presentation: Next-gen Haskell Compilation Techniques

[Sebastian Graf]

Hi Csaba,

Thanks for your presentation, that's a nice high-level overview of what
you're up to.

A few thoughts:

   - Whole-program optimization sounds great, but also very ambitious,
   given the amount of code GHC generates today. I'd be amazed to see advances
   in that area, though, and your >100-module CFA performance incites hope!
   - I wonder if going through GRIN results in a more efficient mapping to
   hardware. I recently found that the code GHC generates is dominated by
   administrative traffic from and to the heap [1]. I suspect that you can
   have big wins here if you manage to convey better call stack, heap and
   alias information to LLVM.
   - The Control Analysis+specialisation approach sounds pretty similar to
   doing Constructor Specialisation [2] for Lambdas (cf. 6.2) if you also
   inline the function for which you specialise afterwards. I sunk many hours
   into making that work reliably, fast and without code bloat in the past, to
   no avail. Frankly, if you can do it in GRIN, I don't see why we couldn't do
   it in Core. But maybe we can learn from the GRIN implementation afterwards
   and maybe rethink SpecConstr. Maybe the key is not to inline the function
   for which we specialise? But then you don't gain that much...
   - I follow the Counting Immutable Beans [3] stuff quite closely
   (Sebastian is a colleague of mine) and hope that it is applicable to
   Haskell some day. But I think using Perceus, like any purely RC-based
   memory management scheme, means that you can't have cycles in your heap, so
   no loopy thunks (such as constant-space `ones = 1:ones`) and mutability. I
   think that makes a pretty huge difference for many use cases. Sebastian
   also told me that they have to adapt their solutions to the cycle
   restriction from time to time, so far always successfully. But it comes at
   a cost: You have to adapt the code you want to write into a form that works.

I only read the slides, apologies if some of my points were invalidated by
something you said.

Keep up the good work!
Cheers,
Sebastian

[1] https://gitlab.haskell.org/ghc/ghc/-/issues/19113
[2]
https://www.microsoft.com/en-us/research/wp-content/uploads/2016/07/spec-constr.pdf
[3] https://arxiv.org/abs/1908.05647

Am So., 10. Jan. 2021 um 00:31 Uhr schrieb Csaba Hruska <
csaba.hruska at gmail.com>:

> Hello,
> I did an online presentation about Haskell related (futuristic)
> compilation techniques.
> The application of these methods is also the main motivation of my work
> with the grin compiler project and ghc-wpc.
>
> video: https://www.youtube.com/watch?v=jyaR8E325ok
> slides:
> https://docs.google.com/presentation/d/1g_-bHgeD7lV4AYybnvjgkWa9GKuP6QFUyd26zpqXssQ/edit?usp=sharing
>
> Regards,
> Csaba
> _______________________________________________
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> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
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