cmd/compile: package-level variable initialization order looks not correct

[zigo101]

Go version

go version go1.22.1 linux/amd64

Output of go env in your module/workspace:

.

What did you do?

package main

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

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

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

What did you see happen?

1 0

What did you expect to see?

1 1

[ianlancetaylor]

CC @griesemer

gccgo prints 1 1.

[griesemer]

It looks like the type checker is computing the correct order. Here is the initorder trace for the above code:

== initOrder ==
Computing initialization order for package p ("p")

Object dependency graph:
        x has no dependencies
        a depends on
                foo
        b depends on
                x
        foo depends on
                x
        main depends on
                a
                b

Transposed object dependency graph (functions eliminated):
        x depends on 0 nodes
                b is dependent
                a is dependent
        a depends on 1 nodes
        b depends on 1 nodes

Processing nodes:
        x (src pos 1) depends on 0 nodes now
        a (src pos 2) depends on 0 nodes now
        b (src pos 3) depends on 0 nodes now

Initialization order:
        x = 0
        a = foo()
        b = x

[randall77]

Yes, this looks like cmd/compile/internal/staticinit not getting it right somehow.
Initorder is generally computed and passed through unified IR export/import correctly. But then we don't use it? Something is odd here.

[kungfukennyg]

I'd like to dig further into this issue if no one else is claiming it.

[mainjzb]

change to this

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

func foo() int {
	x = x + 1       // diferent
	return x
}

func main() {
	println(a, b)
        // 1 1 # go1.22
        // 1 0 # go1.21
}

[cuonglm]

This is a bug in staticinit, where the compiler try optimizing:

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

to:

var x = 0
var a = ~r0
var b = 0

without recognizing that the optimization is unsafe, because foo was inlined.

[gopherbot]

Change https://go.dev/cl/575175 mentions this issue: cmd/compile: add missing OINLCAll case in mayModifyPkgVar