Structures with fixed buffers sometimes misclassified in SystemV eightbyte classification

[jkoritzinsky]

The struct below is misclassified by the SystemV classifier:

[StructLayout(LayoutKind.Sequential)]
public unsafe struct FixedBufferClassificationTest
{
    public fixed int arr[3];
    public float f;
}

The two eightbytes should both be classified as INTEGER eightbytes; however, the runtime sees the type above as equivalent to the following:

[StructLayout(LayoutKind.Sequential)]
public unsafe struct FixedBufferClassificationTest
{
    [StructLayout(LayoutKind.Sequential, Size=12)]
    private struct FixedBufferType
    {
         private int i;
    }
    [FixedBufferAttribute(typeof(int),3)]
    public FixedBufferType arr;
    public float f;
}

Since the classifier has no special knowledge of the concept of fixed buffers, the classifier marks the first four bytes as INTEGER, the next 8 as NO_CLASS, and the final four as SSE. This results in the second eightbyte being classified as SSE instead of INTEGER.

[janvorli]

@jkoritzinsky just to make sure I understand it correctly - does it means that for fixed buffers, we don't classify each entry of the fixed array but just the first one?

[jkoritzinsky]

Yep. We only classify the first field since there’s only one field in metadata.