1 /*
2 * Copyright (c) 2001, 2017, 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.inline.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc/shared/collectedHeap.hpp"
29 #include "gc/shared/collectedHeap.inline.hpp"
30 #include "gc/shared/gcTimer.hpp"
31 #include "gc/shared/gcTraceTime.inline.hpp"
32 #include "gc/shared/referencePolicy.hpp"
33 #include "gc/shared/referenceProcessor.inline.hpp"
34 #include "logging/log.hpp"
35 #include "memory/allocation.hpp"
36 #include "memory/resourceArea.hpp"
37 #include "oops/oop.inline.hpp"
38 #include "runtime/java.hpp"
39 #include "runtime/jniHandles.hpp"
40
41 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
42 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
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-decreasing 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 if (is_server_compilation_mode_vm()) {
61 _default_soft_ref_policy = new LRUMaxHeapPolicy();
62 } else {
63 _default_soft_ref_policy = new LRUCurrentHeapPolicy();
64 }
65 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
66 vm_exit_during_initialization("Could not allocate reference policy object");
67 }
68 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
69 RefDiscoveryPolicy == ReferentBasedDiscovery,
70 "Unrecognized RefDiscoveryPolicy");
71 }
72
73 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
74 #ifdef ASSERT
75 // Verify that we're not currently discovering refs
76 assert(!_discovering_refs, "nested call?");
77
78 if (check_no_refs) {
79 // Verify that the discovered lists are empty
80 verify_no_references_recorded();
81 }
82 #endif // ASSERT
83
84 // Someone could have modified the value of the static
85 // field in the j.l.r.SoftReference class that holds the
86 // soft reference timestamp clock using reflection or
87 // Unsafe between GCs. Unconditionally update the static
88 // field in ReferenceProcessor here so that we use the new
89 // value during reference discovery.
90
91 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
92 _discovering_refs = true;
93 }
94
95 ReferenceProcessor::ReferenceProcessor(MemRegion span,
96 bool mt_processing,
97 uint mt_processing_degree,
98 bool mt_discovery,
99 uint mt_discovery_degree,
100 bool atomic_discovery,
101 BoolObjectClosure* is_alive_non_header) :
102 _discovering_refs(false),
103 _enqueuing_is_done(false),
104 _is_alive_non_header(is_alive_non_header),
105 _processing_is_mt(mt_processing),
106 _next_id(0)
107 {
108 _span = span;
109 _discovery_is_atomic = atomic_discovery;
110 _discovery_is_mt = mt_discovery;
111 _num_q = MAX2(1U, mt_processing_degree);
112 _max_num_q = MAX2(_num_q, mt_discovery_degree);
113 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
114 _max_num_q * number_of_subclasses_of_ref(), mtGC);
115
116 if (_discovered_refs == NULL) {
117 vm_exit_during_initialization("Could not allocated RefProc Array");
118 }
119 _discoveredSoftRefs = &_discovered_refs[0];
120 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
121 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
122 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
123
124 // Initialize all entries to NULL
125 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
126 _discovered_refs[i].set_head(NULL);
127 _discovered_refs[i].set_length(0);
128 }
129
130 setup_policy(false /* default soft ref policy */);
131 }
132
133 #ifndef PRODUCT
134 void ReferenceProcessor::verify_no_references_recorded() {
135 guarantee(!_discovering_refs, "Discovering refs?");
136 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
137 guarantee(_discovered_refs[i].is_empty(),
138 "Found non-empty discovered list at %u", i);
139 }
140 }
141 #endif
142
143 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
144 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
145 if (UseCompressedOops) {
146 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
147 } else {
148 f->do_oop((oop*)_discovered_refs[i].adr_head());
149 }
150 }
151 }
152
153 void ReferenceProcessor::update_soft_ref_master_clock() {
154 // Update (advance) the soft ref master clock field. This must be done
155 // after processing the soft ref list.
156
157 // We need a monotonically non-decreasing time in ms but
158 // os::javaTimeMillis() does not guarantee monotonicity.
159 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
160 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
161 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
162
163 NOT_PRODUCT(
164 if (now < _soft_ref_timestamp_clock) {
165 log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
166 _soft_ref_timestamp_clock, now);
167 }
168 )
169 // The values of now and _soft_ref_timestamp_clock are set using
170 // javaTimeNanos(), which is guaranteed to be monotonically
171 // non-decreasing provided the underlying platform provides such
172 // a time source (and it is bug free).
173 // In product mode, however, protect ourselves from non-monotonicity.
174 if (now > _soft_ref_timestamp_clock) {
175 _soft_ref_timestamp_clock = now;
176 java_lang_ref_SoftReference::set_clock(now);
177 }
178 // Else leave clock stalled at its old value until time progresses
179 // past clock value.
