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

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Message-ID: <47B99D64.3090103@tiscali.nl>
Date:	Mon, 18 Feb 2008 15:59:48 +0100
From:	Roel Kluin <12o3l@...cali.nl>
To:	David Howells <dhowells@...hat.com>
CC:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>,
	Adrian Bunk <bunk@...nel.org>,
	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

David Howells wrote:
> Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com> wrote:
> 
>> Hence shouldn't we ask the gcc people what's the purpose of
>> __builtin_expect(), if it doesn't live up to its promise?
> 
> __builtin_expect() is useful on FRV where you _have_ to give each branch and
> conditional branch instruction a measure of probability whether the branch
> will be taken.
> 
> David

I was wondering whether some of the uses of likely illustrated below are
incorrect or not useful.

x = likely(X) || Y

for ( ... ; likely(...); ... )

while ( likely(X) )

if ( unlikely(X) &&/|| likely(Y) )

if ( X &&/|| unlikely(Y) ) 

return likely(X);

return likely(X) ? a : b;
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