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 "memory/allocation.hpp"
35 #include "oops/oop.inline.hpp"
36 #include "runtime/java.hpp"
37 #include "runtime/jniHandles.hpp"
38
39 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
40 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
41 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
42
43 void referenceProcessor_init() {
44 ReferenceProcessor::init_statics();
45 }
46
47 void ReferenceProcessor::init_statics() {
48 // We need a monotonically non-decreasing time in ms but
49 // os::javaTimeMillis() does not guarantee monotonicity.
50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
51
52 // Initialize the soft ref timestamp clock.
53 _soft_ref_timestamp_clock = now;
54 // Also update the soft ref clock in j.l.r.SoftReference
55 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
56
57 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
58 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
59 NOT_COMPILER2(LRUCurrentHeapPolicy());
60 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
61 vm_exit_during_initialization("Could not allocate reference policy object");
62 }
63 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
64 RefDiscoveryPolicy == ReferentBasedDiscovery,
65 "Unrecognized RefDiscoveryPolicy");
66 }
67
68 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
69 #ifdef ASSERT
70 // Verify that we're not currently discovering refs
71 assert(!_discovering_refs, "nested call?");
72
73 if (check_no_refs) {
74 // Verify that the discovered lists are empty
75 verify_no_references_recorded();
76 }
77 #endif // ASSERT
78
79 // Someone could have modified the value of the static
80 // field in the j.l.r.SoftReference class that holds the
81 // soft reference timestamp clock using reflection or
82 // Unsafe between GCs. Unconditionally update the static
83 // field in ReferenceProcessor here so that we use the new
84 // value during reference discovery.
85
86 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
87 _discovering_refs = true;
88 }
89
90 ReferenceProcessor::ReferenceProcessor(MemRegion span,
91 bool mt_processing,
92 uint mt_processing_degree,
93 bool mt_discovery,
94 uint mt_discovery_degree,
95 bool atomic_discovery,
96 BoolObjectClosure* is_alive_non_header) :
97 _discovering_refs(false),
98 _enqueuing_is_done(false),
99 _is_alive_non_header(is_alive_non_header),
100 _processing_is_mt(mt_processing),
101 _next_id(0)
102 {
103 _span = span;
104 _discovery_is_atomic = atomic_discovery;
105 _discovery_is_mt = mt_discovery;
106 _num_q = MAX2(1U, mt_processing_degree);
107 _max_num_q = MAX2(_num_q, mt_discovery_degree);
108 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
109 _max_num_q * number_of_subclasses_of_ref(), mtGC);
110
111 if (_discovered_refs == NULL) {
112 vm_exit_during_initialization("Could not allocated RefProc Array");
113 }
114 _discoveredSoftRefs = &_discovered_refs[0];
115 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
116 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
117 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
118 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
119
120 // Initialize all entries to NULL
121 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
122 _discovered_refs[i].set_head(NULL);
123 _discovered_refs[i].set_length(0);
124 }
125
126 setup_policy(false /* default soft ref policy */);
127 }
128
129 #ifndef PRODUCT
130 void ReferenceProcessor::verify_no_references_recorded() {
131 guarantee(!_discovering_refs, "Discovering refs?");
132 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
133 guarantee(_discovered_refs[i].is_empty(),
134 "Found non-empty discovered list");
135 }
136 }
137 #endif
138
139 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
140 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
141 if (UseCompressedOops) {
142 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
143 } else {
144 f->do_oop((oop*)_discovered_refs[i].adr_head());
145 }
146 }
147 }
148
149 void ReferenceProcessor::update_soft_ref_master_clock() {
150 // Update (advance) the soft ref master clock field. This must be done
151 // after processing the soft ref list.
152
153 // We need a monotonically non-decreasing time in ms but
154 // os::javaTimeMillis() does not guarantee monotonicity.
155 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
156 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
157 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
158
159 NOT_PRODUCT(
160 if (now < _soft_ref_timestamp_clock) {
161 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
162 _soft_ref_timestamp_clock, now);
163 }
164 )
165 // The values of now and _soft_ref_timestamp_clock are set using
166 // javaTimeNanos(), which is guaranteed to be monotonically
167 // non-decreasing provided the underlying platform provides such
168 // a time source (and it is bug free).
169 // In product mode, however, protect ourselves from non-monotonicity.
170 if (now > _soft_ref_timestamp_clock) {
171 _soft_ref_timestamp_clock = now;
172 java_lang_ref_SoftReference::set_clock(now);
173 }
174 // Else leave clock stalled at its old value until time progresses
175 // past clock value.
