bug#77924: 31.0.50; [Feature branch] Change marker implementation

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: pipcet@protonmail.com,  monnier@iro.umontreal.ca,
>   77924@debbugs.gnu.org,  stefankangas@gmail.com
> Date: Fri, 25 Apr 2025 13:40:47 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Actually, I thought about maintaining the number of markers in the
> > vector, so there would be no need to detect that by the MARKERP test.
> 
> Not sure how that would work, but maybe you are thinking of the LRU list
> in igc.

We were talking about different aspects, sorry.

Regarding the issue with a marker vector where some slots are free, we
could simply compare with NULL.  Or we could keep a pointer to the
next non-free slot.  Not sure what should be cheaper, since iteration
over all the markers is quite frequent, whereas adding and removing
markers is probably less so.

> Iteration is done by stepping over all _ and M in order (= fast), and
> only giving out at the M entries. And as a small optimization for the
> case of over-allocation, I remember the max index of an M that was ever
> used.

Compacting the vector would also be possible, and I'm not sure it will
be too expensive, since memcpy is quite fast on modern machines.

All of this is not very important, except that we are talking about
simplifications of FOR_EACH_MARKER macro, so if any of this
facilitates a simpler macro without hampering performance, I'd suggest
to consider it seriously.