golang_str: Speedup utf-8 decoding a bit on py2
We recently moved our custom UTF-8 encoding/decoding routines to Cython. Now we can start taking speedup advantage on C level to make our own UTF-8 decoder a bit less horribly slow on py2:
name old time/op new time/op delta
stddecode 752ns ± 0% 743ns ± 0% -1.19% (p=0.000 n=9+10)
udecode 216µs ± 0% 75µs ± 0% -65.19% (p=0.000 n=9+10)
stdencode 328ns ± 2% 327ns ± 1% ~ (p=0.252 n=10+9)
bencode 34.1µs ± 1% 32.1µs ± 1% -5.92% (p=0.000 n=10+10)So it is ~ 3x speedup for u(), but still significantly slower compared to std unicode.decode('utf-8').
Only low-hanging fruit here to make _utf_decode_rune a bit more prompt, since it sits in the most inner loop. In the future _utf8_decode_surrogateescape might be reworked as well to avoid constructing resulting unicode via py-level list of py-unicode character objects. And similarly for _utf8_encode_surrogateescape.
On py3 the performance of std and u/b decode/encode is approximately the same.
/cc @jerome