Thank Brian Candler for answering, this is the first time I use golang-nuts
so I accidentally sent you a private reply, I hope you don't mind.
Thanks Keith Randall for referring the issue and the CL, I didn't know if
there was one already. I'll join there.
On Friday 27 September 2024 at 05:33:35 UTC+7 Keith Randall wrote:
> There is an issue and CL for this ("this" = keeping the backing store
> alive when cloning a zero-length slice). See:
>
> https://github.com/golang/go/issues/68488
> https://go-review.googlesource.com/c/go/+/598875
>
> On Thursday, September 26, 2024 at 6:59:49 AM UTC-7 Brian Candler wrote:
>
>> FWIW, I found this thread:
>> https://groups.google.com/g/golang-nuts/c/QFiyvup5UuY/m/xBalv9Y3CAAJ
>> which says experimentally (albeit without showing code or results) that
>> append(S(nil), s...)
>>
>> append(s[:0:0], s...)
>> are faster than some other methods of cloning a slice, when s is "large".
>>
>> On Thursday 26 September 2024 at 14:55:22 UTC+1 Brian Candler wrote:
>>
>>> > I want to know the reason behind the decision of using *append(s[:0:0],
>>> s...)* over the previous code
>>>
>>> It would be helpful to identify specifically the "previous code" you're
>>> comparing against. Looking in git history I find this commit from about a
>>> year ago:
>>>
>>> commit b581e447394b4ba7a08ea64b214781cae0f4ef6c
>>> Author: Brad Fitzpatrick <brad...@golang.org>
>>> Date: Sat Aug 19 09:08:38 2023 -0700
>>>
>>> slices: simplify Clone a bit
>>>
>>> No need for an explicit nil check. Slicing the input slice
>>> down to zero capacity also preserves nil.
>>>
>>> Change-Id: I1f53cc485373d0e65971cd87b6243650ac72612c
>>> Reviewed-on: https://go-review.googlesource.com/c/go/+/521037
>>> Run-TryBot: Brad Fitzpatrick <brad...@golang.org>
>>> Reviewed-by: Dmitri Shuralyov <dmit...@google.com>
>>> TryBot-Result: Gopher Robot <go...@golang.org>
>>> Reviewed-by: Ian Lance Taylor <ia...@google.com>
>>>
>>> diff --git a/src/slices/slices.go b/src/slices/slices.go
>>> index a4d9f7e3f5..252a8eecfc 100644
>>> --- a/src/slices/slices.go
>>> +++ b/src/slices/slices.go
>>> @@ -333,11 +333,8 @@ func Replace[S ~[]E, E any](s S, i, j int, v ...E)
>>> S {
>>> // Clone returns a copy of the slice.
>>> // The elements are copied using assignment, so this is a shallow clone.
>>> func Clone[S ~[]E, E any](s S) S {
>>> - // Preserve nil in case it matters.
>>> - if s == nil {
>>> - return nil
>>> - }
>>> - return append(S([]E{}), s...)
>>> + // The s[:0:0] preserves nil in case it matters.
>>> + return append(s[:0:0], s...)
>>> }
>>>
>>> Is that the change you're referring to?
>>>
>>> The comment says that "slicing the input slice down to zero capacity
>>> also preserves nil", which I confirm:
>>> https://go.dev/play/p/W21qUffeSpg
>>>
>>> Therefore, it's an explicit goal of the code to preserve nilness ("in
>>> case it matters"), which your alternative of *append(S(nil), s...)* would
>>> not do. I do agree that for most practical purposes a nil slice and a
>>> zero-length, zero-capacity slice are more or less interchangeable, but it
>>> *is* possible to distinguish them:
>>> https://go.dev/play/p/Irxuq6pbv4X
>>> ... and therefore some code might depend on this (perhaps a
>>> serialization library?). It's user-visible, so it's safest to clone like
>>> with like.
>>>
>>> Apart from that, your issue seems to be: cloning an empty slice with the
>>> new code keeps a reference to the original slice backing array, albeit with
>>> zero len and cap. It can never overwrite the original backing slice, but it
>>> *can* prevent the original backing array being freed. Is that a correct
>>> summary?
>>>
>>> It seems to me that it would be pretty perverse to take a large slice,
>>> slice it down to zero len, and then ask for it to be cloned; your example
>>> doesn't seem like a real-world use case.
>>>
>>> On Thursday 26 September 2024 at 13:29:59 UTC+1 Hikmatulloh Hari Mukti
>>> (Hari) wrote:
>>>
>>>> Hi gophers, I want to know the reason behind the decision of using
>>>> *append(s[:0:0],
>>>> s...)* over the previous code since the two code return different
>>>> slice when dealing slice with zero len. The previous code will return
>>>> brand
>>>> new slice with size zero, while the current code return an empty slice
>>>> that's still pointing to the previous array. And also, why not maybe using
>>>> *append(S(nil),
>>>> s...)* instead? This will return nil when dealing with zero len slice
>>>> though, but what's the potential problem that it will cause?
>>>>
>>>> I don't know if this can be considered for a problem, but here is my
>>>> concern for the current code, *append(s[:0:0], s...)* :
>>>>
>>>> If we try to create slices from an array pool to reduce allocation by
>>>> using append, and many our slices turned out to be zero, slices.Clone will
>>>> return slice that still pointing to array in the pool. If we try creating
>>>> many of them concurrently, (if I understand it correctly) the pool may try
>>>> to create many array objects as the object retrieved from Get may haven't
>>>> been Put back to the pool. Those array objects can only be
>>>> garbage-collected after those slices are no longer used / reachable and if
>>>> it's an array of a big struct, wouldn't it might potentially pressure the
>>>> memory?
>>>>
>>>> Here is just a pseudo-code for illustration only. I think the array
>>>> generated by pool will only be garbage-collected once *ch* is consumed
>>>> and the slices are no longer used:
>>>>
>>>> var pool = sync.Pool{New: func() any { return &[255]bigstruct{} }}
>>>> var ch = make(chan []bigstruct, 1000)
>>>> for i := 0; i < 1000; i++ {
>>>> go func() {
>>>> arr := pool.Get().(*[255]bigstruct)
>>>> defer pool.Put(arr)
>>>> s := arr[:0]
>>>> ch <- slices.Clone(s) // slice points to arr
>>>> }()
>>>> }
>>>>
>>>>
>>>> CMIIW and thank you!
>>>>
>>>>
>>>>
>>>>
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