Enable Vector256<T> on AVX CPUs

[fiigii]

Enable Vector256<T> on AVX CPUs (Sandy Bridge and Ivy Bridge).

Fix #15519. OSX CI looks like running on an old machine sometimes.

[fiigii]

@jkotas @CarolEidt @BruceForstall PTAL

[fiigii]

I have tested with COMPlus_EnableAVX2=0, it works fine now.

[CarolEidt]

This could be confusing, so I would suggest adding a comment to the effect that in the case of AVX without AVX2, Vector<T> will be limited to 16 bytes, even though the full 32 byte AVX register length is supported for the HW intrinsics.

[CarolEidt]

LGTM, but I think a comment would help clarify.

[fiigii]

@CarolEidt Thank you for the suggestion. I added comments for getSIMDVectorRegisterByteLength and maxSIMDStructBytes to clarify their usage.

[CarolEidt]

Excellent - thanks!

[BruceForstall]

@fiigii @CarolEidt Is there an issue to track how we test all the new SIMD modes?

E.g., we currently have the following SIMD-specific modes in Jenkins:

['COMPlus_EnableAVX' : '0', 'COMPlus_EnableSSE3_4' : '0'],
['COMPlus_FeatureSIMD' : '0'],

(from netci.groovy)

What more do we need?

Are the CI machines even capable of testing this stuff?

[fiigii]

Is there an issue to track how we test all the new SIMD modes?

Not yet.
After #15514 get merged, HW intrinsic tests require COMPlus_EnableIncompleteISAClass=1.

[fiigii]

I think we need the three CI jobs (at least) with Debug and Checked build.

['COMPlus_EnableIncompleteISAClass' : '1'], 
['COMPlus_EnableIncompleteISAClass' : '1', 'COMPlus_EnableAVX' : '0'], // non-Vex-encoding
['COMPlus_EnableIncompleteISAClass' : '1', 'COMPlus_EnableAVX2' : '0'], // for Sandy Bridge and Ivy Bridge
['COMPlus_EnableIncompleteISAClass' : '1', 'COMPlus_FeatureSIMD' : '0'], // No SIMD

[CarolEidt]

What more do we need?

I think that we will need to have the ability to turn off all the various COMPlus_EnableXXX flags. At least for the time being, they won't have any impact except on the tests that explicitly test for them, so they should be selectively applied, however that might be made to work.

Are the CI machines even capable of testing this stuff?

I believe that most of our CI machines now have AVX2. However, I don't think any of the tests require it, so if we happened to run the tests on, say, a non-AVX2 machine, the tests run with COMPlus_EnableAVX2=0 would simply be duplicative of those with it on.

[BruceForstall]

Unrelated: @fiigii These tests are failing in the CI:

JIT_HardwareIntrinsics._IsSupported_ro_IsSupported_ro_._IsSupported_ro_IsSupported_ro_cmd
JIT_HardwareIntrinsics._IsSupported_r_IsSupported_r_._IsSupported_r_IsSupported_r_cmd

in the x64 Checked Windows job with COMPlus_FeatureSIMD=0. Is this due to one of your changes? If so, can you please fix ASAP?

See:
https://ci.dot.net/job/dotnet_coreclr/job/master/job/jitstress/job/x64_checked_windows_nt_jitnosimd/39

[fiigii]

@BruceForstall the current "IsSupported" tests make no sense with those environment variables, so I deleted them in #15514.