Re: Some experience with the igc branch

[Gerd Möllmann]

Eli Zaretskii <eliz@gnu.org> writes:

> So there are Lisp objects allocated by alloc_impl, roots allocated
> via MPS, and data allocated by xmalloc that MPS doesn't know about, is
> that correct?

Correct.

I'd perhaps change "roots allocated" to "roots declared". One simply
tells MPS a range of memory [start, end] is a root of type so-and-so
(ambiguous, exact). The memory itself can come from xmalloc for example.

>> > Once again, I think this is very important for future maintenance.  I
>> > feel that this barrier thing in MPS introduces significant
>> > complications into reasoning about safety of C-level changes.
>> > Previously, we only had the mark bit to worry about if we wanted to
>> > access Lisp objects during GC (see gc_asize, for example), but now we
>> > have a much larger problem, AFAIU.  How do we manage that for the next
>> > 40 years?
>> 
>> These problems do not exist. The barriers are transparent for the
>> application, except in vary special circumstances, namely this shit
>> signal handler.
>
> But I _am_ talking about this "shit signal handler".  I'm trying to
> understand how would I go about reasoning whether accessing specpdl
> from the signal handler is okay.  Is that because I'm supposed to know
> that the specpdl stack is a root?  If so, I'd need to figure out that
> for every datum the handler accesses, no?

That's right, but perhaps it helps that anything that can end up being
Lisp_Object is not a root. And only those can be affected by barriers.

> I guess I'm yearning for some commentary in igc.c, not unlike what you
> wrote in xdisp.c at the time, which would explain the basics, like
> what are roots, what's the purpose of all those root_create_SOMETHING
> functions, what's the difference between exact and ambiguous roots,
> etc.  Because currently we are not too spoiled by comments in igc.c.

Would a reference to the MPS Guide

  https://memory-pool-system.readthedocs.io/en/latest/guide/index.html#

help?