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 /*

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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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  * This code is free software; you can redistribute it and/or modify it

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  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

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  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 #include "precompiled.hpp"

 #include "memory/allocation.inline.hpp"

 #include "memory/cardTableRS.hpp"

 #include "memory/genCollectedHeap.hpp"

 #include "memory/generation.hpp"

 #include "memory/space.hpp"

 #include "oops/oop.inline.hpp"

 #include "runtime/java.hpp"

 #include "runtime/os.hpp"

 #include "utilities/macros.hpp"

 #if INCLUDE_ALL_GCS

 #include "gc_implementation/g1/concurrentMark.hpp"

 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"

 #endif // INCLUDE_ALL_GCS

 CardTableRS::CardTableRS(MemRegion whole_heap,

                          int max_covered_regions) :

   GenRemSet(),

   _cur_youngergen_card_val(youngergenP1_card),

   _regions_to_iterate(max_covered_regions - 1)

 {

 #if INCLUDE_ALL_GCS

   if (UseG1GC) {

       _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,

                                                   max_covered_regions);

   } else {

     _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);

   }

 #else

   _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);

 #endif

   set_bs(_ct_bs);

   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,

                          mtGC, 0, AllocFailStrategy::RETURN_NULL);

   if (_last_cur_val_in_gen == NULL) {

     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");

   }

   for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {

     _last_cur_val_in_gen[i] = clean_card_val();

   }

   _ct_bs->set_CTRS(this);

 }

 CardTableRS::~CardTableRS() {

   if (_ct_bs) {

     delete _ct_bs;

     _ct_bs = NULL;

   }

   if (_last_cur_val_in_gen) {

     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);

   }

 }

 void CardTableRS::resize_covered_region(MemRegion new_region) {

   _ct_bs->resize_covered_region(new_region);

 }

 jbyte CardTableRS::find_unused_youngergenP_card_value() {

   for (jbyte v = youngergenP1_card;

        v < cur_youngergen_and_prev_nonclean_card;

        v++) {

     bool seen = false;

     for (int g = 0; g < _regions_to_iterate; g++) {

       if (_last_cur_val_in_gen[g] == v) {

         seen = true;

         break;

       }

     }

     if (!seen) return v;

   }

   ShouldNotReachHere();

   return 0;

 }

 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {

   // Parallel or sequential, we must always set the prev to equal the

   // last one written.

   if (parallel) {

     // Find a parallel value to be used next.

     jbyte next_val = find_unused_youngergenP_card_value();

     set_cur_youngergen_card_val(next_val);

   } else {

     // In an sequential traversal we will always write youngergen, so that

     // the inline barrier is  correct.

     set_cur_youngergen_card_val(youngergen_card);

   }

 }

 void CardTableRS::younger_refs_iterate(Generation* g,

                                        OopsInGenClosure* blk) {

   _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();

   g->younger_refs_iterate(blk);

 }

 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {

   if (_is_par) {

     return clear_card_parallel(entry);

   } else {

     return clear_card_serial(entry);

   }

 }

 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {

   while (true) {

     // In the parallel case, we may have to do this several times.

     jbyte entry_val = *entry;

     assert(entry_val != CardTableRS::clean_card_val(),

            "We shouldn't be looking at clean cards, and this should "

            "be the only place they get cleaned.");

     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)

         || _ct->is_prev_youngergen_card_val(entry_val)) {

       jbyte res =

         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);

       if (res == entry_val) {

         break;

       } else {

         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,

                "The CAS above should only fail if another thread did "

                "a GC write barrier.");

       }

     } else if (entry_val ==

                CardTableRS::cur_youngergen_and_prev_nonclean_card) {

       // Parallelism shouldn't matter in this case.  Only the thread

       // assigned to scan the card should change this value.

       *entry = _ct->cur_youngergen_card_val();

       break;

     } else {

       assert(entry_val == _ct->cur_youngergen_card_val(),

              "Should be the only possibility.");

       // In this case, the card was clean before, and become

       // cur_youngergen only because of processing of a promoted object.

       // We don't have to look at the card.

       return false;

     }

   }

   return true;

 }

 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {

   jbyte entry_val = *entry;

   assert(entry_val != CardTableRS::clean_card_val(),

          "We shouldn't be looking at clean cards, and this should "

          "be the only place they get cleaned.");

   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,

          "This should be possible in the sequential case.");

   *entry = CardTableRS::clean_card_val();

   return true;

 }

 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(

   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :

     _dirty_card_closure(dirty_card_closure), _ct(ct) {

     // Cannot yet substitute active_workers for n_par_threads

     // in the case where parallelism is being turned off by

     // setting n_par_threads to 0.

