From: Michael Veksler <VEKSLER@il.ibm.com>
To: Morten Welinder <mwelinder@gmail.com>
Cc: Joe Buck <Joe.Buck@synopsys.COM>, gcc@gcc.gnu.org, galibert@pobox.com
Subject: Re: signed is undefined and has been since 1992 (in GCC)
Date: Tue, 28 Jun 2005 18:44:00 -0000 [thread overview]
Message-ID: <OF6615BBE2.375F4841-ONC225702E.00662EBD-C225702E.0066FAC5@il.ibm.com> (raw)
In-Reply-To: <118833cc050628095953822bab@mail.gmail.com>
Morten Welinder wrote on 28/06/2005 19:59:10:
> > In particular, a very large number of C and C++ programs are written
> > with the assumptions:
>
> >- signed and unsigned types are modulo, except in loop induction
> > variables where it's bad taste
>
> Well, as demonstrated by INT_MIN/-1, gcc has NEVER fulfilled such
assumptions
> on i86 and, quite likely, neither has or will any other compiler.
> The runtime
> penalty would be too big and hurt performance numbers.
>
> What I believe you can find examples of is that the more restricted claim
of
> "addition and perhaps subtraction of signed numbers is modulo" is being
> assumed. That's cheap since (for 2-complement) signed addition is the
same
> operation as unsigned addition.
>
> Morten
This is problematic as Joe Buck has shown:
; /* int a, b, c; */
; if (b > 0) {
; a = b + c;
; int count=0;
; for (int i = c; i <= a; i++)
; count++;
; some_func(count);
; }
Can be optimized to
; if (b > 0)
; some_func(b+1);
Only if you assume int never overflows. Requiring operator++
to overflow will prohibit this optimization.
next prev parent reply other threads:[~2005-06-28 18:44 UTC|newest]
Thread overview: 119+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-06-28 16:59 Morten Welinder
2005-06-28 17:23 ` Olivier Galibert
2005-06-28 18:44 ` Michael Veksler [this message]
[not found] <2382433.1119938227627.JavaMail.root@dtm1eusosrv72.dtm.ops.eu.uu.net>
2005-06-28 19:44 ` Toon Moene
-- strict thread matches above, loose matches on Subject: below --
2005-06-28 17:41 Paul Schlie
2005-06-28 4:08 Andrew Pinski
2005-06-28 4:20 ` Michael Veksler
2005-06-28 9:49 ` Robert Dewar
2005-06-28 4:34 ` Gabriel Dos Reis
2005-06-28 4:50 ` Andrew Pinski
2005-06-28 5:13 ` Gabriel Dos Reis
2005-06-28 5:34 ` Andrew Pinski
2005-06-28 6:01 ` Gabriel Dos Reis
2005-06-28 9:18 ` Robert Dewar
2005-06-28 11:50 ` Gabriel Dos Reis
2005-06-28 12:07 ` Robert Dewar
2005-06-28 12:33 ` Gabriel Dos Reis
2005-06-28 12:57 ` Robert Dewar
2005-06-28 13:19 ` Gabriel Dos Reis
2005-06-28 22:58 ` Georg Bauhaus
2005-06-28 23:53 ` Gabriel Dos Reis
2005-06-29 0:27 ` Robert Dewar
2005-06-29 0:43 ` Gabriel Dos Reis
2005-06-29 0:48 ` Robert Dewar
2005-06-29 1:14 ` Gabriel Dos Reis
2005-06-29 1:21 ` Diego Novillo
2005-06-29 2:19 ` Marcin Dalecki
2005-06-29 3:13 ` Scott Robert Ladd
2005-06-28 14:24 ` Olivier Galibert
2005-06-28 14:28 ` Jonathan Wilson
2005-06-28 14:42 ` Olivier Galibert
2005-06-28 14:39 ` Dave Korn
2005-06-28 14:52 ` Olivier Galibert
