# Symmetric ABI Christof Petig, ~~July~~ October 2024 Updates: highlighted by 💡 --- ## Motivation - Combine C++ and Rust with `Result`, `Option`, `String` and `Vec`, also `Stream` and `Future` - More details in presentation at https://bytecodealliance.zulipchat.com/#narrow/stream/438936-SIG-Embedded/topic/Inaugural.20Meeting/near/445576299 --- ## Side constraints - Avoid dynamic memory allocations for Functional Safety compliance - Minimize copying (overhead) --- # Native compilation - Use `wit-bindgen` for language bindings - Modify binding to generate mangled symbols e.g. `autosarX3AaraX2FcoreX00initialize` for `autosar:ara/core` . `initialize` - Normal component when compiled to wasm32 --- ## Drawbacks - Needs host side mesh to connect components - Duplicate Symbols complicate linking - `wasm32` vs `x86_64` pointer size --- ## Can we omit the need for a mesh? - 🤔 - ABI would need to become symmetric - Imported functions args view, return owned - Exported functions args owned, return view + `cabi_post_` - 💡 Second plan - Use import ABI also for exports - Extend handles to pointer size --- ### Resources - Use the imported ABI also for exported resources - `resource-drop` replaces `dtor` - `resource-new`, `resource-rep` hidden - ~~Always pass `id`, never `rep`~~ - 💡 make `rep` and `id` identical --- ### Drawbacks - API / ABI mismatch mandates unnecessary allocations in the generated code - Solution: ~~Caller provided buffers?~~ 💡 New, [simplified](https://github.com/cpetig/wit-bindgen/blob/main/crates/cpp/DESIGN.md), C++ API - 💡~~Result ownership is too complicated~~ ~~- Vec+String view only, `own<resource>` is passed~~ --- ### Example code https://github.com/cpetig/wit-bindgen/tree/main/crates/cpp/tests/meshless_strings https://github.com/cpetig/wit-bindgen/tree/main/crates/cpp/tests/meshless_resources --- ### 💡 Unit tests https://github.com/cpetig/wit-bindgen/tree/main/crates/cpp/tests/symmetric_tests modified versions of wit-bindgen runtime tests --- ## Can also provide - Stable Rust to Rust binary ABI for native plugin architectures - Direct linking of components into a single module or executable --- ## Future plans - 💡~~Caller provided buffers~~ ~~[Lazy value lowering #383](https://github.com/WebAssembly/component-model/issues/383)~~ ~~towards zero allocations per call~~ - ~~Still working on `heapless::String/Vec` API~~ - [Flat data types](https://github.com/WebAssembly/component-model/issues/398) - Integrate with Joel's async prototype - zero allocations become tough --- ### 💡 Flat data types - Another ABI variant 🤦 or canonical option - Works well with zero copy, shared memory, data broadcasting - Idea from [iceoryx2](https://github.com/eclipse-iceoryx/iceoryx2) - Enables also network transmission and disk storage --- ### 💡 Embedded specific - Migration path from native to wasm - Mixing native and wasm possible - No distinction between host and guest --- ## 💡 Outlook - New [BuildTargets.md](https://github.com/WebAssembly/component-model/pull/378/files) (`cm32p2|` prefix) - Works well for AUTOSAR demo - Streams and futures very important