Objects layout and tagging scheme

[Dmitry Antipov]

Any non-trivial GC design which I can imagine requires extra per-object 
information.
It may be just a few bits, bit this is a problem anyway because some objects
(conses at first) has no free space already so their mark bits are allocated
from separate bitmaps. Symbols and misc objects are free from these problems,
but vectors and string are affected. So I would like to raise the following
question: if I need, for example, 2 extra bits for each object, where to get 
them?

Dmitry