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