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  1. #include <chrono>
  2. #include <iostream>
  3. #include <numeric>
  4. #include <vector>
  5.  
  6. constexpr int square( int x )
  7. {
  8. 	return x * x;
  9. }
  10.  
  11. constexpr int times_two( int x )
  12. {
  13. 	return 2 * x;
  14. }
  15.  
  16. // map ((^2) . (^2)) $ [1,2,3]
  17. int manual_fusion( const std::vector<int>& xs )
  18. {
  19. 	std::vector<int> zs;
  20. 	zs.reserve( xs.size() );
  21. 	for ( int x : xs )
  22. 	{
  23. 		zs.push_back( square( times_two( x ) ) );
  24. 	}
  25. 	return zs[0];
  26. }
  27.  
  28. // map (^2) . map (^2) $ [1,2,3]
  29. int two_loops( const std::vector<int>& xs )
  30. {
  31. 	std::vector<int> ys;
  32. 	ys.reserve( xs.size() );
  33. 	for ( int x : xs )
  34. 	{
  35. 		ys.push_back( times_two( x ) );
  36. 	}
  37.  
  38. 	std::vector<int> zs;
  39. 	zs.reserve( ys.size() );
  40. 	for ( int y : ys )
  41. 	{
  42. 		zs.push_back( square( y ) );
  43. 	}
  44. 	return zs[0];
  45. }
  46.  
  47. // map (^2) . map (^2) $ [1,2,3]
  48. int two_loops_in_place( const std::vector<int>& xs )
  49. {
  50. 	std::vector<int> ys;
  51. 	ys.reserve( xs.size() );
  52. 	for ( int x : xs )
  53. 	{
  54. 		ys.push_back( times_two( x ) );
  55. 	}
  56.  
  57. 	for ( int& y : ys )
  58. 	{
  59. 		y = square( y );
  60. 	}
  61. 	return ys[0];
  62. }
  63.  
  64. template <typename F>
  65. void test( F f )
  66. {
  67. 	const std::vector<int> xs( 100000000, 42 );
  68.  
  69. 	const auto start_time = std::chrono::high_resolution_clock::now();
  70. 	const auto result = f( xs );
  71. 	const auto end_time = std::chrono::high_resolution_clock::now();
  72.  
  73. 	const auto elapsed = end_time - start_time;
  74. 	const auto elapsed_us = std::chrono::duration_cast<std::chrono::microseconds>(elapsed).count();
  75. 	std::cout << elapsed_us / 1000 << " ms - " << result << std::endl;
  76. }
  77.  
  78. int main()
  79. {
  80. 	test( manual_fusion );
  81. 	test( two_loops );
  82. 	test( two_loops_in_place );
  83. }
Success #stdin #stdout 1.64s 3472KB
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311 ms - 7056
615 ms - 7056
359 ms - 7056
https://ideone.com/I17kdT
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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