Mmm... I might be able to provide some insight... For the C language it is not possible to reorder fields generally at compile time. This is because if a user defines a struct and the struct is passed to a library that was pre-compiled, then the library and your application hold a different representation for the struct.

You can at link-time be able to change some structure definitions if you know the structure definition is not escaping your application (i.e. it is not passed to a library). There are possibly other things that might hinder the reordering of the fields, for example casting or placing a structure in a union. If you are casting a structure to another, you might be expecting that the underlying memory to be represented in a specific way in order for the cast to make sense. The same thing happens when you place a struct in a union.