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

From: Masahiro Yamada
Date: Mon Aug 19 2019 - 06:53:00 EST

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

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@xxxxxxxxxxxxx>

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).
-#define inline inline __attribute__((__always_inline__)) __gnu_inline \
- __maybe_unused notrace
+#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace
-#define inline inline __gnu_inline \
- __maybe_unused notrace
+#define inline inline __gnu_inline notrace

#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