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                          int max_covered_regions) :
  43   GenRemSet(),
  44   _cur_youngergen_card_val(youngergenP1_card),
  45   _regions_to_iterate(max_covered_regions - 1)
  46 {
  47 #if INCLUDE_ALL_GCS
  48   if (UseG1GC) {
  49       _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,
  50                                                   max_covered_regions);
  51   } else {
  52     _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
  53   }
  54 #else
  55   _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
  56 #endif
  57   _ct_bs->initialize();
  58   set_bs(_ct_bs);
  59   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
  60   // (which always is 2), but GenCollectedHeap has not been initialized yet.
  61   int max_gens = 2;
  62   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
  63                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  64   if (_last_cur_val_in_gen == NULL) {
  65     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  66   }
  67   for (int i = 0; i < max_gens + 1; i++) {
  68     _last_cur_val_in_gen[i] = clean_card_val();
  69   }
  70   _ct_bs->set_CTRS(this);
  71 }
  72 
  73 CardTableRS::~CardTableRS() {
  74   if (_ct_bs) {
  75     delete _ct_bs;
  76     _ct_bs = NULL;
  77   }
  78   if (_last_cur_val_in_gen) {
  79     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);
  80   }
  81 }
  82 
  83 void CardTableRS::resize_covered_region(MemRegion new_region) {
  84   _ct_bs->resize_covered_region(new_region);
  85 }
  86 
  87 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  88   for (jbyte v = youngergenP1_card;
  89        v < cur_youngergen_and_prev_nonclean_card;
  90        v++) {
  91     bool seen = false;
  92     for (int g = 0; g < _regions_to_iterate; g++) {
  93       if (_last_cur_val_in_gen[g] == v) {
  94         seen = true;
  95         break;
  96       }
  97     }
  98     if (!seen) return v;
  99   }
 100   ShouldNotReachHere();
 101   return 0;
 102 }
 103 
 104 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
 105   // Parallel or sequential, we must always set the prev to equal the
 106   // last one written.
 107   if (parallel) {
 108     // Find a parallel value to be used next.
 109     jbyte next_val = find_unused_youngergenP_card_value();
 110     set_cur_youngergen_card_val(next_val);
 111 
 112   } else {
 113     // In an sequential traversal we will always write youngergen, so that
 114     // the inline barrier is  correct.
 115     set_cur_youngergen_card_val(youngergen_card);
 116   }
 117 }
 118 
 119 void CardTableRS::younger_refs_iterate(Generation* g,
 120                                        OopsInGenClosure* blk) {
 121   _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
 122   g->younger_refs_iterate(blk);
 123 }
 124 
 125 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 126   if (_is_par) {
 127     return clear_card_parallel(entry);
 128   } else {
 129     return clear_card_serial(entry);
 130   }
 131 }
 132 
 133 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 134   while (true) {
 135     // In the parallel case, we may have to do this several times.
 136     jbyte entry_val = *entry;
 137     assert(entry_val != CardTableRS::clean_card_val(),
 138            "We shouldn't be looking at clean cards, and this should "
 139            "be the only place they get cleaned.");
 140     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 141         || _ct->is_prev_youngergen_card_val(entry_val)) {
 142       jbyte res =
 143         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 144       if (res == entry_val) {
 145         break;
 146       } else {
 147         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 148                "The CAS above should only fail if another thread did "
 149                "a GC write barrier.");
 150       }
 151     } else if (entry_val ==
 152                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 153       // Parallelism shouldn't matter in this case.  Only the thread
 154       // assigned to scan the card should change this value.
 155       *entry = _ct->cur_youngergen_card_val();
 156       break;
 157     } else {
 158       assert(entry_val == _ct->cur_youngergen_card_val(),
 159              "Should be the only possibility.");
 160       // In this case, the card was clean before, and become
 161       // cur_youngergen only because of processing of a promoted object.
 162       // We don't have to look at the card.
 163       return false;
 164     }
 165   }
 166   return true;
 167 }
 168 
 169 
 170 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 171   jbyte entry_val = *entry;
 172   assert(entry_val != CardTableRS::clean_card_val(),
 173          "We shouldn't be looking at clean cards, and this should "
 174          "be the only place they get cleaned.");
 175   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 176          "This should be possible in the sequential case.");
 177   *entry = CardTableRS::clean_card_val();
 178   return true;
 179 }
 180 
 181 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 182   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
 183     _dirty_card_closure(dirty_card_closure), _ct(ct) {
 184     // Cannot yet substitute active_workers for n_par_threads
 185     // in the case where parallelism is being turned off by
 186     // setting n_par_threads to 0.
