[swift-users] Counting in Threads

[Philippe Hausler]

Or allocate a pointer for it. The other alternative would be NSLock but that might be swatting flies with sledgehammers. We had a similar issue implementing NSLock in swift-corelibs-foundation where taking the address of a pthread mutex corrupted the structure for when another thread would attempt to grab it.

Sent from my iPhone

> On Oct 12, 2016, at 4:45 PM, Shawn Erickson <shawnce at gmail.com> wrote:
> 
> 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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>>> 
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