1 /*
  2  * Copyright (c) 2001, 2019, 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.
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 23  */
 24 
 25 #include "precompiled.hpp"
 26 #include "classfile/classLoaderDataGraph.hpp"
 27 #include "gc/shared/cardTableRS.hpp"
 28 #include "gc/shared/genCollectedHeap.hpp"
 29 #include "gc/shared/genOopClosures.hpp"
 30 #include "gc/shared/generation.hpp"
 31 #include "gc/shared/space.inline.hpp"
 32 #include "memory/allocation.inline.hpp"
 33 #include "memory/iterator.inline.hpp"
 34 #include "oops/access.inline.hpp"
 35 #include "oops/oop.inline.hpp"
 36 #include "runtime/atomic.hpp"
 37 #include "runtime/java.hpp"
 38 #include "runtime/os.hpp"
 39 #include "utilities/macros.hpp"
 40 
 41 class HasAccumulatedModifiedOopsClosure : public CLDClosure {
 42   bool _found;
 43  public:
 44   HasAccumulatedModifiedOopsClosure() : _found(false) {}
 45   void do_cld(ClassLoaderData* cld) {
 46     if (_found) {
 47       return;
 48     }
 49 
 50     if (cld->has_accumulated_modified_oops()) {
 51       _found = true;
 52     }
 53   }
 54   bool found() {
 55     return _found;
 56   }
 57 };
 58 
 59 bool CLDRemSet::mod_union_is_clear() {
 60   HasAccumulatedModifiedOopsClosure closure;
 61   ClassLoaderDataGraph::cld_do(&closure);
 62 
 63   return !closure.found();
 64 }
 65 
 66 
 67 class ClearCLDModUnionClosure : public CLDClosure {
 68  public:
 69   void do_cld(ClassLoaderData* cld) {
 70     if (cld->has_accumulated_modified_oops()) {
 71       cld->clear_accumulated_modified_oops();
 72     }
 73   }
 74 };
 75 
 76 void CLDRemSet::clear_mod_union() {
 77   ClearCLDModUnionClosure closure;
 78   ClassLoaderDataGraph::cld_do(&closure);
 79 }
 80 
 81 CardTable::CardValue CardTableRS::find_unused_youngergenP_card_value() {
 82   for (CardValue v = youngergenP1_card;
 83        v < cur_youngergen_and_prev_nonclean_card;
 84        v++) {
 85     bool seen = false;
 86     for (int g = 0; g < _regions_to_iterate; g++) {
 87       if (_last_cur_val_in_gen[g] == v) {
 88         seen = true;
 89         break;
 90       }
 91     }
 92     if (!seen) {
 93       return v;
 94     }
 95   }
 96   ShouldNotReachHere();
 97   return 0;
 98 }
 99 
100 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
101   // Parallel or sequential, we must always set the prev to equal the
102   // last one written.
103   if (parallel) {
104     // Find a parallel value to be used next.
105     jbyte next_val = find_unused_youngergenP_card_value();
106     set_cur_youngergen_card_val(next_val);
107 
108   } else {
109     // In an sequential traversal we will always write youngergen, so that
110     // the inline barrier is  correct.
111     set_cur_youngergen_card_val(youngergen_card);
112   }
113 }
114 
115 void CardTableRS::younger_refs_iterate(Generation* g,
116                                        OopsInGenClosure* blk,
117                                        uint n_threads) {
118   // The indexing in this array is slightly odd. We want to access
119   // the old generation record here, which is at index 2.
120   _last_cur_val_in_gen[2] = cur_youngergen_card_val();
121   g->younger_refs_iterate(blk, n_threads);
122 }
123 
124 inline bool ClearNoncleanCardWrapper::clear_card(CardValue* entry) {
125   if (_is_par) {
126     return clear_card_parallel(entry);
127   } else {
128     return clear_card_serial(entry);
129   }
130 }
131 
132 inline bool ClearNoncleanCardWrapper::clear_card_parallel(CardValue* entry) {
133   while (true) {
134     // In the parallel case, we may have to do this several times.
