// super super slow sgemm kernel by heehoon

#define TS 	32
#define WPT	8
#define RTS	4

__kernel void sgemm(__global float *A, __global float *B, __global float *C, int M, int N, int K) {

    // Thread identifiers
    const int row = get_local_id(0); // Local row ID (max: TS)
    const int col = get_local_id(1); // Local col ID (max: TS/WPT == RTS)
    const int globalRow = TS*get_group_id(0) + row; // Row ID of C (0..M)
    const int globalCol = TS*get_group_id(1) + col; // Col ID of C (0..N)

    // Local memory to fit a tile of TS*TS elements of A and B
    __local float Asub[TS][TS];
    __local float Bsub[TS][TS];

    // Initialise the accumulation registers
    float acc[WPT];
    for (int w=0; w<WPT; w++) {
        acc[w] = 0.0f;
    }
    
    // Loop over all tiles
    const int numTiles = (K + TS - 1)/TS;
    for (int t=0; t<numTiles; t++) {

        // Load one tile of A and B into local memory
        for (int w=0; w<WPT; w++) {
            const int tiledRow = TS*t + row;
            const int tiledCol = TS*t + col;
            if(globalRow + w*RTS >= M || tiledCol >= K) {
	        Asub[row + w*RTS][col] = 0.0f;
	    } else {
	        Asub[row + w*RTS][col] = A[(globalRow + w*RTS)*K + tiledCol];
	    }
	    if(tiledRow + w*RTS >= K || globalCol >= N) {
	        Bsub[row + w*RTS][col] = 0.0f;
	    } else {
                Bsub[row + w*RTS][col] = B[(tiledRow + w*RTS)*N + globalCol];
	    }
        }

        // Synchronise to make sure the tile is loaded
        barrier(CLK_LOCAL_MEM_FENCE);

        // Perform the computation for a single tile
        for (int k=0; k<TS; k++) {
            for (int w=0; w<WPT; w++) {
                acc[w] += Asub[row + w*RTS][k] * Bsub[k][col];
            }
        }

        // Synchronise before loading the next tile
        barrier(CLK_LOCAL_MEM_FENCE);
    }

    // Store the final results in C
    for (int w=0; w<WPT; w++) {
	if(globalRow + w*RTS >= M || globalCol >= N) continue;     
        C[(globalRow + w*RTS)*N + globalCol] = acc[w];
    }
}