具有共享内存的指针算法

Question

我不明白以下几行究竟发生了什么：

1. unsigned char *membershipChanged = (unsigned char *)sharedMemory;和

2. float *clusters = (float *)(sharedMemory + blockDim.x);

我认为在＃1 sharedMemory中有效地重命名为membershipChanged，但为什么要将blockDim添加到sharedMemory指针。这个地址指向哪里？

sharedMemory是使用extern __shared__ char sharedMemory[];

创建的

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我在CUDA kmeans implementation中找到的代码。

void find_nearest_cluster(int numCoords,
                          int numObjs,
                          int numClusters,
                          float *objects,           //  [numCoords][numObjs]
                          float *deviceClusters,    //  [numCoords][numClusters]
                          int *membership,          //  [numObjs]
                          int *intermediates)
{
extern __shared__ char sharedMemory[];

//  The type chosen for membershipChanged must be large enough to support
//  reductions! There are blockDim.x elements, one for each thread in the
//  block.
unsigned char *membershipChanged = (unsigned char *)sharedMemory;
float *clusters = (float *)(sharedMemory + blockDim.x);

membershipChanged[threadIdx.x] = 0;

//  BEWARE: We can overrun our shared memory here if there are too many
//  clusters or too many coordinates!
for (int i = threadIdx.x; i < numClusters; i += blockDim.x) {
    for (int j = 0; j < numCoords; j++) {
        clusters[numClusters * j + i] = deviceClusters[numClusters * j + i];
    }
}
.....

Answer 1

sharedMemory + blockDim.x点距离共享内存区域的基础blockDim.x个字节。

您可能会执行此类操作的原因是在共享内存中进行子分配。包含find_nearest_cluster的内核的启动站点为内核动态分配一定量的共享存储。该代码暗示两个逻辑上不同的数组驻留在sharedMemory - membershipChanged和clusters指向的共享存储中。指针算术只是获取指向第二个数组的指针的一种方法。