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 /*

  * Copyright (c) 2001, 2012, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  *

  */

 #include "precompiled.hpp"

 #include "gc_implementation/parallelScavenge/cardTableExtension.hpp"

 #include "gc_implementation/parallelScavenge/gcTaskManager.hpp"

 #include "gc_implementation/parallelScavenge/parallelScavengeHeap.hpp"

 #include "gc_implementation/parallelScavenge/psTasks.hpp"

 #include "gc_implementation/parallelScavenge/psYoungGen.hpp"

 #include "oops/oop.inline.hpp"

 #include "oops/oop.psgc.inline.hpp"

 // Checks an individual oop for missing precise marks. Mark

 // may be either dirty or newgen.

 class CheckForUnmarkedOops : public OopClosure {

  private:

   PSYoungGen*         _young_gen;

   CardTableExtension* _card_table;

   HeapWord*           _unmarked_addr;

   jbyte*              _unmarked_card;

  protected:

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

     oop obj = oopDesc::load_decode_heap_oop(p);

     if (_young_gen->is_in_reserved(obj) &&

         !_card_table->addr_is_marked_imprecise(p)) {

       // Don't overwrite the first missing card mark

       if (_unmarked_addr == NULL) {

         _unmarked_addr = (HeapWord*)p;

         _unmarked_card = _card_table->byte_for(p);

       }

     }

   }

  public:

   CheckForUnmarkedOops(PSYoungGen* young_gen, CardTableExtension* card_table) :

     _young_gen(young_gen), _card_table(card_table), _unmarked_addr(NULL) { }

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

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

   bool has_unmarked_oop() {

     return _unmarked_addr != NULL;

   }

 };

 // Checks all objects for the existance of some type of mark,

 // precise or imprecise, dirty or newgen.

 class CheckForUnmarkedObjects : public ObjectClosure {

  private:

   PSYoungGen*         _young_gen;

   CardTableExtension* _card_table;

  public:

   CheckForUnmarkedObjects() {

     ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();

     assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");

     _young_gen = heap->young_gen();

     _card_table = (CardTableExtension*)heap->barrier_set();

     // No point in asserting barrier set type here. Need to make CardTableExtension

     // a unique barrier set type.

   }

   // Card marks are not precise. The current system can leave us with

   // a mismash of precise marks and beginning of object marks. This means

   // we test for missing precise marks first. If any are found, we don't

   // fail unless the object head is also unmarked.

   virtual void do_object(oop obj) {

     CheckForUnmarkedOops object_check(_young_gen, _card_table);

     obj->oop_iterate_no_header(&object_check);

     if (object_check.has_unmarked_oop()) {

       assert(_card_table->addr_is_marked_imprecise(obj), "Found unmarked young_gen object");

     }

   }

 };

 // Checks for precise marking of oops as newgen.

 class CheckForPreciseMarks : public OopClosure {

  private:

   PSYoungGen*         _young_gen;

   CardTableExtension* _card_table;

  protected:

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

     oop obj = oopDesc::load_decode_heap_oop_not_null(p);

     if (_young_gen->is_in_reserved(obj)) {

       assert(_card_table->addr_is_marked_precise(p), "Found unmarked precise oop");

       _card_table->set_card_newgen(p);

     }

   }

  public:

   CheckForPreciseMarks( PSYoungGen* young_gen, CardTableExtension* card_table ) :

     _young_gen(young_gen), _card_table(card_table) { }

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

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

 };

 // We get passed the space_top value to prevent us from traversing into

 // the old_gen promotion labs, which cannot be safely parsed.

 // Do not call this method if the space is empty.

