Re: MPS: weak hash tables

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 02 Jul 2024 16:34:40 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: gerd.moellmann@gmail.com, eller.helmut@gmail.com, emacs-devel@gnu.org
> 
> > > 1. mangle all Lisp_Objects to pointers or fixnums when storing them in a weak hash table, and unmangle them upon retrieval
> > > 2. not use 32-bit x86 machines
> > > 3. modify MPS
> > > 4. throw caution to the wind and just hope it works
> > I don't understand why (1) is needed. Lisp objects are already
> > pointers in disguise, so what exactly is the problem here?
> 
> They need to be aligned for MPS to understand they're pointers; they're unaligned, except for symbols which aren't pointers in the first place.  In essence, MPS was focusing on the wrong language (for us).

The pointers we hide in Lisp objects are already aligned.  Why cannot
we use them directly?

> > I also don't understand how come an optimization turned out to do us
> > some harm.
> 
> Isn't that quite common, really?

Not in my book, no.

> Is it okay if I install the changes for Ihor's bugs first, then fix IA32? I still need to do some work to make sure they build without any unnecessary overhead in the !HAVE_MPS case.

It doesn't matter in what order things are fixed as long as they are
fixed.