DependencyTools misses a simple opportunity for parallelisation? #3224

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1. mn416

   changed the title [-]DependecyTools misses a simple opportunity for parallelisation?[/-] [+]DependencyTools misses a simple opportunity for parallelisation?[/+] on Nov 17, 2025
2. mn416 commented on Nov 18, 2025

   

   mn416

   on Nov 18, 2025

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   Another slightly curious case that's cropped up during testing:

   subroutine double_loop(arr)
     integer, dimension(:,:), intent(inout) :: arr
     integer :: i, j, k
   
     do i = 1, size(arr, 2), 1
       do j = 1, size(arr, 1), 1
           arr(j,i) = 0
       enddo
       do k = 1, size(arr, 1), 1
           arr(k,i) = arr(k,i) + 1
       enddo
     enddo
   end subroutine double_loop

   Here, the i loop is not parallelised by ParallelLoopTrans.
3. LonelyCat124 commented on Nov 21, 2025

   

   LonelyCat124

   on Nov 21, 2025

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   from psyclone.psyir.transformations import OMPLoopTrans, TransformationError
   from psyclone.psyir.nodes import Loop, Routine

   def trans(psyir):
   for loop in psyir.walk(Loop):
   OMPLoopTrans(omp_directive="paralleldo").apply(loop)

   @mn416 what PSyclone version did you test with? Using exactly this I get:

   module subroutine_example
     implicit none
     public
   
     contains
     subroutine main()
       integer :: i
       real, dimension(10) :: re_m
       real, dimension(10) :: im_m
   
       !$omp parallel do default(shared) private(i) schedule(auto)
       do i = 1, 10, 1
         call sub(re_m(i), im_m(i))
       enddo
       !$omp end parallel do
   
     end subroutine main
     pure subroutine sub(a, b)
       real, intent(inout) :: a
       real, intent(inout) :: b
   
   
     end subroutine sub
   
   end module subroutine_example
   

   I tested with current master
4. LonelyCat124 commented on Nov 21, 2025

   

   LonelyCat124

   on Nov 21, 2025

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   The second one does still fail though - this is the error I get:

   psyclone.psyir.transformations.transformation_error.TransformationError: Transformation Error: Loop cannot be parallelised because:
   Error: The write access to 'arr(j,i)' and the read access to 'arr(k,i)' are dependent and cannot be parallelised. Variable: 'arr'.
   Consider using the "ignore_dependencies_for" transformation option if this is a false dependency
   Consider using the "array_privatisation" transformation option if this is a write-write dependency
   

   I suspect this is not so easy to solve (since j/k are modified inside the outer loop) and probably we just expect ignore_dependencies_for or force to be the solution if the outer loop needs to be parallelised over at the moment. I think this outer/inner loop behaviour we could benefit from improving in a few places, but its not so straightforward to know what the solution is? I guess we could in theory resolve that index fully (i.e. we have accesses to arr(:,i) for each value of i, but I'm not sure how this stuff works in detail at the moment).
5. mn416 commented on Nov 21, 2025

   

   mn416

   on Nov 21, 2025

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   Thanks @LonelyCat124. My version of PSyclone is a month old and I confirm that the first example works on master. I notice @sergisiso has made changes to DependencyTools since then, which must have fixed it. Great!

   Regarding the second example, I am also not overly familiar with how DependencyTools works but I found it surprising that it can parallelise the i loop if either of the two inner loops is present, but not both. Maybe it is just a tricky case, in which case please close the issue. (The analysis in #3213 can handle it but, I imagine, it works in quite a different way.)