1 /*
   2  * Copyright (c) 2001, 2014, 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 "memory/allocation.inline.hpp"
  27 #include "memory/cardTableRS.hpp"
  28 #include "memory/genCollectedHeap.hpp"
  29 #include "memory/generation.hpp"
  30 #include "memory/space.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   guarantee(Universe::heap()->kind() == CollectedHeap::GenCollectedHeap, "sanity");
  42   _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  43   _ct_bs->initialize();
  44   set_bs(_ct_bs);
  45   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,
  46                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  47   if (_last_cur_val_in_gen == NULL) {
  48     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  49   }
  50   for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
  51     _last_cur_val_in_gen[i] = clean_card_val();
  52   }
  53   _ct_bs->set_CTRS(this);
  54 }
  55 
  56 CardTableRS::~CardTableRS() {
  57   if (_ct_bs) {
  58     delete _ct_bs;
  59     _ct_bs = NULL;
  60   }
  61   if (_last_cur_val_in_gen) {
  62     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
  63   }
  64 }
  65 
  66 void CardTableRS::resize_covered_region(MemRegion new_region) {
  67   _ct_bs->resize_covered_region(new_region);
  68 }
  69 
  70 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  71   for (jbyte v = youngergenP1_card;
  72        v < cur_youngergen_and_prev_nonclean_card;
  73        v++) {
  74     bool seen = false;
  75     for (int g = 0; g < _regions_to_iterate; g++) {
  76       if (_last_cur_val_in_gen[g] == v) {
  77         seen = true;
  78         break;
  79       }
  80     }
  81     if (!seen) return v;
  82   }
  83   ShouldNotReachHere();
  84   return 0;
  85 }
  86 
  87 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
  88   // Parallel or sequential, we must always set the prev to equal the
  89   // last one written.
  90   if (parallel) {
  91     // Find a parallel value to be used next.
  92     jbyte next_val = find_unused_youngergenP_card_value();
  93     set_cur_youngergen_card_val(next_val);
  94 
  95   } else {
  96     // In an sequential traversal we will always write youngergen, so that
  97     // the inline barrier is  correct.
  98     set_cur_youngergen_card_val(youngergen_card);
  99   }
 100 }
 101 
 102 void CardTableRS::younger_refs_iterate(Generation* g,
 103                                        OopsInGenClosure* blk) {
 104   _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
 105   g->younger_refs_iterate(blk);
 106 }
 107 
 108 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 109   if (_is_par) {
 110     return clear_card_parallel(entry);
 111   } else {
 112     return clear_card_serial(entry);
 113   }
 114 }
 115 
 116 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 117   while (true) {
 118     // In the parallel case, we may have to do this several times.
 119     jbyte entry_val = *entry;
 120     assert(entry_val != CardTableRS::clean_card_val(),
 121            "We shouldn't be looking at clean cards, and this should "
 122            "be the only place they get cleaned.");
 123     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 124         || _ct->is_prev_youngergen_card_val(entry_val)) {
 125       jbyte res =
 126         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 127       if (res == entry_val) {
 128         break;
 129       } else {
 130         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 131                "The CAS above should only fail if another thread did "
 132                "a GC write barrier.");
 133       }
 134     } else if (entry_val ==
 135                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 136       // Parallelism shouldn't matter in this case.  Only the thread
 137       // assigned to scan the card should change this value.
 138       *entry = _ct->cur_youngergen_card_val();
 139       break;
 140     } else {
 141       assert(entry_val == _ct->cur_youngergen_card_val(),
 142              "Should be the only possibility.");
 143       // In this case, the card was clean before, and become
 144       // cur_youngergen only because of processing of a promoted object.
 145       // We don't have to look at the card.
 146       return false;
 147     }
 148   }
 149   return true;
 150 }
 151 
 152 
 153 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 154   jbyte entry_val = *entry;
 155   assert(entry_val != CardTableRS::clean_card_val(),
 156          "We shouldn't be looking at clean cards, and this should "
 157          "be the only place they get cleaned.");
 158   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 159          "This should be possible in the sequential case.");
 160   *entry = CardTableRS::clean_card_val();
 161   return true;
 162 }
 163 
 164 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 165   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
 166     _dirty_card_closure(dirty_card_closure), _ct(ct) {
 167     // Cannot yet substitute active_workers for n_par_threads
 168     // in the case where parallelism is being turned off by
 169     // setting n_par_threads to 0.
