Parallel nested loops (Monte Carlo Simulation) using GCD in Swift

Question

I want to reduce the overhead of the MonteCarlo Simulation in Swift language by performing concurrent programming on the outer loop while the inner loop would be executed normally. I got some info from the post

Using Grand Central Dispatch in Swift to parallelize and speed up “for" loops?, but seems to be outdated and not applying to swift 3. My code is as follows:

//constants
    let outerCount = 10
    let innerCount = 300
    //variables
    var firstMove = -1
    var num = -1
    var sum = [Int](repeating: 0, count: 9)
    var wincount = [[Int]](repeating:[Int](repeating:0, count:9),count:10)
    var candidateCount = 0
    var candidate = [Int](repeating: 0, count: 9)
    var maxWins = -2147483648
    var winners = [Int](repeating: 0, count: outerCount)

    //codeRiddle to solve

    winners.withUnsafeMutableBufferPointer { winnersPtr -> Void in

        DispatchQueue.concurrentPerform(iterations: outerCount) {
            outerIdx -> Void in

            for _ in 0...innerCount
            {
                firstMove = -1
                for _ in 0...8
                {
                    if firstMove == -1
                    {
                        firstMove = Int(arc4random()%UInt32(9))
                    }
                }
                num = Int(arc4random()%UInt32(3))
                if num == 0
                {
                    wincount[outerIdx][firstMove] += 0
                }
                else if num == 1
                {
                    wincount[outerIdx][firstMove] += -10
                }
                else
                {
                    wincount[outerIdx][firstMove] += 1
                }
            }
        }
    }

    for i in 0...8
    {
        for j in 0...9
        {
            sum[i] += wincount[j][i]
        }
    }

    for  i in 0...8
    {
        if sum[i] > maxWins
        {
            maxWins = sum[i]
        }
    }

    for i in 0...8
    {
        if sum[i] == maxWins
        {
            candidate[candidateCount] = i
            candidateCount += 1
        }
    }
    print(candidate[Int(arc4random()%UInt32(candidateCount))])