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.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 #if INCLUDE_ALL_GCS
  37 #include "gc_implementation/g1/concurrentMark.hpp"
  38 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
  39 #endif // INCLUDE_ALL_GCS
  40 
  41 CardTableRS::CardTableRS(MemRegion whole_heap) :
  42   GenRemSet(),
  43   _cur_youngergen_card_val(youngergenP1_card)
  44 {
  45 #if INCLUDE_ALL_GCS
  46   if (UseG1GC) {
  47       _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap);
  48   } else {
  49     _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  50   }
  51 #else
  52   _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  53 #endif
  54   _ct_bs->initialize();
  55   set_bs(_ct_bs);
  56   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,
  57                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  58   if (_last_cur_val_in_gen == NULL) {
  59     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  60   }
  61   for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
  62     _last_cur_val_in_gen[i] = clean_card_val();
  63   }
  64   _ct_bs->set_CTRS(this);
  65 }
  66 
  67 CardTableRS::~CardTableRS() {
  68   if (_ct_bs) {
  69     delete _ct_bs;
  70     _ct_bs = NULL;
  71   }
  72   if (_last_cur_val_in_gen) {
  73     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
  74   }
  75 }
  76 
  77 void CardTableRS::resize_covered_region(MemRegion new_region) {
  78   _ct_bs->resize_covered_region(new_region);
  79 }
  80 
  81 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  82   for (jbyte 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) return v;
  93   }
  94   ShouldNotReachHere();
  95   return 0;
  96 }
  97 
  98 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
  99   // Parallel or sequential, we must always set the prev to equal the
 100   // last one written.
 101   if (parallel) {
 102     // Find a parallel value to be used next.
 103     jbyte next_val = find_unused_youngergenP_card_value();
 104     set_cur_youngergen_card_val(next_val);
 105 
 106   } else {
 107     // In an sequential traversal we will always write youngergen, so that
 108     // the inline barrier is  correct.
 109     set_cur_youngergen_card_val(youngergen_card);
 110   }
 111 }
 112 
 113 void CardTableRS::younger_refs_iterate(Generation* g,
 114                                        OopsInGenClosure* blk) {
 115   _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
 116   g->younger_refs_iterate(blk);
 117 }
 118 
 119 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 120   if (_is_par) {
 121     return clear_card_parallel(entry);
 122   } else {
 123     return clear_card_serial(entry);
 124   }
 125 }
 126 
 127 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 128   while (true) {
 129     // In the parallel case, we may have to do this several times.
 130     jbyte entry_val = *entry;
 131     assert(entry_val != CardTableRS::clean_card_val(),
 132            "We shouldn't be looking at clean cards, and this should "
 133            "be the only place they get cleaned.");
 134     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 135         || _ct->is_prev_youngergen_card_val(entry_val)) {
 136       jbyte res =
 137         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 138       if (res == entry_val) {
 139         break;
 140       } else {
 141         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 142                "The CAS above should only fail if another thread did "
 143                "a GC write barrier.");
 144       }
 145     } else if (entry_val ==
 146                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 147       // Parallelism shouldn't matter in this case.  Only the thread
 148       // assigned to scan the card should change this value.
 149       *entry = _ct->cur_youngergen_card_val();
 150       break;
 151     } else {
 152       assert(entry_val == _ct->cur_youngergen_card_val(),
 153              "Should be the only possibility.");
 154       // In this case, the card was clean before, and become
 155       // cur_youngergen only because of processing of a promoted object.
 156       // We don't have to look at the card.
 157       return false;
 158     }
 159   }
 160   return true;
 161 }
 162 
 163 
 164 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 165   jbyte entry_val = *entry;
 166   assert(entry_val != CardTableRS::clean_card_val(),
 167          "We shouldn't be looking at clean cards, and this should "
 168          "be the only place they get cleaned.");
 169   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 170          "This should be possible in the sequential case.");
 171   *entry = CardTableRS::clean_card_val();
 172   return true;
 173 }
 174 
 175 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 176   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
 177     _dirty_card_closure(dirty_card_closure), _ct(ct) {
 178     // Cannot yet substitute active_workers for n_par_threads
 179     // in the case where parallelism is being turned off by
 180     // setting n_par_threads to 0.
