[go]go接口裝箱-其它類型轉換爲interface類型

Interface實現原理分析

具體類型轉換成接口類型

到此已經知道什麼是接口以及接口的底層結構，那麼當具體類型賦值給接口類型時，是如何進行轉換的？再來看下2.3中的示例

package main

import "fmt"

type Runner interface {
  Run()
}

type Person struct {
  Name string
}

func (p Person) Run() {
  fmt.Printf("%s is running\n", p.Name)
}

func main() {
  var r Runner
  r = Person{Name: "song_chh"}
  r.Run()
}

通過Go提供的工具生成彙編代碼

go tool compile -S interface.go

只截取與第19行相關的代碼

0x001d 00029 (interface.go:19)  PCDATA  $2, $0
0x001d 00029 (interface.go:19)  PCDATA  $0, $1
0x001d 00029 (interface.go:19)  XORPS  X0, X0
0x0020 00032 (interface.go:19)  MOVUPS  X0, ""..autotmp_1+32(SP)
0x0025 00037 (interface.go:19)  PCDATA  $2, $1
0x0025 00037 (interface.go:19)  LEAQ  go.string."song_chh"(SB), AX
0x002c 00044 (interface.go:19)  PCDATA  $2, $0
0x002c 00044 (interface.go:19)  MOVQ  AX, ""..autotmp_1+32(SP)
0x0031 00049 (interface.go:19)  MOVQ  $8, ""..autotmp_1+40(SP)
0x003a 00058 (interface.go:19)  PCDATA  $2, $1
0x003a 00058 (interface.go:19)  LEAQ  go.itab."".Person,"".Runner(SB), AX
0x0041 00065 (interface.go:19)  PCDATA  $2, $0
0x0041 00065 (interface.go:19)  MOVQ  AX, (SP)
0x0045 00069 (interface.go:19)  PCDATA  $2, $1
0x0045 00069 (interface.go:19)  PCDATA  $0, $0
0x0045 00069 (interface.go:19)  LEAQ  ""..autotmp_1+32(SP), AX
0x004a 00074 (interface.go:19)  PCDATA  $2, $0
0x004a 00074 (interface.go:19)  MOVQ  AX, 8(SP)
0x004f 00079 (interface.go:19)  CALL  runtime.convT2I(SB)
0x0054 00084 (interface.go:19)  MOVQ  16(SP), AX
0x0059 00089 (interface.go:19)  PCDATA  $2, $2
0x0059 00089 (interface.go:19)  MOVQ  24(SP), CX

可以看到，編譯器在構造itab後調用runtime.convT2I(SB)轉換函數，看下函數的實現

//runtime/iface.go
func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
  t := tab._type
  if raceenabled {
    raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I))
  }
  if msanenabled {
    msanread(elem, t.size)
  }
  x := mallocgc(t.size, t, true)
  typedmemmove(t, x, elem)
  i.tab = tab
  i.data = x
  return
}

首先根據類型大小調用mallocgc申請一塊內存空間，將elem指針的內容拷貝到新空間，將tab賦值給iface的tab，將新內存指針賦值給iface的data，這樣一個iface就創建完成

將示例代碼稍作更改，使用結構體指針類型的變量賦值給接口變量

r = &Person{Name: "song_chh"}

再次通過工具生成彙編代碼

go tool compile -S interface.go

0x001d 00029 (interface.go:19)  PCDATA  $2, $1
0x001d 00029 (interface.go:19)  PCDATA  $0, $0
0x001d 00029 (interface.go:19)  LEAQ  type."".Person(SB), AX
0x0024 00036 (interface.go:19)  PCDATA  $2, $0
0x0024 00036 (interface.go:19)  MOVQ  AX, (SP)
0x0028 00040 (interface.go:19)  CALL  runtime.newobject(SB)
0x002d 00045 (interface.go:19)  PCDATA  $2, $2
0x002d 00045 (interface.go:19)  MOVQ  8(SP), DI
0x0032 00050 (interface.go:19)  MOVQ  $8, 8(DI)
0x003a 00058 (interface.go:19)  PCDATA  $2, $-2
0x003a 00058 (interface.go:19)  PCDATA  $0, $-2
0x003a 00058 (interface.go:19)  CMPL  runtime.writeBarrier(SB), $0
0x0041 00065 (interface.go:19)  JNE  105
0x0043 00067 (interface.go:19)  LEAQ  go.string."song_chh"(SB), AX
0x004a 00074 (interface.go:19)  MOVQ  AX, (DI)

