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

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

[tkoenig at gcc dot 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.

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

[tkoenig at gcc dot gnu.org]

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

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |13.0
         Depends on|                            |34678
           Keywords|                            |wrong-code
             Blocks|                            |105105

--- Comment #1 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
As long as PR 36678 is not fixed, I see one possible solution in
putting a memory barrier after ieee_set_rounding_mode.

This is a rather big hammer, but as long as the middle-end issue
is not fixed, I do not see an alternative.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678
[Bug 34678] Optimization generates incorrect code with -frounding-math option
(#pragma STDC FENV_ACCESS not implemented)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105105
[Bug 105105] [Meta] Fortran IEEE support

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

[tkoenig at gcc dot gnu.org]

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

--- Comment #2 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
(In reply to Thomas Koenig from comment #1)
> As long as PR 36678

That should be PR 34678 .

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

[tkoenig at gcc dot gnu.org]

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

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-01-07
           Assignee|unassigned at gcc dot gnu.org      |tkoenig at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

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

[tkoenig at gcc dot gnu.org]

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

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Assignee|tkoenig at gcc dot gnu.org         |unassigned at gcc dot gnu.org

--- Comment #3 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Seems to be much more complicated than I thought, see the thrad starting at
https://gcc.gnu.org/pipermail/gcc-patches/2023-January/609532.html