[swift-users] Huge difference in speed in Array<Class>/Array<Struct> on OSX/Ubuntu platform with same? toolchain

Tue Feb 2 15:19:13 CST 2016

> On Jan 29, 2016, at 11:27 AM, Joe Groff via swift-users <swift-users at swift.org> wrote:
> 
> Arrays of class type have additional overhead on Apple platforms due to the NSArray interoperability. You can use ContiguousArray<Class> if you don't need to interoperate with Objective-C to get more consistent performance.
> 

Using ContiguousArray is a very good idea. You can find other optimization tips and tricks here:

https://github.com/apple/swift/blob/master/docs/OptimizationTips.rst

-Nadav

> -Joe
> 
>> On Jan 28, 2016, at 10:30 PM, Sergey Kuratov via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> Hello,
>> I've just started to work with Swift (have C++ background) and try to make some tests to avoid problems in design. But from beginning I met very disappointing case.
>> 
>> 1. So I have 2 similar computers:
>> - Ubuntu 15.10 with clang and Swift-2.2 dev 2016.01.25 for Ubuntu 15.10
>> - iMac with OSX 10.11 and  Swift-2.2 dev 2016.01.25 fro OSX (switched to this toolchain)
>> 
>> 2. Took extremely simple source code from n-body sample <http://benchmarksgame.alioth.debian.org/u64q/program.php?test=nbody&lang=swift&id=3>
>> 
>> 3. Set Release version with -Ounchecked -whole-module-optimization for both computers and get results:
>> 
>> Ubuntu - 8.4 sec
>> OSX - 167.3 sec
>> Was demolished by results
>> 
>> 4. Changed "class Body" to "struct Body" and get another results:
>> 
>> Ubuntu - 12.2 sec
>> OSX - 10.0 sec
>> Again was deeply moved by results
>> 
>> 5. Added
>> let arrPtr = UnsafeMutablePointer<Body>(bodies)
>> before loops and used arrPtr instead of bodies in loops. Got new result:
>> 
>> For "class Body" variant:
>> Ubuntu - 7.3 sec
>> OSX - 11.1 sec
>> 
>> For "struct Body" variant:
>> Ubuntu - 6.7 sec
>> OSX - 8.8 sec
>> 
>> I believe something wrong with Swift compiler/optimizer if for the same x64 processor same toolchain generate very different code. Especially in 1 case with "class Body" on OSX platform - I guess it's bug. Can somebody comment results ?
>> 
>> PS: Forgot to say - similar Java program n-body in Java <http://benchmarksgame.alioth.debian.org/u64q/program.php?test=nbody&lang=java&id=2> takes 6.5 sec on Ubuntu and 9.2 sec on OSX
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