> On Dec 22, 2017, at 7:09 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>
> On Fri, Dec 22, 2017 at 6:12 PM, Chris Lattner <clatt...@nondot.org
> <mailto:clatt...@nondot.org>> wrote:
>
>> On Dec 22, 2017, at 1:03 PM, Xiaodi Wu <xiaodi...@gmail.com
>> <mailto:xiaodi...@gmail.com>> wrote:
>>
>>> In short, respectfully request that you at least add this approach to the
>>> "alternatives considered” section.
>>
>> So, does anyone have any strong objections to Chris’s proposal?
>>
>> From an implementation standpoint, reworking the parser to parse
>> @available(inlinable) and @available(fixedContents) or whatever would be
>> straightforward. I would still like to punt the version range part of this
>> to a future proposal, though.
>>
>>
>> I wish I had more time to compose a fully thought-out reply, but that's not
>> going to happen in a little while because of outside constraints, so I'll
>> spill a few thoughts here:
>
> No rush, no worries, enjoy the holiday!
>
>> I'm not a great fan of the @available(inlinable) notation.
>>
>> For one, I have a hard time reasoning how Swift would behave when
>> inlinability is tied to OS version. In this example, if the *app* (as
>> opposed to the library) is compiled (as opposed to run) on iOS 16+, then the
>> *library method* would potentially be emitted into the app, but if compiled
>> on iOS 15 it wouldn't? Huh?
>
> No: availability information kicks in based on what you are *deploying* to,
> not what you’re compiling on.
>
> I expect that this stuff will be extremely rarely used in practice, but
> here’s an example:
>
> iOS15 declares this public:
>
> public void foo() {
> bar()
> }
>
> iOS16 wants to promote foo to inlinable, but knows that the inlined body
> doesn’t work with iOS15, because iOS15 needs the call to bar to happen (for
> whatever reason)
>
> @available(inlinable: iOS16)
> public void foo() {
> // nothing needed on iOS16 or later.
> }
>
> Deployment platform makes more sense, but I still can't envision a real use
> case. What sorts of `bar()` would hypothetically be necessary for iOS 15 but
> not 16? Why would a third-party library need to increase its inlining
> availability for an app based on deployment platform?
A better example would be if bar() was itself only available in iOS 16:
@available(iOS 15)
@available(inlinable: iOS 16)
public func foo() {
bar()
}
@available(iOS 16)
public func bar() { … }
Suppose your app calls foo() and deploys to iOS 15. Then you cannot inline
foo(), because bar() does not exist on iOS 15. (Presumably, foo() had a
different implementation on iOS 15). But if you’re deploying to iOS 16, all is
well, and you can inline foo(), which results in your app directly calling
bar().
> I'm quite sure that the reason you inverted your "abiPublic" example is
> because of the same issue. Intuitively, you would want to mark something as
> "available" in version N and then maybe some special kind of "available" in
> version N+1 (which @available(inlinable) would be). But
> @available(linkerSymbol), as you spell it, suffers from a similar problem to
> that of @available(unavailable): it's _not_ a special kind of API
> availability, but rather indicates that something is less-than-available.
> That is, you would use it to indicate that something is available as ABI but
> not as API. In that sense, it extends the "mess" we have with
> @available(unavailable).
I don’t think it’s quite the same thing as @available(unavailable). An
@available(abiPublic) symbol would still be declared to have internal
visibility, so in this case the @available attribute makes it strictly more
visible than it would be without. We’re not going to spell it as
‘@available(abiPublic) public’, which indeed would be confusing because the
symbol is not actually public at the source level.
Slava
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