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

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Date:   Mon, 19 Aug 2019 09:09:20 -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>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.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: Re: [PATCH] kbuild: enable unused-function warnings for W= build
 with Clang

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@...ionext.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