Re: [dpdk-dev] Build is broken in dpdk-next-net

["Wiles, Keith" <keith.wiles@intel.com>]

> On Apr 3, 2018, at 5:41 AM, Neil Horman <nhorman@tuxdriver.com> wrote:
> 
> On Mon, Apr 02, 2018 at 09:25:15AM -0700, Stephen Hemminger wrote:
>> On Sat, 31 Mar 2018 14:48:55 -0400
>> Neil Horman <nhorman@tuxdriver.com> wrote:
>> 
>>> On Sat, Mar 31, 2018 at 06:21:41PM +0200, Gaëtan Rivet wrote:
>>>> On Sat, Mar 31, 2018 at 11:27:55AM -0400, Neil Horman wrote:  
>>>>> On Sat, Mar 31, 2018 at 05:09:47PM +0200, Gaëtan Rivet wrote:  
>>>>>> On Sat, Mar 31, 2018 at 09:33:43AM -0400, Neil Horman wrote:  
>>>>>>> On Fri, Mar 30, 2018 at 10:47:09PM +0800, Tonghao Zhang wrote:  
>>>>>>>> I rebuild it on ubuntu 17.10 and cash it. I use the 'RTE_SET_USED' to fix it.
>>>>>>>> 
>>>>>>>> 
>>>>>>>> diff --git a/lib/librte_vhost/fd_man.c b/lib/librte_vhost/fd_man.c
>>>>>>>> index 771675718..f11803191 100644
>>>>>>>> --- a/lib/librte_vhost/fd_man.c
>>>>>>>> +++ b/lib/librte_vhost/fd_man.c
>>>>>>>> @@ -279,7 +279,8 @@ fdset_pipe_read_cb(int readfd, void *dat __rte_unused,
>>>>>>>>                   int *remove __rte_unused)
>>>>>>>> {
>>>>>>>>        char charbuf[16];
>>>>>>>> -       read(readfd, charbuf, sizeof(charbuf));
>>>>>>>> +       int r = read(readfd, charbuf, sizeof(charbuf));
>>>>>>>> +       RTE_SET_USED(r);
>>>>>>>> }
>>>>>>>> 
>>>>>>>> void
>>>>>>>> @@ -319,5 +320,6 @@ fdset_pipe_init(struct fdset *fdset)
>>>>>>>> void
>>>>>>>> fdset_pipe_notify(struct fdset *fdset)
>>>>>>>> {
>>>>>>>> -       write(fdset->u.writefd, "1", 1);
>>>>>>>> +       int r = write(fdset->u.writefd, "1", 1);
>>>>>>>> +       RTE_SET_USED(r);
>>>>>>>> }
>>>>>>>> 
>>>>>>> 
>>>>>>> A better option might be to use _Pragma
>>>>>>> 
>>>>>>> Something like this perhaps
>>>>>>> 
>>>>>>> #define ALLOW_UNUSED(x) \
>>>>>>> _Pragma(push) \
>>>>>>> _Pragma(diagnostic ignored "-Wunused-result") \
>>>>>>> #x;\
>>>>>>> _Pragma(pop)
>>>>>>> 
>>>>>>> This is of course untested, so it probably needs some tweaking, but this method
>>>>>>> avoids the need to declare an additional stack variable, which i don't think can
>>>>>>> be eliminated due to the cast.  I believe that this method should also work
>>>>>>> accross compilers (the gcc and clang compilers support this, and i think the
>>>>>>> intel compiler should as well)
>>>>>>> 
>>>>>>> Neil
>>>>>>> 
>>>>>> 
>>>>>> It would be nice to avoid the definition of a useless variable.
>>>>>> An alternative could be
>>>>>> 
>>>>>>   if (read() < 0) {
>>>>>>       /* Failure here is acceptable for such and such reason. */
>>>>>>   }
>>>>>> 
>>>>>> to ensure all-around compatibility, and the definition or another macro.
>>>>>> Just a suggestion.
>>>>>> 
>>>>> That would be a good alternative, but I think its effectiveness is dependent on
>>>>> when the compiler does with the return value check. Without any code inside the
>>>>> conditional, the compiler may optimize the check out, meaning the warning will
>>>>> still be asserted.  If it doesn't optimize the check out, then you have a
>>>>> useless compare and jump instruction left in the code path.
>>>>> 
>>>>> Best
>>>>> Neil
>>>>> 
>>>> 
>>>> I tested quickly, I see no difference with the three methods:  
>>> 
>>> gcc seems to be sufficiently smart to optimize out the conditional, clang not so
>>> much:
>>> 
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <unistd.h>
>>> 
>>> __attribute__((warn_unused_result))
