寻找优化此代码的方法

Question

The following code is part of an edge detect program:

void detect_optimized(int width, int height, int threshold)
{
    int x, y;
    int tmp;`
    int w = width--;
    int h = height--;

for (y = 1; y < w; y++)
    for (x = 1; x < h; x++)
    {
        tmp = mask_product(mask,a,x,y,0);
        if (tmp>255)
            tmp = 255;
        if (tmp<threshold)
            tmp = 0;
        c[x][y][0] = 255-tmp;

        tmp = mask_product(mask,a,x,y,1);
        if (tmp>255)
            tmp = 255;
        if (tmp<threshold)
            tmp = 0;
        c[x][y][1] = 255-tmp;

        tmp = mask_product(mask,a,x,y,2);
        if (tmp>255)
            tmp = 255;
        if (tmp<threshold)
            tmp = 0;
        c[x][y][2] = 255-tmp;
    }
}

I have been trying to implement blocking with the following code but I am not sure how to use it in this case.

Answer 1

You can swap the loops to get a better cache utilization. This should speedup your code significantly (especially for large data).

for (x = 1; x < h; x++)
    for (y = 1; y < w; y++)

Another substantial benefit can be reached by distributing the loop iterations over multiple threads to exploit multicore architectures. Using OpenMP this is can be reached with a single compiler directive as follows.

#pragma omp parallel for private(y, tmp)
for (x = 1; x < h; x++)
    for (y = 1; y < w; y++)

Other optimizations are usually done by the compiler. Make sure to use appropriate compiler flags like -O2 and don't bother with low level adaptation yourself.

Answer 2

Offer the following candidate

1. Avoid if() s at the price of * . Various pipelined platforms will benefit.
2. Swap x,y order
3. Decrement so end of loop test is against 0.
4. Avoid recomputing c[x][y]

Assume need to go though all colors.

Of course, YMMV.

for (x = h-1; x > 0; x--) {
  byte *p = &c[x][w-1][NUM_COLORS-1];

  for (y = w-1; y > 0; y--) {
    for (int z = NUM_COLORS-1; z >= 0; z--) {
      int tmp = mask_product(mask,a,x,y,z);
      *p = (255 - tmp*(tmp>=threshold))*(tmp <=255);
      p--;
     }
  }
}