bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: sebastien@chapu.is, 31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 22 Apr 2019 01:17:09 +0300
> 
> On 21.04.2019 15:15, Eli Zaretskii wrote:
> 
> > Then maybe we should look for a faster JSON parsing library, because
> > libjansson functions appear in the profile as high or higher than our
> > decoding routines.
> 
> Not in my profile.

No?  I have these near the top in mine:

  +   7.73%  emacs.jansson  emacs.jansson      [.] stream_get.part.3
  +   3.61%  emacs.jansson  emacs.jansson      [.] strbuffer_append_bytes
  +   3.47%  emacs.jansson  emacs.jansson      [.] lex_scan
  +   1.75%  emacs.jansson  emacs.jansson      [.] lex_get_save
  +   1.58%  emacs.jansson  emacs.jansson      [.] string_get

AFAICT, they are all from libjansson.  In this particular profile,
decode_coding_utf_8 looks like this:

  +   4.45%  emacs.jansson  emacs.jansson      [.] decode_coding_utf_8

> Again, if make_specified_string takes 50% of the runtime, even if 
> libjansson (or some other JSON lib) worked instantly, we'd only get a 2x 
> improvement at most.

I'm not sure it's 50%, as in my profile I have only this:

  +   1.41%  emacs.jansson  emacs.jansson      [.] make_uninit_multibyte_string

Please don't forget that json-parse-string first _encodes_ the input
string in UTF-8, then passes it to libjansson, then _decodes_ each
string element it receives back.  So we do some non-trivial processing
besides make_specified_string, and removing decoding only saves part
of that.