[Bug fortran/69368] [6 Regression] spec2006 test case 416.gamess fails with the g++ 6.0 compiler starting with r232508

[dominiq at lps dot ens.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69368

--- Comment #61 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Another oddity of the "optimization" introduced by r232508 is the following
test (borrowed from https://gcc.gnu.org/ml/gcc-patches/2016-02/msg01356.html)

[Book15] f90/bug% cat pr69368_1_a.f90
      SUBROUTINE FOO
      IMPLICIT DOUBLE PRECISION (X)
      INTEGER J
!      COMMON /MYCOMMON / X(1031)
      COMMON /MYCOMMON / X(1)
      DO 10 J=1,1024
         X(J+1)=X(J+7)
  10  CONTINUE
      RETURN
      END
[Book15] f90/bug% cat pr69368_1_b.f90
      IMPLICIT DOUBLE PRECISION (X)
      COMMON /MYCOMMON/ X(1031)
      DO I=1,1031
      X(I)=I
      END DO
      call FOO()
      print *, X(1025)
      IF (X(1025).NE.1031) CALL ABORT
      END

The test fails if pr69368_1_a.f90 is compiled with -O2 (FOO is compiled as a
simple RETURN). Now if I replace

      COMMON /MYCOMMON / X(1)

with

      COMMON /MYCOMMON / X(2)

I get at compile tile with -O2

pr69368_1_a_1.f90:7:0:

          X(J+1)=X(J+7)

Warning: iteration 1 invokes undefined behavior
[-Waggressive-loop-optimizations]
pr69368_1_a_1.f90:6:0:

       DO 10 J=1,1024

note: within this loop

and the test succeeds. Any reason why there is no warning for "COMMON /MYCOMMON
/ X(1)"?

Also the following invalid code

[Book15] f90/bug% cat pr69368_1_c.f90
      SUBROUTINE FOO(X)
      IMPLICIT DOUBLE PRECISION (X)
      INTEGER J
      DIMENSION X(1)
      DO 10 J=1,1024
         X(J+1)=X(J+7)
  10  CONTINUE
      RETURN
      END
[Book15] f90/bug% cat pr69368_1_d.f90
      IMPLICIT DOUBLE PRECISION (X)
      DIMENSION X(1031)
      DO I=1,1031
      X(I)=I
      END DO
      call FOO(X)
      print *, X(1025)
      IF (X(1025).NE.1031.0) CALL ABORT
      END

gives the expected result.