Porting Swift to WebAssembly

Event: WebAssembly Live! Extended https://www.webassembly.live/

WebAssembly is now a compilation target of Swift. SwiftWasm project compiles Swift into Wasm using LLVM. It was an interesting challenge because Swift uses some architecture-dependent techniques. This talk covers the story of bringing Swift to WebAssembly: Porting the Swift compiler, interacting with JavaScript, interoperation of async API using JavaScript event loop.

Transcript

1. Porting Swift to WebAssembly
   WebAssembly Live! Extended
   @kateinoigakukun
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2. Who am I
   Yuta Saito (a.k.a @kateinoigakukun)
   • Co-maintainer of SwiftWasm
   • Working at Merpay, Inc.
   • Love Swift
   !
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3. Motivation: Why Swift?
   • Safe
   • Flexible
   • Not only for iOS application
   • Multi-paradigm
   • Compiled Language
   • Seamless C-interoperability
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4. Motivation: Why porting to WebAssembly?
   • Portable
   • Access to Web developer community
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5. The all story started with an issue
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6. The story of SwiftWasm
   • Apr, 2019: Initial efforts made by @patcheng, @maxdesiatov
   and @zhuowei
   • Setting up basic build system for cross-compiling for Wasm
   • Build stdlib using WASI and wasi-libc for POSIX APIs.
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7. The story of SwiftWasm
   • Sep, 2019: Started hunting Wasm specific bugs around compiler
   • Relative Pointer
   • Swift Calling Convention
   • etc...
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8. Relative Pointer
   • Offset is 32-bit, saving lots of space on 64-bit platforms.
   • Calculated by PC-relative relocation designed for jumping.
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9. Issue with Relative Pointer
   No PC-relative relocation like R_X86_64_PC32 at that time
   WASM_RELOC(R_WASM_TABLE_NUMBER_LEB, 20)
   WASM_RELOC(R_WASM_MEMORY_ADDR_TLS_SLEB, 21)
   WASM_RELOC(R_WASM_FUNCTION_OFFSET_I64, 22)
   +WASM_RELOC(R_WASM_MEMORY_ADDR_LOCREL_I32, 23) // S + A - P
   https://reviews.llvm.org/D96659 9
   

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10. Swift Calling Convention
    declare swiftcc void @foo(i32, i32)
    @data = global i8* bitcast (void (i32, i32)* @foo to i8*)
    define swiftcc void @bar() {
    %1 = load i8*, i8** @data
    %2 = bitcast i8* %1 to void (i32, i32, i32, i32)*
    ; Crash on WebAssembly!
    ; >> call_indirect to a signature that does not match
    call swiftcc void %2(i32 1, i32 2, i32 swiftself 0, swifterror 0)
    %3 = bitcast i8* %1 to void (i32, i32, i32)*
    ; Crash on WebAssembly!
    ; >> call_indirect to a signature that does not match
    call swiftcc void %3(i32 1, i32 2, i32 swiftself 0)
    ret void
    }
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11. Swift Calling Convention
    ; lowered as:
    ; declare swiftcc void @foo(i32, i32, swiftself i32, swifterror i32);
    declare swiftcc void @foo(i32, i32)
    @data = global i8* bitcast (void (i32, i32)* @foo to i8*)
    define void @main() {
    %1 = load i8*, i8** @data
    %2 = bitcast i8* %1 to void (i32, i32, i32, i32)*
    call swiftcc void %2(i32 1, i32 2, i32 swiftself 0, swifterror 0)
    %3 = bitcast i8* %1 to void (i32, i32, i32)*
    ; lowered as:
    ; call swiftcc void %2(i32 1, i32 2, i32 swiftself 0, i32 swifterror 0)
    call swiftcc void %3(i32 1, i32 2, i32 swiftself 0)
    ret void
    }
    https://reviews.llvm.org/D76049 11
    

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12. The story of SwiftWasm
    • Jan, 2020: Develop WebAssembly debugger
    https://github.com/kateinoigakukun/wasminspect 12
    

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13. The story of SwiftWasm
    • Mar, 2020: Passing all standard library test suites
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14. The story of SwiftWasm
    • Mar, 2020: Continuous
    Integration and Delivery
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15. The story of SwiftWasm
    • Apr, 2020: Published JavaScriptKit: JavaScript interoperation
    • Jul, 2020: Published Tokamak: SwiftUI compatible UI framework
    for Wasm
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16. Tokamak: SwiftUI compatible UI framework for Wasm
    import TokamakDOM
    struct Counter: View {
    @State var count: Int = 0
    var body: some View {
    VStack {
    Text("\(count)")
    HStack {
    Button("Reset") { count = 0 }
    Button("Increment") { count += 1 }
    }
    }
    }
    }
    Try it out!: https://pad.swiftwasm.org/ 16
    

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17. The story of SwiftWasm
    • Nov, 2020: Swift cross-module DCE to reduce binary size
    • Jan, 2021: Experimental concurrency support with JS event loop
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18. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").bject!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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19. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").object!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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20. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").object!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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21. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").object!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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22. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").object!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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23. import JavaScriptKit
    let fetch = JSObject.global.fetch.function!.async
    func printZen() async throws {
    let result = try await fetch("https://api.github.com/zen").object!
    let text = try await result.asyncing.text!()
    print(text)
    }
    JavaScriptEventLoop.runAsync {
    try! await printZen()
    }
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24. Future Works
    • WASI-less "bare metal" target
    • Performance and Binary size improvement
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25. Contributions are welcome!
    • Website: https://swiftwasm.org
    • GitHub: https://github.com/swiftwasm
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26. Thanks!
    • All SwiftWasm contributors and sponsors
    • The Swift community
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