Getting Under Swift’s Skin

Staying on the surface, we deal with the Swift language, runtime, and standard library all the time. In this talk, we’ll go one small level down and look underneath the surface: Swift Intermediate Language (SIL), reflection, and debug-enabled toolchains. What can we learn, why should we bother, and most importantly, how can it help improve our programming lives?

Transcript

1. @gregheo
   NSMeetup
   February 7, 2018
   

   View Slide

2. Stack views
   Auto Layout
   Frame calculations
   UIViews
   CALayers
   Core Animation
   OpenGL
   Metal
   putpixel()
   OLEDs
   Individual photons
   Quantum physics
   GUI toolkits
   Standard library
   Unix processes
   fork vs exec
   bash
   Kernel modules
   Operating systems
   Linkers
   Compilers
   Assembly
   Toggle switches
   emacs
   

   View Slide

3. View Slide

4. — Jesse Squires
   

   View Slide

5. Swift
   Runtime
   Instances
   Standard
   Library
   SIL Reflection
   Debug
   Toolchains
   

   View Slide

6. View Slide

7. SIL IR Object
   CodeGen
   IRGen
   AST
   Sema
   SILGen
   Parse
   

   View Slide

8. View Slide

9. 00 e3
   8e c0
   〇〇〇
   〇〇〇〇〇〇〇〇
   〇〇〇〇〇〇
   〇〇〇〇
   add bl,ah
   mov es,eax
   

   View Slide

10. ASM Binary
    

    View Slide

11. ASM
    C Binary
    C++
    

    View Slide

12. ASM
    C Binary
    C++
    Obj-C
    

    View Slide

13. ASM
    C Binary
    C++
    Obj-C
    

    View Slide

14. ASM
    C Binary
    C++
    IR
    Obj-C
    

    View Slide

15. printf(“hello world!”);
    subq $32, %rsp
    leaq L_.str(%rip), %rax
    movl $0, -4(%rbp)
    movl %edi, -8(%rbp)
    movq %rsi, -16(%rbp)
    movq %rax, %rdi
    movb $0, %al
    callq_printf
    define i32 @main(i32, i8**) #0 {
    %3 = alloca i32, align 4
    %4 = alloca i32, align 4
    %5 = alloca i8**, align 8
    store i32 0, i32* %3, align 4
    store i32 %0, i32* %4, align 4
    store i8** %1, i8*** %5, align 8
    %6 = call i32 (i8*, …)

    @printf(i8* getelementptr inbounds…
    ret i32 0
    }
    

    View Slide

16. ASM
    C Binary
    C++
    IR
    Obj-C
    

    View Slide

17. SIL IR …
    

    View Slide

18. View Slide

19. class NSMeetup {
    func sayHello() {
    print("Hello")
    }
    }
    let m = NSMeetup()
    m.sayHello()
    

    View Slide

20. sil_stage canonical
    import Builtin
    import Swift
    import SwiftShims
    // m
    sil_global hidden [let] @_T06simple1mAA8NSMeetupCv : $NSMeetup
    // _swiftEmptyArrayStorage
    sil_global [serialized] @_swiftEmptyArrayStorage :
    $_SwiftEmptyArrayStorage
    // main
    sil @main : $@convention(c) (Int32,
    UnsafeMutablePointer>>) -> Int32 {
    bb0(%0 : $Int32, %1 :
    

    View Slide

21. sil_vtable NSMeetup {
    #NSMeetup.sayHello!1: (NSMeetup) -> () -> () :
    _T06simple8NSMeetupC8sayHelloyyF // NSMeetup.sayHello()
    #NSMeetup.init!initializer.1: (NSMeetup.Type) -> () -> NSMeetup :
    _T06simple8NSMeetupCACycfc // NSMeetup.init()
    #NSMeetup.deinit!deallocator: _T06simple8NSMeetupCfD //
    NSMeetup.__deallocating_deinit
    }
    sil_vtable _ContiguousArrayStorage {
    #_SwiftNativeNSArrayWithContiguousStorage.withUnsafeBufferOfObjects!1
    (_SwiftNativeNSArrayWithContiguousStorage) ->
    ((UnsafeBufferPointer) throws -> R) throws -> R :
    _T0s27_ContiguousArrayStorageBaseC25withUnsafeBufferOfObjectsxxSRyyXlGK
    KlF // _ContiguousArrayStorageBase.withUnsafeBufferOfObjects(_:)
    #_ContiguousArrayStorageBase._withVerbatimBridgedUnsafeBuffer!1:
    (_ContiguousArrayStorageBase) -> ((UnsafeBufferPointer)
    

    View Slide

22. // NSMeetup.sayHello()
    sil hidden @_T06simple8NSMeetupC8sayHelloyyF : $@convention(method)
    (@guaranteed NSMeetup) -> () {
    // %0 // user: %1
    bb0(%0 : $NSMeetup):
    debug_value %0 : $NSMeetup, let, name "self", argno 1 // id: %1
    // function_ref print(_:separator:terminator:)
    %2 = function_ref @_T0s5printySayypGd_SS9separatorSS10terminatortF :
    $@convention(thin) (@owned Array, @owned String, @owned String) ->
    () // user: %23
    %3 = integer_literal $Builtin.Word, 1 // user: %5
    // function_ref specialized _allocateUninitializedArray(_:)
    %4 = function_ref
    @_T0s27_allocateUninitializedArraySayxG_BptBwlFyp_Tgq5 :
    $@convention(thin) (Builtin.Word) -> (@owned Array,
    Builtin.RawPointer) // user: %5
    %5 = apply %4(%3) : $@convention(thin) (Builtin.Word) -> (@owned
    