首先編譯器通過type."".Person(SB)獲取Person結構體類型，作爲參數調用runtime.newobject()函數，同樣的在源碼中查看函數定義

import "unsafe"

// runtime/malloc.go

// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function
func newobject(typ *_type) unsafe.Pointer {
  return mallocgc(typ.size, typ, true)
}

newobject以*Person作爲入參，創建新的Person結構體指針，之後由編譯器設置值，iface由編譯器直接生成
除了convT2I函數外，其實在runtime/runtime.go文件中，還有很多轉換函數的定義

// Non-empty-interface to non-empty-interface conversion.
func convI2I(typ *byte, elem any) (ret any)

// Specialized type-to-interface conversion.
// These return only a data pointer.
func convT16(val any) unsafe.Pointer     // val must be uint16-like (same size and alignment as a uint16)
func convT32(val any) unsafe.Pointer     // val must be uint32-like (same size and alignment as a uint32)
func convT64(val any) unsafe.Pointer     // val must be uint64-like (same size and alignment as a uint64 and contains no pointers)
func convTstring(val any) unsafe.Pointer // val must be a string
func convTslice(val any) unsafe.Pointer  // val must be a slice

// Type to empty-interface conversion.
func convT2E(typ *byte, elem *any) (ret any)
func convT2Enoptr(typ *byte, elem *any) (ret any)

// Type to non-empty-interface conversion.   
func convT2I(tab *byte, elem *any) (ret any)        //for the general case
func convT2Inoptr(tab *byte, elem *any) (ret any)   //for structs that do not contain pointers

convI2I用於接口轉換成另一個接口時調用，在3.4會進行講解

convT2Inoptr用於變量內部不含指針的轉換，noptr可以理解爲no pointer，轉換過程與convT2I類似

convT16、convT32、convT64、convTstring 和 convTslice是針對簡單類型轉接口的特例優化，有興趣的可以看下函數實現的源碼，因爲這幾個函數內容相似，這裏就簡單介紹下convT64

//runtime/iface.go
func convT64(val uint64) (x unsafe.Pointer) {
  if val == 0 {
    x = unsafe.Pointer(&zeroVal[0])
  } else {
    x = mallocgc(8, uint64Type, false)
    *(*uint64)(x) = val
  }
  return
}

相較於convT2系列函數，缺少typedmemmove和memmove函數的調用，減少內存拷貝。另外如果變量值爲該類型的零值，則不會調用 mallocgc 去申請一塊新內存，而是直接返回指向zeroVal[0]的指針

再來看下，空接口轉換函數convT2E

func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
  if raceenabled {
    raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E))
  }
  if msanenabled {
    msanread(elem, t.size)
  }
  x := mallocgc(t.size, t, true)
  // TODO: We allocate a zeroed object only to overwrite it with actual data.
  // Figure out how to avoid zeroing. Also below in convT2Eslice, convT2I, convT2Islice.
  typedmemmove(t, x, elem)
  e._type = t
  e.data = x
  return
}

接口與接口的轉換

如果某個類型實現多個接口，那接口直接是如何進行轉換的，還是先看一段示例：

package main

import "fmt"

type Runner interface {
  Run()
  Say()
}

type Sayer interface {
  Say()
}

type Person struct {
  Name string
}

func (p Person) Run() {
  fmt.Printf("%s is running\n", p.Name)
}

func (p Person) Say() {
  fmt.Printf("hello, %s", p.Name)
}

func main() {
  var r Runner
  r = Person{Name: "song_chh"}

  var s Sayer
  s = r
  s.Say()
}

增加Sayer接口定義，包含Say()方法，在main函數中聲明一個Sayer變量，並將Runner接口變量賦值給Sayer變量。因爲Person實現了Say()方法，所以說Person既實現了是Runner接口，又實現了Sayer接口

