bug#33014: 26.1.50; 27.0.50; Fatal error after re-evaluating a thread's function

[Eli Zaretskii]

> From: Gemini Lasswell <gazally@runbox.com>
> Cc: 33014@debbugs.gnu.org
> Date: Fri, 19 Oct 2018 13:05:19 -0700
> 
> > If this is the case, then I think we should protect the definition of
> > a running function from GC, in some way, either by making sure it is
> > referenced by some stack-based Lisp object, even in heavily optimized
> > code (e.g., by using 'volatile' qualifiers); or by some other method
> > that will ensure that definition is marked and not swept.
> 
> Maybe code optimizers have improved over the last 17 years?

I think a much more significant factor is that modern processors have
many more registers to use.

> I have patched Emacs with a 'volatile' on the definition of 'fun' in
> Ffuncall, and so far haven't managed to reproduce the bug with it:

Thanks.  This needs a comment for why we do something strange like
that, but otherwise, if no one has better ideas in a week's time,
let's install this.