[Haskell-cafe] A GC'd heap using weak pointers

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

This would be well recieved on ghc-devs mailing list. Adding to cc.

On 6 October 2015 at 15:55, Thomas Koster <tkoster at gmail.com> wrote:

> Good afternoon list,
>
> I am implementing a simple, interpreted language that needs a garbage-
> collected heap for storage. Like most discussions on memory management,
> I use the term "heap" by analogy - the actual data structure is not
> necessarily a true heap.
>
> My Heap type is basically a map of "addresses" (Int) to values. Values
> may themselves contain addresses, perhaps recursively to each other
> (like letrec). The extra field in Heap stores the next unused address,
> used to allocate fresh slots, as you can see in "heapAlloc".
>
> > type Address = Int
> > data Value = ...things with Addresses...
> > data Heap = Heap !Address (Map Address Value)
> >
> > heapAlloc :: Heap -> (Address, Heap)
> > heapAlloc (Heap nextFreeAddr binds) =
> >   (nextFreeAddr, Heap (succ nextFreeAddr) binds)
>
> There is also a stack of roots and an expression under evaluation.
>
> > type Stack = [...things with Addresses...]
> > data Expr = ...things with Addresses...
>
> The function I need to write is:
>
> > heapGC :: [Address] -> Heap -> Heap
> > heapGC roots heap = ...collect unreachable values from heap...
>
> Rather than re-invent this particularly complex wheel, I am wondering
> if I could use GHC weak pointers and have the GHC runtime collect values
> in my heap for me. Something like this:
>
> > type Address = Int
> > data Reference = Reference Address
> > data Value = ...things with References...
> > data Heap = Heap !Address (Map Address (Weak Value))
> >
> > heapAlloc :: Heap -> (Reference, Heap)
> > heapAlloc (Heap nextFreeAddr binds) =
> >   (Reference nextFreeAddr, Heap (succ nextFreeAddr) binds)
> >
> > heapStore :: Reference -> Value -> Heap -> IO Heap
> > heapStore ref@(Reference addr) val (Heap nextFreeAddr binds) = do
> >   weakVal <- mkWeak ref val Nothing
> >   return $ Heap nextFreeAddr (Map.insert addr weakVal binds)
>
> Note the new type, Reference. This replaces Address everywhere except
> in the heap's internal structure, which continues to use Address.
> Reference values are opaque, unique and obtained from the heap using
> heapAlloc.
>
> The idea is that only two things should cause Values to stay alive:
> 1. reachable holders of Reference (by GHC's definition of "reachable"),
> 2. ordinary Haskell closures arising from stepping the interpreter.
> "Being in the Heap" should not be enough to keep a Value alive. Then,
> my heapGC function should only have to prune dead addresses from the
> map.
>
> I am having trouble getting a proof-of-concept working (I am unable to
> observe any finalizers running, ever), but before I get into that, is
> this idea sound? Has somebody else implemented this in a library
> already?
>
> Thanks,
> Thomas Koster
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>



-- 
Regards

Sumit Sahrawat
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