 // It is a waste to start tasks and get here only to

 // do no work.  If this method needs to be called

 // when the space is empty, fix the calculation of

 // end_card to allow sp_top == sp->bottom().

 void CardTableExtension::scavenge_contents_parallel(ObjectStartArray* start_array,

                                                     MutableSpace* sp,

                                                     HeapWord* space_top,

                                                     PSPromotionManager* pm,

                                                     uint stripe_number,

                                                     uint stripe_total) {

   int ssize = 128; // Naked constant!  Work unit = 64k.

   int dirty_card_count = 0;

   // It is a waste to get here if empty.

   assert(sp->bottom() < sp->top(), "Should not be called if empty");

   oop* sp_top = (oop*)space_top;

   jbyte* start_card = byte_for(sp->bottom());

   jbyte* end_card   = byte_for(sp_top - 1) + 1;

   oop* last_scanned = NULL; // Prevent scanning objects more than once

   // The width of the stripe ssize*stripe_total must be

   // consistent with the number of stripes so that the complete slice

   // is covered.

   size_t slice_width = ssize * stripe_total;

   for (jbyte* slice = start_card; slice < end_card; slice += slice_width) {

     jbyte* worker_start_card = slice + stripe_number * ssize;

     if (worker_start_card >= end_card)

       return; // We're done.

     jbyte* worker_end_card = worker_start_card + ssize;

     if (worker_end_card > end_card)

       worker_end_card = end_card;

     // We do not want to scan objects more than once. In order to accomplish

     // this, we assert that any object with an object head inside our 'slice'

     // belongs to us. We may need to extend the range of scanned cards if the

     // last object continues into the next 'slice'.

     //

     // Note! ending cards are exclusive!

     HeapWord* slice_start = addr_for(worker_start_card);

     HeapWord* slice_end = MIN2((HeapWord*) sp_top, addr_for(worker_end_card));

 #ifdef ASSERT

     if (GCWorkerDelayMillis > 0) {

       // Delay 1 worker so that it proceeds after all the work

       // has been completed.

       if (stripe_number < 2) {

         os::sleep(Thread::current(), GCWorkerDelayMillis, false);

       }

     }

 #endif

     // If there are not objects starting within the chunk, skip it.

     if (!start_array->object_starts_in_range(slice_start, slice_end)) {

       continue;

     }

     // Update our beginning addr

     HeapWord* first_object = start_array->object_start(slice_start);

     debug_only(oop* first_object_within_slice = (oop*) first_object;)

     if (first_object < slice_start) {

       last_scanned = (oop*)(first_object + oop(first_object)->size());

       debug_only(first_object_within_slice = last_scanned;)

       worker_start_card = byte_for(last_scanned);

     }

     // Update the ending addr

     if (slice_end < (HeapWord*)sp_top) {

       // The subtraction is important! An object may start precisely at slice_end.

       HeapWord* last_object = start_array->object_start(slice_end - 1);

       slice_end = last_object + oop(last_object)->size();

       // worker_end_card is exclusive, so bump it one past the end of last_object's

       // covered span.

       worker_end_card = byte_for(slice_end) + 1;

       if (worker_end_card > end_card)

         worker_end_card = end_card;

     }

     assert(slice_end <= (HeapWord*)sp_top, "Last object in slice crosses space boundary");

     assert(is_valid_card_address(worker_start_card), "Invalid worker start card");

     assert(is_valid_card_address(worker_end_card), "Invalid worker end card");

     // Note that worker_start_card >= worker_end_card is legal, and happens when

     // an object spans an entire slice.

     assert(worker_start_card <= end_card, "worker start card beyond end card");

     assert(worker_end_card <= end_card, "worker end card beyond end card");

     jbyte* current_card = worker_start_card;

     while (current_card < worker_end_card) {

       // Find an unclean card.

       while (current_card < worker_end_card && card_is_clean(*current_card)) {

         current_card++;

       }

       jbyte* first_unclean_card = current_card;

       // Find the end of a run of contiguous unclean cards

       while (current_card < worker_end_card && !card_is_clean(*current_card)) {

         while (current_card < worker_end_card && !card_is_clean(*current_card)) {

           current_card++;

         }

         if (current_card < worker_end_card) {

           // Some objects may be large enough to span several cards. If such

           // an object has more than one dirty card, separated by a clean card,

           // we will attempt to scan it twice. The test against "last_scanned"

           // prevents the redundant object scan, but it does not prevent newly

           // marked cards from being cleaned.

