7007068: G1: refine the BOT during evac failure handling

Summary: During evacuation failure handling we refine the BOT to reflect the location of all the objects in the regions we scan. The changeset includes some minor cleanup: a) non-product print_on() method on the G1 BOT class, b) added more complete BOT verification during heap / region verification, c) slight modification to the BOT set up for humongous regions to be more consistent with the BOT set up during evac failure handling, and d) removed a couple of unused methods.
Reviewed-by: johnc, ysr

7 files changed, 244 insertions, 164 deletions

diff --git a/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp b/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp
index 270e1fa3d..d16685a7e 100644
--- a/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp
+++ b/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -222,7 +222,7 @@ void G1BlockOffsetArray::split_block(HeapWord* blk, size_t blk_size,
 
 // Action_mark - update the BOT for the block [blk_start, blk_end).
 //               Current typical use is for splitting a block.
-// Action_single - udpate the BOT for an allocation.
+// Action_single - update the BOT for an allocation.
 // Action_verify - BOT verification.
 void G1BlockOffsetArray::do_block_internal(HeapWord* blk_start,
                                            HeapWord* blk_end,
@@ -331,47 +331,6 @@ G1BlockOffsetArray::mark_block(HeapWord* blk_start, HeapWord* blk_end) {
   do_block_internal(blk_start, blk_end, Action_mark);
 }
 
-void G1BlockOffsetArray::join_blocks(HeapWord* blk1, HeapWord* blk2) {
-  HeapWord* blk1_start = Universe::heap()->block_start(blk1);
-  HeapWord* blk2_start = Universe::heap()->block_start(blk2);
-  assert(blk1 == blk1_start && blk2 == blk2_start,
-         "Must be block starts.");
-  assert(blk1 + _sp->block_size(blk1) == blk2, "Must be contiguous.");
-  size_t blk1_start_index = _array->index_for(blk1);
-  size_t blk2_start_index = _array->index_for(blk2);
-  assert(blk1_start_index <= blk2_start_index, "sanity");
-  HeapWord* blk2_card_start = _array->address_for_index(blk2_start_index);
-  if (blk2 == blk2_card_start) {
-    // blk2 starts a card.  Does blk1 start on the prevous card, or futher
-    // back?
-    assert(blk1_start_index < blk2_start_index, "must be lower card.");
-    if (blk1_start_index + 1 == blk2_start_index) {
-      // previous card; new value for blk2 card is size of blk1.
-      _array->set_offset_array(blk2_start_index, (u_char) _sp->block_size(blk1));
-    } else {
-      // Earlier card; go back a card.
-      _array->set_offset_array(blk2_start_index, N_words);
-    }
-  } else {
-    // blk2 does not start a card.  Does it cross a card?  If not, nothing
-    // to do.
-    size_t blk2_end_index =
-      _array->index_for(blk2 + _sp->block_size(blk2) - 1);
-    assert(blk2_end_index >= blk2_start_index, "sanity");
-    if (blk2_end_index > blk2_start_index) {
-      // Yes, it crosses a card.  The value for the next card must change.
-      if (blk1_start_index + 1 == blk2_start_index) {
-        // previous card; new value for second blk2 card is size of blk1.
-        _array->set_offset_array(blk2_start_index + 1,
-                                 (u_char) _sp->block_size(blk1));
-      } else {
-        // Earlier card; go back a card.
-        _array->set_offset_array(blk2_start_index + 1, N_words);
-      }
-    }
-  }
-}
-
 HeapWord* G1BlockOffsetArray::block_start_unsafe(const void* addr) {
   assert(_bottom <= addr && addr < _end,
          "addr must be covered by this Array");
@@ -580,15 +539,50 @@ void G1BlockOffsetArray::alloc_block_work2(HeapWord** threshold_, size_t* index_
 #endif
 }
 