176 }
177
178 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
179 size_t total = 0;
180 for (uint i = 0; i < _max_num_q; ++i) {
181 total += lists[i].length();
182 }
183 return total;
184 }
185
186 static void log_ref_count(size_t count, bool doit) {
187 if (doit) {
188 gclog_or_tty->print(", " SIZE_FORMAT " refs", count);
189 }
190 }
191
192 class GCRefTraceTime : public StackObj {
193 GCTraceTimeImpl _gc_trace_time;
194 public:
195 GCRefTraceTime(const char* title, bool doit, GCTimer* timer, size_t count) :
196 _gc_trace_time(title, doit, false, timer) {
197 log_ref_count(count, doit);
198 }
199 };
200
201 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
202 BoolObjectClosure* is_alive,
203 OopClosure* keep_alive,
204 VoidClosure* complete_gc,
205 AbstractRefProcTaskExecutor* task_executor,
206 GCTimer* gc_timer) {
207
208 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
209 // Stop treating discovered references specially.
210 disable_discovery();
211
212 // If discovery was concurrent, someone could have modified
213 // the value of the static field in the j.l.r.SoftReference
214 // class that holds the soft reference timestamp clock using
215 // reflection or Unsafe between when discovery was enabled and
216 // now. Unconditionally update the static field in ReferenceProcessor
217 // here so that we use the new value during processing of the
218 // discovered soft refs.
219
220 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
221
222 bool trace_time = PrintGCDetails && PrintReferenceGC;
223
224 // Include cleaners in phantom statistics. We expect Cleaner
225 // references to be temporary, and don't want to deal with
226 // possible incompatibilities arising from making it more visible.
227 ReferenceProcessorStats stats(
228 total_count(_discoveredSoftRefs),
229 total_count(_discoveredWeakRefs),
230 total_count(_discoveredFinalRefs),
231 total_count(_discoveredPhantomRefs) + total_count(_discoveredCleanerRefs));
232
233 // Soft references
234 {
235 GCRefTraceTime tt("SoftReference", trace_time, gc_timer, stats.soft_count());
236 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
237 is_alive, keep_alive, complete_gc, task_executor);
238 }
239
240 update_soft_ref_master_clock();
241
242 // Weak references
243 {
244 GCRefTraceTime tt("WeakReference", trace_time, gc_timer, stats.weak_count());
245 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
246 is_alive, keep_alive, complete_gc, task_executor);
247 }
248
249 // Final references
250 {
251 GCRefTraceTime tt("FinalReference", trace_time, gc_timer, stats.final_count());
252 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
253 is_alive, keep_alive, complete_gc, task_executor);
254 }
255
256 // Phantom references
257 {
258 GCRefTraceTime tt("PhantomReference", trace_time, gc_timer, stats.phantom_count());
259 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
260 is_alive, keep_alive, complete_gc, task_executor);
261
262 // Process cleaners, but include them in phantom timing. We expect
263 // Cleaner references to be temporary, and don't want to deal with
264 // possible incompatibilities arising from making it more visible.
265 process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
266 is_alive, keep_alive, complete_gc, task_executor);
267 }
268
269 // Weak global JNI references. It would make more sense (semantically) to
270 // traverse these simultaneously with the regular weak references above, but
271 // that is not how the JDK1.2 specification is. See #4126360. Native code can
272 // thus use JNI weak references to circumvent the phantom references and
273 // resurrect a "post-mortem" object.
274 {
275 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
276 NOT_PRODUCT(log_ref_count(count_jni_refs(), trace_time);)
277 if (task_executor != NULL) {
278 task_executor->set_single_threaded_mode();
279 }
280 process_phaseJNI(is_alive, keep_alive, complete_gc);
281 }
282
283 return stats;
284 }
285
286 #ifndef PRODUCT
287 // Calculate the number of jni handles.
288 uint ReferenceProcessor::count_jni_refs() {
289 class AlwaysAliveClosure: public BoolObjectClosure {
290 public:
291 virtual bool do_object_b(oop obj) { return true; }
292 };
293
294 class CountHandleClosure: public OopClosure {
295 private:
296 int _count;
297 public:
298 CountHandleClosure(): _count(0) {}
299 void do_oop(oop* unused) { _count++; }
300 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
301 int count() { return _count; }
302 };
303 CountHandleClosure global_handle_count;
304 AlwaysAliveClosure always_alive;
305 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
306 return global_handle_count.count();
307 }
308 #endif
309
310 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
311 OopClosure* keep_alive,
312 VoidClosure* complete_gc) {
313 JNIHandles::weak_oops_do(is_alive, keep_alive);
314 complete_gc->do_void();
315 }
316
317
318 template <class T>
319 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
320 AbstractRefProcTaskExecutor* task_executor) {
321
322 // Remember old value of pending references list
323 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
324 T old_pending_list_value = *pending_list_addr;
325
326 // Enqueue references that are not made active again, and
327 // clear the decks for the next collection (cycle).
328 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
329 // Do the post-barrier on pending_list_addr missed in
330 // enqueue_discovered_reflist.
331 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
332
333 // Stop treating discovered references specially.
334 ref->disable_discovery();
335
336 // Return true if new pending references were added
337 return old_pending_list_value != *pending_list_addr;
338 }
339
340 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
341 if (UseCompressedOops) {
342 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
343 } else {
344 return enqueue_discovered_ref_helper<oop>(this, task_executor);
345 }
346 }
347
348 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
349 HeapWord* pending_list_addr) {
350 // Given a list of refs linked through the "discovered" field
351 // (java.lang.ref.Reference.discovered), self-loop their "next" field
352 // thus distinguishing them from active References, then
353 // prepend them to the pending list.
