[swift-evolution] [Idea] A 128-bit unsigned integer value type

[Dave Abrahams]

on Sat May 21 2016, Károly Lőrentey <swift-evolution at swift.org> wrote:

> On 2016-05-17 14:30:36 +0000, Muse M via swift-evolution said:
>
>> Most programming languages does have 128-bit of type except Swift.
>
> I don't think there is huge demand for a full-blown Int128 type in the
> standard library, but it would be nice to have double-width
> multiplication & division methods in Swift's existing integer types. I
> assume LLVM has intrinsics for this that compile down to single
> instructions on x86_64.
>
> This would speed up some 64-bit fixed-point operations with 128-bit
> temporaries (such as Mach timestamp scaling arithmetic) and allow pure
> Swift arbitrary-precision integer packages to provide faster
> operations.
>
> I currently need to emulate full-width multiplication/division with
> code such as this:
>
> https://github.com/lorentey/BigInt/blob/master/Sources/BigDigit.swift#L102-L177 
>
> A 128/64=64 full-width division done this way requires 2 single-width
> divisions and 6 multiplications. I'd love to replace this with a
> single full-width x86 DIV instruction.

One reason we don't have Int128 everywhere is that we don't currently
have the necessary primitives on 32-bit systems.

However, when the integers protocol proposal that Max Moiseev is working on
comes out, it will include an implementation of a 

  DoublePrecision<T:Integer>

either as public API or in the tests (I forget which).  The intention is
that on 32-bit platforms we can eventually define Int128 as a typealias
for DoublePrecision<Int64>.  We can also introduce Int256, for which I
know there is an audience.

-- 
-Dave