     _is_par = (SharedHeap::heap()->n_par_threads() > 0);

     assert(!_is_par ||

            (SharedHeap::heap()->n_par_threads() ==

             SharedHeap::heap()->workers()->active_workers()), "Mismatch");

 }

 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {

   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;

 }

 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {

   assert(mr.word_size() > 0, "Error");

   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");

   // mr.end() may not necessarily be card aligned.

   jbyte* cur_entry = _ct->byte_for(mr.last());

   const jbyte* limit = _ct->byte_for(mr.start());

   HeapWord* end_of_non_clean = mr.end();

   HeapWord* start_of_non_clean = end_of_non_clean;

   while (cur_entry >= limit) {

     HeapWord* cur_hw = _ct->addr_for(cur_entry);

     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {

       // Continue the dirty range by opening the

       // dirty window one card to the left.

       start_of_non_clean = cur_hw;

     } else {

       // We hit a "clean" card; process any non-empty

       // "dirty" range accumulated so far.

       if (start_of_non_clean < end_of_non_clean) {

         const MemRegion mrd(start_of_non_clean, end_of_non_clean);

         _dirty_card_closure->do_MemRegion(mrd);

       }

       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary

       if (is_word_aligned(cur_entry)) {

         jbyte* cur_row = cur_entry - BytesPerWord;

         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row()) {

           cur_row -= BytesPerWord;

         }

         cur_entry = cur_row + BytesPerWord;

         cur_hw = _ct->addr_for(cur_entry);

       }

       // Reset the dirty window, while continuing to look

       // for the next dirty card that will start a

       // new dirty window.

       end_of_non_clean = cur_hw;

       start_of_non_clean = cur_hw;

     }

     // Note that "cur_entry" leads "start_of_non_clean" in

     // its leftward excursion after this point

     // in the loop and, when we hit the left end of "mr",

     // will point off of the left end of the card-table

     // for "mr".

     cur_entry--;

   }

   // If the first card of "mr" was dirty, we will have

   // been left with a dirty window, co-initial with "mr",

   // which we now process.

   if (start_of_non_clean < end_of_non_clean) {

     const MemRegion mrd(start_of_non_clean, end_of_non_clean);

     _dirty_card_closure->do_MemRegion(mrd);

   }

 }

 // clean (by dirty->clean before) ==> cur_younger_gen

 // dirty                          ==> cur_youngergen_and_prev_nonclean_card

 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card

 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card

 // cur-younger-gen                ==> cur_younger_gen

 // cur_youngergen_and_prev_nonclean_card ==> no change.

 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {

   jbyte* entry = ct_bs()->byte_for(field);

   do {

     jbyte entry_val = *entry;

     // We put this first because it's probably the most common case.

     if (entry_val == clean_card_val()) {

       // No threat of contention with cleaning threads.

       *entry = cur_youngergen_card_val();

       return;

     } else if (card_is_dirty_wrt_gen_iter(entry_val)

                || is_prev_youngergen_card_val(entry_val)) {

       // Mark it as both cur and prev youngergen; card cleaning thread will

       // eventually remove the previous stuff.

       jbyte new_val = cur_youngergen_and_prev_nonclean_card;

       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);

       // Did the CAS succeed?

       if (res == entry_val) return;

       // Otherwise, retry, to see the new value.

       continue;

     } else {

       assert(entry_val == cur_youngergen_and_prev_nonclean_card

              || entry_val == cur_youngergen_card_val(),

              "should be only possibilities.");

       return;

     }

   } while (true);

 }

 void CardTableRS::younger_refs_in_space_iterate(Space* sp,

                                                 OopsInGenClosure* cl) {

   const MemRegion urasm = sp->used_region_at_save_marks();

 #ifdef ASSERT

   // Convert the assertion check to a warning if we are running

   // CMS+ParNew until related bug is fixed.

