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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/40907] -O2 generates incorrect code on i386
Date: Wed, 29 Jul 2009 18:13:00 -0000 [thread overview]
Message-ID: <20090729181251.17574.qmail@sourceware.org> (raw)
In-Reply-To: <bug-40907-18000@http.gcc.gnu.org/bugzilla/>
------- Comment #5 from rguenth at gcc dot gnu dot org 2009-07-29 18:12 -------
long lo_b_2268X;
long lo_a_2267X;
long lo_c_2271X;
...
lo_a_2267X = 65535 & a_1962X;
lo_b_2268X = 65535 & b_2266X;
...
lo_c_2271X = ((lo_a_2267X) * (lo_b_2268X));
...
if ((536870911 < lo_c_2271X)) {
...
else {
if ((lo_c_2271X < 0)) {
as these are all signed quantities the multiplication in
lo_c_2271X = ((lo_a_2267X) * (lo_b_2268X)) if overflowing invokes
undefined behavior according to the C language standard. Thus
the compiler assumes that multiplying two positive signed values
is always positive.
Use -fwrapv if you want twos-complement semantics for signed integers.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907
prev parent reply other threads:[~2009-07-29 18:13 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-07-29 17:35 [Bug c/40907] New: " sperber at deinprogramm dot de
2009-07-29 17:37 ` [Bug c/40907] " sperber at deinprogramm dot de
2009-07-29 17:40 ` pinskia at gcc dot gnu dot org
2009-07-29 17:41 ` pinskia at gcc dot gnu dot org
2009-07-29 17:42 ` sperber at deinprogramm dot de
2009-07-29 18:13 ` rguenth at gcc dot gnu dot org [this message]
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