Go Data Structures: Interfaces

Go's interfaces—static,checked at compile time,dynamic when asked for—are,for me,the most exciting part of Go from a language design point of view. If I could export one feature of Go into other languages,it would be interfaces.

This post is my take on the implementation of interface values in the “gc” compilers: 6g,8g,and 5g. Over at Airs,Ian Lance Taylor has writtentwopostsabout the implementation of interface values ingccgo. The implementations are more alike than different: the biggest difference is that this post has pictures.

Before looking at the implementation,let's get a sense of what it must support.

Usage

Go's interfaces let you useduck typinglike you would in a purely dynamic language like Python but still have the compiler catch obvious mistakes like passing anintwhere an object with aReadmethod was expected,or like calling theReadmethod with the wrong number of arguments. To use interfaces,first define the interface type (say,ReadCloser):

type ReadCloser interface {
    Read(b []byte) (n int,err os.Error)
    Close()
}

and then define your new function as taking aReadCloser. For example,this function callsReadrepeatedly to get all the data that was requested and then callsClose:

func ReadAndClose(r ReadCloser,buf []byte) (n int,err os.Error) {
    for len(buf) > 0 && err == nil {
        var nr int
        nr,err = r.Read(buf)
        n += nr
        buf = buf[nr:]
    }
    r.Close()
    return
}

The code that callsReadAndClosecan pass a value of any type as long as it hasReadandClosemethods with the right signatures. And,unlike in languages like Python,if you pass a value with the wrong type,you get an error at compile time,not run time.

Interfaces aren't restricted to static checking,though. You can check dynamically whether a particular interface value has an additional method. For example:

type Stringer interface {
    String() string
}

func ToString(any interface{}) string {
    if v,ok := any.(Stringer); ok {
        return v.String()
    }
    switch v := any.(type) {
    case int:
        return strconv.Itoa(v)
    case float:
        return strconv.Ftoa(v,'g',-1)
    }
    return "???"
}

The valueanyhas static typeinterface{},meaning no guarantee of any methods at all: it could contain any type. The “comma ok” assignment inside theifstatement asks whether it is possible to convertanyto an interface value of typeStringer,which has the methodString. If so,the body of that statement calls the method to obtain a string to return. Otherwise,theswitchpicks off a few basic types before giving up. This is basically a stripped down version of what thefmt packagedoes. (Theifcould be replaced by addingcase Stringer:at the top of theswitch,but I used a separate statement to draw attention to the check.)

As a simple example,let's consider a 64-bit integer type with aStringmethod that prints the value in binary and a trivialGetmethod:

type Binary uint64

func (i Binary) String() string {
    return strconv.Uitob64(i.Get(),2)
}

func (i Binary) Get() uint64 {
    return uint64(i)
}

A value of typeBinarycan be passed toToString,which will format it using theStringmethod,even though the program never says thatBinaryintends to implementStringer. There's no need: the runtime can see thatBinaryhas aBinaryhas never heard ofStringer.

These examples show that even though all the implicit conversions are checked at compile time,explicit interface-to-interface conversions can inquire about method sets at run time. “Effective Go” has more details about and examples of how interface values can be used.

Interface Values

Languages with methods typically fall into one of two camps: prepare tables for all the method calls statically (as in C++ and Java),or do a method lookup at each call (as in Smalltalk and its many imitators,JavaScript and Python included) and add fancy caching to make that call efficient. Go sits halfway between the two: it has method tables but computes them at run time. I don't know whether Go is the first language to use this technique,but it's certainly not a common one. (I'd be interested to hear about earlier examples; leave a comment below.)

As a warmup,a value of typeBinaryis just a 64-bit integer made up of two 32-bit words (like in thelast post,we'll assume a 32-bit machine; this time memory grows down instead of to the right):

Interface values are represented as a two-word pair giving a pointer to information about the type stored in the interface and a pointer to the associated data. Assigningbto an interface value of typeStringersets both words of the interface value.

The first word in the interface value points at what I call an interface table or itable (pronounced i-table; in theruntime sources,the C implementation name isItab). The itable begins with some metadata about the types involved and then becomes a list of function pointers. Note that the itable corresponds to theinterface type,not the dynamic type. In terms of our example,the itable forStringerholding typeBinarylists the methods used to satisfyString:Binary's other methods (Get) make no appearance in the itable.

