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 #if defined(COMPILER2) || INCLUDE_JVMCI
58 _default_soft_ref_policy = new LRUMaxHeapPolicy();
59 #else
60 _default_soft_ref_policy = new LRUCurrentHeapPolicy();
61 #endif
62 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
63 vm_exit_during_initialization("Could not allocate reference policy object");
64 }
65 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
66 RefDiscoveryPolicy == ReferentBasedDiscovery,
67 "Unrecognized RefDiscoveryPolicy");
68 }
69
70 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
71 #ifdef ASSERT
72 // Verify that we're not currently discovering refs
73 assert(!_discovering_refs, "nested call?");
74
75 if (check_no_refs) {
76 // Verify that the discovered lists are empty
77 verify_no_references_recorded();
78 }
79 #endif // ASSERT
80
81 // Someone could have modified the value of the static
82 // field in the j.l.r.SoftReference class that holds the
83 // soft reference timestamp clock using reflection or
84 // Unsafe between GCs. Unconditionally update the static
85 // field in ReferenceProcessor here so that we use the new
86 // value during reference discovery.
87
88 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
89 _discovering_refs = true;
90 }
91
92 ReferenceProcessor::ReferenceProcessor(MemRegion span,
93 bool mt_processing,
94 uint mt_processing_degree,
95 bool mt_discovery,
96 uint mt_discovery_degree,
97 bool atomic_discovery,
98 BoolObjectClosure* is_alive_non_header) :
99 _discovering_refs(false),
100 _enqueuing_is_done(false),
101 _is_alive_non_header(is_alive_non_header),
102 _processing_is_mt(mt_processing),
103 _next_id(0)
104 {
105 _span = span;
106 _discovery_is_atomic = atomic_discovery;
107 _discovery_is_mt = mt_discovery;
108 _num_q = MAX2(1U, mt_processing_degree);
109 _max_num_q = MAX2(_num_q, mt_discovery_degree);
110 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
111 _max_num_q * number_of_subclasses_of_ref(), mtGC);
112
113 if (_discovered_refs == NULL) {
114 vm_exit_during_initialization("Could not allocated RefProc Array");
115 }
116 _discoveredSoftRefs = &_discovered_refs[0];
117 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
118 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
119 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
120 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
121
122 // Initialize all entries to NULL
123 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
124 _discovered_refs[i].set_head(NULL);
125 _discovered_refs[i].set_length(0);
126 }
127
128 setup_policy(false /* default soft ref policy */);
129 }
130
131 #ifndef PRODUCT
132 void ReferenceProcessor::verify_no_references_recorded() {
133 guarantee(!_discovering_refs, "Discovering refs?");
134 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
135 guarantee(_discovered_refs[i].is_empty(),
136 "Found non-empty discovered list");
137 }
138 }
139 #endif
140
141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
142 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
143 if (UseCompressedOops) {
144 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
145 } else {
146 f->do_oop((oop*)_discovered_refs[i].adr_head());
147 }
148 }
149 }
150
151 void ReferenceProcessor::update_soft_ref_master_clock() {
152 // Update (advance) the soft ref master clock field. This must be done
153 // after processing the soft ref list.
154
155 // We need a monotonically non-decreasing time in ms but
156 // os::javaTimeMillis() does not guarantee monotonicity.
157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
160
161 NOT_PRODUCT(
162 if (now < _soft_ref_timestamp_clock) {
163 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
164 _soft_ref_timestamp_clock, now);
165 }
166 )
167 // The values of now and _soft_ref_timestamp_clock are set using
168 // javaTimeNanos(), which is guaranteed to be monotonically
169 // non-decreasing provided the underlying platform provides such
170 // a time source (and it is bug free).
171 // In product mode, however, protect ourselves from non-monotonicity.
172 if (now > _soft_ref_timestamp_clock) {
173 _soft_ref_timestamp_clock = now;
174 java_lang_ref_SoftReference::set_clock(now);
175 }
176 // Else leave clock stalled at its old value until time progresses
177 // past clock value.
178 }
179
180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
181 size_t total = 0;
182 for (uint i = 0; i < _max_num_q; ++i) {
183 total += lists[i].length();
184 }
185 return total;
186 }
187
188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
189 BoolObjectClosure* is_alive,
190 OopClosure* keep_alive,
191 VoidClosure* complete_gc,
192 AbstractRefProcTaskExecutor* task_executor,
193 GCTimer* gc_timer,
194 GCId gc_id) {
195
196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
197 // Stop treating discovered references specially.
