1 /*
2 * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc/shared/collectedHeap.hpp"
29 #include "gc/shared/collectedHeap.inline.hpp"
30 #include "gc/shared/gcTimer.hpp"
31 #include "gc/shared/gcTraceTime.hpp"
32 #include "gc/shared/referencePolicy.hpp"
33 #include "gc/shared/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
37
38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
40 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
41
42 void referenceProcessor_init() {
43 ReferenceProcessor::init_statics();
44 }
45
46 void ReferenceProcessor::init_statics() {
47 // We need a monotonically non-decreasing time in ms but
48 // os::javaTimeMillis() does not guarantee monotonicity.
49 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
50
51 // Initialize the soft ref timestamp clock.
52 _soft_ref_timestamp_clock = now;
53 // Also update the soft ref clock in j.l.r.SoftReference
54 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
55
56 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
57 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
58 NOT_COMPILER2(LRUCurrentHeapPolicy());
59 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
60 vm_exit_during_initialization("Could not allocate reference policy object");
61 }
62 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
63 RefDiscoveryPolicy == ReferentBasedDiscovery,
64 "Unrecognized RefDiscoveryPolicy");
65 }
66
67 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
68 #ifdef ASSERT
69 // Verify that we're not currently discovering refs
70 assert(!_discovering_refs, "nested call?");
71
72 if (check_no_refs) {
73 // Verify that the discovered lists are empty
74 verify_no_references_recorded();
75 }
76 #endif // ASSERT
77
78 // Someone could have modified the value of the static
79 // field in the j.l.r.SoftReference class that holds the
80 // soft reference timestamp clock using reflection or
81 // Unsafe between GCs. Unconditionally update the static
82 // field in ReferenceProcessor here so that we use the new
83 // value during reference discovery.
84
85 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
86 _discovering_refs = true;
87 }
88
89 ReferenceProcessor::ReferenceProcessor(MemRegion span,
90 bool mt_processing,
91 uint mt_processing_degree,
92 bool mt_discovery,
93 uint mt_discovery_degree,
94 bool atomic_discovery,
95 BoolObjectClosure* is_alive_non_header) :
96 _discovering_refs(false),
97 _enqueuing_is_done(false),
98 _is_alive_non_header(is_alive_non_header),
99 _processing_is_mt(mt_processing),
100 _next_id(0)
101 {
102 _span = span;
103 _discovery_is_atomic = atomic_discovery;
104 _discovery_is_mt = mt_discovery;
105 _num_q = MAX2(1U, mt_processing_degree);
106 _max_num_q = MAX2(_num_q, mt_discovery_degree);
107 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
108 _max_num_q * number_of_subclasses_of_ref(), mtGC);
109
110 if (_discovered_refs == NULL) {
111 vm_exit_during_initialization("Could not allocated RefProc Array");
112 }
113 _discoveredSoftRefs = &_discovered_refs[0];
114 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
115 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
116 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
117 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
118
119 // Initialize all entries to NULL
120 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
121 _discovered_refs[i].set_head(NULL);
122 _discovered_refs[i].set_length(0);
123 }
124
125 setup_policy(false /* default soft ref policy */);
126 }
127
128 #ifndef PRODUCT
129 void ReferenceProcessor::verify_no_references_recorded() {
130 guarantee(!_discovering_refs, "Discovering refs?");
131 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
132 guarantee(_discovered_refs[i].is_empty(),
133 "Found non-empty discovered list");
134 }
135 }
136 #endif
137
138 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
139 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
140 if (UseCompressedOops) {
141 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
142 } else {
143 f->do_oop((oop*)_discovered_refs[i].adr_head());
144 }
145 }
146 }
147
148 void ReferenceProcessor::update_soft_ref_master_clock() {
149 // Update (advance) the soft ref master clock field. This must be done
150 // after processing the soft ref list.
151
152 // We need a monotonically non-decreasing time in ms but
153 // os::javaTimeMillis() does not guarantee monotonicity.
154 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
155 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
156 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
157
158 NOT_PRODUCT(
159 if (now < _soft_ref_timestamp_clock) {
160 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
161 _soft_ref_timestamp_clock, now);
162 }
163 )
164 // The values of now and _soft_ref_timestamp_clock are set using
165 // javaTimeNanos(), which is guaranteed to be monotonically
166 // non-decreasing provided the underlying platform provides such
167 // a time source (and it is bug free).
168 // In product mode, however, protect ourselves from non-monotonicity.
169 if (now > _soft_ref_timestamp_clock) {
170 _soft_ref_timestamp_clock = now;
171 java_lang_ref_SoftReference::set_clock(now);
172 }
173 // Else leave clock stalled at its old value until time progresses
174 // past clock value.
