[swift-users] [swift-evolution] How does "Sequence.joined" work?

[Félix Cloutier]

The benefit that standard library containers have over containers from other modules is that they're optimized as if they were part of your own module, so you can get the same thing by including the collection classes in your own executable instead of linking with the module. It's my understanding that the @_transparent attribute could surface to the public side with module format stability.

Either way, the issue of cross-module optimizations is separate from COW mechanics, and it was certainly not my goal to distract anyone from the main topic by including an after-thought reference to an algorithmically impressive package. The important takeaway is that COW semantics don't rely on special features, joined() doesn't have to go out of its way to ensure that new storage isn't allocated, and the one important library feature behind these containers, isKnownUniquelyReferenced, lives in broad daylight.

> Le 8 août 2017 à 22:56, Taylor Swift <kelvin13ma at gmail.com> a écrit :
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> 
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> On Wed, Aug 9, 2017 at 1:50 AM, Rob Mayoff <mayoff at dqd.com <mailto:mayoff at dqd.com>> wrote:
> On Tue, Aug 8, 2017 at 11:51 PM, Taylor Swift via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
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> 
> On Wed, Aug 9, 2017 at 12:29 AM, Félix Cloutier via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Yes, exactly. An Array<T> is a struct wrapper for a reference type representing storage. Mutating functions first check if they own the only reference to the storage using isKnownUniquelyReferenced <https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced>. If not, they make a fresh copy before applying the mutating operation.
> 
> There's no difference for `let` arrays. Access control is enforced at compile-time through Array's design: the compiler will prevent you from calling `mutating` functions on `let` structs, and Array is careful to not expose functionality that could modify its storage outside of `mutating` functions.
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> There is no secret. Anyone could implement the same thing only using publicly available and documented compiler features. In fact, it's been done already for some very powerful collections <https://github.com/lorentey/BTree>.
> 
> This isn’t entirely true. That BTree module readme seems to contain a lot of unsubstantiated hyperbole. It’s possible to implement a classic red-black tree in Swift that performs better than a sorted Array, down to about n = 1,500 items, not n = 100,000 items as it claims. (Actually, heap allocators these days are good enough that performance is on par with Array all the way down to n = 1.) Red-Black trees are slow when distributed as packages because of the crossmodule optimization boundary. (This also means the BTree module is much slower than Array for most reasonable n.) It’s possible to write modules using compiler attributes that mitigate this slowdown (reclaiming over 50% of lost performance) but it’s hacky and forces you to design your libraries like the standard library (meaning: ugly underscored properties everywhere and everything is public). And these features aren’t “publicly available” or documented at all. 
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> This seems harsh. I didn't notice Félix making any claims about BTree's performance. The necessary API for implementing COW is indisputably public and documented:
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> https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced <https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced>
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> I really didn’t mean it to be harsh (sorry if it sounded that way 🙁), it’s just that people tend to be overly optimistic about the performance that can be achieved with custom Collection packages. It’s true that you can imitate the functionality of stdlib Collection types with public and documented Swift features, but such custom Collections (when distributed as packages) are almost never effective at improving an application’s performance due to the huge constant factor of cross module calls, unless the library author was willing to make use of undocumented compiler features.
> 

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