Nested Loops in OpenACC - c

Question

I'm trying to parallelize a nested for loop with OpenACC. I don't understand why my code isn't working correctly, The following is the relevant part of my code:
int edgedetect_laplace(int height, // I: image height
int width, // I: image width
gray_t image[height][width], // I: input image
gray_t new_image[height][width]) // O: output image
{
// just for reproducable checksums...
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
new_image[i][j] = 0;
}
}
#pragma acc data copyin(image[:height][:width]) copyout(new_image[:height][:width])
{
#pragma acc parallel
{
#pragma acc loop
for (int i = 1; i < height - 1; i++)
{
#pragma acc loop
for (int j = 1; j < width - 1; ++j)
{
// apply laplace operator
unsigned int val = 4 * image[i][j] - image[i - 1][j] - image[i + 1][j] - image[i][j - 1] - image[i][j + 1];
/* store calculated value (map to correct range) */
new_image[i][j] = min(val, GRAY_MAX);
}
}
}
}
printf("time Laplace edge detection: %.6f s\n", t1 - t0);
unsigned long cs = checksum(height, width, new_image);
if (cs != REFERENCE_CHECKSUM_LAPLACE)
printf("\t error checksum Laplace: expected %lu, seen %lu\n", REFERENCE_CHECKSUM_LAPLACE, cs);
else
printf("checksum Laplace OK : %lu\n", cs);
return 0;
}
I have run the program sequentially and calculated the checksum to test if my parallelized version runs correctly. However, it does not (I'm getting a different checksum) and I don't see why.

Answer 1

It might be because you're assigning values to the interior of "new_image" but copying out the whole array. Since it's not initialized on the device, this means the halos will contain garbage values when copied back. Try using "copy" instead of "copyout" so "new_image" is initialized, or just copyout the interior elements.
If that's not the issue, please provide a minimally reproducing example.