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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. int msb(int x) {
  5.     union { double a; int b[2]; };
  6.     a = x;
  7.     return (b[1] >> 20) - 1023;
  8. }
  9.  
  10. int highestOneBit(int i) {
  11.     // HD, Figure 3-1
  12.     i |= (i >>  1);
  13.     i |= (i >>  2);
  14.     i |= (i >>  4);
  15.     i |= (i >>  8);
  16.     i |= (i >> 16);
  17.     return i - ((unsigned)i >> 1);
  18. }
  19.  
  20. int main() {
  21.     double t1, t2;
  22.     const int lim = 200 * 1000 * 1000;
  23.     {
  24.     	t1 = clock();
  25.     	for (int i = 0; i < lim; i++) {
  26. 			int t = msb(i);
  27. 			if (t == -42) return 1;
  28. 		}
  29. 		t2 = clock();
  30. 		cout << (t2 - t1) / 1000.0 << endl;
  31.     }
  32.     {
  33.     	t1 = clock();
  34.     	for (int i = 0; i < lim; i++) {
  35. 			int t = highestOneBit(i);
  36. 			if (t == -42) return 1;
  37. 		}
  38. 		t2 = clock();
  39. 		cout << (t2 - t1) / 1000.0 << endl;
  40.     }
  41. }
  42.  
Success #stdin #stdout 2.47s 3296KB
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https://ideone.com/KdjzTB
language:
C++ (gcc 8.3)
created:
10 years ago
visibility:
public

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