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 "gc/shared/cardTableRS.hpp"
  27 #include "gc/shared/genCollectedHeap.hpp"
  28 #include "gc/shared/generation.hpp"
  29 #include "gc/shared/space.inline.hpp"
  30 #include "memory/allocation.inline.hpp"
  31 #include "oops/oop.inline.hpp"
  32 #include "runtime/atomic.inline.hpp"
  33 #include "runtime/java.hpp"
  34 #include "runtime/os.hpp"
  35 #include "utilities/macros.hpp"
  36 
  37 CardTableRS::CardTableRS(MemRegion whole_heap) :
  38   GenRemSet(),
  39   _cur_youngergen_card_val(youngergenP1_card)
  40 {
  41   _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  42   _ct_bs->initialize();
  43   set_bs(_ct_bs);
  44   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
  45   // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
  46   uint max_gens = 2;
  47   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
  48                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  49   if (_last_cur_val_in_gen == NULL) {
  50     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  51   }
  52   for (uint i = 0; i < max_gens + 1; i++) {
  53     _last_cur_val_in_gen[i] = clean_card_val();
  54   }
  55   _ct_bs->set_CTRS(this);
  56 }
  57 
  58 CardTableRS::~CardTableRS() {
  59   if (_ct_bs) {
  60     delete _ct_bs;
  61     _ct_bs = NULL;
  62   }
  63   if (_last_cur_val_in_gen) {
  64     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
  65   }
  66 }
  67 
  68 void CardTableRS::resize_covered_region(MemRegion new_region) {
  69   _ct_bs->resize_covered_region(new_region);
  70 }
  71 
  72 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  73   for (jbyte v = youngergenP1_card;
  74        v < cur_youngergen_and_prev_nonclean_card;
  75        v++) {
  76     bool seen = false;
  77     for (int g = 0; g < _regions_to_iterate; g++) {
  78       if (_last_cur_val_in_gen[g] == v) {
  79         seen = true;
  80         break;
  81       }
  82     }
  83     if (!seen) return v;
  84   }
  85   ShouldNotReachHere();
  86   return 0;
  87 }
  88 
  89 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
  90   // Parallel or sequential, we must always set the prev to equal the
  91   // last one written.
  92   if (parallel) {
  93     // Find a parallel value to be used next.
  94     jbyte next_val = find_unused_youngergenP_card_value();
  95     set_cur_youngergen_card_val(next_val);
  96 
  97   } else {
  98     // In an sequential traversal we will always write youngergen, so that
  99     // the inline barrier is  correct.
 100     set_cur_youngergen_card_val(youngergen_card);
 101   }
 102 }
 103 
 104 void CardTableRS::younger_refs_iterate(Generation* g,
 105                                        OopsInGenClosure* blk,
 106                                        uint n_threads) {
 107   // The indexing in this array is slightly odd. We want to access
 108   // the old generation record here, which is at index 2.
 109   _last_cur_val_in_gen[2] = cur_youngergen_card_val();
 110   g->younger_refs_iterate(blk, n_threads);
 111 }
 112 
 113 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 114   if (_is_par) {
 115     return clear_card_parallel(entry);
 116   } else {
 117     return clear_card_serial(entry);
 118   }
 119 }
 120 
 121 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 122   while (true) {
 123     // In the parallel case, we may have to do this several times.
 124     jbyte entry_val = *entry;
 125     assert(entry_val != CardTableRS::clean_card_val(),
 126            "We shouldn't be looking at clean cards, and this should "
 127            "be the only place they get cleaned.");
 128     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 129         || _ct->is_prev_youngergen_card_val(entry_val)) {
 130       jbyte res =
 131         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 132       if (res == entry_val) {
 133         break;
 134       } else {
 135         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 136                "The CAS above should only fail if another thread did "
 137                "a GC write barrier.");
 138       }
 139     } else if (entry_val ==
 140                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 141       // Parallelism shouldn't matter in this case.  Only the thread
 142       // assigned to scan the card should change this value.
 143       *entry = _ct->cur_youngergen_card_val();
 144       break;
 145     } else {
 146       assert(entry_val == _ct->cur_youngergen_card_val(),
 147              "Should be the only possibility.");
 148       // In this case, the card was clean before, and become
 149       // cur_youngergen only because of processing of a promoted object.
 150       // We don't have to look at the card.
