[flang][openmp] Bad code for index variable used in "shared" clause

[psteinfeld]

Here's a reproducer:

program bug
  implicit none
  integer :: i, j
  i = 0
  j = 0
  !$omp parallel shared(i)
    !$omp sections
      do i=1,10
        j = j + i
      end do
    !$omp end sections
  !$omp end parallel
  print *, "i: ", i
  if ( i==11 ) then
    print *,'ok'
  else
    print *,'fail, i: ', i
  end if
end program

When I compile this program with the -fopenmp option and execute it, I get the output:

 i:  0
 fail, i:  0

When similarly compiled with gfortran and executed, I get:

 ok

[Leporacanthicus]

So I'm struggling with how to solve this - it seems like I can't find the relevant information where I need it.

The loop variable is basically always privatized in the function genLoopVariableAddress in Bridge.cpp:1333. I tried fixing this in a few different ways, none of which works really well:

• adding a line to say - only do this when marked OmpPrivate - that works for this test-case, but since the same code is used outside of OpenMP loops, it completely fails in those cases.
• adding a line to NOT do the private variant of the variable if it's marked OmpShared. Initially this didn't work, because, apparently, shared variables are just variables that aren't marked with private or any other special attribute. I added code to set the flag. But from what I can tell, it sets a variable in a scope that is not available to the do-loop. (This mainly involves OmpAttributeVistitor::Post in resolve-directive.cpp:2036 and the lambda function makeSharedSymbol).
• instead of setting OmpShared on the new variable created by makeSharedSymbol, set the lastDeclSymbol or symbol->getUltimate(). This works for the issue's test-case, but about 18 tests fail, mostly because there's extra Shared in some places, but some tests also say "You can't have Shared and at the same time". I tried some further fixing to "only do this when we really need to". I went round so many circles of trying different things I think there's a groove (cartoon style thing) from trying different thing, and it's either not fixing the issue here, or it's causing other issues elswhere.
• I looked at trying to figure out from the current DoConstruct, if it was inside a OMP Parallel, but I can't figure out how to get that information - this may be one of those where it's very easy, if you just know to call the right function - just I couldn't find that one.
• I also tried looking at the Omp parallel directive and see if there's a do-loop in there. It doesn't seem like a great solution, and definitely not easy to do [as there's so many other things that could be between the parallel and relevant do-loop.

At this point, I'm out of good ideas how to solve this issue - I think the correct solution would be to basically move the code in Bridge.cpp that generates the local "private" variable, but that also seems quite complex and would probably involve implementing thre last part above to ensure we know when to do this and when to not do it.

Open to suggestions and happy to answer questions as to what I've tried (although I'm not entirely sure I can provide patches for all of the alternatives).

[tblah]

@Leporacanthicus how does this look? main...tblah:llvm-project:ecclescake/shared_loop_var

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Pete Steinfeld (psteinfeld)

Here's a reproducer: ``` program bug implicit none integer :: i, j i = 0 j = 0 !$omp parallel shared(i) !$omp sections do i=1,10 j = j + i end do !$omp end sections !$omp end parallel print *, "i: ", i if ( i==11 ) then print *,'ok' else print *,'fail, i: ', i end if end program ``` When I compile this program with the `-fopenmp` option and execute it, I get the output: ``` i: 0 fail, i: 0 ``` When similarly compiled with gfortran and executed, I get: ``` ok ```

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Pete Steinfeld (psteinfeld)

Here's a reproducer: ``` program bug implicit none integer :: i, j i = 0 j = 0 !$omp parallel shared(i) !$omp sections do i=1,10 j = j + i end do !$omp end sections !$omp end parallel print *, "i: ", i if ( i==11 ) then print *,'ok' else print *,'fail, i: ', i end if end program ``` When I compile this program with the `-fopenmp` option and execute it, I get the output: ``` i: 0 fail, i: 0 ``` When similarly compiled with gfortran and executed, I get: ``` ok ```