175 }
176
177 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
178 size_t total = 0;
179 for (uint i = 0; i < _max_num_q; ++i) {
180 total += lists[i].length();
181 }
182 return total;
183 }
184
185 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
186 BoolObjectClosure* is_alive,
187 OopClosure* keep_alive,
188 VoidClosure* complete_gc,
189 AbstractRefProcTaskExecutor* task_executor,
190 GCTimer* gc_timer,
191 GCId gc_id) {
192
193 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
194 // Stop treating discovered references specially.
195 disable_discovery();
196
197 // If discovery was concurrent, someone could have modified
198 // the value of the static field in the j.l.r.SoftReference
199 // class that holds the soft reference timestamp clock using
200 // reflection or Unsafe between when discovery was enabled and
201 // now. Unconditionally update the static field in ReferenceProcessor
202 // here so that we use the new value during processing of the
203 // discovered soft refs.
204
205 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
206
207 bool trace_time = PrintGCDetails && PrintReferenceGC;
208
209 // Soft references
210 size_t soft_count = 0;
211 {
212 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
213 soft_count =
214 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
215 is_alive, keep_alive, complete_gc, task_executor);
216 }
217
218 update_soft_ref_master_clock();
219
220 // Weak references
221 size_t weak_count = 0;
222 {
223 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
224 weak_count =
225 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
226 is_alive, keep_alive, complete_gc, task_executor);
227 }
228
229 // Final references
230 size_t final_count = 0;
231 {
232 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
233 final_count =
234 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
235 is_alive, keep_alive, complete_gc, task_executor);
236 }
237
238 // Phantom references
239 size_t phantom_count = 0;
240 {
241 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
242 phantom_count =
243 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
244 is_alive, keep_alive, complete_gc, task_executor);
245
246 }
247
248 // Cleaners
249 size_t cleaner_count = 0;
250 {
251 GCTraceTime tt("Cleaners", trace_time, false, gc_timer, gc_id);
252 cleaner_count =
253 process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
254 is_alive, keep_alive, complete_gc, task_executor);
255 }
256
257 // Weak global JNI references. It would make more sense (semantically) to
258 // traverse these simultaneously with the regular weak references above, but
259 // that is not how the JDK1.2 specification is. See #4126360. Native code can
260 // thus use JNI weak references to circumvent the phantom references and
261 // resurrect a "post-mortem" object.
262 size_t jni_weak_ref_count = 0;
263 {
264 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
265 if (task_executor != NULL) {
266 task_executor->set_single_threaded_mode();
267 }
268 jni_weak_ref_count =
269 process_phaseJNI(is_alive, keep_alive, complete_gc);
270 }
271
272 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count, cleaner_count, jni_weak_ref_count);
273 }
274
275 #ifndef PRODUCT
276 // Calculate the number of jni handles.
277 uint ReferenceProcessor::count_jni_refs() {
278 class AlwaysAliveClosure: public BoolObjectClosure {
279 public:
280 virtual bool do_object_b(oop obj) { return true; }
281 };
282
283 class CountHandleClosure: public OopClosure {
284 private:
285 int _count;
286 public:
287 CountHandleClosure(): _count(0) {}
288 void do_oop(oop* unused) { _count++; }
289 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
290 int count() { return _count; }
291 };
292 CountHandleClosure global_handle_count;
293 AlwaysAliveClosure always_alive;
294 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
295 return global_handle_count.count();
296 }
297 #endif
298
299 size_t ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
300 OopClosure* keep_alive,
301 VoidClosure* complete_gc) {
302 DEBUG_ONLY(size_t check_count = count_jni_refs();)
303 size_t count = JNIHandles::weak_oops_do(is_alive, keep_alive);
304 assert(count == check_count, "Counts didn't match");
305 complete_gc->do_void();
306 if (PrintGCDetails && PrintReferenceGC) {
307 gclog_or_tty->print(", " SIZE_FORMAT " refs", count);
308 }
309 return count;
310 }
311
312
313 template <class T>
314 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
315 AbstractRefProcTaskExecutor* task_executor) {
316
317 // Remember old value of pending references list
318 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
319 T old_pending_list_value = *pending_list_addr;
320
321 // Enqueue references that are not made active again, and
322 // clear the decks for the next collection (cycle).
323 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
324 // Do the post-barrier on pending_list_addr missed in
325 // enqueue_discovered_reflist.
326 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
327
328 // Stop treating discovered references specially.
329 ref->disable_discovery();
330
331 // Return true if new pending references were added
332 return old_pending_list_value != *pending_list_addr;
333 }
334
335 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
336 if (UseCompressedOops) {
337 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
338 } else {
339 return enqueue_discovered_ref_helper<oop>(this, task_executor);
340 }
341 }
342
343 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
344 HeapWord* pending_list_addr) {
345 // Given a list of refs linked through the "discovered" field
346 // (java.lang.ref.Reference.discovered), self-loop their "next" field
347 // thus distinguishing them from active References, then
348 // prepend them to the pending list.