 151       return false;
 152     }
 153   }
 154   return true;
 155 }
 156 
 157 
 158 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 159   jbyte entry_val = *entry;
 160   assert(entry_val != CardTableRS::clean_card_val(),
 161          "We shouldn't be looking at clean cards, and this should "
 162          "be the only place they get cleaned.");
 163   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 164          "This should be possible in the sequential case.");
 165   *entry = CardTableRS::clean_card_val();
 166   return true;
 167 }
 168 
 169 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 170   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
 171     _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
 172 }
 173 
 174 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 175   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 176 }
 177 
 178 // The regions are visited in *decreasing* address order.
 179 // This order aids with imprecise card marking, where a dirty
 180 // card may cause scanning, and summarization marking, of objects
 181 // that extend onto subsequent cards.
 182 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 183   assert(mr.word_size() > 0, "Error");
 184   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 185   // mr.end() may not necessarily be card aligned.
 186   jbyte* cur_entry = _ct->byte_for(mr.last());
 187   const jbyte* limit = _ct->byte_for(mr.start());
 188   HeapWord* end_of_non_clean = mr.end();
 189   HeapWord* start_of_non_clean = end_of_non_clean;
 190   while (cur_entry >= limit) {
 191     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 192     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 193       // Continue the dirty range by opening the
 194       // dirty window one card to the left.
 195       start_of_non_clean = cur_hw;
 196     } else {
 197       // We hit a "clean" card; process any non-empty
 198       // "dirty" range accumulated so far.
 199       if (start_of_non_clean < end_of_non_clean) {
 200         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 201         _dirty_card_closure->do_MemRegion(mrd);
 202       }
 203 
 204       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 205       if (is_word_aligned(cur_entry)) {
 206         jbyte* cur_row = cur_entry - BytesPerWord;
 207         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 208           cur_row -= BytesPerWord;
 209         }
 210         cur_entry = cur_row + BytesPerWord;
 211         cur_hw = _ct->addr_for(cur_entry);
 212       }
 213 
 214       // Reset the dirty window, while continuing to look
 215       // for the next dirty card that will start a
 216       // new dirty window.
 217       end_of_non_clean = cur_hw;
 218       start_of_non_clean = cur_hw;
 219     }
 220     // Note that "cur_entry" leads "start_of_non_clean" in
 221     // its leftward excursion after this point
 222     // in the loop and, when we hit the left end of "mr",
 223     // will point off of the left end of the card-table
 224     // for "mr".
 225     cur_entry--;
 226   }
 227   // If the first card of "mr" was dirty, we will have
 228   // been left with a dirty window, co-initial with "mr",
 229   // which we now process.
 230   if (start_of_non_clean < end_of_non_clean) {
 231     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 232     _dirty_card_closure->do_MemRegion(mrd);
 233   }
 234 }
 235 
 236 // clean (by dirty->clean before) ==> cur_younger_gen
 237 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 238 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 239 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 240 // cur-younger-gen                ==> cur_younger_gen
 241 // cur_youngergen_and_prev_nonclean_card ==> no change.
 242 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 243   jbyte* entry = ct_bs()->byte_for(field);
 244   do {
 245     jbyte entry_val = *entry;
 246     // We put this first because it's probably the most common case.
 247     if (entry_val == clean_card_val()) {
 248       // No threat of contention with cleaning threads.
 249       *entry = cur_youngergen_card_val();
 250       return;
 251     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 252                || is_prev_youngergen_card_val(entry_val)) {
 253       // Mark it as both cur and prev youngergen; card cleaning thread will
 254       // eventually remove the previous stuff.
 255       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 256       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 257       // Did the CAS succeed?
 258       if (res == entry_val) return;
 259       // Otherwise, retry, to see the new value.
 260       continue;
 261     } else {
 262       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 263              || entry_val == cur_youngergen_card_val(),
 264              "should be only possibilities.");
 265       return;
 266     }
 267   } while (true);
 268 }
 269 
 270 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 271                                                 OopsInGenClosure* cl,
 272                                                 uint n_threads) {
 273   const MemRegion urasm = sp->used_region_at_save_marks();
 274 #ifdef ASSERT
 275   // Convert the assertion check to a warning if we are running
 276   // CMS+ParNew until related bug is fixed.
