1 /*
   2  * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/javaClasses.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc_implementation/shared/gcTimer.hpp"
  29 #include "gc_implementation/shared/gcTraceTime.hpp"
  30 #include "gc_interface/collectedHeap.hpp"
  31 #include "gc_interface/collectedHeap.inline.hpp"
  32 #include "memory/referencePolicy.hpp"
  33 #include "memory/referenceProcessor.hpp"
  34 #include "oops/oop.inline.hpp"
  35 #include "runtime/java.hpp"
  36 #include "runtime/jniHandles.hpp"



  37 
  38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  39 
  40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  42 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  43 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  44 
  45 void referenceProcessor_init() {
  46   ReferenceProcessor::init_statics();
  47 }
  48 
  49 void ReferenceProcessor::init_statics() {
  50   // We need a monotonically non-deccreasing time in ms but
  51   // os::javaTimeMillis() does not guarantee monotonicity.
  52   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  53 
  54   // Initialize the soft ref timestamp clock.
  55   _soft_ref_timestamp_clock = now;
  56   // Also update the soft ref clock in j.l.r.SoftReference
  57   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  58 
  59   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  60   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  61                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  62   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  63     vm_exit_during_initialization("Could not allocate reference policy object");
  64   }
  65   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  66             RefDiscoveryPolicy == ReferentBasedDiscovery,
  67             "Unrecongnized RefDiscoveryPolicy");
  68   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  69 }
  70 
  71 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  72 #ifdef ASSERT
  73   // Verify that we're not currently discovering refs
  74   assert(!verify_disabled || !_discovering_refs, "nested call?");
  75 
  76   if (check_no_refs) {
  77     // Verify that the discovered lists are empty
  78     verify_no_references_recorded();
  79   }
  80 #endif // ASSERT
  81 
  82   // Someone could have modified the value of the static
  83   // field in the j.l.r.SoftReference class that holds the
  84   // soft reference timestamp clock using reflection or
  85   // Unsafe between GCs. Unconditionally update the static
  86   // field in ReferenceProcessor here so that we use the new
  87   // value during reference discovery.
  88 
  89   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  90   _discovering_refs = true;
  91 }
  92 
  93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  94                                        bool      mt_processing,
  95                                        uint      mt_processing_degree,
  96                                        bool      mt_discovery,
  97                                        uint      mt_discovery_degree,
  98                                        bool      atomic_discovery,
  99                                        BoolObjectClosure* is_alive_non_header)  :
 100   _discovering_refs(false),
 101   _enqueuing_is_done(false),
 102   _is_alive_non_header(is_alive_non_header),
 103   _processing_is_mt(mt_processing),
 104   _next_id(0)
 105 {
 106   _span = span;
 107   _discovery_is_atomic = atomic_discovery;
 108   _discovery_is_mt     = mt_discovery;
 109   _num_q               = MAX2(1U, mt_processing_degree);
 110   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 111   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 112             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 113 
 114   if (_discovered_refs == NULL) {
 115     vm_exit_during_initialization("Could not allocated RefProc Array");
 116   }
 117   _discoveredSoftRefs    = &_discovered_refs[0];
 118   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 119   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 120   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 121   _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
 122 
 123   // Initialize all entries to NULL
 124   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 125     _discovered_refs[i].set_head(NULL);
 126     _discovered_refs[i].set_length(0);
 127   }
 128 
 129   setup_policy(false /* default soft ref policy */);
 130 }
 131 
 132 #ifndef PRODUCT
 133 void ReferenceProcessor::verify_no_references_recorded() {
 134   guarantee(!_discovering_refs, "Discovering refs?");
 135   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 136     guarantee(_discovered_refs[i].is_empty(),
 137               "Found non-empty discovered list");
 138   }
 139 }
 140 #endif
 141 
 142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 143   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 144     if (UseCompressedOops) {
 145       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 146     } else {
 147       f->do_oop((oop*)_discovered_refs[i].adr_head());
 148     }
 149   }
 150 }
 151 
 152 void ReferenceProcessor::update_soft_ref_master_clock() {
 153   // Update (advance) the soft ref master clock field. This must be done
 154   // after processing the soft ref list.
 155 
 156   // We need a monotonically non-deccreasing time in ms but
 157   // os::javaTimeMillis() does not guarantee monotonicity.
 158   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 159   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 160   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 161 
 162   NOT_PRODUCT(
 163   if (now < _soft_ref_timestamp_clock) {
 164     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 165             _soft_ref_timestamp_clock, now);
 166   }
 167   )
 168   // The values of now and _soft_ref_timestamp_clock are set using
 169   // javaTimeNanos(), which is guaranteed to be monotonically
 170   // non-decreasing provided the underlying platform provides such
 171   // a time source (and it is bug free).
 172   // In product mode, however, protect ourselves from non-monotonicty.
 173   if (now > _soft_ref_timestamp_clock) {
 174     _soft_ref_timestamp_clock = now;
 175     java_lang_ref_SoftReference::set_clock(now);
 176   }
 177   // Else leave clock stalled at its old value until time progresses
 178   // past clock value.
 179 }
 180 
 181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 182   size_t total = 0;
 183   for (uint i = 0; i < _max_num_q; ++i) {
 184     total += lists[i].length();
 185   }
 186   return total;
 187 }
 188 
 189 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 190   BoolObjectClosure*           is_alive,
 191   OopClosure*                  keep_alive,
 192   VoidClosure*                 complete_gc,
 193   AbstractRefProcTaskExecutor* task_executor,
 194   GCTimer*                     gc_timer,
 195   GCId                         gc_id) {
 196   NOT_PRODUCT(verify_ok_to_handle_reflists());
 197 
 198   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 199   // Stop treating discovered references specially.
