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

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Message-ID: <20080218141340.GB667@cs181133002.pp.htv.fi>
Date:	Mon, 18 Feb 2008 16:13:40 +0200
From:	Adrian Bunk <bunk@...nel.org>
To:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
Cc:	Michael Ellerman <michael@...erman.id.au>,
	Roel Kluin <12o3l@...cali.nl>,
	lkml <linux-kernel@...r.kernel.org>, cbe-oss-dev@...abs.org,
	linuxppc-dev@...abs.org, Willy Tarreau <w@....eu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y

On Mon, Feb 18, 2008 at 03:01:35PM +0100, Geert Uytterhoeven wrote:
> On Mon, 18 Feb 2008, Adrian Bunk wrote:
> > 
> > This means it generates faster code with a current gcc for your platform.
> > 
> > But a future gcc might e.g. replace the whole loop with a division
> > (gcc SVN head (that will soon become gcc 4.3) already does 
> > transformations like replacing loops with divisions [1]).
> 
> Hence shouldn't we ask the gcc people what's the purpose of __builtin_expect(),
> if it doesn't live up to its promise?

That's a different issue.

My point here is that we do not know how the latest gcc available in the 
year 2010 might transform this code, and how a likely/unlikely placed 
there might influence gcc's optimizations then.

If this is in hotpath code with a measurable speedup when using 
likely/unlikely with a current gcc then it's worth it.

But otherwise it brings no real advantage today and the longterm effects 
are not predictable.

> With kind regards,
> 
> Geert Uytterhoeven

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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