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

[Eli Zaretskii <eliz@gnu.org>]

> From: Pip Cet <pipcet@gmail.com>
> Date: Sat, 30 May 2020 13:29:33 +0000
> Cc: eggert@cs.ucla.edu, 41321@debbugs.gnu.org, 
> 	Stefan Monnier <monnier@iro.umontreal.ca>
> 
> > > > So this alignment requirement is only due to pthreads being used?
> > >
> > > I'm not sure what you're asking. Obviously there are systems on which
> > > unaligned accesses will fault or be very slow indeed, so we need to
> > > make sure, say, pure space allocations are aligned somehow. That
> > > requires a LISP_ALIGNMENT of 8. Everything beyond that is only for
> > > performance, pthreads, and SIMD types.
> >
> > If the system guarantees 4-byte alignment from malloc (and/or a
> > similar alignment of the runtime C stack), then using that doesn't
> > trigger problems related to unaligned accesses, right?  So let me
> > rephrase: why isn't 4-byte alignment "good enough" for us on systems
> > where malloc and the runtime stack are guaranteed to be thus aligned?
> 
> (The runtime stack isn't relevant, as far as I can tell, since we walk
> that in 4-byte steps on such systems anyway.)

I think it might be relevant for stack-based Lisp objects (if we keep
requiring that Lisp objects are 8-byte aligned on 32-bit platforms).

> You're correct that on such a system, we could get away with a
> LISP_ALIGNMENT of 4, but a LISP_ALIGNMENT of 8 wouldn't hurt either.

That's for sure.  I just wondered why did we start requiring 8-byte
alignment back when we did.  Perhaps someone still remembers the
reason.