GC changes

[Joern Rennecke]

> 
> Tim Josling wrote:
> > 2. What is a good way to make things GC proof for a while until
> > they get into the main tree structure? At the moment I define
> > various roots to handle the intermediate allocations. 
> 
> You might try implementing conservative roots.  I think it's quite easy
> to get access to all the pointers stored in registers and on the stack:
> just call setjmp.  As a comment in some code says, "Callee-saved
> registers are recorded in buf [setjmp(buf)]; caller-saved registers are
> on the stack already.".

That is not safe.  You can't predict what kind of address arithmetic the
compiler will make.  For example, strength reduction could cause a pointer
to be pointing into the middle of an array, or worse, somewhere beyond it
(incorporating some constant adjustments).