Fwd: Restricted sums in BoxedRep

[David Feuer]

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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