--- a/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp	Tue Nov 27 14:20:21 2012 +0100
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp	Tue Nov 27 14:11:37 2012 -0800
@@ -302,16 +302,28 @@
   for (size_t c = start_card + 1; c <= end_card; c++ /* yeah! */) {
     u_char entry = _array->offset_array(c);
     if (c - start_card > BlockOffsetArray::power_to_cards_back(1)) {
-      guarantee(entry > N_words, "Should be in logarithmic region");
+      guarantee(entry > N_words,
+                err_msg("Should be in logarithmic region - "
+                        "entry: " UINT32_FORMAT ", "
+                        "_array->offset_array(c): " UINT32_FORMAT ", "
+                        "N_words: " UINT32_FORMAT,
+                        entry, _array->offset_array(c), N_words));
     }
     size_t backskip = BlockOffsetArray::entry_to_cards_back(entry);
     size_t landing_card = c - backskip;
     guarantee(landing_card >= (start_card - 1), "Inv");
     if (landing_card >= start_card) {
-      guarantee(_array->offset_array(landing_card) <= entry, "monotonicity");
+      guarantee(_array->offset_array(landing_card) <= entry,
+                err_msg("Monotonicity - landing_card offset: " UINT32_FORMAT ", "
+                        "entry: " UINT32_FORMAT,
+                        _array->offset_array(landing_card), entry));
     } else {
       guarantee(landing_card == start_card - 1, "Tautology");
-      guarantee(_array->offset_array(landing_card) <= N_words, "Offset value");
+      // Note that N_words is the maximum offset value
+      guarantee(_array->offset_array(landing_card) <= N_words,
+                err_msg("landing card offset: " UINT32_FORMAT ", "
+                        "N_words: " UINT32_FORMAT,
+                        _array->offset_array(landing_card), N_words));
     }
   }
 }
@@ -536,17 +548,27 @@
   // The offset can be 0 if the block starts on a boundary.  That
   // is checked by an assertion above.
   size_t start_index = _array->index_for(blk_start);
-  HeapWord* boundary    = _array->address_for_index(start_index);
+  HeapWord* boundary = _array->address_for_index(start_index);
   assert((_array->offset_array(orig_index) == 0 &&
           blk_start == boundary) ||
           (_array->offset_array(orig_index) > 0 &&
          _array->offset_array(orig_index) <= N_words),
-         "offset array should have been set");
+         err_msg("offset array should have been set - "
+                  "orig_index offset: " UINT32_FORMAT ", "
+                  "blk_start: " PTR_FORMAT ", "
+                  "boundary: " PTR_FORMAT,
+                  _array->offset_array(orig_index),
+                  blk_start, boundary));
   for (size_t j = orig_index + 1; j <= end_index; j++) {
     assert(_array->offset_array(j) > 0 &&
            _array->offset_array(j) <=
              (u_char) (N_words+BlockOffsetArray::N_powers-1),
-           "offset array should have been set");
+           err_msg("offset array should have been set - "
+                   UINT32_FORMAT " not > 0 OR "
+                   UINT32_FORMAT " not <= " UINT32_FORMAT,
+                   _array->offset_array(j),
+                   _array->offset_array(j),
+                   (u_char) (N_words+BlockOffsetArray::N_powers-1)));
   }
 #endif
 }

--- a/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp	Tue Nov 27 14:20:21 2012 +0100
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp	Tue Nov 27 14:11:37 2012 -0800
@@ -78,7 +78,9 @@
   virtual void resize(size_t new_word_size) = 0;
 
   virtual void set_bottom(HeapWord* new_bottom) {
-    assert(new_bottom <= _end, "new_bottom > _end");
+    assert(new_bottom <= _end,
+           err_msg("new_bottom (" PTR_FORMAT ") > _end (" PTR_FORMAT ")",
+                   new_bottom, _end));
     _bottom = new_bottom;
     resize(pointer_delta(_end, _bottom));
   }
@@ -134,29 +136,42 @@
   VirtualSpace _vs;
   u_char* _offset_array;          // byte array keeping backwards offsets
 
+  void check_index(size_t index, const char* msg) const {
+    assert(index < _vs.committed_size(),
+           err_msg("%s - "
+                   "index: " SIZE_FORMAT ", _vs.committed_size: " SIZE_FORMAT,
+                   msg, index, _vs.committed_size()));
+  }
+
+  void check_offset(size_t offset, const char* msg) const {
+    assert(offset <= N_words,
+           err_msg("%s - "
+                   "offset: " UINT32_FORMAT", N_words: " UINT32_FORMAT,
+                   msg, offset, N_words));
+  }
+
   // Bounds checking accessors:
   // For performance these have to devolve to array accesses in product builds.
   u_char offset_array(size_t index) const {
-    assert(index < _vs.committed_size(), "index out of range");
+    check_index(index, "index out of range");
     return _offset_array[index];
   }
 
   void set_offset_array(size_t index, u_char offset) {
-    assert(index < _vs.committed_size(), "index out of range");
-    assert(offset <= N_words, "offset too large");
+    check_index(index, "index out of range");
+    check_offset(offset, "offset too large");
     _offset_array[index] = offset;
   }
 
