From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id A3A583854178; Fri, 18 Nov 2022 23:32:58 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org A3A583854178
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1668814378;
	bh=081lorjc72idEzUHFBwe8929GQKJEcAjdc+LxAhp/gQ=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=UeQAyLeBwVRNIWrvvk4qfKrbETP6YpDldXTayggIPDoYAWPRR9DROOYI/2MMKEi55
	 WMsZ3JkGkYb+e92dz5FkmxWaInJoMDz+5blfPB8mSx/rDbtCAQHOG4xz4PZRb034ZT
	 nv6hY/2tYlBzmXj/00dPgu1h09gjFm59lKMgbBrk=
From: "sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/107753] gfortran returns NaN in complex divisions
 (x+x*I)/(x+x*I) and (x+x*I)/(x-x*I)
Date: Fri, 18 Nov 2022 23:32:58 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: fortran
X-Bugzilla-Version: 12.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: sgk at troutmask dot apl.washington.edu
X-Bugzilla-Status: WAITING
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-107753-4-0VJfOELvLU@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-107753-4@http.gcc.gnu.org/bugzilla/>
References: <bug-107753-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107753

--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> -=
--
On Fri, Nov 18, 2022 at 11:24:29PM +0000, sgk at troutmask dot
apl.washington.edu wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107753
>=20
> --- Comment #5 from Steve Kargl <sgk at troutmask dot apl.washington.edu>=
 ---
> On Fri, Nov 18, 2022 at 10:05:21PM +0000, kargl at gcc dot gnu.org wrote:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107753
> >=20
> > --- Comment #4 from kargl at gcc dot gnu.org ---
> > (In reply to anlauf from comment #3)
> > > I guess the reporter assumes that gcc uses a clever algorithm like Sm=
ith's
> > > to handle such extreme cases of complex division.  Not sure if that o=
ne is
> > > available by some compilation flag, and I think it would impact perfo=
rmance.
> > >=20
> > > In any case, if the reporter wants to get robust results and in a por=
table
> > > way, I would advise him to change/fix his algorithm accordingly.  It =
appears
> > > that a few other compilers behave here like gfortran.
> >=20
> > It's likely coming from the middle-end where gcc.info has
> > the option
> >=20
> > '-fcx-fortran-rules'
> >      Complex multiplication and division follow Fortran rules.  Range
> >      reduction is done as part of complex division, but there is no
> >      checking whether the result of a complex multiplication or division
> >      is 'NaN + I*NaN', with an attempt to rescue the situation in that
> >      case.
>=20
> Does anyone know what is meant by "Fortran rules"?  F66 does not
> have any particular algorithm specified.  I'll look at F77 shortly.
>=20

I add the subroutine

   subroutine ohno
      complex(dp), parameter :: a =3D cmplx(huge(1.d0),huge(1.d0),dp)
      complex(dp), parameter :: b =3D a / a
      write(*,*) a
      write(*,*) b
   end subroutine ohno=20


% gfortran -o z a.f90 && ./z
   (1.79769313486231571E+308,1.79769313486231571E+308)
                              (NaN,0.0000000000000000)
   (1.79769313486231571E+308,1.79769313486231571E+308)
               (1.0000000000000000,0.0000000000000000)

The last two lines are from ohno.=