 200   disable_discovery();
 201 
 202   // If discovery was concurrent, someone could have modified
 203   // the value of the static field in the j.l.r.SoftReference
 204   // class that holds the soft reference timestamp clock using
 205   // reflection or Unsafe between when discovery was enabled and
 206   // now. Unconditionally update the static field in ReferenceProcessor
 207   // here so that we use the new value during processing of the
 208   // discovered soft refs.
 209 
 210   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 211 
 212   bool trace_time = PrintGCDetails && PrintReferenceGC;
 213 
 214   // Soft references
 215   size_t soft_count = 0;
 216   {
 217     GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
 218     soft_count =
 219       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 220                                  is_alive, keep_alive, complete_gc, task_executor);
 221   }
 222 
 223   update_soft_ref_master_clock();
 224 
 225   // Weak references
 226   size_t weak_count = 0;
 227   {
 228     GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
 229     weak_count =
 230       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 231                                  is_alive, keep_alive, complete_gc, task_executor);
 232   }
 233 
 234   // Final references
 235   size_t final_count = 0;
 236   {
 237     GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
 238     final_count =
 239       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 240                                  is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   // Phantom references
 244   size_t phantom_count = 0;
 245   {
 246     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
 247     phantom_count =
 248       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 249                                  is_alive, keep_alive, complete_gc, task_executor);
 250 
 251     // Process cleaners, but include them in phantom statistics.  We expect
 252     // Cleaner references to be temporary, and don't want to deal with
 253     // possible incompatibilities arising from making it more visible.
 254     phantom_count +=
 255       process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
 256                                  is_alive, keep_alive, complete_gc, task_executor);
 257   }
 258 
 259   // Weak global JNI references. It would make more sense (semantically) to
 260   // traverse these simultaneously with the regular weak references above, but
 261   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 262   // thus use JNI weak references to circumvent the phantom references and
 263   // resurrect a "post-mortem" object.
 264   {
 265     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
 266     if (task_executor != NULL) {
 267       task_executor->set_single_threaded_mode();
 268     }
 269     process_phaseJNI(is_alive, keep_alive, complete_gc);
 270   }
 271 
 272   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 273 }
 274 
 275 #ifndef PRODUCT
 276 // Calculate the number of jni handles.
 277 uint ReferenceProcessor::count_jni_refs() {
 278   class CountHandleClosure: public OopClosure {
 279   private:
 280     int _count;
 281   public:
 282     CountHandleClosure(): _count(0) {}
 283     void do_oop(oop* unused)       { _count++; }
 284     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 285     int count() { return _count; }
 286   };
 287   CountHandleClosure global_handle_count;
 288   JNIHandles::weak_oops_do(&global_handle_count);
 289   return global_handle_count.count();
 290 }
 291 #endif
 292 
 293 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 294                                           OopClosure*        keep_alive,
 295                                           VoidClosure*       complete_gc) {
 296 #ifndef PRODUCT
 297   if (PrintGCDetails && PrintReferenceGC) {
 298     unsigned int count = count_jni_refs();
 299     gclog_or_tty->print(", %u refs", count);
 300   }
 301 #endif
 302   JNIHandles::weak_oops_do(is_alive, keep_alive);

 303   complete_gc->do_void();
 304 }
 305 
 306 
 307 template <class T>
 308 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 309                                    AbstractRefProcTaskExecutor* task_executor) {
 310 
 311   // Remember old value of pending references list
 312   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 313   T old_pending_list_value = *pending_list_addr;
 314 
 315   // Enqueue references that are not made active again, and
 316   // clear the decks for the next collection (cycle).
 317   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 318   // Do the post-barrier on pending_list_addr missed in
 319   // enqueue_discovered_reflist.
 320   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 321 
 322   // Stop treating discovered references specially.
 323   ref->disable_discovery();
 324 
 325   // Return true if new pending references were added
 326   return old_pending_list_value != *pending_list_addr;
 327 }
 328 
 329 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 330   NOT_PRODUCT(verify_ok_to_handle_reflists());
 331   if (UseCompressedOops) {
 332     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 333   } else {
 334     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 335   }
 336 }
 337 
 338 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 339                                                     HeapWord* pending_list_addr) {
 340   // Given a list of refs linked through the "discovered" field
 341   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 342   // thus distinguishing them from active References, then
 343   // prepend them to the pending list.
 344   //
 345   // The Java threads will see the Reference objects linked together through
 346   // the discovered field. Instead of trying to do the write barrier updates
 347   // in all places in the reference processor where we manipulate the discovered
 348   // field we make sure to do the barrier here where we anyway iterate through
 349   // all linked Reference objects. Note that it is important to not dirty any
 350   // cards during reference processing since this will cause card table
 351   // verification to fail for G1.
 352   //
 353   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 354   // the "next" field is used to chain the pending list, not the discovered
 355   // field.
 356   if (TraceReferenceGC && PrintGCDetails) {
 357     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 358                            INTPTR_FORMAT, (address)refs_list.head());
 359   }
 360 
 361   oop obj = NULL;
 362   oop next_d = refs_list.head();
 363   if (pending_list_uses_discovered_field()) { // New behavior
 364     // Walk down the list, self-looping the next field
 365     // so that the References are not considered active.
 366     while (obj != next_d) {
 367       obj = next_d;
 368       assert(obj->is_instanceRef(), "should be reference object");
 369       next_d = java_lang_ref_Reference::discovered(obj);
 370       if (TraceReferenceGC && PrintGCDetails) {
 371         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 372                                (void *)obj, (void *)next_d);
 373       }
 374       assert(java_lang_ref_Reference::next(obj) == NULL,
 375              "Reference not active; should not be discovered");
 376       // Self-loop next, so as to make Ref not active.
