1 /*
   2  * Copyright (c) 2001, 2017, 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.hpp"
  33 #include "runtime/java.hpp"
  34 #include "runtime/os.hpp"
  35 #include "utilities/macros.hpp"
  36 
  37 class HasAccumulatedModifiedOopsClosure : public CLDClosure {
  38   bool _found;
  39  public:
  40   HasAccumulatedModifiedOopsClosure() : _found(false) {}
  41   void do_cld(ClassLoaderData* cld) {
  42     if (_found) {
  43       return;
  44     }
  45 
  46     if (cld->has_accumulated_modified_oops()) {
  47       _found = true;
  48     }
  49   }
  50   bool found() {
  51     return _found;
  52   }
  53 };
  54 
  55 bool CLDRemSet::mod_union_is_clear() {
  56   HasAccumulatedModifiedOopsClosure closure;
  57   ClassLoaderDataGraph::cld_do(&closure);
  58 
  59   return !closure.found();
  60 }
  61 
  62 
  63 class ClearCLDModUnionClosure : public CLDClosure {
  64  public:
  65   void do_cld(ClassLoaderData* cld) {
  66     if (cld->has_accumulated_modified_oops()) {
  67       cld->clear_accumulated_modified_oops();
  68     }
  69   }
  70 };
  71 
  72 void CLDRemSet::clear_mod_union() {
  73   ClearCLDModUnionClosure closure;
  74   ClassLoaderDataGraph::cld_do(&closure);
  75 }
  76 
  77 
  78 CardTableRS::CardTableRS(MemRegion whole_heap) :
  79   _bs(NULL),
  80   _cur_youngergen_card_val(youngergenP1_card)
  81 {
  82   _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
  83   _ct_bs->initialize();
  84   set_bs(_ct_bs);
  85   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
  86   // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
  87   uint max_gens = 2;
  88   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
  89                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
  90   if (_last_cur_val_in_gen == NULL) {
  91     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
  92   }
  93   for (uint i = 0; i < max_gens + 1; i++) {
  94     _last_cur_val_in_gen[i] = clean_card_val();
  95   }
  96   _ct_bs->set_CTRS(this);
  97 }
  98 
  99 CardTableRS::~CardTableRS() {
 100   if (_ct_bs) {
 101     delete _ct_bs;
 102     _ct_bs = NULL;
 103   }
 104   if (_last_cur_val_in_gen) {
 105     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
 106   }
 107 }
 108 
 109 void CardTableRS::resize_covered_region(MemRegion new_region) {
 110   _ct_bs->resize_covered_region(new_region);
 111 }
 112 
 113 jbyte CardTableRS::find_unused_youngergenP_card_value() {
 114   for (jbyte v = youngergenP1_card;
 115        v < cur_youngergen_and_prev_nonclean_card;
 116        v++) {
 117     bool seen = false;
 118     for (int g = 0; g < _regions_to_iterate; g++) {
 119       if (_last_cur_val_in_gen[g] == v) {
 120         seen = true;
 121         break;
 122       }
 123     }
 124     if (!seen) {
 125       return v;
 126     }
 127   }
 128   ShouldNotReachHere();
 129   return 0;
 130 }
 131 
 132 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
 133   // Parallel or sequential, we must always set the prev to equal the
 134   // last one written.
 135   if (parallel) {
 136     // Find a parallel value to be used next.
 137     jbyte next_val = find_unused_youngergenP_card_value();
 138     set_cur_youngergen_card_val(next_val);
 139 
 140   } else {
 141     // In an sequential traversal we will always write youngergen, so that
 142     // the inline barrier is  correct.
 143     set_cur_youngergen_card_val(youngergen_card);
 144   }
 145 }
 146 
 147 void CardTableRS::younger_refs_iterate(Generation* g,
 148                                        OopsInGenClosure* blk,
 149                                        uint n_threads) {
 150   // The indexing in this array is slightly odd. We want to access
 151   // the old generation record here, which is at index 2.
