From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-help-return-39421-listarch-gcc-help=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 4041 invoked by alias); 4 Dec 2009 20:34:32 -0000
Received: (qmail 4029 invoked by uid 22791); 4 Dec 2009 20:34:30 -0000
X-SWARE-Spam-Status: No, hits=-2.5 required=5.0 	tests=AWL,BAYES_00
X-Spam-Check-By: sourceware.org
Received: from gate.crashing.org (HELO gate.crashing.org) (63.228.1.57)     by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 04 Dec 2009 20:34:25 +0000
Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) 	by gate.crashing.org (8.14.1/8.13.8) with ESMTP id nB4KYLOC002796; 	Fri, 4 Dec 2009 14:34:21 -0600
Received: (from apache@localhost) 	by gate.crashing.org (8.14.1/8.14.1/Submit) id nB4KYKvo002795; 	Fri, 4 Dec 2009 21:34:20 +0100
Received: from 84.105.60.153         (SquirrelMail authenticated user segher)         by gate.crashing.org with HTTP;         Fri, 4 Dec 2009 21:34:20 +0100 (CET)
Message-ID: <55182.84.105.60.153.1259958860.squirrel@gate.crashing.org>
In-Reply-To: <4B1963DC.7080108@redhat.com>
References: <5c6f2a5d0911291151j46dc9d28h568b87f57228b28e@mail.gmail.com>     <87ljhpt5lu.fsf@mid.deneb.enyo.de>     <5c6f2a5d0911291400t63b84bdak3c37b8523040c50c@mail.gmail.com>     <87fx7w4iyi.fsf@mid.deneb.enyo.de>     <fa28b9250911300554q63725492ndcf6952b1ab9c2a@mail.gmail.com>     <5c6f2a5d0911300726r7b902fdeuf905f698c55407f2@mail.gmail.com>     <87my23nasp.fsf@mid.deneb.enyo.de>     <29bd08b70911301639u70eeb87xa49e6c92740817c2@mail.gmail.com>     <5c6f2a5d0912010300k49636f40l693db2c206dfc3d9@mail.gmail.com>     <87ljhm8lga.fsf@mid.deneb.enyo.de> <4B163525.8010104@redhat.com>     <873a3qvaan.fsf@mid.deneb.enyo.de> <4B195E8A.90304@redhat.com>     <87ljhituan.fsf@mid.deneb.enyo.de> <4B1963DC.7080108@redhat.com>
Date: Fri, 04 Dec 2009 21:04:00 -0000
Subject: Re: Efficient detection of signed overflow?
From: "Segher Boessenkool" <segher@kernel.crashing.org>
To: "Andrew Haley" <aph@redhat.com>
Cc: "Florian Weimer" <fw@deneb.enyo.de>, "Mark Dickinson" <dickinsm@gmail.com>,         "Lawrence Crowl" <crowl@google.com>, "me22" <me22.ca@gmail.com>,         "GCC-help" <gcc-help@gcc.gnu.org>
User-Agent: SquirrelMail/1.4.10a-1.fc6
MIME-Version: 1.0
Content-Type: text/plain;charset=iso-8859-1
Content-Transfer-Encoding: 8bit
X-IsSubscribed: yes
Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-help.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-help@gcc.gnu.org>
Sender: gcc-help-owner@gcc.gnu.org
X-SW-Source: 2009-12/txt/msg00108.txt.bz2

>>> The test was, if I recall correctly
>>>
>>>   x = a + b;
>>>   if ((x ^ a) & (x ^ b)) < 0)
>>>
>>> all you have to do is convert everything to unsigned values, then
>>>
>>>   ux = ua + ub;
>>>   if ((ux ^ ua) & (ux ^ ub)) & (unsigned)INT_MIN))
>>>     goto deal_with_overflow;
>>>   // we now know there is no overflow
>>>   x = ux;
>>>
>>> which is exactly the same test as before, but perfectly compliant.
>>
>> The comment is wrong.  The code checks for signed overflow, but the
>> following assignment still overflwos when ux is larger than INT_MAX.
>> So this version is usable exactly under the same circumstances as the
>> first one.
>
> Ahhh, I see.  Hmm, there must be a decent way to do this.

Oh, it needs to be absolutely portable?  Well, you're already
assuming two's complement.

if (ux <= LONG_MAX) /* or whatever the type was */
  x = ux;
else {
  x = ux + LONG_MIN;
  x += LONG_MIN;
}

should do the trick I think?


Segher