Swift解惑

 struct HeapObject {
    var Kind: UInt64
    var refcount: UInt64
 }
 
 // 负责包装的结构体,也就是用来包装捕获需要更新的值
 struct Box {
    var refCounted: HeapObject
    // 这个捕获的值的类型根据捕获的值进行分配，此处规范操作是写泛型
    // var value: Int
    var value: <T>
}

// The members of the HeapObject header that are not shared by a
// standard Objective-C instance
#define SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS       \
  InlineRefCounts refCounts

/// The Swift heap-object header.
/// This must match RefCountedStructTy in IRGen.
struct HeapObject {
  /// This is always a valid pointer to a metadata object.
  HeapMetadata const *__ptrauth_objc_isa_pointer metadata;

  SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS;

#ifndef __swift__
  HeapObject() = default;

  // Initialize a HeapObject header as appropriate for a newly-allocated object.
  constexpr HeapObject(HeapMetadata const *newMetadata) 
    : metadata(newMetadata)
    , refCounts(InlineRefCounts::Initialized)
  { }
  
  // Initialize a HeapObject header for an immortal object
  constexpr HeapObject(HeapMetadata const *newMetadata,
                       InlineRefCounts::Immortal_t immortal)
  : metadata(newMetadata)
  , refCounts(InlineRefCounts::Immortal)
  { }

#ifndef NDEBUG
  void dump() const SWIFT_USED;
#endif

#endif // __swift__
};

  //Objects initially start with no side table. They can gain a side table when:
  //* a weak reference is formed and pending future implementation:
  //* strong RC or unowned RC overflows (inline RCs will be small on 32-bit)
  //* associated object storage is needed on an object
  //* etc
  //Gaining a side table entry is a one-way operation; an object with a side 
  //table entry never loses it. This prevents some thread races.

  //Strong and unowned variables point at the object.
  //Weak variables point at the object's side table.


  //Storage layout:

  HeapObject {
    isa
    InlineRefCounts {
      atomic<InlineRefCountBits> {
        strong RC + unowned RC + flags
        OR
        HeapObjectSideTableEntry*
      }
    }
  }

  HeapObjectSideTableEntry {
    SideTableRefCounts {
      object pointer
      atomic<SideTableRefCountBits> {
        strong RC + unowned RC + weak RC + flags
      }
    }   
  }

  Object lifecycle state machine:

  LIVE without side table
  The object is alive.
  Object's refcounts are initialized as 1 strong, 1 unowned, 1 weak.
  No side table. No weak RC storage.
  Strong variable operations work normally. 
  Unowned variable operations work normally.
  Weak variable load can't happen.
  Weak variable store adds the side table, becoming LIVE with side table.
  When the strong RC reaches zero deinit() is called and the object 
    becomes DEINITING.

  LIVE with side table
  Weak variable operations work normally.
  Everything else is the same as LIVE.

  DEINITING without side table
  deinit() is in progress on the object.
  Strong variable operations have no effect.
  Unowned variable load halts in swift_abortRetainUnowned().
  Unowned variable store works normally.
  Weak variable load can't happen.
  Weak variable store stores nil.
  When deinit() completes, the generated code calls swift_deallocObject. 
    swift_deallocObject calls canBeFreedNow() checking for the fast path 
    of no weak or unowned references. 
    If canBeFreedNow() the object is freed and it becomes DEAD. 
    Otherwise, it decrements the unowned RC and the object becomes DEINITED.

  DEINITING with side table
  Weak variable load returns nil. 
  Weak variable store stores nil.
  canBeFreedNow() is always false, so it never transitions directly to DEAD.
  Everything else is the same as DEINITING.

  DEINITED without side table
  deinit() has completed but there are unowned references outstanding.
  Strong variable operations can't happen.
  Unowned variable store can't happen.
  Unowned variable load halts in swift_abortRetainUnowned().
  Weak variable operations can't happen.
  When the unowned RC reaches zero, the object is freed and it becomes DEAD.

  DEINITED with side table
  Weak variable load returns nil.
  Weak variable store can't happen.
  When the unowned RC reaches zero, the object is freed, the weak RC is 
    decremented, and the object becomes FREED.
  Everything else is the same as DEINITED.

  FREED without side table
  This state never happens.

