Re: [dpdk-dev] [PATCH] ixgbe: fix build with gcc 4.4

[Vlad Zolotarov <vladz@cloudius-systems.com>]

On 04/14/15 18:28, Thomas Monjalon wrote:
> 2015-04-14 18:21, Vlad Zolotarov:
>> On 04/14/15 18:13, Thomas Monjalon wrote:
>>> 2015-04-14 17:59, Vlad Zolotarov:
>>>> On 04/14/15 17:17, Thomas Monjalon wrote:
>>>>> 2015-04-14 16:38, Vlad Zolotarov:
>>>>>> On 04/14/15 16:06, Ananyev, Konstantin wrote:
>>>>>>> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com]
>>>>>>>> On 04/14/15 12:31, Thomas Monjalon wrote:
>>>>>>>>> -	struct rte_eth_dev_info dev_info = { 0 };
>>>>>>>>> +	struct rte_eth_dev_info dev_info = { .max_rx_queues = 0 };
>>>>>>>> Hmmm... Unless I miss something this and one above would zero only a
>>>>>>>> single field - "max_rx_queues"; and would leave the rest uninitialized.
>>>>>>>> The original code intend to zero the whole struct. The alternative to
>>>>>>>> the original lines could be usage of memset().
>>>>>>> As I understand, in that case compiler had to set all non-explicitly initialised members to 0.
>>>>>>> So I think we are ok here.
>>>>>> Yeah, I guess it does zero-initializes the rest
>>>>>> (https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html) however I
>>>>>> don't understand how the above change fixes the error if it complains
>>>>>> about the dev_info.driver_name?
>>>>> As only 1 field is required, I chose the one which should not be removed
>>>>> from this structure in the future.
>>>>>
>>>>>> What I'm trying to say - the proposed fix is completely unclear and
>>>>>> confusing. Think of somebody reading this line in a month from today -
>>>>>> he wouldn't get a clue why is it there, why to explicitly set
>>>>>> max_rx_queues to zero and leave the rest be zeroed automatically... Why
>>>>>> to add such artifacts to the code instead of just zeroing the struct
>>>>>> with a memset() and putting a good clear comment above it explaining why
>>>>>> we use a memset() and not and initializer?
>>>>> We can make it longer yes.
>>>>> I think you agree we should avoid extra lines if not needed.
>>>>> In this case, when reading "= { .field = 0 }", it seems clear our goal
>>>>> is to zero the structure (it is to me).
>>>> I'm sorry but it's not clear to me at all since the common C practice
>>>> for zeroing the struct would be
>>>>
>>>> struct st a = {0};
>>>>
>>>> Like in the lines u are changing. The lines as above are clearly should
>>>> not be commented and are absolutely clear.
>>>> The lines u are adding on the other hand are absolutely unclear and
>>>> confusing outside the gcc bug context. Therefore it should be clearly
>>>> stated so in a form of comment. Otherwise somebody (like myself) may see
>>>> this and immediately fix it back (as it should be).
>>>>
>>>>> I thought it is a basic C practice.
>>>> I doubt that. ;) Explained above.
>>>>
>>>>> You should try "git grep '\.[^ ]\+ *= *0 *}'" to be convinced that we are
>>>>> not going to comment each occurence of this coding style.
>>>>> But it must be explained in the coding style document. Agree?
>>>> OMG! This is awful! I think everybody agrees that this is a workaround
>>>> and has nothing to do with a codding style (it's an opposite to a style
>>>> actually). I don't know where this should be explained, frankly.
>>> Once we assert we want to support this buggy compiler, the workarounds
>>> are automatically parts of the coding style.
>> It'd rather not... ;)
>>
>>> I don't know how to deal differently with this constraint.
>> Add -Wno-missing-braces compilation option for compiler versions below
>> 4.7. U (and me and I guess most other developers) compile DPDK code with
>> a newer compiler thus the code would be properly inspected with these
>> compilers and we may afford to be less restrictive with compilation
>> warnings with legacy compiler versions...
> You're right.
> I will test it and submit a v2.
> Then I could use the above grep command to replace other occurences of this
> workaround.

U read my mind!.. ;)

>
>>>> Getting back to the issue - I'm a bit surprised since I use this kind of
>>>> initializer ({0}) in a C code for quite a long time - long before 2012.
>>>> I'd like to understand what is a problem with this specific gcc version.
>>>> This seems to trivial. I'm surprised CentOS has a gcc version with this
>>>> kind of bugs.
>>> Each day brings its surprise :)
>