 170     _is_par = (GenCollectedHeap::heap()->n_par_threads() > 0);
 171     assert(!_is_par ||
 172            (GenCollectedHeap::heap()->n_par_threads() ==
 173             GenCollectedHeap::heap()->workers()->active_workers()), "Mismatch");
 174 }
 175 
 176 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 177   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 178 }
 179 
 180 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 181   assert(mr.word_size() > 0, "Error");
 182   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 183   // mr.end() may not necessarily be card aligned.
 184   jbyte* cur_entry = _ct->byte_for(mr.last());
 185   const jbyte* limit = _ct->byte_for(mr.start());
 186   HeapWord* end_of_non_clean = mr.end();
 187   HeapWord* start_of_non_clean = end_of_non_clean;
 188   while (cur_entry >= limit) {
 189     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 190     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 191       // Continue the dirty range by opening the
 192       // dirty window one card to the left.
 193       start_of_non_clean = cur_hw;
 194     } else {
 195       // We hit a "clean" card; process any non-empty
 196       // "dirty" range accumulated so far.
 197       if (start_of_non_clean < end_of_non_clean) {
 198         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 199         _dirty_card_closure->do_MemRegion(mrd);
 200       }
 201 
 202       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 203       if (is_word_aligned(cur_entry)) {
 204         jbyte* cur_row = cur_entry - BytesPerWord;
 205         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 206           cur_row -= BytesPerWord;
 207         }
 208         cur_entry = cur_row + BytesPerWord;
 209         cur_hw = _ct->addr_for(cur_entry);
 210       }
 211 
 212       // Reset the dirty window, while continuing to look
 213       // for the next dirty card that will start a
 214       // new dirty window.
 215       end_of_non_clean = cur_hw;
 216       start_of_non_clean = cur_hw;
 217     }
 218     // Note that "cur_entry" leads "start_of_non_clean" in
 219     // its leftward excursion after this point
 220     // in the loop and, when we hit the left end of "mr",
 221     // will point off of the left end of the card-table
 222     // for "mr".
 223     cur_entry--;
 224   }
 225   // If the first card of "mr" was dirty, we will have
 226   // been left with a dirty window, co-initial with "mr",
 227   // which we now process.
 228   if (start_of_non_clean < end_of_non_clean) {
 229     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 230     _dirty_card_closure->do_MemRegion(mrd);
 231   }
 232 }
 233 
 234 // clean (by dirty->clean before) ==> cur_younger_gen
 235 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 236 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 237 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 238 // cur-younger-gen                ==> cur_younger_gen
 239 // cur_youngergen_and_prev_nonclean_card ==> no change.
 240 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 241   jbyte* entry = ct_bs()->byte_for(field);
 242   do {
 243     jbyte entry_val = *entry;
 244     // We put this first because it's probably the most common case.
 245     if (entry_val == clean_card_val()) {
 246       // No threat of contention with cleaning threads.
 247       *entry = cur_youngergen_card_val();
 248       return;
 249     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 250                || is_prev_youngergen_card_val(entry_val)) {
 251       // Mark it as both cur and prev youngergen; card cleaning thread will
 252       // eventually remove the previous stuff.
 253       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 254       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 255       // Did the CAS succeed?
 256       if (res == entry_val) return;
 257       // Otherwise, retry, to see the new value.
 258       continue;
 259     } else {
 260       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 261              || entry_val == cur_youngergen_card_val(),
 262              "should be only possibilities.");
 263       return;
 264     }
 265   } while (true);
 266 }
 267 
 268 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 269                                                 OopsInGenClosure* cl) {
 270   const MemRegion urasm = sp->used_region_at_save_marks();
 271 #ifdef ASSERT
 272   // Convert the assertion check to a warning if we are running
 273   // CMS+ParNew until related bug is fixed.
 274   MemRegion ur    = sp->used_region();
 275   assert(ur.contains(urasm) || (UseConcMarkSweepGC),
 276          err_msg("Did you forget to call save_marks()? "
 277                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 278                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
 279                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
 280   // In the case of CMS+ParNew, issue a warning
 281   if (!ur.contains(urasm)) {
 282     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 283     warning("CMS+ParNew: Did you forget to call save_marks()? "
 284             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 285             "[" PTR_FORMAT ", " PTR_FORMAT ")",
 286              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 287     MemRegion ur2 = sp->used_region();
 288     MemRegion urasm2 = sp->used_region_at_save_marks();
 289     if (!ur.equals(ur2)) {
 290       warning("CMS+ParNew: Flickering used_region()!!");
 291     }
 292     if (!urasm.equals(urasm2)) {
 293       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 294     }
 295     ShouldNotReachHere();
 296   }
 297 #endif
 298   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
 299 }
 300 
 301 void CardTableRS::clear_into_younger(Generation* old_gen) {
 302   assert(old_gen->level() == 1, "Should only be called for the old generation");
 303   // The card tables for the youngest gen need never be cleared.
