[Bug fortran/81839] New: [7/8 Regression] doduc.f90 undefined behavior warning

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81839

            Bug ID: 81839
           Summary: [7/8 Regression] doduc.f90 undefined behavior warning
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

Compiling doduc.f90 from the pb11 benchmark suite results in the
warning

$ gfortran -O  -Waggressive-loop-optimizations doduc.f90 
doduc.f90:3587:0:

             QUGii(ij) = 0.

Warning: iteration 2 invokes undefined behavior
[-Waggressive-loop-optimizations]
doduc.f90:3586:0:

          DO ij = 1 , 3

note: within this loop

The loop itself looks innocent enough:

      ELSE
         DO ij = 1 , 3
            QUGii(ij) = 0.
            QCOni(ij) = 0.
         ENDDO
         GOTO 600