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

[Eli Zaretskii]

> Cc: Andrea Corallo <akrl@sdf.org>, Eli Zaretskii <eliz@gnu.org>,
>  41357@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sun, 17 May 2020 12:01:40 -0700
> 
> On 5/17/20 10:00 AM, Pip Cet wrote:
> > I don't think that platform even has callee-saved registers?
> 
> Eli's platform is 32-bit Microsoft Windows, and W32 has four callee-save
> registers (ebx, esi, edi, ebp) not counting esp and eip which are of course
> callee-save by definition. So the problem could at least in theory be 
> occurring
> on his platform, depending on the compiler and its options.

I've seen the same problem on 64-bit Windows as well, in Emacs
compiled with a different (newer) version of GCC.  I don't think this
has anything to do with how many registers are there.  I also never
before saw these problems, so this is most definitely due to some
recent changes, I just cannot yet figure out which ones.