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parallel-processingopenmp
Vaqif Qurbanov
Vaqif Qurbanov

Reputation: 11

How can i make this loop parallel in OpenMP?

Hi everyone I got a question at the exam that I could not solve about parallel programming. Can someone help me with this .

Question: For the following code segment, use OpenMP pragmas to make the loop parallel, or explain why the code segment is not suitable for parallel execution:

flag = 0
for(i=0;(i<n) & (!flag);i++){
    a[i] = 2.3 *i;
    if(a[i]<b[i])flag = 1;
}

Upvotes: 0

Views: 135

Answers (1)

Jim Cownie
Jim Cownie

Reputation: 2859

As written the loop cannot trivially be parallelised with OpenMP because the test-expr in the loop (i.e. (i<n) & !flag) does not conform with the OpenMP restrictions :-

test-expr 
    One of the following:
        var relational-op ub
        ub relational-op var
    relational-op 
        One of the following: <, <=, >, >=, !=

(OpenMP standard). At a semantic level this is because the test on flag prevents the loop iteration count from being determinable at entry to the loop, which is what OpenMP requires.

In recent OpenMP standards there is a cancel construct which could be used here, something like (untested, uncompiled code)

bool flag = false;

#pragma omp parallel for 
for(int i=0;(i<n);i++){
    a[i] = 2.3 *i;
    if (a[i]<b[i]) {
#pragma omp atomic write
        flag = true;
    }
#pragma omp cancel for if (flag)
}

However it seems unlikely that a loop with this little work in it will be profitable for parallelisation in a real code (rather than an exam question).

Upvotes: 1

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