[Bug middle-end/81889] New: [7 Regression] bogus warnings with -Wmaybe-uninitialized -O3

["janus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81889

            Bug ID: 81889
           Summary: [7 Regression] bogus warnings with
                    -Wmaybe-uninitialized -O3
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janus at gcc dot gnu.org
  Target Milestone: ---

Please consider this simple Fortran test case:


module m

   type t
      integer, dimension(:), pointer :: list
   end type

contains

   subroutine s(n, p, Y)
      integer, intent(in) :: n
      type(t) :: p
      real, dimension(:) :: Y

      real, dimension(1:16) :: xx

      if (n > 3) then
         xx(1:n) = 0.
         print *, xx(1:n)
      else
         xx(1:n) = Y(p%list(1:n))
         print *, sum(xx(1:n))
      end if

   end subroutine

end module



When compiled with "gfortran-7 -Wmaybe-uninitialized -O3 -c", it yields:

maybe_uninit.f90:21:0:

          print *, sum(xx(1:n))

Warning: ‘xx[3]’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[4]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[5]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[6]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[7]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[8]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[9]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[10]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[11]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[12]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[13]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[14]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
maybe_uninit.f90:21:0: Warning: ‘xx[15]’ may be used uninitialized in this
function [-Wmaybe-uninitialized]


Obviously no value that is used is actually uninitialized. gfortran 6 and
earlier does not show these warnings. Also decreasing the optimization level
makes them disappear.