2005-06-28 15:01 ` Robert Dewar
2005-06-28 15:04 ` Andrew Haley
2005-06-28 17:18 ` Olivier Galibert
2005-06-28 17:36 ` Dave Korn
2005-06-28 18:02 ` Olivier Galibert
2005-06-28 18:36 ` Dave Korn
2005-06-28 18:56 ` Gabriel Dos Reis
2005-06-28 19:10 ` Olivier Galibert
2005-06-28 19:13 ` Andrew Pinski
2005-06-28 19:20 ` Robert Dewar
2005-06-28 21:48 ` Joe Buck
2005-06-28 19:25 ` Gabriel Dos Reis
2005-06-28 19:32 ` Robert Dewar
2005-06-28 19:48 ` Gabriel Dos Reis
2005-06-28 20:37 ` Robert Dewar
2005-06-28 20:58 ` Gabriel Dos Reis
2005-06-28 21:57 ` Robert Dewar
2005-06-28 21:44 ` Joe Buck
2005-06-28 21:50 ` Olivier Galibert
2005-06-28 21:59 ` Robert Dewar
2005-06-28 18:52 ` Robert Dewar
2005-06-28 19:17 ` Olivier Galibert
2005-06-28 19:21 ` Robert Dewar
2005-06-28 20:18 ` Paul Koning
2005-06-28 20:24 ` Robert Dewar
2005-06-28 21:41 ` Joe Buck
2005-06-28 21:53 ` Michael Veksler
2005-06-28 23:05 ` Michael Veksler
2005-07-02 17:15 ` Florian Weimer
2005-07-02 18:59 ` Gabriel Dos Reis
2005-07-02 23:20 ` Robert Dewar
2005-07-03 0:07 ` Gabriel Dos Reis
2005-07-03 9:49 ` Robert Dewar
2005-07-02 23:12 ` Nicholas Nethercote
2005-07-02 23:20 ` Robert Dewar
2005-07-03 0:13 ` Gabriel Dos Reis
2005-07-03 9:54 ` Robert Dewar
2005-07-03 10:02 ` Florian Weimer
2005-07-03 10:10 ` Robert Dewar
2005-07-03 12:01 ` Gabriel Dos Reis
2005-07-14 7:21 ` Marc Espie
2005-07-02 17:06 ` Florian Weimer
2005-06-28 17:51 ` Joe Buck
2005-06-28 18:21 ` Gabriel Dos Reis
2005-06-28 18:53 ` Robert Dewar
2005-06-28 18:28 ` Olivier Galibert
2005-06-28 18:38 ` Dave Korn
2005-06-28 18:50 ` Robert Dewar
2005-06-28 19:02 ` Gabriel Dos Reis
2005-06-28 19:17 ` Robert Dewar
2005-06-28 19:43 ` Gabriel Dos Reis
2005-06-28 20:31 ` Robert Dewar
2005-06-28 20:51 ` Gabriel Dos Reis
2005-06-28 20:59 ` Robert Dewar
2005-06-28 21:20 ` Gabriel Dos Reis
2005-06-28 21:27 ` Paul Koning
2005-06-28 21:39 ` Andreas Schwab
2005-06-28 21:35 ` Joe Buck
2005-06-28 22:09 ` Joseph S. Myers
2005-06-28 22:16 ` Falk Hueffner
2005-06-29 6:59 ` Eric Botcazou
2005-06-28 22:19 ` Robert Dewar
2005-06-28 16:42 ` Joe Buck
2005-06-28 17:10 ` Dave Korn
2005-06-28 17:21 ` Joe Buck
2005-06-28 22:41 ` Georg Bauhaus
2005-06-28 14:47 ` Gabriel Dos Reis
2005-06-28 16:38 ` Joe Buck
2005-06-28 21:59 ` Mike Stump
2005-06-28 13:47 ` Gabriel Paubert
2005-06-28 13:52 ` Andrew Pinski
2005-06-28 14:33 ` Robert Dewar
2005-06-28 12:08 ` Robert Dewar
2005-06-28 12:34 ` Gabriel Dos Reis
2005-06-28 7:25 ` Michael Veksler
2005-06-28 16:32 ` Joe Buck
2005-06-28 16:56 ` Joe Buck
2005-06-28 17:03 ` Gabriel Dos Reis
2005-06-28 17:34 ` Joe Buck
2005-06-28 18:09 ` Gabriel Dos Reis
2005-06-28 17:35 ` Diego Novillo
2005-06-28 6:55 ` Steven Bosscher
2005-06-28 7:20 ` Michael Veksler
2005-06-28 7:39 ` Falk Hueffner
2005-06-28 12:08 ` Gabriel Dos Reis
2005-06-28 12:01 ` Gabriel Dos Reis
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