-void
-G1BlockOffsetArray::set_for_starts_humongous(HeapWord* new_end) {
-  assert(_end ==  new_end, "_end should have already been updated");
+bool
+G1BlockOffsetArray::verify_for_object(HeapWord* obj_start,
+                                      size_t word_size) const {
+  size_t first_card = _array->index_for(obj_start);
+  size_t last_card = _array->index_for(obj_start + word_size - 1);
+  if (!_array->is_card_boundary(obj_start)) {
+    // If the object is not on a card boundary the BOT entry of the
+    // first card should point to another object so we should not
+    // check that one.
+    first_card += 1;
+  }
+  for (size_t card = first_card; card <= last_card; card += 1) {
+    HeapWord* card_addr = _array->address_for_index(card);
+    HeapWord* block_start = block_start_const(card_addr);
+    if (block_start != obj_start) {
+      gclog_or_tty->print_cr("block start: "PTR_FORMAT" is incorrect - "
+                             "card index: "SIZE_FORMAT" "
+                             "card addr: "PTR_FORMAT" BOT entry: %u "
+                             "obj: "PTR_FORMAT" word size: "SIZE_FORMAT" "
+                             "cards: ["SIZE_FORMAT","SIZE_FORMAT"]",
+                             block_start, card, card_addr,
+                             _array->offset_array(card),
+                             obj_start, word_size, first_card, last_card);
+      return false;
+    }
+  }
+  return true;
+}
 
-  // The first BOT entry should have offset 0.
-  _array->set_offset_array(_array->index_for(_bottom), 0);
-  // The rest should point to the first one.
-  set_remainder_to_point_to_start(_bottom + N_words, new_end);
+#ifndef PRODUCT
+void
+G1BlockOffsetArray::print_on(outputStream* out) {
+  size_t from_index = _array->index_for(_bottom);
+  size_t to_index = _array->index_for(_end);
+  out->print_cr(">> BOT for area ["PTR_FORMAT","PTR_FORMAT") "
+                "cards ["SIZE_FORMAT","SIZE_FORMAT")",
+                _bottom, _end, from_index, to_index);
+  for (size_t i = from_index; i < to_index; ++i) {
+    out->print_cr("  entry "SIZE_FORMAT_W(8)" | "PTR_FORMAT" : %3u",
+                  i, _array->address_for_index(i),
+                  (uint) _array->offset_array(i));
+  }
 }
+#endif // !PRODUCT
 
 //////////////////////////////////////////////////////////////////////
 // G1BlockOffsetArrayContigSpace
@@ -641,10 +635,20 @@ void G1BlockOffsetArrayContigSpace::zero_bottom_entry() {
 }
 
 void
-G1BlockOffsetArrayContigSpace::set_for_starts_humongous(HeapWord* new_end) {
-  G1BlockOffsetArray::set_for_starts_humongous(new_end);
+G1BlockOffsetArrayContigSpace::set_for_starts_humongous(HeapWord* new_top) {
+  assert(new_top <= _end, "_end should have already been updated");
+
+  // The first BOT entry should have offset 0.
+  zero_bottom_entry();
+  initialize_threshold();
+  alloc_block(_bottom, new_top);
+ }
 
-  // Make sure _next_offset_threshold and _next_offset_index point to new_end.
-  _next_offset_threshold = new_end;
-  _next_offset_index     = _array->index_for(new_end);
+#ifndef PRODUCT
+void
+G1BlockOffsetArrayContigSpace::print_on(outputStream* out) {
+  G1BlockOffsetArray::print_on(out);
+  out->print_cr("  next offset threshold: "PTR_FORMAT, _next_offset_threshold);
+  out->print_cr("  next offset index:     "SIZE_FORMAT, _next_offset_index);
 }
+#endif // !PRODUCT
diff --git a/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp b/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp
index 82568dddd..b6a42c73e 100644
--- a/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp
+++ b/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -352,11 +352,6 @@ public:
   // The following methods are useful and optimized for a
   // general, non-contiguous space.
 
