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Beginner

OpenMP Task Dependency with ifort 15

I am currently trying to use the *task* construct of OpenMP 4.0 including the *depend* statement for my Fortran codes. Therefore, I create the following example, which should fill up the first row of a matrix with the numbers 1 to M by a task and fill up the remaining elements by a task each whenever the element in the first row is ready. This results in the following piece of code:

 

     PROGRAM OMP_TEST         
        IMPLICIT NONE 
    
        INTEGER K,L 
        INTEGER M
        PARAMETER (M = 8)
        INTEGER A(M,M) 
    
        A(1:M, 1:M) = 0
    
        !$omp parallel 
        !$omp single
        DO L=1, M 
            !$omp task depend(out:A(1,L)) default(shared) 
            A(1,L) = L
            !$omp end task 
            DO K = 2, M 
                !$omp task depend(in:A(1,L)) default(shared)
                A(K,L) = A(1,L) 
                !$omp end task
            END DO 
        END DO 
        !$omp taskwait 
        !$omp end single 
        !$omp end parallel
    
        DO K =1 , M 
            WRITE(*,*) A(K,1:M) 
        END DO 
    END PROGRAM 

Compile with the Intel Fortran 15 compiler, which is according to the documentation aware of the *depend* statement. But the result printed to the screen is different at every execution. Even the initial zeros of the matrix stay at some positions. For example:

   

           1           2           3           4           5           6
           7           8
           0           0           0           0           0           0
           0           0
           0           0           3           4           0           0
           0           8
           1           0           3           4           0           6
           0           8
           1           0           3           4           5           6
           0           8
           1           2           3           4           5           6
           7           8
           0           2           3           4           5           6
           7           0
           1           2           3           4           5           6
           0           8


Why does the dependencies between the task do not work correctly as I expect it such that the values 1 to 8 are in each row?

 

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2 Replies
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Employee

I see similar results with

I see similar results with more than two threads. It appears to be a defect. I escalated to Development and will let you know what I hear back.

(Internal tracking id: DPD200413379)

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Beginner

I tried the same example with

I tried the same example with icc 15 and ported the code to C and the same happens.

Here is the code in this case:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int M = 16; 
    int i,j; 
    int A[M*M]; 

    for (i = 0; i < M; i++) {
        for (j = 0; j < M; j++) {
            A[j*M+i] = 0; 
        }
    }

     #pragma omp parallel 
    {
        #pragma omp single 
        {
            for (j = 0; j < M; j++) {
                #pragma omp task depend(out:A[M*j]) default(shared) 
                {
                    A[M*j] = (j+1); 
                }
                for (i = 1; i < M; i++) {
                    #pragma omp task depend(in:A[M*j]) default(shared) 
                    A[i+j*M] = A[M*j];  
                }
            }
        }
        #pragma omp taskwait 
    }

    for (i = 0; i < M; i++) {
        for (j = 0; j < M; j++) {
            printf("%4d", A[i+M*j]);
        }
        printf("\n");
    }


    return 0;
}

 

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