OpenMP reduction with custom types

baka_para.cpp

// ref: https://stackoverflow.com/a/39990387
#include <iostream>
#include <vector>

#include <omp.h>

struct Value {
  Value(void) : x(-1) {}
  Value(float x) : x(x) {}
  float x;
};

const Value &max(const Value &a, const Value &b) { return a.x > b.x ? a : b; }

auto prepare(void) -> std::vector<std::vector<Value>> {
  std::vector<std::vector<Value>> x;

  for (size_t i = 0; i < 10; i++) {
    x.push_back(std::vector<Value>(10, Value(0.0)));
  }
  return x;
}

auto main() -> int {
  auto x = prepare();
  Value max_value(-1);
#pragma omp declare reduction(maxVal:Value : omp_out = max(omp_out, omp_in))
#pragma omp parallel for collapse(2) reduction(maxVal : max_value)
  for (size_t i = 0; i < 10; i++) {
    for (size_t j = 0; j < 100; j++) {
      max_value.x = i * 100 + j;
    }
  }

  std::cout << max_value.x << std::endl;
}

#if 0
~/Documents/misc/omp_example
❯ /usr/local/bin/g++-9 baka_para.cpp -std=c++14 -fopenmp -lpthread -O0 -g

~/Documents/misc/omp_example
❯ ./a.out
999
#endif

baka_para_oldOMP.cpp

// If your OpenMP <= 3.1
// https://stackoverflow.com/a/39993717
#include <iostream>
#include <vector>

#include <omp.h>

struct Value {
  Value(void) : x(-1) {}
  Value(float x) : x(x) {}
  float x;
};

const Value &max(const Value &a, const Value &b) { return a.x > b.x ? a : b; }

auto prepare(void) -> std::vector<std::vector<Value>> {
  std::vector<std::vector<Value>> x;

  for (size_t i = 0; i < 10; i++) {
    x.push_back(std::vector<Value>(10, Value(0.0)));
  }
  return x;
}

auto main() -> int {
  auto x = prepare();
  Value max_value(-1);
#pragma omp parallel
  {
    Value thread_max_value;

#pragma omp for collapse(2)
    for (size_t i = 0; i < 10; i++) {
      for (size_t j = 0; j < 100; j++) {
        if (i * 100 + j > thread_max_value.x) {
          thread_max_value.x = i * 100 + j;
        }
      }
    }

#pragma omp critical
    {
      if (thread_max_value.x > max_value.x) {
        max_value.x = thread_max_value.x;
      }
    }
  }

  std::cout << max_value.x << std::endl;
}

#if 0
~/Documents/misc/omp_example
❯ /usr/local/bin/g++-9 baka_para_oldOMP.cpp -std=c++14 -fopenmp -lpthread -O0 -g

~/Documents/misc/omp_example
❯ ./a.out
999
#endif

On mac, GCC9