[Bug middle-end/34678] Optimization generates incorrect code with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

[tkoenig at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678

--- Comment #46 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Fortran gets this right:

$ cat set_rounding_mode.f90
module x
  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]
    integer :: i
    do i=1,4
       call ieee_set_rounding_mode (mode(i))
       a(i) = b + c
    end do
  end subroutine foo
end module x

program main
  use x
  real(kind=wp), dimension(4) :: a
  call foo(a, 0.1_wp, 0.2_wp)
  print *,a
end program main
$ gfortran -O3 set_rounding_mode.f90
$ ./a.out
  0.30000000000000004       0.29999999999999999       0.30000000000000004      
0.29999999999999999