golang：如何有效地模拟联合类型

Question

众所周知，go 没有联合类型，只能通过接口模拟。

我尝试了两种方法来模拟联合，但结果远不如C。

package main

import (

    "fmt"

    "time"

)

type U interface {

    i32() int32

    i16() int16

}

type i32 int32

func (u i32) i32() int32 {

    return int32(u)

}

func (u i32) i16() int16 {

    return int16(u)

}

type i16 int16

func (u i16) i32() int32 {

    return int32(u)

}

func (u i16) i16() int16 {

    return int16(u)

}

func test() (total int64) {

    type A struct {

        t int32

        u interface{}

    }

    a := [...]A{{1, int32(100)}, {2, int16(3)}}

    for i := 0; i < 5000000000; i++ {

        p := &a[i%2]

        switch p.t {

        case 1:

            total += int64(p.u.(int32))

        case 2:

            total += int64(p.u.(int16))

        }

    }

    return

}

func test2() (total int64) {

    type A struct {

        t int32

        u U

    }

    a := [...]A{{1, i32(100)}, {2, i16(3)}}

    for i := 0; i < 5000000000; i++ {

        p := &a[i%2]

        switch p.t {

        case 1:

            total += int64(p.u.i32())

        case 2:

            total += int64(p.u.i16())

        }

    }

    return

}

type testfn func() int64

func run(f testfn) {

    ts := time.Now()

    total := f()

    te := time.Now()

    fmt.Println(total)

    fmt.Println(te.Sub(ts))

}

func main() {

    run(test)

    run(test2)

}

结果：

257500000000

1m23.508223094s

257500000000

34.95081661s

方法方式更好，类型转换方式花费更多的CPU时间。

Answer 1

您可以使用数组将单个表示int32为两个int16s，然后按照 Rob Pike 的建议将它们组合起来：func test3() (total int64) {&nbsp; &nbsp; type A struct {&nbsp; &nbsp; &nbsp; &nbsp; t int32&nbsp; &nbsp; &nbsp; &nbsp; u [2]int16&nbsp; &nbsp; }&nbsp; &nbsp; a := [...]A{&nbsp; &nbsp; &nbsp; &nbsp; {1, [2]int16{100, 0}},&nbsp; &nbsp; &nbsp; &nbsp; {2, [2]int16{3, 0}},&nbsp; &nbsp; }&nbsp; &nbsp; for i := 0; i < N; i++ {&nbsp; &nbsp; &nbsp; &nbsp; p := &a[i%2]&nbsp; &nbsp; &nbsp; &nbsp; switch p.t {&nbsp; &nbsp; &nbsp; &nbsp; case 1:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; total += int64(p.u[0]<<0 | p.u[1]<<8)&nbsp; &nbsp; &nbsp; &nbsp; case 2:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; total += int64(p.u[0])&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; return}使用原始的 Go 编译器，它的运行速度比 C 版本慢约 2 倍，而使用 gccgo (-O3) 时，它的运行速度与 C 一样快。但请注意，这种方法假定小端整数。您需要切换大端架构的转换顺序。此外，如果您需要从字节切片解码结构，您应该真正使用encoding/binary. 创建这个库是为了在字节序列和其他类型之间进行转换。

Answer 2

联合可能包含数字类型和八位字节字符串，因此我尝试使用字节切片作为值容器并unsafe.Pointer根据具体类型使用它。func test3() (total int64) {&nbsp; &nbsp; type A struct {&nbsp; &nbsp; &nbsp; &nbsp; t int32&nbsp; &nbsp; &nbsp; &nbsp; u []byte&nbsp; &nbsp; }&nbsp; &nbsp;&nbsp; &nbsp; a := [...]A{{1, make([]byte, 8)}, {2, make([]byte, 8)}}&nbsp; &nbsp; *(*int32)(unsafe.Pointer(&a[0].u)) = 100&nbsp;&nbsp; &nbsp; *(*int16)(unsafe.Pointer(&a[1].u)) = 3&nbsp;&nbsp; &nbsp; for i := 0; i < 5000000000; i++ {&nbsp; &nbsp; &nbsp; &nbsp; p := &a[i%2]&nbsp; &nbsp; &nbsp; &nbsp; switch p.t {&nbsp; &nbsp; &nbsp; &nbsp; case 1:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; total += int64(*(*int32)(unsafe.Pointer(&p.u)))&nbsp; &nbsp; &nbsp; &nbsp; case 2:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; total += int64(*(*int16)(unsafe.Pointer(&p.u)))&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp;&nbsp; &nbsp; }&nbsp; &nbsp;&nbsp; &nbsp; return}结果：$ go run union.go25750000000012.844752701s$ go run -compiler gccgo -gccgoflags -O3 union.go2575000000006.640667s它是最好的版本吗？