How to speed up matrix multiplication in C++?

by

Speaking of speed-up, your function will be more cache-friendly if you swap the order of the k and j loop iterations:

matrix mult_std(matrix a, matrix b) {
 matrix c(a.dim(), false, false);
 for (int i = 0; i < a.dim(); i++)
  for (int k = 0; k < a.dim(); k++)
   for (int j = 0; j < a.dim(); j++)  // swapped order
    c(i,j) += a(i,k) * b(k,j);

 return c;
}

That’s because a k index on the inner-most loop will cause a cache miss in b on every iteration. With j as the inner-most index, both c and b are accessed contiguously, while a stays put.

More Related Contents:

  1. sizeof works unusual at c++ after array creation [duplicate]
  2. Why is sizeof giving incorrect number of bytes in C++?
  3. If statement – C++
  4. Accessing an array out of bounds gives no error, why?
  5. Passing a 2D array to a C++ function
  6. Why does cout print char arrays differently from other arrays?
  7. Address of an array
  8. Why does pointer decay take priority over a deduced template?
  9. Sort a 2D array in C++ using built in functions(or any other method)?
  10. How to initialize a vector in C++ [duplicate]
  11. Is there a function to copy an array in C/C++?
  12. How do I initialize a member array with an initializer_list?
  13. Why does this delay-loop start to run faster after several iterations with no sleep?
  14. Range-based for loop on a dynamic array?
  15. C/C++ efficient bit array
  16. The intersection of two sorted arrays
  17. Casting double array to a struct of doubles
  18. Pointer interconvertibility vs having the same address
  19. Pointer to array with const qualifier in C & C++
  20. c++ sort with structs
  21. How does *(&arr + 1) – arr give the length in elements of array arr?
  22. How does std::end know the end of an array?
  23. Assign array to array
  24. Is std::array movable?
  25. Pointer to array of unspecified size “(*p)[]” illegal in C++ but legal in C
  26. c++ expression must have a constant value
  27. Parse string containing numbers into integer array
  28. difference between pointer to an array and pointer to the first element of an array
  29. Array of size defined by not constant variable
  30. int *array = new int[n]; what is this function actually doing?

Leave a Comment