349 //
350 // The Java threads will see the Reference objects linked together through
351 // the discovered field. Instead of trying to do the write barrier updates
352 // in all places in the reference processor where we manipulate the discovered
353 // field we make sure to do the barrier here where we anyway iterate through
354 // all linked Reference objects. Note that it is important to not dirty any
355 // cards during reference processing since this will cause card table
356 // verification to fail for G1.
357 if (TraceReferenceGC && PrintGCDetails) {
358 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
359 INTPTR_FORMAT, p2i(refs_list.head()));
360 }
361
362 oop obj = NULL;
363 oop next_d = refs_list.head();
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 p2i(obj), p2i(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 }
391
392 // Parallel enqueue task
393 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
394 public:
395 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
396 DiscoveredList discovered_refs[],
397 HeapWord* pending_list_addr,
398 int n_queues)
399 : EnqueueTask(ref_processor, discovered_refs,
400 pending_list_addr, n_queues)
401 { }
402
403 virtual void work(unsigned int work_id) {
404 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
405 // Simplest first cut: static partitioning.
406 int index = work_id;
407 // The increment on "index" must correspond to the maximum number of queues
408 // (n_queues) with which that ReferenceProcessor was created. That
409 // is because of the "clever" way the discovered references lists were
410 // allocated and are indexed into.
411 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
412 for (int j = 0;
413 j < ReferenceProcessor::number_of_subclasses_of_ref();
414 j++, index += _n_queues) {
415 _ref_processor.enqueue_discovered_reflist(
416 _refs_lists[index], _pending_list_addr);
417 _refs_lists[index].set_head(NULL);
418 _refs_lists[index].set_length(0);
419 }
420 }
421 };
422
423 // Enqueue references that are not made active again
424 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
425 AbstractRefProcTaskExecutor* task_executor) {
426 if (_processing_is_mt && task_executor != NULL) {
427 // Parallel code
428 RefProcEnqueueTask tsk(*this, _discovered_refs,
429 pending_list_addr, _max_num_q);
430 task_executor->execute(tsk);
431 } else {
432 // Serial code: call the parent class's implementation
433 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
434 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
435 _discovered_refs[i].set_head(NULL);
436 _discovered_refs[i].set_length(0);
437 }
438 }
439 }
440
441 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
442 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
443 oop discovered = java_lang_ref_Reference::discovered(_ref);
444 assert(_discovered_addr && discovered->is_oop_or_null(),
445 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
446 _next = discovered;
447 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
448 _referent = java_lang_ref_Reference::referent(_ref);
449 assert(Universe::heap()->is_in_reserved_or_null(_referent),
450 "Wrong oop found in java.lang.Reference object");
451 assert(allow_null_referent ?
452 _referent->is_oop_or_null()
453 : _referent->is_oop(),
454 err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
455 (allow_null_referent ? " or NULL" : ""),
456 p2i(_referent)));
457 }
458
459 void DiscoveredListIterator::remove() {
460 assert(_ref->is_oop(), "Dropping a bad reference");
461 oop_store_raw(_discovered_addr, NULL);
462
463 // First _prev_next ref actually points into DiscoveredList (gross).
464 oop new_next;
465 if (_next == _ref) {
466 // At the end of the list, we should make _prev point to itself.
467 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
468 // and _prev will be NULL.
469 new_next = _prev;
470 } else {
471 new_next = _next;
472 }
473 // Remove Reference object from discovered list. Note that G1 does not need a
474 // pre-barrier here because we know the Reference has already been found/marked,
475 // that's how it ended up in the discovered list in the first place.
476 oop_store_raw(_prev_next, new_next);
477 NOT_PRODUCT(_removed++);
478 _refs_list.dec_length(1);
479 }
480
481 void DiscoveredListIterator::clear_referent() {
482 oop_store_raw(_referent_addr, NULL);
483 }
484
485 // NOTE: process_phase*() are largely similar, and at a high level
486 // merely iterate over the extant list applying a predicate to
487 // each of its elements and possibly removing that element from the
488 // list and applying some further closures to that element.
489 // We should consider the possibility of replacing these
490 // process_phase*() methods by abstracting them into
491 // a single general iterator invocation that receives appropriate
492 // closures that accomplish this work.
493
494 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
495 // referents are not alive, but that should be kept alive for policy reasons.
496 // Keep alive the transitive closure of all such referents.
497 void
498 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
499 ReferencePolicy* policy,
500 BoolObjectClosure* is_alive,
501 OopClosure* keep_alive,
502 VoidClosure* complete_gc) {
503 assert(policy != NULL, "Must have a non-NULL policy");
504 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
505 // Decide which softly reachable refs should be kept alive.
