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

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Date:   Mon, 19 Aug 2019 19:51:38 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     linux-kbuild@...r.kernel.org
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Borislav Petkov <bp@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Paul Burton <paul.burton@...s.com>,
        Xiaozhou Liu <liuxiaozhou@...edance.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

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@...ionext.com>
---

 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