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Re: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n

To: David Daney <ddaney.cavm@gmail.com>
Subject: Re: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n
From: "Pinski, Andrew" <Andrew.Pinski@caviumnetworks.com>
Date: Mon, 30 Sep 2013 17:15:53 +0000
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Cc: Ralf Baechle <ralf@linux-mips.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Markos Chandras <Markos.Chandras@imgtec.com>, "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>, "Pinski, Andrew" <Andrew.Pinski@caviumnetworks.com>, John Crispin <blogic@openwrt.org>
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Thread-topic: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n
> On Sep 30, 2013, at 9:20 AM, "David Daney" <ddaney.cavm@gmail.com> wrote:

> 
>> On 09/30/2013 07:56 AM, Ralf Baechle wrote:
>> Lately I received several patches for build issues that only strike if
>> CONFIG_BUG is disabled.  Here's a test case extracted from one of them:
>> 
>> /*
>>  * Definition of BUG taken from asm-generic/bug.h for the CONFIG_BUG=n case
>>  */
>> #define BUG()    do {} while(0)
>> 
>> int foo(int arg)
>> {
>>    int res;
>> 
>>    if (arg == 1)
>>        res = 23;
>>    else if (arg == 2)
>>        res = 42;
>>    else
>>        BUG();
>> 
>>    return res;
>> }
>> 
>> [ralf@h7 ~]$ gcc -O2 -Wall -c bug.c
>> bug.c: In function ‘foo’:
>> bug.c:17:2: warning: ‘res’ may be used uninitialized in this function 
>> [-Wmaybe-uninitialized]
>>   return res;
>>   ^
>> 
>> It's fairly obvious to see what's happening here - GCC doesn't know that
>> the else case can not be reached, thus razorsharply concludes that res
>> may be used uninitialized.
>> 
>> There several locations where MIPS - possibly other architectures as well -
>> is affected by this.
>> 
>> I think the definition of BUG should be changed to something like
>> 
>> #define BUG()    unreachable()
>> 16304
>> unreachable() will depending on the compiler being used, expand either
>> into a call to __builtin_unreachable() or where that function is
>> unavailable, into do {} while (1).
> 
> The *only* reason we have CONFIG_BUG=n is to reduce code size.
> 
> Sticking in that empty loop, negates the entire point.
> 
> IMHO: We should do one of:
> o Make CONFIG_BUG=y mandatory
> o Ignore the warnings.
> o Fix the warning sites so they quit Warning.
> 
> So I don't think the patch is really an improvement over the status quo.

What about using __builtin_unreachable when we can but turn off warnings and 
use do{}while(0) when __builtin_unreachable does not exist?  This seems the 
both worlds.  Newer compilers produce better code with unreachable anyways.

Thanks,
Andrew


> 
> David Daney
>> 
>> __builtin_unreachable() was introduce for GCC 4.5.0.
>> 
>> This means there'd be minor bloat for antique compilers - but probably
>> even better code generation for compilers supporting __builtin_unreachable().
>> 
>>   Ralf
>> 
>> Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
>> 
>>  include/asm-generic/bug.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
>> index 7d10f96..6f78771 100644
>> --- a/include/asm-generic/bug.h
>> +++ b/include/asm-generic/bug.h
>> @@ -108,7 +108,7 @@ extern void warn_slowpath_null(const char *file, const 
>> int line);
>> 
>>  #else /* !CONFIG_BUG */
>>  #ifndef HAVE_ARCH_BUG
>> -#define BUG() do {} while(0)
>> +#define BUG() unreachable()
>>  #endif
>> 
>>  #ifndef HAVE_ARCH_BUG_ON
>> 
>> ----- End forwarded message -----
>> 
>>   Ralf
> 
> 

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