Incremental mode now works for x64. The only difference

src/ia32/macro-assembler-ia32.cc

 // Copyright 2011 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 2338 matching lines @@
 }
 
 
 void MacroAssembler::GetMarkBits(Register addr_reg,
                                  Register bitmap_reg,
                                  Register mask_reg) {
   ASSERT(!AreAliased(addr_reg, bitmap_reg, mask_reg, ecx));
   mov(bitmap_reg, Operand(addr_reg));
   and_(bitmap_reg, ~Page::kPageAlignmentMask);
   mov(ecx, Operand(addr_reg));
-  shr(ecx, Bitmap::kBitsPerCellLog2);
+  int shift =
+      Bitmap::kBitsPerCellLog2 + kPointerSizeLog2 - Bitmap::kBytesPerCellLog2;
+  shr(ecx, shift);
   and_(ecx,
-       (Page::kPageAlignmentMask >> Bitmap::kBitsPerCellLog2) &
-           ~(kPointerSize - 1));
+       (Page::kPageAlignmentMask >> shift) & ~(Bitmap::kBytesPerCell - 1));
 
   add(bitmap_reg, Operand(ecx));
   mov(ecx, Operand(addr_reg));
   shr(ecx, kPointerSizeLog2);
   and_(ecx, (1 << Bitmap::kBitsPerCellLog2) - 1);
   mov(mask_reg, Immediate(1));
   shl_cl(mask_reg);
 }
 
 
@@ skipping 37 matching lines @@
 
   // Value is a data object, and it is white.  Mark it black.  Since we know
   // that the object is white we can make it black by flipping one bit.
   or_(Operand(bitmap_scratch, MemoryChunk::kHeaderSize), mask_scratch);
   bind(&done);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32