198 disable_discovery();
199
200 // If discovery was concurrent, someone could have modified
201 // the value of the static field in the j.l.r.SoftReference
202 // class that holds the soft reference timestamp clock using
203 // reflection or Unsafe between when discovery was enabled and
204 // now. Unconditionally update the static field in ReferenceProcessor
205 // here so that we use the new value during processing of the
206 // discovered soft refs.
207
208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
209
210 bool trace_time = PrintGCDetails && PrintReferenceGC;
211
212 // Soft references
213 size_t soft_count = 0;
214 {
215 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
216 soft_count =
217 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
220
221 update_soft_ref_master_clock();
222
223 // Weak references
224 size_t weak_count = 0;
225 {
226 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
227 weak_count =
228 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
229 is_alive, keep_alive, complete_gc, task_executor);
230 }
231
232 // Final references
233 size_t final_count = 0;
234 {
235 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
236 final_count =
237 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
238 is_alive, keep_alive, complete_gc, task_executor);
239 }
240
241 // Phantom references
242 size_t phantom_count = 0;
243 {
244 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
245 phantom_count =
246 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
247 is_alive, keep_alive, complete_gc, task_executor);
248
249 // Process cleaners, but include them in phantom statistics. We expect
250 // Cleaner references to be temporary, and don't want to deal with
251 // possible incompatibilities arising from making it more visible.
252 phantom_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 {
263 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
264 if (task_executor != NULL) {
265 task_executor->set_single_threaded_mode();
266 }
267 process_phaseJNI(is_alive, keep_alive, complete_gc);
268 }
269
270 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
271 }
272
273 #ifndef PRODUCT
274 // Calculate the number of jni handles.
275 uint ReferenceProcessor::count_jni_refs() {
276 class AlwaysAliveClosure: public BoolObjectClosure {
277 public:
278 virtual bool do_object_b(oop obj) { return true; }
279 };
280
281 class CountHandleClosure: public OopClosure {
282 private:
283 int _count;
284 public:
285 CountHandleClosure(): _count(0) {}
286 void do_oop(oop* unused) { _count++; }
287 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
288 int count() { return _count; }
289 };
290 CountHandleClosure global_handle_count;
291 AlwaysAliveClosure always_alive;
292 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
293 return global_handle_count.count();
294 }
295 #endif
296
297 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
298 OopClosure* keep_alive,
299 VoidClosure* complete_gc) {
300 #ifndef PRODUCT
301 if (PrintGCDetails && PrintReferenceGC) {
302 unsigned int count = count_jni_refs();
303 gclog_or_tty->print(", %u refs", count);
304 }
305 #endif
306 JNIHandles::weak_oops_do(is_alive, keep_alive);
307 complete_gc->do_void();
308 }
309
310
311 template <class T>
312 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
313 AbstractRefProcTaskExecutor* task_executor) {
314
315 // Remember old value of pending references list
316 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
317 T old_pending_list_value = *pending_list_addr;
318
319 // Enqueue references that are not made active again, and
320 // clear the decks for the next collection (cycle).
321 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
322 // Do the post-barrier on pending_list_addr missed in
323 // enqueue_discovered_reflist.
324 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
325
326 // Stop treating discovered references specially.
327 ref->disable_discovery();
328
329 // Return true if new pending references were added
330 return old_pending_list_value != *pending_list_addr;
331 }
332
333 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
334 if (UseCompressedOops) {
335 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
336 } else {
337 return enqueue_discovered_ref_helper<oop>(this, task_executor);
338 }
339 }
340
341 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
342 HeapWord* pending_list_addr) {
343 // Given a list of refs linked through the "discovered" field
344 // (java.lang.ref.Reference.discovered), self-loop their "next" field
345 // thus distinguishing them from active References, then
346 // prepend them to the pending list.
347 //
348 // The Java threads will see the Reference objects linked together through
349 // the discovered field. Instead of trying to do the write barrier updates
350 // in all places in the reference processor where we manipulate the discovered
351 // field we make sure to do the barrier here where we anyway iterate through
352 // all linked Reference objects. Note that it is important to not dirty any
353 // cards during reference processing since this will cause card table
354 // verification to fail for G1.
355 if (TraceReferenceGC && PrintGCDetails) {
356 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
357 INTPTR_FORMAT, p2i(refs_list.head()));
358 }
359
360 oop obj = NULL;
361 oop next_d = refs_list.head();
362 // Walk down the list, self-looping the next field
363 // so that the References are not considered active.
364 while (obj != next_d) {
365 obj = next_d;
366 assert(obj->is_instanceRef(), "should be reference object");
367 next_d = java_lang_ref_Reference::discovered(obj);
368 if (TraceReferenceGC && PrintGCDetails) {
369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
370 p2i(obj), p2i(next_d));
371 }
372 assert(java_lang_ref_Reference::next(obj) == NULL,
373 "Reference not active; should not be discovered");
374 // Self-loop next, so as to make Ref not active.