354 //
355 // The Java threads will see the Reference objects linked together through
356 // the discovered field. Instead of trying to do the write barrier updates
357 // in all places in the reference processor where we manipulate the discovered
358 // field we make sure to do the barrier here where we anyway iterate through
359 // all linked Reference objects. Note that it is important to not dirty any
360 // cards during reference processing since this will cause card table
361 // verification to fail for G1.
362 if (TraceReferenceGC && PrintGCDetails) {
363 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
364 INTPTR_FORMAT, p2i(refs_list.head()));
365 }
366
367 oop obj = NULL;
368 oop next_d = refs_list.head();
369 // Walk down the list, self-looping the next field
370 // so that the References are not considered active.
371 while (obj != next_d) {
372 obj = next_d;
373 assert(obj->is_instanceRef(), "should be reference object");
374 next_d = java_lang_ref_Reference::discovered(obj);
375 if (TraceReferenceGC && PrintGCDetails) {
376 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
377 p2i(obj), p2i(next_d));
378 }
379 assert(java_lang_ref_Reference::next(obj) == NULL,
380 "Reference not active; should not be discovered");
381 // Self-loop next, so as to make Ref not active.
382 java_lang_ref_Reference::set_next_raw(obj, obj);
383 if (next_d != obj) {
384 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
385 } else {
386 // This is the last object.
387 // Swap refs_list into pending_list_addr and
388 // set obj's discovered to what we read from pending_list_addr.
389 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
390 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
391 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
392 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
393 }
394 }
395 }
396
397 // Parallel enqueue task
398 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
399 public:
400 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
401 DiscoveredList discovered_refs[],
402 HeapWord* pending_list_addr,
403 int n_queues)
404 : EnqueueTask(ref_processor, discovered_refs,
405 pending_list_addr, n_queues)
406 { }
407
408 virtual void work(unsigned int work_id) {
409 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
410 // Simplest first cut: static partitioning.
411 int index = work_id;
412 // The increment on "index" must correspond to the maximum number of queues
413 // (n_queues) with which that ReferenceProcessor was created. That
414 // is because of the "clever" way the discovered references lists were
415 // allocated and are indexed into.
416 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
417 for (int j = 0;
418 j < ReferenceProcessor::number_of_subclasses_of_ref();
419 j++, index += _n_queues) {
420 _ref_processor.enqueue_discovered_reflist(
421 _refs_lists[index], _pending_list_addr);
422 _refs_lists[index].set_head(NULL);
423 _refs_lists[index].set_length(0);
424 }
425 }
426 };
427
428 // Enqueue references that are not made active again
429 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
430 AbstractRefProcTaskExecutor* task_executor) {
431 if (_processing_is_mt && task_executor != NULL) {
432 // Parallel code
433 RefProcEnqueueTask tsk(*this, _discovered_refs,
434 pending_list_addr, _max_num_q);
435 task_executor->execute(tsk);
436 } else {
437 // Serial code: call the parent class's implementation
438 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
439 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
440 _discovered_refs[i].set_head(NULL);
441 _discovered_refs[i].set_length(0);
442 }
443 }
444 }
445
446 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
447 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
448 oop discovered = java_lang_ref_Reference::discovered(_ref);
449 assert(_discovered_addr && discovered->is_oop_or_null(),
450 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
451 _next = discovered;
452 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
453 _referent = java_lang_ref_Reference::referent(_ref);
454 assert(Universe::heap()->is_in_reserved_or_null(_referent),
455 "Wrong oop found in java.lang.Reference object");
456 assert(allow_null_referent ?
457 _referent->is_oop_or_null()
458 : _referent->is_oop(),
459 err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
460 (allow_null_referent ? " or NULL" : ""),
461 p2i(_referent)));
462 }
463
464 void DiscoveredListIterator::remove() {
465 assert(_ref->is_oop(), "Dropping a bad reference");
466 oop_store_raw(_discovered_addr, NULL);
467
468 // First _prev_next ref actually points into DiscoveredList (gross).
469 oop new_next;
470 if (_next == _ref) {
471 // At the end of the list, we should make _prev point to itself.
472 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
473 // and _prev will be NULL.
474 new_next = _prev;
475 } else {
476 new_next = _next;
477 }
478 // Remove Reference object from discovered list. Note that G1 does not need a
479 // pre-barrier here because we know the Reference has already been found/marked,
480 // that's how it ended up in the discovered list in the first place.
481 oop_store_raw(_prev_next, new_next);
482 NOT_PRODUCT(_removed++);
483 _refs_list.dec_length(1);
484 }
485
486 void DiscoveredListIterator::clear_referent() {
487 oop_store_raw(_referent_addr, NULL);
488 }
489
490 // NOTE: process_phase*() are largely similar, and at a high level
491 // merely iterate over the extant list applying a predicate to
492 // each of its elements and possibly removing that element from the
493 // list and applying some further closures to that element.
