## Implementing GC in Wasmtime Ivan Mikushin --- ## Wasmtime A fast and secure runtime for WebAssembly. A [Bytecode Alliance](https://bytecodealliance.org/) project. --- ## What is Wasm GC? Wasm GC (Garbage Collection) is a [proposal](https://github.com/WebAssembly/gc) to enable WebAssembly to natively support garbage collected languages. It introduces: - **New types:** New heap types and new ways to define types. - **New instructions:** ~30 new instructions to manipulate reference types. ---- ### Reference Types WAT syntax: `(ref null? <heap-type>)` ---- ### Heap Types Heap types classify reference types. - **Abstract:** `any`, `eq`, `array`, `struct`, `func`, `extern` - **Unboxed scalars:** `i31` - **Bottom:** `none`, `noextern`, `nofunc` - **User-defined (indexed):** Arrays, Structs, Subtypes ---- ### Already existing heap types - `extern` and `func` -- via [reference-types](https://github.com/WebAssembly/reference-types), - `(ref null? $t)` -- via [function-references](https://github.com/WebAssembly/function-references) ---- ### Type Hierarchy <div style="background-color: beige; padding: 10px">  </div> (from Chromium's [Wasm GC proposal: Spec for Prototype Implementation v.6](https://bit.ly/3cWcm6Q)) ---- ### Shorthands - `funcref` == `(ref null func)` - `externref` == `(ref null extern)` - `anyref` == `(ref null any)` - **`nullref`** == `(ref null `**`none`**`)` - **`nullexternref`** == `(ref null `**`noextern`**`)` - **`nullfuncref`** == `(ref null `**`nofunc`**`)` - `eqref` == `(ref null eq)` - `structref` == `(ref null struct)` - `arrayref` == `(ref null array)` - `i31ref` == `(ref null i31)` ---- ### Type Definitions - **Subtype** -- a single type, defined as a subtype of other types. - **Recursive type group** -- a group of mutually recursive types that can refer to each other. ---- ### New Instructions The Wasm GC proposal introduces approximately 30 new instructions, including: - **`i31.*`**: For creating and manipulating "unboxed scalars" (31-bit integer values). - **`array.*`** and **`struct.*`**: For creating and manipulating arrays and structs. - **`ref.*`**: For casting and testing values of reference types. - **`br_on_*`**: For conditional branching depending on values of reference types. - **`extern.*`**: For external reference conversion. --- ## Implementation Plan 1. Wasm GC Support in wasm-tools: - wasmparser / wasmprinter - wasm-encoder - wasm-smith 2. Begin implementation in Wasmtime - Enable GC support flags - Incrementally enable GC spec tests 3. Enable the proposal in the fuzz targets. 4. Expose the proposal’s new functionality in Wasmtime API. [Detailed implementation plan](https://hackmd.io/@imikushin/HyoMh-Lzh) --- ## Current Status (what's already done) - Wasm GC Support in wasm-tools: - **wasmparser, wasmprinter:** Implemented new type support up to user defined structs, and `i31.*` instructions. - **wasm-encoder:** Implemented only necessary things for now: so that it compiles --- ## Wrap up - **Wasm GC spec defines:** - reference types hierarchy - instructions to work with new types - **Current focus:** - enable support in wasm-tools - **Implementation in Wasmtime:** - can start after wasmparser support is complete --- ## Thank you! :cat: -- @imikushin