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 <[email protected]>
---
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
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 <[email protected]>
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/[email protected]/
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
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 <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
-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
Hi Nathan,
On Tue, Aug 20, 2019 at 1:09 AM Nathan Chancellor
<[email protected]> 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 <[email protected]>
>
> 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/[email protected]/
>
> 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
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
> <[email protected]> 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 <[email protected]>
> >
> > 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/[email protected]/
> >
> > 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