[Bug middle-end/109128] New: [Offload][OpenMP][OpenACC] Static linking with unused offload function will lead to mismatch number of offload fn/symbols

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109128

            Bug ID: 109128
           Summary: [Offload][OpenMP][OpenACC] Static linking with unused
                    offload function will lead to mismatch number of
                    offload fn/symbols
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: openacc, openmp, wrong-code
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

gfortran -fopenmp one.f90
ar rcv libone.a one.o
ranlib libone.a
gfortran -fopenmp two.f90 -L. -lone

With GCN, it prints:
    libgomp: Cannot map target functions or variables (expected 0, have 2)

The issue seems to be related to the linker removing the the symbol from
__OFFLOAD_TABLE__ (i.e. from the .gnu.offload_funcs sections, which has
first/last the__offload_func_table and __offload_funcs_end symbols, whose
pointer is put into the __OFFLOAD_TABLE__).

But the removed symbol is still visible to the offload compiler, i.e. it ends
up in the GOMP_register_var call as generated by
gcc/config/{gcn,nvptx}/mkoffload.cc


It is not quite clear which permutations work or cause problems.
It seems as if the common block (both files have then '.comm' sections) is
important. Whether 'libone.a' also fails or only '-L. -lone' seems to be
inconsistent (some minor details). Additionally, having a libgomp library call
also in the main program is also required.


==> one.f90 <==
module m
  implicit none
  integer :: my_var
  common /my_common/ my_var
contains
  integer function unused_func(n) result(res)
     integer :: n
     !$omp target map(from:res) firstprivate(n)
       res = 5*n
     !$omp end target
  end
end module m

==> two.f90 <==
use m
implicit none
integer :: A
!$omp target enter data map(A)
end