506 while (iter.has_next()) {
507 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
508 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
509 if (referent_is_dead &&
510 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
511 if (TraceReferenceGC) {
512 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
513 p2i(iter.obj()), iter.obj()->klass()->internal_name());
514 }
515 // Remove Reference object from list
516 iter.remove();
517 // keep the referent around
518 iter.make_referent_alive();
519 iter.move_to_next();
520 } else {
521 iter.next();
522 }
523 }
524 // Close the reachable set
525 complete_gc->do_void();
526 NOT_PRODUCT(
527 if (PrintGCDetails && TraceReferenceGC) {
528 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
529 " discovered Refs by policy, from list " INTPTR_FORMAT,
530 iter.removed(), iter.processed(), p2i(refs_list.head()));
531 }
532 )
533 }
534
535 // Traverse the list and remove any Refs that are not active, or
536 // whose referents are either alive or NULL.
537 void
538 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
539 BoolObjectClosure* is_alive,
540 OopClosure* keep_alive) {
541 assert(discovery_is_atomic(), "Error");
542 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
543 while (iter.has_next()) {
544 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
545 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
546 assert(next == NULL, "Should not discover inactive Reference");
547 if (iter.is_referent_alive()) {
548 if (TraceReferenceGC) {
549 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
550 p2i(iter.obj()), iter.obj()->klass()->internal_name());
551 }
552 // The referent is reachable after all.
553 // Remove Reference object from list.
554 iter.remove();
555 // Update the referent pointer as necessary: Note that this
556 // should not entail any recursive marking because the
557 // referent must already have been traversed.
558 iter.make_referent_alive();
559 iter.move_to_next();
560 } else {
561 iter.next();
562 }
563 }
564 NOT_PRODUCT(
565 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
566 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
567 " Refs in discovered list " INTPTR_FORMAT,
568 iter.removed(), iter.processed(), p2i(refs_list.head()));
569 }
570 )
571 }
572
573 void
574 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
575 BoolObjectClosure* is_alive,
576 OopClosure* keep_alive,
577 VoidClosure* complete_gc) {
578 assert(!discovery_is_atomic(), "Error");
579 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
580 while (iter.has_next()) {
581 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
582 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
583 oop next = java_lang_ref_Reference::next(iter.obj());
584 if ((iter.referent() == NULL || iter.is_referent_alive() ||
585 next != NULL)) {
586 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
587 // Remove Reference object from list
588 iter.remove();
589 // Trace the cohorts
590 iter.make_referent_alive();
591 if (UseCompressedOops) {
592 keep_alive->do_oop((narrowOop*)next_addr);
593 } else {
594 keep_alive->do_oop((oop*)next_addr);
595 }
596 iter.move_to_next();
597 } else {
598 iter.next();
599 }
600 }
601 // Now close the newly reachable set
602 complete_gc->do_void();
603 NOT_PRODUCT(
604 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
605 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
606 " Refs in discovered list " INTPTR_FORMAT,
607 iter.removed(), iter.processed(), p2i(refs_list.head()));
608 }
609 )
610 }
611
612 // Traverse the list and process the referents, by either
613 // clearing them or keeping them (and their reachable
614 // closure) alive.
615 void
616 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
617 bool clear_referent,
618 BoolObjectClosure* is_alive,
619 OopClosure* keep_alive,
620 VoidClosure* complete_gc) {
621 ResourceMark rm;
622 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
623 while (iter.has_next()) {
624 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
625 if (clear_referent) {
626 // NULL out referent pointer
627 iter.clear_referent();
628 } else {
629 // keep the referent around
630 iter.make_referent_alive();
631 }
632 if (TraceReferenceGC) {
633 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
634 clear_referent ? "cleared " : "",
635 p2i(iter.obj()), iter.obj()->klass()->internal_name());
636 }
637 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
638 iter.next();
639 }
640 // Close the reachable set
641 complete_gc->do_void();
642 }
643
644 void
645 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
646 oop obj = NULL;
647 oop next = refs_list.head();
648 while (next != obj) {
649 obj = next;
650 next = java_lang_ref_Reference::discovered(obj);
651 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
652 }
653 refs_list.set_head(NULL);
654 refs_list.set_length(0);
655 }
656
657 void ReferenceProcessor::abandon_partial_discovery() {
658 // loop over the lists
659 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
660 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
661 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
662 }
663 clear_discovered_references(_discovered_refs[i]);
664 }
665 }
666
667 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
668 public:
669 RefProcPhase1Task(ReferenceProcessor& ref_processor,
670 DiscoveredList refs_lists[],
671 ReferencePolicy* policy,
672 bool marks_oops_alive)
673 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
674 _policy(policy)
675 { }
676 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
677 OopClosure& keep_alive,
678 VoidClosure& complete_gc)
679 {
680 Thread* thr = Thread::current();
681 int refs_list_index = ((WorkerThread*)thr)->id();
682 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
683 &is_alive, &keep_alive, &complete_gc);
684 }
685 private:
686 ReferencePolicy* _policy;
687 };
688
689 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
690 public:
691 RefProcPhase2Task(ReferenceProcessor& ref_processor,
692 DiscoveredList refs_lists[],
693 bool marks_oops_alive)
694 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
695 { }
696 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
697 OopClosure& keep_alive,
698 VoidClosure& complete_gc)
699 {
700 _ref_processor.process_phase2(_refs_lists[i],
701 &is_alive, &keep_alive, &complete_gc);
702 }
703 };
704
705 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
706 public:
707 RefProcPhase3Task(ReferenceProcessor& ref_processor,
708 DiscoveredList refs_lists[],
709 bool clear_referent,
710 bool marks_oops_alive)
711 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
712 _clear_referent(clear_referent)
713 { }
714 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
715 OopClosure& keep_alive,
716 VoidClosure& complete_gc)
717 {
718 // Don't use "refs_list_index" calculated in this way because
719 // balance_queues() has moved the Ref's into the first n queues.
