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From: Matthias Kaehlcke <mka@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Masahiro Yamada" <yamada.masahiro@socionext.com>,
	"Doug Anderson" <dianders@chromium.org>,
	"Michal Marek" <mmarek@suse.com>,
	"Linux Kbuild mailing list" <linux-kbuild@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Grant Grundler" <grundler@chromium.org>,
	"Greg Hackmann" <ghackmann@google.com>,
	"Michael Davidson" <md@google.com>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Alexander Potapenko" <glider@google.com>,
	"Bernhard Rosenkränzer" <bero@linaro.org>
Subject: Re: [PATCH 2/2] kbuild: clang: Disable the 'duplicate-decl-specifier' warning
Date: Wed, 21 Jun 2017 09:58:32 -0700	[thread overview]
Message-ID: <20170621165832.GB52338@google.com> (raw)
In-Reply-To: <CAK8P3a1QOz6qQdQux4mFXfwadhosHUgw0Ypwmz5tjZOWcV+0qQ@mail.gmail.com>

El Wed, Jun 21, 2017 at 12:11:55PM +0200 Arnd Bergmann ha dit:

> On Wed, Jun 21, 2017 at 11:11 AM, Masahiro Yamada
> <yamada.masahiro@socionext.com> wrote:
> > 2017-05-24 17:21 GMT+09:00 Arnd Bergmann <arnd@arndb.de>:
> >> On Wed, May 24, 2017 at 2:04 AM, Matthias Kaehlcke <mka@chromium.org> wrote:
> >>> El Wed, May 17, 2017 at 11:45:29AM -0700 Matthias Kaehlcke ha dit:
> >>>> El Wed, May 17, 2017 at 09:35:57AM +0200 Arnd Bergmann ha dit:
> >>>> > On Tue, May 16, 2017 at 11:41 PM, Doug Anderson <dianders@chromium.org> wrote:
> >>> It seems the duplicate-decl-specifier warning targets specifically C89:
> >>>
> >>> "The same type qualifier shall not appear more than once in the same
> >>> specifier list or qualifier list, either directly or via one or more
> >>> typedefs."
> >>>
> >>> C89 (6.5.3)
> >>>
> >>> gcc also raises a warning when '-pedantic' is specified and
> >>> -std=gnu89/c89 (or unspecified), but not with -std=gnu99/c99.
> >>>
> >>> This bug might help to shed more light on this:
> >>> https://bugs.llvm.org/show_bug.cgi?id=32985
> >>
> >> I also notice that neither compiler differentiates between a)
> >>
> >> typedef const int cint;
> >> const cint i;
> >>
> >> and b)
> >> const int i;
> >> const typeof(a) j;
> >>
> >> I would have expected a warning for a) but not b), but both 'clang --std=gnu89'
> >> and 'gcc --pedantic --std=gnu89' warn about both of b as well, and don't warn
> >> for newer standards.
> >>
> >>        Arnd
> >
> >
> >
> >
> > I think we agreed to apply 1/2.
> >
> > How about 2/2?
> >
> > I think we mostly discussed preferable behavior of -Wduplicate-decl-specifier,
> > but we did not come up with an idea to solve the problem for
> > already shipped clang versions.
> > (BTW, we have not defined the minimal supported version of clang yet.)

I think it will have to be a future version. There is still an issue
affecting at least llist_for_each_entry_safe(), where clang optimizes
away a check for check for a NULL pointer. For gcc this optimization
is switched off with -fno-delete-null-pointer-check, clang currently
does not have this flag or an equivalent.

For now a workaround like this is needed for newer kernels:

https://android-git.linaro.org/kernel/hikey-clang.git/commit/?h=android-hikey-linaro-4.9-clang&id=4f3c3c1e7b153e333603be74d786d79bb872e8ff

For arm64 at least one other clang fix is missing, to make
-mgeneral-regs-only consistent with gcc
(https://bugs.llvm.org/show_bug.cgi?id=30792)

> I see that container_of() has been modified in linux-next and no longer adds
> the 'const' keyword, do we actually still need the patch?

There is still (at least) the case of const arrays passed to
MODULE_DEVICE_TABLE.

  reply	other threads:[~2017-06-21 16:58 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-21 21:39 [PATCH 0/2] kbuild: clang: Disable spurious warnings Matthias Kaehlcke
2017-04-21 21:39 ` [PATCH 1/2] kbuild: clang: Disable 'address-of-packed-member' warning Matthias Kaehlcke
2017-04-30 13:59   ` Masahiro Yamada
2017-05-02  1:23     ` Matthias Kaehlcke
2017-05-06 16:52       ` Masahiro Yamada
2017-05-08 23:18         ` Matthias Kaehlcke
2017-05-16  6:31           ` Masahiro Yamada
2017-05-16 21:32   ` Doug Anderson
2017-06-22  1:19     ` Masahiro Yamada
2017-04-21 21:39 ` [PATCH 2/2] kbuild: clang: Disable the 'duplicate-decl-specifier' warning Matthias Kaehlcke
2017-04-30 14:01   ` Masahiro Yamada
2017-05-04 19:50     ` Matthias Kaehlcke
2017-05-08  8:29       ` Masahiro Yamada
2017-05-16 21:41   ` Doug Anderson
2017-05-17  7:35     ` Arnd Bergmann
2017-05-17 18:45       ` Matthias Kaehlcke
2017-05-24  0:04         ` Matthias Kaehlcke
2017-05-24  8:21           ` Arnd Bergmann
2017-06-21  9:11             ` Masahiro Yamada
2017-06-21 10:11               ` Arnd Bergmann
2017-06-21 16:58                 ` Matthias Kaehlcke [this message]
2017-06-21 17:59                   ` Arnd Bergmann
2017-06-21 21:19                     ` Matthias Kaehlcke
2017-07-31 16:35                     ` Matthias Kaehlcke

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