<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/138471>138471</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [flang] bug passing non-contiguous array to MPI procedure
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            flang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          nncarlson
      </td>
    </tr>
</table>

<pre>
    I've run into errors with flang 20.1.0 when passing a non-contiguous dummy array to MPI procedures (MPICH 4.3.0), where it needs to be doing copy-in/copy-out. It appears that the address of the initial array element is being passed instead of the address of a _contiguous copy_ of the array. I'm not sure if this is a bug with flang, or a problem with MPICH where it doesn't know how to provide a correct `mpi` module for flang.

I've pared it down to a small (serial) reproducer that doesn't use MPI at all, but mimics the `mpi` module from MPICH 4.3.0. There are two files `mpi_bcast.c`:
```C
// Dummy version of MPI_Bcast to dump the received buffer
#include <stdio.h>
void mpi_bcast_(void *buffer, int *count) {
  float *vector = (float*) buffer;
 printf("     in mpi_bcast: buffer=");
  for (int i = 0; i < *count; ++i) {
    printf(" %f", vector[i]);
  }
  printf("\n");
}
```
and the main program  `flang-20240504.F90`:

```fortran
module mpi_dummy

  !! Actual form of the interface from the MPICH 4.3.0 mpi module
  interface MPI_Bcast
    subroutine MPI_Bcast(buffer, count)!, datatype, root, comm, ierror)
      implicit none
#ifdef __flang__
      !DIR$ IGNORE_TKR buffer
#endif
#ifdef __INTEL_COMPILER
      !DEC$ ATTRIBUTES NO_ARG_CHECK :: buffer
#endif
#ifdef __GFORTRAN__
      !GCC$ ATTRIBUTES NO_ARG_CHECK :: buffer
#endif
      real :: buffer
      integer :: count
 !integer :: datatype
      !integer :: root
      !integer :: comm
 !integer :: ierror
    end subroutine
  end interface

end module

program main

  use mpi_dummy

  real :: x(5)
  x = [1,2,3,4,5]
  print *, 'x=', x
  call bcast(x(1::2))

contains

 subroutine bcast(buffer)
    real, intent(inout) :: buffer(:)
    print *, '  in bcast: buffer=', buffer
    ! A CONTIGUOUS COPY OF BUFFER MUST BE PASSED HERE
    call MPI_Bcast(buffer, size(buffer))
  end subroutine

end program
```
To compile:
```sh
clang -c mpi_bcast.c
flang flang-20240504.F90 mpi_bcast.o
```
The expected output from running should be
```
$ ./a.out
 x= 1. 2. 3. 4. 5.
   in bcast: buffer= 1. 3. 5.
     in mpi_bcast: buffer= 1.000000 3.000000 5.000000
```
But with flang I'm getting
```
$ ./a.out
 x= 1. 2. 3. 4. 5.
   in bcast: buffer= 1. 3. 5.
     in mpi_bcast: buffer= 1.000000 2.000000 3.000000
```
Both Intel ifx and gfortran produce the expected results.
</pre>
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