720 // Thread* thr = Thread::current();
721 // int refs_list_index = ((WorkerThread*)thr)->id();
722 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
723 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
724 &is_alive, &keep_alive, &complete_gc);
725 }
726 private:
727 bool _clear_referent;
728 };
729
730 // Balances reference queues.
731 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
732 // queues[0, 1, ..., _num_q-1] because only the first _num_q
733 // corresponding to the active workers will be processed.
734 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
735 {
736 // calculate total length
737 size_t total_refs = 0;
738 if (TraceReferenceGC && PrintGCDetails) {
739 gclog_or_tty->print_cr("\nBalance ref_lists ");
740 }
741
742 for (uint i = 0; i < _max_num_q; ++i) {
743 total_refs += ref_lists[i].length();
744 if (TraceReferenceGC && PrintGCDetails) {
745 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
746 }
747 }
748 if (TraceReferenceGC && PrintGCDetails) {
749 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
750 }
751 size_t avg_refs = total_refs / _num_q + 1;
752 uint to_idx = 0;
753 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
754 bool move_all = false;
755 if (from_idx >= _num_q) {
756 move_all = ref_lists[from_idx].length() > 0;
757 }
758 while ((ref_lists[from_idx].length() > avg_refs) ||
759 move_all) {
760 assert(to_idx < _num_q, "Sanity Check!");
761 if (ref_lists[to_idx].length() < avg_refs) {
762 // move superfluous refs
763 size_t refs_to_move;
764 // Move all the Ref's if the from queue will not be processed.
765 if (move_all) {
766 refs_to_move = MIN2(ref_lists[from_idx].length(),
767 avg_refs - ref_lists[to_idx].length());
768 } else {
769 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
770 avg_refs - ref_lists[to_idx].length());
771 }
772
773 assert(refs_to_move > 0, "otherwise the code below will fail");
774
775 oop move_head = ref_lists[from_idx].head();
776 oop move_tail = move_head;
777 oop new_head = move_head;
778 // find an element to split the list on
779 for (size_t j = 0; j < refs_to_move; ++j) {
780 move_tail = new_head;
781 new_head = java_lang_ref_Reference::discovered(new_head);
782 }
783
784 // Add the chain to the to list.
785 if (ref_lists[to_idx].head() == NULL) {
786 // to list is empty. Make a loop at the end.
787 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
788 } else {
789 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
790 }
791 ref_lists[to_idx].set_head(move_head);
792 ref_lists[to_idx].inc_length(refs_to_move);
793
794 // Remove the chain from the from list.
795 if (move_tail == new_head) {
796 // We found the end of the from list.
797 ref_lists[from_idx].set_head(NULL);
798 } else {
799 ref_lists[from_idx].set_head(new_head);
800 }
801 ref_lists[from_idx].dec_length(refs_to_move);
802 if (ref_lists[from_idx].length() == 0) {
803 break;
804 }
805 } else {
806 to_idx = (to_idx + 1) % _num_q;
807 }
808 }
809 }
810 #ifdef ASSERT
811 size_t balanced_total_refs = 0;
812 for (uint i = 0; i < _max_num_q; ++i) {
813 balanced_total_refs += ref_lists[i].length();
814 if (TraceReferenceGC && PrintGCDetails) {
815 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
816 }
817 }
818 if (TraceReferenceGC && PrintGCDetails) {
819 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
820 gclog_or_tty->flush();
821 }
822 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
823 #endif
824 }
825
826 void ReferenceProcessor::balance_all_queues() {
827 balance_queues(_discoveredSoftRefs);
828 balance_queues(_discoveredWeakRefs);
829 balance_queues(_discoveredFinalRefs);
830 balance_queues(_discoveredPhantomRefs);
831 balance_queues(_discoveredCleanerRefs);
832 }
833
834 size_t
835 ReferenceProcessor::process_discovered_reflist(
836 DiscoveredList refs_lists[],
837 ReferencePolicy* policy,
838 bool clear_referent,
839 BoolObjectClosure* is_alive,
840 OopClosure* keep_alive,
841 VoidClosure* complete_gc,
842 AbstractRefProcTaskExecutor* task_executor)
843 {
844 bool mt_processing = task_executor != NULL && _processing_is_mt;
845 // If discovery used MT and a dynamic number of GC threads, then
846 // the queues must be balanced for correctness if fewer than the
847 // maximum number of queues were used. The number of queue used
848 // during discovery may be different than the number to be used
849 // for processing so don't depend of _num_q < _max_num_q as part
850 // of the test.
