1 /*
   2  * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "gc/parallel/cardTableExtension.hpp"
  27 #include "gc/parallel/gcTaskManager.hpp"
  28 #include "gc/parallel/parallelScavengeHeap.hpp"
  29 #include "gc/parallel/psPromotionManager.inline.hpp"
  30 #include "gc/parallel/psScavenge.hpp"
  31 #include "gc/parallel/psTasks.hpp"
  32 #include "gc/parallel/psYoungGen.hpp"
  33 #include "oops/oop.inline.hpp"
  34 #include "runtime/prefetch.inline.hpp"
  35 
  36 // Checks an individual oop for missing precise marks. Mark
  37 // may be either dirty or newgen.
  38 class CheckForUnmarkedOops : public OopClosure {
  39  private:
  40   PSYoungGen*         _young_gen;
  41   CardTableExtension* _card_table;
  42   HeapWord*           _unmarked_addr;
  43 
  44  protected:
  45   template <class T> void do_oop_work(T* p) {
  46     oop obj = oopDesc::load_decode_heap_oop(p);
  47     if (_young_gen->is_in_reserved(obj) &&
  48         !_card_table->addr_is_marked_imprecise(p)) {
  49       // Don't overwrite the first missing card mark
  50       if (_unmarked_addr == NULL) {
  51         _unmarked_addr = (HeapWord*)p;
  52       }
  53     }
  54   }
  55 
  56  public:
  57   CheckForUnmarkedOops(PSYoungGen* young_gen, CardTableExtension* card_table) :
  58     _young_gen(young_gen), _card_table(card_table), _unmarked_addr(NULL) { }
  59 
  60   virtual void do_oop(oop* p)       { CheckForUnmarkedOops::do_oop_work(p); }
  61   virtual void do_oop(narrowOop* p) { CheckForUnmarkedOops::do_oop_work(p); }
  62 
  63   bool has_unmarked_oop() {
  64     return _unmarked_addr != NULL;
  65   }
  66 };
  67 
  68 // Checks all objects for the existence of some type of mark,
  69 // precise or imprecise, dirty or newgen.
  70 class CheckForUnmarkedObjects : public ObjectClosure {
  71  private:
  72   PSYoungGen*         _young_gen;
  73   CardTableExtension* _card_table;
  74 
  75  public:
  76   CheckForUnmarkedObjects() {
  77     ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  78     _young_gen = heap->young_gen();
  79     _card_table = barrier_set_cast<CardTableExtension>(heap->barrier_set());
  80     // No point in asserting barrier set type here. Need to make CardTableExtension
  81     // a unique barrier set type.
  82   }
  83 
  84   // Card marks are not precise. The current system can leave us with
  85   // a mismatch of precise marks and beginning of object marks. This means
  86   // we test for missing precise marks first. If any are found, we don't
  87   // fail unless the object head is also unmarked.
  88   virtual void do_object(oop obj) {
  89     CheckForUnmarkedOops object_check(_young_gen, _card_table);
  90     obj->oop_iterate_no_header(&object_check);
  91     if (object_check.has_unmarked_oop()) {
  92       guarantee(_card_table->addr_is_marked_imprecise(obj), "Found unmarked young_gen object");
  93     }
  94   }
  95 };
  96 
  97 // Checks for precise marking of oops as newgen.
  98 class CheckForPreciseMarks : public OopClosure {
  99  private:
 100   PSYoungGen*         _young_gen;
 101   CardTableExtension* _card_table;
 102 
 103  protected:
 104   template <class T> void do_oop_work(T* p) {
 105     oop obj = oopDesc::load_decode_heap_oop_not_null(p);
 106     if (_young_gen->is_in_reserved(obj)) {
 107       assert(_card_table->addr_is_marked_precise(p), "Found unmarked precise oop");
 108       _card_table->set_card_newgen(p);
 109     }
 110   }
 111 
 112  public:
 113   CheckForPreciseMarks( PSYoungGen* young_gen, CardTableExtension* card_table ) :
 114     _young_gen(young_gen), _card_table(card_table) { }
 115 
 116   virtual void do_oop(oop* p)       { CheckForPreciseMarks::do_oop_work(p); }
 117   virtual void do_oop(narrowOop* p) { CheckForPreciseMarks::do_oop_work(p); }
 118 };
 119 
 120 // We get passed the space_top value to prevent us from traversing into
 121 // the old_gen promotion labs, which cannot be safely parsed.
