共享内存的指针运算

Question

我不明白以下几行到底发生了什么：

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

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

我假设 in #1sharedMemory有效地重命名为membershipChanged，但为什么要将 the 添加blockDim到sharedMemory指针中。这个地址指向哪里？

sharedMemory创建于 extern __shared__ char sharedMemory[];

---

我在CUDA kmeans 实现中找到的代码。

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指向blockDim.x远离共享内存区域基址的字节。

您可能会执行此类操作的原因是在共享内存中进行二次分配。内核的启动站点包括find_nearest_cluster为内核动态分配一定量的共享存储。该代码意味着两个逻辑上不同的数组驻留在sharedMemory-- membershipChanged、 和所指向的共享存储中clusters。指针算术只是获取指向第二个数组的指针的一种方法。