[swift-dev] Question about size of Character type

[Slava Pestov]

> On Aug 19, 2016, at 2:04 PM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
> 
> 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

Hi Jordan,

I asked about a related issue, which is that RawPointer only has 1 extra inhabitant instead of 4096. You guys said you wanted non-zero integers to round-trip through RawPointer. It seems that declaring the high bits of a RawPointer as spare bits would cause the same problem as allowing more extra inhabitants.

Also I don’t think alignment is the answer here, RawPointer should be able to represent a char *, where you have no low spare bits.

Slava

> 
> 
>> On Aug 19, 2016, at 13:30, Ole Begemann via swift-dev <swift-dev at swift.org <mailto: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 <https://github.com/apple/swift/blob/master/stdlib/public/core/Character.swift>
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