 377       java_lang_ref_Reference::set_next_raw(obj, obj);
 378       if (next_d != obj) {
 379         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 380       } else {
 381         // This is the last object.
 382         // Swap refs_list into pending_list_addr and
 383         // set obj's discovered to what we read from pending_list_addr.
 384         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 385         // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 386         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 387         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 388       }
 389     }
 390   } else { // Old behaviour
 391     // Walk down the list, copying the discovered field into
 392     // the next field and clearing the discovered field.
 393     while (obj != next_d) {
 394       obj = next_d;
 395       assert(obj->is_instanceRef(), "should be reference object");
 396       next_d = java_lang_ref_Reference::discovered(obj);
 397       if (TraceReferenceGC && PrintGCDetails) {
 398         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 399                                (void *)obj, (void *)next_d);
 400       }
 401       assert(java_lang_ref_Reference::next(obj) == NULL,
 402              "The reference should not be enqueued");
 403       if (next_d == obj) {  // obj is last
 404         // Swap refs_list into pendling_list_addr and
 405         // set obj's next to what we read from pending_list_addr.
 406         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 407         // Need oop_check on pending_list_addr above;
 408         // see special oop-check code at the end of
 409         // enqueue_discovered_reflists() further below.
 410         if (old == NULL) {
 411           // obj should be made to point to itself, since
 412           // pending list was empty.
 413           java_lang_ref_Reference::set_next(obj, obj);
 414         } else {
 415           java_lang_ref_Reference::set_next(obj, old);
 416         }
 417       } else {
 418         java_lang_ref_Reference::set_next(obj, next_d);
 419       }
 420       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 421     }
 422   }
 423 }
 424 
 425 // Parallel enqueue task
 426 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 427 public:
 428   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 429                      DiscoveredList      discovered_refs[],
 430                      HeapWord*           pending_list_addr,
 431                      int                 n_queues)
 432     : EnqueueTask(ref_processor, discovered_refs,
 433                   pending_list_addr, n_queues)
 434   { }
 435 
 436   virtual void work(unsigned int work_id) {
 437     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 438     // Simplest first cut: static partitioning.
 439     int index = work_id;
 440     // The increment on "index" must correspond to the maximum number of queues
 441     // (n_queues) with which that ReferenceProcessor was created.  That
 442     // is because of the "clever" way the discovered references lists were
 443     // allocated and are indexed into.
 444     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 445     for (int j = 0;
 446          j < ReferenceProcessor::number_of_subclasses_of_ref();
 447          j++, index += _n_queues) {
 448       _ref_processor.enqueue_discovered_reflist(
 449         _refs_lists[index], _pending_list_addr);
 450       _refs_lists[index].set_head(NULL);
 451       _refs_lists[index].set_length(0);
 452     }
 453   }
 454 };
 455 
 456 // Enqueue references that are not made active again
 457 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 458   AbstractRefProcTaskExecutor* task_executor) {
 459   if (_processing_is_mt && task_executor != NULL) {
 460     // Parallel code
 461     RefProcEnqueueTask tsk(*this, _discovered_refs,
 462                            pending_list_addr, _max_num_q);
 463     task_executor->execute(tsk);
 464   } else {
 465     // Serial code: call the parent class's implementation
 466     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 467       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 468       _discovered_refs[i].set_head(NULL);
 469       _discovered_refs[i].set_length(0);
 470     }
 471   }
 472 }
 473 
 474 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 475   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 476   oop discovered = java_lang_ref_Reference::discovered(_ref);
 477   assert(_discovered_addr && discovered->is_oop_or_null(),
 478          "discovered field is bad");
 479   _next = discovered;
 480   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 481   _referent = java_lang_ref_Reference::referent(_ref);
 482   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 483          "Wrong oop found in java.lang.Reference object");
 484   assert(allow_null_referent ?
 485              _referent->is_oop_or_null()
 486            : _referent->is_oop(),
 487          "bad referent");
 488 }
 489 
 490 void DiscoveredListIterator::remove() {
 491   assert(_ref->is_oop(), "Dropping a bad reference");
 492   oop_store_raw(_discovered_addr, NULL);
 493 
 494   // First _prev_next ref actually points into DiscoveredList (gross).
 495   oop new_next;
 496   if (_next == _ref) {
 497     // At the end of the list, we should make _prev point to itself.
 498     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 499     // and _prev will be NULL.
 500     new_next = _prev;
 501   } else {
 502     new_next = _next;
 503   }
 504   // Remove Reference object from discovered list. Note that G1 does not need a
 505   // pre-barrier here because we know the Reference has already been found/marked,
 506   // that's how it ended up in the discovered list in the first place.
 507   oop_store_raw(_prev_next, new_next);
 508   NOT_PRODUCT(_removed++);
 509   _refs_list.dec_length(1);
 510 }
 511 
 512 // Make the Reference object active again.
 513 void DiscoveredListIterator::make_active() {
 514   // The pre barrier for G1 is probably just needed for the old
 515   // reference processing behavior. Should we guard this with
 516   // ReferenceProcessor::pending_list_uses_discovered_field() ?
 517   if (UseG1GC) {
 518     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 519     if (UseCompressedOops) {
 520       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 521     } else {
 522       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 523     }
 524   }
 525   java_lang_ref_Reference::set_next_raw(_ref, NULL);
 526 }
 527 
 528 void DiscoveredListIterator::clear_referent() {
 529   oop_store_raw(_referent_addr, NULL);
 530 }
 531 
 532 // NOTE: process_phase*() are largely similar, and at a high level
 533 // merely iterate over the extant list applying a predicate to
 534 // each of its elements and possibly removing that element from the
 535 // list and applying some further closures to that element.
