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I wrote these two programs to test OMP PARALLEL:

1 : my_idx is declared at the beginning of the program and after a private copy for eah thread is done.

program my_pgm

    use omp_lib
    implicit none
    
    integer :: my_idx
    
    CALL OMP_set_dynamic(.FALSE.)
    CALL OMP_set_num_threads(2)
    
    !$OMP PARALLEL DEFAULT(SHARED) PRIVATE(my_idx)
 
    DO my_idx = 0, 3
        WRITE(*,*) 'my_idx , thread_num, ',my_idx,omp_get_thread_num()
    END DO
    
    !$OMP END PARALLEL
    
end program my_pgm

2 : my_idx is declared in a BLOCK after !$OMP PARALLEL

program my_pgm

    use omp_lib
    implicit none
        
    CALL OMP_set_dynamic(.FALSE.)
    CALL OMP_set_num_threads(2)
    
    !$OMP PARALLEL DEFAULT(SHARED)
    
    BLOCK

        integer :: my_idx
 
        DO my_idx = 0, 3
            WRITE(*,*) 'my_idx , thread_num, ',my_idx,omp_get_thread_num()
        END DO
    
    END BLOCK
    
    !$OMP END PARALLEL
    
end program my_pgm

What are the pros and the cons of these two programs ? Or are they equivalent ?

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  • 2
    Did you see any differences in your tests? I highly doubt you would see any difference. The generated Assemblies for your programs are identical, at least by gfortran: godbolt.org/z/7W5c6nM5M , godbolt.org/z/a6MjYhcs6 Other compilers (like Intel) treat block constructs slightly differently. But again that should not lead to tangible differences. Commented Apr 3 at 16:35
  • @Scientist. Thanks a lot. Commented Apr 3 at 16:44
  • 1
    All loop iteration variables in a parallel region are private by default. So, you don't even need the explicit private clause in this case. Commented Apr 3 at 17:59
  • 1
    However I would much, much prefer to see default(none) and you've missed out the what is often the best way of enforcing the scoping - putting the parallel region around a call to a procedure Commented Apr 3 at 18:15
  • @IanBush. Thanks a lot. You answered to a question before I asked Commented Apr 3 at 18:54
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