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cmd/compile: optimisations change the behaviour of *struct{} #8938

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ainar-g opened this issue Oct 15, 2014 · 2 comments
Closed

cmd/compile: optimisations change the behaviour of *struct{} #8938

ainar-g opened this issue Oct 15, 2014 · 2 comments
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@ainar-g
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ainar-g commented Oct 15, 2014 

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
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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
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rsc commented Oct 15, 2014

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.

@bradfitz bradfitz modified the milestone: Go1.5 Dec 16, 2014
@rsc rsc removed accepted labels Apr 14, 2015
@rsc rsc modified the milestones: Unplanned, Go1.5 May 19, 2015
@rsc rsc changed the title cmd/gc: optimisations change the behaviour of *struct{} cmd/compile: optimisations change the behaviour of *struct{} Jun 8, 2015
@rsc rsc removed their assignment Jun 23, 2022
@golang golang locked and limited conversation to collaborators Jun 23, 2023
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@bradfitz @josharian @rsc @ainar-g @griesemer @gopherbot