[Haskell] GHC 64bit?

[Simon Marlow]

> alex:
> > Is there a maximum memory GHC can use/reach?
> > Specifically, can GHC address more than 4gb of
> > memory?
> 
> SimonM may want to comment, but at the moment I think GHC is 
> limited to
> 4G, but only due to lack of 64bit machines/demand on the developers.
> 
> If you look in ghc/rts/MBlock.h you'll see some references to this.
> 
> I *think* it would be easy to overcome this limit.
> 
> (this should really be on glasgow-haskell-users)

In principle, there's no problem.  In practice, GHC's storage manager
needs a (fast) function of type

   Ptr a -> Bool

to indicate whether a particular address is part of the dynamic heap or
not.  This is currently implemented as a bytemap, which is 4k long on a
32-bit machine.  To cover the whole of a 64-bit address space, this
method isn't practical.  Nevertheless, we can extend the bytemap to
cover a lot more real memory, as long as we have a way to map from
virtual memory addresses to elements of the bytemap - this is the tricky
bit.  So far we haven't tackled this problem in a general way.

Cheers,
	Simon