linux-kernel - Re: [PATCH] BUG(): CONFIG_BUG=n version of BUG() should be unreachable()

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Message-Id: <201001051235.45568.arnd@arndb.de>
Date:	Tue, 5 Jan 2010 12:35:44 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Alexander Beregalov <a.beregalov@...il.com>,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	sam@...nborg.org, dhowells@...hat.com
Subject: Re: [PATCH] BUG(): CONFIG_BUG=n version of BUG() should be unreachable()

On Monday 04 January 2010, David Daney wrote:
> Arnd Bergmann wrote:
> > The alternative "do { } while (1)" is not ideal, because an
> > endless loop still requires more code (typically one instruction)
> > than doing nothing at all.
> > 
> 
> Well "do { } while (1)" is exactly the expansion of unreachable() for 
> GCC < 4.5.  Since GCC-4.5 has not been released yet, most people will 
> get these extra looping instructions if you change BUG in this way.

Yes, that is why I wrote the final paragraph, saying
 
> > If there are only than a handful of places that actually cause a warning,
> > using "do { } while (0)" (or __builtin_unreachable where available) and
> > fixing up the code using it might be better.

	Arnd
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