The document discusses value semantics vs reference semantics in Swift. It explains that value types (structs, enums, tuples) use value semantics where copies are made, while reference types (classes) use reference semantics where copies of references are made but the instances are shared. It provides examples to illustrate how value types stored on the stack are copied, while reference types stored on the heap are referenced. The document also covers topics like identity vs equality, mutability, reference counting, and static vs dynamic method dispatch in Swift.

1. Swift

2. 2016 Let'Swift
' '
Value Semantics vs. Reference Semantics
Swift Type
...
!

3. ' , .'
,
.
.

4. ' ' .
.

5. .

6. Value Semantics & Reference Semantics
Value Semantics
Copy-by-Value :
(Identity) < (Equality)
Struct, Enum, Tuple Objective-C Swift Value Type
Swift class ReferenceType
ValueType

7. Stack
Heap -> Reference Counting
Value Semantics & Reference Semantics
Value Type

8. Struct Value Type
struct SomeStruct {
var value = 10
}
func foo() {
var bar1 = SomeStruct()
var bar2 = bar1
bar2.value = 11
}
foo()
Value Semantics & Reference Semantics

9. Struct Value Type
struct SomeStruct {
var value = 10
}
func foo() {
var bar1 = SomeStruct()
var bar2 = bar1
bar2.value = 11
}
foo()
Value Semantics & Reference Semantics
bar2 : (7FFEEFBFF4B0) : 0B 00 00 00
00 00 00 00
bar1 : (7FFEEFBFF4B8) : 0A 00 00 00
00 00 00 00

10. Struct Value Type
struct SomeStruct {
var value = 10
}
func foo() {
var bar1 = SomeStruct()
var bar2 = bar1
bar2.value = 11
}
foo()
bar2 : (7FFEEFBFF4B0) : 0B 00 00 00
00 00 00 00
bar1 : (7FFEEFBFF4B8) : 0A 00 00 00
00 00 00 00!
Value Semantics & Reference Semantics
Stack 8 Byte
bar1.value // 10
bar2.value // 11
I

11. class SomeClass {
var value = 10
}
func foo() {
var bar1 = SomeClass()
var bar2 = bar1
bar2.value = 11
}
foo()
Value Semantics & Reference Semantics
Class Value Type

12. Value Semantics & Reference Semantics
Class Value Type
Stack 8 Byte
class SomeClass {
var value = 10
}
func foo() {
var bar1 = SomeClass()
var bar2 = bar1
bar2.value = 11
}
foo()
bar2 : (7FFEEFBFF4A0) : 20 FF 0A 03
01 00 00 00
bar1 : (7FFEEFBFF4A8) : 20 FF 0A 03
01 00 00 00
(1030AFF20) : 80 8F 00 00 01 00 00 00
02 00 00 00 02 00 00 00
0B 00 00 00 00 00 00 00
a

13. class SomeClass {
var value = 10
}
func foo() {
var bar1 = SomeClass()
var bar2 = bar1
bar2.value = 11
}
foo()
bar2 : (7FFEEFBFF4A0) : 20 FF 0A 03
01 00 00 00
bar1 : (7FFEEFBFF4A8) : 20 FF 0A 03
01 00 00 00
(1030AFF20) : 80 8F 00 00 01 00 00 00
02 00 00 00 02 00 00 00
0B 00 00 00 00 00 00 00
Value Semantics & Reference Semantics
Class Value Type
0x1030aff20
11
??
bar1.value // 11
bar2.value // 11

14. Identity( ) Equality( ) :
Swift Equatable : Equality . (==)
Value Semantics & Reference Semantics
Value Semantics : ' '

15. ,
Thread -> Thread Safe
Value Semantics & Reference Semantics
Value Type Thread

16. ' '(constant time)
Value Semantics & Reference Semantics
?
Value Type Heap Value Type
Int, Double String, Array, Dictionary
' '
' ' +
Copy-on-Write : , Value Semantics
1

