PseudoCustomAttribute.GetStructLayoutCustomAttribute returns incorrect data

[tannergooding]

PseudoCustomAttribute.GetStructLayoutCustomAttribute currently returns incorrect values for various types. See the implementation here: https://github.com/dotnet/coreclr/blob/72d49127a0c25e4b931c81e621c2411bfb6633a5/src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs#L2058-L2096

Namely, for the packing it currently changes 0 to 8; but it doesn't take into account any changes that the VM may have made for a given types data.

I would expect that this API either return the exact values in metadata or it should return the correct values computed by the VM at runtime.

An example of where this is problematic is for types like float it reports 8, when the actual packing is 4. Likewise, for Vector128<>, the packing of 16 is preserved by the VM. Vector<T> is another case where it currently reports Pack = 8, but Size = 0.

[steveharter]

Propose moving to future; not in scope for 3.0 unless someone picks this up.

[steveharter]

Moving to future based on schedule + priority.

[buyaa-n]

I would expect that this API either return the exact values in metadata or it should return the correct values computed by the VM at runtime.

Isn't that will be a breaking change? Also I wonder why it is written this way frist place, tagging @jkotas as he might know more about this code

[jkotas]

Also I wonder why it is written this way first place

The comment explains it: The developer writing this code thought that changing the packing from 0 to 8 is better for some reason. FWIW: This code was checked in on 2003/1/14.

Isn't that will be a breaking change?

Every bug fix is technically a breaking change.

I think it would be fine to delete the adjustment and just return the raw value in metadata. If you are going to fix this, make sure to fix it in all places:

src\coreclr\nativeaot\System.Private.Reflection.Core\src\System\Reflection\Runtime\TypeInfos\RuntimeNamedTypeInfo.cs(163):// Metadata parameter checking should not have allowed 0 for packing size.
src\coreclr\System.Private.CoreLib\src\System\Reflection\RuntimeCustomAttributeData.cs(1876):// Metadata parameter checking should not have allowed 0 for packing size.
src\libraries\System.Reflection.MetadataLoadContext\src\System\Reflection\TypeLoading\Types\RoDefinitionType.cs(173):// Metadata parameter checking should not have allowed 0 for packing size.
src\mono\System.Private.CoreLib\src\System\RuntimeType.Mono.cs(2348):// Metadata parameter checking should not have allowed 0 for packing size.