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

[Eli Zaretskii]

> Cc: pipcet@gmail.com, 41321@debbugs.gnu.org, monnier@iro.umontreal.ca
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 30 May 2020 15:18:53 -0700
> 
> On 5/30/20 12:33 PM, Eli Zaretskii wrote:
> 
> > Posix may require it, but do we actually know of any supported
> > important platforms where this happens?
> 
> > But your proposal is also less efficient, isn't it?   If so, its more
> > general nature comes at a price.
> 
> 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.