big arrays [was Re: bug found]

[Matthew Fluet]

> > What we really want is to be able to distinguish between normal
> > objects, stacks/conts, and arrays from a pointer to the first data
> > word of the object and to be able to distinguish between object
> > headers and array lengths from a pointer to the word preceeding the
> > first data word of an object.
> 
> Yes.
> 
> I believe my proposal does this.  Here it is.

Looks good to me.

> I believe it is more important to make GC_getHeader fast than toData
> fast, since the former happens once per pointer during a GC while the
> latter happens only once per object.

I agree.