 122 
 123 // Do not call this method if the space is empty.
 124 // It is a waste to start tasks and get here only to
 125 // do no work.  If this method needs to be called
 126 // when the space is empty, fix the calculation of
 127 // end_card to allow sp_top == sp->bottom().
 128 
 129 void CardTableExtension::scavenge_contents_parallel(ObjectStartArray* start_array,
 130                                                     MutableSpace* sp,
 131                                                     HeapWord* space_top,
 132                                                     PSPromotionManager* pm,
 133                                                     uint stripe_number,
 134                                                     uint stripe_total) {
 135   int ssize = 128; // Naked constant!  Work unit = 64k.
 136   int dirty_card_count = 0;
 137 
 138   // It is a waste to get here if empty.
 139   assert(sp->bottom() < sp->top(), "Should not be called if empty");
 140   oop* sp_top = (oop*)space_top;
 141   jbyte* start_card = byte_for(sp->bottom());
 142   jbyte* end_card   = byte_for(sp_top - 1) + 1;
 143   oop* last_scanned = NULL; // Prevent scanning objects more than once
 144   // The width of the stripe ssize*stripe_total must be
 145   // consistent with the number of stripes so that the complete slice
 146   // is covered.
 147   size_t slice_width = ssize * stripe_total;
 148   for (jbyte* slice = start_card; slice < end_card; slice += slice_width) {
 149     jbyte* worker_start_card = slice + stripe_number * ssize;
 150     if (worker_start_card >= end_card)
 151       return; // We're done.
 152 
 153     jbyte* worker_end_card = worker_start_card + ssize;
 154     if (worker_end_card > end_card)
 155       worker_end_card = end_card;
 156 
 157     // We do not want to scan objects more than once. In order to accomplish
 158     // this, we assert that any object with an object head inside our 'slice'
 159     // belongs to us. We may need to extend the range of scanned cards if the
 160     // last object continues into the next 'slice'.
 161     //
 162     // Note! ending cards are exclusive!
 163     HeapWord* slice_start = addr_for(worker_start_card);
 164     HeapWord* slice_end = MIN2((HeapWord*) sp_top, addr_for(worker_end_card));
 165 
 166 #ifdef ASSERT
 167     if (GCWorkerDelayMillis > 0) {
 168       // Delay 1 worker so that it proceeds after all the work
 169       // has been completed.
 170       if (stripe_number < 2) {
 171         os::sleep(Thread::current(), GCWorkerDelayMillis, false);
 172       }
 173     }
 174 #endif
 175 
 176     // If there are not objects starting within the chunk, skip it.
 177     if (!start_array->object_starts_in_range(slice_start, slice_end)) {
 178       continue;
 179     }
 180     // Update our beginning addr
 181     HeapWord* first_object = start_array->object_start(slice_start);
 182     debug_only(oop* first_object_within_slice = (oop*) first_object;)
 183     if (first_object < slice_start) {
 184       last_scanned = (oop*)(first_object + oop(first_object)->size());
 185       debug_only(first_object_within_slice = last_scanned;)
 186       worker_start_card = byte_for(last_scanned);
 187     }
 188 
 189     // Update the ending addr
 190     if (slice_end < (HeapWord*)sp_top) {
 191       // The subtraction is important! An object may start precisely at slice_end.
 192       HeapWord* last_object = start_array->object_start(slice_end - 1);
 193       slice_end = last_object + oop(last_object)->size();
 194       // worker_end_card is exclusive, so bump it one past the end of last_object's
 195       // covered span.
