Fix bug in X64 RSet code. Optimize IA32 version.

src/spaces-inl.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 109 matching lines @@
 
   Address rset_address =
       page->address() + kRSetOffset + (bit_offset / kBitsPerInt) * kIntSize;
   // The remembered set address is either in the normal remembered set range
   // of a page or else we have a large object page.
   ASSERT((page->RSetStart() <= rset_address && rset_address < page->RSetEnd())
          || page->IsLargeObjectPage());
 
   if (rset_address >= page->RSetEnd()) {
     // We have a large object page, and the remembered set address is actually
-    // past the end of the object.  The address of the remembered set in this
-    // case is the extra remembered set start address at the address of the
-    // end of the object:
+    // past the end of the object.
+

[William Hesse, 2009/08/05 10:17:50]

Rather than saying "FixedArray" everywhere in comments and identifier names,
could we just say "the large object"?  And mention once that it is a FixedArray?

+    // The first part of the remembered set is still located at the start of
+    // the page, but anything after kRSetEndOffset must be relocated to after
+    // the FixedArray, i.e. after
     //   (page->ObjectAreaStart() + object size)
-    // plus the offset of the computed remembered set address from the start
-    // of the object:
-    //   (rset_address - page->ObjectAreaStart()).
-    // Ie, we can just add the object size.
-    // In the X64 architecture, the remembered set ends before the object start,
-    // so we need to add an additional offset, from rset end to object start
+    // We do that by adding the difference between the normal RSet's end and
+    // the object's end.
     ASSERT(HeapObject::FromAddress(address)->IsFixedArray());
-    rset_address += kObjectStartOffset - kRSetEndOffset +
+    int fixedarray_length =
         FixedArray::SizeFor(Memory::int_at(page->ObjectAreaStart()
                                            + Array::kLengthOffset));
+    rset_address += kObjectStartOffset - kRSetEndOffset + fixedarray_length;
   }
   return rset_address;
 }
 
 
 void Page::SetRSet(Address address, int offset) {
   uint32_t bitmask = 0;
   Address rset_address = ComputeRSetBitPosition(address, offset, &bitmask);
   Memory::uint32_at(rset_address) |= bitmask;
 
@@ skipping 203 matching lines @@
   ASSERT(space->low() <= alloc_info->top
          && alloc_info->top <= space->high()
          && alloc_info->limit == space->high());
 #endif
   return obj;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_INL_H_