diff mbox series

kbuild: enable unused-function warnings for W= build with Clang

Message ID 20190819105138.5053-1-yamada.masahiro@socionext.com
State Superseded
Headers show
Series kbuild: enable unused-function warnings for W= build with Clang | expand

Commit Message

Masahiro Yamada Aug. 19, 2019, 10:51 a.m. UTC 
GCC and Clang have different policy for -Wunused-function; GCC does
not report unused-function warnings at all for the functions marked
as 'static inline'. Clang does report unused-function warnings if they
are defined in source files instead of headers.

We could use Clang for detecting unused functions, but it has been
suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
warning for unused static inline functions").

So, we never notice left-over code if functions in .c files are
marked as inline.

Let's remove __maybe_unused from the inline macro. As always, it is
not a good idea to sprinkle warnings for the normal build. So, these
warnings will be shown for the W= build.

If you contribute to code clean-up, please run "make CC=clang W=1"
and check -Wunused-function warnings. You will find lots of unused
functions.

Some of them are false-positives because the call-sites are disabled
by #ifdef. I do not like to abuse the inline keyword for suppressing
unused-function warnings because it might affect the compiler's
optimization. When I need to fix unused-functions warnings, I prefer
adding #ifdef or __maybe_unused to function definitions.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

---

 include/linux/compiler_types.h | 10 ++--------
 scripts/Makefile.extrawarn     |  1 +
 2 files changed, 3 insertions(+), 8 deletions(-)

-- 
2.17.1

Comments

Nathan Chancellor Aug. 19, 2019, 4:09 p.m. UTC | #1 
On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC does

> not report unused-function warnings at all for the functions marked

> as 'static inline'. Clang does report unused-function warnings if they

> are defined in source files instead of headers.

> 

> We could use Clang for detecting unused functions, but it has been

> suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress

> warning for unused static inline functions").

> 

> So, we never notice left-over code if functions in .c files are

> marked as inline.

> 

> Let's remove __maybe_unused from the inline macro. As always, it is

> not a good idea to sprinkle warnings for the normal build. So, these

> warnings will be shown for the W= build.

> 

> If you contribute to code clean-up, please run "make CC=clang W=1"

> and check -Wunused-function warnings. You will find lots of unused

> functions.

> 

> Some of them are false-positives because the call-sites are disabled

> by #ifdef. I do not like to abuse the inline keyword for suppressing

> unused-function warnings because it might affect the compiler's

> optimization. When I need to fix unused-functions warnings, I prefer

> adding #ifdef or __maybe_unused to function definitions.

> 

> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>


So if I understand everything correctly, this change allows us to start
finding unused static inline functions with clang at W=1 but disables
-Wunused-function by default... I am not sure that is a good tradeoff
as I am pretty sure that W=1 is fairly noisy for clang although I
haven't checked lately. I'd argue most regular developers do not build
with W=1 meaning -Wunused-function generally will not be run with clang
at all, missing stuff like this:

https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/

https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/

Furthermore, per the documemtation [1], -Wno-unused-function will also
disable -Wunneeded-internal-declaration, which can help find bugs like
commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use
mlxplat_mlxcpld_msn201x_items").

[1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function

Is there a way to conditionally remove __maybe_unused from the inline
defintion so that we keep the current behavior but we can still
selectively find potentially unused functions?

Cheers,
Nathan
Kees Cook Aug. 19, 2019, 4:10 p.m. UTC | #2 
On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC does

> not report unused-function warnings at all for the functions marked

> as 'static inline'. Clang does report unused-function warnings if they

> are defined in source files instead of headers.

> 

> We could use Clang for detecting unused functions, but it has been

> suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress

> warning for unused static inline functions").

> 

> So, we never notice left-over code if functions in .c files are

> marked as inline.

> 

> Let's remove __maybe_unused from the inline macro. As always, it is

> not a good idea to sprinkle warnings for the normal build. So, these

> warnings will be shown for the W= build.

> 

> If you contribute to code clean-up, please run "make CC=clang W=1"

> and check -Wunused-function warnings. You will find lots of unused

> functions.

> 

> Some of them are false-positives because the call-sites are disabled

> by #ifdef. I do not like to abuse the inline keyword for suppressing

> unused-function warnings because it might affect the compiler's

> optimization. When I need to fix unused-functions warnings, I prefer

> adding #ifdef or __maybe_unused to function definitions.