180 }
181
182 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
183 size_t total = 0;
184 for (uint i = 0; i < _max_num_q; ++i) {
185 total += lists[i].length();
186 }
187 return total;
188 }
189
190 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
191 BoolObjectClosure* is_alive,
192 OopClosure* keep_alive,
193 VoidClosure* complete_gc,
194 AbstractRefProcTaskExecutor* task_executor,
195 ReferenceProcessorPhaseTimes* phase_times) {
196 double start_time = os::elapsedTime();
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 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
213 total_count(_discoveredWeakRefs),
214 total_count(_discoveredFinalRefs),
215 total_count(_discoveredPhantomRefs));
216
217 // Soft references
218 {
219 RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
220 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
221 is_alive, keep_alive, complete_gc, task_executor, phase_times);
222 }
223
224 update_soft_ref_master_clock();
225
226 // Weak references
227 {
228 RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
229 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
230 is_alive, keep_alive, complete_gc, task_executor, phase_times);
231 }
232
233 // Final references
234 {
235 RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
236 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
237 is_alive, keep_alive, complete_gc, task_executor, phase_times);
238 }
239
240 // Phantom references
241 {
242 RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
243 process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
244 is_alive, keep_alive, complete_gc, task_executor, phase_times);
245 }
246
247 // Weak global JNI references. It would make more sense (semantically) to
248 // traverse these simultaneously with the regular weak references above, but
249 // that is not how the JDK1.2 specification is. See #4126360. Native code can
250 // thus use JNI weak references to circumvent the phantom references and
251 // resurrect a "post-mortem" object.
252 {
253 GCTraceTime(Debug, gc, ref) tt("JNI Weak Reference", phase_times->gc_timer());
254 if (task_executor != NULL) {
255 task_executor->set_single_threaded_mode();
256 }
257 process_phaseJNI(is_alive, keep_alive, complete_gc);
258 }
259
260 phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
261
262 log_develop_trace(gc, ref)("JNI Weak Reference count: " SIZE_FORMAT, count_jni_refs());
263
264 return stats;
265 }
266
267 #ifndef PRODUCT
268 // Calculate the number of jni handles.
269 size_t ReferenceProcessor::count_jni_refs() {
270 class CountHandleClosure: public OopClosure {
271 private:
272 size_t _count;
273 public:
274 CountHandleClosure(): _count(0) {}
275 void do_oop(oop* unused) { _count++; }
276 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
277 size_t count() { return _count; }
278 };
279 CountHandleClosure global_handle_count;
280 JNIHandles::weak_oops_do(&global_handle_count);
281 return global_handle_count.count();
282 }
283 #endif
284
285 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
286 OopClosure* keep_alive,
287 VoidClosure* complete_gc) {
288 JNIHandles::weak_oops_do(is_alive, keep_alive);
289 complete_gc->do_void();
290 }
291
292 void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor,
293 ReferenceProcessorPhaseTimes* phase_times) {
294 // Enqueue references that are not made active again, and
295 // clear the decks for the next collection (cycle).
296 enqueue_discovered_reflists(task_executor, phase_times);
297
298 // Stop treating discovered references specially.
299 disable_discovery();
300 }
301
302 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) {
303 // Given a list of refs linked through the "discovered" field
304 // (java.lang.ref.Reference.discovered), self-loop their "next" field
305 // thus distinguishing them from active References, then
306 // prepend them to the pending list.
307 //
308 // The Java threads will see the Reference objects linked together through
309 // the discovered field. Instead of trying to do the write barrier updates
310 // in all places in the reference processor where we manipulate the discovered
311 // field we make sure to do the barrier here where we anyway iterate through
312 // all linked Reference objects. Note that it is important to not dirty any
313 // cards during reference processing since this will cause card table
314 // verification to fail for G1.
315 log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list));
316
317 oop obj = NULL;
318 oop next_d = refs_list.head();
319 // Walk down the list, self-looping the next field
320 // so that the References are not considered active.
321 while (obj != next_d) {
322 obj = next_d;
323 assert(obj->is_instance(), "should be an instance object");
324 assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
325 next_d = java_lang_ref_Reference::discovered(obj);
326 log_develop_trace(gc, ref)(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));
327 assert(java_lang_ref_Reference::next(obj) == NULL,
328 "Reference not active; should not be discovered");
329 // Self-loop next, so as to make Ref not active.
330 java_lang_ref_Reference::set_next_raw(obj, obj);
331 if (next_d != obj) {
332 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
333 } else {
334 // This is the last object.
335 // Swap refs_list into pending list and set obj's
336 // discovered to what we read from the pending list.
