RFC: Refactoring SystemABI

[Maurizio Cimadamore]

On 23/11/2018 01:47, Maurizio Cimadamore wrote:
> What is it that bothers you about this approach? 

Let me clarify - I like that you are thinking of ways to reduce the 
surface of the API - but it seems that moving a method (getReceiver) 
from UpcallHandle to SystemABI doesn't buy all that much.

I personally prefer having a simple UpcallHandle lookup method in the 
ABI, and then a way to get form the Handle to the receiver as it seems 
more composable this way. Right now, true, we always need to do both 
things at the same time (to speed up things in the upcall machinery), 
but maybe having a 'are you a stub' predicate will turn out handy even 
on its own (in case the user wants to write some pointer-walking code).

But, if we really wanted to save some API estate, I think a better 
approach (if viable) would be to completely hide the stub detection 
logic. As you said very clearly yourself, we have this issue:

"There is a shortcut being taken when native code gives us a Pointer 
back to one of our upcall stubs, and we want to call it from Java. In 
that case we don't really need to call into native, to call our own 
upcall stub to then call back into Java to call our target method."

This is the reason behind the 'isStub/getReceiver' logic. But let's 
analyze this use case more in detail: we have a pointer to function, and 
we want to do a downcall using that pointer as the entry point. But this 
means that, in order to do the downcall, we will call 
SystemABI.downcallHandle (again). So, I think it is theoretically 
possible that the ABI would say: "hey, I know this guy!" and instead of 
giving you a MethodHandle that 'goes down' it gives you a MethodHandle 
that stays in Java-land. And the binder would not even know.

If my conjecture is correct, then we just need your minimal UpcallHandle 
interface (pointer + free) and no getReceiver/getFunctorObject at all. 
And, if you pull on this some more, you realize that UpcallHandle is not 
even needed: just use a Pointer, and add a 
'freeUpcallHandle(Pointer<?>)' to the SystemABI. That'd be sweet.

Thoughts?

Maurizio