494 // We should consider the possibility of replacing these
495 // process_phase*() methods by abstracting them into
496 // a single general iterator invocation that receives appropriate
497 // closures that accomplish this work.
498
499 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
500 // referents are not alive, but that should be kept alive for policy reasons.
501 // Keep alive the transitive closure of all such referents.
502 void
503 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
504 ReferencePolicy* policy,
505 BoolObjectClosure* is_alive,
506 OopClosure* keep_alive,
507 VoidClosure* complete_gc) {
508 assert(policy != NULL, "Must have a non-NULL policy");
509 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
510 // Decide which softly reachable refs should be kept alive.
511 while (iter.has_next()) {
512 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
513 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
514 if (referent_is_dead &&
515 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
516 if (TraceReferenceGC) {
517 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
518 p2i(iter.obj()), iter.obj()->klass()->internal_name());
519 }
520 // Remove Reference object from list
521 iter.remove();
522 // keep the referent around
523 iter.make_referent_alive();
524 iter.move_to_next();
525 } else {
526 iter.next();
527 }
528 }
529 // Close the reachable set
530 complete_gc->do_void();
531 NOT_PRODUCT(
532 if (PrintGCDetails && TraceReferenceGC) {
533 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
534 " discovered Refs by policy, from list " INTPTR_FORMAT,
535 iter.removed(), iter.processed(), p2i(refs_list.head()));
536 }
537 )
538 }
539
540 // Traverse the list and remove any Refs that are not active, or
541 // whose referents are either alive or NULL.
542 void
543 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
544 BoolObjectClosure* is_alive,
545 OopClosure* keep_alive) {
546 assert(discovery_is_atomic(), "Error");
547 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
548 while (iter.has_next()) {
549 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
550 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
551 assert(next == NULL, "Should not discover inactive Reference");
552 if (iter.is_referent_alive()) {
553 if (TraceReferenceGC) {
554 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
555 p2i(iter.obj()), iter.obj()->klass()->internal_name());
556 }
557 // The referent is reachable after all.
558 // Remove Reference object from list.
559 iter.remove();
560 // Update the referent pointer as necessary: Note that this
561 // should not entail any recursive marking because the
562 // referent must already have been traversed.
563 iter.make_referent_alive();
564 iter.move_to_next();
565 } else {
566 iter.next();
567 }
568 }
569 NOT_PRODUCT(
570 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
571 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
572 " Refs in discovered list " INTPTR_FORMAT,
573 iter.removed(), iter.processed(), p2i(refs_list.head()));
574 }
575 )
576 }
577
578 void
579 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
580 BoolObjectClosure* is_alive,
581 OopClosure* keep_alive,
582 VoidClosure* complete_gc) {
583 assert(!discovery_is_atomic(), "Error");
584 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
585 while (iter.has_next()) {
586 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
587 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
588 oop next = java_lang_ref_Reference::next(iter.obj());
589 if ((iter.referent() == NULL || iter.is_referent_alive() ||
590 next != NULL)) {
591 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
592 // Remove Reference object from list
593 iter.remove();
594 // Trace the cohorts
595 iter.make_referent_alive();
596 if (UseCompressedOops) {
597 keep_alive->do_oop((narrowOop*)next_addr);
598 } else {
599 keep_alive->do_oop((oop*)next_addr);
600 }
601 iter.move_to_next();
602 } else {
603 iter.next();
604 }
605 }
606 // Now close the newly reachable set
607 complete_gc->do_void();
608 NOT_PRODUCT(
609 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
610 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
611 " Refs in discovered list " INTPTR_FORMAT,
612 iter.removed(), iter.processed(), p2i(refs_list.head()));
613 }
614 )
615 }
616
617 // Traverse the list and process the referents, by either
618 // clearing them or keeping them (and their reachable
619 // closure) alive.
