int newValue = (oldValue & ~MARK_BIT_MASK) | Plan.getInitialHeaderValue(size);
int newValue = (oldValue & ~GC_BITS_MASK) | Plan.getInitialHeaderValue(size);
  public static void initializeHeader(Object ref, Object[] tib, int size,				      boolean isScalar)    throws VM_PragmaUninterruptible, VM_PragmaInline {    int oldValue = VM_ObjectModel.readAvailableBitsWord(ref);    int newValue = (oldValue & ~MARK_BIT_MASK) | Plan.getInitialHeaderValue(size);    VM_ObjectModel.writeAvailableBitsWord(ref, newValue);  }