   MemRegion ur    = sp->used_region();

   assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),

          err_msg("Did you forget to call save_marks()? "

                  "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "

                  "[" PTR_FORMAT ", " PTR_FORMAT ")",

                  urasm.start(), urasm.end(), ur.start(), ur.end()));

   // In the case of CMS+ParNew, issue a warning

   if (!ur.contains(urasm)) {

     assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");

     warning("CMS+ParNew: Did you forget to call save_marks()? "

             "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "

             "[" PTR_FORMAT ", " PTR_FORMAT ")",

              urasm.start(), urasm.end(), ur.start(), ur.end());

     MemRegion ur2 = sp->used_region();

     MemRegion urasm2 = sp->used_region_at_save_marks();

     if (!ur.equals(ur2)) {

       warning("CMS+ParNew: Flickering used_region()!!");

     }

     if (!urasm.equals(urasm2)) {

       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");

     }

     ShouldNotReachHere();

   }

 #endif

   _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);

 }

 void CardTableRS::clear_into_younger(Generation* old_gen) {

   assert(old_gen->level() == 1, "Should only be called for the old generation");

   // The card tables for the youngest gen need never be cleared.

   // There's a bit of subtlety in the clear() and invalidate()

   // methods that we exploit here and in invalidate_or_clear()

   // below to avoid missing cards at the fringes. If clear() or

   // invalidate() are changed in the future, this code should

   // be revisited. 20040107.ysr

   clear(old_gen->prev_used_region());

 }

 void CardTableRS::invalidate_or_clear(Generation* old_gen) {

   assert(old_gen->level() == 1, "Should only be called for the old generation");

   // Invalidate the cards for the currently occupied part of

   // the old generation and clear the cards for the

   // unoccupied part of the generation (if any, making use

   // of that generation's prev_used_region to determine that

   // region). No need to do anything for the youngest

   // generation. Also see note#20040107.ysr above.

   MemRegion used_mr = old_gen->used_region();

   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);

   if (!to_be_cleared_mr.is_empty()) {

     clear(to_be_cleared_mr);

   }

   invalidate(used_mr);

 }

 class VerifyCleanCardClosure: public OopClosure {

 private:

   HeapWord* _boundary;

   HeapWord* _begin;

   HeapWord* _end;

 protected:

   template <class T> void do_oop_work(T* p) {

     HeapWord* jp = (HeapWord*)p;

     assert(jp >= _begin && jp < _end,

            err_msg("Error: jp " PTR_FORMAT " should be within "

                    "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",

                    jp, _begin, _end));

     oop obj = oopDesc::load_decode_heap_oop(p);

     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,

               err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "

                       "clean card crosses boundary" PTR_FORMAT,

                       (HeapWord*)obj, jp, _boundary));

   }

 public:

   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :

     _boundary(b), _begin(begin), _end(end) {

     assert(b <= begin,

            err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,

                    b, begin));

     assert(begin <= end,

            err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,

                    begin, end));

   }

   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }

   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }

 };

 class VerifyCTSpaceClosure: public SpaceClosure {

 private:

   CardTableRS* _ct;

   HeapWord* _boundary;

 public:

   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :

     _ct(ct), _boundary(boundary) {}

   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }

 };

 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {

   CardTableRS* _ct;

 public:

   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}

   void do_generation(Generation* gen) {

     // Skip the youngest generation.

     if (gen->level() == 0) return;

     // Normally, we're interested in pointers to younger generations.

     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());

     gen->space_iterate(&blk, true);

   }

 };

 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {

   // We don't need to do young-gen spaces.

   if (s->end() <= gen_boundary) return;

   MemRegion used = s->used_region();

   jbyte* cur_entry = byte_for(used.start());

   jbyte* limit = byte_after(used.last());

   while (cur_entry < limit) {

     if (*cur_entry == CardTableModRefBS::clean_card) {

       jbyte* first_dirty = cur_entry+1;

       while (first_dirty < limit &&

              *first_dirty == CardTableModRefBS::clean_card) {

         first_dirty++;

       }

       // If the first object is a regular object, and it has a

       // young-to-old field, that would mark the previous card.

       HeapWord* boundary = addr_for(cur_entry);

       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);

       HeapWord* boundary_block = s->block_start(boundary);

       HeapWord* begin = boundary;             // Until proven otherwise.

