如何使用OpenACC优化矩阵乘法？

#include<stdio.h>
#include <stdlib.h>
#include <time.h>
#include <openacc.h>

#define N 1000

int main() {
    clock_t start, end;
    double cpu_time_used;
    float *a, *b, *c;

    a = (float *)malloc(N * N * sizeof(float));
    b = (float *)malloc(N * N * sizeof(float));
    c = (float *)malloc(N * N * sizeof(float));

    // 初始化数组
    for (int i = 0; i < N * N; i++) {
        a[i] = rand() % 10;
        b[i] = rand() % 10;
        c[i] = 0;
    }

    // 开始计时
    start = clock();

    // 使用OpenACC加速矩阵乘法
    #pragma acc parallel loop copyin(a[0:N*N], b[0:N*N]) copyout(c[0:N*N])
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            float sum = 0;
            for (int k = 0; k < N; k++) {
                sum += a[i * N + k] * b[k * N + j];
            }
            c[i * N + j] = sum;
        }
    }

    // 结束计时
    end = clock();
    cpu_time_used = ((double)(end - start)) / CLOCKS_PER_SEC;

    printf("矩阵乘法完成，耗时：%f秒\n", cpu_time_used);

    // 释放内存
    free(a);
    free(b);
    free(c);

    return 0;
}