135     CardValue entry_val = *entry;
136     assert(entry_val != CardTableRS::clean_card_val(),
137            "We shouldn't be looking at clean cards, and this should "
138            "be the only place they get cleaned.");
139     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
140         || _ct->is_prev_youngergen_card_val(entry_val)) {
141       CardValue res =
142         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
143       if (res == entry_val) {
144         break;
145       } else {
146         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
147                "The CAS above should only fail if another thread did "
148                "a GC write barrier.");
149       }
150     } else if (entry_val ==
151                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
152       // Parallelism shouldn't matter in this case.  Only the thread
153       // assigned to scan the card should change this value.
154       *entry = _ct->cur_youngergen_card_val();
155       break;
156     } else {
157       assert(entry_val == _ct->cur_youngergen_card_val(),
158              "Should be the only possibility.");
159       // In this case, the card was clean before, and become
160       // cur_youngergen only because of processing of a promoted object.
161       // We don't have to look at the card.
162       return false;
163     }
164   }
165   return true;
166 }
167 
168 
169 inline bool ClearNoncleanCardWrapper::clear_card_serial(CardValue* entry) {
170   CardValue entry_val = *entry;
171   assert(entry_val != CardTableRS::clean_card_val(),
172          "We shouldn't be looking at clean cards, and this should "
173          "be the only place they get cleaned.");
174   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
175          "This should be possible in the sequential case.");
176   *entry = CardTableRS::clean_card_val();
177   return true;
178 }
179 
180 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
181   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
182     _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
183 }
184 
185 bool ClearNoncleanCardWrapper::is_word_aligned(CardTable::CardValue* entry) {
186   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
187 }
188 
189 // The regions are visited in *decreasing* address order.
190 // This order aids with imprecise card marking, where a dirty
191 // card may cause scanning, and summarization marking, of objects
192 // that extend onto subsequent cards.
193 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
194   assert(mr.word_size() > 0, "Error");
195   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
196   // mr.end() may not necessarily be card aligned.
197   CardValue* cur_entry = _ct->byte_for(mr.last());
198   const CardValue* limit = _ct->byte_for(mr.start());
199   HeapWord* end_of_non_clean = mr.end();
200   HeapWord* start_of_non_clean = end_of_non_clean;
201   while (cur_entry >= limit) {
202     HeapWord* cur_hw = _ct->addr_for(cur_entry);
203     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
204       // Continue the dirty range by opening the
205       // dirty window one card to the left.
206       start_of_non_clean = cur_hw;
207     } else {
208       // We hit a "clean" card; process any non-empty
209       // "dirty" range accumulated so far.
210       if (start_of_non_clean < end_of_non_clean) {
211         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
212         _dirty_card_closure->do_MemRegion(mrd);
213       }
214 
215       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
216       if (is_word_aligned(cur_entry)) {
217         CardValue* cur_row = cur_entry - BytesPerWord;
218         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row_val()) {
219           cur_row -= BytesPerWord;
220         }
221         cur_entry = cur_row + BytesPerWord;
222         cur_hw = _ct->addr_for(cur_entry);
223       }
224 
225       // Reset the dirty window, while continuing to look
226       // for the next dirty card that will start a
227       // new dirty window.
228       end_of_non_clean = cur_hw;
229       start_of_non_clean = cur_hw;
230     }
231     // Note that "cur_entry" leads "start_of_non_clean" in
232     // its leftward excursion after this point
233     // in the loop and, when we hit the left end of "mr",
234     // will point off of the left end of the card-table
235     // for "mr".
236     cur_entry--;
237   }
238   // If the first card of "mr" was dirty, we will have
239   // been left with a dirty window, co-initial with "mr",
240   // which we now process.
