Vectorize Convert.ToBase64String

[EgorBo]

Use Utf8's version which is already vectorized for both x86 and arm and covert it to UTF16. Up to 3.5x faster for large inputs.

Benchmarks:

public IEnumerable<byte[]> TestData()
{
    int[] lengths = { 65, 100, 1000, 4096 };
    foreach (var length in lengths)
        yield return Enumerable.Range(0, length).Select(i => (byte)i).ToArray();
}

[Benchmark]
[ArgumentsSource(nameof(TestData))]
public string ToBase64String(byte[] data) => Convert.ToBase64String(data);

|         Method |        Job |                   Toolchain |       data |        Mean |
|--------------- |----------- |---------------------------- |----------- |------------:|
| ToBase64String | Job-OMUPEN |      \Core_Root\corerun.exe |   Byte[65] |    65.19 ns |
| ToBase64String | Job-BBSFOG | \Core_Root_base\corerun.exe |   Byte[65] |    82.50 ns |

| ToBase64String | Job-OMUPEN |      \Core_Root\corerun.exe |  Byte[100] |    66.05 ns |
| ToBase64String | Job-BBSFOG | \Core_Root_base\corerun.exe |  Byte[100] |   121.54 ns |

| ToBase64String | Job-OMUPEN |      \Core_Root\corerun.exe | Byte[1000] |   398.56 ns |
| ToBase64String | Job-BBSFOG | \Core_Root_base\corerun.exe | Byte[1000] | 1,124.47 ns |

| ToBase64String | Job-OMUPEN |      \Core_Root\corerun.exe | Byte[4096] | 1,320.25 ns |
| ToBase64String | Job-BBSFOG | \Core_Root_base\corerun.exe | Byte[4096] | 4,507.10 ns |

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

Thanks!

[stephentoub]

A bunch of related test failures

[stephentoub]

I've done that before :-)

[a74nh]

Is there anything still to do for Arm64 on this? I looked at the patch and couldn't see anything X64 specific in it.

[EgorBo]

Is there anything still to do for Arm64 on this? I looked at the patch and couldn't see anything X64 specific in it.

Initially, I submitted an x64-only implementation but then switched to the utf8 one + extension to utf16, so don't worry 🙂

[a74nh]

Initially, I submitted an x64-only implementation but then switched to the utf8 one + extension to utf16, so don't worry 🙂

Ok, that's fine then.

[EgorBo]

Improvements on Linux-x64 dotnet/perf-autofiling-issues#6731
Improvements on Alpine-x64 dotnet/perf-autofiling-issues#6743

[stephentoub]

@EgorBo, do we need something similar to this change for Convert.ToBase64CharArray?

[EgorBo]

@stephentoub makes sense!

[dakersnar]

x86 improvements dotnet/perf-autofiling-issues#6952

[EgorBo]

Arm64 improvements: dotnet/perf-autofiling-issues#6983, dotnet/perf-autofiling-issues#6977

[DrewScoggins]

ubuntu x64 improvements: dotnet/perf-autofiling-issues#6960