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

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Date:   Wed, 7 Apr 2021 16:49:08 +0200
From:   Marek Behún <kabel@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
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, 7 Apr 2021 09:14:29 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> 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

You can look at the current definition in this patch
https://git.kernel.org/pub/scm/linux/kernel/git/kabel/linux.git/commit/?h=marvell10g-updates&id=a4ba5e6563ac4d9e352f55fbae8431339001acf1

And the previous patch, adding variadic-macro.h
https://git.kernel.org/pub/scm/linux/kernel/git/kabel/linux.git/commit/?h=marvell10g-updates&id=d5f8438024b688e96bdd16349f717e5469183362

I fear it won't be possible to expand a macro in such a way to
initialize each member only once, without giving it the number of array
members it has to fill as a constant, i.e. if the bitmap is 100 bits on
a 32 bit machine, it has to fill up to 4 longs, so we would need to
give 4 as an argument:
  ... = INITIALIZE_BITMAP(4, ...);
but DIV_ROUND_UP(100, BITS_PER_LONG) won't work.

Another way around this is to use _Pragma to disable this specific
warning for a specific part of code. Unfortunately it seems that this
_Pragma operator cannot be used withing the designated initializer, it
has to be outside the expression declaring the variable, i.e.
  _Pragma("GCC diagnostic ignored \"-Woverride-init\"")
  ... = INITIALIZE_BITMAP(...);

What I am frustrated about is why doesn't the compiler have the option
to warn only if designated initializer initializes the same member to a
different value...

Marek