Question : What is the most performant way to represent constant System.Runtime.Intrinsics.VectorXXX<T>?

[RamType0]

When we want some constant vector,what is the most performant way?
(e.g. static Vector256<int> Stair => Vector256.Create(1,2,3,4,5,6,7,8);,
static readonly Vector256<int> Stair = Vector256.Create(1,2,3,4,5,6,7,8);)
I hope it documented on MSDN.

[Dotnet-GitSync-Bot]

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[gfoidl]

Beginning with .NET 5 just use VectorXYZ.Create with constant values. E.g.

private void MethodThatUsesVectors()
{
    Vector128<int> constVec = Vector128.Create(1, 2, 3, 4);
}

Then the JIT will emit a "constant" which is then loaded from memory.
#35857 introduced that.

Before .NET 5 there are two patterns:

private static readonly Vector128<int> s_constVec1 = Vector128.Create(1, 2, 3, 4);

Or if the constant vector is of a type with size of one byte (so byte, sbyte, ...):

private void MethodThatUsesVectors()
{
    ReadOnlySpan<byte> constantData = new byte[] {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
    Vector128<byte> constantVec = Unsafe.ReadUnaligned<Vector128<byte>>(ref MemoryMarshal.GetReference(constantData));
}

This version uses C#'s ability to read ROS from static data segment of the assembly. See https://vcsjones.dev/2019/02/01/csharp-readonly-span-bytes-static/ for further infos.

Always measure your goals, and don't just blindly rely on advices, as there may be some corner-cases, etc.

I hope it documented on MSDN.

Good idea to have this infos consolidated at one place.
Side note: it's now called Microsoft Docs.

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