17. Struct Array .
?
Value Semantics & Reference Semantics

18. func foo() {
let iterationCounts = [10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000]
for iterationCount in iterationCounts {
print("Iteration Count : (iterationCount)")
var array = [Int]()
for number in 0 ..< iterationCount {
array.append(number)
}
var startTime = CFAbsoluteTimeGetCurrent()
let array2 = array
print(" : (CFAbsoluteTimeGetCurrent() - startTime)")
startTime = CFAbsoluteTimeGetCurrent()
var array3 = array
array[0] = 1
print(" : (CFAbsoluteTimeGetCurrent() - startTime)")
print("------------------------")
}
}
foo()

19. func foo() {
let iterationCounts = [10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000]
for iterationCount in iterationCounts {
print("Iteration Count : (iterationCount)")
var array = [Int]()
for number in 0 ..< iterationCount {
array.append(number)
}
var startTime = CFAbsoluteTimeGetCurrent()
let array2 = array
print(" : (CFAbsoluteTimeGetCurrent() - startTime)")
startTime = CFAbsoluteTimeGetCurrent()
var array3 = array
array[0] = 1
print(" : (CFAbsoluteTimeGetCurrent() - startTime)")
print("------------------------")
}
}
foo()
n

20. Iteration Count : 10
: 1.0728836059570312e-06
: 2.205371856689453e-05
------------------------
Iteration Count : 100
: 0.0
: 9.5367431640625e-07
------------------------
Iteration Count : 1000
: 9.5367431640625e-07
: 2.0265579223632812e-06
------------------------
Iteration Count : 10000
: 2.0265579223632812e-06
: 3.898143768310547e-05
------------------------
Iteration Count : 100000
: 3.0994415283203125e-06
: 0.0003249645233154297
------------------------
Iteration Count : 1000000
: 4.0531158447265625e-06
: 0.0033320188522338867
------------------------
Iteration Count : 10000000
: 2.9802322387695312e-06
: 0.03259694576263428
------------------------
Iteration Count : 100000000
: 2.0265579223632812e-06
: 0.34505295753479004
------------------------
1μs
2μs
2μs
0.3s
10
1

21. Array ' ' .
Copy-on-Write ,
.
, .
Heap Value Type , Copy-on-Write
.
f
rnr
rn

22. Swift Array Struct , Struct Value Type ,
Stack Heap ?
Array Heap ?
Value Semantics & Reference Semantics

23. func foo() {
var array = [1, 2, 3, 4, 5]
withUnsafeBytes(of: &array) { print($0) }
}
foo()

24. func foo() {
var array = [1, 2, 3, 4, 5]
withUnsafeBytes(of: &array) { print($0) }
}
foo()
UnsafeRawBufferPointer(start: 0x00007ffeefbff4a8, count: 8)
???
40 Byte

25. func foo() {
var array = [1, 2, 3, 4, 5]
withUnsafeBytes(of: &array) { print($0) }
}
foo()
UnsafeRawBufferPointer(start: 0x00007ffeefbff4a8, count: 8)
array : (7FFEEFBFF4A8) : 00 25 53 00 01 00 00 00

26. func foo() {
var array = [1, 2, 3, 4, 5]
withUnsafeBytes(of: &array) { print($0) }
}
foo()
UnsafeRawBufferPointer(start: 0x00007ffeefbff4a8, count: 8)
array : (7FFEEFBFF4A8) : 00 25 53 00 01 00 00 00
(100532500) : A0 19 B3 94 FF 7F 00 00
02 00 00 00 00 00 00 00
05 00 00 00 00 00 00 00
0A 00 00 00 00 00 00 00
01 00 00 00 00 00 00 00
02 00 00 00 00 00 00 00
03 00 00 00 00 00 00 00
04 00 00 00 00 00 00 00
05 00 00 00 00 00 00 00
Heap

27. Array Struct Heap ,
.

28. Value Semantics & Reference Semantics
Reference Type Immutable ?
Value Type , Thread-Safe
Reference Type Immutable ?

29. Value Semantics & Reference Semantics
Reference Type Immutable ?
Reference Type Immutable Thread
Foundation
(NSAttributedString <-> NSMutableAttributedString )
Immutable
Mutable

