Re: Performance improvements for src/port/snprintf.c

I benchmarked this, using your testbed and comparing to libc sprintf 
(Ubuntu GLIBC 2.27-0ubuntu3) and another implementation I know [1], all 
compiled with gcc 5.4.0 with -O2. I used bigger decimals in one of the 
formats, but otherwise they are the same as yours. Here is the table of 
conversion time relative to libc:

format                                 pg      stb
("%2$.*3$f %1$d\n", 42, 123.456, 2)    1.03    -
("%.*g", 15, 123.456)                  1.08    0.31
("%10d", 15)                           0.63    0.52
("%s", "012345678900123456789001234    2.06    6.20
("%d 012345678900123456789001234567    2.03    1.81
("%1$d 0123456789001234567890012345    1.34    -
("%d %d", 845879348, 994502893)        1.97    0.59

Surprisingly, our implementation is twice faster than libc on "%10d". 
Stb is faster than we are with floats, but it uses its own algorithm for 
that. It is also faster with decimals, probably because it uses a 
two-digit lookup table, not one-digit like we do. Unfortunately it 
doesn't support dollars.

1. https://github.com/nothings/stb/blob/master/stb_sprintf.h

-- 
Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company