warp shuffle实验

实验一、

__shfl_sync(unsigned mask, T var, int srcLane, int width=warpSize);
mask 是参与的线程掩码，如0xffffffff，var 是待广播的值，srclane 是被广播的 laneid
__shfl_sync(0xffffffff, value, 2) Broadcast of a single value across a warp
Synchronize all threads in warp, and get “value” from lane 2

#include 
__global__ void bcast(int arg)
{
    int laneId = threadIdx.x & 0x1f;
    int value;
    
    if(laneId == 2) {
        value = arg;
    }
    value = __shfl_sync(0xffffffff, value, 2);
    if(value != arg)
     {
        printf("%d", laneId);
     }
}
int main() {
    bcast<<< 1, 32 >>>(1234);
    cudaDeviceSynchronize();

    return 0;
}

没有输出，说明将1234通过laneId = 2广播到所有laneId

实验二、

能实现前缀和，
thread n = 前 n + 1个thread和

#include 

__global__ void scan4() {
    int laneId = threadIdx.x & 0x1f;
    int value = 31 - laneId;

    // Loop to accumulate scan within my partition.
    // Scan requires log2(n) == 3 steps for 8 threads
    // It works by an accumulated sum up the warp
    // by 1, 2, 4, 8 etc. steps.
    for (int i=1; i<=4; i*=2) {
        int n = __shfl_up_sync(0xffffffff, value, i, 8);
        if ((laneId & 7) >= i)
            value += n;
    }

    printf("Thread %d final value = %d\n", threadIdx.x, value);
}

int main() {
    scan4<<< 1, 32 >>>();
    cudaDeviceSynchronize();
    return 0;
}

实验三、

Reduction across a warp
规约

#include 

__global__ void warpReduce() {
    int laneId = threadIdx.x & 0x1f;
    // Seed starting value as inverse lane ID
    int value = 31 - laneId;

    // Use XOR mode to perform butterfly reduction
    for (int i=16; i>=1; i/=2)
        value += __shfl_xor_sync(0xffffffff, value, i, 32);

    // "value" now contains the sum across all threads
    printf("Thread %d final value = %d\n", threadIdx.x, value);
}

int main() {
    warpReduce<<< 1, 32 >>>();
    cudaDeviceSynchronize();
    return 0;
}