620 void
621 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
622 bool clear_referent,
623 BoolObjectClosure* is_alive,
624 OopClosure* keep_alive,
625 VoidClosure* complete_gc) {
626 ResourceMark rm;
627 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
628 while (iter.has_next()) {
629 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
630 if (clear_referent) {
631 // NULL out referent pointer
632 iter.clear_referent();
633 } else {
634 // keep the referent around
635 iter.make_referent_alive();
636 }
637 if (TraceReferenceGC) {
638 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
639 clear_referent ? "cleared " : "",
640 p2i(iter.obj()), iter.obj()->klass()->internal_name());
641 }
642 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
643 iter.next();
644 }
645 // Close the reachable set
646 complete_gc->do_void();
647 }
648
649 void
650 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
651 oop obj = NULL;
652 oop next = refs_list.head();
653 while (next != obj) {
654 obj = next;
655 next = java_lang_ref_Reference::discovered(obj);
656 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
657 }
658 refs_list.set_head(NULL);
659 refs_list.set_length(0);
660 }
661
662 void ReferenceProcessor::abandon_partial_discovery() {
663 // loop over the lists
664 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
665 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
666 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
667 }
668 clear_discovered_references(_discovered_refs[i]);
669 }
670 }
671
672 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
673 public:
674 RefProcPhase1Task(ReferenceProcessor& ref_processor,
675 DiscoveredList refs_lists[],
676 ReferencePolicy* policy,
677 bool marks_oops_alive)
678 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
679 _policy(policy)
680 { }
681 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
682 OopClosure& keep_alive,
683 VoidClosure& complete_gc)
684 {
685 Thread* thr = Thread::current();
686 int refs_list_index = ((WorkerThread*)thr)->id();
687 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
688 &is_alive, &keep_alive, &complete_gc);
689 }
690 private:
691 ReferencePolicy* _policy;
692 };
693
694 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
695 public:
696 RefProcPhase2Task(ReferenceProcessor& ref_processor,
697 DiscoveredList refs_lists[],
698 bool marks_oops_alive)
699 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
700 { }
701 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
702 OopClosure& keep_alive,
703 VoidClosure& complete_gc)
704 {
705 _ref_processor.process_phase2(_refs_lists[i],
706 &is_alive, &keep_alive, &complete_gc);
707 }
708 };
709
710 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
711 public:
712 RefProcPhase3Task(ReferenceProcessor& ref_processor,
713 DiscoveredList refs_lists[],
714 bool clear_referent,
715 bool marks_oops_alive)
716 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
717 _clear_referent(clear_referent)
718 { }
719 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
720 OopClosure& keep_alive,
721 VoidClosure& complete_gc)
722 {
723 // Don't use "refs_list_index" calculated in this way because
724 // balance_queues() has moved the Ref's into the first n queues.
725 // Thread* thr = Thread::current();
726 // int refs_list_index = ((WorkerThread*)thr)->id();
727 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
728 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
729 &is_alive, &keep_alive, &complete_gc);
730 }
731 private:
732 bool _clear_referent;
733 };
734
735 // Balances reference queues.
736 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
737 // queues[0, 1, ..., _num_q-1] because only the first _num_q
738 // corresponding to the active workers will be processed.
739 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
740 {
741 // calculate total length
742 size_t total_refs = 0;
743 if (TraceReferenceGC && PrintGCDetails) {
744 gclog_or_tty->print_cr("\nBalance ref_lists ");
745 }
746
747 for (uint i = 0; i < _max_num_q; ++i) {
748 total_refs += ref_lists[i].length();
749 if (TraceReferenceGC && PrintGCDetails) {
750 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
751 }
752 }
753 if (TraceReferenceGC && PrintGCDetails) {
754 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
755 }
756 size_t avg_refs = total_refs / _num_q + 1;
757 uint to_idx = 0;
758 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
759 bool move_all = false;
760 if (from_idx >= _num_q) {
761 move_all = ref_lists[from_idx].length() > 0;
762 }
763 while ((ref_lists[from_idx].length() > avg_refs) ||
764 move_all) {
765 assert(to_idx < _num_q, "Sanity Check!");
766 if (ref_lists[to_idx].length() < avg_refs) {
767 // move superfluous refs
768 size_t refs_to_move;
769 // Move all the Ref's if the from queue will not be processed.
770 if (move_all) {
771 refs_to_move = MIN2(ref_lists[from_idx].length(),
772 avg_refs - ref_lists[to_idx].length());
773 } else {
774 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
775 avg_refs - ref_lists[to_idx].length());
776 }
777
778 assert(refs_to_move > 0, "otherwise the code below will fail");
779
780 oop move_head = ref_lists[from_idx].head();
781 oop move_tail = move_head;
782 oop new_head = move_head;
783 // find an element to split the list on
784 for (size_t j = 0; j < refs_to_move; ++j) {
785 move_tail = new_head;
786 new_head = java_lang_ref_Reference::discovered(new_head);
787 }
788
789 // Add the chain to the to list.
790 if (ref_lists[to_idx].head() == NULL) {
791 // to list is empty. Make a loop at the end.
792 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
793 } else {
794 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
795 }
796 ref_lists[to_idx].set_head(move_head);
797 ref_lists[to_idx].inc_length(refs_to_move);
798
799 // Remove the chain from the from list.
800 if (move_tail == new_head) {
801 // We found the end of the from list.