 536 // We should consider the possibility of replacing these
 537 // process_phase*() methods by abstracting them into
 538 // a single general iterator invocation that receives appropriate
 539 // closures that accomplish this work.
 540 
 541 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 542 // referents are not alive, but that should be kept alive for policy reasons.
 543 // Keep alive the transitive closure of all such referents.
 544 void
 545 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 546                                    ReferencePolicy*   policy,
 547                                    BoolObjectClosure* is_alive,
 548                                    OopClosure*        keep_alive,
 549                                    VoidClosure*       complete_gc) {
 550   assert(policy != NULL, "Must have a non-NULL policy");
 551   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 552   // Decide which softly reachable refs should be kept alive.
 553   while (iter.has_next()) {
 554     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 555     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 556     if (referent_is_dead &&
 557         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 558       if (TraceReferenceGC) {
 559         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 560                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 561       }
 562       // Remove Reference object from list
 563       iter.remove();
 564       // Make the Reference object active again
 565       iter.make_active();
 566       // keep the referent around
 567       iter.make_referent_alive();
 568       iter.move_to_next();
 569     } else {
 570       iter.next();
 571     }
 572   }
 573   // Close the reachable set
 574   complete_gc->do_void();
 575   NOT_PRODUCT(
 576     if (PrintGCDetails && TraceReferenceGC) {
 577       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 578         "discovered Refs by policy, from list " INTPTR_FORMAT,
 579         iter.removed(), iter.processed(), (address)refs_list.head());
 580     }
 581   )
 582 }
 583 
 584 // Traverse the list and remove any Refs that are not active, or
 585 // whose referents are either alive or NULL.
 586 void
 587 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 588                              BoolObjectClosure* is_alive,
 589                              OopClosure*        keep_alive) {
 590   assert(discovery_is_atomic(), "Error");
 591   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 592   while (iter.has_next()) {
 593     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 594     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 595     assert(next == NULL, "Should not discover inactive Reference");
 596     if (iter.is_referent_alive()) {
 597       if (TraceReferenceGC) {
 598         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 599                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 600       }
 601       // The referent is reachable after all.
 602       // Remove Reference object from list.
 603       iter.remove();
 604       // Update the referent pointer as necessary: Note that this
 605       // should not entail any recursive marking because the
 606       // referent must already have been traversed.
 607       iter.make_referent_alive();
 608       iter.move_to_next();
 609     } else {
 610       iter.next();
 611     }
 612   }
 613   NOT_PRODUCT(
 614     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 615       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 616         "Refs in discovered list " INTPTR_FORMAT,
 617         iter.removed(), iter.processed(), (address)refs_list.head());
 618     }
 619   )
 620 }
 621 
 622 void
 623 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 624                                                   BoolObjectClosure* is_alive,
 625                                                   OopClosure*        keep_alive,
 626                                                   VoidClosure*       complete_gc) {
 627   assert(!discovery_is_atomic(), "Error");
 628   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 629   while (iter.has_next()) {
 630     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 631     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 632     oop next = java_lang_ref_Reference::next(iter.obj());
 633     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 634          next != NULL)) {
 635       assert(next->is_oop_or_null(), "bad next field");
 636       // Remove Reference object from list
 637       iter.remove();
 638       // Trace the cohorts
 639       iter.make_referent_alive();
 640       if (UseCompressedOops) {
 641         keep_alive->do_oop((narrowOop*)next_addr);
 642       } else {
 643         keep_alive->do_oop((oop*)next_addr);
 644       }
 645       iter.move_to_next();
 646     } else {
 647       iter.next();
 648     }
 649   }
 650   // Now close the newly reachable set
 651   complete_gc->do_void();
 652   NOT_PRODUCT(
 653     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 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 // Traverse the list and process the referents, by either
 662 // clearing them or keeping them (and their reachable
 663 // closure) alive.
 664 void
 665 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 666                                    bool               clear_referent,
 667                                    BoolObjectClosure* is_alive,
 668                                    OopClosure*        keep_alive,
 669                                    VoidClosure*       complete_gc) {
 670   ResourceMark rm;
 671   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 672   while (iter.has_next()) {
 673     iter.update_discovered();
 674     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 675     if (clear_referent) {
 676       // NULL out referent pointer
 677       iter.clear_referent();
 678     } else {
 679       // keep the referent around
 680       iter.make_referent_alive();
 681     }
 682     if (TraceReferenceGC) {
 683       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 684                              clear_referent ? "cleared " : "",
 685                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 686     }
 687     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 688     iter.next();
 689   }
 690   // Remember to update the next pointer of the last ref.
 691   iter.update_discovered();
 692   // Close the reachable set
 693   complete_gc->do_void();
 694 }
 695 
 696 void
 697 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 698   oop obj = NULL;
 699   oop next = refs_list.head();
 700   while (next != obj) {
 701     obj = next;
 702     next = java_lang_ref_Reference::discovered(obj);
 703     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 704   }
 705   refs_list.set_head(NULL);
 706   refs_list.set_length(0);
 707 }
 708 
 709 void
 710 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 711   clear_discovered_references(refs_list);
 712 }
 713 
 714 void ReferenceProcessor::abandon_partial_discovery() {
 715   // loop over the lists
 716   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 717     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 718       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 719     }
 720     abandon_partial_discovered_list(_discovered_refs[i]);
 721   }
 722 }
 723 
 724 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 725 public:
 726   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 727                     DiscoveredList      refs_lists[],
 728                     ReferencePolicy*    policy,
 729                     bool                marks_oops_alive)
 730     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 731       _policy(policy)
 732   { }
 733   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 734                     OopClosure& keep_alive,
 735                     VoidClosure& complete_gc)
 736   {
 737     Thread* thr = Thread::current();
 738     int refs_list_index = ((WorkerThread*)thr)->id();
 739     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 740                                   &is_alive, &keep_alive, &complete_gc);
 741   }
 742 private:
 743   ReferencePolicy* _policy;
 744 };
 745 
 746 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 747 public:
 748   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 749                     DiscoveredList      refs_lists[],
 750                     bool                marks_oops_alive)
 751     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 752   { }
 753   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 754                     OopClosure& keep_alive,
 755                     VoidClosure& complete_gc)
 756   {
 757     _ref_processor.process_phase2(_refs_lists[i],
 758                                   &is_alive, &keep_alive, &complete_gc);
 759   }
 760 };
 761 
 762 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 763 public:
 764   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 765                     DiscoveredList      refs_lists[],
 766                     bool                clear_referent,
 767                     bool                marks_oops_alive)
 768     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 769       _clear_referent(clear_referent)
 770   { }
 771   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 772                     OopClosure& keep_alive,
 773                     VoidClosure& complete_gc)
 774   {
 775     // Don't use "refs_list_index" calculated in this way because
 776     // balance_queues() has moved the Ref's into the first n queues.
