Re: speed up text_position() for utf-8

I wrote:

> 0001 puts the main implementation of pg_utf_mblen() into an inline
> function and uses this in pg_mblen(). This is somewhat faster in the
> strpos tests, so that gives some measure of the speedup expected for
> other callers. Text search seems to call this a lot, so this might
> have noticeable benefit.
>
> 0002 refactors text_position_get_match_pos() to use
> pg_mbstrlen_with_len(). This itself is significantly faster when
> combined with 0001, likely because the latter can inline the call to
> pg_mblen(). The intention is to speed up more than just text_position.
>
> 0003 explicitly specializes for the inline version of pg_utf_mblen()
> into pg_mbstrlen_with_len(), but turns out to be almost as slow as
> master for ascii. It doesn't help if I undo the previous change in
> pg_mblen(), and I haven't investigated why yet.
>
> 0002 looks good now, but the experience with 0003 makes me hesitant to
> propose this seriously until I can figure out what's going on there.
>
> The test is as earlier, a worst-case substring search, times in milliseconds.
>
> patch | no match | ascii | multibyte
> --------+----------+-------+-----------
> PG11 | 1220 | 1220 | 1150
> master | 385 | 2420 | 1980
> 0001 | 390 | 2180 | 1670
> 0002 | 389 | 1330 | 1100
> 0003 | 391 | 2100 | 1360

I tried this test on a newer CPU, and 0003 had no regression. Both
systems used gcc 11.2. Rather than try to figure out why an experiment
had unexpected behavior, I plan to test 0001 and 0002 on a couple
different compilers/architectures and call it a day. It's also worth
noting that 0002 by itself seemed to be decently faster on the newer
machine, but not as fast as 0001 and 0002 together.

Looking at the assembly, pg_mblen is inlined into
pg_mbstrlen_[with_len] and pg_mbcliplen, so the specialization for
utf-8 in 0001 would be inlined in the other 3 as well. That's only a
few bytes, so I think it's fine.

--
John Naylor
EDB: http://www.enterprisedb.com