[Bug middle-end/70626] New: bogus results in 'acc parallel loop' reductions

["cesar at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70626

            Bug ID: 70626
           Summary: bogus results in 'acc parallel loop' reductions
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: openacc
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: cesar at gcc dot gnu.org
          Reporter: cesar at gcc dot gnu.org
                CC: tschwinge at gcc dot gnu.org
  Target Milestone: ---

Created attachment 38233
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38233&action=edit
test case

Currently, given a combined acc parallel loop of the form

  #pragma acc parallel loop reduction(+:var)

all of the front ends will split that loop as

  #pragma acc parallel
  #pragma acc loop reduction(+:var)

This is bad because the gimplifier will not assign an implicit present_or_copy
clause for the reduction variable 'var', instead 'var' gets transferred via
firstprivate. One solution here is to teach the front ends to attach the
reduction clause to both directives, instead of just the split acc loop.