How to speed up Code executions with help of Pragma in C/C++

Question

Following are the Optimization:

1. O1: Optimizing compilation at O1 includes more time and memory to break down larger functions. The compiler makes an attempt to reduce both code and execution time. At O1 hardly any optimizations produce great results, but O1 is a setback for an attempt for better optimizations.
3. // C++ program to calculate the Prime 
4. // Numbers upto 10000000 using Sieve 
5. // of Eratosthenes with O1 optimization 
7. // To see the working of controlled 
8. // optimization "O1" 
9. #pragma GCC optimize("O1") 
11. #include <cmath> 
12. #include <iostream> 
13. #include <vector> 
14. #define N 10000005 
15. using namespace std; 
17. // Boolean array for Prime Number 
18. vector<bool> prime(N, true); 
20. // Seive implemented to find Prime 
21. // Number 
22. void sieveOfEratosthenes() 
23. { 
24. for (int i = 2; i <= sqrt(N); ++i) { 
25. if (prime[i]) { 
26. for (int j = i * i; j <= N; j += i) { 
27. prime[j] = false; 
28. } 
29. } 
30. } 
31. } 
33. // Driver Code 
34. int main() 
35. { 
36. // Intialise clock to calculate 
37. // time required to execute without 
38. // optimization 
39. clock_t start, end; 
41. // Start clock 
42. start = clock(); 
44. // Function call to find Prime Numbers 
45. sieveOfEratosthenes(); 
47. // End clock 
48. end = clock(); 
50. // Calculate the time difference 
51. double time_taken 
52. = double(end - start) 
53. / double(CLOCKS_PER_SEC); 
55. // Print the Calculated execution time 
56. cout << "Execution time: " << time_taken 
57. << " secs."; 
59. return 0; 
60. } 
61. O2: Optimizing compilation at O2 optimize to a greater extent. As compared to O1, this option increases both compilation time and the performance of the generated code. O2 turns on all optimization flags specified by O1.