 777     // Thread* thr = Thread::current();
 778     // int refs_list_index = ((WorkerThread*)thr)->id();
 779     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 780     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 781                                   &is_alive, &keep_alive, &complete_gc);
 782   }
 783 private:
 784   bool _clear_referent;
 785 };
 786 
 787 // Balances reference queues.
 788 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 789 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 790 // corresponding to the active workers will be processed.
 791 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 792 {
 793   // calculate total length
 794   size_t total_refs = 0;
 795   if (TraceReferenceGC && PrintGCDetails) {
 796     gclog_or_tty->print_cr("\nBalance ref_lists ");
 797   }
 798 
 799   for (uint i = 0; i < _max_num_q; ++i) {
 800     total_refs += ref_lists[i].length();
 801     if (TraceReferenceGC && PrintGCDetails) {
 802       gclog_or_tty->print("%d ", ref_lists[i].length());
 803     }
 804   }
 805   if (TraceReferenceGC && PrintGCDetails) {
 806     gclog_or_tty->print_cr(" = %d", total_refs);
 807   }
 808   size_t avg_refs = total_refs / _num_q + 1;
 809   uint to_idx = 0;
 810   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 811     bool move_all = false;
 812     if (from_idx >= _num_q) {
 813       move_all = ref_lists[from_idx].length() > 0;
 814     }
 815     while ((ref_lists[from_idx].length() > avg_refs) ||
 816            move_all) {
 817       assert(to_idx < _num_q, "Sanity Check!");
 818       if (ref_lists[to_idx].length() < avg_refs) {
 819         // move superfluous refs
 820         size_t refs_to_move;
 821         // Move all the Ref's if the from queue will not be processed.
 822         if (move_all) {
 823           refs_to_move = MIN2(ref_lists[from_idx].length(),
 824                               avg_refs - ref_lists[to_idx].length());
 825         } else {
 826           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 827                               avg_refs - ref_lists[to_idx].length());
 828         }
 829 
 830         assert(refs_to_move > 0, "otherwise the code below will fail");
 831 
 832         oop move_head = ref_lists[from_idx].head();
 833         oop move_tail = move_head;
 834         oop new_head  = move_head;
 835         // find an element to split the list on
 836         for (size_t j = 0; j < refs_to_move; ++j) {
 837           move_tail = new_head;
 838           new_head = java_lang_ref_Reference::discovered(new_head);
 839         }
 840 
 841         // Add the chain to the to list.
 842         if (ref_lists[to_idx].head() == NULL) {
 843           // to list is empty. Make a loop at the end.
 844           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 845         } else {
 846           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 847         }
 848         ref_lists[to_idx].set_head(move_head);
 849         ref_lists[to_idx].inc_length(refs_to_move);
 850 
 851         // Remove the chain from the from list.
 852         if (move_tail == new_head) {
 853           // We found the end of the from list.
 854           ref_lists[from_idx].set_head(NULL);
 855         } else {
 856           ref_lists[from_idx].set_head(new_head);
 857         }
 858         ref_lists[from_idx].dec_length(refs_to_move);
 859         if (ref_lists[from_idx].length() == 0) {
 860           break;
 861         }
 862       } else {
 863         to_idx = (to_idx + 1) % _num_q;
 864       }
 865     }
 866   }
 867 #ifdef ASSERT
 868   size_t balanced_total_refs = 0;
 869   for (uint i = 0; i < _max_num_q; ++i) {
 870     balanced_total_refs += ref_lists[i].length();
 871     if (TraceReferenceGC && PrintGCDetails) {
 872       gclog_or_tty->print("%d ", ref_lists[i].length());
 873     }
 874   }
 875   if (TraceReferenceGC && PrintGCDetails) {
 876     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 877     gclog_or_tty->flush();
 878   }
 879   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 880 #endif
 881 }
 882 
 883 void ReferenceProcessor::balance_all_queues() {
 884   balance_queues(_discoveredSoftRefs);
 885   balance_queues(_discoveredWeakRefs);
 886   balance_queues(_discoveredFinalRefs);
 887   balance_queues(_discoveredPhantomRefs);
 888   balance_queues(_discoveredCleanerRefs);
 889 }
 890 
 891 size_t
 892 ReferenceProcessor::process_discovered_reflist(
 893   DiscoveredList               refs_lists[],
 894   ReferencePolicy*             policy,
 895   bool                         clear_referent,
 896   BoolObjectClosure*           is_alive,
 897   OopClosure*                  keep_alive,
 898   VoidClosure*                 complete_gc,
 899   AbstractRefProcTaskExecutor* task_executor)
 900 {
 901   bool mt_processing = task_executor != NULL && _processing_is_mt;
 902   // If discovery used MT and a dynamic number of GC threads, then
 903   // the queues must be balanced for correctness if fewer than the
 904   // maximum number of queues were used.  The number of queue used
 905   // during discovery may be different than the number to be used
 906   // for processing so don't depend of _num_q < _max_num_q as part
 907   // of the test.
