Go 接口实现原理【高阶篇】:type _interface struct

The Internal Definition Of Interface Types

​https://www.tapirgames.com/blog/golang-interface-implementation​

All interface types have the same internal definition:

type _interface struct {
dynamicTypeInfo *_implementation
dynamicValue unsafe.Pointer // unsafe.Pointer means
// *ArbitraryType in Go.}

The internal ​​_implementation​​ type is declared like

type _implementation struct {
itype *_type // the interface type.
dtype *_type // the dynamic type, which must implement itype.
methods []*_func // the methods which are defined on dtype
// and each of them implements a
// corresponding method declared in itype.}

From the definitions, we know that each interface value contains two pointer fields. The ​​dynamicValue​​​ field stores the dynamic value information, and the ​​dynamicTypeInfo​​​ field stores the implementation information. ​​dynamicTypeInfo.itype​​​ stores the type information of the interface value and ​​dynamicTypeInfo.dtype​​ stores the type information of the dynamic value.

The ​​dynamicTypeInfo​​​ field of an interface value may be ​​nil​​​, which means nothing is stored in the interface value. For this case, the ​​dynamicValue​​​ field must be also ​​nil​​​. We can also say the dynamic value of the interface value is untyped ​​nil​​ for this case.

For the official Go compiler and runtime, a non-nil ​​dynamicValue​​ field value may store

  • the address of the dynamic value if the dynamic type is not a pointer type, or
  • the dynamic value itself if the dynamic type is a pointer type.

Surely, it is not essential to make the exception for pointer dynamic values. This is just a compiler optimization. We can get why it is an optimization in following sections. (BTW, about more current and future optimizations in the official interface implementation, please read this article:http://commaok.xyz/post/interface-allocs/.)

Other involved internal types are declared like:

type _func struct {
name string
methodSig uint // two methods with the same signature have
// the same signature id. Receiver parameter
// doesn't contribute to this signature.
funcSig uint // receiver parameter accounts to this signature.

// other information ...}type _type struct {
name string // type name
id uint32 // each type has unique id
flags uint32 // comparable? isPointer?
size uintptr // value size in bytes
kind uint8 //
methods []*_func // the methods are sorted
// by methodSig and name.
// more information ...}const (
// flags
TypeFlag_Comparable = 1 << 0
TypeFlag_IsPointer = 1 << 1
TypeFlag_IsString = 1 << 2)func (t *_type) IsComparable() bool {
return t.flags & TypeFlag_Comparable != 0}func (t *_type) IsPointer() bool {
return t.flags & TypeFlag_IsPointer != 0}func (t *_type) IsString() bool {
return t.flags & TypeFlag_IsString != 0}

Some fields of above types are not listed, for they are unrelated to this article.

Here is the function to get an ​​_implementation​​ value from an interface type and a non-interface type:

// global tablevar cachedImpls = map[uint64]*_implementation{}// itype must be an interface type and// dtype must be a non-interface type.// Return nil if dtype doesn't implement itype.// Must not return nil if dtype implements itype.func getImpl (itype *_type, dtype *_type) *_implementation {
var key = uint64(itype.id) << 32 | uint64(dtype.id)
var impl = cachedImpls[key]
if impl == nil {
// for each (dtype, itype) pair, the implementation
// method table is only calculated most once at
// run time. The calculation result will be cached.

var numMethods = len(itype.methods)
var methods = make([]*_func, numMethods)

// find every implemented methods.
// The methods of itype and dtype are both sorted
// by methodSig and name.
var n = 0
var i = 0
for _, im := range itype.methods {
for i < len(dtype.methods) {
tm := dtype.methods[i]
i++

// Here, for simplicity, assume
// all methods are exported.

if tm.methodSig < im.methodSig {
continue
}
if tm.methodSig > im.methodSig {
// im method is not implemented
return nil
}
if tm.name < im.name {
continue
}
if tm.name > im.name {
// im method is not implemented
return nil
}

methods[n] = tm
n++
break
}
}

// dtype doesn't implement all methods of itype
if n < numMethods {
return nil
}

// dtype implements itype.
// create and cache the implementation.
impl = &_implementation{
dtype: dtype,
itype: itype,
methods: methods,
}
cachedImpls[key] = impl }

return impl}

This function will be called in the value conversions explained in following sections.

