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In-Reply-To: <8081f60d-ef19-14a5-a589-874afc050d94@roeck-us.net>
From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Fri, 19 Oct 2018 11:30:55 -0400
Message-ID: <CADnq5_PFdgZFfejWKdt6jp-Fc1sHDgX6F8kcCpAO4BmpDHHsEQ@mail.gmail.com>
Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Christian Koenig <Christian.Koenig@amd.com>, peng.hao2@zte.com.cn,
        Dave Airlie <airlied@linux.ie>,
        LKML <linux-kernel@vger.kernel.org>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>,
        "Deucher, Alexander" <Alexander.Deucher@amd.com>,
        Martin Peres <martin.peres@linux.intel.com>
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On Fri, Oct 19, 2018 at 9:31 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> 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.

Daniel accidentally replied to the wrong thread.  Please ignore.

Alex

>
> 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
> >
>
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