From mboxrd@z Thu Jan  1 00:00:00 1970
From: "J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: MSEC_TO_JIFFIES is messed up...
Date: Wed, 12 May 2004 17:39:13 -0400
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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>,
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To: Davide Libenzi <davidel@xmailserver.org>
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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