-  // The given arguments are required to be the starts of adjacent ("blk1"
-  // before "blk2") well-formed blocks covered by "this".  After this call,
-  // they should be considered to form one block.
-  virtual void join_blocks(HeapWord* blk1, HeapWord* blk2);
-
   // Given a block [blk_start, blk_start + full_blk_size), and
   // a left_blk_size < full_blk_size, adjust the BOT to show two
   // blocks [blk_start, blk_start + left_blk_size) and
@@ -429,6 +424,12 @@ public:
     verify_single_block(blk, blk + size);
   }
 
+  // Used by region verification. Checks that the contents of the
+  // BOT reflect that there's a single object that spans the address
+  // range [obj_start, obj_start + word_size); returns true if this is
+  // the case, returns false if it's not.
+  bool verify_for_object(HeapWord* obj_start, size_t word_size) const;
+
   // Verify that the given block is before _unallocated_block
   inline void verify_not_unallocated(HeapWord* blk_start,
                                      HeapWord* blk_end) const {
@@ -444,7 +445,7 @@ public:
 
   void check_all_cards(size_t left_card, size_t right_card) const;
 
-  virtual void set_for_starts_humongous(HeapWord* new_end);
+  virtual void print_on(outputStream* out) PRODUCT_RETURN;
 };
 
 // A subtype of BlockOffsetArray that takes advantage of the fact
@@ -494,7 +495,9 @@ class G1BlockOffsetArrayContigSpace: public G1BlockOffsetArray {
   HeapWord* block_start_unsafe(const void* addr);
   HeapWord* block_start_unsafe_const(const void* addr) const;
 
-  virtual void set_for_starts_humongous(HeapWord* new_end);
+  void set_for_starts_humongous(HeapWord* new_top);
+
+  virtual void print_on(outputStream* out) PRODUCT_RETURN;
 };
 
 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1BLOCKOFFSETTABLE_HPP
diff --git a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
index 030aecade..a91b0679b 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -3856,13 +3856,15 @@ private:
   size_t _next_marked_bytes;
   OopsInHeapRegionClosure *_cl;
 public:
-  RemoveSelfPointerClosure(G1CollectedHeap* g1, OopsInHeapRegionClosure* cl) :
-    _g1(g1), _cm(_g1->concurrent_mark()),  _prev_marked_bytes(0),
+  RemoveSelfPointerClosure(G1CollectedHeap* g1, HeapRegion* hr,
+                           OopsInHeapRegionClosure* cl) :
+    _g1(g1), _hr(hr), _cm(_g1->concurrent_mark()),  _prev_marked_bytes(0),
     _next_marked_bytes(0), _cl(cl) {}
 
   size_t prev_marked_bytes() { return _prev_marked_bytes; }
   size_t next_marked_bytes() { return _next_marked_bytes; }
 
+  // <original comment>
   // The original idea here was to coalesce evacuated and dead objects.
   // However that caused complications with the block offset table (BOT).
   // In particular if there were two TLABs, one of them partially refined.
@@ -3871,15 +3873,24 @@ public:
   // of TLAB_2. If the last object of the TLAB_1 and the first object
   // of TLAB_2 are coalesced, then the cards of the unrefined part
   // would point into middle of the filler object.
-  //
   // The current approach is to not coalesce and leave the BOT contents intact.
+  // </original comment>
+  //
+  // We now reset the BOT when we start the object iteration over the
+  // region and refine its entries for every object we come across. So
+  // the above comment is not really relevant and we should be able
+  // to coalesce dead objects if we want to.
   void do_object(oop obj) {
+    HeapWord* obj_addr = (HeapWord*) obj;
+    assert(_hr->is_in(obj_addr), "sanity");
+    size_t obj_size = obj->size();
+    _hr->update_bot_for_object(obj_addr, obj_size);
     if (obj->is_forwarded() && obj->forwardee() == obj) {
       // The object failed to move.
       assert(!_g1->is_obj_dead(obj), "We should not be preserving dead objs.");
       _cm->markPrev(obj);
       assert(_cm->isPrevMarked(obj), "Should be marked!");
-      _prev_marked_bytes += (obj->size() * HeapWordSize);
+      _prev_marked_bytes += (obj_size * HeapWordSize);
       if (_g1->mark_in_progress() && !_g1->is_obj_ill(obj)) {
         _cm->markAndGrayObjectIfNecessary(obj);
       }
@@ -3901,7 +3912,7 @@ public:
     } else {
       // The object has been either evacuated or is dead. Fill it with a
       // dummy object.
-      MemRegion mr((HeapWord*)obj, obj->size());
+      MemRegion mr((HeapWord*)obj, obj_size);
       CollectedHeap::fill_with_object(mr);
       _cm->clearRangeBothMaps(mr);
     }
@@ -3921,10 +3932,13 @@ void G1CollectedHeap::remove_self_forwarding_pointers() {
   HeapRegion* cur = g1_policy()->collection_set();
   while (cur != NULL) {
     assert(g1_policy()->assertMarkedBytesDataOK(), "Should be!");
+    assert(!cur->isHumongous(), "sanity");
 
