I need an unused bit in the bidirectionnal layout object

Hi,

I write a mark-compact gc for SableVM. In order to mark an object, I
use a bit in the header. A problem occurs while a bidirectionnal
layout object has no bit unused. Is there a possibility to change the
use of some others bits to free a bit.

If it is not possible, can I use a bit in the start_offset of the object.

Sebastien