linux-kernel - Re: [RFC] div64

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Message-Id: <20070306.160507.88477212.davem@davemloft.net>
Date:	Tue, 06 Mar 2007 16:05:07 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] div64_64 support

From: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Tue, 6 Mar 2007 16:00:55 -0800

> On Wed, 7 Mar 2007 00:24:35 +0200
> Sami Farin <7atbggg02@...akemail.com> wrote:
> 
> > On Tue, Mar 06, 2007 at 23:53:49 +0200, Sami Farin wrote:
> > ...
> > > And I found bug in gcc-4.1.2, it gave 0 for ncubic results
> > > when doing 1000 loops test... gcc-4.0.3 works.
> > 
> > Found it.
> > 
> > --- cbrt-test.c~	2007-03-07 00:20:54.735248105 +0200
> > +++ cbrt-test.c	2007-03-07 00:21:03.964864343 +0200
> > @@ -209,7 +209,7 @@
> >  
> >  	__asm__("bsrl %1,%0\n\t"
> >  		"cmovzl %2,%0"
> > -		: "=&r" (r) : "rm" (x), "rm" (-1));
> > +		: "=&r" (r) : "rm" (x), "rm" (-1) : "memory");
> >  	return r+1;
> >  }
> >  
> > Now Linux 2.6 does not have "memory" in fls, maybe it causes
> > some gcc funnies some people are seeing.
> > 
> 
> That code was copy-paste from:
> 	include/asm-x86_64/bitops.h
> 
> So shouldn't both fls() and ffs() be fixed there as well?

This code doesn't modify memory behind gcc's back, so this
"memory" clobber shouldn't really be needed.
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