Go: cmd/compile: empty struct addresses are not equal

[mnhkahn]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

GOARCH="amd64"
GOBIN="/vagrant/gopath/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/vagrant/gopath"
GORACE=""
GOROOT="/vagrant/go"
GOTOOLDIR="/vagrant/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build779422972=/tmp/go-build"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

var a, b struct{}
fmt.Println(&a == &b) // true

code link: https://play.golang.org/p/zUnW3LHCCbi.

What did you expect to see?

Print true.

What did you see instead?

Print false. Go with version 1.3.3 prints true but 1.9 print false. Is this reasonable or a bug?

[geekhckr]

You are comparing addresses of empty structs. If you compare values, then it returns true.
Try
fmt.Println(a == b) // true

[davecheney]

@mvdan the OPs example prints true in previous versions of Go. They want to know if this change was intentional, or a bug.

[mvdan]

I did not get that impression from reading the post, my apologies.

[ALTree]

The title is misleading. You're not comparing empty structs, you are comparing the addresses of two struct{} variables.

Also your expectations are incorrect. The two addresses does not need to be equal. The spec says:

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

So both true and false are correct answers.

[ALTree]

I'd argue that it doesn't really matter whether the change (which btw was introduced in go1.6, the first version that prints false) was introduced intentionally or as a side effect of new optimizations in the compiler, since both answers are allowed by the spec, but I'm labelling this as needsInvestigation in case there's actually something unexpected going on.

[mindlesslcc]

Two distinct zero-size variables may have the same address in memory.
https://golang.org/ref/spec#Size_and_alignment_guarantees

[bradfitz]

Yes, this is working as intended per the spec, as others have pointed out.