[RFC][Runtime] Bring `PackedFunc` into TVM Object System

[areusch]

hi @cyx thanks for this proposal! I have a couple questions for you on it.

First, could you elaborate a bit more on the use case you have in mind?

@cyx

allows TVM containers to carry PackedFuncs,

it seems like this might be one of them. Given we can already pass PackedFunc as arguments to other PackedFunc and return them, it seems like one of the big changes here is making the TVM data structures more expressive. I can certainly think of some good use cases for storing function pointers in a map, but I wonder about passing such a data structure at an API boundary.

@cyx

ABI stable data structure

Could you say what you mean/imply by “ABI stable” here? Are you separately compiling a library which needs to invoke PackedFunc? Is there a reason you prefer not to include the PackedFunc library inside that one? (just playing devils’ advocate here–iiuc it seems like another solution is to extract include/tvm/runtime/packed_func.h into a separate library to make it easy to implement PackedFunc stubs to a separate library)

@cyx

Instead, the developer-facing class PackedFunc remains responsible for creating and managing the object, and for properly setting its content.

Could you elaborate? Would there be operator PackedFuncObj?

@cyx

class PackedFuncObj : public runtime::Object {
  using FCallPacked = void(TVMArgs, TVMRetValue*);
  FCallPacked* f_call_packed_;
};

Sort of the inverse of my last question–suppose I have e.g. a Map whose values are PackedFuncObj. How can I invoke the PackedFunc or reconstruct? The function pointer seems to be private in this definition.