Per-generation lists of weak pointers

[Edward Z. Yang]

I was under the impression that foreign pointers finalizers were only
ordered with respect to multiple finalizers on a single object.  So if
you can show your implementation preserves same-object ordering, that
should be sufficient. (Nota bene: I haven't CR'd your code.)

Edward

Excerpts from Akio Takano's message of Mon Mar 11 03:17:48 -0700 2013:
> Hi,
> 
> I'm working on implementing per-generation lists of weak pointers to
> speed up garbage collection in programs that allocate a lot of weak
> pointers. I have a patch [1] that validates and gives a 3x speed up on
> a benchmark. However I'd like to ask for some advise before finishing
> and submitting the patch.
> 
> [1] https://github.com/takano-akio/ghc/commit/c7345c68eaa1e7f9572e693b5e352e386df7d680
> 
> The problem is that since my patch splits the weak pointer list
> between generations, it no longer maintains the right order of weak
> pointers. This could cause finalizers added with
> addForeignPtrFinalizer to run in the wrong order.
> 
> I can think of one way to fix it; to make sure that when a WEAK object
> gets promoted, it is always added to the front of the new list. So my
> questions are:
> 
> - Would it be a correct fix?
> - If so, is it an acceptable fix? For example, is it too fragile a
> reasoning to rely on?
> 
> Thank you in advance,
> 
> Takano Akio
>