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

[Eli Zaretskii <eliz@gnu.org>]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: bug-gnu-emacs@gnu.org, eggert@cs.ucla.edu
> Date: Sun, 17 May 2020 17:45:28 +0000
> 
> Lisp_Object
> foo (void)
> {
>   /* 'res' goes in a callee saved reg  */
>   Lisp_Object res = build_string ("bar");
>   [...]
>   /* LTO inline the following as "flush_stack_call_func (mark_threads_callback, NULL);" */
>   mark_threads ();
>   [...]
>   gc_sweep ();
> 
>   /* The string pointed by 'res' was garbage collected.  */
>   return res;
> }

But mark_threads etc. (GC in general) isn't called from functions like
your 'foo.  It is more like this:

Lisp_Object
foo (void)
{
  /* 'res' goes in a callee saved reg  */
  Lisp_Object res = build_string ("bar");
  [...]
  call_something ();
  [...]

}

call_something (void)
{
  [...]
  garbage_collect ();
  [...]
}

Which is quite different, AFAIU, wrt stack usage.

Or maybe I don't understand how "callee saved registers" work.