Incremental mode now works for x64. The only difference

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
===================================================================
--- src/ia32/macro-assembler-ia32.cc	(revision 8221)
+++ src/ia32/macro-assembler-ia32.cc	(working copy)
@@ -2356,10 +2356,11 @@
   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));