On Wed, 7 Apr 2021 09:14:29 +0200
Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Apr 7, 2021 at 2:24 AM Marek Behún <kabel@xxxxxxxxxx> 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