In any Go program, at run time, all ​​_implementation​​​ values are cached and stored in a global map and all ​​_type​​ values are stored in a global immutable array.

As the blank interface type ​​interface{}​​ is used popular in Go programming, the official Go compiler uses a different and more efficient underlying definition for the blank interface than other interface types:

// blank interfacestruct {
dynamicType *_type // the dynamic type
dynamicValuePtr unsafe.Pointer // points to the dynamic value}

To make the explainations simpler, following sections will treat blank interface types as general interface types.

Convert Non-Interface Values To Interface Types

Here is the internal function to convert a non-interface value to an interface type:

// To call this function, compilers must assure // 1\. itype is an interface type.// 2\. dtype is nil or a non-interface type and implements itype.// p must be nil if dtype is nil.// p is a pointer stores the address of a value of dtype if dtype// is not a pointer type. p stores the value of a value of dtype// if dtype is a pointer type.func t2i (itype *_type, dtype *_type, p unsafe.Pointer) _interface {
// dtype is nil means the non-interface value is untyped nil
if dtype == nil {
return _interface {
dynamicValue: nil,
dynamicTypeInfo: nil,
}
}

// the pointer dynamic value optimization, no need to
// allocate the extra memory block.
if dtype.IsPointer() {
return _interface {
dynamicValue: p,
dynamicTypeInfo: getImpl(dtype, itype),
}
}

// for non-pointer dynamic value, runtime must
// allocate an extra memory block to store a copy
// of the non-pointer value.
var t = memoryAlloc(dtype)
memoryCopy(t, p, dtype.size)
return _interface {
dynamicValue: t,
dynamicTypeInfo: getImpl(dtype, itype),
}}

Compilers will insert a call of this function before

  • assigning a non-interface value to an interface value, to convert the non-interface value to the type of the interface value.
  • comparing a non-interface value with an interface value, to convert the non-interface value to the type of the interface value.

Convert Interface Values To Other Interface Types

Here is the internal function to convert an interface value to an interface type:

// To call this function, compilers must assure // 1\. itype is an interface type.// 2\. the dynamic value of ivalue is untyped nil//    or the dynamic type of ivalue implements ivalue.//    (the method set of the dynamic type of ivalue must//    must be a super set of the method set of itype).func i2i (itype *_type, ivalue _interface) _interface {
// the dynamic value of ivalue is untyped nil.
if ivalue.dynamicTypeInfo == nil {
return _interface {
dynamicValue: nil,
dynamicTypeInfo: nil,
} // <=> return ivalue
}

// compilers should avoid calling this function
// for this case.
if ivalue.dynamicTypeInfo.itype == itype {
return ivalue // return a copy of ivalue.
}

// Convert the dynamic value of ivalue to itype.
// Here, the returned interface value and ivalue
// will share the same extra memory block which
// stores the dyanmic value if the dynamic value
// is not a pointer.
return _interface {
dynamicValue: ivalue.dynamicValue,
dynamicTypeInfo: getImpl(
ivalue.dynamicTypeInfo.dtype,
itype,
), // here, the getImpl call never return nil.
}}

Compilers will call this function before

  • assigning an interface value to another interface value, to convert the first interface value to the type of the second interface value.
  • comparing an interface value with another interface value, to convert the first interface value to the type of the second interface value.

Compilers should translate converting an interface value to its own type as a no-op.

Assign Interface Values

In an interface value assignment, the destination value must be an interface value, and the type of the source value must implement the destination interface type. The source value may be either a non-interface value or an interface value. As above two sections mentioned, compilers will convert the source value to the destination interface type before the assignment. So in the final assignment, the source value and the destination value have the same type, the destination interface type.

