[swift-evolution] Improved value and move semantics

[Chris Lattner]

On Aug 3, 2016, at 7:57 PM, Joe Groff <jgroff at apple.com> wrote:
>>>> 
>>>> a. We indirect automatically based on some heuristic, as an
>>>> optimization.
>> 
>> I weakly disagree with this, because it is important that we provide a predictable model.  I’d rather the user get what they write, and tell people to write ‘indirect’ as a performance tuning option.  “Too magic” is bad.
> 
> I think 'indirect' structs with a heuristic default are important to the way people are writing Swift in practice. We've seen many users fully invest in value semantics types, because they wants the benefits of isolated state, without appreciating the code size and performance impacts. Furthermore, implementing 'indirect' by hand is a lot of boilerplate. Putting indirectness entirely in users' hands feels to me a lot like the "value if word sized, const& if struct" heuristics C++ makes you internalize, since there are similar heuristics where 'indirect' is almost always a win in Swift too.

I understand with much of your motivation, but I still disagree with your conclusion.  I see this as exactly analogous to the situation and discussion when we added indirect to enums.  At the time, some argued for a magic model where the compiler figured out what to do in the most common “obvious” cases.  

We agreed to use our current model though because:
1) Better to be explicit about allocations & indirection that implicit.  
2) The compiler can guide the user in the “obvious” case to add the keyword with a fixit, preserving the discoverability / ease of use.
3) When indirection is necessary, there are choices to make about where the best place to do it is.
4) In the most common case, the “boilerplate” is a single “indirect” keyword added to the enum decl itself.  In the less common case, you want the “boilerplate” so that you know where the indirections are happening.

Overall, I think this model has worked well for enums and I’m still very happy with it.  If you generalize it to structs, you also have to consider that this should be part of a larger model that includes better support for COW.  I think it would be really unfortunate to “magically indirect” struct, when the right answer may actually be to COW them instead.  I’d rather have a model where someone can use:

// simple, predictable, always inline, slow in some cases.
struct S1 { … }  

And then upgrade to one of:

indirect struct S2 {…}
cow struct S3 { … } 

Depending on the structure of their data.  In any case, to reiterate, this really isn’t the time to have this debate, since it is clearly outside of stage 1.

-Chris