241   if (start_of_non_clean < end_of_non_clean) {
242     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
243     _dirty_card_closure->do_MemRegion(mrd);
244   }
245 }
246 
247 // clean (by dirty->clean before) ==> cur_younger_gen
248 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
249 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
250 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
251 // cur-younger-gen                ==> cur_younger_gen
252 // cur_youngergen_and_prev_nonclean_card ==> no change.
253 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
254   volatile CardValue* entry = byte_for(field);
255   do {
256     CardValue entry_val = *entry;
257     // We put this first because it's probably the most common case.
258     if (entry_val == clean_card_val()) {
259       // No threat of contention with cleaning threads.
260       *entry = cur_youngergen_card_val();
261       return;
262     } else if (card_is_dirty_wrt_gen_iter(entry_val)
263                || is_prev_youngergen_card_val(entry_val)) {
264       // Mark it as both cur and prev youngergen; card cleaning thread will
265       // eventually remove the previous stuff.
266       CardValue new_val = cur_youngergen_and_prev_nonclean_card;
267       CardValue res = Atomic::cmpxchg(new_val, entry, entry_val);
268       // Did the CAS succeed?
269       if (res == entry_val) return;
270       // Otherwise, retry, to see the new value.
271       continue;
272     } else {
273       assert(entry_val == cur_youngergen_and_prev_nonclean_card
274              || entry_val == cur_youngergen_card_val(),
275              "should be only possibilities.");
276       return;
277     }
278   } while (true);
279 }
280 
281 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
282                                                 OopsInGenClosure* cl,
283                                                 uint n_threads) {
284   verify_used_region_at_save_marks(sp);
285 
286   const MemRegion urasm = sp->used_region_at_save_marks();
287   non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
288 }
289 
290 #ifdef ASSERT
291 void CardTableRS::verify_used_region_at_save_marks(Space* sp) const {
292   MemRegion ur    = sp->used_region();
293   MemRegion urasm = sp->used_region_at_save_marks();
294 
295   assert(ur.contains(urasm),
296          "Did you forget to call save_marks()? "
297          "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
298          "[" PTR_FORMAT ", " PTR_FORMAT ")",
299          p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
300 }
301 #endif
302 
303 void CardTableRS::clear_into_younger(Generation* old_gen) {
304   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
305          "Should only be called for the old generation");
306   // The card tables for the youngest gen need never be cleared.
307   // There's a bit of subtlety in the clear() and invalidate()
308   // methods that we exploit here and in invalidate_or_clear()
309   // below to avoid missing cards at the fringes. If clear() or
310   // invalidate() are changed in the future, this code should
311   // be revisited. 20040107.ysr
312   clear(old_gen->prev_used_region());
313 }
314 
315 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
316   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
317          "Should only be called for the old generation");
318   // Invalidate the cards for the currently occupied part of
319   // the old generation and clear the cards for the
320   // unoccupied part of the generation (if any, making use
321   // of that generation's prev_used_region to determine that
322   // region). No need to do anything for the youngest
323   // generation. Also see note#20040107.ysr above.
