A Vision for WebAssembly Support in Swift

visions/webassembly.md

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+# A Vision for WebAssembly Support in Swift
+
+## Introduction
+
+WebAssembly (abbreviated [Wasm](https://webassembly.github.io/spec/core/intro/introduction.html#wasm)) is a virtual
+machine instruction set focused on portability, security, and high performance. It is vendor-neutral, designed and
+developed by [W3C](https://w3.org). An implementation of a WebAssembly virtual machine is usually called a
+*WebAssembly runtime*.
+
+One prominent spec-compliant implementation of a Wasm runtime in Swift is [WasmKit](https://github.com/swiftwasm/WasmKit).
+It is available as a Swift package, supports multiple host platforms, and has a simple API for interaction with guest
+Wasm modules.
+
+An application compiled to a Wasm module can run on any platform that has a Wasm runtime available. Despite its origins
+in the browser, it is a general-purpose technology that has use cases in client-side and server-side applications and
+services. WebAssembly support in Swift makes the language more appealing in those settings, and also brings it to the
+browser where it previously wasn't available at all. It facilitates a broader adoption of Swift in more environments
+and contexts.
+
+The WebAssembly instruction set has useful properties from a security perspective, as it has no interrupts or
+peripheral access instructions. Access to the underlying system is always done by calling explicitly imported
+functions, implementations for which are provided by an imported WebAssembly module or a WebAssembly runtime itself.
+The runtime has full control over interactions of the virtual machine with the outside world.
+
+WebAssembly code and data live in completely separate address spaces, with all executable code in a given module loaded
+and validated by the runtime upfront. Combined with the lack of "jump to address" and a limited set of control flow
+instructions that require explicit labels in the same function body, this makes a certain class of attacks impossible to
+execute in a correctly implemented spec-compliant WebAssembly runtime.
+
+### WebAssembly System Interface and the Component Model
+
+The WebAssembly virtual machine has no in-built support for I/O; instead, a Wasm module's access to I/O is dependent
+entirely upon the runtime that executes it.
+
+A standardized set of APIs implemented by a Wasm runtime for interaction with the host operating system is called
+[WebAssembly System Interface (WASI)](https://wasi.dev). [WASI libc](https://github.com/WebAssembly/wasi-libc) is a
+layer on top of WASI that Swift apps compiled to Wasm can already use thanks to C interop. The current implementation
+of Swift stdlib and runtime for `wasm32-unknown-wasi` triple is based on this C library. It is important for WASI
+support in Swift to be as complete as possible to ensure portability of Swift code in the broader Wasm ecosystem.
+
+In the last few years, the W3C WebAssembly Working Group considered multiple proposals for improving the WebAssembly
+[type system](https://github.com/webassembly/interface-types) and
+[module linking](https://github.com/webassembly/module-linking). These were later subsumed into a combined
+[Component Model](https://component-model.bytecodealliance.org) proposal thanks to the ongoing work on
+[WASI Preview 2](https://github.com/WebAssembly/WASI/blob/main/preview2/README.md), which served as playground for
+the new design.
+
+The Component Model defines these core concepts:
+
+- A *component* is a composable container for one or more WebAssembly modules that have a predefined interface;
+- *WebAssembly Interface Types (WIT) language* allows defining contracts between components;
+- *Canonical ABI* is an ABI for types defined by WIT and used by component interfaces in the Component Model.
+
+Preliminary support for WIT has been implemented in
+[the `wit-tool` subcommand](https://github.com/swiftwasm/WasmKit/blob/0.0.3/Sources/WITTool/WITTool.swift) of the
+WasmKit CLI. Users of this tool can generate `.wit` files from Swift declarations, and vice versa: Swift bindings from
+`.wit` files.
+
+## Use Cases
+
+We can't anticipate every possible application Swift developers are going to create with Wasm, but we can provide a few
+examples of its possible adoption in the Swift toolchain itself. To quote
+[a GSoC 2024 idea](https://www.swift.org/gsoc2024/#building-swift-macros-with-webassembly):
+
+> WebAssembly could provide a way to build Swift macros into binaries that can be distributed and run anywhere,
+> eliminating the need to rebuild them continually.
