Stream: Coq devs & plugin devs

Topic: NbE and the implementation of the VM


view this post on Zulip Jason Gross (Sep 08 2020 at 01:26):

I have just had a look at https://hal.inria.fr/inria-00434283/document (Efficient Normalization by Evaluation by Mathieu Boespflug). It looks like it might be a proposal for a significantly less finicky implementation of the VM, but I can't quite tell. Has the knowledge of this document been integrated into Coq already? What is the current state of things?

view this post on Zulip Enrico Tassi (Sep 08 2020 at 05:18):

Native compute is related to nbe, I don't know about this paper exactly cc @Maxime Dénès

view this post on Zulip Pierre-Marie Pédrot (Sep 08 2020 at 06:34):

Both the Coq VM and native compilation are a form of NbE, and I don't see in this paper anything outstandingly different from what we do.

view this post on Zulip Pierre-Marie Pédrot (Sep 08 2020 at 06:35):

Given the author I wonder whether it's not even indirectly linked to the implementation of the VM.

view this post on Zulip Maxime Dénès (Sep 08 2020 at 06:51):

I implemented native_compute after an internship with Mathieu :)

view this post on Zulip Maxime Dénès (Sep 08 2020 at 06:51):

so it is more than related to this paper

view this post on Zulip Jason Gross (Sep 08 2020 at 18:03):

The paper says:

The machine of Grégoire and Leroy (2002) which COQ sometimes uses for the conversion test should also be mentioned here. Theirs is a modified and formalized version of a bytecode interpreter for OCaml (the ZAM), to do normalization via reduction to weak head normal form along with a readback phase to restart weak reduction under binders. Whilst offering striking similarities to NbE, including in its reuse of existing evaluators, one important difference lies in the fact that the implementation of the underlying evaluator needs to be modified, whereas the objective of NbE, here and elsewhere, is to get away without looking under the hood. As a side effect, NbE affords more freedom of choice regarding which evaluator to choose, allowing for instance to trade off minimizing the trusted base for better performance.

The principal extension made to the ZAM to normalize COQ terms is the introduction of accumulators, which represent applications of free variables to a number of terms. Embedding this construct within the virtual machine avoids having to do case analysis at every application to discriminate between function applications and applications of neutral terms. We show that with the simple optimization of Section 3.1, the overhead of this case analysis is very small in practise.

So I'm wondering if the recent troubles with needing to change the accumulator tags, etc (I've seen a bunch of issues/PRs about this recently, IIRC), could be avoided with this paper?

view this post on Zulip Maxime Dénès (Sep 11 2020 at 21:47):

I don't recall what was the status of benchmarks when this paper was written, but when we worked more on this, we showed that having a natural representation for function application did matter in terms of performances (not only you avoid the case analysis, but you also give the compiler more opportunities for applying optimizations, etc).

view this post on Zulip Maxime Dénès (Sep 11 2020 at 21:48):

So in theory, the handling of tags as done by the VM is not necessary (you can implement the same calculus with an ADT of terms and case analyses for applications), but if you care about performance, the discrimination using tags is better.

view this post on Zulip Maxime Dénès (Sep 11 2020 at 21:49):

That being said, I'm sure there are some solutions to the issues that are a good compromise. I haven't had the time to review the PRs yet, but will do it soon with Benjamin Grégoire.

view this post on Zulip Bas Spitters (Sep 12 2020 at 08:43):

With Jon, we wrote a short note about NbE and gluing
https://arxiv.org/abs/1809.08646
Gluing has recently become a popular tool in type theory. E.g.
https://drops.dagstuhl.de/opus/volltexte/2019/10532/pdf/LIPIcs-FSCD-2019-25.pdf
https://www.jonmsterling.com/pdfs/sa20.pdf


Last updated: Oct 16 2021 at 02:03 UTC