 196       worker_end_card = byte_for(slice_end) + 1;
 197 
 198       if (worker_end_card > end_card)
 199         worker_end_card = end_card;
 200     }
 201 
 202     assert(slice_end <= (HeapWord*)sp_top, "Last object in slice crosses space boundary");
 203     assert(is_valid_card_address(worker_start_card), "Invalid worker start card");
 204     assert(is_valid_card_address(worker_end_card), "Invalid worker end card");
 205     // Note that worker_start_card >= worker_end_card is legal, and happens when
 206     // an object spans an entire slice.
 207     assert(worker_start_card <= end_card, "worker start card beyond end card");
 208     assert(worker_end_card <= end_card, "worker end card beyond end card");
 209 
 210     jbyte* current_card = worker_start_card;
 211     while (current_card < worker_end_card) {
 212       // Find an unclean card.
 213       while (current_card < worker_end_card && card_is_clean(*current_card)) {
 214         current_card++;
 215       }
 216       jbyte* first_unclean_card = current_card;
 217 
 218       // Find the end of a run of contiguous unclean cards
 219       while (current_card < worker_end_card && !card_is_clean(*current_card)) {
 220         while (current_card < worker_end_card && !card_is_clean(*current_card)) {
 221           current_card++;
 222         }
 223 
 224         if (current_card < worker_end_card) {
 225           // Some objects may be large enough to span several cards. If such
 226           // an object has more than one dirty card, separated by a clean card,
 227           // we will attempt to scan it twice. The test against "last_scanned"
 228           // prevents the redundant object scan, but it does not prevent newly
 229           // marked cards from being cleaned.
 230           HeapWord* last_object_in_dirty_region = start_array->object_start(addr_for(current_card)-1);
 231           size_t size_of_last_object = oop(last_object_in_dirty_region)->size();
 232           HeapWord* end_of_last_object = last_object_in_dirty_region + size_of_last_object;
 233           jbyte* ending_card_of_last_object = byte_for(end_of_last_object);
 234           assert(ending_card_of_last_object <= worker_end_card, "ending_card_of_last_object is greater than worker_end_card");
 235           if (ending_card_of_last_object > current_card) {
 236             // This means the object spans the next complete card.
 237             // We need to bump the current_card to ending_card_of_last_object
 238             current_card = ending_card_of_last_object;
 239           }
 240         }
 241       }
 242       jbyte* following_clean_card = current_card;
 243 
 244       if (first_unclean_card < worker_end_card) {
 245         oop* p = (oop*) start_array->object_start(addr_for(first_unclean_card));
 246         assert((HeapWord*)p <= addr_for(first_unclean_card), "checking");
 247         // "p" should always be >= "last_scanned" because newly GC dirtied
 248         // cards are no longer scanned again (see comment at end
 249         // of loop on the increment of "current_card").  Test that
 250         // hypothesis before removing this code.
 251         // If this code is removed, deal with the first time through
 252         // the loop when the last_scanned is the object starting in
 253         // the previous slice.
 254         assert((p >= last_scanned) ||
 255                (last_scanned == first_object_within_slice),
 256                "Should no longer be possible");
 257         if (p < last_scanned) {
 258           // Avoid scanning more than once; this can happen because
 259           // newgen cards set by GC may a different set than the
 260           // originally dirty set
 261           p = last_scanned;
 262         }
 263         oop* to = (oop*)addr_for(following_clean_card);
 264 
 265         // Test slice_end first!
 266         if ((HeapWord*)to > slice_end) {
 267           to = (oop*)slice_end;
 268         } else if (to > sp_top) {
 269           to = sp_top;
 270         }
 271 
 272         // we know which cards to scan, now clear them
 273         if (first_unclean_card <= worker_start_card+1)
 274           first_unclean_card = worker_start_card+1;
 275         if (following_clean_card >= worker_end_card-1)
 276           following_clean_card = worker_end_card-1;
 277 
 278         while (first_unclean_card < following_clean_card) {
 279           *first_unclean_card++ = clean_card;
 280         }
 281 
 282         const int interval = PrefetchScanIntervalInBytes;
 283         // scan all objects in the range
 284         if (interval != 0) {
 285           while (p < to) {
 286             Prefetch::write(p, interval);
 287             oop m = oop(p);
 288             assert(m->is_oop_or_null(), err_msg("Expected an oop or NULL for header field at " PTR_FORMAT, p2i(m)));
 289             pm->push_contents(m);
 290             p += m->size();
 291           }
 292           pm->drain_stacks_cond_depth();
 293         } else {
 294           while (p < to) {
 295             oop m = oop(p);
 296             assert(m->is_oop_or_null(), err_msg("Expected an oop or NULL for header field at " PTR_FORMAT, p2i(m)));
 297             pm->push_contents(m);
 298             p += m->size();
 299           }
 300           pm->drain_stacks_cond_depth();
 301         }
 302         last_scanned = p;
 303       }
 304       // "current_card" is still the "following_clean_card" or
 305       // the current_card is >= the worker_end_card so the
 306       // loop will not execute again.
