[Bug tree-optimization/54767] [4.7/4.8 Regression] Incorrect code generated with "-O2 -fcheck=bounds"

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54767

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
          Component|fortran                     |tree-optimization
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-15 10:49:49 UTC ---
I don't see anything wrong by what VRP does:

Visiting statement:
prephitmp_2 = _79;

Found new range for prephitmp_2: [_79, _79]

Simulating statement (from ssa_edges): iaii_23 = PHI <iaii_1(11),
prephitmp_2(10)>

Visiting PHI node: iaii_23 = PHI <iaii_1(11), prephitmp_2(10)>

    Argument #0 (11 -> 12 executable)
        iaii_1
        Value: [_79, _79]

    Argument #1 (10 -> 12 executable)
        prephitmp_2
        Value: [_79, _79]  EQUIVALENCES: { _79 } (1 elements)
Meeting
  [_79, _79]
and
  [_79, _79]  EQUIVALENCES: { _79 } (1 elements)
to
  [_79, _79]
Found new range for iaii_23: [_79, _79]

Simulating statement (from ssa_edges): _35 = prephitmp_2 - iaii_23;

Visiting statement:
_35 = prephitmp_2 - iaii_23;

Found new range for _35: [0, 0]

Visiting statement:
ivvv.5_36 = _35 + 1;

Found new range for ivvv.5_36: [1, 1]


Now changed behavior is from:

Visiting PHI node: iaii_1 = PHI <iaii_18(D)(4), iaii_48(17)>

    Argument #0 (4 -> 5 executable)
        iaii_18(D)
        Value: UNDEFINED

    Argument #1 (17 -> 5 executable)
        iaii_48
        Value: [_79, _79]
Meeting
  UNDEFINED
and
  [_79, _79]
to
  [_79, _79]
Found new range for iaii_1: [_79, _79]

and this is all in a cycle.


I think this may be mixing _79 from different iterations.  Reduced testcase,
maybe easier to look at:

SUBROUTINE XXX (IL, IU)
  implicit none
  integer, INTENT(IN) :: IL, IU

  integer :: NXX (3) = (/ 0, 1, 2 /)
  integer :: ivvv, ia, ja, iaii
  logical :: qop

  QOP=.FALSE.

  DO IA=IL,IU
    JA=NXX(IA)
    IF (.NOT. QOP .and. JA.GT.0) THEN
      IAII=IA
      QOP=.TRUE.
    ENDIF

    IF (QOP) THEN
      ivvv=IA-IAII+1       ! mis-compiled
    ENDIF
  ENDDO

  IF (ivvv.NE.2) THEN
    call abort
  ENDIF
END subroutine

program p
  implicit none
  CALL XXX (1, 3)
end