Ah yes. In my research work I extended Mutable Constructor fields to allow a mutable array field to help with that problem. This allowed me to have a Nil constructor in a sum type and a Node constructor with normal fields as well as an array of mutable fields. Pointer tagging worked as expected so a case match on a dereference of a field would not need to follow the pointer and no intermediate objects were between Node objects.
On Thu, Oct 15, 2020 at 4:08 PM David Feuer <david.fe...@gmail.com> wrote: > This might be lost in the noise for MVar and TVar, but for arrays, there > are tremendous advantages to cutting out the extra heap objects. > > On Thu, Oct 15, 2020, 1:10 PM Ryan Yates <fryguy...@gmail.com> wrote: > >> I haven't been following this discussion too closely, but in my research >> work I found that some of the benefits that I wanted in this direction were >> already there with pointer tagging. >> >> On Thu, Oct 15, 2020 at 12:59 PM David Feuer <david.fe...@gmail.com> >> wrote: >> >>> Yes, that's something quite different. We'd need a whole different heap >>> object type for such MVars and TVars. My approach covers the case where the >>> unboxed thing can only take on a few values, for some value of "a few" >>> which, depending on implementation, may or may not be very small. If the >>> nulls point to actual heap objects in pre-allocated pinned memory (say), >>> then up to 64 or so might be reasonable. If they point to "invalid" address >>> space, then the numbers could go up a good bit. >>> >>> On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald < >>> carter.schonw...@gmail.com> wrote: >>> >>>> Indeed, I mean things that aren’t pointery, and could be represented by >>>> a tvar paired with a mutable byte array or mvar with mutable byte array, >>>> but which we’d want considered as a single heap object from the rts/gc >>>> perspective. >>>> >>>> On Thu, Oct 15, 2020 at 11:58 AM David Feuer <david.fe...@gmail.com> >>>> wrote: >>>> >>>>> Sorry, unlifted, not unboxed... >>>>> >>>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer <david.fe...@gmail.com> >>>>> wrote: >>>>> >>>>>> Putting unboxed things in TVar, MVar, etc., is part of Andrew >>>>>> Martin's accepted BoxedRep proposal. >>>>>> >>>>>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald < >>>>>> carter.schonw...@gmail.com> wrote: >>>>>> >>>>>>> A related idea that came up recently and is perhaps simpler ties >>>>>>> into this via the lens of having unboxed Mvars/tvars (even if it’s >>>>>>> restricted to just things we can embed in a word64#) >>>>>>> >>>>>>> This came up in >>>>>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, >>>>>>> where viktor had millions of independent mvars holding what’s >>>>>>> essentially a >>>>>>> strict unit ()! >>>>>>> >>>>>>> The motivation in this later scenario is that in high concurrency >>>>>>> settings, the less trivial stuff the gc needs to trace under updates, >>>>>>> the >>>>>>> better ghc scales. >>>>>>> >>>>>>> This may not be a use case david has in mind, but certainly seems >>>>>>> related. >>>>>>> >>>>>>> Phrased more succinctly: gc perf dominates large heap / many core >>>>>>> computation in Haskell via sensitivity to allocation volume / mutation >>>>>>> volume (to ensure generational hypothesis stays valid), and providing >>>>>>> tools >>>>>>> to incrementally reduce the pressure with local changes would be good. >>>>>>> >>>>>>> So I’d propose / suggest that a baby step towards what david asks >>>>>>> would be for us to work out some manner of unboxed tvar/mvar ref >>>>>>> machinery >>>>>>> that supports unboxed values. >>>>>>> >>>>>>> >>>>>>> >>>>>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger < >>>>>>> klebinger.andr...@gmx.at> wrote: >>>>>>> >>>>>>>> From a implementors perspective my main questions would be: >>>>>>>> >>>>>>>> * How big is the benefit in practice? How many use cases are there? >>>>>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...) >>>>>>>> >>>>>>>> The details of how this would be exposed to a user would are >>>>>>>> important. >>>>>>>> But if the costs are too high for the drawbacks then it becomes a >>>>>>>> moot point. >>>>>>>> >>>>>>>> >>>>>>>> David Feuer schrieb am 14.10.2020 um 22:21: >>>>>>>> >>>>>>>> Forwarded from Andrew Martin below. I think we want more than just >>>>>>>> Maybe (more than one null), but the nesting I described is certainly >>>>>>>> more >>>>>>>> convenience than necessity. >>>>>>>> >>>>>>>> ---------- Forwarded message --------- >>>>>>>> From: Andrew Martin <andrew.thadd...