blake2-rfc is slightly faster than the portable implementation

[oconnor663]

https://github.com/cesarb/blake2-rfc

I measure it to be about 2% faster than portable.rs. Not yet sure why, though it might be using some SIMD under the covers, or maybe getting optimized to SSE2 by the compiler.

However, the relationship is reversed if I set RUSTFLAGS="-C target-cpu=native -C target-feature=-avx2". No idea why. Again, still a small difference. Notably, both implementations tank their performance if I allow them to use AVX2.

[oconnor663]

I thought it might be because blake2-rfc was getting autovectorized, but looking at the output of cargo asm that doesn't seem to be the case. So I'm still not sure where the difference comes from.

[oconnor663]

When I try it on ARM I get the opposite result. Should look at 32-bit ARM at some point.

[LuoZijun]

@oconnor663 i got same performance (vs blake2-rfc)

# code copy from https://github.com/shadowsocks/crypto2/tree/dev/src/hash/blake2b
git clone https://github.com/LuoZijun/test_blake2b/
cargo bench

[Ujang360]

@oconnor663 I tried with ARM Neoverse N1, blake2-rfc is slightly faster.

The code:
https://github.com/gemtek-indonesia/blake2b256-bench/blob/249cac1bf8788c224f45990d607c4b510a92c862/src/main.rs#L103-L134

And compiled it with:

RUSTFLAGS="-C target-cpu=native -C codegen-units=1" cargo build --release