* Re: [PATCH kbuild] Makefile.extrawarn: disable -Woverride-init in W=1
2021-04-07 7:14 ` Arnd Bergmann
@ 2021-04-07 14:49 ` Marek Behún
2021-04-07 20:44 ` Marek Behún
1 sibling, 0 replies; 4+ messages in thread
From: Marek Behún @ 2021-04-07 14:49 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Linux Kbuild mailing list, Nathan Chancellor, Masahiro Yamada,
Andrew Lunn, Networking
On Wed, 7 Apr 2021 09:14:29 +0200
Arnd Bergmann <arnd@arndb.de> wrote:
> On Wed, Apr 7, 2021 at 2:24 AM Marek Behún <kabel@kernel.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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH kbuild] Makefile.extrawarn: disable -Woverride-init in W=1
2021-04-07 7:14 ` Arnd Bergmann
2021-04-07 14:49 ` Marek Behún
@ 2021-04-07 20:44 ` Marek Behún
1 sibling, 0 replies; 4+ messages in thread
From: Marek Behún @ 2021-04-07 20:44 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Linux Kbuild mailing list, Nathan Chancellor, Masahiro Yamada,
Andrew Lunn, Networking
On Wed, 7 Apr 2021 09:14:29 +0200
Arnd Bergmann <arnd@arndb.de> wrote:
> On Wed, Apr 7, 2021 at 2:24 AM Marek Behún <kabel@kernel.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
^ permalink raw reply [flat|nested] 4+ messages in thread