30. Value Semantics & Reference Semantics
Reference Type Immutable ?
Immutable
var array = NSArray()
for number in [1, 2, 3] {
array = array.adding(number) as NSArray
}
GO

31. Value Semantics & Reference Semantics
Reference Type Immutable ?
Immutable
var array = NSArray()
for number in [1, 2, 3] {
array = array.adding(number) as NSArray
}
var array = NSMutableArray()
for number in [1, 2, 3] {
array.add(number)
}
i

32. Value Semantics & Reference Semantics
Reference Type Immutable ?
API
// Mutable API
car.dashboard.speed = 100
// Immutable API
car.dashboard = Car.Dashboard(speed: 100)
// car = Car(dashboard: Car.Dashboard(speed: 100))
TO
of
Dashboard

33. Value Semantics & Reference Semantics
Class .
Identity Equality
Objective-C : Cocoa / Cocoa Touch Objective-C
Indirect Storage : Struct
100P t

34. Memory Allocation Reference Counting Method Dispatch
Stack
Heap
Yes
No
Static
Dynamic

35. Heap
Memory Fragmentation -
Thread-Safe : lock ->
Stack : ,
Heap

36. String
Heap
Value Type Heap
Heap

37. Heap
//
let key = "(color)(theme)(selected)"
//
enum Color {}
enum Theme {}
struct Attribute: Hashable {
var color: Color
var theme: Theme
var selected: Bool
}
let key = Attribute(color: color, theme: theme, selected: selected)
string
let L
t Dictionary Key Hashable
iii
in j
it

38. Reference Counting
Reference Counting
Reference Counting . Reference Type
Reference Counting Thread-Safe , Atomic

39. Reference Counting
Reference Counting
class A {}
func foo(_ a: A) {}
let a0 = A()
var a1: A? = a0
foo(a0)
a1 = nil
l

40. Reference Counting
Reference Counting
class A {}
func foo(_ a: A) {
// retain(a) : 3
// release(a) : 2
}
let a0 = A()
// alloc(A): 1
var a1: A? = a0
// retain(a0) : 2
foo(a0)
a1 = nil
// release(a1): 1
retain release
.

41. Reference Counting
Reference Counting
for _ in 0 ..< 1000000 {
foo(c0)
}
retain release ->

42. Reference Counting
CFGetRetainCount Reference Count .
( Reference Count .)

43. Reference Counting
class A {
deinit { print("deinit") }
}
func foo(_ a: A?) {
print(CFGetRetainCount(a))
}
var a0: A? = A()
print(CFGetRetainCount(a0))
var a1: A? = a0
print(CFGetRetainCount(a0))
foo(a0)
print(CFGetRetainCount(a0))
a1 = nil
print(CFGetRetainCount(a0))
a0 = nil
Reference Count
retain 1 ,
release 1 .
2
3
4
3
2
deinit
ref I
refI2
ref
refI 2
ref I print I
refiO

44. Strong De Ht i RC 0
child
R I
4
parentRL 2
ParentRC I
parentdeinit t RC I

45. Weak Optional i 1
RC I
person deinitU RC 0
Apartment tenant
t
myParent weak
RC 0 Child
Parent RC 0
4 nil
Unowned Non Optional
Weak
Customer 1 0 nil
CreditCardcustomer
nil
T

46. Static Method Dispatch
->
:

48. struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
drawPoint(point)

49. drawPoint(_:)
point.draw()
struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
point.draw()

50. draw()
print(point.x, point.y)
struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
print(point.x, point.y)

51. struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
print(point.x, point.y)
struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
drawPoint(point)
drawPoint(point) print(point.x, point.y) .
e

52. struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
print(point.x, point.y)
struct Point {
var x: CGFloat
var y: CGFloat
func draw() {
print(x, y)
}
}
func drawPoint(_ point: Point) {
point.draw()
}
let point = Point(x: 0, y: 0)
drawPoint(point)

53. Dynamic Method Dispatch
.

54. Dynamic Method Dispatch
class Drawable {
func draw() {}
}
class Point: Drawable {
var x: CGFloat
var y: CGFloat
// init...
override func draw() { ... }
}
class Line: Drawable {
var x1: CGFloat
var y1: CGFloat
var x2: CGFloat
var y2: CGFloat
// init...
override func draw() { ... }
}
func draw(_ drawable: Drawable) {
drawable.draw()
}

