[Bug libstdc++/30482] New: <complex> division by 0

[Bug libstdc++/30482] New: <complex> division by 0

[jakub at gcc dot gnu dot org]

http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=221319
raises a question whether:
std::complex<double> (1.0, 1.0) / 0.0
and
std::complex<double> (1.0, 1.0) / std::complex<double> (0.0, 0.0)
can or can't be NaN.  In C say:
#define I (__extension__ 1.0iF)
_Complex double d = 1.0 + 0.0 * I, e, f, z = 0.0 + 0.0 * I;

int
main (void)
{
  e = d / 0.0;
  f = d / z;
  __builtin_printf ("%g %g %g %g\n", __real__ e, __imag__ e, __real__ f,
__imag__ f);
  return 0;
}
in -std=c99 always prints inf (where complex inf is even when just one of the
parts is inf), no matter what optimization options, in C89 the first division
when optimizing is usually optimized into the simpler __real__ d / 0.0,
__imag__ d / 0.0 division and thus gives inf too, but the latter and when not
optimizing
even the first one gives NaN.

What do we want in C++?  C++ doesn't use flag_complex_method = 2, so unless
that changes, only the division by double (rather than std::complex <double>)
could generally (when not optimizing) return inf - with a help in the
specialization, see the above URL.


-- 
           Summary: <complex> division by 0
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jakub at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30482

[Bug libstdc++/30482] <complex> division by 0

[pcarlini at suse dot de]

------- Comment #1 from pcarlini at suse dot de  2007-01-17 04:48 -------
Jakub, a few observations: the first one, we have got other PRs about complex
math (e.g., 24581, 28408), I don't know if you are aware of that, it would be
great if you could have a unified look. Also, I don't think we want to change
the library here (like in you link), I think we discussed this kind of solution
for such troubles other times and we came to the conclusion that really these
are compiler issues, because we are relying in a very straightforward way on
the complex extension (maybe Gaby has something to add). Also, as you may also
find in some of the existing audit trails and past discussions on gcc@, people
is particularly troubled by the large differences in behavior when optimizing
and not, I think we should tackle that issue.

Ah, maybe the most important point for this specific PR: I think we *do* want a
behavior of the complex division in C++ consistent with C99 (and LIA-2, which,
AFAIK, is consistent with C99), for many different reasons, among other things,
the new standard will be "based" on C99, not C89, probably Gaby has something
to add. Unfortunately, I'm afraid the way we are implementing C99 vs complex in
the middle-end is buggy, see 24581, Joseph' comments in particular, but
consistency between the C99 and the C++ math seems to me an improvement anyway.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gdr at integrable-solutions
                   |                            |dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30482

[Bug libstdc++/30482] <complex> division by 0

[pcarlini at suse dot de]

-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |pcarlini at suse dot de
                   |dot org                     |
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-07-05 22:28:18
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30482

[Bug middle-end/30482] <complex> division by 0

[paolo at gcc dot gnu dot org]

------- Comment #2 from paolo at gcc dot gnu dot org  2007-07-11 10:24 -------
Subject: Bug 30482

Author: paolo
Date: Wed Jul 11 10:23:56 2007
New Revision: 126546

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=126546
Log:
2007-07-11  Paolo Carlini  <pcarlini@suse.de>

        PR middle-end/30482
        * c-opts.c (c_common_post_options): Do not change flag_complex_method
        conditional to flag_isoc99.
        (c_common_init_options): Do it here, unconditionally.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-opts.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30482

[Bug middle-end/30482] <complex> division by 0

[pcarlini at suse dot de]

------- Comment #3 from pcarlini at suse dot de  2007-07-11 10:24 -------
Fixed.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30482