1 /*
   2  * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "gc/shared/cardTableRS.hpp"
  27 #include "gc/shared/genCollectedHeap.hpp"
  28 #include "gc/shared/generation.hpp"
  29 #include "gc/shared/space.inline.hpp"
  30 #include "memory/allocation.inline.hpp"
  31 #include "oops/oop.inline.hpp"
  32 #include "runtime/atomic.inline.hpp"
  33 #include "runtime/java.hpp"
  34 #include "runtime/os.hpp"
  35 #include "utilities/macros.hpp"
  36 
  37 CardTableRS::CardTableRS(MemRegion whole_heap) :
  38   GenRemSet(),
  39   _cur_youngergen_card_val(youngergenP1_card)
  40 {
  41   _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  42   _ct_bs->initialize();
  43   set_bs(_ct_bs);
  44   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
  45   // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
  46   uint max_gens = 2;
  47   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
  48                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  49   if (_last_cur_val_in_gen == NULL) {
  50     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  51   }
  52   for (uint i = 0; i < max_gens + 1; i++) {
  53     _last_cur_val_in_gen[i] = clean_card_val();
  54   }
  55   _ct_bs->set_CTRS(this);
  56 }
  57 
  58 CardTableRS::~CardTableRS() {
  59   if (_ct_bs) {
  60     delete _ct_bs;
  61     _ct_bs = NULL;
  62   }
  63   if (_last_cur_val_in_gen) {
  64     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
  65   }
  66 }
  67 
  68 void CardTableRS::resize_covered_region(MemRegion new_region) {
  69   _ct_bs->resize_covered_region(new_region);
  70 }
  71 
  72 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  73   for (jbyte v = youngergenP1_card;
  74        v < cur_youngergen_and_prev_nonclean_card;
  75        v++) {
  76     bool seen = false;
  77     for (int g = 0; g < _regions_to_iterate; g++) {
  78       if (_last_cur_val_in_gen[g] == v) {
  79         seen = true;
  80         break;
  81       }
  82     }
  83     if (!seen) {
  84       return v;
  85     }
  86   }
  87   ShouldNotReachHere();
  88   return 0;
  89 }
  90 
  91 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
  92   // Parallel or sequential, we must always set the prev to equal the
  93   // last one written.
  94   if (parallel) {
  95     // Find a parallel value to be used next.
  96     jbyte next_val = find_unused_youngergenP_card_value();
  97     set_cur_youngergen_card_val(next_val);
  98 
  99   } else {
 100     // In an sequential traversal we will always write youngergen, so that
 101     // the inline barrier is  correct.
 102     set_cur_youngergen_card_val(youngergen_card);
 103   }
 104 }
 105 
 106 void CardTableRS::younger_refs_iterate(Generation* g,
 107                                        OopsInGenClosure* blk,
 108                                        uint n_threads) {
 109   // The indexing in this array is slightly odd. We want to access
 110   // the old generation record here, which is at index 2.
 111   _last_cur_val_in_gen[2] = cur_youngergen_card_val();
 112   g->younger_refs_iterate(blk, n_threads);
 113 }
 114 
 115 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 116   if (_is_par) {
 117     return clear_card_parallel(entry);
 118   } else {
 119     return clear_card_serial(entry);
 120   }
 121 }
 122 
 123 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 124   while (true) {
 125     // In the parallel case, we may have to do this several times.
 126     jbyte entry_val = *entry;
 127     assert(entry_val != CardTableRS::clean_card_val(),
 128            "We shouldn't be looking at clean cards, and this should "
 129            "be the only place they get cleaned.");
 130     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 131         || _ct->is_prev_youngergen_card_val(entry_val)) {
 132       jbyte res =
 133         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 134       if (res == entry_val) {
 135         break;
 136       } else {
 137         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 138                "The CAS above should only fail if another thread did "
 139                "a GC write barrier.");
 140       }
 141     } else if (entry_val ==
 142                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 143       // Parallelism shouldn't matter in this case.  Only the thread
 144       // assigned to scan the card should change this value.
 145       *entry = _ct->cur_youngergen_card_val();
 146       break;
 147     } else {
 148       assert(entry_val == _ct->cur_youngergen_card_val(),
 149              "Should be the only possibility.");
 150       // In this case, the card was clean before, and become
 151       // cur_youngergen only because of processing of a promoted object.
 152       // We don't have to look at the card.
