0x00 相关概念

 在go语言里面interface用于定义一组行为，一个具体的类型只要实现了这一组行为，那么就可以称这个具体的类型实现了这个接口。下面是interface结构体

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type interfacetype struct {
	typ     _type    		// 通用底层数据结构，任何一个类型的底层实现，都基于_type这一数据结构来实现
	pkgpath name      		// 这是一个指向typename结构的地址
	mhdr    []imethod       // 这个里面就是interface所定义的方法
}

imethod结构体定义如下:

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type imethod struct {
	name nameOff 		// int32类型，相对于firstmoduledata.type的偏移量，计算后的地址指向一个type name结构体
	ityp typeOff        // int32类型，相对于firstmoduledata.type的偏移量，方法声明的信息，计算后执行一个func type结构体
}

 在go语言里面interface和其他类型之间的映射关系是通过interface映射表来实现的(itab)(有点类似C++里面的vtab), itab的结构体如下：

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type itab struct {
	inter *interfacetype   			//  指向一个interface类型的定义
	_type *_type  					// 指向一个普通数据类型，另外这个普通类型_type 还需要实现上inter接口所有的方法
	hash  uint32 // copy of _type.hash. Used for type switches.
	_     [4]byte
	// fun 指向一组方法，	指向的上具体类型_type(上面第二个字段)的方法
	// 这个指针数组是可以变化的，编译器负责填充，在下面的例子我们将会看到这一点
	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
}

0x01 接口调用过程分析

 一段简单的demo代码如下：

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package main

type I interface{
	DoSomeWork()
}

type T struct{
	a int
}

func(t *T)DoSomeWork(){

}


func main(){
	t := &T{}
	i := I(t)
	print(i)
}

生成AST语法树如下:

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before (*T).DoSomeWork
after walk (*T).DoSomeWork

before main
.   DCL l(17)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS l(17) colas(1) tc(1)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T
.   .   PTRLIT l(17) esc(no) ld(1) tc(1) PTR64-*main.T
.   .   .   STRUCTLIT l(17) tc(1) main.T
.   .   .   .   TYPE <S> l(17) tc(1) implicit(1) type=PTR64-*main.T PTR64-*main.T

.   DCL l(18)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I

.   AS l(18) tc(1)
.   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS l(18) colas(1) tc(1)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   .   CONVIFACE l(18) tc(1) main.I
.   .   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

.   VARKILL l(18) tc(1)
.   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

.   PRINT l(19) tc(1)
.   PRINT-list
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
after walk main
.   DCL l(17)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS-init
.   .   AS l(17) tc(1)
.   .   .   NAME-main.autotmp_0002 u(1) a(1) l(17) x(0+0) class(PAUTO) esc(N) tc(1) used(1) main.T

.   .   AS l(17) tc(1)
.   .   .   NAME-main.autotmp_0001 u(1) a(1) l(17) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T
.   .   .   ADDR u(2) l(17) tc(1) PTR64-*main.T
.   .   .   .   NAME-main.autotmp_0002 u(1) a(1) l(17) x(0+0) class(PAUTO) esc(N) tc(1) used(1) main.T

.   .   AS u(2) l(17) tc(1)
.   .   .   IND u(2) l(17) tc(1) main.T
.   .   .   .   NAME-main.autotmp_0001 u(1) a(1) l(17) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T
.   AS u(100) l(17) tc(1)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T
.   .   NAME-main.autotmp_0001 u(1) a(1) l(17) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

.   DCL l(18)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I

.   AS u(2) l(18) tc(1)
.   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS-init
.   .   AS l(18) tc(1)
.   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   NAME-go.itab.*"".T."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) used(1) PTR64-*uint8

.   .   IF l(18) tc(1)
.   .   IF-test
.   .   .   EQ l(18) tc(1) bool
.   .   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   .   LITERAL-nil u(1) a(1) l(18) tc(1) PTR64-*uint8
.   .   IF-body
.   .   .   AS l(18) tc(1)
.   .   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   .   CALLFUNC u(100) l(18) tc(1) PTR64-*byte
.   .   .   .   .   NAME-runtime.typ2Itab u(1) a(1) l(2) x(0+0) class(PFUNC) tc(1) used(1) FUNC-funcSTRUCT-(FIELD-
.   .   .   .   .   .   NAME-runtime.typ·2 u(1) a(1) l(2) x(0+0) class(PPARAM) f(1) PTR64-*byte PTR64-*byte, FIELD-
.   .   .   .   .   .   NAME-runtime.typ2·3 u(1) a(1) l(2) x(8+0) class(PPARAM) f(1) PTR64-*byte PTR64-*byte, FIELD-
.   .   .   .   .   .   NAME-runtime.cache·4 u(1) a(1) l(2) x(16+0) class(PPARAM) f(1) PTR64-*PTR64-*byte PTR64-*PTR64-*byte) PTR64-*byte
.   .   .   .   CALLFUNC-list
.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(0+0) tc(1) runtime.typ·2 G0 PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*uint8
.   .   .   .   .   .   .   NAME-type.*"".T u(1) a(1) l(11) x(0+0) class(PEXTERN) tc(1) uint8

