From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753995AbdEQSpe (ORCPT ); Wed, 17 May 2017 14:45:34 -0400 Received: from mail-pf0-f182.google.com ([209.85.192.182]:32779 "EHLO mail-pf0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752832AbdEQSpb (ORCPT ); Wed, 17 May 2017 14:45:31 -0400 Date: Wed, 17 May 2017 11:45:29 -0700 From: Matthias Kaehlcke To: Arnd Bergmann Cc: Doug Anderson , Masahiro Yamada , Michal Marek , Linux Kbuild mailing list , "linux-kernel@vger.kernel.org" , Grant Grundler , Greg Hackmann , Michael Davidson Subject: Re: [PATCH 2/2] kbuild: clang: Disable the 'duplicate-decl-specifier' warning Message-ID: <20170517184529.GG141096@google.com> References: <20170421213931.155210-1-mka@chromium.org> <20170421213931.155210-3-mka@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 wrote: > > Hi > > > > On Fri, Apr 21, 2017 at 2:39 PM, Matthias Kaehlcke 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 *" variable for their internal use which triggers a > >> "duplicate 'const' specifier" warning if the is already const > >> qualified. > >> > >> Wording-mostly-from: Michael Davidson > >> Signed-off-by: Matthias Kaehlcke > >> --- > >> 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.