Re: [PATCH] amdgpu/gmc : fix compile warning

[Guenter Roeck <linux@roeck-us.net>]

On 10/19/2018 01:53 AM, Daniel Vetter wrote:
> On Mon, Oct 08, 2018 at 06:13:56PM +0000, Koenig, Christian wrote:
>> Am 08.10.2018 um 19:46 schrieb Guenter Roeck:
>>> On Mon, Oct 08, 2018 at 05:22:24PM +0000, Koenig, Christian wrote:
>>>> Am 08.10.2018 um 17:57 schrieb Deucher, Alexander:
>>>>>>>> One thing I found missing in the discussion was the reference to the
>>>>>>>> C standard.
>>>>>>>> The C99 standard states in section 6.7.8 (Initialization) clause 19:
>>>>>>>> "... all
>>>>>>>> subobjects that are not initialized explicitly shall be initialized
>>>>>>>> implicitly the same as objects that have static storage duration".
>>>>>>>> Clause 21 makes further reference to partial initialization,
>>>>>>>> suggesting the same. Various online resources, including the gcc
>>>>>>>> documentation, all state the same. I don't find any reference to a
>>>>>>>> partial initialization which would leave members of a structure
>>>>>>>> undefined. It would be interesting for me to understand how and why
>>>>>>>> this does not apply here.
>>>>>>>>
>>>>>>>> In this context, it is interesting that the other 48 instances of the
>>>>>>>> { { 0 } } initialization in the same driver don't raise similar
>>>>>>>> concerns, nor seemed to have caused any operational problems.
>>>>>>> Feel free to provide patches to replace those with memset().
>>>>>>>
>>>>>> Not me. As I see it, the problem, if it exists, would be a violation of the C
>>>>>> standard. I don't believe hacking around bad C compilers. I would rather
>>>>>> blacklist such compilers.
>>>> Well then you would need to blacklist basically all gcc variants of the
>>>> last decade or so.
>>>>
>>>> Initializing only known members of structures is a perfectly valid
>>>> optimization and well known issue when you then compare the structure
>>>> with memcpy() or use the bytes for hashing or something similar.
>>>>
>>> Isn't that about padding ? That is a completely different issue.
>>
>> Correct, yes. But that is the reason why I recommend using memset() for
>> zero initialization.
>>
>> See we don't know the inner layout of the structure, could be another
>> structure or an union.
>>
>> If it's a structure everything is fine because if you initialize one
>> structure member all other get their default type (whatever that means),
>> but if it's an union.....
>>
>> Not sure if compilers still react allergic to that, but its the status
>> I've learned the hard way when the C99 standard came out and it still
>> seems like people are working around that so I recommend everybody to
>> stick with memset().
> 
> Went boom:
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=108490
> 

What went boom ? This patch wasn't accepted, and I don't immediately see
the correlation of the suggested revert with the rejected patch.

Guenter

> Can we revert?
> 
> Also, can we properly igt this so that intel-gfx-ci could test this before
> it's all fireworks?
> 
> Thanks, Daniel
>