Re: MPS: weak hash tables

[Pip Cet <pipcet@protonmail.com>]

On Tuesday, July 2nd, 2024 at 05:47, Gerd Möllmann <gerd.moellmann@gmail.com> wrote:
> Pip Cet pipcet@protonmail.com writes:
> 
> > Also, any opinions on :weakness 'key-or-value? Can we just not support it and use strong hash tables instead?
> 
> 
> So, after some more reading, I think I have an opinion. Let me read what
> it is :-)
> 
> The design supports 1 vector of weak references, and 1 vector of
> strong references. So either keys or values can be weak but not both.

Why not? I may be missing something obvious, but the scan method is allowed to modify the scanned object, right? We're doing that in the weak-key case anyway.

So what I'm currently doing is creating a single vector containing all keys, then all values, for key-and-value hash tables. If the key gets splatted, the value gets splatted right away and everything works as it should. If the value gets splatted, we've already decided to keep alive the key, but that's okay as it's only a weak reference.

> Let's say that W(O) returns true when object O is references from weak
> references only. AFAICT, the implementation of W in MPS requires
> allocation with rank weak to get weak references.

That's my understanding too.

> key/value pairs are then removed from a hash table when
> 
> Weakness Predicate
> ----------------------------------------
> key W(key)
> value W(value)
> key-and-value W(key) and W(value),
> key-or-value W(key) or W(value).

Hmm. I think I have key-and-value and key-or-value reversed, then...


> I'd say that neither key-and-value nor key-and-value can be
> implemented because both require 2 weak parts for implementing W.

Sorry, I still don't understand why.

> In Emacs itself, I see from git grep that :weakness key, value, and t
> are being used, where t means key-and-value. Haven't checked for which
> use cases t is used, though.

What surprised me is that there are plenty of hash tables that are both weak and use equal as a predicate. That doesn't make much sense to me...

> Anyway, we're better off than before :-).

On 64-bit systems. 32-bit systems are still broken, I'm afraid, and while my patch fixes the crash I've seen on i386 Debian, Eli's crash looked very different and may not be fixed by it.

Also, there's the whole caution thing about weak objects containing only unaligned words or words pointing directly to a base object, which is only relevant on Unix/i386, IIRC. (MPS emulates instructions to simulate fine-grained barriers, which is a really cool idea; I'd still like an option to turn it off though...). That would mean we have to replace Lisp_Objects and use the ptr member of our union (and that's the reason I'm using fixnums rather than plain integers for the hash).

Anyway, thanks a lot, so far. Sorry I'm being dense about the key-and-value thing.

Pip