375 java_lang_ref_Reference::set_next_raw(obj, obj);
376 if (next_d != obj) {
377 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
378 } else {
379 // This is the last object.
380 // Swap refs_list into pending_list_addr and
381 // set obj's discovered to what we read from pending_list_addr.
382 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
383 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
384 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
385 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
386 }
387 }
388 }
389
390 // Parallel enqueue task
391 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
392 public:
393 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
394 DiscoveredList discovered_refs[],
395 HeapWord* pending_list_addr,
396 int n_queues)
397 : EnqueueTask(ref_processor, discovered_refs,
398 pending_list_addr, n_queues)
399 { }
400
401 virtual void work(unsigned int work_id) {
402 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
403 // Simplest first cut: static partitioning.
404 int index = work_id;
405 // The increment on "index" must correspond to the maximum number of queues
406 // (n_queues) with which that ReferenceProcessor was created. That
407 // is because of the "clever" way the discovered references lists were
408 // allocated and are indexed into.
409 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
410 for (int j = 0;
411 j < ReferenceProcessor::number_of_subclasses_of_ref();
412 j++, index += _n_queues) {
413 _ref_processor.enqueue_discovered_reflist(
414 _refs_lists[index], _pending_list_addr);
415 _refs_lists[index].set_head(NULL);
416 _refs_lists[index].set_length(0);
417 }
418 }
419 };
420
421 // Enqueue references that are not made active again
422 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
423 AbstractRefProcTaskExecutor* task_executor) {
424 if (_processing_is_mt && task_executor != NULL) {
425 // Parallel code
426 RefProcEnqueueTask tsk(*this, _discovered_refs,
427 pending_list_addr, _max_num_q);
428 task_executor->execute(tsk);
429 } else {
430 // Serial code: call the parent class's implementation
431 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
432 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
433 _discovered_refs[i].set_head(NULL);
434 _discovered_refs[i].set_length(0);
435 }
436 }
437 }
438
439 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
440 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
441 oop discovered = java_lang_ref_Reference::discovered(_ref);
442 assert(_discovered_addr && discovered->is_oop_or_null(),
443 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
444 _next = discovered;
445 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
446 _referent = java_lang_ref_Reference::referent(_ref);
447 assert(Universe::heap()->is_in_reserved_or_null(_referent),
448 "Wrong oop found in java.lang.Reference object");
449 assert(allow_null_referent ?
450 _referent->is_oop_or_null()
451 : _referent->is_oop(),
452 err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
453 (allow_null_referent ? " or NULL" : ""),
454 p2i(_referent)));
455 }
456
457 void DiscoveredListIterator::remove() {
458 assert(_ref->is_oop(), "Dropping a bad reference");
459 oop_store_raw(_discovered_addr, NULL);
460
461 // First _prev_next ref actually points into DiscoveredList (gross).
462 oop new_next;
463 if (_next == _ref) {
464 // At the end of the list, we should make _prev point to itself.
465 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
466 // and _prev will be NULL.
467 new_next = _prev;
468 } else {
469 new_next = _next;
470 }
471 // Remove Reference object from discovered list. Note that G1 does not need a
472 // pre-barrier here because we know the Reference has already been found/marked,
473 // that's how it ended up in the discovered list in the first place.
474 oop_store_raw(_prev_next, new_next);
475 NOT_PRODUCT(_removed++);
476 _refs_list.dec_length(1);
477 }
478
479 void DiscoveredListIterator::clear_referent() {
480 oop_store_raw(_referent_addr, NULL);
481 }
482
483 // NOTE: process_phase*() are largely similar, and at a high level
484 // merely iterate over the extant list applying a predicate to
485 // each of its elements and possibly removing that element from the
486 // list and applying some further closures to that element.
487 // We should consider the possibility of replacing these
488 // process_phase*() methods by abstracting them into
489 // a single general iterator invocation that receives appropriate
490 // closures that accomplish this work.
491
492 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
493 // referents are not alive, but that should be kept alive for policy reasons.
494 // Keep alive the transitive closure of all such referents.
495 void
496 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
497 ReferencePolicy* policy,
498 BoolObjectClosure* is_alive,
499 OopClosure* keep_alive,
500 VoidClosure* complete_gc) {
501 assert(policy != NULL, "Must have a non-NULL policy");
502 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
503 // Decide which softly reachable refs should be kept alive.
