bug#44674: 28.0.50; Adding current-cpu-time for performance tests

[Eli Zaretskii]

> Date: Mon, 16 Nov 2020 19:28:24 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: mattiase@acm.org, p.stephani2@gmail.com, 44674@debbugs.gnu.org
> 
> > What do you mean by "origin" and by "calling 'system'"?
> 
> Quoting https://pubs.opengroup.org/onlinepubs/9699919799/:
> 
>           The clock() function shall return the implementation's best
>           approximation to the processor time used by the process
>           since the beginning of an implementation-defined era
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>         related only to the process invocation.
> 
> How do you write portable Lisp code that returns consistent results
> based on such shaky foundations?

Sorry, forgot to answer the 'system' part.  The issue there is whether
calling 'system' (and in general waiting for sub-processes to exit) is
counted against the process's use of CPU or not.  AFAIK, this is not
well defined, either.