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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. const int BUFFER_SIZE = 50000000;
  5. double* list = new double[BUFFER_SIZE];
  6.  
  7. double tightLoop() {
  8.  
  9. 	// VERSION 1
  10. 	// double sum = 0.0;
  11. 	// for (int i = 0; i < BUFFER_SIZE; ++i) {
  12. 	// 	sum += list[i];
  13. 	// }
  14. 	// return sum;
  15.  
  16.     // VERSION 2. 7x faster!	
  17. 	double sum = 0.0;
  18. 	for (int i = 0; i < BUFFER_SIZE; i+=4) {
  19. 		sum += list[i];
  20. 		sum += list[i + 1];
  21. 		sum += list[i + 2];
  22. 		sum += list[i + 3];
  23. 	}
  24. 	return sum;
  25.  
  26. 	// VERSION 3. No more gain than simple loop unrolling
  27. 	// double sum1 = 0.0, sum2 = 0.0, sum3 = 0.0, sum4 = 0.0;
  28. 	// for (int i = 0; i < BUFFER_SIZE; i += 4) {
  29. 	// 	sum1 += list[i];
  30. 	// 	sum2 += list[i + 1];
  31. 	// 	sum3 += list[i + 2];
  32. 	// 	sum4 += list[i + 3];
  33. 	// }
  34. 	// return (sum1 + sum2) + (sum3 + sum4);
  35. }
  36.  
  37. int main() {
  38.  
  39. 	double force = 0.0;
  40. 	force += tightLoop();
  41. 	force += tightLoop();
  42. 	force += tightLoop();
  43. 	force += tightLoop();
  44. 	force += tightLoop();
  45. 	force += tightLoop();
  46. 	force += tightLoop();
  47. 	cout << force;
  48. 	delete[] list;
  49. 	return 0;
  50. }
Success #stdin #stdout 0.17s 3296KB
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https://ideone.com/xqXjns
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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