 153       return false;
 154     }
 155   }
 156   return true;
 157 }
 158 
 159 
 160 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 161   jbyte entry_val = *entry;
 162   assert(entry_val != CardTableRS::clean_card_val(),
 163          "We shouldn't be looking at clean cards, and this should "
 164          "be the only place they get cleaned.");
 165   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 166          "This should be possible in the sequential case.");
 167   *entry = CardTableRS::clean_card_val();
 168   return true;
 169 }
 170 
 171 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 172   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
 173     _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
 174 }
 175 
 176 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 177   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 178 }
 179 
 180 // The regions are visited in *decreasing* address order.
 181 // This order aids with imprecise card marking, where a dirty
 182 // card may cause scanning, and summarization marking, of objects
 183 // that extend onto subsequent cards.
 184 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 185   assert(mr.word_size() > 0, "Error");
 186   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 187   // mr.end() may not necessarily be card aligned.
 188   jbyte* cur_entry = _ct->byte_for(mr.last());
 189   const jbyte* limit = _ct->byte_for(mr.start());
 190   HeapWord* end_of_non_clean = mr.end();
 191   HeapWord* start_of_non_clean = end_of_non_clean;
 192   while (cur_entry >= limit) {
 193     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 194     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 195       // Continue the dirty range by opening the
 196       // dirty window one card to the left.
 197       start_of_non_clean = cur_hw;
 198     } else {
 199       // We hit a "clean" card; process any non-empty
 200       // "dirty" range accumulated so far.
 201       if (start_of_non_clean < end_of_non_clean) {
 202         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 203         _dirty_card_closure->do_MemRegion(mrd);
 204       }
 205 
 206       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 207       if (is_word_aligned(cur_entry)) {
 208         jbyte* cur_row = cur_entry - BytesPerWord;
 209         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 210           cur_row -= BytesPerWord;
 211         }
 212         cur_entry = cur_row + BytesPerWord;
 213         cur_hw = _ct->addr_for(cur_entry);
 214       }
 215 
 216       // Reset the dirty window, while continuing to look
 217       // for the next dirty card that will start a
 218       // new dirty window.
 219       end_of_non_clean = cur_hw;
 220       start_of_non_clean = cur_hw;
 221     }
 222     // Note that "cur_entry" leads "start_of_non_clean" in
 223     // its leftward excursion after this point
 224     // in the loop and, when we hit the left end of "mr",
 225     // will point off of the left end of the card-table
 226     // for "mr".
 227     cur_entry--;
 228   }
 229   // If the first card of "mr" was dirty, we will have
 230   // been left with a dirty window, co-initial with "mr",
 231   // which we now process.
 232   if (start_of_non_clean < end_of_non_clean) {
 233     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 234     _dirty_card_closure->do_MemRegion(mrd);
 235   }
 236 }
 237 
 238 // clean (by dirty->clean before) ==> cur_younger_gen
 239 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 240 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 241 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 242 // cur-younger-gen                ==> cur_younger_gen
 243 // cur_youngergen_and_prev_nonclean_card ==> no change.
 244 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 245   jbyte* entry = _ct_bs->byte_for(field);
 246   do {
 247     jbyte entry_val = *entry;
 248     // We put this first because it's probably the most common case.
 249     if (entry_val == clean_card_val()) {
 250       // No threat of contention with cleaning threads.
 251       *entry = cur_youngergen_card_val();
 252       return;
 253     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 254                || is_prev_youngergen_card_val(entry_val)) {
 255       // Mark it as both cur and prev youngergen; card cleaning thread will
 256       // eventually remove the previous stuff.
 257       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 258       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 259       // Did the CAS succeed?
 260       if (res == entry_val) return;
 261       // Otherwise, retry, to see the new value.
 262       continue;
 263     } else {
 264       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 265              || entry_val == cur_youngergen_card_val(),
 266              "should be only possibilities.");
 267       return;
 268     }
 269   } while (true);
 270 }
 271 
 272 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 273                                                 OopsInGenClosure* cl,
 274                                                 uint n_threads) {
 275   const MemRegion urasm = sp->used_region_at_save_marks();
 276 #ifdef ASSERT
 277   // Convert the assertion check to a warning if we are running
 278   // CMS+ParNew until related bug is fixed.