  FREED with side table
  The object is freed but there are weak refs to the side table outstanding.
  Strong variable operations can't happen.
  Unowned variable operations can't happen.
  Weak variable load returns nil.
  Weak variable store can't happen.
  When the weak RC reaches zero, the side table entry is freed and 
    the object becomes DEAD.

  DEAD
  The object and its side table are gone.

store %6 to %3 : $*Array<Any>                   // id: %7
%8 = alloc_stack $Any                           // users: %17, %15, %11
%9 = integer_literal $Builtin.Int64, 1          // user: %10
%10 = struct $Int (%9 : $Builtin.Int64)         // user: %12
%11 = init_existential_addr %8 : $*Any, $Int    // user: %12
store %10 to %11 : $*Int                        // id: %12
%13 = begin_access [modify] [dynamic] %3 : $*Array<Any> // users: %16, %15
// function_ref Array.append(_:)
%14 = function_ref @Swift.Array.append(__owned A) -> () : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> () // user: %15
%15 = apply %14<Any>(%8, %13) : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> ()
end_access %13 : $*Array<Any>                   // id: %16
dealloc_stack %8 : $*Any                        // id: %17
%18 = alloc_stack $Any                          // users: %32, %30, %19
%19 = init_existential_addr %18 : $*Any, $(Int, Int) // users: %21, %20
%20 = tuple_element_addr %19 : $*(Int, Int), 0  // user: %24
%21 = tuple_element_addr %19 : $*(Int, Int), 1  // user: %27
%22 = integer_literal $Builtin.Int64, 100       // user: %23
%23 = struct $Int (%22 : $Builtin.Int64)        // user: %24
store %23 to %20 : $*Int                        // id: %24
%25 = integer_literal $Builtin.Int64, 200       // user: %26
%26 = struct $Int (%25 : $Builtin.Int64)        // user: %27
store %26 to %21 : $*Int                        // id: %27
%28 = begin_access [modify] [dynamic] %3 : $*Array<Any> // users: %31, %30
// function_ref Array.append(_:)
%29 = function_ref @Swift.Array.append(__owned A) -> () : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> () // user: %30
%30 = apply %29<Any>(%18, %28) : $@convention(method) <τ_0_0> (@in τ_0_0, @inout Array<τ_0_0>) -> ()
end_access %28 : $*Array<Any>                   // id: %31
dealloc_stack %18 : $*Any                       // id: %32
%33 = alloc_stack $Any                          // users: %43, %41, %37
%34 = metatype $@thick XX.Type                  // user: %36
// function_ref XX.__allocating_init()
%35 = function_ref @A.XX.__allocating_init() -> A.XX : $@convention(method) (@thick XX.Type) -> @owned XX // user: %36
%36 = apply %35(%34) : $@convention(method) (@thick XX.Type) -> @owned XX // user: %38
%37 = init_existential_addr %33 : $*Any, $XX    // user: %38
store %36 to %37 : $*XX                         // id: %38
%39 = begin_access [modify] [dynamic] %3 : $*Array<Any> // users: %42, %41

// 1.泛型
func tFoo<T: Equatable>() -> T {
    return 42 as! T
}

// 2.some
func someFoo() -> some Equatable {
    return 42
}

// 3.any
func anyFoo() -> any Equatable {
    return 42
}

 // ✅ No compile error when changing the underlying data type
 var myCar: any Vehicle = Car()
 myCar = Bus()
 myCar = Car()
 ​
  // 🔴 Compile error in Swift 5.7: Use of protocol 'Vehicle' as a type must be written 'any Vehicle' 
 func wash(_ vehicle: Vehicle)  {
     // Wash the given vehicle
 }
 ​
 // ✅ No compile error in Swift 5.7
 func wash(_ vehicle: any Vehicle)  {
     // Wash the given vehicle
 }

 // 🔴 Compile error in Swift 5.7: Use of protocol 'Vehicle' as a type must be written 'any Vehicle'
// 一个函数不能返回多种类型结果，而`some`在编译期就可以确定类型，所以编译失败
 func createVehicle(isPublicTransport: Bool) -> some Vehicle {
    if isPublicTransport {
      return Bus()
    } else {
      return Car()
    }
 }
 ​
 // ✅ No compile error when returning different kind of concrete type 
func createAnyVehicle(isPublicTransport: Bool) -> any Vehicle {
    if isPublicTransport {
      return Bus()
    } else {
      return Car()
    }
}

sil_stage canonical

import Builtin
import Swift
import SwiftShims

import Foundation

@objc @_inheritsConvenienceInitializers class Foo : NSObject {
  @objc func bar()
  override dynamic init()
  @objc deinit
}

