[avx2] Optimize byte packing during decoding

[jethrogb]

During decoding, decode_avx2 returns an 32-byte value along with a 32-bit mask indicating which bytes are valid (i.e. not decoded from whitespace). Currently, these are packed using a simple loop over the bytes. There are likely more efficient ways to do this. (On AVX-512, you'd use the VPCOMPRESSB instruction, but that's not available here)

[TheIronBorn]

You can emulate VPCOMPRESS with PSHUFB and a lookup table of shuffle control masks indexed by the bitmask.

Here's a library for that purpose https://github.com/lemire/simdprune/. It includes methods for various memory needs (a 16-bit mask means a 1 MiB table if unoptimized)

[jethrogb]

That looks like an good direction to explore. Note that PSHUFB works on 16 bytes so you'd still need to re-pack the output of 2 PSHUFB calls.