55. Dynamic Method Dispatch
class Drawable {
func draw() {}
}
class Point: Drawable {
var x: CGFloat
var y: CGFloat
// init...
override func draw() { ... }
}
class Line: Drawable {
var x1: CGFloat
var y1: CGFloat
var x2: CGFloat
var y2: CGFloat
// init...
override func draw() { ... }
}
func draw(_ drawable: Drawable) {
drawable.draw()
}
Point.draw?
Line.draw?
Drawable.draw?

56. Dynamic Method Dispatch
1.
2. V-Table
3. V-Table .
.

57. Dynamic Method Dispatch
Thread-Safe
Memory Allocation
,

58. Objective-C Method Dispatch
Objective-C :
// 1
[object foo:parameter];
// 2 : 1
objc_msgSend(object, @selector(foo:), parameter);
->

59. Static Method Dispatch
[final, private ]
[dynamic ]
[Objective-C ]
Objective-C Runtime
[WMO(Whole Module Optimization) ]
Xcode 8

60. Whole-Module Optimization
Module(Swift )
-
Xcode 8
Single-File Optimization
Whole-Module Optimization

61. final class vs. class
class final ?

62. final class SomeClass {
var foo = 3
func bar() {}
}
sil_vtable SomeClass {
#SomeClass.init!allocator.1: (SomeClass.Type) -> () ->
SomeClass : @$s4main9SomeClassCACycfC //
SomeClass.__allocating_init()
#SomeClass.deinit!deallocator.1: @$s4main9SomeClassCfD
// SomeClass.__deallocating_deinit
}

63. class SomeClass {
var foo = 3
func bar() {}
}
class DerivedSomeClass: SomeClass {}
sil_vtable SomeClass {
#SomeClass.foo!getter.1: (SomeClass) -> () ->
Int : @$s4main9SomeClassC3fooSivg //
SomeClass.foo.getter
#SomeClass.foo!setter.1: (SomeClass) -> (Int)
-> () : @$s4main9SomeClassC3fooSivs //
SomeClass.foo.setter
#SomeClass.foo!modify.1: (SomeClass) -> () ->
() : @$s4main9SomeClassC3fooSivM//
SomeClass.foo.modify
#SomeClass.bar!1: (SomeClass) -> () -> () :
@$s4main9SomeClassC3baryyF // SomeClass.bar()
#SomeClass.init!allocator.1: (SomeClass.Type)
-> () -> SomeClass : @$s4main9SomeClassCACycfC
// SomeClass.__allocating_init()
#SomeClass.deinit!deallocator.1:
@$s4main9SomeClassCfD //
SomeClass.__deallocating_deinit
}
alerts

64. class SomeClass {
var foo = 3
func bar() {}
}
class DerivedSomeClass: SomeClass {}
sil_vtable DerivedSomeClass {
#SomeClass.foo!getter.1: (SomeClass) -> () -> Int
: @$s4main9SomeClassC3fooSivg [inherited] //
SomeClass.foo.getter
#SomeClass.foo!setter.1: (SomeClass) -> (Int) ->
() : @$s4main9SomeClassC3fooSivs [inherited] //
SomeClass.foo.setter
#SomeClass.foo!modify.1: (SomeClass) -> () ->
() : @$s4main9SomeClassC3fooSivM [inherited] //
SomeClass.foo.modify
#SomeClass.bar!1: (SomeClass) -> () -> () :
@$s4main9SomeClassC3baryyF [inherited]//
SomeClass.bar()
#SomeClass.init!allocator.1: (SomeClass.Type) ->
() -> SomeClass : @$s4main16DerivedSomeClassCACycfC
[override] // DerivedSomeClass.__allocating_init()
#DerivedSomeClass.deinit!deallocator.1:
@$s4main16DerivedSomeClassCfD //
DerivedSomeClass.__deallocating_deinit
}
i

65. V-Table , final class V-Table
, class V-Table .
var stored property getter, setter, modify
, final class V-Table
.
.

66. final method vs. normal method
final ?

