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0051-PackedFunc-as-Object.md

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  • Feature Name: PackedFunc as Object
  • Start Date: 2022-01-01
  • RFC PR: #51
  • GitHub Issue: apache/tvm#10032

1. Summary

This RFC allows developers to use PackedFunc as TVM objects, which completes the last missing step of TVM runtime object system; and stabilizes the PackedFunc into a layout-stable TVM object, which makes PackedFunc shareable across C++ DLL boundary.

2. Motivation

Historically, several fundamental data structures in TVM are not part of the runtime object system, namely NDArray (not object), Module (not object), String (not exist), Array (not in runtime), Map (not in runtime), PackedFunc (not yet an object).

The rationale of the original design is mainly for simplicity, which is desirable for the usecases as a monolithic compiler. As time goes on, the community has come to realize the fact that the object system should be inclusive enough and by design allow more convenient integration with vendor libraries. Therefore, as part of the effort in TVM refactoring and TVM Unity, recent work strives to re-implement these core data structures to be consistent with the runtime object protocol with stable ABI guarantee, and thus could be passed across the DLL boundary.

As the central piece of the TVM ecosystem, this proposal focuses on making PackedFunc a TVM object. By doing so, it completes the last missing piece of the object ecosystem, allows TVM containers to carry PackedFuncs.

In addition, the original design uses a std::function to store callable objects, which is not able to be passed across the DLL boundary. However, this proposal deprecates the original design, and introduces a layout-stable one, which enables PackedFuncs to be passed across the DLL boundary to bring convenience to the vendor library integration.

3. Guide-level introduction

This is mainly a developer-facing feature, and thus there is no sensible change to the existing functionalities to the end users, who are still supposed to use the same PackedFunc API.

Only one major object is introduced, PackedFuncObj, a TVM object in the runtime system (detailed in the next section) which is an ABI stable data structure for packed functions that could be shared across language and DLL boundary.

To avoid API misuse from developers, the PackedFuncObj cannot be created or manipulated directly, and the specialization of its creation make_object<PackedFuncObj> will be deleted for safety. Instead, the developer-facing class PackedFunc remains responsible for creating and managing the object, and for properly setting its content.

In the future, it’s possible to incrementally add more information into PackedFuncObj to better help debugging and error reporting.

Note: This RFC doesn’t change any of the existing functionality, including C ABI or PackedFunc’s C++ API. Any modification to the C ABI is out of scope of this RFC. And this RFC does not create new ABIs, just refactors existing ones.

4. Reference-level introduction

As introduced below, the RFC introduces a new class:

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

A templated subclass is introduces to do the type-erasing trick:

template <typename TCallable>
class PackedFuncSubObj : public PackedFuncObj {
  TCallable callable_;
};

The PackedFuncObj inherits an intrusive reference counter and an object deleter from the runtime::Object. Besides, with the inheritance trick on PackedFuncSubObj, the field callable_ is introduced to store the content of the callable object, which can be a function pointer, a struct/class, an anonymous lambda function or any other object.

To make the change minimal, PackedFuncObj is not designed to be serializable, and doesn’t support TVM’s native reflection. Copying the type-erased object is strictly prohibited for now for simplicity, and instead copying the PackedFunc is implemented as a straightforward increment to the reference counter by 1.

5. Drawbacks

Just like every change to the runtime, the proposed change could slightly affect runtime’s binary size. The effect, depending on the compiler, could be positive or negative.

Overall, given that it brings significantly better experience as stated in the previous sections, we believe the benefits outweighs the potential drawback.

6. Rationale and alternatives

This refactoring is the last missing piece of effort that brings core data structures of the TVM runtime into the ABI-stable TVM runtime.

Alternatively, one might argue that it’s not important whether PackedFunc should be a TVM object or not; however, it significantly brings negative impact when TVM object system is used across the DLL boundary, or putting PackedFunc into TVM containers.

7. Prior Art

NDArray and Module are brought into the object system according to RFC Issue #4286.

Containers, including String, Array and Map, are discussed in the forum thread and brought into the object system. The String part is introduced by PR #4628, Array in PR #5585, and Map in PR #5740.

DGL, one of the most popular frameworks for distributed graph neural network training, adopts TVM’s object and FFI system.

8. Unresolved questions

This RFC only introduces C++ ABI for invoking a PackedFunc, which might have some limitation when linking artifacts compiled by different compilers. In the future, more effort should be invested into the design of a stable C ABI when two PackedFuncs come from different TVM runtime.

9. Future possibilities

Based on similar metaprogramming tricks, it’s possible to extract the function signatures of TypedPackedFunc and to make error reporting more readable.