Re: last_marked array is now ifdef'ed away

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Fri, 13 Sep 2024 16:43:49 +0200
> Cc: emacs-devel@gnu.org
> 
> 12 sep. 2024 kl. 11.19 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> >> Commit 7a8798de95a5 (from Apr 4 2022) made the last_marked[] array and
> >> the related machinery of tracing the marked objects #ifdef'ed away by
> >> default:
> >> 
> >>  +/* Whether to remember a few of the last marked values for debugging.  */
> >>  +#define GC_REMEMBER_LAST_MARKED 0
> 
> Yes, debugging code in the inner loop of one of the hottest paths of the GC probably shouldn't be on by default. I've done a large amount of intricate GC debugging and never felt a need for it, but it is easy enough to enable when wanted.

But if a user reports a crash inside GC, that's basically the only
trace we have to try and understand what caused the crash, especially
if the debug info was stripped.  That's why this code was always on by
default.

> We could hitch it to another existing config option like ENABLE_CHECKING, or even make this an independent config option, but there's value in not having to reconfigure and recompile everything when turning it on so keeping it localised in the source seems the better choice.
> 
> We could also change the `#if` to `if` to reduce the risk of bitrot. I'll do that later.

My main concern is not bitrot, it's actually being able to debug
crashes inside GC.

I think we should turn it on again, and remove the conditions.