67. class SomeClass {
final func foo() {}
}
sil_vtable SomeClass {
#SomeClass.init!allocator.1:
(SomeClass.Type) -> () -> SomeClass :
@$s4main9SomeClassCACycfC //
SomeClass.__allocating_init()
#SomeClass.deinit!deallocator.1:
@$s4main9SomeClassCfD//
SomeClass.__deallocating_deinit
}
fool

68. sil_vtable SomeClass {
#SomeClass.foo!1: (SomeClass) -> () ->
() : @$s4main9SomeClassC3fooyyF //
SomeClass.foo()
#SomeClass.init!allocator.1:
(SomeClass.Type) -> () -> SomeClass :
@$s4main9SomeClassCACycfC //
SomeClass.__allocating_init()
#SomeClass.deinit!deallocator.1:
@$s4main9SomeClassCfD//
SomeClass.__deallocating_deinit
}
class SomeClass {
func foo() {}
}

69. V-Table , final
V-Table ,
V-Table .
.

70. Memory Allocation Reference Counting Method Dispatch
Stack
Heap
Yes
No
Static
Dynamic

71. Memory Allocation
Heap Stack
Heap Thread-Safe

72. Reference Counting
Thread-Safe , Atomic

73. Method Dispatch

74. Swift
Class
Class
Memory Allocation Heap
Reference Counting Yes
Method Dispatch Dynamic (V-Table)
Reference Semantics

75. Swift
Class
Final Class
Memory Allocation Heap
Reference Counting Yes
Method Dispatch Static

76. Swift
Struct
Reference Type Struct
Memory Allocation Stack
Reference Counting No
Method Dispatch Static

77. Swift
Struct
Reference Type Struct
Memory Allocation Stack
Reference Counting
Yes
(Reference Type )
Method Dispatch Static

78. Swift
Struct
Reference Type Struct
struct Label {
var text: String
var font: UIFont
}
// 1
let font = UIFont.systemFont(ofSize: 15)
// 2
let label = Label(text: "msg", font: font)
// 3
let label2 = label
UIFont Class (Reference Type) -> Reference Counting
String Heap -> Reference Counting

79. Swift
Struct
Reference Type Struct
struct Label {
var text: String
var font: UIFont
}
// 1
let font = UIFont.systemFont(ofSize: 15)
// 2
let label = Label(text: "msg", font: font)
// 3
let label2 = label
...
refCount 3
...
HeapStack
...font
text(storage)
font
text(storage)
font
...
refCount 2
...
label
label2

80. Swift
Struct
Reference Type
struct HTTPRequest {
var `protocol`: String
var domain: String
var path: String
var filename: String
var `extension`: String
var query: [String: String]
var httpMethod: String
var httpVersion: String
var httpHost: String
}

81. Swift
Struct
Reference Type
enum HTTPMethod {
case get
case post
case put
case delete
}
enum HTTPVersion {
case v1
case v2
}
struct HTTPRequest {
var urlString: String
var httpMethod: HTTPMethod
var httpVersion: HTTPVersion
var httpHost: String
}
struct HTTPRequest {
var `protocol`: String
var domain: String
var path: String
var filename: String
var `extension`: String
var query: [String: String]
var httpMethod: String
var httpVersion: String
var httpHost: String
}

82. Swift
Protocol
Protocol
Value Type -> Value Semantics
Protocol-Oriented Programming

83. Swift
Protocol
Protocol Value Type
protocol Drawable { func draw() }
struct Point: Drawable { func draw() { ... } }
struct Line: Drawable { func draw() { ... } }
let drawables: [Drawable] = [Point(), Line()]
for drawable in drawables {
drawable.draw()
}

84. Swift
Protocol
Protocol Type : Memory Allocation
Reference Type (8Byte)
Value Type Stack

85. Swift
Protocol
Protocol Type : Memory Allocation
protocol Drawable {}
struct Point: Drawable {
var a: Int
}
struct Line: Drawable {
var x: Int
var y: Int
}
let drawables: [Drawable] =
[Point(), Line()]
refCount
HeapStack
...
storage
drawables
a x
y
8 Byte
16 Byte

