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

[Eli Zaretskii <eliz@gnu.org>]

> From: Gemini Lasswell <gazally@runbox.com>
> Cc: 33014@debbugs.gnu.org
> Date: Sun, 14 Oct 2018 12:29:42 -0700
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > How do we know there's a reference to that vector on thread 7's stack?
> > Could it be that there is no reference at all?
> 
> Yes it could be that the reference is getting optimized out.  I asked
> gdb for more detail on the stack frames for exec_byte_code and
> funcall_lambda, and the arguments referring to the byte-code object and
> its components do appear to be optimized out, see below.  I also tried
> adding 'volatile' to the declaration of the local variable 'fun' in
> Ffuncall, and that made the bug go away.

"Optimized out" is GDB's way of saying it's confused by the complex
way a variable's location changes as the program counter advances.  It
doesn't mean the variable is lost, just that GDB lost its track.

> Is there anything else I should be looking at before concluding that
> this is the problem?  And if it is, what is the best way to fix it?

There's the question Andreas asked, we should look into that, I think.