+
+This can be applicable not only to Swift macros, but also for the evaluation of SwiftPM manifests and plugins.
+
+In the context of Swift developer tools, arbitrary code execution during build time can be virtualized with Wasm.
+While Swift macros, SwiftPM manifests, and plugins are sandboxed on Darwin platforms, with Wasm we can provide stronger
+security guarantees on other platforms that have a compatible Wasm runtime available.
+
+The WebAssembly instruction set is designed with performance in mind. A WebAssembly module can be JIT-compiled or
+compiled on a client machine to an optimized native binary ahead of time. With recently accepted proposals to the Wasm
+specification it now supports features such as SIMD, atomics, multi-threading, and more. A WebAssembly runtime can
+generate a restricted subset of native binary code that implements these features with little performance overhead.
+
+Adoption of Wasm in developer tools does not imply unavoidable performance overhead. With security guarantees that
+virtualization brings, there's no longer a need to spawn a separate process for each Swift compiler and SwiftPM
+plugin/manifest invocation. Virtualized Wasm binaries can run in the host process of a Wasm runtime, removing the
+overhead of new process setup and IPC infrastructure.
+
+## Goals
+
+As of March 2024 all patches necessary for basic Wasm and WASI support have been merged to the Swift toolchain and
+core libraries. Based on this, we propose a high-level roadmap for WebAssembly support and adoption in the Swift
+ecosystem:
+
+1. Make it easier to evaluate and adopt Wasm with increased API coverage for this platform in the Swift core libraries.
+As a virtualized embeddable platform, not all system APIs are always available or easy to port to WASI. For example,
+multi-threading, file system access, and localization need special support in Wasm runtimes and a certain amount of
+consideration from a developer adopting these APIs.
+
+2. Improve support for cross-compilation in Swift and SwiftPM. We can simplify versioning, installation, and overall
+management of Swift SDKs for cross-compilation in general, which is beneficial not only for WebAssembly, but for all
+platforms.
+
+3. Continue work on Wasm Component Model support in Swift as the Component Model proposal is stabilized. Ensure
+that future versions of WASI are available to Swift developers targeting Wasm.
+
+4. Make interoperability with Wasm components as smooth as C and C++ interop already is for Swift. With a formal
+specification for Canonical ABI progressing, this will become more achievable with time.
+
+### Proposed Language Features
+
+In our work on Wasm support in Swift, we experimented with a few function attributes that could be considered
+as pitches and eventually Swift Evolution proposals, if the community is interested in their wider adoption.
+These attributes allow easier interoperation between Swift code and other Wasm modules linked with it by a Wasm
+runtime.
+
+## Platform-specific Considerations
+
+### Debugging
+
+Debugging Wasm modules is challenging because Wasm does not expose ways to introspect and control the execution of
+a Wasm module instance, so a debugger cannot be built on top of Wasm itself. Special support from the Wasm execution
+engine is necessary for debugging.
+
+The current state of debugging tools in the Wasm ecosystem is not as mature as other platforms, but there are two
+main directions:
+
+1. LLDB debugger with Wasm runtime supporting GDB Remote Serial Protocol [1]
+2. Wasm runtime with a built-in debugger [2]
+
+The first approach provides an almost equivalent experience to existing debugging workflows on other platforms. It
+can utilize LLDB's Swift support, remote metadata inspection, and serialized Swift module information. However, since
+Wasm is a Harvard architecture and has no way to allocate executable memory space at runtime, implementing expression
+evaluation with JIT in user space is challenging. In other words, gdb stub in Wasm engines need tricky implementations
+or need to extend the GDB Remote Serial Protocol.