337 oop old = Universe::swap_reference_pending_list(refs_list.head());
338 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
339 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
340 }
341 }
342 }
343
344 // Parallel enqueue task
345 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
346 public:
347 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
348 DiscoveredList discovered_refs[],
349 int n_queues,
350 ReferenceProcessorPhaseTimes* phase_times)
351 : EnqueueTask(ref_processor, discovered_refs, n_queues, phase_times)
352 { }
353
354 virtual void work(unsigned int work_id) {
355 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefEnqueue, _phase_times, work_id);
356
357 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
358 // Simplest first cut: static partitioning.
359 int index = work_id;
360 // The increment on "index" must correspond to the maximum number of queues
361 // (n_queues) with which that ReferenceProcessor was created. That
362 // is because of the "clever" way the discovered references lists were
363 // allocated and are indexed into.
364 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
365 for (int j = 0;
366 j < ReferenceProcessor::number_of_subclasses_of_ref();
367 j++, index += _n_queues) {
368 _ref_processor.enqueue_discovered_reflist(_refs_lists[index]);
369 _refs_lists[index].set_head(NULL);
370 _refs_lists[index].set_length(0);
371 }
372 }
373 };
374
375 // Enqueue references that are not made active again
376 void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor* task_executor,
377 ReferenceProcessorPhaseTimes* phase_times) {
378
379 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
380 total_count(_discoveredWeakRefs),
381 total_count(_discoveredFinalRefs),
382 total_count(_discoveredPhantomRefs));
383
384 RefProcEnqueueTimeTracker tt(phase_times, stats);
385
386 if (_processing_is_mt && task_executor != NULL) {
387 // Parallel code
388 RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_q, phase_times);
389 task_executor->execute(tsk);
390 } else {
391 // Serial code: call the parent class's implementation
392 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
393 enqueue_discovered_reflist(_discovered_refs[i]);
394 _discovered_refs[i].set_head(NULL);
395 _discovered_refs[i].set_length(0);
396 }
397 }
398 }
399
400 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
401 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
402 oop discovered = java_lang_ref_Reference::discovered(_ref);
403 assert(_discovered_addr && discovered->is_oop_or_null(),
404 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
405 _next = discovered;
406 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
407 _referent = java_lang_ref_Reference::referent(_ref);
408 assert(Universe::heap()->is_in_reserved_or_null(_referent),
409 "Wrong oop found in java.lang.Reference object");
410 assert(allow_null_referent ?
411 _referent->is_oop_or_null()
412 : _referent->is_oop(),
413 "Expected an oop%s for referent field at " PTR_FORMAT,
414 (allow_null_referent ? " or NULL" : ""),
415 p2i(_referent));
416 }
417
418 void DiscoveredListIterator::remove() {
419 assert(_ref->is_oop(), "Dropping a bad reference");
420 oop_store_raw(_discovered_addr, NULL);
421
422 // First _prev_next ref actually points into DiscoveredList (gross).
423 oop new_next;
424 if (_next == _ref) {
425 // At the end of the list, we should make _prev point to itself.
426 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
427 // and _prev will be NULL.
428 new_next = _prev;
429 } else {
430 new_next = _next;
431 }
432 // Remove Reference object from discovered list. Note that G1 does not need a
433 // pre-barrier here because we know the Reference has already been found/marked,
434 // that's how it ended up in the discovered list in the first place.
435 oop_store_raw(_prev_next, new_next);
436 NOT_PRODUCT(_removed++);
437 _refs_list.dec_length(1);
438 }
439
440 void DiscoveredListIterator::clear_referent() {
441 oop_store_raw(_referent_addr, NULL);
442 }
443
444 // NOTE: process_phase*() are largely similar, and at a high level
445 // merely iterate over the extant list applying a predicate to
446 // each of its elements and possibly removing that element from the
447 // list and applying some further closures to that element.
448 // We should consider the possibility of replacing these
449 // process_phase*() methods by abstracting them into
450 // a single general iterator invocation that receives appropriate
451 // closures that accomplish this work.
452
453 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
454 // referents are not alive, but that should be kept alive for policy reasons.
455 // Keep alive the transitive closure of all such referents.
456 void
457 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
458 ReferencePolicy* policy,
459 BoolObjectClosure* is_alive,
460 OopClosure* keep_alive,
461 VoidClosure* complete_gc) {
462 assert(policy != NULL, "Must have a non-NULL policy");
463 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
464 // Decide which softly reachable refs should be kept alive.
465 while (iter.has_next()) {
466 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
467 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
468 if (referent_is_dead &&
469 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
470 log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
471 p2i(iter.obj()), iter.obj()->klass()->internal_name());
472 // Remove Reference object from list
473 iter.remove();
474 // keep the referent around
475 iter.make_referent_alive();
476 iter.move_to_next();
477 } else {
478 iter.next();
479 }
480 }
481 // Close the reachable set
482 complete_gc->do_void();
483 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
484 iter.removed(), iter.processed(), p2i(&refs_list));
485 }
486
487 // Traverse the list and remove any Refs that are not active, or
488 // whose referents are either alive or NULL.