 307       assert((current_card == following_clean_card) ||
 308              (current_card >= worker_end_card),
 309         "current_card should only be incremented if it still equals "
 310         "following_clean_card");
 311       // Increment current_card so that it is not processed again.
 312       // It may now be dirty because a old-to-young pointer was
 313       // found on it an updated.  If it is now dirty, it cannot be
 314       // be safely cleaned in the next iteration.
 315       current_card++;
 316     }
 317   }
 318 }
 319 
 320 // This should be called before a scavenge.
 321 void CardTableExtension::verify_all_young_refs_imprecise() {
 322   CheckForUnmarkedObjects check;
 323 
 324   ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
 325   PSOldGen* old_gen = heap->old_gen();
 326 
 327   old_gen->object_iterate(&check);
 328 }
 329 
 330 // This should be called immediately after a scavenge, before mutators resume.
 331 void CardTableExtension::verify_all_young_refs_precise() {
 332   ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
 333   PSOldGen* old_gen = heap->old_gen();
 334 
 335   CheckForPreciseMarks check(
 336     heap->young_gen(),
 337     barrier_set_cast<CardTableExtension>(heap->barrier_set()));
 338 
 339   old_gen->oop_iterate_no_header(&check);
 340 
 341   verify_all_young_refs_precise_helper(old_gen->object_space()->used_region());
 342 }
 343 
 344 void CardTableExtension::verify_all_young_refs_precise_helper(MemRegion mr) {
 345   CardTableExtension* card_table =
 346     barrier_set_cast<CardTableExtension>(ParallelScavengeHeap::heap()->barrier_set());
 347 
 348   jbyte* bot = card_table->byte_for(mr.start());
 349   jbyte* top = card_table->byte_for(mr.end());
 350   while(bot <= top) {
 351     assert(*bot == clean_card || *bot == verify_card, "Found unwanted or unknown card mark");
 352     if (*bot == verify_card)
 353       *bot = youngergen_card;
 354     bot++;
 355   }
 356 }
 357 
 358 bool CardTableExtension::addr_is_marked_imprecise(void *addr) {
 359   jbyte* p = byte_for(addr);
 360   jbyte val = *p;
 361 
 362   if (card_is_dirty(val))
 363     return true;
 364 
 365   if (card_is_newgen(val))
 366     return true;
 367 
 368   if (card_is_clean(val))
 369     return false;
 370 
 371   assert(false, "Found unhandled card mark type");
 372 
 373   return false;
 374 }
 375 
 376 // Also includes verify_card
 377 bool CardTableExtension::addr_is_marked_precise(void *addr) {
 378   jbyte* p = byte_for(addr);
 379   jbyte val = *p;
 380 
 381   if (card_is_newgen(val))
 382     return true;
 383 
 384   if (card_is_verify(val))
 385     return true;
 386 
 387   if (card_is_clean(val))
 388     return false;
 389 
 390   if (card_is_dirty(val))
 391     return false;
 392 
 393   assert(false, "Found unhandled card mark type");
 394 
 395   return false;
 396 }
 397 
 398 // Assumes that only the base or the end changes.  This allows indentification
 399 // of the region that is being resized.  The
 400 // CardTableModRefBS::resize_covered_region() is used for the normal case
 401 // where the covered regions are growing or shrinking at the high end.
