Why Does Our Swift Code Work? Understanding Swift Through Platforms, Runtime, and the Compiler

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4. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
5. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
6. iOS Windows watchOS Linux visionOS Wasm macOS Embedded System tvOS Android
7. Motivation Why go deeper into Swift? Swift is growing into a true multi-platform language Platform-native development benefits from knowing what's underneath It also shapes your Developer Experience in meaningful ways
8. Platform constraints APIs arrive with OS versions — not all users are on the latest Your deployment target caps which APIs you can call @available #available // Deployment target: iOS 15 @Observable class Model { var count = 0 } ✕ 'Observable' is only available in iOS 17 or newer
9. Platform constraints APIs arrive with OS versions — not all users are on the latest Your deployment target caps which APIs you can call @available #available // Deployment target: iOS 15 @available(iOS 17, *) @Observable class Model { var count = 0 }
10. Invisible runtime costs Many codes are inserted by the compiler — not written by you Example: ARC A lot of Invisible reference counting code injected at compile time Looking at generated code helps improving your app performance let dog = Dog() let dog = Dog() let friend = dog friend.bark() strong_retain %2 let friend = dog friend.bark() strong_release %2
12. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
13. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
14. Swift on Apple Platform App
15. Swift on Apple Platform App OS
16. Swift on Apple Platform App OS Runtime
17. What is Runtime The engine beneath every Swift program The program that handles Swift's runtime behavior The heart that makes Swift feel like Swift on every platform
18. Compiler and Runtime Compiler paves a safe road Runtime the truck driver on road
19. Calls at Runtime Swift Code Runtime Ships with OS
20. Calls at Runtime Swift Code Runtime Creating an instance swift_allocObject Reference counting (ARC) swift_retain / release as? / is casts swift_dynamicCast async / await Concurrency runtime Ships with OS
21. Before iOS 12.2
22. Before iOS 12.2 App A App B App C Compiled code Compiled code Compiled code Swift Runtime Swift Runtime Swift Runtime Apple Platform
23. After iOS 12.2 App A App B App C Compiled code Compiled code Compiled code Apple Platform Swift Runtime (Dynamic Library)
24. App App calls newAPI() calls newAPI() iOS 18 Runtime iOS 17 Runtime existingAPI() existingAPI() anotherAPI() anotherAPI() ✓ newAPI() ✕ newAPI()
25. Static Library App App Extension Compiled .swift code Extension code Library Library
26. Dynamic Library App App Extension Compiled .swift code Extension code Loaded at app launch Dynamic Library
27. Launch Time Cost Static launch main() Dynamic launch dyld: load .dylib main() Longer launch time Meet mergeable libraries WWDC23
28. After iOS 12.2 With ABI Stability (SE-0260) App A App B App C Compiled code (Swift 5) Compiled code (Swift 5.1) Compiled code (Swift 6) Apple Platform Swift 6.0 Runtime (Dynamic Library)
29. After iOS 12.2 With ABI Stability App A App B App C Compiled code (Swift 5) Compiled code (Swift 5.1) Compiled code (Swift 6) Stable ABI (From Swift 5.0) Apple Platform Swift 6.0 Runtime (Dynamic Library)
30. What is ABI Stability A stable contract between Swift binaries and the runtime Defines fixed contract of Application Binary Interface (ABI) for Swift Allows the OS to provide Swift Runtime loaded by all application
31. Beyond ABI Stability Module Stability from Swift 5.1 Module Stability (from Swift 5.1) is introduced Allows importing libraries (.framework) built with different compiler versions
32. Before Module stability Import-level compatibility at build time App Framework Built with Swift 5.1 .swiftmodule Built with Swift 5.1 ✓ import Framework
33. Before Module stability Import-level compatibility at build time App Framework Built with Swift 5.1 .swiftmodule Built with Swift 5.1.2 ✕ import Framework module compiled with Swift 5.1 cannot be imported by the Swift 5.1.2 compiler
34. After Module stability Import-level compatibility at build time App Framework Built with Swift 5.1 .swiftmodule Built with Swift 5.1 ✓ import Framework
35. After Module stability Import-level compatibility at build time App Framework Built with Swift 5.1 .swiftinterface Built with Swift 5.1.2 ✓ import Framework Set BUILD_LIBRARY_FOR_DISTRIBUTION = YES
36. 🌴💃
37. 🙅
38. A note for maintainers Don t too reply on Module Stability Stability keeps old binaries working It's a safety net, not a goal ’ Keep your libraries / frameworks updated