-    RemoveSelfPointerClosure rspc(_g1h, cl);
     if (cur->evacuation_failed()) {
       assert(cur->in_collection_set(), "bad CS");
+      RemoveSelfPointerClosure rspc(_g1h, cur, cl);
+
+      cur->reset_bot();
       cl->set_region(cur);
       cur->object_iterate(&rspc);
 
@@ -3989,15 +4003,6 @@ void G1CollectedHeap::drain_evac_failure_scan_stack() {
   }
 }
 
-void G1CollectedHeap::handle_evacuation_failure(oop old) {
-  markOop m = old->mark();
-  // forward to self
-  assert(!old->is_forwarded(), "precondition");
-
-  old->forward_to(old);
-  handle_evacuation_failure_common(old, m);
-}
-
 oop
 G1CollectedHeap::handle_evacuation_failure_par(OopsInHeapRegionClosure* cl,
                                                oop old) {
diff --git a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
index 0cec92564..54827d1fa 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -826,7 +826,6 @@ protected:
   void finalize_for_evac_failure();
 
   // An attempt to evacuate "obj" has failed; take necessary steps.
-  void handle_evacuation_failure(oop obj);
   oop handle_evacuation_failure_par(OopsInHeapRegionClosure* cl, oop obj);
   void handle_evacuation_failure_common(oop obj, markOop m);
 
diff --git a/src/share/vm/gc_implementation/g1/heapRegion.cpp b/src/share/vm/gc_implementation/g1/heapRegion.cpp
index ab0226683..dc2799e66 100644
--- a/src/share/vm/gc_implementation/g1/heapRegion.cpp
+++ b/src/share/vm/gc_implementation/g1/heapRegion.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -386,26 +386,27 @@ void HeapRegion::calc_gc_efficiency() {
 }
 // </PREDICTION>
 
-void HeapRegion::set_startsHumongous(HeapWord* new_end) {
+void HeapRegion::set_startsHumongous(HeapWord* new_top, HeapWord* new_end) {
   assert(end() == _orig_end,
          "Should be normal before the humongous object allocation");
   assert(top() == bottom(), "should be empty");
+  assert(bottom() <= new_top && new_top <= new_end, "pre-condition");
 
   _humongous_type = StartsHumongous;
   _humongous_start_region = this;
 
   set_end(new_end);
-  _offsets.set_for_starts_humongous(new_end);
+  _offsets.set_for_starts_humongous(new_top);
 }
 
-void HeapRegion::set_continuesHumongous(HeapRegion* start) {
+void HeapRegion::set_continuesHumongous(HeapRegion* first_hr) {
   assert(end() == _orig_end,
          "Should be normal before the humongous object allocation");
   assert(top() == bottom(), "should be empty");
-  assert(start->startsHumongous(), "pre-condition");
+  assert(first_hr->startsHumongous(), "pre-condition");
 
