This site suggests a version using withUnsafeMutableBufferPointer:
http://blog.human-friendly.com/swift-arrays-are-not-threadsafe
let nbrOfThreads = 8
let step = 2
let itemsPerThread = number * step
let bitLimit = nbrOfThreads * itemsPerThread
var bitfield = [Bool](count: bitLimit, repeatedValue: false)
let queue = dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_HIGH, 0 );
bitfield.withUnsafeMutableBufferPointer { (inout bitfieldBuffer :
UnsafeMutableBufferPointer<Bool>) -> () in
dispatch_apply( Int(nbrOfThreads), queue ) { ( idx: size_t) -> Void in
let startIndex = itemsPerThread * Int(idx)
let endIndex = min( startIndex + itemsPerThread, bitLimit )
if talk { print("Thread[\(idx)] does \(startIndex) ..<
\(endIndex)") }
var currIndex = startIndex
while( currIndex < endIndex )
{
bitfieldBuffer[currIndex] = true
currIndex += step
}
}
}
On Tue, Sep 13, 2016 at 1:52 AM, Gerriet M. Denkmann <gerr...@mdenkmann.de>
wrote:
>
> > On 12 Sep 2016, at 22:49, Jens Alfke <j...@mooseyard.com> wrote:
> >
> >
> >> On Sep 12, 2016, at 6:42 AM, Gerriet M. Denkmann <gerr...@mdenkmann.de>
> wrote:
> >>
> >> So: is the code ok and the compiler broken in Debug mode?
> >> Or is the code fundamentally wrong and that it works in Release is just
> a fluke?
>
> I tried a variation of my function. This does not crash (not even in Debug
> builds), but takes twice the memory and is about four times slower:
>
> func markAndTell_2( talk: Bool, number: Int) -> [Bool]
> {
> [...]
> let bitLimit = nbrOfThreads * itemsPerThread
> var bitfield = [Bool](count: bitLimit, repeatedValue: false)
>
> var outputSlice0: ArraySlice<Bool> = []
> [...]
> var outputSlice7: ArraySlice<Bool> = []
>
> let queue = dispatch_get_global_queue(
> DISPATCH_QUEUE_PRIORITY_HIGH, 0 );
> dispatch_apply( Int(nbrOfThreads), queue,
> { ( idx: size_t) -> Void in
>
> let startIndex = itemsPerThread * Int(idx)
> let endIndex = min( startIndex + itemsPerThread,
> bitLimit )
>
> var tempSlice: ArraySlice<Bool> = bitfield[
> startIndex ..< endIndex ]
>
> var currIndex = startIndex
> while( currIndex < endIndex )
> {
> tempSlice[currIndex] = true
> currIndex += step
> }
>
> switch(idx)
> {
> case 0: outputSlice0 = tempSlice
> [...]
> case 7: outputSlice7 = tempSlice
> default: print("Thread[\(idx)] does not
> know where to put its slice")
> }
> }
> )
>
> bitfield = []
> bitfield += outputSlice0
> [...]
> bitfield += outputSlice7
>
> return bitfield
> }
>
> Gerriet.
>
>
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