489 void
490 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
491 BoolObjectClosure* is_alive,
492 OopClosure* keep_alive) {
493 assert(discovery_is_atomic(), "Error");
494 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
495 while (iter.has_next()) {
496 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
497 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
498 assert(next == NULL, "Should not discover inactive Reference");
499 if (iter.is_referent_alive()) {
500 log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
501 p2i(iter.obj()), iter.obj()->klass()->internal_name());
502 // The referent is reachable after all.
503 // Remove Reference object from list.
504 iter.remove();
505 // Update the referent pointer as necessary: Note that this
506 // should not entail any recursive marking because the
507 // referent must already have been traversed.
508 iter.make_referent_alive();
509 iter.move_to_next();
510 } else {
511 iter.next();
512 }
513 }
514 NOT_PRODUCT(
515 if (iter.processed() > 0) {
516 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
517 " Refs in discovered list " INTPTR_FORMAT,
518 iter.removed(), iter.processed(), p2i(&refs_list));
519 }
520 )
521 }
522
523 void
524 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
525 BoolObjectClosure* is_alive,
526 OopClosure* keep_alive,
527 VoidClosure* complete_gc) {
528 assert(!discovery_is_atomic(), "Error");
529 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
530 while (iter.has_next()) {
531 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
532 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
533 oop next = java_lang_ref_Reference::next(iter.obj());
534 if ((iter.referent() == NULL || iter.is_referent_alive() ||
535 next != NULL)) {
536 assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
537 // Remove Reference object from list
538 iter.remove();
539 // Trace the cohorts
540 iter.make_referent_alive();
541 if (UseCompressedOops) {
542 keep_alive->do_oop((narrowOop*)next_addr);
543 } else {
544 keep_alive->do_oop((oop*)next_addr);
545 }
546 iter.move_to_next();
547 } else {
548 iter.next();
549 }
550 }
551 // Now close the newly reachable set
552 complete_gc->do_void();
553 NOT_PRODUCT(
554 if (iter.processed() > 0) {
555 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
556 " Refs in discovered list " INTPTR_FORMAT,
557 iter.removed(), iter.processed(), p2i(&refs_list));
558 }
559 )
560 }
561
562 // Traverse the list and process the referents, by either
563 // clearing them or keeping them (and their reachable
564 // closure) alive.
565 void
566 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
567 bool clear_referent,
568 BoolObjectClosure* is_alive,
569 OopClosure* keep_alive,
570 VoidClosure* complete_gc) {
571 ResourceMark rm;
572 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
573 while (iter.has_next()) {
574 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
575 if (clear_referent) {
576 // NULL out referent pointer
577 iter.clear_referent();
578 } else {
579 // keep the referent around
580 iter.make_referent_alive();
581 }
582 log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
583 clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
584 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
585 iter.next();
586 }
587 // Close the reachable set
588 complete_gc->do_void();
589 }
590
591 void
592 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
593 oop obj = NULL;
594 oop next = refs_list.head();
595 while (next != obj) {
596 obj = next;
597 next = java_lang_ref_Reference::discovered(obj);
598 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
599 }
600 refs_list.set_head(NULL);
601 refs_list.set_length(0);
602 }
603
604 void ReferenceProcessor::abandon_partial_discovery() {
605 // loop over the lists
606 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
607 if ((i % _max_num_q) == 0) {
608 log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
609 }
610 clear_discovered_references(_discovered_refs[i]);
611 }
612 }
613
614 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
615 DiscoveredList* list = NULL;
616
617 switch (type) {
618 case REF_SOFT:
619 list = _discoveredSoftRefs;
620 break;
621 case REF_WEAK:
622 list = _discoveredWeakRefs;
623 break;
624 case REF_FINAL:
625 list = _discoveredFinalRefs;
626 break;
627 case REF_PHANTOM:
628 list = _discoveredPhantomRefs;
629 break;
630 case REF_OTHER:
631 case REF_NONE:
632 default:
633 ShouldNotReachHere();
634 }
635 return total_count(list);
636 }
637
638 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
639 public:
640 RefProcPhase1Task(ReferenceProcessor& ref_processor,