 304   // There's a bit of subtlety in the clear() and invalidate()
 305   // methods that we exploit here and in invalidate_or_clear()
 306   // below to avoid missing cards at the fringes. If clear() or
 307   // invalidate() are changed in the future, this code should
 308   // be revisited. 20040107.ysr
 309   clear(old_gen->prev_used_region());
 310 }
 311 
 312 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 313   assert(old_gen->level() == 1, "Should only be called for the old generation");
 314   // Invalidate the cards for the currently occupied part of
 315   // the old generation and clear the cards for the
 316   // unoccupied part of the generation (if any, making use
 317   // of that generation's prev_used_region to determine that
 318   // region). No need to do anything for the youngest
 319   // generation. Also see note#20040107.ysr above.
 320   MemRegion used_mr = old_gen->used_region();
 321   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 322   if (!to_be_cleared_mr.is_empty()) {
 323     clear(to_be_cleared_mr);
 324   }
 325   invalidate(used_mr);
 326 }
 327 
 328 
 329 class VerifyCleanCardClosure: public OopClosure {
 330 private:
 331   HeapWord* _boundary;
 332   HeapWord* _begin;
 333   HeapWord* _end;
 334 protected:
 335   template <class T> void do_oop_work(T* p) {
 336     HeapWord* jp = (HeapWord*)p;
 337     assert(jp >= _begin && jp < _end,
 338            err_msg("Error: jp " PTR_FORMAT " should be within "
 339                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 340                    p2i(jp), p2i(_begin), p2i(_end)));
 341     oop obj = oopDesc::load_decode_heap_oop(p);
 342     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 343               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 344                       "clean card crosses boundary" PTR_FORMAT,
 345                       p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
 346   }
 347 
 348 public:
 349   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 350     _boundary(b), _begin(begin), _end(end) {
 351     assert(b <= begin,
 352            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 353                    p2i(b), p2i(begin)));
 354     assert(begin <= end,
 355            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 356                    p2i(begin), p2i(end)));
 357   }
 358 
 359   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 360   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 361 };
 362 
 363 class VerifyCTSpaceClosure: public SpaceClosure {
 364 private:
 365   CardTableRS* _ct;
 366   HeapWord* _boundary;
 367 public:
 368   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 369     _ct(ct), _boundary(boundary) {}
 370   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 371 };
 372 
 373 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 374   CardTableRS* _ct;
 375 public:
 376   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 377   void do_generation(Generation* gen) {
 378     // Skip the youngest generation.
 379     if (gen->level() == 0) return;
 380     // Normally, we're interested in pointers to younger generations.
 381     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 382     gen->space_iterate(&blk, true);
 383   }
 384 };
 385 
 386 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 387   // We don't need to do young-gen spaces.
 388   if (s->end() <= gen_boundary) return;
 389   MemRegion used = s->used_region();
 390 
 391   jbyte* cur_entry = byte_for(used.start());
 392   jbyte* limit = byte_after(used.last());
 393   while (cur_entry < limit) {
 394     if (*cur_entry == CardTableModRefBS::clean_card) {
 395       jbyte* first_dirty = cur_entry+1;
 396       while (first_dirty < limit &&
 397              *first_dirty == CardTableModRefBS::clean_card) {
 398         first_dirty++;
 399       }
 400       // If the first object is a regular object, and it has a
 401       // young-to-old field, that would mark the previous card.
 402       HeapWord* boundary = addr_for(cur_entry);
 403       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 404       HeapWord* boundary_block = s->block_start(boundary);
 405       HeapWord* begin = boundary;             // Until proven otherwise.
 406       HeapWord* start_block = boundary_block; // Until proven otherwise.
 407       if (boundary_block < boundary) {
 408         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 409           oop boundary_obj = oop(boundary_block);
 410           if (!boundary_obj->is_objArray() &&
 411               !boundary_obj->is_typeArray()) {
 412             guarantee(cur_entry > byte_for(used.start()),
 413                       "else boundary would be boundary_block");
 414             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
 415               begin = boundary_block + s->block_size(boundary_block);
 416               start_block = begin;
 417             }
 418           }
 419         }
 420       }
 421       // Now traverse objects until end.