39. Swift on Linux Static Runtime Dynamic Runtime Executable Executable Compiled .swift code Compiled .swift code libswiftCore.a (Runtime & Stdlib) Loaded at launch libswiftCore.so (Runtime & Stdlib) Linux
40. Swift on Wasm Wasm Module (.wasm) Compiled .swift code libswiftCore.a (Runtime & Stdlib) JavaScriptKit JavaScript host Browser / Node.js / Bun / Deno Wasm Runtime (Browser / WASI) Introduction to WebAssembly for Swift Developers SwiftLeeds 2024
41. Swift on Android .apk libMyLibrary.so Compiled .swift code Java / Kotlin swift-java (JNI) libswiftCore.a (Runtime & Stdlib) Android classes.dex (ART)
42. Embedded Swift App Firmware Compiled .swift code Specialized app code Type metadata ✕ Type metadata Stdlib Stdlib subset Swift Runtime ✕ Swift Runtime iOS Bare metal / MCU
43. Embedded Swift: Language Subset ✓ Available ✕ Not available Generics Reflection (Mirror) Protocols Existentials (any P) Enums with associated values Metatypes / Untyped throws Optionals / Tuples Codable Classes & inheritance Printing arbitrary types Runtime polymorphism weak / unowned references Arrays (copy-on-write) Non-final generic class methods Library Evolution / ObjC interop
44. Embedded Swift: Language Subset ✓ Available ✕ Not available Generics Reflection (Mirror) Protocols Codable Enums with associated values Printing arbitrary types Optionals / Tuples weak / unowned references Classes & inheritance Non-final generic class methods Runtime polymorphism Library Evolution / ObjC interop Arrays (copy-on-write) Existentials (any P) Metatypes / Untyped throws
45. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
46. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
47. Compiler and Runtime Compiler paves a safe road Runtime the truck driver on road
48. Swift Compiler Pipeline .swift Parse Sema AST Typechecked AST SILGen SIL Optimize Canonical SIL Raw SIL IRGen LLVM IR CodeGen Object file (.o) Linker Executable
49. Swift Compiler Pipeline .swift Parse Sema AST Typechecked AST SILGen SIL Optimize Canonical SIL Raw SIL IRGen LLVM IR CodeGen Object file (.o) Linker Executable
50. Type Check Cost Example: Code that relies heavily on type inference can slow the compiler This is where you have room to optimize
51. Too complex expression Type inference hits its time limit let x = Optional(1).map { $0 + 0.5 } .map { $0 * 2 } .map { $0 + 1 } .map { $0 / 2 } ✕ The compiler is unable to type-check this expression in reasonable time
52. Break it up let x = Optional(1).map { $0 + 0.5 } ✕ .map { $0 * 2 } .map { $0 + 1 } .map { $0 / 2 } let x: Double? = Optional(1).map { $0 + 0.5 } ✕ 1.8 s .map { $0 * 2 } .map { $0 + 1 } .map { $0 / 2 } let a = Optional(1).map { $0 + 0.5 } let b = a.map { $0 * 2 } let c = b.map { $0 + 1 } let x = c.map { $0 / 2 } ✓ 0.06 s
53. Measure type checking Slow even without the error Find hot spots, cut build time -Xfrontend -warn-long-function-bodies=100 -Xfrontend -warn-long-expression-type-checking=100
54. Swift Compiler Pipeline .swift Parse Sema AST Typechecked AST SILGen SIL Optimize Canonical SIL Raw SIL IRGen LLVM IR CodeGen Object file (.o) Linker Executable
55. Swift Compiler Pipeline .swift Parse Sema AST Typechecked AST SILGen SIL Optimize Canonical SIL Raw SIL IRGen LLVM IR CodeGen Object file (.o) Linker Executable
56. SIL Swift Intermediate Language Intermediate language specialized for Swift Understands Swift semantics Runs Swift-specific optimizations
57. SIL Optimizer Optimizations and diagnostics on SIL Sits between SILGen and IRGen Optimize — inlining, specialization, ARC … Diagnose — Definite init / unreachable code / overflow even at -Onone
58. SIL Optimizations / Generic specialization / Inlining / Devirtualization / Common subexpression elimination / ARC optimization / Closure specialization / Dead code elimination / Existential specialization / SROA / Copy propagation / Stack promotion / Constant propagation / Dead object elimination / Mem2Reg / Object outlining / String optimization / Simplify CFG / COW optimization / Alloc box to stack / Redundant load elimination / Release devirtualization / Exclusivity enforcement
59. SIL Optimizations / Generic specialization / Inlining / Devirtualization / Common subexpression elimination / ARC optimization / Closure specialization / Dead code elimination / Existential specialization / SROA / Copy propagation / Stack promotion / Constant propagation / Dead object elimination / Mem2Reg / Object outlining / String optimization / Simplify CFG / COW optimization / Alloc box to stack / Redundant load elimination / Release devirtualization / Exclusivity enforcement