324   MemRegion used_mr = old_gen->used_region();
325   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
326   if (!to_be_cleared_mr.is_empty()) {
327     clear(to_be_cleared_mr);
328   }
329   invalidate(used_mr);
330 }
331 
332 
333 class VerifyCleanCardClosure: public BasicOopIterateClosure {
334 private:
335   HeapWord* _boundary;
336   HeapWord* _begin;
337   HeapWord* _end;
338 protected:
339   template <class T> void do_oop_work(T* p) {
340     HeapWord* jp = (HeapWord*)p;
341     assert(jp >= _begin && jp < _end,
342            "Error: jp " PTR_FORMAT " should be within "
343            "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
344            p2i(jp), p2i(_begin), p2i(_end));
345     oop obj = RawAccess<>::oop_load(p);
346     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
347               "pointer " PTR_FORMAT " at " PTR_FORMAT " on "
348               "clean card crosses boundary" PTR_FORMAT,
349               p2i(obj), p2i(jp), p2i(_boundary));
350   }
351 
352 public:
353   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
354     _boundary(b), _begin(begin), _end(end) {
355     assert(b <= begin,
356            "Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
357            p2i(b), p2i(begin));
358     assert(begin <= end,
359            "Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
360            p2i(begin), p2i(end));
361   }
362 
363   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
364   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
365 };
366 
367 class VerifyCTSpaceClosure: public SpaceClosure {
368 private:
369   CardTableRS* _ct;
370   HeapWord* _boundary;
371 public:
372   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
373     _ct(ct), _boundary(boundary) {}
374   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
375 };
376 
377 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
378   CardTableRS* _ct;
379 public:
380   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
381   void do_generation(Generation* gen) {
382     // Skip the youngest generation.
383     if (GenCollectedHeap::heap()->is_young_gen(gen)) {
384       return;
385     }
386     // Normally, we're interested in pointers to younger generations.
387     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
388     gen->space_iterate(&blk, true);
389   }
390 };
391 
392 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
393   // We don't need to do young-gen spaces.
394   if (s->end() <= gen_boundary) return;
395   MemRegion used = s->used_region();
396 
397   CardValue* cur_entry = byte_for(used.start());
398   CardValue* limit = byte_after(used.last());
399   while (cur_entry < limit) {
400     if (*cur_entry == clean_card_val()) {
401       CardValue* first_dirty = cur_entry+1;
402       while (first_dirty < limit &&
403              *first_dirty == clean_card_val()) {
404         first_dirty++;
405       }
406       // If the first object is a regular object, and it has a
407       // young-to-old field, that would mark the previous card.
408       HeapWord* boundary = addr_for(cur_entry);
409       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
410       HeapWord* boundary_block = s->block_start(boundary);
411       HeapWord* begin = boundary;             // Until proven otherwise.
412       HeapWord* start_block = boundary_block; // Until proven otherwise.
413       if (boundary_block < boundary) {
414         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
415           oop boundary_obj = oop(boundary_block);
416           if (!boundary_obj->is_objArray() &&
417               !boundary_obj->is_typeArray()) {
418             guarantee(cur_entry > byte_for(used.start()),
419                       "else boundary would be boundary_block");
420             if (*byte_for(boundary_block) != clean_card_val()) {
421               begin = boundary_block + s->block_size(boundary_block);
422               start_block = begin;
423             }
424           }
425         }
426       }
427       // Now traverse objects until end.
428       if (begin < end) {
429         MemRegion mr(begin, end);
430         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
431         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
432           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
433             oop(cur)->oop_iterate(&verify_blk, mr);
434           }
435         }
436       }
437       cur_entry = first_dirty;
438     } else {
439       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
440       // is a transient value, that cannot be in the card table
441       // except during GC, and thus assert that:
442       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
443       //        "Illegal CT value");
444       // That however, need not hold, as will become clear in the
445       // following...
446 
447       // We'd normally expect that if we are in the parallel case,
448       // we can't have left a prev value (which would be different
449       // from the current value) in the card table, and so we'd like to
450       // assert that:
451       // guarantee(cur_youngergen_card_val() == youngergen_card
452       //           || !is_prev_youngergen_card_val(*cur_entry),
453       //           "Illegal CT value");
454       // That, however, may not hold occasionally, because of
455       // CMS or MSC in the old gen. To wit, consider the
456       // following two simple illustrative scenarios:
457       // (a) CMS: Consider the case where a large object L
458       //     spanning several cards is allocated in the old
459       //     gen, and has a young gen reference stored in it, dirtying
460       //     some interior cards. A young collection scans the card,
461       //     finds a young ref and installs a youngergenP_n value.
