OpenMP not supporting break in loop

Question

I am optimizing some code with OpenMP. If NO_VALUE is met in a loop, I would like it to break. However, compiler tells me this is not allowed with openMP. How could I handle this?

#pragma omp parallel
{
     #pragma omp for  reduction(+:functionEvaluation)   schedule(dynamic)   nowait
     for (int j=m_colStart;j<m_colEnd+1;j++)
          {
               double d = command_->execute().toDouble();
               if(d==NO_VALUE)
               {
                    functionEvaluation  = NO_VALUE;
                                                   break;
                }
                else
                {
                    functionEvaluation += d;
                }
                delete command_;
            }
 }

How could I work around? thanks!

score 10 · Accepted Answer · answered Apr 17 '12 at 11:30

There is no problem with Qt.
you cannot have a break in a loop that has been parallelized using openMP.

Reasoning behind this : Lets say you have a loop with 10 iterations. Now, you want to break the loop at iteration 5. You say,

if (iteration==5) 
    break;

Now come to the parallel context, you have created 10 threads and each do their respective iteration in parallel. So when thread5 reaches certain condition you want all other threads to

determine this
undo their output
Do not process anymore iterations
do this dynamically and for all different scheduling policies.

as you can see, this is not possible/practical in most circumstances. Hence OpenMP has disallowed the use of break in a parallel for loop.

Now, say you know that a certain condition for breaking is very rare in your loop and you are hell bent on having a break in your parallel loop.

Thread i 
Check input condition
If input leads to breaking flag a marker break_loop
******** barrier across all threads *************
if flag break_loop is true
then
   discontinue this operation
   mark a variable
   discontinue all the remaining iterations
endif
do your normal work

This is just a framework. You will have to look at corner cases, locks, etc. For this you can have each thread check their input before iteration. If their input condition leads to breaking of loop, they set a con

@dlib “OpenMP” is written with a capital “O”. I’m just commenting because you persistently (and consistently) write it with a lower-case “o”. — Konrad Rudolph, Apr 17 '12 at 13:06

score 0 · Answer 2 · answered Apr 30 '12 at 13:43

0

openmp doesn't support break in for clause.

answered Apr 30 '12 at 13:43

yyfn

737
4
4

just kidding, but why not SET THE LOOP VARIABLE TO THE CONDITION WHERE IT STOPS - 1 ? – Yvain Jun 24 '19 at 17:18

score 0 · Answer 3 · answered Jun 28 '19 at 23:43

I'm adding this as a proper answer; think about this: you got a loop with a condition to stop, for instance: for(int a=0; a<10; a++) ...

if, while in the loop, you just set a to 9, and execute a continue statement, it will break out the loop implicitly.

OpenMP not supporting break in loop

3 Answers3