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

[Pip Cet]

On Mon, May 25, 2020 at 3:14 PM Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Pip Cet <pipcet@gmail.com>
> > Date: Mon, 25 May 2020 06:40:11 +0000
> > Cc: 41321@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
> >
> > What are you actually planning to do?
> Not
> much: I intend to continue running Emacs under GDB and collect data
> about the crashes until either I figure out what causes the crashes,
> or the crashes disappear (which would mean the problem was fixed
> indirectly by some other change).

(Or directly, of course. I still believe my "theory" about your bug is correct.)

> > I think we should work around the mingw bug on both the master and
> > emacs-27 branches.
>
> That depends on what the proposed solution or workaround will be.

For emacs-27, reducing the alignment requirement in
maybe_lisp_pointer: that will only make us check more pointers, not
fewer, so while it is a GC change it's one that makes sense.

For master, I'd consider setting LISP_ALIGNMENT to 8 on the mingw32
platform, where memory is already scarce. I don't trust the alleged
performance hit of 20%, so we might have to collect some actual
performance data. But we definitely need to make strings aligned to
LISP_ALIGNMENT, one way or the other, because that's the original
reason for maybe_mark_pointer.