 152   _last_cur_val_in_gen[2] = cur_youngergen_card_val();
 153   g->younger_refs_iterate(blk, n_threads);
 154 }
 155 
 156 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 157   if (_is_par) {
 158     return clear_card_parallel(entry);
 159   } else {
 160     return clear_card_serial(entry);
 161   }
 162 }
 163 
 164 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 165   while (true) {
 166     // In the parallel case, we may have to do this several times.
 167     jbyte entry_val = *entry;
 168     assert(entry_val != CardTableRS::clean_card_val(),
 169            "We shouldn't be looking at clean cards, and this should "
 170            "be the only place they get cleaned.");
 171     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 172         || _ct->is_prev_youngergen_card_val(entry_val)) {
 173       jbyte res =
 174         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 175       if (res == entry_val) {
 176         break;
 177       } else {
 178         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 179                "The CAS above should only fail if another thread did "
 180                "a GC write barrier.");
 181       }
 182     } else if (entry_val ==
 183                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 184       // Parallelism shouldn't matter in this case.  Only the thread
 185       // assigned to scan the card should change this value.
 186       *entry = _ct->cur_youngergen_card_val();
 187       break;
 188     } else {
 189       assert(entry_val == _ct->cur_youngergen_card_val(),
 190              "Should be the only possibility.");
 191       // In this case, the card was clean before, and become
 192       // cur_youngergen only because of processing of a promoted object.
 193       // We don't have to look at the card.
 194       return false;
 195     }
 196   }
 197   return true;
 198 }
 199 
 200 
 201 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 202   jbyte entry_val = *entry;
 203   assert(entry_val != CardTableRS::clean_card_val(),
 204          "We shouldn't be looking at clean cards, and this should "
 205          "be the only place they get cleaned.");
 206   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 207          "This should be possible in the sequential case.");
 208   *entry = CardTableRS::clean_card_val();
 209   return true;
 210 }
 211 
 212 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 213   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
 214     _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
 215 }
 216 
 217 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 218   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 219 }
 220 
 221 // The regions are visited in *decreasing* address order.
 222 // This order aids with imprecise card marking, where a dirty
 223 // card may cause scanning, and summarization marking, of objects
 224 // that extend onto subsequent cards.
 225 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 226   assert(mr.word_size() > 0, "Error");
 227   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 228   // mr.end() may not necessarily be card aligned.
 229   jbyte* cur_entry = _ct->byte_for(mr.last());
 230   const jbyte* limit = _ct->byte_for(mr.start());
 231   HeapWord* end_of_non_clean = mr.end();
 232   HeapWord* start_of_non_clean = end_of_non_clean;
 233   while (cur_entry >= limit) {
 234     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 235     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 236       // Continue the dirty range by opening the
 237       // dirty window one card to the left.
 238       start_of_non_clean = cur_hw;
 239     } else {
 240       // We hit a "clean" card; process any non-empty
 241       // "dirty" range accumulated so far.
 242       if (start_of_non_clean < end_of_non_clean) {
 243         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 244         _dirty_card_closure->do_MemRegion(mrd);
 245       }
 246 
 247       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 248       if (is_word_aligned(cur_entry)) {
 249         jbyte* cur_row = cur_entry - BytesPerWord;
 250         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {
 251           cur_row -= BytesPerWord;
 252         }
 253         cur_entry = cur_row + BytesPerWord;
 254         cur_hw = _ct->addr_for(cur_entry);
 255       }
 256 
 257       // Reset the dirty window, while continuing to look
 258       // for the next dirty card that will start a
 259       // new dirty window.
 260       end_of_non_clean = cur_hw;
 261       start_of_non_clean = cur_hw;
 262     }
 263     // Note that "cur_entry" leads "start_of_non_clean" in
 264     // its leftward excursion after this point
 265     // in the loop and, when we hit the left end of "mr",
 266     // will point off of the left end of the card-table
 267     // for "mr".
