Re: [dpdk-dev] error: value computed is not used

["Qiu, Michael" <michael.qiu@intel.com>]

On 12/15/2014 6:55 PM, Thomas Monjalon wrote:
> 2014-12-08 15:26, Wodkowski, PawelX:
>> From: Qiu, Michael
>>> On 2014/12/8 19:00, Wodkowski, PawelX wrote:
>>>>> lib/librte_pmd_enic/enic_main.c: In function 'enic_set_rsskey':
>>>>> lib/librte_pmd_enic/enic_main.c:862:2: error: value computed is not used
>>>>>
>>>>> I dig out that, it was ome issue of  the macros rte_memcpy()
>>>>> #define rte_memcpy(dst, src, n)              \
>>>>>         ((__builtin_constant_p(n)) ?          \
>>>>>         memcpy((dst), (src), (n)) :          \
>>>>>         rte_memcpy_func((dst), (src), (n)))
>>>>>
>>>>> When I use only (n) instead of (__builtin_constant_p(n), it will pass( I
>>>>> know that it was incorrect, just a experiment).
>>>>>
>>>>> But I try to use inline function instead of macros:
>>>>> static inline void * rte_memcpy(void *dst, const void *src, size_t n)
>>>>> {
>>>>>         return __builtin_constant_p(n) ? memcpy(dst, src, n) :
>>>>>                                          rte_memcpy_func(dst, src, n);
>>>>> }
>>>>>
>>>>> It will pass:), and works, this could be one potential workaround fix.
>>>>>
>>>>> Who knows why? The root cause is what?
>>>>>
>>>>> I've no idea about this.
>>>>>
>>>> I got the same issue while ago. I don't remember exactly everything
>>>> but my conclusion was that there was some bug in compiler. I think,
>>>> when 'n' I constant and/or small compiler is inlining memcpy and throwing
>>>> everything else (including returned value). In that case error is not
>>>> produced (I think this is a bug in compiler). In other case it is computing
>>>> some value calling memcpy or rte_ memcpy and you should at least
>>>> explicitly throw it away by casting to void. I like solution with static
>>> Actually, I try to pass "n" as a Int value like 4, it still report this
>>> error :)
>> My workaround was:
>> (void) rte_memcpy(...);
>>
>> But this is only a workaround.
> It's not so bad.
>
>>>> inline but someone else should spoke about possible side effects.
>>> Yes, but as I know inline is better than macros.
> From the GCC manual:
> "
> You may use this built-in function in either a macro or an inline function.
> However, if you use it in an inlined function and pass an argument of the
> function as the argument to the built-in, GCC never returns 1 when you call
> the inline function with a string constant or compound literal and does not
> return 1 when you pass a constant numeric value to the inline function unless
> you specify the -O option.
> "
>
> It seems the "inline fix" cannot be used.

Actually, it can be used and work, as -O option is always specified(I've
test before).

But it should be a issue and not safe.

Thanks,
Michael
> I'm going to send a patch with Pawel's workaround.
>