504 while (iter.has_next()) {
505 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
506 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
507 if (referent_is_dead &&
508 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
509 if (TraceReferenceGC) {
510 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
511 p2i(iter.obj()), iter.obj()->klass()->internal_name());
512 }
513 // Remove Reference object from list
514 iter.remove();
515 // keep the referent around
516 iter.make_referent_alive();
517 iter.move_to_next();
518 } else {
519 iter.next();
520 }
521 }
522 // Close the reachable set
523 complete_gc->do_void();
524 NOT_PRODUCT(
525 if (PrintGCDetails && TraceReferenceGC) {
526 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
527 " discovered Refs by policy, from list " INTPTR_FORMAT,
528 iter.removed(), iter.processed(), p2i(refs_list.head()));
529 }
530 )
531 }
532
533 // Traverse the list and remove any Refs that are not active, or
534 // whose referents are either alive or NULL.
535 void
536 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
537 BoolObjectClosure* is_alive,
538 OopClosure* keep_alive) {
539 assert(discovery_is_atomic(), "Error");
540 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
541 while (iter.has_next()) {
542 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
543 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
544 assert(next == NULL, "Should not discover inactive Reference");
545 if (iter.is_referent_alive()) {
546 if (TraceReferenceGC) {
547 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
548 p2i(iter.obj()), iter.obj()->klass()->internal_name());
549 }
550 // The referent is reachable after all.
551 // Remove Reference object from list.
552 iter.remove();
553 // Update the referent pointer as necessary: Note that this
554 // should not entail any recursive marking because the
555 // referent must already have been traversed.
556 iter.make_referent_alive();
557 iter.move_to_next();
558 } else {
559 iter.next();
560 }
561 }
562 NOT_PRODUCT(
563 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
564 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
565 " Refs in discovered list " INTPTR_FORMAT,
566 iter.removed(), iter.processed(), p2i(refs_list.head()));
567 }
568 )
569 }
570
571 void
572 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
573 BoolObjectClosure* is_alive,
574 OopClosure* keep_alive,
575 VoidClosure* complete_gc) {
576 assert(!discovery_is_atomic(), "Error");
577 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
578 while (iter.has_next()) {
579 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
580 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
581 oop next = java_lang_ref_Reference::next(iter.obj());
582 if ((iter.referent() == NULL || iter.is_referent_alive() ||
583 next != NULL)) {
584 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
585 // Remove Reference object from list
586 iter.remove();
587 // Trace the cohorts
588 iter.make_referent_alive();
589 if (UseCompressedOops) {
590 keep_alive->do_oop((narrowOop*)next_addr);
591 } else {
592 keep_alive->do_oop((oop*)next_addr);
593 }
594 iter.move_to_next();
595 } else {
596 iter.next();
597 }
598 }
599 // Now close the newly reachable set
600 complete_gc->do_void();
601 NOT_PRODUCT(
602 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
603 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
604 " Refs in discovered list " INTPTR_FORMAT,
605 iter.removed(), iter.processed(), p2i(refs_list.head()));
606 }
607 )
608 }
609
610 // Traverse the list and process the referents, by either
611 // clearing them or keeping them (and their reachable
612 // closure) alive.
613 void
614 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
615 bool clear_referent,
616 BoolObjectClosure* is_alive,
617 OopClosure* keep_alive,
618 VoidClosure* complete_gc) {
619 ResourceMark rm;
620 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
621 while (iter.has_next()) {
622 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
623 if (clear_referent) {
624 // NULL out referent pointer
625 iter.clear_referent();
626 } else {
627 // keep the referent around
628 iter.make_referent_alive();
629 }
630 if (TraceReferenceGC) {
631 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
632 clear_referent ? "cleared " : "",
633 p2i(iter.obj()), iter.obj()->klass()->internal_name());
634 }
635 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
636 iter.next();
637 }
638 // Close the reachable set
639 complete_gc->do_void();
640 }
641
642 void
643 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
644 oop obj = NULL;
645 oop next = refs_list.head();
646 while (next != obj) {
647 obj = next;
648 next = java_lang_ref_Reference::discovered(obj);
649 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
650 }
651 refs_list.set_head(NULL);
652 refs_list.set_length(0);
653 }
654
655 void ReferenceProcessor::abandon_partial_discovery() {
656 // loop over the lists
657 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
658 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
659 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
660 }
661 clear_discovered_references(_discovered_refs[i]);
662 }
663 }
664
665 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
666 public:
667 RefProcPhase1Task(ReferenceProcessor& ref_processor,
668 DiscoveredList refs_lists[],
669 ReferencePolicy* policy,
670 bool marks_oops_alive)
671 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
672 _policy(policy)
673 { }
674 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
675 OopClosure& keep_alive,
676 VoidClosure& complete_gc)
677 {
678 Thread* thr = Thread::current();
679 int refs_list_index = ((WorkerThread*)thr)->id();
680 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
681 &is_alive, &keep_alive, &complete_gc);
682 }
683 private:
684 ReferencePolicy* _policy;
685 };
686
687 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
688 public:
689 RefProcPhase2Task(ReferenceProcessor& ref_processor,
690 DiscoveredList refs_lists[],
691 bool marks_oops_alive)
692 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
693 { }
694 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
695 OopClosure& keep_alive,
696 VoidClosure& complete_gc)
697 {
698 _ref_processor.process_phase2(_refs_lists[i],
699 &is_alive, &keep_alive, &complete_gc);
700 }
701 };
702
703 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
704 public:
705 RefProcPhase3Task(ReferenceProcessor& ref_processor,
706 DiscoveredList refs_lists[],
707 bool clear_referent,
708 bool marks_oops_alive)
709 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
710 _clear_referent(clear_referent)
711 { }
712 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
713 OopClosure& keep_alive,
714 VoidClosure& complete_gc)
715 {
716 // Don't use "refs_list_index" calculated in this way because
717 // balance_queues() has moved the Ref's into the first n queues.
