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

From: Daniel Vetter <daniel@ffwll.ch>
To: "Koenig, Christian" <Christian.Koenig@amd.com>
Cc: Guenter Roeck <linux@roeck-us.net>,
	Peng Hao <peng.hao2@zte.com.cn>,
	"airlied@linux.ie" <airlied@linux.ie>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"Deucher, Alexander" <Alexander.Deucher@amd.com>,
	Martin Peres <martin.peres@linux.intel.com>
Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
Date: Fri, 19 Oct 2018 10:53:08 +0200	[thread overview]
Message-ID: <20181019085308.GY31561@phenom.ffwll.local> (raw)
In-Reply-To: <4a1faa3b-8c42-b742-9b55-9d2711f7ecc1@amd.com>

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

Can we revert?

Also, can we properly igt this so that intel-gfx-ci could test this before
it's all fireworks?

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch