[dpdk-dev] Build is broken in dpdk-next-net
Wiles, Keith
keith.wiles at intel.com
Tue Apr 3 15:14:06 CEST 2018
> On Apr 3, 2018, at 5:41 AM, Neil Horman <nhorman at 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 at 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 at neilslaptop ~]$ gcc -g -Wunused-result -Werror ./test.c
>>> [nhorman at neilslaptop ~]$ objdump -d -S a.out > ./results
>>> [nhorman at 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 $0x0,%eax
>>> }
>>> 400559: 5d pop %rbp
>>> 40055a: c3 retq
>>> 40055b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
>>>
>>>
>>> [nhorman at neilslaptop ~]$ clang -g -Wunused-result -Werror ./test.c
>>> [nhorman at neilslaptop ~]$ objdump -d -S a.out > ./results
>>> [nhorman at 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
More information about the dev
mailing list