That's very good to know. Thanks, Ian.
Unfortunately if I use this KeepAlive-based fix, p escapes and so the
function now allocates. I guess I'll stick with the original version
from my first email.
Does this indicate a shortcoming of either compiler support for
KeepAlive or escape analysis in general?
Caleb
On Thu, Mar 23, 2017 at 10:26 AM, Ian Lance Taylor <i...@golang.org> wrote:
> On Thu, Mar 23, 2017 at 9:16 AM, Caleb Spare <cesp...@gmail.com> wrote:
>>
>> Brief follow-up: does the seeming validity of the code rely at all on
>> the fact that the indicated line is written as a single line? What if,
>> instead, a *StringHeader var were extracted?
>>
>> func stringToSliceUnsafe(s string) []uint64 {
>> var v []uint64
>> h := (*reflect.StringHeader)(unsafe.Pointer(&s)) // <--
>> sh := (*reflect.SliceHeader)(unsafe.Pointer(&v))
>> sh.Data = h.Data
>> sh.Len = h.Len >> 3
>> sh.Cap = h.Len >> 3
>> return v
>> }
>>
>> (Play link: https://play.golang.org/p/BmGtYTsGNY)
>>
>> Does h keep s alive? A strict reading of rule 6 doesn't seem to say
>> that keeping a *StringHeader or *SliceHeader around keeps the
>> underlying string/slice alive (but it's sort of implied by the rule 6
>> example code, which doesn't refer to s after converting it to a
>> *StringHeader).
>
> That is an interesting point. I don't think there is anything keeping
> s alive here. I think this isn't quite the same as the example in the
> docs, because that example is assuming that you are doing to use s
> after setting the fields--why else would you be doing that? In this
> case it does seem theoretically possible that s could be freed between
> the assignment to h and the use of h.Data. With the current and
> foreseeable toolchains it's a purely theoretical problem, since there
> is no point there where the goroutine could be preempted and the fact
> that s is no longer referenced be detected. But as a theoretical
> problem it does seem real. One fix would be something like
> p := &s
> h := (*reflect.StringHeader)(unsafe.Pointer(p))
> sh := (*reflect.SliceHeader)(unsafe.Pointer(&v))
> sh.Data = h.Data
> sh.Len = ...
> sh.Cap = ...
> runtime.KeepAlive(p)
>
> Ian
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