[flang-commits] [flang] [flang][Semantics] Introduce `-Wpass-global-variable` warning (PR #160324)

[via flang-commits]

foxtran wrote:

>  It is perfectly legit and indeed expected to pass global values to procedure calls.

For some cases, it may lead to undefined behaviour. For example, for the following pieces of code:

main.f90:
```fortran
program main
  implicit none
  common /mem/ imem
  integer :: imem
  imem = 0
  call test(imem)
end program main

subroutine update_imem
  implicit none
  common /mem/ imem
  integer :: imem
  imem = 30
end subroutine update_imem
```

test.f90:
```
subroutine test(imem)
  integer :: imem
  print *, imem
  call update_imem()
  print *, imem
end subroutine test
```

No opts:
```console
$ flang test.f90 main.f90 -O0 -o a.exe && ./a.exe
 0
 30
```

With -O3:
```
$ flang test.f90 main.f90 -O3 -o a.exe && ./a.exe
 0
 0
```

With this PR, one gets:
```console
$ flang -Wpass-global-variable -fsyntax-only *.f90
./main.f90:6:13: warning: Passing global variable 'imem' from COMMON 'mem' as function argument [-Wpass-global-variable]
    call test(imem)
              ^^^^
```


>  I think this would be a very noisy warning.

Now, it is not. I have reduced some possible invalid cases that, unfortunately, used a lot, but in general they should not give a problem. And in some valid contexts it also does not give warnings. I have played with GAMESS(US), MRCC and xtb and it does not give a lot of warnings.

https://github.com/llvm/llvm-project/pull/160324