How can I express "@inline(always) but not when -Osize"?

As part of my work on GitHub - swift-dns/swift-endpoint: A highly-optimized library containing types representing an endpoint, such as DomainName and IPv4/v6Address · GitHub parts of which might soon make it to a swiftlang repo under supervision of the Networking workgroup [related post], I'm trying to make the ip-related implementations even more performant than the overkill they already are. Those who have worked on performance or studied it a bit know that performance is not always as straig...