 187     _is_par = (SharedHeap::heap()->n_par_threads() > 0);
 188     assert(!_is_par ||
 189            (SharedHeap::heap()->n_par_threads() ==
 190             SharedHeap::heap()->workers()->active_workers()), "Mismatch");
 191 }
 192 
 193 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 194   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 195 }
 196 
 197 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 198   assert(mr.word_size() > 0, "Error");
 199   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 200   // mr.end() may not necessarily be card aligned.
 201   jbyte* cur_entry = _ct->byte_for(mr.last());
 202   const jbyte* limit = _ct->byte_for(mr.start());
 203   HeapWord* end_of_non_clean = mr.end();
 204   HeapWord* start_of_non_clean = end_of_non_clean;
 205   while (cur_entry >= limit) {
 206     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 207     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 208       // Continue the dirty range by opening the
 209       // dirty window one card to the left.
 210       start_of_non_clean = cur_hw;
 211     } else {
 212       // We hit a "clean" card; process any non-empty
 213       // "dirty" range accumulated so far.
 214       if (start_of_non_clean < end_of_non_clean) {
 215         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 216         _dirty_card_closure->do_MemRegion(mrd);
 217       }
 218 
 219       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 220       if (is_word_aligned(cur_entry)) {
 221         jbyte* cur_row = cur_entry - BytesPerWord;
 222         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 223           cur_row -= BytesPerWord;
 224         }
 225         cur_entry = cur_row + BytesPerWord;
 226         cur_hw = _ct->addr_for(cur_entry);
 227       }
 228 
 229       // Reset the dirty window, while continuing to look
 230       // for the next dirty card that will start a
 231       // new dirty window.
 232       end_of_non_clean = cur_hw;
 233       start_of_non_clean = cur_hw;
 234     }
 235     // Note that "cur_entry" leads "start_of_non_clean" in
 236     // its leftward excursion after this point
 237     // in the loop and, when we hit the left end of "mr",
 238     // will point off of the left end of the card-table
 239     // for "mr".
 240     cur_entry--;
 241   }
 242   // If the first card of "mr" was dirty, we will have
 243   // been left with a dirty window, co-initial with "mr",
 244   // which we now process.
 245   if (start_of_non_clean < end_of_non_clean) {
 246     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 247     _dirty_card_closure->do_MemRegion(mrd);
 248   }
 249 }
 250 
 251 // clean (by dirty->clean before) ==> cur_younger_gen
 252 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 253 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 254 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 255 // cur-younger-gen                ==> cur_younger_gen
 256 // cur_youngergen_and_prev_nonclean_card ==> no change.
 257 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 258   jbyte* entry = ct_bs()->byte_for(field);
 259   do {
 260     jbyte entry_val = *entry;
 261     // We put this first because it's probably the most common case.
 262     if (entry_val == clean_card_val()) {
 263       // No threat of contention with cleaning threads.
 264       *entry = cur_youngergen_card_val();
 265       return;
 266     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 267                || is_prev_youngergen_card_val(entry_val)) {
 268       // Mark it as both cur and prev youngergen; card cleaning thread will
 269       // eventually remove the previous stuff.
 270       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 271       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 272       // Did the CAS succeed?
 273       if (res == entry_val) return;
 274       // Otherwise, retry, to see the new value.
 275       continue;
 276     } else {
 277       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 278              || entry_val == cur_youngergen_card_val(),
 279              "should be only possibilities.");
 280       return;
 281     }
 282   } while (true);
 283 }
 284 
 285 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 286                                                 OopsInGenClosure* cl) {
 287   const MemRegion urasm = sp->used_region_at_save_marks();
 288 #ifdef ASSERT
 289   // Convert the assertion check to a warning if we are running
 290   // CMS+ParNew until related bug is fixed.
 291   MemRegion ur    = sp->used_region();
 292   assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
 293          err_msg("Did you forget to call save_marks()? "
 294                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 295                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
 296                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
 297   // In the case of CMS+ParNew, issue a warning
 298   if (!ur.contains(urasm)) {
 299     assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
 300     warning("CMS+ParNew: Did you forget to call save_marks()? "
 301             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 302             "[" PTR_FORMAT ", " PTR_FORMAT ")",
 303              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 304     MemRegion ur2 = sp->used_region();
 305     MemRegion urasm2 = sp->used_region_at_save_marks();
 306     if (!ur.equals(ur2)) {
 307       warning("CMS+ParNew: Flickering used_region()!!");
 308     }
 309     if (!urasm.equals(urasm2)) {
 310       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 311     }
 312     ShouldNotReachHere();
 313   }
 314 #endif
 315   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
 316 }
 317 
 318 void CardTableRS::clear_into_younger(Generation* old_gen) {
 319   assert(old_gen->level() == 1, "Should only be called for the old generation");
 320   // The card tables for the youngest gen need never be cleared.