851 bool must_balance = _discovery_is_mt;
852
853 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
854 must_balance) {
855 balance_queues(refs_lists);
856 }
857
858 size_t total_list_count = total_count(refs_lists);
859
860 if (PrintReferenceGC && PrintGCDetails) {
861 gclog_or_tty->print(", " SIZE_FORMAT " refs", total_list_count);
862 }
863
864 // Phase 1 (soft refs only):
865 // . Traverse the list and remove any SoftReferences whose
866 // referents are not alive, but that should be kept alive for
867 // policy reasons. Keep alive the transitive closure of all
868 // such referents.
869 if (policy != NULL) {
870 if (mt_processing) {
871 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
872 task_executor->execute(phase1);
873 } else {
874 for (uint i = 0; i < _max_num_q; i++) {
875 process_phase1(refs_lists[i], policy,
876 is_alive, keep_alive, complete_gc);
877 }
878 }
879 } else { // policy == NULL
880 assert(refs_lists != _discoveredSoftRefs,
881 "Policy must be specified for soft references.");
882 }
883
884 // Phase 2:
885 // . Traverse the list and remove any refs whose referents are alive.
886 if (mt_processing) {
887 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
888 task_executor->execute(phase2);
889 } else {
890 for (uint i = 0; i < _max_num_q; i++) {
891 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
892 }
893 }
894
895 // Phase 3:
896 // . Traverse the list and process referents as appropriate.
897 if (mt_processing) {
898 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
899 task_executor->execute(phase3);
900 } else {
901 for (uint i = 0; i < _max_num_q; i++) {
902 process_phase3(refs_lists[i], clear_referent,
903 is_alive, keep_alive, complete_gc);
904 }
905 }
906
907 return total_list_count;
908 }
909
910 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
911 uint id = 0;
912 // Determine the queue index to use for this object.
913 if (_discovery_is_mt) {
914 // During a multi-threaded discovery phase,
915 // each thread saves to its "own" list.
916 Thread* thr = Thread::current();
917 id = thr->as_Worker_thread()->id();
918 } else {
919 // single-threaded discovery, we save in round-robin
920 // fashion to each of the lists.
921 if (_processing_is_mt) {
922 id = next_id();
923 }
924 }
925 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
926
927 // Get the discovered queue to which we will add
928 DiscoveredList* list = NULL;
929 switch (rt) {
930 case REF_OTHER:
931 // Unknown reference type, no special treatment
932 break;
933 case REF_SOFT:
934 list = &_discoveredSoftRefs[id];
935 break;
936 case REF_WEAK:
937 list = &_discoveredWeakRefs[id];
938 break;
939 case REF_FINAL:
940 list = &_discoveredFinalRefs[id];
941 break;
942 case REF_PHANTOM:
943 list = &_discoveredPhantomRefs[id];
944 break;
945 case REF_CLEANER:
946 list = &_discoveredCleanerRefs[id];
947 break;
948 case REF_NONE:
949 case REF_JNI:
950 // we should not reach here if we are an InstanceRefKlass
951 default:
952 guarantee(false, err_msg("rt should not be %d", rt));
953 }
954 if (TraceReferenceGC && PrintGCDetails) {
955 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
956 }
957 return list;
958 }
959
960 inline void
961 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
962 oop obj,
963 HeapWord* discovered_addr) {
964 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
965 // First we must make sure this object is only enqueued once. CAS in a non null
966 // discovered_addr.
967 oop current_head = refs_list.head();
968 // The last ref must have its discovered field pointing to itself.
969 oop next_discovered = (current_head != NULL) ? current_head : obj;
970
971 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
972 NULL);
973 if (retest == NULL) {
974 // This thread just won the right to enqueue the object.
975 // We have separate lists for enqueueing, so no synchronization
976 // is necessary.
977 refs_list.set_head(obj);
978 refs_list.inc_length(1);
979
980 if (TraceReferenceGC) {
981 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
982 p2i(obj), obj->klass()->internal_name());
983 }
984 } else {
985 // If retest was non NULL, another thread beat us to it:
986 // The reference has already been discovered...
987 if (TraceReferenceGC) {
988 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
989 p2i(obj), obj->klass()->internal_name());
990 }
991 }
992 }
993
994 #ifndef PRODUCT
995 // Non-atomic (i.e. concurrent) discovery might allow us
996 // to observe j.l.References with NULL referents, being those
997 // cleared concurrently by mutators during (or after) discovery.