 181     _is_par = (SharedHeap::heap()->n_par_threads() > 0);
 182     assert(!_is_par ||
 183            (SharedHeap::heap()->n_par_threads() ==
 184             SharedHeap::heap()->workers()->active_workers()), "Mismatch");
 185 }
 186 
 187 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 188   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 189 }
 190 
 191 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 192   assert(mr.word_size() > 0, "Error");
 193   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 194   // mr.end() may not necessarily be card aligned.
 195   jbyte* cur_entry = _ct->byte_for(mr.last());
 196   const jbyte* limit = _ct->byte_for(mr.start());
 197   HeapWord* end_of_non_clean = mr.end();
 198   HeapWord* start_of_non_clean = end_of_non_clean;
 199   while (cur_entry >= limit) {
 200     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 201     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 202       // Continue the dirty range by opening the
 203       // dirty window one card to the left.
 204       start_of_non_clean = cur_hw;
 205     } else {
 206       // We hit a "clean" card; process any non-empty
 207       // "dirty" range accumulated so far.
 208       if (start_of_non_clean < end_of_non_clean) {
 209         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 210         _dirty_card_closure->do_MemRegion(mrd);
 211       }
 212 
 213       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 214       if (is_word_aligned(cur_entry)) {
 215         jbyte* cur_row = cur_entry - BytesPerWord;
 216         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 217           cur_row -= BytesPerWord;
 218         }
 219         cur_entry = cur_row + BytesPerWord;
 220         cur_hw = _ct->addr_for(cur_entry);
 221       }
 222 
 223       // Reset the dirty window, while continuing to look
 224       // for the next dirty card that will start a
 225       // new dirty window.
 226       end_of_non_clean = cur_hw;
 227       start_of_non_clean = cur_hw;
 228     }
 229     // Note that "cur_entry" leads "start_of_non_clean" in
 230     // its leftward excursion after this point
 231     // in the loop and, when we hit the left end of "mr",
 232     // will point off of the left end of the card-table
 233     // for "mr".
 234     cur_entry--;
 235   }
 236   // If the first card of "mr" was dirty, we will have
 237   // been left with a dirty window, co-initial with "mr",
 238   // which we now process.
 239   if (start_of_non_clean < end_of_non_clean) {
 240     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 241     _dirty_card_closure->do_MemRegion(mrd);
 242   }
 243 }
 244 
 245 // clean (by dirty->clean before) ==> cur_younger_gen
 246 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 247 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 248 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 249 // cur-younger-gen                ==> cur_younger_gen
 250 // cur_youngergen_and_prev_nonclean_card ==> no change.
 251 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 252   jbyte* entry = ct_bs()->byte_for(field);
 253   do {
 254     jbyte entry_val = *entry;
 255     // We put this first because it's probably the most common case.
 256     if (entry_val == clean_card_val()) {
 257       // No threat of contention with cleaning threads.
 258       *entry = cur_youngergen_card_val();
 259       return;
 260     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 261                || is_prev_youngergen_card_val(entry_val)) {
 262       // Mark it as both cur and prev youngergen; card cleaning thread will
 263       // eventually remove the previous stuff.
 264       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 265       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 266       // Did the CAS succeed?
 267       if (res == entry_val) return;
 268       // Otherwise, retry, to see the new value.
 269       continue;
 270     } else {
 271       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 272              || entry_val == cur_youngergen_card_val(),
 273              "should be only possibilities.");
 274       return;
 275     }
 276   } while (true);
 277 }
 278 
 279 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 280                                                 OopsInGenClosure* cl) {
 281   const MemRegion urasm = sp->used_region_at_save_marks();
 282 #ifdef ASSERT
 283   // Convert the assertion check to a warning if we are running
 284   // CMS+ParNew until related bug is fixed.
 285   MemRegion ur    = sp->used_region();
 286   assert(ur.contains(urasm) || (UseConcMarkSweepGC),
 287          err_msg("Did you forget to call save_marks()? "
 288                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 289                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
 290                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
 291   // In the case of CMS+ParNew, issue a warning
 292   if (!ur.contains(urasm)) {
 293     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 294     warning("CMS+ParNew: Did you forget to call save_marks()? "
 295             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 296             "[" PTR_FORMAT ", " PTR_FORMAT ")",
 297              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 298     MemRegion ur2 = sp->used_region();
 299     MemRegion urasm2 = sp->used_region_at_save_marks();
 300     if (!ur.equals(ur2)) {
 301       warning("CMS+ParNew: Flickering used_region()!!");
 302     }
 303     if (!urasm.equals(urasm2)) {
 304       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 305     }
 306     ShouldNotReachHere();
 307   }
 308 #endif
 309   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
 310 }
 311 
 312 void CardTableRS::clear_into_younger(Generation* old_gen) {
 313   assert(old_gen->level() == 1, "Should only be called for the old generation");
 314   // The card tables for the youngest gen need never be cleared.
