linux-kernel - Re: [PATCH 2/2] kbuild: clang: Disable the 'duplicate-decl-specifier' warning

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Date:   Tue, 23 May 2017 17:04:19 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Doug Anderson <dianders@...omium.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Grant Grundler <grundler@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Michael Davidson <md@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH 2/2] kbuild: clang: Disable the
 'duplicate-decl-specifier' warning

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@...omium.org> wrote:
> > > Hi
> > >
> > > On Fri, Apr 21, 2017 at 2:39 PM, Matthias Kaehlcke <mka@...omium.org> wrote:
> > >> clang generates plenty of these warnings in different parts of the code.
> > >> They are mostly caused by container_of() and other macros which declare
> > >> a "const <type> *" variable for their internal use which triggers a
> > >> "duplicate 'const' specifier" warning if the <type> is already const
> > >> qualified.
> > >>
> > >> Wording-mostly-from: Michael Davidson <md@...gle.com>
> > >> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > >> ---
> > >>  Makefile | 1 +
> > >>  1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/Makefile b/Makefile
> > >> index df5abf346354..6cd6d428db43 100644
> > >> --- a/Makefile
> > >> +++ b/Makefile
> > >> @@ -704,6 +704,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, unused-variable)
> > >>  KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
> > >>  KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> > >>  KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> > >> +KBUILD_CFLAGS += $(call cc-disable-warning, duplicate-decl-specifier)
> > >>  # Quiet clang warning: comparison of unsigned expression < 0 is always false
> > >>  KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
> > >>  # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > >
> > > It seems like gcc 7 may have the same warning.  Specifically I saw a
> > > patch fly by from Arnd, which you can find in Mark Brown's tree now:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git/commit/?h=topic/rt5614&id=03ba791df98d15d07ea74075122af71e35c7611c
> > >
> > >
> > > +Arnd since he may be trying to solve the same issues?
> > 
> > gcc-7 has a warning option with the same name, and I think I have
> > fixed all the occurrences we got in mainline (some patches my still
> > be in flight). However, it seems that only clang warns about
> > 'const typeof(type)' with 'type' being already const.
> > 
> > I have not looked at clang warnings in a while, how many of these do
> > we get overall (aside from container_of)? We might be able
> > to turn off this particular warning by sprinkling in
> > '_Pragma("clang diagnostic push") _Pragma("clang diagnostic
> > ignored \"-Wduplicate-decl-specifier\"")' inside of the macro
> > (not sure if clang allows it there, gcc-4.4 and earlier I think did
> > not).
> > 
> > It might also be useful to open a bug against clang so they can
> > change it in future releases, as the gcc behavior seems more
> > sensible in this instance.
> 
> I asked our toolchain folks to follow up with the clang devs. They
> asked me for a simple test case, to my suprise clang didn't raise a
> warning when building this:
> 
> static const int x;
> static const typeof(x) y;
> 
> It turns out that the warning is only raised when -std=gnu89 (and
> potentially others) is set, which is the case of the
> kernel. Definitely looks like this should be fixed in clang.

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