 422       if (begin < end) {
 423         MemRegion mr(begin, end);
 424         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 425         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 426           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 427             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 428           }
 429         }
 430       }
 431       cur_entry = first_dirty;
 432     } else {
 433       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 434       // is a transient value, that cannot be in the card table
 435       // except during GC, and thus assert that:
 436       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 437       //        "Illegal CT value");
 438       // That however, need not hold, as will become clear in the
 439       // following...
 440 
 441       // We'd normally expect that if we are in the parallel case,
 442       // we can't have left a prev value (which would be different
 443       // from the current value) in the card table, and so we'd like to
 444       // assert that:
 445       // guarantee(cur_youngergen_card_val() == youngergen_card
 446       //           || !is_prev_youngergen_card_val(*cur_entry),
 447       //           "Illegal CT value");
 448       // That, however, may not hold occasionally, because of
 449       // CMS or MSC in the old gen. To wit, consider the
 450       // following two simple illustrative scenarios:
 451       // (a) CMS: Consider the case where a large object L
 452       //     spanning several cards is allocated in the old
 453       //     gen, and has a young gen reference stored in it, dirtying
 454       //     some interior cards. A young collection scans the card,
 455       //     finds a young ref and installs a youngergenP_n value.
 456       //     L then goes dead. Now a CMS collection starts,
 457       //     finds L dead and sweeps it up. Assume that L is
 458       //     abutting _unallocated_blk, so _unallocated_blk is
 459       //     adjusted down to (below) L. Assume further that
 460       //     no young collection intervenes during this CMS cycle.
 461       //     The next young gen cycle will not get to look at this
 462       //     youngergenP_n card since it lies in the unoccupied
 463       //     part of the space.
 464       //     Some young collections later the blocks on this
 465       //     card can be re-allocated either due to direct allocation
 466       //     or due to absorbing promotions. At this time, the
 467       //     before-gc verification will fail the above assert.
 468       // (b) MSC: In this case, an object L with a young reference
 469       //     is on a card that (therefore) holds a youngergen_n value.
 470       //     Suppose also that L lies towards the end of the used
 471       //     the used space before GC. An MSC collection
 472       //     occurs that compacts to such an extent that this
 473       //     card is no longer in the occupied part of the space.
 474       //     Since current code in MSC does not always clear cards
 475       //     in the unused part of old gen, this stale youngergen_n
 476       //     value is left behind and can later be covered by
 477       //     an object when promotion or direct allocation
 478       //     re-allocates that part of the heap.
 479       //
 480       // Fortunately, the presence of such stale card values is
 481       // "only" a minor annoyance in that subsequent young collections
 482       // might needlessly scan such cards, but would still never corrupt
 483       // the heap as a result. However, it's likely not to be a significant
 484       // performance inhibitor in practice. For instance,
 485       // some recent measurements with unoccupied cards eagerly cleared
 486       // out to maintain this invariant, showed next to no
 487       // change in young collection times; of course one can construct
 488       // degenerate examples where the cost can be significant.)
 489       // Note, in particular, that if the "stale" card is modified
 490       // after re-allocation, it would be dirty, not "stale". Thus,
 491       // we can never have a younger ref in such a card and it is
 492       // safe not to scan that card in any collection. [As we see
 493       // below, we do some unnecessary scanning
 494       // in some cases in the current parallel scanning algorithm.]
 495       //
 496       // The main point below is that the parallel card scanning code
 497       // deals correctly with these stale card values. There are two main
 498       // cases to consider where we have a stale "younger gen" value and a
 499       // "derivative" case to consider, where we have a stale
 500       // "cur_younger_gen_and_prev_non_clean" value, as will become
 501       // apparent in the case analysis below.
 502       // o Case 1. If the stale value corresponds to a younger_gen_n
 503       //   value other than the cur_younger_gen value then the code
 504       //   treats this as being tantamount to a prev_younger_gen
 505       //   card. This means that the card may be unnecessarily scanned.
 506       //   There are two sub-cases to consider:
 507       //   o Case 1a. Let us say that the card is in the occupied part
 508       //     of the generation at the time the collection begins. In
 509       //     that case the card will be either cleared when it is scanned
 510       //     for young pointers, or will be set to cur_younger_gen as a
 511       //     result of promotion. (We have elided the normal case where
 512       //     the scanning thread and the promoting thread interleave
 513       //     possibly resulting in a transient
 514       //     cur_younger_gen_and_prev_non_clean value before settling
 515       //     to cur_younger_gen. [End Case 1a.]
