Compile your Swift code into WebAssembly

SwiftcKaigi #1 https://iosdiscord.connpass.com/event/142582/

Transcript

1. Compile your Swift code into WebAssembly SwiftcKaigi #1 @kateinoigakukun 1
2. Introduction @kateinoigakukun • Intern at Merpay Expert Team • Love Swift ! 2
3. What is WebAssembly? WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/ C++/Rust, enabling deployment on the web for client and server applications1. 1 https://webassembly.org/ 3
4. https://hacks.mozilla.org/2019/03/standardizing-wasi-a-webassembly-system-interface/ 4
5. https://hacks.mozilla.org/2019/03/standardizing-wasi-a-webassembly-system-interface/ 5
6. Swift on browser? 6
7. SwiftWasm Project https://swiftwasm.org • Supporting WebAssembly for Swift • Fork and apply patches to • apple/swift • apple/llvm-project • etc.. 7
8. Build Pipeline • LLVM supports WebAssembly target 8
9. It's easy to compile Swift into WebAssembly 9
10. It's easy very hard to compile Swift into WebAssembly 10
11. What's hard? 1. Swift heavily depends on libc 2. Swift runtime needs some linker supports 3. Swift uses special calling convention 11
12. Swift heavily depends on libc 12
13. Swift heavily depends on libc 13
14. print public func print( _ items: Any..., separator: String = " ", terminator: String = "\n" ) { var output = _Stdout() _print(items, separator: separator, terminator: terminator, to: &output) } https://github.com/apple/swift/blob/master/stdlib/public/core/Print.swift#L52-L66 14
15. print internal struct _Stdout: TextOutputStream { ... internal mutating func write(_ string: String) { if string.isEmpty { return } var string = string _ = string.withUTF8 { utf8 in _swift_stdlib_fwrite_stdout(utf8.baseAddress!, 1, utf8.count) } } } https://github.com/apple/swift/blob/master/stdlib/public/stubs/LibcShims.cpp#L50-L55 15
16. _swift_stdlib_fwrite_stdout #include <stdio.h> SWIFT_RUNTIME_STDLIB_INTERNAL __swift_size_t swift::_swift_stdlib_fwrite_stdout(const void *ptr, __swift_size_t size, __swift_size_t nitems) { return fwrite(ptr, size, nitems, stdout); } https://github.com/apple/swift/blob/master/stdlib/public/stubs/LibcShims.cpp#L50-L55 16
17. WebAssembly System Interface (WASI) • A system interface for conceptual OS on WebAssembly • Focus on portability and security https://github.com/WebAssembly/WASI 17
18. wasi-libc CraneStation/wasi-libc • A libc implementation using WASI and musl • Can be built as static library 18
19. Runtime dependency on WebAssembly 19
20. What's hard? 1. Swift heavily depends on libc 2. Swift runtime needs some linker supports 3. Swift uses special calling convention 20
21. Relative Pointer 21
22. Relative Pointer • Reduce binary size • Minimize load time relocation 22
23. How Relative Pointer generated 1 . Compiler emits LLVM IR using sub instruction on global variable to calculate the difference between two addresses. 23
24. $ swiftc -emit-ir relative_pointer.swift struct Animal {} let type = Animal.self 24
25. @"$s16relative_pointer6AnimalVMF" = internal constant { i32, i32, i16, i16, i32 } { i32 trunc ( i64 sub ( i64 ptrtoint (<{ i8, i32, i8 }>* @"symbolic _____ 16relative_pointer6AnimalV" to i64), i64 ptrtoint ({ i32, i32, i16, i16, i32 }* @"$s16relative_pointer6AnimalVMF" to i64) ) to i32 ), i32 0, i16 0, i16 12, i32 0 }, section "__TEXT,__swift5_fieldmd, regular, no_dead_strip", align 4 25
26. How Relative Pointer generated 2 . LLVM transforms the sub instruction into RELOC_SUBTRACTOR2 linker command. $ swiftc -c relative_pointer.swift $ objdump -x relative_pointer.o ... RELOCATION RECORDS FOR [__swift5_fieldmd]: 0000000000000000 X86_64_RELOC_SUBTRACTOR _symbolic _____ 16relative_pointer6AnimalV-_$s16relative_pointer6AnimalVMF 0000000000000000 X86_64_RELOC_UNSIGNED _symbolic _____ 16relative_pointer6AnimalV ... 2 for MachO format. 26
27. How Relative Pointer generated 3 . Linker links the object files and handle RELOC_SUBTRACTOR. Only linker can calculate the difference because compiler can't know where the global variable will be located. $ swiftc relative_pointer.o $ objdump -x relative_pointer No RELOCATION RECORDS !! 27
28. How Relative Pointer generated 4 . No need to be relocated by program loader. Make program launch time fast ! 28
29. Relative Pointer doesn't work on WebAssembly 29
30. wasm-ld • A linker project for WebAssembly • A part of LLVM project • Doesn't support RELOC_SUBTRACTOR yet. 30
31. Use absolute pointer instead @zhuowei tried to use absolute pointer instead of relative pointer. const uint32_t offset = ...; #ifdef __wasm__ const Pointee *address = reinterpret_cast<Pointee *>(offset); #else const Pointee *address = reinterpret_cast<Pointee *>(baseAddress + offset); #endif 31
32. What's hard? 1. Swift heavily depends on libc 2. Swift runtime needs some linker supports 3. Swift uses special calling convention 32
