Re: Time to merge scratch/correct-warning-pos into master, perhaps?

[Alan Mackenzie]

Hello, Eli.

On Fri, Feb 04, 2022 at 20:54:20 +0200, Eli Zaretskii wrote:
> > Date: Fri, 4 Feb 2022 18:31:01 +0000
> > Cc: gregory@heytings.org, monnier@iro.umontreal.ca, mattiase@acm.org,
> >   larsi@gnus.org, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > > Can you explain why we need to complicate EQ so much to account for
> > > symbols-with-pos?

> > There are four possible ways the two Lisp objects might match - each
> > object can be either a bare symbol, or a symbol with pos, giving four
> > possibilities.  It's worth noting that the last three possibilities are
> > cut off completely by the check on symbols-with-pos-enabled, except
> > during compilation.

> I don't see how the number of combinations is relevant.  If all you
> need to compare is the symbol part, then whether there's something
> after it in memory is not interesting and doesn't affect comparison.

Yes, that's true.

> > Specific questions:

> > >   . comparison of a symbol and symbol-with-pos should only compare the
> > >     symbol part, and disregard the "pos" part, right?

Yes.

> > >   . if so, couldn't we implement symbol-with-pos as a struct that has
> > >     the symbol at its beginning, and then use memcmp to compare only
> > >     that part?

> > > Or what am I missing?

> > I don't think we can get anywhere that way.  What gets compared in EQ
> > are the Lisp_Object's, which are typically already in processor
> > registers.  I don't think we typically have a pointer to memory
> > containing the symbol with position.

> Sorry, I don't understand how this answers my questions.

OK, I don't think we can improve the performance by the use of a struct
containing the symbol followed by the position.  I'm not sure what more I
can add.

> > Don't forget that the position attaches to the symbol OCCURRENCE,
> > not the symbol itself.

> I don't understand what I need not to forget, please elaborate.

In the moderate past, that thing, the (lack of) possibility of attaching
the position to the symbol, caused me a lack of clarity.  I forgot many
times that we need to deal with symbol occurrences, not symbols.

We cannot extend struct Lisp_Symbol with the position, since a single
symbol typically occurs several times in different positions.  Rather, I
think we would need to extend the Lisp_Object in this fashion.

> What is there in a symbol-with-pos except the symbol and the position?

There is the symbol, the position, and a pseudovector header.

> (And please be more cooperative and forthcoming, since my questions
> are meant to help make this feature less expensive, otherwise there's
> a real chance of the feature to be removed, something I hope you don't
> want.)

Sorry, I was doing my best to answer what I thought you were asking.  I
wasn't trying to be evasive.

-- 
Alan Mackenzie (Nuremberg, Germany).