 321   // There's a bit of subtlety in the clear() and invalidate()
 322   // methods that we exploit here and in invalidate_or_clear()
 323   // below to avoid missing cards at the fringes. If clear() or
 324   // invalidate() are changed in the future, this code should
 325   // be revisited. 20040107.ysr
 326   clear(old_gen->prev_used_region());
 327 }
 328 
 329 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 330   assert(old_gen->level() == 1, "Should only be called for the old generation");
 331   // Invalidate the cards for the currently occupied part of
 332   // the old generation and clear the cards for the
 333   // unoccupied part of the generation (if any, making use
 334   // of that generation's prev_used_region to determine that
 335   // region). No need to do anything for the youngest
 336   // generation. Also see note#20040107.ysr above.
 337   MemRegion used_mr = old_gen->used_region();
 338   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 339   if (!to_be_cleared_mr.is_empty()) {
 340     clear(to_be_cleared_mr);
 341   }
 342   invalidate(used_mr);
 343 }
 344 
 345 
 346 class VerifyCleanCardClosure: public OopClosure {
 347 private:
 348   HeapWord* _boundary;
 349   HeapWord* _begin;
 350   HeapWord* _end;
 351 protected:
 352   template <class T> void do_oop_work(T* p) {
 353     HeapWord* jp = (HeapWord*)p;
 354     assert(jp >= _begin && jp < _end,
 355            err_msg("Error: jp " PTR_FORMAT " should be within "
 356                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 357                    p2i(jp), p2i(_begin), p2i(_end)));
 358     oop obj = oopDesc::load_decode_heap_oop(p);
 359     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 360               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 361                       "clean card crosses boundary" PTR_FORMAT,
 362                       p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
 363   }
 364 
 365 public:
 366   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 367     _boundary(b), _begin(begin), _end(end) {
 368     assert(b <= begin,
 369            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 370                    p2i(b), p2i(begin)));
 371     assert(begin <= end,
 372            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 373                    p2i(begin), p2i(end)));
 374   }
 375 
 376   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 377   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 378 };
 379 
 380 class VerifyCTSpaceClosure: public SpaceClosure {
 381 private:
 382   CardTableRS* _ct;
 383   HeapWord* _boundary;
 384 public:
 385   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 386     _ct(ct), _boundary(boundary) {}
 387   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 388 };
 389 
 390 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 391   CardTableRS* _ct;
 392 public:
 393   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 394   void do_generation(Generation* gen) {
 395     // Skip the youngest generation.
 396     if (gen->level() == 0) return;
 397     // Normally, we're interested in pointers to younger generations.
 398     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 399     gen->space_iterate(&blk, true);
 400   }
 401 };
 402 
 403 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 404   // We don't need to do young-gen spaces.
 405   if (s->end() <= gen_boundary) return;
 406   MemRegion used = s->used_region();
 407 
 408   jbyte* cur_entry = byte_for(used.start());
 409   jbyte* limit = byte_after(used.last());
 410   while (cur_entry < limit) {
 411     if (*cur_entry == CardTableModRefBS::clean_card) {
 412       jbyte* first_dirty = cur_entry+1;
 413       while (first_dirty < limit &&
 414              *first_dirty == CardTableModRefBS::clean_card) {
 415         first_dirty++;
 416       }
 417       // If the first object is a regular object, and it has a
 418       // young-to-old field, that would mark the previous card.
 419       HeapWord* boundary = addr_for(cur_entry);
 420       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 421       HeapWord* boundary_block = s->block_start(boundary);
 422       HeapWord* begin = boundary;             // Until proven otherwise.
 423       HeapWord* start_block = boundary_block; // Until proven otherwise.
 424       if (boundary_block < boundary) {
 425         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 426           oop boundary_obj = oop(boundary_block);
 427           if (!boundary_obj->is_objArray() &&
 428               !boundary_obj->is_typeArray()) {
 429             guarantee(cur_entry > byte_for(used.start()),
 430                       "else boundary would be boundary_block");
 431             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
 432               begin = boundary_block + s->block_size(boundary_block);
 433               start_block = begin;
 434             }
 435           }
 436         }
 437       }
 438       // Now traverse objects until end.
