Performance Issue with Union

I have a Union inside a Struct (with other primitive fields). The struct I implements Structure.ByReference and initialize it with a Pointer.

Inside the Union there is a few Structure with different size.

It works correctly when I do a read() on the outer Structure after setting the Union to the correct type. However the performance is 2-3x slower than when I process the Union separately.

By separately I mean I need to break the Union into normal Structure and dispatch them based on input information and init with a Pointer and an offset. The faster method however is not good since the offset in 32-bit machine and 64-bit machine is different.

So I guess the problem is the offset calculation internally of JNA. I am not sure if this is a known problem or I was doing it the wrong way. I tested with JNA 4.0.0 from the download link.

Reproduce the report with JNA 4.0.0 using an outer Structure.ByReference containing a Union of differently sized Structures, then compare read() performance with separate Structures initialized from a Pointer and offset. Test on both 32-bit and 64-bit systems; done means identifying whether internal offset calculation causes the slowdown or documenting that the behavior is expected.