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

[Andrew Pinski]

On Feb 16, 2008 9:58 AM, Willy Tarreau <w@xxxxxx> 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.

-- Pinski
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