[Bug middle-end/82601] New: missing uninitialized warning with -O0 / -Og

[janus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82601

            Bug ID: 82601
           Summary: missing uninitialized warning with -O0 / -Og
           Product: gcc
           Version: 8.0
            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: ---

Consider this simple Fortran test case:


program uninit

   integer :: p,q
   p = -1
   call sub(p, q)
   if (p<q) print *,"..."

contains

   subroutine sub(i, o)
      integer, intent(in) :: i
      integer, intent(out) :: o
      if (i<0) then
         print *, "..."
      else
         o = 1
      end if
   end subroutine

end


With -Wall and -O1 (or higher), all recent gfortran versions print:

    if (p<q) print *,"..."

Warning: ‘q’ is used uninitialized in this function [-Wuninitialized]


However, with -O0 or -Og, no warning is issued.

It should be possible to detect that 'o' may be uninitialized by looking only
at the subroutine itself (without any information about the caller).