linux-kernel - Re: [PATCH] x86: apic: convert BUG() to BUG

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Message-ID: <alpine.LFD.2.00.0909122336330.12448@eddie.linux-mips.org>
Date:	Sat, 12 Sep 2009 23:51:40 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Daniel Walker <dwalker@...o99.com>
cc:	Cyrill Gorcunov <gorcunov@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Julia Lawall <julia@...u.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: apic: convert BUG() to BUG_ON()

On Sat, 12 Sep 2009, Daniel Walker wrote:

> For one it condenses duplicate code (i.e. the if()). If the BUG_ON()
> macro gets updated with something new, all the users get the updates
> automatically. The other thing is your re-using potentially more
> advanced code that's inside the macro. In this case it's fairly trivial,
> 
> #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)
> 
> So we're getting the benefit on the new "unlikely" in the apic code.
> unlikely/likely calls will usually allow the compiler to create smaller,
> and or, more optimized code. 

 For non-x86 platforms the use of the BUG_ON() macro may result in more 
efficient code GCC may not be able to optimise to with if (...) BUG();.  
For example the macro may expand to inline assembly with a conditional 
trap instruction GCC would not emit for an if () clause.  While GCC does 
have a __builtin_trap() intrinsic that could be optimised if alone in a 
conditional block, such usage may not be frequent enough for a dedicated 
optimisation to be provided and build-time efficiency of the compiler does 
matter too, so such an optimisation might be of too questionable a value 
to incur an additional performance hit for the compiler.

 Just a general note on patches of this kind, or to put it short, yes I 
agree it's a good idea.

  Maciej
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