extern __shared__ memory

The extern __shared__ declaration in C++ is used to allocate memory that is shared across all threads in a CUDA kernel. It allows for the efficient sharing of data among threads in a block. When this declaration is used, the memory is allocated dynamically and is shared among all threads in a block. This allows for the threads within a block to cooperate and communicate by sharing data during the execution of the kernel. The use of extern __shared__ memory can help optimize the performance of CUDA kernels by reducing the need to access global memory, which tends to be slower. This type of memory allocation is particularly useful when dealing with parallel processing tasks where threads need to exchange data efficiently.