[flang][openmp] complains about incorrect OpenMP reduction if variable is used before parallel block

[aitap]

Originally reported at R-package-devel for 092b6c5, here's the compilation log and the full source code. Can also be reproduced for e107c94 (main at the time of writing).

I was able to reduce this to the following example:

program main
    implicit none

    real :: sum = 0
    integer :: i, n = 10

    sum = sum + 1. ! <-- error here if followed by OpenMP reduction

    !$OMP PARALLEL DO default(none) PRIVATE(i) SHARED(n) REDUCTION(+:sum) SCHEDULE(Dynamic,1)
    do i=1,n
        sum = sum + real(i)
    end do
    !$OMP END PARALLEL DO

    print *, sum
end

% flang-new -c -o /dev/null src.f90 -fopenmp
error: loc("src.f90":7:5): 'omp.reduction' op must be used within an operation supporting reduction clause interface
error: verification of lowering to FIR failed

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: None (aitap)

Originally reported at [R-package-devel](https://stat.ethz.ch/pipermail/r-package-devel/2023q4/009787.html) for 092b6c5, here's [the compilation log](http://web.archive.org/web/20231031154710/https://www.stats.ox.ac.uk/pub/bdr/clang17/frailtypack.log) and [the full source code](https://cran.r-project.org/src/contrib/Archive/frailtypack/frailtypack_3.5.0.tar.gz). Can also be reproduced for e107c94 (`main` at the time of writing).

I was able to reduce this to the following example:

program main
    implicit none

    real :: sum = 0
    integer :: i, n = 10

    sum = sum + 1. ! <-- error here if followed by OpenMP reduction

    !$OMP PARALLEL DO default(none) PRIVATE(i) SHARED(n) REDUCTION(+:sum) SCHEDULE(Dynamic,1)
    do i=1,n
        sum = sum + real(i)
    end do
    !$OMP END PARALLEL DO

    print *, sum
end

% flang-new -c -o /dev/null src.f90 -fopenmp
error: loc("src.f90":7:5): 'omp.reduction' op must be used within an operation supporting reduction clause interface
error: verification of lowering to FIR failed

[kiranchandramohan]

Thank you. We are working on reduction handling and will come to this soon.

[llvmbot]

@llvm/issue-subscribers-bug

Author: None (aitap)

Originally reported at [R-package-devel](https://stat.ethz.ch/pipermail/r-package-devel/2023q4/009787.html) for 092b6c5, here's [the compilation log](http://web.archive.org/web/20231031154710/https://www.stats.ox.ac.uk/pub/bdr/clang17/frailtypack.log) and [the full source code](https://cran.r-project.org/src/contrib/Archive/frailtypack/frailtypack_3.5.0.tar.gz). Can also be reproduced for e107c94 (`main` at the time of writing).

I was able to reduce this to the following example:

program main
    implicit none

    real :: sum = 0
    integer :: i, n = 10

    sum = sum + 1. ! <-- error here if followed by OpenMP reduction

    !$OMP PARALLEL DO default(none) PRIVATE(i) SHARED(n) REDUCTION(+:sum) SCHEDULE(Dynamic,1)
    do i=1,n
        sum = sum + real(i)
    end do
    !$OMP END PARALLEL DO

    print *, sum
end

% flang-new -c -o /dev/null src.f90 -fopenmp
error: loc("src.f90":7:5): 'omp.reduction' op must be used within an operation supporting reduction clause interface
error: verification of lowering to FIR failed

[kiranchandramohan]

This works now with #80019