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

[Chris Lattner]

> On Oct 4, 2017, at 9:36 AM, Joe Groff <jgroff at apple.com> wrote:
> 
>>>> It wouldn't avoid the complexity, because we want the "non-ABI, always-emit-into-client" behavior for the standard library. For the soon-to-be-ABI-stable libraries where @inlinable even matters, such as the standard library and Apple SDK overlays, there's pretty much perfect overlap between things we want to inline and things we don't want to take up binary space and ABI surface in binaries, so the behavior Slava proposes seems like the right default. 
>>> 
>>> I disagree.  The semantics being proposed perfectly overlap with the transitional plan for overlays (which matters for the next few years), but they are the wrong default for anything other than overlays and the wrong thing for long term API evolution over the next 20 years.
>> 
>> Can you elaborate on this? If inlinable functions have public entry points, the version in the framework may or may not be called… because of SIL serialization and inlining. Since the existence of the public entry point doesn’t offer much of a guarantee, it seems desirable to not have the public entry point. For example if the inlinable function is not used elsewhere in the framework, we wouldn’t have to emit it at all. This might make the standard library smaller for instance.
>> 
>> However I’m still waiting for Dave or Jordan to chime in with the original justification for the ‘always emit into client’ behavior. IIRC there was a resilience-related argument too, but I don’t remember what it is now.
> 
> The suggestion to have this semantics was originally my fault, I believe, and it arose from the observation that if we have 'inlinable' backed by a symbol in the binary, then we'd also want the 'must be emitted by client' attribute. I think 'must be emitted by client' is going to almost always be preferable for an inlinable function, though, so it's better to have the single attribute with this behavior, only constrained by backward deployment.

What is the use case of “must be emitted by client” attribute?  If I imagine that the Swift 5 standard library is shipped in the OS, I can see cases where deprecated/legacy shims for Swift3/4 compatibility would be emitted into the client but not shipped in the OS.  Those seem relatively obscure though.

The other issue is that if/when the stdlib and overlays start shipping in the OS, that backward deployment will require them to be statically linked into the app (optionally with an arclite style dynamic fallback approach).  This problem seems orthogonal to the discussion though.

-Chris

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