 315   // There's a bit of subtlety in the clear() and invalidate()
 316   // methods that we exploit here and in invalidate_or_clear()
 317   // below to avoid missing cards at the fringes. If clear() or
 318   // invalidate() are changed in the future, this code should
 319   // be revisited. 20040107.ysr
 320   clear(old_gen->prev_used_region());
 321 }
 322 
 323 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 324   assert(old_gen->level() == 1, "Should only be called for the old generation");
 325   // Invalidate the cards for the currently occupied part of
 326   // the old generation and clear the cards for the
 327   // unoccupied part of the generation (if any, making use
 328   // of that generation's prev_used_region to determine that
 329   // region). No need to do anything for the youngest
 330   // generation. Also see note#20040107.ysr above.
 331   MemRegion used_mr = old_gen->used_region();
 332   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 333   if (!to_be_cleared_mr.is_empty()) {
 334     clear(to_be_cleared_mr);
 335   }
 336   invalidate(used_mr);
 337 }
 338 
 339 
 340 class VerifyCleanCardClosure: public OopClosure {
 341 private:
 342   HeapWord* _boundary;
 343   HeapWord* _begin;
 344   HeapWord* _end;
 345 protected:
 346   template <class T> void do_oop_work(T* p) {
 347     HeapWord* jp = (HeapWord*)p;
 348     assert(jp >= _begin && jp < _end,
 349            err_msg("Error: jp " PTR_FORMAT " should be within "
 350                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 351                    p2i(jp), p2i(_begin), p2i(_end)));
 352     oop obj = oopDesc::load_decode_heap_oop(p);
 353     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 354               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 355                       "clean card crosses boundary" PTR_FORMAT,
 356                       p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
 357   }
 358 
 359 public:
 360   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 361     _boundary(b), _begin(begin), _end(end) {
 362     assert(b <= begin,
 363            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 364                    p2i(b), p2i(begin)));
 365     assert(begin <= end,
 366            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 367                    p2i(begin), p2i(end)));
 368   }
 369 
 370   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 371   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 372 };
 373 
 374 class VerifyCTSpaceClosure: public SpaceClosure {
 375 private:
 376   CardTableRS* _ct;
 377   HeapWord* _boundary;
 378 public:
 379   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 380     _ct(ct), _boundary(boundary) {}
 381   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 382 };
 383 
 384 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 385   CardTableRS* _ct;
 386 public:
 387   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 388   void do_generation(Generation* gen) {
 389     // Skip the youngest generation.
 390     if (gen->level() == 0) return;
 391     // Normally, we're interested in pointers to younger generations.
 392     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 393     gen->space_iterate(&blk, true);
 394   }
 395 };
 396 
 397 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 398   // We don't need to do young-gen spaces.
 399   if (s->end() <= gen_boundary) return;
 400   MemRegion used = s->used_region();
 401 
 402   jbyte* cur_entry = byte_for(used.start());
 403   jbyte* limit = byte_after(used.last());
 404   while (cur_entry < limit) {
 405     if (*cur_entry == CardTableModRefBS::clean_card) {
 406       jbyte* first_dirty = cur_entry+1;
 407       while (first_dirty < limit &&
 408              *first_dirty == CardTableModRefBS::clean_card) {
 409         first_dirty++;
 410       }
 411       // If the first object is a regular object, and it has a
 412       // young-to-old field, that would mark the previous card.
 413       HeapWord* boundary = addr_for(cur_entry);
 414       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 415       HeapWord* boundary_block = s->block_start(boundary);
 416       HeapWord* begin = boundary;             // Until proven otherwise.
 417       HeapWord* start_block = boundary_block; // Until proven otherwise.
 418       if (boundary_block < boundary) {
 419         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 420           oop boundary_obj = oop(boundary_block);
 421           if (!boundary_obj->is_objArray() &&
 422               !boundary_obj->is_typeArray()) {
 423             guarantee(cur_entry > byte_for(used.start()),
 424                       "else boundary would be boundary_block");
 425             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
 426               begin = boundary_block + s->block_size(boundary_block);
 427               start_block = begin;
 428             }
 429           }
 430         }
 431       }
 432       // Now traverse objects until end.
