[swift-evolution] Open Issues Affecting Standard Library API Stability

Karl razielim at gmail.com
Wed Jul 6 20:15:53 CDT 2016

> On 7 Jul 2016, at 02:06, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> On Wed, Jul 6, 2016 at 4:21 AM, Karl <razielim at gmail.com> wrote:
>> I had a PR open for this which added a Collection specialisation, but you
>> said this could be handled in a more general way which allows for more
>> optimised mutations. I’m curious how this would work; how does
>> `RangeReplaceableCollection.replaceSubrange<S:Sequence>(range:, with: S)`
>> call a more optimised implementation if S also conforms to Collection, if
>> not by adding a specialisation?
> The RRC can call into S.someCustomizationPoint(), which will
> initialize a region of memory in the most efficient way possible,
> since it has complete knowledge about the memory layout of S.
> Dmitri
> -- 
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/

Sorry, it’s been a little while since I looked at it (https://github.com/apple/swift/pull/3067 <https://github.com/apple/swift/pull/3067>). Actually, the issue is with the append() function specifically - it only has a Sequence specialisation. That’s the point; we were losing type information and not calling the more optimised replaceSubrange — which is exactly the specialisation point you are talking about, Dmitry (I think everything funnels down in to replaceSubrange).

I must have misunderstood what you were saying at the time. I’ll have to test, but I think it’s still an issue. I thought there was some more general work on RRC planned, so when I saw the bullet I thought maybe that was it.


I was trying to initialise a String.CharacterView - depending on whether I initialised it with another CharacterView, or created it empty and appended the other CharacterView, I would get huge performance differences, as the other CharacterView would get split up and a new Character created and appended for every character in the string (which had like 10,000 characters). It’s a subtle bug and a bit nasty when it hits you.


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