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

[Eli Zaretskii]

> Cc: akrl@sdf.org, bug-gnu-emacs@gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sun, 17 May 2020 12:05:25 -0700
> 
> On 5/17/20 10:24 AM, Eli Zaretskii wrote:
> > I struggle to see
> > how a Lisp object could appear in a register without leaving any trace
> > on the stack
> 
> Quite easily. It happens all the time. If I do something like this:
> 
>     Lisp_Object a = Fcons (b, c);
>     f (x, y);
>     return a;

And where's GC in this picture?  If it's called directly from 'f', can
you show me such code in Emacs?  Then we could disassembly it and see
what we've got.

Usually the code that calls GC is much deeper, and thus the chance of
that temporary to stay in a register is very small, to say the least.

> The proposed fix is harmless

Yeah, right.  Sorry, I don't buy this.  Too many gray hair from such
assumptions.