718 // Thread* thr = Thread::current();
719 // int refs_list_index = ((WorkerThread*)thr)->id();
720 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
721 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
722 &is_alive, &keep_alive, &complete_gc);
723 }
724 private:
725 bool _clear_referent;
726 };
727
728 // Balances reference queues.
729 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
730 // queues[0, 1, ..., _num_q-1] because only the first _num_q
731 // corresponding to the active workers will be processed.
732 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
733 {
734 // calculate total length
735 size_t total_refs = 0;
736 if (TraceReferenceGC && PrintGCDetails) {
737 gclog_or_tty->print_cr("\nBalance ref_lists ");
738 }
739
740 for (uint i = 0; i < _max_num_q; ++i) {
741 total_refs += ref_lists[i].length();
742 if (TraceReferenceGC && PrintGCDetails) {
743 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
744 }
745 }
746 if (TraceReferenceGC && PrintGCDetails) {
747 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
748 }
749 size_t avg_refs = total_refs / _num_q + 1;
750 uint to_idx = 0;
751 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
752 bool move_all = false;
753 if (from_idx >= _num_q) {
754 move_all = ref_lists[from_idx].length() > 0;
755 }
756 while ((ref_lists[from_idx].length() > avg_refs) ||
757 move_all) {
758 assert(to_idx < _num_q, "Sanity Check!");
759 if (ref_lists[to_idx].length() < avg_refs) {
760 // move superfluous refs
761 size_t refs_to_move;
762 // Move all the Ref's if the from queue will not be processed.
763 if (move_all) {
764 refs_to_move = MIN2(ref_lists[from_idx].length(),
765 avg_refs - ref_lists[to_idx].length());
766 } else {
767 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
768 avg_refs - ref_lists[to_idx].length());
769 }
770
771 assert(refs_to_move > 0, "otherwise the code below will fail");
772
773 oop move_head = ref_lists[from_idx].head();
774 oop move_tail = move_head;
775 oop new_head = move_head;
776 // find an element to split the list on
777 for (size_t j = 0; j < refs_to_move; ++j) {
778 move_tail = new_head;
779 new_head = java_lang_ref_Reference::discovered(new_head);
780 }
781
782 // Add the chain to the to list.
783 if (ref_lists[to_idx].head() == NULL) {
784 // to list is empty. Make a loop at the end.
785 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
786 } else {
787 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
788 }
789 ref_lists[to_idx].set_head(move_head);
790 ref_lists[to_idx].inc_length(refs_to_move);
791
792 // Remove the chain from the from list.
793 if (move_tail == new_head) {
794 // We found the end of the from list.
