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: Wed, 27 May 2020 10:53:22 -0700
> 
> The source code says
> 
>    for (i = 0; i < size; i++)
>       foo (AREF (obj, i));
> 
> This is the last reference to obj, so the compiler reuses the register R 
> holding
> obj, and has that register R contain &XVECTOR (obj)->contents[0], &XVECTOR
> (obj)->contents[1], etc. each time through the loop, and transforms the call
> into foo (*R) as an optimization. When foo calls the garbage collector,
> maybe_lisp_pointer (R) can be false because R doesn't point directly at a Lisp
> object: it points somewhere into the middle of a Lisp object and R's value is
> not GC-aligned.

For this to cause trouble, you'd need to arrange for no other
reference to obj, neither anywhere else up the callstack, nor from
another object we will mark.