60. SILGen func greet() { let dog = Dog() let friend = dog friend.bark() }
61. Raw SIL Raw SIL from swiftc -emit-silgen
62. Raw SIL Raw SIL from swiftc -emit-silgen sil hidden [ossa] @$s4demo5greetyyF : $@convention(thin) () -> () { bb0: %0 = metatype $@thick Dog.Type // function_ref Dog.__allocating_init() %1 = function_ref @$s4demo3DogCACycfC : $@convention(method) (@thick Dog.Type) -> @owned Dog %2 = apply %1(%0) : $@convention(method) (@thick Dog.Type) -> @owned Dog %3 = move_value [lexical] [var_decl] %2 debug_value %3, let, name "dog" %5 = begin_borrow %3 %6 = copy_value %5 %7 = move_value [lexical] [var_decl] %6 debug_value %7, let, name "friend" end_borrow %5 %10 = begin_borrow %7 // function_ref Dog.bark() %11 = function_ref @$s4demo3DogC4barkyyF : $@convention(method) (@guaranteed Dog) -> () %12 = apply %11(%10) : $@convention(method) (@guaranteed Dog) -> () end_borrow %10 destroy_value %7 destroy_value %3 %16 = tuple () return %16 }
63. Canonical SIL Canonical SIL from swiftc -emit-sil sil hidden @$s4demo5greetyyF : $@convention(thin) () -> () { bb0: %0 = metatype $@thick Dog.Type // function_ref Dog.__allocating_init() %1 = function_ref @$s4demo3DogCACycfC : $@convention(method) (@thick Dog.Type) -> @owned Dog %2 = apply %1(%0) : $@convention(method) (@thick Dog.Type) -> @owned Dog debug_value %2, let, name "dog" strong_retain %2 debug_value %2, let, name "friend" // function_ref Dog.bark() %6 = function_ref @$s4demo3DogC4barkyyF : $@convention(method) (@guaranteed Dog) -> () %7 = apply %6(%2) : $@convention(method) (@guaranteed Dog) -> () strong_release %2 strong_release %2 %10 = tuple () return %10 }
64. Canonical SIL Canonical SIL from swiftc -emit-sil sil hidden @$s4demo5greetyyF : $@convention(thin) () -> () { bb0: %0 = metatype $@thick Dog.Type // function_ref Dog.__allocating_init() %1 = function_ref @$s4demo3DogCACycfC : $@convention(method) (@thick Dog.Type) -> @owned Dog %2 = apply %1(%0) : $@convention(method) (@thick Dog.Type) -> @owned Dog debug_value %2, let, name "dog" strong_retain %2 debug_value %2, let, name "friend" // function_ref Dog.bark() %6 = function_ref @$s4demo3DogC4barkyyF : $@convention(method) (@guaranteed Dog) -> () %7 = apply %6(%2) : $@convention(method) (@guaranteed Dog) -> () strong_release %2 strong_release %2 %10 = tuple () return %10 }
65. SIL Optimizations Redundant ARC pairs removed SemanticARCOpts ARCSequenceOpts func greet() { let dog = Dog() let friend = dog swift_retain(dog) friend.bark() swift_release(friend) swift_release(dog) }
66. Don't over-trust the optimizer Optimizations are best-effort Results change across compilers Express performance in source code and inspect it
67. ARC has a cost Retain / release calls remain — even at -O Invisible in your code func register(_ dog: Dog) { kennel.append(dog) } Impacts performance sil hidden @$s4demo8registeryyAA3DogCnF : $@convention(thin) (@owned Dog) -> () bb0(%0 : @noImplicitCopy @_eagerMove $Dog): strong_retain %0 ... // Array.append %42 = apply %40(%39, %41, %5) ...
68. Ownership Transfer Ownership by consuming Move ownership to the callee consuming func register(_ dog: consuming Dog) { kennel.append(dog) } sil hidden @$s4demo8registeryyAA3DogCnF : $@convention(thin) (@owned Dog) -> () bb0(%0 : @noImplicitCopy @_eagerMove $Dog): strong_retain %0 ... // Array.append %42 = apply %40(%39, %41, %5) ...
69. .swift Parse Sema AST Typechecked AST SILGen SIL Optimize Canonical SIL Raw SIL IRGen LLVM IR CodeGen Object file (.o) Linker Executable
70. Swift Optimizing (at Compiler World) NSSpain 2020
71. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
72. Motivation Mechanism of Swift with Runtime What Compiler really do? To become friendly with Swift
73. Go deeper The details are worth knowing Every new Swift feature has a design reason behind it Runtime and compiler knowledge helps you see those reasons Use this as your lens — and enjoy the next feature even more
74. Example: Behind some and any some shipped alongside SwiftUI — Opaque Result Types (SE-0244) Overcomes several limitations of existing existential types Keyword: Existential Container Deep Dive into "any" and "some" SwiftHeroes 2023
75. Example: Build Artifact Caching Swift frameworks and build outputs can be cached effectively Many tools exist to take advantage of this Try exploring them with the knowledge from today Tuist tuist/Tuist Scipio giginet/Scipio
76. Understand from deep side of Swift Optimize by lens of Runtime and Compiler Enjoy your Swift life
77. X (Twitter): https://x.com/___freddi___ GitHub: https://github.com/freddi-kit