462       //     L then goes dead. Now a CMS collection starts,
463       //     finds L dead and sweeps it up. Assume that L is
464       //     abutting _unallocated_blk, so _unallocated_blk is
465       //     adjusted down to (below) L. Assume further that
466       //     no young collection intervenes during this CMS cycle.
467       //     The next young gen cycle will not get to look at this
468       //     youngergenP_n card since it lies in the unoccupied
469       //     part of the space.
470       //     Some young collections later the blocks on this
471       //     card can be re-allocated either due to direct allocation
472       //     or due to absorbing promotions. At this time, the
473       //     before-gc verification will fail the above assert.
474       // (b) MSC: In this case, an object L with a young reference
475       //     is on a card that (therefore) holds a youngergen_n value.
476       //     Suppose also that L lies towards the end of the used
477       //     the used space before GC. An MSC collection
478       //     occurs that compacts to such an extent that this
479       //     card is no longer in the occupied part of the space.
480       //     Since current code in MSC does not always clear cards
481       //     in the unused part of old gen, this stale youngergen_n
482       //     value is left behind and can later be covered by
483       //     an object when promotion or direct allocation
484       //     re-allocates that part of the heap.
485       //
486       // Fortunately, the presence of such stale card values is
487       // "only" a minor annoyance in that subsequent young collections
488       // might needlessly scan such cards, but would still never corrupt
489       // the heap as a result. However, it's likely not to be a significant
490       // performance inhibitor in practice. For instance,
491       // some recent measurements with unoccupied cards eagerly cleared
492       // out to maintain this invariant, showed next to no
493       // change in young collection times; of course one can construct
494       // degenerate examples where the cost can be significant.)
495       // Note, in particular, that if the "stale" card is modified
496       // after re-allocation, it would be dirty, not "stale". Thus,
497       // we can never have a younger ref in such a card and it is
498       // safe not to scan that card in any collection. [As we see
499       // below, we do some unnecessary scanning
500       // in some cases in the current parallel scanning algorithm.]
501       //
502       // The main point below is that the parallel card scanning code
503       // deals correctly with these stale card values. There are two main
504       // cases to consider where we have a stale "young gen" value and a
505       // "derivative" case to consider, where we have a stale
506       // "cur_younger_gen_and_prev_non_clean" value, as will become
507       // apparent in the case analysis below.
508       // o Case 1. If the stale value corresponds to a younger_gen_n
509       //   value other than the cur_younger_gen value then the code
510       //   treats this as being tantamount to a prev_younger_gen
511       //   card. This means that the card may be unnecessarily scanned.
512       //   There are two sub-cases to consider:
513       //   o Case 1a. Let us say that the card is in the occupied part
514       //     of the generation at the time the collection begins. In
515       //     that case the card will be either cleared when it is scanned
516       //     for young pointers, or will be set to cur_younger_gen as a
517       //     result of promotion. (We have elided the normal case where
518       //     the scanning thread and the promoting thread interleave
519       //     possibly resulting in a transient
520       //     cur_younger_gen_and_prev_non_clean value before settling
521       //     to cur_younger_gen. [End Case 1a.]
522       //   o Case 1b. Consider now the case when the card is in the unoccupied
523       //     part of the space which becomes occupied because of promotions
524       //     into it during the current young GC. In this case the card
525       //     will never be scanned for young references. The current
526       //     code will set the card value to either
527       //     cur_younger_gen_and_prev_non_clean or leave
528       //     it with its stale value -- because the promotions didn't
529       //     result in any younger refs on that card. Of these two
530       //     cases, the latter will be covered in Case 1a during
531       //     a subsequent scan. To deal with the former case, we need
532       //     to further consider how we deal with a stale value of
533       //     cur_younger_gen_and_prev_non_clean in our case analysis
534       //     below. This we do in Case 3 below. [End Case 1b]
535       //   [End Case 1]
536       // o Case 2. If the stale value corresponds to cur_younger_gen being
537       //   a value not necessarily written by a current promotion, the
538       //   card will not be scanned by the younger refs scanning code.
