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Also update the notes on the relative pointers in WebAssembly to reflect the current implementation.
62 lines
2.8 KiB
Markdown
62 lines
2.8 KiB
Markdown
# WebAssembly support in Swift
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WebAssembly is a platform that significantly differs from hardware platforms that Swift already supports.
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While it's a virtual machine, there are considerations to be taken into account when targeting it:
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* WebAssembly is still at an early stage, so many features you'd be expect from other platforms are not
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available yet, specifically:
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1. `wasm64` variant is not specified yet, only the 32-bit `wasm32` variant is supported in WebAssembly
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hosts such as browsers.
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2. While a preview of multi-threading and atomics is available in some browsers and stand-alone
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WebAssembly hosts, [the corresponding proposal](https://github.com/WebAssembly/threads/) haven't
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formally reached the implementation phase yet.
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The multi-threading feature is available in `wasm32-unknown-wasip1-threads` target, but it's not
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in `wasm32-unknown-wasi` target.
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3. Dynamic linking is not formally specified and tooling for it is not available yet.
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* Binary size is a high priority requirement. Since WebAssembly payloads are usually served in browsers,
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one wouldn't want end users to download multi-megabyte binaries.
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## Building Swift SDK for WebAssembly
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The [Swift SDK](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0387-cross-compilation-destinations.md)
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for WebAssembly is built using the following command:
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```bash
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./utils/build-script --build-wasm-stdlib
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```
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This command will build the Swift compiler for the host platform and then build the Swift standard library
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for WebAssembly targets. The resulting Swift SDK `.artifactbundle` will be placed in the `../swift-sdk-generator/Bundles`
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directory.
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## Building Swift SDK for WebAssembly without building the compiler
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Building the Swift compiler is a time-consuming process. If you only want to build the Swift standard library
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with pre-built Swift compiler, you can use the following command:
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```console
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$ SWIFT_TOOLS_PATH=path/to/swift-development-snapshot/usr/bin
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$ ./utils/build-script \
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--skip-build-llvm \
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--skip-build-swift \
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--skip-build-cmark \
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--build-wasm-stdlib \
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--native-swift-tools-path="$SWIFT_TOOLS_PATH" \
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--native-clang-tools-path="$SWIFT_TOOLS_PATH" \
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--native-llvm-tools-path="$SWIFT_TOOLS_PATH"
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```
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## Notes on the implementation
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Here we're describing some decisions that were made while developing
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the implementation.
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### Relative Pointers
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Relative pointers are used in Swift runtime, but currently it's not feasible to use them for some cases
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where the pointee is a function pointer. The reason is that WebAssembly has a separate address space for
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functions and data, and the offset bwtween a function and a data pointer cannot be defined. Therefore,
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we have to use absolute pointers for function pointers in WebAssembly (see `include/swift/ABI/CompactFunctionPointer.h`
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for more details).
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