   _humongous_type = ContinuesHumongous;
-  _humongous_start_region = start;
+  _humongous_start_region = first_hr;
 }
 
 bool HeapRegion::claimHeapRegion(jint claimValue) {
@@ -782,9 +783,6 @@ void HeapRegion::verify(bool allow_dirty) const {
   verify(allow_dirty, /* use_prev_marking */ true, /* failures */ &dummy);
 }
 
-#define OBJ_SAMPLE_INTERVAL 0
-#define BLOCK_SAMPLE_INTERVAL 100
-
 // This really ought to be commoned up into OffsetTableContigSpace somehow.
 // We would need a mechanism to make that code skip dead objects.
 
@@ -795,83 +793,125 @@ void HeapRegion::verify(bool allow_dirty,
   *failures = false;
   HeapWord* p = bottom();
   HeapWord* prev_p = NULL;
-  int objs = 0;
-  int blocks = 0;
   VerifyLiveClosure vl_cl(g1, use_prev_marking);
   bool is_humongous = isHumongous();
+  bool do_bot_verify = !is_young();
   size_t object_num = 0;
   while (p < top()) {
-    size_t size = oop(p)->size();
-    if (is_humongous != g1->isHumongous(size)) {
+    oop obj = oop(p);
+    size_t obj_size = obj->size();
+    object_num += 1;
+
+    if (is_humongous != g1->isHumongous(obj_size)) {
       gclog_or_tty->print_cr("obj "PTR_FORMAT" is of %shumongous size ("
                              SIZE_FORMAT" words) in a %shumongous region",
-                             p, g1->isHumongous(size) ? "" : "non-",
-                             size, is_humongous ? "" : "non-");
+                             p, g1->isHumongous(obj_size) ? "" : "non-",
+                             obj_size, is_humongous ? "" : "non-");
        *failures = true;
+       return;
     }
-    object_num += 1;
-    if (blocks == BLOCK_SAMPLE_INTERVAL) {
-      HeapWord* res = block_start_const(p + (size/2));
-      if (p != res) {
-        gclog_or_tty->print_cr("offset computation 1 for "PTR_FORMAT" and "
-                               SIZE_FORMAT" returned "PTR_FORMAT,
-                               p, size, res);
-        *failures = true;
-        return;
-      }
-      blocks = 0;
-    } else {
-      blocks++;
+
+    // If it returns false, verify_for_object() will output the
+    // appropriate messasge.
+    if (do_bot_verify && !_offsets.verify_for_object(p, obj_size)) {
+      *failures = true;
+      return;
     }
-    if (objs == OBJ_SAMPLE_INTERVAL) {
-      oop obj = oop(p);
-      if (!g1->is_obj_dead_cond(obj, this, use_prev_marking)) {
-        if (obj->is_oop()) {
-          klassOop klass = obj->klass();
-          if (!klass->is_perm()) {
-            gclog_or_tty->print_cr("klass "PTR_FORMAT" of object "PTR_FORMAT" "
-                                   "not in perm", klass, obj);
-            *failures = true;
-            return;
-          } else if (!klass->is_klass()) {
-            gclog_or_tty->print_cr("klass "PTR_FORMAT" of object "PTR_FORMAT" "
-                                   "not a klass", klass, obj);
+
+    if (!g1->is_obj_dead_cond(obj, this, use_prev_marking)) {
+      if (obj->is_oop()) {
+        klassOop klass = obj->klass();
+        if (!klass->is_perm()) {
+          gclog_or_tty->print_cr("klass "PTR_FORMAT" of object "PTR_FORMAT" "
+                                 "not in perm", klass, obj);
+          *failures = true;
+          return;
+        } else if (!klass->is_klass()) {
+          gclog_or_tty->print_cr("klass "PTR_FORMAT" of object "PTR_FORMAT" "
+                                 "not a klass", klass, obj);
+          *failures = true;
+          return;
+        } else {
+          vl_cl.set_containing_obj(obj);
+          obj->oop_iterate(&vl_cl);