795 ref_lists[from_idx].set_head(NULL);
796 } else {
797 ref_lists[from_idx].set_head(new_head);
798 }
799 ref_lists[from_idx].dec_length(refs_to_move);
800 if (ref_lists[from_idx].length() == 0) {
801 break;
802 }
803 } else {
804 to_idx = (to_idx + 1) % _num_q;
805 }
806 }
807 }
808 #ifdef ASSERT
809 size_t balanced_total_refs = 0;
810 for (uint i = 0; i < _max_num_q; ++i) {
811 balanced_total_refs += ref_lists[i].length();
812 if (TraceReferenceGC && PrintGCDetails) {
813 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
814 }
815 }
816 if (TraceReferenceGC && PrintGCDetails) {
817 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
818 gclog_or_tty->flush();
819 }
820 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
821 #endif
822 }
823
824 void ReferenceProcessor::balance_all_queues() {
825 balance_queues(_discoveredSoftRefs);
826 balance_queues(_discoveredWeakRefs);
827 balance_queues(_discoveredFinalRefs);
828 balance_queues(_discoveredPhantomRefs);
829 balance_queues(_discoveredCleanerRefs);
830 }
831
832 size_t
833 ReferenceProcessor::process_discovered_reflist(
834 DiscoveredList refs_lists[],
835 ReferencePolicy* policy,
836 bool clear_referent,
837 BoolObjectClosure* is_alive,
838 OopClosure* keep_alive,
839 VoidClosure* complete_gc,
840 AbstractRefProcTaskExecutor* task_executor)
841 {
842 bool mt_processing = task_executor != NULL && _processing_is_mt;
843 // If discovery used MT and a dynamic number of GC threads, then
844 // the queues must be balanced for correctness if fewer than the
845 // maximum number of queues were used. The number of queue used
846 // during discovery may be different than the number to be used
847 // for processing so don't depend of _num_q < _max_num_q as part
848 // of the test.
849 bool must_balance = _discovery_is_mt;
850
851 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
852 must_balance) {
853 balance_queues(refs_lists);
854 }
855
856 size_t total_list_count = total_count(refs_lists);
857
858 if (PrintReferenceGC && PrintGCDetails) {
859 gclog_or_tty->print(", " SIZE_FORMAT " refs", total_list_count);
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 return total_list_count;
906 }
907
908 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
909 uint id = 0;
910 // Determine the queue index to use for this object.
911 if (_discovery_is_mt) {
912 // During a multi-threaded discovery phase,
913 // each thread saves to its "own" list.
914 Thread* thr = Thread::current();
915 id = thr->as_Worker_thread()->id();
916 } else {
917 // single-threaded discovery, we save in round-robin
918 // fashion to each of the lists.
919 if (_processing_is_mt) {
920 id = next_id();
921 }
922 }
923 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
924
925 // Get the discovered queue to which we will add
926 DiscoveredList* list = NULL;
927 switch (rt) {
928 case REF_OTHER:
929 // Unknown reference type, no special treatment
930 break;
931 case REF_SOFT:
932 list = &_discoveredSoftRefs[id];
933 break;
934 case REF_WEAK:
935 list = &_discoveredWeakRefs[id];
936 break;
937 case REF_FINAL:
938 list = &_discoveredFinalRefs[id];
939 break;
940 case REF_PHANTOM:
941 list = &_discoveredPhantomRefs[id];
942 break;
943 case REF_CLEANER:
944 list = &_discoveredCleanerRefs[id];
945 break;
946 case REF_NONE:
947 // we should not reach here if we are an InstanceRefKlass
948 default:
949 ShouldNotReachHere();
950 }
951 if (TraceReferenceGC && PrintGCDetails) {
952 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
953 }
954 return list;
955 }
956
957 inline void
958 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
959 oop obj,
960 HeapWord* discovered_addr) {
961 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
962 // First we must make sure this object is only enqueued once. CAS in a non null
963 // discovered_addr.
964 oop current_head = refs_list.head();
965 // The last ref must have its discovered field pointing to itself.
966 oop next_discovered = (current_head != NULL) ? current_head : obj;
967
968 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
969 NULL);
970 if (retest == NULL) {
971 // This thread just won the right to enqueue the object.
972 // We have separate lists for enqueueing, so no synchronization
973 // is necessary.
974 refs_list.set_head(obj);
975 refs_list.inc_length(1);
976
977 if (TraceReferenceGC) {
978 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
979 p2i(obj), obj->klass()->internal_name());
980 }
981 } else {
982 // If retest was non NULL, another thread beat us to it:
983 // The reference has already been discovered...
984 if (TraceReferenceGC) {
985 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
986 p2i(obj), obj->klass()->internal_name());
987 }
988 }
989 }
990
991 #ifndef PRODUCT
992 // Non-atomic (i.e. concurrent) discovery might allow us
993 // to observe j.l.References with NULL referents, being those
994 // cleared concurrently by mutators during (or after) discovery.
995 void ReferenceProcessor::verify_referent(oop obj) {
996 bool da = discovery_is_atomic();
997 oop referent = java_lang_ref_Reference::referent(obj);
998 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
999 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1000 INTPTR_FORMAT " during %satomic discovery ",
1001 p2i(referent), p2i(obj), da ? "" : "non-"));
1002 }
1003 #endif
1004
1005 // We mention two of several possible choices here:
1006 // #0: if the reference object is not in the "originating generation"
1007 // (or part of the heap being collected, indicated by our "span"
1008 // we don't treat it specially (i.e. we scan it as we would
1009 // a normal oop, treating its references as strong references).
