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

[Paul Eggert]

On 5/27/20 8:26 AM, Eli Zaretskii wrote:
> We used to rely on 8-byte alignment on those systems, and I don't see
> any reason not to continue relying on that and punishing those
> systems' performance.  What would we gain?

In looking into this more, it appears that the maybe_lisp_pointer idea is wrong,
in that compilers can make pointers into a Lisp object while losing the address
of the original object (and we've seen them do this) and there's no guarantee
that these sub-pointers are GCALIGNED. This sort of failure should be quite rare
but can cause crashes such as the one you observed. I am looking into a fix and
plan to apply it to master (I've already installed some minor glitches I
observed on the way); we can then talk about what to do with emacs-27.