@gmail.com> >>>>>>>> Date: Wed, Oct 14, 2020, 4:14 PM >>>>>>>> Subject: Re: Restricted sums in BoxedRep >>>>>>>> To: David Feuer <david.fe...@gmail.com> >>>>>>>> >>>>>>>> >>>>>>>> You'll have to forward this to the ghc-devs list to share it with >>>>>>>> others since I'm not currently subscribed to it, but I've had this same >>>>>>>> thought before. It is discussed at >>>>>>>> https://github.com/andrewthad/impure-containers/issues/12. Here's >>>>>>>> the relevant excerpt: >>>>>>>> >>>>>>>>> Relatedly, I was thinking the other day that after finishing >>>>>>>>> implementing >>>>>>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst, >>>>>>>>> I should really look at seeing if it's possible to add this >>>>>>>>> maybe-of-a-lifted value trick straight to GHC. I think that with: >>>>>>>>> >>>>>>>>> data RuntimpRep >>>>>>>>> = BoxedRep Levity >>>>>>>>> | MaybeBoxedRep Levity >>>>>>>>> | IntRep >>>>>>>>> | ... >>>>>>>>> >>>>>>>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE >>>>>>>>> ('MaybeBoxedRep v) >>>>>>>>> >>>>>>>>> This doesn't have the nesting issues because the kind system >>>>>>>>> prevents nesting. But anyway, back to the original question. I would >>>>>>>>> recommend not using Maybe.Unsafe and using unpacked-maybe >>>>>>>>> instead. The latter is definitely safe, and it only costs an extra >>>>>>>>> machine >>>>>>>>> word of space in each data constructor it gets used in, and it doesn't >>>>>>>>> introduce more indirections. >>>>>>>>> >>>>>>>> >>>>>>>> On Tue, Oct 13, 2020 at 5:47 PM David Feuer <david.fe...@gmail.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> Null pointers are widely known to be a lousy language feature in >>>>>>>>> general, but there are certain situations where they're *really* >>>>>>>>> useful for >>>>>>>>> compact representation. For example, we define >>>>>>>>> >>>>>>>>> newtype TMVar a = TMVar (TVar (Maybe a)) >>>>>>>>> >>>>>>>>> We don't, however, actually use the fact that (Maybe a) is lifted. >>>>>>>>> So we could represent this much more efficiently using something like >>>>>>>>> >>>>>>>>> newtype TMVar a = TMVar (TVar a) >>>>>>>>> >>>>>>>>> where Nothing is represented by a distinguished "null" pointer. >>>>>>>>> >>>>>>>>> While it's possible to implement this sort of thing in user code >>>>>>>>> (with lots of fuss and care), it's not very nice at all. What I'd >>>>>>>>> really >>>>>>>>> like to be able to do is represent certain kinds of sums like this >>>>>>>>> natively. >>>>>>>>> >>>>>>>>> Now that we're getting BoxedRep, I think we can probably make it >>>>>>>>> happen. The trick is to add a special Levity constructor representing >>>>>>>>> sums >>>>>>>>> of particular shapes. Specifically, we can represent a type like this >>>>>>>>> if it >>>>>>>>> is a possibly-nested sum which, when flattened into a single sum, >>>>>>>>> consists >>>>>>>>> of some number of nullary tuples and at most one Lifted or Unlifted >>>>>>>>> type. >>>>>>>>> Then we can have (inline) primops to convert between the BoxedRep and >>>>>>>>> the >>>>>>>>> sum-of-sums representations. >>>>>>>>> >>>>>>>>> Anyone have thoughts on details for what the Levity constructor >>>>>>>>> arguments might look like? >>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> -Andrew Thaddeus Martin >>>>>>>> >>>>>>>> >>>>>>>> _______________________________________________ >>>>>>>> ghc-devs mailing >>>>>>>> listghc-devs@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >>>>>>>> >>>>>>>> >>>>>>>> _______________________________________________ >>>>>>>> ghc-devs mailing list >>>>>>>> ghc-devs@haskell.org >>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >>>>>>>> >>>>>>> _______________________________________________ >>> ghc-devs mailing list >>> ghc-devs@haskell.org >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >>> >>
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