       HeapWord* start_block = boundary_block; // Until proven otherwise.

       if (boundary_block < boundary) {

         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {

           oop boundary_obj = oop(boundary_block);

           if (!boundary_obj->is_objArray() &&

               !boundary_obj->is_typeArray()) {

             guarantee(cur_entry > byte_for(used.start()),

                       "else boundary would be boundary_block");

             if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {

               begin = boundary_block + s->block_size(boundary_block);

               start_block = begin;

             }

           }

         }

       }

       // Now traverse objects until end.

       if (begin < end) {

         MemRegion mr(begin, end);

         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);

         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {

           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {

             oop(cur)->oop_iterate_no_header(&verify_blk, mr);

           }

         }

       }

       cur_entry = first_dirty;

     } else {

       // We'd normally expect that cur_youngergen_and_prev_nonclean_card

       // is a transient value, that cannot be in the card table

       // except during GC, and thus assert that:

       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,

       //        "Illegal CT value");

       // That however, need not hold, as will become clear in the

       // following...

       // We'd normally expect that if we are in the parallel case,

       // we can't have left a prev value (which would be different

       // from the current value) in the card table, and so we'd like to

       // assert that:

       // guarantee(cur_youngergen_card_val() == youngergen_card

       //           || !is_prev_youngergen_card_val(*cur_entry),

       //           "Illegal CT value");

       // That, however, may not hold occasionally, because of

       // CMS or MSC in the old gen. To wit, consider the

       // following two simple illustrative scenarios:

       // (a) CMS: Consider the case where a large object L

       //     spanning several cards is allocated in the old

       //     gen, and has a young gen reference stored in it, dirtying

       //     some interior cards. A young collection scans the card,

       //     finds a young ref and installs a youngergenP_n value.

       //     L then goes dead. Now a CMS collection starts,

       //     finds L dead and sweeps it up. Assume that L is

       //     abutting _unallocated_blk, so _unallocated_blk is

       //     adjusted down to (below) L. Assume further that

       //     no young collection intervenes during this CMS cycle.

       //     The next young gen cycle will not get to look at this

       //     youngergenP_n card since it lies in the unoccupied

       //     part of the space.

       //     Some young collections later the blocks on this

       //     card can be re-allocated either due to direct allocation

       //     or due to absorbing promotions. At this time, the

       //     before-gc verification will fail the above assert.

       // (b) MSC: In this case, an object L with a young reference

       //     is on a card that (therefore) holds a youngergen_n value.

       //     Suppose also that L lies towards the end of the used

       //     the used space before GC. An MSC collection

       //     occurs that compacts to such an extent that this

       //     card is no longer in the occupied part of the space.

       //     Since current code in MSC does not always clear cards

       //     in the unused part of old gen, this stale youngergen_n

       //     value is left behind and can later be covered by

       //     an object when promotion or direct allocation

       //     re-allocates that part of the heap.

       //

       // Fortunately, the presence of such stale card values is

       // "only" a minor annoyance in that subsequent young collections

       // might needlessly scan such cards, but would still never corrupt

       // the heap as a result. However, it's likely not to be a significant

       // performance inhibitor in practice. For instance,

       // some recent measurements with unoccupied cards eagerly cleared

       // out to maintain this invariant, showed next to no

       // change in young collection times; of course one can construct

       // degenerate examples where the cost can be significant.)

       // Note, in particular, that if the "stale" card is modified

       // after re-allocation, it would be dirty, not "stale". Thus,

       // we can never have a younger ref in such a card and it is

       // safe not to scan that card in any collection. [As we see

       // below, we do some unnecessary scanning

       // in some cases in the current parallel scanning algorithm.]

       //

       // The main point below is that the parallel card scanning code

       // deals correctly with these stale card values. There are two main

       // cases to consider where we have a stale "younger gen" value and a

       // "derivative" case to consider, where we have a stale

       // "cur_younger_gen_and_prev_non_clean" value, as will become

       // apparent in the case analysis below.

       // o Case 1. If the stale value corresponds to a younger_gen_n

       //   value other than the cur_younger_gen value then the code

       //   treats this as being tantamount to a prev_younger_gen

       //   card. This means that the card may be unnecessarily scanned.

       //   There are two sub-cases to consider:

       //   o Case 1a. Let us say that the card is in the occupied part

       //     of the generation at the time the collection begins. In

       //     that case the card will be either cleared when it is scanned

       //     for young pointers, or will be set to cur_younger_gen as a

       //     result of promotion. (We have elided the normal case where

       //     the scanning thread and the promoting thread interleave

       //     possibly resulting in a transient

       //     cur_younger_gen_and_prev_non_clean value before settling

       //     to cur_younger_gen. [End Case 1a.]

