[swift-dev] Question about size of Character type

[Jordan Rose]

We have an old Radar about this, rdar://problem/16754935 <rdar://problem/16754935>. It's probably just a case we're missing in enum layout. My guess is that it's because we don't have a whole spare bit in a RawPointer, but we should be able to pick some up either from alignment or from ABI knowledge.

Jordan


> On Aug 19, 2016, at 13:30, Ole Begemann via swift-dev <swift-dev at swift.org> wrote:
> 
> The Character type is essentially defined like this [1]:
> 
> struct Character {
>    enum Representation {
>        // A _StringBuffer whose first grapheme cluster is self.
>        // NOTE: may be more than 1 Character long.
>        case large(_StringBuffer._Storage)
>        case small(Builtin.Int63)
>    }
> 
>    var _representation: Representation
> }
> 
> Note the type of the associated value for `case .small`, `Builtin.Int63`. Presumably, this is so that the type fits into a single word, including the single bit that is needed for the enum case.
> 
> However, `MemoryLayout<Character>.size` returns 9. Given the above, I would have expected 8 bytes.
> 
> Why? Is this a potential optimization that hasn't been implemented? Or am I missing something?
> 
> Thanks
> Ole
> 
> [1]: https://github.com/apple/swift/blob/master/stdlib/public/core/Character.swift
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