> On Aug 5, 2016, at 9:42 AM, KS Sreeram via swift-users > <swift-users@swift.org> wrote: > > >> On 05-Aug-2016, at 1:19 PM, Daniel Vollmer <li...@maven.de> wrote: >> >> I’m by no means a Swift expert (not even a user at the moment), but I don't >> see a way to do this either. The best that comes to mind is initializing a >> ContiguousArray with the sufficient capacity as size, using >> withUnsafeBufferPointer to overwrite elements, and then use replaceAll() with >> an empty collection to remove the excess size. > > The only initializer that could help with this is: > init(count:repeatedValue:). Unfortunately, this means the buffer is written > to twice - once in the initializer, and a second time to actually fill in the > data. It’s not efficient. > >> >> I'd think that just reserving enough capacity and then appending the elements >> one-by-one will be (at least?) as efficient: The byte is always copied >> anyway, >> and in this case you wouldn’t have to default-construct the capacity >> beforehand. > > This too isn’t efficient because each append call will incur an the cost of > checking for sufficient capacity and also incrementing the size. The > additional penalty could dwarf the cost of actually copying a byte! I don’t > think the compiler is capable of removing this inefficiency.
cc-ing Arnold, who owns this part of the optimizer. IIRC we are able to hoist the capacity check in 'append' loops in some cases. -Joe _______________________________________________ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