> 

> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>


Reviewed-by: Kees Cook <keescook@chromium.org>


-Kees

> ---

> 

>  include/linux/compiler_types.h | 10 ++--------

>  scripts/Makefile.extrawarn     |  1 +

>  2 files changed, 3 insertions(+), 8 deletions(-)

> 

> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h

> index 599c27b56c29..14de8d0162fb 100644

> --- a/include/linux/compiler_types.h

> +++ b/include/linux/compiler_types.h

> @@ -130,10 +130,6 @@ struct ftrace_likely_data {

>  

>  /*

>   * Force always-inline if the user requests it so via the .config.

> - * GCC does not warn about unused static inline functions for

> - * -Wunused-function.  This turns out to avoid the need for complex #ifdef

> - * directives.  Suppress the warning in clang as well by using "unused"

> - * function attribute, which is redundant but not harmful for gcc.

>   * Prefer gnu_inline, so that extern inline functions do not emit an

>   * externally visible function. This makes extern inline behave as per gnu89

>   * semantics rather than c99. This prevents multiple symbol definition errors

> @@ -143,11 +139,9 @@ struct ftrace_likely_data {

>   * (which would break users of __always_inline).

>   */

>  #if !defined(CONFIG_OPTIMIZE_INLINING)

> -#define inline inline __attribute__((__always_inline__)) __gnu_inline \

> -	__maybe_unused notrace

> +#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace

>  #else

> -#define inline inline                                    __gnu_inline \

> -	__maybe_unused notrace

> +#define inline inline                                    __gnu_inline notrace

>  #endif

>  

>  #define __inline__ inline

> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn

> index a74ce2e3c33e..92f542797e03 100644

> --- a/scripts/Makefile.extrawarn

> +++ b/scripts/Makefile.extrawarn

> @@ -70,5 +70,6 @@ KBUILD_CFLAGS += -Wno-initializer-overrides

>  KBUILD_CFLAGS += -Wno-format

>  KBUILD_CFLAGS += -Wno-sign-compare

>  KBUILD_CFLAGS += -Wno-format-zero-length

> +KBUILD_CFLAGS += -Wno-unused-function

>  endif

>  endif

> -- 

> 2.17.1

> 


-- 
Kees Cook
Masahiro Yamada Aug. 19, 2019, 4:58 p.m. UTC | #3 
Hi Nathan,

On Tue, Aug 20, 2019 at 1:09 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>

> On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:

> > GCC and Clang have different policy for -Wunused-function; GCC does

> > not report unused-function warnings at all for the functions marked

> > as 'static inline'. Clang does report unused-function warnings if they

> > are defined in source files instead of headers.

> >

> > We could use Clang for detecting unused functions, but it has been

> > suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress

> > warning for unused static inline functions").

> >

> > So, we never notice left-over code if functions in .c files are

> > marked as inline.

> >

> > Let's remove __maybe_unused from the inline macro. As always, it is

> > not a good idea to sprinkle warnings for the normal build. So, these

> > warnings will be shown for the W= build.

> >

> > If you contribute to code clean-up, please run "make CC=clang W=1"

> > and check -Wunused-function warnings. You will find lots of unused

> > functions.

> >

> > Some of them are false-positives because the call-sites are disabled

> > by #ifdef. I do not like to abuse the inline keyword for suppressing

> > unused-function warnings because it might affect the compiler's

> > optimization. When I need to fix unused-functions warnings, I prefer

> > adding #ifdef or __maybe_unused to function definitions.

> >

> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

>

> So if I understand everything correctly, this change allows us to start

> finding unused static inline functions with clang at W=1 but disables

> -Wunused-function by default... I am not sure that is a good tradeoff

> as I am pretty sure that W=1 is fairly noisy for clang although I

> haven't checked lately. I'd argue most regular developers do not build

> with W=1 meaning -Wunused-function generally will not be run with clang

> at all, missing stuff like this:



Try "git log --grep=W=1"

Some people are making efforts to fix W=1 warnings.
I believe somebody will start to remove unused static inline functions.



>

> https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/

>

> https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/

>

> Furthermore, per the documemtation [1], -Wno-unused-function will also

> disable -Wunneeded-internal-declaration, which can help find bugs like

> commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use

> mlxplat_mlxcpld_msn201x_items").

>

> [1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function



How about this?

KBUILD_CFLAGS += -Wno-unused-function
KBUILD_CFLAGS += -Wunneeded-internal-declaration



> Is there a way to conditionally remove __maybe_unused from the inline

> defintion so that we keep the current behavior but we can still

> selectively find potentially unused functions?


It would be possible by tweaking include/linux/compiler_types.h
but I am not a big fan of uglifying the 'inline' replacement any more.


