linux-kernel - Re: [PATCH] amdgpu/gmc : fix compile warning

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Date:   Mon, 8 Oct 2018 10:46:28 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Koenig, Christian" <Christian.Koenig@....com>
Cc:     "Deucher, Alexander" <Alexander.Deucher@....com>,
        Peng Hao <peng.hao2@....com.cn>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH] amdgpu/gmc : fix compile warning

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.

Guenter