998 void ReferenceProcessor::verify_referent(oop obj) {
999 bool da = discovery_is_atomic();
1000 oop referent = java_lang_ref_Reference::referent(obj);
1001 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1002 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1003 INTPTR_FORMAT " during %satomic discovery ",
1004 p2i(referent), p2i(obj), da ? "" : "non-"));
1005 }
1006 #endif
1007
1008 // We mention two of several possible choices here:
1009 // #0: if the reference object is not in the "originating generation"
1010 // (or part of the heap being collected, indicated by our "span"
1011 // we don't treat it specially (i.e. we scan it as we would
1012 // a normal oop, treating its references as strong references).
1013 // This means that references can't be discovered unless their
1014 // referent is also in the same span. This is the simplest,
1015 // most "local" and most conservative approach, albeit one
1016 // that may cause weak references to be enqueued least promptly.
1017 // We call this choice the "ReferenceBasedDiscovery" policy.
1018 // #1: the reference object may be in any generation (span), but if
1019 // the referent is in the generation (span) being currently collected
1020 // then we can discover the reference object, provided
1021 // the object has not already been discovered by
1022 // a different concurrently running collector (as may be the
1023 // case, for instance, if the reference object is in CMS and
1024 // the referent in DefNewGeneration), and provided the processing
1025 // of this reference object by the current collector will
1026 // appear atomic to every other collector in the system.
1027 // (Thus, for instance, a concurrent collector may not
1028 // discover references in other generations even if the
1029 // referent is in its own generation). This policy may,
1030 // in certain cases, enqueue references somewhat sooner than
1031 // might Policy #0 above, but at marginally increased cost
1032 // and complexity in processing these references.
1033 // We call this choice the "RefeferentBasedDiscovery" policy.
1034 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1035 // Make sure we are discovering refs (rather than processing discovered refs).
1036 if (!_discovering_refs || !RegisterReferences) {
1037 return false;
1038 }
1039 // We only discover active references.
1040 oop next = java_lang_ref_Reference::next(obj);
1041 if (next != NULL) { // Ref is no longer active
1042 return false;
1043 }
1044
1045 HeapWord* obj_addr = (HeapWord*)obj;
1046 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1047 !_span.contains(obj_addr)) {
1048 // Reference is not in the originating generation;
1049 // don't treat it specially (i.e. we want to scan it as a normal
1050 // object with strong references).
1051 return false;
1052 }
1053
1054 // We only discover references whose referents are not (yet)
1055 // known to be strongly reachable.
1056 if (is_alive_non_header() != NULL) {
1057 verify_referent(obj);
1058 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1059 return false; // referent is reachable
1060 }
1061 }
1062 if (rt == REF_SOFT) {
1063 // For soft refs we can decide now if these are not
1064 // current candidates for clearing, in which case we
1065 // can mark through them now, rather than delaying that
1066 // to the reference-processing phase. Since all current
1067 // time-stamp policies advance the soft-ref clock only
1068 // at a full collection cycle, this is always currently
1069 // accurate.
1070 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1071 return false;
1072 }
1073 }
1074
1075 ResourceMark rm; // Needed for tracing.
1076
1077 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1078 const oop discovered = java_lang_ref_Reference::discovered(obj);
1079 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1080 if (discovered != NULL) {
1081 // The reference has already been discovered...
1082 if (TraceReferenceGC) {
1083 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1084 p2i(obj), obj->klass()->internal_name());
1085 }
1086 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1087 // assumes that an object is not processed twice;
1088 // if it's been already discovered it must be on another
1089 // generation's discovered list; so we won't discover it.
1090 return false;
1091 } else {
1092 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1093 "Unrecognized policy");
1094 // Check assumption that an object is not potentially
1095 // discovered twice except by concurrent collectors that potentially
1096 // trace the same Reference object twice.
1097 assert(UseConcMarkSweepGC || UseG1GC,
1098 "Only possible with a concurrent marking collector");
1099 return true;
1100 }
1101 }
1102
1103 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1104 verify_referent(obj);
1105 // Discover if and only if EITHER:
1106 // .. reference is in our span, OR
1107 // .. we are an atomic collector and referent is in our span
1108 if (_span.contains(obj_addr) ||
1109 (discovery_is_atomic() &&
1110 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1111 // should_enqueue = true;
1112 } else {
1113 return false;
1114 }
1115 } else {
1116 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1117 _span.contains(obj_addr), "code inconsistency");
1118 }
1119
1120 // Get the right type of discovered queue head.
1121 DiscoveredList* list = get_discovered_list(rt);
1122 if (list == NULL) {
1123 return false; // nothing special needs to be done
1124 }
1125
1126 if (_discovery_is_mt) {
1127 add_to_discovered_list_mt(*list, obj, discovered_addr);
1128 } else {
1129 // We do a raw store here: the field will be visited later when processing
1130 // the discovered references.
1131 oop current_head = list->head();
1132 // The last ref must have its discovered field pointing to itself.
