[swift-dev] Implementation of swift's value types

[Andrew Trick]

> On Jul 14, 2016, at 12:39 PM, Johannes Neubauer via swift-dev <swift-dev at swift.org> wrote:
> 
> Dear Devs,
> 
> I saw the WWDC 2016 video [Understanding Swift Performance][0] as well as some others regarding value types in swift of WWDC2015. I think there are a few ambiguities which make it hard both to decide which weapon to choose and to give proposition how to evolve this implementations to the better (swift-evolution).
> 
> Is there a place, where low-level decisions in the language are documented? Is there an adequate place/forum where we can ask questions regarding low-level implementations? It would be great if you add a (moderated) comment section to each of the WWDC videos, so that we can discuss the contents (with transcript-like links) as well as an errata section containing a list of Apple-Approved mistakes/ambiguities in a given video/talk.
> 
> # So, here come my questions
> 
> In the talk [Understanding Swift Performance][0] Kyle says, that value types are put stored in the stack and copied. He uses a point and a line which are both copied. Lateron Arnold uses a similar example with protocols. Then the Existential Container is used, which either uses the value buffer (for small values like `Point`) or allocates some memory on the heap and adds a pointer to this value (e.g. for a `Line`):
> 
> 1. If I have an object (instance of a class) in a variable (or a container like an array) of a protocol type, will it be stored into an Existential Container, too? Or are reference types always stored as a reference (storing it in an Existential Container makes more sense to me).

In that case the value stored in the existential container is just the reference. There’s no additional heap allocation.

Promoting objects to the stack is a separate thing that I think will only happen if the existential container is already optimized away.

> 2. If I use a variable of the concrete type (although it implements a protocol), will it always be copied (no matter its size) or does the compiler choose an existential container if it is greater than some given size (or perhaps even always, because it gives a good tradeoff?)

A value's representation starts off as the representation for whatever type is in the declaration. For something declared as a struct, it’s irrelevant whether the type also conforms to a protocol. We don’t yet provide a language feature or optimization for indirect copy-on-write struct storage.

> 3. Then Arnold says that four existential containers pointing to the same value (greater than the value buffer) use 4 heap allocations. He proposes copy-on-write (using a local storage class), but **does he mean this should be implemented by hand or is this an optimization that the swift compiler does on its own?** The issue here is, that the switch between "swift subscript" for showing an abstraction of internals and real swift code that one should write is sometimes not clear. Doing this by hand has some clear disadvantages as this would add a reference to each usage of `Line` (and reference counting) even in the first examples of Kyle. Doing this as a compiler optimization would allow to use a struct in different scenarios and always the best tradeoff is used. Otherwise, I would perhaps even need to create two different types for different situations and choose it wisely. This would add a big burden on the developer.

It needs be done by hand because the compiler doesn’t do it. The compiler *should* do it in the sense that it would make the world a better place.

> 4. If Arnold really means *manually* (see *3.*) and reference types are not stored in existential containers (see *1.*) the slides are wrong, because there a existential container is still used and the instance on the heap is named `Line` instead of `Line._storage`. So what is the case?

I don’t have the slides in front of me, but the instance is always on the heap (modulo stack promotion, which doesn’t happen for existentials). The reference to the instance is in the container.

> 5. The implementations of `String` and `Array` seem to follow the copy-on-write strategy "manually", but I think they do that because this behavior is wanted even if the values would be copied automatically (if this is true, the answer for *3.* would be *manually*). Or am I wrong here?

There’s no observable behavior to copy-on-write other than the program running much faster.

> 6. Is the Value-Witness-Table a (kind of) dictionary for all values of a given value type (bigger than the value buffer), so that you do not store any value twice on the heap, but share the reference? If this is the case the answer of *3.* should be *automatically*. But then again, the "manual" implementation of `String` and `Array` (see *4.*) make no sense anymore, does it? Or are `Array` and `String` implemented only on the lower-level and the copy-on-write implementation is not visible in their Swift implementation?

A value witness table is a dictionary for all values of a type regardless of whether it fits in a buffer. The keys are operations that can be done to any value (copy/destroy), the implementation for that type knows where the value is stored.

> 7. If you want to have a reference-type (like `NSData`) with value semantics, then I need to implement my own copy-on-write of course, but if I want to have it only on the swift-value-type level the compiler should be able to do it all by itself, shouldn't it?

Someone else can provide a better answer. I’ll say that the compiler *should* be able to do it in the sense that the world would be better off for it…

-Andy

> I read some [posts like this one][1] describing how Swift implements value types in a manner, that is conflicting with some of the things Kyle and Arnold said on WWDC 2016 (see above). Did Swift’s implementation change here between v2 and v3 or what do you think? The articles interpretation of the changes of the memory address (and the padding ints for the address struct; see the post) suggest, that always an existential container is used for structs (see *2.*) and copy-on-write is done automatically (see *3.*)…
> 
> It would be great, if someone could give me the answers to these questions :). Thanks in advance.
> 
> All the best
> Johannes
> 
> [0]: https://developer.apple.com/videos/play/wwdc2016/416/
> [1]: https://www.raywenderlich.com/112029/reference-value-types-in-swift-part-2
> 
> --
> Dr. Johannes Neubauer
> E-Mail: neubauer at kingsware.de
> WWW   : http://www.kingsware.de
> 
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