Re: MPS: weak hash tables

[Eli Zaretskii]

> 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.