641 DiscoveredList refs_lists[],
642 ReferencePolicy* policy,
643 bool marks_oops_alive,
644 ReferenceProcessorPhaseTimes* phase_times)
645 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
646 _policy(policy)
647 { }
648 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
649 OopClosure& keep_alive,
650 VoidClosure& complete_gc)
651 {
652 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
653
654 _ref_processor.process_phase1(_refs_lists[i], _policy,
655 &is_alive, &keep_alive, &complete_gc);
656 }
657 private:
658 ReferencePolicy* _policy;
659 };
660
661 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
662 public:
663 RefProcPhase2Task(ReferenceProcessor& ref_processor,
664 DiscoveredList refs_lists[],
665 bool marks_oops_alive,
666 ReferenceProcessorPhaseTimes* phase_times)
667 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
668 { }
669 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
670 OopClosure& keep_alive,
671 VoidClosure& complete_gc)
672 {
673 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
674
675 _ref_processor.process_phase2(_refs_lists[i],
676 &is_alive, &keep_alive, &complete_gc);
677 }
678 };
679
680 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
681 public:
682 RefProcPhase3Task(ReferenceProcessor& ref_processor,
683 DiscoveredList refs_lists[],
684 bool clear_referent,
685 bool marks_oops_alive,
686 ReferenceProcessorPhaseTimes* phase_times)
687 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
688 _clear_referent(clear_referent)
689 { }
690 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
691 OopClosure& keep_alive,
692 VoidClosure& complete_gc)
693 {
694 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
695
696 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
697 &is_alive, &keep_alive, &complete_gc);
698 }
699 private:
700 bool _clear_referent;
701 };
702
703 #ifndef PRODUCT
704 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
705 if (!log_is_enabled(Trace, gc, ref)) {
706 return;
707 }
708
709 stringStream st;
710 for (uint i = 0; i < active_length; ++i) {
711 st.print(SIZE_FORMAT " ", ref_lists[i].length());
712 }
713 log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
714 #ifdef ASSERT
715 for (uint i = active_length; i < _max_num_q; i++) {
716 assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
717 ref_lists[i].length(), i);
718 }
719 #endif
720 }
721 #endif
722
723 void ReferenceProcessor::set_active_mt_degree(uint v) {
724 _num_q = v;
725 _next_id = 0;
726 }
727
728 // Balances reference queues.
729 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
730 // queues[0, 1, ..., _num_q-1] because only the first _num_q
731 // corresponding to the active workers will be processed.
732 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
733 {
734 // calculate total length
735 size_t total_refs = 0;
736 log_develop_trace(gc, ref)("Balance ref_lists ");
737
738 for (uint i = 0; i < _max_num_q; ++i) {
739 total_refs += ref_lists[i].length();
740 }
741 log_reflist_counts(ref_lists, _max_num_q, total_refs);
742 size_t avg_refs = total_refs / _num_q + 1;
743 uint to_idx = 0;
744 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
745 bool move_all = false;
746 if (from_idx >= _num_q) {
747 move_all = ref_lists[from_idx].length() > 0;
748 }
749 while ((ref_lists[from_idx].length() > avg_refs) ||
750 move_all) {
751 assert(to_idx < _num_q, "Sanity Check!");
752 if (ref_lists[to_idx].length() < avg_refs) {
753 // move superfluous refs
754 size_t refs_to_move;
755 // Move all the Ref's if the from queue will not be processed.
756 if (move_all) {
757 refs_to_move = MIN2(ref_lists[from_idx].length(),
758 avg_refs - ref_lists[to_idx].length());
759 } else {
760 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
761 avg_refs - ref_lists[to_idx].length());
762 }
763
764 assert(refs_to_move > 0, "otherwise the code below will fail");
765
766 oop move_head = ref_lists[from_idx].head();
767 oop move_tail = move_head;
768 oop new_head = move_head;
769 // find an element to split the list on
770 for (size_t j = 0; j < refs_to_move; ++j) {
771 move_tail = new_head;
772 new_head = java_lang_ref_Reference::discovered(new_head);
773 }
774
775 // Add the chain to the to list.
776 if (ref_lists[to_idx].head() == NULL) {
777 // to list is empty. Make a loop at the end.
778 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
779 } else {
780 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
781 }
782 ref_lists[to_idx].set_head(move_head);
783 ref_lists[to_idx].inc_length(refs_to_move);
784
785 // Remove the chain from the from list.
786 if (move_tail == new_head) {
787 // We found the end of the from list.
