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.