> On Sep 30, 2013, at 9:20 AM, "David Daney" <ddaney.cavm@xxxxxxxxx> 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@xxxxxxxxxxxxxx>
>>
>> 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
>
>