For the current official Go compiler/runtime, there are just two copies for the two fields, ​​dynamicValue​​​ and ​​dynamicTypeInfo​​​, in the final assignment. So if the dynamic value is non-pointer, the underlying dynamic value memory block, which address is stored in the ​​dynamicValue​​ field, will be shared between the destination and source interface values. However, this should be viewed as an optimization. Other compilers may not adopt this optimization.

Compare Interface Values

There are three comparison circumstances involving interface values:

  • interface value vs. interface value.
  • interface value vs. non-interface value.
  • interface value vs. untyped​​nil​​.

A good compiler should treat the three circumstances differently to get better program performance. Here, for simplicity, we assume non-interface and untyped ​​nil​​​ values will be converted to ​​interface{}​​ type before making the comparisons. So all comparisons involving interface values can be viewed as comparing two interface values.

Here is the internal function to compare two interface values:

func iCompare (ivalue1 _interface, ivalue2 _interface) bool {
// untyped nil is only equal to untyped nil.
if ivalue1.dynamicTypeInfo == nil {
return ivalue2.dynamicTypeInfo == nil
}
if ivalue2.dynamicTypeInfo == nil {
return false
}

// panic on incomparable dynamic values.
if ! ivalue1.dynamicTypeInfo.dtype.IsComparable() {
panic(ivalue1.dynamicTypeInfo.dtype.name +
" is incomparable")
}
if ! ivalue2.dynamicTypeInfo.dtype.IsComparable() {
panic(ivalue2.dynamicTypeInfo.dtype.name +
" is incomparable")
}

// return false if dynamic type is not the same.
if ivalue1.dynamicTypeInfo.dtype !=
ivalue2.dynamicTypeInfo.dtype {
return false
}

// optimization: early return.
if ivalue1.dynamicValue == ivalue2.dynamicValue {
return true
}

// special case: string comparison
if ivalue1.dynamicTypeInfo.dtype.IsString() {
return stringCompare(
*(*string)(ivalue1.dynamicValue),
*(*string)(ivalue2.dynamicValue),
)
}

// general case: compare all bytes in dynamic value
// memory blocks one by one.
return memoryCompare(
ivalue1.dynamicValue,
ivalue2.dynamicValue,
ivalue1.dynamicTypeInfo.dtype.size,
)}

This article will not explain how two strings are compared.

Type Assert To Non-Interface Types

Here is the internal function to assert an interface value to a non-interface type:

// To call this function, compilers must assure // 1\. dtype is a non-interface type.// 2\. outP is nil or stores the address of a value of dtype.// 3\. outOk is nil or stores the address of a bool value.func assertI2T (ivalue _interface, dtype *_type,
outP *unsafe.Pointer, outOk *bool) {
// dynamic value is untype nil.
if ivalue.dynamicTypeInfo == nil {
// if okay is not present, panic.
if outOk == nil {
panic("interface is nil, not " + dtype.name)
}

// return (zero value, false)
*outOk = false
if outP != nil {
if dtype.IsPointer() {
*outP = nil
} else {
memoryReset(*outP, dtype.size)
}
}

return
}

// assersion fails.
if ivalue.dynamicTypeInfo.dtype != dtype {
// if ok is not present, panic.
if outOk == nil {
panic("interface is " +
ivalue.dynamicTypeInfo.dtype.name +
", not " + dtype.name)
}

// return (zero value, false)
*outOk = false
if outP != nil {
if dtype.IsPointer() {
*outP = nil
} else {
memoryReset(*outP, dtype.size)
}
}

return
}

// assersion succeeds.

if outOk != nil {
*outOk = true
}
if outP == nil {
return
}
// copy dynamic value.
if dtype.IsPointer() {
*outP = ivalue.dynamicValue } else {
memoryCopy(*outP, ivalue.dynamicValue, dtype.size)
}}

