[swift-users] Counting in Threads

[Daniel Dunbar]

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.
> 
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users