linux-kernel - Re: [PATCH] perf tools: Prevent from BITS_PER

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Message-ID: <4B3C8677.3030606@goop.org>
Date:	Thu, 31 Dec 2009 22:09:43 +1100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] perf tools: Prevent from BITS_PER_LONG redefinition

On 12/31/2009 08:45 AM, Frederic Weisbecker wrote:
>> You can use if() with a constant expression instead of #if.
>>      
>
>
> I did not write this code. But yes you're right, although I
> think CPP is more suitable here because fls() can be called
> from fastpath and this conditional build makes one check less
> and lesser i-cache footprint.
>    

Constant if()s are removed at compile time, so there should be no 
runtime overhead.  Constant if()s are generally preferable to #if 
because the compile will statically check the other code branch, even if 
it never gets executed, which helps prevent it from rotting.

     J

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