.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(8+0) tc(1) runtime.typ2·3 G0 PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*uint8
.   .   .   .   .   .   .   NAME-type."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) uint8

.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(16+0) tc(1) runtime.cache·4 G0 PTR64-*PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*PTR64-*uint8
.   .   .   .   .   .   .   NAME-go.itab.*"".T."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) used(1) PTR64-*uint8
.   AS u(100) l(18) tc(1)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   .   EFACE u(2) l(18) tc(1) main.I
.   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

.   VARKILL l(18) tc(1)
.   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

.   EMPTY-init
.   .   CALLFUNC u(100) l(19) tc(1)
.   .   .   NAME-runtime.printiface u(1) a(1) l(2) x(0+0) class(PFUNC) tc(1) used(1) FUNC-funcSTRUCT-(FIELD-main.I)
.   .   CALLFUNC-list
.   .   .   AS u(1) l(19) tc(1)
.   .   .   .   INDREG-SP a(1) l(19) x(0+0) tc(1) main.I
.   .   .   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   EMPTY u(100) l(19) tc(1)

before init
.   IF l(20) tc(1)
.   IF-test
.   .   NE l(20) tc(1) bool
.   .   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   .   LITERAL-0 u(1) a(1) l(20) tc(1) uint8
.   IF-body
.   .   IF l(20) tc(1)
.   .   IF-test
.   .   .   EQ l(20) tc(1) bool
.   .   .   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   .   .   LITERAL-2 u(1) a(1) l(20) tc(1) uint8
.   .   IF-body
.   .   .   RETURN l(20) tc(1)

.   .   CALLFUNC l(20) tc(1)
.   .   .   NAME-runtime.throwinit u(1) a(1) l(2) x(0+0) class(PFUNC) tc(1) used(1) FUNC-funcSTRUCT-()

.   AS l(20) tc(1)
.   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   LITERAL-1 u(1) a(1) l(20) tc(1) uint8

.   AS l(20) tc(1)
.   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   LITERAL-2 u(1) a(1) l(20) tc(1) uint8

.   RETURN l(20) tc(1)
after walk init
.   IF l(20) tc(1)
.   IF-test
.   .   NE u(2) l(20) tc(1) bool
.   .   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   .   LITERAL-0 u(1) a(1) l(20) tc(1) uint8
.   IF-body
.   .   IF l(20) tc(1)
.   .   IF-test
.   .   .   EQ u(2) l(20) tc(1) bool
.   .   .   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   .   .   LITERAL-2 u(1) a(1) l(20) tc(1) uint8
.   .   IF-body
.   .   .   RETURN l(20) tc(1)

.   .   CALLFUNC u(100) l(20) tc(1)
.   .   .   NAME-runtime.throwinit u(1) a(1) l(2) x(0+0) class(PFUNC) tc(1) used(1) FUNC-funcSTRUCT-()

.   AS u(2) l(20) tc(1)
.   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   LITERAL-1 u(1) a(1) l(20) tc(1) uint8

.   AS u(2) l(20) tc(1)
.   .   NAME-main.initdone· u(1) a(1) l(20) x(0+0) class(PEXTERN) tc(1) used(1) uint8
.   .   LITERAL-2 u(1) a(1) l(20) tc(1) uint8

.   RETURN l(20) tc(1)

before I.DoSomeWork
.   CALLINTER l(22) tc(1)
.   .   DOTINTER l(22) x(0+0) tc(1) FUNC-methodSTRUCT-(FIELD-PTR64-*STRUCT-struct {}) funcSTRUCT-()
.   .   .   NAME-main..this u(1) a(1) g(1) l(22) x(0+0) class(PPARAM) f(1) tc(1) used(1) main.I
.   .   .   NAME-main.DoSomeWork u(1) a(1) l(22) x(0+0)
after walk I.DoSomeWork
.   CALLINTER u(100) l(22) tc(1)
.   .   DOTINTER u(2) l(22) x(0+0) tc(1) FUNC-methodSTRUCT-(FIELD-PTR64-*STRUCT-struct {}) funcSTRUCT-()
.   .   .   NAME-main..this u(1) a(1) g(1) l(22) x(0+0) class(PPARAM) f(1) tc(1) used(1) main.I
.   .   .   NAME-main.DoSomeWork u(1) a(1) l(22) x(0+0)

before main节点树
先看分析下第一节点DCL第二个节点AS两个节点

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DCL l(17)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS l(17) colas(1) tc(1)
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T
.   .   PTRLIT l(17) esc(no) ld(1) tc(1) PTR64-*main.T
.   .   .   STRUCTLIT l(17) tc(1) main.T
.   .   .   .   TYPE <S> l(17) tc(1) implicit(1) type=PTR64-*main.T PTR64-*main.T

