linux-kernel - Re: [PATCH] [v2] Kbuild: move to -std=gnu11

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Message-ID: <CAK7LNATbX3TfETQTAr=e5kQLMDSXSn_KetDKTAaeZSq9k_70Uw@mail.gmail.com>
Date:   Tue, 1 Mar 2022 02:07:08 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        llvm@...ts.linux.dev, Jonathan Corbet <corbet@....net>,
        Federico Vaga <federico.vaga@...a.pv.it>,
        Alex Shi <alexs@...nel.org>, Hu Haowen <src.res@...il.cn>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc-tw-discuss@...ts.sourceforge.net,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        greybus-dev@...ts.linaro.org, linux-staging@...ts.linux.dev,
        linux-btrfs@...r.kernel.org, Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] [v2] Kbuild: move to -std=gnu11

On Mon, Feb 28, 2022 at 8:25 PM Mark Rutland <mark.rutland@....com> wrote:
>
> Hi Arnd,
>
> This is great!
>
> On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > During a patch discussion, Linus brought up the option of changing
> > the C standard version from gnu89 to gnu99, which allows using variable
> > declaration inside of a for() loop. While the C99, C11 and later standards
> > introduce many other features, most of these are already available in
> > gnu89 as GNU extensions as well.
> >
> > An earlier attempt to do this when gcc-5 started defaulting to
> > -std=gnu11 failed because at the time that caused warnings about
> > designated initializers with older compilers. Now that gcc-5.1 is the
> > minimum compiler version used for building kernels, that is no longer a
> > concern. Similarly, the behavior of 'inline' functions changes between
> > gnu89 and gnu11, but this was taken care of by defining 'inline' to
> > include __attribute__((gnu_inline)) in order to allow building with
> > clang a while ago.
> >
> > One minor issue that remains is an added gcc warning for shifts of
> > negative integers when building with -Werror, which happens with the
> > 'make W=1' option, as well as for three drivers in the kernel that always
> > enable -Werror, but it was only observed with the i915 driver so far.
> > To be on the safe side, add -Wno-shift-negative-value to any -Wextra
> > in a Makefile.
> >
> > Nathan Chancellor reported an additional -Wdeclaration-after-statement
> > warning that appears in a system header on arm, this still needs a
> > workaround.
>
> FWIW, I had a go at moving to c99 a few weeks ago (to be able to use
> for-loop-declarations in some concurrency primitives), and when I tried, I also
> saw declaration-after-statement warnings when building modpost.c, which is easy
> enough to fix:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=treewide/gnu99&id=505775bd6fd0bc1883f3271f826963066bbdc194
>


I do not understand this statement:

"Usually such warnings are implciitly enabled as part of `-std=gnu89`,
 and in preparation for changing the standard used, this patch explciitly
enales the warnings with `-Wdeclaration-after-statement`, which takes
effect regardless of which version of the C standard is in use."



modpost is already built with -std=gnu89.

If  Wdeclaration-after-statement is implied by gnu89,
why did nobody notice this before?


-- 
Best Regards
Masahiro Yamada