788 ref_lists[from_idx].set_head(NULL);
789 } else {
790 ref_lists[from_idx].set_head(new_head);
791 }
792 ref_lists[from_idx].dec_length(refs_to_move);
793 if (ref_lists[from_idx].length() == 0) {
794 break;
795 }
796 } else {
797 to_idx = (to_idx + 1) % _num_q;
798 }
799 }
800 }
801 #ifdef ASSERT
802 size_t balanced_total_refs = 0;
803 for (uint i = 0; i < _num_q; ++i) {
804 balanced_total_refs += ref_lists[i].length();
805 }
806 log_reflist_counts(ref_lists, _num_q, balanced_total_refs);
807 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
808 #endif
809 }
810
811 void ReferenceProcessor::balance_all_queues() {
812 balance_queues(_discoveredSoftRefs);
813 balance_queues(_discoveredWeakRefs);
814 balance_queues(_discoveredFinalRefs);
815 balance_queues(_discoveredPhantomRefs);
816 }
817
818 void ReferenceProcessor::process_discovered_reflist(
819 DiscoveredList refs_lists[],
820 ReferencePolicy* policy,
821 bool clear_referent,
822 BoolObjectClosure* is_alive,
823 OopClosure* keep_alive,
824 VoidClosure* complete_gc,
825 AbstractRefProcTaskExecutor* task_executor,
826 ReferenceProcessorPhaseTimes* phase_times)
827 {
828 bool mt_processing = task_executor != NULL && _processing_is_mt;
829
830 phase_times->set_processing_is_mt(mt_processing);
831
832 // If discovery used MT and a dynamic number of GC threads, then
833 // the queues must be balanced for correctness if fewer than the
834 // maximum number of queues were used. The number of queue used
835 // during discovery may be different than the number to be used
836 // for processing so don't depend of _num_q < _max_num_q as part
837 // of the test.
838 bool must_balance = _discovery_is_mt;
839
840 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
841 must_balance) {
842 RefProcBalanceQueuesTimeTracker tt(phase_times);
843 balance_queues(refs_lists);
844 }
845
846 // Phase 1 (soft refs only):
847 // . Traverse the list and remove any SoftReferences whose
848 // referents are not alive, but that should be kept alive for
849 // policy reasons. Keep alive the transitive closure of all
850 // such referents.
851 if (policy != NULL) {
852 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
853
854 if (mt_processing) {
855 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
856 task_executor->execute(phase1);
857 } else {
858 for (uint i = 0; i < _max_num_q; i++) {
859 process_phase1(refs_lists[i], policy,
860 is_alive, keep_alive, complete_gc);
861 }
862 }
863 } else { // policy == NULL
864 assert(refs_lists != _discoveredSoftRefs,
865 "Policy must be specified for soft references.");
866 }
867
868 // Phase 2:
869 // . Traverse the list and remove any refs whose referents are alive.
870 {
871 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
872
873 if (mt_processing) {
874 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
875 task_executor->execute(phase2);
876 } else {
877 for (uint i = 0; i < _max_num_q; i++) {
878 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
879 }
880 }
881 }
882
883 // Phase 3:
884 // . Traverse the list and process referents as appropriate.
885 {
886 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
887
888 if (mt_processing) {
889 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
890 task_executor->execute(phase3);
891 } else {
892 for (uint i = 0; i < _max_num_q; i++) {
893 process_phase3(refs_lists[i], clear_referent,
894 is_alive, keep_alive, complete_gc);
895 }
896 }
897 }
898 }
899
900 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
901 uint id = 0;
902 // Determine the queue index to use for this object.
903 if (_discovery_is_mt) {
904 // During a multi-threaded discovery phase,
905 // each thread saves to its "own" list.
906 Thread* thr = Thread::current();
907 id = thr->as_Worker_thread()->id();
908 } else {
909 // single-threaded discovery, we save in round-robin
910 // fashion to each of the lists.
911 if (_processing_is_mt) {
912 id = next_id();
913 }
914 }
915 assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q);
916
917 // Get the discovered queue to which we will add
918 DiscoveredList* list = NULL;
919 switch (rt) {
920 case REF_OTHER:
921 // Unknown reference type, no special treatment
922 break;
923 case REF_SOFT:
924 list = &_discoveredSoftRefs[id];
925 break;
926 case REF_WEAK:
927 list = &_discoveredWeakRefs[id];
928 break;
929 case REF_FINAL:
930 list = &_discoveredFinalRefs[id];
931 break;
932 case REF_PHANTOM:
933 list = &_discoveredPhantomRefs[id];
934 break;
935 case REF_NONE:
936 // we should not reach here if we are an InstanceRefKlass
937 default:
938 ShouldNotReachHere();
939 }
940 log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
941 return list;
942 }
943
944 inline void
945 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
946 oop obj,
947 HeapWord* discovered_addr) {
948 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
949 // First we must make sure this object is only enqueued once. CAS in a non null
950 // discovered_addr.
951 oop current_head = refs_list.head();
952 // The last ref must have its discovered field pointing to itself.
953 oop next_discovered = (current_head != NULL) ? current_head : obj;
954
955 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
956 NULL);
957 if (retest == NULL) {
958 // This thread just won the right to enqueue the object.
959 // We have separate lists for enqueueing, so no synchronization
960 // is necessary.
961 refs_list.set_head(obj);
962 refs_list.inc_length(1);
963
964 log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
965 p2i(obj), obj->klass()->internal_name());
966 } else {
967 // If retest was non NULL, another thread beat us to it:
968 // The reference has already been discovered...
