linux-kernel - Re: [PATCH] ARM: Convert BUG() to use unreachable()

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Message-ID: <20091216135849.GC3674@pengutronix.de>
Date:	Wed, 16 Dec 2009 14:58:49 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: Convert BUG() to use unreachable()

Hallo,

On Thu, Dec 10, 2009 at 09:55:51AM -0800, David Daney wrote:
> Russell King - ARM Linux wrote:
>> On Tue, Dec 08, 2009 at 10:55:38AM +0100, Uwe Kleine-König wrote:
>>> Use the new unreachable() macro instead of for(;;);
>>
>> Have you investigated what effect this has on generated code?
>
> Yes.
>
> Pre GCC-4.5 the generated code should be identical as 'unreachable()'  
> just expands to 'for(;;);' in this case.
>
> Post GCC-4.5 the generated code should be smaller.
I don't have a toolchain using gcc 4.5.

What should we do with this patch?  I think in theory the patch is OK.
And for pre gcc-4.5 it should not make any difference as we have in
include/linux/compiler-gcc4.h:

#if __GNUC_MINOR__ >= 5
...
#define unreachable() __builtin_unreachable()
#endif

and in include/linux/compiler.h:

#ifndef unreachable
# define unreachable() do { } while (1)
#endif

So the only impact if that

	do { } while (1)

is used instead of

	for(;;)

.  My toolchain (based on 4.3.2) produces the same object files with and
without the patch.

Best regards
Uwe

-- 
Pengutronix e.K.                              | Uwe Kleine-König            |
Industrial Linux Solutions                    | http://www.pengutronix.de/  |
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