Messages in this thread | | | From | "Koenig, Christian" <> | Subject | Re: [PATCH] amdgpu/gmc : fix compile warning | Date | Mon, 8 Oct 2018 17:22:24 +0000 |
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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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