969 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
970 p2i(obj), obj->klass()->internal_name());
971 }
972 }
973
974 #ifndef PRODUCT
975 // Non-atomic (i.e. concurrent) discovery might allow us
976 // to observe j.l.References with NULL referents, being those
977 // cleared concurrently by mutators during (or after) discovery.
978 void ReferenceProcessor::verify_referent(oop obj) {
979 bool da = discovery_is_atomic();
980 oop referent = java_lang_ref_Reference::referent(obj);
981 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
982 "Bad referent " INTPTR_FORMAT " found in Reference "
983 INTPTR_FORMAT " during %satomic discovery ",
984 p2i(referent), p2i(obj), da ? "" : "non-");
985 }
986 #endif
987
988 // We mention two of several possible choices here:
989 // #0: if the reference object is not in the "originating generation"
990 // (or part of the heap being collected, indicated by our "span"
991 // we don't treat it specially (i.e. we scan it as we would
992 // a normal oop, treating its references as strong references).
993 // This means that references can't be discovered unless their
994 // referent is also in the same span. This is the simplest,
995 // most "local" and most conservative approach, albeit one
996 // that may cause weak references to be enqueued least promptly.
997 // We call this choice the "ReferenceBasedDiscovery" policy.
998 // #1: the reference object may be in any generation (span), but if
999 // the referent is in the generation (span) being currently collected
1000 // then we can discover the reference object, provided
1001 // the object has not already been discovered by
1002 // a different concurrently running collector (as may be the
1003 // case, for instance, if the reference object is in CMS and
1004 // the referent in DefNewGeneration), and provided the processing
1005 // of this reference object by the current collector will
1006 // appear atomic to every other collector in the system.
1007 // (Thus, for instance, a concurrent collector may not
1008 // discover references in other generations even if the
1009 // referent is in its own generation). This policy may,
1010 // in certain cases, enqueue references somewhat sooner than
1011 // might Policy #0 above, but at marginally increased cost
1012 // and complexity in processing these references.
1013 // We call this choice the "RefeferentBasedDiscovery" policy.
1014 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1015 // Make sure we are discovering refs (rather than processing discovered refs).
1016 if (!_discovering_refs || !RegisterReferences) {
1017 return false;
1018 }
1019 // We only discover active references.
1020 oop next = java_lang_ref_Reference::next(obj);
1021 if (next != NULL) { // Ref is no longer active
1022 return false;
1023 }
1024
1025 HeapWord* obj_addr = (HeapWord*)obj;
1026 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1027 !_span.contains(obj_addr)) {
1028 // Reference is not in the originating generation;
1029 // don't treat it specially (i.e. we want to scan it as a normal
1030 // object with strong references).
1031 return false;
1032 }
1033
1034 // We only discover references whose referents are not (yet)
1035 // known to be strongly reachable.
1036 if (is_alive_non_header() != NULL) {
1037 verify_referent(obj);
1038 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1039 return false; // referent is reachable
1040 }
1041 }
1042 if (rt == REF_SOFT) {
1043 // For soft refs we can decide now if these are not
1044 // current candidates for clearing, in which case we
1045 // can mark through them now, rather than delaying that
1046 // to the reference-processing phase. Since all current
1047 // time-stamp policies advance the soft-ref clock only
1048 // at a full collection cycle, this is always currently
1049 // accurate.
1050 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1051 return false;
1052 }
1053 }
1054
1055 ResourceMark rm; // Needed for tracing.
1056
1057 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1058 const oop discovered = java_lang_ref_Reference::discovered(obj);
1059 assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1060 if (discovered != NULL) {
1061 // The reference has already been discovered...
1062 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1063 p2i(obj), obj->klass()->internal_name());
1064 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1065 // assumes that an object is not processed twice;
1066 // if it's been already discovered it must be on another
1067 // generation's discovered list; so we won't discover it.
1068 return false;
1069 } else {
1070 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1071 "Unrecognized policy");
1072 // Check assumption that an object is not potentially
1073 // discovered twice except by concurrent collectors that potentially
1074 // trace the same Reference object twice.
1075 assert(UseConcMarkSweepGC || UseG1GC,
1076 "Only possible with a concurrent marking collector");
1077 return true;
1078 }
1079 }
1080
1081 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1082 verify_referent(obj);
1083 // Discover if and only if EITHER:
1084 // .. reference is in our span, OR
1085 // .. we are an atomic collector and referent is in our span
1086 if (_span.contains(obj_addr) ||
1087 (discovery_is_atomic() &&
1088 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1089 // should_enqueue = true;
1090 } else {
1091 return false;
1092 }
1093 } else {
1094 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1095 _span.contains(obj_addr), "code inconsistency");
1096 }
1097
1098 // Get the right type of discovered queue head.
