On 5/27/20 11:31 PM, Pip Cet wrote:
> I hope you're right, in that compilers will support GC better before
> they move on to clever optimizations that break it :-)

After looking into it, I decided it wasn't worth the hassle of treating pointers
just past the end of a Lisp object as pointing into the object. Although such
pointers can exist, I can't think of a realistic-with-today's-compilers scenario
at the machine level where (1) a pointer like that will exist, (2) no pointers
into the middle or start of the object will exist, and (3) the object might be
accessed later. In contrast we have seen scenarios with pointers into the middle
of Lisp objects.

With that in mind, attached is a proposed patch to master that I hope deals with
some of the more-serious problems mentioned so far in this thread, in particular
the problem with Lisp_Object representations of symbols being split into two
registers in a --with-wide-int build. I haven't tested this as much as I'd like,
but I need to turn my attention to sleep and work and so this is a good place to
broadcast a checkpoint.

This patch doesn't address the LISP_ALIGNMENT issues you mentioned, both in
lisp.h and in the pdumper; I can work on that soon, I think.

PS. Thanks for helping bring this problem to our attention; it's been fun to
look into it.