Re: last_marked array is now ifdef'ed away

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Sat, 14 Sep 2024 15:50:03 -0700
> Cc: emacs-devel@gnu.org
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I'm quite confident that I want this be turned on again.
> 
> Since this affects performance by quite a bit (4.5-6.5 %, reportedly),
> how about the suggestion to "hitch it to another existing config option
> like ENABLE_CHECKING, or even make this an independent config option"?

This feature is useless if it is optional, because its main (perhaps
the only) use is when Emacs crashes during GC on a user machine,
especially without enough debug info.  Crashes during GC are rarely if
at all reproducible, so the only hope to investigate them is to look
at the objects recorded in the last_marked array, and asking users to
rebuild with some non-default option will not help.

If we are going to make it a compilation option, we might as well
remove it entirely.

5% of slowdown during GC (if that's a well-established number) is a
small price to pay for the possibility of investigating rare crashes.
A typical GC cycle takes tens or hundreds of milliseconds, so these
few percents are just a couple of msec.