 402 // The method resize_covered_region_by_end() is analogous to
 403 // CardTableModRefBS::resize_covered_region() but
 404 // for regions that grow or shrink at the low end.
 405 void CardTableExtension::resize_covered_region(MemRegion new_region) {
 406 
 407   for (int i = 0; i < _cur_covered_regions; i++) {
 408     if (_covered[i].start() == new_region.start()) {
 409       // Found a covered region with the same start as the
 410       // new region.  The region is growing or shrinking
 411       // from the start of the region.
 412       resize_covered_region_by_start(new_region);
 413       return;
 414     }
 415     if (_covered[i].start() > new_region.start()) {
 416       break;
 417     }
 418   }
 419 
 420   int changed_region = -1;
 421   for (int j = 0; j < _cur_covered_regions; j++) {
 422     if (_covered[j].end() == new_region.end()) {
 423       changed_region = j;
 424       // This is a case where the covered region is growing or shrinking
 425       // at the start of the region.
 426       assert(changed_region != -1, "Don't expect to add a covered region");
 427       assert(_covered[changed_region].byte_size() != new_region.byte_size(),
 428         "The sizes should be different here");
 429       resize_covered_region_by_end(changed_region, new_region);
 430       return;
 431     }
 432   }
 433   // This should only be a new covered region (where no existing
 434   // covered region matches at the start or the end).
 435   assert(_cur_covered_regions < _max_covered_regions,
 436     "An existing region should have been found");
 437   resize_covered_region_by_start(new_region);
 438 }
 439 
 440 void CardTableExtension::resize_covered_region_by_start(MemRegion new_region) {
 441   CardTableModRefBS::resize_covered_region(new_region);
 442   debug_only(verify_guard();)
 443 }
 444 
 445 void CardTableExtension::resize_covered_region_by_end(int changed_region,
 446                                                       MemRegion new_region) {
 447   assert(SafepointSynchronize::is_at_safepoint(),
 448     "Only expect an expansion at the low end at a GC");
 449   debug_only(verify_guard();)
 450 #ifdef ASSERT
 451   for (int k = 0; k < _cur_covered_regions; k++) {
 452     if (_covered[k].end() == new_region.end()) {
 453       assert(changed_region == k, "Changed region is incorrect");
 454       break;
 455     }
 456   }
 457 #endif
 458 
 459   // Commit new or uncommit old pages, if necessary.
 460   if (resize_commit_uncommit(changed_region, new_region)) {
 461     // Set the new start of the committed region
 462     resize_update_committed_table(changed_region, new_region);
 463   }
 464 
 465   // Update card table entries
 466   resize_update_card_table_entries(changed_region, new_region);
 467 
 468   // Update the covered region
 469   resize_update_covered_table(changed_region, new_region);
 470 
 471   if (TraceCardTableModRefBS) {
 472     int ind = changed_region;
 473     gclog_or_tty->print_cr("CardTableModRefBS::resize_covered_region: ");
 474     gclog_or_tty->print_cr("  "
 475                   "  _covered[%d].start(): " INTPTR_FORMAT
 476                   "  _covered[%d].last(): " INTPTR_FORMAT,
 477                   ind, p2i(_covered[ind].start()),
 478                   ind, p2i(_covered[ind].last()));
 479     gclog_or_tty->print_cr("  "
 480                   "  _committed[%d].start(): " INTPTR_FORMAT
 481                   "  _committed[%d].last(): " INTPTR_FORMAT,
 482                   ind, p2i(_committed[ind].start()),
 483                   ind, p2i(_committed[ind].last()));
 484     gclog_or_tty->print_cr("  "
 485                   "  byte_for(start): " INTPTR_FORMAT
 486                   "  byte_for(last): " INTPTR_FORMAT,
 487                   p2i(byte_for(_covered[ind].start())),
 488                   p2i(byte_for(_covered[ind].last())));
 489     gclog_or_tty->print_cr("  "
 490                   "  addr_for(start): " INTPTR_FORMAT
 491                   "  addr_for(last): " INTPTR_FORMAT,
 492                   p2i(addr_for((jbyte*) _committed[ind].start())),
 493                   p2i(addr_for((jbyte*) _committed[ind].last())));
 494   }
 495   debug_only(verify_guard();)
 496 }
 497 
 498 bool CardTableExtension::resize_commit_uncommit(int changed_region,
 499                                                 MemRegion new_region) {
 500   bool result = false;
 501   // Commit new or uncommit old pages, if necessary.
