[flang][polymorphic] Type descriptors are not emitted during separate compilation

[rofirrim]

I'm aware the polymorphic support is still experimental but I think it is useful to have this recorded.

Consider the two following files:

! mymod.f90
module mymod
  implicit none
  type, abstract :: myfoo
    integer :: x
  end type myfoo
end module mymod

! mymod2.f90
module mymod2
  use mymod, only: myfoo
  implicit none
  contains
    subroutine mysub(myparam)
      implicit none
      class(myfoo), pointer :: myparam

      nullify(myparam)
    end subroutine mysub
end module mymod2

If we compile them separatedly, mymod2.f90 will crash while generating FIR because the type descriptor of myfoo has not been emitted and nullify needs it.

$ flang-new -flang-experimental-polymorphism -c mymod.f90
$ flang-new -flang-experimental-polymorphism -c mymod2.f90
error: loc("/home/rferrer/fio/upstream/llvm-install/tmp/mymod2.f90":11:7): no type descriptor found for NULLIFY
LLVM ERROR: aborting

I looked into it a bit and IIUC, it seems that the type descriptor is emitted as part of lowerModuleDeclScope in the Bridge. If the two files together are compiled as a single file (e.g. adding an include 'mymod.f90' at the beginning of mymod2.f90) then the crash does not happen because the descriptor is indeed emitted.

In a separate compilation setting, AFAICT the module is loaded and parsed and its names resolved but I don't think we synthesise enough information so the type descriptors get emitted as a consequence of a use-statement. One thing I saw is that the descriptors (and other runtime info) are already emitted as weak global objects so emitting repeated definitions in different object files should not be a problem.

Any hints on how we could address this? I wonder if it makes sense to check use-associations to derived types and emit their descriptors if they haven't been emitted yet?

[llvmbot]

@llvm/issue-subscribers-flang-ir

[clementval]

The problem is coming from the fact that we try to find the global before we emit it. I'll have a patch that will fix this issue up for review tomorrow.

[rofirrim]

The problem is coming from the fact that we try to find the global before we emit it. I'll have a patch that will fix this issue up for review tomorrow.

Thanks a lot Valentin!

[clementval]

https://reviews.llvm.org/D154982

[llvmbot]

@llvm/issue-subscribers-flang-runtime

[llvmbot]

@llvm/issue-subscribers-flang-ir

[clementval]

@rofirrim The patch has landed upstream. Please confirm it solved your issue.

[rofirrim]

Hi @clementval, yes it fixes my issue.

Thanks again for the prompt fix.

[clementval]

Thanks for confirming and reporting the issue!