 268     cur_entry--;
 269   }
 270   // If the first card of "mr" was dirty, we will have
 271   // been left with a dirty window, co-initial with "mr",
 272   // which we now process.
 273   if (start_of_non_clean < end_of_non_clean) {
 274     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 275     _dirty_card_closure->do_MemRegion(mrd);
 276   }
 277 }
 278 
 279 // clean (by dirty->clean before) ==> cur_younger_gen
 280 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 281 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 282 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 283 // cur-younger-gen                ==> cur_younger_gen
 284 // cur_youngergen_and_prev_nonclean_card ==> no change.
 285 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 286   volatile jbyte* entry = _ct_bs->byte_for(field);
 287   do {
 288     jbyte entry_val = *entry;
 289     // We put this first because it's probably the most common case.
 290     if (entry_val == clean_card_val()) {
 291       // No threat of contention with cleaning threads.
 292       *entry = cur_youngergen_card_val();
 293       return;
 294     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 295                || is_prev_youngergen_card_val(entry_val)) {
 296       // Mark it as both cur and prev youngergen; card cleaning thread will
 297       // eventually remove the previous stuff.
 298       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 299       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 300       // Did the CAS succeed?
 301       if (res == entry_val) return;
 302       // Otherwise, retry, to see the new value.
 303       continue;
 304     } else {
 305       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 306              || entry_val == cur_youngergen_card_val(),
 307              "should be only possibilities.");
 308       return;
 309     }
 310   } while (true);
 311 }
 312 
 313 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 314                                                 OopsInGenClosure* cl,
 315                                                 uint n_threads) {
 316   const MemRegion urasm = sp->used_region_at_save_marks();
 317 #ifdef ASSERT
 318   // Convert the assertion check to a warning if we are running
 319   // CMS+ParNew until related bug is fixed.
 320   MemRegion ur    = sp->used_region();
 321   assert(ur.contains(urasm) || (UseConcMarkSweepGC),
 322          "Did you forget to call save_marks()? "
 323          "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 324          "[" PTR_FORMAT ", " PTR_FORMAT ")",
 325          p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 326   // In the case of CMS+ParNew, issue a warning
 327   if (!ur.contains(urasm)) {
 328     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 329     log_warning(gc)("CMS+ParNew: Did you forget to call save_marks()? "
 330                     "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 331                     "[" PTR_FORMAT ", " PTR_FORMAT ")",
 332                     p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 333     MemRegion ur2 = sp->used_region();
 334     MemRegion urasm2 = sp->used_region_at_save_marks();
 335     if (!ur.equals(ur2)) {
 336       log_warning(gc)("CMS+ParNew: Flickering used_region()!!");
 337     }
 338     if (!urasm.equals(urasm2)) {
 339       log_warning(gc)("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 340     }
 341     ShouldNotReachHere();
 342   }
 343 #endif
 344   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
 345 }
 346 
 347 void CardTableRS::clear_into_younger(Generation* old_gen) {
 348   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 349          "Should only be called for the old generation");
 350   // The card tables for the youngest gen need never be cleared.
 351   // There's a bit of subtlety in the clear() and invalidate()
 352   // methods that we exploit here and in invalidate_or_clear()
 353   // below to avoid missing cards at the fringes. If clear() or
 354   // invalidate() are changed in the future, this code should
 355   // be revisited. 20040107.ysr
 356   clear(old_gen->prev_used_region());
 357 }
 358 
 359 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 360   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 361          "Should only be called for the old generation");
 362   // Invalidate the cards for the currently occupied part of
 363   // the old generation and clear the cards for the
 364   // unoccupied part of the generation (if any, making use
 365   // of that generation's prev_used_region to determine that
 366   // region). No need to do anything for the youngest
 367   // generation. Also see note#20040107.ysr above.
