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

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

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.
I don't know how to deal differently with this constraint.

> 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 :)