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

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Date:	Mon, 14 Sep 2009 10:43:51 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Daniel Walker <dwalker@...o99.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()

[Maciej W. Rozycki - Sat, Sep 12, 2009 at 11:51:40PM +0100]
| 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
| 

Actually this is quite a good candidate for commit message :)

	-- Cyrill
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