Date: Sat, 9 Aug 2003 10:13:46 +0200
From: Willy Tarreau <willy@w.ods.org>
To: Jamie Lokier <jamie@shareable.org>
Cc: Albert Cahalan <albert@users.sourceforge.net>,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>,
       chip@pobox.com
Subject: Re: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers
Message-ID: <20030809081346.GC29616@alpha.home.local>
References: <1060087479.796.50.camel@cube> <20030809002117.GB26375@mail.jlokier.co.uk>
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On Sat, Aug 09, 2003 at 01:21:17AM +0100, Jamie Lokier wrote:
> Albert Cahalan wrote:
> > // tell gcc what to expect:   if(unlikely(err)) die(err);
> > #define likely(x)       __builtin_expect(!!(x),1)
> > #define unlikely(x)     __builtin_expect(!!(x),0)
> > #define expected(x,y)   __builtin_expect((x),(y))
> 
> You may want to check that GCC generates the same code as for
> 
> 	#define likely(x)	__builtin_expect((x),1)
> 	#define unlikely(x)	__builtin_expect((x),0)
> 
> I tried this once, and I don't recall the precise result but I do
> recall it generating different code (i.e. not optimal for one of the
> definitions).

anyway, in __builtin_expect(!!(x),0) there is a useless conversion, because
it's totally equivalent to __builtin_expect((x),0) (how could !!x be 0 if x
isn't ?), but I'm pretty sure that the compiler will add the test.

Willy

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