 908   bool must_balance = _discovery_is_mt;
 909 
 910   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 911       must_balance) {
 912     balance_queues(refs_lists);
 913   }
 914 
 915   size_t total_list_count = total_count(refs_lists);
 916 
 917   if (PrintReferenceGC && PrintGCDetails) {
 918     gclog_or_tty->print(", %u refs", total_list_count);
 919   }
 920 
 921   // Phase 1 (soft refs only):
 922   // . Traverse the list and remove any SoftReferences whose
 923   //   referents are not alive, but that should be kept alive for
 924   //   policy reasons. Keep alive the transitive closure of all
 925   //   such referents.
 926   if (policy != NULL) {
 927     if (mt_processing) {
 928       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 929       task_executor->execute(phase1);
 930     } else {
 931       for (uint i = 0; i < _max_num_q; i++) {
 932         process_phase1(refs_lists[i], policy,
 933                        is_alive, keep_alive, complete_gc);
 934       }
 935     }
 936   } else { // policy == NULL
 937     assert(refs_lists != _discoveredSoftRefs,
 938            "Policy must be specified for soft references.");
 939   }
 940 
 941   // Phase 2:
 942   // . Traverse the list and remove any refs whose referents are alive.
 943   if (mt_processing) {
 944     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 945     task_executor->execute(phase2);
 946   } else {
 947     for (uint i = 0; i < _max_num_q; i++) {
 948       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 949     }
 950   }
 951 
 952   // Phase 3:
 953   // . Traverse the list and process referents as appropriate.
 954   if (mt_processing) {
 955     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 956     task_executor->execute(phase3);
 957   } else {
 958     for (uint i = 0; i < _max_num_q; i++) {
 959       process_phase3(refs_lists[i], clear_referent,
 960                      is_alive, keep_alive, complete_gc);
 961     }
 962   }
 963 
 964   return total_list_count;
 965 }
 966 
 967 void ReferenceProcessor::clean_up_discovered_references() {
 968   // loop over the lists
 969   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 970     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 971       gclog_or_tty->print_cr(
 972         "\nScrubbing %s discovered list of Null referents",
 973         list_name(i));
 974     }
 975     clean_up_discovered_reflist(_discovered_refs[i]);
 976   }
 977 }
 978 
 979 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 980   assert(!discovery_is_atomic(), "Else why call this method?");
 981   DiscoveredListIterator iter(refs_list, NULL, NULL);
 982   while (iter.has_next()) {
 983     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 984     oop next = java_lang_ref_Reference::next(iter.obj());
 985     assert(next->is_oop_or_null(), "bad next field");
 986     // If referent has been cleared or Reference is not active,
 987     // drop it.
 988     if (iter.referent() == NULL || next != NULL) {
 989       debug_only(
 990         if (PrintGCDetails && TraceReferenceGC) {
 991           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 992             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 993             " and referent: " INTPTR_FORMAT,
 994             (void *)iter.obj(), (void *)next, (void *)iter.referent());
 995         }
 996       )
 997       // Remove Reference object from list
 998       iter.remove();
 999       iter.move_to_next();
1000     } else {
1001       iter.next();
1002     }
1003   }
1004   NOT_PRODUCT(
1005     if (PrintGCDetails && TraceReferenceGC) {
1006       gclog_or_tty->print(
1007         " Removed %d Refs with NULL referents out of %d discovered Refs",
1008         iter.removed(), iter.processed());
1009     }
1010   )
1011 }
1012 
1013 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1014   uint id = 0;
1015   // Determine the queue index to use for this object.
1016   if (_discovery_is_mt) {
1017     // During a multi-threaded discovery phase,
1018     // each thread saves to its "own" list.
1019     Thread* thr = Thread::current();
1020     id = thr->as_Worker_thread()->id();
1021   } else {
1022     // single-threaded discovery, we save in round-robin
1023     // fashion to each of the lists.
1024     if (_processing_is_mt) {
1025       id = next_id();
1026     }
1027   }
1028   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1029 
1030   // Get the discovered queue to which we will add
1031   DiscoveredList* list = NULL;
1032   switch (rt) {
1033     case REF_OTHER:
1034       // Unknown reference type, no special treatment
1035       break;
1036     case REF_SOFT:
1037       list = &_discoveredSoftRefs[id];
1038       break;
1039     case REF_WEAK:
1040       list = &_discoveredWeakRefs[id];
1041       break;
1042     case REF_FINAL:
1043       list = &_discoveredFinalRefs[id];
1044       break;
1045     case REF_PHANTOM:
1046       list = &_discoveredPhantomRefs[id];
1047       break;
1048     case REF_CLEANER:
1049       list = &_discoveredCleanerRefs[id];
1050       break;
1051     case REF_NONE:
1052       // we should not reach here if we are an InstanceRefKlass
1053     default:
1054       ShouldNotReachHere();
1055   }
1056   if (TraceReferenceGC && PrintGCDetails) {
1057     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1058   }
1059   return list;
1060 }
1061 
1062 inline void
1063 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1064                                               oop             obj,
1065                                               HeapWord*       discovered_addr) {
1066   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1067   // First we must make sure this object is only enqueued once. CAS in a non null
1068   // discovered_addr.
1069   oop current_head = refs_list.head();
1070   // The last ref must have its discovered field pointing to itself.
1071   oop next_discovered = (current_head != NULL) ? current_head : obj;
1072 
1073   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1074                                                     NULL);
1075   if (retest == NULL) {
1076     // This thread just won the right to enqueue the object.
