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.
>>>> Anyway, I fixed up the code in our tree (with { }), so I'll leave it
>>>> up to you folks to decide what if anything to do about it.
>>> Well considering the known problems with {} initialization I'm
>>> certainly rejecting all patches which turns memset() into {}.
>>>
>> Please point me to specific instances of this problem.
See here for a good example of how people try to avoid issues:
https://cgit.freedesktop.org/mesa/mesa/commit/?id=9422999e4041d4e984acbd2f44813d5928d20f18
> I think there are a number of places in DC (drivers/gpu/drm/amd/display) where we applied the original proposed solution before realizing that it would only initialize the first element. It would be nice to get them fixed up.
Yes, agree.
Christian.
>
> Alex
>
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