Part of the beauty of how optionals are implemented in Swift is that the compiler doesn’t have to do any magic w.r.t. optionals besides a bit of syntactic sugar (`T?` -> `Optional<T>`, `if let x` -> `if let case .some(x)`, auto-boxing when necessary, etc.). - I strongly dislike the idea of special-casing optionals just to save a Byte. - Optionals were presented as explicitly removing the need for such a sentinel value in the first place. - There are reasonable cases where such a bit pattern is reasonably necessary to the data (e.g. bit fields, RSSI, IP addresses, etc.) and removing that value would force ugly workarounds and/or moving to a larger int size because of an ill-advised implementation detail. - If performance or memory is so critical to your specific use case, use a non-optional and your own sentinel value. It’s likely no less efficient than having the compiler do it that way.
(more below) > On Oct 18, 2016, at 11:17 AM, Guoye Zhang via swift-evolution > <swift-evolution@swift.org> wrote: > > Currently, Swift Int family and UInt family have compact representations that > utilize all available values, which is inherited from C. However, it is > horribly inefficient to implement optional integers. It takes double the > space to store [Int?] than to store [Int] because of alignment. > > I propose to ban the top value in Int/UInt which is 0xFFFF... in hex. Int > family would lose its smallest value, and UInt family would lose its largest > value. Top value is reserved for nil in optionals. An additional benefit is > that negating an Int would never crash. > > Interacting with C/Obj-C is a major concern, but since we are already > importing some of the unsigned integers as Int which loses half the values, I’d argue those imports are bugs and should be fixed to the correct signedness. > one value is not such big a drawback. Unless you happen to need all $width bits. > Alternatively, we could leave current behavior as CInt/CUInt. Converting them > to the new Int?/UInt? doesn't generate any instructions since the invalid > value already represents nil. Trying to convert an invalid value like that crashes in most of Swift. > > With optional integers improved, we could implement safe arithmetic > efficiently, or even revisit lenient subscript proposals, I don’t see how losing a particular value has any effect on either of those, but it’s possible there’s some theory or implementation detail I’m not aware of. > but they are not in the scope of this pitch. Float/Double optionals could > also be improved with the similar idea. (Isn't signaling nan the same as nil) > Nested optionals such as "Int??" are still bloated, but I don't think they > are widely used. > > So what do you think? Can we break C compatibility a bit for better Swift > types? We can, and do. C.f. structs, non-@objc classes, and enums not RawRepresentable with a C-compatible entity. If anything, this breaks compatibility with the rest of Swift. > > - Guoye > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
