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

[Pip Cet]

On Sat, May 30, 2020 at 6:04 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
> On 5/30/20 6:29 AM, Pip Cet wrote:
> > I'm surprised, but glad that you think so. Patch for emacs-27 attached.
> That patch is on the right track but it's not clear whether it will
> cause GC to fail to mark some objects that it should, both because it
> omits mark_maybe_object on platforms like x86 --with-wide-int where
> alignof (void *) < sizeof (Lisp_Object), and because it skips
> mark_maybe_pointer on more-typical platforms where alignof (void *) ==
> sizeof (Lisp_Object).

I've thought about this for a while, but I fail to see the problem
with my patch. mark_maybe_object is unnecessary on x86
--with-wide-int, and mark_maybe_pointer (off + lispsym) is unnecessary
on platforms that don't rip apart our precious Lisp_Objects. The other
call to mark_maybe_pointer isn't skipped.

I still think we ought to use yours (and accept a ~25% performance
penalty in this particular loop on Eli's platform), but include a
comment like the one I had in mine. It might hide further bugs, but
that's probably what we want to do on emacs-27.

Proposed patch attached.

0001-Be-more-aggressive-in-marking-objects-during-GC-bug-.patch
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