Re: speed up text_position() for utf-8

Attached is a short patch series to develop some ideas of inlining
pg_utf_mblen().

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

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