 439       if (begin < end) {
 440         MemRegion mr(begin, end);
 441         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 442         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 443           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 444             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 445           }
 446         }
 447       }
 448       cur_entry = first_dirty;
 449     } else {
 450       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 451       // is a transient value, that cannot be in the card table
 452       // except during GC, and thus assert that:
 453       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 454       //        "Illegal CT value");
 455       // That however, need not hold, as will become clear in the
 456       // following...
 457 
 458       // We'd normally expect that if we are in the parallel case,
 459       // we can't have left a prev value (which would be different
 460       // from the current value) in the card table, and so we'd like to
 461       // assert that:
 462       // guarantee(cur_youngergen_card_val() == youngergen_card
 463       //           || !is_prev_youngergen_card_val(*cur_entry),
 464       //           "Illegal CT value");
 465       // That, however, may not hold occasionally, because of
 466       // CMS or MSC in the old gen. To wit, consider the
 467       // following two simple illustrative scenarios:
 468       // (a) CMS: Consider the case where a large object L
 469       //     spanning several cards is allocated in the old
 470       //     gen, and has a young gen reference stored in it, dirtying
 471       //     some interior cards. A young collection scans the card,
 472       //     finds a young ref and installs a youngergenP_n value.
 473       //     L then goes dead. Now a CMS collection starts,
 474       //     finds L dead and sweeps it up. Assume that L is
 475       //     abutting _unallocated_blk, so _unallocated_blk is
 476       //     adjusted down to (below) L. Assume further that
 477       //     no young collection intervenes during this CMS cycle.
 478       //     The next young gen cycle will not get to look at this
 479       //     youngergenP_n card since it lies in the unoccupied
 480       //     part of the space.
 481       //     Some young collections later the blocks on this
 482       //     card can be re-allocated either due to direct allocation
 483       //     or due to absorbing promotions. At this time, the
 484       //     before-gc verification will fail the above assert.
 485       // (b) MSC: In this case, an object L with a young reference
 486       //     is on a card that (therefore) holds a youngergen_n value.
 487       //     Suppose also that L lies towards the end of the used
 488       //     the used space before GC. An MSC collection
 489       //     occurs that compacts to such an extent that this
 490       //     card is no longer in the occupied part of the space.
 491       //     Since current code in MSC does not always clear cards
 492       //     in the unused part of old gen, this stale youngergen_n
 493       //     value is left behind and can later be covered by
 494       //     an object when promotion or direct allocation
 495       //     re-allocates that part of the heap.
 496       //
 497       // Fortunately, the presence of such stale card values is
 498       // "only" a minor annoyance in that subsequent young collections
 499       // might needlessly scan such cards, but would still never corrupt
 500       // the heap as a result. However, it's likely not to be a significant
 501       // performance inhibitor in practice. For instance,
 502       // some recent measurements with unoccupied cards eagerly cleared
 503       // out to maintain this invariant, showed next to no
 504       // change in young collection times; of course one can construct
 505       // degenerate examples where the cost can be significant.)
 506       // Note, in particular, that if the "stale" card is modified
 507       // after re-allocation, it would be dirty, not "stale". Thus,
 508       // we can never have a younger ref in such a card and it is
 509       // safe not to scan that card in any collection. [As we see
 510       // below, we do some unnecessary scanning
 511       // in some cases in the current parallel scanning algorithm.]
 512       //
 513       // The main point below is that the parallel card scanning code
 514       // deals correctly with these stale card values. There are two main
 515       // cases to consider where we have a stale "younger gen" value and a
 516       // "derivative" case to consider, where we have a stale
 517       // "cur_younger_gen_and_prev_non_clean" value, as will become
 518       // apparent in the case analysis below.
 519       // o Case 1. If the stale value corresponds to a younger_gen_n
 520       //   value other than the cur_younger_gen value then the code
 521       //   treats this as being tantamount to a prev_younger_gen
 522       //   card. This means that the card may be unnecessarily scanned.
 523       //   There are two sub-cases to consider:
 524       //   o Case 1a. Let us say that the card is in the occupied part
 525       //     of the generation at the time the collection begins. In
 526       //     that case the card will be either cleared when it is scanned
 527       //     for young pointers, or will be set to cur_younger_gen as a
 528       //     result of promotion. (We have elided the normal case where
 529       //     the scanning thread and the promoting thread interleave
 530       //     possibly resulting in a transient
 531       //     cur_younger_gen_and_prev_non_clean value before settling
 532       //     to cur_younger_gen. [End Case 1a.]
