Fwd: Restricted sums in BoxedRep

Ryan Yates fryguybob at gmail.com
Thu Oct 15 17:10:16 UTC 2020

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.feuer at 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.schonwald at 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.feuer at gmail.com>
>> wrote:
>>> Sorry, unlifted, not unboxed...
>>> On Thu, Oct 15, 2020, 11:57 AM David Feuer <david.feuer at 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.schonwald at 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.andreas at 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.thaddeus at gmail.com>
>>>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>>>> Subject: Re: Restricted sums in BoxedRep
>>>>>> To: David Feuer <david.feuer at 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.feuer at 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
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