wip: Bitmap operations SIMD logic refactor

[PaulusParssinen]

wip, todo: pr description

Now we can do BITOPs on when running Garnet inside Android VMs!

screen-20250316-174515.mp4

TODO:

• Add Vector512 path, maybe,
• Add some BDN micro-benchmarks to exercise only the bitmap operation logic
• Add tests exercising >2 key SIMD logic. Envs: DOTNET_EnableHWIntrinsic=0 + DOTNET_MaxVectorTBitWidth=128/256/512

Follow-up work:

• Refactor BITCOUNT logic similarly to be cross-platform (Platform independent bitcount operation #1034) and to follow "modern" .NET SIMD practices

[prvyk]

It would be nice if all three paths were tested. In practice, all of Github's x86 based runners will have Vector256.IsHardwareAcceleated as true, not testing the other paths by default.

(If Garnet had an arm64 runner - which it should anyway - then I strongly suspect it would have Vector128.IsHardwareAccelerated as true.)

[PaulusParssinen]

I agree, it would be nice. I would prefer to have "outerloop" test runs that either manually and/or scheduled with DOTNET_EnableAVX2=0 or DOTNET_EnableHWIntrinsic=0 environment variables for all of the tests.

ISimdVector in future .NET versions is gonna push this testing effort back to .NET runtime team hopefully, as the 128 & 256 & other paths are just Vector### templates and if there was non-typo bug in them, it would be bug in the .NET BCL.

[prvyk]

We can do the same TestProcess solution #1041 ended up with for these 2-3 specific tests (it can be generalized a bit further, but we just need to test the paths once or twice, so adding the same using var p = TestProcess and env.Add() isn't much).

It's also possible to do what the Vector documentation suggests to do with BDN and allow an option to run the entire suit with acceleration off (I suspect these hardware accelerations affect other parts of the program, via library code or JIT).

It would also make sense to have a regular ARM runner, as ARM has other differences like memory model which may be important (the BITCOUNT portable code issue wasn't detected since no runner - despite Garnet offering binaries for these platforms...).