop obj) {
1132   bool da = discovery_is_atomic();
1133   oop referent = java_lang_ref_Reference::referent(obj);
1134   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1135          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1136                  INTPTR_FORMAT " during %satomic discovery ",
1137                  (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1138 }
1139 #endif
1140 
1141 // We mention two of several possible choices here:
1142 // #0: if the reference object is not in the "originating generation"
1143 //     (or part of the heap being collected, indicated by our "span"
1144 //     we don't treat it specially (i.e. we scan it as we would
1145 //     a normal oop, treating its references as strong references).
1146 //     This means that references can't be enqueued unless their
1147 //     referent is also in the same span. This is the simplest,
1148 //     most "local" and most conservative approach, albeit one
1149 //     that may cause weak references to be enqueued least promptly.
1150 //     We call this choice the "ReferenceBasedDiscovery" policy.
1151 // #1: the reference object may be in any generation (span), but if
1152 //     the referent is in the generation (span) being currently collected
1153 //     then we can discover the reference object, provided
1154 //     the object has not already been discovered by
1155 //     a different concurrently running collector (as may be the
1156 //     case, for instance, if the reference object is in CMS and
1157 //     the referent in DefNewGeneration), and provided the processing
1158 //     of this reference object by the current collector will
1159 //     appear atomic to every other collector in the system.
1160 //     (Thus, for instance, a concurrent collector may not
1161 //     discover references in other generations even if the
1162 //     referent is in its own generation). This policy may,
1163 //     in certain cases, enqueue references somewhat sooner than
1164 //     might Policy #0 above, but at marginally increased cost
1165 //     and complexity in processing these references.
1166 //     We call this choice the "RefeferentBasedDiscovery" policy.
1167 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1168   // We enqueue references only if we are discovering refs
1169   // (rather than processing discovered refs).
1170   if (!_discovering_refs || !RegisterReferences) {
1171     return false;
1172   }
1173   // We only enqueue active references.
1174   oop next = java_lang_ref_Reference::next(obj);
1175   if (next != NULL) {
1176     return false;
1177   }
1178 
1179   HeapWord* obj_addr = (HeapWord*)obj;
1180   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1181       !_span.contains(obj_addr)) {
1182     // Reference is not in the originating generation;
1183     // don't treat it specially (i.e. we want to scan it as a normal
1184     // object with strong references).
1185     return false;
1186   }
1187 
1188   // We only enqueue references whose referents are not (yet) strongly
1189   // reachable.
1190   if (is_alive_non_header() != NULL) {
1191     verify_referent(obj);
1192     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1193       return false;  // referent is reachable
1194     }
1195   }
1196   if (rt == REF_SOFT) {
1197     // For soft refs we can decide now if these are not
1198     // current candidates for clearing, in which case we
1199     // can mark through them now, rather than delaying that
1200     // to the reference-processing phase. Since all current
1201     // time-stamp policies advance the soft-ref clock only
1202     // at a major collection cycle, this is always currently
1203     // accurate.
1204     if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1205       return false;
1206     }
1207   }
1208 
1209   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1210   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1211   assert(discovered->is_oop_or_null(), "bad discovered field");
1212   if (discovered != NULL) {
1213     // The reference has already been discovered...
1214     if (TraceReferenceGC) {
1215       gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1216                              obj, obj->blueprint()->internal_name());
1217     }
1218     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1219       // assumes that an object is not processed twice;
1220       // if it's been already discovered it must be on another
1221       // generation's discovered list; so we won't discover it.
1222       return false;
1223     } else {
1224       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1225              "Unrecognized policy");
1226       // Check assumption that an object is not potentially
1227       // discovered twice except by concurrent collectors that potentially
1228       // trace the same Reference object twice.
1229       assert(UseConcMarkSweepGC || UseG1GC,
1230              "Only possible with a concurrent marking collector");
1231       return true;
1232     }
1233   }
1234 
1235   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1236     verify_referent(obj);
1237     // enqueue if and only if either:
1238     // reference is in our span or
1239     // we are an atomic collector and referent is in our span
1240     if (_span.contains(obj_addr) ||
1241         (discovery_is_atomic() &&
1242          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1243       // should_enqueue = true;
1244     } else {
1245       return false;
1246     }
1247   } else {
1248     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1249            _span.contains(obj_addr), "code inconsistency");
1250   }
1251 
1252   // Get the right type of discovered queue head.
1253   DiscoveredList* list = get_discovered_list(rt);
1254   if (list == NULL) {
1255     return false;   // nothing special needs to be done
1256   }
1257 
1258   if (_discovery_is_mt) {
1259     add_to_discovered_list_mt(*list, obj, discovered_addr);
1260   } else {
1261     // If "_discovered_list_needs_barrier", we do write barriers when
1262     // updating the discovered reference list.  Otherwise, we do a raw store
1263     // here: the field will be visited later when processing the discovered
1264     // references.
1265     oop current_head = list->head();
1266     // As in the case further above, since we are over-writing a NULL
1267     // pre-value, we can safely elide the pre-barrier here for the case of G1.
1268     assert(discovered == NULL, "control point invariant");
1269     if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1
1270       if (UseCompressedOops) {
1271         _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1272       } else {
1273         _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1274       }
1275       guarantee(false, "Need to check non-G1 collector");
1276     }
1277     oop_store_raw(discovered_addr, current_head);
1278     if (_discovered_list_needs_barrier) {
1279       _bs->write_ref_field((void*)discovered_addr, current_head);
1280     }
1281     list->set_head(obj);
1282     list->inc_length(1);
1283 
1284     if (TraceReferenceGC) {
1285       gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
1286                                 obj, obj->blueprint()->internal_name());
1287     }
1288   }
1289   assert(obj->is_oop(), "Enqueued a bad reference");
1290   verify_referent(obj);
1291   return true;
1292 }
1293 
1294 // Preclean the discovered references by removing those
1295 // whose referents are alive, and by marking from those that
1296 // are not active. These lists can be handled here
1297 // in any order and, indeed, concurrently.
