[swift-dev] Why are BinaryFloatingPoint's RawSignificand and RawExponent different type?

[Jens Persson]

I understand.
It's just very tempting to try and use the new static computed properties
for eg 23 and 52 etc.
I guess I'll just have to write a lot of boilerplate, or perhaps a protocol
that is just implemented by Double and Float (that will be very similar to
BinaryFloatingPoint in a lot of ways).
/Jens

On Sat, Aug 27, 2016 at 1:25 AM, Stephen Canon <scanon at apple.com> wrote:

> This doesn’t really scale up very well, though.  BinaryFloatingPoint needs
> to also be able to model e.g. Float2048 or similar; we generally don't want
> to require that RawExponent to be the same type as RawSignificand (which I
> think is what you’re really suggesting), because in typical bignum usage
> significands are much larger than exponents.
>
> It sounds like maybe you actually want to be operating directly on
> bitPatterns, rather than the abstract fields of the types.
>
> – Steve
>
> On Aug 26, 2016, at 7:21 PM, Jens Persson <jens at bitcycle.com> wrote:
>
> Oh, to more directly answer your question: I don't like having to create a
> UInt (UInt64) value when all my bit manipulaton code happens in UInt32 (for
> Float) for example.
>
> The most probable context for using these computed properties and types of
> BinaryFloatingPoint is one in which specific fixed width types really
> matters a lot (look at the name of the protocol and the properties and
> assocated types we are talking about).
>
> /Jens
>
>
> On Sat, Aug 27, 2016 at 1:15 AM, Jens Persson <jens at bitcycle.com> wrote:
>
>> Reason for asking is that I have this:
>>
>> extension Double {
>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>         let bitPattern = s | (1023 << 52)
>>         self = unsafeBitCast(bitPattern, to: Double.self) - 1.0
>>     }
>> }
>> extension Float {
>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>         let bitPattern = s | (127 << 23)
>>         self = unsafeBitCast(bitPattern, to: Float.self) - 1.0
>>     }
>> }
>>
>> But they would be better as:
>> extension BinaryFloatingPoint {
>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>         ... problems here, have to try casting things into
>> RawSignificand's type ...
>>     }
>> }
>>
>> Please have a go at that and perhaps you see what I mean or you will come
>> up with a nice solution that I have missed. (Speed is very important btw.)
>>
>> /Jens
>>
>>
>> On Sat, Aug 27, 2016 at 1:02 AM, Stephen Canon <scanon at apple.com> wrote:
>>
>>> > On Aug 26, 2016, at 6:06 PM, Jens Persson via swift-dev <
>>> swift-dev at swift.org> wrote:
>>> >
>>> > I can understand why
>>> > Double.RawSignificand is UInt64
>>> > and
>>> > Float.RawSignificand is UInt32
>>> >
>>> > But I can't understand why both
>>> > Double.RawExponent
>>> > and
>>> > Float.RawExponent
>>> > should be UInt.
>>> >
>>> > Why aren't they also just UInt64 and UInt32, resp.?
>>>
>>> Let me flip the question: why would they be UInt64 and UInt32?  Absent a
>>> reason to prefer a specific fixed-with type, Swift integers should
>>> generally default to being [U]Int (and ideally Int, but RawExponent is
>>> Unsigned).
>>>
>>> – Steve
>>
>>
>>
>
>
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