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

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

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...

>
>> 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 :)
>