[swift-evolution] Pitch: Cross-module inlining and specialization
jgroff at apple.com
Wed Oct 4 11:36:23 CDT 2017
> On Oct 3, 2017, at 9:59 PM, Slava Pestov <spestov at apple.com> wrote:
>>> On Oct 3, 2017, at 9:56 PM, Chris Lattner <clattner at nondot.org> wrote:
>>>> On Oct 3, 2017, at 9:50 AM, Joe Groff <jgroff at apple.com> wrote:
>>>>> On Oct 2, 2017, at 10:58 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> We have discussed adding a "versioned @inlinable" variant that preserves the public entry point for older clients, while making the declaration inlinable for newer clients. This will likely be a separate proposal and discussion.
>>>> 5) It eliminates this complexity.
>>> 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.
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