 279   MemRegion ur    = sp->used_region();
 280   assert(ur.contains(urasm) || (UseConcMarkSweepGC),
 281          err_msg("Did you forget to call save_marks()? "
 282                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 283                  "[" PTR_FORMAT ", " PTR_FORMAT ")",
 284                  p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
 285   // In the case of CMS+ParNew, issue a warning
 286   if (!ur.contains(urasm)) {
 287     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 288     warning("CMS+ParNew: Did you forget to call save_marks()? "
 289             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 290             "[" PTR_FORMAT ", " PTR_FORMAT ")",
 291              p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 292     MemRegion ur2 = sp->used_region();
 293     MemRegion urasm2 = sp->used_region_at_save_marks();
 294     if (!ur.equals(ur2)) {
 295       warning("CMS+ParNew: Flickering used_region()!!");
 296     }
 297     if (!urasm.equals(urasm2)) {
 298       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 299     }
 300     ShouldNotReachHere();
 301   }
 302 #endif
 303   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
 304 }
 305 
 306 void CardTableRS::clear_into_younger(Generation* old_gen) {
 307   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 308          "Should only be called for the old generation");
 309   // The card tables for the youngest gen need never be cleared.
 310   // There's a bit of subtlety in the clear() and invalidate()
 311   // methods that we exploit here and in invalidate_or_clear()
 312   // below to avoid missing cards at the fringes. If clear() or
 313   // invalidate() are changed in the future, this code should
 314   // be revisited. 20040107.ysr
 315   clear(old_gen->prev_used_region());
 316 }
 317 
 318 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 319   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 320          "Should only be called for the old generation");
 321   // Invalidate the cards for the currently occupied part of
 322   // the old generation and clear the cards for the
 323   // unoccupied part of the generation (if any, making use
 324   // of that generation's prev_used_region to determine that
 325   // region). No need to do anything for the youngest
 326   // generation. Also see note#20040107.ysr above.
 327   MemRegion used_mr = old_gen->used_region();
 328   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 329   if (!to_be_cleared_mr.is_empty()) {
 330     clear(to_be_cleared_mr);
 331   }
 332   invalidate(used_mr);
 333 }
 334 
 335 
 336 class VerifyCleanCardClosure: public OopClosure {
 337 private:
 338   HeapWord* _boundary;
 339   HeapWord* _begin;
 340   HeapWord* _end;
 341 protected:
 342   template <class T> void do_oop_work(T* p) {
 343     HeapWord* jp = (HeapWord*)p;
 344     assert(jp >= _begin && jp < _end,
 345            err_msg("Error: jp " PTR_FORMAT " should be within "
 346                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 347                    p2i(jp), p2i(_begin), p2i(_end)));
 348     oop obj = oopDesc::load_decode_heap_oop(p);
 349     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 350               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 351                       "clean card crosses boundary" PTR_FORMAT,
 352                       p2i(obj), p2i(jp), p2i(_boundary)));
 353   }
 354 
 355 public:
 356   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 357     _boundary(b), _begin(begin), _end(end) {
 358     assert(b <= begin,
 359            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 360                    p2i(b), p2i(begin)));
 361     assert(begin <= end,
 362            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 363                    p2i(begin), p2i(end)));
 364   }
 365 
 366   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 367   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 368 };
 369 
 370 class VerifyCTSpaceClosure: public SpaceClosure {
 371 private:
 372   CardTableRS* _ct;
 373   HeapWord* _boundary;
 374 public:
 375   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 376     _ct(ct), _boundary(boundary) {}
 377   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 378 };
 379 
 380 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 381   CardTableRS* _ct;
 382 public:
 383   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 384   void do_generation(Generation* gen) {
 385     // Skip the youngest generation.
 386     if (GenCollectedHeap::heap()->is_young_gen(gen)) {
 387       return;
 388     }
 389     // Normally, we're interested in pointers to younger generations.
 390     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 391     gen->space_iterate(&blk, true);
 392   }
 393 };
 394 
 395 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 396   // We don't need to do young-gen spaces.
 397   if (s->end() <= gen_boundary) return;
 398   MemRegion used = s->used_region();
 399 
 400   jbyte* cur_entry = byte_for(used.start());
 401   jbyte* limit = byte_after(used.last());
 402   while (cur_entry < limit) {
 403     if (*cur_entry == clean_card_val()) {
 404       jbyte* first_dirty = cur_entry+1;
 405       while (first_dirty < limit &&
 406              *first_dirty == clean_card_val()) {
 407         first_dirty++;
 408       }
 409       // If the first object is a regular object, and it has a
 410       // young-to-old field, that would mark the previous card.
