[clang] [flang] [flang][Driver] Let the linker fail on multiple definitions of main() (PR #73124)

[Michael Klemm via cfe-commits]

mjklemm wrote:

Thanks for the reproducer.

> The error shows up when linking a C program with a Fortran shared library, so maybe you weren't enabling building shared libraries?

I was building OpenBLAS using the "old" Makefile-based build.  There the issue indeed does not happen.  When I switched to your CMake configuration line, I was able to reproduce the problem.

The solution is to add `-fno-fortran-main` to the linker options via `CMAKE_SHARED_LINKER_FLAGS`.  This will need PR https://github.com/llvm/llvm-project/pull/74139 land first.  But this option will be a good way to control if the flang compiler should attempt linking in the `main` stub from its library.

It seems like `flang-new` when being used as a linker with `-shared` included Fortran_main in the shared library.  This seems wrong to me.  The option `-fno-fortran-main` avoids this.  I'm pondering if `-shared` is buggy here.  It will require a bit more digging on my end to figure that out.


https://github.com/llvm/llvm-project/pull/73124