-- 
Best Regards
Masahiro Yamada
Nathan Chancellor Aug. 20, 2019, 2:25 a.m. UTC | #4 
On Tue, Aug 20, 2019 at 01:58:26AM +0900, Masahiro Yamada wrote:
> Hi Nathan,

> 

> On Tue, Aug 20, 2019 at 1:09 AM Nathan Chancellor

> <natechancellor@gmail.com> wrote:

> >

> > On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:

> > > GCC and Clang have different policy for -Wunused-function; GCC does

> > > not report unused-function warnings at all for the functions marked

> > > as 'static inline'. Clang does report unused-function warnings if they

> > > are defined in source files instead of headers.

> > >

> > > We could use Clang for detecting unused functions, but it has been

> > > suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress

> > > warning for unused static inline functions").

> > >

> > > So, we never notice left-over code if functions in .c files are

> > > marked as inline.

> > >

> > > Let's remove __maybe_unused from the inline macro. As always, it is

> > > not a good idea to sprinkle warnings for the normal build. So, these

> > > warnings will be shown for the W= build.

> > >

> > > If you contribute to code clean-up, please run "make CC=clang W=1"

> > > and check -Wunused-function warnings. You will find lots of unused

> > > functions.

> > >

> > > Some of them are false-positives because the call-sites are disabled

> > > by #ifdef. I do not like to abuse the inline keyword for suppressing

> > > unused-function warnings because it might affect the compiler's

> > > optimization. When I need to fix unused-functions warnings, I prefer

> > > adding #ifdef or __maybe_unused to function definitions.

> > >

> > > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

> >

> > So if I understand everything correctly, this change allows us to start

> > finding unused static inline functions with clang at W=1 but disables

> > -Wunused-function by default... I am not sure that is a good tradeoff

> > as I am pretty sure that W=1 is fairly noisy for clang although I

> > haven't checked lately. I'd argue most regular developers do not build

> > with W=1 meaning -Wunused-function generally will not be run with clang

> > at all, missing stuff like this:

> 

> 

> Try "git log --grep=W=1"

> 

> Some people are making efforts to fix W=1 warnings.

> I believe somebody will start to remove unused static inline functions.


Yes, it could be a good way to get people involved with working with
clang.

> 

> 

> 

> >

> > https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/

> >

> > https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/

> >

> > Furthermore, per the documemtation [1], -Wno-unused-function will also

> > disable -Wunneeded-internal-declaration, which can help find bugs like

> > commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use

> > mlxplat_mlxcpld_msn201x_items").

> >

> > [1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function

> 

> 

> How about this?

> 

> KBUILD_CFLAGS += -Wno-unused-function

> KBUILD_CFLAGS += -Wunneeded-internal-declaration


Yes, that would work.

> 

> 

> 

> > Is there a way to conditionally remove __maybe_unused from the inline

> > defintion so that we keep the current behavior but we can still

> > selectively find potentially unused functions?

> 

> It would be possible by tweaking include/linux/compiler_types.h

> but I am not a big fan of uglifying the 'inline' replacement any more.


I agree that ugly is not ideal but I think it is even less ideal to
weaken the default set of warnings for clang... Merely food for thought
though.

Cheers,
Nathan
diff mbox series

Patch

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 599c27b56c29..14de8d0162fb 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -130,10 +130,6 @@  struct ftrace_likely_data {
 
 /*
  * Force always-inline if the user requests it so via the .config.
- * GCC does not warn about unused static inline functions for
- * -Wunused-function.  This turns out to avoid the need for complex #ifdef
- * directives.  Suppress the warning in clang as well by using "unused"
- * function attribute, which is redundant but not harmful for gcc.
  * Prefer gnu_inline, so that extern inline functions do not emit an
  * externally visible function. This makes extern inline behave as per gnu89
  * semantics rather than c99. This prevents multiple symbol definition errors
@@ -143,11 +139,9 @@  struct ftrace_likely_data {
  * (which would break users of __always_inline).
  */
 #if !defined(CONFIG_OPTIMIZE_INLINING)
-#define inline inline __attribute__((__always_inline__)) __gnu_inline \
-	__maybe_unused notrace
+#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace
 #else
-#define inline inline                                    __gnu_inline \
-	__maybe_unused notrace
+#define inline inline                                    __gnu_inline notrace
 #endif
 
 #define __inline__ inline
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index a74ce2e3c33e..92f542797e03 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -70,5 +70,6 @@  KBUILD_CFLAGS += -Wno-initializer-overrides
 KBUILD_CFLAGS += -Wno-format
 KBUILD_CFLAGS += -Wno-sign-compare
 KBUILD_CFLAGS += -Wno-format-zero-length
+KBUILD_CFLAGS += -Wno-unused-function
 endif
 endif