 502   MemRegion cur_committed = _committed[changed_region];
 503   assert(_covered[changed_region].end() == new_region.end(),
 504     "The ends of the regions are expected to match");
 505   // Extend the start of this _committed region to
 506   // to cover the start of any previous _committed region.
 507   // This forms overlapping regions, but never interior regions.
 508   HeapWord* min_prev_start = lowest_prev_committed_start(changed_region);
 509   if (min_prev_start < cur_committed.start()) {
 510     // Only really need to set start of "cur_committed" to
 511     // the new start (min_prev_start) but assertion checking code
 512     // below use cur_committed.end() so make it correct.
 513     MemRegion new_committed =
 514         MemRegion(min_prev_start, cur_committed.end());
 515     cur_committed = new_committed;
 516   }
 517 #ifdef ASSERT
 518   ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
 519   assert(cur_committed.start() ==
 520     (HeapWord*) align_size_up((uintptr_t) cur_committed.start(),
 521                               os::vm_page_size()),
 522     "Starts should have proper alignment");
 523 #endif
 524 
 525   jbyte* new_start = byte_for(new_region.start());
 526   // Round down because this is for the start address
 527   HeapWord* new_start_aligned =
 528     (HeapWord*)align_size_down((uintptr_t)new_start, os::vm_page_size());
 529   // The guard page is always committed and should not be committed over.
 530   // This method is used in cases where the generation is growing toward
 531   // lower addresses but the guard region is still at the end of the
 532   // card table.  That still makes sense when looking for writes
 533   // off the end of the card table.
 534   if (new_start_aligned < cur_committed.start()) {
 535     // Expand the committed region
 536     //
 537     // Case A
 538     //                                          |+ guard +|
 539     //                          |+ cur committed +++++++++|
 540     //                  |+ new committed +++++++++++++++++|
 541     //
 542     // Case B
 543     //                                          |+ guard +|
 544     //                        |+ cur committed +|
 545     //                  |+ new committed +++++++|
 546     //
 547     // These are not expected because the calculation of the
 548     // cur committed region and the new committed region
 549     // share the same end for the covered region.
 550     // Case C
 551     //                                          |+ guard +|
 552     //                        |+ cur committed +|
 553     //                  |+ new committed +++++++++++++++++|
 554     // Case D
 555     //                                          |+ guard +|
 556     //                        |+ cur committed +++++++++++|
 557     //                  |+ new committed +++++++|
 558 
 559     HeapWord* new_end_for_commit =
 560       MIN2(cur_committed.end(), _guard_region.start());
 561     if(new_start_aligned < new_end_for_commit) {
 562       MemRegion new_committed =
 563         MemRegion(new_start_aligned, new_end_for_commit);
 564       os::commit_memory_or_exit((char*)new_committed.start(),
 565                                 new_committed.byte_size(), !ExecMem,
 566                                 "card table expansion");
 567     }
 568     result = true;
 569   } else if (new_start_aligned > cur_committed.start()) {
 570     // Shrink the committed region
 571 #if 0 // uncommitting space is currently unsafe because of the interactions
 572       // of growing and shrinking regions.  One region A can uncommit space
 573       // that it owns but which is being used by another region B (maybe).
 574       // Region B has not committed the space because it was already
 575       // committed by region A.
 576     MemRegion uncommit_region = committed_unique_to_self(changed_region,
 577       MemRegion(cur_committed.start(), new_start_aligned));
 578     if (!uncommit_region.is_empty()) {
 579       if (!os::uncommit_memory((char*)uncommit_region.start(),
 580                                uncommit_region.byte_size())) {
 581         // If the uncommit fails, ignore it.  Let the
 582         // committed table resizing go even though the committed
 583         // table will over state the committed space.