// Foo.bar()
sil hidden @Foo.Foo.bar() -> () : $@convention(method) (@guaranteed Foo) -> () {
// %0 "self"                                      // user: %1
bb0(%0 : $Foo):
  debug_value %0 : $Foo, let, name "self", argno 1, implicit // id: %1
  %2 = integer_literal $Builtin.Word, 1           // user: %4
  // function_ref _allocateUninitializedArray<A>(_:)
  %3 = function_ref @Swift._allocateUninitializedArray<A>(Builtin.Word) -> ([A], Builtin.RawPointer) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // user: %4
  %4 = apply %3<Any>(%2) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // users: %6, %5
  %5 = tuple_extract %4 : $(Array<Any>, Builtin.RawPointer), 0 // user: %17
  %6 = tuple_extract %4 : $(Array<Any>, Builtin.RawPointer), 1 // user: %7
  %7 = pointer_to_address %6 : $Builtin.RawPointer to [strict] $*Any // user: %14
  %8 = string_literal utf8 "Hello from Swift!"    // user: %13
  %9 = integer_literal $Builtin.Word, 17          // user: %13
  %10 = integer_literal $Builtin.Int1, -1         // user: %13
  %11 = metatype $@thin String.Type               // user: %13
  // function_ref String.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:)
  %12 = function_ref @Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String // user: %13
  %13 = apply %12(%8, %9, %10, %11) : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String // user: %15
  %14 = init_existential_addr %7 : $*Any, $String // user: %15
  store %13 to %14 : $*String                     // id: %15
  // function_ref _finalizeUninitializedArray<A>(_:)
  %16 = function_ref @Swift._finalizeUninitializedArray<A>(__owned [A]) -> [A] : $@convention(thin) <τ_0_0> (@owned Array<τ_0_0>) -> @owned Array<τ_0_0> // user: %17
  %17 = apply %16<Any>(%5) : $@convention(thin) <τ_0_0> (@owned Array<τ_0_0>) -> @owned Array<τ_0_0> // users: %26, %23
  // function_ref default argument 1 of print(_:separator:terminator:)
  %18 = function_ref @default argument 1 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %19
  %19 = apply %18() : $@convention(thin) () -> @owned String // users: %25, %23
  // function_ref default argument 2 of print(_:separator:terminator:)
  %20 = function_ref @default argument 2 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: %21
  %21 = apply %20() : $@convention(thin) () -> @owned String // users: %24, %23
  // function_ref print(_:separator:terminator:)
  %22 = function_ref @Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> () // user: %23
  %23 = apply %22(%17, %19, %21) : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> ()
  release_value %21 : $String                     // id: %24
  release_value %19 : $String                     // id: %25
  release_value %17 : $Array<Any>                 // id: %26
  %27 = tuple ()                                  // user: %28
  return %27 : $()                                // id: %28
} // end sil function 'Foo.Foo.bar() -> ()'

// @objc Foo.bar()
sil private [thunk] @@objc Foo.Foo.bar() -> () : $@convention(objc_method) (Foo) -> () {
// %0                                             // users: %4, %3, %1
bb0(%0 : $Foo):
  strong_retain %0 : $Foo                         // id: %1
  // function_ref Foo.bar()
  %2 = function_ref @Foo.Foo.bar() -> () : $@convention(method) (@guaranteed Foo) -> () // user: %3
  %3 = apply %2(%0) : $@convention(method) (@guaranteed Foo) -> () // user: %5
  strong_release %0 : $Foo                        // id: %4
  return %3 : $()                                 // id: %5
} // end sil function '@objc Foo.Foo.bar() -> ()'

sil_vtable Foo {
  #Foo.bar: (Foo) -> () -> () : @Foo.Foo.bar() -> ()	// Foo.bar()
  #Foo.deinit!deallocator: @Foo.Foo.__deallocating_deinit	// Foo.__deallocating_deinit
}