heap headers

[Stephen Weeks]

> > On an unrelated note, does any code besides the GC ever look at the headers
> > of heap objects?  I.e., suppose I went to a generational GC and I wanted
> > small things ('a * 'b and 'a list where 'a and 'b require indirection) to be
> > stored in older generations without the 1 word header.  Would that work or would
> > some generated code get screwed up.
> 
> None of the generated code looks at headers, IIRC.  All of the code that does is 
> in gc.c, I think.  The only other thing I can think of besides the gc is
> saveWorld and size.

I just remebered that size does rely on having a mark bit in each header word so 
that it can do a dfs traversal of an object.  I like having size around, so
maybe we could think of a fix.