[Users] Fortran OpenMP parallel do

[Erik Schnetter]

On Oct 30, 2013, at 16:49 , Roland Haas <roland.haas at physics.gatech.edu> wrote:

> Signed PGP part
> Hello all,
> 
> > On Wed, Oct 30, 2013 at 08:54:08AM -0400, Yosef Zlochower wrote:
> >> I though that for C code, the (outermost) loop index was
> >> automatically private. Has this changed in the openmp standard?
> Philipp and I did some more reading of the OpenMP specs
> (http://www.openmp.org/mp-documents/spec30.pdf‎) and while we could
> not find a helpful definition of "associated loops" (since those are
> the ones whose controlling variables are automatically private in
> FOTRAN) we *did* find an example in the specs that does exactly what
> we do. Example A.7.1f (on page 168) has:
> $OMP DO
>      DO 300 I = 1,10
>        DO 300 J = 1,10
>          CALL WORK(I,J)
> 300  CONTINUE
> !$OMP ENDDO
> which would indicate that that *both* nested loops are seen as one and
> that both i and j are private. Note that the 2.0 specs (which have no
> COLLAPSE yet) also speak of do_loops and inner and outer loops so this
> all points towards the standard requiring that all nested loops in
> FORTRAN have private loop variables.

In this case, i and j are both private since the OpenMP parallel construct is in the routine calling this routine, so that the routine-local declarations of i and j make them private (since declared inside the parallel region). This has nothing to do with the loops; only the i loop is parallelized, while the j loop is not.

However, I stand corrected: Apparently, in Fortran all loop indices are private, as described in the example following the one you mention.

-erik

> HOWEVER we certainly should NOT do this in C since no such guarantee
> is given there and I believe we SHOULD do as Erik suggested and
> explicitly declare all loop control variables as private (in FORTRAN)
> since the standard is confusing and possibly misleading. Appendix A.8
> of the specs give more information on this.
> 
> > Having said that, assume a construct like:
> >
> > #pragma omp parallel for for (int i=0; i<N; i++) { }
> >
> > Can you here actually specify private(i)? 'i' isn't even declared
> > at the point of the pragma yet.
> This is fine and actually the recommended method since it declares "i"
> inside of the parallel region so it is automatically private. It is
> (up to the lifetime of "i" after the loop ends) the same as:
> #pragma omp parallel for
> {
> int i;
> for (i=0; i<N; i++)
> {
> }
> }
> 
> Yours,
> Roland
> 
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-- 
Erik Schnetter <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/

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