第一个节点DCL 声明了一个变量main.t，变量类型是一个64位指向main.T的指针，
第二个节点AS 节点将一个main.T实力的指针赋值给main.T
这两段的伪代码如下:

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var main.t *main.T
main.t = &main.T{}

接着看下第三节点DCL和第四第五个AS节点：

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.   DCL l(18)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I

.   AS l(18) tc(1)
.   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T
.   .   NAME-main.t u(1) a(1) g(1) l(17) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) PTR64-*main.T

.   AS l(18) colas(1) tc(1)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   .   CONVIFACE l(18) tc(1) main.I
.   .   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

第三个节点DCL声明了一个变量main.i, 类型是main.I
第四个节点AS节点创建一个临时对象main.autotmp_000，类型是*main.T, 并把main.t 赋值给main.autotmp_000
第五个节点AS节点，创建一个特殊的节点CONVIFACE,并且把这个节点赋值给main.i
这三个节点伪代码如下:

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var main.i main.I
var main.autotmp_000 = main.t
main.i = CONVIFACE(main.autotmp_000)

看下编译器处理后的第二个版本的节点树

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 AS-init
.   .   AS l(18) tc(1)
.   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   NAME-go.itab.*"".T."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) used(1) PTR64-*uint8

.   .   IF l(18) tc(1)
.   .   IF-test
.   .   .   EQ l(18) tc(1) bool
.   .   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   .   LITERAL-nil u(1) a(1) l(18) tc(1) PTR64-*uint8
.   .   IF-body
.   .   .   AS l(18) tc(1)
.   .   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   .   CALLFUNC u(100) l(18) tc(1) PTR64-*byte
.   .   .   .   .   NAME-runtime.typ2Itab u(1) a(1) l(2) x(0+0) class(PFUNC) tc(1) used(1) FUNC-funcSTRUCT-(FIELD-
.   .   .   .   .   .   NAME-runtime.typ·2 u(1) a(1) l(2) x(0+0) class(PPARAM) f(1) PTR64-*byte PTR64-*byte, FIELD-
.   .   .   .   .   .   NAME-runtime.typ2·3 u(1) a(1) l(2) x(8+0) class(PPARAM) f(1) PTR64-*byte PTR64-*byte, FIELD-
.   .   .   .   .   .   NAME-runtime.cache·4 u(1) a(1) l(2) x(16+0) class(PPARAM) f(1) PTR64-*PTR64-*byte PTR64-*PTR64-*byte) PTR64-*byte
.   .   .   .   CALLFUNC-list
.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(0+0) tc(1) runtime.typ·2 G0 PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*uint8
.   .   .   .   .   .   .   NAME-type.*"".T u(1) a(1) l(11) x(0+0) class(PEXTERN) tc(1) uint8

.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(8+0) tc(1) runtime.typ2·3 G0 PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*uint8
.   .   .   .   .   .   .   NAME-type."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) uint8

.   .   .   .   .   AS u(2) l(18) tc(1)
.   .   .   .   .   .   INDREG-SP a(1) l(18) x(16+0) tc(1) runtime.cache·4 G0 PTR64-*PTR64-*byte
.   .   .   .   .   .   ADDR u(2) a(1) l(18) tc(1) PTR64-*PTR64-*uint8
.   .   .   .   .   .   .   NAME-go.itab.*"".T."".I u(1) a(1) l(18) x(0+0) class(PEXTERN) tc(1) used(1) PTR64-*uint8
.   AS u(100) l(18) tc(1)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   .   EFACE u(2) l(18) tc(1) main.I
.   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

在AS-init节点里面主要干了以下事情：临时创建一个对象main.autotmp_0003，并且把go.itab.*"".T."".I赋值给main.autotmp_0003, 然后判断这个对象是否为空，如果为空则调用runtime.typ2Itab方法

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func typ2Itab(t *_type, inter *interfacetype, cache **itab) *itab {
	tab := getitab(inter, t, false)
	atomicstorep(unsafe.Pointer(cache), unsafe.Pointer(tab))
	return tab
}

