[Bug fortran/17927] New: Math error in simple divide operation

[Bug fortran/17927] New: Math error in simple divide operation

[jvdelisle at verizon dot net]

The following code produces erroneous results with:

GNU Fortran 95 (GCC 4.0.0 20041010 (experimental))
Copyright (C) 2003 Free Software Foundation, Inc.

program real_test

real(kind=4) :: little
real(kind=8) :: big

little=1.0/3.0
big=2.0/3.0

print *, little
print *, big

print *, '1/3 and 2/3?'

end program real_test

The output is:

  0.3333333
  0.666666686534882
 1/3 and 2/3?

-- 
           Summary: Math error in simple divide operation
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jvdelisle at verizon dot net
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-11 05:28 -------
Confirmed, this is a front-end problem, we are not getting enough precission for some reason (really I 
don't know the rules of fortran).  I will note the equivalent  C code works just fine on the same build of 
the compiler so it is not a bug in the rest of GCC (aka fold and real.c).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |wrong-code
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-11 05:28:50
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[tobi at gcc dot gnu dot org]

------- Additional Comments From tobi at gcc dot gnu dot org  2004-10-11 15:54 -------
gfortran uses its own constant folder, which uses the mpfr library. Apparently
the precision is too low for double precision. Output from -fdump-parse-tree
includes:
      ASSIGN big 6.66666686534882e-1_8
So it's indeed the frontend's fault.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[tobi at gcc dot gnu dot org]

------- Additional Comments From tobi at gcc dot gnu dot org  2004-10-11 16:00 -------
Actually, looking at this again, gfortran's behavior is correct.

If you had written
 big = 2.0_8 / 3.0_8
you would have gotten the result you were expecting.

I think, adding a warning would be the best route for "fixing" this problem.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[jvdelisle at verizon dot net]

------- Additional Comments From jvdelisle at verizon dot net  2004-10-12 04:57 -------
Subject: Re:  Math error in simple divide operation

On Mon, 2004-10-11 at 09:00, tobi at gcc dot gnu dot org wrote:
> ------- Additional Comments From tobi at gcc dot gnu dot org  2004-10-11 16:00 -------
> Actually, looking at this again, gfortran's behavior is correct.
> 
> If you had written
>  big = 2.0_8 / 3.0_8
> you would have gotten the result you were expecting.
> 
> I think, adding a warning would be the best route for "fixing" this problem.

I tried this same program with g95 before I submitted a bug and got what
I thought was the correct results without needing the additional _8 . 
Do the standards specify that default precision for constants is kind=4?

I suppose then that a warning would be good when mixing precisions in
assignments.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[anlauf at hep dot tu-darmstadt dot de]

------- Additional Comments From anlauf at hep dot tu-darmstadt dot de  2004-10-12 06:55 -------
(In reply to comment #4)
> Subject: Re:  Math error in simple divide operation
> 
> I suppose then that a warning would be good when mixing precisions in
> assignments.

With Fortran 77, the great free tool ftnchek assisted in finding these
problems and gave a warning that the r.h.s. result was promoted to a variable
of higher precision.  I am not aware of a similar free tool for Fortran 90.

With the gfortran parser in place, this is a good opportunity to fill this gap.
On the other hand, there are lots of real bugs waiting to be fixed... ;-)

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[tobi at gcc dot gnu dot org]

------- Additional Comments From tobi at gcc dot gnu dot org  2004-10-12 21:26 -------
(In reply to comment #4)
> I tried this same program with g95 before I submitted a bug and got what
> I thought was the correct results without needing the additional _8 . 
> Do the standards specify that default precision for constants is kind=4?

Yes. A real constant is of type default real, if no kind suffix is given. Of
course, the compiler is free to evaluate mathematical expressions any way it
wants, as long as the result is a "mathematical approximation" to the
expression, so g95 is not wrong. I assume that this difference between both
compilers was added when Steve Kargl changed constant folding to use mpfr and
the correct bit widths in all operations, a few months back.

> I suppose then that a warning would be good when mixing precisions in
> assignments.

I agree, but I also agree with Harald Anlauf :-)

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[tobi at gcc dot gnu dot org]

------- Additional Comments From tobi at gcc dot gnu dot org  2004-10-21 15:25 -------
We actually do have a warning for this: -Wconversion, which is currently not
enabled with -Wall. I'm closing this bug, but if you are in favor of adding this
warning to -Wall, Jerry, feel free to propose a patch to this effect. See options.c.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927

[Bug fortran/17927] Math error in simple divide operation

[tobi at gcc dot gnu dot org]

------- Additional Comments From tobi at gcc dot gnu dot org  2005-01-06 14:27 -------
Looks like I didn't close this bug, when I said I would.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17927