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 3A523385801E; Thu, 17 Mar 2022 14:03:17 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 3A523385801E
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/104937] wrong code with _Complex division and
 -fsanitize=undefined
Date: Thu, 17 Mar 2022 14:03:16 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
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-104937-4-o4FcReuOhi@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-104937-4@http.gcc.gnu.org/bugzilla/>
References: <bug-104937-4@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
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 17 Mar 2022 14:03:17 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104937

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
What we could for _Complex int just use the standard
(a + b*i) / (c + d*i) =3D (a*c + b*d) / (c*c + d*d) + ((b*c - a*d) / (c*c +
d*d))*i
which would be 6 multiplications and 2 divisions instead of the current
3 multiplications and 3 divisions.  But in order to avoid overflows we'd ne=
ed
to do that all in bigger precision.  c*c + d*d for _Complex int is in [0,
LONG_LONG_MAX+1ULL] range, a*c+b*d in [-LONG_LONG_MAX+2*INT_MAX+2,
LONG_LONG_MAX+1ULL] range etc., so perhaps with some special cases we fit i=
nto
2*prec type.
For _Complex unsigned we'd also need to watch for b*c being smaller than a*d
and handle that by doing - ((a*d - b*c) / (c*c + d*d)) instead.=