           HeapWord* last_object_in_dirty_region = start_array->object_start(addr_for(current_card)-1);

           size_t size_of_last_object = oop(last_object_in_dirty_region)->size();

           HeapWord* end_of_last_object = last_object_in_dirty_region + size_of_last_object;

           jbyte* ending_card_of_last_object = byte_for(end_of_last_object);

           assert(ending_card_of_last_object <= worker_end_card, "ending_card_of_last_object is greater than worker_end_card");

           if (ending_card_of_last_object > current_card) {

             // This means the object spans the next complete card.

             // We need to bump the current_card to ending_card_of_last_object

             current_card = ending_card_of_last_object;

           }

         }

       }

       jbyte* following_clean_card = current_card;

       if (first_unclean_card < worker_end_card) {

         oop* p = (oop*) start_array->object_start(addr_for(first_unclean_card));

         assert((HeapWord*)p <= addr_for(first_unclean_card), "checking");

         // "p" should always be >= "last_scanned" because newly GC dirtied

         // cards are no longer scanned again (see comment at end

         // of loop on the increment of "current_card").  Test that

         // hypothesis before removing this code.

         // If this code is removed, deal with the first time through

         // the loop when the last_scanned is the object starting in

         // the previous slice.

         assert((p >= last_scanned) ||

                (last_scanned == first_object_within_slice),

                "Should no longer be possible");

         if (p < last_scanned) {

           // Avoid scanning more than once; this can happen because

           // newgen cards set by GC may a different set than the

           // originally dirty set

           p = last_scanned;

         }

         oop* to = (oop*)addr_for(following_clean_card);

         // Test slice_end first!

         if ((HeapWord*)to > slice_end) {

           to = (oop*)slice_end;

         } else if (to > sp_top) {

           to = sp_top;

         }

         // we know which cards to scan, now clear them

         if (first_unclean_card <= worker_start_card+1)

           first_unclean_card = worker_start_card+1;

         if (following_clean_card >= worker_end_card-1)

           following_clean_card = worker_end_card-1;

         while (first_unclean_card < following_clean_card) {

           *first_unclean_card++ = clean_card;

         }

         const int interval = PrefetchScanIntervalInBytes;

         // scan all objects in the range

         if (interval != 0) {

           while (p < to) {

             Prefetch::write(p, interval);

             oop m = oop(p);

             assert(m->is_oop_or_null(), "check for header");

             m->push_contents(pm);

             p += m->size();

           }

           pm->drain_stacks_cond_depth();

         } else {

           while (p < to) {

             oop m = oop(p);

             assert(m->is_oop_or_null(), "check for header");

             m->push_contents(pm);

             p += m->size();

           }

           pm->drain_stacks_cond_depth();

         }

         last_scanned = p;

       }

       // "current_card" is still the "following_clean_card" or

       // the current_card is >= the worker_end_card so the

       // loop will not execute again.

       assert((current_card == following_clean_card) ||

              (current_card >= worker_end_card),

         "current_card should only be incremented if it still equals "

         "following_clean_card");

       // Increment current_card so that it is not processed again.

       // It may now be dirty because a old-to-young pointer was

       // found on it an updated.  If it is now dirty, it cannot be

       // be safely cleaned in the next iteration.

       current_card++;

     }

   }

 }

 // This should be called before a scavenge.

 void CardTableExtension::verify_all_young_refs_imprecise() {

   CheckForUnmarkedObjects check;

   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();

   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");

   PSOldGen* old_gen = heap->old_gen();

   old_gen->object_iterate(&check);

 }

 // This should be called immediately after a scavenge, before mutators resume.

 void CardTableExtension::verify_all_young_refs_precise() {

   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();

   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");

   PSOldGen* old_gen = heap->old_gen();

   CheckForPreciseMarks check(heap->young_gen(), (CardTableExtension*)heap->barrier_set());

   old_gen->oop_iterate_no_header(&check);

   verify_all_young_refs_precise_helper(old_gen->object_space()->used_region());

 }

 void CardTableExtension::verify_all_young_refs_precise_helper(MemRegion mr) {

   CardTableExtension* card_table = (CardTableExtension*)Universe::heap()->barrier_set();

