# 8.4.4 编译日志

编译器提供了参数来反馈编译过程中的各种优化：

> ```
> -json string
> version,file for JSON compiler/optimizer detail output
> -m	print optimization decisions
> ```

我们可以通过以上参数来查看编译器所做的各种优化，包括内联在内。将下列代码保存为 `main.go`:

```go
package main

func C() {
    println("C")
    D()
}

func D() {
    println("D")
    C()
}

func main() {
    C()
}
```

通过命令 `go tool compile -G=3 -m -json "0,file:///tmp/main.json" main.go` 进行编译，可以看到如下输出：

> ```
> main.go:3:6: can inline C
> main.go:8:6: can inline D
> main.go:10:3: inlining call to C
> main.go:13:6: can inline main
> main.go:14:3: inlining call to C
> main.go:14:3: inlining call to D
> ```

文件 `/tmp/main.json` 也会包含相关信息。其中 `-m` 对应的数字越大，日志就越详细，例如我们通过命令 `go tool compile -G=3 -m=2 -json "0,file:///tmp/main.json" main.go` 编译时，得到的结果如下：

> ```
> main.go:3:6: can inline C with cost 61 as: func() { println(string(“C”)); D() }
> main.go:8:6: can inline D with cost 65 as: func() { println(string(“D”)); C() }
> main.go:10:3: inlining call to C func() { println(string(“C”)); D() }
> main.go:13:6: can inline main with cost 63 as: func() { C() }
> main.go:14:3: inlining call to C func() { println(string(“C”)); D() }
> main.go:14:3: inlining call to D func() { println(string(“D”)); C() }
> main.go:14:3: cannot inline C into main: repeated recursive cycle
> ```

 其中 `-m -m` 与 `-m=2` 效果一样，当 `-m=4` 时，日志还会打印出内联前后的 IR Tree 结构，读者可以自己尝试。


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