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

[Paul Eggert]

On 5/30/20 11:21 AM, Eli Zaretskii wrote:

> on current emacs-27, both LISP_ALIGNMENT
> and alignof(max_align_t) are 16 in my builds.  And I still don't
> understand why using LISP_ALIGNMENT of 8 is not right in this case (on
> emacs-27).

You're right that LISP_ALIGNMENT is 16 on your host on the emacs-27 
branch, because alignof (max_align_t) is 16 there. And you're also right 
that setting LISP_ALIGNMENT to be 8 on your host would fix the marker 
bug you observed there, because it would work around your host's bug 
where malloc returns a pointer that is not a multiple of 
alignof(max_align_t). However, C and POSIX allow platforms where 
LISP_ALIGNMENT should be greater than 8, or (if !USE_LSB_TAG) should be 
less than 8, so I'd be leery about changing LISP_ALIGNMENT on any host 
that doesn't have your host's idiosyncrasies. And that specific 
workaround should not be needed anyway if we install the emacs-27 patch 
that I have most-recently suggested (or Pip Cet's very-similar recent 
patch), since this patch solves the problem in a more-general way that 
should help to prevent more bugs like this one.