 533       //   o Case 1b. Consider now the case when the card is in the unoccupied
 534       //     part of the space which becomes occupied because of promotions
 535       //     into it during the current young GC. In this case the card
 536       //     will never be scanned for young references. The current
 537       //     code will set the card value to either
 538       //     cur_younger_gen_and_prev_non_clean or leave
 539       //     it with its stale value -- because the promotions didn't
 540       //     result in any younger refs on that card. Of these two
 541       //     cases, the latter will be covered in Case 1a during
 542       //     a subsequent scan. To deal with the former case, we need
 543       //     to further consider how we deal with a stale value of
 544       //     cur_younger_gen_and_prev_non_clean in our case analysis
 545       //     below. This we do in Case 3 below. [End Case 1b]
 546       //   [End Case 1]
 547       // o Case 2. If the stale value corresponds to cur_younger_gen being
 548       //   a value not necessarily written by a current promotion, the
 549       //   card will not be scanned by the younger refs scanning code.
 550       //   (This is OK since as we argued above such cards cannot contain
 551       //   any younger refs.) The result is that this value will be
 552       //   treated as a prev_younger_gen value in a subsequent collection,
 553       //   which is addressed in Case 1 above. [End Case 2]
 554       // o Case 3. We here consider the "derivative" case from Case 1b. above
 555       //   because of which we may find a stale
 556       //   cur_younger_gen_and_prev_non_clean card value in the table.
 557       //   Once again, as in Case 1, we consider two subcases, depending
 558       //   on whether the card lies in the occupied or unoccupied part
 559       //   of the space at the start of the young collection.
 560       //   o Case 3a. Let us say the card is in the occupied part of
 561       //     the old gen at the start of the young collection. In that
 562       //     case, the card will be scanned by the younger refs scanning
 563       //     code which will set it to cur_younger_gen. In a subsequent
 564       //     scan, the card will be considered again and get its final
 565       //     correct value. [End Case 3a]
 566       //   o Case 3b. Now consider the case where the card is in the
 567       //     unoccupied part of the old gen, and is occupied as a result
 568       //     of promotions during thus young gc. In that case,
 569       //     the card will not be scanned for younger refs. The presence
 570       //     of newly promoted objects on the card will then result in
 571       //     its keeping the value cur_younger_gen_and_prev_non_clean
 572       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 573       //   [End Case 3]
 574       //
 575       // (Please refer to the code in the helper class
 576       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 577       //
 578       // The informal arguments above can be tightened into a formal
 579       // correctness proof and it behooves us to write up such a proof,
 580       // or to use model checking to prove that there are no lingering
 581       // concerns.
 582       //
 583       // Clearly because of Case 3b one cannot bound the time for
 584       // which a card will retain what we have called a "stale" value.
 585       // However, one can obtain a Loose upper bound on the redundant
 586       // work as a result of such stale values. Note first that any
 587       // time a stale card lies in the occupied part of the space at
 588       // the start of the collection, it is scanned by younger refs
 589       // code and we can define a rank function on card values that
 590       // declines when this is so. Note also that when a card does not
 591       // lie in the occupied part of the space at the beginning of a
 592       // young collection, its rank can either decline or stay unchanged.
 593       // In this case, no extra work is done in terms of redundant
 594       // younger refs scanning of that card.
 595       // Then, the case analysis above reveals that, in the worst case,
 596       // any such stale card will be scanned unnecessarily at most twice.
 597       //
 598       // It is nonetheless advisable to try and get rid of some of this
 599       // redundant work in a subsequent (low priority) re-design of
 600       // the card-scanning code, if only to simplify the underlying
 601       // state machine analysis/proof. ysr 1/28/2002. XXX
 602       cur_entry++;
 603     }
 604   }
 605 }
 606 
 607 void CardTableRS::verify() {
 608   // At present, we only know how to verify the card table RS for
 609   // generational heaps.
 610   VerifyCTGenClosure blk(this);
 611   CollectedHeap* ch = Universe::heap();
 612 
 613   if (ch->kind() == CollectedHeap::GenCollectedHeap) {
 614     GenCollectedHeap::heap()->generation_iterate(&blk, false);
 615     _ct_bs->verify();
 616     }
 617   }
 618 
 619 
 620 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
 621   if (!mr.is_empty()) {
 622     jbyte* cur_entry = byte_for(mr.start());
 623     jbyte* limit = byte_after(mr.last());
 624     // The region mr may not start on a card boundary so
 625     // the first card may reflect a write to the space
 626     // just prior to mr.
 627     if (!is_aligned(mr.start())) {
 628       cur_entry++;
 629     }
 630     for (;cur_entry < limit; cur_entry++) {
 631       guarantee(*cur_entry == CardTableModRefBS::clean_card,
 632                 "Unexpected dirty card found");
 633     }
 634   }
 635 }