    View Slide

23. $ xcrun swift-demangle

    _T0s5printySayypGd_SS9separatorSS10terminatortF

    Swift.print(Any...,

    separator: Swift.String,

    terminator: Swift.String) -> ()
    

    View Slide

24. retain_value %6 : $Array // id: %7
    %8 = tuple_extract %5 : $(Array, Builtin.RawPointer), 1 // user:
    %10
    release_value %5 : $(Array, Builtin.RawPointer) // id: %9
    %10 = pointer_to_address %8 : $Builtin.RawPointer to [strict] $*Any /
    user: %11
    %11 = init_existential_addr %10 : $*Any, $String // user: %18
    %12 = string_literal utf8 "Hello" // user: %17
    %13 = integer_literal $Builtin.Word, 5 // user: %17
    %14 = integer_literal $Builtin.Int1, -1 // user: %17
    %15 = metatype $@thin String.Type // user: %17
    // function_ref
    String.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:)
    %16 = function_ref
    @_T0S2SBp21_builtinStringLiteral_Bw17utf8CodeUnitCountBi1_7isASCIItcfC
    $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1,
    @thin String.Type) -> @owned String // user: %17
    %17 = apply %16(%12, %13, %14, %15) : $@convention(method)
    

    View Slide

25. class NSMeetup {
    func sayHello() {
    print("Hello")
    }
    }
    // let m = NSMeetup()

    var m = NSMeetup()

    m.sayHello()
    

    View Slide

26. %9 = load %8 : $*NSMeetup
    %10 = class_method %9 : $NSMeetup, #NSMeetup.sayHello!1 : (NSMeetup) ->
    () -> (), $@convention(method) (@guaranteed NSMeetup) -> ()
    %11 = apply %10(%9) : $@convention(method) (@guaranteed NSMeetup) -> ()
    

    View Slide

27. %8 = begin_access [read] [dynamic] %3 : $*NSMeetup
    %9 = load %8 : $*NSMeetup
    strong_retain %9 : $NSMeetup
    end_access %8 : $*NSMeetup
    %10 = class_method %9 : $NSMeetup, #NSMeetup.sayHello!1 : (NSMeetup) ->
    () -> (), $@convention(method) (@guaranteed NSMeetup) -> ()
    %11 = apply %10(%9) : $@convention(method) (@guaranteed NSMeetup) -> ()
    strong_release %9 : $NSMeetup
    

    View Slide

28. call void @llvm.lifetime.start(i64 -1, i8* %13)
    call void @swift_beginAccess(i8* bitcast (%TSi*
    @_T011simpleprint1aSiv to i8*), [24 x i8]* %access-scratch, i64
    0, i8* null) #6
    %14 = load i64, i64* getelementptr inbounds (%TSi, %TSi*
    @_T011simpleprint1aSiv, i32 0, i32 0), align 8
    call void @swift_endAccess([24 x i8]* %access-scratch) #6
    %15 = bitcast [24 x i8]* %access-scratch to i8*
    call void @llvm.lifetime.end(i64 -1, i8* %15)
    %._value = getelementptr inbounds %TSi, %TSi* %12, i32 0, i32
    0
    store i64 %14, i64* %._value, align 8
    %16 = call swiftcc { i64, i64, i64 }
    @_T0s5printySayypGd_SS9separatorSS10terminatortFfA0_()
    %17 = extractvalue { i64, i64, i64 } %16, 0
    %18 = extractvalue { i64, i64, i64 } %16, 1
    %19 = extractvalue { i64, i64, i64 } %16, 2
    

    View Slide

29. call void @llvm.lifetime.start(i64 -1, i8* %13)
    call void @swift_beginAccess(i8* bitcast (%TSi*
    @_T011simpleprint1aSiv to i8*), [24 x i8]* %access-scratch, i64
    0, i8* null) #6

    

    call void @#__tsan_read8(i8* bitcast (%TSi*
    @_T011simpleprint1aSiv to i8*))
    %14 = load i64, i64* getelementptr inbounds (%TSi, %TSi*
    @_T011simpleprint1aSiv, i32 0, i32 0), align 8
    call void @swift_endAccess([24 x i8]* %access-scratch) #6
    %15 = bitcast [24 x i8]* %access-scratch to i8*
    call void @llvm.lifetime.end(i64 -1, i8* %15)
    %._value = getelementptr inbounds %TSi, %TSi* %12, i32 0, i32
    0
    store i64 %14, i64* %._value, align 8
    %16 = call swiftcc { i64, i64, i64 }
    @_T0s5printySayypGd_SS9separatorSS10terminatortFfA0_()
    

    View Slide

30. • Your code, lowered
    • ARC & memory management
    • Change code; see diffs on SIL
    

    View Slide

31. View Slide

32. Queries Modifications Construction
    

    View Slide

33. Reflection Demo
    

    View Slide

34. • Objective-C runtime functions
    • Mirror for “read-only reflection”
    • Direct pointer access
    • More advanced reflection to come?
    

    View Slide

35. ASM
    C Binary
    C++
    IR
    Obj-C
    

    View Slide

36. Language Standard Library
    

    View Slide

37. Standard library demo
    

    View Slide

38. • Standard library is open source!
    • Standard library code in the debugger
    • Running custom Xcode toolchains
    

    View Slide

39. Swift
    Runtime
    Instances
    Standard
    Library
    SIL Reflection
    Debug
    Toolchains
    

    View Slide

40. So What?
    

    View Slide

41. View Slide

42. View Slide

43. View Slide

44. https://gregheo.com
    

    View Slide

45. Getting Under

    Swift’s Skin @gregheo
    NSMeetup
    February 7, 2018
    

    View Slide