From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Vetter Subject: Re: [PATCH] amdgpu/gmc : fix compile warning Date: Fri, 19 Oct 2018 10:53:08 +0200 Message-ID: <20181019085308.GY31561@phenom.ffwll.local> References: <022e41c0-8465-dc7a-a45c-64187ecd9684@amd.com> <4772f72c-6018-3556-6324-5f49faa00073@roeck-us.net> <4da23fcb-4a94-2695-ad80-929025e84bd2@gmail.com> <74078dc6-ef08-586b-fd58-51eb2c0b5060@roeck-us.net> <20181008174628.GB11442@roeck-us.net> <4a1faa3b-8c42-b742-9b55-9d2711f7ecc1@amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4a1faa3b-8c42-b742-9b55-9d2711f7ecc1@amd.com> Sender: linux-kernel-owner@vger.kernel.org To: "Koenig, Christian" Cc: Guenter Roeck , Peng Hao , "airlied@linux.ie" , "linux-kernel@vger.kernel.org" , "amd-gfx@lists.freedesktop.org" , "dri-devel@lists.freedesktop.org" , "Deucher, Alexander" , Martin Peres List-Id: dri-devel@lists.freedesktop.org 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