[Bug fortran/69360] New: loop optimization produces invalid code when a common array has dimension 1 in some files

["genericsoma at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69360

            Bug ID: 69360
           Summary: loop optimization produces invalid code when a common
                    array has dimension 1 in some files
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: genericsoma at gmail dot com
  Target Milestone: ---

I'm not sure if this bug is a valid one, as it depends on two common blocks
declared with different array size. This happens in out legacy code which had
no such problem in version 4.4.3 of gfortran. The versions which were found to
have problem are 4.8.2 on Linux and 5.2.0 on Windows using MSYS2/MinGW-w64.

Basically, when compiling with -O1 optimization flag, it appears as if the line
"e(kr)=e(kr)+e(k)" is optimized out. -O0 produces expected behaviour.

init.f:

        subroutine init
        common/e/e(6)
        do i=1,6
          e(i)=i
        end do
        end

test.f:

        program test
        common/e/e(1)
        call init
        do i=1,3
          kr=i*2-1
          k=i*2
          e(kr)=e(kr)+e(k)
          e(k)=1.d0
        end do
        print *,e(1)
        end

Compiled with

gfortran -O1 com.f test.f

outputs

   1.00000000,

while compiled with

gfortran -O0 com.f test.f

outputs

   3.00000000.

Changing "e(k)=1.d0" to "if (e(k).gt.0.d0) e(k)=1.d0" also eliminates the
problem.