[swift-users] Counting in Threads

[Philippe Hausler]

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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> 
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