[System.Runtime.Intrinsics] API suggestion: Introduce an intrinsic all bits set field to Vector intrinsic types

[john-h-k]

Currently we have Vector{nnn}<T>.Zero type which the JIT recognises and transforms into a xorps or similar. It would be useful to have a .AllBitsSet property, which is the inverse, having all bits set. The JIT could ideally generate this to be
cmpps <dest>, xmm0, xmm0, 0x0 which is recognised as dependency breaking on modern intel architectures for minimal cost. An all-ones value is useful, and can be frequently used for masking and mixing elements alongside the all-zero value.

Rational and Usage

It is commonly used, for example it can be used as Xor(left, AllBitsSet) to implement a Not using SIMD, as well as frequently for masking and mixing elements, e.g with Shuffle/And/Or

Proposed API

public struct Vector64<T> where T : struct
{
    public static Vector64<T> AllBitsSet { [Intrinsic] get; }
}

public struct Vector128<T> where T : struct
{
    public static Vector128<T> AllBitsSet { [Intrinsic] get; }
}

public struct Vector256<T> where T : struct
{
    public static Vector256<T> AllBitsSet { [Intrinsic] get; }
}

Details

It should be implemented as an intrinsic which results in

[v]cmpps xmmN, xmmN, xmmN

which is recognised as dependency breaking on Haswell+ archs. Software fallback is relatively simple, effectively Vector128.Create(-1).As<int, T>()

Open Questions

Is the name ok?

Pull Request

None yet

Updates

[GrabYourPitchforks]

@tannergooding Weren't you looking into this a while back? Am I misremembering?

[tannergooding]

Yes, and I agree this would be something useful to expose. However, One is not an appropriate name since that might imply the wrong thing.

Something that conveys AllBitsSet would be better, but I'm not sure I have a good suggestion.

[GrabYourPitchforks]

API review - AllBitsSet seems like a decent name given the scenario.

Suggestion: also rename the internal member Vector<T>.AllOnes at the same time so that if it does become public in the future we have consistent naming already.

[Gnbrkm41]

Note: it's PCMPEQ[D/Q/W/B] that are dependency breaking idioms.

[tannergooding]

This is being handled in #33924, I've co-assigned it to @Gnbrkm41 for the time being.