X64: Move remembered set to a safe location on x64 platform.

src/spaces-inl.h

Index: src/spaces-inl.h
===================================================================
--- src/spaces-inl.h	(revision 2316)
+++ src/spaces-inl.h	(working copy)
@@ -93,17 +93,21 @@
 
 
 void Page::ClearRSet() {
-#ifndef V8_HOST_ARCH_64_BIT
   // This method can be called in all rset states.
   memset(RSetStart(), 0, kRSetEndOffset - kRSetStartOffset);
-#endif
 }
 
 
-// Give an address a (32-bits):
+// Given an address (32-bits), separate its bits into:
 // | page address | words (6) | bit offset (5) | pointer alignment (2) |
 // The rset address is computed as:
 //    page_address + words * 4
+// For a 64-bit address, if it is:
+// | page address | quadwords(5) | bit offset(5) | pointer alignment (3) |
+// The reset address is computed as:

[Lasse Reichstein, 2009/07/01 12:30:18]

reset -> rset (or RSet?)

+//    page_address + words * 4 + kRSetOffset.

[Lasse Reichstein, 2009/07/01 12:30:18]

words -> quadwords

+// The rset is accessed as 32-bit words, and bit offsets in a 32-bit word,
+// even on the X64 architecture.

[Lasse Reichstein, 2009/07/01 12:30:18]

We could probably speed up a little by loading 64 bits at a time when scanning
for non-null bits.

 
 Address Page::ComputeRSetBitPosition(Address address, int offset,
                                      uint32_t* bitmask) {
@@ -115,7 +119,7 @@
   *bitmask = 1 << (bit_offset % kBitsPerInt);
 
   Address rset_address =
-      page->address() + (bit_offset / kBitsPerInt) * kIntSize;
+      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())
@@ -131,8 +135,10 @@
     // 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
     ASSERT(HeapObject::FromAddress(address)->IsFixedArray());
-    rset_address +=
+    rset_address += kObjectStartOffset - kRSetEndOffset +
         FixedArray::SizeFor(Memory::int_at(page->ObjectAreaStart()
                                            + Array::kLengthOffset));
   }
@@ -160,14 +166,9 @@
 
 
 bool Page::IsRSetSet(Address address, int offset) {
-#ifdef V8_HOST_ARCH_64_BIT
-  // TODO(X64): Reenable when RSet works.
-  return true;
-#else  // V8_HOST_ARCH_64_BIT
   uint32_t bitmask = 0;
   Address rset_address = ComputeRSetBitPosition(address, offset, &bitmask);
   return (Memory::uint32_at(rset_address) & bitmask) != 0;
-#endif  // V8_HOST_ARCH_64_BIT
 }