1099 DiscoveredList* list = get_discovered_list(rt);
1100 if (list == NULL) {
1101 return false; // nothing special needs to be done
1102 }
1103
1104 if (_discovery_is_mt) {
1105 add_to_discovered_list_mt(*list, obj, discovered_addr);
1106 } else {
1107 // We do a raw store here: the field will be visited later when processing
1108 // the discovered references.
1109 oop current_head = list->head();
1110 // The last ref must have its discovered field pointing to itself.
1111 oop next_discovered = (current_head != NULL) ? current_head : obj;
1112
1113 assert(discovered == NULL, "control point invariant");
1114 oop_store_raw(discovered_addr, next_discovered);
1115 list->set_head(obj);
1116 list->inc_length(1);
1117
1118 log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1119 }
1120 assert(obj->is_oop(), "Discovered a bad reference");
1121 verify_referent(obj);
1122 return true;
1123 }
1124
1125 bool ReferenceProcessor::has_discovered_references() {
1126 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1127 if (!_discovered_refs[i].is_empty()) {
1128 return true;
1129 }
1130 }
1131 return false;
1132 }
1133
1134 // Preclean the discovered references by removing those
1135 // whose referents are alive, and by marking from those that
1136 // are not active. These lists can be handled here
1137 // in any order and, indeed, concurrently.
1138 void ReferenceProcessor::preclean_discovered_references(
1139 BoolObjectClosure* is_alive,
1140 OopClosure* keep_alive,
1141 VoidClosure* complete_gc,
1142 YieldClosure* yield,
1143 GCTimer* gc_timer) {
1144
1145 // Soft references
1146 {
1147 GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1148 for (uint i = 0; i < _max_num_q; i++) {
1149 if (yield->should_return()) {
1150 return;
1151 }
1152 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1153 keep_alive, complete_gc, yield);
1154 }
1155 }
1156
1157 // Weak references
1158 {
1159 GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1160 for (uint i = 0; i < _max_num_q; i++) {
1161 if (yield->should_return()) {
1162 return;
1163 }
1164 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1165 keep_alive, complete_gc, yield);
1166 }
1167 }
1168
1169 // Final references
1170 {
1171 GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1172 for (uint i = 0; i < _max_num_q; i++) {
1173 if (yield->should_return()) {
1174 return;
1175 }
1176 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1177 keep_alive, complete_gc, yield);
1178 }
1179 }
1180
1181 // Phantom references
1182 {
1183 GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1184 for (uint i = 0; i < _max_num_q; i++) {
1185 if (yield->should_return()) {
1186 return;
1187 }
1188 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1189 keep_alive, complete_gc, yield);
1190 }
1191 }
1192 }
1193
1194 // Walk the given discovered ref list, and remove all reference objects
1195 // whose referents are still alive, whose referents are NULL or which
1196 // are not active (have a non-NULL next field). NOTE: When we are
1197 // thus precleaning the ref lists (which happens single-threaded today),
1198 // we do not disable refs discovery to honor the correct semantics of
1199 // java.lang.Reference. As a result, we need to be careful below
1200 // that ref removal steps interleave safely with ref discovery steps
1201 // (in this thread).
1202 void
1203 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1204 BoolObjectClosure* is_alive,
1205 OopClosure* keep_alive,
1206 VoidClosure* complete_gc,
1207 YieldClosure* yield) {
1208 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1209 while (iter.has_next()) {
1210 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1211 oop obj = iter.obj();
1212 oop next = java_lang_ref_Reference::next(obj);
1213 if (iter.referent() == NULL || iter.is_referent_alive() ||
1214 next != NULL) {
1215 // The referent has been cleared, or is alive, or the Reference is not
1216 // active; we need to trace and mark its cohort.
1217 log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1218 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1219 // Remove Reference object from list
1220 iter.remove();
1221 // Keep alive its cohort.
1222 iter.make_referent_alive();
1223 if (UseCompressedOops) {
1224 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1225 keep_alive->do_oop(next_addr);
1226 } else {
1227 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1228 keep_alive->do_oop(next_addr);
1229 }
1230 iter.move_to_next();
1231 } else {
1232 iter.next();
1233 }
1234 }
1235 // Close the reachable set
1236 complete_gc->do_void();
1237
1238 NOT_PRODUCT(
1239 if (iter.processed() > 0) {
1240 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1241 iter.removed(), iter.processed(), p2i(&refs_list));
1242 }
1243 )
1244 }
1245
1246 const char* ReferenceProcessor::list_name(uint i) {
1247 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1248 "Out of bounds index");
1249
1250 int j = i / _max_num_q;
1251 switch (j) {
1252 case 0: return "SoftRef";
1253 case 1: return "WeakRef";
1254 case 2: return "FinalRef";
1255 case 3: return "PhantomRef";
1256 }
1257 ShouldNotReachHere();
1258 return NULL;
1259 }