Re: MPS signals and Emacs

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: eggert@cs.ucla.edu,  eller.helmut@gmail.com,  emacs-devel@gnu.org
> Date: Mon, 22 Apr 2024 12:22:25 +0200
> 
> Now a GC happens. We can copy objects from a pages in from-space to a
> page in to-soace, and everything works as in the copying collector, in
> principle. At the end, we just switch what we consider to-space and
> from-space. Maybe a 1 in the page info data structure formerly stood for
> to-space and 0 for to-space. Now exchange the meaning of 0 and 1.
> 
> Ok, I hope. Now say we have an ambiguous reference to an object O in
> page P. We can copy the objects as above, but not O, which we leave
> where it is. At the end of the GC, we move P to to-space by simply
> changing its number in the page info data. Mission accomplished.

I don't understand this part:

  At the end of the GC, we move P to to-space by simply changing its
  number in the page info data.

What is "its number" here?  Since all the live objects that could be
copied were copied from P to to-space, we now have a single object O
in P (which is in from-space) and all the rest in to-space.  Now comes
that magic sentence where I got lost, and somehow "moves P to
to-space".  How does it do that?  And why move P to to-space in the
first place, when all of its objects except O are already copied to
to-space?