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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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.

> >> 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 :)