 411       HeapWord* boundary = addr_for(cur_entry);
 412       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 413       HeapWord* boundary_block = s->block_start(boundary);
 414       HeapWord* begin = boundary;             // Until proven otherwise.
 415       HeapWord* start_block = boundary_block; // Until proven otherwise.
 416       if (boundary_block < boundary) {
 417         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 418           oop boundary_obj = oop(boundary_block);
 419           if (!boundary_obj->is_objArray() &&
 420               !boundary_obj->is_typeArray()) {
 421             guarantee(cur_entry > byte_for(used.start()),
 422                       "else boundary would be boundary_block");
 423             if (*byte_for(boundary_block) != clean_card_val()) {
 424               begin = boundary_block + s->block_size(boundary_block);
 425               start_block = begin;
 426             }
 427           }
 428         }
 429       }
 430       // Now traverse objects until end.
 431       if (begin < end) {
 432         MemRegion mr(begin, end);
 433         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 434         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 435           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 436             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 437           }
 438         }
 439       }
 440       cur_entry = first_dirty;
 441     } else {
 442       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 443       // is a transient value, that cannot be in the card table
 444       // except during GC, and thus assert that:
 445       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 446       //        "Illegal CT value");
 447       // That however, need not hold, as will become clear in the
 448       // following...
 449 
 450       // We'd normally expect that if we are in the parallel case,
 451       // we can't have left a prev value (which would be different
 452       // from the current value) in the card table, and so we'd like to
 453       // assert that:
 454       // guarantee(cur_youngergen_card_val() == youngergen_card
 455       //           || !is_prev_youngergen_card_val(*cur_entry),
 456       //           "Illegal CT value");
 457       // That, however, may not hold occasionally, because of
 458       // CMS or MSC in the old gen. To wit, consider the
 459       // following two simple illustrative scenarios:
 460       // (a) CMS: Consider the case where a large object L
 461       //     spanning several cards is allocated in the old
 462       //     gen, and has a young gen reference stored in it, dirtying
 463       //     some interior cards. A young collection scans the card,
 464       //     finds a young ref and installs a youngergenP_n value.
 465       //     L then goes dead. Now a CMS collection starts,
 466       //     finds L dead and sweeps it up. Assume that L is
 467       //     abutting _unallocated_blk, so _unallocated_blk is
 468       //     adjusted down to (below) L. Assume further that
 469       //     no young collection intervenes during this CMS cycle.
 470       //     The next young gen cycle will not get to look at this
 471       //     youngergenP_n card since it lies in the unoccupied
 472       //     part of the space.
 473       //     Some young collections later the blocks on this
 474       //     card can be re-allocated either due to direct allocation
 475       //     or due to absorbing promotions. At this time, the
 476       //     before-gc verification will fail the above assert.
 477       // (b) MSC: In this case, an object L with a young reference
 478       //     is on a card that (therefore) holds a youngergen_n value.
 479       //     Suppose also that L lies towards the end of the used
 480       //     the used space before GC. An MSC collection
 481       //     occurs that compacts to such an extent that this
 482       //     card is no longer in the occupied part of the space.
 483       //     Since current code in MSC does not always clear cards
 484       //     in the unused part of old gen, this stale youngergen_n
 485       //     value is left behind and can later be covered by
 486       //     an object when promotion or direct allocation
 487       //     re-allocates that part of the heap.
 488       //
 489       // Fortunately, the presence of such stale card values is
 490       // "only" a minor annoyance in that subsequent young collections
 491       // might needlessly scan such cards, but would still never corrupt
 492       // the heap as a result. However, it's likely not to be a significant
 493       // performance inhibitor in practice. For instance,
 494       // some recent measurements with unoccupied cards eagerly cleared
 495       // out to maintain this invariant, showed next to no
 496       // change in young collection times; of course one can construct
 497       // degenerate examples where the cost can be significant.)
 498       // Note, in particular, that if the "stale" card is modified
 499       // after re-allocation, it would be dirty, not "stale". Thus,
 500       // we can never have a younger ref in such a card and it is
 501       // safe not to scan that card in any collection. [As we see
 502       // below, we do some unnecessary scanning
 503       // in some cases in the current parallel scanning algorithm.]
 504       //
 505       // The main point below is that the parallel card scanning code
 506       // deals correctly with these stale card values. There are two main
 507       // cases to consider where we have a stale "young gen" value and a
 508       // "derivative" case to consider, where we have a stale
 509       // "cur_younger_gen_and_prev_non_clean" value, as will become
 510       // apparent in the case analysis below.
 511       // o Case 1. If the stale value corresponds to a younger_gen_n
 512       //   value other than the cur_younger_gen value then the code
 513       //   treats this as being tantamount to a prev_younger_gen
 514       //   card. This means that the card may be unnecessarily scanned.
