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

From: Willy Tarreau <[email-masked]>
Date: Sun, 17 Feb 2008 11:08:55 +0100

On Sun, Feb 17, 2008 at 01:45:23AM -0800, Andrew Pinski wrote:
> On Feb 16, 2008 9:58 AM, Willy Tarreau <[email-masked]> 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