[PATCH v2 2/5] benchtests: Add memset zero fill benchtest

[naohirot@fujitsu.com]

Hi Wilco, Noah,

> > There may be miscomminuation.
> > The * 16 is already in the outer loop (1).
> 
> The outer loop is in test_main, and it determines 'n' in do_one_test:
> 
>   for (i = ...)
>     {
>       do_test (&json_ctx, 0, c, i);
>     }
> 
> > Let me copy the code from the mail [1] I put in the previouse mail [2].
> 
> The key issue is that this loop:
> 
>       for (j = 0; j < 16; j++)
>         CALL (impl, s + n * j, c2, n);
> 
> is equivalent to:
> 
> CALL (impl, s, c2, n * 16);
> 
> The loop we really want is something like bench-memset-large:
> 
>   CALL (impl, s, c, n);
>   TIMING_NOW (start);
>   for (i = 0; i < iters; ++i)
>     {
>       CALL (impl, s, c, n);
>     }
>   TIMING_NOW (stop);
> 
> This repeats CALL on data of size 'n' after an initial warmup of the caches.
> 
> > It doesn't matter what kind of memset is called, but matters the
> > function name in the code so that we can understand it is not mesured.
> 
> Then using the standard name 'memset' would be best.
> 

OK, I understood, thanks.

Taking Noah's comment [1] into account, the final code should be like
the below. Can we agree with this code?

Two results, two loop version in the mail [1] and one loop version
below, are almost same in case of __memset_generic on a64fx as
shown in the graph [2].

-----
static void
__attribute__((noinline, noclone))
do_one_test (json_ctx_t *json_ctx, impl_t *impl, CHAR *s,
             int c1 __attribute ((unused)), int c2 __attribute ((unused)),
             size_t n)
{
  size_t i, iters = 32;
  timing_t start, stop, cur, latency = 0;

  CALL (impl, s, c2, n); // warm up

  for (i = 0; i < iters; i++)
    {
      memset (s, c1, n); // alternation

      TIMING_NOW (start);

      CALL (impl, s, c2, n);

      TIMING_NOW (stop);
      TIMING_DIFF (cur, start, stop);
      TIMING_ACCUM (latency, cur);
    }

  json_element_double (json_ctx, (double) latency / (double) iters);
}
-----

[1] https://sourceware.org/pipermail/libc-alpha/2021-July/129486.html
[2] https://drive.google.com/file/d/1bptHqg5vvFAGoYgoR3w_pvclXFSP8Sr0/view?usp=sharing

Thanks.
Naohiro