   // FIX ME ASSERT HERE

   jbyte* bot = card_table->byte_for(mr.start());

   jbyte* top = card_table->byte_for(mr.end());

   while(bot <= top) {

     assert(*bot == clean_card || *bot == verify_card, "Found unwanted or unknown card mark");

     if (*bot == verify_card)

       *bot = youngergen_card;

     bot++;

   }

 }

 bool CardTableExtension::addr_is_marked_imprecise(void *addr) {

   jbyte* p = byte_for(addr);

   jbyte val = *p;

   if (card_is_dirty(val))

     return true;

   if (card_is_newgen(val))

     return true;

   if (card_is_clean(val))

     return false;

   assert(false, "Found unhandled card mark type");

   return false;

 }

 // Also includes verify_card

 bool CardTableExtension::addr_is_marked_precise(void *addr) {

   jbyte* p = byte_for(addr);

   jbyte val = *p;

   if (card_is_newgen(val))

     return true;

   if (card_is_verify(val))

     return true;

   if (card_is_clean(val))

     return false;

   if (card_is_dirty(val))

     return false;

   assert(false, "Found unhandled card mark type");

   return false;

 }

 // Assumes that only the base or the end changes.  This allows indentification

 // of the region that is being resized.  The

 // CardTableModRefBS::resize_covered_region() is used for the normal case

 // where the covered regions are growing or shrinking at the high end.

 // The method resize_covered_region_by_end() is analogous to

 // CardTableModRefBS::resize_covered_region() but

 // for regions that grow or shrink at the low end.

 void CardTableExtension::resize_covered_region(MemRegion new_region) {

   for (int i = 0; i < _cur_covered_regions; i++) {

     if (_covered[i].start() == new_region.start()) {

       // Found a covered region with the same start as the

       // new region.  The region is growing or shrinking

       // from the start of the region.

       resize_covered_region_by_start(new_region);

       return;

     }

     if (_covered[i].start() > new_region.start()) {

       break;

     }

   }

   int changed_region = -1;

   for (int j = 0; j < _cur_covered_regions; j++) {

     if (_covered[j].end() == new_region.end()) {

       changed_region = j;

       // This is a case where the covered region is growing or shrinking

       // at the start of the region.

       assert(changed_region != -1, "Don't expect to add a covered region");

       assert(_covered[changed_region].byte_size() != new_region.byte_size(),

         "The sizes should be different here");

       resize_covered_region_by_end(changed_region, new_region);

       return;

     }

   }

   // This should only be a new covered region (where no existing

   // covered region matches at the start or the end).

   assert(_cur_covered_regions < _max_covered_regions,

     "An existing region should have been found");

   resize_covered_region_by_start(new_region);

 }

 void CardTableExtension::resize_covered_region_by_start(MemRegion new_region) {

   CardTableModRefBS::resize_covered_region(new_region);

   debug_only(verify_guard();)

 }

 void CardTableExtension::resize_covered_region_by_end(int changed_region,

                                                       MemRegion new_region) {

   assert(SafepointSynchronize::is_at_safepoint(),

     "Only expect an expansion at the low end at a GC");

   debug_only(verify_guard();)

 #ifdef ASSERT

   for (int k = 0; k < _cur_covered_regions; k++) {

     if (_covered[k].end() == new_region.end()) {

       assert(changed_region == k, "Changed region is incorrect");

       break;

     }

   }

 #endif

   // Commit new or uncommit old pages, if necessary.

   if (resize_commit_uncommit(changed_region, new_region)) {

     // Set the new start of the committed region

     resize_update_committed_table(changed_region, new_region);

   }

   // Update card table entries

   resize_update_card_table_entries(changed_region, new_region);

   // Update the covered region

   resize_update_covered_table(changed_region, new_region);

   if (TraceCardTableModRefBS) {

     int ind = changed_region;

     gclog_or_tty->print_cr("CardTableModRefBS::resize_covered_region: ");

     gclog_or_tty->print_cr("  "

