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

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Date:   Wed, 7 Apr 2021 22:44:06 +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

Arnd,

since it is possible to create a macro which will expand N times if N
is a preprocessor numeric constant, i.e.
  EXPAND_N_TIMES(3, macro, args...)
would expand to
  macro(1, args...) macro(2, args...) macro(3, args...)

But the first argument to this EXPAND_N_TIMES macro would have to be a
number when preprocessing, so no expression via division, nor enums.

Example:

  The PHY_INTERFACE_MODE_* constants are defined via enum, and
  the last is PHY_INTERFACE_MODE_MAX.

  We could then implement bitmap initializers for PHY_INTERFACE_MODE
  bitmap in the following way:

  enum {
    ...
    PHY_INTERFACE_MODE_MAX
  };

  /* assume PHY_INTERFACE_MODE_MASK has value 50, so 2 longs on 32-bit
   * and 1 long on 64-bit. These have to be direct constant, no expressions
   * allowed. If more PHY_INTERFACE_MODE_* constants are added to the enum
   * above, the following must be changed accordingly. The static_assert
   * below guards against invalid value.
   */

  #define PHY_INTERFACE_BITMAP_LONGS_64  1
  #define PHY_INTERFACE_BITMAP_LONGS_32  2

  /* check if PHY_INTERFACE_BITMAP_LONGS_* have correct values */
  static_assert(PHY_INTERFACE_BITMAP_LONGS_64 ==
                DIV_ROUND_UP(PHY_INTERFACE_MODE_MASK, 64));
  static_assert(PHY_INTERFACE_BITMAP_LONGS_32 ==
                DIV_ROUND_UP(PHY_INTERFACE_MODE_MASK, 32));

  #define DECLARE_PHY_INTERFACE_MASK(name) \
    DECLARE_BITMAP(name, PHY_INTERFACE_MODE_MAX)

  #define INIT_PHY_INTERFACE_MASK(...) \
    INITIALIZE_BITMAP(PHY_INTERFACE_BITMAP_LONGS, ##__VA_ARGS__)

What do you think?

Marek