86. Swift
Protocol
Protocol Type : Method Dispatch
Class -> V-Table
Protocol -> V-Table

87. Swift
Protocol
Existential Container
Protocol Type
5 Word (64bit -> 1 Word == 8 Byte)
- 3 Word : Value Buffer
- 2 Word : Buffer (VWT / PWT )
Protocol Type Existential Container
Protocol 3 Word ,
- 3 Word : Existential Container Value Buffer
- 3 Word : Heap ,
Value Buffer

88. Swift
Protocol
Value Witness Table (VWT)
Existential Container /
allocate / copy / destruct / deallocate

89. Swift
Protocol
Value Witness Table (VWT)
3 Word VWT
allocate : X
copy : Existential Container
destruct : Existential Container
deallocate : X

90. Swift
Protocol
Value Witness Table (VWT)
3 Word VWT
allocate : Heap /
Existential Container
copy :
destruct :
deallocate : / Existential Container

91. Swift
Protocol
Protocol Witness Table (PWT)
Method Dispatch :
Protocol PWT

92. Swift
Protocol
Copy
Existential Container
3 Word

93. Swift
Protocol
Copy
3 Word
Existential Container
a: 10
b: 11
-
vwt
pwt
a: 10
b: 11
-
vwt
pwt
Stack

94. Swift
Protocol
Copy
3 Word
: Existential Container Heap
: Heap
-> Existential Container
* Reference Counting ! Reference Count

95. Swift
Protocol
Copy
ref
-
-
vwt
pwt
ref
-
-
vwt
pwt
Stack
a: 10
b: 11
c: 12
d: 13
Heap
3 Word :

96. Swift
Protocol
Copy
ref
-
-
vwt
pwt
ref
-
-
vwt
pwt
Stack
a: 10
b: 11
c: 12
d: 13
Heap
3 Word :
a: 10
b: 11
c: 12
d: 13

97. Swift
Protocol
Indirect Storage
Value Type Reference Type
Value Semantics Copy-on-Write
isKnownUniquelyReferenced(_:)
if !isKnownUniquelyReferenced(&object) {
storage = Storage(storage)
}

98. Swift
Protocol
Existential Container
Protocol Type
Class
- Heap (3 Word )
- Dynamic Method Dispatch
- Class : V-Table
- Protocol : PWT

99. Swift
Protocol
3 Word Protocol Type
Memory Allocation Stack (Existential Container)
Reference Counting No
Method Dispatch Dynamic (PWT)

100. Swift
Protocol
3 Word Protocol Type
Memory Allocation
Stack (Existential Container)
Heap
Reference Counting No
Method Dispatch Dynamic (PWT)

101. Swift
Protocol Type Type ?
Protocol Type Type .

102. protocol SomeProtocol {
var a: Int { get }
var b: Int { get }
}
struct SomeStruct: SomeProtocol {
var a: Int = 10
var b: Int = 11
}
func foo() {
var someStruct1 = SomeStruct()
var someStruct2: SomeProtocol = SomeStruct()
withUnsafeBytes(of: &someStruct1) { print($0) }
withUnsafeBytes(of: &someStruct2) { print($0) }
}
foo()

103. UnsafeRawBufferPointer(start: 0x00007ffeefbff4a0, count: 16)
UnsafeRawBufferPointer(start: 0x00007ffeefbff478, count: 40)
someStruct2 : (7FFEEFBFF478) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
B8 84 00 00 01 00 00 00
38 84 00 00 01 00 00 00
someStruct1 : (7FFEEFBFF4A0) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00

104. sil_witness_table hidden SomeStruct: SomeProtocol module main {
method #SomeProtocol.a!getter.1: <Self where Self : SomeProtocol>
(Self) -> () -> Int :
@$s4main10SomeStructVAA0B8ProtocolA2aDP1aSivgTW // protocol
witness for SomeProtocol.a.getter in conformance SomeStruct
method #SomeProtocol.b!getter.1: <Self where Self : SomeProtocol>
(Self) -> () -> Int :
@$s4main10SomeStructVAA0B8ProtocolA2aDP1bSivgTW // protocol
witness for SomeProtocol.b.getter in conformance SomeStruct
}