1077     // We have separate lists for enqueueing, so no synchronization
1078     // is necessary.
1079     refs_list.set_head(obj);
1080     refs_list.inc_length(1);
1081 
1082     if (TraceReferenceGC) {
1083       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1084                              (void *)obj, obj->klass()->internal_name());
1085     }
1086   } else {
1087     // If retest was non NULL, another thread beat us to it:
1088     // The reference has already been discovered...
1089     if (TraceReferenceGC) {
1090       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1091                              (void *)obj, obj->klass()->internal_name());
1092     }
1093   }
1094 }
1095 
1096 #ifndef PRODUCT
1097 // Non-atomic (i.e. concurrent) discovery might allow us
1098 // to observe j.l.References with NULL referents, being those
1099 // cleared concurrently by mutators during (or after) discovery.
1100 void ReferenceProcessor::verify_referent(oop obj) {
1101   bool da = discovery_is_atomic();
1102   oop referent = java_lang_ref_Reference::referent(obj);
1103   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1104          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1105                  INTPTR_FORMAT " during %satomic discovery ",
1106                  (void *)referent, (void *)obj, da ? "" : "non-"));
1107 }
1108 #endif
1109 
1110 // We mention two of several possible choices here:
1111 // #0: if the reference object is not in the "originating generation"
1112 //     (or part of the heap being collected, indicated by our "span"
1113 //     we don't treat it specially (i.e. we scan it as we would
1114 //     a normal oop, treating its references as strong references).
1115 //     This means that references can't be discovered unless their
1116 //     referent is also in the same span. This is the simplest,
1117 //     most "local" and most conservative approach, albeit one
1118 //     that may cause weak references to be enqueued least promptly.
1119 //     We call this choice the "ReferenceBasedDiscovery" policy.
1120 // #1: the reference object may be in any generation (span), but if
1121 //     the referent is in the generation (span) being currently collected
1122 //     then we can discover the reference object, provided
1123 //     the object has not already been discovered by
1124 //     a different concurrently running collector (as may be the
1125 //     case, for instance, if the reference object is in CMS and
1126 //     the referent in DefNewGeneration), and provided the processing
1127 //     of this reference object by the current collector will
1128 //     appear atomic to every other collector in the system.
1129 //     (Thus, for instance, a concurrent collector may not
1130 //     discover references in other generations even if the
1131 //     referent is in its own generation). This policy may,
1132 //     in certain cases, enqueue references somewhat sooner than
1133 //     might Policy #0 above, but at marginally increased cost
1134 //     and complexity in processing these references.
1135 //     We call this choice the "RefeferentBasedDiscovery" policy.
1136 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1137   // Make sure we are discovering refs (rather than processing discovered refs).
1138   if (!_discovering_refs || !RegisterReferences) {
1139     return false;
1140   }
1141   // We only discover active references.
1142   oop next = java_lang_ref_Reference::next(obj);
1143   if (next != NULL) {   // Ref is no longer active
1144     return false;
1145   }
1146 
1147   HeapWord* obj_addr = (HeapWord*)obj;
1148   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1149       !_span.contains(obj_addr)) {
1150     // Reference is not in the originating generation;
1151     // don't treat it specially (i.e. we want to scan it as a normal
1152     // object with strong references).
1153     return false;
1154   }
1155 
1156   // We only discover references whose referents are not (yet)
1157   // known to be strongly reachable.
1158   if (is_alive_non_header() != NULL) {
1159     verify_referent(obj);
1160     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1161       return false;  // referent is reachable
1162     }
1163   }
1164   if (rt == REF_SOFT) {
1165     // For soft refs we can decide now if these are not
1166     // current candidates for clearing, in which case we
1167     // can mark through them now, rather than delaying that
1168     // to the reference-processing phase. Since all current
1169     // time-stamp policies advance the soft-ref clock only
1170     // at a major collection cycle, this is always currently
1171     // accurate.
1172     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1173       return false;
1174     }
1175   }
1176 
1177   ResourceMark rm;      // Needed for tracing.
1178 
1179   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1180   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1181   assert(discovered->is_oop_or_null(), "bad discovered field");
1182   if (discovered != NULL) {
1183     // The reference has already been discovered...
1184     if (TraceReferenceGC) {
1185       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1186                              (void *)obj, obj->klass()->internal_name());
1187     }
1188     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1189       // assumes that an object is not processed twice;
1190       // if it's been already discovered it must be on another
1191       // generation's discovered list; so we won't discover it.
1192       return false;
1193     } else {
1194       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1195              "Unrecognized policy");
1196       // Check assumption that an object is not potentially
1197       // discovered twice except by concurrent collectors that potentially
1198       // trace the same Reference object twice.
1199       assert(UseConcMarkSweepGC || UseG1GC,
1200              "Only possible with a concurrent marking collector");
1201       return true;
1202     }
1203   }
1204 
1205   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1206     verify_referent(obj);
1207     // Discover if and only if EITHER:
1208     // .. reference is in our span, OR
1209     // .. we are an atomic collector and referent is in our span
1210     if (_span.contains(obj_addr) ||
1211         (discovery_is_atomic() &&
1212          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1213       // should_enqueue = true;
1214     } else {
1215       return false;
1216     }
1217   } else {
1218     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1219            _span.contains(obj_addr), "code inconsistency");
1220   }
1221 
1222   // Get the right type of discovered queue head.
1223   DiscoveredList* list = get_discovered_list(rt);
1224   if (list == NULL) {
1225     return false;   // nothing special needs to be done
1226   }
1227 
1228   if (_discovery_is_mt) {
1229     add_to_discovered_list_mt(*list, obj, discovered_addr);
1230   } else {
1231     // We do a raw store here: the field will be visited later when processing
1232     // the discovered references.
