netdev - Re: [PATCH kbuild] Makefile.extrawarn: disable -Woverride-init in W=1

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Message-ID: <CAK8P3a0_ruZSMv-kLMY7Jja7wq0K3aNNDviYqQPmN-3UayiHaQ@mail.gmail.com>
Date:   Wed, 7 Apr 2021 09:14:29 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Marek Behún <kabel@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH kbuild] Makefile.extrawarn: disable -Woverride-init in W=1

On Wed, Apr 7, 2021 at 2:24 AM Marek Behún <kabel@...nel.org> wrote:
>
> The -Wextra flag enables -Woverride-init in newer versions of GCC.
>
> This causes the compiler to warn when a value is written twice in a
> designated initializer, for example:
>   int x[1] = {
>     [0] = 3,
>     [0] = 3,
>   };
>
> Note that for clang, this was disabled from the beginning with
> -Wno-initializer-overrides in commit a1494304346a3 ("kbuild: add all
> Clang-specific flags unconditionally").
>
> This prevents us from implementing complex macros for compile-time
> initializers.

I think this is generally a useful warning, and it has found a number
of real bugs. I would want this to be enabled in both gcc and clang
by default, and I have previously sent both bugfixes and patches to
disable it locally.

> For example a macro of the form INITIALIZE_BITMAP(bits...) that can be
> used as
>   static DECLARE_BITMAP(bm, 64) = INITIALIZE_BITMAP(0, 1, 32, 33);
> can only be implemented by allowing a designated initializer to
> initialize the same members multiple times (because the compiler
> complains even if the multiple initializations initialize to the same
> value).

We don't have this kind of macro at the moment, and this may just mean
you need to try harder to come up with a definition that only initializes
each member once if you want to add this.

How do you currently define it?

            Arnd