 433       if (begin < end) {
 434         MemRegion mr(begin, end);
 435         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 436         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 437           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 438             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 439           }
 440         }
 441       }
 442       cur_entry = first_dirty;
 443     } else {
 444       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 445       // is a transient value, that cannot be in the card table
 446       // except during GC, and thus assert that:
 447       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 448       //        "Illegal CT value");
 449       // That however, need not hold, as will become clear in the
 450       // following...
 451 
 452       // We'd normally expect that if we are in the parallel case,
 453       // we can't have left a prev value (which would be different
 454       // from the current value) in the card table, and so we'd like to
 455       // assert that:
 456       // guarantee(cur_youngergen_card_val() == youngergen_card
 457       //           || !is_prev_youngergen_card_val(*cur_entry),
 458       //           "Illegal CT value");
 459       // That, however, may not hold occasionally, because of
 460       // CMS or MSC in the old gen. To wit, consider the
 461       // following two simple illustrative scenarios:
 462       // (a) CMS: Consider the case where a large object L
 463       //     spanning several cards is allocated in the old
 464       //     gen, and has a young gen reference stored in it, dirtying
 465       //     some interior cards. A young collection scans the card,
 466       //     finds a young ref and installs a youngergenP_n value.
 467       //     L then goes dead. Now a CMS collection starts,
 468       //     finds L dead and sweeps it up. Assume that L is
 469       //     abutting _unallocated_blk, so _unallocated_blk is
 470       //     adjusted down to (below) L. Assume further that
 471       //     no young collection intervenes during this CMS cycle.
 472       //     The next young gen cycle will not get to look at this
 473       //     youngergenP_n card since it lies in the unoccupied
 474       //     part of the space.
 475       //     Some young collections later the blocks on this
 476       //     card can be re-allocated either due to direct allocation
 477       //     or due to absorbing promotions. At this time, the
 478       //     before-gc verification will fail the above assert.
 479       // (b) MSC: In this case, an object L with a young reference
 480       //     is on a card that (therefore) holds a youngergen_n value.
 481       //     Suppose also that L lies towards the end of the used
 482       //     the used space before GC. An MSC collection
 483       //     occurs that compacts to such an extent that this
 484       //     card is no longer in the occupied part of the space.
 485       //     Since current code in MSC does not always clear cards
 486       //     in the unused part of old gen, this stale youngergen_n
 487       //     value is left behind and can later be covered by
 488       //     an object when promotion or direct allocation
 489       //     re-allocates that part of the heap.
 490       //
 491       // Fortunately, the presence of such stale card values is
 492       // "only" a minor annoyance in that subsequent young collections
 493       // might needlessly scan such cards, but would still never corrupt
 494       // the heap as a result. However, it's likely not to be a significant
 495       // performance inhibitor in practice. For instance,
 496       // some recent measurements with unoccupied cards eagerly cleared
 497       // out to maintain this invariant, showed next to no
 498       // change in young collection times; of course one can construct
 499       // degenerate examples where the cost can be significant.)
 500       // Note, in particular, that if the "stale" card is modified
 501       // after re-allocation, it would be dirty, not "stale". Thus,
 502       // we can never have a younger ref in such a card and it is
 503       // safe not to scan that card in any collection. [As we see
 504       // below, we do some unnecessary scanning
 505       // in some cases in the current parallel scanning algorithm.]
 506       //
 507       // The main point below is that the parallel card scanning code
 508       // deals correctly with these stale card values. There are two main
 509       // cases to consider where we have a stale "younger gen" value and a
 510       // "derivative" case to consider, where we have a stale
 511       // "cur_younger_gen_and_prev_non_clean" value, as will become
 512       // apparent in the case analysis below.
 513       // o Case 1. If the stale value corresponds to a younger_gen_n
 514       //   value other than the cur_younger_gen value then the code
 515       //   treats this as being tantamount to a prev_younger_gen
 516       //   card. This means that the card may be unnecessarily scanned.
 517       //   There are two sub-cases to consider:
 518       //   o Case 1a. Let us say that the card is in the occupied part
 519       //     of the generation at the time the collection begins. In
 520       //     that case the card will be either cleared when it is scanned
 521       //     for young pointers, or will be set to cur_younger_gen as a
 522       //     result of promotion. (We have elided the normal case where
 523       //     the scanning thread and the promoting thread interleave
 524       //     possibly resulting in a transient
 525       //     cur_younger_gen_and_prev_non_clean value before settling
 526       //     to cur_younger_gen. [End Case 1a.]