 515       //   There are two sub-cases to consider:
 516       //   o Case 1a. Let us say that the card is in the occupied part
 517       //     of the generation at the time the collection begins. In
 518       //     that case the card will be either cleared when it is scanned
 519       //     for young pointers, or will be set to cur_younger_gen as a
 520       //     result of promotion. (We have elided the normal case where
 521       //     the scanning thread and the promoting thread interleave
 522       //     possibly resulting in a transient
 523       //     cur_younger_gen_and_prev_non_clean value before settling
 524       //     to cur_younger_gen. [End Case 1a.]
 525       //   o Case 1b. Consider now the case when the card is in the unoccupied
 526       //     part of the space which becomes occupied because of promotions
 527       //     into it during the current young GC. In this case the card
 528       //     will never be scanned for young references. The current
 529       //     code will set the card value to either
 530       //     cur_younger_gen_and_prev_non_clean or leave
 531       //     it with its stale value -- because the promotions didn't
 532       //     result in any younger refs on that card. Of these two
 533       //     cases, the latter will be covered in Case 1a during
 534       //     a subsequent scan. To deal with the former case, we need
 535       //     to further consider how we deal with a stale value of
 536       //     cur_younger_gen_and_prev_non_clean in our case analysis
 537       //     below. This we do in Case 3 below. [End Case 1b]
 538       //   [End Case 1]
 539       // o Case 2. If the stale value corresponds to cur_younger_gen being
 540       //   a value not necessarily written by a current promotion, the
 541       //   card will not be scanned by the younger refs scanning code.
 542       //   (This is OK since as we argued above such cards cannot contain
 543       //   any younger refs.) The result is that this value will be
 544       //   treated as a prev_younger_gen value in a subsequent collection,
 545       //   which is addressed in Case 1 above. [End Case 2]
 546       // o Case 3. We here consider the "derivative" case from Case 1b. above
 547       //   because of which we may find a stale
 548       //   cur_younger_gen_and_prev_non_clean card value in the table.
 549       //   Once again, as in Case 1, we consider two subcases, depending
 550       //   on whether the card lies in the occupied or unoccupied part
 551       //   of the space at the start of the young collection.
 552       //   o Case 3a. Let us say the card is in the occupied part of
 553       //     the old gen at the start of the young collection. In that
 554       //     case, the card will be scanned by the younger refs scanning
 555       //     code which will set it to cur_younger_gen. In a subsequent
 556       //     scan, the card will be considered again and get its final
 557       //     correct value. [End Case 3a]
 558       //   o Case 3b. Now consider the case where the card is in the
 559       //     unoccupied part of the old gen, and is occupied as a result
 560       //     of promotions during thus young gc. In that case,
 561       //     the card will not be scanned for younger refs. The presence
 562       //     of newly promoted objects on the card will then result in
 563       //     its keeping the value cur_younger_gen_and_prev_non_clean
 564       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 565       //   [End Case 3]
 566       //
 567       // (Please refer to the code in the helper class
 568       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 569       //
 570       // The informal arguments above can be tightened into a formal
 571       // correctness proof and it behooves us to write up such a proof,
 572       // or to use model checking to prove that there are no lingering
 573       // concerns.
 574       //
 575       // Clearly because of Case 3b one cannot bound the time for
 576       // which a card will retain what we have called a "stale" value.
 577       // However, one can obtain a Loose upper bound on the redundant
 578       // work as a result of such stale values. Note first that any
 579       // time a stale card lies in the occupied part of the space at
 580       // the start of the collection, it is scanned by younger refs
 581       // code and we can define a rank function on card values that
 582       // declines when this is so. Note also that when a card does not
 583       // lie in the occupied part of the space at the beginning of a
 584       // young collection, its rank can either decline or stay unchanged.
 585       // In this case, no extra work is done in terms of redundant
 586       // younger refs scanning of that card.
 587       // Then, the case analysis above reveals that, in the worst case,
 588       // any such stale card will be scanned unnecessarily at most twice.
 589       //
 590       // It is nonetheless advisable to try and get rid of some of this
 591       // redundant work in a subsequent (low priority) re-design of
 592       // the card-scanning code, if only to simplify the underlying
 593       // state machine analysis/proof. ysr 1/28/2002. XXX
 594       cur_entry++;
 595     }
 596   }
 597 }
 598 
 599 void CardTableRS::verify() {
 600   // At present, we only know how to verify the card table RS for
 601   // generational heaps.
 602   VerifyCTGenClosure blk(this);
 603   GenCollectedHeap::heap()->generation_iterate(&blk, false);
 604   _ct_bs->verify();
 605 }