[Arm64] MultiplyHigh

[echesakov]

Exposing smulh/umulh as intrinsics on Arm64

class ArmBase.Arm64
{
  /// <summary>
  ///   A64: SMULH Xd, Xn, Xm
  /// </summary>
  public static long MultiplyHigh(long left, long right);

  /// <summary>
  ///   A64: UMULH Xd, Xn, Xm
  /// </summary>
  public static ulong MultiplyHigh(ulong left, ulong right);
}

would allow to implement System.Math.BigMul as

  low = a * b;
  return ArmBase.Arm64.MultiplyHigh(a, b);

cc @CarolEidt @tannergooding @TamarChristinaArm

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[terrajobst]

Video

Looks good as proposed:

namespace System.Runtime.Intrinsics.Arm
{
    public abstract class ArmBase
    {
        public abstract class Arm64
        {
            public static long MultiplyHigh(long left, long right);
            public static ulong MultiplyHigh(ulong left, ulong right);
        }
    }
}