 277   MemRegion ur    = sp->used_region();
 278   assert(ur.contains(urasm) || (UseConcMarkSweepGC),
 279          err_msg("Did you forget to call save_marks()? "
 280                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 281                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
 282                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
 283   // In the case of CMS+ParNew, issue a warning
 284   if (!ur.contains(urasm)) {
 285     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 286     warning("CMS+ParNew: Did you forget to call save_marks()? "
 287             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 288             "[" PTR_FORMAT ", " PTR_FORMAT ")",
 289              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 290     MemRegion ur2 = sp->used_region();
 291     MemRegion urasm2 = sp->used_region_at_save_marks();
 292     if (!ur.equals(ur2)) {
 293       warning("CMS+ParNew: Flickering used_region()!!");
 294     }
 295     if (!urasm.equals(urasm2)) {
 296       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 297     }
 298     ShouldNotReachHere();
 299   }
 300 #endif
 301   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
 302 }
 303 
 304 void CardTableRS::clear_into_younger(Generation* old_gen) {
 305   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 306          "Should only be called for the old generation");
 307   // The card tables for the youngest gen need never be cleared.
 308   // There's a bit of subtlety in the clear() and invalidate()
 309   // methods that we exploit here and in invalidate_or_clear()
 310   // below to avoid missing cards at the fringes. If clear() or
 311   // invalidate() are changed in the future, this code should
 312   // be revisited. 20040107.ysr
 313   clear(old_gen->prev_used_region());
 314 }
 315 
 316 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 317   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 318          "Should only be called for the old generation");
 319   // Invalidate the cards for the currently occupied part of
 320   // the old generation and clear the cards for the
 321   // unoccupied part of the generation (if any, making use
 322   // of that generation's prev_used_region to determine that
 323   // region). No need to do anything for the youngest
 324   // generation. Also see note#20040107.ysr above.
 325   MemRegion used_mr = old_gen->used_region();
 326   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 327   if (!to_be_cleared_mr.is_empty()) {
 328     clear(to_be_cleared_mr);
 329   }
 330   invalidate(used_mr);
 331 }
 332 
 333 
 334 class VerifyCleanCardClosure: public OopClosure {
 335 private:
 336   HeapWord* _boundary;
 337   HeapWord* _begin;
 338   HeapWord* _end;
 339 protected:
 340   template <class T> void do_oop_work(T* p) {
 341     HeapWord* jp = (HeapWord*)p;
 342     assert(jp >= _begin && jp < _end,
 343            err_msg("Error: jp " PTR_FORMAT " should be within "
 344                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 345                    p2i(jp), p2i(_begin), p2i(_end)));
 346     oop obj = oopDesc::load_decode_heap_oop(p);
 347     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 348               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 349                       "clean card crosses boundary" PTR_FORMAT,
 350                       p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
 351   }
 352 
 353 public:
 354   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 355     _boundary(b), _begin(begin), _end(end) {
 356     assert(b <= begin,
 357            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 358                    p2i(b), p2i(begin)));
 359     assert(begin <= end,
 360            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 361                    p2i(begin), p2i(end)));
 362   }
 363 
 364   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 365   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 366 };
 367 
 368 class VerifyCTSpaceClosure: public SpaceClosure {
 369 private:
 370   CardTableRS* _ct;
 371   HeapWord* _boundary;
 372 public:
 373   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 374     _ct(ct), _boundary(boundary) {}
 375   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 376 };
 377 
 378 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 379   CardTableRS* _ct;
 380 public:
 381   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 382   void do_generation(Generation* gen) {
 383     // Skip the youngest generation.
 384     if (GenCollectedHeap::heap()->is_young_gen(gen)) {
 385       return;
 386     }
 387     // Normally, we're interested in pointers to younger generations.
 388     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 389     gen->space_iterate(&blk, true);
 390   }
 391 };
 392 
 393 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 394   // We don't need to do young-gen spaces.
 395   if (s->end() <= gen_boundary) return;
 396   MemRegion used = s->used_region();
 397 
 398   jbyte* cur_entry = byte_for(used.start());
 399   jbyte* limit = byte_after(used.last());
 400   while (cur_entry < limit) {
 401     if (*cur_entry == CardTableModRefBS::clean_card) {
 402       jbyte* first_dirty = cur_entry+1;
 403       while (first_dirty < limit &&
 404              *first_dirty == CardTableModRefBS::clean_card) {
 405         first_dirty++;
 406       }
 407       // If the first object is a regular object, and it has a
 408       // young-to-old field, that would mark the previous card.
