From owner-freebsd-i386@FreeBSD.ORG  Thu Feb 20 03:10:01 2014
Return-Path: <owner-freebsd-i386@FreeBSD.ORG>
Delivered-To: freebsd-i386@smarthost.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id AB729EB1
 for <freebsd-i386@smarthost.ysv.freebsd.org>;
 Thu, 20 Feb 2014 03:10:01 +0000 (UTC)
Received: from freefall.freebsd.org (freefall.freebsd.org
 [IPv6:2001:1900:2254:206c::16:87])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (No client certificate requested)
 by mx1.freebsd.org (Postfix) with ESMTPS id 924FD15F4
 for <freebsd-i386@smarthost.ysv.freebsd.org>;
 Thu, 20 Feb 2014 03:10:01 +0000 (UTC)
Received: from freefall.freebsd.org (localhost [127.0.0.1])
 by freefall.freebsd.org (8.14.7/8.14.7) with ESMTP id s1K3A1bG027673
 for <freebsd-i386@freefall.freebsd.org>; Thu, 20 Feb 2014 03:10:01 GMT
 (envelope-from gnats@freefall.freebsd.org)
Received: (from gnats@localhost)
 by freefall.freebsd.org (8.14.8/8.14.8/Submit) id s1K3A1OG027672;
 Thu, 20 Feb 2014 03:10:01 GMT (envelope-from gnats)
Date: Thu, 20 Feb 2014 03:10:01 GMT
Message-Id: <201402200310.s1K3A1OG027672@freefall.freebsd.org>
To: freebsd-i386@FreeBSD.org
Cc: 
From: Bruce Evans <brde@optusnet.com.au>
Subject: Re: i386/186848: CLANG/LLVM code generation bug with optimisation
 on i386
X-BeenThere: freebsd-i386@freebsd.org
X-Mailman-Version: 2.1.17
Precedence: list
Reply-To: Bruce Evans <brde@optusnet.com.au>
List-Id: I386-specific issues for FreeBSD <freebsd-i386.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-i386>,
 <mailto:freebsd-i386-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-i386/>
List-Post: <mailto:freebsd-i386@freebsd.org>
List-Help: <mailto:freebsd-i386-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-i386>,
 <mailto:freebsd-i386-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 20 Feb 2014 03:10:01 -0000

The following reply was made to PR i386/186848; it has been noted by GNATS.

From: Bruce Evans <brde@optusnet.com.au>
To: David Hines <freebsd@dph.fluff.org>
Cc: freebsd-gnats-submit@freebsd.org, freebsd-i386@freebsd.org
Subject: Re: i386/186848: CLANG/LLVM code generation bug with optimisation
 on i386
Date: Thu, 20 Feb 2014 14:06:55 +1100 (EST)

 On Mon, 17 Feb 2014, David Hines wrote:
 
 >> Description:
 > "cc -O -o clang_bug-O clang_bug.c" generates incorrect code on i386, with the attached sample code. Without the "-O", or on an amd64 install the problem does not occur.
 
 This is a bug in clang_bug.c.  It's behaviour is undefined.
 
 > Xunion
 > X{
 > X    int i;
 > X} u;
 > X
 > X
 > Xint
 > Xmain(int argc, char *argv[])
 > X{
 > X    int j = 1;
 > X
 > X    for (u.i = 1;  u.i += u.i;  ++j)
 > X	;
 > X    printf("An int has %d bits\n", j);
 > X
 > X    return 0;
 > X}
 
 Undefined behaviour occurs when the addition overflows.  clang somehow
 notices this (u.i starts as 1, and repeated doublings of it cannot
 make it 0 unless overflow occurs).  The addition does in fact overflow.
 The result can be anything.
 
 Some people (not me) don't like clang not giving the "normal" behaviour
 on overflow.
 
 Bruce