> On 4. Aug 2017, at 20:15, John McCall via swift-evolution 
> <swift-evolution@swift.org> wrote:
> 
>> 
>> On Aug 4, 2017, at 1:19 PM, Félix Cloutier via swift-evolution 
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>> 
>> That's not a concern with the `let` case that Robert brought up, since you 
>> can't mutate a `let` array at all.
>> 
>> The big thing is that unconstrained escape analysis is uncomputable. Since 
>> Swift array storage is COW, any function that receives the array as a 
>> parameter is allowed to take a reference on its storage. If the storage 
>> lives on the stack and that reference outlives the stack frame, you've got a 
>> problem. It's the same problem that motivated @escaping for closures.
>> 
>> You could allow storage to be on the stack by forcing user to make a 
>> pessimistic copy, which is possibly not an improvement.
> 
> Right.  I think maybe the name people keeping using for this feature is 
> misleading; a better name would be "inline arrays" or "directly-stored 
> arrays".  Having a fixed size is a necessary condition for storing the array 
> elements directly, but the people asking for this feature are really asking 
> for the different representation, not just the ability to statically 
> constrain the size of an array.
> 
> That representation difference comes with a lot of weaknesses and trade-offs, 
> but it's also useful sometimes.
> 
> John.
> 

Right, and the question I’ve been asking (indirectly) is: why is this only 
useful for arrays? Doesn’t it really apply to any value-type which allocates 
storage which it manages with COW semantics (e.g. Dictionary, Set, Data, your 
own custom types…)? Really, we want to inform the compiler that the 
dynamically-allocated memory is part of the value - and if it sees that the 
storage is only allocated once, it should be allowed to allocate that storage 
inline with the value, on the stack.

As I understand it, the only problem with this is when a function takes such a 
value as a parameter and assigns it to some escaping reference (an ivar, 
global, or capturing it inside an escaping closure).

So why can’t such assignments simply check if the value has inline storage and 
copy it to the heap if necessary? The compiler should be able to optimise the 
function so the check (which is really cheap anyway) only needs to happen once 
per function. Because the entire type has value semantics, we can substitute 
the original value with the copy for the rest of the function (preventing 
further copies down the line).


// Module one

import ModuleTwo

func doSomething() {

    let values = (0..<5).map { _ in random() }    // allocated inline, since 
the size can never change
    ModuleTwo.setGlobalItems(values)              // passes a stack-allocated 
array to the (opaque) function
}

// Module two

var GlobalItems = [Int]()
var MoreGlobalItems = [Int]()

func setGlobalItems(_ newItems: [Int]) {

    GlobalItems = newItems          // assignment to escaping reference: checks 
for inline storage, copies to heap if needed

    // all references to ‘newItems’ from this point refer to the copy known to 
be on the heap

    MoreGlobalItems = newItems  // we already have a known out-of-line copy of 
the value; no checks or copying needed
}

// To make it more explicit...

func setGlobalItems_explicit(_ newItems: [Int]) {

    let newItems_heap = newItems.backing.isAllocatedInline ? 
newItems(withBacking: newItems.backing.clone()) : newItems
    GlobalItems       = newItems_heap
    MoreGlobalItems   = newItems_heap
}


This would require some special language support for values that allocate 
memory which is managed as COW.

- Karl

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