105. Swift
3 Word /
.

106. protocol SmallProtocol {
var a: Int { get }
var b: Int { get }
var c: Int { get }
}
protocol LargeProtocol {
var a: Int { get }
var b: Int { get }
var c: Int { get }
var d: Int { get }
}
struct SmallStruct: SmallProtocol {
var a: Int = 10
var b: Int = 11
var c: Int = 12
}
struct LargeStruct: LargeProtocol {
var a: Int = 10
var b: Int = 11
var c: Int = 12
var d: Int = 13
}
func foo() {
var smallStruct: SmallProtocol = SmallStruct()
var largeStruct: LargeProtocol = LargeStruct()
withUnsafeBytes(of: &smallStruct) { print($0) }
withUnsafeBytes(of: &largeStruct) { print($0) }
}
foo()

107. largeStruct : (7FFEEFBFF460) : 60 D1 54 00 01 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
90 85 00 00 01 00 00 00
88 84 00 00 01 00 00 00
smallStruct : (7FFEEFBFF488) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
10 85 00 00 01 00 00 00
68 84 00 00 01 00 00 00

108. (10054D160) : 50 84 00 00 01 00 00 00
02 00 00 00 00 00 00 00
0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
0D 00 00 00 00 00 00 00

109. Swift
3 Word / Copy
Copy .

110. Swift
3 Word / Copy
Copy .

111. protocol SmallProtocol {
var a: Int { get set }
var b: Int { get set }
var c: Int { get set }
}
struct SmallStruct: SmallProtocol {
var a: Int = 10
var b: Int = 11
var c: Int = 12
}
func foo() {
var bar1: SmallProtocol = SmallStruct()
var bar2 = bar1
// breakpoint 1
bar2.a = 11
// breakpoint 2
withUnsafeBytes(of: &bar1) { print($0) }
withUnsafeBytes(of: &bar2) { print($0) }
}
foo()
3 Word Protocol Type

112. // breakpoint 1
bar2 : (7FFEEFBFF460) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
28 85 00 00 01 00 00 00
50 84 00 00 01 00 00 00
bar1 : (7FFEEFBFF488) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
28 85 00 00 01 00 00 00
50 84 00 00 01 00 00 00
3 Word Protocol Type
// breakpoint 2
bar2 : (7FFEEFBFF460) : 0B 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
28 85 00 00 01 00 00 00
50 84 00 00 01 00 00 00
bar1 : (7FFEEFBFF488) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
28 85 00 00 01 00 00 00
50 84 00 00 01 00 00 00

113. protocol LargeProtocol {
var a: Int { get set }
var b: Int { get set }
var c: Int { get set }
var d: Int { get set }
}
struct LargeStruct: LargeProtocol {
var a: Int = 10
var b: Int = 11
var c: Int = 12
var d: Int = 13
}
func foo() {
var bar1: LargeProtocol = LargeStruct()
var bar2 = bar1
// breakpoint 1
bar2.a = 11
// breakpoint 2
withUnsafeBytes(of: &bar1) { print($0) }
withUnsafeBytes(of: &bar2) { print($0) }
}
foo()
3 Word Protocol Type

114. // breakpoint 1
bar2 : (7FFEEFBFF460) : 90 B2 B3 03 01 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
48 85 00 00 01 00 00 00
80 84 00 00 01 00 00 00
bar1 : (7FFEEFBFF488) : 90 B2 B3 03 01 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
48 85 00 00 01 00 00 00
80 84 00 00 01 00 00 00
3 Word Protocol Type
(103B3B290) : 0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
0D 00 00 00 00 00 00 00

115. // breakpoint 2
bar2 : (7FFEEFBFF460) : 70 B8 B3 03 01 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
48 85 00 00 01 00 00 00
80 84 00 00 01 00 00 00
bar1 : (7FFEEFBFF488) : 90 B2 B3 03 01 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
48 85 00 00 01 00 00 00
80 84 00 00 01 00 00 00
3 Word Protocol Type
(103B3B290) : 68 84 00 00 01 00 00 00
02 00 00 00 00 00 00 00
0A 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
0D 00 00 00 00 00 00 00
(103B3B870) : 78 19 B3 94 FF 7F 00 00
02 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00
0D 00 00 00 00 00 00 00

