> On Nov 6, 2017, at 1:04 AM, John McCall via swift-dev <swift-dev@swift.org>
> wrote:
>
>> On Nov 6, 2017, at 3:46 AM, Kelvin Ma via swift-dev <swift-dev@swift.org>
>> wrote:
>> i noticed that this enum takes up 9 bytes of storage
>>
>> 1> enum QuantizationTable
>> 2. {
>> 3. case q8 (UnsafeMutablePointer<UInt8>),
>> 4. q16(UnsafeMutablePointer<UInt16>)
>> 5. }
>> 6> MemoryLayout<QuantizationTable>.size
>> $R0: Int = 9
>>
>> but when i make it optional, it takes up 10
>>
>> 7> MemoryLayout<QuantizationTable?>.size
>> $R1: Int = 10
>>
>> can’t the compiler just tack the nil case onto the original enum as a third
>> case? after all, this works fine
>>
>> 1> enum QuantizationTable
>> 2. {
>> 3. case q8 (UnsafeMutablePointer<UInt8>),
>> 4. q16(UnsafeMutablePointer<UInt16>),
>> 5. none
>> 6. }
>> 7> MemoryLayout<QuantizationTable>.size
>> $R0: Int = 9
>>
>> the way i’m using it atm it all gets padded to 16 anyway so i don’t care
>> that much but is this something that’s being looked at?
>
> We could, yes. Our experience with enum layout optimization is that it's one
> of those things that doesn't seem to bottom out: you can always keep
> improving it in one way or another.
>
> It is an important goal of our ABI stability plan that future versions of
> Swift will be able to improve layouts for types that aren't required to
> interoperate with older versions, crucially including the private and
> internal types in a module.
In this specific case, I'm a bit surprised our existing mechanisms didn't avoid
spilling a byte here—I thought we considered the tag byte of an enum to have
spare bits, which should have let us use a spare bit from QuantizationTable to
represent nil in QuantizationTable?. Seems bug-worthy to me. (I would say
there's still a good amount of slack in our current implementation to be
tightened up before we start invoking the full employment theorem or halting
problem anyway.)
-Joe
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