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

[David Daney <ddaney.cavm@gmail.com>]

On 09/30/2013 10:15 AM, Pinski, Andrew wrote:
>> 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.
>

Simply not true.

do{}while(0) is a NOP it is no more useful than an ';' statement.  It 
doesn't serve as a magic uninitialized variable hiding mechanism.

David Daney


> 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
>>
>>
>
>