base64 encoding with simd-support

[gfoidl]

I've prototyped base64 encoding with SSE2 / SSSE3 support here.
The input, that can't be processed via simd, is handled with the current default encoded.

Numbers look quite promising:

Method	N	Mean	Error	StdDev	Scaled	ScaledSD
Default	16	39.04 ns	0.7766 ns	0.6884 ns	1.00	0.00
Simd_Sse2	16	30.03 ns	0.4058 ns	0.3168 ns	0.77	0.02
Default	1000	1,072.48 ns	8.5875 ns	8.0327 ns	1.00	0.00
Simd_Sse2	1000	497.02 ns	4.4778 ns	4.1886 ns	0.46	0.01

The code is based and inspired on https://github.com/aklomp/base64 wich is licensed under BSD 2-Clause "Simplified" License. So can this code be used here in corefx under the terms of the license?

Should I pursue this approach and finally submit a PR?

An AVX2 variant was also prototyped, but some intrinsics like PermuteVar8x32 are WIP, so AVX2 wasn't pursued further, but it can be on the landscape too.

In my prototype right now there is only encoding. But for decoding Klomp has code too, so it could be incorporated, so that both encoding and decoding could be done with simd-support.

This is somewhat tracked in https://github.com/dotnet/coreclr/issues/15506#issuecomment-351498311

/cc: @ahsonkhan

[danmoseley]

@tannergooding

[tannergooding]

Implementation looks good. It might be nice to wait on taking this (if we do) until after we finish the API cleanup for the HWIntrinsics (some design decisions were made at the last API review, that we need to respond to).

[gfoidl]

@tannergooding thanks. So let's wait for the cleanup, then we can come back to productize this.

[gfoidl]

FYI in the WIP-project https://github.com/gfoidl/Base64 SSSE3 / AVX2 is used for encoding / decoding, and speedups for encoding up to 12 are possible (for datalength=1000). For decoding ~6x speedup is possbile (has to check for invalid inputs).

Although this project now exists, I still would like to take this here*, so that corefx can benefit from these improvements in base64 encoding / decoding.

The project also supports char-buffers as sink / source, and base64Url encoding, so I see no "conflict" with an implementation here in corefx.

* still aware of https://github.com/dotnet/corefx/issues/32365#issuecomment-423257658 as this comment should just be an informational note, not to produce any pressure / hurry... (especially for myself 😉)