 527       //   o Case 1b. Consider now the case when the card is in the unoccupied
 528       //     part of the space which becomes occupied because of promotions
 529       //     into it during the current young GC. In this case the card
 530       //     will never be scanned for young references. The current
 531       //     code will set the card value to either
 532       //     cur_younger_gen_and_prev_non_clean or leave
 533       //     it with its stale value -- because the promotions didn't
 534       //     result in any younger refs on that card. Of these two
 535       //     cases, the latter will be covered in Case 1a during
 536       //     a subsequent scan. To deal with the former case, we need
 537       //     to further consider how we deal with a stale value of
 538       //     cur_younger_gen_and_prev_non_clean in our case analysis
 539       //     below. This we do in Case 3 below. [End Case 1b]
 540       //   [End Case 1]
 541       // o Case 2. If the stale value corresponds to cur_younger_gen being
 542       //   a value not necessarily written by a current promotion, the
 543       //   card will not be scanned by the younger refs scanning code.
 544       //   (This is OK since as we argued above such cards cannot contain
 545       //   any younger refs.) The result is that this value will be
 546       //   treated as a prev_younger_gen value in a subsequent collection,
 547       //   which is addressed in Case 1 above. [End Case 2]
 548       // o Case 3. We here consider the "derivative" case from Case 1b. above
 549       //   because of which we may find a stale
 550       //   cur_younger_gen_and_prev_non_clean card value in the table.
 551       //   Once again, as in Case 1, we consider two subcases, depending
 552       //   on whether the card lies in the occupied or unoccupied part
 553       //   of the space at the start of the young collection.
 554       //   o Case 3a. Let us say the card is in the occupied part of
 555       //     the old gen at the start of the young collection. In that
 556       //     case, the card will be scanned by the younger refs scanning
 557       //     code which will set it to cur_younger_gen. In a subsequent
 558       //     scan, the card will be considered again and get its final
 559       //     correct value. [End Case 3a]
 560       //   o Case 3b. Now consider the case where the card is in the
 561       //     unoccupied part of the old gen, and is occupied as a result
 562       //     of promotions during thus young gc. In that case,
 563       //     the card will not be scanned for younger refs. The presence
 564       //     of newly promoted objects on the card will then result in
 565       //     its keeping the value cur_younger_gen_and_prev_non_clean
 566       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 567       //   [End Case 3]
 568       //
 569       // (Please refer to the code in the helper class
 570       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 571       //
 572       // The informal arguments above can be tightened into a formal
 573       // correctness proof and it behooves us to write up such a proof,
 574       // or to use model checking to prove that there are no lingering
 575       // concerns.
 576       //
 577       // Clearly because of Case 3b one cannot bound the time for
 578       // which a card will retain what we have called a "stale" value.
 579       // However, one can obtain a Loose upper bound on the redundant
 580       // work as a result of such stale values. Note first that any
 581       // time a stale card lies in the occupied part of the space at
 582       // the start of the collection, it is scanned by younger refs
 583       // code and we can define a rank function on card values that
 584       // declines when this is so. Note also that when a card does not
 585       // lie in the occupied part of the space at the beginning of a
 586       // young collection, its rank can either decline or stay unchanged.
 587       // In this case, no extra work is done in terms of redundant
 588       // younger refs scanning of that card.
 589       // Then, the case analysis above reveals that, in the worst case,
 590       // any such stale card will be scanned unnecessarily at most twice.
 591       //
 592       // It is nonetheless advisable to try and get rid of some of this
 593       // redundant work in a subsequent (low priority) re-design of
 594       // the card-scanning code, if only to simplify the underlying
 595       // state machine analysis/proof. ysr 1/28/2002. XXX
 596       cur_entry++;
 597     }
 598   }
 599 }
 600 
 601 void CardTableRS::verify() {
 602   // At present, we only know how to verify the card table RS for
 603   // generational heaps.
 604   VerifyCTGenClosure blk(this);
 605   CollectedHeap* ch = Universe::heap();
 606 
 607   if (ch->kind() == CollectedHeap::GenCollectedHeap) {
 608     GenCollectedHeap::heap()->generation_iterate(&blk, false);
 609     _ct_bs->verify();
 610     }
 611   }