Is this because the go compiler optimized the code?

Question

package main

import "time"

func main() {
    i := 1
    go func() {
        for {
            i++
        }
    }()
    <-time.After(1 * time.Second)
    println(i)
}

The output is always 1.

However it's absolutely that 1s is enough for the for loop to go over many many times.

I think the i in the closure is the i in the main func.

See the code below.

package main

import "time"

func main() {
    i := 1
    go func() {
        for {
            i++
            println("+1")
        }
    }()
    <-time.After(1 * time.Second)
    println(i)
}

After many lines of "+1", the output is exactly a great number as expected.

Answer 1

The Go Memory Model

Version of May 31, 2014

Introduction

The Go memory model specifies 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.

Advice

Programs that modify data being simultaneously accessed by multiple goroutines must serialize such access.

To serialize access, protect the data with channel operations or other synchronization primitives such as those in the sync and sync/atomic packages.

If you must read the rest of this document to understand the behavior of your program, you are being too clever.

Don't be clever.

Synchronization

var a string

func hello() {
  go func() { a = "hello" }()
  print(a)
}

the assignment to a is not followed by any synchronization event, so it is not guaranteed to be observed by any other goroutine. In fact, an aggressive compiler might delete the entire go statement.

---

The assignment to i, via increment i++ (i = i + 1), is not followed by any synchronization event, so it is not guaranteed to be observed by any other goroutine. In fact, an aggressive compiler might delete the entire i++ statement.

For example,

package main

import "time"

func main() {
    i := 1
    go func() {
        for {
            i++
        }
    }()
    <-time.After(1 * time.Millisecond)
    println(i)
}

Output:

1

The goroutine reduces to:

"".main.func1 STEXT nosplit size=2 args=0x8 locals=0x0
    0x0000 00000 (elide.go:7)   TEXT    "".main.func1(SB), NOSPLIT, $0-8
    0x0000 00000 (elide.go:7)   FUNCDATA    $0, gclocals·2a5305abe05176240e61b8620e19a815(SB)
    0x0000 00000 (elide.go:7)   FUNCDATA    $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x0000 00000 (elide.go:9)   JMP 0

To the compiler,

for {
    i++
}

can be implemented by incrementing a register forever, essentially a no-op for loop.

for { }

After inserting a print statement,

package main

import "time"

func main() {
    i := 1
    go func() {
        for {
            i++
            println("+1")
        }
    }()
    <-time.After(1 * time.Millisecond)
    println(i)
}

Output:

+1
+1
<< SNIP >>
+1
+1
432

The goroutine expands to,

"".main.func1 STEXT size=81 args=0x8 locals=0x18
    0x0000 00000 (elide.go:7)   TEXT    "".main.func1(SB), $24-8
    0x0000 00000 (elide.go:7)   MOVQ    (TLS), CX
    0x0009 00009 (elide.go:7)   CMPQ    SP, 16(CX)
    0x000d 00013 (elide.go:7)   JLS 74
    0x000f 00015 (elide.go:7)   SUBQ    $24, SP
    0x0013 00019 (elide.go:7)   MOVQ    BP, 16(SP)
    0x0018 00024 (elide.go:7)   LEAQ    16(SP), BP
    0x001d 00029 (elide.go:7)   FUNCDATA    $0, gclocals·a36216b97439c93dafebe03e7f0808b5(SB)
    0x001d 00029 (elide.go:7)   FUNCDATA    $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
    0x001d 00029 (elide.go:8)   MOVQ    "".&i+32(SP), AX
    0x0022 00034 (elide.go:9)   INCQ    (AX)
    0x0025 00037 (elide.go:10)  PCDATA  $0, $0
    0x0025 00037 (elide.go:10)  CALL    runtime.printlock(SB)
    0x002a 00042 (elide.go:10)  LEAQ    go.string."+1\n"(SB), AX
    0x0031 00049 (elide.go:10)  MOVQ    AX, (SP)
    0x0035 00053 (elide.go:10)  MOVQ    $3, 8(SP)
    0x003e 00062 (elide.go:10)  PCDATA  $0, $0
    0x003e 00062 (elide.go:10)  CALL    runtime.printstring(SB)
    0x0043 00067 (elide.go:10)  PCDATA  $0, $0
    0x0043 00067 (elide.go:10)  CALL    runtime.printunlock(SB)
    0x0048 00072 (elide.go:9)   JMP 29
    0x004a 00074 (elide.go:9)   NOP
    0x004a 00074 (elide.go:7)   PCDATA  $0, $-1
    0x004a 00074 (elide.go:7)   CALL    runtime.morestack_noctxt(SB)
    0x004f 00079 (elide.go:7)   JMP 0

The increased complexity of the goroutine means that the compiler no longer considers dedicating a register to the value of i. The in-memory value of i is incremented, which makes the updates visible, with a data race, to the main goroutine.

==================
WARNING: DATA RACE

Read at 0x00c420094000 by 
main goroutine:
  main.main()
      /home/peter/gopath/src/lucky.go:14 +0xac

Previous write at 0x00c420094000 by 
goroutine 5:
  main.main.func1()
      /home/peter/gopath/src/lucky.go:9 +0x4e

Goroutine 5 (running) created at:
  main.main()
      /home/peter/gopath/src/lucky.go:7 +0x7a
==================

---

For your expected result, add some synchronization,

package main

import (
    "sync"
    "time"
)

func main() {
    mx := new(sync.Mutex)
    i := 1
    go func() {
        for {
            mx.Lock()
            i++
            mx.Unlock()
        }
    }()
    <-time.After(1 * time.Second)
    mx.Lock()
    println(i)
    mx.Unlock()
}

Output:

41807838