[swift-evolution] Yet another fixed-size array spitball session

[Charlie Monroe]

> On May 31, 2017, at 4:16 PM, Jean-Daniel <mailing at xenonium.com> wrote:
> 
>> 
>> Le 31 mai 2017 à 01:47, Anders Kierulf <anders at smartgo.com <mailto:anders at smartgo.com>> a écrit :
>> 
>>> 
>>> On May 30, 2017, at 3:36 PM, Jean-Daniel via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> 
>>>> Le 30 mai 2017 à 12:42, Charlie Monroe via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>>> 
>>>> There was someone a few weeks ago trying to port his Go game to Swift from (I believe) C++ and found out that the lack of fixed-size arrays was causing the move-computing algorithm to slow down significantly.
>>>> 
>>>> This is due to fixed arrays being able to live on stack, while "normal Array" is dynamically allocated on heap, etc.
>>> 
>>> Really ? Isn’t it due to the value semantic of swift arrays ?
>>> 
>>> If this is the former, its algorithm can probably be tweak to reuse the array and require less allocations.
>>> 
>>> Unless if you algorithm is eager in memory allocation/deallocation, you shouldn't get a significant difference between static array and dynamic array.
>> 
>> Eliminating the dynamic allocations and extra indirections caused by the Swift array implementation can make a huge difference, not just in itself, but it also gives the compiler more opportunities to optimize the code.
> 
> I don’t see how static sized array would solve that issue.
> 
> If swift wants to keep value semantic it has to support COW or a similar machinery. 
> If you want to avoid COW and force the array to be allocated on the stack, it will impose swift to copy it at each function boundary (there is no @unescape for array to tell the compiler it can safely pass a reference).
> 
> If what you want is just a preallocated array with reference semantic, wrap it in a class. You should get the same speed up than when using imported C arrays.

Creating an Array in Swift immediately makes a heap allocation. You then get COW semantics, but you already made a heap allocation (unless the array is empty).  

https://github.com/apple/swift/blob/master/stdlib/public/core/ContiguousArrayBuffer.swift#L208 <https://github.com/apple/swift/blob/master/stdlib/public/core/Arrays.swift.gyb#L946>

With sized arrays, they can live on stack and the compiler can access the elements by referencing the stack frame (quick access), vs. the array, where it needs to dereference the pointer. Not to mention that the compiler can actually make some elements accessible via registers.

Might seem like a small gain, but if you are making something that needs to compute really fast, even getting 2x speed is fast (and accessing heap via Array is more than 2x compared to accessing something on the stack). Not to mention if you create a lot of arrays (e.g. matrix computations), creating a static-sized array means just adding the size to the stack pointer.


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