1010 // This means that references can't be discovered unless their
1011 // referent is also in the same span. This is the simplest,
1012 // most "local" and most conservative approach, albeit one
1013 // that may cause weak references to be enqueued least promptly.
1014 // We call this choice the "ReferenceBasedDiscovery" policy.
1015 // #1: the reference object may be in any generation (span), but if
1016 // the referent is in the generation (span) being currently collected
1017 // then we can discover the reference object, provided
1018 // the object has not already been discovered by
1019 // a different concurrently running collector (as may be the
1020 // case, for instance, if the reference object is in CMS and
1021 // the referent in DefNewGeneration), and provided the processing
1022 // of this reference object by the current collector will
1023 // appear atomic to every other collector in the system.
1024 // (Thus, for instance, a concurrent collector may not
1025 // discover references in other generations even if the
1026 // referent is in its own generation). This policy may,
1027 // in certain cases, enqueue references somewhat sooner than
1028 // might Policy #0 above, but at marginally increased cost
1029 // and complexity in processing these references.
1030 // We call this choice the "RefeferentBasedDiscovery" policy.
1031 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1032 // Make sure we are discovering refs (rather than processing discovered refs).
1033 if (!_discovering_refs || !RegisterReferences) {
1034 return false;
1035 }
1036 // We only discover active references.
1037 oop next = java_lang_ref_Reference::next(obj);
1038 if (next != NULL) { // Ref is no longer active
1039 return false;
1040 }
1041
1042 HeapWord* obj_addr = (HeapWord*)obj;
1043 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1044 !_span.contains(obj_addr)) {
1045 // Reference is not in the originating generation;
1046 // don't treat it specially (i.e. we want to scan it as a normal
1047 // object with strong references).
1048 return false;
1049 }
1050
1051 // We only discover references whose referents are not (yet)
1052 // known to be strongly reachable.
1053 if (is_alive_non_header() != NULL) {
1054 verify_referent(obj);
1055 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1056 return false; // referent is reachable
1057 }
1058 }
1059 if (rt == REF_SOFT) {
1060 // For soft refs we can decide now if these are not
1061 // current candidates for clearing, in which case we
1062 // can mark through them now, rather than delaying that
1063 // to the reference-processing phase. Since all current
1064 // time-stamp policies advance the soft-ref clock only
1065 // at a full collection cycle, this is always currently
1066 // accurate.
1067 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1068 return false;
1069 }
1070 }
1071
1072 ResourceMark rm; // Needed for tracing.
1073
1074 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1075 const oop discovered = java_lang_ref_Reference::discovered(obj);
1076 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1077 if (discovered != NULL) {
1078 // The reference has already been discovered...
1079 if (TraceReferenceGC) {
1080 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1081 p2i(obj), obj->klass()->internal_name());
1082 }
1083 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1084 // assumes that an object is not processed twice;
1085 // if it's been already discovered it must be on another
1086 // generation's discovered list; so we won't discover it.
1087 return false;
1088 } else {
1089 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1090 "Unrecognized policy");
1091 // Check assumption that an object is not potentially
1092 // discovered twice except by concurrent collectors that potentially
1093 // trace the same Reference object twice.
1094 assert(UseConcMarkSweepGC || UseG1GC,
1095 "Only possible with a concurrent marking collector");
1096 return true;
1097 }
1098 }
1099
1100 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1101 verify_referent(obj);
1102 // Discover if and only if EITHER:
1103 // .. reference is in our span, OR
1104 // .. we are an atomic collector and referent is in our span
1105 if (_span.contains(obj_addr) ||
1106 (discovery_is_atomic() &&
1107 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1108 // should_enqueue = true;
1109 } else {
1110 return false;
1111 }
1112 } else {
1113 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1114 _span.contains(obj_addr), "code inconsistency");
1115 }
1116
1117 // Get the right type of discovered queue head.
1118 DiscoveredList* list = get_discovered_list(rt);
1119 if (list == NULL) {
1120 return false; // nothing special needs to be done
1121 }
1122
1123 if (_discovery_is_mt) {
1124 add_to_discovered_list_mt(*list, obj, discovered_addr);
1125 } else {
1126 // We do a raw store here: the field will be visited later when processing
1127 // the discovered references.
1128 oop current_head = list->head();
1129 // The last ref must have its discovered field pointing to itself.