 368   MemRegion used_mr = old_gen->used_region();
 369   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 370   if (!to_be_cleared_mr.is_empty()) {
 371     clear(to_be_cleared_mr);
 372   }
 373   invalidate(used_mr);
 374 }
 375 
 376 
 377 class VerifyCleanCardClosure: public OopClosure {
 378 private:
 379   HeapWord* _boundary;
 380   HeapWord* _begin;
 381   HeapWord* _end;
 382 protected:
 383   template <class T> void do_oop_work(T* p) {
 384     HeapWord* jp = (HeapWord*)p;
 385     assert(jp >= _begin && jp < _end,
 386            "Error: jp " PTR_FORMAT " should be within "
 387            "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 388            p2i(jp), p2i(_begin), p2i(_end));
 389     oop obj = oopDesc::load_decode_heap_oop(p);
 390     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 391               "pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 392               "clean card crosses boundary" PTR_FORMAT,
 393               p2i(obj), p2i(jp), p2i(_boundary));
 394   }
 395 
 396 public:
 397   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 398     _boundary(b), _begin(begin), _end(end) {
 399     assert(b <= begin,
 400            "Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 401            p2i(b), p2i(begin));
 402     assert(begin <= end,
 403            "Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 404            p2i(begin), p2i(end));
 405   }
 406 
 407   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 408   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 409 };
 410 
 411 class VerifyCTSpaceClosure: public SpaceClosure {
 412 private:
 413   CardTableRS* _ct;
 414   HeapWord* _boundary;
 415 public:
 416   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 417     _ct(ct), _boundary(boundary) {}
 418   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 419 };
 420 
 421 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 422   CardTableRS* _ct;
 423 public:
 424   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 425   void do_generation(Generation* gen) {
 426     // Skip the youngest generation.
 427     if (GenCollectedHeap::heap()->is_young_gen(gen)) {
 428       return;
 429     }
 430     // Normally, we're interested in pointers to younger generations.
 431     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 432     gen->space_iterate(&blk, true);
 433   }
 434 };
 435 
 436 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 437   // We don't need to do young-gen spaces.
 438   if (s->end() <= gen_boundary) return;
 439   MemRegion used = s->used_region();
 440 
 441   jbyte* cur_entry = byte_for(used.start());
 442   jbyte* limit = byte_after(used.last());
 443   while (cur_entry < limit) {
 444     if (*cur_entry == clean_card_val()) {
 445       jbyte* first_dirty = cur_entry+1;
 446       while (first_dirty < limit &&
 447              *first_dirty == clean_card_val()) {
 448         first_dirty++;
 449       }
 450       // If the first object is a regular object, and it has a
 451       // young-to-old field, that would mark the previous card.
 452       HeapWord* boundary = addr_for(cur_entry);
 453       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 454       HeapWord* boundary_block = s->block_start(boundary);
 455       HeapWord* begin = boundary;             // Until proven otherwise.
 456       HeapWord* start_block = boundary_block; // Until proven otherwise.
 457       if (boundary_block < boundary) {
 458         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 459           oop boundary_obj = oop(boundary_block);
 460           if (!boundary_obj->is_objArray() &&
 461               !boundary_obj->is_typeArray()) {
 462             guarantee(cur_entry > byte_for(used.start()),
 463                       "else boundary would be boundary_block");
 464             if (*byte_for(boundary_block) != clean_card_val()) {
 465               begin = boundary_block + s->block_size(boundary_block);
 466               start_block = begin;
 467             }
 468           }
 469         }
 470       }
 471       // Now traverse objects until end.
 472       if (begin < end) {
 473         MemRegion mr(begin, end);
 474         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 475         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 476           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 477             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 478           }
 479         }
 480       }
 481       cur_entry = first_dirty;
 482     } else {
 483       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 484       // is a transient value, that cannot be in the card table
 485       // except during GC, and thus assert that:
 486       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 487       //        "Illegal CT value");
 488       // That however, need not hold, as will become clear in the
 489       // following...