+          if (vl_cl.failures()) {
             *failures = true;
+          }
+          if (G1MaxVerifyFailures >= 0 &&
+              vl_cl.n_failures() >= G1MaxVerifyFailures) {
             return;
-          } else {
-            vl_cl.set_containing_obj(obj);
-            obj->oop_iterate(&vl_cl);
-            if (vl_cl.failures()) {
-              *failures = true;
-            }
-            if (G1MaxVerifyFailures >= 0 &&
-                vl_cl.n_failures() >= G1MaxVerifyFailures) {
-              return;
-            }
           }
-        } else {
-          gclog_or_tty->print_cr(PTR_FORMAT" no an oop", obj);
-          *failures = true;
-          return;
         }
+      } else {
+        gclog_or_tty->print_cr(PTR_FORMAT" no an oop", obj);
+        *failures = true;
+        return;
       }
-      objs = 0;
-    } else {
-      objs++;
     }
     prev_p = p;
-    p += size;
+    p += obj_size;
   }
-  HeapWord* rend = end();
-  HeapWord* rtop = top();
-  if (rtop < rend) {
-    HeapWord* res = block_start_const(rtop + (rend - rtop) / 2);
-    if (res != rtop) {
-        gclog_or_tty->print_cr("offset computation 2 for "PTR_FORMAT" and "
-                               PTR_FORMAT" returned "PTR_FORMAT,
-                               rtop, rend, res);
+
+  if (p != top()) {
+    gclog_or_tty->print_cr("end of last object "PTR_FORMAT" "
+                           "does not match top "PTR_FORMAT, p, top());
+    *failures = true;
+    return;
+  }
+
+  HeapWord* the_end = end();
+  assert(p == top(), "it should still hold");
+  // Do some extra BOT consistency checking for addresses in the
+  // range [top, end). BOT look-ups in this range should yield
+  // top. No point in doing that if top == end (there's nothing there).
+  if (p < the_end) {
+    // Look up top
+    HeapWord* addr_1 = p;
+    HeapWord* b_start_1 = _offsets.block_start_const(addr_1);
+    if (b_start_1 != p) {
+      gclog_or_tty->print_cr("BOT look up for top: "PTR_FORMAT" "
+                             " yielded "PTR_FORMAT", expecting "PTR_FORMAT,
+                             addr_1, b_start_1, p);
+      *failures = true;
+      return;
+    }
+
+    // Look up top + 1
+    HeapWord* addr_2 = p + 1;
+    if (addr_2 < the_end) {
+      HeapWord* b_start_2 = _offsets.block_start_const(addr_2);
+      if (b_start_2 != p) {
+        gclog_or_tty->print_cr("BOT look up for top + 1: "PTR_FORMAT" "
+                               " yielded "PTR_FORMAT", expecting "PTR_FORMAT,
+                               addr_2, b_start_2, p);
         *failures = true;
         return;
+      }
+    }
+
+    // Look up an address between top and end
+    size_t diff = pointer_delta(the_end, p) / 2;
+    HeapWord* addr_3 = p + diff;
+    if (addr_3 < the_end) {
+      HeapWord* b_start_3 = _offsets.block_start_const(addr_3);
+      if (b_start_3 != p) {
+        gclog_or_tty->print_cr("BOT look up for top + diff: "PTR_FORMAT" "
+                               " yielded "PTR_FORMAT", expecting "PTR_FORMAT,
+                               addr_3, b_start_3, p);
+        *failures = true;
+        return;
+      }
+    }
+
+    // Loook up end - 1
+    HeapWord* addr_4 = the_end - 1;
+    HeapWord* b_start_4 = _offsets.block_start_const(addr_4);
+    if (b_start_4 != p) {
+      gclog_or_tty->print_cr("BOT look up for end - 1: "PTR_FORMAT" "
+                             " yielded "PTR_FORMAT", expecting "PTR_FORMAT,
+                             addr_4, b_start_4, p);
+      *failures = true;
+      return;
     }
   }
 
