Cool trick, but personally I don’t trust C bitfields. When I need something like that, I usually create C++ class or C# structure with a single private uint64 field, and public methods to extract or manipulate the logical fields.

Because the class/structure only has a single uint64 field, the compilers are likely to pass value in a single general-purpose register. I believe that’s unlikely to happen for a structure with bit fields.

If you target AVX2 or newer you also have BMI1 and BMI2, intrinsics like bextr and bzhi are probably faster than whatever codes compilers are generating for bit fields.

Binary compatibility of bit fields is a moot point, using them at the API surface across compilers or languages is not ideal. A structure with a single uint64 field is very compatible.