[Bug fortran/57160] New: short-circuit IF only with -ffrontend-optimize

[Bug fortran/57160] New: short-circuit IF only with -ffrontend-optimize

[Joost.VandeVondele at mat dot ethz.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57160

             Bug #: 57160
           Summary: short-circuit IF only with -ffrontend-optimize
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Joost.VandeVondele@mat.ethz.ch


it would be nice to have the short-circuit evaluation of IF statements only if
-ffrontend-optimize is in place. It would help to capture (at -O0) bugs like:

MODULE M1
 TYPE T1
   LOGICAL :: T=.TRUE.
 END TYPE T1
CONTAINS
 SUBROUTINE S1(m)
   TYPE(T1), POINTER :: m
   IF (ASSOCIATED(m) .AND. m%T) THEN
    WRITE(6,*) "X"
   ENDIF
 END SUBROUTINE
END MODULE

USE M1
 TYPE(T1), POINTER :: m=>NULL()
 CALL S1(m)
END

where I would like the code to segfault.

[Bug fortran/57160] short-circuit IF only with -ffrontend-optimize

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57160

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |burnus at gcc dot gnu.org

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-05-04 19:23:08 UTC ---
GCC (the middle end) has TRUTH_AND_EXPR (matching Fortran's .AND.) and
TRUTH_ANDIF_EXPR (matching C's &&) - besides the IAND/& which is BIT_AND_EXPR.

Currently, the code generation directly translates all .AND. into
TRUTH_AND_EXPR. Hence, the middle end/target-code generation might decide to
evaluate "A.AND.B" as "A andif B", "B andif A" or both as "A and B". That's
really outside the scope of the Fortran front end.

What you would like is that both A and B are /always/ evaluated with .AND.
That's quite some work with little gain. As I know for experience, the current
TRUTH_AND_EXPR does no short-circuit evaluation in the given order - I already
had segfaults for code similar to your's.

* * *

As a side note, see http://www.j3-fortran.org/doc/year/13/13-234.txt for a
proposal for the next Fortran standard which allows to explicitly require
short-circuit evaluation, using:

if ( IF (ASSOCIATED(m)) THEN (m%T) ELSE (.false.) ) then
  write (6,*) s1(m)
end if

Or even as:
  write (6,*) IF (ASSOCIATED(m)) THEN ( IF(m%T)then("X")ELSE("") ) ELSE ("")

[Bug fortran/57160] short-circuit IF only with -ffrontend-optimize

[tkoenig at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57160

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tkoenig at gcc dot gnu.org

--- Comment #2 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
I would really prefer if there was a compile-time warning for

   IF (ASSOCIATED(m) .AND. m%T) THEN

which might be doable.

Same thing for

   if (x /= 0 .and. 2/x > 0) then

[Bug fortran/57160] short-circuit IF only with -ffrontend-optimize

[dominiq at lps dot ens.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57160

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2014-12-21
     Ever confirmed|0                           |1

--- Comment #3 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Is there still some interest for this PR?

[Bug fortran/57160] short-circuit IF only with -ffrontend-optimize

[Joost.VandeVondele at mat dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57160

Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
                 CC|                            |Joost.VandeVondele at mat dot ethz
                   |                            |.ch

--- Comment #4 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
(In reply to Dominique d'Humieres from comment #3)
> Is there still some interest for this PR?

Yes, seems still nice.