執行命令

go tool compile -S interface.go

截取32行彙編代碼

0x0062 00098 (interface.go:32)  PCDATA  $2, $3
0x0062 00098 (interface.go:32)  LEAQ    type."".Sayer(SB), DX
0x0069 00105 (interface.go:32)  PCDATA  $2, $2
0x0069 00105 (interface.go:32)  MOVQ    DX, (SP) 
0x006d 00109 (interface.go:32)  MOVQ    AX, 8(SP)
0x0072 00114 (interface.go:32)  PCDATA  $2, $0
0x0072 00114 (interface.go:32)  MOVQ    CX, 16(SP)
0x0077 00119 (interface.go:32)  CALL    runtime.convI2I(SB)
0x007c 00124 (interface.go:32)  MOVQ    24(SP), AX
0x0081 00129 (interface.go:32)  PCDATA  $2, $2
0x0081 00129 (interface.go:32)  MOVQ    32(SP), CX

可以看到在執行期間，調用runtime.convI2I進行接口轉換，接下來看下源代碼

func convI2I(inter *interfacetype, i iface) (r iface) {
  tab := i.tab
  if tab == nil {
    return
  }
  if tab.inter == inter {
    r.tab = tab
    r.data = i.data
    return
  }
  r.tab = (inter, tab._type, false)
  r.data = i.data
  return
}

函數參數inter表示接口的類型，由編譯器生成，即type."".Sayer(SB)，i 是綁定實體的接口， r 是轉換後新的接口，如果要轉換的接口是同一類型，則直接把 i 的tab和data給新接口 r ，將 r 返回。如果要轉換的接口不是同一類型，則通過getitab生成一個新的tab複製給r.tab，然後將 r 返回

那麼具體來看一下getitab這個函數，還是先看源碼

func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
  if len(inter.mhdr) == 0 {
    throw("internal error - misuse of itab")
  }

  // easy case
  if typ.tflag&tflagUncommon == 0 {
    if canfail {
      return nil
    }
    name := inter.typ.nameOff(inter.mhdr[0].name)
    panic(&TypeAssertionError{nil, typ, &inter.typ, name.name()})
  }

  var m *itab

  // First, look in the existing table to see if we can find the itab we need.
  // This is by far the most common case, so do it without locks.
  // Use atomic to ensure we see any previous writes done by the thread
  // that updates the itabTable field (with atomic.Storep in itabAdd).
  t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
  if m = t.find(inter, typ); m != nil {
    goto finish
  }

  // Not found.  Grab the lock and try again.
  lock(&itabLock)
  if m = itabTable.find(inter, typ); m != nil {
    unlock(&itabLock)
    goto finish
  }

  // Entry doesn't exist yet. Make a new entry & add it.
  m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*sys.PtrSize, 0, &memstats.other_sys))
  m.inter = inter
  m._type = typ
  m.init()
  itabAdd(m)
  unlock(&itabLock)
finish:
  if m.fun[0] != 0 {
    return m
  }
  if canfail {
    return nil
  }
  // this can only happen if the conversion
  // was already done once using the , ok form
  // and we have a cached negative result.
  // The cached result doesn't record which
  // interface function was missing, so initialize
  // the itab again to get the missing function name.
  panic(&TypeAssertionError{concrete: typ, asserted: &inter.typ, missingMethod: m.init()})
}

首先，用 t 保存全局itabTable的地址，使用t.find進行查找，這個是通用的查找

如果沒有查找到，就會上鎖，重新使用itabTable.find進行查找

再沒有找到，就會根據具體類型typ和接口類型inter生成一個itab，並將這個新生成的itab添加到全局的itabTable中。如果具體類型並沒有實現接口，根據canfail值返回nil或者painc