cmd/go: code/instruction with side effects is disappeared

[xiaopengli89]

What version of Go are you using (go version)?

$ go version
go version go1.14.5 linux/amd64

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/tora/.cache/go-build"
GOENV="/home/tora/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/tora/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/tora/code/go/proj/foo/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build205500719=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"fmt"
	"runtime"
	"time"
)

var a int32 = 0

func main() {
	runtime.GOMAXPROCS(2)
	go func() {
		for {
			a = 5
			// If uncomment code below, everything is ok
			// runtime.Gosched()
		}
	}()
	go func() {
		for {
			fmt.Println("a:", a)
		}
	}()
	time.Sleep(200 * time.Second)
}

What did you expect to see?

Finally a: 5 should appear on the screen

What did you see instead?

Always a: 0 appears on the screen, and a = 5 instruction is disappeared. Is this the expected behavior?

0000000000491980 <main.main.func1>:
  491980:       90                      nop
  491981:       eb fd                   jmp    491980 <main.main.func1>

[randall77]

Yes, this is allowed. There is no synchronization operation that would guarantee that the second goroutine would see the effects of the first one.

In the compiler, this happens because the memory state generated by the a=5 statement is never used, so it gets dead-code eliminated. That's why adding runtime.Gosched fixes it, because that call uses the aforementioned memory state.