--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/memory/cardTableRS.cpp Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,568 @@
+/*
+ * Copyright 2001-2006 Sun Microsystems, Inc. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * 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.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ *
+ */
+
+# include "incls/_precompiled.incl"
+# include "incls/_cardTableRS.cpp.incl"
+
+CardTableRS::CardTableRS(MemRegion whole_heap,
+ int max_covered_regions) :
+ GenRemSet(&_ct_bs),
+ _ct_bs(whole_heap, max_covered_regions),
+ _cur_youngergen_card_val(youngergenP1_card)
+{
+ _last_cur_val_in_gen = new jbyte[GenCollectedHeap::max_gens + 1];
+ if (_last_cur_val_in_gen == NULL) {
+ vm_exit_during_initialization("Could not 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);
+}
+
+void CardTableRS::resize_covered_region(MemRegion new_region) {
+ _ct_bs.resize_covered_region(new_region);
+}
+
+jbyte CardTableRS::find_unused_youngergenP_card_value() {
+ GenCollectedHeap* gch = GenCollectedHeap::heap();
+ for (jbyte v = youngergenP1_card;
+ v < cur_youngergen_and_prev_nonclean_card;
+ v++) {
+ bool seen = false;
+ for (int g = 0; g < gch->n_gens()+1; 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);
+}
+
+class ClearNoncleanCardWrapper: public MemRegionClosure {
+ MemRegionClosure* _dirty_card_closure;
+ CardTableRS* _ct;
+ bool _is_par;
+private:
+ // Clears the given card, return true if the corresponding card should be
+ // processed.
+ bool clear_card(jbyte* entry) {
+ if (_is_par) {
+ 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;
+ } else {
+ 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;
+ }
+ }
+
+public:
+ ClearNoncleanCardWrapper(MemRegionClosure* dirty_card_closure,
+ CardTableRS* ct) :
+ _dirty_card_closure(dirty_card_closure), _ct(ct) {
+ _is_par = (SharedHeap::heap()->n_par_threads() > 0);
+ }
+ void do_MemRegion(MemRegion mr) {
+ // We start at the high end of "mr", walking backwards
+ // while accumulating a contiguous dirty range of cards in
+ // [start_of_non_clean, end_of_non_clean) which we then
+ // process en masse.
+ HeapWord* end_of_non_clean = mr.end();
+ HeapWord* start_of_non_clean = end_of_non_clean;
+ jbyte* entry = _ct->byte_for(mr.last());
+ const jbyte* first_entry = _ct->byte_for(mr.start());
+ while (entry >= first_entry) {
+ HeapWord* cur = _ct->addr_for(entry);
+ if (!clear_card(entry)) {
+ // We hit a clean card; process any non-empty
+ // dirty range accumulated so far.
+ if (start_of_non_clean < end_of_non_clean) {
+ MemRegion mr2(start_of_non_clean, end_of_non_clean);
+ _dirty_card_closure->do_MemRegion(mr2);
+ }
+ // Reset the dirty window while continuing to
+ // look for the next dirty window to process.
+ end_of_non_clean = cur;
+ start_of_non_clean = end_of_non_clean;
+ }
+ // Open the left end of the window one card to the left.
+ start_of_non_clean = cur;
+ // Note that "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".
+ 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) {
+ MemRegion mr2(start_of_non_clean, end_of_non_clean);
+ _dirty_card_closure->do_MemRegion(mr2);
+ }
+ }
+};
+// 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(oop* 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) {
+ DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, _ct_bs.precision(),
+ cl->gen_boundary());
+ ClearNoncleanCardWrapper clear_cl(dcto_cl, this);
+
+ _ct_bs.non_clean_card_iterate(sp, sp->used_region_at_save_marks(),
+ dcto_cl, &clear_cl, false);
+}
+
+void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) {
+ GenCollectedHeap* gch = GenCollectedHeap::heap();
+ // Generations younger than gen have been evacuated. We can clear
+ // card table entries for gen (we know that it has no pointers
+ // to younger gens) and for those below. The card tables for
+ // the youngest gen need never be cleared, and those for perm gen
+ // will be cleared based on the parameter clear_perm.
