I'm pretty sure new example is not valid Go code to begin with. You violate
the rules on converting from uintptr to unsafe.Pointer. If you read the
rules at https://pkg.go.dev/unsafe#Pointer, a unitptr can not *generally *be
converted to an unsafe.Pointer. Since this code is not valid, the question
is pretty much moot.
On Thursday, September 23, 2021 at 2:23:47 AM UTC-4 chole...@gmail.com
wrote:
> Thanks, I see. How about this one? I use a temp variable to store sh.Data,
> sh/ds/data are all no longer mentioned at the point when unsafe.Slice is
> called. Do I need to use runtime.KeepAlive?
>
> package main
>
> import (
> "fmt"
> "reflect"
> "runtime"
> "unsafe"
> )
>
> var b []byte
>
> func f(data interface{}) {
> switch ds := data.(type) {
> case []int32:
> sh := (*reflect.SliceHeader)(unsafe.Pointer(&ds))
> l := len(ds)
> d := sh.Data
> runtime.GC()
> b = unsafe.Slice((*byte)(unsafe.Pointer(d)), l*4)
> }
> // runtime.KeepAlive(data)
> }
>
> func main() {
> f([]int32{1, 2})
> runtime.GC()
> fmt.Println(b)
> }
>
> 在2021年9月23日星期四 UTC+8 上午6:11:48<Ian Lance Taylor> 写道：
>
>> On Wed, Sep 22, 2021 at 1:22 AM Cholerae Hu <chole...@gmail.com> wrote:
>> >
>> > https://play.golang.org/p/WJTH-lUukeb
>> >
>> > Do I need to uncomment line 23? IMHO, variable data and ds will not be
>> used after line 16, so they can be collected in runtime.GC. But running it
>> with gccheckmark=1 and clobberfree=1 doesn't show any problems.
>> >
>> > Are there any documents about the details or behavior of liveness
>> analysis?
>>
>> I don't see any reason why that program should need runtime.KeepAlive.
>>
>> The local variable sh will point to data (aka ds), so the object will
>> stay alive. This is not different than p := &data. After that point,
>> if data is no longer mentioned, then the variable data is no longer
>> live, but the value that p points to will remain alive.
>>
>> Ian
>>
>
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