                   "  _covered[%d].start(): " INTPTR_FORMAT

                   "  _covered[%d].last(): " INTPTR_FORMAT,

                   ind, _covered[ind].start(),

                   ind, _covered[ind].last());

     gclog_or_tty->print_cr("  "

                   "  _committed[%d].start(): " INTPTR_FORMAT

                   "  _committed[%d].last(): " INTPTR_FORMAT,

                   ind, _committed[ind].start(),

                   ind, _committed[ind].last());

     gclog_or_tty->print_cr("  "

                   "  byte_for(start): " INTPTR_FORMAT

                   "  byte_for(last): " INTPTR_FORMAT,

                   byte_for(_covered[ind].start()),

                   byte_for(_covered[ind].last()));

     gclog_or_tty->print_cr("  "

                   "  addr_for(start): " INTPTR_FORMAT

                   "  addr_for(last): " INTPTR_FORMAT,

                   addr_for((jbyte*) _committed[ind].start()),

                   addr_for((jbyte*) _committed[ind].last()));

   }

   debug_only(verify_guard();)

 }

 bool CardTableExtension::resize_commit_uncommit(int changed_region,

                                                 MemRegion new_region) {

   bool result = false;

   // Commit new or uncommit old pages, if necessary.

   MemRegion cur_committed = _committed[changed_region];

   assert(_covered[changed_region].end() == new_region.end(),

     "The ends of the regions are expected to match");

   // Extend the start of this _committed region to

   // to cover the start of any previous _committed region.

   // This forms overlapping regions, but never interior regions.

   HeapWord* min_prev_start = lowest_prev_committed_start(changed_region);

   if (min_prev_start < cur_committed.start()) {

     // Only really need to set start of "cur_committed" to

     // the new start (min_prev_start) but assertion checking code

     // below use cur_committed.end() so make it correct.

     MemRegion new_committed =

         MemRegion(min_prev_start, cur_committed.end());

     cur_committed = new_committed;

   }

 #ifdef ASSERT

   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();

   assert(cur_committed.start() ==

     (HeapWord*) align_size_up((uintptr_t) cur_committed.start(),

                               os::vm_page_size()),

     "Starts should have proper alignment");

 #endif

   jbyte* new_start = byte_for(new_region.start());

   // Round down because this is for the start address

   HeapWord* new_start_aligned =

     (HeapWord*)align_size_down((uintptr_t)new_start, os::vm_page_size());

   // The guard page is always committed and should not be committed over.

   // This method is used in cases where the generation is growing toward

   // lower addresses but the guard region is still at the end of the

   // card table.  That still makes sense when looking for writes

   // off the end of the card table.

   if (new_start_aligned < cur_committed.start()) {

     // Expand the committed region

     //

     // Case A

     //                                          |+ guard +|

     //                          |+ cur committed +++++++++|

     //                  |+ new committed +++++++++++++++++|

     //

     // Case B

     //                                          |+ guard +|

     //                        |+ cur committed +|

     //                  |+ new committed +++++++|

     //

     // These are not expected because the calculation of the

     // cur committed region and the new committed region

     // share the same end for the covered region.

     // Case C

     //                                          |+ guard +|

     //                        |+ cur committed +|

     //                  |+ new committed +++++++++++++++++|

     // Case D

     //                                          |+ guard +|

     //                        |+ cur committed +++++++++++|

     //                  |+ new committed +++++++|

     HeapWord* new_end_for_commit =

       MIN2(cur_committed.end(), _guard_region.start());

     if(new_start_aligned < new_end_for_commit) {

       MemRegion new_committed =

         MemRegion(new_start_aligned, new_end_for_commit);

       if (!os::commit_memory((char*)new_committed.start(),

                              new_committed.byte_size())) {

         vm_exit_out_of_memory(new_committed.byte_size(), OOM_MMAP_ERROR,

                               "card table expansion");

       }

     }

     result = true;

   } else if (new_start_aligned > cur_committed.start()) {

     // Shrink the committed region

 #if 0 // uncommitting space is currently unsafe because of the interactions

       // of growing and shrinking regions.  One region A can uncommit space

       // that it owns but which is being used by another region B (maybe).

