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

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Date:	Sat, 12 Sep 2009 11:20:41 -0700
From:	Daniel Walker <dwalker@...o99.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	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, 2009-09-12 at 22:05 +0400, Cyrill Gorcunov wrote:

> 
> Hi Daniel,
> 
> I believe having a changelog like
> 
> 	Use short form of "if() BUG()" sequence
> 
> would be better perhaps? Since "Coccinelle's BUG_ON semantic patch"
> somehow doesn't describe why it's done.
> 
> Don't get me wrong please. It's trivial and seen from patch
> itself _why_ it's done though changelog doesn't say the same.
> 
> Perhaps I'm too nagging :) Feel free to ignore me.

Not nagging, I wondered myself what the benefit was when I ran
Coccinelle.

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. 

So there are at least two benefits, and I don't see any downside to it.

Daniel

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