       //   o Case 1b. Consider now the case when the card is in the unoccupied

       //     part of the space which becomes occupied because of promotions

       //     into it during the current young GC. In this case the card

       //     will never be scanned for young references. The current

       //     code will set the card value to either

       //     cur_younger_gen_and_prev_non_clean or leave

       //     it with its stale value -- because the promotions didn't

       //     result in any younger refs on that card. Of these two

       //     cases, the latter will be covered in Case 1a during

       //     a subsequent scan. To deal with the former case, we need

       //     to further consider how we deal with a stale value of

       //     cur_younger_gen_and_prev_non_clean in our case analysis

       //     below. This we do in Case 3 below. [End Case 1b]

       //   [End Case 1]

       // o Case 2. If the stale value corresponds to cur_younger_gen being

       //   a value not necessarily written by a current promotion, the

       //   card will not be scanned by the younger refs scanning code.

       //   (This is OK since as we argued above such cards cannot contain

       //   any younger refs.) The result is that this value will be

       //   treated as a prev_younger_gen value in a subsequent collection,

       //   which is addressed in Case 1 above. [End Case 2]

       // o Case 3. We here consider the "derivative" case from Case 1b. above

       //   because of which we may find a stale

       //   cur_younger_gen_and_prev_non_clean card value in the table.

       //   Once again, as in Case 1, we consider two subcases, depending

       //   on whether the card lies in the occupied or unoccupied part

       //   of the space at the start of the young collection.

       //   o Case 3a. Let us say the card is in the occupied part of

       //     the old gen at the start of the young collection. In that

       //     case, the card will be scanned by the younger refs scanning

       //     code which will set it to cur_younger_gen. In a subsequent

       //     scan, the card will be considered again and get its final

       //     correct value. [End Case 3a]

       //   o Case 3b. Now consider the case where the card is in the

       //     unoccupied part of the old gen, and is occupied as a result

       //     of promotions during thus young gc. In that case,

       //     the card will not be scanned for younger refs. The presence

       //     of newly promoted objects on the card will then result in

       //     its keeping the value cur_younger_gen_and_prev_non_clean

       //     value, which we have dealt with in Case 3 here. [End Case 3b]

       //   [End Case 3]

       //

       // (Please refer to the code in the helper class

       // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)

       //

       // The informal arguments above can be tightened into a formal

       // correctness proof and it behooves us to write up such a proof,

       // or to use model checking to prove that there are no lingering

       // concerns.

       //

       // Clearly because of Case 3b one cannot bound the time for

       // which a card will retain what we have called a "stale" value.

       // However, one can obtain a Loose upper bound on the redundant

       // work as a result of such stale values. Note first that any

       // time a stale card lies in the occupied part of the space at

       // the start of the collection, it is scanned by younger refs

       // code and we can define a rank function on card values that

       // declines when this is so. Note also that when a card does not

       // lie in the occupied part of the space at the beginning of a

       // young collection, its rank can either decline or stay unchanged.

       // In this case, no extra work is done in terms of redundant

       // younger refs scanning of that card.

       // Then, the case analysis above reveals that, in the worst case,

       // any such stale card will be scanned unnecessarily at most twice.

       //

       // It is nonethelss advisable to try and get rid of some of this

       // redundant work in a subsequent (low priority) re-design of

       // the card-scanning code, if only to simplify the underlying

       // state machine analysis/proof. ysr 1/28/2002. XXX

       cur_entry++;

     }

   }

 }

 void CardTableRS::verify() {

   // At present, we only know how to verify the card table RS for

   // generational heaps.

   VerifyCTGenClosure blk(this);

   CollectedHeap* ch = Universe::heap();

   if (ch->kind() == CollectedHeap::GenCollectedHeap) {

     GenCollectedHeap::heap()->generation_iterate(&blk, false);

     _ct_bs->verify();

     }

   }

 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {

   if (!mr.is_empty()) {

     jbyte* cur_entry = byte_for(mr.start());

     jbyte* limit = byte_after(mr.last());

     // The region mr may not start on a card boundary so

     // the first card may reflect a write to the space

     // just prior to mr.

     if (!is_aligned(mr.start())) {

       cur_entry++;

     }

     for (;cur_entry < limit; cur_entry++) {

       guarantee(*cur_entry == CardTableModRefBS::clean_card,

                 "Unexpected dirty card found");

     }

   }

 }