   void set_offset_array(size_t index, HeapWord* high, HeapWord* low) {
-    assert(index < _vs.committed_size(), "index out of range");
+    check_index(index, "index out of range");
     assert(high >= low, "addresses out of order");
-    assert(pointer_delta(high, low) <= N_words, "offset too large");
+    check_offset(pointer_delta(high, low), "offset too large");
     _offset_array[index] = (u_char) pointer_delta(high, low);
   }
 
   void set_offset_array(HeapWord* left, HeapWord* right, u_char offset) {
-    assert(index_for(right - 1) < _vs.committed_size(),
-           "right address out of range");
+    check_index(index_for(right - 1), "right address out of range");
     assert(left  < right, "Heap addresses out of order");
     size_t num_cards = pointer_delta(right, left) >> LogN_words;
     if (UseMemSetInBOT) {
@@ -171,7 +186,7 @@
   }
 
   void set_offset_array(size_t left, size_t right, u_char offset) {
-    assert(right < _vs.committed_size(), "right address out of range");
+    check_index(right, "right index out of range");
     assert(left <= right, "indexes out of order");
     size_t num_cards = right - left + 1;
     if (UseMemSetInBOT) {
@@ -186,11 +201,10 @@
   }
 
   void check_offset_array(size_t index, HeapWord* high, HeapWord* low) const {
-    assert(index < _vs.committed_size(), "index out of range");
+    check_index(index, "index out of range");
     assert(high >= low, "addresses out of order");
-    assert(pointer_delta(high, low) <= N_words, "offset too large");
-    assert(_offset_array[index] == pointer_delta(high, low),
-           "Wrong offset");
+    check_offset(pointer_delta(high, low), "offset too large");
+    assert(_offset_array[index] == pointer_delta(high, low), "Wrong offset");
   }
 
   bool is_card_boundary(HeapWord* p) const;
@@ -481,7 +495,6 @@
                       blk_start, blk_end);
   }
 
-
  public:
   G1BlockOffsetArrayContigSpace(G1BlockOffsetSharedArray* array, MemRegion mr);
 

--- a/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.inline.hpp	Tue Nov 27 14:20:21 2012 +0100
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.inline.hpp	Tue Nov 27 14:11:37 2012 -0800
@@ -49,16 +49,17 @@
   char* pc = (char*)p;
   assert(pc >= (char*)_reserved.start() &&
          pc <  (char*)_reserved.end(),
-         "p not in range.");
+         err_msg("p (" PTR_FORMAT ") not in reserved [" PTR_FORMAT ", " PTR_FORMAT ")",
+                 p, (char*)_reserved.start(), (char*)_reserved.end()));
   size_t delta = pointer_delta(pc, _reserved.start(), sizeof(char));
   size_t result = delta >> LogN;
-  assert(result < _vs.committed_size(), "bad index from address");
+  check_index(result, "bad index from address");
   return result;
 }
 
 inline HeapWord*
 G1BlockOffsetSharedArray::address_for_index(size_t index) const {
-  assert(index < _vs.committed_size(), "bad index");
+  check_index(index, "index out of range");
   HeapWord* result = _reserved.start() + (index << LogN_words);
   assert(result >= _reserved.start() && result < _reserved.end(),
          err_msg("bad address from index result " PTR_FORMAT

--- a/hotspot/src/share/vm/memory/space.cpp	Tue Nov 27 14:20:21 2012 +0100
+++ b/hotspot/src/share/vm/memory/space.cpp	Tue Nov 27 14:11:37 2012 -0800
@@ -790,7 +790,9 @@
 
 // Very general, slow implementation.
 HeapWord* ContiguousSpace::block_start_const(const void* p) const {
-  assert(MemRegion(bottom(), end()).contains(p), "p not in space");
+  assert(MemRegion(bottom(), end()).contains(p),
+         err_msg("p (" PTR_FORMAT ") not in space [" PTR_FORMAT ", " PTR_FORMAT ")",
+                  p, bottom(), end()));
   if (p >= top()) {
     return top();
   } else {
@@ -800,19 +802,27 @@
       last = cur;
       cur += oop(cur)->size();
     }
-    assert(oop(last)->is_oop(), "Should be an object start");
+    assert(oop(last)->is_oop(),
+           err_msg(PTR_FORMAT " should be an object start", last));
     return last;
   }
 }
 
 size_t ContiguousSpace::block_size(const HeapWord* p) const {
-  assert(MemRegion(bottom(), end()).contains(p), "p not in space");
+  assert(MemRegion(bottom(), end()).contains(p),
+         err_msg("p (" PTR_FORMAT ") not in space [" PTR_FORMAT ", " PTR_FORMAT ")",
+                  p, bottom(), end()));
   HeapWord* current_top = top();
-  assert(p <= current_top, "p is not a block start");
-  assert(p == current_top || oop(p)->is_oop(), "p is not a block start");
-  if (p < current_top)
+  assert(p <= current_top,
+         err_msg("p > current top - p: " PTR_FORMAT ", current top: " PTR_FORMAT,
+                  p, current_top));
+  assert(p == current_top || oop(p)->is_oop(),
+         err_msg("p (" PTR_FORMAT ") is not a block start - "
+                 "current_top: " PTR_FORMAT ", is_oop: %s",
+                 p, current_top, BOOL_TO_STR(oop(p)->is_oop())));
+  if (p < current_top) {
     return oop(p)->size();
-  else {
+  } else {
     assert(p == current_top, "just checking");
     return pointer_delta(end(), (HeapWord*) p);
   }