真正工作的函数是getitab(inter, t, false)方法
getitab函数如下：

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

	// easy case
	x := typ.x
	if x == nil {
		if canfail {
			return nil
		}
		i := (*imethod)(add(unsafe.Pointer(inter), unsafe.Sizeof(interfacetype{})))
		panic(&TypeAssertionError{"", *typ._string, *inter.typ._string, *i.name})
	}

	// compiler has provided some good hash codes for us.
	h := inter.typ.hash
	h += 17 * typ.hash
	// TODO(rsc): h += 23 * x.mhash ?
	h %= hashSize

	// look twice - once without lock, once with.
	// common case will be no lock contention.
	var m *itab
	var locked int
	for locked = 0; locked < 2; locked++ {
		if locked != 0 {
			lock(&ifaceLock)
		}
		for m = (*itab)(atomicloadp(unsafe.Pointer(&hash[h]))); m != nil; m = m.link {
			if m.inter == inter && m._type == typ {
				if m.bad != 0 {
					m = nil
					if !canfail {
						// 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 jump
						// down to the interface check, which will
						// do more work but give a better error.
						goto search
					}
				}
				if locked != 0 {
					unlock(&ifaceLock)
				}
				return m
			}
		}
	}
	// 上面我们提过itab.fun这个指针数组的大小是动态
	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr))*ptrSize, 0, &memstats.other_sys))
	m.inter = inter
	m._type = typ

search:
	// both inter and typ have method sorted by name,
	// and interface names are unique,
	// so can iterate over both in lock step;
	// the loop is O(ni+nt) not O(ni*nt).
	ni := len(inter.mhdr)
	nt := len(x.mhdr)
	j := 0
	for k := 0; k < ni; k++ {
		// 这里面有两层循环，目的是遍历interface定义的所有的方法
		// 在遍历具体类型所定义的方法
		// 如果匹配到，则把它放到interface的mhdr里面。
		i := (*imethod)(add(unsafe.Pointer(inter), unsafe.Sizeof(interfacetype{})+uintptr(k)*unsafe.Sizeof(imethod{})))
		iname := i.name
		ipkgpath := i.pkgpath
		itype := i._type
		for ; j < nt; j++ {
			t := (*method)(add(unsafe.Pointer(x), unsafe.Sizeof(uncommontype{})+uintptr(j)*unsafe.Sizeof(method{})))
			if t.mtyp == itype && t.name == iname && t.pkgpath == ipkgpath {
				if m != nil {
					*(*unsafe.Pointer)(add(unsafe.Pointer(m), unsafe.Sizeof(itab{})+uintptr(k)*ptrSize)) = t.ifn
				}
				goto nextimethod
			}
		}
		// didn't find method
		if !canfail {
			if locked != 0 {
				unlock(&ifaceLock)
			}
			panic(&TypeAssertionError{"", *typ._string, *inter.typ._string, *iname})
		}
		m.bad = 1
		break
	nextimethod:
	}
	if locked == 0 {
		gothrow("invalid itab locking")
	}
	m.link = hash[h]
	atomicstorep(unsafe.Pointer(&hash[h]), unsafe.Pointer(m))
	unlock(&ifaceLock)
	if m.bad != 0 {
		return nil
	}
	return m
}

再看下最后的AS节点：

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.   AS u(100) l(18) tc(1)
.   .   NAME-main.i u(1) a(1) g(2) l(18) x(0+0) class(PAUTO) f(1) ld(1) tc(1) used(1) main.I
.   .   EFACE u(2) l(18) tc(1) main.I
.   .   .   NAME-main.autotmp_0003 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*uint8
.   .   .   NAME-main.autotmp_0000 u(1) a(1) l(18) x(0+0) class(PAUTO) esc(N) tc(1) used(1) PTR64-*main.T

这里将EFACE节点赋给main.i变量，EFACE里面包含了一个指向autotmp_0003变量(autotmp_0003就是上面生成itab的那个变量) 和一个指针变量autotmp_0000(这个其实就是*main.T)
因此main.i里面其实就包含了两部分

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type iface struct {
    tab *itab
    data unsafe.Pointer
}

0x02 总结

 当通过接口调用方法的时候大体过程如下：

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var t *T
var i *I
var tmp *T
tmp = t
i = interface{
    tab: getitab()
    data: tmp
}

从上面伪代码可以看出

  1. interface自己本身就是一个变量
  2. interface有两部分，第一部分是一个itab映射表，一个是指向具体对象的指针

0x03 参考

  1. 《go语言核心编程》
  2. golang AST简介
  3. drive into compiler