539       //   (This is OK since as we argued above such cards cannot contain
540       //   any younger refs.) The result is that this value will be
541       //   treated as a prev_younger_gen value in a subsequent collection,
542       //   which is addressed in Case 1 above. [End Case 2]
543       // o Case 3. We here consider the "derivative" case from Case 1b. above
544       //   because of which we may find a stale
545       //   cur_younger_gen_and_prev_non_clean card value in the table.
546       //   Once again, as in Case 1, we consider two subcases, depending
547       //   on whether the card lies in the occupied or unoccupied part
548       //   of the space at the start of the young collection.
549       //   o Case 3a. Let us say the card is in the occupied part of
550       //     the old gen at the start of the young collection. In that
551       //     case, the card will be scanned by the younger refs scanning
552       //     code which will set it to cur_younger_gen. In a subsequent
553       //     scan, the card will be considered again and get its final
554       //     correct value. [End Case 3a]
555       //   o Case 3b. Now consider the case where the card is in the
556       //     unoccupied part of the old gen, and is occupied as a result
557       //     of promotions during thus young gc. In that case,
558       //     the card will not be scanned for younger refs. The presence
559       //     of newly promoted objects on the card will then result in
560       //     its keeping the value cur_younger_gen_and_prev_non_clean
561       //     value, which we have dealt with in Case 3 here. [End Case 3b]
562       //   [End Case 3]
563       //
564       // (Please refer to the code in the helper class
565       // ClearNonCleanCardWrapper and in CardTable for details.)
566       //
567       // The informal arguments above can be tightened into a formal
568       // correctness proof and it behooves us to write up such a proof,
569       // or to use model checking to prove that there are no lingering
570       // concerns.
571       //
572       // Clearly because of Case 3b one cannot bound the time for
573       // which a card will retain what we have called a "stale" value.
574       // However, one can obtain a Loose upper bound on the redundant
575       // work as a result of such stale values. Note first that any
576       // time a stale card lies in the occupied part of the space at
577       // the start of the collection, it is scanned by younger refs
578       // code and we can define a rank function on card values that
579       // declines when this is so. Note also that when a card does not
580       // lie in the occupied part of the space at the beginning of a
581       // young collection, its rank can either decline or stay unchanged.
582       // In this case, no extra work is done in terms of redundant
583       // younger refs scanning of that card.
584       // Then, the case analysis above reveals that, in the worst case,
585       // any such stale card will be scanned unnecessarily at most twice.
586       //
587       // It is nonetheless advisable to try and get rid of some of this
588       // redundant work in a subsequent (low priority) re-design of
589       // the card-scanning code, if only to simplify the underlying
590       // state machine analysis/proof. ysr 1/28/2002. XXX
591       cur_entry++;
592     }
593   }
594 }
595 
596 void CardTableRS::verify() {
597   // At present, we only know how to verify the card table RS for
598   // generational heaps.
