[swift-evolution] Improved value and move semantics

Matthew Johnson matthew at anandabits.com
Wed Aug 3 08:41:25 CDT 2016

> On Aug 2, 2016, at 4:54 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> on Tue Aug 02 2016, Brent Royal-Gordon <brent-AT-architechies.com> wrote:
>>> On Aug 2, 2016, at 12:06 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> If it says that, it's... not quite right.  There are things we could do
>>> to make some value copies more optimal.  For example, any value type
>>> containing multiple class references—or multiple other value types (such
>>> as arrays or strings or dictionaries) that contain class references—will
>>> cost more to copy than a single class reference does.  At the cost of
>>> some allocation and indirection, we could reduce the copying cost of
>>> such values.  It's an optimization we've considered making, but haven't
>>> prioritized.  
>>> You can put a CoW wrapper around your value to do it manually.  I hacked
>>> one up using ManagedBuffer for someone at WWDC but I don't seem to have
>>> saved the code, sadly.
>> Slightly off-topic, but one day I would like to see `indirect` turned
>> into a generalized COW feature:
>> * `indirect` can only be applied to a value type (or at least to a
>> type with `mutating` members, so reference types would have to gain
>> those).
>> * The value type is boxed in a reference type.
>> * Any use of a mutating member (and thus, use of the setter) is
>> guarded with `isKnownUniquelyReferenced` and a copy.
>> * `indirect` can be applied to an enum case with a payload (the
>> payload is boxed), a stored property (the value is boxed), or a type
>> (the entire type is boxed).
>> Then you can just slap `indirect` on a struct whose copying is too
>> complicated and let Swift transparently COW it for you. (And it would
>> also permit recursive structs and other such niceties.)
> My vision for this feature is:
> a. We indirect automatically based on some heuristic, as an
>   optimization.
> b. We allow you to indirect manually.
> c. We provide an attribute that suppresses automatic indirection to
>   whatever depth possible given resilience boundaries.

This all sounds great.  Does any of this fit into Swift 4 (either phase 1 or phase 2)?  It seems like at least the automatic part would have ABI impact.

> -- 
> -Dave
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