`DOTNET_PreferredVectorBitWidth` is not respected by `Vector<T>`

[neon-sunset]

Description

It appears that Vector<T> matches native vector width regardless of the value specified in environment variable DOTNET_PreferredVectorBitWidth.

Reproduction Steps

Given dotnet run -c Release and the following code:

using System.Numerics;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics;

Print(Vector128.IsHardwareAccelerated);
Print(Vector256.IsHardwareAccelerated);
Print(Vector512.IsHardwareAccelerated);
Print(Vector<byte>.Count * 8);

static void Print(object expr,
    [CallerArgumentExpression(nameof(expr))] string text = "")
{
    Console.WriteLine($"{text}: {expr}");
}

Expected behavior

none

Vector128.IsHardwareAccelerated: True
Vector256.IsHardwareAccelerated: True
Vector512.IsHardwareAccelerated: False
Vector<byte>.Count * 8: 256

$env:DOTNET_PreferredVectorBitWidth=128

Vector128.IsHardwareAccelerated: True
Vector256.IsHardwareAccelerated: False
Vector512.IsHardwareAccelerated: False
Vector<byte>.Count * 8: 128

Actual behavior

none

Vector128.IsHardwareAccelerated: True
Vector256.IsHardwareAccelerated: True
Vector512.IsHardwareAccelerated: False
Vector<byte>.Count * 8: 256

$env:DOTNET_PreferredVectorBitWidth=128

Vector128.IsHardwareAccelerated: True
Vector256.IsHardwareAccelerated: False
Vector512.IsHardwareAccelerated: False
Vector<byte>.Count * 8: 256 <-- unexpected

Regression?

Doesn't seem so

Known Workarounds

Disable platform-specific ISA with e.g. DOTNET_EnableAVX=0.

Configuration

.NET SDK:
 Version:           9.0.100-preview.7.24365.16
 Commit:            365af5faed
 Workload version:  9.0.100-manifests.d91b6c9f
 MSBuild version:   17.12.0-preview-24365-02+977d5ccf6

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.22631
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.100-preview.7.24365.16\

Other information

No response

[EgorBo]

For Vector<T> you need to play with DOTNET_MaxVectorTBitWidth env var. Although, I agree that UX is not great, DOTNET_PreferredVectorBitWidth=128 should probably also changer Vector<T> cc @tannergooding

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