bug#41357: 28.0.50; GC may miss to mark calle safe register content

[Paul Eggert]

On 5/17/20 10:04 AM, Eli Zaretskii wrote:
> I cannot easily
> correlate your description of using registers with what happens when
> we call Lisp or bytecode.

His description is generic: it applies regardless of whether the garbage
collector is called from C code (in his branch, generated from Lisp code) or
from the interpreter (either in his branch or in the emacs-27 branch) as it is
executing Lisp code or bytecode.

It's a low-level problem in which the garbage collector is not seeing some
objects that it should see, because at the machine level the object addresses
are in registers that the garbage collector hasn't saved and thus won't see when
it scans memory.

A serious and insidious bug in our existing system, in other words.