cmd/compile: optimisations change the behaviour of *struct{}

[ainar-g]

What does 'go version' print?
go version go1.3.3 linux/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1. Get http://play.golang.org/p/mQAu2o6wEb.
2. Uncomment the Println before the if.
3. Run.

What happened?
When compiled as usual, this prints "Not Equals". But when compiled with
-gcflags '-N' it prints "Equals".

What should have happened instead?
It should have printed "Equals" in both cases, as it does when fmt.Println(a)
is commented, or if b is printed as well.

Please provide any additional information below.
The behaviour is the same on Go 1.2. The tip version (go version devel +fb3d6c1631c3 Wed
Oct 15 14:33:52 2014 -0400 linux/amd64) prints "Not Equals" both with
optimisations and without them.

[griesemer]

Comment 1:

Per the spec:
"Pointers to distinct zero-size variables may or may not be equal."
(http://tip.golang.org/ref/spec#Comparison_operators)
Technically this is not an error.
Whether or not a given implementation should be consistent with respect to this is a
different question.
Leaving for rsc.

Labels changed: added release-none, repo-main.

Owner changed to @rsc.

[rsc]

Comment 2:

It's not an error, and it has been this way since at least Go 1.1 if not earlier. I
don't think it's worth changing, certainly not now. Don't use equality for pointers to
zero-size values and expect it to mean anything.
I guess we could make all non-nil pointers to zero-size values equal. We can see the
type at compile time.

Labels changed: added release-go1.5, removed release-none.

Status changed to Accepted.