802 ref_lists[from_idx].set_head(NULL);
803 } else {
804 ref_lists[from_idx].set_head(new_head);
805 }
806 ref_lists[from_idx].dec_length(refs_to_move);
807 if (ref_lists[from_idx].length() == 0) {
808 break;
809 }
810 } else {
811 to_idx = (to_idx + 1) % _num_q;
812 }
813 }
814 }
815 #ifdef ASSERT
816 size_t balanced_total_refs = 0;
817 for (uint i = 0; i < _max_num_q; ++i) {
818 balanced_total_refs += ref_lists[i].length();
819 if (TraceReferenceGC && PrintGCDetails) {
820 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
821 }
822 }
823 if (TraceReferenceGC && PrintGCDetails) {
824 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
825 gclog_or_tty->flush();
826 }
827 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
828 #endif
829 }
830
831 void ReferenceProcessor::balance_all_queues() {
832 balance_queues(_discoveredSoftRefs);
833 balance_queues(_discoveredWeakRefs);
834 balance_queues(_discoveredFinalRefs);
835 balance_queues(_discoveredPhantomRefs);
836 balance_queues(_discoveredCleanerRefs);
837 }
838
839 void ReferenceProcessor::process_discovered_reflist(
840 DiscoveredList refs_lists[],
841 ReferencePolicy* policy,
842 bool clear_referent,
843 BoolObjectClosure* is_alive,
844 OopClosure* keep_alive,
845 VoidClosure* complete_gc,
846 AbstractRefProcTaskExecutor* task_executor)
847 {
848 bool mt_processing = task_executor != NULL && _processing_is_mt;
849 // If discovery used MT and a dynamic number of GC threads, then
850 // the queues must be balanced for correctness if fewer than the
851 // maximum number of queues were used. The number of queue used
852 // during discovery may be different than the number to be used
853 // for processing so don't depend of _num_q < _max_num_q as part
854 // of the test.
855 bool must_balance = _discovery_is_mt;
856
857 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
858 must_balance) {
859 balance_queues(refs_lists);
860 }
861
862 // Phase 1 (soft refs only):
863 // . Traverse the list and remove any SoftReferences whose
864 // referents are not alive, but that should be kept alive for
865 // policy reasons. Keep alive the transitive closure of all
866 // such referents.
867 if (policy != NULL) {
868 if (mt_processing) {
869 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
870 task_executor->execute(phase1);
871 } else {
872 for (uint i = 0; i < _max_num_q; i++) {
873 process_phase1(refs_lists[i], policy,
874 is_alive, keep_alive, complete_gc);
875 }
876 }
877 } else { // policy == NULL
878 assert(refs_lists != _discoveredSoftRefs,
879 "Policy must be specified for soft references.");
880 }
881
882 // Phase 2:
883 // . Traverse the list and remove any refs whose referents are alive.
884 if (mt_processing) {
885 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
886 task_executor->execute(phase2);
887 } else {
888 for (uint i = 0; i < _max_num_q; i++) {
889 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
890 }
891 }
892
893 // Phase 3:
894 // . Traverse the list and process referents as appropriate.
895 if (mt_processing) {
896 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
897 task_executor->execute(phase3);
898 } else {
899 for (uint i = 0; i < _max_num_q; i++) {
900 process_phase3(refs_lists[i], clear_referent,
901 is_alive, keep_alive, complete_gc);
902 }
903 }
904 }
905
906 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
907 uint id = 0;
908 // Determine the queue index to use for this object.
909 if (_discovery_is_mt) {
910 // During a multi-threaded discovery phase,
911 // each thread saves to its "own" list.
912 Thread* thr = Thread::current();
913 id = thr->as_Worker_thread()->id();
914 } else {
915 // single-threaded discovery, we save in round-robin
916 // fashion to each of the lists.
917 if (_processing_is_mt) {
918 id = next_id();
919 }
920 }
921 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
922
923 // Get the discovered queue to which we will add
924 DiscoveredList* list = NULL;
925 switch (rt) {
926 case REF_OTHER:
927 // Unknown reference type, no special treatment
928 break;
929 case REF_SOFT:
930 list = &_discoveredSoftRefs[id];
931 break;
932 case REF_WEAK:
933 list = &_discoveredWeakRefs[id];
934 break;
935 case REF_FINAL:
936 list = &_discoveredFinalRefs[id];
937 break;
938 case REF_PHANTOM:
939 list = &_discoveredPhantomRefs[id];
940 break;
941 case REF_CLEANER:
942 list = &_discoveredCleanerRefs[id];
943 break;
944 case REF_NONE:
945 // we should not reach here if we are an InstanceRefKlass
946 default:
947 ShouldNotReachHere();
948 }
949 if (TraceReferenceGC && PrintGCDetails) {
950 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
951 }
952 return list;
953 }
954
955 inline void
956 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
957 oop obj,
958 HeapWord* discovered_addr) {
959 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
960 // First we must make sure this object is only enqueued once. CAS in a non null
961 // discovered_addr.
962 oop current_head = refs_list.head();
963 // The last ref must have its discovered field pointing to itself.
964 oop next_discovered = (current_head != NULL) ? current_head : obj;
965
966 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
967 NULL);
968 if (retest == NULL) {
969 // This thread just won the right to enqueue the object.
970 // We have separate lists for enqueueing, so no synchronization
971 // is necessary.
972 refs_list.set_head(obj);
973 refs_list.inc_length(1);
974
975 if (TraceReferenceGC) {
976 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
977 p2i(obj), obj->klass()->internal_name());
978 }
979 } else {
980 // If retest was non NULL, another thread beat us to it:
981 // The reference has already been discovered...
982 if (TraceReferenceGC) {
983 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
984 p2i(obj), obj->klass()->internal_name());
985 }
986 }
987 }
988
989 #ifndef PRODUCT
990 // Non-atomic (i.e. concurrent) discovery might allow us
991 // to observe j.l.References with NULL referents, being those
992 // cleared concurrently by mutators during (or after) discovery.
