[swift-users] Counting in Threads

[Daniel Dunbar]

Not in this case, today:
--
$ cat x.swift 
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)
   }
}

$ swiftc -S -o - x.swift -O | grep '.globl	__TFC4main11AtomicInt329incrementfT_T_' -A16
	.globl	__TFC4main11AtomicInt329incrementfT_T_
	.align	4, 0x90
__TFC4main11AtomicInt329incrementfT_T_:
	.cfi_startproc
	pushq	%rbp
Ltmp4:
	.cfi_def_cfa_offset 16
Ltmp5:
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
Ltmp6:
	.cfi_def_cfa_register %rbp
	leaq	16(%rdi), %rsi
	movl	$1, %edi
	popq	%rbp
	jmp	_OSAtomicAdd32
	.cfi_endproc
--

 - Daniel

> On Oct 12, 2016, at 8:31 AM, Philippe Hausler <phausler at apple.com> 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 <mailto: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 <mailto: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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>>> swift-users at swift.org <mailto:swift-users at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
>> 
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