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Date: Sat, 12 Sep 2009 22:49:29 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Daniel Walker <dwalker@fifo99.com>
Cc: Ingo Molnar <mingo@elte.hu>, Julia Lawall <julia@diku.dk>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: apic: convert BUG() to BUG_ON()
Message-ID: <20090912184929.GB4893@lenovo>
References: <1252777220-30796-1-git-send-email-dwalker@fifo99.com> <20090912180527.GA4893@lenovo> <1252779641.28368.81.camel@desktop>
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[Daniel Walker - Sat, Sep 12, 2009 at 11:20:41AM -0700]
| 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. 

I would not relay on "unlikely" much especially in apic code.
Though on some platforms this apic_id_registered returns plain 1
which could (didn't check myself) bring in some optimization
(to be fair -- I can't imagine what could be optimized
 in this particular case, especially since we may have locked
 operations on mem write).

So I consider it as code shrinking on source level only :)

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