[vm/ffi] Allow struct classes to override sizeOf

[sjindel-google]

Since all operations on structs are dispatched to the getters/settings (instance methods which can already be overriden) and the auto-generated #sizeOf, we can allow users to define custom struct classes by overriding sizeOf.

This would allow, for example, defining structs with different behavior (e.g. different fields) on different platforms.

[dcharkes]

This would allow, for example, defining structs with different behavior (e.g. different fields) on different platforms.

This would not allow people to access those fields right? Only to allocate the correct amount of memory and use elementAt. Or is there a way we can enable accessing fields which have different offsets or might not exist on various platforms?

[sjindel-google]

You could write the fields by hand, or delegate them to different (generated) implementations depending on the platform.

[mkustermann]

By allowing user-defined sizeOf, we also allow user-defined layout (i.e. number of fields, type of fields, offset/alignment of fields).

In order to generate the right machine code for Dart -> C and C -> Dart trampolines the VM has to know the exact struct layout for passing structs by value for the current ABI.

How would such struct classes work with by-value calls?

[sjindel-google]

I had imagined that there might be some other functions in addition to sizeOf which describe the struct layout. But it's a very explicit and heavy-weight solution.

[mkustermann]

IMHO we should remove this from the ffi MVP project and consider closing if we don't intend to implement this.