System.Numerics.Vectors reference assembly has reordered fields compared to runtime implementation

Description

We have a tool that generates some code based on a C# reference file. This file uses System.Numerics.Vectors for its math types. When we parse the Vector4 and Quaternion structs, Roslyn returns the components in WXYZ order. We thought this looked strange and maybe a bug on our side at first, because on C++ WindowsNumerics.h uses XYZW, but indeed it seems it’s not us and there’s a discrepancy here (I’ll take Quaternion as example):

• The core implementation uses XYZW: https://github.com/dotnet/runtime/blob/e64bc548c609455652fcd4107f1f4a2ac3084ff3/src/libraries/System.Private.CoreLib/src/System/Numerics/Quaternion.cs#L11-L25

• The reference library uses WXYZ:

  

  I assume that this is why Roslyn returns the fields in this order when calling ITypeSymbol.GetMembers().OfType<IFieldSymbol>().

• Incidentally, the official Microsoft docs probably follow the reference library and list WXYZ: https://docs.microsoft.com/en-us/dotnet/api/system.numerics.quaternion?view=net-5.0

The actual layout is important for us to decide if we generate some code for a blittable type or not (we assume Vector4 and Quaternion are, but previously we couldn’t consider them blittable because we used WXYZ internally whereas they are projected in C++ in WindowsNumerics.h as XYZW). As @tannergooding pointed on another project comment we assume that the layout of those types is Sequential (should be the default for structs) and will not change since they’re intrinsics.

Configuration

.NET 5.0 (but was already in .NET Core 3.x) Windows 10 x64 (though it shouldn’t matter)

Regression?

Probably not.

Other information

Copying some internal analysis from @tannergooding for reference (and tagging @ericstj as requested):

These are autogenerated by GenAPI https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.GenAPI/Program.cs which uses CCI. At least browsing through the code that writes the fields (https://github.com/dotnet/arcade/blob/4873d157a8f34f8cc7e28b3f9938b32c642ef542/src/Microsoft.Cci.Extensions/Writers/CSharp/CSDeclarationWriter.cs#L182-L187) and which traverses the fields (https://github.com/dotnet/arcade/blob/4873d157a8f34f8cc7e28b3f9938b32c642ef542/src/Microsoft.Cci.Extensions/Writers/CSharp/CSharpWriter.cs#L155) it looks like we are reordering the members here: https://github.com/dotnet/arcade/blob/4873d157a8f34f8cc7e28b3f9938b32c642ef542/src/Microsoft.Cci.Extensions/Writers/CSharp/CSharpWriter.cs#L188 and that is likely causing the difference.

[ericstj]

@tannergooding no issue fixing manually for .NET 7. I suspect the GenAPI change for this isn’t too difficult so it won’t be much in the way of dead code. @ViktorHofer are you tracking this for a rule in the new API compat as well? cc @smasher164

[tannergooding]

LayoutKind.Explicit probably also needs to be considered, but I don’t think we have any such types today.