       // Region B has not committed the space because it was already

       // committed by region A.

     MemRegion uncommit_region = committed_unique_to_self(changed_region,

       MemRegion(cur_committed.start(), new_start_aligned));

     if (!uncommit_region.is_empty()) {

       if (!os::uncommit_memory((char*)uncommit_region.start(),

                                uncommit_region.byte_size())) {

         // If the uncommit fails, ignore it.  Let the

         // committed table resizing go even though the committed

         // table will over state the committed space.

       }

     }

 #else

     assert(!result, "Should be false with current workaround");

 #endif

   }

   assert(_committed[changed_region].end() == cur_committed.end(),

     "end should not change");

   return result;

 }

 void CardTableExtension::resize_update_committed_table(int changed_region,

                                                        MemRegion new_region) {

   jbyte* new_start = byte_for(new_region.start());

   // Set the new start of the committed region

   HeapWord* new_start_aligned =

     (HeapWord*)align_size_down((uintptr_t)new_start,

                              os::vm_page_size());

   MemRegion new_committed = MemRegion(new_start_aligned,

     _committed[changed_region].end());

   _committed[changed_region] = new_committed;

   _committed[changed_region].set_start(new_start_aligned);

 }

 void CardTableExtension::resize_update_card_table_entries(int changed_region,

                                                           MemRegion new_region) {

   debug_only(verify_guard();)

   MemRegion original_covered = _covered[changed_region];

   // Initialize the card entries.  Only consider the

   // region covered by the card table (_whole_heap)

   jbyte* entry;

   if (new_region.start() < _whole_heap.start()) {

     entry = byte_for(_whole_heap.start());

   } else {

     entry = byte_for(new_region.start());

   }

   jbyte* end = byte_for(original_covered.start());

   // If _whole_heap starts at the original covered regions start,

   // this loop will not execute.

   while (entry < end) { *entry++ = clean_card; }

 }

 void CardTableExtension::resize_update_covered_table(int changed_region,

                                                      MemRegion new_region) {

   // Update the covered region

   _covered[changed_region].set_start(new_region.start());

   _covered[changed_region].set_word_size(new_region.word_size());

   // reorder regions.  There should only be at most 1 out

   // of order.

   for (int i = _cur_covered_regions-1 ; i > 0; i--) {

     if (_covered[i].start() < _covered[i-1].start()) {

         MemRegion covered_mr = _covered[i-1];

         _covered[i-1] = _covered[i];

         _covered[i] = covered_mr;

         MemRegion committed_mr = _committed[i-1];

       _committed[i-1] = _committed[i];

       _committed[i] = committed_mr;

       break;

     }

   }

 #ifdef ASSERT

   for (int m = 0; m < _cur_covered_regions-1; m++) {

     assert(_covered[m].start() <= _covered[m+1].start(),

       "Covered regions out of order");

     assert(_committed[m].start() <= _committed[m+1].start(),

       "Committed regions out of order");

   }

 #endif

 }

 // Returns the start of any committed region that is lower than

 // the target committed region (index ind) and that intersects the

 // target region.  If none, return start of target region.

 //

 //      -------------

 //      |           |

 //      -------------

 //              ------------

 //              | target   |

 //              ------------

 //                               -------------

 //                               |           |

 //                               -------------

 //      ^ returns this

 //

 //      -------------

 //      |           |

 //      -------------

 //                      ------------

 //                      | target   |

 //                      ------------

 //                               -------------

 //                               |           |

 //                               -------------

 //                      ^ returns this

 HeapWord* CardTableExtension::lowest_prev_committed_start(int ind) const {

   assert(_cur_covered_regions >= 0, "Expecting at least on region");

   HeapWord* min_start = _committed[ind].start();

   for (int j = 0; j < ind; j++) {

     HeapWord* this_start = _committed[j].start();

     if ((this_start < min_start) &&

         !(_committed[j].intersection(_committed[ind])).is_empty()) {

        min_start = this_start;

     }

   }

   return min_start;

 }