linux-kernel - Re: [Cbe-oss-dev] [PATCH 1/3] Fix Unlikely(x) == y

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Message-ID: <20080217100855.GA5135@1wt.eu>
Date:	Sun, 17 Feb 2008 11:08:55 +0100
From:	Willy Tarreau <w@....eu>
To:	Andrew Pinski <pinskia@...il.com>
Cc:	Arjan van de Ven <arjan@...radead.org>, linuxppc-dev@...abs.org,
	Roel Kluin <12o3l@...cali.nl>, cbe-oss-dev@...abs.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [Cbe-oss-dev] [PATCH 1/3] Fix Unlikely(x) == y

On Sun, Feb 17, 2008 at 01:45:23AM -0800, Andrew Pinski wrote:
> On Feb 16, 2008 9:58 AM, Willy Tarreau <w@....eu> wrote:
> > Last but not least, gcc 4 tends to emit stupid checks, to the point that I
> > have replaced unlikely(x) with (x) in my code when gcc >= 4 is detected. What
> > I observe is that the following code :
> >
> >     if (unlikely(p == NULL)) ...
> >
> > often gets coded like this :
> >
> >     reg1 = (p == NULL)
> >     if (reg1 != 0) ...
> >
> > ... which clobbers reg1 for nothing and performs a double test.
> 
> This really only can happen in GCC 4.0.x and 4.1.x and cannot happen
> for 4.2 or 4.3 really because of the way __builtin_expect is handled
> for those two.

Happy to know that, thanks for the info Andrew!

Willy

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