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

Uwe Kleine-König
2009-12-16T13:58:58Z

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

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