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

[Andrea Corallo]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andrea Corallo <akrl@sdf.org>
>> Cc: bug-gnu-emacs@gnu.org, eggert@cs.ucla.edu
>> Date: Sun, 17 May 2020 16:40:09 +0000
>> 
>> I think this is a real bug that we have in the codebase (emacs-27
>> included).
>
> Maybe it's so, but your explanation makes sense only in the context of
> calling a machine-language function.  When we call Lisp or bytecode,
> the machine-level operation is very different, and I cannot easily
> correlate your description of using registers with what happens when
> we call Lisp or bytecode.  Sorry for my misunderstanding.

That is correct, but I don't think we need bytecode to come into play
here to have the problem.

If a C function caller of 'flush_stack_call_func' allocates a
Lisp_Object in a temp variable and the compiler decide to keep this in a
callee saved reg while 'flush_stack_call_func' is called this will be
garbage collected unexpectedly.

Am I wrong?

  Andrea

-- 
akrl@sdf.org