Re: MPS image cache

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: eller.helmut@gmail.com,  emacs-devel@gnu.org
> Date: Sun, 05 May 2024 16:32:02 +0200
> 
> I think it's a matter of personal priorities. It guess it can't hurt to
> say explicity what these are.
> 
> 1. Make progress fast, so that I can assess for myself if it's worth
> continuing. With an Emacs up time of ca. 8h now, I tend to think so.
> When it would reach say 5 days or so, we'd be in the realm of what
> master survives for me on macOS.
> 
> 2. Make it work for me in my personal fork (CL packages) which is macOS
> only anyway, and is basically for me alone only.
> 
> 3. GNU. And I'm neither a maintainer of anything, nor want to maintain
> anything in the GNU sense.

And my impression was that we are trying to advance on 3 most
important Emacs platforms in parallel, with the purpose of adding this
to Emacs at some point.  Apologies for my misunderstanding.

> >> >> We allocate a vector of N pointers. The N is found in the header when we
> >> >> scan it. It's the size of the MPS object we scan minus the header
> >> >> divided by how large a pointer is.
> >> >
> >> > OK, but why does the struct have to fit in a single word?
> >> 
> >> It doesn't, it the other way around. I'm using nwords - 1 for the size
> >> of the vector, and the static_assert is intended to fire if nwords - 1
> >> is not right.
> >
> > OK, then my question now is "why does it have to be nwords-1?"
> 
> I suspect that I'll answer something you don't ask again: We allocate a
> vector of N elements. We don't store N directly somewhere, but we have
> an igc_header from which we can compute N. So I compute it as nwords -
> 1. If that's not right, I static_assert, and leave it to the interested
> party to do something.

I fixed the code according to my understanding, please take a look and
tell if I missed something (since the new function does not seem to be
used yet for the two caches you mentioned, it is hard to know whether
I violated some assumptions).

Thanks.