linux-kernel - Re: [PATCH v2] kbuild: enable unused-function warnings for W= build with Clang

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Message-ID: <20190827192811.GA24626@archlinux-threadripper>
Date:   Tue, 27 Aug 2019 12:28:11 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: enable unused-function warnings for W= build
 with Clang

On Tue, Aug 27, 2019 at 07:36:21PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC never
> reports unused-function warnings for 'static inline' functions whereas
> Clang reports them if they are defined in source files instead of
> included headers although it has been suppressed since commit
> abb2ea7dfd82 ("compiler, clang: suppress warning for unused static
> inline functions").
> 
> We often miss to remove unused functions where 'static inline' is used
> in .c files since there is no tool to detect them. Unused code remains
> until somebody notices. For example, commit 075ddd75680f ("regulator:
> core: remove unused rdev_get_supply()").
> 
> Let's remove __maybe_unused from the inline macro to allow Clang to
> start finding unused static inline functions. As always, it is not a
> good idea to sprinkle warnings for the normal build, so I added
> -Wno-unsued-function for no W= build.
> 
> Per the documentation [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"). (pointed out by Nathan Chancellor)
> I added -Wunneeded-internal-declaration to address it.
> 
> 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 is intended to be a hint for the
> compiler's optimization. I prefer __maybe_unused or #ifdef around the
> definition.
> 
> [1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>

I am still not a big fan of this as I think it weakens clang as a
standalone compiler but the change itself looks good so if it is going
in anyways:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

I'm sure Nick would like to weigh in as well before this gets merged.