1298 void ReferenceProcessor::preclean_discovered_references(
1299   BoolObjectClosure* is_alive,
1300   OopClosure* keep_alive,
1301   VoidClosure* complete_gc,
1302   YieldClosure* yield,
1303   bool should_unload_classes) {
1304 
1305   NOT_PRODUCT(verify_ok_to_handle_reflists());
1306 
1307 #ifdef ASSERT
1308   bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1309                                CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1310                                ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1311                                  UseConcMarkSweepGC && should_unload_classes;
1312   RememberKlassesChecker mx(must_remember_klasses);
1313 #endif
1314   // Soft references
1315   {
1316     TraceTime tt("Preclean SoftReferences", 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(_discoveredSoftRefs[i], is_alive,
1323                                   keep_alive, complete_gc, yield);
1324     }
1325   }
1326 
1327   // Weak references
1328   {
1329     TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1330               false, gclog_or_tty);
1331     for (int i = 0; i < _num_q; i++) {
1332       if (yield->should_return()) {
1333         return;
1334       }
1335       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1336                                   keep_alive, complete_gc, yield);
1337     }
1338   }
1339 
1340   // Final references
1341   {
1342     TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1343               false, gclog_or_tty);
1344     for (int i = 0; i < _num_q; i++) {
1345       if (yield->should_return()) {
1346         return;
1347       }
1348       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1349                                   keep_alive, complete_gc, yield);
1350     }
1351   }
1352 
1353   // Phantom references
1354   {
1355     TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1356               false, gclog_or_tty);
1357     for (int i = 0; i < _num_q; i++) {
1358       if (yield->should_return()) {
1359         return;
1360       }
1361       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1362                                   keep_alive, complete_gc, yield);
1363     }
1364   }
1365 }
1366 
1367 // Walk the given discovered ref list, and remove all reference objects
1368 // whose referents are still alive, whose referents are NULL or which
1369 // are not active (have a non-NULL next field). NOTE: When we are
1370 // thus precleaning the ref lists (which happens single-threaded today),
1371 // we do not disable refs discovery to honour the correct semantics of
1372 // java.lang.Reference. As a result, we need to be careful below
1373 // that ref removal steps interleave safely with ref discovery steps
1374 // (in this thread).
1375 void
1376 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1377                                                 BoolObjectClosure* is_alive,
1378                                                 OopClosure*        keep_alive,
1379                                                 VoidClosure*       complete_gc,
1380                                                 YieldClosure*      yield) {
1381   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1382   while (iter.has_next()) {
1383     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1384     oop obj = iter.obj();
1385     oop next = java_lang_ref_Reference::next(obj);
1386     if (iter.referent() == NULL || iter.is_referent_alive() ||
1387         next != NULL) {
1388       // The referent has been cleared, or is alive, or the Reference is not
1389       // active; we need to trace and mark its cohort.
1390       if (TraceReferenceGC) {
1391         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1392                                iter.obj(), iter.obj()->blueprint()->internal_name());
1393       }
1394       // Remove Reference object from list
1395       iter.remove();
1396       // Keep alive its cohort.
1397       iter.make_referent_alive();
1398       if (UseCompressedOops) {
1399         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1400         keep_alive->do_oop(next_addr);
1401       } else {
1402         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1403         keep_alive->do_oop(next_addr);
1404       }
1405       iter.move_to_next();
1406     } else {
1407       iter.next();
1408     }
1409   }
1410   // Close the reachable set
1411   complete_gc->do_void();
1412 
1413   NOT_PRODUCT(
1414     if (PrintGCDetails && PrintReferenceGC) {
1415       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1416         "Refs in discovered list " INTPTR_FORMAT,
1417         iter.removed(), iter.processed(), (address)refs_list.head());
1418     }
1419   )
1420 }
1421 
1422 const char* ReferenceProcessor::list_name(int i) {
1423    assert(i >= 0 && i <= _max_num_q * subclasses_of_ref, "Out of bounds index");
1424    int j = i / _max_num_q;
1425    switch (j) {
1426      case 0: return "SoftRef";
1427      case 1: return "WeakRef";
1428      case 2: return "FinalRef";
1429      case 3: return "PhantomRef";
1430    }
1431    ShouldNotReachHere();
1432    return NULL;
1433 }
1434 
1435 #ifndef PRODUCT
1436 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1437   // empty for now
1438 }
1439 #endif
1440 
1441 void ReferenceProcessor::verify() {
1442   guarantee(sentinel_ref() != NULL && sentinel_ref()->is_oop(), "Lost _sentinelRef");
1443 }
1444 
1445 #ifndef PRODUCT
1446 void ReferenceProcessor::clear_discovered_references() {
1447   guarantee(!_discovering_refs, "Discovering refs?");
1448   for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1449     oop obj = _discoveredSoftRefs[i].head();
1450     while (obj != sentinel_ref()) {
1451       oop next = java_lang_ref_Reference::discovered(obj);
1452       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
1453       obj = next;
1454     }
1455     _discoveredSoftRefs[i].set_head(sentinel_ref());
1456     _discoveredSoftRefs[i].set_length(0);
1457   }
1458 }
1459 #endif // PRODUCT