+
+The second approach embeds the debugger within the Wasm engine. In scenarios where the Wasm engine is embedded as a
+guest in another host engine (e.g. within a Web Browser), this approach allows seamless debugging experiences with the
+host language by integrating with the host debugger. For example, in cases where JavaScript and Wasm call frames
+are interleaved, the debugger works well in both contexts without switching tools. Debugging tools like Chrome DevTools
+can use DWARF information embedded in Wasm file to provide debugging support. However, supporting Swift-specific
+metadata information and JIT-based expression evaluation will require integrating LLDB's Swift plugin with these
+debuggers in some way.
+
+Given that both approaches can utilize DWARF information, the Swift toolchain should generate as much DWARF information
+as possible for what's statically known. Also, making Swift type information DWARF-friendly is desirable, since the
+current debug information generated by the Swift compiler assumes that debuggers can understand certain Swift
+primitives. E.g. `Swift.Int` is currently represented as a `DW_TAG_structure_type` node without any member, but ideally
+such types should be represented as `DW_TAG_base_type` nodes with `DW_AT_encoding` attribute or provide a debug summary
+string by LLDB Summary Strings).
+
+[1]: https://github.com/llvm/llvm-project/pull/77949
+[2]: https://github.com/ChromeDevTools/devtools-frontend/blob/main/extensions/cxx_debugging
+
+### Multi-threading and Concurrency
+
+WebAssembly has [atomic operations in the instruction set](https://github.com/WebAssembly/threads) (only sequential
+consistency is supported), but it does not have a built-in way to create threads. Instead, it relies on the host
+environment to provide multi-threading support. This means that multi-threading in Wasm is dependent on the Wasm runtime
+that executes a module. There are two proposals to standardize ways to create threads in Wasm:
+
+(1) [wasi-threads](https://github.com/WebAssembly/wasi-threads), which is already supported by some toolchains,
+runtimes, and libraries but has been superseded;
+
+(2) The new [shared-everything-threads](https://github.com/WebAssembly/shared-everything-threads) proposal is still
+in the early stages, but is expected to be the future of multi-threading in Wasm.
+
+Swift currently supports two threading models in Wasm: single-threaded (`wasm32-unknown-wasi`) and multi-threaded
+using wasi-threads (`wasm32-unknown-wasip1-threads`). Despite the latter supporting multi-threading, Swift Concurrency
+defaults to a cooperative single-threaded executor due to the lack of wasi-threads support in libdispatch. Preparing
+for the shared-everything-threads proposal is crucial to ensure that Swift Concurrency can adapt to future
+multi-threading standards in Wasm.
+
+### 64-bit address space
+
+WebAssembly currently uses a 32-bit address space, but [64-bit address space](https://github.com/WebAssembly/memory64/)
+proposal is already in the implementation phase.
+
+Swift supports 64-bit pointers on other platforms where available, however WebAssembly is the first platform where
+relative reference from data to code is not allowed. Alternative solutions like image-base relative addressing or
+"small code model" for fitting 64-bit pointer in 32-bit are unavailable, at least for now. This means that we need
+cooperation from the WebAssembly toolchain side or different memory layout in Swift metadata to support 64-bit linear
+memory support in WebAssembly.
+
+### Shared libraries
+
+There are two approaches to using shared libraries in the WebAssembly ecosystem:
+
+1. [Emscripten-style dynamic linking](https://emscripten.org/docs/compiling/Dynamic-Linking.html)
+2. [Component Model-based "ahead-of-time" linking](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Linking.md)
+
+Emscripten-style dynamic linking is a traditional way to use shared libraries in WebAssembly, where the host
+environment provides non-standard dynamic loading capabilities.
+
+The latter approach cannot fully replace the former, as it is unable to handle dynamic loading of shared libraries at
+runtime, but it is more portable way to distribute programs linked with shared libraries, as it does not require the
+host environment to provide any special capabilities except for Component Model support.
+
+Support for shared libraries in Swift for Wasm is mostly about making sure that Swift programs can be compiled in
+position-independent code mode and linked with shared libraries by following the corresponding dynamic linking ABI.
+