 409       HeapWord* boundary = addr_for(cur_entry);
 410       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 411       HeapWord* boundary_block = s->block_start(boundary);
 412       HeapWord* begin = boundary;             // Until proven otherwise.
 413       HeapWord* start_block = boundary_block; // Until proven otherwise.
 414       if (boundary_block < boundary) {
 415         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 416           oop boundary_obj = oop(boundary_block);
 417           if (!boundary_obj->is_objArray() &&
 418               !boundary_obj->is_typeArray()) {
 419             guarantee(cur_entry > byte_for(used.start()),
 420                       "else boundary would be boundary_block");
 421             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
 422               begin = boundary_block + s->block_size(boundary_block);
 423               start_block = begin;
 424             }
 425           }
 426         }
 427       }
 428       // Now traverse objects until end.
 429       if (begin < end) {
 430         MemRegion mr(begin, end);
 431         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 432         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 433           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 434             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 435           }
 436         }
 437       }
 438       cur_entry = first_dirty;
 439     } else {
 440       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 441       // is a transient value, that cannot be in the card table
 442       // except during GC, and thus assert that:
 443       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 444       //        "Illegal CT value");
 445       // That however, need not hold, as will become clear in the
 446       // following...
 447 
 448       // We'd normally expect that if we are in the parallel case,
 449       // we can't have left a prev value (which would be different
 450       // from the current value) in the card table, and so we'd like to
 451       // assert that:
 452       // guarantee(cur_youngergen_card_val() == youngergen_card
 453       //           || !is_prev_youngergen_card_val(*cur_entry),
 454       //           "Illegal CT value");
 455       // That, however, may not hold occasionally, because of
 456       // CMS or MSC in the old gen. To wit, consider the
 457       // following two simple illustrative scenarios:
 458       // (a) CMS: Consider the case where a large object L
 459       //     spanning several cards is allocated in the old
 460       //     gen, and has a young gen reference stored in it, dirtying
 461       //     some interior cards. A young collection scans the card,
 462       //     finds a young ref and installs a youngergenP_n value.
 463       //     L then goes dead. Now a CMS collection starts,
 464       //     finds L dead and sweeps it up. Assume that L is
 465       //     abutting _unallocated_blk, so _unallocated_blk is
 466       //     adjusted down to (below) L. Assume further that
 467       //     no young collection intervenes during this CMS cycle.
 468       //     The next young gen cycle will not get to look at this
 469       //     youngergenP_n card since it lies in the unoccupied
 470       //     part of the space.
 471       //     Some young collections later the blocks on this
 472       //     card can be re-allocated either due to direct allocation
 473       //     or due to absorbing promotions. At this time, the
 474       //     before-gc verification will fail the above assert.
 475       // (b) MSC: In this case, an object L with a young reference
 476       //     is on a card that (therefore) holds a youngergen_n value.
 477       //     Suppose also that L lies towards the end of the used
 478       //     the used space before GC. An MSC collection
 479       //     occurs that compacts to such an extent that this
 480       //     card is no longer in the occupied part of the space.
 481       //     Since current code in MSC does not always clear cards
 482       //     in the unused part of old gen, this stale youngergen_n
 483       //     value is left behind and can later be covered by
 484       //     an object when promotion or direct allocation
 485       //     re-allocates that part of the heap.
 486       //
 487       // Fortunately, the presence of such stale card values is
 488       // "only" a minor annoyance in that subsequent young collections
 489       // might needlessly scan such cards, but would still never corrupt
 490       // the heap as a result. However, it's likely not to be a significant
 491       // performance inhibitor in practice. For instance,
 492       // some recent measurements with unoccupied cards eagerly cleared
 493       // out to maintain this invariant, showed next to no
 494       // change in young collection times; of course one can construct
 495       // degenerate examples where the cost can be significant.)
 496       // Note, in particular, that if the "stale" card is modified
 497       // after re-allocation, it would be dirty, not "stale". Thus,
 498       // we can never have a younger ref in such a card and it is
 499       // safe not to scan that card in any collection. [As we see
 500       // below, we do some unnecessary scanning
 501       // in some cases in the current parallel scanning algorithm.]
 502       //
 503       // The main point below is that the parallel card scanning code
 504       // deals correctly with these stale card values. There are two main
 505       // cases to consider where we have a stale "younger gen" value and a
 506       // "derivative" case to consider, where we have a stale
 507       // "cur_younger_gen_and_prev_non_clean" value, as will become
 508       // apparent in the case analysis below.
 509       // o Case 1. If the stale value corresponds to a younger_gen_n
 510       //   value other than the cur_younger_gen value then the code
 511       //   treats this as being tantamount to a prev_younger_gen
 512       //   card. This means that the card may be unnecessarily scanned.
