From: "J. Bruce Fields" <bfields@fieldses.org>
To: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>, Jeff Garzik <jgarzik@pobox.com>,
Greg KH <greg@kroah.com>, Andrew Morton <akpm@osdl.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Netdev <netdev@oss.sgi.com>
Subject: Re: MSEC_TO_JIFFIES is messed up...
Date: Wed, 12 May 2004 17:39:13 -0400 [thread overview]
Message-ID: <20040512213913.GA16658@fieldses.org> (raw)
In-Reply-To: <Pine.LNX.4.58.0405121400360.11950@bigblue.dev.mdolabs.com>
On Wed, May 12, 2004 at 02:01:58PM -0700, Davide Libenzi wrote:
> On Wed, 12 May 2004, Ingo Molnar wrote:
> > the compiler cannot discard the multiplication and the division from the
> > following:
> >
> > x * 1000 / 1000
> >
> > due to overflows.
>
> $ cat foo.c
>
> int foo(int i) {
>
>
> return i * 1000 / 1000;
> }
If gcc really optimizes that to just the identity function, then surely
that's a gcc bug? Multiplication is left-associative, so i * 1000 /
1000 = (i * 1000) / 1000, but (i * 1000) should be zero for any i
divisible by i^(sizeof(int) - 12).
It shouldn't be able to optimize out the 1000 here for exactly the same
reason it shouldn't be able to optimize out the shifts in, e.g.,
i << 12 >> 12
--Bruce Fields
next prev parent reply other threads:[~2004-05-12 21:39 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20040512020700.6f6aa61f.akpm@osdl.org>
[not found] ` <20040512181903.GG13421@kroah.com>
2004-05-12 18:42 ` MSEC_TO_JIFFIES is messed up Jeff Garzik
2004-05-12 19:33 ` Ingo Molnar
2004-05-12 19:47 ` Valdis.Kletnieks
2004-05-12 19:56 ` Davide Libenzi
2004-05-12 20:07 ` Valdis.Kletnieks
2004-05-12 20:28 ` Ingo Molnar
2004-05-12 20:35 ` Ingo Molnar
2004-05-12 20:50 ` Ingo Molnar
2004-05-12 21:03 ` Valdis.Kletnieks
2004-05-12 21:33 ` Davide Libenzi
2004-05-12 21:07 ` Andrew Morton
2004-05-12 21:17 ` Ingo Molnar
2004-05-12 22:18 ` William Lee Irwin III
2004-05-13 17:38 ` Paul Wagland
2004-05-13 19:11 ` Andrew Morton
2004-05-13 19:19 ` Jeff Garzik
2004-05-13 22:40 ` Andrew Morton
2004-05-13 22:40 ` Andrew Morton
2004-05-13 22:41 ` Andrew Morton
2004-05-13 23:02 ` Jeff Garzik
2004-05-13 19:50 ` Paul Wagland
2004-05-12 23:33 ` Peter Williams
2004-05-12 19:49 ` Davide Libenzi
2004-05-12 20:03 ` Ingo Molnar
2004-05-12 20:18 ` Valdis.Kletnieks
2004-05-12 20:20 ` Andrew Morton
2004-05-12 20:24 ` Jeff Garzik
2004-05-12 20:35 ` Andrew Morton
2004-05-12 20:44 ` Jeff Garzik
2004-05-12 21:03 ` Sridhar Samudrala
2004-05-12 20:32 ` Greg KH
2004-05-12 20:38 ` William Lee Irwin III
2004-05-12 20:47 ` Andrew Morton
2004-05-12 20:58 ` Jeff Garzik
2004-05-12 20:59 ` William Lee Irwin III
2004-05-12 20:55 ` Ingo Molnar
2004-05-12 21:01 ` Davide Libenzi
2004-05-12 21:12 ` Ingo Molnar
2004-05-12 21:40 ` Davide Libenzi
2004-05-12 21:49 ` Zan Lynx
2004-05-12 22:05 ` Roland Dreier
2004-05-12 21:56 ` Zan Lynx
2004-05-12 21:39 ` J. Bruce Fields [this message]
2004-05-12 21:55 ` Andreas Schwab
2004-05-12 22:07 ` J. Bruce Fields
2004-05-16 3:48 ` Chris Wedgwood
2004-05-16 12:10 ` Paul Wagland
2004-05-12 20:17 ` Jeff Garzik
2004-05-12 20:54 ` Bill Rugolsky Jr.
2004-05-12 22:44 ` Bill Rugolsky Jr.
2004-05-12 20:40 Jan Olderdissen
2004-05-12 20:46 ` Jeff Garzik
2004-05-12 20:49 ` Andreas Schwab
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