Type Assert To Interface Types

Here is the internal function to assert an interface value to an interface type:

// To call this function, compilers must assure // 1\. itype is an interface type.// 2\. outI is nil or stores the address of a value of itype.// 3\. outOk is nil or stores the address of a bool value.func assertI2I (ivalue _interface, itype *_type,
outI *_interface, outOk *bool) {
// dynamic value is untype nil.
if ivalue.dynamicTypeInfo == nil {
// if ok is not present, panic.
if outOk == nil {
panic("interface is nil, not " + itype.name)
}

*outOk = false
if outI == nil {
*outI = _interface {
dynamicValue: nil,
dynamicTypeInfo: nil,
}
}

return
}

// check whether or not the dynamic type implements itype
var impl = getImpl(itype, ivalue.dynamicTypeInfo.dtype)

// assersion fails.
if impl == nil {
// if ok is not present, panic.
if outOk == nil {
panic("interface is " +
ivalue.dynamicTypeInfo.dtype.name +
", not " + itype.name)
}

// return (zero value, false)
*outOk = false
if outI != nil {
*outI = _interface {
dynamicValue: nil,
dynamicTypeInfo: nil,
}
}

return
}

// assersion succeeds.

if outI == nil {
*outOk = true
}
if outI != nil {
*outI = _interface {
dynamicValue: ivalue.dynamicValue,
dynamicTypeInfo: impl,
}
}}

If the type of the interface value is the asserted interface type, compilers should simply return the interface value.

Call Interface Methods

For an interface value ​​i​​, a call of its nth method (by the order after sorted)

... = i.Method_n(...)

will be translated to

if i.dynamicTypeInfo == nil {
panic("runtime error: nil pointer dereference")}if i.dynamicTypeInfo.dtype.IsPointer() {
... = _call(i.dynamicTypeInfo.methods[n], i.dynamicValue, ...)} else {
... = _call(i.dynamicTypeInfo.methods[n],
*(*unsafe.Pointer)(i.dynamicValue), ...)}

The ​​interfacetype​​ structure

Finally, here's the ​​interfacetype​​ structure (src/runtime/type.go):

type interfacetype struct { // 80 bytes on a 64bit arch
typ _type
pkgpath name
mhdr []imethod
}

type imethod struct {
name nameOff
ityp typeOff
}

As mentioned, an ​​interfacetype​​​ is just a wrapper around a ​​_type​​​ with some extra interface-specific metadata added on top.
In the current implementation, this metadata is mostly composed of a list of offsets that points to the respective names and types of the methods exposed by the interface (​​​[]imethod​​).

Conclusion

Here's an overview of what an ​​iface​​ looks like when represented with all of its sub-types inlined; this hopefully should help connect all the dots:

type iface struct { // `iface`
tab *struct { // `itab`
inter *struct { // `interfacetype`
typ struct { // `_type`
size uintptr
ptrdata uintptr
hash uint32
tflag tflag
align uint8
fieldalign uint8
kind uint8
alg *typeAlg
gcdata *byte
str nameOff
ptrToThis typeOff
}
pkgpath name
mhdr []struct { // `imethod`
name nameOff
ityp typeOff
}
}
_type *struct { // `_type`
size uintptr
ptrdata uintptr
hash uint32
tflag tflag
align uint8
fieldalign uint8
kind uint8
alg *typeAlg
gcdata *byte
str nameOff
ptrToThis typeOff
}
hash uint32
_ [4]byte
fun [1]uintptr
}
data unsafe.Pointer
}

learn more:
​​​https://github.com/teh-cmc/go-internals​​​https://github.com/teh-cmc/go-internals/blob/master/chapter2_interfaces/README.md
https://research.swtch.com/interfaces
https://go.dev/doc/effective_go#interfaces_and_types
https://www.tapirgames.com/blog/golang-interface-implementation