[swift-evolution] Swift ABI Stability Manifesto

[Dave Abrahams]

on Thu Jan 26 2017, Michael Ilseman <swift-evolution at swift.org> wrote:

> Weak references will most likely not be bitwise-movable on all platforms.
>
> In order for a weak reference to be nil-ed out, the weak reference
> itself must, conceptually, be referenced. There may be many convoluted
> schemes that can serve as alternatives, 
> but a straight-forward (and pragmatic) approach would make the weak
> reference address-only. 

I don't think adding a level of indirection is convoluted, but it does
bring its own problems.

> That is, something elsewhere has taken and stored its address so that
> it can nil-out the weak reference when the objects that it references
> is destroyed. At that point, it can’t just be bitwise-moved, as that
> would result in a dangling reference to the weak reference. One
> approach to implementing this is a global table mapping class instance
> addresses to weak reference addresses that need to be nil-ed
> out. Alternatively, the class instances could store a list of the
> addresses of weak reference to themselves. Either way, if the address
> of the weak reference changes, something somewhere else in memory must
> be updated. Thus, they are not bitwise-movable.
>
> This is not true for unowned references, which can be implemented without their address being taken,
> e.g. by a separate ref count.
>
>> On Jan 25, 2017, at 10:30 PM, Guillaume Lessard <glessard at tffenterprises.com> wrote:
>> 
>> 
>>> On 25 janv. 2017, at 14:16, Michael Ilseman via swift-evolution <swift-evolution at swift.org>
> wrote:
>>> 
>>> An example of a type that is neither trivial nor bitwise movable is
>>> a struct containing a weak reference. Weak references are tracked
>>> in a side table so that they can be nil-ed out when the referenced
>>> object is destroyed. When moving an object of such type from one
>>> address to another, the side table must be updated to refer to the
>>> weak reference's new address.
>> 
>> Isn’t this specifically a description of @objc weak references? Would weak references with no
> compatibility mode (e.g. on Linux) be bitwise-movable?
>> 
>> Thanks for the illuminating document!
>> 
>> Guillaume Lessard
>> 
>
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-- 
-Dave