1233     oop current_head = list->head();
1234     // The last ref must have its discovered field pointing to itself.
1235     oop next_discovered = (current_head != NULL) ? current_head : obj;
1236 
1237     assert(discovered == NULL, "control point invariant");
1238     oop_store_raw(discovered_addr, next_discovered);
1239     list->set_head(obj);
1240     list->inc_length(1);
1241 
1242     if (TraceReferenceGC) {
1243       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1244                                 (void *)obj, obj->klass()->internal_name());
1245     }
1246   }
1247   assert(obj->is_oop(), "Discovered a bad reference");
1248   verify_referent(obj);
1249   return true;
1250 }
1251 
1252 // Preclean the discovered references by removing those
1253 // whose referents are alive, and by marking from those that
1254 // are not active. These lists can be handled here
1255 // in any order and, indeed, concurrently.
1256 void ReferenceProcessor::preclean_discovered_references(
1257   BoolObjectClosure* is_alive,
1258   OopClosure* keep_alive,
1259   VoidClosure* complete_gc,
1260   YieldClosure* yield,
1261   GCTimer* gc_timer,
1262   GCId     gc_id) {
1263 
1264   NOT_PRODUCT(verify_ok_to_handle_reflists());
1265 
1266   // Soft references
1267   {
1268     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1269               false, gc_timer, gc_id);
1270     for (uint i = 0; i < _max_num_q; i++) {
1271       if (yield->should_return()) {
1272         return;
1273       }
1274       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1275                                   keep_alive, complete_gc, yield);
1276     }
1277   }
1278 
1279   // Weak references
1280   {
1281     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1282               false, gc_timer, gc_id);
1283     for (uint i = 0; i < _max_num_q; i++) {
1284       if (yield->should_return()) {
1285         return;
1286       }
1287       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1288                                   keep_alive, complete_gc, yield);
1289     }
1290   }
1291 
1292   // Final references
1293   {
1294     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1295               false, gc_timer, gc_id);
1296     for (uint i = 0; i < _max_num_q; i++) {
1297       if (yield->should_return()) {
1298         return;
1299       }
1300       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1301                                   keep_alive, complete_gc, yield);
1302     }
1303   }
1304 
1305   // Phantom references
1306   {
1307     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1308               false, gc_timer, gc_id);
1309     for (uint i = 0; i < _max_num_q; i++) {
1310       if (yield->should_return()) {
1311         return;
1312       }
1313       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1314                                   keep_alive, complete_gc, yield);
1315     }
1316 
1317     // Cleaner references.  Included in timing for phantom references.  We
1318     // expect Cleaner references to be temporary, and don't want to deal with
1319     // possible incompatibilities arising from making it more visible.
1320     for (uint i = 0; i < _max_num_q; i++) {
1321       if (yield->should_return()) {
1322         return;
1323       }
1324       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1325                                   keep_alive, complete_gc, yield);
1326     }
1327   }
1328 }
1329 
1330 // Walk the given discovered ref list, and remove all reference objects
1331 // whose referents are still alive, whose referents are NULL or which
1332 // are not active (have a non-NULL next field). NOTE: When we are
1333 // thus precleaning the ref lists (which happens single-threaded today),
1334 // we do not disable refs discovery to honour the correct semantics of
1335 // java.lang.Reference. As a result, we need to be careful below
1336 // that ref removal steps interleave safely with ref discovery steps
1337 // (in this thread).
1338 void
1339 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1340                                                 BoolObjectClosure* is_alive,
1341                                                 OopClosure*        keep_alive,
1342                                                 VoidClosure*       complete_gc,
1343                                                 YieldClosure*      yield) {
1344   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1345   while (iter.has_next()) {
1346     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1347     oop obj = iter.obj();
1348     oop next = java_lang_ref_Reference::next(obj);
1349     if (iter.referent() == NULL || iter.is_referent_alive() ||
1350         next != NULL) {
1351       // The referent has been cleared, or is alive, or the Reference is not
1352       // active; we need to trace and mark its cohort.
1353       if (TraceReferenceGC) {
1354         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1355                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1356       }
1357       // Remove Reference object from list
1358       iter.remove();
1359       // Keep alive its cohort.
1360       iter.make_referent_alive();
1361       if (UseCompressedOops) {
1362         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1363         keep_alive->do_oop(next_addr);
1364       } else {
1365         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1366         keep_alive->do_oop(next_addr);
1367       }
1368       iter.move_to_next();
1369     } else {
1370       iter.next();
1371     }
1372   }
1373   // Close the reachable set
1374   complete_gc->do_void();
1375 
1376   NOT_PRODUCT(
1377     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1378       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1379         "Refs in discovered list " INTPTR_FORMAT,
1380         iter.removed(), iter.processed(), (address)refs_list.head());
1381     }
1382   )
1383 }
1384 
1385 const char* ReferenceProcessor::list_name(uint i) {
1386    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1387           "Out of bounds index");
1388 
1389    int j = i / _max_num_q;
1390    switch (j) {
1391      case 0: return "SoftRef";
1392      case 1: return "WeakRef";
1393      case 2: return "FinalRef";
1394      case 3: return "PhantomRef";
1395      case 4: return "CleanerRef";
1396    }
1397    ShouldNotReachHere();
1398    return NULL;
1399 }
1400 
1401 #ifndef PRODUCT
1402 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1403   // empty for now
1404 }
1405 #endif
1406 
1407 #ifndef PRODUCT
1408 void ReferenceProcessor::clear_discovered_references() {
1409   guarantee(!_discovering_refs, "Discovering refs?");
1410   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1411     clear_discovered_references(_discovered_refs[i]);
1412   }
1413 }
1414 
1415 #endif // PRODUCT
--- EOF ---