 490 
 491       // We'd normally expect that if we are in the parallel case,
 492       // we can't have left a prev value (which would be different
 493       // from the current value) in the card table, and so we'd like to
 494       // assert that:
 495       // guarantee(cur_youngergen_card_val() == youngergen_card
 496       //           || !is_prev_youngergen_card_val(*cur_entry),
 497       //           "Illegal CT value");
 498       // That, however, may not hold occasionally, because of
 499       // CMS or MSC in the old gen. To wit, consider the
 500       // following two simple illustrative scenarios:
 501       // (a) CMS: Consider the case where a large object L
 502       //     spanning several cards is allocated in the old
 503       //     gen, and has a young gen reference stored in it, dirtying
 504       //     some interior cards. A young collection scans the card,
 505       //     finds a young ref and installs a youngergenP_n value.
 506       //     L then goes dead. Now a CMS collection starts,
 507       //     finds L dead and sweeps it up. Assume that L is
 508       //     abutting _unallocated_blk, so _unallocated_blk is
 509       //     adjusted down to (below) L. Assume further that
 510       //     no young collection intervenes during this CMS cycle.
 511       //     The next young gen cycle will not get to look at this
 512       //     youngergenP_n card since it lies in the unoccupied
 513       //     part of the space.
 514       //     Some young collections later the blocks on this
 515       //     card can be re-allocated either due to direct allocation
 516       //     or due to absorbing promotions. At this time, the
 517       //     before-gc verification will fail the above assert.
 518       // (b) MSC: In this case, an object L with a young reference
 519       //     is on a card that (therefore) holds a youngergen_n value.
 520       //     Suppose also that L lies towards the end of the used
 521       //     the used space before GC. An MSC collection
 522       //     occurs that compacts to such an extent that this
 523       //     card is no longer in the occupied part of the space.
 524       //     Since current code in MSC does not always clear cards
 525       //     in the unused part of old gen, this stale youngergen_n
 526       //     value is left behind and can later be covered by
 527       //     an object when promotion or direct allocation
 528       //     re-allocates that part of the heap.
 529       //
 530       // Fortunately, the presence of such stale card values is
 531       // "only" a minor annoyance in that subsequent young collections
 532       // might needlessly scan such cards, but would still never corrupt
 533       // the heap as a result. However, it's likely not to be a significant
 534       // performance inhibitor in practice. For instance,
 535       // some recent measurements with unoccupied cards eagerly cleared
 536       // out to maintain this invariant, showed next to no
 537       // change in young collection times; of course one can construct
 538       // degenerate examples where the cost can be significant.)
 539       // Note, in particular, that if the "stale" card is modified
 540       // after re-allocation, it would be dirty, not "stale". Thus,
 541       // we can never have a younger ref in such a card and it is
 542       // safe not to scan that card in any collection. [As we see
 543       // below, we do some unnecessary scanning
 544       // in some cases in the current parallel scanning algorithm.]
 545       //
 546       // The main point below is that the parallel card scanning code
 547       // deals correctly with these stale card values. There are two main
 548       // cases to consider where we have a stale "young gen" value and a
 549       // "derivative" case to consider, where we have a stale
 550       // "cur_younger_gen_and_prev_non_clean" value, as will become
 551       // apparent in the case analysis below.
 552       // o Case 1. If the stale value corresponds to a younger_gen_n
 553       //   value other than the cur_younger_gen value then the code
 554       //   treats this as being tantamount to a prev_younger_gen
 555       //   card. This means that the card may be unnecessarily scanned.
 556       //   There are two sub-cases to consider:
 557       //   o Case 1a. Let us say that the card is in the occupied part
 558       //     of the generation at the time the collection begins. In
 559       //     that case the card will be either cleared when it is scanned
 560       //     for young pointers, or will be set to cur_younger_gen as a
 561       //     result of promotion. (We have elided the normal case where
 562       //     the scanning thread and the promoting thread interleave
 563       //     possibly resulting in a transient
 564       //     cur_younger_gen_and_prev_non_clean value before settling
 565       //     to cur_younger_gen. [End Case 1a.]
