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

[Philipp Stephani <p.stephani2@gmail.com>]

Am Mo., 16. Nov. 2020 um 02:08 Uhr schrieb Stefan Monnier
<monnier@iro.umontreal.ca>:

> +#ifdef CLOCKS_PER_SEC
> +DEFUN ("current-cpu-time", Fcurrent_cpu_time, Scurrent_cpu_time, 0, 0, 0,
> +       doc: /* Return the current CPU time along with its resolution.
> +The return value is a pair (CPU-TICKS . TICKS-PER-SEC).
> +The CPU-TICKS counter can wrap around, so values cannot be meaningfully
> +compared if too much time has passed between them.  */)
> +  (void)
> +{
> +  return Fcons (make_int (clock ()), make_int (CLOCKS_PER_SEC));
> +}
> +#endif

Looks useful!

It might be beneficial to use the higher-resolution clock_gettime with
CLOCK_PROCESS_CPUTIME_ID if available and only fall back to clock()
otherwise.

While there, consider also implementing a function to read the
CLOCK_MONOTONIC clock – that should be preferred over the realtime
clock for stopwatch-style measurements.