cuda-samples/Samples/matrixMulDrv/matrixMul_kernel.cu

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/* Matrix multiplication: C = A * B.
 * Device code.
 */

#ifndef _MATRIXMUL_KERNEL_H_
#define _MATRIXMUL_KERNEL_H_

#include <stdio.h>

#define AS(i, j) As[i][j]
#define BS(i, j) Bs[i][j]

////////////////////////////////////////////////////////////////////////////////
//! Matrix multiplication on the device: C = A * B
//! wA is A's width and wB is B's width
////////////////////////////////////////////////////////////////////////////////
template <int block_size, typename size_type>
__device__ void matrixMul(float *C, float *A, float *B, size_type wA,
                          size_type wB) {
  // Block index
  size_type bx = blockIdx.x;
  size_type by = blockIdx.y;

  // Thread index
  size_type tx = threadIdx.x;
  size_type ty = threadIdx.y;

  // Index of the first sub-matrix of A processed by the block
  size_type aBegin = wA * block_size * by;

  // Index of the last sub-matrix of A processed by the block
  size_type aEnd = aBegin + wA - 1;

  // Step size used to iterate through the sub-matrices of A
  size_type aStep = block_size;

  // Index of the first sub-matrix of B processed by the block
  size_type bBegin = block_size * bx;

  // Step size used to iterate through the sub-matrices of B
  size_type bStep = block_size * wB;

  // Csub is used to store the element of the block sub-matrix
  // that is computed by the thread
  float Csub = 0;

  // Loop over all the sub-matrices of A and B
  // required to compute the block sub-matrix
  for (size_type a = aBegin, b = bBegin; a <= aEnd; a += aStep, b += bStep) {
    // Declaration of the shared memory array As used to
    // store the sub-matrix of A
    __shared__ float As[block_size][block_size];

    // Declaration of the shared memory array Bs used to
    // store the sub-matrix of B
    __shared__ float Bs[block_size][block_size];

    // Load the matrices from device memory
    // to shared memory; each thread loads
    // one element of each matrix
    AS(ty, tx) = A[a + wA * ty + tx];
    BS(ty, tx) = B[b + wB * ty + tx];

    // Synchronize to make sure the matrices are loaded
    __syncthreads();

    // Multiply the two matrices together;
    // each thread computes one element
    // of the block sub-matrix
#pragma unroll

    for (size_type k = 0; k < block_size; ++k) Csub += AS(ty, k) * BS(k, tx);

    // Synchronize to make sure that the preceding
    // computation is done before loading two new
    // sub-matrices of A and B in the next iteration
    __syncthreads();
  }

  // Write the block sub-matrix to device memory;
  // each thread writes one element
  size_type c = wB * block_size * by + block_size * bx;
  C[c + wB * ty + tx] = Csub;
}

// C wrappers around our template kernel
extern "C" __global__ void matrixMul_bs16_64bit(float *C, float *A, float *B,
                                                size_t wA, size_t wB) {
  matrixMul<16, size_t>(C, A, B, wA, wB);
}
extern "C" __global__ void matrixMul_bs32_64bit(float *C, float *A, float *B,
                                                size_t wA, size_t wB) {
  matrixMul<32, size_t>(C, A, B, wA, wB);
}

#endif  // #ifndef _MATRIXMUL_KERNEL_H_