cmd/compile: package-level variable initialization with constant dependencies doesn't match order specified in Go spec

[bigwhite]

Go version

go 1.22.0

Output of go env in your module/workspace:

$go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/tonybai/Library/Caches/go-build'
GOENV='/Users/tonybai/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/tonybai/Go/pkg/mod'
GONOPROXY='bitbucket.org/bigwhite/t'
GOOS='darwin'
GOPATH='/Users/tonybai/Go'
GOPROXY='https://goproxy.cn'
GOROOT='/Users/tonybai/.bin/go1.22.0'
GOSUMDB='off'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/tonybai/.bin/go1.22.0/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cz/sbj5kg2d3m3c6j650z0qfm800000gn/T/go-build1615674647=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I have example below:

// main.go

package main

import (
	"fmt"
)

var (
	v0 = constInitCheck()
	v1 = variableInit("v1")
	v2 = variableInit("v2")
)

const (
	c1 = "c1"
	c2 = "c2"
)

func constInitCheck() string {
	if c1 != "" {
		fmt.Println("main: const c1 has been initialized")
	}
	if c1 != "" {
		fmt.Println("main: const c2 has been initialized")
	}
	return ""
}

func variableInit(name string) string {
	fmt.Printf("main: var %s has been initialized\n", name)
	return name
}

func init() {
	fmt.Println("main: first init func invoked")
}

func init() {
	fmt.Println("main: second init func invoked")
}

func main() {
	// do nothing
}

What did you see happen?

Compile and run the code in go 1.22.0，I got this output:

$go run main.go
main: var v1 has been initialized
main: var v2 has been initialized
main: const c1 has been initialized
main: const c2 has been initialized
main: first init func invoked
main: second init func invoked

What did you expect to see?

According to the latest go spec:

Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in declaration order which has no dependencies on uninitialized variables.

More precisely, a package-level variable is considered ready for initialization if it is not yet initialized and either has no [initialization expression](https://go.dev/ref/spec#Variable_declarations) or its initialization expression has no dependencies on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization.

v0、v1 and v2 all have initializaiton expression , and should be considered as "not ready for initialization"。their init order should be v0 -> v1 -> v2。but the real init order is v1 -> v2 -> v0。

[bigwhite]

If we place const declaration block before var declaration block, the init order will become v0 -> v1 -> v2：

$go run main.go // go1.22.0
main: const c1 has been initialized
main: const c2 has been initialized
main: var v1 has been initialized
main: var v2 has been initialized
main: first init func invoked
main: second init func invoked

[seankhliao]

constInitCheck() has a dependency on c1 and c2, which are uninitialized at that point.
variableInit has no dependency. this all appears consistent with the current spec.

[bigwhite]

@seankhliao thanks for instant reply. Go spec says "no dependencies on uninitialized variables" , but c1 and c2 are uninitialized constants. there is a little bit of inconsistency.

[randall77]

This looks like it changed between 1.21 and 1.22.
I agree with the OP, constants are not variables.
@gri

[randall77]

@griesemer

[gopherbot]

Change https://go.dev/cl/575075 mentions this issue: cmd/compile: put constants before variables in initialization order

[randall77]

Bisect points to https://go-review.googlesource.com/c/go/+/517617, switching to use types2 for init order, which makes sense. types2 always had this bug.
FYI @mdempsky

[zigo101]

[edit]: BTW, should this be backported? As it may cause some wrong initial values: https://go.dev/play/p/2HA4DR3_cjD

---

Just a curious BTW question: why is a initialized after b?
By my understanding of the spec, the order should be inverted.

package main

var x = 0
var a = foo()
var b = x

func foo() int {
	x++
	return x
}

func main() {
	println(a, b) // 1 0
}

[ianlancetaylor]

@go101 Thanks. Would you mind opening a separate issue for that? The gc compiler prints 1, 0 while gccgo prints 1, 1.