For this version of your program with
type S struct{ B byte }
There is an allocation,
// example.go
package main
import (
"runtime"
"unsafe"
)
type S struct{ B byte }
func main() {
var stats runtime.MemStats
runtime.ReadMemStats(&stats)
println(stats.Mallocs)
var x S
runtime.ReadMemStats(&stats)
println(stats.Mallocs)
_ = *ref(x)
runtime.ReadMemStats(&stats)
println(stats.Mallocs)
println(unsafe.Sizeof(x))
}
func ref(z S) *S {
return &z
}
Output:
$ go version
go version devel +bc29313 Fri Apr 14 16:47:25 2017 +0000 linux/amd64
$ go run -gcflags '-N -l -m' example.go
# command-line-arguments
./example.go:27:9: &z escapes to heap
./example.go:26:16: moved to heap: z
./example.go:14:23: main &stats does not escape
./example.go:17:23: main &stats does not escape
./example.go:20:23: main &stats does not escape
83
83
84
1
$
Peter
On Saturday, April 15, 2017 at 12:14:48 PM UTC-4, peterGo wrote:
>
> 刘桂祥,
>
> Size and alignment guarantees
> https://golang.org/ref/spec#Size_and_alignment_guarantees
>
> A struct or array type has size zero if it contains no fields (or
> elements, respectively) that have a size greater than zero. Two distinct
> zero-size variables may have the same address in memory.
>
> You have
>
> type S struct{}
>
> Why do you expect runtime.MemStats to show an allocation for zero bytes?
>
> Peter
>
> On Saturday, April 15, 2017 at 10:12:34 AM UTC-4, 刘桂祥 wrote:
>>
>>
>> go build -gcflags '-N -l' example.go && ./example
>> If I use this to compile example.go it shound disable function inline and
>> the compile code should call ref() in example.go and will call mallocgc
>> runtimestats.Mallocs can see it but it is not.
>>
>> 在 2017年4月14日星期五 UTC+8下午10:01:25，Ian Lance Taylor写道：
>>>
>>> On Thu, Apr 13, 2017 at 10:45 PM, 刘桂祥 <liuguix...@gmail.com> wrote:
>>> > go1.7.1
>>> >
>>> > go run -gcflags '-N -l -m' example.go
>>>
>>> It is the -m flag that is generating the output you see.
>>>
>>> If there is some other bug, please let us know what it is.
>>>
>>> Ian
>>>
>>> > 在 2017年4月14日星期五 UTC+8下午1:18:26，Ian Lance Taylor写道：
>>> >>
>>> >> On Thu, Apr 13, 2017 at 8:33 PM, 刘桂祥 <liuguix...@gmail.com> wrote:
>>> >> > // example.go
>>> >> > package main
>>> >> >
>>> >> >
>>> >> > import "runtime"
>>> >> >
>>> >> >
>>> >> > type S struct{}
>>> >> >
>>> >> >
>>> >> > func main() {
>>> >> > var stats runtime.MemStats
>>> >> >
>>> >> >
>>> >> > runtime.ReadMemStats(&stats)
>>> >> > println(stats.Mallocs)
>>> >> > var x S
>>> >> > runtime.ReadMemStats(&stats)
>>> >> > println(stats.Mallocs)
>>> >> > _ = *ref(x)
>>> >> > runtime.ReadMemStats(&stats)
>>> >> > println(stats.Mallocs)
>>> >> > }
>>> >> >
>>> >> >
>>> >> > func ref(z S) *S {
>>> >> > return &z
>>> >> > }
>>> >> >
>>> >> > go build -gcflags '-N -l' example.go && ./example
>>> >> > or
>>> >> > go run -gcflags '-N -l' example.go
>>> >> >
>>> >> > results:
>>> >> > ./example.go:21: &z escapes to heap
>>> >> > ./example.go:20: moved to heap: z
>>> >> > ./example.go:10: main &stats does not escape
>>> >> > ./example.go:13: main &stats does not escape
>>> >> > ./example.go:16: main &stats does not escape
>>> >> > 61
>>> >> > 61
>>> >> > 61
>>> >>
>>> >> That looks like the output from -gcflags="-m -l". Are you really
>>> sure
>>> >> you typed -N? What version of Go are you using?
>>> >>
>>> >> Ian
>>> >
>>> > --
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>>> Groups
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>>>
>>
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