993 void ReferenceProcessor::verify_referent(oop obj) {
994 bool da = discovery_is_atomic();
995 oop referent = java_lang_ref_Reference::referent(obj);
996 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
997 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
998 INTPTR_FORMAT " during %satomic discovery ",
999 p2i(referent), p2i(obj), da ? "" : "non-"));
1000 }
1001 #endif
1002
1003 // We mention two of several possible choices here:
1004 // #0: if the reference object is not in the "originating generation"
1005 // (or part of the heap being collected, indicated by our "span"
1006 // we don't treat it specially (i.e. we scan it as we would
1007 // a normal oop, treating its references as strong references).
1008 // This means that references can't be discovered unless their
1009 // referent is also in the same span. This is the simplest,
1010 // most "local" and most conservative approach, albeit one
1011 // that may cause weak references to be enqueued least promptly.
1012 // We call this choice the "ReferenceBasedDiscovery" policy.
1013 // #1: the reference object may be in any generation (span), but if
1014 // the referent is in the generation (span) being currently collected
1015 // then we can discover the reference object, provided
1016 // the object has not already been discovered by
1017 // a different concurrently running collector (as may be the
1018 // case, for instance, if the reference object is in CMS and
1019 // the referent in DefNewGeneration), and provided the processing
1020 // of this reference object by the current collector will
1021 // appear atomic to every other collector in the system.
1022 // (Thus, for instance, a concurrent collector may not
1023 // discover references in other generations even if the
1024 // referent is in its own generation). This policy may,
1025 // in certain cases, enqueue references somewhat sooner than
1026 // might Policy #0 above, but at marginally increased cost
1027 // and complexity in processing these references.
1028 // We call this choice the "RefeferentBasedDiscovery" policy.
1029 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1030 // Make sure we are discovering refs (rather than processing discovered refs).
1031 if (!_discovering_refs || !RegisterReferences) {
1032 return false;
1033 }
1034 // We only discover active references.
1035 oop next = java_lang_ref_Reference::next(obj);
1036 if (next != NULL) { // Ref is no longer active
1037 return false;
1038 }
1039
1040 HeapWord* obj_addr = (HeapWord*)obj;
1041 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1042 !_span.contains(obj_addr)) {
1043 // Reference is not in the originating generation;
1044 // don't treat it specially (i.e. we want to scan it as a normal
1045 // object with strong references).
1046 return false;
1047 }
1048
1049 // We only discover references whose referents are not (yet)
1050 // known to be strongly reachable.
1051 if (is_alive_non_header() != NULL) {
1052 verify_referent(obj);
1053 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1054 return false; // referent is reachable
1055 }
1056 }
1057 if (rt == REF_SOFT) {
1058 // For soft refs we can decide now if these are not
1059 // current candidates for clearing, in which case we
1060 // can mark through them now, rather than delaying that
1061 // to the reference-processing phase. Since all current
1062 // time-stamp policies advance the soft-ref clock only
1063 // at a full collection cycle, this is always currently
1064 // accurate.
1065 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1066 return false;
1067 }
1068 }
1069
1070 ResourceMark rm; // Needed for tracing.
1071
1072 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1073 const oop discovered = java_lang_ref_Reference::discovered(obj);
1074 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1075 if (discovered != NULL) {
1076 // The reference has already been discovered...
1077 if (TraceReferenceGC) {
1078 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1079 p2i(obj), obj->klass()->internal_name());
1080 }
1081 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1082 // assumes that an object is not processed twice;
1083 // if it's been already discovered it must be on another
1084 // generation's discovered list; so we won't discover it.
1085 return false;
1086 } else {
1087 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1088 "Unrecognized policy");
1089 // Check assumption that an object is not potentially
1090 // discovered twice except by concurrent collectors that potentially
1091 // trace the same Reference object twice.
1092 assert(UseConcMarkSweepGC || UseG1GC,
1093 "Only possible with a concurrent marking collector");
1094 return true;
1095 }
1096 }
1097
1098 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1099 verify_referent(obj);
1100 // Discover if and only if EITHER:
1101 // .. reference is in our span, OR
1102 // .. we are an atomic collector and referent is in our span
1103 if (_span.contains(obj_addr) ||
1104 (discovery_is_atomic() &&
1105 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1106 // should_enqueue = true;
1107 } else {
1108 return false;
1109 }
1110 } else {
1111 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1112 _span.contains(obj_addr), "code inconsistency");
1113 }
1114
1115 // Get the right type of discovered queue head.
1116 DiscoveredList* list = get_discovered_list(rt);
1117 if (list == NULL) {
1118 return false; // nothing special needs to be done
1119 }
1120
1121 if (_discovery_is_mt) {
1122 add_to_discovered_list_mt(*list, obj, discovered_addr);
1123 } else {
1124 // We do a raw store here: the field will be visited later when processing
1125 // the discovered references.