 516       //   o Case 1b. Consider now the case when the card is in the unoccupied
 517       //     part of the space which becomes occupied because of promotions
 518       //     into it during the current young GC. In this case the card
 519       //     will never be scanned for young references. The current
 520       //     code will set the card value to either
 521       //     cur_younger_gen_and_prev_non_clean or leave
 522       //     it with its stale value -- because the promotions didn't
 523       //     result in any younger refs on that card. Of these two
 524       //     cases, the latter will be covered in Case 1a during
 525       //     a subsequent scan. To deal with the former case, we need
 526       //     to further consider how we deal with a stale value of
 527       //     cur_younger_gen_and_prev_non_clean in our case analysis
 528       //     below. This we do in Case 3 below. [End Case 1b]
 529       //   [End Case 1]
 530       // o Case 2. If the stale value corresponds to cur_younger_gen being
 531       //   a value not necessarily written by a current promotion, the
 532       //   card will not be scanned by the younger refs scanning code.
 533       //   (This is OK since as we argued above such cards cannot contain
 534       //   any younger refs.) The result is that this value will be
 535       //   treated as a prev_younger_gen value in a subsequent collection,
 536       //   which is addressed in Case 1 above. [End Case 2]
 537       // o Case 3. We here consider the "derivative" case from Case 1b. above
 538       //   because of which we may find a stale
 539       //   cur_younger_gen_and_prev_non_clean card value in the table.
 540       //   Once again, as in Case 1, we consider two subcases, depending
 541       //   on whether the card lies in the occupied or unoccupied part
 542       //   of the space at the start of the young collection.
 543       //   o Case 3a. Let us say the card is in the occupied part of
 544       //     the old gen at the start of the young collection. In that
 545       //     case, the card will be scanned by the younger refs scanning
 546       //     code which will set it to cur_younger_gen. In a subsequent
 547       //     scan, the card will be considered again and get its final
 548       //     correct value. [End Case 3a]
 549       //   o Case 3b. Now consider the case where the card is in the
 550       //     unoccupied part of the old gen, and is occupied as a result
 551       //     of promotions during thus young gc. In that case,
 552       //     the card will not be scanned for younger refs. The presence
 553       //     of newly promoted objects on the card will then result in
 554       //     its keeping the value cur_younger_gen_and_prev_non_clean
 555       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 556       //   [End Case 3]
 557       //
 558       // (Please refer to the code in the helper class
 559       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 560       //
 561       // The informal arguments above can be tightened into a formal
 562       // correctness proof and it behooves us to write up such a proof,
 563       // or to use model checking to prove that there are no lingering
 564       // concerns.
 565       //
 566       // Clearly because of Case 3b one cannot bound the time for
 567       // which a card will retain what we have called a "stale" value.
 568       // However, one can obtain a Loose upper bound on the redundant
 569       // work as a result of such stale values. Note first that any
 570       // time a stale card lies in the occupied part of the space at
 571       // the start of the collection, it is scanned by younger refs
 572       // code and we can define a rank function on card values that
 573       // declines when this is so. Note also that when a card does not
 574       // lie in the occupied part of the space at the beginning of a
 575       // young collection, its rank can either decline or stay unchanged.
 576       // In this case, no extra work is done in terms of redundant
 577       // younger refs scanning of that card.
 578       // Then, the case analysis above reveals that, in the worst case,
 579       // any such stale card will be scanned unnecessarily at most twice.
 580       //
 581       // It is nonetheless advisable to try and get rid of some of this
 582       // redundant work in a subsequent (low priority) re-design of
 583       // the card-scanning code, if only to simplify the underlying
 584       // state machine analysis/proof. ysr 1/28/2002. XXX
 585       cur_entry++;
 586     }
 587   }
 588 }
 589 
 590 void CardTableRS::verify() {
 591   // At present, we only know how to verify the card table RS for
 592   // generational heaps.
 593   VerifyCTGenClosure blk(this);
 594   CollectedHeap* ch = Universe::heap();
 595 
 596   if (ch->kind() == CollectedHeap::GenCollectedHeap) {
 597     GenCollectedHeap::heap()->generation_iterate(&blk, false);
 598     _ct_bs->verify();
 599     }
 600   }