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cmd/compile: recognize x[:] of global array x as static data #7651

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mpvl opened this issue Mar 27, 2014 · 6 comments

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@mpvl
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commented Mar 27, 2014

If an array is defined in the global scope and concerted to a slice in an unused struct
literal, it will not get garbage collected by the linker.

See code below to reproduce the problem.

go version devel +0134c7020c40 Wed Mar 26 15:23:31 2014 -0700 darwin/amd64

===BEGIN===
package main

func main() {
}

type foo struct {
    arr []int
}

var f = foo{
    arr: a[:], // Remove this line and a will get collected.
}

var a = [1000111]int{3} // Removing the 3 will cause the array to be collected.

===END===
@rsc

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commented Mar 27, 2014

Comment 1:

Are you saying that you have to remove both lines to have 'a' be removed as dead code by
the linker, or does removing either one suffice?
@gopherbot

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commented Mar 30, 2014

Comment 2 by mpvl@audacitycapital.com:

No, either one works.
More importantly.  Just writing var a as a slice works as well (see code below).  So
there is a workaround. 
// This works:
var f = foo{
    arr: a, 
}
var a = []int{3}
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commented Apr 24, 2014

Comment 4:

Removing the use of a[:] lets the linker drop the variable, no matter what. That slice
operation happens in a generated func init either way. If we recognized that in the
'init->static' converter it would always get collected. I've changed the bug report to
track that.
The confusion about {} vs {3} happens because if you use {} the binary gets smaller,
because 'a' is moved into the bss. But it's still there if and only if a[:] is used in
the original.
Here are four test cases showing the variations from the original report:
g% cat x1.go # original report
package main
func main() {}
type foo struct { arr []int }
var f = foo{arr: a[:]}
var a = [1000111]int{3}
g% cat x2.go # {3} -> {}
package main
func main() {}
type foo struct { arr []int }
var f = foo{arr: a[:]}
var a = [1000111]int{}
g% cat x3.go # foo{a[:]} -> foo{}
package main
func main() {}
type foo struct { arr []int }
var f = foo{}
var a = [1000111]int{3}
g% cat x4.go # {3} -> {}, foo{a[:]} -> foo{}
package main
func main() {}
type foo struct { arr []int }
var f = foo{}
var a = [1000111]int{}
# a still here for both {} and {3}. difference in size is initialized data vs bss.
g% go build x1.go && echo $(go tool nm x1 | grep main.a) $(ls -l x1)
55020 D main.a -rwxr-xr-x 1 rsc 5000 8490784 Apr 24 17:29 x1
g% go build x2.go && echo $(go tool nm x2 | grep main.a) $(ls -l x2)
75ce0 B main.a -rwxr-xr-x 1 rsc 5000 487200 Apr 24 17:29 x2
# removing use of a removes from binary (no nm output)
g% go build x3.go && echo $(go tool nm x3 | grep main.a) $(ls -l x3)
-rwxr-xr-x 1 rsc 5000 483024 Apr 24 17:30 x3
g% go build x4.go && echo $(go tool nm x4 | grep main.a) $(ls -l x4)
-rwxr-xr-x 1 rsc 5000 483024 Apr 24 17:30 x4

Labels changed: added release-go1.4, removed release-go1.3maybe.

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commented Sep 15, 2014

Comment 5:

Labels changed: added release-go1.5, removed release-go1.4.

@griesemer

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commented Oct 1, 2014

Comment 6:

Labels changed: added repo-main.

@mpvl mpvl added accepted labels Oct 1, 2014

@bradfitz bradfitz modified the milestone: Go1.5 Dec 16, 2014

@bradfitz bradfitz removed the release-go1.5 label Dec 16, 2014

@rsc rsc removed accepted labels Apr 14, 2015

@rsc rsc changed the title cmd/ld: recognize x[:] of global array x as static data cmd/gc: recognize x[:] of global array x as static data Apr 26, 2015

@rsc rsc changed the title cmd/gc: recognize x[:] of global array x as static data cmd/compile: recognize x[:] of global array x as static data Jun 8, 2015

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commented Jun 29, 2015

Too late for Go 1.5.

@rsc rsc modified the milestones: Go1.6Early, Go1.5 Jun 29, 2015

@rsc rsc modified the milestones: Unplanned, Go1.6Early Nov 4, 2015

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