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

[Guenter Roeck]

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