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

[Pip Cet]

On Sat, May 30, 2020 at 10:23 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
> Oh, I see I am being the dense one. I was thinking based on some of my
> master-branch improvements. One option is to do away with
> mark_maybe_object entirely, so that one needn't deal with looking at
> each part of the stack twice (this is for efficiency).

Yes, I thought you'd already done that on master. I must not have been
keeping up with the patches.

Much as I like thinking about putting symbols in the rbtree twice and
walking it smartly to retrieve up to two overlapping nodes, I suspect
there are much easier ways of fixing this, at least on 64-bit
architectures. We could make sure, for example, that all symbol blocks
come after lispsym in memory, and store lispsym - address in the
Lisp_Object. Those values would then fall outside the 48-bit space of
actually valid x86_64 addresses, so we could get away with
mark_maybe_pointer (word < 0 ? lispsym - word : word) on that
architecture.

> In emacs-27 the patch you proposed earlier is probably OK, though I
> haven't had time to think through all the possibilities.

I was just curious. I think we should go with your latest patch.