 513       //   There are two sub-cases to consider:
 514       //   o Case 1a. Let us say that the card is in the occupied part
 515       //     of the generation at the time the collection begins. In
 516       //     that case the card will be either cleared when it is scanned
 517       //     for young pointers, or will be set to cur_younger_gen as a
 518       //     result of promotion. (We have elided the normal case where
 519       //     the scanning thread and the promoting thread interleave
 520       //     possibly resulting in a transient
 521       //     cur_younger_gen_and_prev_non_clean value before settling
 522       //     to cur_younger_gen. [End Case 1a.]
 523       //   o Case 1b. Consider now the case when the card is in the unoccupied
 524       //     part of the space which becomes occupied because of promotions
 525       //     into it during the current young GC. In this case the card
 526       //     will never be scanned for young references. The current
 527       //     code will set the card value to either
 528       //     cur_younger_gen_and_prev_non_clean or leave
 529       //     it with its stale value -- because the promotions didn't
 530       //     result in any younger refs on that card. Of these two
 531       //     cases, the latter will be covered in Case 1a during
 532       //     a subsequent scan. To deal with the former case, we need
 533       //     to further consider how we deal with a stale value of
 534       //     cur_younger_gen_and_prev_non_clean in our case analysis
 535       //     below. This we do in Case 3 below. [End Case 1b]
 536       //   [End Case 1]
 537       // o Case 2. If the stale value corresponds to cur_younger_gen being
 538       //   a value not necessarily written by a current promotion, the
 539       //   card will not be scanned by the younger refs scanning code.
 540       //   (This is OK since as we argued above such cards cannot contain
 541       //   any younger refs.) The result is that this value will be
 542       //   treated as a prev_younger_gen value in a subsequent collection,
 543       //   which is addressed in Case 1 above. [End Case 2]
 544       // o Case 3. We here consider the "derivative" case from Case 1b. above
 545       //   because of which we may find a stale
 546       //   cur_younger_gen_and_prev_non_clean card value in the table.
 547       //   Once again, as in Case 1, we consider two subcases, depending
 548       //   on whether the card lies in the occupied or unoccupied part
 549       //   of the space at the start of the young collection.
 550       //   o Case 3a. Let us say the card is in the occupied part of
 551       //     the old gen at the start of the young collection. In that
 552       //     case, the card will be scanned by the younger refs scanning
 553       //     code which will set it to cur_younger_gen. In a subsequent
 554       //     scan, the card will be considered again and get its final
 555       //     correct value. [End Case 3a]
 556       //   o Case 3b. Now consider the case where the card is in the
 557       //     unoccupied part of the old gen, and is occupied as a result
 558       //     of promotions during thus young gc. In that case,
 559       //     the card will not be scanned for younger refs. The presence
 560       //     of newly promoted objects on the card will then result in
 561       //     its keeping the value cur_younger_gen_and_prev_non_clean
 562       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 563       //   [End Case 3]
 564       //
 565       // (Please refer to the code in the helper class
 566       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 567       //
 568       // The informal arguments above can be tightened into a formal
 569       // correctness proof and it behooves us to write up such a proof,
 570       // or to use model checking to prove that there are no lingering
 571       // concerns.
 572       //
 573       // Clearly because of Case 3b one cannot bound the time for
 574       // which a card will retain what we have called a "stale" value.
 575       // However, one can obtain a Loose upper bound on the redundant
 576       // work as a result of such stale values. Note first that any
 577       // time a stale card lies in the occupied part of the space at
 578       // the start of the collection, it is scanned by younger refs
 579       // code and we can define a rank function on card values that
 580       // declines when this is so. Note also that when a card does not
 581       // lie in the occupied part of the space at the beginning of a
 582       // young collection, its rank can either decline or stay unchanged.
 583       // In this case, no extra work is done in terms of redundant
 584       // younger refs scanning of that card.
 585       // Then, the case analysis above reveals that, in the worst case,
 586       // any such stale card will be scanned unnecessarily at most twice.
 587       //
 588       // It is nonetheless advisable to try and get rid of some of this
 589       // redundant work in a subsequent (low priority) re-design of
 590       // the card-scanning code, if only to simplify the underlying
 591       // state machine analysis/proof. ysr 1/28/2002. XXX
 592       cur_entry++;
 593     }
 594   }
 595 }
 596 
 597 void CardTableRS::verify() {
 598   // At present, we only know how to verify the card table RS for
 599   // generational heaps.
 600   VerifyCTGenClosure blk(this);
 601   GenCollectedHeap::heap()->generation_iterate(&blk, false);
 602   _ct_bs->verify();
 603 }