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CUDA: atomicAdd takes too much time, serializing threads
__global__ void mykernel(..., unsigned int *gColCnt) {
    ...

    __shared__ unsigned int sColCnt;
    __shared__ unsigned int sIndex;

    if (threadIdx.x == 0) {
        sColCnt = 0;
    }

    __syncthreads();

    unsigned int index = 0;
    if (colliding)
        index = atomicAdd(&sColCnt, 1); //!!Time Consuming!!

    __syncthreads();

    if (threadIdx.x == 0)
        sIndex = atomicAdd(gColCnt, sColCnt);

    __syncthreads();

    if (sColCnt + sIndex > outputSize) { //output buffer is not enough
        //printf("Exceeds outputsize: %d + %d > %dn", sColCnt, sIndex, outputSize);
        return;
    }

    if (colliding) {
        output[sIndex + index] = make_uint2(startId, toId);
    }
}