Intriguing Pointer Equality Behavior on Zero-Width Types

[izzii (Yuto Ichikawa)]

I’d like to report an intriguing behavior regarding the pointer equality of zero-width types. Apologies if this is trivial or a known issue.

The two codes below appear to be identical, yet the results of &a == &b differ. It seems predictable that “variables go into separate addresses on the stack, unless pointers are needed”, as opposed to “variables escape to the same address on the heap, when pointers are needed,”.

// go1.20.5 darwin/arm64

// go run input.go
func main() {
	var a, b struct{}
	fmt.Println(&a == &b) // false
}

// go build -gcflags="-m -N -l" -o output input.go
// # command-line-arguments
// ./input.go:9:13: ... argument does not escape
// ./input.go:9:17: &a == &b escapes to heap

// go1.20.5 darwin/arm64

// go run input.go
func main() {
	var a, b struct{}
	fmt.Println(&a == &b)  // true
	fmt.Printf("%p\n", &a) // 0x1172f60 *depends on env
	fmt.Printf("%p\n", &b) // 0x1172f60 *depends on env
}

// go build -gcflags="-m -N -l" -o output input.go
// # command-line-arguments
// ./input.go:8:6: moved to heap: a
// ./input.go:8:9: moved to heap: b
// ./input.go:9:13: ... argument does not escape
// ./input.go:9:17: &a == &b escapes to heap
// ./input.go:10:12: ... argument does not escape
// ./input.go:11:12: ... argument does not escape

Is this defined behavior, undefined behavior, or a bug?
Please provide any insights or feedback that you might have.
Thanks.

[skillian (Sean Killian)]

The comparison operators section of the language specification says this about pointers:

Pointers to distinct zero-size variables may or may not be equal.

And the last sentence of the language specification says:

Two distinct zero-size variables may have the same address in memory.

But that doesn’t mean that they have to. I’m not aware if anywhere the criteria for when that happens is specified; I suspect it is an implementation detail of the compiler.

[izzii (Yuto Ichikawa)]

@skillian

I’m glad to find out that it’s clearly written in the specifications that I want to know!
According to this text, it seems like this behavior is not defined.

From now on, I’m going to make sure to check the language specifications myself.
Thank you!