1133 oop next_discovered = (current_head != NULL) ? current_head : obj;
1134
1135 assert(discovered == NULL, "control point invariant");
1136 oop_store_raw(discovered_addr, next_discovered);
1137 list->set_head(obj);
1138 list->inc_length(1);
1139
1140 if (TraceReferenceGC) {
1141 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1142 p2i(obj), obj->klass()->internal_name());
1143 }
1144 }
1145 assert(obj->is_oop(), "Discovered a bad reference");
1146 verify_referent(obj);
1147 return true;
1148 }
1149
1150 // Preclean the discovered references by removing those
1151 // whose referents are alive, and by marking from those that
1152 // are not active. These lists can be handled here
1153 // in any order and, indeed, concurrently.
1154 void ReferenceProcessor::preclean_discovered_references(
1155 BoolObjectClosure* is_alive,
1156 OopClosure* keep_alive,
1157 VoidClosure* complete_gc,
1158 YieldClosure* yield,
1159 GCTimer* gc_timer,
1160 GCId gc_id) {
1161
1162 // Soft references
1163 {
1164 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1165 false, gc_timer, gc_id);
1166 for (uint i = 0; i < _max_num_q; i++) {
1167 if (yield->should_return()) {
1168 return;
1169 }
1170 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1171 keep_alive, complete_gc, yield);
1172 }
1173 }
1174
1175 // Weak references
1176 {
1177 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1178 false, gc_timer, gc_id);
1179 for (uint i = 0; i < _max_num_q; i++) {
1180 if (yield->should_return()) {
1181 return;
1182 }
1183 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1184 keep_alive, complete_gc, yield);
1185 }
1186 }
1187
1188 // Final references
1189 {
1190 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1191 false, gc_timer, gc_id);
1192 for (uint i = 0; i < _max_num_q; i++) {
1193 if (yield->should_return()) {
1194 return;
1195 }
1196 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1197 keep_alive, complete_gc, yield);
1198 }
1199 }
1200
1201 // Phantom references
1202 {
1203 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1204 false, gc_timer, gc_id);
1205 for (uint i = 0; i < _max_num_q; i++) {
1206 if (yield->should_return()) {
1207 return;
1208 }
1209 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1210 keep_alive, complete_gc, yield);
1211 }
1212
1213 // Cleaner references. Included in timing for phantom references. We
1214 // expect Cleaner references to be temporary, and don't want to deal with
1215 // possible incompatibilities arising from making it more visible.
1216 for (uint i = 0; i < _max_num_q; i++) {
1217 if (yield->should_return()) {
1218 return;
1219 }
1220 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1221 keep_alive, complete_gc, yield);
1222 }
1223 }
1224 }
1225
1226 // Walk the given discovered ref list, and remove all reference objects
1227 // whose referents are still alive, whose referents are NULL or which
1228 // are not active (have a non-NULL next field). NOTE: When we are
1229 // thus precleaning the ref lists (which happens single-threaded today),
1230 // we do not disable refs discovery to honor the correct semantics of
1231 // java.lang.Reference. As a result, we need to be careful below
1232 // that ref removal steps interleave safely with ref discovery steps
1233 // (in this thread).
1234 void
1235 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1236 BoolObjectClosure* is_alive,
1237 OopClosure* keep_alive,
1238 VoidClosure* complete_gc,
1239 YieldClosure* yield) {
1240 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1241 while (iter.has_next()) {
1242 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1243 oop obj = iter.obj();
1244 oop next = java_lang_ref_Reference::next(obj);
1245 if (iter.referent() == NULL || iter.is_referent_alive() ||
1246 next != NULL) {
1247 // The referent has been cleared, or is alive, or the Reference is not
1248 // active; we need to trace and mark its cohort.
1249 if (TraceReferenceGC) {
1250 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1251 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1252 }
1253 // Remove Reference object from list
1254 iter.remove();
1255 // Keep alive its cohort.
1256 iter.make_referent_alive();
1257 if (UseCompressedOops) {
1258 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1259 keep_alive->do_oop(next_addr);
1260 } else {
1261 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1262 keep_alive->do_oop(next_addr);
1263 }
1264 iter.move_to_next();
1265 } else {
1266 iter.next();
1267 }
1268 }
1269 // Close the reachable set
1270 complete_gc->do_void();
1271
1272 NOT_PRODUCT(
1273 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1274 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1275 " Refs in discovered list " INTPTR_FORMAT,
1276 iter.removed(), iter.processed(), p2i(refs_list.head()));
1277 }
1278 )
1279 }
1280
1281 const char* ReferenceProcessor::list_name(uint i) {
1282 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1283 "Out of bounds index");
1284
1285 int j = i / _max_num_q;
1286 switch (j) {
1287 case 0: return "SoftRef";
1288 case 1: return "WeakRef";
1289 case 2: return "FinalRef";
1290 case 3: return "PhantomRef";
1291 case 4: return "CleanerRef";
1292 }
1293 ShouldNotReachHere();
1294 return NULL;
1295 }
1296