API Proposal: Avx.Zeroupper

[benaadams]

The Jit normally issues vzeroupper commands at entry and exit of methods, however when when mixing Avx and Sse code in the same method it is helpful to manually issue vzeroupper at the transitions:

Proposed api

public partial abstract class Avx : Sse42
{
    /// <summary>
    /// void _mm256_zeroupper (void)
    ///   VZEROUPPER
    /// </summary>
    public static void ZeroUpper();
    
    /// <summary>
    /// void _mm256_zeroall (void)
    ///   VZEROALL
    /// </summary>
    public static void ZeroAll();
}

Also added vzeroall for discussion.

Use case: dotnet/coreclr#22187 (comment)

/cc @CarolEidt @fiigii @tannergooding @jkotas

[benaadams]

from: @mikedn dotnet/coreclr#22187 (comment)

Does it need a VZEROUPPER here?

No, that's for mixing non-VEX SSE code with AVX code. That's not possible in .NET, except at method boundaries.