~ $ go help environment
...
  GOARCH
    The architecture, or processor, for which to compile code.
    Examples are amd64, 386, arm, ppc64.
...
  GOOS
    The operating system for which to compile code.
    Examples are linux, darwin, windows, netbsd.
...
  CGO_ENABLED
    Whether the cgo command is supported. Either 0 or 1.

export CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build main.go
export CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build main.go

export CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 go build main.go
export CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build main.go

SET CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 go build main.go
SET CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build main.go

// +build

package main
?
// 引用了Go所能支持的所有架構(gòu)體系庫代碼，根據(jù)GOARCH選擇對應(yīng)的體系代碼
import (  
  "cmd/compile/internal/amd64"
  "cmd/compile/internal/arm"
  "cmd/compile/internal/arm64"
 ....
  "cmd/compile/internal/x86"
...
)
?
// 初始化代碼
var archInits = map[string]func(*gc.Arch){
  "386":      x86.Init,
  "amd64":    amd64.Init,
  "arm":      arm.Init,
  "arm64":    arm64.Init,
...
}
?
func main() {
  // disable timestamps for reproducible output
  log.SetFlags(0)
  log.SetPrefix("compile: ")
?
// 通過objabi.GOARCH選擇對應(yīng)的架構(gòu)體系
  archInit, ok := archInits[objabi.GOARCH]
...
  gc.Main(archInit)
...
}

var (
  defaultGOROOT string // set by linker
?
...
  GOROOT   = envOr("GOROOT", defaultGOROOT)
  GOARCH   = envOr("GOARCH", defaultGOARCH)
  GOOS     = envOr("GOOS", defaultGOOS)
...
)

// Code generated by go tool dist; DO NOT EDIT.
?
package objabi
?
import "runtime"
?
...
const defaultGOOS = runtime.GOOS
const defaultGOARCH = runtime.GOARCH
...

// GOARCH is the running program's architecture target:
// one of 386, amd64, arm, s390x, and so on.
const GOARCH string = sys.GOARCH

// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
?
// 我的機(jī)器平臺是amd64，且未對GOARCH的值做修改。因此這里的構(gòu)建標(biāo)簽是amd64.
// +build amd64
?
package sys
?
const GOARCH = `amd64`

package main
?
import "fmt"
?
func main() {
  fmt.Println("mike")
  speak("hello")
}

//+build speak
?
package main
?
func speak(s string) {
  println("speak:", s)
}

$ go build -o main
$ ./main

$ go build -tags speak -o main
$ ./main