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POKnow the Block ID in CUDA from a given 2D offset
 

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  1. POKnow the Block ID in CUDA from a given 2D offset
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    <p>i've trying to calculate the blockIdx.x and blockIdx.y from a given offset in CUDA but i'm totally mind-blocked. The idea is read data from shared memory when possible and from global memory in other case.</p> <p>In example, if I've a 1D array of 64 elements and I configure a kernel with 16x1 threads (4 blocks in total) each thread can access to a position using:</p> <pre><code>int idx = blockDim.x*blockIdx.x + threadIdx.x </code></pre> <p>and i can easily get the blockIdx.x of a given index value from the idx as </p> <pre><code>int blockNumber = idx / blockDim.x; </code></pre> <p>but in a 2D scenario with 8x8 elements and a kernel configuration of 4x4 threads (2x2 blocks in total) each thread accesses to a position using:</p> <pre><code>int x = threadIdx.x + blockIdx.x * blockDim.x; int y = threadIdx.y + blockIdx.y * blockDim.y; int pitch = blockDim.x * gridDim.x; int idx = x + y * pitch; int sharedMemIndex = threadIdx.x+threadIdx.y+BLOCK_DIM_X; __shared_block[sharedMemIndex] = fromGlobalMemory[idx]; __syncthreads(); // ... some operations int unknow_index = __shared_block[sharedMemIndex]; if ( unknow_index within this block? ) // ... read from shared memory else // ... read from global memory </code></pre> <p>How can i know the Block ID.x and ID.y at a given idx? i.e. index 34 and 35 are in block (0, 1) and index 36 in block (1, 1). So, if a thread in block (0, 1) read a value of index 35, that thread will know that the value is within its block and will read it from shared memory. The index 35 value will be in stored in the position 11 of the shared memory of the block (0. 1).</p> <p>Thanks in advance!</p>
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    1. COI don't understand this question at all. Any thread, in any kernel always has blockIdx.{xyz} available. Why would you ever need to try and calculate it?
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    2. COImage you need to do some loads from memory. The first one load value_index_1 = array[idx] and the second loads value_index_2 = array[value_index]. So, if value_index_1 is in the block, i can load from shared memory, else i need to load from global memory.
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    3. COYou still don't need the to compute the block index value for that. You can compare the difference between a given threads id within the block and the block size to the global thread ID and the index in question. That will tell you whether the index lies within the range of shared memory within the block.
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