| Message ID | 20210407002450.10015-1-kabel@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [kbuild] Makefile.extrawarn: disable -Woverride-init in W=1 | expand |
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
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
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
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn index d53825503874..cf7bc1eec5e3 100644 --- a/scripts/Makefile.extrawarn +++ b/scripts/Makefile.extrawarn @@ -36,6 +36,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation) KBUILD_CFLAGS += -Wno-missing-field-initializers KBUILD_CFLAGS += -Wno-sign-compare KBUILD_CFLAGS += -Wno-type-limits +KBUILD_CFLAGS += $(call cc-disable-warning, override-init) KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
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. 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). Disable the -Woverride-init flag. Signed-off-by: Marek Behún <kabel@kernel.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Nathan Chancellor <natechancellor@gmail.com> Cc: Masahiro Yamada <masahiroy@kernel.org> Cc: Andrew Lunn <andrew@lunn.ch> Cc: netdev@vger.kernel.org --- scripts/Makefile.extrawarn | 1 + 1 file changed, 1 insertion(+)