推奨されるコーディング スタイルの例 - 2023.2 日本語

単純な読み出し/書き込みの推論

INFO: [HLS 214-115] Burst read of variable length and bit width 32 has been inferred on port 'gmem'
INFO: [HLS 214-115] Burst write of variable length and bit width 32 has been inferred on port 'gmem' (./src/vadd.cpp:75:9). 

この例のコードは次のとおりです。

/****** BEGIN EXAMPLE *******/

#define DATA_SIZE 2048
// Define internal buffer max size
#define BURSTBUFFERSIZE 256

//TRIPCOUNT identifiers
const unsigned int c_min = 1;
const unsigned int c__max = BURSTBUFFERSIZE;
const unsigned int c_chunk_sz = DATA_SIZE;

extern "C" {
void vadd(int *a, int size, int inc_value) {
    // Map pointer a to AXI4-master interface for global memory access
#pragma HLS INTERFACE mode=m_axi port=a  offset=slave bundle=gmem max_read_burst_length=256 max_write_burst_length=256
    // We also need to map a and return to a bundled axilite slave interface
#pragma HLS INTERFACE mode=s_axilite port=a bundle=control
#pragma HLS INTERFACE mode=s_axilite port=size bundle=control
#pragma HLS INTERFACE mode=s_axilite port=inc_value bundle=control
#pragma HLS INTERFACE mode=s_axilite port=return bundle=control

    int burstbuffer[BURSTBUFFERSIZE];

    // Per iteration of this loop perform BURSTBUFFERSIZE vector addition
    for (int i = 0; i < size; i += BURSTBUFFERSIZE) {
#pragma HLS LOOP_TRIPCOUNT min=c_min*c_min max=c_chunk_sz*c_chunk_sz/(c_max*c_max)
        int chunk_size = BURSTBUFFERSIZE;
        //boundary checks
        if ((i + BURSTBUFFERSIZE) > size)
            chunk_size = size - i;

        // Use a for loop to create a burst access to memory
        // memcpy is not recommended
        // memcpy(burstbuffer, &a[i], chunk_size * sizeof(int));
        for (i=0; i < chunk_size; i++) {
            burstbuffer[i] = a[i];
        }

    // Calculate and write results to global memory, the sequential write in a for loop can be 
    // inferred as a memory burst access
    calc_write:
        for (int j = 0; j < chunk_size; j++) {
           #pragma HLS LOOP_TRIPCOUNT min=c_size_max max=c_chunk_sz
           #pragma HLS PIPELINE II=1
            burstbuffer[j] = burstbuffer[j] + inc_value;
            a[i + j] = burstbuffer[j];
        }
    }
}

バースト間のパイプライン処理

for(int x=0; x < k; ++x) {
   int off = f(x);
   for(int i = 0; i < N; ++i) {
      #pragma HLS PIPELINE II=1
      ... = gmem[off + i];
   }
}

外側ループはパイプライン処理されていないことに注意してください。つまり、バースト内のパイプライン処理はありますが、バースト間のパイプライン処理はないということです。

for(int x=0; x < k; ++x) {
   #pragma HLS PIPELINE II=N
   int off = f(x);
   for(int i = 0; i < N; ++i) {
      #pragma HLS UNROLL
      ... = gmem[off + i];
   }
}

2 次元配列の行データのアクセス

INFO: [HLS 214-115] Burst read of length 256 and bit width 512 has been inferred on port 'gmem' (./src/row_array_2d.cpp:43:5)
INFO: [HLS 214-115] Burst write of length 256 and bit width 512 has been inferred on port 'gmem' (./src/row_array_2d.cpp:56:5)

この例では、512 のビット幅が達成されます。これは、上記の単純な例で達成された 32 ビットよりも効率的です。ポート幅の自動変更 に説明するように、バーストのビット幅を広げるのもバースト最適化の別の方法です。

この例のコードは次のとおりです。

/****** BEGIN EXAMPLE *******/
// Parameters Description:
//         NUM_ROWS:            matrix height
//         WORD_PER_ROW:        number of words in a row
//         BLOCK_SIZE:          number of words in an array
#define NUM_ROWS   64
#define WORD_PER_ROW 64
#define BLOCK_SIZE (WORD_PER_ROW*NUM_ROWS)

// Default datatype is integer
typedef int DTYPE;
typedef hls::stream<DTYPE> my_data_fifo;

// Read data function: reads data from global memory
void read_data(DTYPE *inx, my_data_fifo &inFifo) {
read_loop_i:
    for (int i = 0; i < NUM_ROWS; ++i) {
    read_loop_jj:
        for (int jj = 0; jj < WORD_PER_ROW; ++jj) {
           #pragma HLS PIPELINE II=1
            inFifo << inx[WORD_PER_ROW * i + jj];
            ;
        }
    }
}

// Write data function - writes results to global memory
void write_data(DTYPE *outx, my_data_fifo &outFifo) {
write_loop_i:
    for (int i = 0; i < NUM_ROWS; ++i) {
    write_loop_jj:
        for (int jj = 0; jj < WORD_PER_ROW; ++jj) {
           #pragma HLS PIPELINE II=1
            outFifo >> outx[WORD_PER_ROW * i + jj];
        }
    }
}

// Compute function is pretty simple because this example is focused on efficient 
// memory access pattern.
void compute(my_data_fifo &inFifo, my_data_fifo &outFifo, int alpha) {
compute_loop_i:
    for (int i = 0; i < NUM_ROWS; ++i) {
    compute_loop_jj:
        for (int jj = 0; jj < WORD_PER_ROW; ++jj) {
           #pragma HLS PIPELINE II=1
            DTYPE inTmp;
            inFifo >> inTmp;
            DTYPE outTmp = inTmp * alpha;
            outFifo << outTmp;
        }
    }
}

extern "C" {
    void row_array_2d(DTYPE *inx, DTYPE *outx, int alpha) {
        // AXI master interface
    #pragma HLS INTERFACE mode=m_axi port = inx offset = slave bundle = gmem
    #pragma HLS INTERFACE mode=m_axi port = outx offset = slave bundle = gmem
        // AXI slave interface
    #pragma HLS INTERFACE mode=s_axilite port = inx bundle = control
    #pragma HLS INTERFACE mode=s_axilite port = outx bundle = control
    #pragma HLS INTERFACE mode=s_axilite port = alpha bundle = control
    #pragma HLS INTERFACE mode=s_axilite port = return bundle = control

        my_data_fifo inFifo;
        // By default the FIFO depth is 2, user can change the depth by using 
        // #pragma HLS stream variable=inFifo depth=256
        my_data_fifo outFifo;

        // Dataflow enables task level pipelining, allowing functions and loops to execute 
        // concurrently. For more details please refer to UG902.
    #pragma HLS DATAFLOW
        // Read data from each row of 2D array
        read_data(inx, inFifo);
        // Do computation with the acquired data
        compute(inFifo, outFifo, alpha);
        // Write data to each row of 2D array
        write_data(outx, outFifo);
        return;
    }
}