 584       }
 585     }
 586 #else
 587     assert(!result, "Should be false with current workaround");
 588 #endif
 589   }
 590   assert(_committed[changed_region].end() == cur_committed.end(),
 591     "end should not change");
 592   return result;
 593 }
 594 
 595 void CardTableExtension::resize_update_committed_table(int changed_region,
 596                                                        MemRegion new_region) {
 597 
 598   jbyte* new_start = byte_for(new_region.start());
 599   // Set the new start of the committed region
 600   HeapWord* new_start_aligned =
 601     (HeapWord*)align_size_down((uintptr_t)new_start,
 602                              os::vm_page_size());
 603   MemRegion new_committed = MemRegion(new_start_aligned,
 604     _committed[changed_region].end());
 605   _committed[changed_region] = new_committed;
 606   _committed[changed_region].set_start(new_start_aligned);
 607 }
 608 
 609 void CardTableExtension::resize_update_card_table_entries(int changed_region,
 610                                                           MemRegion new_region) {
 611   debug_only(verify_guard();)
 612   MemRegion original_covered = _covered[changed_region];
 613   // Initialize the card entries.  Only consider the
 614   // region covered by the card table (_whole_heap)
 615   jbyte* entry;
 616   if (new_region.start() < _whole_heap.start()) {
 617     entry = byte_for(_whole_heap.start());
 618   } else {
 619     entry = byte_for(new_region.start());
 620   }
 621   jbyte* end = byte_for(original_covered.start());
 622   // If _whole_heap starts at the original covered regions start,
 623   // this loop will not execute.
 624   while (entry < end) { *entry++ = clean_card; }
 625 }
 626 
 627 void CardTableExtension::resize_update_covered_table(int changed_region,
 628                                                      MemRegion new_region) {
 629   // Update the covered region
 630   _covered[changed_region].set_start(new_region.start());
 631   _covered[changed_region].set_word_size(new_region.word_size());
 632 
 633   // reorder regions.  There should only be at most 1 out
 634   // of order.
 635   for (int i = _cur_covered_regions-1 ; i > 0; i--) {
 636     if (_covered[i].start() < _covered[i-1].start()) {
 637         MemRegion covered_mr = _covered[i-1];
 638         _covered[i-1] = _covered[i];
 639         _covered[i] = covered_mr;
 640         MemRegion committed_mr = _committed[i-1];
 641       _committed[i-1] = _committed[i];
 642       _committed[i] = committed_mr;
 643       break;
 644     }
 645   }
 646 #ifdef ASSERT
 647   for (int m = 0; m < _cur_covered_regions-1; m++) {
 648     assert(_covered[m].start() <= _covered[m+1].start(),
 649       "Covered regions out of order");
 650     assert(_committed[m].start() <= _committed[m+1].start(),
 651       "Committed regions out of order");
 652   }
 653 #endif
 654 }
 655 
 656 // Returns the start of any committed region that is lower than
 657 // the target committed region (index ind) and that intersects the
 658 // target region.  If none, return start of target region.
 659 //
 660 //      -------------
 661 //      |           |
 662 //      -------------
 663 //              ------------
 664 //              | target   |
 665 //              ------------
 666 //                               -------------
 667 //                               |           |
 668 //                               -------------
 669 //      ^ returns this
 670 //
 671 //      -------------
 672 //      |           |
 673 //      -------------
 674 //                      ------------
 675 //                      | target   |
 676 //                      ------------
 677 //                               -------------
 678 //                               |           |
 679 //                               -------------
 680 //                      ^ returns this
 681 
 682 HeapWord* CardTableExtension::lowest_prev_committed_start(int ind) const {
 683   assert(_cur_covered_regions >= 0, "Expecting at least on region");
 684   HeapWord* min_start = _committed[ind].start();
 685   for (int j = 0; j < ind; j++) {
 686     HeapWord* this_start = _committed[j].start();
 687     if ((this_start < min_start) &&
 688         !(_committed[j].intersection(_committed[ind])).is_empty()) {
 689        min_start = this_start;
 690     }
 691   }
 692   return min_start;
 693 }