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

[Pip Cet <pipcet@gmail.com>]

On Sat, May 30, 2020 at 9:08 AM Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Pip Cet <pipcet@gmail.com>
> > Date: Sat, 30 May 2020 07:19:18 +0000
> > Cc: eggert@cs.ucla.edu, 41321@debbugs.gnu.org,
> >       Stefan Monnier <monnier@iro.umontreal.ca>
> >
> > > Btw, can someone remind me why we started requiring non-default
> > > alignment from Lisp objects?
> >
> > max_align_t was changed to include a float128 type, and
> > alignof(float128) == 16 on x86, even though virtually all x86 systems
> > are configured to allow unaligned accesses.
>
> I understand that part, but my question was why, even before the
> change in max_align_t, did we start requiring 8-byte alignment on
> systems where that is not automatically guaranteed?

I don't know. As I said, I think that was always buggy on pdumper
systems, though the bug was very subtle. My guess is it predates
pdumper, at which time it was a valid optimization.

> > If I understand Paul's concerns correctly, he believes it's possible a
> > system will once again come into use in which atomic accesses only
> > work for offsets aligned to, say, 32 bytes. Since pthread variables
> > require atomic accesses, such a platform would see weird crashes if a
> > pthread inside a Lisp_Vector wasn't aligned to 32 bytes.
>
> 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.

> > > Also, given the fact that in the crashing case the 2 32-bit parts of a
> > > Lisp object were pushed onto the stack non-contiguously, will fixing
> > > the alignment alone cause those Lisp objects to be marked?
> >
> > Yes. The lower 32-bit part was ignored because its value wasn't
> > 16-byte aligned, not because its stack location wasn't 8-byte aligned.
>
> Right, but I'm talking about marking.  AFAIU, when scanning the stack
> finds a value that looks like a Lisp object, we mark that object.

And if we find a value that looks like a pointer to a Lisp structure,
as the lower half of a non-symbol Lisp_Object does, we mark the
corresponding Lisp object.

> If
> the two 32-bit parts of the object are non-contiguous, will we be able
> to recognize such an object, and will we be able to mark it correctly,
> and if so, how?  IOW, don't we need the upper 32-bit (which encodes
> the object type) for the purposes of marking it?

For everything but symbols, we don't, mark_maybe_pointer called on the
low 32 bits suffices. For symbols, mark_maybe_pointer needs to be
changed to also check the pointer at <low 32-bit word> + &lispsym.