[swift-evolution] [swift-evolution-announce] [Review] SE-0067: Enhanced Floating Point Protocols

[Jordan Rose]

> On Apr 22, 2016, at 7:13, Stephen Canon <scanon at apple.com> wrote:
> 
>>   /// A signaling NaN (not-a-number).
>>   @warn_unused_result
>>   static func signalingNaN: Self { get }
>> 
>> I’m not sure it really makes sense for a Bignum / APFloat type to support such a value. But really I think this is just underspecified. What does it mean, in terms of this protocol and its uses, for a NaN to be signaling? Is it just a specific “color" of NaN, with no semantic requirements other than being distinguishable?
> 
> There are a variety of means that a softfloat type could use to implement signaling NaNs.  Here are two of the simpler ones:
> 
> (a) if running on HW with hard-float support, use native-precision hard-float instructions to set flags as needed.
> (b) provide operation variants that take an inout flags / parameter:
> 
> 	mutating func add(rhs: Self, inout flags: Flags)
> 

Sorry, I think my question is more general than this. "Signaling" is a term of art in IEEE 754, but it doesn't actually mean anything in Swift, and none of the operations in the protocol have defined semantics for "signaling". Maybe that's the real question: what must each of the operations of FloatingPoint and BinaryFloatingPoint do when operating on an SNaN? a QNaN?

(I know what IEEE 754 says. We need the same rules in Swift semantics: "throws", "traps", "returns an unspecified result".)

Jordan
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