Why Does My Goroutine Incrementing a Variable Produce Unexpected Results?

Is This a Result of Compiler Optimization?

In this code snippet, a goroutine is launched and repeatedly increments the variable i:

<code class="go">package main

import "time"

func main() {
    i := 1
    go func() {
        for {
            i++
        }
    }()
    <p>However, the output is always 1. This behavior can be attributed to the Go memory model and the specific implementation of this code.</p>
<h3 id="The-Go-Memory-Model">The Go Memory Model</h3>
<p>The Go memory model defines the conditions under which reads of a variable in one goroutine can be guaranteed to observe values produced by writes to the same variable in a different goroutine. It emphasizes the importance of synchronization for concurrent access to shared data.</p>
<h3 id="Omitting-Synchronization">Omitting Synchronization</h3>
<p>In the given code:</p>
<ul>
<li>The assignment to i (i.e., i  ) is not followed by any synchronization event, indicating that changes may not be immediately visible to other goroutines.</li>
<li>The compiler may optimize this loop optimization by simplifying it to a no-op.</li>
</ul>
<h3 id="Optimization-by-the-Compiler">Optimization by the Compiler</h3>
<p>An aggressive compiler might delete the i   statement, effectively reducing the goroutine to:</p>
<pre class="brush:php;toolbar:false"><code class="go">for {}</code>

Example with Synchronization

To demonstrate that the issue stems from the lack of synchronization, consider the following code:

<code class="go">package main

import (
    "sync"
    "time"
)

func main() {
    mx := new(sync.Mutex)
    i := 1
    go func() {
        for {
            mx.Lock()
            i++
            mx.Unlock()
        }
    }()
    <p>In this case, the output is no longer 1, but a large number, as expected. The sync.Mutex provides synchronization and ensures that both goroutines access i in a controlled manner, allowing the goroutine to increment i and making the changes visible to the main routine.</p></code>

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