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

[Pip Cet]

On Sun, May 17, 2020 at 4:47 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
> On 5/17/20 9:40 AM, Andrea Corallo wrote:
> > I think this is a real bug that we have in the codebase (emacs-27
> > included).
> Thanks for all the detective work! Your analysis is correct and your patch 
> looks
> good.

That's my impression as well.

> The only question in my mind is whether to install the patch into the emacs-27
> branch or the master branch. Given Eli's problems with stability in emacs-27
> (see Bug#41321), I'm inclined to think the former, as the bug could explain 
> the
> problems Eli is observing.

I don't think that platform even has callee-saved registers? But I
think the fix should go on the emacs-27 branch. It's a bad bug and
sheer luck that Fgarbage_collect on my platform (using this specific
compiler, etc.) pushes all callee-saved registers. We shouldn't rely
on such lucks on all platforms.