33. Swift Calling Convention on WebAssembly • Almost works even on stack-based machine • swifterror attribute breaks calling convention on WebAssembly 33
34. Function conversion (Int?) -> Void to (Int) -> Void func f(_ a: (Int) -> Void) -> Void { a(1) } func g(_ b: (Int?) -> Void) -> Void { f(b) } g { _ in } 34
35. Function conversion (Int?) -> Void to (Int) -> Void func f(_ a: (Int) -> Void) -> Void { a(1) } func g(_ b: (Int?) -> Void) -> Void { func thunk(_ x: Int) -> Void { // 0. Capture `b` // 1. Transform arguments let y = Optional<Int>.some(x) // 2. Apply return b(y) } f(thunk) } g { _ in } 35
36. Function conversion (Int) -> Void to (Int) throws -> Void func f(_ a: (Int) throws -> Void) -> Void { try! a(1) } func g(_ b: (Int) -> Void) -> Void { f(b) } g { _ in } 36
37. swiftc -emit-ir Experiment.swift 37
38. define i32 @main(i32, i8**) #0 { entry: call swiftcc void @"g"( i8* bitcast (void (i64)* @"closure" to i8*), %swift.opaque* null ) ret i32 0 } define hidden swiftcc void @"f"(i8*, %swift.opaque*) #0 { entry: ... %err = alloca swifterror %swift.error*, align 8 store %swift.error* null, %swift.error** %err, align 8 %2 = bitcast i8* %0 to void (i64, %swift.refcounted*, %swift.error**)* call swiftcc void %2(i64 1, %swift.error** swifterror %err) } define hidden swiftcc void @"g"(i8*, %swift.opaque*) #0 { entry: call swiftcc void @"f"(i8* %0, %swift.opaque* %1) ret void } define internal swiftcc void @"closure"(i64) #0 { entry: ret void } 38
39. define i32 @main(i32, i8**) #0 { entry: call swiftcc void @"g"( i8* bitcast (void (i64)* @"closure" to i8*), %swift.opaque* null ) ret i32 0 } define hidden swiftcc void @"f"(i8*, %swift.opaque*) #0 { entry: ... %err = alloca swifterror %swift.error*, align 8 store %swift.error* null, %swift.error** %err, align 8 %2 = bitcast i8* %0 to void (i64, %swift.refcounted*, %swift.error**)* call swiftcc void %2(i64 1, %swift.error** swifterror %err) } define hidden swiftcc void @"g"(i8*, %swift.opaque*) #0 { entry: call swiftcc void @"f"(i8* %0, %swift.opaque* %1) ret void } define internal swiftcc void @"closure"(i64) #0 { entry: ret void } 38
40. define i32 @main(i32, i8**) #0 { entry: call swiftcc void @"g"( i8* bitcast (void (i64)* @"closure" to i8*), %swift.opaque* null ) ret i32 0 } define hidden swiftcc void @"f"(i8*, %swift.opaque*) #0 { entry: ... %err = alloca swifterror %swift.error*, align 8 store %swift.error* null, %swift.error** %err, align 8 %2 = bitcast i8* %0 to void (i64, %swift.refcounted*, %swift.error**)* call swiftcc void %2(i64 1, %swift.error** swifterror %err) } define hidden swiftcc void @"g"(i8*, %swift.opaque*) #0 { entry: call swiftcc void @"f"(i8* %0, %swift.opaque* %1) ret void } define internal swiftcc void @"closure"(i64) #0 { entry: ret void } 38
41. define i32 @main(i32, i8**) #0 { entry: call swiftcc void @"g"( i8* bitcast (void (i64)* @"closure" to i8*), %swift.opaque* null ) ret i32 0 } define hidden swiftcc void @"f"(i8*, %swift.opaque*) #0 { entry: ... %err = alloca swifterror %swift.error*, align 8 store %swift.error* null, %swift.error** %err, align 8 %2 = bitcast i8* %0 to void (i64, %swift.refcounted*, %swift.error**)* call swiftcc void %2(i64 1, %swift.error** swifterror %err) } define hidden swiftcc void @"g"(i8*, %swift.opaque*) #0 { entry: call swiftcc void @"f"(i8* %0, %swift.opaque* %1) ret void } define internal swiftcc void @"closure"(i64) #0 { entry: ret void } 38
42. Runtime (Swift like language) func f(_ a: (Int, inout Error) throws -> Void) -> Void { var error: Error = nullptr a(1, &error) if error != nullptr { fatalError() } } func g(_ b: (Int) -> Void) -> Void { f(b) } func closure(_: Int) -> Void {} g(closure) 39
43. swifterror This attribute is motivated to model and optimize Swift error handling3. • Place the argument in a specific register • No effect to stack • ABI compatible with one which doesn't have swifterror 3 https://llvm.org/docs/LangRef.html#parameter-attributes 40
44. swifterror on WebAssembly WebAssembly runtime checks number of arguments for indirect call for security purpose. But swifterror needs a variadic error parameter https://github.com/WebAssembly/design/blob/master/Rationale.md#indirect-calls 41
45. Solutions There are two ways • Add swifterror to all functions • Use thunk function 42
46. Use thunk function func f(_ a: (Int) throws -> Void) -> Void { try! a(1) } func g(_ b: (Int) -> Void) -> Void { func thunk(_ x: Int) throws -> Void { return b(x) } f(thunk) } g { _ in } 43
47. Still... • Many crashes related with metadata • No debugger for Swift on WebAssembly • Large binary (Hello, world binary is over 5 MB) • etc... 44
48. Conclusion • Supporting new platform is hard but fun • It's a chance to learn Swift compiler techniques 45