116. Swift
Protocol
3 Word Protocol Type vs. Indirect Storage
3 Word Protocol Type Indirect Storage
Heap
Reference Counting : X
Copy-on-Write -> Value Semantics
Heap
Reference Counting : O
Value Semantics
Copy-on-Write

117. Swift
Generics
3 Word Protocol Type vs. Indirect Storage
protocol Drawable { func draw() }
struct Point: Drawable { func draw() { ... } }
struct Line: Drawable { func draw() { ... } }
func draw<T: Drawable>(_ drawable: T) {
drawable.draw()
}
draw(Point())
draw(Line())
Generic Type T Drawable
Existential Container
T :
->

118. Swift
Generics
3 Word Protocol Type vs. Indirect Storage
protocol Drawable { func draw() }
struct Point: Drawable { func draw() { ... } }
struct Line: Drawable { func draw() { ... } }
func draw<T: Drawable>(_ drawable: T) {
drawable.draw()
}
draw(Point())
draw(Line())
Generic Specialization
Existential Container
->
func draw(_ point: Point) {
point.draw()
}
func draw(_ line: Line) {
line.draw()
}
draw(Point())
draw(Line())

119. Swift
?
.

120. protocol SomeProtocol {
var a: Int { get }
}
struct SomeStruct1: SomeProtocol {
var a: Int = 10
}
struct SomeStruct2: SomeProtocol {
var a: Int = 11
var b: Int = 12
}
func bar<T: SomeProtocol>(_ bar: T) {
var bar = bar
print(type(of: bar))
withUnsafeBytes(of: &bar) { print($0) }
}
func foo() {
bar(SomeStruct1())
bar(SomeStruct2())
}
foo()

121. SomeStruct1
UnsafeRawBufferPointer(start: 0x00007ffeefbff380, count: 8)
SomeStruct2
UnsafeRawBufferPointer(start: 0x00007ffeefbff380, count: 16)
(7FFEEFBFF380) : 0A 00 00 00 00 00 00 00
(7FFEEFBFF380) : 0B 00 00 00 00 00 00 00
0C 00 00 00 00 00 00 00

122. Swift
Generics
Generics (3 Word Protocol Type)
Memory Allocation Stack (Existential Container)
Reference Counting No
Method Dispatch Dynamic (PWT)

123. Swift
Generics
Generics (3 Word Protocol Type)
Memory Allocation
Stack (Existential Container)
Heap
Reference Counting No
Method Dispatch Dynamic (PWT)

124. Swift
Generics
Generics (Struct)
Memory Allocation Stack
Reference Counting No
Method Dispatch Static

125. Swift
Generics
Generics (Class)
Memory Allocation Heap
Reference Counting Yes
Method Dispatch Dynamic (V-Table)

126. Swift
Generics
(Static Polymorphism)
(Specialization)

127. Value Type Protocol Type
/

128. Struct : Value Semantics
Class : Identity / OOP / Objective-C Compatible
Generics : Static Polymorphism
Protocol : Dynamic Polymorphism

129. Struct Reference Type
- Value Type
- Reference Counting
Protocol Type Struct
- Indirect Storage Struct
- Mutable Copy-on-Write
Method Dispatch : Dynamic -> Static
- final / private
- dynamic X
- Objective-C
- WMO

130. .

131. Reference
[Swift : Value , Protocol ], , 2016.
https://academy.realm.io/kr/posts/letswift-swift-performance/
[Cocoa Internals], , 2017.
[Swift Copy-on-Write (CoW)], OAKSONG, 2018.
https://oaksong.github.io/2018/01/06/copy-on-write/
[Whole-Module Optimization in Swift 3], Erik Eckstein, 2016
https://swift.org/blog/whole-module-optimizations/
https://github.com/presto95/Study/blob/master/Swift/Whole-
Module%20Optimization.md

132. .