599   VerifyCTGenClosure blk(this);
600   GenCollectedHeap::heap()->generation_iterate(&blk, false);
601   CardTable::verify();
602 }
603 
604 CardTableRS::CardTableRS(MemRegion whole_heap, bool scanned_concurrently) :
605   CardTable(whole_heap, scanned_concurrently),
606   _cur_youngergen_card_val(youngergenP1_card),
607   // LNC functionality
608   _lowest_non_clean(NULL),
609   _lowest_non_clean_chunk_size(NULL),
610   _lowest_non_clean_base_chunk_index(NULL),
611   _last_LNC_resizing_collection(NULL)
612 {
613   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
614   // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
615   uint max_gens = 2;
616   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(CardValue, max_gens + 1,
617                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
618   if (_last_cur_val_in_gen == NULL) {
619     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
620   }
621   for (uint i = 0; i < max_gens + 1; i++) {
622     _last_cur_val_in_gen[i] = clean_card_val();
623   }
624 }
625 
626 CardTableRS::~CardTableRS() {
627   if (_last_cur_val_in_gen) {
628     FREE_C_HEAP_ARRAY(CardValue, _last_cur_val_in_gen);
629     _last_cur_val_in_gen = NULL;
630   }
631   if (_lowest_non_clean) {
632     FREE_C_HEAP_ARRAY(CardArr, _lowest_non_clean);
633     _lowest_non_clean = NULL;
634   }
635   if (_lowest_non_clean_chunk_size) {
636     FREE_C_HEAP_ARRAY(size_t, _lowest_non_clean_chunk_size);
637     _lowest_non_clean_chunk_size = NULL;
638   }
639   if (_lowest_non_clean_base_chunk_index) {
640     FREE_C_HEAP_ARRAY(uintptr_t, _lowest_non_clean_base_chunk_index);
641     _lowest_non_clean_base_chunk_index = NULL;
642   }
643   if (_last_LNC_resizing_collection) {
644     FREE_C_HEAP_ARRAY(int, _last_LNC_resizing_collection);
645     _last_LNC_resizing_collection = NULL;
646   }
647 }
648 
649 void CardTableRS::initialize() {
650   CardTable::initialize();
651   _lowest_non_clean =
652     NEW_C_HEAP_ARRAY(CardArr, _max_covered_regions, mtGC);
653   _lowest_non_clean_chunk_size =
654     NEW_C_HEAP_ARRAY(size_t, _max_covered_regions, mtGC);
655   _lowest_non_clean_base_chunk_index =
656     NEW_C_HEAP_ARRAY(uintptr_t, _max_covered_regions, mtGC);
657   _last_LNC_resizing_collection =
658     NEW_C_HEAP_ARRAY(int, _max_covered_regions, mtGC);
659 
660   for (int i = 0; i < _max_covered_regions; i++) {
661     _lowest_non_clean[i] = NULL;
662     _lowest_non_clean_chunk_size[i] = 0;
663     _last_LNC_resizing_collection[i] = -1;
664   }
665 }
666 
667 bool CardTableRS::card_will_be_scanned(CardValue cv) {
668   return card_is_dirty_wrt_gen_iter(cv) || is_prev_nonclean_card_val(cv);
669 }
670 
671 bool CardTableRS::card_may_have_been_dirty(CardValue cv) {
672   return
673     cv != clean_card &&
674     (card_is_dirty_wrt_gen_iter(cv) ||
675      CardTableRS::youngergen_may_have_been_dirty(cv));
676 }
677 
678 void CardTableRS::non_clean_card_iterate_possibly_parallel(
679   Space* sp,
680   MemRegion mr,
681   OopsInGenClosure* cl,
682   CardTableRS* ct,
683   uint n_threads)
684 {
685   if (!mr.is_empty()) {
686     if (n_threads > 0) {
687       non_clean_card_iterate_parallel_work(sp, mr, cl, ct, n_threads);
688     } else {
689       // clear_cl finds contiguous dirty ranges of cards to process and clear.
690 
691       // This is the single-threaded version used by DefNew.
692       const bool parallel = false;
693 
694       DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, precision(), cl->gen_boundary(), parallel);
695       ClearNoncleanCardWrapper clear_cl(dcto_cl, ct, parallel);
696 
697       clear_cl.do_MemRegion(mr);
698     }
699   }
700 }
701 
702 void CardTableRS::non_clean_card_iterate_parallel_work(Space* sp, MemRegion mr,
703                                                        OopsInGenClosure* cl, CardTableRS* ct,
704                                                        uint n_threads) {
705   fatal("Parallel gc not supported here.");
706 }
707 
708 bool CardTableRS::is_in_young(oop obj) const {
709   return GenCollectedHeap::heap()->is_in_young(obj);
710 }