>>> int wur(void)
>>> {
>>> 	printf("CALLING WUR!\n");
>>>        return read(0, NULL, 0);
>>> }
>>> 
>>> #define UNUSED_RESULT(x) if (x) {}
>>> 
>>> int main(void)
>>> {
>>> 	UNUSED_RESULT(wur());
>>>        return 0;
>>> }
>>> 
>>> [nhorman@neilslaptop ~]$ gcc -g -Wunused-result -Werror ./test.c
>>> [nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
>>> [nhorman@neilslaptop ~]$ cat results
>>> ... 
>>> 000000000040054b <main>:
>>> 
>>> #define UNUSED_RESULT(x) if (x) {}
>>> 
>>> int main(void)
>>> {
>>>  40054b:       55                      push   %rbp
>>>  40054c:       48 89 e5                mov    %rsp,%rbp
>>>        UNUSED_RESULT(wur());
>>>  40054f:       e8 d3 ff ff ff          callq  400527 <wur>
>>>        return 0;
>>>  400554:       b8 00 00 00 00          mov    $0x0,%eax
>>> }
>>>  400559:       5d                      pop    %rbp
>>>  40055a:       c3                      retq
>>>  40055b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>>> 
>>> 
>>> [nhorman@neilslaptop ~]$ clang -g -Wunused-result -Werror ./test.c
>>> [nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
>>> [nhorman@neilslaptop ~]$ cat results 
>>> ...
>>> 0000000000400570 <main>:
>>> }
>>> 
>>> #define UNUSED_RESULT(x) if (x) {}
>>> 
>>> int main(void)
>>> {
>>>  400570:       55                      push   %rbp
>>>  400571:       48 89 e5                mov    %rsp,%rbp
>>>  400574:       48 83 ec 10             sub    $0x10,%rsp
>>>  400578:       c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
>>>        UNUSED_RESULT(wur());
>>>  40057f:       e8 ac ff ff ff          callq  400530 <wur>
>>>  400584:       83 f8 00                cmp    $0x0,%eax
>>>  400587:       0f 84 05 00 00 00       je     400592 <main+0x22>
>>>  40058d:       e9 00 00 00 00          jmpq   400592 <main+0x22>
>>>  400592:       31 c0                   xor    %eax,%eax
>>>        return 0;
>>>  400594:       48 83 c4 10             add    $0x10,%rsp
>>>  400598:       5d                      pop    %rbp
>>>  400599:       c3                      retq
>>>  40059a:       66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
>>> 
>>> 
>>> There is an additional compare and two jump statements there.  I'm sure
>>> eventually most compilers will figure out how to eliminate this, and it might
>>> even do so now with the right optimization flags, but I think its best to just
>>> organize the source such that no conditional branching is implied.  Assuming the
>>> intel compiler supports it (which I think it should, can someone with access to
>>> it confirm), the _Pragma utility is probably the most clear way to do that.
>>> 
>>> Regards
>>> Neil
>> 
>> 
>> Rather than wallpapering over the unused result, why not do real error checking?
>> If the program was run in a non-Linux environment (such as WSL etc), maybe an error
>> could occur. Best to return an error; or at least call rte_exit().
>> 
> Thats a fair point, but I think there are legitimate situations where the return
> value of a function is really a don't care state.  In those, it doesn't hurt to
> have a proscribed method of ignoring said result.

Providing a standard solution for developers to ignore a returned value is a good thing as it clearly provides the reader a hint the value should be ignored as Neil is pointing out. We need to make sure the code is readable and understandable by non-experts of DPDK.
> 
> Neil
> 

Regards,
Keith