[swift-evolution] Pitch: Cross-module inlining and specialization

[Slava Pestov]

> On Oct 3, 2017, at 10:32 PM, Jonas B <bobergj at gmail.com> wrote:
> 
> 
>> On 4 Oct 2017, at 13:36, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> 
>>> On Oct 3, 2017, at 9:14 PM, Jonas B via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 
>>> Now I understand that this use-case is deferred for a later separate discussion, but my point here is that the name and the semantics of this attribute should be somewhat “forward-compatilble” with this use-case. “ inlinable” does not sound appropriate, because we don’t want to “inline” (in the C/C++ meaning) declarations into each usage site.
>>> Instead we want to compile the annotated parts of -all linked modules- as one unit. Basically, for those parts, the module name would just function like a C++ namespace - an input to the symbol name mangling, and then the whole thing could be whole-module-optimized together.
>> 
>> Yeah, @inlinable does not actually force any kind of inlining to be performed — it declared that the SIL for the function body should be serialized as part of the module.
>> 
>>> 
>>> This touches upon another comment someone made previously in this discussion - that access level and compiler visibility should be separate concepts. Because not just public methods, also private methods should be subject to this. 
>> 
>> The undocumented @_versioned attribute is currently used to make something visible to the compiler without making it visible in the language. It sounds like there’s some interest in documenting this attribute too — can someone suggest a better name than @_versioned? If we converge on a design here I can incorporate that into the proposal, relaxing the restriction that @inlinable functions can only reference other public functions.
>> 
>> Slava
> 
> 
> It’s not totally clear to me what @_versioned is supposed to do. Well, it’s kind of clear that if something less-than-public in module A is declared @_versioned then it’s visible to the compiler when compiling module B (which imports module A). But does @_versioned imply @inlineable? If not, what’s the use case for declaring something @_versioned but not @inlineable? Giving some more information to the optimiser without introducing ABI fragility? Why not always do that then?
> 

@_versioned makes a symbol visible externally without making it visible from the language. There is no requirement that a @_versioned thing is @inlinable. It is used when you want to reference an internal function from an inlinable function. Eg,

internal func myImplDetail() { … }

@inlinable public func myPublicFunction() { myImplDetail() } // error!

—

@_versioned internal func myImplDetail() { … }

@inlinable public func myPublicFunction() { myImplDetail() } // OK

Slava
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