为什么此代码未定义行为？ [重复]

Question

This question already has an answer here:

• Is this because the go compiler optimized the code? 2 answers

package main

import "time"
import "fmt"

var i int = 0

func loopInc() {
    for {
        i++
    }
}

func main() {
    go loopInc()
    <-time.After(1 * time.Millisecond)
    fmt.Println(i)
}

This code would always print 0.

I dumped the executable file and found that the go my loopInc function was compiled to a do-nothing loop. i++ doesn't exist in it.

Why?

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Answer 1

It's all explained in the Go Memory Model.

In this example, there is not a happens before relationship between incrementing i and and printing i. Because there is not a happens before relationship, the main goroutine may or may not see the changes to i.

The compiler is allowed to optimize away i++ because there's no guarantee that the changes to i will be observed.