 566       //   o Case 1b. Consider now the case when the card is in the unoccupied
 567       //     part of the space which becomes occupied because of promotions
 568       //     into it during the current young GC. In this case the card
 569       //     will never be scanned for young references. The current
 570       //     code will set the card value to either
 571       //     cur_younger_gen_and_prev_non_clean or leave
 572       //     it with its stale value -- because the promotions didn't
 573       //     result in any younger refs on that card. Of these two
 574       //     cases, the latter will be covered in Case 1a during
 575       //     a subsequent scan. To deal with the former case, we need
 576       //     to further consider how we deal with a stale value of
 577       //     cur_younger_gen_and_prev_non_clean in our case analysis
 578       //     below. This we do in Case 3 below. [End Case 1b]
 579       //   [End Case 1]
 580       // o Case 2. If the stale value corresponds to cur_younger_gen being
 581       //   a value not necessarily written by a current promotion, the
 582       //   card will not be scanned by the younger refs scanning code.
 583       //   (This is OK since as we argued above such cards cannot contain
 584       //   any younger refs.) The result is that this value will be
 585       //   treated as a prev_younger_gen value in a subsequent collection,
 586       //   which is addressed in Case 1 above. [End Case 2]
 587       // o Case 3. We here consider the "derivative" case from Case 1b. above
 588       //   because of which we may find a stale
 589       //   cur_younger_gen_and_prev_non_clean card value in the table.
 590       //   Once again, as in Case 1, we consider two subcases, depending
 591       //   on whether the card lies in the occupied or unoccupied part
 592       //   of the space at the start of the young collection.
 593       //   o Case 3a. Let us say the card is in the occupied part of
 594       //     the old gen at the start of the young collection. In that
 595       //     case, the card will be scanned by the younger refs scanning
 596       //     code which will set it to cur_younger_gen. In a subsequent
 597       //     scan, the card will be considered again and get its final
 598       //     correct value. [End Case 3a]
 599       //   o Case 3b. Now consider the case where the card is in the
 600       //     unoccupied part of the old gen, and is occupied as a result
 601       //     of promotions during thus young gc. In that case,
 602       //     the card will not be scanned for younger refs. The presence
 603       //     of newly promoted objects on the card will then result in
 604       //     its keeping the value cur_younger_gen_and_prev_non_clean
 605       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 606       //   [End Case 3]
 607       //
 608       // (Please refer to the code in the helper class
 609       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
 610       //
 611       // The informal arguments above can be tightened into a formal
 612       // correctness proof and it behooves us to write up such a proof,
 613       // or to use model checking to prove that there are no lingering
 614       // concerns.
 615       //
 616       // Clearly because of Case 3b one cannot bound the time for
 617       // which a card will retain what we have called a "stale" value.
 618       // However, one can obtain a Loose upper bound on the redundant
 619       // work as a result of such stale values. Note first that any
 620       // time a stale card lies in the occupied part of the space at
 621       // the start of the collection, it is scanned by younger refs
 622       // code and we can define a rank function on card values that
 623       // declines when this is so. Note also that when a card does not
 624       // lie in the occupied part of the space at the beginning of a
 625       // young collection, its rank can either decline or stay unchanged.
 626       // In this case, no extra work is done in terms of redundant
 627       // younger refs scanning of that card.
 628       // Then, the case analysis above reveals that, in the worst case,
 629       // any such stale card will be scanned unnecessarily at most twice.
 630       //
 631       // It is nonetheless advisable to try and get rid of some of this
 632       // redundant work in a subsequent (low priority) re-design of
 633       // the card-scanning code, if only to simplify the underlying
 634       // state machine analysis/proof. ysr 1/28/2002. XXX
 635       cur_entry++;
 636     }
 637   }
 638 }
 639 
 640 void CardTableRS::verify() {
 641   // At present, we only know how to verify the card table RS for
 642   // generational heaps.
 643   VerifyCTGenClosure blk(this);
 644   GenCollectedHeap::heap()->generation_iterate(&blk, false);
 645   _ct_bs->verify();
 646 }