@@ -880,12 +920,6 @@ void HeapRegion::verify(bool allow_dirty,
                            "but has "SIZE_FORMAT", objects",
                            bottom(), end(), object_num);
     *failures = true;
-  }
-
-  if (p != top()) {
-    gclog_or_tty->print_cr("end of last object "PTR_FORMAT" "
-                           "does not match top "PTR_FORMAT, p, top());
-    *failures = true;
     return;
   }
 }
diff --git a/src/share/vm/gc_implementation/g1/heapRegion.hpp b/src/share/vm/gc_implementation/g1/heapRegion.hpp
index f11c6a3a3..4e5f2224b 100644
--- a/src/share/vm/gc_implementation/g1/heapRegion.hpp
+++ b/src/share/vm/gc_implementation/g1/heapRegion.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -173,6 +173,19 @@ class G1OffsetTableContigSpace: public ContiguousSpace {
   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* end);
 
   virtual void print() const;
+
+  void reset_bot() {
+    _offsets.zero_bottom_entry();
+    _offsets.initialize_threshold();
+  }
+
+  void update_bot_for_object(HeapWord* start, size_t word_size) {
+    _offsets.alloc_block(start, word_size);
+  }
+
+  void print_bot_on(outputStream* out) {
+    _offsets.print_on(out);
+  }
 };
 
 class HeapRegion: public G1OffsetTableContigSpace {
@@ -404,13 +417,35 @@ class HeapRegion: public G1OffsetTableContigSpace {
     return _humongous_start_region;
   }
 
-  // Causes the current region to represent a humongous object spanning "n"
-  // regions.
-  void set_startsHumongous(HeapWord* new_end);
-
-  // The regions that continue a humongous sequence should be added using
-  // this method, in increasing address order.
-  void set_continuesHumongous(HeapRegion* start);
+  // Makes the current region be a "starts humongous" region, i.e.,
+  // the first region in a series of one or more contiguous regions
+  // that will contain a single "humongous" object. The two parameters
+  // are as follows:
+  //
+  // new_top : The new value of the top field of this region which
+  // points to the end of the humongous object that's being
+  // allocated. If there is more than one region in the series, top
+  // will lie beyond this region's original end field and on the last
+  // region in the series.
+  //
+  // new_end : The new value of the end field of this region which
+  // points to the end of the last region in the series. If there is
+  // one region in the series (namely: this one) end will be the same
+  // as the original end of this region.
+  //
+  // Updating top and end as described above makes this region look as
+  // if it spans the entire space taken up by all the regions in the
+  // series and an single allocation moved its top to new_top. This
+  // ensures that the space (capacity / allocated) taken up by all
+  // humongous regions can be calculated by just looking at the
+  // "starts humongous" regions and by ignoring the "continues
+  // humongous" regions.
+  void set_startsHumongous(HeapWord* new_top, HeapWord* new_end);
+
+  // Makes the current region be a "continues humongous'
+  // region. first_hr is the "start humongous" region of the series
+  // which this region will be part of.
+  void set_continuesHumongous(HeapRegion* first_hr);
 
   // If the region has a remembered set, return a pointer to it.
   HeapRegionRemSet* rem_set() const {
diff --git a/src/share/vm/gc_implementation/g1/heapRegionSeq.cpp b/src/share/vm/gc_implementation/g1/heapRegionSeq.cpp
index 336ba21c4..a116d9d81 100644
--- a/src/share/vm/gc_implementation/g1/heapRegionSeq.cpp
+++ b/src/share/vm/gc_implementation/g1/heapRegionSeq.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -144,7 +144,7 @@ HeapRegionSeq::alloc_obj_from_region_index(int ind, size_t word_size) {
     // will also update the BOT covering all the regions to reflect
     // that there is a single object that starts at the bottom of the
     // first region.
-    first_hr->set_startsHumongous(new_end);
+    first_hr->set_startsHumongous(new_top, new_end);
 
     // Then, if there are any, we will set up the "continues
     // humongous" regions.