[libcxx-commits] [PATCH] D71525: [libc++] Fix typo in std::midpoint

[Ruslan Baratov via Phabricator via libcxx-commits]

ruslo added a comment.

> there is no visible error because the difference is between (a/2 + b/2) and (a/2 + b) when (a > hi && b < lo)

You can see the difference if you run the benchmark:

  #include <iostream>
  #include <numeric>
  #include <vector>
  
  using namespace std;
  
  double midpoint_sum(const vector<double>& v1, const vector<double>& v2) {
    double res{0.0};
    for (size_t i = 0; i < v1.size(); ++i) {
      res += midpoint(v1[i], v2[i]);
    }
    return res;
  }
  
  template <class T>
  void print_duration(T start, T end) {
    auto diff = end - start;
    cout << chrono::duration_cast<chrono::milliseconds>(diff).count() << " ms\n";
  }
  
  int main() {
    using clock = chrono::high_resolution_clock;
  
    size_t n = 500 * 1000 * 1000;
  
    double a = numeric_limits<double>::min() * 1.99;  // a < lo
    double b = numeric_limits<double>::max() / 1.99;  // b > hi
  
    vector<double> v_a(n, a);
    vector<double> v_b(n, b);
  
    auto start = clock::now();
    double r_a = midpoint_sum(v_a, v_b);
    auto end = clock::now();
    print_duration(start, end);
  
    start = clock::now();
    double r_b = midpoint_sum(v_b, v_a);
    end = clock::now();
    print_duration(start, end);
  
    // Use results
    cout << "Result: " << r_a << '\n';
    cout << "Result: " << r_b << '\n';
  }

Original libc++:

  1388 ms
  21910 ms

Patched libc++:

  1390 ms
  1393 ms


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D71525