bug#63225: Compiling regexp patterns (and REGEXP_CACHE_SIZE in search.c)

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 63225@debbugs.gnu.org
> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 2 May 2023 16:33:58 +0200
> 
> Indeed it sounds like you are suffering from regexp cache thrashing. I'm attaching two patches: one to measure the cache miss rate, and one that allows the regexp cache size to be changed at run time.

Thanks, but the new primitives need to be documented (why do the doc
strings say "internal use only"?), and also we should include the
cache size in the output of memory-report.

> +static void
> +set_regexp_cache_size (int n)
> +{
> +  for (int i = 0; i < regexp_cache_size; i++)
> +    xfree (searchbufs[i].buf.buffer);
> +
> +  size_t bytes = n * sizeof *searchbufs;
> +  searchbufs = xrealloc (searchbufs, bytes);
> +  memset (searchbufs, 0, bytes);
> +  regexp_cache_size = n;
> +

Should this first check that the new size is identical to the old one,
and if so, do nothing?  Come to think of this, do we really need to
realloc if the new size is smaller?  And why zero out the cache when
changing the size?

> +DEFUN ("regexp-cache-size", Fregexp_cache_size, Sregexp_cache_size,
> +       0, 0, 0,
> +       doc: /* Current regexp cache size.  Internal use only.  */)
> +  (void)
> +{
> +  return make_int (regexp_cache_size);
> +}

Since the size of the cache is a fixnum, why not use make_fixnum here?
it's a tad faster.