<charconv>: Investigate using KholdStare/qnd-integer-parsing-experiments SIMD

[StephanTLavavej]

See "Faster Integer Parsing" and KholdStare/qnd-integer-parsing-experiments.

KholdStare has graciously licensed this code under the Boost Software License. 😸

[sylveon]

uhhh from_chars takes an explicit length?

[KholdStare]

Hi,

I've made a new commit to the repo with some preliminary SIMD verification: KholdStare/qnd-integer-parsing-experiments@205fa91

Unfortunately the instructions that would be perfect don't exist so I had to cobble some things together. No we can parse from 1 to 16 chars with SIMD, with input validation. The two "wishlist" instructions would be "count trailing zeros" for __m128i type, and a left shift with a non-immediate value. I had to emulate both, and unfortunately there is one branch now. The running time is ~2.5 ns for any number of digits up to 16, up from 0.75ns for exactly 16 digits.

I'll continue thinking about other ways to do the same thing, with some other SSE instructions - I'm sure there's a way to get that lower.

[Kristine1975]

re. validation: this function from Daniel Lemire's fast_float library (Apache 2.0) validates 8 characters at once. It's only for base 10, though:

https://github.com/lemire/fast_float/blob/7eae925b51fd0f570ccd5c880c12e3e27a23b86f/include/fast_float/ascii_number.h#L27