At the time of writing, StableHLO is ready to supersede MHLO/HLO as compiler interface. It can be produced by TensorFlow, JAX and PyTorch, it can be consumed by XLA and IREE, and it has all the public features provided by MHLO/HLO as well as additional functionality.
This document describes the next steps for the StableHLO project, categorizing the ongoing work reflected in the issue tracker and arranging this work into planned deliverables.
Milestones
In 2023, we are planning two big milestones: 1) StableHLO v0.9 which will provide an initial version of the opset and initial compatibility guarantees, 2) StableHLO v1.0 which will implement high-priority improvements and start providing full compatibility guarantees.
StableHLO v0.9 will mirror MHLO/HLO, augmented with a specification for statically-shaped ops and initial compatibility guarantees. Per the compatibility RFC, this release will provide 1 month of forward and backward compatibility. These modest guarantees will enable gaining experience with dialect evolution and allow some time for cleanup before full guarantees go into effect. We are planning to release StableHLO v0.9 in Q1 2023.
StableHLO v1.0 will implement high-priority improvements, including cleaning up the frontend contract (with the goal that StableHLO programs only include ops from the StableHLO dialect, rather than today's mixture of dialects and unregistered attributes) and providing a reference implementation. We are planning to release StableHLO v1.0 in H2 2023.
Workstreams
In order to organize the development towards the aforementioned milestones, we have categorized the tickets in the issue tracker into multiple workstreams and tied these workstreams to the milestones. A limited number of tickets (less than 10%) are not assigned to any specific workstream and aren't part of any specific milestone.
(P0) The Compatibility Implementation workstream is dedicated to implementing the compatibility RFC along with a compatibility test suite. Most of this work is expected to be completed in StableHLO v0.9, and the rest will be done in StableHLO v1.0.
(P0) The Frontend Contract workstream consists of implementing 100% of the features which are used by StableHLO frontends but are not yet in the StableHLO specification. The goal of this workstream is to ensure that StableHLO programs only include ops from the StableHLO dialect, rather than today's mixture of dialects and unregistered attributes. We are planning to complete all or almost all work in this workstream in StableHLO v1.0.
(P0) The Reference Implementation workstream organizes the work on implementing an interpreter for 100% of StableHLO ops as defined in the StableHLO specification. We are planning to complete all or almost all work in this workstream in StableHLO v1.0.
(P0) The Documentation workstream is dedicated to providing all the information that StableHLO producers or consumers might need. The StableHLO specification is a major deliverable, as well as a reference for the StableHLO API and StableHLO serialization format. Critical pieces of the workstream will be delivered in StableHLO v1.0, with lower-priority items addressed on a rolling basis.
(P1) The Conformance Suite workstream consists of delivering a test suite that compares a reference implementation with implementations provided by StableHLO backends. Tests for the reference implementation will provide a conformance suite of sorts, so this workstream does not have P0 priority. However, further augmenting this suite with additional interesting test cases will likely be a useful area for future work.
(P1) The Specification Compliance workstream ensures that 100% of StableHLO ops are implemented in the StableHLO dialect as defined in the StableHLO specification. The StableHLO dialect is already reasonably compliant, so this workstream does not have P0 priority, but a lot of minor items still remain (especially in the corner cases of verifier implementation) and will be addressed on a rolling basis.
(P1) The New Features workstream concludes the StableHLO roadmap and consists of a ragtag collection of new functionality for the StableHLO opset (not the StableHLO dialect or the StableHLO bindings - that would be other workstreams). A few of these new features are planned for delivery in StableHLO v1.0, but the majority are currently lower-priority items which are not part of any specific milestone.
(P1) The Public API workstream is dedicated to delivering C/C++/Python bindings for the StableHLO dialect. Existing C++/Python bindings are already fairly reasonable, so this workstream does not have P0 priority. However, quite a bit of work still remains to be done, especially around providing stability for these bindings - which is something that is not currently covered by the compatibility RFC but will likely be a useful area for future work.