Question

for (int i = 0; i < 5000; i++)
   for (int j = 0; j < 5000; j++)
   {
      for (int ii = 0; ii < 20; ii++)
          for (int jj = 0; jj < 20; jj++)
           {
               int num = matBigger[i+ii][j+jj];
               // Extract range from this.
               int low = num & 0xff;
               int high = num >> 8;
               if (low < matSmaller[ii][jj] && matSmaller[ii][jj] > high)
                  // match found
           }
   }

The machine is x86_64, 32kb L1 cahce, 256 Kb L2 cache.

Any pointers on how can I possibly optimize this code?

EDIT Some background to the original problem : Fastest way to Find a m x n submatrix in M X N matrix

Answer 1

Some basic advice:

1. Profile it, so you can learn where the hot-spots are.
2. Think about cache locality, and the addresses resulting from your loop order.
3. Use more const in the innermost scope, to hint more to the compiler.
4. Try breaking it up so you don’t compute high if the low test is failing.
5. Try maintaining the offset into matBigger and matSmaller explicitly, to the innermost stepping into a simple increment.