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runtime: KeepAlive causes its argument to escape #19687

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cespare opened this issue Mar 24, 2017 · 3 comments
Closed

runtime: KeepAlive causes its argument to escape #19687

cespare opened this issue Mar 24, 2017 · 3 comments
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FrozenDueToAge Performance
Milestone
Go1.9Maybe

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@cespare
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cespare commented Mar 24, 2017 •
edited

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

I see this on Go 1.8 and also tip. The examples below were run with tip.

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

linux/amd64

See original context here: https://groups.google.com/d/msg/golang-nuts/dcjzJy-bSpw/6yntq3OVBQAJ

It seems that calling runtime.KeepAlive causes the argument to escape:

package main

import "runtime"

func f(s string) {
	runtime.KeepAlive(s)
}

func main() {
	f("hello")
}

// ./test.go:5:6: can inline f
// ./test.go:9:6: can inline main
// ./test.go:10:3: inlining call to f
// ./test.go:6:19: s escapes to heap
// ./test.go:5:10: leaking param: s
// ./test.go:10:3: s escapes to heap

Bizarrely, if I change the definition of KeepAlive to give the parameter a name:

//go:noinline
func KeepAlive(v interface{}) {} // instead of func KeepAlive(interface{}) {}

then the argument doesn't escape:

package main

import "runtime"

func f(s string) {
	runtime.KeepAlive(s)
}

func main() {
	f("hello")
}

// ./test.go:5:6: can inline f
// ./test.go:9:6: can inline main
// ./test.go:10:3: inlining call to f
// ./test.go:5:10: f s does not escape
// ./test.go:6:19: f s does not escape
// ./test.go:10:3: main s does not escape

@randall77 @ianlancetaylor
@dmac
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dmac commented Mar 24, 2017

Here's a minimal example that I believe exhibits the same issue:

package main

//go:noinline
func foo(*int) {}

func main() {
	n := 1
	foo(&n)
}

// ./test.go:8: &n escapes to heap
// ./test.go:7: moved to heap: n
package main

//go:noinline
func foo(p *int) {}

func main() {
	n := 1
	foo(&n)
}

// ./test.go:4: foo p does not escape
// ./test.go:8: main &n does not escape

@bradfitz bradfitz added this to the Go1.9Maybe milestone Mar 24, 2017
@randall77 randall77 self-assigned this Mar 24, 2017
@josharian
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cc @dr2chase

@gopherbot
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CL https://golang.org/cl/38600 mentions this issue.

@golang golang locked and limited conversation to collaborators Mar 24, 2018
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@bradfitz @josharian @dmac @cespare @randall77 @gopherbot