带有break语句的并行OpenMP循环 [英] Parallel OpenMP loop with break statement

问题描述

我知道您不能为OpenMP循环使用break语句，但是我想知道是否有任何变通办法，同时仍然可以从并行性中受益.基本上，我有一个"for"循环，该循环遍历一个大向量的元素，以寻找一个满足特定条件的元素.但是，只有一个元素可以满足条件，因此一旦找到，我们就可以跳出循环了，在此先感谢

I know that you cannot have a break statement for an OpenMP loop, but I was wondering if there is any workaround while still the benefiting from parallelism. Basically I have 'for' loop, that loops through the elements of a large vector looking for one element that satisfies a certain condition. However there is only one element that will satisfy the condition so once that is found we can break out of the loop, Thanks in advance

for(int i = 0; i <= 100000; ++i)
  {
    if(element[i] ...)
     {
          ....
          break;
      }
   }

推荐答案

您可以尝试使用while循环手动执行openmp for循环的操作:

You could try to manually do what the openmp for loop does, using a while loop:

const int N = 100000;
std::atomic<bool> go(true);
uint give = 0;

#pragma omp parallel
{
    uint i, stop;

    #pragma omp critical
    {
        i = give;
        give += N/omp_get_num_threads();
        stop = give;

        if(omp_get_thread_num() == omp_get_num_threads()-1)
            stop = N;
    } 


    while(i < stop && go)
    {
        ...
        if(element[i]...)
        {
            go = false;
        }
        i++;
    }
}

这样，您必须在每个循环中测试执行"，但这没什么大不了的.更重要的是，这将对应于静态" omp for循环，仅当您可以期望所有迭代花费相似的时间时，此功能才有用.否则，可能已经完成了3个线程，而一个线程还有一半要完成...

This way you have to test "go" each cycle, but that should not matter that much. More important is that this would correspond to a "static" omp for loop, which is only useful if you can expect all iterations to take a similar amount of time. Otherwise, 3 threads may be already finished while one still has halfway to got...

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