Go 接口实现原理高阶篇

Posted 2022-06-14 禅与计算机程序设计艺术

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 != 0func (t *_type) IsPointer() bool 
    return t.flags & TypeFlag_IsPointer != 0func (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

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