1130 oop next_discovered = (current_head != NULL) ? current_head : obj;
1131
1132 assert(discovered == NULL, "control point invariant");
1133 oop_store_raw(discovered_addr, next_discovered);
1134 list->set_head(obj);
1135 list->inc_length(1);
1136
1137 if (TraceReferenceGC) {
1138 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1139 p2i(obj), obj->klass()->internal_name());
1140 }
1141 }
1142 assert(obj->is_oop(), "Discovered a bad reference");
1143 verify_referent(obj);
1144 return true;
1145 }
1146
1147 // Preclean the discovered references by removing those
1148 // whose referents are alive, and by marking from those that
1149 // are not active. These lists can be handled here
1150 // in any order and, indeed, concurrently.
1151 void ReferenceProcessor::preclean_discovered_references(
1152 BoolObjectClosure* is_alive,
1153 OopClosure* keep_alive,
1154 VoidClosure* complete_gc,
1155 YieldClosure* yield,
1156 GCTimer* gc_timer,
1157 GCId gc_id) {
1158
1159 // Soft references
1160 {
1161 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1162 false, gc_timer, gc_id);
1163 for (uint i = 0; i < _max_num_q; i++) {
1164 if (yield->should_return()) {
1165 return;
1166 }
1167 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1168 keep_alive, complete_gc, yield);
1169 }
1170 }
1171
1172 // Weak references
1173 {
1174 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1175 false, gc_timer, gc_id);
1176 for (uint i = 0; i < _max_num_q; i++) {
1177 if (yield->should_return()) {
1178 return;
1179 }
1180 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1181 keep_alive, complete_gc, yield);
1182 }
1183 }
1184
1185 // Final references
1186 {
1187 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1188 false, gc_timer, gc_id);
1189 for (uint i = 0; i < _max_num_q; i++) {
1190 if (yield->should_return()) {
1191 return;
1192 }
1193 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1194 keep_alive, complete_gc, yield);
1195 }
1196 }
1197
1198 // Phantom references
1199 {
1200 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1201 false, gc_timer, gc_id);
1202 for (uint i = 0; i < _max_num_q; i++) {
1203 if (yield->should_return()) {
1204 return;
1205 }
1206 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1207 keep_alive, complete_gc, yield);
1208 }
1209
1210 // Cleaner references. Included in timing for phantom references. We
1211 // expect Cleaner references to be temporary, and don't want to deal with
1212 // possible incompatibilities arising from making it more visible.
1213 for (uint i = 0; i < _max_num_q; i++) {
1214 if (yield->should_return()) {
1215 return;
1216 }
1217 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1218 keep_alive, complete_gc, yield);
1219 }
1220 }
1221 }
1222
1223 // Walk the given discovered ref list, and remove all reference objects
1224 // whose referents are still alive, whose referents are NULL or which
1225 // are not active (have a non-NULL next field). NOTE: When we are
1226 // thus precleaning the ref lists (which happens single-threaded today),
1227 // we do not disable refs discovery to honor the correct semantics of
1228 // java.lang.Reference. As a result, we need to be careful below
1229 // that ref removal steps interleave safely with ref discovery steps
1230 // (in this thread).
1231 void
1232 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1233 BoolObjectClosure* is_alive,
1234 OopClosure* keep_alive,
1235 VoidClosure* complete_gc,
1236 YieldClosure* yield) {
1237 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1238 while (iter.has_next()) {
1239 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1240 oop obj = iter.obj();
1241 oop next = java_lang_ref_Reference::next(obj);
1242 if (iter.referent() == NULL || iter.is_referent_alive() ||
1243 next != NULL) {
1244 // The referent has been cleared, or is alive, or the Reference is not
1245 // active; we need to trace and mark its cohort.
1246 if (TraceReferenceGC) {
1247 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1248 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1249 }
1250 // Remove Reference object from list
1251 iter.remove();
1252 // Keep alive its cohort.
1253 iter.make_referent_alive();
1254 if (UseCompressedOops) {
1255 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1256 keep_alive->do_oop(next_addr);
1257 } else {
1258 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1259 keep_alive->do_oop(next_addr);
1260 }
1261 iter.move_to_next();
1262 } else {
1263 iter.next();
1264 }
1265 }
1266 // Close the reachable set
1267 complete_gc->do_void();
1268
1269 NOT_PRODUCT(
1270 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1271 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1272 " Refs in discovered list " INTPTR_FORMAT,
1273 iter.removed(), iter.processed(), p2i(refs_list.head()));
1274 }
1275 )
1276 }
1277
1278 const char* ReferenceProcessor::list_name(uint i) {
1279 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1280 "Out of bounds index");
1281
1282 int j = i / _max_num_q;
1283 switch (j) {
1284 case 0: return "SoftRef";
1285 case 1: return "WeakRef";
1286 case 2: return "FinalRef";
1287 case 3: return "PhantomRef";
1288 case 4: return "CleanerRef";
1289 }
1290 ShouldNotReachHere();
1291 return NULL;
1292 }
1293