Optimize System.Collections.BitArray using arm64 intrinsics

[BruceForstall]

This item tracks the conversion of the System.Collections.BitArray class to use arm64 intrinsics.

Related: #33308

[Gnbrkm41]

I think I can put up a PR for this, but we seem to be lacking a couple instructions that would be nice to have:

1. Unloading values of single SIMD register where the vector contains a single element (e.g. Vector64 returned from AddAcross) to non-vector registers - vduph_laneq_s16 etc?
2. Unloading values of single SIMD register to a memory location ("Store") - vst1q_s16 etc?

[echesakov]

@Gnbrkm41

Unloading values of single SIMD register where the vector contains a single element (e.g. Vector64 returned from AddAcross) to non-vector registers - vduph_laneq_s16 etc?

I believe this should be supported via ToScalar() method. cc @tannergooding to confirm

Unloading values of single SIMD register to a memory location ("Store") - vst1q_s16 etc?

I am working on these #33461 #33535

[Gnbrkm41]

Ah, did not notice ToScalar extension method 🙂. I did manage to work around using a different approach which probably is better.

It still would be nice to have those exposed as intrinsics for feature parity, though.

[tannergooding]

It still would be nice to have those exposed as intrinsics for feature parity, though.

For some cases, there isn't a corresponding intrinsic on the native side. Where there is, we also expose it there (for example int Sse2.ConvertToInt32(Vector128<int> value) on the x86 side, which corresponds to MOVD reg/m32, xmm).

[echesakov]

It still would be nice to have those exposed as intrinsics for feature parity, though.

The similar functionality will be exposed as a part of #24588 - I am working on this at the moment

[echesakov]

@Gnbrkm41 FYI, I just merged Store intrinsics implementation (#33535), so you should be unblocked now.

Let me know if you need some help with running you pr on an arm64 device/collecting jit disasm for this issue.