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

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Message-ID: <20080216102957.56986e03@laptopd505.fenrus.org>
Date:	Sat, 16 Feb 2008 10:29:57 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Willy Tarreau <w@....eu>
Cc:	Roel Kluin <12o3l@...cali.nl>, geoffrey.levand@...sony.com,
	linuxppc-dev@...abs.org, cbe-oss-dev@...abs.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y

On Sat, 16 Feb 2008 18:58:49 +0100
> > If you think unlikely() means something else, we should fix what it
> > maps to towards gcc ;) (to.. be empty ;)
> 
> eventhough the gcc docs say it's just a hint to help the compiler
> optimize the branch it takes by default, I too have noticed that it
> more often does bad than good. Code gets completely reordered and
> even sometimes partially duplicated (especially when the branch is a
> return).
> 
> 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)) ...

this is pure bad since GCC already assumes that NULL checks are unlikely,
and with the unlikely() code needing to normalize stuff... it will generate 
worse code for sure yes.

> 
> often gets coded like this :
> 
>     reg1 = (p == NULL)
>     if (reg1 != 0) ...
> 
> ... which clobbers reg1 for nothing and performs a double test.
> 
> But yes, I assumed that the author considered its use to be
> legitimate (I've not looked at the code). Maybe you're right and it
> should be removed, but in this case we would need a large audit of
> the abuses of unlikely()...

no argument.. how about we start with all the cases where the author just got it 
entirely wrong ... like the ones from this patch ;)


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