+ // 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
+ Generation* g = gen;
+ for(Generation* prev_gen = gch->prev_gen(g);
+ prev_gen != NULL;
+ g = prev_gen, prev_gen = gch->prev_gen(g)) {
+ MemRegion to_be_cleared_mr = g->prev_used_region();
+ clear(to_be_cleared_mr);
+ }
+ // Clear perm gen cards if asked to do so.
+ if (clear_perm) {
+ MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region();
+ clear(to_be_cleared_mr);
+ }
+}
+
+void CardTableRS::invalidate_or_clear(Generation* gen, bool younger,
+ bool perm) {
+ GenCollectedHeap* gch = GenCollectedHeap::heap();
+ // For each generation gen (and younger and/or perm)
+ // invalidate the cards for the currently occupied part
+ // of that 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.
+ Generation* g = gen;
+ for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
+ g = prev_gen, prev_gen = gch->prev_gen(g)) {
+ MemRegion used_mr = g->used_region();
+ MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
+ if (!to_be_cleared_mr.is_empty()) {
+ clear(to_be_cleared_mr);
+ }
+ invalidate(used_mr);
+ if (!younger) break;
+ }
+ // Clear perm gen cards if asked to do so.
+ if (perm) {
+ g = gch->perm_gen();
+ MemRegion used_mr = g->used_region();
+ MemRegion to_be_cleared_mr = g->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 {
+ HeapWord* boundary;
+ HeapWord* begin; HeapWord* end;
+public:
+ void do_oop(oop* p) {
+ HeapWord* jp = (HeapWord*)p;
+ if (jp >= begin && jp < end) {
+ guarantee(*p == NULL || (HeapWord*)p < boundary
+ || (HeapWord*)(*p) >= boundary,
+ "pointer on clean card crosses boundary");
+ }
+ }
+ VerifyCleanCardClosure(HeapWord* b, HeapWord* _begin, HeapWord* _end) :
+ boundary(b), begin(_begin), end(_end) {}
+};
+
+class VerifyCTSpaceClosure: public SpaceClosure {
+ CardTableRS* _ct;
+ HeapWord* _boundary;
+public:
+ VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
+ _ct(ct), _boundary(boundary) {}
+ 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.
+ HeapWord* cur = start_block;
+ VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
+ while (cur < end) {
+ if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
+ oop(cur)->oop_iterate(&verify_blk);
+ }
+ cur += s->block_size(cur);
+ }
+ 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();
+ // We will do the perm-gen portion of the card table, too.
+ Generation* pg = SharedHeap::heap()->perm_gen();
+ HeapWord* pg_boundary = pg->reserved().start();
+
+ if (ch->kind() == CollectedHeap::GenCollectedHeap) {
+ GenCollectedHeap::heap()->generation_iterate(&blk, false);
+ _ct_bs.verify();
+
+ // If the old gen collections also collect perm, then we are only
+ // interested in perm-to-young pointers, not perm-to-old pointers.
+ GenCollectedHeap* gch = GenCollectedHeap::heap();
+ CollectorPolicy* cp = gch->collector_policy();
+ if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) {
+ pg_boundary = gch->get_gen(1)->reserved().start();
+ }
+ }
+ VerifyCTSpaceClosure perm_space_blk(this, pg_boundary);
+ SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true);
+}
+
+
+void CardTableRS::verify_empty(MemRegion mr) {
+ if (!mr.is_empty()) {
+ jbyte* cur_entry = byte_for(mr.start());
+ jbyte* limit = byte_after(mr.last());
+ for (;cur_entry < limit; cur_entry++) {
+ guarantee(*cur_entry == CardTableModRefBS::clean_card,
+ "Unexpected dirty card found");
+ }
+ }
+}