bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: sebastien@chapu.is, yyoncho@gmail.com, 31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 22 Apr 2019 15:58:57 +0300
> 
> On 22.04.2019 15:07, Eli Zaretskii wrote:
> > I don't know why.  In my profiles I enlarged the iteration count to
> > 100, and also enlarged the sampling frequency.  Maybe that's the
> > reason.
> 
> I posted my invocations here, with all arguments. I didn't see yours.

They are the same.  The difference is that you visit the file inside
the benchmark loop, whereas I left only the conversion inside the
loop.  AFAIU, doing so produces a better measurement of the JSON
parsing part alone.

> > The percentage taken by decode_coding_utf_8 didn't change as
> > result, FWIW.
> 
> Maybe it's not the only bottleneck. What about produce_chars? And 
> malloc, of course. Here's the top of my profiling report again:

produce_chars is a subroutine of any decoding routine, including
decode_coding_utf_8, so I added that to the score.  In my profile,
produce_chars took 3.77%.