[swift-users] Counting in Threads

[Shawn Erickson]

So we would have to drop down to C code, etc. to safely leverage OSAtomic?

On Wed, Oct 12, 2016 at 8:32 AM Philippe Hausler via swift-users <
swift-users at swift.org> wrote:

> I was under the impression that taking the address was more than a single
> load instruction and would emit a placeholder invalid value: which would
> make that technically unsafe in a threaded context.
>
> Sent from my iPhone
>
> On Oct 12, 2016, at 8:18 AM, Daniel Dunbar via swift-users <
> swift-users at swift.org> wrote:
>
> I suspect one of the actual compiler people might tell me I shouldn't
> trust this, but in practice it works:
> --
> import Darwin.C
>
> public class AtomicInt32 {
>    public fileprivate (set) var value : Int32 = 0
>
>    /// Create a new atomic integer with the specified initial value.
>    public init(_ value: Int32 = 0) {
>        self.value = value
>    }
>
>    /// Add one to the value.
>    public func increment () {
>        OSAtomicIncrement32(&value)
>    }
> }
>
> public func +=(int: AtomicInt32, value: Int32) {
>    OSAtomicAdd32(value, &int.value)
> }
> --
>
> Would also love to know if compiler guarantees I *can* trust this.
>
> Note that this has to be a class for this to be in any way safe, which
> means it is also rather inefficient if the use case was having a lot of
> them.
>
> - Daniel
>
> On Oct 12, 2016, at 12:47 AM, Gerriet M. Denkmann via swift-users <
> swift-users at swift.org> wrote:
>
>
>
> How to translate this to Swift:
>
>
> __block atomic_uint_fast64_t counter = ATOMIC_VAR_INIT(0);
>
> dispatch_apply( nbrInterations, queue, ^void(size_t idx)
>
>    {
>
>        uint64_t tCount = 0;
>
>        ... do some counting ...
>
>        atomic_fetch_add_explicit( &counter, tCount, memory_order_relaxed );
>
>    }
>
> )
>
>
> Currently I am using:
>
>
> var counter: UInt64 = 0
>
> let dsema = DispatchSemaphore(value: 1)
>
> DispatchQueue.concurrentPerform( iterations: nbrInterations )
>
> { ( idx: size_t) -> Void in
>
>
>
>    var tCount: UInt64 = 0
>
>    ... do some counting ...
>
>    _ = dsema.wait(timeout: .distantFuture)
>
>    counter += tCount;
>
>    dsema.signal()
>
> }
>
>
> Is there a better way?
>
>
> Gerriet.
>
>
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