[Bug fortran/108329] New: IEEE_SET_ROUNDING_MODE ineffective with common subexpression elimination

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108329

            Bug ID: 108329
           Summary: IEEE_SET_ROUNDING_MODE ineffective with common
                    subexpression elimination
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

Split from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678#c47 .

The test case

$ cat y.f90
module y
  implicit none
  integer, parameter :: wp = selected_real_kind(15)
contains
  subroutine foo(a,b,c)
    use ieee_arithmetic
    real(kind=wp), dimension(4), intent(out) :: a
    real(kind=wp), intent(in) :: b, c
    type (ieee_round_type), dimension(4), parameter :: mode = &
         [ieee_nearest, ieee_to_zero, ieee_up, ieee_down]
    call ieee_set_rounding_mode (mode(1))
    a(1) = b + c
    call ieee_set_rounding_mode (mode(2))
    a(2) = b + c
    call ieee_set_rounding_mode (mode(3))
    a(3) = b + c
    call ieee_set_rounding_mode (mode(4))
    a(4) = b + c
  end subroutine foo
end module y

program main
  use y
  real(kind=wp), dimension(4) :: a
  call foo(a, 0.1_wp, 0.2_wp)
  print *,a
end program main
$ gfortran -O  y.f90 && ./a.out
  0.30000000000000004       0.30000000000000004       0.30000000000000004      
0.30000000000000004     
$ gfortran y.f90 && ./a.out
  0.30000000000000004       0.29999999999999999       0.30000000000000004      
0.29999999999999999

shows that common subexpression removal causes the addition to be performed
only once.