[swift-evolution] [Review #2] SE-0067: Enhanced Floating Point Protocols

[Xiaodi Wu]

Unrelated to this discussion about comparison operators, please remember to
put back .abs() or .absoluteValue() somewhere in FloatingPoint. Thanks!


On Tue, Apr 26, 2016 at 12:07 PM, Nicola Salmoria via swift-evolution <
swift-evolution at swift.org> wrote:

> If performance is the only reasons to have a separate method, perhaps it
> might make sense to have a note about this in the isLessThanOrEqualTo
> description, and provide a default implementation that calls down on isLess
> and isEqual.
>
> Nicola
>
>
> On Tue, Apr 26, 2016 at 4:59 PM, Stephen Canon <scanon at apple.com> wrote:
>
>> On Apr 26, 2016, at 10:03 AM, Nicola Salmoria via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>> it should be enough to do
>>
>> func <= <T: FloatingPoint>(lhs: T, rhs: T) -> Bool
>> {
>> return lhs.isLess(than: rhs) || lhs.isEqual(to: rhs)
>> }
>>
>> Or is there some case of x, y where (x <= y) is true but (x < y) and (x
>> == y) are both false?
>>
>>
>> No, these are always equivalent.
>>
>> However, there is a problem with this approach: the compiler knows about
>> the built-in floating-point types (Float, Double, Float80) and their
>> aliases (CGFloat, etc) and can fuse the two comparisons into a single
>> hardware operation.  However, for soft-float types, without a
>> lessThanOrEqual method, there is no single comparison that the compiler
>> could fuse these to.
>>
>> In some cases, if they are simple enough, the compiler may be able to
>> actually fuse the bodies of the two comparison operations, but we can’t
>> reasonably depend on that happening or even being possible in all cases.
>> So for performance, it’s important that there be an actual less than or
>> equal to method.
>>
>> – Steve
>>
>
>
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