bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 31 May 2020 18:48:28 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 41321@debbugs.gnu.org, monnier@iro.umontreal.ca, pipcet@gmail.com
> 
> > Sure. Compared to simply making LISP_ALIGNMENT = 8 as a workaround 
> > (which is not correct as noted above but which fixes the lost-marker 
> > bug), the proposed patch is about a 1% CPU-time hit in my usual 
> > benchmark (make compile-always) on a 32-bit platform compiled with 
> > --with-wide-int (this is Ubuntu 18.04.4, gcc -m32, Xeon E3-1225 v2). We 
> > can surely speed this up with some cost in complexity (that's what I was 
> > working on on the master branch), but for emacs-27 I thought that 
> > reliability took precedence over 1% performance improvements.
> > 
> > I expect that most of the performance hit is not due to the 
> > LISP_ALIGNMENT thing, it's due to the "you have to check pointers three 
> > times" thing. In my master-branch draft I'm working on getting this down 
> > to "you have to check pointers an average of 1+epsilon times" for some 
> > suitable value of epsilon. I don't know yet what epsilon will be. But 
> > anyway, this is only about improving that 1% performance hit.
> 
> OK, then let's get this change into emacs-27, and thanks.

FTR, I'm now running Emacs 27.0.91 pretest patched with Paul's changes
in commit  68b6dad1d8e22fe700871c9a5a18da3dd496cc8a.