1126 oop current_head = list->head();
1127 // The last ref must have its discovered field pointing to itself.
1128 oop next_discovered = (current_head != NULL) ? current_head : obj;
1129
1130 assert(discovered == NULL, "control point invariant");
1131 oop_store_raw(discovered_addr, next_discovered);
1132 list->set_head(obj);
1133 list->inc_length(1);
1134
1135 if (TraceReferenceGC) {
1136 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1137 p2i(obj), obj->klass()->internal_name());
1138 }
1139 }
1140 assert(obj->is_oop(), "Discovered a bad reference");
1141 verify_referent(obj);
1142 return true;
1143 }
1144
1145 // Preclean the discovered references by removing those
1146 // whose referents are alive, and by marking from those that
1147 // are not active. These lists can be handled here
1148 // in any order and, indeed, concurrently.
1149 void ReferenceProcessor::preclean_discovered_references(
1150 BoolObjectClosure* is_alive,
1151 OopClosure* keep_alive,
1152 VoidClosure* complete_gc,
1153 YieldClosure* yield,
1154 GCTimer* gc_timer) {
1155
1156 // Soft references
1157 {
1158 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1159 false, gc_timer);
1160 for (uint i = 0; i < _max_num_q; i++) {
1161 if (yield->should_return()) {
1162 return;
1163 }
1164 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1165 keep_alive, complete_gc, yield);
1166 }
1167 }
1168
1169 // Weak references
1170 {
1171 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1172 false, gc_timer);
1173 for (uint i = 0; i < _max_num_q; i++) {
1174 if (yield->should_return()) {
1175 return;
1176 }
1177 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1178 keep_alive, complete_gc, yield);
1179 }
1180 }
1181
1182 // Final references
1183 {
1184 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1185 false, gc_timer);
1186 for (uint i = 0; i < _max_num_q; i++) {
1187 if (yield->should_return()) {
1188 return;
1189 }
1190 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1191 keep_alive, complete_gc, yield);
1192 }
1193 }
1194
1195 // Phantom references
1196 {
1197 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1198 false, gc_timer);
1199 for (uint i = 0; i < _max_num_q; i++) {
1200 if (yield->should_return()) {
1201 return;
1202 }
1203 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1204 keep_alive, complete_gc, yield);
1205 }
1206
1207 // Cleaner references. Included in timing for phantom references. We
1208 // expect Cleaner references to be temporary, and don't want to deal with
1209 // possible incompatibilities arising from making it more visible.
1210 for (uint i = 0; i < _max_num_q; i++) {
1211 if (yield->should_return()) {
1212 return;
1213 }
1214 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1215 keep_alive, complete_gc, yield);
1216 }
1217 }
1218 }
1219
1220 // Walk the given discovered ref list, and remove all reference objects
1221 // whose referents are still alive, whose referents are NULL or which
1222 // are not active (have a non-NULL next field). NOTE: When we are
1223 // thus precleaning the ref lists (which happens single-threaded today),
1224 // we do not disable refs discovery to honor the correct semantics of
1225 // java.lang.Reference. As a result, we need to be careful below
1226 // that ref removal steps interleave safely with ref discovery steps
1227 // (in this thread).
1228 void
1229 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1230 BoolObjectClosure* is_alive,
1231 OopClosure* keep_alive,
1232 VoidClosure* complete_gc,
1233 YieldClosure* yield) {
1234 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1235 while (iter.has_next()) {
1236 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1237 oop obj = iter.obj();
1238 oop next = java_lang_ref_Reference::next(obj);
1239 if (iter.referent() == NULL || iter.is_referent_alive() ||
1240 next != NULL) {
1241 // The referent has been cleared, or is alive, or the Reference is not
1242 // active; we need to trace and mark its cohort.
1243 if (TraceReferenceGC) {
1244 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1245 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1246 }
1247 // Remove Reference object from list
1248 iter.remove();
1249 // Keep alive its cohort.
1250 iter.make_referent_alive();
1251 if (UseCompressedOops) {
1252 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1253 keep_alive->do_oop(next_addr);
1254 } else {
1255 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1256 keep_alive->do_oop(next_addr);
1257 }
1258 iter.move_to_next();
1259 } else {
1260 iter.next();
1261 }
1262 }
1263 // Close the reachable set
1264 complete_gc->do_void();
1265
1266 NOT_PRODUCT(
1267 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1268 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1269 " Refs in discovered list " INTPTR_FORMAT,
1270 iter.removed(), iter.processed(), p2i(refs_list.head()));
1271 }
1272 )
1273 }
1274
1275 const char* ReferenceProcessor::list_name(uint i) {
1276 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1277 "Out of bounds index");
1278
1279 int j = i / _max_num_q;
1280 switch (j) {
1281 case 0: return "SoftRef";
1282 case 1: return "WeakRef";
1283 case 2: return "FinalRef";
1284 case 3: return "PhantomRef";
1285 case 4: return "CleanerRef";
1286 }
1287 ShouldNotReachHere();
1288 return NULL;
1289 }
1290