Date: Sun, 17 Feb 2008 11:08:55 +0100
From: Willy Tarreau <w@1wt.eu>
To: Andrew Pinski <pinskia@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>, linuxppc-dev@ozlabs.org,
       Roel Kluin <12o3l@tiscali.nl>, cbe-oss-dev@ozlabs.org,
       lkml <linux-kernel@vger.kernel.org>
Subject: Re: [Cbe-oss-dev] [PATCH 1/3] Fix Unlikely(x) == y
Message-ID: <20080217100855.GA5135@1wt.eu>
References: <47B70A61.9030306@tiscali.nl> <20080216092552.325e5726@laptopd505.fenrus.org> <20080216173315.GU8953@1wt.eu> <20080216094226.1e8eede1@laptopd505.fenrus.org> <20080216175849.GA25636@1wt.eu> <de8d50360802170145u4bd60f76x9662b66c63bdf41d@mail.gmail.com>
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On Sun, Feb 17, 2008 at 01:45:23AM -0800, Andrew Pinski wrote:
> On Feb 16, 2008 9:58 AM, Willy Tarreau <w@1wt.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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