The second word in the interface value points at the actual data,in this case a copy ofb. The assignmentvar s Stringer = bmakes a copy ofbrather than point atbfor the same reason thatvar c uint64 = bmakes a copy: ifblater changes,monospace">sandcare supposed to have the original value,not the new one. Values stored in interfaces might be arbitrarily large,but only one word is dedicated to holding the value in the interface structure,so the assignment allocates a chunk of memory on the heap and records the pointer in the one-word slot. (There's an obvious optimization when the value does fit in the slot; we'll get to that later.)

To check whether an interface value holds a particular type,as in thetype switchabove,the Go compiler generates code equivalent to the C expressions.tab->typeto obtain the type pointer and check it against the desired type. If the types match,the value can be copied by by dereferencings.data.

To calls.String(),the Go compiler generates code that does the equivalent of the C expressions.tab->fun[0](s.data): it calls the appropriate function pointer from the itable,passing the interface value's data word as the function's first (in this example,only) argument. You can see this code if you run8g -S x.go(details at the bottom of this post). Note that the function in the itable is being passed the 32-bit pointer from the second word of the interface value,not the 64-bit value it points at. In general,the interface call site doesn't know the meaning of this word nor how much data it points at. Instead,the interface code arranges that the function pointers in the itable expect the 32-bit representation stored in the interface values. Thus the function pointer in this example is(*Binary).StringnotBinary.String.

The example we're considering is an interface with just one method. An interface with more methods would have more entries in thefunlist at the bottom of the itable.

Computing the Itable

Now we know what the itables look like,but where do they come from? Go's dynamic type conversions mean that it isn't reasonable for the compiler or linker to precompute all possible itables: there are too many (interface type,concrete type) pairs,and most won't be needed. Instead,the compiler generates a type description structure for each concrete type likeBinaryorintorfunc(map[int]string). Among other metadata,the type description structure contains a list of the methods implemented by that type. Similarly,the compiler generates a (different) type description structure for each interface type likeStringer; it too contains a method list. The interface runtime computes the itable by looking for each method listed in the interface type's method table in the concrete type's method table. The runtime caches the itable after generating it,so that this correspondence need only be computed once.

In our simple example,the method table forStringerhas one method,while the table forBinaryhas two methods. In general there might benimethods for the interface type andntmethods for the concrete type. The obvious search to find the mapping from interface methods to concrete methods would takeO(ni×nt) time,but we can do better. By sorting the two method tables and walking them simultaneously,we canbuild the mappinginO(ni+nt) time instead.

Memory Optimizations

The space used by the implementation described above can be optimized in two complementary ways.

First,if the interface type involved is empty—it has no methods—then the itable serves no purpose except to hold the pointer to the original type. In this case,the itable can be dropped and the value can point at the type directly:

Whether an interface type has methods is a static property—either the type in the source code saysinterface{}or it saysinterace{ methods... }—so the compiler knows which representation is in use at each point in the program.

Second,if the value associated with the interface value can fit in a single machine word,there's no need to introduce the indirection or the heap allocation. If we defineBinary32to be likeBinarybut implemented as auint32,it could be stored in an interface value by keeping the actual value in the second word:

1   var any interface{}  // initialized elsewhere
2   s := any.(Stringer)  // dynamic conversion
3   for i := 0; i < 100; i++ {
4       fmt.Println(s.String())
5   }

package main

import (
 "fmt"
 "strconv"
)

type Stringer interface {
 String() string
}

type Binary uint64

func (i Binary) String() string {
 return strconv.Uitob64(i.Get(),2)
}

func (i Binary) Get() uint64 {
 return uint64(i)
}

func main() {
 b := Binary(200)
 s := Stringer(b)
 fmt.Println(s.String())
}

0045 (x.go:25) LEAL    s+-24(SP),BX
0046 (x.go:25) MOVL    4(BX),BP
0047 (x.go:25) MOVL    BP,(SP)
0048 (x.go:25) MOVL    (BX),BX
0049 (x.go:25) MOVL    20(BX),BX
0050 (x.go:25) CALL,BX