NAG Fortran support broken in v5.0.8

[mathomp4]

Background information

What version of Open MPI are you using? (e.g., v4.1.6, v5.0.1, git branch name and hash, etc.)

v5.0.8

Describe how Open MPI was installed (e.g., from a source/distribution tarball, from a git clone, from an operating system distribution package, etc.)

Installed via the tarfile.

Please describe the system on which you are running

• Operating system/version: macOS 15.6.1
• Computer hardware: M2 Pro
• Network type: Internal (ethernet?)

---

Details of the problem

I believe https://github.com/open-mpi/ompi/pull/13240 broke NAG support in Open MPI. The issue is that the new Fortran code used in configure:

program falignment
   CHARACTER A,B
   COMMON /AA/A
   COMMON /BB/B
   OPEN(UNIT=10, FILE="conftestval")
   if (LOC(A) > LOC(B)) then
      write (10,'(I5)') LOC(A)-LOC(B)
   else
      write (10,'(I5)') LOC(B)-LOC(A)
   endif
   CLOSE(10)

end program

uses LOC() and LOC() is not part of the Fortran standard, but rather a (common) Fortran extension. And NAG Fortran is a stickler for the Standard and will not support Fortran extensions if the Standard has a way to do something. When NAG tries to compile this code:

❯ nagfor test.F90
NAG Fortran Compiler Release 7.2(Shin-Urayasu) Build 7236
Obsolescent: test.F90, line 3: COMMON statement
Obsolescent: test.F90, line 4: COMMON statement
[NAG Fortran Compiler normal termination, 2 warnings]
Undefined symbols for architecture arm64:
  "_loc_", referenced from:
      _falignment_ in test.079142.o
      _falignment_ in test.079142.o
      _falignment_ in test.079142.o
      _falignment_ in test.079142.o
      _falignment_ in test.079142.o
      _falignment_ in test.079142.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This can be "duplicated" with GNU, say, if you ask for strict Fortran:

❯ gfortran-15 -std=f2023 test.F90
test.F90:4:14:

    4 |    COMMON /BB/B
      |              1
Warning: Fortran 2018 obsolescent feature: COMMON block at (1)
test.F90:3:14:

    3 |    COMMON /AA/A
      |              1
Warning: Fortran 2018 obsolescent feature: COMMON block at (1)
Undefined symbols for architecture arm64:
  "_loc_", referenced from:
      _MAIN__ in cca9Zeyr.o
      _MAIN__ in cca9Zeyr.o
      _MAIN__ in cca9Zeyr.o
      _MAIN__ in cca9Zeyr.o
      _MAIN__ in cca9Zeyr.o
      _MAIN__ in cca9Zeyr.o
ld: symbol(s) not found for architecture arm64
collect2: error: ld returned 1 exit status

One path forward is C_LOC, et al:

program falignment
   use iso_c_binding, only: C_LOC, C_INTPTR_T
   implicit none
   CHARACTER A,B
   COMMON /AA/A
   COMMON /BB/B
   OPEN(UNIT=10, FILE="conftestval")
   if (CLOC(A) > CLOC(B)) then
      write (10,'(I5)') CLOC(A)-CLOC(B)
   else
      write (10,'(I5)') CLOC(B)-CLOC(A)
   endif
   CLOSE(10)

   contains

      function CLOC(X) result(res)
         CHARACTER, INTENT(IN), TARGET :: X
         INTEGER(C_INTPTR_T) :: res
         res = transfer(C_LOC(X), res)
      end function CLOC

end program

though I guess now you require iso_c_binding support (and Open MPI might want to support older compilers?).

In my testing, this new code does seem to duplicate with GNU:

❯ rm -f conftestval && gfortran-15 test.F90 && ./a.out && cat conftestval
   16
❯ rm -f conftestval && gfortran-15 test_iso.F90 && ./a.out && cat conftestval
   16

and NAG now works:

❯ rm -f conftestval && nagfor test_iso.F90 && ./a.out && cat conftestval
NAG Fortran Compiler Release 7.2(Shin-Urayasu) Build 7236
Obsolescent: test_iso.F90, line 5: COMMON statement
Obsolescent: test_iso.F90, line 6: COMMON statement
[NAG Fortran Compiler normal termination, 2 warnings]
    1

I guess this is saying NAG doesn't align common blocks?